Commissioning of a displacement-controlled knee wear simulator and exploration of some issues related to the lubricant

A six-station displacement-controlled knee simulator with separately controlled left (L) and right (R) banks (three wear implants per bank) was commissioned for a total of three million cycles (Mc) following ISO 14243-3. A commissioning protocol was applied to compare the polyethylene wear among the six wear stations by exchanging the implants between wear stations. Changes in lubricant characteristics during wear testing, such as polypeptide degradation, low-molecular-weight polypeptide concentration, and possible microbial contamination were also assessed. The total mean wear rate for the implants was 23.60 +/- 1.96 mm3/Mc and this was of a similar magnitude to the mean wear rate for the same implant tested under similar conditions by DePuy Orthopaedics Inc. (Warsaw, IN). Repeated run-in wear was observed when the implants were exchanged between wear stations, suggesting that implants should be subjected to the same wear station throughout the duration of a wear test. The total polypeptide degradation for the implants measured 30.53 +/- 3.96 percent; the low-molecular-weight polypeptide concentration of the "used" lubricant for implants (0.131 +/- 0.012 g/L) was 3.3 times greater than the mean polypeptide concentration of the fresh, "unused" lubricant (0.039 +/- 0.004 g/L). This increase in low-molecular weight polypeptide concentration was suggested to be attributable to protein shear in the articulation of the implant, the circulation of the lubricant, and some proteolytic activity. Sodium azide was ineffective in maintaining a sterile environment for wear testing as a single, highly motile Gram-negative micro-organism was identified in the lubricant from wear tests.     

Design forms of total knee replacement

The starting point of this article is a general design criterion applicable to all types of total knee replacement. This criterion is then expanded upon to provide more specifics of the required kinematics, and the forces which the total knee must sustain. A characteristic which differentiates total knees is the amount of constraint which is required, and whether the constraint is translational or rotational. The different forms of total knee replacement are described in terms of these constraints, starting with the least constrained unicompartments to the almost fully constrained fixed and rotating hinges. Much attention is given to the range of designs in between these two extreme types, because they constitute by far the largest in usage. This category includes condylar replacements where the cruciate ligaments are preserved or resected, posterior cruciate substituting designs and mobile bearing knees. A new term, 'guided motion knees', is applied to the growing number of designs which control the kinematics by the use of intercondylar cams or specially shaped and even additional bearing surfaces. The final section deals with the selection of an appropriate design of total knee for specific indications based on the design characteristics.     

Reduction of total knee replacement wear with vitamin E blended highly cross-linked ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is a common bearing component in total knee replacement (TKR) implants, and its susceptibility to wear continues to be the long-term limiting factor in the life of these implants. This study hypothesized that in TKR systems, a highly cross-linked (HXL) UHMWPE blended with vitamin E will result in reduced wear as compared to a direct compression-moulded (DCM) UHMWPE. A wear simulation study was conducted using an asymmetric lateral pivoting '3D Knee' design to compare the two inserts. The highly cross-linked UHMWPE was aged prior to the testing and force-controlled wear testing was carried out for 5 million cycles using a load-controlled ISO-14243 standard at a frequency of 1 Hz on both groups. Gravimetric measurements of DCM UHMWPE (4.4 +/- 3.0 mg/million cycles) and HXL UHMWPE with vitamin E (1.9 +/- 1.9 mg/million cycles) showed significant statistical differences (p < 0.01) between the wear rates. Wear modes and surface roughness for both groups revealed no significant dissimilarities.     

Femoral stem wear in cemented total hip replacement

The great success of cemented total hip replacement to treat patients with endstage osteoarthritis and osteonecrosis has been well documented. However, its long-term survivorship has been compromised by progressive development of aseptic loosening, and few hip prostheses could survive beyond 25 years. Aseptic loosening is mainly attributed to bone resorption which is activated by an in-vivo macrophage response to particulate debris generated by wear of the hip prosthesis. Theoretically, wear can occur not only at the articulating head-cup interface but also at other load-bearing surfaces, such as the stem-cement interface. Recently, great progress has been made in reducing wear at the head-cup interface through the introduction of new materials and improved manufacture; consequently femoral stem wear is considered to be playing an increasingly significant role in the overall wear of cemented total hip replacement. In this review article, the clinical incidences of femoral stem wear are comprehensively introduced, and its significance is highlighted as a source of generation of wear debris and corrosion products. Additionally, the relationship between femoral stem surface finish and femoral stem wear is discussed and the primary attempts to reproduce femoral stem wear through in-vitro wear testing are summarized. Furthermore, the initiation and propagation processes of femoral stem wear are also proposed and a better understanding of the issue is considered to be essential to reduce femoral stem wear and to improve the functionality of cemented total hip replacement.     

Wear and conformity in total knee replacement

The wear on the plastic components of removed condylar replacement total knee prostheses and on total hip prostheses was studied using light microscopy, scanning electron microscopy and surface profilometry. The wear phenomena observed were smearing and stretching of the surface, cracking, “roll” formation, pitting and three-body abrasion. The type of wear apparently depended upon the stresses and the conformity between the two joint components. With high conformity there was a greater tendency for smearing and stretching, with cracking in highly stressed areas. With low conformity, e.g. with the condylar knees employing fairly flat tibial plateaux, cracking and pitting were the most common features. In medium conformity situations, roll formation and general surface disruption, with some cracking, were observed. The three-body abrasion, characterized by numerous shreds of plastic on the surface, was sometimes associated with entrapped particles of methyl methacrylate cement. Fluctuating load and motion were considered to be fundamental to the formation of the various surface phenomena. The phenomena were more or less reproduced in experiments using a special purpose test machine. The specimens were polished metal cylinders in a hemi-cylindrical plastic trough, the conformity being varied in different tests. The motion was cyclic to and fro, and the load was cyclic off and on. On the basis of these experiments and from studies of removed prostheses, conclusions about the type of wear occurring and the likelihood of severe wear were correlated with the design of the bearing (conformity) and the service conditions (stress).     

Biological reactions to wear debris in total joint replacement

The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, evidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an increasing need to implant hip prostheses in the younger, more active patient the need to understand the mechanisms of failure and to develop artificial hip joints using alternative materials have become major issues in the orthopaedic community. This review focuses initially on our current understanding of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosening. While the precise mechanisms of osteolysis induced by UHMWPE wear debris have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 micron) for macrophage activation. The review then considers whether the problem of wear-debris-induced osteolysis may be overcome with the use of new generation metal-on-metal or ceramic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of the wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the characteristics of the wear particles generated in vivo before predictions can be made. It is concluded that the pre-clinical testing of any new materials for joint replacement must include an analysis of the wear particle characteristics and their biological reactivity in addition to the usual assessment of wear.     

Development of a new type of corrosive wear tester

A new type of corrosive wear tester has been developed, on which both static and transient corrosive wear tests can be performed. By combining the tester with an electrochemical measuring system and a dynamic strain gauge, studies can be done in fields such as polarisation curve measurement under conditions of corrosive wear, surface state and wear coefficient measurement, and dynamic observations of current under static potential, as well as the study of surface film rupture and recovery. Leakage of the solution was considered throughout the design, and was addressed by avoiding passing the rotating shaft through the electrolytic cell.     

Towards a standard in assessment of bone cutting for total knee replacement

Arthroplasty outcome is influenced by the 'quality' of bone preparation for implant insertion. Surgeons face increasing choices of technique and instrumentation, yet clinical scoring methods assess the overall outcome and patient satisfaction but not the bone cuts directly. 'Quality management' of bone reshaping is needed to evaluate different bone cutting methods and computer assisted orthopaedic surgery (CAOS) systems. Analyses and experiments in this study were formulated for measurement and computation of four quantitative characteristics of bone preparation 'quality' and produced a highly condensed index for each. These represented (a) surface finish of cuts, (b) implant fit/looseness possible with the cut shape, (c) implant location/misalignment, and (d) accuracy of individual planar cuts. Assessment of synthetic bone cuts verified the robustness of the method for wide application in arthroplasty intraoperatively, in vitro and for comparing navigation systems.     

Reproduction of fretting wear at the stem-cement interface in total hip replacement

The stem-cement interface experiences fretting wear in vivo due to low-amplitude oscillatory micromotion under physiological loading, as a consequence it is considered to play an important part in the overall wear of cemented total hip replacement. Despite its potential significance, in-vitro simulation to reproduce fretting wear has seldom been attempted and even then with only limited success. In the present study, fretting wear was successfully reproduced at the stem-cement interface through an in-vitro wear simulation, which was performed in part with reference to ISO 7206-4: 2002. The wear locations compared well with the results of retrieval studies. There was no evidence of bone cement transfer films on the stem surface and no fatigue cracks in the cement mantle. The cement surface was severely damaged in those areas in contact with the fretting zones on the stem surface, with retention of cement debris in the micropores. Furthermore, it was suggested that these micropores contributed to initiation and propagation of fretting wear. This study gave scope for further comparative study of the influence of stem geometry, stem surface finish, and bone cement brand on generation of fretting wear.     

Long-term implant-bone fixation of the femoral component in total knee replacement

Success of total knee replacement (TKR) depends on the prosthetic design. Aseptic loosening of the femoral component is a significant failure mode that has received little attention. Despite the clinical relevance of failures, no protocol is available to test long-term implant-bone fixation of TKR in vitro. The scope of this work was to develop and validate a protocol to assess pre-clinically the fixation of TKR femoral components. An in vitro protocol was designed to apply a simplified but relevant loading profile using a 6-degrees-of-freedom knee simulator for 1,000000 cycles. Implant-bone inducible micromotions and permanent migrations were measured at three locations throughout the test. After test completion, fatigue damage in the cement was quantified. The developed protocol was successfully applied to a commercial TKR. Additional tests were performed to exclude artefacts due to swelling or creep of the composite femur models. The components migrated distally; they tilted towards valgus in the frontal plane and in extension in the sagittal plane. The migration patterns were consistent with clinical roentgen-stereophotogrammetric recordings with TKR. Additional indicators were proposed that could quantify the tendency to loosen/stabilize. The type and amount of damage found in the cement, as well as the migration patterns, were consistent with clinical experience with the specific TKR investigated. The proposed pre-clinical test yielded repeatable results, which were consistent with the clinical literature. Therefore, its relevance and reliability was proved.     

Analysis and modelling of wear of cobalt-chrome alloys in a pin-on-plate test for a metal-on-metal total hip replacement

The complex interaction between wear and bearing surfaces of two contacting solids was investigated in this study, with particular reference to the use of metal-on-metal material combinations for artificial hip joint replacements. The contact mechanics model was coupled with the wear model and solved simultaneously as a function of time for a simple case of a uniaxial pin-on-plate wear test. Both a spherical pin and a flat-ended spherical pin were considered. It was shown that the contact pressure between the pin and the plate was substantially reduced by the wear process, particularly during the initial running-in period and for the spherical pin. The theoretical prediction of the worn profiles of the pin and the plate was found to be in good agreement with previous experimental measurements by Tipper et al. in 1999.     

An objective tool for assessing the outcome of total knee replacement surgery

The need for an objective tool to assess the outcome of total knee replacement (TKR) surgery is widely recognized. This study investigates the potential of an objective diagnostic tool for assessing the outcome of TKR surgery based on motion analysis techniques. The diagnostic tool has two main elements: collection of data using motion analysis, and the assessment of knee function using a classifier that is based around the Dempster-Shafer theory of evidence. The tool was used to analyse the knee function of nine TKR subjects preoperatively and at three stages post-operatively. Using important measurable characteristics of the knee, the tool was able to establish the level of benefit achieved by surgery and to enable a comparison of subjects. No subject recovered normal knee function following TKR surgery. This has important implications for knee implant designs.     

New joints for the Millennium: wear control in total replacement hip joints

Hip joint replacement is described as the greatest achievement in orthopaedic surgery in the twentieth century. The field has been dominated for some forty years by implants based upon metallic femoral heads and stems and polymeric acetabular cups. At the dawn of the new Millennium, many alternative materials and designs are now being proposed or evaluated. The reasons for these developments and the current contributions of engineering science and tribology to advances in hip replacement are discussed. Illustrations are presented of the significant changes being proposed or introduced. While the new designs of total hip replacements offer exciting engineering contributions to the future of joint replacement, the long-term benefits to patients will depend upon the biological response to the new devices.     

Improvement in the assessment of wear of total knee replacements using coordinate-measuring machine techniques

Total joint replacement is one of the most common elective surgical procedures performed worldwide, with an estimate of 1.5x 10(6) operations performed annually. Currently joint replacements are expected to function for 10-15 years; however, with an increase in life expectancy, and a greater call for knee replacement due to increased activity levels, there is a requirement to improve their function to offer longer-term improved quality of life for patients. Wear analysis of total joint replacements has long been an important means in determining failure mechanisms and improving longevity of these devices. The effectiveness of the coordinate-measuring machine (CMM) technique for assessing volumetric material loss during simulated life testing of a replacement knee joint has been proved previously by the present authors. The purpose of the current work is to present an improvement to this method for situations where no pre-wear data are available. To validate the method, simulator tests were run and gravimetric measurements taken throughout the test, such that the components measured had a known wear value. The implications of the results are then discussed in terms of assessment of joint functionality and development of standardized CMM-based product standards. The method was then expanded to allow assessment of clinically retrieved bearings so as to ascertain a measure of true clinical wear.     

A study of the wear of some materials in connection with total hip replacement

Failure of total hip prostheses due to wear is examined. It is concluded that wearing out of these devices should not be a problem. However, it is desirable to look for materials of improved wear resistance due to possibilities of long-term response to wear debris.A series of experiments is described to evaluate the wear resistance of candidate materials on an annular wear tester. The results indicate that the wear resistance of ultra high molecular weight polyethylene may be improved by increase in molecular weight, by irradiation or by the use of fillers. Pyrolytic carbon containing silicon is also a good candidate.     

Increased total knee arthroplasty ultra-high molecular weight polyethylene wear using a clinically relevant hyaluronic acid simulator lubricant

In this study, osteoarthritic and periprosthetic synovial fluid samples were rheologically and biochemically compared to develop a hyaluronic acid (HA) supplemented bovine serum (BS) lubricant that mimicked the properties of human joint synovial fluid. The effect of this BS + HA lubricant (50 per cent bovine calf serum + 1.5 g/l HA) on the wear rate of ultra-high molecular weight polyethylene (UHMWPE) during a total knee replacement wear test was then investigated. In conjunction with biochemical similarities, the rheological analysis showed that the BS + HA lubricant viscosity was not statistically different to aspirated total knee arthroplasty (TKA) revision joint fluid viscosity over a range of physiologic shear rates. Gravimetric results at 5 million wear testing cycles showed that the BS + HA lubricant produced an average of 6.88 times more UHMWPE wear than 50 per cent bovine serum lubricant alone. The BS + HA lubricated CoCr femoral component surfaces revealed pitting and surface roughening that was not observed using standard bovine serum only lubricants, but that was similar to the metallic surface corrosion observed on in vivo CoCr femoral component retrievals. These findings support the hypothesis that the addition of HA to simulator lubricant is capable of producing CoCr femoral component surface damage similar to that observed in vivo.     

The biomechanics of a validated finite element model of stress shielding in a novel hybrid total knee replacement

This study proposes a novel hybrid total knee replacement (TKR) design to improve stress transfer to bone in the distal femur and, thereby, reduce stress shielding and consequent bone loss. Three-dimensional finite element (FE) models were developed for a standard and a hybrid TKR and validated experimentally. The Duracon knee system (Stryker Canada) was the standard TKR used for the FE models and for the experimental tests. The FE hybrid device was identical to the standard TKR, except that it had an interposing layer of carbon fibre-reinforced polyamide 12 lining the back of the metallic femoral component. A series of experimental surface strain measurements were then taken to validate the FE model of the standard TKR at 3000 N of axial compression and at 0 degreeof knee flexion. Comparison of surface strain values from FE analysis with experiments demonstrated good agreement, yielding a high Pearson correlation coefficient of R(2)= 0.94. Under a 3000N axial load and knee flexion angles simulating full stance (0O degree, heel strike (200 degrees, and toe off (600 degrees during normal walking gait, the FE model showed considerable changes in maximum Von Mises stress in the region most susceptible to stress shielding (i.e. the anterior region, just behind the flange of the femoral implant). Specifically, going from a standard to a hybrid TKR caused an increase in maximum stress of 87.4 per cent (O0 degree from 0.15 to 0.28 MPa), 68.3 per cent (200 degrees from 1.02 to 1.71 MPa), and 12.6 per cent (600 degrees from 2.96 to 3.33 MPa). This can potentially decrease stress shielding and subsequent bone loss and knee implant loosening. This is the first report to propose and biomechanically to assess a novel hybrid TKR design that uses a layer of carbon fibrereinforced polyamide 12 to reduce stress shielding.     

A computerized bioskills system for surgical skills training in total knee replacement

Although all agree that the results of total knee replacement (TKR) are primarily determined by surgical skill, there are few satisfactory alternatives to the 'apprenticeship' model of surgical training. A system capable of evaluating errors of instrument alignment in TKR has been developed and demonstrated. This system also makes it possible quantitatively to assess the source of errors in final component position and limb alignment. This study demonstrates the use of a computer-based system to analyse the surgical skills in TKR through detailed quantitative analysis of the technical accuracy of each step of the procedure. Twelve surgeons implanted a posterior-stabilized TKR in 12 fresh cadavers using the same set of surgical instruments. During each procedure, the position and orientation of the femur, tibia, each surgical instrument, and the trial components were measured with an infrared coordinate measurement system. Through analysis of these data, the sources and relative magnitudes of errors in position and alignment of each instrument were determined, as well as its contribution to the final limb alignment, component positioning and ligament balance. Perfect balancing of the flexion and extension gaps was uncommon (0/15). Under standardized loading, the opening of the joint laterally exceeded the opening medially by an average of approximately 4 mm in both extension (4.1 +/- 2.1 mm) and flexion (3.8 +/- 3.4 mm). In addition, the overall separation of the femur and the tibia was greater in flexion than extension by an average of 4.6 mm. The most significant errors occurred in locating the anterior/posterior position of the entry point in the distal femur (SD = 8.4 mm) and the correct rotational alignment of the tibial tray (SD = 13.2 degrees). On a case-by-case basis, the relative contributions of errors in individual instrument alignments to the final limb alignment and soft tissue balancing were identified. The results indicate that discrete steps in the surgical procedure make the largest contributions to the ultimate alignment and laxity of the prosthetic knee. Utilization of this method of analysis and feedback in orthopaedic training is expected rapidly to enhance surgical skills without the risks of patient exposure.     

Computational development of a polyethylene wear model for the articular and backside surfaces in modular total knee replacements

A computational model to predict polyethylene wear in modular total knee replacements was developed. The results from knee simulator wear tests were implemented with finite element simulations to identify the wear factors of Archard's wear law. The calculated wear factor for the articular and backside surface was 1.03±0.22×10⁻⁷ mm³/Nm and 2.43±0.52×10⁻¹⁰ mm³/Nm, respectively. The difference in wear factors was attributed to differences in wear mode and wear mechanisms between the articular (mainly two-body rolling/sliding wear mode with an abrasive/adhesive wear mechanism) and the backside surfaces (mainly fretting wear mode with an adhesive wear mechanism).