Purpose: Study experimental instability by ligament section leaving intact all the periarticular elements.

Material and methods: Sixteen fresh cadaver shoulders were studied. Dissection was achieved via an axillary approach isolating the ligaments without muscle section. Instability was classified in five stages: 0) stable, 1) drawer and sulcus, 2) subluxation: the head crossed the glenoid border but remained in the plane of the glenoid, 3) reversible dislocation: the head was dislocated by returned spontaneously into place when the arm was left to hang along the body, 4) permanent dislocation. The ligaments were sectioned in the following order: 1) betrween 7h and 5h, 2) between 5h and 2h, 3) between 1h and 11h. Instability was tested with usual manœuvres: drawer, sulcus, hyperabduction test, provoked dislocation in elevation and maximal external rotation, downward pressure in the axis of the humerus.

Results: Dissection of the ligaments produced class 1 instability in 0% of the shoulders, Section between 7h and 5h (anterior part of the inferior glenohumeral ligament) yielded class 2 instability in 12 cases, and class 3 instability in six. The hyperabduction test was positive in all shoulders. Section between 5h and 3h (middle glenohumeral ligament) produced class 3 instability in all the shoulders but never permanent dislocation. To obtain class 4 instabilty, section between 1h and 11h (superior glenohumeral ligament) was required. Section of the cuff was not necessary to obtain permanent dislocation.

Discussion: The role of the superior glenohumeral ligament in the production of shoulder instability has not been detailed to date. Closure of the rotator interval, proposed by Nobuhar and by Field, corresponds to retightening this ligament. The function of the superior glenohumeral ligament should be taken into account during the treatment of shoulder instability.