Characterisation of wear debris from UHMWPE on zirconia ceramic, metal-on-metal and alumina ceramic-on-ceramic hip prostheses generated in a physiological anatomical hip joint simulator

There is currently much interest in the characterisation of wear debris from different types of artificial hip joints. There have been numerous studies on the wear of UHMWPE in hip joint simulators, but relatively few studies on the wear of alternative materials such as metal-on-metal (MOM) and ceramic-on-ceramic (COC). The aim of this study was to compare the wear volumes and wear debris generated from zirconia ceramic-on-UHMWPE, MOM and COC hip joints under identical conditions in the same hip joint simulator.

All prostheses showed an initial higher ‘bedding in’ wear rate, which was followed by a lower steady state wear rate. The zirconia ceramic-on UHMWPE prostheses showed the highest wear rates (31±4.0 mm³/million cycles), followed by the MOM (1.23±0.5 mm/million cycles), with the COC prostheses showing significantly (P<0.01) lower wear rates at 0.05±0.02 mm³/million cycles. The mode (±95% confidence limits) of the size distribution of the UHMWPE wear debris was 300±200, 30±2.25 nm for the metal particles, and 9±0.5 nm for the ceramic wear particles. The UHMWPE particles were significantly larger (P<0.05) than the metal and ceramic wear particles, and the metal particles were significantly larger (P<0.05) than the ceramic wear particles. A variety of morphologies and sizes were observed for the UHMWPE wear particles, including submicrometer granules and large flakes in excess of 50 μm. However, the wear particles generated in both the MOM and COC articulations were very uniform in size and oval or round in shape.

This investigation has demonstrated substantial differences in volumetric wear. The in vitro wear rates for the zirconia-on-UHMWPE and MOM are comparable with clinical studies and the UHMWPE and metal wear particles were similar to the wear debris isolated from retrieved tissues. However, the alumina/alumina wear rate was lower than some clinical retrieval studies, and the severe wear patterns and micrometer-sized particles described in vivo were not reproduced here.

This study revealed significant differences in the wear volumes and particle sizes from the three different prostheses. In addition, this study has shown that the alternative bearing materials such as MOM and COC may offer a considerable advantage over the more traditional articulations which utilise UHMWPE as a bearing material, both in terms of wear volume and osteolytic potential.     

Three-body wear of UHMWPE acetabular cups by PMMA particles against CoCr, alumina and zirconia heads in a hip joint simulator

Three-body abrasive wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by loose polymethyl methacrylate (PMMA) bone cement particles is an important mechanism responsible for elevated wear debris generation in total hip arthroplasty. The resistance of the femoral head material to third-body damage has been considered critical for the wear performance of the polyethylene component. This study examines the effect of loose bone cement particles on the wear rate of UHMWPE acetabular cups against both metal and ceramic counterfaces in a hip joint simulator. Against the CoCr head, the UHMWPE cup showed a strong dependence of wear rate on the concentration of the PMMA particles in the lubricant. At a concentration less than 5 g/l, the presence of the PMMA particles had no detrimental effect on the wear rate; higher concentrations of the PMMA particles greater than 5 g/l led to an accelerated wear of the acetabular cups. Mild scratching damage was observed on the CoCr heads after testing with all PMMA-containing lubricants. However, no increased UHMWPE wear rate was found against these damaged femoral heads in a fresh lubricant without PMMA particles, indicating that femoral head scratching was not a major cause for the elevated wear observed under the three-body abrasive conditions. Against both alumina and zirconia ceramic heads, the wear rate of the UHMWPE was independent of the concentration of the PMMA particles. It was observed that a significant portion of the CoCr heads was covered with loose patches of PMMA particles. The higher the concentration of the PMMA particles, the greater the area of the head covered with PMMA particles. The attachment of PMMA particles to the ceramic heads was much reduced compared to the CoCr heads. It is therefore concluded that ceramic femoral heads are effective against potential run-away wear of the UHMWPE acetabular cups when an excessive amount of loose PMMA particles are present in the lubricant.     

Pemberton手术联合股骨近端截骨治疗先天性髋关节脱位

目的探讨儿童先天性髋关节脱位用Pemberton手术联合股骨近端截骨治疗的临床效果。方法采用Mckav的临床评价标准和WilletX线测量方法,对笔者1997年12月-2008年5月所做的190例（212个髋）Pemberton手术进行回顾性研究。手术年龄1．5-8岁（平均4．2岁）,男29例,女161例,男女比例1：5．55,左侧141例,右侧27例,双侧22例。结果其中181例获得随诊,随诊时间1～8年,平均29．6月,优183例,良33例,差8例,优良率96．4％,髋臼指数由术前30&#176;-45&#176;（平均36&#176;）减少到术后4&#176;-15&#176;,平均10&#176;。结论Pemberton手术联合股骨近端截骨疗效确定,是治疗儿童先天性髋关节脱位的理想术式之一。     

Transmission electron microscopy analysis of worn alumina hip replacement prostheses

Explanted worn alumina orthopaedic hip replacements show characteristic wear regions, ranging from severe wear, dominated by intergranular fracture, to regions in which minimal damage has occurred during articulation. The surface damage accumulation mechanisms are complex and not fully understood. This paper presents a detailed transmission electron microscopy (TEM) study of the surface damage accumulation mechanisms following in vitro tested worn alumina hip replacement prostheses. TEM of focused ion beam cross-section samples indicated extensive surface dislocation activity, which is restricted in the outer grain layer. Except for one example of basal slip, all slips were found to be on pyramidal planes. Both inter- and transgranular cracks were observed in regions of high wear. Grooves, largely associated with third-body abrasion, were generally associated with extensive dislocation activity. Three types of wear debris were seen from the worn surface, namely: granular wear debris, nanocrystalline wear debris and oblong wear debris. Wear debris were shown to arise from grain pull-out and severe plastic deformation at the surface. The observations allow a mechanistic model of the damage accumulations leading to wear and ultimately failure.     

Performance Characterization of Modified ASTM F75 Alloy for Hip Implant Application

ASTM F75 is a low-carbon CoCrMo alloy which has been used as hip implant material for decades,but the ASTM F75 implants can fail when the femoral head and the a...     

The acting wear mechanisms on metal-on-metal hip joint bearings: in vitro results

Metal-on-metal (MOM) hip joint bearings are currently under discussion as alternatives to metal-on-polymer (MOP) bearings. Some criteria under scrutiny are the wear resistance, the influence of wear particles on the surrounding tissue, as well as the frictional torque. In order to understand and control the wear behavior of such a bearing a close correlation between the microstructures of the alloys used and the acting wear mechanisms has to be found. Thus, commercially available CoCrMo-balls were tested against self mating concave pins in a physiological fluid at 37°C under reciprocating sliding wear (1 Hz). The compressive load was 750 N (body weight). For 2×10⁶ cycles tests were carried out continuously and with periodically occurring resting periods. On the basis of the observed wear appearances the acting wear mechanisms are defined and evaluated as to their contribution to the wear behavior. Due to the high local contact stresses surface fatigue prevails initially. Cr– and Mo–carbides are fractured and torn off the surfaces bringing about additional surface fatigue by indentations and initiating abrasion. The weight loss can be predominately attributed to these mechanically dominated wear mechanisms. In a parallel occurring tribochemical reaction layers are generated from denatured proteins. These adhere rigidly to the surfaces and cover parts of the contacting surfaces avoiding adhesion. Thus, the wear behavior is mainly influenced by the alternating balance between surface fatigue and abrasion on the one side and by tribochemical reactions on the other side.     

Comparison of Diamond‐Like‐Carbon and Alumina‐Oxide articulating with Polyethylene in Total Hip Arthroplasty

To reduce wear in joint bearings of total hip arthroplasty (THA) is the most important issue for improving long term results and implant survival. Due to low wear rates and excellent tribological features in simulator tests Diamond‐Like‐Carbon coating (DLC) of femoral balls is still discussed as an alternative articulation in THA. This clinical prospective study compares survivorship of DLC‐coated femoral heads and of Aluminia‐Oxide‐ (Al₂O₃) heads articulating with Polyethylene (PE). Over a period of two years 101 THA with DLC‐coated heads and PE cups (DLC‐group) and another 101 THA consisting of Al₂O₃ heads (Al₂O₃‐group) and PE cups as well were implanted. Both articulations were based on the same type of cementless hip joint prosthesis. All hips were implanted by one surgeon in consecutive series consisting of 51 Al₂O₃ and 101 DLC‐articulations and further 50 Al₂O₃. All perioperative and follow‐up data was processed with SPSS®. Survival of THA in both groups was evaluated according to Kaplan‐Meier survivorship analysis with an intervall of 90 months (range:78‐101). Qualitative surface analysis was performed in nineteen retrieved DLC‐heads which were revised for aseptic loosening using field scanning electron microscopy (FE‐SEM, XL 30 SFEG Philips, Eindoven NL). 178 patients (88.2 %) were evaluated for follow‐up. Fourteen patients died meantimes (nine DLC, five Al₂O₃) with the implant components not revised. Ten patients (five DLC, five Al₂O₃) were lost to follow‐up. Both groups were comparable regarding patient age, weight and indications for THA with a normal distribution. Survivorship analysis for aseptic loosening 8.5 years following implantation resulted in a significant difference between both groups with a 54 % survival for DLC/PE compared to 88 % for Al₂O₃/PE bearings (p <0.001). No correlation to variables as age, gender or bodyweight could be detected. Surfaces of nineteen retrieved DLC‐heads showed numerous smallest pits of the diamond‐carbon layers in different quantity. SEM showed delamination of the carbon layer which caused excessive debris of polyethylene and in some cases even of the metallic substrate of the heads. Despite modern manufacturing technology and excellent experimental results for its tribochemical characteristics and wear, even “new” DLC‐coating of femoral heads is to be considered critically due to very high rates of clinical failure.     

Modified cemented stem fixation in hip arthroplasty with computer based analysis – Experimental results

27 cemented stems were implanted in fresh‐frozen human cadaver femora with third generation antegrade cementing technique and with prechilled vacuum‐mixed and pre‐pressurized Palacos R bone cement. 3 groups with different implantation techniques were compared according to the achieved cement mantle thickness distribution. Group 1 (modified stem fixation): 7 canulated stems inserted over guidetubes that were fixed in distal synthetic plugs (3 Willert CF‐30 and 4 flanged Charnley stems). The guidetubes served simultaneously for stem centralization and evacuation of the intramedullary cavity. Group 2: 12 stems with distal centralizers with fins (4 flanged Charnley, 8 anatomic cemented AC), and Group 3: 8 stems without centralizer (4 Willert CF‐30 and 4 flanged Charnley). For analysis, each femur was cut into mean 25 cross‐sections (range, 22 ‐ 31). Contact radiographs were taken and digitized with a sampling distance of 0.12mm (204.8 dpi). Using a custom‐made computer program (FemStat 1.001), the cement mantle thickness was determined at mean 650 implant surface points per cross section. Results: The percentage of critical cement mantle thickness was lower for the canulated stems with guidetube. Here 14.1 % of the measurements were smaller than 2mm. For the stems with distal centralizer and without centralizer this was 28.3 % and 28.2 % (p = 0.003 and p = 0.002, U‐test). Cement mantles smaller than 1mm were also reduced for group 1 (2.8 % compared with 4.7 % and 7.5 %) but this was not significant (p = 0.384 and p = 0.094). The stems without centralizers had only slightly inferior cement mantles if compared to stems with distal centralizers with fins (for < 2mm p = 0.571, for < 1mm p = 0.305). Comparing different cementing techniques for one constant type of prosthesis (Charnley) the canulated stems with guide tubes provided a significantly better cement mantle thickness than the stems with distal centralizers (for < 2mm 10.3 % compared with 26.6 %, p = 0.029; for < 1mm 1.5 % compared with 3.4 %, p = 0.343). Distal centralizers failed, as the intramedullary cavity is elliptic in cross section and the gaps between the wings caused malposition. For all stems in group 3 improvement of the cement mantle thickness was predominantly achieved in the distal and middle third of the femur. Conclusion: The implantation technique with canulated stems inserted over a guidetube allows superior stem positioning leading to a more favourable cement mantle.     

Shape optimization of uncemented hip prostheses

A computational model to obtain optimized geometries for the femoral component of hip prosthesis is presented. Using structural optimization techniques, the objective is to determine the shape of uncemented stems that maximize initial stability and improve performance. To accomplish this, the optimization problem is formulated by the minimization of the contact stresses and relative displacement on bone-stem interface. Design variables are geometric parameters that characterize selected cross sections. These parameters are subject to a set of linear geometric constraints in order to obtain clinically admissible geometries. Furthermore, a multiple load formulation is used to incorporate different daily life activities. Optimization results are useful to design new stems or, if integrated in an appropriate computer-aided design (CAD) system, to design custom-made hip prostheses. In the later case, the model is able to include personalized information such as patient's femur geometry and therefore personalized geometric constraints and optimization parameters.     

Comparative study on the wear behaviour of different conventional and cross-linked polyethylenes for total hip replacement

In this study, five different types of conventional and cross-linked polyethylene (XLPE) (γ-sterilised PE GUR1020, EtO-sterilised PE GUR1020, γ-sterilised PE GUR1050, EtO-sterilised XLPE GUR1020, EtO-sterilised XLPE GUR1050) acetabular cups were tested on a hip joint simulator run for 5 million cycles in order to compare the relative long-term wear resistance in relation to material properties (PE grade, conventional or cross-linked) and sterilisation method (EtO treatment or γ-irradiation).Gravimetric measurements revealed significant differences between the wear behaviours of the five sets of acetabular cups. Weight loss was found to decrease along the series: γ-sterilised PE GUR1020>EtO-sterilised PE GUR1020>γ-sterilised PE GUR1050>EtO-sterilised XLPE GUR1050>EtO-sterilised XLPE GUR1020. The wear results were discussed in relation to the crystallinity degree of the cups which was determined by micro-Raman spectroscopy coupled to partial least-squares analysis. Within both conventional and cross-linked PE series, it appeared that higher crystallinity samples (i.e. γ-sterilised PE GUR1020 and EtO-sterilised XLPE GUR1050, respectively) were characterised by higher wear rates. The higher weight loss observed for PE GUR1020 was explained in relation to its lower molecular weight with respect to PE GUR1050. Raman analysis showed that wear testing did not significantly modify the crystallinity degree of any of the tested acetabular cups. The most worn cup, i.e. γ-sterilised PE GUR1020, appeared the most homogeneously polished upon wear testing, as confirmed by the lowest standard deviation associated to the crystallinity value recorded in the centre of the cup. The results of this investigation have clearly shown a dramatic wear reduction in favour of the cross-linked polyethylene.