The effect of different assembly loads on taper junction fretting wear in total hip replacements

Variability in the magnitude of the impaction force applied by surgeons to assemble a hip prosthetic head to a femoral stem could be a cause of increased wear in taper junctions. This study investigates the effect of varying the magnitude of the assembly force on fretting wear at the taper over a 10 year period using a 3D finite element model and wear algorithm. It is demonstrated that an increase in assembly force results in a reduction in fretting wear and it is recommended that surgeons should apply an impact force of at least 4 kN to minimise wear rates. The wear patterns and wear rates presented are comparable with observation and measurement of those seen in retrieved prostheses

A COMPUTATIONAL APPROACH TO FRETTING WEAR PREDICTION IN TOTAL HIP REPLACEMENTS

A challenge in engineering coupling design is the understanding of performance of contact geometry for a given application. “Wear” is one of a number of mechanical failures that can occur in mechanical coupling design. “Fretting wear” occurs where surfaces in contact are subjected to oscillating load and very small relative motion over a period of time. Fretting has been observed in many mechanical interactions and is known to be a reason for failure in many designs.Recent evidence suggests that fretting wear occurs at the taper junction of modular total hip replacements and leads to failure of the implants. Experimental testing to determine the wear behaviour that occurs in mechanical devices is time consuming, expensive and complicated. Computational wear modelling is an alternative method which is faster and cheaper than real testing and can be used in addition to testing to help improve component design and enhance wear characteristics. Developing an algorithm that can accurately predict fretting wear considering linear wear, volumetric wear and surface wear damage is the main focus of this thesis.The thesis proposes a new computational methodology incorporating published wear laws into commercial finite element code to predict fretting wear which could occur at the taper junction of total hip replacements. The assessment of wear in this study is solely based on mechanical wear (fretting) as being the primary mechanism causing surface damage. The method is novel in that it simulates the weakening of the initial taper ‘fixation’ (created at impaction of the head onto the stem in surgery) due to the wearing process. The taper fixation is modelled using a contact analysis with overlapped meshes at the taper junction. The reduction in fixation is modelled by progressive removal of the overlap between components based on calculated wear depth and material loss.The method has been used for three different studies to determine surface wear damage, linear and volumetric wear rates that could occur at taper junction of total hip replacements over time. The results obtained are consistent with those found from observation and measurement of retrieved prostheses. The fretting wear analysis approach has been shown to model the evolution of wear effectively; however, it has been shown that accurate, quantitative values for wear are critically dependant on mesh refinement, wear fraction and scaling factor, wear coefficient used and knowledge of the device loading history. The numerical method presented could be used to consider the effect of design changes and clinical technique on subsequent fretting wear in modular prosthetic devices or other mechanically coupled designs

A computational approach to fretting wear prediction at the head-stem taper junction of total hip replacements

Wear is one of the main reasons for failure of modular total hip replacements. Recent evidence suggests that fretting wear occurs at the taper junction which provides fixation between the prosthesis femoral head and stem components. The fine metallic wear debris that is released can lead to adverse soft-tissue reactions which can necessitate a revision surgery. The present study proposes a computational methodology utilising an energy wear law and a 3D finite element model to predict fretting wear at the taper junction. The method is novel in that it simulates the weakening of the initial taper ‘fixation’ (created at impaction of the head onto the stem in surgery) due to the wearing process. The taper fixation is modelled using a contact analysis with overlapped meshes at the taper junction. The reduction in fixation is modelled by progressive removal of the overlap between components based on calculated wear. The fretting wear analysis approach has been shown to model the evolution of wear effectively; however, it has been shown that accurate, quantitative values for wear are critically dependant on mesh refinement, wear scaling factor and fraction, wear coefficient used and knowledge of the device loading history. The method has been implemented with a 3D finite element model of the taper junction of a commercial total hip replacement. This has been used to determine taper wear patterns, wear damage and wear rates which have been shown to be consistant with those found from observation and measurement of retrieved prostheses. The numerical method could be used to consider the effect of design changes and clinical technique on subsequent fretting wear in modular prosthetic devices

A large taper mismatch is one of the key factors behind high wear rates and failure at the taper junction of total hip replacements: A finite element wear analysis.

Total hip replacement (THR) is one of the most successful orthopaedic surgeries; however, failures can occur due to adverse reactions to wear debris. Recently, a large number of failures linked to the release of metal particles from the taper junction between femoral head and femoral stem have been reported. One possible reason for this may be design variations such as taper mismatches associated with the taper and trunnion angles. Could a large taper mismatch lead to inappropriate contact mechanics and increase relative micromotion and thus wear? In this study, 3D finite element (FE) models of a commercial THR from a perfectly matched interface to large taper mismatches and a wear algorithm were used to investigate the extent of wear that could occur at this junction and identify the optimum tolerances in order to reduce the wear. A co-ordinate measuring machine (CMM) was used to analyse the wear depth and volumetric wear rate of the tapers of 54 explanted 36mm diameter Cobalt Chromium femoral heads, which had been in service for 5.1 years in average, to validate the FE analyses. It was found that a large taper mismatch (e.g. 9.12´) results in a high wear rate (2.960mm(3) per million load cycles). Such wear rates can have a major negative effect on the clinical outcomes of these implants. It was also found that even a slight reduction in mismatch significantly reduced the magnitude of the wear rates (0.069mm(3) per million load cycles on average for 6´ taper mismatch). It is recommended that the cone angles of femoral head and femoral trunnion should be manufactured to produce a taper mismatch of less than 6´ at the taper junction

Knee wraps are detrimental to the maximal squat performance of powerlifters competing in lower weight classes

The aim of the study was to evaluate the efficacy of knee wraps for competitive powerlifters. To achieve this, an analysis of powerlifting results dating back to 1964 has been conducted. From these results the one repetition max (1RM) squat was evaluated, specifically examining the influence of competitors using knee wraps versus raw (allowing only neoprene knee sleeves). Student’s t-test was used to compare the 1RM squat for male raw competitors (n=270,268) to those using wraps (n=26,576); and likewise for female raw (n=136,530) versus wraps (n=11,468). Overall, the males using wraps yielded significantly higher (p<.05) 1RM squat values (203 kg ± 46.8) than their raw counterparts (195 kg ± 45.7). The females yielded modest, but significantly higher (p<.05) 1RM squat values for raw (112 kg ± 29.3), compared to wraps (111 kg ± 29.8). The results were stratified by weight classes, and it was found that males in heavier classes (105, 120, and 120+ kg) significantly benefited (p<.05) from using knee wraps (+1.3, +4.8, and +6.9 kg respectively). For the lighter weight classes (59, 66, and 74 kg) knee wraps appeared significantly detrimental (p<.05) to the 1RM squat (-8.4, -4.4, and -3.6 kg), respectively. Only the heaviest female weight class (84 kg+) saw a significant benefit (p<.05) in the use of wraps with a net gain in 1RM squat of 4.5 kg. Again, wraps were shown to be significantly detrimental (p<.05) to the lighter weight classes (47, 52, 57, and 63 kg) with a net loss in 1RM squat (-3, -5.3, -3.6, and -3.3 kg), respectively. Considering these findings, it is suggested that only males in the heaviest three weight classes (105, 120, and 120+ kg), as well as females in the heaviest weight class (84+ kg), stand to gain any benefit from the use of knee wraps in competition

The relation between body weight and wear in Total Hip Prosthesis: A finite element study

As the current obesity epidemic grows, an increased number of obese patients undergoing Total Hip Arthroplasty (THA) can be expected in the coming years. The National Health Service of the UK (NHS) recommends that an obese patient should undergo weight loss before THA. It is understood that an increased body weight would increase the wear rates on the prostheses, however, the extent of increased wear and the impact on the longevity of the prosthesis is unclear. The NHS found that 45% of THA failures in 2019 were caused by wear which led to a multitude of failures such as infection, aseptic loosening and dislocation such that a revision surgery is then needed. In this study, a finite element model was created to model a walking cycle and a newly developed wear algorithm was used to perform a series of computational wear analyses to investigate the effect of different patient weights on the evolution of wear in THAs up to 5 million cycles. The wear rates shown in this study are closely comparable to previous literature. The XLPE volumetric wear rates were found to be between 15 – 35mm3/yr (range: 1.5 – 57.6mm3/yr) and femoral head taper surface volumetric wear rates were between 0.174 – 0.225mm3/yr (range: 0.01 – 3.15mm3/yr). The results also showed that an increased weight of 140kg can increase the metallic wear by 26% and polyethylene wear by 30% when compared to 100kg body weight. As increased wear can lead to a multitude of failure such as aseptic loosening, dislocation and metallosis, from this study, it is recommended that obese patients undergo recommended weight loss and maintain this lesser weight to reduce wear and prolong the life of the THA

The impact of femoral head size on the wear evolution at contacting surfaces of total hip prostheses: A finite element analysis.

Total Hip Arthroplasty has been a revolutionary technique in restoring mobility to patients with damaged hip joints. The introduction of modular components of the hip prosthesis allowed for bespoke solutions based on the requirements of the patient. The femoral stem is designed with a conical trunnion to allow for assembly of different femoral head sizes based on surgical requirements. The femoral head diameters for a metal-on-polyethylene hip prosthesis have typically ranged between 22 mm and 36 mm and are typically manufactured using Cobalt-Chromium alloy. A smaller femoral head diameter is associated with lower wear of the polyethylene, however, there is a higher risk of dislocation. In this study, a finite element model of a standard commercial hip arthroplasty prosthesis was modelled with femoral head diameters ranging from 22 mm to 36 mm to investigate the wear evolution and material loss at both contacting surfaces (acetabular cup and femoral stem trunnion). The finite element model, coupled with a validated in-house wear algorithm modelled a human walking for 10 million steps. The results have shown that as the femoral head size increased, the amount of wear on all contacting surfaces increased. As the femoral head diameter increased from 22 mm to 36 mm, the highly cross-linked polyethylene (XLPE) volumetric wear increased by 61% from 98.6 mm3 to 159.5 mm3 while the femoral head taper surface volumetric wear increased by 21% from 4.18 mm3 to 4.95 mm3. This study has provided an insight into the amount of increased wear as the femoral head size increased which can highlight the life span of these prostheses in the human body