Due to their superior wear characteristics, oxidized Zr-2.5Nb heads are used with hip stems made of conventional orthopaedic alloys. Galvanic interactions between Zr-2.5Nb (Zr) and Ti-6Al-4V (Ti), cobalt-chromium (CoCr), and 316L stainless steel (SS) alloys were evaluated.

Galvanic current density was measured for Zr/Ti,Zr/CoCr, Zr/SS, CoCr/Ti, and CoCr/SS couples under static conditions in aneutral Ringer’s solution and in an acidic (1.7 pH) solution. To simulate fretting, one or both coupled alloys in the neutral solution subsequently were abraded by a bone cement pin (82 MPa Hertzian stress). An extended(7-day) static test in the acidic solution was performed for Zr/SS and CoCr/Ti to simulate crevice conditions. The dissolved metal ion concentration was determined using direct coupled plasma emission spectrometry.

Mean initial current densities of the Zr/SS, SS/CoCr,Zr/CoCr, CoCr/Ti, and Zr/Ti couples were 3.0, 0.36, 0.16, 0.05, and 0.04μA/cm2, respectively, in the neutral solution, and 0.57, −0.29, 0.04, 0.02, and 0.03 μA/cm2, respectively, in the acidic solution (positive when first alloy was anode). Within 30 minutes, all values decreased below 0.02μA/cm2. The current densities increased by orders of magnitude under fretting conditions. When both alloys were abraded, the highest values were minus;677 and 464 μA/cm2 for CoCr/Ti and Zr/SS, respectively. In the extended static test of Zr/SS, the mean total metal ion concentration decreased from 8.15 mg/L when the alloys were uncoupled to 4.50 mg/L(p=0.007) when they were coupled. For CoCr/Ti, the change from 1.28 to 1.72mg/L when the alloys were coupled was not statistically significant(p=0.22).

With its strong tendency to passivate, the Zr alloy produced galvanic interactions within the range observed with conventional alloy couples. Its anodic characteristic helped protect SS in a galvanic couple.

Oxidized Zirconium (OxZr), metallic zirconium alloy oxidized to form a ceramic surface, was developed as an alternative bearing material to cobalt-chrome (CoCr) alloy for improvements in roughening resistance, frictional behavior, and biocompatibility without a risk of brittle fracture. Knee simulator testing without intentional addition of abrasives demonstrated that the ultra-high molecular weight polyethylene (UHMWPE) wear rate was 85% less with OxZr than with CoCr femorals. The relative performance of articulating materials can change when tested under abrasive conditions, so test protocols were investigated with abrasives added directly to the simulator test lubricant.

Testing was conducted on a six-station, four-axis, physiological knee simulator. OxZr and CoCr medium-sized, cruciate-retaining, femoral components were tested against UHMWPE tibial inserts sterilized by ethylene oxide. Alumina powder was mixed into 50% bovine serum lubricant at a concentration of 0.2 mg/cm3. Tests were conducted with different powder sizes in the range of 0.3 to 150 μm. Measurements included tibial insert weight and femoral surface roughness.

The lubricant in CoCr tests became opaque with gray debris while the femoral condyles became scratched. In contrast, the lubricant in OxZr tests remained normal (as in tests without abrasives), and femoral condyle scratching was much less severe. Despite these obvious effects, the UHMWPE wear produced by each material did not increase appreciably over that of tests without abrasives, with OxZr maintaining a wear rate about 85% less than for CoCr. It was noted that the scratches were aligned, or became realigned, with the translation motion and had little evidence of the swirls or cross-hatching often observed on retrieved components. Previous testing indicates that UHMWPE wear increases significantly only if scratches are oblique to the sliding direction. Thus, a test technique that produces scratches with more clinically relevant orientations is needed for a performance comparison between femoral materials under abrasive conditions.