Tuesday, January 1, 2013

Stabilizing effect of negative intraarticular pressure

The role of negative intraarticular pressure and the long head of biceps tendon on passive stability of the glenohumeral joint

These authors examined the effect of venting the glenohumeral joint on its translational laxity. They found that the joint was most lax in its mid range of motion, at 30° GH abduction. Venting of the joint increased translations in all positions and directions (mean ± standard error of the mean), the greatest difference was 12.5 (3.9) mm in the anterior-posterior direction and 7.5 (3.9) mm in the SI direction.

They concluded that negative intraarticular pressure contributes significantly to overall passive stability of the glenohumeral joint.

These results are similar to those we published in 1991. We noted that 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. 

We measured the translational laxity of 8 anatomic specimen shoulders (aged 57-87 years, including six contralateral pairs) 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. 

On this basis we proposed the contribution of limited joint volume to the passive stability of the glenohumeral joint as shown in Practical Evaluation and Management of the Shoulder.

The authors of the article also found that tensioning the biceps tendon contributed to joint stability. This is another example of shoulder stabilization through the concavity compression mechanism.


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