Time-dependent fixation and implantation forces for a femoral knee component--an in vitro study

Implant survival rate is a primary concern for individuals receiving a primary total knee arthroplasty. Loosening is the primary reason for revision surgery and was therefore the focus of the current study. To better understand the mechanics of implant fixation, the time-dependent fixation of a femoral knee component was measured in vitro on three cadaveric femurs. The fixation of each femoral knee component was measured with strain gauged implants for at least 10min on each femoral component. Additionally, impaction forces were measured during the implantation of each component. These forces were 2-6 times less than previously reported. The implantation impact forces were higher for the bones with higher bone density. Power law regressions were fit to the absolute value of the principal strains measured on the components over time to quantify the relaxation of the bone. The average power coefficient value for the three bones was lower for the bones with higher bone density. The average power coefficient value for the maximum principal strains was significantly higher than that of the minimum principal strains in each bone. The results were extrapolated to approximate the fixation strength at 9 months after implantation. In this time period the strain was predicted to decrease to between 78 and 91% of the strain 1s after implantation where those with lower bone density will have decreased fixation strength.