Computer simulation of glenohumeral and patellofemoral subluxation. Estimating pathological articular contact.

Analytic stereophotogrammetry and an interactive computer graphics program were used to obtain first order assessments of joint contact patterns in patellofemoral and glenohumeral joints, simulating normal and abnormal articulations. Precise (90 microns accuracy) computer graphic representations of the humeral head, glenoid, patella, and femoral articular surfaces were obtained from cadaver knees and shoulders. These surface representations were then manipulated into an articulated position, and joint contact areas computed by a proximity criterion. Pathologic states were then simulated, and contact recomputed. Simulated glenohumeral subluxations dramatically reduced contact area, and focused it eccentrically on the glenoid rim. Simulated size mismatch of humeral heads to glenoids reduced contact area, producing a pattern of peripheral contact on the glenoid if the humeral head had a larger radius of curvature, and central contact on the glenoid if the humeral head had a smaller radius of curvature. At 30 degrees knee flexion, the patellofemoral joint demonstrated a broad distribution of contact along the distal aspect of the patella and proximal aspect of the trochlea. Simulated lateral tilt (5 degrees) and translation (5 mm) of the patella resulted in shift of the predominant contact area laterally, along with a drastic decrease in the contact area. These results have implications for prosthetic sizing and biomechanical modeling of the glenohumeral and patellofemoral joints, and in selecting models for more rigorous empiric studies of joint contact. Furthermore, this technique allows a first order assessment of the effects of specific surgical reconstructions on articular mechanics.