QUANTITATION OF INSITU CONTACT AREAS AT THE GLENOHUMERAL JOINT - A BIOMECHANICAL STUDY

Glenohumeral arthritis may result from abnormal articular mechanics, and shoulder reconstructive procedures often rely implicitly on the belief that the restoration of normal articular mechanics is required to obtain satisfactory clinical results. Despite this, limited knowledge of normal or pathologic glenohumeral joint articular mechanics and contact is available. This study uses a stereophotogrammetry technique to determine contact areas in normal cadaver glenohumeral joints with intact ligaments and capsule through a large range of motion using simulated forces of the four rotator cuff muscles and three deltoid heads. All shoulders were first elevated to their maximum elevation in the scapular plane at an external rotation (starting rotation = 40 +/- 8-degrees), which allowed each shoulder to attain its maximal elevation in the scapular plane, and then repeated at 20-degrees internal to this rotation. Contact areas consistently increased with increasing elevation until 120-degrees to an average of 5.07 cm2 before decreasing with further increased elevation to an average of 2.59 cm2 at 180-degrees of total arm elevation. At 20-degrees internal to the starting rotation, contact areas reached high values 60-degrees earlier (averaged 4.56 cm2 at 60-degrees of total.arm elevation) and then remained fairly constant through 120-degrees before decreasing with further increased elevation to 2.51 cm2 at 180-degrees total arm elevation. With increasing elevation in the external starting rotation, humeral head contact dramatically migrates from an inferior region to a superocentral-posterior region while glenoid contact shifts posteriorly. When the humeral shaft is positioned 20-degrees internal to the starting rotation, humeral head contact shifts from inferocentral-anterior to superocentral-posterior regions. Simultaneously, a similar posterior shift in glenoid contact is observed. Furthermore, whereas only a small portion of the humeral head surface area is in contact in any given position, contact on the glenoid surface is much more uniformly distributed over its entire articulating surface.