Alistair Elfick

The effect of 'running-in' on the tribology and surface morphology of metal-on-metal Birmingham hip resurfacing device in simulator studies.

Abstract It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 106 cycles for the first 106 cycles of testing to 0.24 mm3 per 106 cycles over the final 2 × 106 cycles of testing. The friction tests suggested boundary lubrication initially, but at 1 × 106 cycles a mixed lubrication regime was evident. By 2 × 106 cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3 × 106 and 5 × 106 cycles. The surface study complements these findings.

Wear in retrieved acetabular components: Effect of femoral head radius and patient parameters

Forty-seven explanted Porous Coated Anatomic (PCA, Howmedica, Rutherford, NJ) cementless acetabular components were acquired at revision surgery. All the components articulated against CoCrMo femoral heads of 32-mm diameter. The penetration depth and angle were measured using the shadowgraph technique. The wear volume was then calculated using Kabos formula. Using weighted linear regression analysis, the mean penetration rate and mean volumetric wear rate were calculated to be 0.23 (SE, 0.03) mm3/y and 96 (SE, 13) mm3/y, respectively. The creep component was not found to be significantly different from zero. The clinical wear factor, k(clinical), for this cohort was also calculated using linear regression analysis but with the assumption that creep was zero. The value found, k(clinical) = 1.93 (SE, 0.29) x 10(-6) mm3/N-m, was similar to those in previous studies involving cemented joints with a 22-mm femoral head diameter. The similar k(clinical) values of these substantially different joint types suggest that the high volumetric wear rate for the PCA joint can be attributed entirely to its larger head size and the younger, more active, patient profile. Fixation technique and metal backing seem not to influence the rate of wear.

Poly(e-caprolactone) as a potential material for a temporary joint spacer

Sepsis of a total joint replacement may not respond to treatment with systemic antibiotics. In these circumstances the treatment is often a two-stage revision of the prosthesis; the infected prosthesis is removed, a period of treatment with an antibiotic-loaded joint spacer ensues, finally a new total joint is implanted once the infection has subsided. A PMMA temporary joint spacer offering a degree of functionality is gaining popularity. Patient activity will cause this temporary implant to wear, releasing potentially damaging PMMA particles into the joint environment. An alternative biodegradable polymer is proposed for use as a temporary joint spacer. This study details initial investigations into the wear behaviour of one such polymer, poly(epsilon-caprolactone). A multi-directional pin-on-plate wear tester was used to assess the performance of poly(epsilon-caprolactone) against a stainless-steel counterface. Two lubricating conditions were studied: distilled water and bovine serum. In water the wear rate of the poly(epsilon-caprolactone), 0.35 x 10(-6) mm3/N m, was comparable to that of polyethylene. However, in bovine serum the wear rate was greater, 18.09 x 10(-6) mm3/Nm. This result compares favourably with PTFE wear rates, suggesting that the wear of poly(epsilon-caprolactone) in this specialised application may be acceptable.

The influence of femoral head surface roughness on the wear of ultrahigh molecular weight polyethylene sockets in cementless total hip replacement

A theoretical relationship was recently proposed relating the wear behavior of polymetric bearing materials articulating against hard counterfaces.(1) This model attempts to predict the influence of surface roughness on wear. Laboratory-based studies have been used to establish the validity of these relationships, but their application to the clinical situation has not been investigated fully. Forty-two retrieved PCA hip joints have been assessed. The total wear volume was calculated from the penetration measured using the shadowgraph method, and roughness of the articulating surfaces was recorded using noncontacting profilometry. The roughness of the explanted femoral heads was observed to increase (median S(a) - 10. 35 nm worn region, 3.05 nm peripheral region), while that of the acetabular liner fell dramatically (median S(a) - 41 nm worn region, 212 nm unworn region). No evidence of a relationship between the topography of the worn regions of the femoral head and that of the acetabular liner could be found. Similarly, the strength of the association between the surface roughness and the clinical wear factor was considerably poorer than that achieved in laboratory experiments. A number of reasons for this observation are proposed. Most deleterious was considered to be the inability of the roughness parameters to describe the damaging features of the surface adequately. Uncertainty as to when the surface of the component degrades during its life serves to introduce further doubt as to the application of the wear models in the clinical environment. In conclusion, this study fails to provide clinical evidence to substantiate the relationship between surface finish and wear rate. The adoption of standardized measurement parameters and techniques would facilitate the direct comparison of joint types and the selection of the most advantageous materials.

The quantitative assessment of UHMWPE wear debris produced in hip simulator testing: the influence of head material and roughness, motion and loading

Abstract Biomaterial wear particles are known to provoke a foreign-body reaction when released from total joint prostheses. Considerable effort is being invested in the search for materials which, by wearing less, will release fewer particles. In this research the role of joint wear simulators is paramount. The wear debris from 26 simulator tests of hip prostheses was extracted from the bovine serum lubricant and sized using a laser diffraction technique. The influence on particle size of a broad range of parameters was examined. The parameters considered included the bedding-in phenomenon, the femoral head material and roughness and physiological versus simplified load cycles and wear paths. The physiological wear simulators produced a similar size distribution of wear particles to that produced by implanted joints. Head material had no effect on this observation. Simplifying the loading cycle or wear path independently made no impact on this finding. This remained the case when simplified load and wear path were combined in one test. The effect of head roughness was pronounced with an increase in minimum particle size with increasing roughness. Joint simulators remain the optimal method for assessing new joint materials and designs. However, their use to characterise joints in terms of wear rate solely should be guarded against. Instead focus should be concentrated on a combination of the size and amount of the wear debris created. In this way the “loosening hazard” of a joint can be distilled.

A novel technique for the detailed size characterization of wear debris

The accurate and detailed characterization of artificial joint wear debris is important in determining both the wear rate of prostheses and understanding the role that the debris plays in the development and progression of aseptic loosening. The novel application of low angle laser light scattering (LALLS) to the particle size characterization of ultra high molecular weight polyethylene (UHMWPE) wear debris is described. The results demonstrate that both ex vivo and in vitro origin wear debris samples, at concentrations typical of those produced via an alkali-digestion retrieval route, can be reproducibly analyzed via LALLS. Because the LALLS route enables particle size analysis of the entire debris sample to be acquired non-destructively and whilst in suspension, artefacts associated with filtering, drying and agglomeration of debris are avoided, in contrast to currently used techniques such as filtration and scanning electron microscopy (SEM) observation. ©2000 Kluwer Academic Publishers

An evaluation of the tribological performance of zirconia and CoCrMo femoral heads

Five new zirconia, five new CoCrMo and three explanted CoCrMo femoral heads were wear-tested in bovine serum for five million cycles using the Durham Hip Joint Wear Simulator. Wear was measured gravimetrically and surface topography with a 3D non-contacting profilometer. This allowed an evaluation of the different head types on UHMWPE acetabular cup wear rates and the effect of roughening of the femoral head on acetabular cup wear. The mean acetabular cup wear rate against the five CoCrMo femoral heads was 40.8 mm3/106 cycles which was significantly higher (p = 0.03) than against zirconia (33.3 mm3/106 cycles). The initial surface roughness of the CoCrMo femoral heads (Ra = 4.6 nm) was statistically significantly higher than for the zirconia heads (Ra = 3.1 nm). Over the wear test the CoCrMo heads got statistically significantly rougher (Ra = 10.5 nm) whilst the zirconia heads showed no statistically signficant change. The three explanted CoCrMo femoral heads had initial mean surface roughness, Ra, values of 19, 24 and 55 nm with corresponding cup wear rates of 97.6, 131.2 and 148.4 mm3/106 cycles respectively. The very high wear rates against the explanted heads highlight the need for scratch resistant femoral head surfaces.

Surface Topography of Retrieved PCA Acetabular Liners: Proposal of a Novel Wear Mechanism

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Tribology of Replacement Hip Joints

The clinical wear rates of current designs of artificial hip joints which pair either stainless steel or CoCrMo alloy femoral heads with ultra high molecular weight polyethylene (UHMWPE) sockets are found to vary from 50-140xa0mm 3 /year depending on the radius of the head of the hip joint. A 28xa0mm diameter CoCrMo femoral head will produce about 62-75xa0mm 3 /year of UHMWPE wear particles. The rate of wear is important because the failure of artificial joints by loosening in the bone, has been shown to occur when the total volume of wear debris reaches about 600xa0mm 3 . Thus the lower the wear rate, the longer a joint will last. These clinical wear rates compare with modern joint simulator results of 60xa0mm 3 /10 6 xa0cycles (over 0 to 2 million cycles) to 48.2xa0mm 3 /10 6 xa0cycles (over 0-5 million cycles). Ceramic heads (zirconia) have a very smooth finish and had UHMWPE wear rates of only 80% of those of CoCrMo alloy which were rougher. Studies of friction in these artificial joints have led to the conclusion that all of these designs operate under mixed lubrication conditions. Thus they wear. In an attempt to reduce this wear, some designs have moved towards generating a full fluid film lubrication mechanism.

A re-appraisal of wear features of acetabular sockets using atomic force microscopy

Abstract The morphological and material features of 10 worn ultra-high molecular weight polyethylene (UHMWPE) acetabular sockets were assessed using atomic force microscopy (AFM) imaging of topography, stiffness and adhesion. Six of the sockets were simulator worn, four with a physiological gait cycle and two under linear motion. The remaining four sockets were explanted cemented joints. Polishing of the UHMWPE was shown in the pole of all cups. The presence of rippling or fatigue cracking within the surface of the UHMWPE at the pole was found to require motion approaching unidirectionality. Fibril formation within the polished region was highly associated with explanted cups and scratching was seen at the pole was found in simulator tested cups. The periphery of the contact region showed polishing of the UHMWPE in the simulator testing but the explanted cups showed highly fibrillar wear areas. Third bodies were positively identified in the periphery of one cup. Evidence in support of current wear models was not forthcoming. An alternative explanation of the formation of fibrils within the polished regions has been proposed; partial particle release is followed by the teasing of a fibril through a rolling action between the two articular surfaces.