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Patellofemoral instability

122 citationsUpdated Sep 2026

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Overview

Patellofemoral instability is a multifactorial condition where each underlying component must be considered to determine appropriate treatment [5]. The 2022 JAAOS review series aimed to synthesize current best practices into a concise, algorithmic approach for the clinical and radiological evaluation and nonsurgical management of this condition [1]. While nonsurgical management is the initial step, surgical intervention remains the mainstay of treatment for recurrent patellofemoral instability that fails conservative measures, with procedure selection guided by underlying pathoanatomic mechanisms [2]. A thorough understanding of etiologies, careful preoperative planning, and meticulous surgical techniques are required to optimize clinical outcomes [3]. For the vast majority of adolescents with recurrent patella instability and no prior surgery, isolated medial patellofemoral reconstruction is a rational, safe, efficient, and evidence-based selection [52].

Medial patellofemoral ligament (MPFL) reconstruction is a primary surgical option, with limited but growing evidence supporting excellent functional outcomes [6]. MPFL reconstruction may serve as a more durable surgical treatment option with lower rates of recurrent instability and failure when compared with other patellofemoral stabilization procedures at up to 10 years after surgery [20]. Isolated MPFL reconstruction with gracilis tendon autograft in appropriately selected patients is an effective long-term treatment for recurrent patellofemoral instability with low rates of recurrence [71]. Allografts may represent a feasible alternative to traditional autograft for MPFL reconstruction in selected patients with patellofemoral instability [146], although the current literature lacks clinical evidence on allograft MPFL reconstruction for adolescent patients with recurrent patellofemoral instability [22].

Despite the efficacy of these procedures, the overall reoperation rate after patellofemoral instability surgery remains high, and any reoperation portends worse patient-reported outcomes [26]. The rate of recurrent instability after a primary patellofemoral stabilizing surgery is 6.5% [10]. Medial patellofemoral ligament reconstruction has a high rate of success for patients with patellofemoral instability; however, the complication rate of 26.1% associated with this procedure is not trivial [27]. Many treatment injuries following patellofemoral instability surgery were in principle preventable [15]. Patellofemoral instability surgery for isolated or combined MPFL reconstruction is safe and substantially improves knee function and patient quality of life and activity levels [16]. Good clinical outcomes are demonstrated after patellar cartilage repair with no evidence of inferior results even in complex cases with the need for additional patellofemoral realignment procedures [153]. A comprehensive approach optimizes functional outcomes in patients with complex patellofemoral instability and proximal tibiofibular joint instability [4]. There is much heterogeneity among clinical case series and lack of comparative studies to allow clear indication for the role of isolated or combined surgical reconstruction in patellar stabilization [40]. Appropriately powered randomized controlled trials are needed to better understand what the adequate indications for surgery in patients with patellar instability and clinical outcomes are [150].

Anatomy & Pathophysiology

Bony Anatomy & Kinematics

The patellofemoral joint comprises the patella and the femoral trochlea, sharing a cartilaginous articulating surface [21]. The patella is a sesamoid bone within the extensor mechanism, functioning to optimize the lever arm of the quadriceps around the knee [21]. This lever arm is greatest at 20 to 30 degrees of flexion [110]. Patellar contact with the trochlea progresses with flexion: the inferior articular surface contacts the trochlea at approximately 20 degrees, the midportion at 60 degrees, and the superior portion at 90 degrees [110]. In extreme flexion beyond 120 degrees, the patella articulates only medially and laterally with the femoral condyles, while the quadriceps tendon articulates with the trochlea [110]. A third medial patellar facet often articulates with the lateral aspect of the medial femoral condyle at flexion angles exceeding 90 degrees [110].

Patellofemoral joint reaction forces range from two to five times body weight during activities of daily living, increasing to seven to eight times body weight during squatting with knee flexion up to 120 degrees [110]. The Q angle, defined as the angle between the extended anatomic axis of the femur and the line connecting the center of the patella to the tibial tubercle, influences stability; limbs with larger Q angles exhibit a greater tendency for lateral patellar subluxation [110]. Normal internal rotation of the tibia relative to the femur during flexion centralizes the tibial tubercle, thereby diminishing the Q angle [110]. Patella alta reduces contact and stability in knee extension and early flexion because the patella does not articulate with the sulcus until higher degrees of flexion are reached [60]. This condition increases the extent of knee flexion required for the patella to engage within the trochlear groove [91]. Radiographic measurements confirm that patellar height is significantly greater in patients with patellar instability than in healthy controls [91]. Furthermore, patella alta is associated with abnormal patellar tracking, characterized by greater lateral shift and tilt in subjects with an increased patellar height index [91].

Trochlear morphology significantly influences patellar stability. The mean trochlear depth in patients with normal anatomy is 4.0 mm, differing by sex with a mean of 3.4 mm in women and 4.2 mm in men [91]. The medial facet contributes 37.4% of the width of the cartilage covering the trochlea, while the lateral facet contributes 62.6% [91]. Decreased trochlear depth reduces the effectiveness of the osteochondral restraint, contributing to patellar instability [91]. At flexion angles greater than 30°, the osteochondral constraint of the trochlea provides primary stability to the patellofemoral joint [91]. Abnormalities in lower extremity alignment influence patellofemoral kinematics; measurements of femoral anteversion and genu valgum are greater in knees with patellar instability than in healthy controls [91]. Increased lateralization of the tibial tuberosity relative to the trochlear groove is a common malalignment pattern, and radiographic measurements of such malalignment correlate significantly with increased lateral shift and tilt of the patella [91].

More than 90% of patellofemoral instability events involve lateral patellar translation, with dislocation almost always occurring towards the lateral side [24]. Medial dislocation is rare, seen only in iatrogenic cases following overzealous lateral release or medial transposition of the patellar tendon [19]. Repeated dislocation damages the contiguous articular surfaces of the patella and femoral condyle, potentially flattening the condyle and facilitating further dislocations [19]. Patellofemoral instability is a complex, multifactorial pathology [12]. Identifying and managing specific anatomic or functional deficits is crucial for directing nonsurgical and surgical treatments [11]. Factors predisposing to dislocation include generalized ligamentous laxity, underdevelopment of the lateral femoral condyle, flattening of the intercondylar groove, maldevelopment of the patella, valgus deformity of the knee, external tibial torsion, and primary muscle defects [19]. Additional risk factors include valgus alignment, increased quadriceps angle, excessive femoral anteversion, excessive external tibial torsion, trochlear dysplasia, patella alta, previous patellar dislocation, skeletal immaturity, and disorders affecting ligamentous laxity such as Ehlers-Danlos or Down syndrome [24].

Altered function of the patellar tendon in anterior cruciate ligament deficiency results in altered patellar tracking and patellofemoral cartilage contact [75]. Recent evidence suggests that recurrent patellofemoral instability causes cartilage degeneration [98]. Surgical restoration of the anatomy and biomechanics of the patellofemoral joint may significantly reduce the risk of osteoarthritis by stopping this degenerative process [98]. Patellofemoral joint cartilage lesions are associated with blunt trauma, lateral patella dislocations, or secondary development from abnormal joint loading [21]. Cartilage fibrillation usually occurs on the medial patellar facet or the median ridge, remains confined to the superficial zones, and generally heals spontaneously [58]. This fibrillation is not a precursor of progressive osteoarthritis in later life [58]. Occasionally, the lateral facet is involved in cartilage fibrillation, a condition described as Ficat’s ‘hyperpression zone’ syndrome, which may be progressive [58].

Ligaments & Soft Tissue Restraints

The medial patellofemoral ligament (MPFL) is a fan-shaped ligament extending from the medial femur to the proximal patella, serving as the primary static restraint to lateral patellar translation within the first 30° of knee flexion [91]. Recent cadaver studies have identified additional fibers extending to the quadriceps tendon, leading to nomenclature such as the medial quadriceps tendon femoral ligament or medial patellofemoral complex [91]. The femoral origin of the MPFL is located 9.5 mm distal and 9.5 mm anterior to the adductor tubercle [91]. Radiographically, this origin correlates with a point 1 mm anterior to a line extending along the posterior cortex, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to the posterior aspect of the Blumensaat line [91]. At its patellar insertion, the MPFL merges with the vastus medialis obliquus (VMO) attachment and extends to the medial border of the patella and vastus intermedius tendon [91]. The average width of the MPFL at its insertion is 30 mm, with 57% of fibers attaching to the patella and the remainder to the quadriceps tendon [91]. The average ultimate strength of the MPFL is 178 N [91].

Distal soft-tissue restraints originate on the distal inferomedial patella and comprise the medial patellotibial ligament and medial patellomeniscal ligament [91]. These ligaments play a role in patellar stability at greater knee flexion angles than the MPFL, restricting lateral patellar translation, tilt, and rotation at 90° knee flexion [91]. In 22 cadaver knees, the MPFL and medial patellotibial ligament demonstrated comparable ultimate loads and stiffness, whereas the medial patellomeniscal ligament exhibited lower failure loads and stiffness [105]. Midsubstance failure was the most common type of failure reported in medial stabilizers of the patella [105]. The VMO provides a dynamic restraint to lateral translation; increasing VMO force decreases maximum lateral patellofemoral contact pressure and lateral patellar shift at multiple flexion angles [91]. Administering a motor branch block to the VMO increases lateral patellar shift during knee extension [91]. A significant delay in VMO activation relative to the vastus lateralis has been found in men who later experienced patellofemoral pain, with measurements of lateral patellar translation and tilt correlating with this delay [91]. An increase in the activation ratio of the vastus lateralis to the vastus medialis is associated with increased lateral patellar tilt [91]. Patellofemoral cartilage stresses are sensitive to variations in vastus medialis forces [32].

Deficits in hip strength, particularly in hip abduction and extension, have been identified in patients with patellofemoral pain compared with asymptomatic controls [32]. Generalized ligamentous laxity is associated with an increased risk of patellar instability [32]. The understanding of the complexity of medial patellar stabilizers continues to evolve [64]. The MPFL is the primary soft tissue restraint to lateral patellar translation [135]. In the setting of recurrent dislocation, the MPFL is often injured at the patellar insertion in skeletally immature patients and may be attenuated or deficient in multiple sites [135]. Common injuries occurring during a patellar instability event include a torn medial patellofemoral ligament and/or medial retinaculum, an avulsion fracture of the medial patella at the MPFL insertion, osteochondral injuries resulting in intra-articular loose bodies, and bone bruising of the patella and lateral femoral condyle [24].

Lateral patellofemoral ligament (LPFL) injury most commonly occurs secondary to iatrogenic disruption [80]. Surgical procedures that violate the deep lateral retinacular layer, such as lateral retinacular release, lateral parapatellar arthrotomy, and aggressive lateral dissection, may compromise LPFL integrity [80]. Medial patellar instability attributable to lateral-sided soft-tissue insufficiency has been described without prior surgery [80]. Traumatic or atraumatic LPFL insufficiency can occur in the absence of prior lateral-sided surgery, most frequently in patients with generalized ligamentous laxity, connective tissue disorders, or significant coronal or rotational malalignment [80]. Acute traumatic medial dislocations have been reported in high-level athletes following direct lateral knee impact or noncontact mechanisms resulting in lateral retinacular disruption [80]. Bone contusion patterns involving the lateral patella and anteromedial femoral condyle are characteristic of acute traumatic medial dislocations [80].

Vascular & Neural Anatomy

The patella possesses an extraosseous blood supply consisting of an anastomotic ring that encircles the bone and receives blood from all the geniculate arteries [51]. The intraosseous blood supply of the patella is damaged during resurfacing, theoretically leaving only the superior lateral genicular artery after surgery [51]. The highest concentration of Type IVa free nerve endings in the knee is found in the quadriceps tendon, with the retinacula and the patellar tendon having the second highest concentrations [93].

Pathophysiology & Clinical Presentation

Patellofemoral disorders present as pain or instability, requiring the clinician to differentiate between the two based on history and physical examination [11]. In 15–20% of cases, mostly in children, the first episode of patellar dislocation is followed by recurrent dislocation or subluxation after minimal stress [19]. Recurrent dislocation is partly due to disruption or stretching of medially based ligamentous structures such as the MPFL [19]. In a significant proportion of cases, there is no history of an acute strain, and the initial episode is thought to have occurred spontaneously [19]. In all cases of recurrent dislocation, one or more predisposing factors are often present [19]. Girls are affected more commonly than boys, and recurrent patellar dislocation is often bilateral [19].

The main complaint of recurrent patellar dislocation is that the knee suddenly gives way and the patient falls, which may be accompanied by pain and sometimes the knee getting stuck in flexion [19]. Although the patella always dislocates laterally, the patient may perceive medial displacement because the uncovered medial femoral condyle stands out prominently [19]. There is usually tenderness on the medial side of the joint during an acute dislocation, and the joint becomes swollen, with aspiration potentially revealing a blood-stained effusion [19]. Between attacks, clinical signs are sparse, but the apprehension test is positive [19]. The patient should be carefully examined for features known to predispose to patellar instability [19].

Imaging plays a critical role in evaluation. X-rays may reveal loose bodies from new or old osteochondral fractures, and a lateral view with the knee in slight flexion may show a high-riding patella [19]. Tangential views can be used to measure the sulcus angle and the congruence angle [19]. MRI is helpful and may show signs of previous patellofemoral soft-tissue disruption on the medial side of the knee or trochlear dysplasia [19]. The main objective in evaluating patellofemoral dysfunction is to identify contributing factors and differentiate between pain and instability [32]. A clear description of symptoms helps distinguish patellar subluxation from giving way due to pain or weakness [32]. The mechanism of injury and the chronicity, number, and type of episodes (dislocation versus subluxation) are important historical elements [32]. Knowledge of earlier treatments and surgical procedures aids in determining treatment options [32].

General ligamentous laxity can be identified using criteria such as the Beighton hypermobility score [32]. Assessment of alignment begins with the patient standing, allowing evaluation of Q angle, genu valgus, and lateralization of the tibial tuberosity [32]. Rotational malalignment due to excessive femoral anteversion is often detected in hip range of motion [32]. So-called squinting patellae are present when the patient stands in neutral position and are associated with excessive femoral anteversion and tibial torsion [32]. Patellar mobility is assessed using the glide test, with translation quantified based on patellar quadrants (25% of the width of the patella) [32]. The apprehension sign can be elicited using a manually directed lateral force on the patella [32]. A positive apprehension test and J sign in the setting of increased lateral glide can represent a loss of patellar stability [32]. A J-sign pattern with more than two quadrants of lateral translation has been correlated with patellar instability [32]. Findings should be correlated with the patient’s description of symptoms to determine whether the patellofemoral joint is functionally unstable [32].

Point tenderness is maximal at the site of the retinacular or ligament tear along its course from the medial epicondyle to the medial patella [24]. An effusion may be subtle or large following a patellar instability event [24]. The apprehension test is typically positive in patellofemoral instability [24]. Evaluation of axial and rotational alignment, including the quadriceps angle, is performed during the physical examination [24]. A J-sign may be appreciated during the physical examination [24]. Plain radiographs may appear normal or may show an osteochondral loose body [24]. Trochlear morphology and patellar height and tilt are evaluated on lateral and sunrise views [24]. Ordering an MRI after a dislocation is controversial but is advised if a tense knee effusion is present without

Classification

WARPS/STAID: The WARPS/STAID classification system was introduced for patients with patellofemoral instability and has established validity and reliability in subjects with the condition [44]. Statistically significant evidence demonstrates that certain demographics and pathoanatomies are more prevalent in each of the WARPS and STAID patellofemoral instability subtypes [68].

Segmental Femoral Torsion: A novel classification system for patients with recurrent patellar subluxation and excessive femoral torsion was established based on segmental femoral torsion analysis [87].

MRI-Based Morphology: An MRI-based classification system utilizing objective measurements of trochlear and patellofemoral morphology was presented, emphasizing a 2-measurement combination of sulcus angle and LPFA that yielded excellent diagnostic accuracy for pediatric patellofemoral instability [116].

Wiberg: The Wiberg classification is a potential risk factor for recurrent lateral patellofemoral instability, though its clinical utility in this population remains unclear [170].

Other Considerations: Patellofemoral instability represents a complex pathology with a considerable number of different influencing factors, requiring a detailed understanding of anatomy and kinematics to identify underlying pathologies [12]. Developing a standard battery of reliable and reproducible radiographic measures of patellofemoral instability is a challenge, and imaging parameters currently cannot be used to predict when to operate, what procedure(s) to perform, or how the patient might do [29]. Common physical examination tests for patellofemoral instability demonstrate weak inter-rater reliability [45]. Considering the multifactorial nature of lateral patellofemoral instability, more comprehensive objective measures with robust reference standards and classifications are essential to correctly assess, diagnose, and treat patients [45]. The Banff Patella Instability Instrument (BPII) 2.0 is a reliable and valid instrument for evaluation of patients who present with patellofemoral instability [70]. Female patients with patellofemoral instability have a higher prevalence of anatomical risk factors and worse outcomes after standard stabilization surgery [17]. Anatomical risk factors for patellofemoral instability develop differently between females and males as femoral width increases in pediatric patients [25]. The rotational and mechanical axis of the entire limb plays an important role in the pathology of patellofemoral instability and should be considered in the evaluation of all patients, particularly those who are skeletally immature [152]. Radiographic measures of tracking and alignment changed with knee motion and, in knees with instability symptoms, were most abnormal at lower flexion angles [13]. Patellofemoral instability injury rates vary by sport, sex, and type of exposure (competition vs practice) among high school athletes [34]. Patients with joint hypermobility syndromes experienced significantly increased rates of patellofemoral instability and subsequent surgery compared to a matched cohort [35]. Medial patellofemoral ligament injury patterns vary in skeletally immature patients compared with skeletally mature patients [37]. This comprehensive approach optimizes functional outcomes in patients with complex patellofemoral instability and proximal tibiofibular joint instability [4]. The clinical relevance of the 2019 study is that it provides clinicians with the best currently available evidence on recurrent patellofemoral instability rates after surgical treatment for patellofemoral instability in skeletally immature patients [7]. Surgical management for patellofemoral instability in the presence of patella alta consistently led to improvement in clinical and functional outcomes, regardless of the type of procedure performed [9]. An individualized 'a la carte' approach addressing specific anatomical risk factors may be more appropriate for female patients with patellofemoral instability [17]. Disorders of the patellofemoral joint are a common source of knee pain and can be broadly classified as soft-tissue abnormalities, patellar instability, and patellofemoral arthritis [18]. The purpose of the 2021 review is to provide a framework for the incorporation of bony procedures to address coronal and transverse plane deformities in the evaluation of patellofemoral instability in the pediatric and adolescent patient [76]. Surgical procedures that violate the deep lateral retinacular layer, such as lateral retinacular release, lateral parapatellar arthrotomy, and aggressive lateral dissection during laterally based procedures, may compromise LPFL integrity [80]. Traumatic or atraumatic LPFL insufficiency can occur in the absence of prior lateral-sided surgery, most frequently in patients with generalized ligamentous laxity, connective tissue disorders (e.g., Ehlers-Danlos Syndrome), or significant coronal or rotational malalignment [80]. Acute traumatic medial dislocations have been reported in high-level athletes following direct lateral knee impact or noncontact mechanisms resulting in lateral retinacular disruption and characteristic bone contusion patterns involving the lateral patella and anteromedial femoral condyle [80]. In a retrospective study of 211 isolated MPFL reconstructions, 20% of the cohort had a TT-TG distance of 20 mm and 73% had a patellar tilt of 20°, and all had good to excellent results after isolated MPFL reconstruction [82]. Patellofemoral instability (PFI) is a condition that leads to recurrent dislocation events and is typically secondary to extensor mechanism malalignment and rotational deformities [83]. A tibial tubercle osteotomy (TTO) can be performed to correct malalignment in patellofemoral instability [83]. The BPII 2.0 was successfully adapted into German and is a reliable and valid instrument for evaluation of German-speaking patients who present with patellofemoral instability [70]. The MPFL has thicker concentrated bands running along its proximal and inferior edges: an inferior-straight bundle and a superior oblique bundle [190]. The angle formed between the 2 bundles of the MPFL is approximately 15° [190]. The MPFL is a single continuous structure with no functional difference between the 2 bands [190]. The medial patellotibial ligament (MPTL) and medial patellomeniscal ligament (MPML) are considered secondary restraints of patellar stability [190]. The contributions of the MPTL and MPML against lateral translation increase from 26% in extension to 46% at 90° of flexion [190]. At 90° of flexion, the MPTL and MPML are responsible for 72% of patellar tilt and 92% of patellar rotation [190]. The surgical combination of trochleaplasty and reconstruction of the medial patellofemoral ligament and vastus medialis oblique muscle offers excellent clinical and radiological results [192]. The overall results of the 2014 study showed significant improvement of the Kujala score in patients with Dejour grades C and D dysplasia [192]. Current evidence advocates use of a new pathologic tibial tubercle–posterior cruciate ligament distance threshold in patients with patellar instability [193].

Clinical Presentation

History and Symptoms

Patellofemoral disorders present clinically as either pain or instability [11]. The primary complaint in recurrent patellar dislocation is that the knee suddenly gives way, causing the patient to fall [19]. Patients may report a noncontact twisting mechanism or a contact with a direct blow as the mechanism of injury [24]. In cases of frank dislocation, the patella may spontaneously reduce or may require positioning with the leg in hip flexion and knee extension for reduction [24]. The main objective of the clinical evaluation is to differentiate between patellofemoral pain and instability based on the patient’s history and physical examination [11]. A clear description of symptoms helps distinguish patellar subluxation from knee buckling caused by pain or weakness [32].

The history must capture the mechanism of injury, chronicity, number of episodes, and type of episodes (dislocation versus subluxation) [32]. For recurrent instability, the history should include what activities provoked symptoms, response to prior treatment, severity of pain and swelling, number of instability episodes, and whether knee function was normal between episodes [54]. Patellofemoral instability is classified descriptively as subluxation or dislocation, and as first-time or recurrent [24]. In 15–20% of cases, mostly in children, the first episode of dislocation is followed by recurrent dislocation or subluxation after minimal stress [19]. Dislocation is almost always towards the lateral side, while medial dislocation is seen only in rare iatrogenic cases following overzealous lateral release or medial transposition of the patellar tendon [19]. Although the patella always dislocates laterally, patients may perceive it as displaced medially because the uncovered medial femoral condyle stands out prominently [19]. Girls are affected more commonly than boys, and the condition is often bilateral [19]. Patellofemoral instability injury rates vary by sport, sex, and type of exposure (competition vs practice) [34]. The incidence of bilateral patellofemoral instability was found to be 37.8% [66].

Physical Examination

An effusion may be subtle or large, and a large effusion may raise suspicion of an osteochondral fracture [24, 54]. The apprehension test is typically positive in patients with patellofemoral instability [19, 24]. Patellar mobility is assessed using the glide test, with translation quantified based on patellar quadrants (25% of the width of the patella) [32]. General ligamentous laxity, which is associated with an increased risk of patellar instability, can be identified using criteria such as the Beighton hypermobility score [32]. Assessment of alignment in a standing position includes evaluation of Q angle, genu valgus, and lateralization of the tibial tuberosity [32]. Rotational malalignment due to excessive femoral anteversion can often be detected in hip range of motion, and "squinting patellae" may be present when the patient stands in a neutral position [32]. Dynamic assessment of the lower extremity includes evaluation of quadriceps strength [32]. Deficits in hip strength, particularly in hip abduction and extension, have been identified in patients with patellofemoral pain compared with asymptomatic control patients [32]. The management of patellofemoral instability requires a synthesis of clinical tests, radiological evaluations, and patient expectations while avoiding over-reliance on strict numerical values [62].

Imaging

After a dislocation, ordering an MRI is controversial but is advised if a tense knee effusion is present without radiographic signs of an osteochondral injury, as this effusion may signify a chondral loose body [24]. Evaluation of the tibial tubercle to trochlear groove (TTTG) distance is part of the imaging workup [24]. Evaluation of skeletal maturity is performed if surgical intervention is planned [24]. There are several bony alterations associated with patellofemoral instability, but data did not show a significantly smaller lateral patellar facet or relative patellar width that could facilitate a patellar dislocation [72]. Signs of patellofemoral dysplasia were found in patients with atraumatic patellar dislocation [53]. Subjects who had dislocated their patella had a flattened trochlear groove, whereas normal knees had a more concave groove [102]. The position of the trochlear groove was more medial in patients with patellofemoral instability compared with normal knees [102]. Groove position was more important than shape when predicting which patellae were likely to dislocate [102].

Risk Factors and Predisposing Conditions

Patellofemoral instability is multifactorial, and each component must be considered in determining treatment [5]. Multiple factors contribute to the stability of the patellofemoral joint, including static restraints, dynamic restraints, osteochondral constraints, and lower extremity alignment [11]. Generalized ligamentous laxity is a factor predisposing to dislocation of the patella [19]. Underdevelopment of the lateral femoral condyle and flattening of the intercondylar groove are factors predisposing to dislocation of the patella [19]. Maldevelopment of the patella, which may be unusually small or seated too high, is a factor predisposing to dislocation of the patella [19]. Valgus deformity of the knee is a factor predisposing to dislocation of the patella [19]. External tibial torsion is a factor predisposing to dislocation of the patella [19]. A primary muscle defect is a factor predisposing to dislocation of the patella [19]. Female sex and skeletal immaturity increase the risk for recurrent instability after primary patellofemoral stabilizing surgery [10]. Anatomical risk factors for patellofemoral instability develop differently between females and males as femoral width increases [25]. Age, body mass index, female gender, and patellofemoral cartilage degeneration predict worse patient outcome after patellofemoral instability surgery [16]. Dynamic Q-angle is increased in patients with chronic patellofemoral instability and correlates positively with femoral torsion [100]. Certain demographics and pathoanatomies are more prevalent in each of the WARPS and STAID patellofemoral instability subtypes [68]. Patellar dislocation is a significant risk factor for patellofemoral arthritis, as nearly half of patients have symptoms and radiographic changes consistent with arthritis at 25 years after lateral patellar dislocation [65].

Associated Injuries

Common injuries occurring during a patellar instability event include a torn medial patellofemoral ligament (MPFL) and/or medial retinaculum (femoral-, midsubstance-, or patellar-based) [24]. Avulsion fracture of the medial patella at the insertion of the MPFL is a common injury occurring during a patellar instability event [24]. Osteochondral injuries resulting in intra-articular loose bodies are common injuries occurring during a patellar instability event [24]. Bone bruising of the patella and lateral femoral condyle is a common injury occurring during a patellar instability event [24]. Chondral defects of the medial facet and the crest of the patella are the most common in patients with recurrent patellofemoral instability [134].

Investigations

History and Physical Examination

The primary objective of evaluating patellofemoral dysfunction is to identify contributing factors and differentiate between pain and instability [32]. Historical elements include the mechanism of injury, chronicity, number of episodes, and the type of episode, distinguishing dislocation from subluxation [32]. In recurrent dislocation, the main complaint is that the knee suddenly gives way and the patient falls, which may be accompanied by pain or the knee getting stuck in flexion [19]. Although the patella always dislocates laterally, patients may perceive medial displacement because the uncovered medial femoral condyle stands out prominently [19]. Between attacks, clinical signs are sparse, but the apprehension test is positive [19].

General ligamentous laxity is associated with an increased risk of patellar instability and can be identified using criteria such as the Beighton hypermobility score [32]. Standing alignment assessment includes evaluation of the Q angle, genu valgus, and lateralization of the tibial tuberosity [32]. Excessive femoral anteversion can be detected through hip range of motion assessment and the presence of squinting patellae in a neutral standing position [32]. Evaluation of axial and rotational alignment is performed during physical examination [24]. The quadriceps angle is assessed during physical examination [24]. A J-sign may be appreciated during physical examination [24].

Dynamic assessment includes evaluation of quadriceps strength, as patellofemoral cartilage stresses are sensitive to variations in vastus medialis forces [32]. Deficits in hip strength, particularly in hip abduction and extension, have been identified in patients with patellofemoral pain compared with asymptomatic controls [32]. The apprehension sign is elicited using a manually directed lateral force on the patella [32]. Clinical findings should be correlated with the patient’s symptom description to determine if the patellofemoral joint is functionally unstable [32].

Imaging

Plain radiography: Objective radiographic measurements remain important for characterizing tibial tuberosity position when planning patellar stabilization surgery [177]. Radiographic measures of tracking and alignment change with knee motion and are most abnormal at lower flexion angles in knees with instability symptoms [13].

MRI: Ordering an MRI after a dislocation is controversial but is advised if a tense knee effusion is present without radiographic signs of osteochondral injury, as the effusion may signify a chondral loose body [24]. MRI demonstrates a comparatively good diagnostic performance for the evaluation of first and recurrent lateral patellar dislocations, supporting its recommendation for cartilage assessment after lateral patellar dislocation [159]. In symptomatic cases with inconspicuous conventional MRI imaging, additional MRI imaging in the axial plane at 20° of knee flexion could be beneficial and useful in clinical daily routine [136]. MRI-based patellar tilt measures are an excellent group of measurements for delineating between controls and those with instability [130]. MRI-based indices frequently indicated preserved patellofemoral engagement despite Caton-Deschamps Index-defined patella alta, underscoring their complementary role [169]. Substantial cartilage damage was present in 56% of patients who underwent isolated MPFL reconstruction for recurrent patellar instability, with medial patellar lesions being the most common [179].

CT: The difference between computed tomography and magnetic resonance imaging measurements of tibiofemoral rotation correlates with the degree of knee flexion during image acquisition [49].

Other Considerations: Evaluation of tibial tubercle to trochlear groove (TTTG) distance is part of the imaging workup [24]. There is a high degree of variability in tibial tuberosity to trochlear groove distance across repeat MRI scans of the same knee in the same patient [160]. A TT-TG distance ≥20 mm was the strongest predictor of recurrent instability, but it was relatively insensitive, and sensitivity may be improved by normalizing the TT-TG distance to patient-specific axial and sagittal plane patellofemoral measurements [184]. Evaluation of skeletal maturity is required if surgical intervention is planned [24]. Arthroscopic assessment of patella tracking is accurate, reproducible, and a knee flexion angle of greater than 44° correlates with patellar instability diagnosis [171]. Using dynamic ultrasound assessment, medial patellofemoral distance significantly increases with disruption of the MPFC [186]. The management of patellofemoral instability requires synthesis of clinical tests, radiological evaluations, and patient expectations while avoiding over-reliance on strict numerical values [62]. Establishing baseline threshold values of preoperative PROs, clinical exam findings, and imaging parameters specific to patellofemoral instability is necessary [47].

Treatment

Non-Operative

Non-operative management is the recommended initial strategy for acute patellar dislocation in patients with anatomically normal knees, provided osteochondral fracture is excluded on MRI [172]. Initial care involves immobilization for comfort, rest, ice, compression, and elevation [24]. Physical therapy is initiated to strengthen the injured extremity and address core and hip weakness [24]. A patellar stabilizing brace may be used for activities of daily living and upon return to play [24]. For medial patellofemoral ligament (MPFL) injuries specifically, non-surgical treatment is the treatment of choice for overlap-region injuries, where it achieves better clinical outcomes, lower instability rates, and improved subjective function compared to non-overlap-region injuries [165]. While active rehabilitation improves patient-reported knee function at one year, it does not protect against persistent patellar instability [42].

Operative

Indications: Surgery is indicated for recurrent instability following unsuccessful nonsurgical treatment [11, 24]. Additional indications include osteochondral injury with loose body, and persistent pain or instability despite conservative management [24]. The specific dynamic and anatomic variations contributing to the patient’s instability must be determined before surgery proceeds [11, 14]. Female patients often present with a higher prevalence of anatomical risk factors and worse outcomes after standard stabilization; therefore, an individualized 'a la carte' approach addressing specific anatomical risk factors is appropriate [17]. Non-anatomical MPFL reconstruction and failure to address relevant pathologies are common causes of treatment failure [15].

Surgical Approach / Technique: MPFL reconstruction is the most popular technique for stabilizing the unstable patella and is an accepted method to restore static medial stabilization in patients who have failed nonoperative management [145, 157]. Isolated MPFL reconstruction provides reliable outcomes, good congruous patellofemoral joint mechanics, and allows most patients to engage in recreational sports two years postoperatively, provided severe trochlear dysplasia is absent [8, 33, 108, 138]. In skeletally immature patients, physeal-sparing MPFL reconstruction is the treatment of choice for recurrent instability, replacing older nonanatomic techniques [163]. Combined MPFL and medial quadriceps tendon femoral ligament (MQTFL) reconstruction using semitendinosus allograft yields low failure rates and improved functional outcomes in pediatric patients with first-time dislocation [148]. Modern synthetic material is a safe and effective alternative for MPFL reconstruction [30].

Implant Selection: Isolated MPFL reconstruction with gracilis tendon autograft is an effective long-term treatment for recurrent patellofemoral instability with low recurrence rates in appropriately selected patients [71]. However, the current literature lacks clinical evidence on allograft MPFL reconstruction in adolescent patients with recurrent patellofemoral instability [22]. Horizontal Y-shaped grafts in anatomical two-bundle MPFL reconstruction confirm good restoration of patellar stability and significant improvement of knee function without special complications [36].

Alignment / Balancing Strategy: The tension of the MPFL construct is set with the patella centered in the trochlea with the knee in 30° of flexion [24]. Graft attachment sites on the patella and femur must be checked to produce an isometric graft with the knee in 0° to 60° of flexion [24]. The femoral attachment can be within 1 to 3 mm from the epiphyseal plate; in younger patients, the site is generally just distal to the physis, while in older adolescents, it can be at or proximal to the physis [24]. Femoral drill tunnels should be aimed away from the physis [24]. Post-repair, the patella should have one to two quadrants of medial and lateral translation similar to the native patella [24]. Flexion past 90° will be difficult if the repair is overtensioned [24]. Excessive lateral release can result in iatrogenic medial instability or dislocation [24]. Malpositioning of the femoral tunnel and securing the graft with excessive tension are associated with medial patellofemoral articular overload, iatrogenic medial subluxation, and recurrent lateral instability [145]. Intraoperative fluoroscopy is useful for achieving optimal tunnel positioning [145]. Confirmation of normal patellar translation and full knee range of motion is recommended before final graft fixation to prevent overtensioning [145].

Bony Procedures and Combined Techniques: MPFL reconstruction combined with tibial tubercle osteotomy (TTO) is an effective and reliable treatment for patellofemoral instability [55]. Some authors report better outcome scores when MPFL reconstruction is combined with TTO in patients with a TTTG distance of 17 to 20 mm, patella alta, and a preoperative J sign [145]. Quadriceps/femoral nerve stimulation intraoperatively may help fine-tune the amount of medialization during TTO [24]. Patellar lateral closing-wedge osteotomy serves as a useful component of an 'à la carte' strategy for severe instability [182]. Femoral sulcus deepening trochleoplasty of Lyon reliably protects against recurrent patellar instability, leading to good postoperative clinical outcomes [185]. Patellofemoral arthrodesis is safe and effective for pain relief in extreme patella alta [155].

Other Considerations: Tibial tubercle osteotomy should be avoided in skeletally immature patients to prevent angular deformity [24]. Procedures that compromise an open physis, such as TTO or MPFL graft transphyseal tunnels, are contraindicated in skeletally immature patients [24]. The risk of patellar fracture from violation of the anterior patellar cortex or large-diameter transverse patellar tunnels has led to the development of numerous alternative graft fixation techniques [145]. In a study of MPFL reconstructions, 10 of 29 femoral tunnels were malpositioned [145]. Femoral tunnel malpositioning of greater than 10 mm was associated with postoperative complications but did not necessarily lead to an unsatisfactory outcome [145]. Higher revision stabilization rates are observed in patients with trochlear dysplasia and patella alta after TTO and MPFL reconstruction [26].

Outcomes and Complications: Studies of MPFL reconstruction report success rates of 80% to 96%, with few occurrences of recurrent instability [145]. The largest study to date reported a 4.6% rate of recurrent dislocation after MPFL reconstruction [145]. In that same study, 14% of patients had a positive apprehension sign and 12% had a flexion deficit of 10° or more [145]. A meta-analysis of 25 studies involving 629 knees found a complication rate of 26.1% (164 knees) [145, 27]. The most common complications were recurrent apprehension (52/164 knees), loss of knee flexion (22), painful hardware (19), and patellar fracture (4) [145]. At 10-year minimum follow-up, one-third of patients have patellofemoral arthritis after isolated MPFL reconstruction using gracilis tendon autograft [71]. MPFL reconstruction is superior to active rehabilitation in protecting against further patella dislocations [42].

Rehabilitation and Post-Operative Care: Immediate weight bearing locked in extension is allowed after surgery [24]. Range of motion should be restricted to 0° to 90° for 4 to 6 weeks postoperatively [24]. Early physical therapy is important for quadriceps activation and range of motion [24]. Return to sports or activities may occur when strength and neuromuscular control have returned, generally around 6 months postoperatively [24]. A patellar stabilizing brace may be used for return to play [24].

Special Populations and Complex Cases: In patients with patellofemoral instability after total knee arthroplasty, a thorough understanding of etiologies, careful preoperative planning, and meticulous surgical techniques optimize clinical outcomes [3]. Indications for MPFL reconstruction following total knee arthroplasty must be based on exact analysis of the reasons for instability [114]. A comprehensive approach optimizes functional outcomes in patients with complex patellofemoral instability and proximal tibiofibular joint instability, such as in Ehlers-Danlos syndrome [4]. Not surgically addressing chondral lesions in patients with overlying meniscus coverage can allow for high functional outcomes and minimal pain after patellar stabilization procedures [181]. Salvage of failed patellofemoral arthroplasty due to instability can be achieved with combined MPFL reconstruction, tibial tubercle osteotomy, and vastus medialis obliquus advancement [124].

Complications

Recurrent Instability and Failure Rates: Recurrent patellofemoral instability remains a significant concern following surgical stabilization. In adolescents, 25% experienced recurrent instability after primary patellar stabilization surgery [78]. Recurrent instability rates after MPFL reconstruction are comparable to those seen with other soft tissue realignment techniques [7]. Green's procedure for patellofemoral instability is associated with a high long-term failure rate and low subjective assessments [74]. While active rehabilitation without MPFL reconstruction improves patient-reported knee function at one year, it does not protect against persistent patellar instability [42]. Subjective outcomes and recurrence rates are worse in patients with trochlear dysplasia or an increased TT-TG distance who undergo isolated MPFL reconstruction [156]. A 2014 systematic review indicated that recurrence rates are higher for MPFL repair (26.9%) or medial retinacular repair/plication (16.5%) compared with MPFL reconstruction (6.6%) [156]. Failure to address all contributing risk factors may lead to redislocation, with reported rates of 42% at 5 years and 52% at 10 years postsurgery [189].

General Complication Rates and Risk Factors: The overall complication rate for medial patellofemoral ligament reconstruction is 26.1%, a figure that is not trivial [27]. In patients younger than 21 years, a 2013 study reported a 16.2% complication rate for MPFL reconstruction [156]. Forty-seven percent of these complications were secondary to technical factors and considered preventable [156]. Female sex and bilateral MPFL reconstruction were identified as risk factors for postoperative complications in this younger demographic [156]. Many complications of patellofemoral instability surgery can be avoided by using appropriate surgical techniques [11]. At short- to mid-term follow-up, 80% of service members undergoing tibial tubercle osteotomy for patellofemoral instability returned to military duty, with a moderate perioperative complication and postoperative instability rate [63].

Technical and Iatrogenic Complications: MPFL femoral tunnel malpositioning, specifically anterior and proximal placement, can lead to disabling symptoms including iatrogenic medial patella subluxation, medial patellofemoral overload, and recurrent lateral instability [73]. Patellofemoral instability may also arise from extensor mechanism imbalance, where the lateral retinaculum is too tight or medial soft tissues are too loose [57]. Medial retinacular laxity can occur due to postoperative rupture of the medial capsular repair, caused by a closure that is too tight or by a traumatic event in the early postoperative period [57]. Suboptimally positioned patellar, femoral, or tibial components may also lead to patellofemoral instability [57]. Excessive lateral patellar facet resection, possible due to the normal asymmetry of the facets, can lead to tilting of the patellar component [57]. Lateral placement of the patellar component fails to reproduce the normal median eminence of the patella and can lead to lateral subluxation in extension [57]. Suboptimal position of the tibial component in an internally rotated position increases the Q angle by moving the tibial tubercle laterally, leading to lateral subluxation [57]. Internal rotation and medial translation of the femoral component move the trochlea more medial relative to the extensor mechanism, leading to lateral subluxation [57]. Distal realignment procedures, such as tibial tubercle osteotomy, should be undertaken only with extreme caution because serious functional loss would result if nonunion of the transferred tibial tubercle occurred [57].

Patellar Fracture and Extensor Mechanism Issues: Patellar fracture after total knee arthroplasty is uncommon, occurring in less than 1% of patients [57]. Patellar fracture has been correlated with multiple factors, including excessive patellar resection, vascular compromise secondary to lateral release, patellar maltracking secondary to component malposition, excessive joint line elevation, knee flexion of more than 115 degrees, trauma, thermal necrosis from PMMA polymerization, and revision total knee arthroplasty [57]. In a series of 1146 total knee arthroplasties, a statistically significant association was found between lateral release and patellar fracture [57]. Patellar thickness and sacrifice of the superior lateral geniculate artery were not associated with patellar fracture after lateral release [57]. Nonunion and hardware failure are frequent after internal fixation of patellar fractures after total knee arthroplasty [57]. Periprosthetic patellar fractures associated with an intact extensor mechanism and stable implant (type I) should be treated nonoperatively with a knee immobilizer or cylinder cast for 6 weeks [57]. Displaced periprosthetic patellar fractures with extensor mechanism discontinuity (type II) should be treated operatively [57]. Loose patellar components (type III) should be excised and not replaced because this may impair fracture healing [57]. Patellectomy and extensor mechanism repair are indicated when extreme comminution or poor bone stock preclude stable bony fixation of periprosthetic patellar fractures [57]. Painful patellofemoral instability can occur secondary to peroperative patellar fracture during bone-patellar tendon-bone autograft harvesting for anterior cruciate ligament reconstruction [81].

Arthrosis and Cartilage Degeneration: A single, first-time or infrequently recurring traumatic lateral patellar dislocation is associated with cartilage injury and gradual deterioration over time, potentially leading to generalized knee osteoarthritis [85]. Patellofemoral instability surgery for isolated or combined MPFL reconstruction is safe and substantially improves knee function and patient quality of life and activity levels, with age, body mass index, female gender, and patellofemoral cartilage degeneration predicting worse patient outcome [16]. Progression of tibiofemoral arthritis is the most common reason for failure of second-generation patellofemoral arthroplasty implants [38]. Inflammatory arthropathies are a contraindication to patellofemoral arthroplasty because of progressive tibiofemoral arthritis and pathologic synovitis [38].

Other Considerations: Complications following surgery for patellofemoral instability include recurrent instability, stiffness, patellar fracture, patellofemoral arthrosis, and persistent pain [156]. First-generation patellofemoral arthroplasty designs failed because of narrow trochlear grooves and high constraint, which often produced maltracking, patellar catching, or persistent anterior knee pain [38]. Patellofemoral complications include patellofemoral instability, patellar fracture, patellar component failure, patellar component loosening, patellar clunk syndrome, and extensor mechanism rupture [57].

Recovery

Outcomes and Recurrence Rates: Post-operative patellofemoral instability occurs in 25% of adolescents following primary patellar stabilization surgery [78] and in 25% of adolescents after primary patellofemoral stabilization surgery [167]. Despite these recurrence rates, patellofemoral instability surgery for isolated or combined medial patellofemoral ligament reconstruction is safe and substantially improves knee function and patient quality of life and activity levels [16]. Medial patellofemoral ligament reconstruction for recurrent patellar instability achieves good clinical results, with a good congruous patellofemoral joint and good knee function [33]. Clinical follow-up of two-bundle medial patellofemoral ligament reconstruction confirms the good restoration of the patellar stability and significant improvement of knee function without special complications [36]. At short- to mid-term follow-up, 80% of service members undergoing tibial tubercle osteotomy for patellofemoral instability returned to military duty with significant improvement in pain scores and a moderate perioperative complication and postoperative instability rate [63]. At the latest follow-up, 80.8% of patients undergoing lateral release and proximal realignment had a good or an excellent clinical result, with only one patient having redislocation of the patella [197]. Favorable patient-reported outcomes were achieved in patients who underwent trochleoplasty for the treatment of patellofemoral instability with severe trochlear dysplasia at final follow-up [201].

Complications and Long-Term Joint Health: The complication rate associated with medial patellofemoral ligament reconstruction is 26.1% [27]. Patellofemoral osteoarthritis developed significantly in 43% of patients at midterm follow-up after Insall's proximal realignment [198]. The development of patellofemoral osteoarthritis at 12-year follow-up after stand-alone lateral condyle-elevating trochlear osteotomy did not exceed the findings from other trochleoplasty case series [199]. The association of definite knee osteoarthritis in medial patellofemoral ligament reconstruction with or without lateral release was small in the long-term follow-up [166].

Risk Factors and Predictors: The Recurrent Instability of the Patella Score provides a statistically based model for prediction of long-term recurrence risk after first-time dislocation [79].

Key Evidence

• [L5] The goal of this review series was to synthesize current best practices into a concise, algorithmic approach for the clinical and radiological evaluation and nonsurgical management of patellofemoral instability. [1] (10.5435/jaaos-d-22-00254)
• [L5] Surgical management is the mainstay of treatment for recurrent patellofemoral instability that fails nonsurgical treatment, with selection guided by underlying pathoanatomic mechanisms. [2] (10.5435/jaaos-d-23-00650)
• [L5] A thorough understanding of the etiologies of patellofemoral instability, careful preoperative planning, and meticulous surgical techniques will optimize clinical outcome. [3] (10.1097/01.blo.0000214415.83593.db)
• [L5] This comprehensive approach optimizes functional outcomes in patients with complex patellofemoral instability and PTFJ. [4] (10.1016/j.eats.2025.103902)
• [L5] Recurrent patellofemoral instability is multifactorial, and each component must be considered in determining treatment. [5] (10.5435/00124635-201101000-00002)
• [L1] There is limited but growing evidence that a medial patellofemoral ligament–based surgical approach to patellofemoral instability leads to excellent functional outcomes. [6] (10.1177/0363546509353132)
• [L1] The clinical relevance of this study is that it provides clinicians with the best currently available evidence on recurrent patellofemoral instability rates after surgical treatment for patellofemoral instability in skeletally immature patients. [7] (10.1007/s00167-019-05656-3)
• [L1] Isolated MPFL reconstruction for recurrent patellofemoral instability provides reliable surgical outcomes. [8] (10.1186/s13018-021-02383-9)
• [L1] Surgical management for patellofemoral instability in the presence of patella alta consistently led to improvement in clinical and functional outcomes, regardless of the type of procedure performed. [9] (10.1177/2325967121999642)
• [L3] The rate of recurrent instability after a primary patellofemoral stabilizing surgery is 6.5%. [10] (10.1177/2325967125s00275)
• [L5] Patellofemoral instability represents a complex pathology with a considerable number of different influencing factors, requiring a detailed understanding of anatomy and kinematics to identify underlying pathologies. [12] (10.1007/s00167-018-4860-3)
• [L3] Radiographic measures of tracking and alignment changed with knee motion and, in knees with instability symptoms, were most abnormal at lower flexion angles. [13] (10.1016/j.arthro.2014.04.036)
• [L5] Patellofemoral syndrome is multifactorial, and the true anatomic reason for each patient's individual pathology must be determined before surgery proceeds. [14] (10.1136/jisakos-2018-000260)
• [L2] Many treatment injuries following patellofemoral instability surgery were in principle preventable. [15] (10.1002/ksa.70493)
• [L3] Patellofemoral instability surgery for isolated or combined MPFL reconstruction is safe and substantially improves knee function and patient quality of life and activity levels. [16] (10.1007/s00167-022-06986-5)
• [L5] Female patients with patellofemoral instability have a higher prevalence of anatomical risk factors and worse outcomes after standard stabilization surgery; therefore, an individualized 'a la carte' approach addressing specific anatomical risk factors may be more appropriate. [17] (10.1016/j.jisako.2024.03.014)
• [L5] Disorders of the patellofemoral joint are a common source of knee pain and can be broadly classified as soft-tissue abnormalities, patellar instability, and patellofemoral arthritis. [18] (10.5435/00124635-199701000-00006)
• [L3] MPFL reconstruction may serve as a more durable surgical treatment option with lower rates of recurrent instability and failure when compared with other patellofemoral stabilization procedures at up to 10 years after surgery. [20] (10.1177/03635465231182143)
• [L5] [21] (10.1007/s00167-016-4103-4)
• [L4] Though patellofemoral instability is common in clinical practice, the current literature lacks clinical evidence on allograft MPFL reconstruction. [22] (10.3390/children10050840)
• [L4] The findings demonstrate that anatomical risk factors for patellofemoral instability develop differently between females and males as femoral width increases. [25] (10.1177/2325967126s00032)
• [L4] The overall reoperation rate after patellofemoral instability surgery remains high, and any reoperation portends worse patient-reported outcomes. [26] (10.1007/s00167-021-06784-5)
• [L1] Medial patellofemoral ligament reconstruction has a high rate of success for patients with patellofemoral instability; however, the complication rate of 26.1% associated with this procedure is not trivial. [27] (10.1177/0363546512442330)
• [L5] Developing a standard battery of reliable and reproducible radiographic measures of patellofemoral instability is a challenge, and imaging parameters currently cannot be used to predict when to operate, what procedure(s) to perform, or how the patient might do. [29] (10.1016/j.arthro.2023.05.026)
• [L2] [30] (10.1007/s00167-017-4711-7)
• [L2] MPFLR for recurrent patellar instability could achieve good clinical results, with a good congruous patellofemoral joint and good knee function. [33] (10.1016/j.arthro.2013.01.030)
• [L3] Patellofemoral instability injury rates vary by sport, sex, and type of exposure (competition vs practice). [34] (10.1177/0363546515577786)
• [L3] Patients with joint hypermobility syndromes experienced significantly increased rates of patellofemoral instability and subsequent surgery compared to a matched cohort. [35] (10.1016/j.asmr.2024.100995)
• [L4] Clinical follow-up confirms the good restoration of the patellar stability and significant improvement of knee function without special complications. [36] (10.1007/s00167-014-3005-6)
• [L4] Medial patellofemoral ligament injury patterns vary in skeletally immature patients compared with skeletally mature patients. [37] (10.1177/0363546512457558)
• [L5] However, there is much heterogeneity among clinical case series and lack of comparative studies to allow clear indication for the role of isolated or combined surgical reconstruction in patellar stabilization. [40] (10.1007/s00167-017-4469-y)
• [L1] Active rehabilitation of the knee without MPFL-R improves patient reported knee function after one year, but does not protect against persistent patellar instability. [42] (10.1007/s00167-022-06934-3)
• [L4] In selected cases, patelloplasty of the medial facet of the patella has no particular morbidity at intermediate follow-up. [43] (10.1016/j.otsr.2011.10.008)
• [L2] This study introduced the WARPS/STAID classification system and established both validity and reliability in subjects with patellofemoral instability. [44] (10.1007/s00167-013-2477-0)
• [L4] [45] (10.1016/j.asmr.2021.01.004)
• [Paper] The indication should be restricted to patients with unsatisfactory intraoperative patellofemoral tracking after trochleoplasty. [46] (10.1016/j.eats.2023.09.023)
• [L2] Furthermore, these findings underscore the need to establish the baseline threshold values of preoperative PROs, clinical exam findings, and imaging parameters specific to patellofemoral instability. [47] (10.1177/2325967126s00378)
• [L3] The difference between imaging modalities correlated with the degree of knee flexion during image acquisition. [49] (10.1177/23259671241304754)
• [L5] For the vast majority of adolescents with recurrent patella instability and no prior surgery, isolated medial patellofemoral reconstruction is a rational, safe, efficient, and evidence-based selection. [52] (10.1016/j.arthro.2023.10.040)
• [L4] Signs of patellofemoral dysplasia were found in patients with atraumatic patellar dislocation. [53] (10.1007/s00167-013-2751-1)
• [L5] [54] (10.1177/2325967117750352)
• [L1] MPFLR and MPFLR + TTO are effective and reliable treatments in the setting of patellofemoral instability. [55] (10.1186/s12891-024-07722-5)
• [L5] The management of patellofemoral instability is a delicate balance between art and science, requiring synthesis of clinical tests, radiological evaluations, and patient expectations while avoiding over-reliance on strict numerical values. [62] (10.1302/0301-620x.105b12.bjj-2023-1026)
• [L4] At short- to mid-term follow-up, 80% of service members undergoing TTO for patellofemoral instability returned to military duty with significant improvement in pain scores and a moderate perioperative complication and postoperative instability rate. [63] (10.1055/s-0037-1603639)
• [L5] The understanding of the complexity of the medial patellar stabilizers continues to evolve. [64] (10.1007/s00167-018-5266-y)
• [L3] Patellar dislocation is a significant risk factor for patellofemoral arthritis, as nearly half of patients have symptoms and radiographic changes consistent with arthritis at 25 years after lateral patellar dislocation. [65] (10.1177/0363546516680604)
• [L3] The incidence of bilateral patellofemoral instability was found to be 37.8%. [66] (10.1177/2325967123s00271)
• [L3] This study demonstrated statistically significant evidence that certain demographics and pathoanatomies are more prevalent in each of the WARPS and STAID patellofemoral instability subtypes. [68] (10.1007/s00167-016-4346-0)
• [L3] The BPII 2.0 was successfully adapted into German and is a reliable and valid instrument for evaluation of German-speaking patients who present with patellofemoral instability. [70] (10.1007/s00167-017-4673-9)
• [L4] Isolated MPFLr with gracilis tendon autograft in appropriately selected patients is an effective long-term treatment for recurrent patellofemoral instability with low rates of recurrence. [71] (10.1016/j.arthro.2022.07.021)
• [L3] There are several bony alterations associated with patellofemoral instability, but our data did not show a significantly smaller lateral patellar facet or relative patellar width that could facilitate a patellar dislocation. [72] (10.1007/s00167-019-05660-7)
• [L4] MPFL femoral tunnel malpositioning (anterior and proximal) can lead to disabling symptoms including iatrogenic medial patella subluxation, medial patellofemoral overload, and recurrent lateral instability. [73] (10.1016/j.arthro.2011.02.014)
• [L4] Green's procedure to stabilize patellofemoral instabilities results in a high failure rate on the long-term and low subjective assessments. [74] (10.1007/s00402-019-03322-4)
• [L4] The altered function of the patellar tendon in anterior cruciate ligament deficiency resulted in an altered patellar tracking and patellofemoral cartilage contact. [75] (10.1177/0363546508314404)
• [L5] The purpose of this paper is to provide a review of the evaluation of patellofemoral instability in the pediatric and adolescent patient with a specific focus on the contributions of coronal and transverse plane deformities, and to provide a framework for the incorporation of bony procedures to address these issues. [76] (10.3390/jcm10143035)
• [L3] In conclusion, 25% of adolescents experienced recurrent instability after primary patellar stabilization surgery. [78] (10.1177/2325967121s00795)
• [L3] This long-term risk stratification holds significant potential clinical utility for determination of patients who are at high risk for recurrent instability after primary patellar dislocation. [79] (10.1016/j.arthro.2018.09.017)
• [L4] [80] (10.2106/jbjs.rvw.26.00085)
• [L5] The originality of the present case report lies in the unusual and late presentation of the complication and the use of sagittal patellar closing osteotomy to correct the patellar mal-union underlying the patellofemoral instability. [81] (10.1016/j.otsr.2012.05.008)
• [L4] [82] (10.1177/0363546519838405)
• [L4] [83] (10.1177/23259671251374307)
• [L4] A single, first-time or infrequently recurring traumatic lateral patellar dislocation is associated with cartilage injury and gradual deterioration over time, potentially leading to generalized knee osteoarthritis. [85] (10.1177/0363546516687549)
• [L3] A novel classification system for patients with recurrent patellar subluxation and excessive femoral torsion based on segmental femoral torsion analysis was established. [87] (10.1186/s13018-024-05123-x)
• [L5] Recent evidence suggests that recurrent patellofemoral instability is causing cartilage degeneration, and stopping this process via surgical restoration of the anatomy and biomechanics of the patellofemoral joint may significantly reduce the risk of osteoarthritis. [98] (10.1016/j.arthro.2022.10.003)
• [L2] [100] (10.1007/s00167-020-06163-6)
• [L2] [102] (10.1097/01.blo.0000203478.27044.9a)
• [L1] MPFL reconstruction is an effective and reliable treatment in the setting of patellofemoral instability. [108] (10.1177/0363546521990004)
• [L4] Its indications must be based on exact analysis of the reasons for the instability. [114] (10.1016/j.otsr.2016.03.018)
• [L3] The authors present an MRI-based classification system utilizing objective measurements of trochlear and patelloemoral morphology, emphasizing a 2-measurement combination of sulcus angle and LPFA that yielded excellent diagnostic accuracy. [116] (10.1177/03635465251411751)
• [Case_report] [124] (10.1016/j.jisako.2025.101063)
• [L3] MRI-based patellar tilt measures proved to be an excellent group of measurements for delineating between controls and those with instability. [130] (10.1177/0363546512472441)
• [L4] Chondral defects of the medial facet and the crest of the patella are the most common in patients with recurrent patellofemoral instability. [134] (10.1186/s13018-022-02911-1)
• [L4] In particular, symptomatic cases with inconspicuous conventional MRI imaging, additional MRI imaging only in the axial plane in a 20° of knee flexion could be beneficial and useful in clinical daily routine. [136] (10.1186/s12891-021-04733-4)
• [L4] Reconstruction of the MPFL is a safe and effective treatment for patellofemoral instability without severe trochlear dysplasia and allows most patients to engage in regular sports activities 2 years postoperatively, at least at a recreational level. [138] (10.1177/0363546514529640)
• [L1] Allografts may represent a feasible alternative to traditional autograft for MPFL reconstruction in selected patients with patellofemoral instability. [146] (10.1007/s00167-021-06569-w)
• [L3] MPFL/MQTFL reconstruction in skeletally immature patients with first-time patellofemoral dislocation demonstrated lower failure rates and improved functional outcomes at a minimum 2-year follow-up compared with nonoperative management. [148] (10.1016/j.arthro.2025.01.021)
• [L2] Appropriately powered randomized controlled trials are needed to better understand what the adequate indications for surgery in patients with patellar instability and clinical outcomes are. [150] (10.1186/s13018-020-02072-z)
• [L5] The authors agree that the rotational and mechanical axis of the entire limb plays an important role in the pathology of patellofemoral instability and should be considered in the evaluation of all patients, particularly those who are skeletally immature. [152] (10.5435/jaaos-d-18-00654)
• [L4] This comprehensive literature review demonstrated good clinical outcomes after patellar cartilage repair with no evidence of inferior results even in complex cases with the need for additional patellofemoral realignment procedures. [153] (10.1007/s00167-021-06728-z)
• [L4] A patellofemoral arthrodesis can be regarded safe and also very effective in the relief of pain in extreme patella alta. [155] (10.1007/s00167-014-3203-2)
• [L5] MPFL reconstruction is an accepted method to restore static medial stabilization for patients with recurrent patellar instability who have failed nonoperative management. [157] (10.1016/j.csm.2014.03.006)
• [L3] This study demonstrated a comparatively good diagnostic performance for MRI in the evaluation of first and recurrent lateral patellar dislocations, and we therefore recommend MRI for the cartilage assessment after a LPD. [159] (10.1186/1471-2474-11-149)
• [L4] These study findings demonstrate a high degree of variability across repeat MRI scans of the same knee in the same patient. [160] (10.1177/2325967126s00363)
• [L5] Physeal-sparing medial patellofemoral ligament reconstruction is the treatment of choice for skeletally immature patients with recurrent instability, replacing older nonanatomic techniques. [163] (10.5435/jaaos-d-17-00255)
• [L4] Non-surgical treatment achieves better clinical outcomes with respect to a lower patellar instability rate and better subjective function for the overlap-region injury of MPFL than for the non-overlap-region injury, and can be considered as treatment of choice for overlap-region injury of MPFL. [165] (10.1007/s00167-012-2020-8)
• [L4] The association of definite knee osteoarthritis in medial patellofemoral ligament reconstruction with or without lateral release was small in the long-term follow-up. [166] (10.1177/0363546507306161)
• [L3] In conclusion, 25% of adolescents experienced post-operative PFI after primary patellofemoral stabilization surgery. [167] (10.1177/2325967121s00508)
• [L4] However, MRI-based indices frequently indicated preserved patellofemoral engagement despite CDI-defined patella alta, underscoring their complementary role. [169] (10.1177/03635465261426564)
• [L3] [170] (10.1177/23259671251407090)
• [L3] This study confirms arthroscopic assessment of patella tracking is accurate, reproducible, and a knee flexion angle of greater than 44° correlates with patellar instability diagnosis. [171] (10.1007/s00167-019-05532-0)
• [L5] Non-operative treatment is usually recommended if patellar dislocation occurs in an anatomically normal knee and osteochondral fracture is ruled out on MRI. [172] (10.1016/j.otsr.2014.12.001)
• [L3] Objective radiographic measurements remain important for characterizing tibial tuberosity position when planning patellar stabilization surgery. [177] (10.1177/2325967123s00225)
• [L3] Substantial cartilage damage was present in 56% of patients who underwent isolated MPFL reconstruction for recurrent patellar instability, with medial patellar lesions being the most common. [179] (10.1016/j.jisako.2025.100862)
• [L3] The present findings suggest that not surgically addressing chondral lesions in patients with overlying meniscus coverage can allow for high functional outcomes and minimal pain after patellar stabilization procedures. [181] (10.1177/2325967125s00244)
• [L4] It serves as a useful component of an 'à la carte' surgical strategy for severe patellofemoral instability. [182] (10.1016/j.otsr.2015.07.019)
• [L3] While a TT-TG distance ≥20 mm was the strongest predictor of recurrent instability, it was relatively insensitive, and sensitivity may be improved by normalizing the TT-TG distance to patient-specific axial and sagittal plane patellofemoral measurements. [184] (10.1007/s00167-017-4752-y)
• [Paper] The procedure reliably protects against recurrent patellar instability, leading to good postoperative clinical outcomes. [185] (10.1016/j.eats.2023.01.007)
• [L5] Using dynamic ultrasound assessment, we found that medial patellofemoral distance significantly increases with disruption of the MPFC. [186] (10.1177/23259671221098748)
• [Case_report] [189] (10.1016/j.jisako.2025.100860)
• [L4] [190] (10.1016/j.injury.2020.06.028)
• [L4] [192] (10.1007/s00167-014-3397-3)
• [L3] [193] (10.1007/s00167-017-4716-2)
• [L3] At the latest followup, 80.8 per cent of the patients had a good or an excellent clinical result; only one patient had redislocation of the patella. [197] (10.2106/00004623-198870060-00010)
• [L4] Patellofemoral osteoarthritis developed significantly in 43% of patients at midterm follow-up after Insall's proximal realignment. [198] (10.1007/s00167-013-2485-0)
• [L4] In contrast to general belief, the development of patellofemoral osteoarthritis at 12-year follow-up did not exceed the findings from other trochleoplasty case series. [199] (10.1007/s00167-017-4602-y)
• [L3] Furthermore, at final follow-up favorable PROs were achieved in patients who underwent trochleoplasty for the treatment of their patellofemoral instability with severe trochlear dysplasia. [201] (10.1177/23259671251321499)

See Also

• MPFL reconstruction
• Anatomy
• Knee osteoarthritis

References

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[179] Articular cartilage damage is frequently noted at the time of medial patellofemoral ligament reconstruction and is associated with age and patellofemoral anatomy. Journal of ISAKOS. 2025. DOI: 10.1016/j.jisako.2025.100862

[181] Poster 149: Traumatic Lateral Femoral Condyle Lesions associated with Patellofemoral Instability Demonstrate Excellent Outcomes With Non-Operative Management and Patellar Stabilization with Meniscal Coverage at 8-Years Follow-Up. Orthopaedic Journal of Sports Medicine. 2025. DOI: 10.1177/2325967125s00244

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[184] Individualizing the tibial tubercle to trochlear groove distance to patient specific anatomy improves sensitivity for recurrent instability. Knee Surgery, Sports Traumatology, Arthroscopy. 2017. DOI: 10.1007/s00167-017-4752-y

[185] The Femoral Sulcus Deepening Trochleoplasty of Lyon. Arthroscopy Techniques. 2023. DOI: 10.1016/j.eats.2023.01.007

[186] Utility of Diagnostic Ultrasound in the Assessment of Patellar Instability. Orthopaedic Journal of Sports Medicine. 2022. DOI: 10.1177/23259671221098748

[189] Concomitant distal femoral osteotomy in managing chronic patellofemoral instability with an associated valgus deformity: A case report and review of literature. Journal of ISAKOS. 2025. DOI: 10.1016/j.jisako.2025.100860

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[192] Reconstructive surgery for patellofemoral joint incongruency. Knee Surgery, Sports Traumatology, Arthroscopy. 2014. DOI: 10.1007/s00167-014-3397-3

[193] Current evidence advocates use of a new pathologic tibial tubercle–posterior cruciate ligament distance threshold in patients with patellar instability. Knee Surgery, Sports Traumatology, Arthroscopy. 2017. DOI: 10.1007/s00167-017-4716-2

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[198] Patellofemoral osteoarthritis after Insall's proximal realignment for recurrent patellar dislocation. Knee Surgery, Sports Traumatology, Arthroscopy. 2013. DOI: 10.1007/s00167-013-2485-0

[199] A stand-alone lateral condyle-elevating trochlear osteotomy leads to high residual instability but no excessive increase in patellofemoral osteoarthritis at 12-year follow-up. Knee Surgery, Sports Traumatology, Arthroscopy. 2017. DOI: 10.1007/s00167-017-4602-y

[201] Evaluating the Outcomes of Trochleoplasty in the Treatment of Patellofemoral Instability in the Adolescent and Young Adult Knee With Severe Trochlear Dysplasia: A Minimum 2-Year Follow-up. Orthopaedic Journal of Sports Medicine. 2025. DOI: 10.1177/23259671251321499