Quantitation of ligament laxity in anterior shoulder instability: an experimental cadaver model

Three groups of cadaver specimens were studied. In group 1 (20 shoulders) glenohumeral ligaments were detached from the humerus until a permanent dislocation of the humeral head occurred in abduction plus external rotation. On the dislocated joint the ligament was reconstructed using a fascia lata lengthening plasty. After the plasty had been completed, the shoulder was reduced and instability checked in the same position. Then the capsule (including the plasty) was harvested and measured. In group 2 (20 shoulders), after the plasty had been completed in the same conditions as above, the capsule was progressively reduced by 2 mm steps until the instability disappeared. Then the capsule (including the plasty) was harvested and measured. In group 3 (12 shoulders), measurements of the head and of the capsule were done. To dislocate the shoulder the section of the three glenohumeral ligaments was required. Lengthening of the capsule in group 1 was 240-250%. In all cases shortening of the capsule led to the stabilization of the shoulder. After stabilization of the shoulder was reached a residual lengthening of 175-185% was recorded. In 3 out of 4 shoulders the amount of capsule shortening required to return to a stable shoulder was between 16 and 18 mm. This experiment did not reproduce the Bankart lesion; therefore it only concerns atraumatic instability. The main limitation of this model is the low lever force that may be used to dislocate the shoulder; consequently the elasticity of the glenohumeral ligament was not taken in account. The experimental values were likely over-estimated. Nevertheless the present results provide useful information for building an experimental model of atraumatic instability of the shoulder.