Os acromiale results from a failure of fusion between 1 or more ossification centers and the acromion process.1 The acromion consists of 4 different ossification centers, which appear by 14 years of age and fuse by age 25 years. The 4 ossification centers are the basi-acromion, meta-acromion, mesoacromion, and pre-acromion (Figure 1). Formation of an os acromiale occurs most often due to failure of fusion between the meta-acromion and mesoacromion. Os acromiale appears to occur in approximately 8% of the population, according to cadaveric studies.2 This anatomic variant occurs more commonly in African-Americans than Caucasians, and shows a preponderance for males over females.3
Plain radiographs are usually adequate for diagnosis. Axillary views are most sensitive for detection, which can be difficult to see on anteroposterior radiographs.4 In os acromiale, the unfused segment is connected to the acromioclavicular joint and the coracoid, which can lead to motion of the segment and impingement of the rotator cuff.2-4 Patients frequently experience localized tenderness and symptomatic pain with signs and symptoms of impingement. Rotator cuff tears may occur secondary to chronic impingement.5
Various forms of repair have been described. A recent meta-analysis showed that internal fixation (60%) was the most common surgical technique reported, followed by excision (27%) and acromioplasty (13%).6 Rotator cuff repair is a common concurrent surgical procedure.7-11 The available literature favors internal fixation through an open technique with or without bone grafting.5,7,8,12-15 Various forms of fixation have been presented in the literature, including Kirschner wire fixation, cannulated screw fixation alone, cannulated screw fixation with FiberWire Suture (Arthrex), and cannulated screw fixation with a stainless steel wire tension band technique. Based on the results of the meta-analysis, surgical fixation with cannulated screws has been shown to lead to a significantly greater rate of radiographic healing (23/24 patients) compared to Kirschner wire fixation (31/49 patients).6 Further, radiographic healing is significantly associated with improved clinical outcomes.12 Removal of symptomatic internal fixation hardware is significantly more common after Kirschner wire fixation cases (88%; 43/49) compared to cannulated screw fixation cases (38%; 9/24). However, hardware issues may also be encountered with screw fixation, with 1 case series reporting a 25% rate of hardware complication.16 The patient provided written informed consent for print and electronic publication of this case report.
CASE REPORT
The patient is a 19-year-old right-hand-dominant woman who injured her right shoulder while diving into the bleachers during a volleyball game 4 years prior to presentation. She suffered a direct blow to her shoulder and immediately became symptomatic. She underwent a long period of nonoperative management, which included physical therapy, strengthening, nonsteroidal anti-inflammatory drug (NSAID) therapy, and narcotic pain medications. Her primary complaints upon presentation were pain with lifting, as well as mechanical symptoms. On examination, the patient had moderate tenderness directly over the acromion. She also had evidence of mild impingement symptoms. Plain radiographs revealed a mesoacromial-type os acromiale clearly seen on the axillary lateral film (Figure 2). She underwent magnetic resonance imaging, which suggested rotator cuff tendinosis and evidence of edema at the os acromiale site. She underwent a diagnostic injection directly into the site of maximal tenderness at the os, which provided complete transient relief of her pain. Despite the transient pain relief, the patient continued to be symptomatic after the local anesthetic effect wore off. Surgical options were then discussed with the patient.
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