Correlating subjective and objective descriptors of ultra high molecular weight wear particles from total joint prostheses.

A total of 750 images of individual ultra-high molecular weight polyethylene (UHMWPE) particles isolated from periprosthetic failed hip, knee, and shoulder arthroplasties were extracted from archival scanning electron micrographs. Particle size and morphology was subsequently analyzed using computerized image analysis software utilizing five descriptors found in ASTM F1877-98, a standard for quantitative description of wear debris. An online survey application was developed to display particle images, and allowed ten respondents to classify particle morphologies according to commonly used terminology as fibers, flakes, or granules. Particles were categorized based on a simple majority of responses. All descriptors were evaluated using a one-way ANOVA and Tukey-Kramer test for all-pairs comparison among each class of particles. A logistic regression model using half of the particles included in the survey was then used to develop a mathematical scheme to predict whether a given particle should be classified as a fiber, flake, or granule based on its quantitative measurements. The validity of the model was then assessed using the other half of the survey particles and compared with human responses. Comparison of the quantitative measurements of isolated particles showed that the morphologies of each particle type classified by respondents were statistically different from one another (p<0.05). The average agreement between mathematical prediction and human respondents was 83.5% (standard error 0.16%). These data suggest that computerized descriptors can be feasibly correlated with subjective terminology, thus providing a basis for a common vocabulary for particle description which can be translated into quantitative dimensions.

The influence of sterilization method on wear performance of the low contact stress total knee system.

Sterilization method changes to the Low Contact Stress (LCS) Total Knee System (DePuy Orthopaedics Inc, Warsaw, Ind) have resulted in progressive improvements in wear resistance. Initially, the system used the traditional gamma in air irradiation sterilization method, but was transitioned to Gas plasma sterilization. The Gas plasma technique did not introduce free radicals that subsequently oxidize with extended shelf aging found with gamma in air irradiation. This oxidation has demonstrated a significant reduction in mechanical properties leading to increased wear. The introduction of Gas plasma was also accompanied by a transition to polyethylene resins that no longer contained calcium stearate, surmised to influence the rate of oxidation. More recently, gamma irradiation and storage of the polyethylene components in an essentially oxygen-free environment imparted by gamma irradiation in a vacuum foil pouch (GVF), has been shown to protect the implant from oxidation. Additionally, the GVF process imparts the beneficial effects of moderate crosslinking that has been demonstrated to improve wear resistance.