Os Acromiale

Os acromiale ("bone of the acromion") is a developmental anomaly. One of the secondary ossification centres of the acromion fails to fuse with the rest of the acromion, and the persistent gap is bridged by fibrocartilaginous rather than bony tissue.

It is neither a fracture nor, in itself, a disease: the majority are incidental and asymptomatic, and it becomes clinically relevant only when it produces symptoms.

Why it matters. It is a recognised but under-appreciated cause of shoulder pain and secondary impingement, and it is easily mistaken for a fracture, which can lead to inappropriate treatment. It must be identified before subacromial surgery, because operating through or around an unstable os without addressing it can lead to persistent symptoms.

How common. The reported prevalence varies widely with the population and the imaging method:

• 1% to 15% is the range generally quoted, and the condition is described as particularly common in people of African descent (You et al., 2019, comprehensive review) DOI
• 6.8% in a prospective series of patients presenting with shoulder pain (Burbank et al., 2007) [DOI not indexed; PMID 17461398]
• 0.7% in a large Korean shoulder-clinic radiographic series, illustrating real ethnic and methodological variation (Kumar et al., 2013) DOI
• 3.4% in a review of more than 1,000 shoulder MRIs, with no significant sex difference and a tendency to right-sided dominance (Rovesta et al., 2017) DOI

The condition is bilateral in around a third of cases.

Development. The acromion forms from four secondary ossification centres that appear in adolescence, around 14-16 years, and normally fuse with one another and with the scapular spine. From anterior to posterior they are the pre-acromion, meso-acromion, meta-acromion and basi-acromion. Fusion is usually complete by 22-25 years.

The deltoid arises from the acromion. The anterior deltoid originates from the anterior acromial border and the lateral third of the clavicle, the middle (acromial) deltoid from the lateral margin and superior surface of the acromion, and the posterior deltoid from the scapular spine. A meso-acromion therefore carries a substantial portion of the anterior and middle deltoid origin, and preserving that origin is what the whole surgical strategy hinges on.

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How it causes symptoms. Symptoms arise only from a mobile unstable fragment, secondary impingement or an associated rotator cuff tear, and the three mechanisms often coexist:

1. Micromotion. The unfused meso-acromion is mobile. The deltoid pulls the fragment downward as it contracts, narrowing the subacromial space and irritating the rotator cuff, and motion at the synchondrosis itself can also be painful.
2. Secondary impingement. The downward-tilting fragment and any reactive osteophytes at the gap margins reduce the space available for supraspinatus, producing classic impingement symptoms.
3. Cuff pathology. Os acromiale is associated with supraspinatus tendinopathy and tears; the strength of that association is discussed under Associations below.

The os is named by which centre has failed to fuse, or more precisely by where the unfused gap lies. The meso-acromion is the most common symptomatic form because it lies under the deforming pull of the deltoid, and it is the one fixed rather than excised when the fragment is large.

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Park's MRI types. Park and colleagues described os acromiale on MRI in patients with rotator cuff disease, typed by which junction had failed to fuse (Park et al., 1994) DOI. Their type A is the meso-/meta-acromion gap of the meso-acromion in the table above.

• Type A: gap between the meso-acromion and meta-acromion; by far the most common (8 of 10 in their series)
• Type B: gap between the pre-acromion and meso-acromion; uncommon
• Type C: gap between the meta-acromion and basi-acromion; uncommon

History. The os is often an incidental, asymptomatic finding on imaging done for another reason. When it is symptomatic the pain is anterior-superior, with pain on overhead activity and sometimes night pain, and it is frequently indistinguishable from ordinary subacromial impingement.

A history of failed impingement treatment, or of persistent symptoms after subacromial surgery, should raise suspicion of a missed os.

Examination. Palpate the acromion carefully.

• Point tenderness directly over the acromial gap is the most useful specific sign
• Impingement signs (Neer, Hawkins-Kennedy) may be positive because of secondary impingement, which is exactly why the os is mislabelled as routine impingement
• Weakness on rotator cuff testing suggests an associated cuff tear
• Examine the contralateral shoulder, given frequent bilaterality

Radiographs. Request a full series of AP, scapular Y and axillary (axial) views. The os is frequently invisible on the AP view and is best seen on the axillary view, where the unfused fragment is demonstrated anterior to the rest of the acromion; in Burbank's series it was seen on the axillary view in every case.

Always request an axillary view (or a scapular Y, axial CT or MRI) when assessing shoulder pain, as it is the single most reliable plain-film way to identify the fusion defect.

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The double-density sign. On the AP view, a double density occurs when the cortical edge of a meso-acromion overlaps the acromial base.

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MRI. MRI shows the transversely oriented fusion defect with characteristic irregular margins. Crucially, it demonstrates bone-marrow oedema and interface oedema around a symptomatic, mobile os, features that help separate a genuinely painful os from a quiet incidental one, and it evaluates the rotator cuff and bursa in the same study.

Alignment. Sagittal MRI also distinguishes an aligned os from a step-off deformity, in which inferior displacement of the fragment can narrow the subacromial space.

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CT. CT best defines the bony anatomy, fragment size and any sclerosis or osteophytes at the gap, which makes it useful for planning fixation versus excision. In difficult or adolescent cases, SPECT-CT can demonstrate increased activity at a symptomatic synchondrosis, supporting it as the pain source.

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Adolescents. Because normal fusion may not complete until 22-25 years, calling an unfused centre an os acromiale in a teenager is contentious.

MRI can still distinguish the two: a true os acromiale shows a transverse interface orientation with irregular margins, whereas a normally developing centre shows an arched interface with lobulated margins.

Using these features, adult cases were diagnosed with 100% accuracy and most adolescents were correctly identified as normal development, although a minority of adolescents did have genuine os acromiale. Diagnosis should therefore be based on imaging features, not age alone (Winfeld et al., 2015) DOI.

Treat the patient, not the X-ray. If the os is incidental, reassure the patient. Before any operation, confirm that the os is the genuine pain source, with point tenderness over it and an injection test.

Conservative care first. For symptomatic os acromiale, non-operative treatment is started first, exactly as for impingement (You et al., 2019) DOI:

• Activity modification, immediately: reduce provocative overhead and loading activities
• NSAIDs, weeks 0-6: for pain and any reactive inflammation
• Physiotherapy, weeks 0-12: rotator cuff and scapular stabiliser strengthening to optimise dynamic control and reduce secondary impingement
• Injection, as required: a subacromial (or peri-os) corticosteroid and local-anaesthetic injection is both therapeutic and diagnostic, because relief of pain localises the os or subacromial space as the pain source and predicts response

A large proportion of patients settle. Surgery is reserved for those who fail an adequate trial, typically several months.

Fix or excise: fragment size decides. Excising a large meso-acromion detaches the deltoid origin it carries, weakening forward flexion and abduction and leaving a poor functional result, so a large mobile fragment is reduced and fixed. Open excision of a large fragment is discouraged because it can cause persistent deltoid dysfunction and weakness of flexion; open excision is reserved for small fragments or salvage after failed fixation (Ortiguera & Buss, 2002) DOI. A small fragment carrying little deltoid can be excised, or resected arthroscopically, with good results. Acromioplasty is different: removing bone from the acromial undersurface for impingement does not detach the deltoid, which is the anatomical reason the two operations differ.

Fixation. For a large, mobile, symptomatic meso-acromion, the aim is bony union with the anterior deltoid origin preserved. Reduction and compression across the prepared synchondrosis convert a mobile fibrocartilaginous gap into a stable bony union.

• Open reduction and internal fixation uses K-wires or cannulated screws with a tension band of wire or suture, often with bone grafting, and gives reliable union and good satisfaction. In a 26-patient series union was 96%, 92% were satisfied and mean time to union was about 4 months; the main drawback was hardware irritation requiring removal in about a third (Peckett et al., 2004) DOI
• Open plate osteosynthesis exposes the entire fragment, compresses the nonunion and secures the acromion with a contoured plate while preserving the deltoid attachment
• Suture fixation avoids hardware problems. Arthroscopic fixation of meso-acromion with polyester sutures achieved bony union in all 10 shoulders at 12-month CT, with improved Constant and VAS scores and no hardware irritation (Guo et al., 2019) DOI
• Arthroscopy-assisted double-button fixation debrides both bone surfaces, closes the gap under compression and secures the reduction without prominent screw heads

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The os as a surgical window. Concurrent rotator cuff repair is performed where a tear coexists. Because the meso-acromion sits over supraspinatus, a mobile os can even be used deliberately as an acromion-splitting approach to expose and repair a massive cuff tear, after which the acromion is re-fixed, analogous to a controlled acromial osteotomy (Richman et al., 1997) DOI.

Excision and arthroscopic resection. For a small fragment unsuitable for reattachment, or after failed fixation, excision or arthroscopic resection is preferred. Arthroscopic subacromial decompression with resection of the unstable meso-acromion can give satisfactory results without the morbidity of open surgery, with faster rehabilitation and the ability to treat coexisting impingement or cuff pathology at the same sitting (Ortiguera & Buss, 2002) DOI.

Arthroscopy is generally favoured, for fixation and resection alike, because of fewer complications, especially in older patients and overhead athletes, in whom concurrent cuff and impingement pathology is common (You et al., 2019) DOI.

Nonunion after fixation. Internal fixation of the os can fail to unite. Nonunion rates were historically a concern, though modern tension-band and suture techniques report union rates of 96-100% in selected series (Peckett 2004; Guo 2019), and bone grafting improves union.

Hardware irritation. The acromion is subcutaneous, so prominent K-wires and screws become symptomatic, hence the removal rate quoted under Fixation; this drove the development of suture-based techniques.

Missed diagnosis and failed surgery. An unrecognised os acromiale is a cause of persistent symptoms after subacromial decompression. Failing to obtain an axillary view, or to address a mobile os, leaves the pain generator untreated.

Operating on the wrong diagnosis. Acromioplasty spares the deltoid origin and still fails to treat a symptomatic os, because the pain is generated at the mobile synchondrosis rather than under the acromion. In Peckett's series all 26 patients had first been treated as impingement, and one had already undergone arthroscopic acromioplasty without benefit before the os was fixed. Taken with Burbank's series above, the recurring error is operating on the wrong diagnosis.

Rotator cuff disease. Os acromiale is reported in association with subacromial impingement and supraspinatus tears. In Park's MRI series every patient had a supraspinatus lesion and 9 of 10 showed osteophytic lipping at the gap margins (Park et al., 1994) DOI. The strength of the cuff-tear association is population-dependent, however: the large Korean series found no statistically significant association between os acromiale and cuff tears (Kumar et al., 2013) DOI. In the exam, state that there is a recognised association with impingement and cuff pathology, but that it is not invariable.

Reverse total shoulder arthroplasty. An unstable os acromiale can become symptomatic or fracture after reverse total shoulder arthroplasty (RTSA) because of altered deltoid tensioning (You et al., 2019) DOI. A reverse prosthesis medialises and distalises the centre of rotation and lengthens and tensions the deltoid. The tightened deltoid pulls harder on its acromial origin, greatly increasing the bending stress across the acromion and across any weak, fibrocartilage-bridged os.

The acromial stress fracture. An acromial or scapular-spine stress fracture is a recognised cause of new pain and loss of active forward elevation with anterior/lateral tilt after an initially successful RTSA, and an unstable os acts as a stress riser that predisposes to it.

The Levy classification of these fractures:

• Type I: anterior acromion, involving only part of the middle deltoid origin
• Type II: posterior acromion or more of the acromion, involving the entire middle deltoid origin
• Type III: the scapular spine

Management is usually non-operative, with immobilisation or bracing, because fixation in thin, osteopenic acromial bone is difficult and frequently fails. The fracture nonetheless often leaves a worse functional result, which is the practical reason to recognise an os acromiale before performing an RTSA.

Before any shoulder surgery. Because the os alters acromial bony anatomy and the deltoid origin, recognising it before any subacromial or arthroplasty surgery is essential to avoid intra-operative surprises and to plan whether to fix, excise or work around it.

There is no single dedicated international guideline for os acromiale; management is guided by the shoulder-impingement and rotator-cuff literature plus the os-specific case series above. The globally consistent principles are:

• Diagnosis depends on the axillary view (or axial CT/MRI). This is emphasised across US, European and Asian series alike, reflecting how often the condition is missed on the AP film (Burbank et al., 2007, PMID 17461398; Rovesta et al., 2017) DOI.
• Prevalence varies markedly by population - quoted at 1-15% generally and higher in people of African descent, but as low as 0.7% in a Korean radiographic series, underlining genuine ethnic and methodological variation rather than one universal figure (Kumar et al., 2013 DOI; You et al., 2019 DOI).
• Conservative treatment first, then size-matched surgery is the universally recommended pathway: fixation for large mobile fragments and excision/arthroscopic resection for small ones, with a global trend toward arthroscopic techniques to reduce morbidity and treat concomitant cuff/impingement pathology (You et al., 2019) DOI.
• Registry data are not specific to os acromiale; the relevant arthroplasty registries (NJR, AJRR, AOANJRR) capture reverse shoulder arthroplasty outcomes, within which an unstable os acromiale is recognised as a potential source of post-arthroplasty acromial complications.

Related pages: Subacromial Impingement for the diagnosis os acromiale is most often mistaken for — five of Burbank's six had been labelled with it — and Rotator Cuff Tears for the pathology that so often coexists, though whether the association is causal is disputed on this page; Reverse Total Shoulder Arthroplasty for the setting in which an unfused acromion becomes a stress-fracture risk; Deltoid for the origin whose preservation decides fixation against excision; and Scapular Dyskinesis for the wider scapular mechanics of a painful shoulder.

All evidence cited on this page is drawn from the referenced articles; please see the linked DOIs for the primary sources.