Ian C. Clarke

Mechanism and clinical significance of wear debris-induced osteolysis.

Loosening of joint replacement components is often multifactorial. The quality of initial fixation is very important to the outcome of the arthroplasty and is often a factor in short-term and long-term failure. This paper discusses another important factor of implant loosening, namely wear debris induced osteolysis. Macrophages activated by the phagocytosis of particulate wear debris are the key cells in this process, which can potentially occur in any implant system regardless of implant design or fixation mode. This is because each implant system creates wear debris from the articulating surfaces and the interfaces. The clinical consequences of wear debris cover a broad spectrum from radiolucencies to massive osteolysis and implant failure. For this reason, the reduction of wear debris should be a primary goal of orthopedic research in the future.

Ultra-low wear rates for rigid-on-rigid bearings in total hip replacements.

Abstract With the increased clinical interest in metal-on-metal and ceramic-on-ceramic total-hip replacements (THRs), the objective of this hip simulator study was to identify the relative wear ranking of three bearing systems, namely CoCr-polyethylene (M-PE), CoCr-CoCr (M-M) and ceramic-on-ceramic (C-C). Volumetric wear rates were used as the method of comparison. The seven THR groupings included one M-PE study, two M-M studies and four C-C studies. Special emphasis was given to defining the ‘run-in’ phase of accelerated wear that rigid-on-rigid bearings generally exhibit. The hypothesis was that characterization of the run-in and steady state wear phases would clarify not only the tribological performance in vitro but also help correlate these in vitro wear rates with the ‘average’ wear rates measured on retrieved implants. The implant systems were studied on multichannel hip simulators using the Paul gait cycle and bovine serum as the lubricant. With 28 mm CoCr heads, the PE (2.5 Mrad/N2) wear rates averaged 13 mm3/106 cycles duration. This was considered a low value compared with the clinical model of 74 mm3/year (for 28 mm heads). Our later studies established that this low laboratory value was a consequence of the serum parameters then in use. The mating CoCr heads (with PE cups) wore at the steady state rate of 0.028 mm3/106 cycles. The concurrently run Metasul™ M-M THRs wore at the steady state rate of 0.119 mm3/106 cycles with high-protein serum. In the second Metasul M-M study with low-protein serum, the THR run-in rate was 2.681 mm3/106 cycles and steady state was 0.977 mm3/106 cycles. At 10 years, these data would predict a 70-fold reduction in M-M wear debris compared with the clinical PE wear model. All M-M implants exhibited biphasic wear trends, with the transition point at 0.5 × 106 cycles between run-in and steady state phases, the latter averaging a 3-fold decrease in wear rate. White surface coatings on implants (coming from the serum solution) were a confounding factor but did not obscure the two orders of magnitude wear performance improvement for CoCr over PE cups. The liners in the alumina head-alumina cup combination wore at the steady state rate of 0.004 mm3/106 cycles over 14 × 106 cycles duration (high-protein serum). The zirconia head-alumina cup THR combination wore at 0.174 and 0.014 mm3/106 cycles for run-in and steady state rates respectively (low-protein serum). The zirconia head and cup THR combination wore slightly higher initially with 0.342 and 0.013 mm3/106 cycles for run-in and steady state rates respectively. Other wear studies have generally predicted catastrophic wear for such zirconia-ceramic combinations. It was noted that the zirconia wear trends were frequently masked by the effects of tenacious white surface coatings. It was possible that these coatings protected the zirconia surfaces somewhat in this simulator study. The experimental ceramic Crystaloy THR had the highest ceramic run-in wear at 0.681 mm3/106 cycles and typical 0.016 mm3/106 cycles for steady state. Since these implants represented the first Crystaloy THR sets made, it was likely that the surface conditions of this high-strength ceramic could be improved in the future. Overall, the ceramic THRs demonstrated three orders of magnitude wear performance improvement over PE cups. With zirconia implants, while the cup wear was sometimes measurable, head wear was seldom discernible, Therefore, we have to be cautious in interpreting such zirconia wear data. Identifying the run=in and steady state wear rates was a valuable step in processing the ceramic wear data and assessing its reliability. Thus, The M-M and C-C THRs have demonstrated two to three orders of reduction in volumetric wear in the laboratory compared with the PE wear standard, which helps to explain the excellent wear performance and minimal osteolysis seen with such implants at retrieval operations.

Correlation of wear debris-induced osteolysis and revision with volumetric wear-rates of polyethylene: a survey of 8 reports in the literature.

This survey focused on clinical reports of polyethylene wear and osteolysis in total hip replacements. With regard to documentation of clinical wear-rates, 57 publications were reduced to an analysis of 8 reports of THR series, including the incidence of osteolysis. A direct correlation was found among volumetric wear rates, incidence of osteolysis and revision rates in THR concepts of the 1983-1987 era. As volumetric wear rate increased, the incidence of osteolysis and revision rates increased. With regard to our grading system for volumetric wear, with follow-up in the 4-15 year range, osteolysis was rare in group A (wear = 0-80 mm 3 /year), ranging from 6% to 31% in group B (wear 40-80 mm 3 /year) and from 21% to 100% in group C (wear > 140 mm 3 /year). With regard to cup design, the optimal low-wear group had mainly cemented polyethylene cups with 22 and 28 mm head sizes. The mid-wear group B had metal-backed cemented and uncemented cups, with 28 mm head size, and the high-risk group C had only uncemented, metalbacked cups, with the highest wear in the 32 mm head size. Less than 10 years of follow-up did not distinguish adequately between different designs of THR, except in a few cases which had early failures due to material or design deficiencies. Overall, the cemented all-polyethylene cup combined with the smaller ball head proved to be better.

Clinical and hip simulator comparisons of ceramic-on-polyethylene and metal-on-polyethylene wear.

The benefit of reduced polyethylene wear with ceramic in hip replacements does not seem to have been universally appreciated. In this current study, wear predictions from laboratory and clinical studies were compared for ceramic-on-polyethylene and cobalt chrome-on-polyethylene combinations. Many laboratory studies included water-based lubrication and linear-tracking mechanisms. Now it is appreciated that these were inappropriate methods, because of a propensity for very low or virtually no polyethylene wear against ceramics in water. Thus, water-based studies predicting a 20- to 80-fold advantage for ceramic-on-polyethylene compared with metal-on-polyethylene clearly were in error. However, serum-based simulator studies with high protein-concentrations generally have shown greater wear with alumina-on-polyethylene than with metal-on-polyethylene. Controversy still remains over the use of such nonphysiologic protein levels. The simulator studies were just beginning to explore the role of serum protein concentrations and the influence on the various wear models. Polyethylene wear with zirconia systems was particularly affected by serum protein concentrations. In one simulator study, use of proteins in the physiologic range resulted in the alumina-on-polyethylene wear rate decreasing to approximately 50% of that of metal-on-polyethylene. In the literature, many hip design and polyethylene variations were reported which confounded the wear analysis. Overall, the clinical data supported the superior performance of ceramic-on-polyethylene systems by a factor of 1.5- to fourfold. However, the amount of supporting data was not large. This summary of laboratory and clinical data indicated that ceramic-on-polyethylene hip replacement systems offered on average a 50% wear reduction from metal-on-polyethylene systems.

ARTICULAR CARTILAGE: A REVIEW AND SCANNING ELECTRON MICROSCOPE STUDY

1. The fibrillar networks of adult human articular cartilage, taken from femoral and acetabular specimens, have been systematically examined by scanning electron microscopy. The internal structures revealed by rupturing the tissue were compared with published findings from transmission electron microscope studies. 2. Though this technique demonstrated the internal fibrillar appearance of cartilage to a remarkable degree, it had several attendant limitations. On final drying, specimens generally exhibited shrinkage which varied within wide limits; this could have altered the internal architecture to some extent. In addition, the rupturing technique, which at the time of this investigation was the only satisfactory method of revealing the fibrillar cartilage structure, may well have had a great influence on the fibril orientations. 3. The fibrils revealed no characteristic collagen periodicity and were considerably thicker than those observed by transmission electron microscopy. It is suggested that a coating of mucin on the collagen fibrils might account for this. 4. At low magnifications the torn layers in the fractured surfaces extended radially from the calcified zone and turned obliquely at or near the articular surface to merge with the distinctly layered superficial zone, thus forming arcade-like structures. That these were not artefacts produced by the fracturing technique was shown by their similarity to the classical arcade pattern of light microscopy. However, the factor which governed the direction of these planes of weakness, be it collagen, mucopolysaccharides or cells, could not be satisfactorily determined. 5. At higher magnifications only three regions of distinct fibrillar organisation could be identified: 1) a surface layer consisting of a random fibrillar network; 2) a superficial zone composed of layers of fibrillar network, intersecting and overlapping in planes parallel to the surface; and 3) elsewhere below the superficial zone a network of virtually random fibrils which extended to the calcified region with apparently little variation in thickness or density. There was little variation from this pattern even in aged fibrillated specimens. 6. At the lower magnification range the scanning electron microscope has revealed the arcade pattern described by light microscopy, while at the higher magnifications the fibrillar organisation as seen by scanning electron microscopy correlated well with the concepts developed by transmission electron microscopy, that is, a random network of fibrils overlaid at the articular surface by a membrane-like system of bundled fibrils. 7. A possible role in the transmission of joint forces is outlined for the above fibrillar organisation.

Fabrication and testing of silicon nitride bearings in total hip arthroplasty: winner of the 2007 "HAP" PAUL Award.

Total hip arthroplasty (THA) bearings were fabricated from silicon nitride (Si(3)N(4)) powder. Mechanical testing showed that Si(3)N(4) had improved fracture toughness and fracture strength over modern alumina (Al(2)O(3)) ceramic. When tested with Si(3)N(4) cups in a hip simulator, both cobalt-chromium (CoCr) and Si(3)N(4) femoral heads produced low wear rates that were comparable to Al(2)O(3)-Al(2)O(3) bearings in THA. This study offers experimental support for a novel metal-ceramic THA bearing couple that combines the reliability of CoCr femoral heads with the wear advantages of ceramic surfaces.

Porous ingrowth fixation of the femoral component in a canine surface replacement of the hip

Prosthetic fixation by growth of bone into porous implants coated with a sintered layer of cobalt chrome beads offers an attractive alternative to acrylic cementation. Experiments on dogs demonstrate that bone ingrowth occurred into the femoral component of a hip surface replacement. Methods of stabilizing prostheses in order to promote bone ingrowth must not interfere with the uniformity of stress transfer at the bone - implant interface. Ingrowth occurred as early as the second week postimplantation. The absence of any fibrous membrane formation in the interface between bone and metal suggests that sintered chrome cobalt is a highly compatible biological system, and that fixation by bone ingrowth is stable and does not promote osteoclasis and progression to a fibrous membrane at the interface.

Hip-simulator ranking of polyethylene wear:Comparisons between ceramic heads of different sizes

We carried out simulator studies on ceramic-polyethylene total-hip combinations to determine the volumetric wear-rates of 22 mm, 26 mm and 28 mm femoral-head sizes. Bovine-serum lubrication and 2 kN peak sinusoidal load-profile were used with polyethylene (UHMWPE) cups. Wear was assessed by gravimetric technique. Precision (9%) was ensured by the use of multiple specimens, multiple wear-events, and the linear-regression method of estimating the average wear trend, thereby reducing the inherent, unpredictable nature of each wear-event. Volumetric wear-rates for polyethylene averaged 23 mm3 per 10(6) cycles for the 22 mm ceramic head and up to 32 mm3 per 10(6) cycles for the 28 mm head. The difference between 22 mm and the larger head-sizes was significant. This may well be the first laboratory confirmation of Charnleys original clinical Low-Friction Arthroplasty concept with regard to wear rate. The wear penalty increased linearly at the rate of 6% to 9% per mm of diameter increase.

THARIES surface replacements: a review of the first 100 cases.

The first 100 cases of the 135 THARIES surface replacement procedures with 4--32 months follow-up, are evaluated in terms of clinical results, radiographic information and complications. The short but detailed follow-up suggests that this procedure was an excellent alternative for the younger and more active population. The overall hip ratings (pain, walking and function) and range of motion are comparable to that of stem-type total hip replacements. When the THARIES and conventional hip results are further compared in 34 patients matched by sex and age for 3 major etiological groups (osteoarthritis, osteonecrosis and congenital hip dysplasia), the interim results are essentially comparable for both types. Three cases in the first 100 have required revision. One patient (no. 1) with osteoporosis, chondrolysis and arthrofibrosis following slipped capital femoral epiphysis had loose femoral and acetabular components 24 months postoperatively. He was revised to a T-28 hip replacement. The polyethylene socket in another patient (no. 4), the first dysplastic hip in this series, was 20 nm uncovered superiorly, became loose and was revised 9 months postoperatively. Now 15 months postoperative with a more medial THARIES acetabulum, the patient continues to have a good result. Another patient (no. 12) with bilateral dysplastic hips became progressively more disabled due to heterotopic bone, which was then excised 18 months postoperatively. Radiographic studies of the THARIES sockets demonstrate radiolucent zones at the cement-bone interfaces of the acetabulum in 88 cases, partial in 51 and complete in 37. Three hips were currently considered to have evidence of progressive socket loosening but are active and asymptomatic. There have been no femoral neck fractures in this series which we attribute to the custom fitting ability inherent in the range of components, the reaming protocol and the various remodelling guides. There have been no prosthetic breakages, subluxations, dislocations or sepsis. The complications observed in this series are minor and comparable to that of many other total hip arthroplasty operations. Nerve palsy and trochanteric separation have not been major problems although one existing peroneal nerve dysfunction and one trochanteric migration emphasize the need to minimize these complications by careful handling of the operative leg, and an accurate trochanteric reattachment technique.

Improved wear performance with crosslinked UHMWPE and zirconia implants in knee simulation

Background Suggestions for improved wear performance of total knee replacements have included replacement of standard CoCr femoral components with ceramic and replacement of 3.5-Mrad ultra-high molecular weight polyethylene (UHMWPE) inserts with 5- or 7-Mrad UHMWPE inserts. The ceramic materials used clinically have included alumina, zirconia ceramic and oxidized zirconium. Patients and method We compared both CoCr and zirconia versions of the Bi-Surface knee replacement in a 6-station knee simulator using alpha calf serum for lubrication (20 mg protein per mL) to evaluate the relative bearing performance. Results We studied the 4-way knee simulation of implant materials: zirconia ceramic, CoCr, 3.5-Mrad UHMWPE, and 7-Mrad UHMWPE. With CoCr femoral components, the 7-Mrad UHMWPE resulted in a 5- to 8-fold reduction in wear compared to the 3.5-Mrad insert. With the 3.5-Mrad insert, the zirconia bearing provided approximately 4-fold wear reduction compared to CoCr. These wear rates with standard UHMWPE were similar to published wear studies on entire knees. With the exception of the CoCr/7-Mrad and ZrO2/3.5-Mrad combinations, the wear differences were statistically significant. Interpretation The ZrO2/7-Mrad UHMWPE combination gave the best performance, with no measurable wear over the 5.5 million cycle test duration.