Ideally, standardized wear testing protocols replicate the in vivo motions and forces of TKR patients. In a previous study with 30 TKR patients, two distinct in vivo gait patterns emerged, one characterized as having low anteroposterior (AP-L) motion and the other high anteroposterior (AP-H) motion. The aim of this study was to determine the effect of the two in vivo-determined gait patterns on total and backside insert wear in comparison with the ISO standard 14243-3. In order to differentiate and accurately quantify topside and backside wear, a novel technique was employed where different lanthanide tracers are incorporated into the polyethylene during manufacture. Components from the Zimmer NexGen CR Knee Replacement System were used. Europium (Eu) and Gadolinium (Gd)-stearates were mechanically mixed with GUR1050 UHMWPE resin to obtain two tracer-UHMWPE resins containing 49.1±1.5 ppm Eu and 68.8±1.6 ppm Gd, respectively. 12 grams of the Eu-doped resin was placed on the bottom, 10 grams of virgin GUR1050 resin was placed in the middle, and 10 grams of Gd-doped resin was placed on the top to mold NexGen CR tibial inserts. The backside was then machined to interlock with the tibial baseplate. The minimum insert thickness was 10 mm. All inserts were packaged in nitrogen and gamma sterilized. The wear test was conducted on a 4-station knee simulator in displacement-control mode. Simulator input was obtained from ISO 14243-3 and from gait of 30 NexGen TKR subjects, previously categorized into low (AP-L) and high (AP-H) anteroposterior motion groups. Per station, each insert was sequentially subjected to ISO, AP-L, AP-H motion for 2 Mc at 1 Hz. Subsequently, the ISO profile was repeated. Tibial inserts were weighed and lubricant samples were taken after every 0.5 Mc interval. Knowing the Eu and Gd concentrations from ICP-MS analysis, and normalizing those to the concentrations in the polyethylene inserts, the localized (Eu – backside; Gd – topside) wear was calculated. Wear particle analysis was conducted following established protocols.Introduction
Materials and Methods
