Os Acromiale

Os acromiale ("bone of the acromion") is a developmental anomaly in which one of the secondary ossification centres of the acromion fails to fuse with the rest of the acromion, leaving a persistent gap bridged by fibrocartilaginous (rather than bony) tissue. It is not a fracture and it is not, in itself, a disease - 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 a recognised cause of failed shoulder surgery when missed.
• 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.

Epidemiology - what the studies show

According to PubMed, the reported prevalence varies widely with population and imaging method:

• The prevalence is generally quoted as 1% to 15%, and is described as particularly common in people of African descent (You et al., 2019, comprehensive review) DOI.
• In a prospective series of patients presenting with shoulder pain, os acromiale was found in 6.8%, was seen on the axillary view in every case, and most of those patients had been labelled with impingement or a cuff tear instead (Burbank et al., 2007) [DOI not indexed; PMID 17461398].
• A large Korean shoulder-clinic radiographic series found a much lower prevalence of 0.7%, illustrating real ethnic and methodological variation, and found no statistically significant association with rotator cuff tears in that population (Kumar et al., 2013) DOI.
• A review of more than 1,000 shoulder MRIs reported a frequency of 3.4%, with no significant sex difference and a tendency to right-sided dominance (Rovesta et al., 2017) DOI.

The headline for exams: prevalence is roughly 1-15%, the meso-acromion is the most common symptomatic type, and the condition is bilateral in around a third of cases.

Normal acromial 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 these are the pre-acromion, meso-acromion, meta-acromion and basi-acromion. Fusion is usually complete by age 22-25 years.

Classification by site

The os is named by which centre has failed to fuse (more precisely, where the unfused gap lies):

• Pre-acromion - failure at the most anterior centre.
• Meso-acromion - the mid-segment is separate; this is the most common symptomatic form because it lies under the deforming pull of the deltoid.
• Meta-acromion - the posterior segment is separate.
• Basi-acromion - rare, at the base near the scapular spine.

Park and colleagues described MR imaging types A, B and C based on which junction had failed: type A (between meso- and meta-acromion) was by far the most common in their series of patients with rotator cuff disease, with type B (pre-/meso-) and type C (meta-/basi-) being uncommon (Park et al., 1994) DOI.

How it causes symptoms

There are three main mechanisms, and they often coexist:

1. Instability / micromotion of the fragment - the unfused meso-acromion is mobile. The deltoid pulls the fragment downward during contraction, narrowing the subacromial space and irritating the rotator cuff. Motion at the synchondrosis itself can also be painful.
2. Secondary subacromial impingement - the downward-tilting fragment and any reactive osteophytes at the gap margins reduce the space available for the supraspinatus, producing classic impingement symptoms.
3. Associated rotator cuff pathology - os acromiale is associated with supraspinatus tendinopathy and tears. In MR series of patients with cuff disease, nearly all os acromiale shoulders had a supraspinatus lesion (tendinitis or tear), and osteophytic lipping was common at the gap margins (Park et al., 1994) DOI.

History

• Often an incidental, asymptomatic finding on imaging done for another reason.
• When symptomatic: anterior-superior shoulder pain, frequently indistinguishable from ordinary subacromial impingement, with pain on overhead activity and sometimes night pain.
• A history of failed impingement treatment or persistent symptoms after subacromial surgery should raise suspicion of a missed os.

Examination

• Point tenderness directly over the acromial gap is the most useful specific sign - palpate the acromion carefully.
• Standard impingement signs may be positive (Neer, Hawkins-Kennedy) because of secondary impingement, which is exactly why it is mislabelled as routine impingement.
• Assess rotator cuff strength - weakness suggests an associated cuff tear.
• Examine the contralateral shoulder, given frequent bilaterality.

Diagnosing os acromiale in adolescents

Because normal fusion may not complete until 22-25 years, calling an unfused centre an os acromiale in a teenager is contentious. According to PubMed, 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 - so diagnosis should be based on imaging features, not age alone (Winfeld et al., 2015) DOI.

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The whole surgical strategy hinges on "preserving the anterior deltoid origin" — worth developing, because the deltoid origin is exactly what a large os carries.

• 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.
• Why you fix a large fragment. Excising a large meso-acromion detaches that deltoid origin, weakening forward flexion and abduction and leaving a poor functional result — so a large mobile fragment is reduced and fixed (preserving the origin) rather than removed. Only a small fragment carrying little deltoid can be safely excised.
• The os as a deliberate surgical window. Because the meso-acromion sits over the supraspinatus, a mobile os can be used as an acromion-splitting approach to expose and repair a massive rotator cuff tear, after which the acromion is re-fixed (analogous to a controlled acromial osteotomy).
• Contrast with acromioplasty. Removing bone from the acromial undersurface for impingement is safe because it does not detach the deltoid; excising the whole os is not — this is the anatomical reason the two operations differ.

• Association with rotator cuff disease: os acromiale is reported in association with subacromial impingement and supraspinatus tears, and reactive osteophytes at the gap margins narrow the subacromial space (Park et al., 1994) DOI. However, the strength of the cuff-tear association is population-dependent - a large Korean series found no statistically significant association between os acromiale and cuff tears (Kumar et al., 2013) DOI. For exams: 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 shoulder arthroplasty because of altered deltoid tensioning - a point flagged in contemporary reviews (You et al., 2019) DOI.
• Pre-operative planning: 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.

The clinical-relevance section and a viva follow-up both raise why an os becomes symptomatic after a reverse total shoulder arthroplasty (RTSA) — here is the mechanism and classification.

• Why RTSA loads the acromion. A reverse prosthesis medialises and distalises the centre of rotation and lengthens and tensions the deltoid. The now-tightened deltoid pulls harder on its acromial origin, greatly increasing the bending stress across the acromion — and across any weak, fibrocartilage-bridged os acromiale.
• The result — an acromial or scapular-spine stress fracture. This is a recognised cause of new pain and loss of active forward elevation with anterior/lateral tilt after an initially successful RTSA; an unstable os acts as a stress riser that predisposes to it.
• Levy classification of post-RTSA acromial/scapular-spine fractures:
  • Type I — anterior acromion (only part of the middle deltoid origin involved).
  • Type II — posterior acromion / more of the acromion (the entire middle deltoid origin).
  • Type III — the scapular spine.
• Management is usually non-operative (immobilisation/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.

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.

According to PubMed, all evidence cited on this page is drawn from the referenced articles; please see the linked DOIs for the primary sources.