Design aspects of compliant, soft layer bearings for an experimental hip prosthesis

Three-year results of a polycarbonate urethane acetabular bearing in total hip arthroplasty

BACKGROUND

Polycarbonate urethane (PCU) is a bearing surface with a lower modulus of elasticity than polyethylene or ceramic and is thought to more closely replicate the tribology of native hyaline cartilage. The purpose of this study was to determine the clinical outcomes with the use of PCU in elective total hip arthroplasty (THA).

METHODS

We carried out a prospective observational study in which 157 patients underwent elective THA with a metal-on-PCU hip system. Patients had radiographic follow-up at 6 months and 3 years after surgery. Oxford Hip Scores and EuroQol scores were obtained annually and Harris Hip Scores were obtained at 6 months and 3 years after surgery.

RESULTS

180 hips were implanted, of which, 149 hips reached 3-year review with no revisions. There was an increase in Harris Hip Scores, Oxford Hip Scores and EuroQol scores (p < 0.001). 12 patients (12 hips) reported painless hip squeaking. There were no dislocations and no other adverse events were reported.

CONCLUSION

Our results showed satisfactory survivorship and improvements in patient reported outcomes with metal on PCU THA. Long-term data are still being collected to confirm these findings. We recommend further tribological research into the squeaking phenomenon we observed.

Study of the polycarbonate-urethane/metal contact in different positions during gait cycle

Nowadays, a growing number of young and more active patients receive hip replacement. More strenuous activities in such patients involve higher friction and wear rates, with friction on the bearing surface being crucial to ensure arthroplasty survival in the long term. Over the last years, the polycarbonate-urethane has offered a feasible alternative to conventional bearings. A finite element model of a healthy hip joint was developed and adjusted to three gait phases (heel strike, mid-stance, and toe-off), serving as a benchmark for the assessment of the results of joint replacement model. Three equivalent models were made with the polycarbonate-urethane Tribofit system implanted, one for each of the three gait phases, after reproducing a virtual surgery over the respective healthy models. Standard body-weight loads were considered: 230% body-weight toe-off, 275% body-weight mid-stance, and 350% body-weight heel strike. Contact pressures were obtained for the different models. When comparing the results corresponding to the healthy model to polycarbonate-urethane joint, contact areas are similar and so contact pressures are within a narrower value range. In conclusion, polycarbonate-urethane characteristics are similar to those of the joint cartilage. So, it is a favorable alternative to traditional bearing surfaces in total hip arthroplasty, especially in young patients.

The use of compliant layer prosthetic components in orthopedic joint repair and replacement: a review

The surgical repair or treatment of degenerative joint disease has traditionally involved the substitution of synthetic materials for one or both surfaces of the joint. Engineering thermoplastics, metals, and ceramics have either been widely accepted or experimentally evaluated for use as bearing surfaces in these prostheses. When engineering thermoplastics are used, the opposing surface is a metal or a ceramic, but metal-on-metal, metal-on-ceramic, and ceramic-on-ceramic have also been used or tested. Researchers have sought the opportunity to utilize materials with compressive mechanical properties more closely matching those of the natural articular cartilage. This review discusses the theory, testing, and application of elastomers for one bearing component of articular joint prostheses.

An investigation into PEEK-on-PEEK as a bearing surface candidate for cervical total disc replacement

BACKGROUND CONTEXT

Total disc replacement (TDR) is a relatively new reconstructive non-fusion technology for the spine that aims to overcome some of the limitations of fusion technology. The first generation artificial discs were mainly based on well-known material combinations from total hip replacement.

PURPOSE

To evaluate the feasibility of PEEK-on-PEEK as a bearing surface material for use in cervical TDR.

STUDY DESIGN

In vitro biotribological study including the assessment of different parameters, including the influence of radial clearance, axial load, and angular motion on the wear performance of a PEEK-on-PEEK bearing.

METHODS

PEEK-on-PEEK bearing couples with ball-and-socket articulation were prepared using standard production methods. Two groups of bearing couples, one with large radial clearances and the other with small radial clearances, were manufactured for testing. Wear tests were performed using a servo-hydraulic wear simulator that allowed controlled motions to be applied in three axes corresponding to flexion-extension, lateral bending, and axial rotation. In addition, a dynamic axial compressive load was applied to simulate the weight of the head. All tests were performed at 37°C (±2°C) in bovine calf serum with a 30 g/l protein concentration. In the first test series, the influence of the radial clearance on the wear behavior was evaluated using the load and motion parameters for cervical TDR as defined in the ISO standard 18192-1. Subsequent tests were performed to check if surface degradation was influenced by thermal effects. Finally, in a third series of tests, the effect of load and motion profiles on wear performance was investigated by applying increased loads and corresponding motion parameters for lumbar TDR as defined in the ISO standard 18192-1.

RESULTS

The measured wear rates for a PEEK-on-PEEK bearing under cervical test conditions were considerably larger than those of similar testing using UHMWPE-on-CoCr and seemed to depend on initial radial clearances. The PEEK bearing surfaces exhibited severe surface degradation for large and small clearances, but the group with smaller clearances seemed to have less severe damage. Thermal effects were not found to play a role in the surface degradation, as the temperature near the bearing surface was measured and found to vary only a few degrees during testing. A change of the wear test parameters to simulate lumbar loading resulted in a considerable wear rate reduction as well as in the preservation of the bearing surfaces.

CONCLUSIONS

It was found that a cervical TDR using a PEEK-on-PEEK bearing may be subjected to severe long-term degradation of the bearing surfaces. In this study, the use of PEEK as an articulation material for cervical TDR was found to be sensitive to loading conditions as well as to the initial clearance of the bearing surfaces. Despite the excellent properties of PEEK as an implant material, its use for articulating surfaces for cervical TDR must be critically reviewed.

Cushion bearings versus large diameter head metal-on-metal bearings in total hip arthroplasty: a short-term metal ion study

INTRODUCTION

Metal-on-metal total hip arthroplasty (MOM THA) has the advantage of replicating the femoral head size, but the postoperative elevation of serum metal ion levels is a cause for concern. Metal-on-polycarbonate-urethane is a new cushion bearing featuring a large diameter metal head coupled with a polycarbonate-urethane liner.

AIM

The aim of this study was to assess and compare serum cobalt (Co) and chromium (Cr) levels in a group of 15 patients treated with a cushion bearing THA system (Group A) and a group of 15 patients treated with a MOM THA system (Group B) at short-term. At a mean follow-up of 27.3 months (18-35 months), in Group A the median Cr and Co serum levels were significantly lower than in Group B, measuring 0.24 μg/L (0.1-2.1 μg/L) and 0.6 μg/L (0.29-2.3 μg/L) compared to 1.3 μg/L (0.1-9 μg/L, p < 0.001) and 2.9 μg/L (0.85-13.8 μg/L, p < 0.001) respectively.

RESULTS

All patients demonstrated an excellent clinical result, as shown by the Harris and Oxford hip scores. The cushion bearing THA studied in this paper showed clinical outcomes similar to the MOM THA bearing, with the advantage of no significant metal ion elevation in the serum.

CONCLUSION

These findings warrant the continued clinical study of compliant bearing options.

Long-term evaluation of a compliant cushion form acetabular bearing for hip joint replacement: a 20 million cycles wear simulation

Soft bearing materials that aim to reproduce the tribological function of the natural joint are gaining popularity as an alternative concept to conventional hard bearing materials in the hip and knee. However, it has not been proven so far that an elastic cushion bearing can be sufficiently durable as a long term (∼20 years) articulating joint prosthesis. The use of new bearing materials should be supported by accurate descriptions of the implant following usage and of the number, volume, and type of wear particles generated. We report on a long-term 20 million cycle (Mc) wear study of a commercial hip replacement system composed of a compliant polycarbonate-urethane (PCU) acetabular liner coupled to a cobalt-chromium alloy femoral head. The PCU liner showed excellent wear characteristics in terms of its low and steady volumetric wear rate (5.8-7.7 mm(3)/Mc) and low particle generation rate (2-3 × 10(6) particles/Mc). The latter is 5-6 orders of magnitude lower than that of highly cross-linked polyethylene and 6-8 orders of magnitude lower than that of metal-on-metal bearings. Microscopic analysis of the implants after the simulation demonstrated a low damage level to the implants' articulating surfaces. Thus, the compliant PCU bearing may provide a substantial advantage over traditional bearing materials.

Evaluation of the wear performance of a polycarbonate-urethane acetabular component in a hip joint simulator and comparison with UHMWPE and cross-linked UHMWPE

Acetabular hip joint components manufactured from gamma-sterilized ultra high molecular weight polyethylene (UHMWPE), gamma cross-linked UHMWPE, or polycarbonate-urethane (PCU) polymers were evaluated in a hip joint simulator, using cobalt alloy femoral components, for at least 5 million cycles. The volume of material losses due to wear was calculated for each type of sample, based upon mass loss measurements, every 500,000 cycles. The loss of material for the conventional UHMWPE was much higher than for the cross-linked UHMWPE, showing about a 70% reduction in wear due to cross-linking. The material loss for the PCU samples appears to have been at least 24% lower than for the cross-linked UHMWPE. Based upon these results, the PCU material seems to have potential for use as an alternative bearing material to UHMWPE for total hip replacement surgeries.

Chondroprotective effects of a polycarbonate-urethane meniscal implant: histopathological results in a sheep model

PURPOSE

injury or loss of the meniscus generally leads to degenerative osteoarthritic changes in the knee joint. However, few surgical options exist for meniscal replacement. The goal of this study was to examine the ability of a non-degradable, anatomically shaped artificial meniscal implant, composed of Kevlar-reinforced polycarbonate-urethane (PCU), to prevent progressive cartilage degeneration following complete meniscectomy.

METHODS

the artificial meniscus was implanted in the knees of mature female sheep following total medial meniscectomy, and the animals were killed at 3- and 6-months post-surgery. Macroscopic analysis and semi-quantitative histological analysis were performed on the cartilage of the operated knee and unoperated contralateral control joint.

RESULTS

the PCU implants remained well secured throughout the experimental period and showed no signs of wear or changes in structural or material properties. Histological analysis showed relatively mild cartilage degeneration that was dominated by loss of proteoglycan content and cartilage structure. However, the total osteoarthritis score did not significantly differ between the control and operated knees, and there were no differences in the severity of degenerative changes between 3 and 6 months post-surgery.

CONCLUSION

current findings provide preliminary evidence for the ability of an artificial PCU meniscal implant to delay or prevent osteoarthritic changes in knee joint following complete medial meniscectomy.

Compliant-layer tibial bearing inserts: Friction testing of different materials and designs for a new generation of prostheses that mimic the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of devices that exhibit good lubricating properties with low friction and low wear could well extend this. A novel tibial bearing design, using polyurethane (PU) as a compliant layer, to mimic the natural joint, has been developed. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the effects of material modulus and surface roughness, together with bearing design parameters such as bearing thickness and conformity, on lubrication. Corethane 80A was the preferred material and was chosen as the compliant layer for subsequent testing. A low surface roughness resulted in lower asperity contact as the asperities were depressed by the pressurized entraining fluid and full-fluid-film lubrication was approached. The three different tibial bearing conformities (low, medium, and high) did not appear to influence the mode of lubrication and all these bearings performed with extremely low friction. Similarly, the bearing thickness effects on lubrication at the levels tested (2 mm, 3 mm, and 4 mm) were minimal, although the effects of layer thickness on interface shear stress could be expected to be significant. This study describes a series of friction tests that have been used to select the most appropriate material and to optimize the design parameters to establish optimum conditions for these compliant layer joints.

Characterisation of Bionate polycarbonate polyurethanes for orthopaedic applications

Two polycarbonate polyurethanes, Bionate 75D and Bionate 80A, have been characterized for application in biomimetic joint replacement systems. Procedures involved measurement of the effects of compounding and moulding on molecular weight, melt rheometry, and mechanical testing using conditioned and aged specimens. The effects of compounding with hydroxyapatite and carbon fibres were also evaluated. With Bionate 75D moulding reduces the molecular weight by 30%. Passing the material through a twin screw extruder without filler has similar molecular weight reduction effects to injection moulding. Inclusion of carbon fibre has little additional effect on molecular weight, although moulding of the fibre filled compound causes some further degradation, and Mw is almost halved compared with the original value. Inclusion of hydroxyapatite reduces Mw in a moulded component to less than a quarter of the original value and some form of chemical interaction between the polymer and filler is presumed. The apparent melt viscosity of the Bionate 75D was reduced by the addition of both carbon fibres and hydroxyapatite and this is thought to arise from reduction in molecular weight during the compounding process and the development of shear planes at the polymer-filler interface. The polymer glass transition temperatures are shifted to slightly higher values by the inclusion of filler. The tensile test results show the reinforcing effect of the carbon fibres, but poor wetting and pull out of the fibres was evident. Water absorption results suggest that the materials stabilise after 2 weeks, but the tensile results indicate that property change occurred between 1 month and 5 months of exposure. However the shape of the stress-strain curves is not altered, but with extended water exposure is translated to lower stress levels.

Friction moments of large metal-on-metal hip joint bearings and other modern designs.Med Eng Phys. 2008;30:1057-1064. [PMID: 18291702 DOI: 10.1016/j.medengphy.2008.01.001]

Modern hip joint replacements are designed to minimise wear problems. The most popular metal-on-polyethylene components are being updated by harder metal and ceramic combinations. However, this has also been shown to influence the friction moments, which could overload the interface between the implant and the body. In this study custom test apparatus was used to measure the joint moments in various modern bearings under simulated physiological joint conditions. The largest moments in serum were measured for large diameter metal-metal bearings (<8 Nm for standard bearings), followed by metal-polyethylene, and the lowest moments were for small diameter ceramic-ceramic and ceramic-metal combinations. Water as a lubricant was found to double the moments in comparison with serum. In metal-metal bearings moments were reduced by increasing loading frequency. Swing phase load and a rest period between load cycles had little effect. The moment magnitudes are within the turn-out capacity measured for press-fit cups and might become critical with higher joint loads.

Compliant layer acetabular cups: friction testing of a range of materials and designs for a new generation of prosthesis that mimics the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of components that exhibit good lubricating properties with low friction and low wear could extend the life of TJRs. A novel acetabular cup design using polyurethane (PU) as a compliant layer (to mimic the natural joint) has been developed. This study describes a series of friction tests that have been used to select the most appropriate material, optimize the design parameters, and fine-tune the manufacturing processes of these joints. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the lubrication of four PU bearing materials. Corethane 80A was the preferred material and was subjected to subsequent testing. Friction tests conducted on acetabular cups, manufactured using Corethane 80A articulating against standard, commercially available femoral heads, demonstrated friction factors approaching those for full-fluid-film lubrication with only approximately 1 per cent asperity contact. As the joint produces these low friction factors within less than half a walking cycle after prolonged periods of loading, start-up friction was not considered to be a critical factor. Cups performed well across the full range of femoral head sizes, but a number of samples manufactured with reduced radial clearances performed with higher than expected friction. This was caused by the femoral head being gripped around the equator by the low clearance cup. To avoid this, the cup design was modified by increasing the flare at the rim. In addition to this the radial clearance was increased. As the material is incompressible, a radial clearance of 0.08 mm was too small for a cup diameter of 32 mm. A clearance of between 0.10 and 0.25 mm produced a performance approaching full-fluid-film lubrication. This series of tests acted as a step towards the optimization of the design of these joints, which has now led to an in vivo ovine model.