Os acromiale

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Os acromiale 

1. Cause and Etiology

Os acromiale is an anatomical variant caused by the failure of fusion of the ossification centers of the acromion process of the scapula during skeletal maturation.

  • The acromion develops from four ossification centers:
    • Pre-acromion
    • Meso-acromion
    • Meta-acromion
    • Basi-acromion (the most medial and proximal)
  • Normally, these centers exist between the ages of 15 and 25. A failure in this fusion process, usually between the meso-acromion and meta-acromion, leads to the formation of a mobile accessory ossicle termed an os acromiale.

2. Pathophysiology

The presence of an unfused acromial segment can alter the biomechanics of the shoulder in the following ways:

  • It may result in micro-motion at the unfused junction during shoulder activity, leading to:
    • Fibrocartilaginous articulation or non-union
    • Bursal inflammation
    • Rotator cuff impingement or tendinopathy
  • In active individuals, particularly overhead athletes, repetitive stress can provoke painful pseudoarthrosis.
  • Os acromiale may predispose patients to subacromial impingement syndrome or contribute to rotator cuff pathology, especially supraspinatus tendon degeneration.

3. Epidemiology

  • Prevalence: ~1–15%, depending on the population and diagnostic method.
    • Higher in African-American populations (~13.2%) compared to Caucasians (~5.8%)
  • Gender: Slightly more common in males
  • Laterality: May be bilateral in up to 30–50% of cases
  • Often found incidentally on imaging, especially in asymptomatic individuals.

4. Clinical Presentation

Most individuals with os acromiale are asymptomatic, but symptomatic cases may present with:

  • Chronic shoulder pain, especially in the anterosuperior region
  • Pain during overhead activities
  • Tenderness to palpation over the acromion
  • Positive Neer and Hawkins-Kennedy tests due to subacromial impingement
  • Weakness or painful arc of motion
  • Symptoms may mimic rotator cuff tear, subacromial bursitis, or acromioclavicular (AC) joint pathology

5. Imaging Features

Plain Radiography:

  • Best seen on axillary or supraspinatus outlet views
  • Appearance: smooth, corticated, separate bone fragment at the acromion
  • Look for a radiolucent line between the ossicle and the acromion body
  • May be overlooked on AP view alone

MRI:


  • Useful for assessing associated soft tissue pathology
    • Rotator cuff tears
    • Bursal inflammation
  • Signal changes at the non-union site:
    • T2 hyperintensity or bone marrow edema suggests symptomatic os acromiale
  • Pseudoarthrosis may show surrounding inflammation

CT Scan:

  • High-resolution CT or 3D-CT:
    • Clearly delineates the non-union line
    • Defines the size and position of the fragment
    • Helps preoperative planning

Ultrasound:

  • Limited utility in diagnosing os acromiale itself
  • May detect associated bursitis or rotator cuff tears

6. Treatment

Conservative Management:

  • Indicated for asymptomatic or mildly symptomatic cases
  • Options:
    • NSAIDs
    • Physical therapy to strengthen the rotator cuff and scapular stabilizers
    • Activity modification
    • Subacromial steroid injection

Surgical Management:

  • Indicated when conservative treatment fails (typically after 3–6 months)
  • Options include:
    • Open or arthroscopic excision of the os acromiale (small, unstable fragments)
    • Internal fixation of the unfused segment:
      • Screws with or without bone grafting
      • Often combined with rotator cuff repair if pathology coexists
    • Acromioplasty (controversial and less preferred in os acromiale)

7. Prognosis

  • Asymptomatic os acromiale generally requires no treatment and has an excellent prognosis.
  • Symptomatic cases respond well to:
    • Conservative treatment in a majority of mild cases
    • Surgical fixation or excision has favorable outcomes, with improvement in pain and function
    • However, nonunion risk exists post-fixation (~10–15%)
  • Prognosis is poorer if coexisting rotator cuff pathology is extensive or if delayed treatment is rendered

Key Differential Diagnoses

  • Acromial fracture (especially post-traumatic)
  • Os subacromiale (rare variant)
  • Calcific tendinitis
  • AC joint osteoarthritis
  • Subacromial impingement syndrome (non-os-related)
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Case study: Chronic Shoulder Pain in a 38-Year-Old Woman
 Os Acromiale

History and Images

  1. A 38-year-old woman presented with chronic, non-traumatic right shoulder pain.

  2. The pain persisted both during physical activity and at rest.

  3. The patient denied any other precipitating factors for the pain.

  4. On physical examination, the pain was localized to the superolateral aspect of the shoulder, with preservation of the range of motion.

  5. The patient underwent radiographic imaging as part of the initial evaluation.

Quiz 1:
What is the primary radiographic finding?
(1) Fracture of the humeral neck
(2) Widening of the acromioclavicular joint
(3) Anterior shoulder dislocation
(4) Severe osteoarthritis of the glenohumeral joint
(5) No radiographic abnormality is visible

Additional Images
The patient subsequently underwent a shoulder MRI for further evaluation. An axial proton density fat-suppressed image selected for assessment is provided below.

Quiz 2

  1. Os acromiale is present.
    (1) True
    (2) False

  2. What subtype does this patient have?
    (1) Pre-acromion
    (2) Meso-acromion
    (3) Meta-acromion
    (4) Indeterminate

  3. MRI findings are suggestive of acute inflammation.
    (1) True
    (2) False

  4. The majority of individuals with this anatomical variant are symptomatic.
    (1) True
    (2) False

Findings and Diagnosis

Radiographic Findings:
Plain radiographs did not identify any acute osseous abnormalities. Although an os acromiale can occasionally be visualized on radiographs, it may be obscured by overlying soft tissues or suboptimal positioning.

MRI Findings:
Axial proton density-weighted images with fat suppression demonstrate a clearly demarcated os acromiale, separated from the remainder of the scapula. The non-union is consistent with the meso-acromion subtype, which is the most common variant. Bone marrow edema and fluid signal within the pseudoarthrosis are evident, suggesting active inflammation. These findings correlate with the patient’s clinical symptoms. This condition is distinct from typical acromioclavicular joint pathology.

Differential Diagnosis

Non-traumatic shoulder pain can arise from a broad range of etiologies involving bone (e.g., degenerative osteoarthritis, congenital variants, subacromial spurs), muscle (e.g., strain, myositis, rotator cuff tendinopathy or tear), and other soft tissue structures (e.g., labral tears). In cases of localized anterosuperior shoulder pain, particular attention should be paid to the acromioclavicular joint and adjacent osseous structures.

Final Diagnosis:
Symptomatic Os Acromiale

Discussion: Os Acromiale

Pathophysiology:
Os acromiale results from the failure of fusion between the acromion and the scapular spine during skeletal development. The non-united segment is held in place by fibrocartilaginous tissue, which, though strong, permits some mobility. This motion may cause repetitive microtrauma and inflammation, potentially leading to pain. Based on the location of the non-union, three main subtypes are recognized: pre-acromion (smallest fragment), meso-acromion (most common), and meta-acromion (largest fragment).

Epidemiology:
Os acromiale is estimated to occur in approximately 8–10% of the adult population. Epidemiological studies suggest a higher prevalence among Black individuals and males, though the underlying mechanism remains unclear. Up to one-third of affected individuals may have bilateral involvement.

Clinical Presentation:
Most individuals with os acromiale are asymptomatic, and the condition is often discovered incidentally on imaging. Recognizing this variant is essential to avoid misdiagnosing it as a fracture. Symptomatic cases typically present with localized pain in the anterosuperior shoulder region, often mimicking acromioclavicular joint pathology due to their close anatomical proximity. In more advanced cases, os acromiale may be associated with rotator cuff tears and subacromial impingement syndrome.

Imaging Features:
If large enough, os acromiale may be visible on radiographs as a separate ossified fragment with cortical margins, distinct from the scapular spine. It is best visualized on axillary or Y-scapular views.
Both CT and MRI can reliably identify os acromiale, but only MRI can detect associated bone marrow edema and surrounding inflammatory changes, key features indicating symptomatic or active disease. This distinction is particularly relevant given the varying surgical approaches for os acromiale, rotator cuff tears, and shoulder impingement in the general population.

Treatment:
Initial management typically involves conservative pain control with non-narcotic analgesics, corticosteroid injections, and physical therapy. For patients who do not respond to conservative therapy and are surgical candidates, preoperative MRI is important to assess for associated rotator cuff pathology.
In cases of pre-acromion subtype, the small size of the fragment allows for straightforward excision without compromising shoulder stability. In contrast, meso-acromion and meta-acromion variants involve larger fragments, making excision less feasible. In such cases, internal fixation is often required to achieve symptomatic relief and structural stability.

Reference

(1)      Eckl L, Scheibel M. Surgical treatment of a symptomatic os acromiale by arthroscopy-assisted double-button fixation: A case report. Arch Orthop Trauma Surg. 2022.

(2)      Hurst SA, Gregory TM, Reilly P. Os acromiale: A review of its incidence, pathophysiology, and clinical management. EFORT Open Rev. 2019;4:525-532.

(3)      Phanichwong P, Apivatgaroon A, Boonsaeng WS. Prevalence of os acromiale in Thai patients with shoulder problems: A magnetic resonance imaging study. Orthop J Sports Med. 2022;10:23259671221078806.

(4)      Spiegl UJ, Millett PJ, Josten C, Hepp P. Optimal management of symptomatic os acromiale: Current perspectives. Orthop Res Rev. 2018;10:1-7.

(5)      You T, Frostick S, Zhang WT, Yin Q. Os Acromiale: Reviews and current perspectives. Orthop Surg. 2019;11:738-744.


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