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
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 is interest in minimally invasive solutions that reduce osteoarthritic symptoms and restore joint mobility in the early stages of cartilage degeneration or damage. The aim of the present study was to evaluate the Biolox®delta alumina-zirconia composite as a counterface for articulation against live cartilage in comparison to the clinically relevant CoCrMo alloy using a highly controlled in vitro ball-on-flat articulation bioreactor that has been shown to rank materials in accord with clinical experience. The four-station bioreactor was housed in an incubator. The dual axis concept of this simulator approximates the rolling-gliding kinematics of the joint. Twelve 32 mm alumina-zirconia composite femoral heads (Biolox®delta, CeramTec GmbH, Germany) and twelve 32 mm CoCrMo femoral heads (Peter Brehm GmbH, Germany) made up the testing groups. Each head articulated against a cartilage disk of 14 mm diam., harvested from six months old steers. Free-swelling control disks were obtained as well. Testing was conducted in Mini ITS medium for three hours daily over 10 days applying a load of 40 N (∼2 MPa). PG/GAG was determined using the dimethylmethylene blue (DMMB) assay. Hydroxyproline was analyzed by high performance liquid chromatography coupled to a mass spectrometer. Additionally, at test conclusion, chondrocyte survival was determined using Live/Dead assay. Histological analysis was performed using a modified Mankin score. The effect of articulating material (ceramic, CoCrMo) on the various outputs of interest was evaluated using ANOVA. Blocking was performed with respect to the animals. The Mankin scores were compared using the Kruskal–Wallis test.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
Currently, there is a focus on the development of novel materials to articulate against cartilage. Such materials should either eliminate or delay the necessity of total joint replacement. While cobalt-chromium (CoCr) alloy is still a material of choice and used for hemi-arthroplasties, spacers, and repair plugs, alternative materials are being studied. Pyrolytic carbon (PyC) is a biocompatible material that has been available since the 1980s. It has been widely and successfully used in small joints of the foot and the hand, but its tribological effects in direct comparison to cobalt-chromium (CoCr) remain to be investigated. A four station simulator (Figure 1), mimicking joint load and motion, was used for testing. The simulator is housed in an incubator, which and provides the necessary environmental conditions for cartilage survival. Articular cartilage disks (14mm in diameter) were obtained from the trochleas of six to eight months old steer for testing and free-swelling controls. Disks (n=8 per material) were placed in porous polyethylene scaffolds within polypropylene cups and mounted onto the simulator to articulate against 28mm balls of either PyC or CoCr. Each ball was pressed onto the cartilage disk with 40N. In order to allow fluidal load support, the contact migrated over the biphasic cartilage with a 5.2 mm excursion. Concomitantly, the ball oscillated with ±30° at 1 Hz. Testing was conducted for three hours per day over 10 days in Mini ITS medium. Media samples were collected at the end of each three hour test. Upon test commencement, media was pooled (days 1, 4, 7, 10) and analyzed for proteoglycans/sGAGs and hydroxyproline. In addition, total material release into media was estimated by determining the dry weight increase of media samples. For this purpose, 1 ml aliquots of fresh and test media were dialyzed, lyophilized and weighed on a high precision balance. Disk morphology and cell viability were histologically examined.Introduction
Methods
A method to extensively cross-link polyethylene for total hip application has been developed and tested in hip wear simulation. Extensively cross-linked polyethylene was prepared by exposing GUR 1050 polyethylene resin to 90 kg to 110 kg of e-beam radiation. For total hip application, the material was evaluated in an AMTI joint simulator in normal debris-free conditions and in a Shorewestern simulator for the adverse condition of added bone cement and aluminum oxide debris. The normal condition testing was conducted to 30 million cycles, while the adverse condition tests were conducted to 5 million cycles. Femoral head sizes from 22 mm to 46 mm were evaluated. The wear performance of extensively cross-linked material was compared to control material (GUR 1050 gamma sterilized in nitrogen). The results demonstrate a significant improvement in wear (greater than 80 percent reduction) of extensively cross-linked GUR 1050 acetabular components compared to the control acetabular components. The adverse condition wear of both materials was greater than the normal wear; however, when compared to the controls, the extensively cross-linked material had improved wear performance in both normal and adverse conditions. The wear of femoral heads larger than normal 32 mm sizes showed accelerated wear in the control material and desirable low wear in the extensively cross-linked condition. The polyethylene particles generated in the wear simulation were of similar size and shape between the extensively cross-linked and controlled polyethylene. As demonstrated in the laboratory simulation, this extensively cross-linked polyethylene has the potential to substantially reduce particular debris generation in total hip applications. A multicenter randomized controlled clinical study of extensively cross-linked and control acetabular components is ongoing.
