THE MICROMOTION OF PEGGED GLENOID COMPONENTS IN TOTAL SHOULDER REPLACEMENTS

To compare the effect of standard non-offset and offset humeral heads on early glenoid loosening. From 2002–2004 in a prospective study, 12 patients with osteoarthritis and intact rotator cuff had a TSR with an offset humeral head matching normal anatomy. Five tantalum markers were introduced into the scapula and acromion, spaced widely apart; four markers were placed in a pegged glenoid component. RSA radiographs were taken postoperatively, three, six, twelve months. The micromotion of this offset head group was compared with a baseline series of TSRs with standard non offset humeral head implants which had previously been measured over 24 months. At 24 months, the largest segment translation for the offset head group was in the anterior/posterior direction with a mean of 0.24mm, the standard head group mean was 0.61mm, t=1.3,P=0.21. Proximal/distal translation gave means of 0.51mm (standard head) and 0.17mm (offset head), t=1.3,P=0.21. Medial/lateral translation was smallest for the offset group with mean 0.16mm this is less than half the standard group mean 0.57mm and is significant, t=1.4,P=0.19. The largest rotation was anteversion/retroversion with a mean of 1.50 (offset) compared to 4.40 (standard), t=2.3, P=0.04. Varus/valgus tilt differed significantly with means 0.90 (offset) and 2.50 (standard), t=3.8,P=0.001; and finally anterior/posterior tilt had smallest means of 0.70 (offset) and 1.00 (standard), t=1.2,P=0.25. Maximum total point motion was also significantly different at 24 months, means of 0.66mm (offset) compared to 1.65mm (standard), t=2.3,P=0.04. Early migration occurs in both groups of pegged components. A principal component micromotion index illustrates the complexity of micromotion. Offset heads move significantly less than the standard type, the significant differences in MTPM movement and in rotation (varus/valgus tilt) suggest that the offset head may decrease eccentric loading