Last Updated on November 28, 2022
Os acromiale is an accessory bone that results from an unfused center of ossification of the acromion of the scapula bone.
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.
Os acromiale is seen in about 8% of the population and 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
These fuse by 15-18 years of age, and these ossification centers fuse to form 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 to the basi-acromial region result in the formation of os acromiale.
Types
There are four types of os acromiale depending on what ossification center has failed to fuse with the basi-acromion. These are
- Meta-acromial
- Meso-acromial
- Pre-acromial
Meta-acromial is the most common.
Symptoms and Signs of Os Acromiale
Os acromiale is usually not symptomatic. In fact, most of these cases are noted as an incidental finding on shoulder x-rays.
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
The pain in os-acromiale may be due to
- Impingement from the unfused segment
- Concomitant rotator cuff tear
- Arthritic changes.
On examination of these symptomatic cases, there could be the following signs
- Localized tenderness on the acromion
- Decreased range of motion
- Signs suggesting impingement
There may or may not be a history of blunt trauma preceding to the pain event.
Rotator cuff deformity has been noted more in people with step-off deformity of the os acromiale.
Importance of Identifying Os Acromiale
In asymptomatic cases also, identification of the condition is very important. It is important to ascertain before any shoulder procedure whether there is os acromiale or not.
- Removal of bone on the undersurface of the acromion for decompression in the presence of an unfused fragment can result in destabilization, pain, and disability.
- Failure of arthroscopic subacromial decompression may occur with its presence
Imaging
X-rays
The unfused anterior acromial ossification center is best seen on axillary views.
Factors favoring the diagnosis of os acromiale over fracture are
- Bilateral occurrence
- Rounded borders with uniform space
- The bony ossification center is even with or above the posterior acromion on the anteroposterior view.
MRI
MRI is able to define the unfused parts better. Fat-suppressed images should be considered alongside traditional MR images.
Technetium Bone Scan
This can be done when the diagnosis remains unclear.
Treatment
Nonoperative Treatment
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.
Implant used can be
- Tension band wiring
- Cannulated Screws
- A deltoid-off approach has been associated with the better outcome
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.
Two types of excision are
- Excision of the acromion
- Excisions of the fragment
Excision can be arthroscopic or open. 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.
ASAD or arthroscopic subacromial decompression is a well-known procedure for subacromial impingement.
In ASAD, impinging structures like inflamed bursa and bone on the under-surface of the are removed.
It can be considered in patients where pain seems due to impingement.
The evidence of the use of ASAD in os acromiale as a primary treatment methodology is low. There are also concerns about converting stable fragments into unstable ones.
References
- Harris JD , Griesser MJ , Jones GL . Systematic review of the surgical treatment for symptomatic os acromiale. Int J Shoulder Surg 2011; 5: 9–16. [Link]
- Ortiguera CJ , Buss DD . Surgical management of the symptomatic os acromiale. J Shoulder Elbow Surg 2002; 11: 521–528 .
- Sammarco VJ. Os acromiale: frequency, anatomy, and clinical implications. J Bone Joint Surg Am 2000; 82: 394–400.
- Anwar I, Amiras D, Khanna M, Walker M . Physes around the shoulder girdle: normal development and injury patterns. Clin Radiol 2016; 71: 702–709.
