Abstract
Introduction
Although cementless press-fit femoral total knee arthroplasty (TKA) components are routinely used in clinical practice, the effect of the interference fit on primary stability is still not well understood. Intuitively, one would expect that a thicker coating and a higher surface roughness lead to a superior fixation. However, during implant insertion, a thicker coating can introduce more damage to the underlying bone, which could adversely influence the primary fixation. Therefore, in the current study, the effect of coating thickness and roughness on primary stability was investigated by measuring the micromotions at the bone-implant interface with experimental testing.
Methods
A previous experimental set-up was used to test 6 pairs of human cadaveric femurs (47–60 years, 5 females) implanted with two femoral component designs with either the standard e.motion (Total Knee System, B. Braun, Germany) interference fit of 350 µm (right femurs) or a novel, thicker interference fit of 700 µm (left femurs). The specimens were placed in a MTS machine (Figure 1) and subjected to the peak loads of normal gait (1960N) and squat (1935N), based on the Orthoload dataset for Average 75.
Varus/valgus moments were incorporated by applying the loads at an offset relative to the center of the implants, leading to a physiological mediolateral load distribution. Under these loads, micromotions at the implant-bone interface were measured using Digital Image Correlation (DIC) at different regions of interest (ROIs – Figure 1). In addition, DIC was used to measure opening and closing of the implant-bone interface in the same ROIs.
Results
After comparing the micromotions and opening of the two implant designs, we found no significant differences between the standard and novel coating. Loading was a significant factor for both opening (P<0.0001) and micromotions (P=0.019), where the squat produced higher micromotions than gait. Opening was seen anteriorly (MA, LA), and was higher during squat. Closing was noticed distally (MD, LD), particularly during gait (Figure 2).
During gait (Figure 3), the highest micromotions were found in the posterior condyles (CM, MP), followed by the medial anterior region (MA). For squat, the largest micromotions were in the anterior flange (ANT), followed by the distal regions (LD, MD).
Discussion
In the current study, the primary stability of the same implant with two different coating thicknesses was evaluated. The results demonstrate that increasing the coating thickness does not automatically influence the primary stability of a femoral TKA component. This is likely due to abrasion and damage of the underlying trabecular during implant insertion, which also was observed in previous experiments.
The exact relation between coating thickness or interference fit and primary implant stability still remains subject to debate. Obviously, the primary implant stability is compromised when the interference fit is too low. However, the current results suggest that there is a threshold beyond which further improvement of the fixation is not possible. The exact magnitude of this threshold is unknown, and may depend on coating characteristics and bone quality, and requires further evaluation, possibly utilizing a hybrid approach of experimental and computational techniques.
