Os Acromiale
- A Report of Seven Cases Treated Operatively-

Abstract

Os acromiale is non-union of one or more ossification centers of the acromion. We will report on seven cases of os acromiale treated operatively during the period between March 2009 and January 2010. The average age was 53 years old (15 to 74). Six shoulders were pointed out incidentally. (Five shoulders were diagnosed with rotator cuff tear (RCT) and one was diagnosed with recurrent dislocation of the shoulder (RDS). The remaining one shoulder was diagnosed on careful examination of shoulder pain. At the operation, instability of the os acromiale was confirmed arthroscopically in all shoulders. Next, they had open reduction and internal fixation with a tension-band procedure. Of those shoulders, sliding method was used in three shoulders and an artificial bonegraft was used in one. Four shoulders had an excision of the non-union site. RCT was treated by arthroscopic rotator cuff repair and RDS was treated by arthroscopic Bankart repair at the same time. We studied the location of unfused area, the rate of bone union and the results of the surgery using JOA score. The unfused area was located in the os meso- acromiale in all shoulders. Six of those shoulders had osseous union, and one shoulder had a non-union due to a distruption of the fixation. Six shoulders that got bone union showed improvement in the JOA score from 63 to 94. When the deltoid muscle contracts, the os acromiale is pulled downward. In this study, two shoulders with osseous continuous in undersurface of the acromion got instability later, as if a chocolate bar cracked. We named this mechanism “Chocolate bar phenomenon”.Inferior displacement of os acromiale onto rotator cuff causes subacrominal impingement, pain and rotator cuff tear. It is necessary to treat every case of os acromiale with fixation.