Louise Jennings

Wear and biological activity of highly crosslinked polyethylene in the hip under low serum protein concentrations.

Abstract Crosslinked ultra-high molecular weight polyethylene (UHMWPE) has been developed and introduced into clinical practice in order to reduce wear in the hip. Zero wear of highly crosslinked UHMWPE in vitro has been reported by some groups using lubricants with high concentrations of serum proteins in hip simulators. In contrast, some clinical studies have reported finite wear rates. The aim of this study was to compare the wear rates, wear surfaces, and wear debris produced by UHMWPE with different levels of crosslinking in a hip joint simulator, with lower, more physiologically relevant concentrations of protein in the lubricant. The UHMWPEs were tested in the Leeds ProSim hip joint simulator against cobalt-chromium (CoCr) femoral heads. The wear particles were isolated and imaged using a field emission gun scanning electron microscope (FEGSEM) at high resolution. The highly crosslinked UHMWPEs had significantly lower wear volumes than the non-crosslinked UHMWPEs. No significant difference was found in the percentage number and percentage volume of the particles in different size ranges from any of the materials. They had similar values of specific biological activity. The functional biological activity (FBA), which takes into account the wear volume and specific biological activity, showed that the highly crosslinked UHMWPEs had lower FBAs due to their lower wear volume.

Effect of cup inclination angle during microseparation and rim loading on the wear of BIOLOX® delta ceramic‐on‐ceramic total hip replacement

Ceramic-on-ceramic (CoC) bearings in total hip replacements (THRs) have shown low wear volumes under standard gait in hip simulator studies. However, clinical reports have indicated variations in wear rates and formation of stripe-like wear area on the ceramic femoral heads. The aim of this study was to investigate the influence of cup inclination angle and microseparation on the wear of CoC bearings in THRs. The six station Leeds II Physiological Anatomical Joint Simulator was used to investigate the wear of 28 mm diameter alumina matrix composite ceramic bearings (BIOLOX® delta). It was shown that increasing the cup inclination angle from 55° to 65° had no significant effect on the wear rate of BIOLOX® delta CoC under both standard gait and microseparation conditions in this in vitro study. Under standard gait conditions, the mean wear rate for both cup inclination angle conditions was very low at 0.05 mm(3)/million cycles. The introduction of microseparation to the standard gait cycle increased the mean wear rates to 0.13 mm(3)/million cycles for the cup inclination angle of 55° and 0.11 mm(3)/million cycles for that of 65°. The level of increased wear with microseparation was not dependent on cup angle. A stripe of wear on the head also formed, with corresponding superior rim wear on the cup. The wear rates obtained were low compared to the HIPed third generation alumina ceramic (BIOLOX® forte) tested under the same adverse conditions (1.84 mm(3)/million cycles). BIOLOX® delta has shown lower wear than previous ceramic materials used in THR under adverse conditions.

The influence of femoral condylar lift-off on the wear of artificial knee joints.

Abstract In vivo fluoroscopic studies of patients with total knee replacements (TKRs) have shown lift-off of the femoral condyles from the tibial insert. This study investigated the influence of femoral condylar lift-off on the ultra-high molecular weight polyethylene (UHMWPE) wear of fixed bearing (FB) and rotating platform mobile bearing (RP MB) total knee replacements, using a physiological knee joint simulator. In the absence of lift-off, the RP MB knees exhibited a lower wear rate of 5.2;;2.2 mm3 per million cycles (mm3/MC) compared with 8.8;;4.8 mm3/MC for the FB knees. The presence of femoral condylar lift-off was found to accelerate the wear of the FB and RP MB knees tested in this study to 16.4;;2.9 and 16.9;;2.9 mm3/MC respectively. For the RP MB knees the increase in wear rate was more marked, resulting in a similar wear rate for both designs of knee under lift-off conditions. In both cases the medial condyle displayed more wear damage. This study has shown that a small amount of abduction/adduction lift-off and medial-lateral shift increases wear and that the increase in wear is design dependent. In this simulator test, lift-off was simulated on every cycle, whereas the amount of wear and effect of lift-off clinically would depend on the frequency of occurrence of lift-off in vivo.

2009 Knee Society Presidential Guest Lecture: Polyethylene Wear in Total Knees

Knee arthroplasties in young and active patients place a substantial increase in the lifetime tribological demand and potential for wear-induced osteolysis. Polyethylene materials have advanced in recent years, reducing the potential for oxidative degradation and delamination failure. It is timely to consider tribological design variables and their potential to reduce surface wear and the long-term risk of osteolysis. The influence of reduced cross shear in rotating platform mobile-bearing knee designs and reduced surface wear area in low conforming fixed-bearing knees has been investigated. A reduction in cross shear substantially reduced wear in both multidirectional pin-on-plate studies and in rotating platform mobile-bearing designs in knee simulator studies. A reduction in bearing surface contact area substantially reduced surface wear in multidirectional pin-on-plate simulations and in low conforming fixed-bearing knee designs in knee simulator studies. This offers potential for a paradigm shift in knee design predicated by enhanced mechanical properties of new polymer materials. We describe two distinct low-wearing tribological design solutions: (1) a rotating platform design solution with reduced cross shear provides reduced wear with conformity and intrinsic stability; and (2) a low conformity fixed bearing with reduced surface area, provides reduced wear, but has less intrinsic stability and requires good soft tissue function.

Effect of conformity and contact stress on wear in fixed-bearing total knee prostheses

Ultra high molecular weight polyethylene (PE) remains the primary bearing surface of choice in total knee replacements (TKR). Wear is controlled by levels of cross-shear motion and contact stress. The aim of this study was to compare the wear of fixed-bearing total knee replacements with curved and flat inserts and to test the hypothesis that the flat inserts which give higher contact stresses and smaller contact areas would lead to lower levels of surface wear. A low-conforming, high contact stress knee with a low-medium level of cross shear resulted in significantly lower wear rates in comparison to a standard cruciate sacrificing fixed-bearing knee. The low wear solution found in the knee simulator was supported by fundamental studies of wear as a function of pressure and cross shear in the pin on plate system. Current designs of fixed-bearing knees do not offer this low wear solution due to their medium cross shear, moderate conformity and medium contact stress.

Computational wear prediction of artificial knee joints based on a new wear law and formulation.

Laboratory joint wear simulator testing has become the standard means for preclinical evaluation of wear resistance of artificial knee joints. Recent simulator designs have been advanced and become successful at reproducing the wear patterns observed in clinical retrievals. However, a single simulator test can be very expensive and take a long time to run. On the other hand computational wear modelling is an alternative attractive solution to these limitations. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. However, all these models have adopted the classical Archards wear law, which was developed for metallic materials, and have selected wear factors arbitrarily. It is known that such an approach is not generally true for polymeric bearing materials and is difficult to implement due to the high dependence of the wear factor on the contact pressure. Therefore, these studies are generally not independent and lack general predictability. The objective of the present study was to develop a new computational wear model for the knee implants, based on the contact area and an independent experimentally determined non-dimensional wear coefficient. The effects of cross-shear and creep on wear predictions were also considered. The predicted wear volume was compared with the laboratory simulation measurements. The model was run under two different kinematic inputs and two different insert designs with curved and custom designed flat bearing surfaces. The new wear model was shown to be capable of predicting the difference of the wear volume and wear pattern between the two kinematic inputs and the two tibial insert designs. Conversely, the wear factor based approach did not predict such differences. The good agreement found between the computational and experimental results, on both the wear scar areas and volumetric wear rates, suggests that the computational wear modelling based on the new wear law and the experimentally calculated non-dimensional wear coefficient should be more reliable and therefore provide a more robust virtual modelling platform.

Wear of 36-mm BIOLOX® delta ceramic-on-ceramic bearing in total hip replacements under edge loading conditions

Ceramic-on-ceramic bearings have become of great interest due to the substantial improvements in the manufacturing techniques and material properties and due to polyethylene wear debris–induced osteolysis and the issues with metal wear debris and ion release by metal-on-metal bearings. Edge loading conditions due to translational malpositioning (microseparation conditions) have been shown to replicate clinically relevant wear mechanisms and increase the wear of ceramic-on-ceramic bearings; thus, it was necessary to test new bearing materials and designs under these adverse conditions. The aim of this study was to assess the effect of increasing head size on the wear of BIOLOX® delta ceramic-on-ceramic bearings under edge loading conditions due to rotational (steep cup inclination angle) and translational (microseparation) malpositioning. In this study, six 36-mm ceramic-on-ceramic bearings (BIOLOX delta, CeramTec, Germany) were tested under standard and edge loading conditions using the Leeds II hip simulator and compared to the 28-mm bearings tested and published previously under identical conditions. The mean wear rate under standard gait conditions was below 0.1 mm3/million cycles for both the 28-mm and the 36-mm ceramic-on-ceramic bearings, and increasing the inclination angle did not affect the wear rates. The introduction of microseparation to the gait cycle increased the wear rate of ceramic-on-ceramic bearing and resulted in stripe wear on the femoral heads. Under microseparation conditions, the wear rate of size 36-mm bearings (0.22 mm3/million cycles) was significantly higher (p = 0.004) than that for size 28-mm bearings (0.13 mm3/million cycles). This was due to the larger contact area for the larger bearings and deprived lubrication under edge loading conditions. The wear rate of BIOLOX delta ceramic-on-ceramic bearings under microseparation conditions was still very low (<0.25 mm3/million cycles) compared to earlier generation ceramic-on-ceramic bearings (BIOLOX forte, 1.84 mm3/million cycles) and other bearing materials such as metal-on-metal bearings (2–8 mm3/million cycles).

Wear and Creep of Highly Crosslinked Polyethylene against Cobalt Chrome and Ceramic Femoral Heads

The wear and creep characteristics of highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against large-diameter (36 mm) ceramic and cobalt chrome femoral heads have been investigated in a physiological anatomical hip joint simulator for 10 million cycles. The crosslinked UHMWPE/ceramic combination showed higher volume deformation due to creep plus wear during the first 2 million cycles, and a steady-state wear rate 40 per cent lower than that of the crosslinked UHMWPE/cobalt chrome combination. Wear particles were isolated and characterized from the hip simulator lubricants. The wear particles were similar in size and morphology for both head materials. The particle isolation methodology used could not detect a statistically significant difference between the particles produced by the cobalt chrome and alumina ceramic femoral heads.

Wear of novel ceramic‐on‐ceramic bearings under adverse and clinically relevant hip simulator conditions

Further development of ceramic materials for total hip replacement aim to increase fracture toughness and further reduce the incidence of bearing fracture. Edge loading due to translational mal positioning (microseparation) has replicated stripe wear, wear rates, and bimodal wear debris observed on retrievals. This method has replicated the fracture of early zirconia ceramic-on-ceramic bearings. This has shown the necessity of introducing microseparation conditions to the gait cycle when assessing the tribological performance of new hip replacement bearings. Two novel ceramic matrix composite materials, zirconia-toughened alumina (ZTA) and alumina-toughened zirconia (ATZ), were developed by Mathys Orthopädie GmbH. In this study, ATZ-on-ATZ and ZTA-on-ZTA bearing combinations were tested and compared with alumina-on-alumina (Al2O3-on-Al2O3) bearings under adverse microseparation and edge loading conditions using the Leeds II physiological anatomical hip joint simulator. The wear rate (±95% confidence limit) of ZTA-on-ZTA was 0.14 ± 0.10 mm(3)/million cycles and that of ATZ-on-ATZ was 0.06 ± 0.004 mm(3)/million cycles compared with a wear rate of 0.74 ± 1.73 mm(3)/million cycles for Al2O3-on-Al2O3 bearings. Stripe wear was evident on all bearing combinations; however, the stripe formed on the ATZ and ZTA femoral heads was thinner and shallower that that formed on the Al2O3 heads. Posttest phase composition measurements for both ATZ and ZTA materials showed no significant change in the monoclinic zirconia content. ATZ-on-ATZ and ZTA-on-ZTA showed superior wear resistance properties when compared with Al2O3-on-Al2O3 under adverse edge loading conditions.

Effect of femoral head size on the wear of metal on metal bearings in total hip replacements under adverse edge‐loading conditions

Abstract Metal-on-metal (MoM) bearings have shown low-wear rates under standard hip simulator conditions; however, retrieval studies have shown large variations in wear rates and mechanisms. High-wear in vivo has caused catastrophic complications and has been associated with steep cup-inclination angle (rotational malpositioning). However, increasing the cup-inclination angle in vitro has not replicated the increases in wear to the same extent as those observed in retrievals. Clinically relevant wear rates, patterns, and particles were observed in vitro for ceramic-on-ceramic bearings when microseparation (translational malpositioning) conditions were introduced into the gait cycle. In the present study, 28 and 36-mm MoM bearings were investigated under adverse conditions. Increasing the cup angle from 45° to 65° resulted in a significant increase in the wear rate of the 28 mm bearings. However, for the 36 mm bearings, head-rim contact did not occur under the steep cup-angle condition, and the wear rate did not increase. The introduction of microseparation to the gait cycle significantly increased the wear rate of the MoM bearings. Cup angle and head size did not influence the wear rate under microseparation conditions. This study indicated that high-in vivo wear rates were associated with edge loading due to rotational malpositioning such as high-cup-inclination angle and translational malpositioning that could occur due to several surgical factors. Translational malpositioning had a more dominant effect on the wear rate. Preclinical simulation testing should be undertaken with translational and rotational malpositioning conditions as well as standard walking cycle conditions defined by the ISO standard.