Abstract
The wear particles released from the polyethylene (PE) tibial insert of modular total knee replacements (TKRs) have been shown to cause wear particle induced osteolysis, which may necessitate revision surgery [1]. Wear occurs at the backside surface of the PE insert of modular TKRs, resulting from the relative movement between the PE insert and the tibial tray [2]. Wear particles generated from the backside surface of the PE insert have been shown to be smaller in size than those originating from the articular surface [1], and may therefore have increased biological activity and osteolytic potential [3-4]. The ability to predict backside micromotion and contact pressure by finite element simulation has previously been demonstrated by O'Brien et al. [6-7]. Although the effect of insert thickness on articular surface contact pressure has been investigated [5], the effects of insert thickness on backside contact pressures, backside micromotion, and wear has not received adequate attention. Brandt et al. [2] has suggested that increased insert thickness was associated with increased backside damage (Fig. 1). In the present study, finite element simulations were conducted using the Sigma - Press Fit Condylar TKR (Sigma-PFC®, DePuy Orthopedics Inc., Warsaw, IN) with inserts of different insert thickness ranging between 5, 10, 15, 20 and 25 mm. The TKRs were simulated under ISO 14343-2 [8]. A non-linear PE material model was implemented by means of the J2-plasticity theory [6] and the effects of insert thickness on backside micromotion and contact pressure were analyzed. At the peak loading of the simulated gait cycle (time=13%), the 5 mm thick PE insert showed a greater backside peak contact pressure than the 25 mm thickness PE insert. Increasing insert thickness from 5 mm to 25 mm lead to approximately 15% greater peak micromotion at the modular interface (Fig. 2). This effect may be attributed to the ability of the PE material to distribute the load more evenly through deformation at the modular interface and reduce micromotion for thinner inserts. It is suggested that increased insert thickness results in increased moments at the modular interface that could lead to increased backside wear in silico. Although an increase in PE insert thickness was only associated with a moderate increase in backside micromotion in the present study, it was deemed likely that backside micromotion could be accelerated for thicker inserts in vivo as the PE locking mechanism has been shown to degrade after extended implantation periods.
