Abstract
‘High flexion’ polyethylene tibial tray inserts are available from total knee replacement (TKR) manufacturers. There is currently no published data available that examines how much extra knee flexion these new implants give or if there are any wear consequences for the change in design. The high flexion inserts are narrower posteriorly than standard inserts and have chamfers anteriorly and on the post in cruciate sacrificing designs.
This prospective randomised controlled trial of 100 patients undergoing posterior stabilised TKR compared knee flexion, measured intra-operatively by a computer navigation system, of the standard and high flexion trial inserts in the same knee. Patients were then randomised to receive either a standard or ‘high flexion’ definitive component and the stability assessed. The post-operative knee flexion of all patients was measured at six months.
High flexion inserts did not give significantly more knee flexion than standard inserts either per-operatively at the trial insert stage, or at six months post-op and resulted in marginally more anterior draw. The average per-operative difference in flexion between standard and high flex inserts measured in the same knee was 3.2° (range -4-18°) The average knee flexion at 6 months post op was 106° for both groups. The average change in knee flexion comparing pre and post op was 2.3° for the high flex group and 0.6° for the standard insert group.
Laboratory Tek scan contact pressure analysis at the surface of the standard and high flexion designs was not significantly different, but the thinner polyethylene of the high flexion design raises questions about wear characteristics. High flexion polyethylene inserts are probably not justified in terms of improved knee flexion, but may be a useful option in certain technical circumstances during TKR such as patella baja or if the patella impinges on the post in deep flexion.
