In situ force distribution in the glenohumeral joint capsule during anterior-posterior loading

Our objective was to examine the function of the glenohumeral capsule and l igaments during application of an anterior-posterior load by directly measu ring the in situ force distribution in these structures as well as the comp liance of the joint. We hypothesized that interaction between different reg ions of the capsule due to its continuous nature results in a complex force distribution throughout the glenohumeral joint capsule. A robotic/universa l force-moment sensor testing system was utilized to determine the force di stribution in the glenohumeral capsule and ligaments of intact shoulder spe cimens and the joint kinematics resulting from the application of external loads at four abduction angles. Our results suggest that the glenohumeral c apsule carries no force when the humeral head is centered in the glenoid wi th the humerus in anatomic rotation. However, once an anterior-posterior lo ad is applied to the joint, the glenohumeral ligaments carry force (during anterior loading, the superior glenohumeral-coracohumeral ligaments carried 26 +/- 16 N at 0 degrees and the anterior band of the inferior glenohumera l ligament carried 30 +/- 21 N at 90 degrees). Therefore, the patient's abi lity to use the arm with the humerus in anatomic rotation should not be lim ited following repair procedures for shoulder instability because the repai red capsuloligamentous structures should not carry force during this motion . Separation of the capsule into its components revealed that forces are be ing transmitted between each region and that the glenohumeral ligaments do not act as traditional ligaments that carry a pure tensile force along thei r length. The interrelationship of thr glenohumeral ligaments forms the bio mechanical basis for the capsular shift procedure. The compliance of the jo int under our loading conditions indicates that the passive properties of t he capsule provide little resistance to motion of the humerus during 10 mm of anterior or posterior translation with anatomic humeral rotation, Finall y, this knowledge also enhances the understanding of arm positioning relati ve to the portion of the glenohumeral capsule that limits translation durin g examination under anesthesia.