Capsulolabral augmentation increases glenohumeral stability in the cadaver shoulder

Abstract

Multidirectional instability is not clearly understood. Excessive capsular laxity has been proposed as the key component. However, because ligaments fail to resist humeral head translation until they are tensioned, glenohumeral instability in the mid range of motion cannot be explained by capsuloligamentous pathology alone. Capsulolabral augmentation is designed to increase glenohumeral stability by 2 separate mechanisms: deepening the glenoid concavity and reducing capsular laxity. This is accomplished by shifting the capsule to buttress the glenoid labrum. Hence, the glenolabral concavity in which the humeral head is stabilized by compression throughout the entire range is enhanced. The purposes of this study were to examine glenolabral depth and glenohumeral stability before and after labral augmentation and to measure the effect of diminished capsular laxity on motion in clinically important positions. We compared glenolabral depth, resistance to humeral head displacement, and glenohumeral range of motion before and after capsulolabral augmentation. Glenolabral depth was measured as the lateral displacement of the center of the humeral head translating from the glenoid fossa. We recorded a mean increase in glenoid depth of 1.9 mm inferiorly, 2.0 mm posteroinferiorly, and 0.9 mm posteriorly (P <.02). Resistance to humeral head displacement was measured by use of the stability ratio, defined as the translatory force required to displace the humeral head divided by the force compressing the humeral head into the glenoid fossa. The mean stability ratio was increased by 0.24 inferiorly and 0.24 posteroinferiorly (P <.02). Motion was measured by achieving 30 degrees and 60 degrees elevation in the 0 degrees, 30 degrees, 60 degrees, and 90 degrees planes of elevation and measuring the extent of possible internal rotation for each of these 8 positions when the capsule was tensioned to exert 1000 N-mm of torque. Reduction of internal rotation in these positions was a mean of 15 degrees at 1000 N-mm of torque. This study demonstrates that humeral head stability within the glenolabral fossa is increased by local capsular augmentation. A simultaneous reduction in capsular laxity is achieved, which partially limits glenohumeral motion. Understanding the biomechanical effect of this procedure helps the physician to establish surgical goals and to explain to patients the rationale of why this procedure may be clinically efficacious.