The specific factors affecting wear of the ultrahigh molecular weight polyethylene (UHMWPE) tibial component of total knee replacements (TKR) are poorly understood. One recent study demonstrated that lower conforming inserts produced less wear in knee simulators. The purpose of this study is to investigate the effect of insert conformity and design on articular surface wear of postmortem retrieved UHMWPE tibial inserts. Nineteen NexGen cruciate-retaining (NexGen CR) and twenty-five NexGen posterior-stabilized (NexGen PS) (Zimmer) UHWMPE tibial inserts were retrieved at postmortem from fifteen and eighteen patients respectively. Articular surfaces were scanned at 100×100μm using a coordinate measuring machine (SmartScope, OGP Inc.). Autonomous mathematical reconstruction of the original surface was used to calculate volume loss and linear penetration maps of the medial and lateral plateaus. Wear rates for the medial, lateral and total articular surface were calculated as the slope of the linear regression line of volume loss against implantation time. Volume loss due to creep was estimated as the regression intercept. Student t-tests were used to check for significant.INTRODUCTION
METHODS
Studies of retrieved TKR components demonstrate that Eleven retrieved ultra-high molecular weight polyethylene (UHMWPE) cruciate-retaining tibial liner components from ten separate patients (implantation time = 8.6±5.6 years) had matching gait trials of normal level walking for each knee. Volume loss on retrieved components was calculated using a coordinate measuring machine and autonomous reconstruction. Motion analysis of normal level walking gait had been conducted between 1986 and 2005 for various previous studies and stored in a consented Human Mechanics Repository, ranging from pre-operative to long-term post- operative testing. Contact location between the femoral component and the tibial component on the medial and lateral plateaus were calculated throughout stance. A previously validated and fine-tuned parametric numerical model was used to calculate TKR contact forces for each gait trial. Vertical contact forces and contact paths on the medial and lateral plateaus were input as normal force and sliding distance to a simplified Archard equation for wear with material wear constant averaged from literature (2.42 × 10−7 mm3/Nm) to compute average wear per gait cycle. Wear rates were calculated using linear regression, and Pearson correlation examined correlations between modeled and measured wear.INTRODUCTION
METHODS
Failure of total knee replacements due to the generation of polyethylene wear debris remains a crucial issue in orthopedics. Unlike the hip, it is difficult to accurately determine knee implant wear rates from retrieved components. Several studies have relied on thickness measurements to estimate penetration, but the complicated geometry of contemporary tibial liners poses a challenge to accurately assess wear. In this study we address the question whether linear penetration can serve as a surrogate measure for volumetric material loss. Eighty-one retrieved UHMWPE NexGen cruciate-retaining tibial liners (Zimmer, Warsaw, IN) with an average time in situ of 5.27±2.89 years were included in the study. Metrology data for the surfaces of the tibial liners were obtained with a coordinate measuring machine (OGP, Rochester, NY). Using a laser scanner with two micrometer depth accuracy, at least 400,000 measurement points were taken by investigator #1. Areal thickness changes were mapped for the lateral and medial sides with the help of an autonomous mathematical reconstruction algorithm and volume loss was calculated based on wear scar area and local thickness change. Investigator #2, blinded from these results, measured the minimum thickness of the medial and lateral tibial plateau using a dial indicator with a spherical tip radius of 3mm. Twenty-three short term retrievals (3 to 4 per implant size), removed due to infection and without any signs of wear, served as “unused” reference. Linear penetration was then calculated by subtracting the minimum thickness of each plateau from the average thickness of the reference components.Introduction
Methods
Wear of the ultra-high molecular weight polyethylene (UHWMPE) component and the subsequent aseptic loosening remains a primary reason for late revision of total knee replacements (TKRs).[1] While improved measurement techniques have provided more quantitative information on the wear of surgically retrieved inserts, it is not well understood how observed damage patterns translate to volume loss of polyethylene in vivo. The overall purpose of this study is to investigate the relationship of damage patterns and volume loss at the articular surface of total knee replacements. We hypothesize that damage patterns are reliable predictors of volume loss. Two different investigators independently analyzed damage patterns and volume loss on 43 revision- and 21 postmortem-retrieved MG II (Zimmer Inc.) tibial UHMWPE components. Areas of damage patterns on the articular surfaces were outlined with a video microscope (SmartScope, OGP) and were separated into four spatially exclusive categories (Fig. 1): delamination, pitting, striations and polishing. Articular surfaces were digitized with a low-incidence laser coordinate measuring machine (SmartScope, OGP). Autonomous reconstruction, a previously described and validated method,[2] calculated volume loss on the medial and lateral sides of each component. To investigate the predictability of volume loss using observed patterns, stepwise linear regression models were rendered in PASW Statistics 18 (SPSS Inc).Introduction
Methods
Wear of the UHMWPE tibial component remains a major reason for aseptic loosening and subsequent revision or failure of TKAs [1]. Many retrieval studies measure surface damage patterns as surrogates for the severity of wear, but little is known about how these patterns relate to the volume of material lost. This study (a) examines the wear rate of a cruciate retaining TKA design and (b) relates observed wear patterns to volume loss on the surface. We hypothesize that damage patterns are good predictors for volumetric wear. 43 revision and 21 postmortem-retrieved MG II (Zimmer Inc.) tibial UHMWPE components were included in this study. Wear scars and damage patterns on the superior articular surfaces were digitized using a video microscope (SmartScope, OGP). Patterns were parsed into four spatially exclusive categories: delamination, polishing, striations and pitting. The surfaces were measured at 100×100µm using a low-incidence laser on a coordinate measuring machine (SmartScope, OGP). Autonomous mathematical reconstruction of the original surface was used [2] to calculate volume changes on the medial and lateral surfaces as an estimate of wear volume [Fig. 1] Total volume loss was calculated within the observed wear scar, and volume loss under each pattern was calculated and normalized to the total volume loss of its insert.Introduction
Methods
Many variables contribute to aseptic loosening, and the release of wear particles is a predominant source of late failure. It has been difficult to measure TKA wear quantitatively from retrieved devices; hence, there is a relative paucity of clinically observed TKA wear rates in the literature. Additionally, little is known about patient factors influencing wear rates. This study (a) establishes a clinically relevant TKA wear rate for a cruciate retaining TKA design and (b) relate those wear readings to gait measures of their hosts. 34 revision- and 11 postmortem-retrieved MG II tibial PE-components were included in the analysis. Wear scars on the articulating surface of the insert were digitized under light microscopy. The geometry of the surfaces was mapped at 100×100 μm using a low-incidence laser. Autonomous mathematical reconstruction of the original surface was used [1], and linear penetration on the medial and lateral surfaces and total wear volume were calculated (Fig-1). For five implants, gait data recorded during 1.5 years after surgery were available. Gait studies were performed using a three-dimensional optoelectronic system for motion capture. Joint kinematics and kinetics were calculated using a six-marker model of the lower extremity [2]. All knee moments are reported in Nm, acting externally at the tibia. Potential linear relationships between wear and moment characteristics were investigated.Introduction:
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