Abstract
INTRODUCTION
Many studies have looked at the effects of titanium tibial baseplates compared to cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different (polished/unpolished) [1,2]. Backside wear may be a function not only of tray material but also of the locking mechanism. The purpose of this study was to evaluate the wear performance of conventional polyethylene inserts when mated with titanium tibial trays or cobalt chrome tibial trays that both have non-polished topside surfaces.
MATERIALS AND METHODS
Three titanium (Ti) trays were used along with three cobalt chrome (CoCr) trays. The Ti trays underwent Type II anodization prior to testing. All trays were Triathlon® design (Stryker Orthopaedics, Mahwah, NJ). Tibial inserts were manufactured from GUR 1020 conventional polyethylene then vacuum/flush packaged and sterilized in nitrogen (30 kGy). Appropriate sized CoCr femoral components articulated against the tibial inserts (Triathlon®, Stryker Orthopaedics, Mahwah, NJ).
Surface roughness of the tibial trays was taken prior to testing using white light interferometry (Zygo Corp, Middlefield, CT). A 6-station knee simulator (MTS, Eden Prairie, MN) was used for testing. Two phases were conducted. The first phase used a normal walking profile, as dictated by ISO 14243-3 [3]. The second phase used waveforms created specifically for stair climbing kinematics. Testing was conducted at a frequency of 1 Hz for 2 million cycles for each test with 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 (protein level = 20 g/l) [4]. The serum solution was replaced and inserts were weighed for gravimetric wear at least every 0.5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts [5]. Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). 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 Ti and CoCr tibial trays (p < 0.01). Figure 2 displays the results of wear testing after 2 million cycles for walking and stair climbing kinematics and showed no significant difference in wear rate between the two tibial trays for either test. The large standard deviation for the CoCr trays during stair climbing kinematics is due to one outlier that had 60% higher wear than the remaining two stations. Without this station, the average of the remaining two CoCr stations was 7.6 mm3/mc, which was similar to the average obtained using Ti tibial trays. Figure 3 shows the backside surfaces of the polyethylene inserts after 2 million cycles using the stair climbing kinematics. Visually, the inserts mated with the Ti trays showed less of a stenciling effect that those mated with CoCr trays. The location of the stenciled area corresponded to the location of the femoral condyle during the loading cycle.
DISCUSSION
Although Ti has different material properties than CoCr, the results of this study show that the wear performance was not adversely affected when Ti tibial trays were substituted for standard CoCr tibial trays under normal walking and stair climbing kinematics. Even though there are differences between the two materials, the documented wear was not significantly different possibly owing to the specific locking mechanism tested.
