Background: The inferior capsular shift procedure is commonly performed for multidirectional instability of the shoulder with excellent clinical results. Hypothesis: The mechanism of this procedure is to increase shoulder stability by changing the responsiveness of intra-articular pressure to downward loading. Study Design: Controlled laboratory study. Methods: In 7 fresh-frozen cadaveric shoulders, inferior capsular shift was simulated by imbricating the anterior and posterior capsule using clamps. The position was monitored using an electromagnetic tracking device with the glenohumeral joint capsule intact, imbricated anteriorly, or imbricated anteriorly and posteriorly and with the inferior load of 0, 0.5, or 1.0 kg. Simultaneously, intra-articular pressure was monitored using a pressure transducer. Saline was injected into the glenohumeral joint to measure capsular volume. The capsule was vented, and the position was again measured. Results: Intra-articular pressure was -71 ± 19, -221 ± 70, and -366 ± 73 cm H2O with 0, 0.5, and 1.0 kg of load, respectively, with the capsule intact. With anterior imbrication, intra-articular pressure decreased to -79 ± 10, -274 ± 103, and -460 ± 135 cm H2O, respectively, and with anterior and posterior imbrication, intra-articular pressure further decreased to -87 ± 16, -308 ± 74, and -548 ± 39 cm H2O, respectively. The volume of the intact shoulder (36 ± 9 mL) significantly decreased to 27 ± 7 mL (75%) with anterior imbrication and to 15 ± 5 mL (42%) with anterior and posterior imbrications (P = .0001). Before venting the capsule, inferior displacement was 5% of the vertical length of the glenoid, even with 1.0 kg of load with any capsular conditions. After venting, the humeral head dislocated inferiorly in all shoulders, even after imbrications. Conclusion: The inferior capsular shift procedure decreases joint volume and increases responsiveness of intra-articular pressure to downward loading. Clinical Relevance: Biomechanical data provide scientific background to the commonly performed procedures of inferior capsular shift and thermal capsular shrinkage.
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