Last Updated on May 12, 2025
Os acromiale refers to an unfused accessory ossification center of the acromion, part of the scapula that extends over the shoulder joint. It arises when one or more developmental ossification centers fail to unite, typically during adolescence.
The acromion is part of the scapula that extends laterally over the shoulder joint. It develops from 4 ossification centers. If one of the four ossification centers of the acromion fails to fuse, an os acromiale results.
While often asymptomatic, os acromiale can lead to shoulder pain, impingement symptoms, or surgical complications if unrecognized.
The prevalence of os acromiale is 1% to 15%, and is quite common in the African American population. [1] It may be bilateral in 60%.
Ossification of Scapula
The scapula bone forms from nine ossification centers, of which 3 form the acromion. These are known as
- Pre-acromion
- Meso-acromion
- Meta-acromion
Fusion typically completes between ages 15 and 18 to form the acromion.
Anatomically, the acromion can be divided into 4 regions
- Pre-acromion
- Meso-acromion
- Meta-acromion
- Basi-acromion
Pre, meso, and meta-acromion fuse with basiacromion to form adult acromion. Failure of any of these to fuse with the basi-acromion results in an os acromiale
Types
There are four types of os acromiale depending on which ossification center has failed to fuse with the basi-acromion. These are
- Meta-acromial
- Meso-acromial
- Pre-acromial
Meta-acromial is the most common [2] and most often associated with clinical symptoms and impingement syndromes.
Clinical Importance
The pain in os-acromiale may be due to
- Impingement from the unfused segment
- Concomitant rotator cuff tear
- Arthritic changes.
Rotator cuff pathology, including tears or tendinosis, is more common when there is a step-off deformity at the os acromiale site.
In asymptomatic cases, identification of the condition is also very important.
Identifying os acromiale is essential during shoulder imaging and especially before surgery. It is important to ascertain before any shoulder procedure whether there is os acromiale or not.
Performing subacromial decompression on a mobile or unfused segment may destabilize the acromion, causing worsening pain or functional limitation.
Symptoms and Signs of Os Acromiale
Os acromiale is usually not symptomatic. Most of these cases are noted as an incidental finding on shoulder x-rays.
Common Symptoms
Some patients may present with non-specific symptoms. These can include
- Shoulder pain localized to the acromion region
- Difficulty in performing overhead activities
- Night-time pain in the shoulder
- Weakness of the shoulder
On examination of these symptomatic cases, there may be
- Localized tenderness on the acromion
- Decreased range of motion
- Signs suggesting impingement
There may or may not be a history of blunt trauma preceding the pain event.
Imaging
X-rays
The unfused anterior acromial ossification center is best seen on axillary views.
Radiographic cues favoring the diagnosis of os acromiale over fracture
o Bilateral occurrence
o Smooth, corticated margins
o Even alignment with or above the posterior acromion on AP view
MRI
Fat-suppressed sequences (e.g., STIR) can help detect marrow edema or inflammation in symptomatic cases. An axial MRI cut may be required for correct diagnosis. [3]
Technetium Bone Scan
This can be done when the diagnosis remains unclear.
Treatment
Conservative Management
Nonoperative treatment or conservative treatment is the first line of treatment. The treatment includes
- Activity modification
- Rest
- Cold or heat application
- Drugs for pain
- Steroids at the site of the injection [Not more than 4 injections in total]
- Physical therapy
- Range of motion exercises
- Strengthening exercises
Conservative treatment should be tried for six weeks to six months.
The patients who do not show satisfactory improvement/ pain relief should be considered for surgical treatment
Operative Treatment
Two-stage Fusion
This treatment can be used in cases with symptomatic lesions with impingement.
The treatment is done in two stages. In the first stage, the fragment is fused with aid of an implant and a bone graft.
The following implants can be used
- Tension band wiring
- Cannulated Screws
- A deltoid-off approach has been associated with better outcomes
In the second stage, acromioplasty is done, where the space beneath the acromion is increased by shaving off the undersurface of the acromion.
Excision
Excisions are done less commonly now as these are often associated with deltoid dysfunction. Smaller fragments excision [such as preacromion] is associated with better results. [4]
Two types of excision are
- Excision of the acromion
- Excisions of the fragment
Excision can be arthroscopic or open. The former is preferred as it causes less soft tissue injury.
Acromionectomy involves the removal of about 80 percent of the acromion. It is also associated with persistent pain.
Arthroscopic Subacromial Decompression and Acromioplasty
This is indicated in impingement with/without rotator cuff tear. [4]
ASAD or arthroscopic subacromial decompression is a well-known procedure for subacromial impingement.
In ASAD, impinging structures like inflamed bursa and bone on the undersurface of are removed.
It can be considered in patients where pain seems due to impingement.
ASAD should be approached cautiously in patients with os acromiale, as it may inadvertently destabilize a previously stable segment. Literature supporting its use as a first-line treatment in this context is limited.
References
- You T, Frostick S, Zhang WT, Yin Q. Os Acromiale: Reviews and Current Perspectives. Orthop Surg. 2019 Oct;11(5):738-744. [PubMed]
- Viner GC, He JK, Brabston EW, Momaya A, Ponce BA. Os acromiale: systematic review of surgical outcomes. J Shoulder Elbow Surg. 2020 Feb;29(2):402-410. [PubMed]
- Rovesta C, Marongiu MC, Corradini A, Torricelli P, Ligabue G. Os acromiale: frequency and a review of 726 shoulder MRI. Musculoskelet Surg. 2017 Dec;101(3):201-205. [Pubmed]
- Stetson WB, McIntyre JA, Mazza GR. Arthroscopic Excision of a Symptomatic Meso-acromiale. Arthrosc Tech. 2017 Feb 13;6(1):e189-e194. [Pubmed]
