Paper 87: What is the Stabilizing Mechanism of the Latarjet Procedure?

Abstract

The Latarjet procedure is one of common ones for patients with anterior instability of the shoulder and its excellent clinical results have been reported. It has also been used as a bone-grafting procedure in patients with a large glenoid defect. “Sling effect of the subscapularis muscle” or “bone block effect of the coracoid process” has been believed as a main stabilizing mechanism of this procedure. However, the true mechanism is still unknown and there have been no biomechanical studies in the literature. The purpose of this study was to clarify the stabilizing mechanism of the Latarjet procedure for patients with anterior instability and ones with a large glenoid defect. Seven fresh-frozen cadaveric shoulders (mean age, 76 years) were investigated. The skin, subcutaneous tissue, and deltoid were removed preserving the rotator cuff muscles. Two arm positions (60° of abduction and maximum external rotation and neutral rotation) were chosen to simulate the terminal and mid-range conditions. With use of a custom multiaxis electromechanical testing machine with a six-degrees-of-freedom load-cell, the humeral head was translated 10 mm in the anterior direction. With a 50-N axial force constantly applied to the humerus in order to keep the humeral head centered in the glenoid fossa, the peak translational force was determined under eight sequential conditions: in the terminal range, 1) with the capsule intact, 2) after the creation of a simulated Bankart lesion, 3) after the Latarjet procedure, 4) after sectioning the conjoint tendon, 5) after sectioning the subscapularis tendon; in the mid-range, 6) after the creation of a large anterior glenoid defect (6mm width), 7) after the Latarjet procedure with and without applied load, 8) after Bankart repair. A 10-N and 2.5-N load was applied to the subscapularis and conjoint tendons, respectively. We followed G Walch's surgical technique. In the terminal range, the peak translational force with intact capsule was 157 ± 17, which significantly decreased down to 53 ± 26 after creating a Bankart lesion (p = 0.0004) and significantly increased to 131 ± 25 after the Latarjet procedure (p = 0.0010). After sectioning the conjoint and subscapularis tendons, the force was 112 ± 24, 107 ± 22 (19% decrease compared to that after the Latarjet procedure) (p < 0.0001), respectively. In the mid-range, the force with intact capsule was 67 ± 8, which significantly decreased to 17 ± 4 after creating a large glenoid defect (p < 0.0001) and did not return (33 ± 4) to the intact condition even after Bankart repair. However, it was significantly increased to 63 ± 6 (p < 0.0001) and 73 ± 4 (p < 0.0001), respectively, after the Latarjet procedure with and without load. Percentage contribution to the increase of the stability was 82% after the Latarjet procedure without load, 18% that with load, respectively. From our results, the stabilizing mechanism of the Latarjet procedure both in the terminal and mid-range motion was demonstrated. In the terminal range, 81% of the stability was contributed by the shortening of the anterior capsule including the anteroinferior glenohumeral ligament (re-tension effect), and the contribution of sling effect which many have believed was only 19%. On the other hand, in the mid-range, 82% of the stability was contributed by reconstruction of the glenoid concavity (glenoid plasty effect) and 18% by sling effect. Thus, it was demonstrated that the Latarjet procedure had a reasonable stabilizing mechanism as a re-stabilizing procedure for shoulders not only with anterior instability but also with a large glenoid defect.