Todd Stewart

Long-term wear of HIPed alumina on alumina bearings for THR under microseparation conditions.

The long term wear and wear debris generated in HIPed alumina on alumina bearings for hip prostheses with microseparation in vitro is compared to standard simulator conditions and ex vivo specimens. Microseparation studies were completed to five million cycles at two severity levels in attempts to rigorously evaluate the long-term tribological performance of the bearings. During the first million cycles (bedding-in) of the microseparation tests characteristic stripe wear was observed on all of the femoral heads with a matching area on the rim of the acetabular inserts. Under mild microseparation conditions an average wear rate of 0.55 mm3/million cycles was observed during the initial million cycles which reduced to a steady state level of 0.1 mm3/million cycles. Under more severe conditions an average wear rate of 4.0 mm3/million cycles was observed during bedding-in which reduced to a steady state level of 1.3 mm3/million cycles. These compare to a bedding-in wear rate of 0.11 mm3/million cycles and steady-state wear rate of 0.05 mm3/million cycles for the same material under normal simulation with no microseparation. Furthermore, under microseparation the wear mechanisms and wear debris were similar to those observed in previous alumina retrieval studies with debris ranging from 10 nm to 1 μm in size.© 2001 Kluwer Academic Publishers

An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Abstract Although the wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/106 cycles) is much lower than the more widely used polyethylene-on-metal bearings, there are concerns about the toxicity of metal wear particles and elevated metal ion levels, both locally and systemically, in the human body. The aim of this study was to investigate the possibility of reducing the volume of wear, the concentration of metal debris and the level of metal ion release through using surfaceengineered femoral heads. Three thick (8-12 μm) coatings (TiN, CrN and CrCN) and one thin (2 μm) coating (diamond-like carbon, DLC), were evaluated on the femoral heads when articulating against high carbon content cobalt-chromium alloy acetabular inserts (HC CoCrMo) and compared with a clinically used MOM cobalt-chromium alloy bearing couple using a physiological anatomical hip joint simulator (Leeds Mark II). This study showed that CrN, CrCN and DLC coatings produced substantially lower wear volumes for both the coated femoral heads and the HC CoCrMo inserts. The TiN coating itself had little wear, but it caused relatively high wear of the HC CoCrMo inserts compared with the other coatings. The majority of the wear debris for all half-coated couples comprised small, 30 nm or less, CoCrMo metal particles. The Co, Cr and Mo ion concentrations released from the bearing couples of CrN-, CrCN- and DLC-coated heads articulating against HC CoCrMo inserts were at least 7 times lower than those released from the clinical MOM prostheses. These surface-engineered femoral heads articulating on HC CoCrMo acetabular inserts produced significantly lower wear volumes and rates, and hence lower volumetric concentrations of wear particles, compared with the clinical MOM prosthesis. The substantially lower ion concentration released by these surface-engineered components provides important evidence to support the clinical application of this technology.

Wear and deformation of ceramic-on-polyethylene total hip replacements with joint laxity and swing phase microseparation

Abstract Wear of polyethylene and the resulting wear debris-induced osteolysis remains a major cause of long-term failure in artificial hip joints. There is interest in understanding engineering and clinical conditions that influence wear rates. Fluoroscopic studies have shown separation of the head and the cup during the swing phase of walking due to joint laxity. In ceramic-on-ceramic hips, joint laxity and microseparation, which leads to contact of the head on the superior rim of the cup, has led to localized damage and increased wear in vivo and in vitro. The aim of this study was to investigate the influence of joint laxity and microseparation on the wear of ceramic on polyethylene artificial hip joints in an in vitro simulator. Microseparation during the swing phase of the walking cycle produced contact of the ceramic head on the rim of the polyethylene acetabular cup that deformed the softer polyethylene cup. No damage to the alumina ceramic femoral head was found. Under standard simulator conditions the volume change of the moderately crosslinked polyethylene cups was 25.6 ± 5.3 mm3/million cycles and this reduced to 5.6 ± 4.2 mm3/million cycles under microseparation conditions. Testing under microseparation conditions caused the rim of the polyethylene cup to deform locally, possibly due to creep, and the volume change of the polyethylene cup when the head relocated was substantially reduced, possibly due to improved lubrication. Joint laxity may be caused by poor soft tissue tension or migration and subsidence of components. In ceramic-on-polyethylene acetabular cups wear was decreased with a small degree of joint laxity, while in contrast in hard-on-hard alumina bearings, microseparation accelerated wear. These findings may have significant implications for the choice of fixation systems to be used for different types of bearing couples.

Severe wear and fracture of zirconia heads against alumina inserts in hip simulator studies with microseparation

The wear of zirconia femoral heads against alumina acetabular inserts with swing-phase microseparation was investigated in a hip joint simulator. Under mild microseparation conditions, the wear was very low, with an average wear rate of 0.05 mm(3)/million cycles reported over 5 million cycles of testing. However, under severe microseparation conditions representative of greater joint laxity, the wear rate of zirconia against alumina increased by 2 orders of magnitude, producing severe wear and, in one case, femoral head fracture. The adverse results of this study indicate that the combination of a zirconia femoral head articulating against an alumina acetabular insert is not recommended for clinical use. The results further raise concerns over the suitability of conventional simulators in evaluating the wear of ceramic hip prostheses.

In vitro investigation of friction under edge-loading conditions for ceramic-on-ceramic total hip prosthesis.

Edge‐loading generates higher wear rates in ceramic‐on‐ceramic total hip prosthesis (THP). To investigate the friction coefficient (FC) in these conditions, three alumina ceramic (Biolox Forte) 32 mm‐diameter components were tested using a hip friction simulator. The cup was positioned with a 75° abduction angle to achieve edge‐loading conditions. The motion was first applied along the edge and then across the edge of the cup. First, tests were conducted under lubricated conditions with 25% bovine serum. Next, to simulate an extremely high contact pressure, the tests were run with the addition of a third body alumina ceramic chip inserted between the edge of the cup and the head. Engineering blue was used to analyze the contact area. Reference values were determined using a 0° cup abduction angle. Edge loading was achieved. The FC increased by three‐ to sixfold when the motion was applied along the edge, and by 70% when the motion was applied across the edge. However, the FC value was still low (about 0.1), which is similar to metal‐on‐metal THP. With the third body alumina ceramic particle inserted, the FC was 26 times higher than in the ideal conditions and intermittent squeaking occurred. High cup abduction angles may generate edge‐loading and an increase in the friction coefficient for ceramic THP.

Experimental and Theoretical Study of the Contact Mechanics of Five Total Knee Joint Replacements

The tibio-femoral contact area in five current popular total knee joint replacements has been measured using pressure-sensitive film under a normal load of 2.5 kN and at several angles of flexion The corresponding maximum contact pressure has been estimated from the measured contact areas and found to exceed the point at which plastic deformation is expected in the ultra-high molecular weight polyethylene (UHMWPE) component particularly at flexion angles near 90°. The measured contact area and the estimated maximum contact stress have been found to be similar in magnitude for all of the five knee joint replacements tested. A significant difference, however, has been found in maximum contact pressure predicted from linear elasticity analysis for the different knee joints. This indicates that varying amounts of plastic deformation occurred in the polyethylene component in the different knee designs. It is important to know the extent of damage as knees with large amounts of plastic deformation are more likely to suffer low cycle fatigue failure. It is therefore concluded that the measurement of contact areas alone can be misleading in the design of and deformation in total knee joint replacements. It is important to modify geometries to reduce the maximum contact stress as predicted from the linear elasticity analysis, to below the linear elastic limit of the plastic component.

Litigation after hip and knee replacement in the national health service

The results of hip and knee replacement surgery are generally regarded as positive for patients. Nonetheless, they are both major operations and have recognised complications. We present a review of relevant claims made to the National Health Service Litigation Authority. Between 1995 and 2010 there were 1004 claims to a value of £41.5 million following hip replacement surgery and 523 claims to a value of £21 million for knee replacement. The most common complaint after hip surgery was related to residual neurological deficit, whereas after knee replacement it was related to infection. Vascular complications resulted in the highest costs per case in each group.Although there has been a large increase in the number of operations performed, there has not been a corresponding relative increase in litigation. The reasons for litigation have remained largely unchanged over time after hip replacement. In the case of knee replacement, although there has been a reduction in claims for infection, there has been an increase in claims for technical errors. There has also been a rise in claims for non-specified dissatisfaction. This information is of value to surgeons and can be used to minimise the potential mismatch between patient expectation, informed consent and outcome.

Bone Anchors or Interference Screws? A Biomechanical Evaluation for Autograft Ankle Stabilization

Background Autograft stabilization uses free semitendinosus tendon grafts to anatomically reconstruct the anterior talofibular ligament. Study aims were to evaluate the biomechanical properties of Mitek GII anchors compared with the Arthrex Bio-Tenodesis Screw for free tendon reconstruction of the anterior talofibular ligament. Null Hypothesis There are no differences in load to failure and percentage specimen elongation at failure between the 2 methods. Study Design Controlled laboratory study using porcine models. Methods Sixty porcine tendon constructs were failure tested. Re-creating the pull of the anterior talofibular ligament, loads were applied at 70° to the bones. Thirty-six tendons were fixed to porcine tali and tested using a single pull to failure; 10 were secured with anchors and No. 2 Ethibond, 10 with anchors and Fiber Wire, 10 with screws and Fiberwire, and 6 with partially gripped screws. Cyclic preloading was conducted on 6 tendons fixed by anchors and on 6 tendons fixed by screws before failure testing. Two groups of 6 components fixed to the fibula were also tested. Results The talus single-pull anchor group produced a mean load of 114 N and elongation of 37% at failure. The talus single-pull screw group produced a mean load of 227 N and elongation of 22% at failure (P < .05). Cyclic preloading at 65% failure load before failure testing produced increases in load and decreases in elongation at failure. Partially gripped screws produced a load of 133 N and elongation of 30% at failure. The fibula model produced significant increases in load to failure for both. The human anterior talofibular ligament has loads of 139 N at failure with instability occurring at 20% elongation. Conclusions Interference screw fixation produced significantly greater failure strength and less elongation at failure than bone anchors. Clinical Relevance The improved biomechanics of interference screws suggests that these may be more suited to in vivo reconstruction of the anterior talofibular ligament than are bone anchors.

Three-dimensional modeling of in vitro hip kinematics under micro-separation regime for ceramic on ceramic total hip prosthesis: An analysis of vibration and noise

Micro-separation corresponds to a medial-lateral hip laxity after total hip replacement (THR). This laxity has been shown to generate higher wear rates and a specific pattern of stripe wear caused by edge loading of the head on the rim of the cup. Recently some authors have implicated edge loading as a source of noise generation and in particular squeaking. The goal of this study was to model hip kinematics under the micro-separation regime in a computational simulation of total hip prosthesis including joint laxity and to analyze the vibration frequencies and the potential for noise generation. A three-dimensional computer model of the Leeds II hip simulator was developed using ADAMS((R)) software, simulating a controlled micro-separation during the swing phase of the walking cycle and replicating the experimental conditions previously reported. There was an excellent correlation between the theoretical values and the experimental values of the medial-lateral separation during the walking cycle. Vibratory frequencies were in the audible zone but were lower in magnitude than those reported clinically in relation to squeaking. Micro-separation and rim loading may explain the generation of some sounds from noisy hips after THR. However, the computational model, and the experimental model of micro-separation were unable to replicate the higher frequency squeaking reported clinically. In contrast, other experimental studies involving normal kinematics in combination with third-body particles have replicated clinically relevant frequencies and noises.

Production tooling for polymer moulding using the RapidSteel process

Presents procedures which were developed for the manufacture of production tooling using the DTM RapidSteel process, and on the methodology adopted in the generation of the procedures. Accuracy and surface finish, the manufacturability of small features, unsupported features, and mechanical strength have all been investigated, as has the capability of the process for generating conformal cooling channels. An overall tool design and manufacture process is presented, and it is concluded that the DTM RapidSteel process is capable of generating conformally cooled production specification tooling, provided that it is accepted that finishing will be required, and that there are limits on small feature manufacture.