Abstract
INTRODUCTION
Tibiofemoral contact at the base of the articular surface spine in posterior-stabilized total knee arthroplasty (TKA) implants can lead to spine fracture [1]. Revision TKA implants also have an articular surface spine to provide sufficient constraint when soft tissues are compromised. While some revision TKA designs have metal reinforcement in the articular surface spine, others rely solely on a polyethylene spine. This study used finite element analysis (FEA) to study the effect of metal reinforcement on stresses in the spine when subjected to posteriorly directed loading.
METHODS
Two clinically successful Zimmer Biomet revision TKA designs were selected; NexGen LCCK with metal reinforcement and all-poly Vanguard SSK. The largest sizes were selected. FEA models consisted of the polyethylene articular surface and a CoCr femoral component; LCCK also included a CoCr metal reinforcement in the spine. A 7° and 0° tibial slope, as well as 3° and 0.7° femoral hyperextension, were used for the LCCK and SSK, respectively. A posteriorly directed load was applied to the spine through the femoral component (Figure 1). The base of the articular surface was constrained. The articular surfaces for both designs are made from different polyethylene materials. However, for the purpose of this study, to isolate the effect of material differences on stresses, both were modeled using conventional GUR1050 nonlinear polyethylene material properties. Femoral component and metal reinforcement were modeled using linear elastic CoCr properties. Additionally, the LCCK was reanalyzed by replacing the metal reinforcement component with polyethylene material, in order to isolate the effect of metal reinforcement for an otherwise equivalent design. Frictional sliding contact was modeled between the spine and femoral/metal reinforcement components. Nonlinear static analyses were performed using Ansys version 17 software and peak von mises stresses in the spine were compared.
RESULTS
Peak von mises stresses were predicted towards the base of the anterior aspect of the spine in both designs (Figure 2). In LCCK, the high stresses were also predicted on the medial and lateral edges of the anterior spine, matching the tibiofemoral contact (Figure 3). The LCCK with metal reinforcement design predicted 14% and 31% lower stress than LCCK and SSK all-poly designs.
DISCUSSION
Clinical reports of spine fracture in TKA highlight the need for further understanding of the biomechanics of spine loading. Here, through comparison of two clinically successful devices, the effect of multiple design factors was quantified. Inclusion of metal reinforcement in the spine, as well as differences in the conforming geometry between the femoral component and the articular surface, resulted in a 31% decrease in polyethylene stress for the LCCK as compared to the all-poly SSK; of which only 16% was attributed to the metal reinforcement. Further improvements to articular surface design, as well as polyethylene material advances, have the potential to result in all-poly designs with strength characteristics equivalent to or exceeding those of designs with metal reinforcement.
