Abstract
INTRODUCTION
Many studies have looked at the effect of titanium versus cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different [1,2]. There may also be subtle locking mechanism design changes [2]. The purpose of this study was to evaluate the wear performance of polyethylene inserts when mated with titanium baseplates to cobalt chrome baseplates, where both have non-polished topside surfaces and an identical locking mechanism.
MATERIALS AND METHODS:
A total of three trays per material were used. The titanium trays are intended for cementless application and include a porous titanium surface on the underside, while the cobalt chrome trays are intended for cemented applications. All trays were Triathlon design (Stryker Orthopaedics, Mahwah, NJ). Tibial inserts were manufactured from GUR 1020 polyethylene then vacuum/flush packaged and sterilized in nitrogen (30 kGy). Cobalt chrome femoral components were articulated against the tibial inserts.
Surface roughness of the baseplates was measured prior to testing using white light interferometry (Zygo, Middlefield, CT). A 6-station knee simulator (MTS, Eden Prairie, MN) was used for testing. A normal walking profile was applied [3]. Testing was conducted for 1 million cycles. A lubricant of Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA was used [4]. The serum solution was replaced and inserts were weighed for wear every 0.5 million cycles. Standard test protocols were used for cleaning, weighing, and assessing the wear loss [5]. Soak control specimens were used to correct for fluid absorption. Statistical analysis was performed using the Student's t-test (p < 0.05).
RESULTS:
White light interferometry measurements (Figure 1) showed a significant difference in surface roughness between the tray materials (p < 0.01). Figure 2 displays the results of wear testing after 1 million cycles, which show no significant difference.
Visually, the backside of all inserts showed mild “stenciling” which corresponded to the location of the femoral condyle during the loading cycle. This surface feature transfer phenomenon was less pronounced with the titanium trays.
DISCUSSION:
Although criticized as a bearing material, the results of testing show no deleterious effect on wear performance when using titanium baseplates in comparison to cobalt chrome. In fact, the inserts mated with titanium baseplates show a slight improvement in wear performance, although the difference is not statistically significant. Visually, the backside surfaces of the polyethylene mated with titanium baseplates showed less stenciling effect, which may be due to the difference in material properties as well as the difference in surface roughness.
In conclusion, the results of our wear simulation show that wear performance was not adversely affected when titanium baseplates were substituted for cobalt chrome baseplates under normal walking kinematics.
