The Influence of Coracoacromial Arch Anatomy on Rotator Cuff Tears

We performed an anatomic study of 140 cadaver shoulders to correlate the influence of the coracoacromial arch anatomy on full-thickness rotator cuff tears (RCTs). The presence, location, and size of RCTs were initially documented. After dissection was complete 14 dimensional parameters, including humeral head size, glenoid size, and location of bony landmarks, were obtained by direct measurement. These data were entered into a computerized data base and 24 additional parameters (lengths, angles, areas) were calculated. The data were then subjected to analysis of variance and paired and unpaired t tests to determine correlation between the multiple parameters, age, sex, and left-to-right variance with RCTs. Three-dimensional computer modeling was then used to investigate the role of humeral head position in defining the available space within the coracoacromial arch (supraspinatus outlet). Overall 20% of the cadaver group exhibited full-thickness RCTs. The age group 60 years and older had a 29% incidence of RCTs compared with 5% in the group less than 60 years of age. The RCT group had a significantly greater anterior projection of the acromion than had the intact group (difference = 3.8 mm, p < 0.007). Acromial tilt was 28.5° in the RCT group and 33.5° in the intact group (p < 0.007). The supraspinatus outlet area was calculated by determining the total coracoacromial arch area and subtracting the area of the humeral head within the coracoacromial arch. The supraspinatus outlet was 22.5% smaller in the RCT group (p < 0.07). By using a series of measured and calculated parameters, we were able to characterize the anatomy of the coracoacromial arch and its relationship with the humeral head and to correlate variations in structure with the presence of RCTs. These findings may aid in our understanding of outlet impingement as a factor in the cause of RCTs.