Total Hip Arthroplasty (THA) devices are now increasingly subjected to a progressively greater range of kinematic and loading regimes from substantially younger and more active patients. In the interest of ensuring adequate THA solutions for all patient groups, THA polyethylene acetabular liner (PE Liner) wear representative of younger, heavier, and more active patients (referred to as HA in this study) warrants further understanding. Previous studies have investigated HA joint related morbidity [1]. Current or past rugby players are more likely to report osteoarthritis, osteoporosis, and joint replacement than a general population. This investigation aimed to provide a preliminary understanding of HA patient specific PE liner tribological performance during Standard Walking (SW) gait in comparison to IS0:14242-1:2014 standardized testing. Nine healthy male subjects volunteered for a gait lab-based study to collect kinematics and loading profiles. Owing to limitations in subject selection, five subjects wore a weighted jacket to increase Body Mass Index ≥30 (BMI). An induced increase in Bodyweight was capped (<30%BW) to avoid significantly effecting gait [3] (mean=11%BW). Six subjects identified as HA per BMI≥30, but with anthropometric ratios indicative of lower body fat as previously detailed by the author [2] (Waist-to-hip circumference ratio and waist circumference-to-height ratio). Three subjects identified as Normal (BMI<25). Instrumented force plate loading profiles were scaled (≈270%BW) in agreement with instrumented hip force data [4]. A previously verified THA (Pinnacle® Marathon® 36×56mm, DePuy Synthes) Finite Element Analysis wear model based on Archard's law and modified time hardening model [5] was used to predict geometrical changes due to wear and deformation, respectively (Figure 1). Subject dependent kinematic and loading conditions were sampled to generate, for both legs, 19 SW simulation runs using a central composite design of response surface method.Introduction
Materials and Methods
Hip arthroplasty is considered common to patients aged 65 and over however, both Jennings, et al., (2012) and Bergmann (2016) found THA patients are substantially younger with more patients expecting to return to preoperative activity levels. With heavier, younger, and often more active patients, devices must be able to support a more demanding loading-regime to meet patient expectations. McClung (2000) demonstrated that obese patients can display lower wear-rates with UHMWPE bearing resulting from post-operative, self-induced reduced ambulatory movement, thus questioning if obese kinematics and loading are indeed the worst-case. Current loading patterns used to test hip implants are governed by ISO 14242-1 (2014). This study aimed to characterize a heavy and active population (referred to as HA) and investigate how the gait profile may differ to the current ISO profile. A comprehensive anthropometric data set of 4082 men (Gordon, CC., et.al., 2014) was used to characterize a HA population. Obese and HA participants were classed as BMI ≥30 however HA participants were identified by applying anthropometric ratios indicative of lower body fat, namely “waist to height” (i.e. WHtR <0.6) and “waist to hip” (i.e. WHpR <0.9).Introduction
Method
To meet the demands of younger more active patients more robust pre-clinical wear testing methods are required, in order to simulate a wider range of activities. A new electromechanical simulator (Simulation Solutions, UK) with a greater range of motion, a driven abduction/adduction axis and improved input kinematic following has been developed to meet these requirements, as well as requirements of the relevant international standards. This study investigated the wear of a fixed bearing total knee replacement using this new electromechanical knee simulator, comparing with previous data from a pneumatic simulator. The wear of six Sigma CR fixed bearing TKRs (DePuy, UK) with curved moderately cross-linked polyethylene inserts (XLK) was determined in pneumatic and electromechanical Prosim knee simulators (Simulation Solutions, UK). Standard gait displacement controlled kinematics were used, with a maximum anterior-posterior displacement of either 10mm (high) or 5mm (intermediate) [1]. The output profiles from the simulators were obtained and compared to the demand input profiles. The lubricant used was 25% new-born calf serum and wear determined gravimetrically. Statistical analysis was performed using the one-way ANOVA with 95% confidence interval and significance was taken at p<0.05.Introduction
Materials/Methods
Moderately crosslinked, thermally treated ultrahigh molecular weight polyethylene (UHMWPE) has to date demonstrated a good balance of wear resistance and mechanical properties. MARATHON™ Polyethylene (DePuySynthes Joint Reconstruction, Warsaw, IN) is made from polyethylene resin GUR 1050, gamma-irradiated at a dose of 5.0 Mrads to create crosslinking of polyethylene, and followed by a remelting process to eliminate free radicals for oxidative stability. 10-year clinical study [1] and laboratory wear simulation tests [2–3] have reported excellent wear performance of the MARATHON poly. There continues to be demand for improved head-to-shell ratio acetabular systems because larger head sizes have the benefits of increased stability and range of motion. The increased head-to-shell ratio is often times achieved by using a reduced liner thickness. One of the clinical concerns of thinner poly liners is the potential for rim fracture, particularly in the occurrences of rim loading or impingement at high cup angles [4–7]. This study investigated the performance of thinner poly liners to the challenge of high angle rim loading and neck-to-liner impingement. Three groups of ETO sterilized MARATHON polyethylene liners (ID/OD: 28/44, 32/48, and 36/52 mm) were paired with matching CoCrMo heads (n=6 each group). To simulate rim loading, liners were assembled in the metal shells tilted at 64° (Figure 1) with sinusoidal loading (0 to 5000N at 3Hz) in a 37°C water bath for 5-million cycles or until component failure, whichever occurred first. For neck-liner impingement testing, metal shells were potted at 54º (in the abduction/adduction plane with a ±10° of motion per ISO 14242–1 [8]) on a hip simulator (n=4 each group) using a physiological loading (max 3000N at 1Hz) for 3-million cycles (Figure 2). The impingement occurred at 64º during the simulated gait cycle (Figure 3). The liners were inspected every million cycles, using a high intensity light to search for signs of crack initiation and/or fractures. Both test methods were validated to be able to replicate liner fractures.Introduction
Materials and Methods
Moderately crosslinked polyethylene maintains a balance of wear resistance and mechanical properties. The GVF poly was manufactured from GUR1020 UHMWPE bars, sealed in vacuumed foil package, and gamma sterilized at 4 Mrads. The MARATHON® polyethylene inserts were manufactured from GUR1050 UHMWPE bars, crosslinked by gamma irradiation at 5 Mrad, and followed by a remelting process that eliminates free radicals. The final sterilization method is gas plasma (GP) or ethylene oxide (EtO). Both methods will not introduce free radicals. Previous studies have shown MARATHON polyethylene (GP sterilized) with 83% lower wear than conventional polyethylene in a simulation test [1], compared to a 10-year clinical study that showed 77% wear reduction [2]. There is no study to compare the wear performance of MARATHON (EtO sterilized) and conventional poly. Four groups of polyethylene inserts (Table 1) were paired with matching femoral heads that were manufactured from CoCrMo (ASTM F1537) with diameters of 28, 32, and 36 mm. The inserts were chosen to have similar thickness at the dome for MARATHON, while for GVF it was the largest head size available. Wear testing was performed on an AMTI Hip Simulator per the ISO 14242-1 standard [3] at 1 Hz using the described inputs (Table 2), which provide a larger range of motion than the ISO standard. The cups were mounted in accordance with ISO 14242-1 using custom fixturing and secured with cement while the femoral heads were mounted on a vertical taper support. Testing was performed in 25% bovine calf serum at 37 ± 2°C. Wear of the inserts was determined gravimetrically. Finally, wear rates were calculated by linear regression and then compared between the groups using ANOVA analysis (α = 0.05).Introduction:
Materials and Methods:
Wear debris induced osteolysis and loosening continue to be causes of clinical failure in total knee replacement (TKR). Laboratory simulation aims to predict the wear of TKR bearings under specific loading and motion conditions. However, the conditions applied may have significant influence on the study outcomes (1) The aim of this study was to examine the influence of femoral setup and kinematic inputs on the wear of a conventional polyethylene fixed bearing TKR through experimental and computational models. Six right Sigma CR fixed bearing TKRs (DePuy Synthes, Leeds, UK) with curved polyethylene inserts (GVF, GUR1020 UHMWPE) were tested in Prosim knee simulator (Simulator Solutions, UK). The femoral bearing was set up with the centre of rotation (CoR) on either on the distal radius of the implant (Distal CoR), as indicated by the device design, or according to the ISO specification (ISO CoR; ISO14243-3). The tests were conducted under ‘High Kinematics’ (2). It was necessary to reverse the direction of the anterior-posterior displacement for the tests conducted with the ISO centre of rotation to maintain the contact region within the insert surface (Reverse High Kinematics). Tests were conducted for three million cycles, lubricated with 25% bovine serum, with wear assessed gravimetrically. The computational wear model for the TKR was based on the contact area and an independent experimentally determined non-dimensional wear coefficient, previously validated against the experimental data (3).Introduction
Methods
Wear of polyethylene continues to be a significant factor in the longevity of total knee replacement (TKR). Moderately cross-linked polyethylene has been employed to reduce the wear of knee prostheses, and more recently anti-oxidants have been introduced to improve the long-term stability of the polyethylene material. This is the initial study of the wear of a new anti-oxidant polyethylene and a new TKR design, which has modified femoral condylar geometry. The wear of a new TKR the Attune knee was investigated using a physiological six station Prosim knee wear simulator (Simulator Solutions, UK). Six mid-size Attune fixed bearing cruciate retaining TKRs (DePuy Inc, Warsaw, USA) were tested for a period of 6 million cycles. The inserts were manufactured from AOX™, a compression moulded GUR1020 polyethylene incorporating Covernox™ solid anti-oxidant. The AOX polymer was irradiated to 8M Rad, to give a moderately cross-linked material. High and intermediate kinematics, under anterior-posterior displacement control were used for this study (McEwen Introduction
Materials and Methods
