Abstract
Introduction
A previous computational study on an all-polymer PEEK-on-UHMWPE total knee replacement implant showed improved periprosthetic bone loading, compared to a conventional implant [1]. That study used a simulated gait cycle to determine distal loading, but a patella was not included. Substantial distal decrease of bone remodeling stimulus was found, in accordance with previous reports [2], but it was not consistent with other clinical and post-mortem DEXA results, which found the largest loss of bone stock in the anterior region [3,4]. As patellofemoral forces are relatively low during gait compared to squatting, we simulated a deep squat, expecting that a high-demand activity would provide similar indications of bone loss as literature [3,4]. Consequently, we applied both high tibiofemoral and patellofemoral loads, to provide more insight in the potential benefits of a new PEEK-Optima® femoral component on periprosthetic bone stock.
Methods
We adopted a deep squat finite element model from Zelle et al. and included quasi-static deep flexion and load sharing at the posterior condyles [6]. A new implant design was inserted, with three variations in material properties: intact, CoCr and PEEK. The stiffness of the femoral elements was mapped from CT and applied to either the cut femur only (CoCr and PEEK) or the entire femoral construct (intact). The strain energy density (SED) was evaluated in the periprosthetic region as a measure for bone remodeling stimulus. To examine the effects of the entire exercise, SED values were integrated over all increments.
Results
During squat the highest SED values were found at the intercondylar region, behind the posterior condyles and behind the anterior flange, extending further inward to the bone. Both the lateral and medial view of the periprosthetic region show markedly different SED patterns from the conventional CoCr implant. Higher values originating proximally extended to lower values in the anterodistal region (Figure 1). However, in the CoCr reconstruction these anterodistal patterns less prominent or even absent. In Figure 2, simulated DEXA images are presented showing the bone remodeling stimulus throughout the periprosthetic volume. Overall, the image for CoCr shows darker areas than PEEK and the reference, further corroborating the findings shown in Figure 1. Moreover, it is visible that the PEEK reconstruction had SED values similar to the reference in the femoral component region.
Discussion
This study has corroborated that the influence of the patella in high-demanding tasks is of great importance to the anterior periprosthetic bone stock [4,5]. The loss of bone remodeling stimulus in the CoCr reconstruction is in accordance with literature findings [2–4]. The resemblance of PEEK to the intact reference suggests that the new PEEK-Optima® femoral component could largely retain the integrity of the periprosthetic bone.
