Influence of component positioning on impingement in conventional total shoulder arthroplasty.

BACKGROUND: Clinical experience suggests that component impingement can lead to eccentric implant loading and thereby cause glenoid loosening in conventional total shoulder arthroplasty. This study tests the hypothesis that certain implant component positioning configurations may lead to impingement within the physiological range of motion.
METHODS: A rigid-body model of the shoulder comprising the scapula and humerus was constructed. Within this 3D model, a commercially available total shoulder arthroplasty implant was positioned according to manufacturer guidelines. The configuration was modified around this default position to investigate the associated angle of inferior and superior impingement during glenohumeral elevation, as well as in lateral impingement during axial rotation at both 0 degrees and 60 degrees of glenohumeral elevation. Glenoid component size, version, inclination and inferior-superior offset as well as humeral component size, torsion, inclination, offset and height were examined. The influence of the humeral calcar anatomy was also investigated.
FINDINGS: Certain implant configurations caused component impingement in the physiological range of motion. The most sensitive parameters affecting impingement were: (1) the inclination of the glenoid component, (2) the inferior-superior position of the humeral component along the resection line and (3) the prominence of the humeral calcar. Glenoid offset and inclination and humeral head offset and height directly affected subacromial impingement.
INTERPRETATION: This study suggests that several intraoperatively adjustable implant positioning parameters can influence the likelihood of implant impingement in conventional total shoulder arthroplasty, and that the geometry of the humeral calcar should be taken into consideration when designing an operative strategy for shoulder joint replacement.