Wear debris from polyethylene tibial inserts has been associated with limited longevity of total knee replacements (TKRs). While material factors were studied extensively and considerable progress has been made, there is little knowledge about surgical factors, particularly on how the wear rate is related to implant positioning. It was the purpose of this study to determine the combined effect of patient and implant positioning factors on the volumetric wear rate of TKRs. Our hypothesis was that implant alignment has a significant impact on the wear rate when controlled for other patient factors. This study included 59 tibial inserts of a cruciate retaining TKR design (Nexgen, Zimmer Inc.). The patients' age, sex, weight, height, and implant size were obtained. All implants were scanned with a coordinate measuring machine. Volumetric wear was determined using an autonomous mathematical reconstruction method (Figure 1). Radiographs were used to determine the anatomic lateral distal femoral angle (aLDFA), anatomic medial proximal tibial angle (aMPTA), femoral tilt angle (FTA) and posterior tibial slope (PTS). Also, the patella position was assessed using the Blackburne-Peel Index (BPI) and the Insall-Salvati Ratio (Figure 2). General linear modeling (SPSS) was conducted in order to determine the most significant patient and implant positioning factors on wear rate.Introduction
Methods
There are increasing reports of total hip replacement (THR) failure due to corrosion within modular taper junctions, and subsequent adverse local tissue reactions (ALTRs) to corrosion products. Modular junction corrosion is a multifactorial problem that depends on material, design, patient and surgical factors. However, the influence of alloy microstructure on corrosion has not been studied sufficiently. Especially for cast CoCrMo, there are concerns regarding microstructure variability with respect to grain size and hard-phase volume fraction. Therefore, it was the goal of this study to (1) identify different types of microstructures in contemporary implants, and (2) determine implications of alloy microstructure on the occurring corrosion modes. Fifteen surgically retrieved femoral stems made from cast CoCrMo alloy were analyzed for this study. Damage on the taper surfaces was investigated by scanning electron microscopy (SEM) and damage was assessed with the Goldberg Score. The alloy microstructure was evaluated by standard metallographic techniques. Alloy samples were sectioned off the femoral stem, and microstructural features were visualized by chemical etching. Cyclic potentio-dynamic polarization tests were carried out with alloy samples from two implants with different commonly occurring types of microstructures. Both had a similar grain size, but type 1 had no hard-phases, where as type 2 exhibited hard-phases along the grain boundaries, as well as intra-granular hard-phase clusters. Tests were performed in bovine serum at 37°C with a saturate calomel reference electrode and a graphite counter electrode. In vitro generated corrosion damage was then compared to in vivo generated damage features on the taper surfaces of the corresponding implants.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
