Abstract
How total knee replacements (TKR) articulate is directly related to their functional and wear performance. Recently, significant interest has concerned the center of axial rotation, or pivot point. Since the tibia exhibits internal rotation with knee flexion, the pivot point describes condylar translations: a medial pivot implies posterior lateral condylar translations with flexion, a lateral pivot implies anterior medial condylar translations with flexion.The purpose of this study was to describe the location of the pivot point, as related to TKR design, in a large number of knees studied under dynamic weight-bearing conditions.
Two hundred and four well functioning TKR’s were studied using fluoroscopy as subjects performed a stair ascent. There were 131 cruciate retaining fixed bearing knees (11 designs), 33 mobile bearing knees (5 designs), and 40 posterior stabilized knees (4 designs). CAD model based shape matching was used to determine 3D knee kinematics and the pivot point location from 21,837 images. The pivot location was described as a percentage of tibial width, −50% (lateral) to +50% (medial).
Posterior stabilized knees exhibited medial pivots (mean +14%, +7% to +30%) while cruciate retaining (mean −9%, −35% to +21%) and mobile bearing knees (mean −20%, −48% to +5%) exhibited lateral pivots on average (p< 0.001).
How a TKR design provides A/P stability dictates the location of its center of axial rotation and the A/P motions of the condyles. As the relationship between constraint and in vivo motions becomes clearer, TKR designs can be enhanced to achieve more favorable functional and wear performance.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
