Tribological assessment of a flexible carbon-fibre-reinforced poly(ether—ether—ketone) acetabular cup articulating against an alumina femoral head

Implementing Machine Learning approaches for accelerated prediction of bone strain in acetabulum of a hip joint

The Finite Element (FE) methods for biomechanical analysis involving implant design and subject parameters for musculoskeletal applications are extensively reported in literature. Such an approach is manually intensive and computationally expensive with longer simulations times. Although Artificial Intelligence (AI) based approaches are implemented to a limited extent in biomechanics, such approaches to predict bone strain in acetabulum of a hip joint, are hardly explored. In this context, the primary objective of this paper is to evaluate machine learning (ML) models in tandem with high-fidelity FEA data for the accelerated prediction of the biomechanical response in the acetabulum of the human hip joint, during the walking gait. The parameters used in the FEA study included the subject weight, number and distribution of fins on the periphery of the acetabular shell, bone condition and phases of the gait cycle. The biomechanical response has also been evaluated using three different acetabular liners, including pre-clinically validated HDPE-20% HA-20% Al2O3, highly-crosslinked ultrahigh molecular weight polyethylene (HC-UHMWPE) and ZrO2-toughened Al2O3 (ZTA). Such parametric variation in FEA analysis, involving 26 variables and a full factorial design resulted in 10,752 datasets for spatially varying bone strains. The bone condition, as opposed to subject weight, was found to play a statistically significant role in determining the strain response in the periprosthetic bone of the acetabulum. While utilising hyperparameter tuning, K-fold cross validation and statistical learning approaches, a number of ML models were trained on the FEA dataset, and the Random Forest model performed the best with a coefficient of determination (R2) value of 0.99/0.97 and Root Mean Square Error (RMSE) of 0.02/0.01 on the training/test dataset. Taken together, this study establishes the potential of ML approach as a fast surrogate of FEA for implant biomechanics analysis, in less than a minute.

Disappointing long-term outcome of THA with carbon-fiber-reinforced poly-ether-ether-ketone (CFR-PEEK) as acetabular insert liner: a prospective study with a mean follow-up of 14.3 years

INTRODUCTION

Insert liner wear of the acetabular component is one of the predictive values for survival of total hip arthroplasties (THAs). This prospective single-centre study was designed to evaluate the follow-up of carbon-fiber-reinforced poly-ether-ether-ketone (CFR-PEEK) insert liner used as bearing in cementless THAs.

METHODS

29 healthy patients with an indication for cementless THA were selected for a CFR-PEEK insert liner and followed over time. All patients received a cementless THA with a CFR-PEEK insert liner used as bearing. At different follow-up moments patients were routinely examined and were analysed using the Oxford Hip Score (OHS), the modified Merle d'Aubigne-Postel (MAP) score, and radiologically. At the follow up moments the plain radiographics where assessed for loosening, cyst formations and wear of the CFR-PEEK liners.

RESULTS

At a mean of 14.3 years follow-up 4 revisions of the acetabular component were performed, resulting in a survival rate of 86.5% (CI 95%, 72.4-96.6). A statistically significant difference in OHS and MAP scores between pre- and postoperative follow-up moments was observed. The acetabular components of the remaining patients showed no radiological abnormalities at 14.3 years follow-up. The overall CFR-PEEK wear was low, with a mean of 0.81 (0.2-1.4) mm wear at 14.3 years follow-up.

CONCLUSIONS

In this series we found an aseptic loosening with unclear reasons in 4 well-positioned acetabular components, hence we do not recommend routine use of CFR-PEEK insert liners as bearing in cementless THAs. All the remaining THAs and acetabular components were in situ without abnormalities at 14.3 years follow-up.

Stress and strain distribution in femoral heads for hip resurfacing arthroplasty with different materials: A finite element analysis

Femoral bone loss due to stress and strain shielding is a common problem in hip resurfacing arthroplasty (HRA), which arises from the different stiffness of implant materials and the adjacent bone. Usually, the implants used in HRA are made of cobalt-chromium alloy (CoCr). As a novel concept, implants may also be made of ceramics, whose stiffness exceeds that of the adjacent bone by a multiple. Therefore, this computational study aimed to evaluate whether poly (ether-ether-ketone) (PEEK) or a hybrid material with a PEEK body and ceramic surface made of alumina toughened zirconia (ATZ) might be more suitable implant alternatives for HRA, as they can avoid stress and strain shielding. A reconstructed model of a human femur with an HRA implant was simulated, whereby the material of the HRA was varied between CoCr, ATZ, zirconia toughened alumina (ZTA), PEEK, and a hybrid PEEK-ATZ material. The implant fixation method also varied (cemented or cementless). The simulated models were compared with an intact model to analyze stress and strain distribution in the femoral head and neck. The strain distribution was evaluated at a total of 30,344 (cemented HRA) and 63,531 (uncemented HRA) nodes in the femoral head and neck region and divided into different strain regions (<400 µm/m: atrophy; 400-3000 μm/m: bone preserving and building; 3000-20,000 μm/m: yielding and >20,000 μm/m fracture). In addition, the mechanical stability of the implants was evaluated. When the material of the HRA implant was simulated as metal or ceramic while evaluating the strains, it was seen that around 22-26% of the analyzed nodes in the femoral head and neck were in an atrophic region, 47-51% were in a preserving or building region, and 27-28% were in a yielding region. In the case of PEEK implant, less than 0.5% of the analyzed nodes were in an atrophic region, 66-69% in a preserving or building region, and 31-34% in a yielding region. The fixation technique also had a small influence. When a hybrid HRA was simulated, the strains at the analyzed nodes depended on the thickness of the ceramic material. In conclusion, the material of the HRA implant was crucial in terms of stress and strain distribution in the adjacent bone. HRA made of PEEK or a hybrid material leads to decisively reduced stress and strain alteration compared to stiffer materials such as CoCr, ATZ, and ZTA. This confirms the potential for reduction in stress and strain shielding in the femoral head with the use of a hybrid material with a PEEK body for HRA.

Nanomechanical analysis of medical grade PEEK and carbon fiber-reinforced PEEK composites

Polyether ether ketone (PEEK) and PEEK composites are viable candidates for orthopedic implants owing to their ability for modulus match of surrounding bone tissue. The structural properties of these systems for load-bearing application in the body can be tailored by incorporating carbon fibers; to this end, polyacrylonitrile (PAN) and pitch fibers are commonly incorporated in the PEEK matrix. Mechanical property optimization for a given medical application requires consideration of carbon fiber type and volume fraction, as well as processing conditions for the composite systems. While much is known about the bulk mechanical properties of PEEK and PEEK composites, little is known about the nanomechanical properties of these systems. Insight into nanoscale behavior can offer valuable information about fiber-matrix interactions that may influence long-term integrity of these biomaterials when used in load bearing medical device applications. In this study, we utilize nanoindentation as a method to characterize mechanical behavior of clinical grade PEEK and PEEK composites. We examine PEEK formulations with pitch and PAN fibers and evaluate a range of thermal treatments known to influence polymer microstructure. We use a conospherical tip of 1.5 μm in radius and a conospherical tip of 20 μm radius to determine indentation modulus over different length scales. We correlate these findings with previous characterization on these same PEEK systems using microindentation. A novelty of this work is that we combine nanoindentation with k-means clustering to quantitatively discern the influence of heat treatment and carbon fiber type on the mechanical behavior of PEEK composites and their constituents. We demonstrate that nanoindentation is an effective characterization tool for discerning fiber-matrix interactions and measuring the mechanical behavior in response to thermal treatment and carbon fiber type in PEEK composites. Nanoindentation is shown to be a viable tool for characterizing complex biomaterials and can serve as an effective technique to guide optimization of microstructures for long-term structural applications in the body.

Influence of the Acetabular Cup Material on the Shell Deformation and Strain Distribution in the Adjacent Bone-A Finite Element Analysis

In total hip arthroplasty, excessive acetabular cup deformations and altered strain distribution in the adjacent bone are potential risk factors for implant loosening. Materials with reduced stiffness might alter the strain distribution less, whereas shell and liner deformations might increase. The purpose of our current computational study was to evaluate whether carbon fiber-reinforced poly-ether-ether-ketones with a Young´s modulus of 15 GPa (CFR-PEEK-15) and 23 GPa (CFR-PEEK-23) might be an alternative shell material compared to titanium in terms of shell and liner deformation, as well as strain distribution in the adjacent bone. Using a finite element analysis, the press-fit implantation of modular acetabular cups with shells made of titanium, CFR-PEEK-15 and CFR-PEEK-23 in a human hemi-pelvis model was simulated. Liners made of ceramic and polyethylene were simulated. Radial shell and liner deformations as well as strain distributions were analyzed. The shells made of CFR-PEEK-15 were deformed most (266.7 µm), followed by CFR-PEEK-23 (136.5 µm) and titanium (54.0 µm). Subsequently, the ceramic liners were radially deformed by up to 4.4 µm and the polyethylene liners up to 184.7 µm. The shell materials slightly influenced the strain distribution in the adjacent bone with CFR-PEEK, resulting in less strain in critical regions (<400 µm/m or >3000 µm/m) and more strain in bone building or sustaining regions (400 to 3000 µm/m), while the liner material only had a minor impact. The superior biomechanical properties of the acetabular shells made of CFR-PEEK could not be determined in our present study.

Finite Element Analysis to Probe the Influence of Acetabular Shell Design, Liner Material, and Subject Parameters on Biomechanical Response in Periprosthetic Bone

The implant stability and biomechanical response of periprosthetic bone in acetabulum around total hip joint replacement (THR) devices depend on a host of parameters, including design of articulating materials, gait cycle and subject parameters. In this study, the impact of shell design (conventional, finned, spiked, and combined design) and liner material on the biomechanical response of periprosthetic bone has been analyzed using finite element (FE) method. Two different liner materials: high density polyethylene-20% hydroxyapatite-20% alumina (HDPE-20%HA-20%Al2O3) and highly cross-linked ultrahigh molecular weight polyethylene (HC-UHMWPE) were used. The subject parameters included bone condition and bodyweight. Physiologically relevant load cases of a gait cycle were considered. The deviation of mechanical condition of the periprosthetic bone due to implantation was least for the finned shell design. No significant deviation was observed at the bone region adjacent to the spikes and the fins. This study recommends the use of the finned design, particularly for weaker bone conditions. For stronger bones, the combined design may also be recommended for higher stability. The use of HC-UHMWPE liner was found to be better for convensional shell design. However, similar biomechanical response was captured in our FE analysis for both the liner materials in case of other shell designs. Overall, the study establishes the biomechanical response of periprosthetic bone in the acetabular with preclinically tested liner materials together with new shell design for different subject conditions.

Wear and Friction of UHMWPE-on-PEEK OPTIMA™

PEEK-OPTIMA™ is being considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacement to provide a metal-free implant. The aim of this study was to investigate the influence of lubricant temperature (standard rig running and elevated temperature (~36 °C)) on the wear of a UHMWPE-on-PEEK OPTIMA™ bearing couple using different lubricant protein concentrations (0%, 2%, 5%, 25% and 90% bovine serum) in a simple geometry pin-on-plate configuration. Friction was also investigated under a single temperature condition for different lubricant protein concentrations. The studies were repeated for UHMWPE-on-cobalt chrome in order to compare relationships with temperature (wear only) and lubricant protein concentration (wear and friction). In low lubricant protein concentrations (≤ 5%) there was no influence of temperature on the wear factors of UHMWPE-on-PEEK. With 25% bovine serum, the wear factor of UHMWPE-on-PEEK reduced by half at elevated temperature. When tested in high protein concentration (90% serum), there was no influence of temperature on the wear factor of UHMWPE-on-PEEK. These temperature dependencies were not the same for UHMWPE-on-cobalt chrome. For both material combinations, there was a trend of decreasing friction with increasing protein concentration once protein was present in the lubricant. This study has shown the importance of the selection of appropriate test conditions when investigating the wear and friction of different materials, in order to minimise test artefacts such as polymer transfer, and protein precipitation and deposition.

Biomechanical behavior of modular acetabular cups made of poly-ether-ether-ketone: A finite element study

After total hip arthroplasty, stress-shielding is a potential risk factor for aseptic loosening of acetabular cups made of metals. This might be avoided by the use of acetabular cups made of implant materials with lower stiffness. The purpose of this numerical study was to determine whether a modular acetabular cup with a shell made of poly-ether-ether-ketone or poly-ether-ether-ketone reinforced with carbon fibers might be an alternative to conventional metallic shells. Therefore, the press-fit implantation of modular cups with shells made of different materials (Ti6Al4V, poly-ether-ether-ketone, and poly-ether-ether-ketone reinforced with carbon fibers) and varying liner materials (ceramics and ultra-high-molecular-weight polyethylene) into an artificial bone cavity was simulated using finite element analysis. The shell material had a major impact on the radial shell deformation determined at the rim of the shell, ranging from 17.9 µm for titanium over 92.2 µm for poly-ether-ether-ketone reinforced with carbon fibers up to 475.9 µm for poly-ether-ether-ketone. Larger radial liner deformations (up to 618.4 µm) occurred in combination with the shells made of poly-ether-ether-ketone compared to titanium and poly-ether-ether-ketone reinforced with carbon fibers. Hence, it can be stated that conventional poly-ether-ether-ketone is not a suitable shell material for modular acetabular cups. However, the radial shell deformation can be reduced if the poly-ether-ether-ketone reinforced with carbon fiber material is used, while deformation of ceramic liners is similar to the deformation in combination with titanium shells.

PEEK and CFR-PEEK as alternative bearing materials to UHMWPE in a fixed bearing total knee replacement: An experimental wear study

New bearing materials for total joint replacement have been explored as the need to improve longevity and enhance performance is driven by the changing demands of the patient demographic. Carbon-reinforced PEEK has demonstrated good wear characteristics in experimental wear simulation in both simple geometry pin-on-plate studies and in total hip joint replacement. Carbon reinforced PEEK CFR-PEEK has the potential to reduce tibial insert thickness and preserve bone in the knee. This study investigated the wear performance of PEEK and CFR-PEEK in a low conformity total knee replacement configuration. Custom-made flat inserts were tested against cobalt-chromium femoral bearings in a knee wear simulation for a period of three million cycles. Wear was assessed gravimetrically at intervals throughout the study. The wear rates of both PEEK and CFR-PEEK were very high and almost two orders of magnitude higher than the wear rate of UHMWPE under comparable conditions. Evidence of mechanical failure of the materials, including surface cracking and delamination was observed in both materials. This study highlights that these materials may not be suitable alternatives for UHMWPE in low-conformity designs.

Analysis of Carbon Fiber Reinforced PEEK Hinge Mechanism Articulation Components in a Rotating Hinge Knee Design: A Comparison of In Vitro and Retrieval Findings

Carbon fiber reinforced poly-ether-ether-ketone (CFR-PEEK) represents a promising alternative material for bushings in total knee replacements, after early clinical failures of polyethylene in this application. The objective of the present study was to evaluate the damage modes and the extent of damage observed on CFR-PEEK hinge mechanism articulation components after in vivo service in a rotating hinge knee (RHK) system and to compare the results with corresponding components subjected to in vitro wear tests. Key question was if there were any similarities or differences between in vivo and in vitro damage characteristics. Twelve retrieved RHK systems after an average of 34.9 months in vivo underwent wear damage analysis with focus on the four integrated CFR-PEEK components and distinction between different damage modes and classification with a scoring system. The analysis included visual examination, scanning electron microscopy, and energy dispersive X-ray spectroscopy, as well as surface roughness and profile measurements. The main wear damage modes were comparable between retrieved and in vitro specimens (n = 3), whereby the size of affected area on the retrieved components showed a higher variation. Overall, the retrieved specimens seemed to be slightly heavier damaged which was probably attributable to the more complex loading and kinematic conditions in vivo.

PEEK-OPTIMA™ as an alternative to cobalt chrome in the femoral component of total knee replacement: A preliminary study

PEEK-OPTIMA^™ (Invibio Ltd, UK) has been considered as an alternative joint arthroplasty bearing material due to its favourable mechanical properties and the biocompatibility of its wear debris. In this study, the potential to use injection moulded PEEK-OPTIMA^™ as an alternative to cobalt chrome in the femoral component of a total knee replacement was investigated in terms of its wear performance. Experimental wear simulation of three cobalt chrome and three PEEK-OPTIMA^™ femoral components articulating against all-polyethylene tibial components was carried out under two kinematic conditions: 3 million cycles under intermediate kinematics (maximum anterior-posterior displacement of 5 mm) followed by 3 million cycles under high kinematic conditions (anterior-posterior displacement 10 mm). The wear of the GUR1020 ultra-high-molecular-weight polyethylene tibial components was assessed by gravimetric analysis; for both material combinations under each kinematic condition, the mean wear rates were low, that is, below 5 mm³/million cycles. Specifically, under intermediate kinematic conditions, the wear rate of the ultra-high-molecular-weight polyethylene tibial components was 0.96 ± 2.26 mm³/million cycles and 2.44 ± 0.78 mm³/million cycle against cobalt chrome and PEEK-OPTIMA^™ implants, respectively (p = 0.06); under high kinematic conditions, the wear rates were 2.23 ± 1.85 mm³/million cycles and 4.44 ± 2.35 mm³/million cycles, respectively (p = 0.03). Following wear simulation, scratches were apparent on the surface of the PEEK-OPTIMA^™ femoral components. The surface topography of the femoral components was assessed using contacting profilometry and showed a statistically significant increase in measured surface roughness of the PEEK-OPTIMA^™ femoral components compared to the cobalt chrome implants. However, this did not appear to influence the wear rate, which remained linear over the duration of the study. These preliminary findings showed that PEEK-OPTIMA^™ gives promise as an alternative bearing material to cobalt chrome alloy in the femoral component of a total knee replacement with respect to wear performance.

Influence of contact pressure, cross-shear and counterface material on the wear of PEEK and CFR-PEEK for orthopaedic applications

Total joint replacement is a successful surgical intervention for the treatment of the degeneration of many joints, particularly the hip and knee. As the demand for joint replacement grows, and the life expectancy of the population increases, the performance requirements of these implants also changes. New materials, to improve longevity and enhance performance have been explored including PEEK and CFR-PEEK.

This study investigated whether CFR-PEEK and PEEK were appropriate materials for total joint replacement by examining wear performance in simple configuration studies articulating against cobalt chrome under a range of cross-shear and contact pressure conditions. Simple geometry pin on plate studies were conducted for one million cycles for each test condition, with the contact pressure and cross-shear conditions representing a range in which the material may need to operate in-vivo.

The wear factor for PEEK was significantly higher than CFR-PEEK and conventional polyethylene under all test conditions. Both PEEK and CFR-PEEK wear were influenced by contact pressure, with the highest wear factors for both materials measured at the highest pressure conditions. PEEK appeared to have a cross-shear dependent wear response, but this was not observed for the CFR-PEEK material.

This study has further characterised the wear performance of two materials that are gaining interest for total joint replacement. The wear performance of the PEEK material showed poorer wear performance compared to polyethylene when articulating with a metal counterface, but the performance of the CFR-PEEK material suggested it may provide a suitable alternative to polyethylene in some applications. The wear performance of CFR-PEEK was poorer than polyethylene when it was used as the plate, when there was translation of the contact zone over the surface of the CFR-PEEK plate. This has implications for applications in low conforming contacts, such as lower conformity knee replacement.

Wear of PEEK-OPTIMA® and PEEK-OPTIMA®-Wear Performance articulating against highly cross-linked polyethylene

The idea of all polymer artificial joints, particularly for the knee and finger, has been raised several times in the past 20 years. This is partly because of weight but also to reduce stress shielding in the bone when stiffer materials such as metals or ceramics are used. With this in mind, pin-on-plate studies of various polyetheretherketone preparations against highly cross-linked polyethylene were conducted to investigate the possibility of using such a combination in the design of a new generation of artificial joints. PEEK-OPTIMA(®) (no fibre) against highly cross-linked polyethylene gave very low wear factors of 0.0384 × 10(-6) mm(3)/N m for the polyetheretherketone pins and -0.025 × 10(-6) mm(3)/N m for the highly cross-linked polyethylene plates. The carbon-fibre-reinforced polyetheretherketone (PEEK-OPTIMA(®)-Wear Performance) also produced very low wear rates in the polyetheretherketone pins but produced very high wear in the highly cross-linked polyethylene, as might have been predicted since the carbon fibres are quite abrasive. When the fibres were predominantly tangential to the sliding plane, the mean wear factor was 0.052 × 10(-6) mm(3)/N m for the pins and 49.3 × 10(-6) mm(3)/N m for the highly cross-linked polyethylene plates; a half of that when the fibres ran axially in the pins (0.138 × 10(-6) mm(3)/N m for the pins and 97.5 × 10(-6) mm/ N m for the cross-linked polyethylene plates). PEEK-OPTIMA(®) against highly cross-linked polyethylene merits further investigation.

Quantification of in vitro wear of a synthetic meniscus implant using gravimetric and micro-CT measurements

A synthetic meniscus implant was recently developed for the treatment of patients with mild to moderate osteoarthritis with knee pain associated with medial joint overload. The implant is distinctively different from most orthopedic implants in its pliable construction, and non-anchored design, which enables implantation through a mini-arthrotomy without disruption to the bone, cartilage, and ligaments. Due to these features, it is important to show that the material and design can withstand knee joint conditions. This study evaluated the long-term performance of this device by simulating loading for a total of 5 million gait cycles (Mc), corresponding to approximately five years of service in-vivo. All five implants remained in good condition and did not dislodge from the joint space during the simulation. Mild abrasion was detected by electron microscopy, but µ-CT scans of the implants confirmed that the damage was confined to the superficial surfaces. The average gravimetric wear rate was 14.5 mg/Mc, whereas volumetric changes in reconstructed µ-CT scans point to an average wear rate of 15.76 mm(3)/Mc (18.8 mg/Mc). Particles isolated from the lubricant had average diameter of 15 µm. The wear performance of this polycarbonate-urethane meniscus implant concept under ISO-14243 loading conditions is encouraging.

The Use of Carbon-Fiber-Reinforced (CFR) PEEK Material in Orthopedic Implants: A Systematic Review

Carbon-fiber-reinforced polyetheretherketone (CFR-PEEK) has been successfully used in orthopedic implants. The aim of this systematic review is to investigate the properties, technical data, and safety of CFR-PEEK biomaterial and to evaluate its potential for new innovation in the design of articulating medical devices. A comprehensive search in PubMed and EMBASE was conducted to identify articles relevant to the outcomes of CFR-PEEK orthopedic implants. The search was also expanded by reviewing the reference sections of selected papers and references and benchmark reports provided by content experts. A total of 23 articles were included in this review. There is limited literature available assessing the performance of CFR-PEEK, specifically as an implant material for arthroplasty systems. Nevertheless, available studies strongly support CFR-PEEK as a promising and suitable material for orthopedic implants because of its biocompatibility, material characteristics, and mechanical durability. Future studies should continue to investigate CFR-PEEK’s potential benefits.

The influence of nominal stress on wear factors of carbon fibre–reinforced polyetheretherketone (PEEK-OPTIMA® Wear Performance) against zirconia toughened alumina (Biolox®delta ceramic)

Carbon fibre–reinforced polyetheretherketone is an attractive alternative to ultra-high-molecular-weight polyethylene in artificial joints, but little has been published on the influence of stress on the wear factor. We know that in ultra-high-molecular-weight polyethylene, the wear factor reduces as the normal stress increases, which is counter-intuitive but very helpful in the case of non-conforming contacts. In this study, carbon fibre–reinforced polyetheretherketone (PEEK-OPTIMA® Wear Performance) has been investigated in a pin-on-plate machine under steady loads and under stresses typical of hip and knee joints. At stresses below about 6 MPa, wear factors are between 10 and a 100 times lower than for ultra-high-molecular-weight polyethylene but at higher stresses the wear factors increase substantially.

Enhancement of CRF-PEEK osseointegration by plasma-sprayed hydroxyapatite: A rabbit model

Carbon-fibre-reinforced polyether ether ketone (CFR-PEEK) exhibits excellent biomechanical properties as it has an elastic modulus similar to bone. However, CFR-PEEK displays inferior biocompatibility compared with titanium alloy and coating techniques are therefore of interest in order to improve integration. In this paper, the early biological response to CFR-PEEK implants, with and without hydroxyapatite coating, was investigated. Furthermore, a hydroxyapatite-coated titanium alloy reference served as a clinically relevant control. The study was conducted in a rabbit model, both in femur trabecular bone as well as in tibia cortical bone. The results demonstrated that an hydroxyapatite coating significantly enhances the bone response to PEEK implants in vivo. Moreover, in cortical bone, hydroxyapatite-coated PEEK implants induced superior bone response compared with hydroxyapatite-coated Ti ones. These results suggest that hydroxyapatite-coated CFR-PEEK is a suitable material for in vivo implantation.

The current state of bearing surfaces in total hip replacement

We reviewed the literature on the currently available choices of bearing surface in total hip replacement (THR). We present a detailed description of the properties of articulating surfaces review the understanding of the advantages and disadvantages of existing bearing couples. Recent technological developments in the field of polyethylene and ceramics have altered the risk of fracture and the rate of wear, although the use of metal-on-metal bearings has largely fallen out of favour, owing to concerns about reactions to metal debris. As expected, all bearing surface combinations have advantages and disadvantages. A patient-based approach is recommended, balancing the risks of different options against an individual’s functional demands.

Cite this article: Bone Joint J 2014;96-B:147–56.

Bone remodelling around cementless composite acetabular components: the effects of implant geometry and implant-bone interfacial conditions

Recent developments in acetabular implants suggest flexible, alternative bearing material that may reduce wear and peri-prosthetic bone resorption. The goal of this study was to investigate the deviations in load transfer and the extent of bone remodelling around composite acetabular components having different geometries, material properties and implant-bone interface conditions, using 3-D FE analysis and bone remodelling algorithm. Variation in prosthesis type and implant-bone interface conditions affected peri-prosthetic strain distribution and bone remodelling. Strain shielding was considerably higher for bonded implant-bone interface condition as compared to debonded implant-bone interface condition. The average bone deformation (0.133mm) for horseshoe-shaped CFR-PEEK (resembling MITCH PCR(TM) cup) was very close to that of the intact acetabulum (0.135mm) at comparable locations. A reduction in bone density of 21-50% was predicted within the acetabulum for the implant resembling Cambridge cup, having bonded interface. For debonded interface condition, bone density increase of ~55% was observed in the supero-posterior part of acetabulum, whereas bone density reductions were low (1-20%) in other locations. Bone density reductions were considerably less (2-4%) for horseshoe-shaped CFR-PEEK component. Moreover, an increase in bone density of 1-87% was predicted around the acetabulum. Compared to the horseshoe-shaped design, the hemispherical design exacerbated bone resorption. Results indicated that the thickness of the acetabular component played a crucial role in the implant induced bone adaptation. The horseshoe-shaped CFR-PEEK component of 3mm thickness seemed a better alternative bearing surface than other designs, with regard to strain shielding, bone deformation and bone remodelling.

Three-year prospective clinical and radiological results of a new flexible horseshoe acetabular cup

We report the three-year results of a new flexible, horseshoe-shaped acetabular cup, with a carbon fibre reinforced polyetheretherketone (CFR-PEEK) bearing surface. The 3 mm thick composite cup is designed to conserve acetabular bone stock and reproduce a near-physiological stress distribution to the adjacent bone. The cup is intended to articulate against a large diameter ceramic femoral head to produce a low-wear bearing couple that generates minimal wear debris. A prospective, two-centre clinical study of the MITCH PCR cup was started in January 2007, to verify its safety and performance. Twenty-five MITCH PCR cups were implanted by three surgeons. There were 12 men and 13 women, with a mean age of 67.9 years (range 57.4 to 74.9). The mean Oxford hip score improved from 19.6 (SD 7.5) preoperatively to 43.5 (SD 7) at 3 years. The mean Harris hip score improved from 52.9 (SD 7) to 91.4 (SD 13.8) and the Euroqol-5D score increased from 62.6 (SD 18.4) to 82.8 (SD 19). One revision of the acetabular cup was undertaken at 21 months for squeaking. This has been investigated and modification of the articular geometry has resolved the problem, on in-vitro testing. Radiological analysis showed good early osseointegration of the MITCH PCR cup. However at three years, five cases of acetabular component migration and calcar resorption were observed. Three patients have subsequently undergone revision of the acetabular cup, at 41, 42 and 50 months respectively. The cause of the osteolysis is the subject of on-going investigation.

Biotribological study of large diameter ceramic-on-CFR-PEEK hip joint including fluid uptake, wear and frictional heating

A novel material combination of a large diameter Biolox(®) Delta zirconia-toughened-alumina (ZTA) head and a pitch-based carbon fibre reinforced poly ether-ether-ketone (CFR-PEEK) MOTIS(®) cup has been studied. The acetabular cups were inclined at three angles and tested using Durham Hip Simulators. The different inclination angles used did not have a significant effect on the wear rates (ANOVA, p = 0.646). Averaged over all cups, the wear rates were calculated to be 0.551 ± 0.115 mm(3)/10(6) cycles and 0.493 ± 0.107 mm(3)/10(6) cycles taking into account two types of soak controls; loaded at room temperature and unloaded at 37 °C respectively. Averaged across all femoral heads, the wear rate was 0.243 ± 0.031 mm(3)/10(6) cycles. The temperature change of the lubricant caused by the frictional heat was measured in situ. Friction factors measured using the Durham Friction Simulator were lower for the worn CFR-PEEK cups compared with unworn. This correlated with the decreased surface roughness. Even though relatively high friction was observed in these hemispherical hard-on-soft bearings, the wear rate is encouragingly low.

Retention force of plastic clips on implant bars: a randomized controlled trial

OBJECTIVES

Retention of overdentures is important for patients' satisfaction. The study tested whether the clinical performance of retentive clips made of poly-ether-ether-ketone (PEEK) is superior to those made of poly-oxy-methylene (POM).

METHODS

A total of 30 patients received complete dentures with round bars (SFI-Bar) on two implants in a chairside technique. Two types of clip matrices (PEEK/POM) were used in a split-mouth technique. Retention forces were measured separately for both materials at baseline when the dentures were inserted and after 1, 3 and 6 months. The measurement was performed extraorally and intraorally by using a measuring stylus equipped, respectively, with an opposing matrix or bar part. Simultaneously, at each point in time the patient and the dentist judged the retention either to be good, or to be too high or too low. Statistical analysis involved performance of global non-parametric testing of dependence of retention force on time and material was performed with Brunner-Langer model; non-parametric 95% confidence intervals (CIs) were calculated.

RESULTS

At baseline the median force for POM matrices was 6.89N (95% CI: 6.50-8.21) and for PEEK matrices 7.17N (95% CI: 6.97-7.93). After 6 months, the retention of POM decreased to 5.53N (95% CI: 4.81-7.00) and of PEEK to 6.42N (95% CI: 5.15-7.51). The retention force changed significantly over time (P = 0.004) without differences between POM and PEEK (P = 0.135). No significant alteration of the retention force over time was measured at the bar (P = 0.289). Retention was estimated to be good with 90% at baseline and with 80% after 6 months, equally by patients and dentist.

CONCLUSIONS

POM as well as PEEK material fulfills the requirements of retentive clips on round bars.

Application of carbon fibers to biomaterials: a new era of nano-level control of carbon fibers after 30-years of development

Carbon fibers are state-of-the-art materials with properties that include being light weight, high strength, and chemically stable, and are applied in various fields including aeronautical science and space science. Investigation of applications of carbon fibers to biomaterials was started 30 or more years ago, and various products have been developed. Because the latest technological progress has realized nano-level control of carbon fibers, applications to biomaterials have also progressed to the age of nano-size. Carbon fibers with diameters in the nano-scale (carbon nanofibers) dramatically improve the functions of conventional biomaterials and make the development of new composite materials possible. Carbon nanofibers also open possibilities for new applications in regenerative medicine and cancer treatment. The first three-dimensional constructions with carbon nanofibers have been realized, and it has been found that the materials could be used as excellent scaffolding for bone tissue regeneration. In this critical review, we summarize the history of carbon fiber application to the biomaterials and describe future perspectives in the new age of nano-level control of carbon fibers (122 references).

Notch sensitivity of PEEK in monotonic tension

Poly(ether-ether-ketone) (PEEK) has been used as a load bearing orthopaedic implant material with clinical success. All of the orthopaedic applications contain stress concentrations (notches) in their design; however, little work has been done to examine the stress-strain behavior of PEEK in the presence of a notch. This work examines both the stress-strain behavior and the fracture behavior of neat PEEK in a uniaxial loaded condition, and in circumferentially grooved round bar specimens with different elastic stress concentration factors. It was found that the material shows ductile necking in the smooth condition and that this is almost completely suppressed in the notched conditions. Additionally, the deformation and fracture micromechanisms changed drastically, from one of plastic deformation and void coalescence to one dominated by crazing and brittle fast fracture. This change in mechanism was explained via Neuber's theory of stresses at a notch.

Wear studies on the likely performance of CFR-PEEK/CoCrMo for use as artificial joint bearing materials

It is well known that a reduction in the volume of wear produced by articulating surfaces in artificial joints is likely to result in a lower incidence of failure due to wear particle induced osteolysis. Therefore, new materials have been introduced in an effort to produce bearing surfaces with lower, more biologically acceptable wear. Polyetheretherketone (PEEK-OPTIMA) has been successfully used in a number of implant applications due to its combination of mechanical strength and biocompatibility. Pin-on-plate wear tests were performed on various combinations of PEEK-OPTIMA and carbon fibre reinforced PEEK-OPTIMA (CFR-PEEK) against various CoCrMo alloys to assess the potential of this material combination for use in orthopaedic implants. The PEEK/low carbon CoCrMo produced the highest wear. CFR-PEEK against high carbon or low carbon CoCrMo provided low wear factors. Pin-on-plate tests performed on ultra-high molecular weight polyethylene (UHMWPE) against CoCrMo (using comparable test conditions) have shown similar or higher wear than that found for CFR-PEEK/CoCrMo. This study gives confidence in the likelihood of this material combination performing well in orthopaedic applications.

Pitch-based carbon-fibre-reinforced poly (ether—ether—ketone) OPTIMA® assessed as a bearing material in a mobile bearing unicondylar knee joint

The introduction of unicondylar knee prostheses has allowed the preservation of the non-diseased compartment of the knee while replacing the diseased or damaged compartment. In an attempt to reduce the likelihood of aseptic loosening, new material combinations have been investigated within the laboratory. Tribological tests (friction, lubrication, and wear) were performed on metal-on-carbon-fibre-reinforced (CFR) poly (ether—ether—ketone) (PEEK) (pitch-based) mobile unicondylar knee prostheses up to 5×106 cycles. Both a loaded soak control and an unloaded soak control (both medial and lateral components) were used to compensate for weight change due to lubricant absorption. For this material combination the loaded soak control gave slightly lower wear for both the medial and the lateral components than did the unloaded soak control. The medial components gave higher steady state wear than the lateral components (1.70 mm3 per 106 cycles compared with 1.02 mm3 per 106 cycles with the loaded soak control). The results show that the CFR PEEK unicondylar knee joints performed well in these wear tests. They gave lower volumetric wear rates than conventional metal-on-ultra-high-molecular-weight polyethylene prostheses have given in the past when tested under similar conditions. The friction tests showed that, at physiological viscosities, these joints operated in the boundary—mixed-lubrication regime. The low wear produced by these joints seems to be a function of the material combination and not of the lubrication regime.