Primary stability of unicompartmental knee arthroplasty under dynamic flexion movement in human femora.

BACKGROUND: The objective of our study was to evaluate the impact of a trabecular stem fixation versus a cortical teeth fixation technique on the primary stability of cemented unicompartmental femoral components under dynamic flexion movement loading conditions in human femora.

METHODS: Ten fresh-frozen human knees of a mean donor age of 73.9years were used to perform medial unicompartmental knee arthroplasty under a less invasive parapatellar surgical approach. The femora were divided into two groups of matched pairs based on comparable trabecular bone mineral density. To assess the primary stability, a new method based on a combination of dynamic flexion movement, double-peak loading simulating stair climbing, kinematic analysis of the femoral component migration relative to the bone and an evaluation of the cement layer by fragments cut through the implant-cement-bone interface in the sagittal plane of the medial condyle was introduced.

FINDINGS: For the "trabecular stem fixation" technique the mean load to failure was 2340 (SD 650) N and for "cortical teeth fixation" it was 1080 (SD 455) N, with a substantially enhanced dynamic fixation strength for the "trabecular stem fixation" (p=0.008). In the distal area the cement layer of the "trabecular stem fixation" showed a significant decreased thickness compared to the "cortical teeth fixation" (p=0.029), while a substantially deeper cement penetration (p=0.044) has been achieved for the "trabecular stem fixation".

INTERPRETATION: From our observations, we conclude that there is a significantly enhanced primary stability with a "trabecular stem fixation" compared to a "cortical teeth fixation" technique of cemented unicompartmental femoral components, in terms of dynamic failure load and migration characteristics.