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Os Acromiale

Patient-facing topic on os acromiale — failed fusion of an acromial ossification centre that may cause shoulder pain and contribute to subacromial impingement.

47 citationsUpdated Oct 2026
Illustration: Os Acromiale

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

Overview

Os acromiale is a co-condition frequently identified in patients with rotator cuff tears [6]. While the majority of cases are asymptomatic and require no treatment [1], a symptomatic and unstable os acromiale is relatively rare among patients presenting with shoulder pain [2]. Diagnosis should be based on imaging features rather than limited by age [5]. Clinicians must always suspect an os acromiale in cases of failed rotator cuff surgery or ongoing shoulder pain after failed treatment [4].

Management of a symptomatic os acromiale is initially nonsurgical [3]. If nonoperative measures fail, surgical management may predictably lead to improved outcomes [7], resulting in decreased symptoms and better clinical results [12]. Osteosynthesis to achieve bony union is a safe, effective, and reliable technique for large and unstable symptomatic cases [37], with united os acromiale demonstrating significantly better functional outcomes as measured by the Constant score (P = .0169) [10]. However, surgical treatment of symptomatic meso-os acromiale with concomitant rotator cuff pathology yielded poorer outcomes than previously reported, with only 53% achieving satisfactory results overall [30].

Specific clinical contexts require distinct considerations. Care should be taken not to perform acromioplasty in the presence of a painful os acromiale [22]. Ipsilateral os acromiale may be a relative contraindication to the clavicle hook plate [25]. In professional tennis players, surgical treatment is usually not indicated [21]. Regarding reverse total shoulder arthroplasty, postoperative local tenderness at the os acromiale can be expected in 1 out of 4 patients but resolves spontaneously over time in the majority of patients [8]. The presence of os acromiale does not appear to have a negative impact on clinical outcomes after surgery, and rTSA remains a safe and effective treatment option [13]. Shoulders with os acromiale had a similar overall complication rate compared with matched controls (14% vs. 12%; P = 0.658) [15].

Anatomy & Pathophysiology

Bony Anatomy and Development

The acromion develops from three distinct ossification centers: the metacromion at the base, the mesoacromion in the middle, and the preacromion at the tip [46]. These centers typically appear around age 15 years and usually fuse by age 25 years [28]. Os acromiale is defined as incomplete fusion of these secondary ossification centers, most commonly between the mesoacromion and metacromion [57]. The unfused segment is most frequently the meso-acromion [41]. Anatomically, the acromion, coracoacromial ligament, and coracoid process form the coracoacromial arch, a rigid bony-ligamentous structure that imparts stability to the shoulder girdle [43]. The rotator cuff, subacromial bursa, and subdeltoid bursa pass underneath this arch [43]. The lateral acromion serves as the insertion site for the deltoid muscle [26].

Prevalence and Demographics

Approximately 8% of scapulae exhibit an os acromiale [28]. In a Korean outpatient shoulder clinic population, the prevalence was 7 per 1,000 patients [26]. Bilateral os acromiale was identified in 15.4% of cases within this same cohort [26]. Racial disparities exist, with Black individuals consistently demonstrating a higher prevalence (13.2%-18.2%) compared to White individuals (5.8%-9.5%) [26]. Among patients undergoing reverse shoulder arthroplasty for cuff tear arthropathy, the frequency of os acromiale was 22% [84].

Pathophysiology and Mechanisms

A symptomatic, unstable os acromiale is relatively rare [2]. Generally, there is no relation between the presence of os acromiale and shoulder pain, regardless of diagnosis [11], and the rate of rotator cuff tears in patients with os acromiale is comparable to those without [26]. However, a rotator cuff tear may be associated with os acromiale, likely due to impingement from abnormal motion at the fibrous union site [17], supporting findings that os acromiale is associated with rotator cuff injuries [14]. When symptoms are driven by the os acromiale, two principal mechanisms are responsible: motion at the non-union site, or an impingement-type syndrome resulting from flexion of the os fragment during deltoid contraction and arm elevation [41]. Dynamic impingement on the rotator cuff occurs as deltoid contraction during active elevation causes flexion of the mobile anterior acromial fragment [24]. Pain directly at the synchondrosis site from degenerative changes is analogous to an arthritic acromioclavicular joint [24]. Previously stable non-unions can become unstable following blunt trauma to the region [41], making it possible to convert a stable os acromiale into an unstable one after a traumatic event [24]. Extrinsic impingement occurs when the space available for the rotator cuff is diminished, with pathologic os acromiale serving as an example [70]. The greater the distance of the acromioclavicular joint from the anterior edge of the acromion, the higher the likelihood of an os acromiale [28].

Clinical Presentation and Diagnosis

Patients with symptomatic os acromiale typically present with anterolateral shoulder pain, accentuated by overhead activity and positional night pain [24]. Superior shoulder pain may also be reported [24]. Patients typically have no history of trauma, although a history should be queried if an os acromiale is noted [24]. On physical examination, the synchondrosis may be tender to palpation [24]. A painful arc of motion is often present [24], and rotator cuff impingement signs are often positive [24]. An unstable os acromiale should always be included in the differential diagnosis for any patient being evaluated for rotator cuff-related symptoms [24]. A very high index of suspicion for unstable os acromiale is warranted in younger patients who present with an atraumatic full-thickness or high-grade bursal-sided rotator cuff tear [24]. Most diagnoses are made incidentally on axillary view radiographs of the shoulder [41], as the axillary lateral radiograph demonstrates the presence of an os acromiale [71].

Classification

Prevalence: The overall frequency of os acromiale in a sample of 1594 skeletons was 8.34% [20]. Os acromiale is present in 8% of subjects [28]. In a Korean patient population visiting the shoulder outpatient clinic, os acromiale was observed in 7 per 1,000 patients [26].

Bilateral Presentation: Bilateral os acromiale was found in 15.4% of cases in a Korean patient population [26].

Subtype Rarity: Meta–os acromiale is the rarest subtype of os acromiale [38].

Acromioclavicular Joint Location: In a cohort of 33 subjects with os acromiale, 18.1% had the acromioclavicular joint lying on the acromion tip and 81.1% had it located distally [28]. In a control group of 211 volunteers, 52.1% had the articular facet of the acromioclavicular joint on the acromion tip and 45.4% had the facet tip located distally [28].

Clinical Presentation

Prevalence and Diagnostic Context

Os acromiale is a relatively common anatomical variant, with an incidence of 1.4% identified in a review of 1800 shoulder girdles [32]. In that same cohort, the lesion was bilateral in 62% of patients [32]. A study of Korean patients visiting a shoulder clinic found bilateral os acromiale in 15.4% of cases [26]. Because os acromiale is often asymptomatic and an incidental finding, determining its contribution to a patient’s symptoms can be challenging [24]. Awareness of the os acromiale in the young athlete, combined with appropriate clinical examination and image studies, is crucial to confirm the diagnosis [35].

History and Risk Factors

Patients typically present with no history of trauma, although converting a stable os acromiale into an unstable os after a traumatic event is possible [24]. If an os acromiale is noted, the patient should be specifically queried about any history of trauma [24]. There should be a very high index of suspicion in younger patients who present with an atraumatic full-thickness or high-grade bursal-sided rotator cuff tear [24]. An os acromiale should be included in the differential diagnosis of the swimming athlete who presents with shoulder pain, particularly in the setting of a failed response to physical therapy, activity modification, or steroid injection [23].

Symptoms and Physical Examination

Patients typically present with typical rotator cuff impingement symptoms, such as anterolateral shoulder pain, accentuated by overhead activity and positional night pain [24]. On physical examination, the synchondrosis may be tender to palpation [24]. A painful arc of motion is often present [24]. Rotator cuff impingement signs are often positive [24]. Weakness of abduction or external rotation may also be present [24].

Pathophysiology and Associations

Dynamic impingement on the rotator cuff by the unstable os acromiale is a believed source of pain; deltoid contraction during active elevation causes flexion of the mobile anterior acromial fragment [24]. Pain directly at the synchondrosis site from degenerative changes, analogous to an arthritic acromioclavicular joint, is another believed source of pain [24]. The synchondrosis of an os acromiale can be injured following trauma, though rarely [39]. The rate of rotator cuff tears for patients with os acromiale was comparable with that of patients without os acromiale, indicating that there is no definitive connection between os acromiale and rotator cuff pathology [26].

Postoperative Considerations

Postoperative local tenderness at the os acromiale can be expected in 1 out of 4 patients after reverse total shoulder arthroplasty but resolves spontaneously over time in the majority of patients [8]. Inferior tilting is observed in approximately one-third of the shoulders after reverse total shoulder arthroplasty and does not seem to change the overall outcome of the procedure [19]. The presence of os acromiale does not appear to have a negative impact on the clinical outcomes after surgery and reverse total shoulder arthroplasty remains a safe and effective treatment option [13].

Investigations

Plain radiography: Standardized plain films are almost always sufficient to garner the information needed for shoulder evaluation [34]. The axillary view is helpful in evaluation of glenoid morphology and provides good visualization of the coracoid process, acromion, and distal clavicle [68]. The scapular Y view provides visualization of the coracoacromial arch and can reveal coracoacromial spurs [68]. This view can also show acromial shape [68]. The synchondrosis of an os acromiale can be injured following trauma, emphasizing the need for appropriate radiographic investigation including axillary views [39]. The greater the distance of the AC joint from the anterior edge of the acromion, the higher the likelihood of an os acromiale [28].

MRI: MRI is the modality of choice for evaluating the rotator cuff, biceps, and subacromial/subdeltoid bursa [67]. T2-weighted MRI provides better visualization of full thickness rotator cuff tears [67]. MRI is useful to identify rotator cuff tears, although accuracy is enhanced by combining the scan with arthrography [60].

Ultrasonography: Ultrasonography is a simple and accurate test for identifying rotator cuff tears and calcific tendinitis [60]. It serves as a low-cost alternative to MRI and arthrography for evaluating both skeletal and soft-tissue structures of the shoulder [67]. Ultrasonography can provide immediate, real-time visualization of the rotator cuff, biceps tendon, and calcific deposits [67]. It can be used to measure the subacromial space and detect atrophy of rotator cuff muscles [67]. Ultrasonography is highly operator dependent and is not as useful for evaluating labral tears or rotator cuff tears that are very small or larger than 3 cm [67].

CT: CT imaging is frequently used to evaluate fractures of the shoulder, to assess for bony lesions in recurrent instability cases, or for preoperative templating for shoulder arthritis [67]. CT with three-dimensional reconstructions is the advanced imaging study of choice for determining the extent of glenoid bone loss in the setting of shoulder instability [68].

Treatment

Non-Operative Management

The provided evidence does not detail specific conservative protocols such as weight loss, physical therapy, NSAIDs, or injections. However, failed non-operative management is implicitly recognized as a prerequisite for surgical intervention in the context of symptomatic os acromiale.

Operative Management

Indications: Surgery is indicated for symptomatic os acromiale, particularly when the fragment is large and unstable. Osteosynthesis to achieve bony union is a safe, effective, and reliable technique for these cases [37].

Surgical Approach / Technique: Arthroscopic internal fixation is a predictable and consistent treatment for symptomatic os acromiale [9]. This approach offers improved visualization of the undersurface of the acromion, allowing excellent reduction, the possibility of treating concomitant shoulder pathologies, and avoidance of the increased morbidity associated with open approaches [27]. The technique requires no special instrumentation and may be performed reproducibly by surgeons familiar with arthroscopic techniques of the shoulder [29].

Implant Selection: Special consideration must be given to the type of tension-band construct used to achieve adequate compression and fixation for meta–os acromiale [38]. While osteosynthesis is established as a reliable method, high-level studies are still needed to compare outcomes between different fixation techniques [37].

Reverse Total Shoulder Arthroplasty (rTSA) Considerations

Preoperative acromial lesions are not a contraindication to reverse shoulder arthroplasty [72]. Inferior tilting is observed in approximately one-third of the shoulders after RTSA and does not seem to change the overall outcome of the procedure [19].

Complications

Rotator Cuff Pathology

An unstable os acromiale must always be included in the differential diagnosis for any patient being evaluated for rotator cuff-related symptoms [24].

Pain and Impingement

The exact mechanism by which an unstable os acromiale causes pain has not been definitively defined [24]. Patients with symptomatic os acromiale may also complain of superior shoulder pain [24]. Rotator cuff impingement signs are often positive in these patients, although concomitant rotator cuff pathology may also be a source of the patient's pain [24]. The synchondrosis may be tender to palpation in patients with symptomatic os acromiale [24].

Reverse Total Shoulder Arthroplasty (rTSA)

Postoperative local tenderness at the os acromiale can be expected in 1 out of 4 patients after rTSA, but it resolves spontaneously over time in the majority of patients [8]. Shoulders with os acromiale had a similar overall complication rate compared with matched controls (14% vs. 12%; P = 0.658) after rTSA [15].

Recovery

Light activity (weeks): The provided evidence does not specify a typical week range for the resumption of desk work, driving, or light activities of daily living.

Full activity (months): The provided evidence does not specify a month range for the return to manual work, sport, or full range of motion and strength.

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

Rehabilitation protocol: Management of symptomatic os acromiale is initially nonsurgical [3]. The technique described for symptomatic os acromiale in athletes is a predictable and consistent treatment [9].

Functional milestones: After 3 to 6 years of follow-up, all 6 shoulders undergoing osteosynthesis of an unstable mesoacromion were rated as excellent according to criteria similar to that of the American Shoulder and Elbow Surgeons [40]. At a two to five year followup, all five cases of unstable mesoacromion fusion had fused [89].

Other Considerations: An os acromiale should be suspected in cases of failed rotator cuff surgery or ongoing shoulder pain after failed treatment [4]. Os acromiale is associated with rotator cuff injuries [14]. Shoulders with os acromiale had a similar overall complication rate compared with matched controls (14% vs. 12%; P = 0.658) after reverse total shoulder arthroplasty [15]. Liberson identified an incidence of os acromiale of 1.4% in 1800 shoulder girdles, with the lesion being bilateral in 62% of patients [32].

Key Evidence

  • [L4] The majority of os acromiale is asymptomatic and requires no treatment. [1] (10.2147/orr.s141480)
  • [L4] While the radiographic appearance of an os acromiale is not uncommon in patients presenting with shoulder pain, a symptomatic, unstable os acromiale is relatively rare. [2] (10.1097/00132589-200412000-00006)
  • [L5] For the symptomatic os acromiale, management is initially nonsurgical. [3] (10.5435/00124635-200601000-00004)
  • [L4] An os acromiale should always be suspected in cases of failed rotator cuff surgery or ongoing shoulder pain after failed treatment. [4] (10.1097/00132589-200609000-00006)
  • [L4] The diagnosis of os acromiale should be based on imaging features and not limited by age. [5] (10.1007/s00256-015-2098-4)
  • [L4] Os acromiale is a co-condition in patients with rotator cuff tear. [6] (10.1080/00016470510030643)
  • [L4] Surgical management of symptomatic os acromiale that has failed nonoperative measures may predictably lead to improved outcomes. [7] (10.4103/0973-6042.80461)
  • [L3] Postoperative local tenderness at the os acromiale can be expected in 1 out of 4 patients but resolves spontaneously over time in the majority of patients. [8] (10.1177/2325967120965131)
  • [L4] This technique is a predictable and consistent treatment for symptomatic os acromiale in this population. [9] (10.1016/j.jse.2005.09.019)
  • [L4] Patients with a united os acromiale had a significantly better functional outcome as measured by the Constant score (P = .0169). [10] (10.1016/s1058-2746(98)90008-8)
  • [L3] There was no relation between the presence of os acromiale and shoulder pain, regardless of diagnosis. [11] (10.1177/23259671221078806)
  • [L4] Operative management of a symptomatic os acromiale that has failed initial nonoperative treatment leads to decreased symptoms and improvement in clinical outcomes. [12] (10.1016/j.jse.2019.05.047)
  • [L4] The presence of os acromiale does not appear to have a negative impact on the clinical outcomes after surgery and rTSA remains a safe and effective treatment option. [13] (10.1016/j.xrrt.2025.01.002)
  • [L3] The study supports previous findings that os acromiale is associated with rotator cuff injuries. [14] (10.1016/j.jseint.2025.05.015)
  • [L3] Shoulders with os acromiale had a similar overall complication rate compared with matched controls (14% vs. 12%; P = 0.658). [15] (10.5435/jaaos-d-25-00614)
  • [L4] A tear of the rotator cuff may often be associated with os acromiale, likely due to impingement from abnormal motion at the fibrous union site. [17] (10.2106/00004623-198466080-00029)
  • [L4] Inferior tilting is observed in approximately one-third of the shoulders after RTSA and does not seem to change the overall outcome of the procedure. [19] (10.1016/j.jse.2017.02.012)
  • [L4] The overall frequency of os acromiale in the sample of 1594 skeletons was 8.34%. [20] (10.1002/oa.877)
  • [L4] Surgical treatment is usually not indicated for os acromiale in the professional tennis player. [21] (10.1177/2325967118773723)
  • [L4] Care should be taken not to perform acromioplasty in the presence of painful os acromiale. [22] (10.1016/j.otsr.2012.10.020)
  • [L5] An os acromiale should be included in the differential diagnosis of the swimming athlete who presents with shoulder pain, particularly in the setting of a failed response to physical therapy, activity modification, or steroid injection. [23] (10.1177/1941738108326705)
  • [L5] [24] (10.5435/jaaos-d-17-00011)
  • [L4] Ipsilateral os acromiale may be a relative contraindication to the clavicle hook plate. [25] (10.1186/s12891-021-04841-1)
  • [L4] [26] (10.4055/cios.2013.5.3.202)
  • [L5] The advantages of this technique are improved visualization of the undersurface of the acromion, allowing excellent reduction, the possibility of treating concomitant shoulder pathologies, and to avoid increased morbidity of the open approach. [27] (10.1097/bte.0000000000000191)
  • [L4] [28] (10.1067/mse.2003.128136)
  • [L4] The surgical technique requires no special instrumentation and may be performed reproducibly by those familiar with arthroscopic techniques of the shoulder. [29] (10.1067/mse.2002.122227)
  • [L4] Surgical treatment of symptomatic meso-os acromiale with concomitant rotator cuff pathology yielded poorer outcomes than previously reported, with only 53% achieving satisfactory results overall. [30] (10.1016/j.jse.2005.08.024)
  • [L4] Awareness of the os acromiale in the young athlete, appropriate clinical examination, and image studies are crucial to confirm diagnosis. [35] (10.1016/j.jseint.2020.02.008)
  • [Paper] Osteosynthesis to achieve bony union is a safe, effective, and reliable technique for the treatment of large and unstable symptomatic os acromiale, though high-level studies are still needed to compare outcomes between different techniques. [37] (10.1016/j.eats.2017.03.025)
  • [L4] Meta–os acromiale is the rarest subtype of os acromiale, and special consideration must be given to the type of tension-band construct used to achieve adequate compression and fixation. [38] (10.1177/03635465211028238)
  • [L4] This case highlights that the synchondrosis of an os acromiale can be injured following trauma, though rarely, and emphasizes the need for appropriate radiographic investigation including axillary views and a flexible surgical approach. [39] (10.1016/j.jse.2008.02.012)
  • [L4] After 3 to 6 years of follow-up, all 6 shoulders were rated as excellent according to criteria similar to that of the American Shoulder and Elbow Surgeons. [40] (10.1016/s1058-2746(99)90004-6)
  • [L4] [41] (10.1302/2058-5241.4.180100)
  • [L3] Preoperative acromial lesions are not a contraindication to reverse shoulder arthroplasty. [72] (10.1016/j.jse.2008.12.002)
  • [L3] [84] (10.1016/j.otsr.2018.11.017)
  • [L4] At a two to five year followup all had fused. [89] (10.1016/s1058-2746(95)80192-8)

See Also

References

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[2] Treatment of the Symptomatic Os Acromiale. Techniques in Shoulder and Elbow Surgery. 2004. DOI: 10.1097/00132589-200412000-00006

[3] Symptomatic Os Acromiale. Journal of the American Academy of Orthopaedic Surgeons. 2006. DOI: 10.5435/00124635-200601000-00004

[4] Management of Os Acromiale. Techniques in Shoulder and Elbow Surgery. 2006. DOI: 10.1097/00132589-200609000-00006

[5] Differentiating os acromiale from normally developing acromial ossification centers using magnetic resonance imaging. Skeletal Radiology. 2015. DOI: 10.1007/s00256-015-2098-4

[6] Rotator cuff tears associated with os acromiale. Acta Orthopaedica. 2005. DOI: 10.1080/00016470510030643

[7] Systematic review of the surgical treatment for symptomatic os acromiale. International Journal of Shoulder Surgery. 2011. DOI: 10.4103/0973-6042.80461

[8] Os Acromiale in Reverse Total Shoulder Arthroplasty: A Cohort Study. Orthopaedic Journal of Sports Medicine. 2020. DOI: 10.1177/2325967120965131

[9] Painful os acromiale (or unfused acromial apophysis) in athletes. Journal of Shoulder and Elbow Surgery. 2006. DOI: 10.1016/j.jse.2005.09.019

[10] Transacromial approach to obtain fusion of unstable os acromiale. Journal of Shoulder and Elbow Surgery. 1998. DOI: 10.1016/s1058-2746(98)90008-8

[11] Prevalence of Os Acromiale in Thai Patients With Shoulder Problems: A Magnetic Resonance Imaging Study. Orthopaedic Journal of Sports Medicine. 2022. DOI: 10.1177/23259671221078806

[12] Os acromiale: systematic review of surgical outcomes. Journal of Shoulder and Elbow Surgery. 2020. DOI: 10.1016/j.jse.2019.05.047

[13] Clinical implications of reverse total shoulder arthroplasty with an os acromiale: a systematic review. JSES Reviews, Reports, and Techniques. 2025. DOI: 10.1016/j.xrrt.2025.01.002

[14] Prevalence and factors associated with os acromiale: a multicenter study. JSES International. 2025. DOI: 10.1016/j.jseint.2025.05.015

[15] The Influence of Os Acromiale on Patient Outcomes After Reverse Total Shoulder Arthroplasty: A Matched Cohort Study. Journal of the American Academy of Orthopaedic Surgeons. 2026. DOI: 10.5435/jaaos-d-25-00614

[17] Rotator cuff tears associated with os acromiale.. The Journal of Bone & Joint Surgery. 1984. DOI: 10.2106/00004623-198466080-00029

[19] Reverse shoulder arthroplasty in patients with os acromiale. Journal of Shoulder and Elbow Surgery. 2017. DOI: 10.1016/j.jse.2017.02.012

[20] The frequency of os acromiale in the Robert J. Terry Collection. International Journal of Osteoarchaeology. 2006. DOI: 10.1002/oa.877

[21] Os Acromiale in Professional Tennis Players. Orthopaedic Journal of Sports Medicine. 2018. DOI: 10.1177/2325967118773723

[22] Os acromiale, a cause of shoulder pain, not to be overlooked. Orthopaedics & Traumatology: Surgery & Research. 2013. DOI: 10.1016/j.otsr.2012.10.020

[23] Os Acromiale as a Cause for Shoulder Pain in a Competitive Swimmer: A Case Report. Sports Health: A Multidisciplinary Approach. 2009. DOI: 10.1177/1941738108326705

[24] Symptomatic, Unstable Os Acromiale. Journal of the American Academy of Orthopaedic Surgeons. 2018. DOI: 10.5435/jaaos-d-17-00011

[25] Os acromiale may be a contraindication of the clavicle hook plate: case reports and literature review. BMC Musculoskeletal Disorders. 2021. DOI: 10.1186/s12891-021-04841-1

[26] The Prevalence of Os Acromiale in Korean Patients Visiting Shoulder Clinic. Clinics in Orthopedic Surgery. 2013. DOI: 10.4055/cios.2013.5.3.202

[27] Arthroscopic Internal Fixation of Symptomatic Os Acromiale. Techniques in Shoulder & Elbow Surgery. 2020. DOI: 10.1097/bte.0000000000000191

[28] Relationship between os acromiale and acromioclavicular joint anatomic position. Journal of Shoulder and Elbow Surgery. 2003. DOI: 10.1067/mse.2003.128136

[29] Surgical management of the symptomatic os acromiale. Journal of Shoulder and Elbow Surgery. 2002. DOI: 10.1067/mse.2002.122227

[30] Surgical treatment of os acromiale with and without associated rotator cuff tears. Journal of Shoulder and Elbow Surgery. 2006. DOI: 10.1016/j.jse.2005.08.024

[32] Types of os acromiale according to Liberson. 2006.

[34] Rockwood And Matsen S The Shoulder. Arthroscopic Management of Prearthritic and Arthritic Conditions of the Shoulder and the Postarthroplasty Shoulder > Radiographic Evaluation.

[35] The unstable os acromiale: a cause of pain in the young athlete. JSES International. 2020. DOI: 10.1016/j.jseint.2020.02.008

[37] Operative Treatment of Symptomatic Meso‐Type Os Acromiale. Arthroscopy Techniques. 2017. DOI: 10.1016/j.eats.2017.03.025

[38] Rare Symptomatic Meta–Os Acromiale in an Athlete. The American Journal of Sports Medicine. 2021. DOI: 10.1177/03635465211028238

[39] Fracture of an os acromiale with associated rupture of the coracoclavicular ligaments. Journal of Shoulder and Elbow Surgery. 2008. DOI: 10.1016/j.jse.2008.02.012

[40] Successful osteosynthesis of an unstable mesoacromion in 6 shoulders: A new technique. Journal of Shoulder and Elbow Surgery. 1999. DOI: 10.1016/s1058-2746(99)90004-6

[41] Os acromiale: a review of its incidence, pathophysiology, and clinical management. EFORT Open Reviews. 2019. DOI: 10.1302/2058-5241.4.180100

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[70] Campbell S Operative Orthopaedics 4 Volume Set. ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION WITH BONE-PATELLAR TENDON-BONE GRAFT > Developmental Stages of Impingement Syndrome.

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[72] Acromial insufficiency in reverse shoulder arthroplasties. Journal of Shoulder and Elbow Surgery. 2009. DOI: 10.1016/j.jse.2008.12.002

[84] Do we need to stabilize and treat the os acromiale when performing reverse shoulder arthroplasty?. Orthopaedics & Traumatology: Surgery & Research. 2019. DOI: 10.1016/j.otsr.2018.11.017

[89] Successful fusion of the unstable mesoacromion: Technique and report of five cases. Journal of Shoulder and Elbow Surgery. 1995. DOI: 10.1016/s1058-2746(95)80192-8

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Considerations for the public: By using one of our public licenses, a licensor grants the public permission to use the licensed material under specified terms and conditions. If the licensor's permission is not necessary for any reason--for example, because of any applicable exception or limitation to copyright--then that use is not regulated by the license. Our licenses grant only permissions under copyright and certain other rights that a licensor has authority to grant. Use of the licensed material may still be restricted for other reasons, including because others have copyright or other rights in the material. A licensor may make special requests, such as asking that all changes be marked or described. Although not required by our licenses, you are encouraged to respect those requests where reasonable. More considerations for the public: wiki.creativecommons.org/Considerations_for_licensees

Creative Commons Attribution-NonCommercial 4.0 International Public License

By exercising the Licensed Rights (defined below), You accept and agree to be bound by the terms and conditions of this Creative Commons Attribution-NonCommercial 4.0 International Public License ("Public License"). To the extent this Public License may be interpreted as a contract, You are granted the Licensed Rights in consideration of Your acceptance of these terms and conditions, and the Licensor grants You such rights in consideration of benefits the Licensor receives from making the Licensed Material available under these terms and conditions.

Section 1 -- Definitions.

a. Adapted Material means material subject to Copyright and Similar Rights that is derived from or based upon the Licensed Material and in which the Licensed Material is translated, altered, arranged, transformed, or otherwise modified in a manner requiring permission under the Copyright and Similar Rights held by the Licensor. For purposes of this Public License, where the Licensed Material is a musical work, performance, or sound recording, Adapted Material is always produced where the Licensed Material is synched in timed relation with a moving image.

b. Adapter's License means the license You apply to Your Copyright and Similar Rights in Your contributions to Adapted Material in accordance with the terms and conditions of this Public License.

c. Copyright and Similar Rights means copyright and/or similar rights closely related to copyright including, without limitation, performance, broadcast, sound recording, and Sui Generis Database Rights, without regard to how the rights are labeled or categorized. For purposes of this Public License, the rights specified in Section 2(b)(1)-(2) are not Copyright and Similar Rights.

d. Effective Technological Measures means those measures that, in the absence of proper authority, may not be circumvented under laws fulfilling obligations under Article 11 of the WIPO Copyright Treaty adopted on December 20, 1996, and/or similar international agreements.

e. Exceptions and Limitations means fair use, fair dealing, and/or any other exception or limitation to Copyright and Similar Rights that applies to Your use of the Licensed Material.

f. Licensed Material means the artistic or literary work, database, or other material to which the Licensor applied this Public License.

g. Licensed Rights means the rights granted to You subject to the terms and conditions of this Public License, which are limited to all Copyright and Similar Rights that apply to Your use of the Licensed Material and that the Licensor has authority to license.

h. Licensor means the individual(s) or entity(ies) granting rights under this Public License.

i. NonCommercial means not primarily intended for or directed towards commercial advantage or monetary compensation. For purposes of this Public License, the exchange of the Licensed Material for other material subject to Copyright and Similar Rights by digital file-sharing or similar means is NonCommercial provided there is no payment of monetary compensation in connection with the exchange.

j. Share means to provide material to the public by any means or process that requires permission under the Licensed Rights, such as reproduction, public display, public performance, distribution, dissemination, communication, or importation, and to make material available to the public including in ways that members of the public may access the material from a place and at a time individually chosen by them.

k. Sui Generis Database Rights means rights other than copyright resulting from Directive 96/9/EC of the European Parliament and of the Council of 11 March 1996 on the legal protection of databases, as amended and/or succeeded, as well as other essentially equivalent rights anywhere in the world.

l. You means the individual or entity exercising the Licensed Rights under this Public License. Your has a corresponding meaning.

Section 2 -- Scope.

a. License grant.

1. Subject to the terms and conditions of this Public License, the Licensor hereby grants You a worldwide, royalty-free, non-sublicensable, non-exclusive, irrevocable license to exercise the Licensed Rights in the Licensed Material to:

a. reproduce and Share the Licensed Material, in whole or in part, for NonCommercial purposes only; and

b. produce, reproduce, and Share Adapted Material for NonCommercial purposes only.

2. Exceptions and Limitations. For the avoidance of doubt, where Exceptions and Limitations apply to Your use, this Public License does not apply, and You do not need to comply with its terms and conditions.

3. Term. The term of this Public License is specified in Section 6(a).

4. Media and formats; technical modifications allowed. The Licensor authorizes You to exercise the Licensed Rights in all media and formats whether now known or hereafter created, and to make technical modifications necessary to do so. The Licensor waives and/or agrees not to assert any right or authority to forbid You from making technical modifications necessary to exercise the Licensed Rights, including technical modifications necessary to circumvent Effective Technological Measures. For purposes of this Public License, simply making modifications authorized by this Section 2(a) (4) never produces Adapted Material.

5. Downstream recipients.

a. Offer from the Licensor -- Licensed Material. Every recipient of the Licensed Material automatically receives an offer from the Licensor to exercise the Licensed Rights under the terms and conditions of this Public License.

b. No downstream restrictions. You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, the Licensed Material if doing so restricts exercise of the Licensed Rights by any recipient of the Licensed Material.

6. No endorsement. Nothing in this Public License constitutes or may be construed as permission to assert or imply that You are, or that Your use of the Licensed Material is, connected with, or sponsored, endorsed, or granted official status by, the Licensor or others designated to receive attribution as provided in Section 3(a)(1)(A)(i).

b. Other rights.

1. Moral rights, such as the right of integrity, are not licensed under this Public License, nor are publicity, privacy, and/or other similar personality rights; however, to the extent possible, the Licensor waives and/or agrees not to assert any such rights held by the Licensor to the limited extent necessary to allow You to exercise the Licensed Rights, but not otherwise.

2. Patent and trademark rights are not licensed under this Public License.

3. To the extent possible, the Licensor waives any right to collect royalties from You for the exercise of the Licensed Rights, whether directly or through a collecting society under any voluntary or waivable statutory or compulsory licensing scheme. In all other cases the Licensor expressly reserves any right to collect such royalties, including when the Licensed Material is used other than for NonCommercial purposes.

Section 3 -- License Conditions.

Your exercise of the Licensed Rights is expressly made subject to the following conditions.

a. Attribution.

1. If You Share the Licensed Material (including in modified form), You must:

a. retain the following if it is supplied by the Licensor with the Licensed Material:

i. identification of the creator(s) of the Licensed Material and any others designated to receive attribution, in any reasonable manner requested by the Licensor (including by pseudonym if designated);

ii. a copyright notice;

iii. a notice that refers to this Public License;

iv. a notice that refers to the disclaimer of warranties;

v. a URI or hyperlink to the Licensed Material to the extent reasonably practicable;

b. indicate if You modified the Licensed Material and retain an indication of any previous modifications; and

c. indicate the Licensed Material is licensed under this Public License, and include the text of, or the URI or hyperlink to, this Public License.

2. You may satisfy the conditions in Section 3(a)(1) in any reasonable manner based on the medium, means, and context in which You Share the Licensed Material. For example, it may be reasonable to satisfy the conditions by providing a URI or hyperlink to a resource that includes the required information.

3. If requested by the Licensor, You must remove any of the information required by Section 3(a)(1)(A) to the extent reasonably practicable.

4. If You Share Adapted Material You produce, the Adapter's License You apply must not prevent recipients of the Adapted Material from complying with this Public License.

Section 4 -- Sui Generis Database Rights.

Where the Licensed Rights include Sui Generis Database Rights that apply to Your use of the Licensed Material:

a. for the avoidance of doubt, Section 2(a)(1) grants You the right to extract, reuse, reproduce, and Share all or a substantial portion of the contents of the database for NonCommercial purposes only;

b. if You include all or a substantial portion of the database contents in a database in which You have Sui Generis Database Rights, then the database in which You have Sui Generis Database Rights (but not its individual contents) is Adapted Material; and

c. You must comply with the conditions in Section 3(a) if You Share all or a substantial portion of the contents of the database.

For the avoidance of doubt, this Section 4 supplements and does not replace Your obligations under this Public License where the Licensed Rights include other Copyright and Similar Rights.

Section 5 -- Disclaimer of Warranties and Limitation of Liability.

a. UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE EXTENT POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS AND AS-AVAILABLE, AND MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND CONCERNING THE LICENSED MATERIAL, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHER. THIS INCLUDES, WITHOUT LIMITATION, WARRANTIES OF TITLE, MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, NON-INFRINGEMENT, ABSENCE OF LATENT OR OTHER DEFECTS, ACCURACY, OR THE PRESENCE OR ABSENCE OF ERRORS, WHETHER OR NOT KNOWN OR DISCOVERABLE. WHERE DISCLAIMERS OF WARRANTIES ARE NOT ALLOWED IN FULL OR IN PART, THIS DISCLAIMER MAY NOT APPLY TO YOU.

b. TO THE EXTENT POSSIBLE, IN NO EVENT WILL THE LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY (INCLUDING, WITHOUT LIMITATION, NEGLIGENCE) OR OTHERWISE FOR ANY DIRECT, SPECIAL, INDIRECT, INCIDENTAL, CONSEQUENTIAL, PUNITIVE, EXEMPLARY, OR OTHER LOSSES, COSTS, EXPENSES, OR DAMAGES ARISING OUT OF THIS PUBLIC LICENSE OR USE OF THE LICENSED MATERIAL, EVEN IF THE LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH LOSSES, COSTS, EXPENSES, OR DAMAGES. WHERE A LIMITATION OF LIABILITY IS NOT ALLOWED IN FULL OR IN PART, THIS LIMITATION MAY NOT APPLY TO YOU.

c. The disclaimer of warranties and limitation of liability provided above shall be interpreted in a manner that, to the extent possible, most closely approximates an absolute disclaimer and waiver of all liability.

Section 6 -- Term and Termination.

a. This Public License applies for the term of the Copyright and Similar Rights licensed here. However, if You fail to comply with this Public License, then Your rights under this Public License terminate automatically.

b. Where Your right to use the Licensed Material has terminated under Section 6(a), it reinstates:

1. automatically as of the date the violation is cured, provided it is cured within 30 days of Your discovery of the violation; or

2. upon express reinstatement by the Licensor.

For the avoidance of doubt, this Section 6(b) does not affect any right the Licensor may have to seek remedies for Your violations of this Public License.

c. For the avoidance of doubt, the Licensor may also offer the Licensed Material under separate terms or conditions or stop distributing the Licensed Material at any time; however, doing so will not terminate this Public License.

d. Sections 1, 5, 6, 7, and 8 survive termination of this Public License.

Section 7 -- Other Terms and Conditions.

a. The Licensor shall not be bound by any additional or different terms or conditions communicated by You unless expressly agreed.

b. Any arrangements, understandings, or agreements regarding the Licensed Material not stated herein are separate from and independent of the terms and conditions of this Public License.

Section 8 -- Interpretation.

a. For the avoidance of doubt, this Public License does not, and shall not be interpreted to, reduce, limit, restrict, or impose conditions on any use of the Licensed Material that could lawfully be made without permission under this Public License.

b. To the extent possible, if any provision of this Public License is deemed unenforceable, it shall be automatically reformed to the minimum extent necessary to make it enforceable. If the provision cannot be reformed, it shall be severed from this Public License without affecting the enforceability of the remaining terms and conditions.

c. No term or condition of this Public License will be waived and no failure to comply consented to unless expressly agreed to by the Licensor.

d. Nothing in this Public License constitutes or may be interpreted as a limitation upon, or waiver of, any privileges and immunities that apply to the Licensor or You, including from the legal processes of any jurisdiction or authority.

Creative Commons is not a party to its public licenses. Notwithstanding, Creative Commons may elect to apply one of its public licenses to material it publishes and in those instances will be considered the “Licensor.” The text of the Creative Commons public licenses is dedicated to the public domain under the CC0 Public Domain Dedication. Except for the limited purpose of indicating that material is shared under a Creative Commons public license or as otherwise permitted by the Creative Commons policies published at creativecommons.org/policies, Creative Commons does not authorize the use of the trademark "Creative Commons" or any other trademark or logo of Creative Commons without its prior written consent including, without limitation, in connection with any unauthorized modifications to any of its public licenses or any other arrangements, understandings, or agreements concerning use of licensed material. For the avoidance of doubt, this paragraph does not form part of the public licenses.

Creative Commons may be contacted at creativecommons.org.