Passive anterior-posterior knee stability after unconstrained unicompartmental arthroplasty

These Passive anterior-posterior (A-P) stability of the knee, measured in terms of joint laxity, is considered important for good clinical outcome following knee arthroplasty. In vitro and in vivo studies measured the laxity at selected joint positions in the intact and replaced knees. However, analyzing the effects of surgical techniques on the joint stability is difficult to implement experimentally. In the present study a mathematical model of the knee with unconstrained unicompartmental arthroplasty was used to study relative translations of the bones during a simulated A-P laxity test over 0-120° flexion. The knee ligaments were modeled as bundles of non-linear elastic fibers. Anatomical data, material properties of ligaments, geometries of the prosthetic components and guidelines for component placement on the bones were taken from literature. The model calculations for tibial translations resulting from ±130 N A-P forces were compared with the experimental measurements of Lo et al. (2010), reported as mean of 14 cadaver knees. Further, the effects of component placement on the bones were also studied. The model calculations agreed in general with the experimental measurements showing similar patterns during flexion. The joint laxity first increased from 0o to about 45° flexion and decreased thereafter. An increase in the A-P force resulted in uniform increase in laxity over flexion. A change of 1 mm in the placement of femoral component affected the laxity by nearly 3 mm near extension. However, this effect of change varied significantly with flexion. Such effects can alter the joint kinematics and may be clinically significant. The analysis has clinical relevance .