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Unicompartmental knee replacement

155 citationsUpdated Sep 2026

For patients: a plain-language version of this topic is available. See the patient guide.

Overview

Unicompartmental knee arthroplasty (UKA) is a mature surgical option that preserves native anatomy and restores function through individualized patient care [3]. It serves as a definitive treatment for appropriately selected patients, including those aged 60 years or younger [12], octogenarians [7], and very elderly individuals [152]. Recent reports demonstrate success in expanding classic indications to younger and heavier patients [4], with expanded indications associated with comparable clinical outcomes and great short-term survivorship [2]. For spontaneous osteonecrosis of the knee, UKA is less destructive to native structure than total knee arthroplasty (TKA) while achieving comparable prognosis with strict selection [9]. The procedure is economically attractive compared to TKA in patients aged 65 years or older [19] and remains a safe alternative in those over 80 years old [24].

Headline outcomes indicate that UKA provides reliable and durable results, with functional recovery continuing beyond 6 months and extending up to 2 years [8]. When correct indications are employed, the Oxford medial UKA maintains a low revision rate through the second decade [11], and recent series using modern techniques show fixed-bearing UKA results as reliable as TKA at 10 years' follow-up [46]. Outcomes may not match primary TKA but tend to exceed revision surgery for previous TKA [15]. Pre-existing patellofemoral osteoarthritis does not affect intermediate-term outcomes or 10-year survivorship in fixed-bearing designs and should not be considered a contraindication [14, 1]. There is no difference in quality-of-life outcomes between mobile and fixed-bearing medial UKAs [10].

Long-term survival depends on patient selection, surgical technique, and surgical experience for both fixed and mobile-bearing designs [25]. Obesity remains a contraindication, and excluding obese patients makes the procedure more predictable and reliable [17]. The Unicompartmental Indication Score (UIS) is positively correlated with postoperative Knee Society Score and patient satisfaction, with a cutoff of 25 points predicting optimal outcomes for pain relief, function, and satisfaction [58]. While UKA accounted for 4.5% of unilateral knee replacements among Medicare beneficiaries from 2000 to 2009 with dramatic use increases [6], implant survivorship remains a limiting factor toward achieving cost-effectiveness in patients aged less than 65 years [33]. Single-stage bilateral UKA is a relevant procedure in selected patients, not associated with higher complication rates compared to two-stage surgery, and results in substantial cost savings [18].

Anatomy & Pathophysiology

Bony Anatomy

The knee joint comprises the distal femur, proximal tibia, and patella [38]. The medial femoral condyle is larger and projects farther posteriorly and distally than the lateral condyle [79]. Conversely, the lateral femoral condyle projects farther anteriorly and is wider in the medial-lateral direction than the medial condyle [79]. The tibial articular surface slopes 7° to 10° in the sagittal plane [79]. Specifically, the posterior slope of the medial tibial plateau averages 10.7°, while the lateral plateau averages 7.2° [94]. The medial tibial plateau is larger than the lateral plateau and is concave in both frontal and sagittal planes [79]. The lateral tibial plateau is smaller, more circular, concave in the frontal plane, and convex in the sagittal plane [79]. Consequently, the medial compartment has a large surface area containing a convex femoral condyle and concave tibial plateau [94]. The lateral compartment has a smaller surface area than the medial compartment and contains a convex femoral condyle and convex lateral plateau in the sagittal plane [94]. The mechanical axis of the lower extremity normally passes just medial to the medial tibial spine [95]. This axis is in 3 degrees of valgus angulation from the vertical axis [95].

Ligaments

The anterior cruciate ligament (ACL) prevents anterior translation and rotation of the tibia on the femur [38]. The posterior cruciate ligament (PCL) prevents posterior subluxation of the tibia on the femur [38]. The medial collateral ligament (MCL) stabilizes the knee against valgus stresses [38]. The lateral collateral ligament is the main stabilizer against varus stress [38]. The ACL is composed of 90% type I collagen and 10% type III collagen [79]. The mean length of the ACL is 33 mm and the mean midsubstance width is 11 mm [79]. The ACL femoral attachment is a semicircular area on the posteromedial aspect of the lateral femoral condyle [79]. The ACL tibial attachment is a broad, irregular, oval-shaped area slightly medial and anterior to the midline between the tibial spinous processes [79]. The ACL consists of anteromedial and posterolateral bundles named according to their tibial insertions [83]. The anteromedial bundle of the ACL is tighter in knee flexion, while the posterolateral bundle is tighter in extension [83]. The PCL has a mean length of 38 mm and a mean width of 13 mm [94]. The PCL femoral attachment is broad and crescent-shaped on the anterolateral aspect of the medial femoral condyle [94]. The PCL tibial insertion is located 10 to 15 mm distal to the joint line on the posterior central sulcus [94]. The PCL anterolateral bundle is tight in knee flexion, while the posteromedial bundle is tight in knee extension [94]. The MCL has superficial and deep portions that stabilize the knee to valgus stresses [38]. The superficial MCL proximal division resists valgus tibial translation and tibial external rotation [80]. The deep MCL resists valgus translation and tibial internal and external rotation [80]. The posterior oblique ligament resists tibial internal rotation, especially in knee extension [80]. The lateral collateral ligament resists varus tibial translation and tibial external rotation, especially at 30 degrees of knee flexion [80]. The popliteus tendon resists tibial external rotation, especially in knee flexion [80]. The popliteofibular ligament is present in 90% of knees and runs from the popliteus tendon to the posterior fibular head [38]. The ACL is typically subjected to peak loads of 170 N during walking and up to 500 N with running [95]. The ultimate strength of the ACL in young patients is approximately 1750 N [95].

Menisci

The menisci are C-shaped fibrocartilaginous disks that provide shock absorption, increase joint congruency, enhance stability, and aid in synovial fluid distribution [38]. The medial meniscus is firmly attached to the joint capsule along its entire peripheral edge [38]. The lateral meniscus is attached to the anterior and posterior capsule but has a region posterolaterally where it is not firmly attached [38]. The medial meniscus has less mobility than the lateral meniscus and is more susceptible to tearing when trapped between the femoral condyle and tibial plateau [38]. The lateral meniscus is larger than the medial meniscus and carries a greater share of the lateral compartment pressure [38]. The medial meniscus has a semicircular shape covering approximately 50% to 60% of the medial tibial plateau in adulthood [90]. The lateral meniscus has a more circular C-shape with symmetric sizes of the anterior and posterior horns [90]. The mean lateral meniscus excursion from knee extension to flexion is 11.2 mm, compared to a mean medial meniscus excursion of 5.1 mm [90]. Menisci bear one-third to one-half of body weight during load transmission [95]. Removal of the meniscus increases contact stresses by up to four times the load transfer to bone [95]. The medial meniscus posterior horn serves as a chock block on the tibial plateau, and its absence increases instability in ACL and PCL deficient knees [89].

Vascular and Nerve Anatomy

The blood supply to the knee is formed from an anastomosis around the knee derived from the descending geniculate artery, superior and inferior geniculate arteries, middle geniculate artery, and anterior tibial recurrent arteries [79]. The middle geniculate artery supplies both the anterior and posterior cruciate ligaments [79]. The knee is innervated by branches of the femoral nerve (L2, L3, L4), obturator nerve (L2, L3, L4), and sciatic nerve (L4, L5, S1, S2) [79]. The posterior articular branch of the tibial nerve is the largest nerve providing innervation of the intra-articular knee [79]. Nerves to the cruciate ligaments contain vasomotor and pain fibers as well as mechanoreceptors involved in proprioception [79].

Kinematics and Biomechanics

The knee is a hinge joint that incorporates both gliding and rolling, which are essential to its kinematics [80]. The "screw-home" mechanism involves the tibia externally rotating 5 degrees in the final 15 degrees of extension [80]. Knee joint surface loads are three times body weight during level walking and up to four times body weight with stair walking [95]. The quadriceps produces maximum anterior force on the tibia at 0 to 60 degrees of knee flexion [95]. The patella has the thickest articular surface in the body, approximately 5 mm in the midportion and 2 mm on the sides [79]. The patella bears half the body weight with normal walking and seven times the body weight with squatting and jogging [95]. In descending stairs, compressive force on the patellofemoral joint reaches two to three times body weight [95].

UKA-Specific Pathophysiology and Kinematics

Unicompartmental knee arthroplasty (UKA) is a logical procedure when the lesion is limited to one femorotibial compartment [77]. The success of UKA is influenced by surgical technique and the choice of proper indication [77]. Indications for UKA include osteoarthritis or osteonecrosis limited to one femorotibial compartment ranked as Ahlback 4, a deformity fully correctible on stress radiographs, and a functional anterior cruciate ligament [77]. UKA cannot restore the functional flexion axis of a living knee to normal, evidenced by significantly lower vertical translation of the clinical epicondylar axis compared to normal knees [60]. The kinematics of the unloaded knee following mobile-bearing UKA closely resemble those of the native knee, although relative medial overstuffing results in a more valgus joint orientation [119]. UKA reliably restores normal knee kinematics and function for arthritis limited to one compartment, but survival rates vary based on patient selection, implant design, and surgeon experience [126]. The in vivo kinematics of bicruciate-retaining total knee arthroplasty reproduces those of normal knees to a lower extent than UKA knees [118]. UKA significantly enhances knee joint kinematics, facilitating the transition from basic to advanced functional activities [108]. Altered contact kinematics and reduced range of contact excursion in UKA knees could lead to excessive cumulative articular surface contact stress, implicated in the pathogenesis of osteoarthritis [127]. Optimal positioning of the tibial component is crucial to restore normal knee kinematics and prevent implant wear and lesions to adjacent compartments [111]. Kinematic characteristics do not change significantly after kinematic alignment UKA [123]. Pre-arthritic/kinematically aligned knees in UKA cohorts had superior outcomes, including higher KOOS Activities of Daily Living and Sport subscales, compared with non-pre-arthritic/kinematically aligned knees at an average of 10 years [131]. Knees that achieved pre-arthritic alignment using the kinematic alignment technique for mobile-bearing UKA demonstrated superior survivorship and subjective postoperative knee function compared with those that did not [146]. The mechanical axis passes medial to the centre of the knee, reflecting the constitutional varus alignment in tibia vara patients undergoing cemented mobile-bearing UKA [115]. Phenotype analysis using the functional knee phenotype system demonstrated a wide diversity of coronal alignment phenotypes among knees with anteromedial osteoarthritis [124]. Knees with anteromedial osteoarthritis show substantial phenotypic variation prior and following medial UKA, challenging the assumption of uniform characteristics among knees with an identical wear pattern [147]. Robotic-assisted, cruciate-sparing bi-UKA maintains the natural anatomy of the knee in the coronal, sagittal, and axial planes better than a mechanically aligned TKA [140]. Lateral UKA is a favorable alternative to distal femoral osteotomy and total knee arthroplasty for isolated lateral compartment osteoarthritis, provided indications are strictly selected and surgical techniques are tailored to the lateral compartment's unique anatomy and biomechanics [47]. Lateral UKA survival has evolved with improved understanding of knee compartment biomechanics, indications, surgical techniques, and implant designs [110]. Despite relevant anatomical and biomechanical contrasts between medial and lateral UKAs, recent literature points to comparable survival rates [148]. There is a clear difference between the kinematics, and prevalence and extent of posterior bearing overhang between medial and lateral Oxford UKRs [133]. Varus placement of the tibial component in Oxford UKA reduces intraoperative bearing mediolateral translation during 0–60° of knee flexion and decreases the risk of impingement with the vertical wall compared with neutral placement [143]. During knee flexion in Oxford UKA, all mobile bearings moved posteriorly, and mobile bearings whose femoral components were set laterally tended to move posteriorly while in contact with the lateral wall [136]. The tibia first technique with tensor measurement allows surgeons to predict final soft tissue tension before femoral osteotomies, helping restore natural knee kinematics [144]. In-vivo medial and lateral collateral elongation following UKA is associated with knee functional scores [134]. Valgus correctability and meniscal extrusion are associated with alignment after UKA and can help predict coronal-plane overcorrection [154]. There is a correlation between varus orientation of the tibial component and maximum total point motion from radiostereometry in UKA [155]. Patients with lateral osteoarthritis of the knee showed an impaired gait with an increased knee abduction and hip adduction angle before mobile-bearing UKA [156]. Minor medial joint line elevation after medial UKA triggers a novel, angle-dependent biomechanical cascade driven by dynamic alignment changes [141]. Although native knee biomechanics are preserved in UKA, younger patients do not seem to perceive this benefit as greater health-related quality of life or patient satisfaction compared to TKA [142]. UKA offers advantages over traditional TKA including being less invasive, preserving bone, and allowing more normal kinematics and range of motion [135]. UKA has evolved into a mature and reliable surgical option that aligns with principles of preservation of native anatomy, restoration of function, and individualized patient care [3]. There may be a difference in knee kinematics where the medial mobile bearing knee closer approximates that of the normal knee, accounting for a significantly lower frequency of radiolucent lines in mobile rather than fixed bearing prosthesis [40]. The authors propose that research and development efforts should focus on addressing aberrant biomechanics through technologies that 'unload' the joint, as this may reverse structural damage or delay the need for invasive joint reconstruction [114].

Classification

Indications and Patient Selection

Unicompartmental knee arthroplasty (UKA) is a mature, reliable surgical option that preserves native anatomy and restores function [3]. It serves as a definitive treatment rather than a pre-total knee arthroplasty (TKA) step, even in younger patients [27]. For medial osteoarthritis, UKA is safe and effective in patients aged 60 years or younger [12], and medial mobile-bearing designs are viable for young patients with anteromedial osteoarthritis [37]. Age-related candidacy declines, yet UKA remains a definitive option for appropriately selected octogenarians [7], with reliable results in this demographic [7]. Long-term data supports durability in older cohorts, with good performance at minimum 21-year follow-up [5].

Pre-existing patellofemoral disease does not contraindicate UKA [1]. It does not affect 10-year survivorship in fixed-bearing implants [1], nor does the grade of associated patellofemoral osteoarthritis negatively impact clinical outcomes, complication rates, or survival after femorotibial UKA [54]. However, a statistically significant association exists between preoperative femorotibial malalignment magnitude and the severity of associated patellofemoral osteoarthritis [54]. Associated patellofemoral osteoarthritis must be treated systematically in a tailored, stepwise fashion based on severity to extend indications [54].

UKA is indicated for spontaneous or secondary osteonecrosis limited to one compartment, offering reliable clinical improvement, minimal complications, and durable survivorship [179]. For isolated compartment spontaneous knee osteonecrosis, 10-year survivorship free of revision is 93% [21], with Kaplan-Meier survivorship at 12 years reaching 96.7% [179]. In a study of 30 knees, reliable pain relief and function improvement were achieved in 96%, and restoration of appropriate lower-limb mechanical axis was achieved in 88% [179]. UKA is less destructive to native knee structure than TKA for spontaneous osteonecrosis but achieves comparable prognosis with strict patient selection [9].

A unicompartmental knee arthroplasty may be preferable for medial knee osteoarthritis before the varus angle exceeds 15º [69]. Expanded indications for UKA are associated with comparable clinical outcomes and great short-term, albeit limited, survivorship [2].

Implant Design and Fixation

Fixed-bearing: Fixed-bearing implants lower the risk of revision (relative risk 0.65 [95% confidence interval 0.48 to 0.86]) and dislocation (relative risk 0.16 [95% confidence interval 0.07 to 0.37]) compared to mobile-bearing implants [52]. They also improve Western Ontario and McMaster University Osteoarthritis Index scores (mean difference 1.44 [95% confidence interval 0.71 to 2.17]) compared to mobile-bearing implants [52].

Mobile-bearing: There is no difference in outcomes between mobile and fixed unicompartmental knee replacements [10]. The debate over mobile-bearing implants in UKA to TKA conversion continues, with no conclusive advantage of one design over another [173].

Unconstrained: Unconstrained unicompartmental knee replacements demonstrate significantly better clinical results and lower revision rates compared to constrained designs [28].

Fixation: At present, standard cemented implants and conventional designs represent the optimal solution [43]. Keys to long-term survival for both fixed and mobile-bearing designs are patient selection, surgical technique, and surgical experience [25].

Survivorship and Outcomes

Recent reports indicate 94% survivorship at 10 years and 90% at 18 years, driven by newer techniques, designs, and improved polyethylene quality [16]. In the hands of high-volume and experienced surgeons, implant survival rates reach 97% at 10 years, 85.9% at 20 years, and 80% at 25 years [54]. When correct indications are employed, the Oxford medial unicompartmental knee arthroplasty maintains a low revision rate through the second decade [11].

Functional recovery continues beyond 6 months and up to 2 years [8]. Outcomes of UKA may not match those of primary TKA but tend to exceed results of revision of a previous TKA [15]. No differences in outcome scores or survivorship exist between patients younger or older than 55 years at minimum 10-year follow-up [165].

Epidemiology and Trends

Although UKA accounted for only 4.5% of unilateral knee replacements among Medicare beneficiaries, its use has increased dramatically [6]. The incidence of UKA is increasing at a more rapid rate than TKA, growing by approximately 30% per year, though it still represents a very small proportion of all knee arthroplasties [22]. Between 1998 and 2005, UKA use increased at a rate nearly triple that of TKA [35]. According to international studies, only 8–15% of patients with knee OA are treated with UKA, though it could be performed in almost 50% of cases [54].

Failure Modes and Revision

The risk for revision is three times higher in UKA compared to TKA [173]. Primary causes of failure include component loosening and progression of arthritis [173]. In one study of medial UKA, seven knees (9.2%) underwent revision for progression of osteoarthritis (n = 3), infection (n = 2), aseptic loosening (n = 1), or persistent symptoms (n = 1) [73]. Two additional minor complications, heterotopic ossification and delayed wound healing, were observed and resolved without revision [73].

Failed UKAs can be successfully converted to TKA with encouraging results [173]. In most cases, the posterior cruciate ligament can be repaired, and bone defects corrected with wedges or cancellous grafts during conversion [173]. Stem tibial extension and metal block augmentation are effective in managing proximal tibial defects, offering stability during the conversion process [173]. The results of UKA revision surgery are superior to those of failed TKA and failed high tibial osteotomy [173].

Diagnostic and Preoperative Assessment

A novel 2.5-dimensional deep learning model for “bone-on-bone” detection on magnetic resonance imaging may enhance diagnostic accuracy and consistency, providing a standardized reference for cartilage assessment and supporting preoperative clinical decision-making [13]. Comparisons between UKA and TKA are open to bias, particularly in registry data, which can be misleading [66]. PASS thresholds for the Oxford Knee Score were calculated at 6 months, 5 years, 10 years, and 15 years for both total and unicompartmental knee arthroplasty [157].

Clinical Presentation

Diagnostic Evaluation

Physical and radiographic evaluation forms the cornerstone of the diagnostic process for knee osteoarthritis and is essential to determine whether a knee with unicompartmental osteoarthritis is indicated for unicompartmental knee arthroplasty [117]. Evaluating the presence of unicompartmental knee osteoarthritis through medical history, physical examination, and imaging contributes to precise patient selection and provides valuable information in surgical decision-making after diagnostic criteria are met [117]. A novel 2.5-dimensional deep learning model for "bone-on-bone" detection on magnetic resonance imaging provides objective, reproducible output that may enhance diagnostic accuracy and consistency for cartilage assessment in medial unicompartmental knee arthroplasty candidates [13].

Pain Characteristics

Preoperative pain location is a poor predictor of outcome after Oxford unicompartmental knee arthroplasty at 1 and 5 years [51]. Localised medial pain should not be a prerequisite to unicompartmental knee arthroplasty, which may be performed in patients with generalised or anterior knee pain [51]. Pre-operative anterior knee pain does not compromise functional outcome or survival and should not be considered a contraindication to mobile-bearing unicompartmental knee arthroplasty [70]. In a cohort of 406 knees undergoing Oxford medial unicompartmental knee arthroplasty, 27% of patients had mixed or generalised pain and 6% had pure anterior knee pain [51].

Patellofemoral Joint Status

The presence of preoperative patello-femoral osteoarthritis did not affect intermediate-term outcomes in patients undergoing unicompartmental knee arthroplasty [14]. Preoperative patello-femoral osteoarthritis should not be considered a contraindication to unicompartmental knee arthroplasty [14].

Ligamentous and Structural Status

Unicompartmental knee arthroplasty combined with anterior cruciate ligament reconstruction can be a valid treatment option for selected patients with combined medial unicompartmental knee osteoarthritis and ACL deficiency [160]. Preoperative bone marrow edema negatively impacted clinical outcomes, especially pain, at a mean of 10 years after unicompartmental knee arthroplasty [68]. Previous ipsilateral knee surgery was associated with a lower objective clinical score and may identify knees requiring more cautious pre-operative counselling [31].

Patient Demographics and Comorbidities

Unicompartmental knee arthroplasty was associated with pain relief and excellent function in a cohort of patients who had been sixty years of age or younger and active at the time of surgery at an average duration of follow-up of eleven years [30]. Medial mobile-bearing unicompartmental arthroplasty should be considered a viable surgical treatment in young patients suffering from anteromedial osteoarthritis of the knee aged less than or equal to 50 years [37]. Unicompartmental knee arthroplasty can be expected to provide reliable and durable results in certain octogenarians and should be regarded as a definitive treatment option in appropriately selected patients of this age group [7]. In patients aged 70 years or more, unicompartmental knee arthroplasty for medial gonarthrosis should be considered an excellent alternative to high tibial osteotomy [59]. Disease state is more important than age in identifying those who will do well after unicompartmental knee arthroplasty [57]. The best way to obtain reliable results from unicompartmental knee arthroplasty is to operate only on those with severe anteromedial disease [57]. Obesity remains a contraindication for unicompartmental knee arthroplasty, and better defining the ideal candidate with this criterion will make the procedure more predictable and reliable [17]. Patients with reduced bone mineral density could safely undergo cementless unicompartmental knee replacement surgery and have similar clinical outcomes to those with normal bone mineral density [36].

Indications and Disease Patterns

Unicompartmental knee arthroplasty is a durable and efficient option to treat patients with unicompartmental osteonecrosis of the knee [48]. The success of unicompartmental knee arthroplasty relies on appropriate patient selection, yet considerable challenges persist in predicting patient-specific success due to variations in patient characteristics and healthcare practices [151].

Investigations

Plain radiography: Weight-bearing AP and lateral radiographs serve as the appropriate initial imaging studies for assessing knee arthritis [98]. A weight-bearing PA (Rosenberg) view in flexion is specifically used to assess cartilage loss from the posterior femur and tibial plateau [98]. Patellofemoral views are employed to evaluate patellofemoral alignment, patellar and trochlear morphology, and patellofemoral arthritis [98]. The Kellgren-Lawrence rating grades the extent of osteoarthritis based on review of the AP knee radiograph [106]. Primary features used for this rating include osteophytes, joint space narrowing, subchondral sclerosis with or without subchondral cysts, and altered shape of periarticular bones [106]. Kellgren-Lawrence Grade 4 is characterized by prominent osteophytes, absent joint space, subchondral bone sclerosis with cysts, and bone deformity with flattening of femoral condyle contours [106]. Knee arthroplasty is recommended when Kellgren-Lawrence Grade 4 findings are present [106]. However, application of isolated radiologic criteria in patients with knee osteoarthritis results in a unicompartmental knee arthroplasty candidacy that is misleadingly high [205, 206]. A machine learning method based on a voting mechanism provides a high-accuracy method for surgical decision-making between total and unicompartmental knee arthroplasty using only standard X-rays [216].

MRI: MRI is indicated when osteonecrosis is suspected in the arthritic patient population [106]. It is not indicated if the joint space is significantly narrowed on radiograph [106]. Assessment of medial cartilage thickness loss using MRI provides additional utility over standard radiographs in preoperative assessments of medial unicompartmental knee arthroplasty patients [190]. A 2.5-dimensional deep learning model for "bone-on-bone" detection on MRI provides objective, reproducible output that may enhance diagnostic accuracy and consistency for cartilage assessment in unicompartmental knee arthroplasty candidates [13]. Preoperative lateral meniscal pathology on MRI does not preclude excellent results in unicompartmental knee arthroplasty for medial osteoarthritis if the patient has no symptoms related to the lateral meniscus lesion [202].

CT: Three-dimensional CT with remodeling is used for preoperative planning for reconstruction associated with dysplasia, post-trauma planning, and complex total knee arthroplasty planning [106].

Bone scan: SPECT bone scan provides information regarding the degree of osteoarthritis in knee weight-bearing compartments for pre-operative planning [203].

Other Considerations: Preoperative radiographic severe knee varus, mild-to-moderate patellofemoral joint osteoarthritis, and malalignment of the patellofemoral joint do not adversely influence early outcomes after fixed-bearing medial robotic-assisted unicompartmental knee arthroplasty [213]. Preoperative patellar tilt angle does not significantly influence clinical outcomes or radiographic parameters following unicompartmental knee arthroplasty [218]. The location and severity of preoperative subchondral bone marrow lesions were not associated with inferior postoperative outcomes after medial unicompartmental knee arthroplasty [50]. Consequently, bone marrow lesions should not be considered a contraindication for medial unicompartmental knee arthroplasty [50]. ACL deficiency is not always a contraindication for medial unicompartmental knee arthroplasty in patients with typical anteromedial osteoarthritis radiographs [189]. Advanced varus deformity associated with medial knee osteoarthritis is a potential predictor of anterior cruciate ligament tear and risk for suitable unicompartmental knee arthroplasty [69]. Unicompartmental knee arthroplasty may be preferable for medial knee osteoarthritis before the varus angle reaches more than 15º [69]. Obesity remains a contraindication for unicompartmental knee arthroplasty [17].

Treatment

Non-Operative

The provided evidence does not detail specific conservative management protocols such as weight loss, physiotherapy, or pharmacological interventions. Surgical indications are defined by the pathoanatomy of the disease and specific clinical criteria rather than a mandated duration of failed non-operative care [130].

Operative

Indications: Unicompartmental knee arthroplasty (UKA) is indicated for osteoarthritis or osteonecrosis limited to one femorotibial compartment, ranked as Ahlback 4, with a deformity that is fully correctible on stress radiographs and a functional anterior cruciate ligament [77]. For spontaneous osteonecrosis of the knee (SONK), UKA is less destructive to native structure than total knee arthroplasty (TKA) but achieves comparable prognosis with strict patient selection [9]. Localised medial pain is not a prerequisite; UKA may be performed in patients with generalised or anterior knee pain, as pre-operative anterior knee pain does not compromise functional outcome or survival for mobile-bearing implants [51, 70]. Preoperative patello-femoral osteoarthritis does not affect intermediate-term outcomes and should not be considered a contraindication [14]. Indications for mobile-bearing UKA should be based on the pathoanatomy of the disease as proposed by Goodfellow et al, rather than the contraindications proposed by Kozinn and Scott [130]. Lateral UKA is a favorable alternative for isolated lateral compartment osteoarthritis, provided indications are strictly selected and techniques are tailored to lateral anatomy [47]. The Unicompartmental Indication Score (UIS) is positively correlated with postoperative Knee Society Score and satisfaction, with a cutoff of 25 points predicting optimal outcomes [58]. Preoperative bone marrow edema negatively impacts clinical outcomes, especially pain, at a mean of 10 years [68]. An objective deep learning model for “bone-on-bone” detection on MRI may enhance diagnostic accuracy and support preoperative decision-making [13].

Surgical Approach / Technique: UKA is a logical procedure when the lesion is limited to one femorotibial compartment, performed through a mini-incision to minimize extensor mechanism disruption and provide quicker recovery [77]. Minimally invasive implantation is associated with better functional results in the immediate postoperative period compared to standard open techniques [194]. However, a substantial complication rate persists despite modifications to improve surgical technique over time [72]. Revision rates for minimally invasive UKA fell from 5.0% to 2.5% and reoperation rates fell from 8.1% to 5.4% when comparing the first half of cases to the second half [72]. Free bone cement fragments after minimally invasive UKA are an underappreciated problem [194]. An extended sagittal saw cut significantly reduces fracture load in cementless UKA compared to cemented tibia plateaus [188].

Implant Selection: Fixed-bearing implants lower the risk of revision (relative risk 0.65 [95% confidence interval 0.48 to 0.86]) and dislocation (relative risk 0.16 [95% confidence interval 0.07 to 0.37]) and improve Western Ontario and McMaster University Osteoarthritis Index scores (mean difference 1.44 [95% CI 0.71 to 2.17]) compared to mobile-bearing implants [52]. Recent reports show encouraging results with 94% survivorship at 10 years and 90% survivorship at 18 years, driven by newer techniques, designs, and improved polyethylene quality [16]. Recent series corresponding to modern use criteria show results as reliable as TKA at 10 years' follow-up [46]. UKA offers optimal survivorship rates of up to 20 years for patients with unicondylar knee degeneration [162]. Fracture of metallic components is an uncommon cause of cemented UKA failure [32].

Alignment / Balancing Strategy: Coronal alignment of the femoral component in fixed-bearing UKA may affect long-term clinical outcomes [169].

Pain Management: Addition of a single preoperative dose of 125 mg systemic methylprednisolone to a multimodal analgesic regime significantly reduced postsurgical pain and opioid consumption and decreased knee swelling in the first 24 h after fast-track UKA [199].

Adjuncts: Robotic-assisted surgery allows surgeons to more accurately and reproducibly plan and achieve operative targets [42]. It offers technological advancements that enhance surgical precision and reduce revision rates compared to manual techniques, though it does not significantly differ in functional outcomes [128].

Setting of Care: Patients can be safely and effectively discharged on the day of surgery after UKA within the United Kingdom NHS, with high levels of satisfaction [34]. Day-of-surgery discharge allows for safe, efficient care of appropriately selected patients [67].

Revision: Conversion of a failed unicondylar knee arthroplasty to a total knee arthroplasty is technically demanding but may be done successfully with careful preoperative planning and possible need for revision techniques [23].

Other Considerations: Periprosthetic fracture following UKA is a rare but devastating complication associated with significant morbidity [20]. Incidences of periprosthetic fractures can range up to 5% [188]. Periprosthetic medial tibia plateau fractures are rare but represent a serious complication [188]. Single-stage bilateral UKA is a relevant procedure in selected patients, not associated with higher complication rates compared to two-stage surgery, and results in substantial cost savings [18]. UKA is an economically attractive alternative in patients sixty-five years of age or older, and modest improvements in implant survivorship could make it cost-effective in younger patients [19]. At an average follow-up of eleven years, UKA was associated with pain relief and excellent function in patients who were sixty years of age or younger and active at the time of surgery [30]. UKA patients reported lower pain scores and required 45% lower opioid medication in the immediate postsurgical period than TKA patients [191]. UKA offers a safe and efficient alternative to osteoarthritis with reduced postoperative pain, less postoperative complication, and revision compared to high tibial osteotomy [207]. High tibial osteotomy is a thoughtful alternative to maintain function and relieve pain in selected cases, while UKA offers early advantages despite potentially lower survivorship than total knee replacement [26].

Complications

Overall Complication Rates: Unicompartmental knee arthroplasty (UKA) is associated with a relatively low incidence of complications [35]. The overall risk of complications for patients undergoing total knee arthroplasty (TKA) was 11.0%, compared with 4.3% for patients undergoing UKA [183]. TKA was associated with increased rates of manipulation, transfusion, intensive care unit admission, and discharge to a rehabilitation facility compared to UKA [183]. TKA had longer hospital stays than UKA, with a mean of 3.3 days versus 2.0 days [183]. There was a trend toward an increased risk of deep infection, readmission, thromboembolic events, and any reoperation after TKA compared to UKA, though these differences were not statistically significant [183]. In a systematic review, overall complications occurred in 4.2% of knees in the robotic UKA group and 5.5% in the manual UKA group [122]. The cumulative incidence of complications following robotic UKA was 4.4% for medial procedures and 8.3% for lateral procedures [122].

Infection (PJI): Periprosthetic joint infection (PJI) after UKA results in substantial morbidity, and patients should be followed closely for aseptic causes of failure in addition to infection recurrence [232]. Survivorship free from persistent PJI at 1 year is 76% following debridement, antibiotics, and implant retention in UKA [220]. Data suggest that a wide exposure and thorough synovectomy should be incorporated during debridement, antibiotics, and implant retention for UKA to improve the likelihood of successful eradication of PJI at the 1-year mark [234]. Intra-articular corticosteroid injection in a UKA doubled the risk of PJI compared to patients who did not receive an injection [233]. There is an urgent and compelling need for future studies to elucidate optimal clinical strategies for PJI management specifically tailored to UKA, given the absence of established guidelines [235].

Revision and Conversion Outcomes: Although outcomes of UKA may not match those of a primary TKA, they tend to exceed results of revision of a previous TKA [15]. Re-revision rates of failed UKA are equivalent to revision rates of primary TKA and substantially better than re-revision rates of revision TKA [210]. Revision free survivorship and arthroplasty related complications at two years following revision UKA are lower than that for primary TKA, but higher than that for aseptic revision TKA [172]. Medical complications following revision UKA are similar to those following primary TKA [172]. The 2-year failure rate was six times higher for UKA-to-UKA revision than after conversion to TKA [219]. Implant survival and clinical outcomes of UKA revisions to TKA were inferior to primary TKA [215]. Aseptic revision from UKA to TKA provides excellent midterm implant survivorship, clinical outcomes, and radiographic results, particularly when performed for the progression of osteoarthritis [222].

Failure Modes and Risk Factors: The most common failure modes for UKA are instability, progression of disease to another compartment, and aseptic loosening of the tibial component [161]. Registry results show that UKAs are revised three times more often and much earlier than TKAs [161]. The most common reason for failure in a series of Oxford UKAs was progression of osteoarthritis in the lateral compartment, occurring in 2.3% of cases [121]. Indications for revision in a series of medial mobile bearing UKAs included aseptic loosening, tibial collapse, mobile bearing dislocation, persistent pain, progression of disease, infection, and tibiofemoral instability [168]. Knee arthroscopy within two years of UKA is associated with an increased rate of conversion to TKA and a higher rate of failure from aseptic loosening [196]. There was no increased incidence of PJI when UKA was staged greater than three months after arthroscopy [171]. Preoperative coronal tibiofemoral subluxation of five mm or greater was associated with a higher risk of conversion to TKA due to aseptic loosening after mobile-bearing UKA [221].

Other Considerations: In a large United States cohort, conversion from UKA to TKA was associated with higher rates of PJI than revision TKA across all follow-up intervals [149]. Based on current literature, conversion from UKA to TKA is associated with a similar surgical site infection or PJI rate compared to primary TKA [223]. The estimated lifetime risk of revision following knee arthroplasty was dependent on patient age and implant type, with unicondylar knee arthroplasty being associated with a higher lifetime revision risk [212]. UKA has higher revision and slightly higher reoperation rates but lower mortality rates than matched TKA [229]. Reported complications were not uncommon, as nearly 40% of patients required a conversion to a TKA in a large United States cohort [225]. Early orthostatic intolerance is uncommon after UKA compared to TKA, suggesting a procedure-specific component [61].

Recovery

Light activity (weeks): The evidence provided does not specify a precise week range for the resumption of desk work, driving, or light activities of daily living. However, UKA patients return to work significantly sooner than total knee arthroplasty patients, although Work Outcome Questionnaire, Work Ability Index, and satisfaction scores are similar between groups [180].

Full activity (months): Most patients can expect to resume physical activity or sports within a short timeframe after knee arthroplasty, particularly to low-impact activities [163]. The majority of patients returned to sports and recreational activity after unicompartmental knee arthroplasty [55]. Higher prearthritic activity is associated with higher long-term activity after medial unicompartmental knee arthroplasty [73].

Complete recovery / outcome plateau (months): The provided evidence does not define a specific month range for the stabilization of pain, strength, and final functional outcomes.

Functional milestones: Both unicompartmental knee arthroplasty and total knee arthroplasty groups showed an overall improvement in WOMAC and Oxford Knee scores over time [182]. Minimally invasive unicompartmental knee arthroplasty allows for rapid recovery, minimal blood loss, and excellent functional outcomes including high flexion and the ability to sit cross-legged [211]. A good restoration of gait was achieved by most unicompartmental knee patients independently of the UKA design, although some abnormalities persisted in muscle activity around the knee [159]. The functional flexion axis of a living knee cannot be restored to normal after medial unicompartmental knee arthroplasty, as evidenced by significantly lower vertical translation of the clinical epicondylar axis compared to normal knees [60].

Other Considerations: These findings underscore the importance of restoring the joint line to achieve optimal outcomes and survival after UKA for medial compartmental knee osteoarthritis [231]. Unicompartmental knee replacements in this relatively older age group of patients performed well at minimum 21-year followup [5]. Seven knees (9.2%) underwent revision for progression of osteoarthritis (n = 3), infection (n = 2), aseptic loosening (n = 1) or persistent symptoms (n = 1) [73]. Two additional minor complications (heterotopic ossification and delayed wound healing) were observed and resolved without the need for revision [73]. Two-staged bicompartmental UKA is a viable less-invasive alternative to TKA conversion for patients with disease progression or ON after primary UKA [178]. Revision surgery for failed medial unicompartmental knee arthroplasty with bone loss using a lateral tibial plateau autograft and cemented posterior-stabilized implant resulted in improved Oxford Knee Scores and satisfactory mid-term outcomes [224].

Key Evidence

• [L3] These patients should not be contraindicated from undergoing unicompartmental knee arthroplasty. [1] (10.1007/s00167-018-5169-y)
• [L3] Expanded indications for unicompartmental knee arthroplasty were associated with comparable clinical outcomes and great short-term, albeit limited, survivorship. [2] (10.1016/j.arth.2025.08.005)
• [L5] Unicompartmental knee arthroplasty has evolved into a mature and reliable surgical option that aligns closely with the principles of modern orthopaedic practice: preservation of native anatomy, restoration of function, and individualized patient care. [3] (10.1007/s00402-026-06409-x)
• [L5] Recent reports have demonstrated success in expanding the classic indications of unicompartmental knee arthroplasty to younger and heavier patients. [4] (10.5435/jaaos-d-24-00380)
• [L4] Unicompartmental knee replacements in this relatively older age group of patients performed well at minimum 21-year followup. [5] (10.1097/01.blo.0000185451.96987.aa)
• [L3] Although unicompartmental knee arthroplasty accounted for only 4.5% of the unilateral knee replacements among Medicare beneficiaries, the use of this procedure has increased dramatically. [6] (10.2106/jbjs.l.00652)
• [L4] The unicompartmental knee arthroplasty can be expected to provide reliable and durable results in certain octogenarians, and should be regarded as a definitive treatment option in appropriately selected patients of this age group. [7] (10.1097/01.blo.0000223983.67325.61)
• [L3] Functional recovery after unicompartmental knee replacement continues beyond 6 months and even up to 2 years. [8] (10.1007/s00167-007-0351-7)
• [L4] Unicompartmental arthroplasty for SONK is less destructive to the native knee structure than total knee arthroplasty but can achieve comparable prognosis with strict patient selection. [9] (10.1155/2021/6614122)
• [L3] There is no difference in outcomes between mobile and fixed unicompartmental knee replacements. [10] (10.1016/j.arth.2012.05.017)
• [L4] Providing correct indications are employed, the Oxford medial unicompartmental knee arthroplasty can have a low revision rate through the second decade after implantation. [11] (10.1007/s11999-010-1506-2)
• [L4] Unicompartmental knee arthroplasty for medial osteoarthritis is a safe, reliable and effective treatment option for patients of 60 years or younger. [12] (10.1007/s00167-022-07029-9)
• [L4] Its objective, reproducible output may enhance diagnostic accuracy and consistency, providing a standardized reference for cartilage assessment and supporting preoperative clinical decision-making for unicompartmental knee arthroplasty. [13] (10.1016/j.arth.2026.05.043)
• [L4] The presence of preoperative patello-femoral osteoarthritis did not affect intermediate-term outcomes in patients undergoing unicompartmental knee arthroplasty, suggesting it should not be considered a contraindication. [14] (10.1016/j.arth.2025.04.070)
• [L5] Although outcomes of unicompartmental knee arthroplasty may not match those of a primary TKA, they tend to exceed results of revision of a previous TKA. [15] (10.5435/jaaos-d-17-00107)
• [L4] More recent reports of unicompartmental knee replacement have been more encouraging with 94% survivorship at 10 years and 90% survivorship at 18 years, driven by newer techniques, designs, and improved polyethylene quality. [16] (10.1097/01.blo.0000151844.03672.0b)
• [L4] Better defining the ideal candidate for unicompartmental knee arthroplasty, with obesity remaining a contraindication, will make this a more predictable and reliable procedure. [17] (10.1097/01.blo.0000187062.65691.e3)
• [L3] Single-stage bilateral unicompartmental knee arthroplasty is a relevant procedure in selected patients, not associated with higher complication rates compared to two-stage surgery, and results in substantial cost savings. [18] (10.1016/j.otsr.2018.01.021)
• [L5] Unicompartmental knee arthroplasty is an economically attractive alternative in patients sixty-five years of age or older, and modest improvements in implant survivorship could make it a cost-effective alternative in younger patients. [19] (10.2106/jbjs.n.00169)
• [L5] Periprosthetic fracture following unicompartmental knee arthroplasty is a rare but devastating complication associated with significant morbidity. [20] (10.1016/j.injury.2024.111654)
• [L3] Unicompartmental knee arthroplasty for isolated compartment spontaneous knee osteonecrosis results in reliable clinical improvement, minimal complications, and durable survivorship free of revision of 93% at ten years. [21] (10.1302/0301-620x.100b4.bjj-2017-1041.r2)
• [L4] The incidence of unicompartmental knee arthroplasty is increasing per year at a more rapid rate than total knee arthroplasty, growing by approximately 30% per year, but still represents a very small proportion of all knee arthroplasties. [22] (10.1016/j.arth.2007.04.012)
• [L4] Conversion of a failed unicondylar knee arthroplasty to a total knee arthroplasty is technically demanding, but may be done successfully with careful preoperative planning and possible need for revision techniques. [23] (10.1097/01.blo.0000214431.19033.fa)
• [L4] Unicompartmental knee arthroplasty is a safe and effective alternative to TKA in patients over 80 years old. [24] (10.1016/j.arth.2026.06.071)
• [L4] Keys to long-term survival of both fixed and mobile-bearing designs in unicompartmental knee arthroplasty are patient selection, surgical technique, and surgical experience. [25] (10.1016/j.arth.2008.06.025)
• [L5] High tibial osteotomy is a thoughtful alternative to maintain function and relieve pain in selected cases, while unicompartmental replacement offers early advantages despite potentially lower survivorship than total knee replacement. [26] (10.1007/s00167-006-0195-6)
• [L3] UKR should be considered to be a definitive knee replacement rather than a Pre-TKR even in the young. [27] (10.1007/s00167-020-05863-3)
• [L4] At an average duration of follow-up of eleven years, unicompartmental knee arthroplasty was associated with pain relief and excellent function in a cohort of patients who had been sixty years of age or younger and active at the time of surgery. [30] (10.2106/00004623-200409001-00004)
• [L3] Previous ipsilateral knee surgery was associated with a lower objective clinical score and may identify knees requiring more cautious pre-operative counselling. [31] (10.1007/s00264-026-07029-1)
• [L4] [32] (10.1007/s00167-012-2062-y)
• [L1] Implant survivorship is a limiting factor toward achieving cost-effective unicompartmental knee arthroplasty in patients aged less than 65 years. [33] (10.1016/j.arth.2016.08.019)
• [L4] Patients can be safely and effectively discharged on the day of surgery after unicompartmental knee arthroplasty within the United Kingdom NHS, with high levels of satisfaction. [34] (10.1302/0301-620x.99b6.bjj-2016-0540.r2)
• [L5] [35] (10.1055/s-0037-1601414)
• [L3] Patients with reduced bone mineral density could safely undergo cementless unicompartmental knee replacement surgery and have similar clinical outcomes to those with normal bone mineral density. [36] (10.1186/s13018-020-1566-2)
• [L4] Medial mobile-bearing unicompartmental arthroplasty should be considered a viable surgical treatment in young patients suffering from anteromedial osteoarthritis of the knee. [37] (10.1016/j.arth.2018.03.069)
• [L1] There may be a difference in knee kinematics where the medial mobile bearing knee closer approximates that of the normal knee, accounting for a significantly lower frequency of radiolucent lines in mobile rather than fixed bearing prosthesis. [40] (10.1016/j.otsr.2009.10.006)
• [L4] Robotic-assisted surgery has become increasingly popular in unicompartmental knee arthroplasty, as it allows surgeons to more accurately and reproducibly plan and achieve operative targets during surgery. [42] (10.1016/j.arth.2018.01.050)
• [L4] At the present time, the standard cemented implants and the conventional designs for unicompartmental knee replacement still represent the optimal solution. [43] (10.1007/s00167-014-3444-0)
• [Paper] Recent series whose indications and technique correspond to modern use criteria have shown results that are as reliable as those of total knee arthroplasty at a 10 years' follow-up. [46] (10.1016/j.otsr.2011.08.003)
• [L5] Lateral unicompartmental knee arthroplasty is a favorable alternative to distal femoral osteotomy and total knee arthroplasty for isolated lateral compartment osteoarthritis, provided indications are strictly selected and surgical techniques are tailored to the lateral compartment's unique anatomy and biomechanics to mitigate complications. [47] (10.1016/j.jisako.2024.100342)
• [L4] UKA is a durable and efficient option to treat patients with unicompartmental ON of the knee. [48] (10.1016/j.arth.2018.11.010)
• [L3] Clinically, these results suggest that bone marrow lesions should not be considered a contraindication for medial unicompartmental knee arthroplasty. [50] (10.1016/j.arth.2016.05.009)
• [L2] [51] (10.1007/s00167-012-2211-3)
• [L1] [52] (10.1016/j.arth.2026.07.011)
• [L4] [54] (10.1016/j.jisako.2024.04.003)
• [L4] The majority of patients returned to sports and recreational activity after unicompartmental knee arthroplasty. [55] (10.1177/0363546507303562)
• [L5] The author concludes that disease state is more important than age in identifying those who will do well after unicompartmental knee arthroplasty, and that the best way to obtain reliable results is to operate only on those with severe anteromedial disease. [57] (10.1097/corr.0000000000000797)
• [L3] The Unicompartmental Indication Score (UIS) is positively correlated with postoperative Knee Society Score and patient satisfaction, with a cutoff of 25 points predicting optimal outcomes in terms of pain relief, function, and satisfaction. [58] (10.1007/s00402-018-3069-8)
• [L3] In patients aged 70 years or more, unicompartmental knee arthroplasty for medial gonarthrosis should be considered an excellent alternative to high tibial osteotomy. [59] (10.1007/s004020050226)
• [L3] The functional flexion axis of a living knee cannot be restored to normal after medial unicompartmental knee arthroplasty, as evidenced by significantly lower vertical translation of the clinical epicondylar axis compared to normal knees. [60] (10.1007/s00167-014-3296-7)
• [L3] Early orthostatic intolerance is uncommon after unicompartmental knee arthroplasty compared to total knee arthroplasty, suggesting a procedure-specific component. [61] (10.1186/s13018-024-04639-6)
• [Paper] Candidacy for medial unicompartmental knee replacement declines with age. [64] (10.1016/j.otsr.2019.11.012)
• [L5] Comparisons between unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA) are open to bias, particularly in registry data, which can be misleading. [66] (10.1302/0301-620x.99b1.bjj-2016-0515.r1)
• [L3] Day of surgery discharge after unicompartmental knee arthroplasty allows for safe, efficient care of appropriately selected patients with high satisfaction. [67] (10.1016/j.arth.2013.08.021)
• [L3] At a mean of 10 years, preoperative bone marrow edema negatively impacted clinical outcomes, especially pain, after unicompartmental knee arthroplasty. [68] (10.1016/j.arth.2022.10.010)
• [L3] A unicompartmental knee arthroplasty may be preferable for medial knee OA before the varus angle reaches more than 15º. [69] (10.1016/j.arth.2025.01.005)
• [L3] Pre-operative anterior knee pain also does not compromise functional outcome or survival and should not be considered a contraindication. [70] (10.1302/0301-620x.99b5.bjj-2016-0695.r2)
• [L3] [72] (10.1016/j.arth.2009.05.011)
• [L4] [73] (10.1002/ksa.70624)
• [L5] [77] (10.1097/blo.0b013e3180986da7)
• [L3] UKA significantly enhances knee joint kinematics, facilitating the transition from basic to advanced functional activities. [108] (10.1186/s13018-025-05662-x)
• [L3] Lateral UKA survival has evolved with improved understanding of knee compartment biomechanics, indications, surgical techniques and implant designs. [110] (10.1002/ksa.12785)
• [L4] Optimal positioning of the tibial component is crucial to restore normal knee kinematics and prevent implant wear and lesions to adjacent compartments. [111] (10.1016/j.otsr.2013.03.004)
• [L5] The authors propose that research and development efforts should focus on addressing aberrant biomechanics through technologies that 'unload' the joint, as this may reverse structural damage, delay the need for invasive joint reconstruction, or obviate the need entirely. [114] (10.1007/s00167-011-1403-6)
• [L2] The mechanical axis passes medial to the centre of the knee, reflecting the constitutional varus alignment. [115] (10.1007/s00264-026-06977-y)
• [L4] [117] (10.1136/jisakos-2016-000102)
• [L3] The in vivo kinematics of BCR-TKA knees reproduces those of normal knees to a lower extent than those of UKA knees. [118] (10.1007/s00167-019-05754-2)
• [L5] The kinematics of the unloaded knee following mobile-bearing unicompartmental knee arthroplasty closely resemble those of the native knee, although relative medial overstuffing results in a more valgus joint orientation. [119] (10.1007/s00402-017-2794-8)
• [L4] [121] (10.1016/j.arth.2018.01.035)
• [L1] [122] (10.1016/j.arth.2026.08.017)
• [L5] However, the kinematic characteristics do not change significantly after KA UKA. [123] (10.1186/s13018-025-05659-6)
• [L3] Phenotype analysis using the functional knee phenotype system demonstrated a wide diversity of coronal alignment phenotypes among knees with anteromedial osteoarthritis. [124] (10.1002/ksa.12043)
• [L4] UKA reliably restores normal knee kinematics and function for arthritis limited to one compartment, but survival rates vary significantly based on patient selection, implant design, and surgeon experience. [126] (10.1055/s-0038-1625961)
• [L4] The altered contact kinematics and reduced range of contact excursion in UKA knees could lead to excessive cumulative articular surface contact stress, which is implicated in the pathogenesis of OA. [127] (10.1177/23259671221150958)
• [L2] Robotic-assisted unicompartmental knee arthroplasty offers technological advancements that enhance surgical precision and reduce revision rates compared to manual techniques, but it does not significantly differ in functional outcomes. [128] (10.1016/j.arth.2024.10.095)
• [L4] This study provides long-term evidence that for mobile-bearing unicompartmental knee arthroplasty the indications should be based on the pathoanatomy of the disease, as proposed by Goodfellow et al and does not support the contraindications proposed by Kozinn and Scott and others. [130] (10.1016/j.arth.2016.12.036)
• [L4] Pre-arthritic/kinematically aligned knees in this cohort had superior outcomes, including higher KOOS Activities of Daily Living and Sport subscales and achievement of the patient acceptable symptom state, compared with non-pre-arthritic/kinematically aligned knees at an average of 10 years. [131] (10.2106/jbjs.21.00801)
• [L2] This demonstrates a clear difference between the kinematics, and prevalence and extent of posterior bearing overhang between medial and lateral OUKRs. [133] (10.1002/jor.24339)
• [L3] These patterns are associated with knee functional scores. [134] (10.1016/j.otsr.2024.104052)
• [L5] It offers advantages over traditional TKA including being less invasive, preserving bone, and allowing more normal kinematics and range of motion. [135] (10.1055/s-0030-1247834)
• [L4] During knee flexion, all mobile bearings moved posteriorly, and mobile bearings whose femoral components were set laterally tended to move posteriorly while in contact with the lateral wall. [136] (10.1007/s00167-018-5064-6)
• [L1] Robotic-assisted, cruciate-sparing bi-UKA maintains the natural anatomy of the knee in the coronal, sagittal, and axial planes better, and may therefore preserve normal joint kinematics, compared with a mechanically aligned TKA. [140] (10.1302/0301-620x.102b11.bjj-2020-1166.r1)
• [Paper] This study reveals a novel, angle-dependent biomechanical cascade triggered by minor JLE, driven by dynamic alignment changes. [141] (10.1186/s13018-026-06982-2)
• [L3] Although native knee biomechanics are preserved, younger patients do not seem to perceive this oft-cited benefit of UKA, as this did not translate into greater health-related quality of life or patient satisfaction compared to TKA. [142] (10.1016/j.arth.2017.09.022)
• [L4] Varus placement of the tibial component reduces intraoperative bearing mediolateral translation during 0–60° of knee flexion and decreases the risk of impingement with the vertical wall, compared with neutral placement along the mechanical axis. [143] (10.1002/ksa.70295)
• [L4] The tibia first technique with the tensor measurement has the advantage that surgeons can predict final soft tissue tension before femoral osteotomies with the prosthetic design and help restore natural knee kinematics, potentially improving implant survival and functional outcomes. [144] (10.1007/s00264-014-2531-7)
• [L3] Using the kinematic alignment technique for mobile-bearing UKA, knees that achieved pre-arthritic alignment demonstrated superior survivorship and subjective postoperative knee function compared with those that did not. [146] (10.1186/s12891-025-08363-y)
• [L3] This variability challenges the assumption of uniform characteristics among knees with an identical wear pattern associated with anteromedial OA and emphasizes the complexity and variability of this specific form of OA. [147] (10.1007/s00167-023-07603-9)
• [L1] Despite relevant anatomical and biomechanical contrasts between medial and lateral UKAs, recent literature points to comparable survival rates. [148] (10.1016/j.jisako.2024.100329)
• [L3] In a large United States cohort, UKA-TKA was associated with higher rates of PJI than TKA-TKA across all follow-up intervals. [149] (10.1016/j.arth.2026.04.041)
• [L5] The success of unicompartmental knee arthroplasty relies on appropriate patient selection, yet considerable challenges persist in predicting patient-specific success due to variations in patient characteristics and healthcare practices. [151] (10.1016/j.jisako.2024.100348)
• [L4] UKA is a viable option for treating unicompartmental knee osteoarthritis in very elderly patients with reduced complications and morbidity, and excellent survivorship when proper indications and accurate technique are used. [152] (10.1007/s00402-015-2323-6)
• [L3] These factors can help predict coronal-plane overcorrection and guide patient selection to avoid progression of lateral knee osteoarthritis. [154] (10.1097/corr.0000000000001260)
• [L2] There is a correlation between varus orientation of the tibial component and maximum total point motion from radiostereometry in unicompartmental knee arthroplasties. [155] (10.1007/s00167-014-3147-6)
• [L2] Patients with lateral osteoarthritis of the knee showed an impaired gait with an increased knee abduction and hip adduction angle. [156] (10.1007/s00167-014-2944-2)
• [L4] PASS thresholds for the Oxford Knee Score were calculated at 6 months, 5 years, 10 years, and 15 years for both total and unicompartmental knee arthroplasty. [157] (10.1002/ksa.70539)
• [L3] A good restoration of gait was achieved by most unicompartmental knee patients independently of the UKA design, although some abnormalities persisted in muscle activity around the knee. [159] (10.1007/s00167-011-1620-z)
• [L4] Unicompartmental knee arthroplasty combined with ACL reconstruction can be a valid treatment option for selected patients, with combined medial unicompartmental knee osteoarthritis and ACL deficiency. [160] (10.1007/s00167-017-4536-4)
• [L4] [161] (10.1302/0301-620x.99b1.bjj-2016-0272)
• [L4] UKA is an excellent treatment option for patients with unicondylar knee degeneration with optimal survivorship rates of up to 20 years. [162] (10.1302/2058-5241.3.170060)
• [L4] Most patients can expect to resume physical activity or sports within a short timeframe after knee arthroplasty, particularly to low-impact activities. [163] (10.1002/ksa.70267)
• [L3] [165] (10.1097/corr.0000000000000737)
• [L4] [168] (10.1016/j.arth.2013.01.005)
• [L3] [169] (10.1016/j.arth.2020.07.070)
• [L3] There was no increased incidence of PJI when UKA was staged greater than three months after arthroscopy. [171] (10.1016/j.arth.2024.10.021)
• [L3] Revision free survivorship and arthroplasty related complications at two years following revision UKA are lower than that for primary TKA, but higher than that for aseptic revision TKA, whereas medical complications are similar to those following primary TKA. [172] (10.1016/j.arth.2024.12.026)
• [L4] [173] (10.1016/j.jisako.2024.100341)
• [L4] Two-staged bicompartmental UKA is a viable less-invasive alternative to TKA conversion for patients with disease progression or ON after primary UKA. [178] (10.1002/ksa.70040)
• [L4] [179] (10.1097/blo.0b013e31812f7821)
• [L3] UKA patients return to work significantly sooner than TKA patients, although WORQ, WAI, and satisfaction scores are similar between groups. [180] (10.1007/s00167-019-05667-0)
• [L3] Both groups showed an overall improvement in WOMAC and Oxford Knee scores over time. [182] (10.1016/j.arth.2013.05.009)
• [L3] [183] (10.1016/j.arth.2012.03.022)
• [L5] [188] (10.1007/s00167-011-1698-3)
• [L3] ACL deficiency is not always a contraindication for medial unicompartmental knee arthroplasty in patients with typical anteromedial osteoarthritis radiographs. [189] (10.1016/j.arth.2020.08.024)
• [L3] Assessment of medial cartilage thickness loss using MRI provides additional utility over standard radiographs in preoperative assessments of medial UKA patients. [190] (10.1002/ksa.12611)
• [L3] UKA patients reported lower pain scores and required 45% lower opioid medication in the immediate postsurgical period than TKA patients. [191] (10.1016/j.arth.2020.04.072)
• [L4] [194] (10.1007/s00167-008-0563-5)
• [L3] Knee arthroscopy within two years of unicompartmental knee arthroplasty is associated with an increased rate of conversion to total knee arthroplasty and a higher rate of failure from aseptic loosening. [196] (10.1016/j.arth.2020.10.060)
• [L2] Addition of a single preoperative dose of 125 mg systemic methylprednisolone to a multimodal analgesic regime significantly reduced postsurgical pain and opioid consumption and decreased knee swelling in the first 24 h after fast-track UKA. [199] (10.1007/s00167-014-3501-8)
• [L3] The results of UKA for medial unicompartmental knee osteoarthritis were excellent regardless of preoperative lateral meniscal pathology in the magnetic resonance imaging, if the patients had no symptoms related to the lateral meniscus lesion. [202] (10.1016/j.arth.2021.03.054)
• [Paper] SPECT bone scan provides very useful information regarding the degree of osteoarthritis in knee weight-bearing compartments for pre-operative planning. [203] (10.1007/s00402-007-0399-3)
• [L3] Application of isolated radiologic criteria in patients with knee OA results in a UKA candidacy that is misleadingly high. [205] (10.1007/s00167-021-06632-6)
• [L4] Application of isolated radiologic criteria in patients with knee OA results in a UKA candidacy is misleadingly high. [206] (10.1177/2325967121s00180)
• [L1] UKA offers a safe and efficient alternative to osteoarthritis reduced postoperative pain, less postoperative complication, and revision. [207] (10.1016/j.arth.2017.10.025)
• [L3] Re-revision rates of failed UKA are equivalent to revision rates of primary TKA and substantially better than re-revision rates of revision TKA. [210] (10.1016/j.arth.2018.03.023)
• [L4] Minimally invasive unicompartmental knee arthroplasty allows for rapid recovery, minimal blood loss, and excellent functional outcomes including high flexion and the ability to sit cross-legged. [211] (10.1016/j.arth.2006.12.109)
• [L3] The estimated lifetime risk of revision following knee arthroplasty was dependent on patient age and implant type, with fully constrained and unicondylar knee arthroplasty being associated with a higher lifetime revision risk. [212] (10.1016/j.arth.2024.11.054)
• [L3] Preoperative radiographic severe knee varus, mild-to-moderate patellofemoral joint osteoarthritis, and malalignment of the patellofemoral joint do not adversely influence early outcomes after fixed-bearing medial robotic-assisted unicompartmental knee arthroplasty. [213] (10.1186/s13018-025-06181-5)
• [L3] Our data suggest that the implant survival and clinical outcomes were inferior to primary TKA. [215] (10.1302/0301-620x.107b12.bjj-2025-0184.r2)
• [L4] This study provides a high-accuracy method for surgical decision-making between TKA and UKA for KOA patients, requiring only standard X-rays and offering potential for clinical application in automated referrals and preoperative planning. [216] (10.1016/j.arth.2025.06.005)
• [L3] Preoperative patellar tilt angle does not significantly influence clinical outcomes or radiographic parameters following unicompartmental knee arthroplasty, as no significant differences were observed between normal and abnormal tilt groups in functional scores or complications. [218] (10.1186/s13018-026-06896-z)
• [L3] The 2-year failure rate was six times higher for UKA-to-UKA revision than after conversion to TKA. [219] (10.1016/j.arth.2025.04.023)
• [L3] Survivorship free from persistent PJI at 1 year is low at 76% but is consistent with similar reports of DAIRs for total knee arthroplasties. [220] (10.1016/j.arth.2020.02.036)
• [L3] Both groups exhibited survivorship higher than 90% at 15 years; however, preoperative coronal tibiofemoral subluxation ≥ five mm was associated with a higher risk of conversion to total knee arthroplasty due to aseptic loosening after mobile-bearing unicompartmental knee arthroplasty. [221] (10.1016/j.arth.2026.01.066)
• [L4] Aseptic revision from UKA to TKA provides excellent midterm implant survivorship, clinical outcomes, and radiographic results, particularly when performed for the progression of osteoarthritis. [222] (10.1002/ksa.70156)
• [L1] Based on current literature, UKA to TKA conversion is associated with a similar surgical site infection/PJI rate compared to primary TKA. [223] (10.1002/ksa.70228)
• [L4] Revision surgery for failed medial unicompartmental knee arthroplasty with bone loss using a lateral tibial plateau autograft and cemented posterior-stabilized implant resulted in improved Oxford Knee Scores and satisfactory mid-term outcomes. [224] (10.1007/s00167-015-3610-z)
• [L3] Reported complications were not uncommon, as nearly 40% of patients required a conversion to a TKA. [225] (10.1016/j.arth.2024.06.048)
• [L3] UKR has higher revision and slightly higher reoperation rates but lower mortality rates than matched TKR. [229] (10.1016/j.arth.2021.09.018)
• [L3] These findings underscore the importance of restoring the joint line to achieve optimal outcomes and survival after UKA for medial compartmental knee osteoarthritis. [231] (10.1016/j.arth.2026.02.042)
• [L4] UKA PJI results in substantial morbidity, and patients should be followed closely for aseptic causes of failure in addition to infection recurrence. [232] (10.1097/corr.0000000000000372)
• [L3] In this study, IACI in a UKA doubled the risk of PJI compared to patients who did not receive an injection. [233] (10.1016/j.arth.2022.12.005)
• [L3] The data suggest that a wide exposure and thorough synovectomy be incorporated during the DAIR UKA to improve the likelihood of successful eradication of PJI at the 1-year mark. [234] (10.1016/j.arth.2024.03.057)
• [L3] Given the absence of established guidelines for PJI management specifically tailored to UKA, there is an urgent and compelling need for future studies to elucidate optimal clinical strategies. [235] (10.5435/jaaos-d-23-01202)

See Also

• Anatomy
• Distal femoral osteotomy
• Knee osteoarthritis
• High tibial osteotomy
• Total Knee Replacement
• Knee arthroscopy

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