The effect of capsular venting on glenohumeral laxity

Anesthetized shoulders are frequently stable against forces applied during drawer and sulcus tests, even though the shoulder muscles are inactive and do not contribute to stability. This passive stability is also evident in the glenohumeral joints of anatomic specimens. The translational laxity of anatomic specimen shoulders was measured, and it was demonstrated that this laxity was substantially increased when air was admitted into the capsule. Eight shoulders, aged 57-87 years, including six contralateral pairs, were analyzed using a six degrees-of-freedom force transducer and a six degrees-of-freedom spatial tracker. Capsules were vented by admitting air ad libitum through an 18-gauge needle. Venting reduced the force necessary to translate the humeral head with respect to the glenoid fossa by an average of 15.3 N (55%) for anterior forces, 10.8 N (43%) for posterior forces, and 19.0 N (57%) for inferior forces. It is likely that passive stability will also be diminished by a similar mechanism in patients with intact but excessively lax capsules. The principle of limited joint volume should be considered and tested when investigating glenohumeral stability.