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Effects of Simulated Scapular Protraction on Anterior Glenohumeral Stability

Orthopaedic Biomechanics Laboratory, Long Beach Veterans Affairs Medical Center, Department of Orthopaedic Surgery, University of California, Irvine, California

* Address correspondence and reprint requests to Thay Q Lee, PhD, Orthopaedic Biomechanics Laboratory, Veterans Affairs Medical Center (151), 5901 East 7th Street, Long Beach, CA 90822

We evaluated the effect of simulated scapular protraction on anterior glenohumeral translation and in situ inferior glenohumeral ligament strain. Five fresh-frozen cadaveric glenohumeral joints were tested in the position of apprehension while simulating scapular protraction using a custom translation testing device and a differential variable reluctance transducer. The results showed a statistically significant decrease in anterior glenohumeral translation as the scapula was increasingly rotated to simulate 20° of scapular protraction. With 15 N of thorax-based, anteriorly directed load, the mean anterior translation was 6.3 ± 1.6 mm (mean ± SEM). With 10° of simulated scapular protraction, translation decreased to 4.1 ± 1.0 mm; with 20° of simulated scapular protraction, it decreased further to 2.5 ± 0.5 mm. For each of the other anteriorly directed loads, translations also decreased when 20° of scapular protraction was simulated. Concurrently for each of the anteriorly directed loads, there was increased in situ strain of the anterior band of the inferior glenohumeral ligament with increased simulated scapular protraction. These results suggest that repetitive or chronic protraction of the scapula may result in excessive strain and, ultimately, insufficiency in the anterior band of the inferior glenohumeral ligament.

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