Background: Factors predisposing to tearing of the rotator cuff are
poorly understood. We have observed that the acromion of patients with a
rotator cuff tear very often appears large on anteroposterior radiographs or
during surgery. The purpose of this study was to quantify the lateral
extension of the acromion in patients with a full-thickness rotator cuff tear
and in patients with an intact rotator cuff.
Methods: The lateral extension of the acromion was assessed on true
anteroposterior radiographs made with the arm in neutral rotation. The
distance from the glenoid plane to the lateral border of the acromion was
divided by the distance from the glenoid plane to the lateral aspect of the
humeral head to calculate the acromion index. This index was determined in a
group of 102 patients (average age, 65.0 years) with a proven full-thickness
rotator cuff tear, in an age and gender-matched group of forty-seven patients
(average age, 63.7 years) with osteoarthritis of the shoulder and an intact
rotator cuff, and in an age and gender-matched control group of seventy
volunteers (average age, 64.4 years) with an intact rotator cuff as
demonstrated by ultrasonography.
Results: The average acromion index (and standard deviation) was
0.73 ± 0.06 in the shoulders with a full-thickness tear, 0.60 ±
0.08 in those with osteoarthritis and an intact rotator cuff, and 0.64
± 0.06 in the asymptomatic, normal shoulders with an intact rotator
cuff. The difference between the index in the shoulders with a full-thickness
supraspinatus tear and the index in those with an intact rotator cuff was
highly significant (p < 0.0001).
Conclusions: A large lateral extension of the acromion appears to be
associated with full-thickness tearing of the rotator cuff.
Level of Evidence: Diagnostic Level IV. See Instructions
to Authors for a complete description of levels of evidence.