Introduction. It is well accepted that larger heads provide more stability in total hip arthroplasty. This is due to an increase in jump height providing increased resistance to subluxation. However, other implant parameters also contribute to the bearing's stability. Specifically, the liner's rim design and the centre of rotation relative to the liner's face. Both these features contribute to define the Cup Articular Arc Angle (CAAA). The CAAA describes the degree of dysplasia of the acetabular liner, and plays an important role in defining the jump height. The aim of this study was to determine the difference in jump height between bearing materials with a commonly used acetabular implant system. Methods. From 3D models of the Trinity acetabular implant system (Corin, UK), the CAAA was measured in CAD software (SolidWorks, Dassault Systems, France) for the ceramic, poly and modular dual mobility (DM) liners, for cup sizes 46mm to 64mm. The most commonly used bearing size was used in the analysis of each cup size. For the ceramic and poly liners, a 36mm bearing was used for cups 50mm and above. For the 46mm and 48mm cups, a 32mm bearing was used. The DM liners were modelled with the largest head size possible. Using a published equation, the jump height was calculated for each of the three bearing materials and each cup size. Cup inclination and anteversion were kept constant. Results. CAAA varied substantially between cup sizes and bearing materials. The mean CAAA for the ceramic, poly and DM bearings were 166°, 175° and 186°, respectively. Consequently, over the entire size range, the ceramic liners had the lowest mean jump height of 12.9mm. In comparison to the ceramic liner, there was a mean 10% increase in jump height when transitioning to a poly (14.2mm), and a further 30% increase when transitioning from a poly to the dual mobility bearing (18.5mm) [Fig.1]. However, the difference in jump heights between bearings was variable, and dependent on cup size. Discussion. It is well understood that increasing head size increases stability in THA. However, other implant design parameters contribute to stability. With this particular implant system, the poly bearing had a greater jump height than the ceramic for cup sizes 50mm and above. The DM bearing improved jump height over the ceramic and poly by a mean of 41% and 30%, respectively. In conclusion, different liners have different design features that affect jump height. Consequently, not all bearings of identical head size are the same. We encourage a dialogue with your implant provider to understand the differences in CAAA between cup sizes and bearing materials. For any figures or tables, please contact the authors directly

Introduction:. Backside wear has been previously reported through in-vitro and in-vivo to have a significant contribution to the total wear in rotating bearing TKRs. The present study investigated the contribution of backside wear to the total wear in the PFC Sigma rotating platform mobile bearing TKR. In addition, the wear results were compared to the computed wear rates of the PFC Sigma fixed bearing TKR, with two different bearing materials. Materials and Methods:. The commercially available PFC Sigma rotating platform mobile bearing and PFC Sigma fixed bearing total knee replacements, size 3 (DePuy, UK) were tested, with either conventional or moderately cross-linked (5 MRad) GUR1020 UHMWPE bearing materials. The computational wear model for the knee implants was based on the contact area and an independent experimentally determined non-dimensional wear coefficient [1,2,3]. The experimental wear test for the mobile bearing was force controlled using the ISO anterior-posterior force (ISO14243-1-2009). However, due to time limitation of the explicit simulation required to run the force controlled model, the simulation was run using the AP displacements taken from the experimental knee simulator which was run under the ISO AP force. The Sigma fixed bearing TKR was run under high level of anterior-posterior displacements (maximum of 10 mm). Results and Discussion:. The rotating platform bearing showed lower wear rates, compared to that of the PFC Sigma fixed bearing, for both conventional and moderately cross-linked UHMWPE bearing materials (Fig. 1). Moreover, the results showed a high contribution of backside wear to the total wear, approximately 1 mm. 3. /million cycles (∼30% of the total wear). The computational wear predictions were in good agreements with the clinical and experimental measurements [4,5]. Contrasting the effect of bearing material on wear prediction, introducing the moderately cross-linked UHMWPE as a bearing material reduced the predicted wear rates by approximately 1 mm. 3. /million cycles in rotating platform bearing, compared to more than 5 mm. 3. /million cycles in PFC fixed bearing TKR. This reduced effect of cross-linking on wear in mobile bearing was mainly attributed to the lower cross-shear ratios in these bearings, compared to fixed bearings, and the less dependency of wear in moderately cross-linked UHMWPE on the degree of cross-shear, compared to conventional UHMWPE. Decreasing the degree of cross-shear from higher values (Sigma curved insert, high kinematic) to lower ones (rotating platform bearing) changed the predicted wear rates from 8.7 to 3.3 and from 3.4 to 2.4 (mm. 3. /million cycles), for conventional and moderately cross-linked UHMWPE materials respectively (Fig. 2). Conclusion:. The modelling confirmed the previous experimental observations of very low wear with the rotating platform knee. The models also determined the level of wear from the backside of the rotating platform knee which was approximately 1 mm. 3. /million cycles. The fixed bearing knee with moderately cross linked polyethylene also showed low wear at approximately 3 mm. 3. /million cycles. These low wear rates were determined under high kinematic walking cycles conditions. Future work will consider additional conditions

Introduction. There is a demand for longer lasting arthroplasty implants driving the investigation of novel material combinations. PEEK has shown promise as an arthroplasty bearing material, with potentially relatively bio inert wear debris [1]. When coupled with an all-polyethylene tibial component this combination shows potential as a metal-free knee. In this study, the suitability of PEEK Optima® as an alternative to cobalt chrome for the femoral component of total knee replacements was assessed using experimental knee wear simulation under two kinematic conditions. Methods. Three cobalt chrome and three injection moulded PEEK Optima® (Invibio Biomaterial Solutions, UK) femoral components of similar geometry and surface roughness (mean surface roughness (Ra) ∼0.02µm) were coupled with all-polyethylene GUR1020 (conventional, unsterilised) tibial components in a 6 station ProSim knee simulator (Simulation Solutions, UK). 3 million cycles (MC) of wear simulation were carried out under intermediate kinematics (maximum anterior-posterior (AP) displacement 5mm) followed by 3MC under high kinematics (AP 10mm) [2] with 25% serum as the lubricant. The wear of the tibial component was assessed gravimetrically. At each measurement point, the surface roughness of the femoral components was determined using contacting profilometry and throughout testing, the bulk lubricant temperature was monitored close to the articulating surfaces. Statistical analysis was carried out using ANOVA, with significance at p<0.05. Results. Figure 1 shows the wear rate of the all-polyethylene tibial components. After 3MC of intermediate kinematics, the mean wear rate of UHMWPE articulating against cobalt chrome was 1.0±2.3mm3/MC and against PEEK was similar (p=0.06) 2.5±0.8mm3/MC. Scratches were apparent on the surface of the PEEK implant in the AP direction significantly (p<0.05) increasing mean surface roughness of the PEEK components (Table 1) compared to pre-test values. The surface topography of the cobalt chrome components (Table 2) was similar to pre-test measurements. Increasing AP displacement caused no significant increase in the wear of the tibial inserts against either material. Under intermediate kinematics, the mean bulk lubricant temperature was 28.0±0.7°C for cobalt chrome and significantly higher (p<0.001) for PEEK, 29.5±0.1°C; kinematic conditions had no effect on the lubricant temperature. Conclusions. This study showed a similar wear rate of all-polyethylene tibial components against PEEK and cobalt chrome femoral components of similar initial surface topography and geometry. Wear simulation with a higher AP displacement did not increase the wear of the polyethylene, in contrast to other designs of knee replacements, potentially due to the low conforming geometry of the implant [3]. The linear scratching on the surface of the PEEK implants did not increase the wear rate of the tibial components and the surface did not deteriorate further between 3 and 6 MC. A higher mean lubricant temperature was measured with PEEK femoral components, which was attributed to the higher friction of the PEEK-PE bearing couple. However it is not known whether this is clinically relevant or an artefact of the continuous running of the simulator. PEEK Optima® shows promise as the femoral component in a metal-free knee

Burroughs et al showed that frictional torque increases with increasing head size in a simple in vitro model and showed differences in frictional torque with different polyethylene materials [1]. Therefore, the purpose of this study was to evaluate the influence of bearing material and bearing size on the frictional torque of hip bearings utilizing a more physiologically relevant hip simulator model. A total of four hip bearing combinations (Crosslinked PE/CoCr, Conventional PE/CoCr, Crosslinked PE/Delta and Alumina /Alumina) with various bearing sizes were evaluated. The sizes tested in this study range from 22 mm to 44 mm; it is important to note that the study only evaluated bearing combinations (size and material combination) currently commercially available. A total of three samples per bearing combination were tested, with the exception of conventional PE, which included a total of 4 samples. A MTS hip joint simulator was used. All components were oriented anatomically with the femoral head mounted below on a rotating angled block which imparts a 23° biaxial rocking motion onto the head. Loading was held constant at each load level (500N, 1000N, 1500N, 2000N, 2450N) for at least two rotational cycles while all 3 axes of load and all 3 axes of moments were measured at 10 khz. Fresh Alpha Calf Fraction serum was utilized as a lubricant. Results show that frictional torque increases with the increase of head size regardless of head material for all polyethylene combinations (p > 0.05), as shown in Figure 1 and 2. However, results showed no change in frictional behavior for the Alumina/Alumina combination regardless of the bearing size. The results of this test did not show any significant difference between crosslinked PE and conventional PE materials for sizes 28 mm and 32 mm when paired against a CoCr head (p > 0.05) (Figure 3). The Alumina/Alumina bearing combination had the lowest frictional torque among all the bearing material combinations evaluated in this study. This data suggests that there is a strong correlation between increased head size and increased frictional torque (R. 2. = 0.6906, 0.8847) for the polyethylenes evaluated here regardless of head material. No correlation can be concluded for the Alumina /Alumina bearing combination (R. 2. = 0.0217). The combination of Alumina /Alumina seems to have the most favorable frictional properties. This data also suggests no effect on frictional properties regardless of the polyethylene material (crosslinked and conventional) for sizes 28 mm and 32 mm. The frictional torque values recorded in this study are different than those published by Burroughs et al [1]. This difference may be attributed to the testing methodology. The current study utilizes a hip simulator, which closely mimics the natural joint providing a more physiologically relevant model whereas the Burroughs et al study utilizes a single axis machine. It is important to understand that frictional behavior in hip bearings may be highly sensitive to bearing clearance, cup thickness, and stiffness, which may outweight the effect of head diameter. Further evaluation is necessary to isolate and investigate those parameters

Several options for high demand/high activity patients for bearings in THA exist. Each of them faces certain known and unknown risks of failure. There is a remarked trend to bigger diameter heads to reduce the incidence of dislocation for such patients. While combinations with hard-on-hard bearings have been used in such incidences, a Polyethylene (PE) option is desirable due to its less sensitivity to edge loading and price.

A highly crosslinked sequentially annealed PE of the 3rd generation was prepared by sequentially crosslinking with appropriate annealing steps with a cumulative dose of 90 kGy and subsequent gas plasma sterilization. The structure of this material was determined using TEM, DSC and SAXS. Free radicals and oxidation was determined by ESR and IR spectroscopy. Mechanical evaluation in the unaged and aged condition were performed by quasi-static, dynamic and functional dynamic tests in comparison with negative controls. Wear testing was performed by ball-on-plate tests and hip joint simulators. PE inserts of various internal diameters up to 44mm and thicknesses of 4-8mm in comparison with a historic inert gas irradiation sterilized PE as negative control. These tests have been carried out at 3 institutions using different set-up and protocols. To look at worst case scenarios the simulator testing was done in an impingement mode and fatigue tests of the thinnest components where performed in 2 different fatigue set-ups up to 10 million cycles.

The structure and crystallinity of the sequentially crosslinked PE were comparable to the controls. The radical concentration was reduced by more than 95% due to the sequential process employed and consequently the oxidation level after artificially aging remained at the level of untreated PE. 5 year storage data confirmed the stability of this polymer. All mechanical testing revealed the maintenance of the properties at the same level as the controls. The screening wear test revealed that the high sliding stress used in this set-up had no effect on the sequentially crosslinked PE even when aged, while the controls showed fatigue wear after a short time of testing.

The decrease in volumetric wear compared to a negative control (28 mm head size) was on average 90% in volumetric independent of the head size and thickness of the PE liner. This result was confirmed by the studies at 2 other institutions with a wear reduction of 86 and 95% respectively. Impingement increased the wear rate marginally, without causing any fractures or failures of the components. The analysis of the wear particles from the simulator studies showed a marked decrease in number with close similarity in appearance and morphology to that from the control tests. Fatigue testing even in a luxation model showed no negative effect on the impact on the rim after 10 million cycles also with the thinnest components.

Highly crosslinked, sequentially annealed PE from the perspective of tribological and fatigue testing can be used safely even in impingement and luxation situations. Other factors in the clinical usage of thin liners may play a role and need to be investigated further.

The use of rotating hinge (RH) prostheses for severe primary as well as revision arthroplasty is widely established. Aim of this study was to investigate long term results of a new RH prosthesis (EnduRo®, B Braun, Germany), which uses carbon-fiber reinforced poly-ether-ether-ketone (CFR PEEK) as a new bearing material, first time used in knee arthroplasty. Fifty-six consecutive patients, who received the EnduRo® RH prosthesis were included in this prospective study: 21 patients (37.5%) received the prosthesis as a primary total knee arthroplasty (TKA) and 35 patients (62.5%) underwent revision total knee arthroplasties (rTKA). Clinical and radiographic examinations were performed preoperatively as well as postoperatively after 3 and 12 months and annually thereafter. Min. Follow up was 7 and mean follow up 9,3 years. Clinical examination included Knee Society Score (KSS), Western Ontario and McMaster Osteoarthritis Index (WOMAC), Oxford Knee Score (OKS), and range of motion (ROM). Competing risk analysis was assessed for survival with respect to indication and failure mode. KSS, WOMAC, OKS, and ROM significantly improved from the preoperative to the follow up investigations (p < 0.0001). There was no difference in clinical outcome between the primary and the revision group. The overall cumulative incidence for revision for any reason was 23.6% and the cumulative incidence for complications associated with failure of the prothesis was 5.6% at 7 years, respectively. Complications occurred more frequently in the revision group (p = 0.002). The evaluated RH prosthesis provided reliable and durable results with a minimum follow-up of 7 years. Prosthesis survival was successful considering the complexity of cases. The use of this RH system in primary patients showed high survival rates. Long-term functional and clinical results proved to be satisfying in both revision and primary cases. No adverse events were associated with the new bearing material CFR-PEEK

Introduction. In an effort to provide a TKA bearing material that balances resistance to wear, mechanical failure and oxidation, manufacturers introduced antioxidant polyethylene. In many designs, this is accomplished through pre-blending the polymer with the antioxidant before consolidation and radiation crosslinking. This study reports the wear performance (in terms of thickness change) of a hindered phenol (PBHP) UHMWPE from analysis of an early series of knee retrievals and explores these questions: 1) What is early-time performance of this new bearing material? 2) Is there a difference in performance between fixed and mobile bearings in this design? 3) How does quantitative surface analysis help understand performance at the insert-tray modular interface?. Methods. A series of 100 consecutive Attune™ knee inserts (DePuy Synthes, Warsaw, IN) received at revision by an IRB approved retrieval laboratory between September 2014 and March 2019 were investigated. In vivo duration was 0–52 months. Both the fixed bearing design (n=74) and the rotating platform mobile bearing design (n=26) were included. Dimensional change was determined by measurement of each insert and compared to the as-manufactured dimensions, provided by the manufacturer. The insert-tray interfaces under the loaded bearing zones were analyzed with light interferometry using an optical surface profiler (NewView™ 7300, Zygo, Middlefield, CT). Statistical analyses to explore relationships between measured variables were conducted using SPSS. Results. Mean total through-thickness change of the inserts was 0.052 mm. Mean rate of thickness change for all inserts having in vivo duration > 12 months was 0.038 mm/year (fixed bearing 0.042, mobile bearing 0.029 mm/year). The rate of thickness change for all inserts showed a decreasing trend with duration that was not statistically significant, (rho -.244, p=.094); however, the mobile bearing cohort alone showed a significant decrease in thickness change rate with duration (rho= −.659; p=.014). Surface roughness (Sa) of the distal surface of the UHMWPE inserts under the bearing areas averaged 1.24 µm (range 0.12 – 8.53) and peak-to-valley height (PV) averaged 27.1 µm (range 4 – 95). Sa and PV both showed a decreasing trend with duration in vivo in the mobile bearing inserts, but that trend did not reach statistical significance (p= 0.05 criterion). Neither Sa nor PV showed correlation with measured thickness change. Discussion. This study indicates that the rate of thickness change of a relatively new antioxidant cross-linked bearing material is very similar to other reported wear rates of crosslinked knee inserts. Lower wear rate of mobile bearing inserts compared to fixed bearings also is consistent with earlier published studies. Direct comparison between quantitative thickness change and objective, quantitative surface metrology on the same series brings new information to the arena of measuring and reporting “wear” of UHMWPE and underscores the importance of the distinction between visual damage and actual thinning of the bearing. The systematic surface analysis of the modular interfaces showing that surface roughness (Sa) and total damage feature topography (PV) trend downward with in vivo duration of mobile bearings supports the hypothesis that relative motion at that interface may ‘polish out’ the surface topography over time. For any figures or tables, please contact authors directly

Introduction. The input mechanical properties of knee replacement bearing materials, such as elastic modulus and Poisson's ratio, significantly contribute to the accuracy of computational models. They should therefore be determined from independent experimental studies, under similar test conditions to the clinical and experimental conditions, to provide reliability to the models. In most cases, the reported values in the literature for the elastic modulus and Poisson's ratio of the bearing materials have been measured under tensile test conditions, in contrast to the compressive operating conditions of the total knee replacements (TKR). This study experimentally determined the elastic modulus and Poisson's ratio of conventional and moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under compressive test conditions. These material parameters will be inputs to future computational models of TKR. Materials/Methods. To determine the Poisson's ratio of the conventional and moderately cross-linked UHMWPE, contact areas of 12mm diameter cylindrical specimens of 10.2mm length were measured experimentally under a compressive displacement of 1mm, at a strain rate of 12mm/min that was held for 10minutes. A computational model was developed in Abaqus, 6.14–1, to simulate this experimental test assuming different values for the Poisson's ratio of the UHMWPE cylindrical specimens. The curve fitted relationship between the computationally predicted contact area and Poisson's ratio was used to calculate the Poisson's ratio of the UHMWPE specimens, using the experimentally measured contact areas. Using a similar approach, the equivalent elastic modulus of the UHMWPE was calculated using the computationally calculated curve fitted contact area-elastic modulus relationship, from the computational simulation of a ball-on-flat compression test, and the experimentally measured contact area from a ball-on-flat dynamic compression test. This experiment used 10mm thick UHMWPE flat specimens against a 63.5mm rigid ball, under a compressive dynamic sinusoidal loading of 250N average load, and 6000 cycles. The applied test conditions maintained the stress level within the reported range for the TKR. Results. The predicted maximum contact stress was 26 and 35 [MPa] for the conventional and moderately cross-linked UHMWPE respectively. The measured Poisson's ratio was 0.33±0.04 (mean ± 95% confidence interval (CI), n=5) and 0.32±0.08 (mean ± 95% CI, n=3) for conventional and moderately cross-linked UHMWPE respectively. The corresponding values for the equivalent elastic modulus were 365±31 and 553±51 [MPa] (mean ± 95% CI, n=3) respectively (Fig.1). Discussion. The Poisson's ratios and elastic moduli for the conventional and moderately cross-linked UHMWPE materials were more than 20% lower than values reported in literature that have been measured under tensile test conditions [1–3]. Computational wear models adopting mechanical properties of the bearing materials delivered under more realistic compressive loading conditions are more appropriate. Conclusion. The current study presented a reverse engineering approach to characterise the mechanical properties of conventional and moderately cross-linked UHMWPE for TKR bearing materials, under realistic compressive test conditions. The measured mechanical properties, were lower than that reported in literature under tensile loading conditions, and should be adopted in future computational models of TKR for a more realistic and robust virtual modelling platform

Background. The current use of a spherical prosthetic humeral head in total shoulder arthroplasty results in an imprecise restoration of the native geometry and improper placement of the center of rotation, maintained in a constant position, in comparison to the native head and regardless of glenoid component conformity. A radially-mismatched spherical head to allow gleno-humeral translation is a trade-off that decreases the contact area on the glenoid component, which may cause glenoid component wear. This finding suggests that the use of a non-spherical head with a more conforming glenoid component may reduce the risk of glenoid component wear by allowing gleno-humeral translation while increasing the contact area. A non-spherical prosthetic head more accurately replicates the head shape, rotational range of motion and gleno-humeral joint kinematics than a spherical prosthetic head, compared with the native humeral head. The combination of inversion of the bearing materials with the non-spherical configuration of the humeral head may thus decrease polyethylene wear. Aim of the present study is to evaluate in vitro wear behaviour of an all-polyethylene elliptical humeral head component against a metallic glenoid component in an anatomic configuration. Material and methods. The prosthetic components tested are from the Mirai. ®. Modular Shoulder System by Permedica S.p.A.. The prosthetic bearing components were tested in their anatomic configuration: the humeral head rubbing against the glenoid inlay, assembled over the glenoid base-plate. The glenoid insert is made of Ti6Al4V alloy coated with TiNbN. The glenoid insert, as the glenoid base-plate have the same shape which reproduce the native shape of the glenoid. Moreover, the glenoid insert has a concave articular surface described by two different radii on orthogonal planes. The vitamin E-blended UHMWPE humeral head is not spherical but elliptic-shaped with an articular surface described by two different profiles in sagittal and coronal plane. The component sizes combination tested have the greatest radial mismatches allowed between humeral head and glenoid insert. The test was performed up to 2.5 million of cycles applying a constant axial load of 756 N. Results. After 2.500.000 cycles the mean mass loss from the humeral head was 0.68 mg. The mean wear rate of the humeral head was 0.28 mg/Mc (SD 0.45 mg/Mc). The surface of the humeral heads showed an elliptical worn area with matt and polished areas with scratching. The surface of the TiNbN-coated glenoid insert counterparts did not show wear signs. Conclusion. The tested prosthetic humeral head has a non-spherical shape with an elliptical base and 2 different radii on sagittal and coronal plane. Also the tested glenoid insert has 2 different radii on sagittal and coronal planes. This components geometry leads to a radial mismatch between head and glenoid on sagittal and coronal planes. A different kinematics, allowing gleno-humeral translation while increasing the contact area, radial mismatch in different planes and the inversion of bearing materials may have a role in reducing component wear and may explain the extremely low wear rate found in the present study

Introduction. PEEK-OPTIMA™ has been considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacements. To better understand the tribology of UHMWPE-on-PEEK-OPTIMA™ and to find the most appropriate environmental conditions under which to test this novel bearing material combination, a series of tests under different protein lubricant concentrations at rig (∼24°C) and elevated temperature (∼35°C) were carried out in simple geometry wear and friction rigs. Under all conditions, the wear of UHMWPE-on-PEEK-OPTIMA™ was compared to UHMWPE-on-cobalt chrome (CoCr). Methods. The pins used were GUR1020 UHMWPE (conventional, non-sterile) and the plate material was either polished CoCr (Ra<0.01µm) or PEEK-OPTIMA (Ra∼0.03µm) provided by Invibio Ltd, UK. The wear simulation was carried out in a six station reciprocating rig. The kinematic conditions were consistent for all tests and reflected the average cross shear and contact pressure (3.2MPa) in a total knee replacement. Tests were carried out at either rig running temperature (∼24°C) or at elevated temperature (∼35°C) and in varying protein lubricant concentrations (0, 2, 5, 25 and 90%). Wear of the UHMWPE pins was determined by gravimetric analysis. The pin-on-plate friction rig study was carried out at rig temperature in 0, 2, 5, 25 and 90% serum and reflected the contact pressure used in the wear tests. Measurements were taken using a piezoelectric sensor and the steady state friction derived. At least 3 repeats were taken for each study, statistical analysis carried out using ANOVA with significance taken at p<0.05. Results and Discussion. The influence of protein lubricant concentration and temperature on the wear of UHMWPE was different for the two bearing couples tested. In low serum concentrations (≤5%), polymer transfer was evident on the surface of the plates suggesting insufficient boundary lubrication and a non-clinically relevant wear mechanism. In 25% serum at rig temperature, the wear factor of UHMWPE-on-PEEK-OPTIMA™ was similar (p>0.05) to UHMWPE-on-CoCr at 2.00×10. −7. ±1.08×10. −7. mm. 3. /Nm and 2.15×10. −7. ±7.44×10. −8. mm. 3. /Nm respectively. Increasing the temperature of the lubricant lowered the wear factor of UHMWPE-on-PEEK-OPTIMA (9.93×10. −8. ±2.96×10. −8. mm. 3. /Nm); there was no influence of temperature on UHMWPE-on-CoCr (1.87×10. −7. ±6.14×10. −8. mm. 3. /Nm). The lower wear rate of the all-polymer couple was attributed to the elevated test temperature coupled with the higher friction of the all-polymer combination causing protein in the lubricant to come out of solution, adhere to the articulating surfaces and protect them from wear. In high serum concentrations (90%), protein deposition was visible on the surface of the plates and protein precipitation was visible in the lubricant. Under all protein lubricant concentrations, the coefficient of friction was higher for the all-polymer bearing couple than for UHMWPE-on-CoCr, at serum concentrations ≥2%, this difference was significant (p>0.05). Conclusion. Environmental conditions such as protein concentration and lubricant temperature influence wear and the effects of these variables can differ for different material combinations. For UHMWPE-on-PEEK-OPTIMA™, testing in 25% bovine serum at rig temperature minimised test artefacts such as polymer transfer, protein deposition and protein precipitation which suggests that these may be appropriate test conditions for this material combination

Hemiarthroplasty is a common procedure that is an attractive alternative to total arthroplasty because it conserves natural tissue, allows for quicker recovery, and has a lower cost. One significant issue with hemiarthroplasties is that they lead to accelerated wear of the opposing native cartilage, likely due to the high stiffness of the implant. The purpose of this study was to investigate the range of currently available biomaterials for hemiarthroplasty applications. We employed a finite-element (FE) model of a radial head implant against the native capitellum as our joint model. The FE model was developed in ABAQUS v6.14 (Dassault Systèmes Simulia Corp., Providence, RI, USA). A solid axisymmetric concave implant with seven different materials and the native radial head were evaluated, six modelled as elastic materials with different Young's moduli (E) and Poisson's Ratios (ν), and one modelled as a Mooney-Rivlin hyperelastic material. The materials investigated were CoCr (E=230 GPa, ν = 0.3), PEEK (E=3.7 GPa, ν = 0.36), HDPE (E=2.7 GPa, ν = 0.42), UHMWPE (E=0.69 GPa, ν = 0.49), Bionate 75D (E=0.288 GPa, ν = 0.39), Bionate 55D (E=0.039 GPa, ν = 0.45), and Bionate 80A (modelled as a Mooney-Rivlin hyperelastic material). A load of 100 N was applied to the radius through the center of rotation representing a typical load through the radius. The variable of interest was articular contact stress on the capitellum. The CoCr implant had a maximum contact stress over 114% higher than the native radial head. By changing the material to lower the stiffness of the implant, the maximum contact stress was 24%, 70%, 105%, 111%, 113%, and 113% higher than the native radial head for Bionate 80A, Bionate 55D, Bionate 75D, UHMWPE, HDPE, and PEEK respectively. This work shows that lowering implant stiffness can reduce the contact stress on cartilage in hemiarthroplasty implants. By changing the material below a Young's modulus of ∼100 MPa elevated stresses on the capitellum can be markedly reduced and hence potentially reduce or prevent degenerative changes of the native articulating cartilage. Low stiffness implant materials are not a novel concept, but to date there have been few that investigate materials (such as Bionate) as a potential load bearing material for implant applications. Further work is required to assess the efficacy of these materials for articular bearing applications

Introduction. The complex process of inflammation and osteolysis due to wear particles still is not understood in detail. So far, Ultra-high-molecular-weight-polyethylene (UHMWPE) is the bearing material of choice in knee arthroplasty and revision knee arthroplasty, but there is a growing demand for alternative bearing materials with improved wear properties. Lately, increasing interest developed in the use of natural and carbon-fiber-reinforced-poly-ether-ether-ketones (CFR-PEEK). While there is a lack of data concerning the effects of CFR-PEEK particles on human tissue, the effects of such wear debris in vitro and in animal studies is controversially discussed. The aim of this study was to analyze human tissue containing CFR-PEEK as well as UHMWPE wear debris. The authors hypothesized no difference between the used biomaterials because of similar size parameters of the wear particles in a prior knee simulator study of this implant. Methods and Materials. Synovial tissue samples of 10 patients while knee revision surgery of a rotating hinge knee implant design (Enduro®, Aesculap, Germany) were achieved. The tibial inserts of this design were made from UHMWPE (GUR 1020), whereas the bushings and flanges are made of CFR-PEEK containing 30% polyacrylonitrile (PAN) based carbon fibers (PEEK-Optima LT1, Invibio Ltd. Thornton-Cleveleys, UK). In a prior in vitro test most of the released CFR-PEEK particles were in a size range between 0.1 and 2μm. The implant survival until revision surgery was 22 (2.5–48 min.-max.) months. As a control synovial tissue out of a patient also got knee revision surgery without any PEEK components. The tissue was fixed with 4% paraformaldehyde, embedded in paraffin, sliced into 2 µm thick sections. stained with hematoxylin and eosin in a standard process. A modified panoptical staining (preincubation in propylenglycol; >3h; 35°C) was also done which stained the UHMWPE particles turquoise. The study was approved by the ethics committee of the local university. Results. Overall, histologically a “wear-type” reaction was seen in the testing and the control group similar as described for other materials in the common literature. In all samples of the testing group the UHMWPE particles were scattered in the tissue similar to the control. Larger UHWMPE particles were incorporated in giant cells. In contrast to these findings, CFR-PEEK particles were not scattered in tissue but located only as conglomerates. In addition, these conglomerates have been found exclusively near to or in vessels. Furthermore, CFR-PEEK particles were collected in macrophages, no CFR-PEEK particles were seen in giant cells. In conclusion, the hypothesis has to be rejected. Interestingely, different behaviour of UHMWPE and PEEK particles has been found in human synovial tissue. This aspect needs further investigation concerning the cytokine expression and also the surface texture of particles. Acknowledgement. This study was supported by Aesculap, Germany

It has been seven years since silicon nitride (Si. 3. N. 4. ) was first proposed as a new bearing material for total hip arthroplasty [1]. Although its introduction into this application has been hampered by regulatory and clinical hurdles, it remains a strong candidate for advancing the state of care in patients undergoing joint replacement. Si. 3. N. 4. has a distinctive set of properties, such as high strength and fracture toughness, inherent phase stability, low wear, scratch resistance, biocompatibility, hydrophilicity, excellent radiographic imaging, and bacterial resistance, many of which are not fully realized with other bioceramics. This combination of properties is desirable for demanding structural implants in the hip, knee and other total joints. Of foremost concern to clinicians is the wear behavior of any new or novel bearing material. Minimization of wear debris and prevention of corresponding osteolytic lesions are essential regardless of whether the artificial implant is articulating against itself, a metallic or polymeric counterpart. In this regard, Si. 3. N. 4. may have a unique advantage. Other bearing couples rely solely on the presence of a biologic lubricating film to minimize erosive wear. However, Si. 3. N. 4. forms a tribochemical film between the articulation surfaces consisting of silicon diimide Si(NH). 2. , silicic acid Si(OH). 4. , and ammonia groups NH. 3. , NH. 4. OH. Depending upon the bearing couple, this tribochemical film generally produces low friction. It is self-replenishing and resorbable, leading to the minimization of wear debris within the joint capsule. In this paper, we will review the essential physical, mechanical, and surface chemistry of Si. 3. N. 4. , and contrast these properties with other available bioceramics. Results from hip simulator testing of Si. 3. N. 4. femoral heads on conventional and highly cross-linked polyethylene will be presented and discussed. Data will demonstrate that various Si. 3. N. 4. bearing couples have wear comparable to other bioceramics. Microscopy and spectroscopic examinations of surfaces will provide a view of the surface stoichiometry and chemical stability of Si. 3. N. 4. in comparison to other bioceramics. Laboratory friction tests will be reported, which show that the tribochemistry of the lubricating film generated by Si. 3. N. 4. favors the use of highly cross-linked polyethylene as a counterface material. Overall results will demonstrate that silicon nitride is poised to become a new generation biomaterial for total joint arthroplasty

Introduction. Silicon nitride (SiN) is a recently introduced bearing material for THR that has shown potential in its bulk form and as a coating material on cobalt-chromium (CoCr) substrates. Previous studies have shown that SiN has low friction characteristics, low wear rates and high mechanical strength. Moreover, it has been shown to have osseointegration properties. However, there is limited evidence to support its biocompatibility as an implant material. The aim of this study was to investigate the responses of peripheral blood mononuclear cells (PBMNCs) isolated from healthy human volunteers and U937 human histiocytes (U937s) to SiN nanoparticles and CoCr wear particles. Methods. SiN nanopowder (<50nm, Sigma UK) and CoCr wear particles (nanoscale, generated in a multidirectional pin-on-plate reciprocator) were heat-treated for 4 h at 180°C and dispersed by sonication for 10 min prior to their use in cell culture experiments. Whole peripheral blood was collected from healthy donors (ethics approval BIOSCI 10–108, University of Leeds). The PBMNCs were isolated using Lymphoprep® as a density gradient medium and incubated for 24 h in 5% (v/v) CO2at 37°C to allow attachment of mononuclear phagocytes. SiN and CoCr particles were then added to the phagocytes at a volume concentration of 50 µm3 particles per cell and cultured for 24 h in RPMI-1640 culture medium in 5% (v/v) CO2 at 37°C. Cells alone were used as a negative control and lipopolysaccharide (LPS; 200ng/ml) was used as a positive control. Cell viability was measured after 24 h by ATPLite assay and tumour necrosis factor alpha (TNF-α) release was measured by sandwich ELISA. U937s were co-cultured with SiN and CoCr particles at doses of 0.05, 0.5, 5 and 50 µm3 particles per cell for 24h in 5% (v/v) CO2 at 37 C. Cells alone were used as a negative control and camptothecin (2 µg/ml) was used as a positive control. Cell viability was measured after 0, 1, 3, 6 and 9 days. Results from cell viability assays and TNF-α response were expressed as mean ±95% confidence limits and the data was analysed using one-way ANOVA and Tukey-Kramer post-hoc analysis. Results and Discussion. At a high volume concentration of particles (50µm3 per cell), SiN did not affect the viability of PBMNCs, while CoCr significantly reduced the viability over a 24 h period [Figure 1A]. Similarly, SiN particles had no effect on the viability of U937s up to 9 days with a range of particle doses (0.05–50 µm3 per cell) [Figure 2A]. In contrast, CoCr particles significantly reduced the viability of U937s after 6 days [Figure 2B]. Additionally, CoCr particles caused significantly elevated levels of pro-inflammatory cytokine TNF-α, whereas no inflammation was associated with SiN particles [Figure 1B]. Conclusion. This study has demonstrated the in-vitro biocompatibility of SiN nanoparticles. Therefore, SiN is a promising orthopaedic bearing material not only due to its suitable mechanical and tribological properties, but also due to its biocompatibility. Acknowledgements. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. GA-310477 LifeLongJoints

To prevent aseptic loosening resulting from osteolysis induced by polyethylene (PE) wear particles in THA, it is necessary to develop a high wear-resistance bearing material. We have investigated the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface. High wear-resistance of PMPC-grafted surface has been revealed in the hip simulator wear test of 20 million cycles. Additionaly, in THA, oxidation degradation induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization is also regarded as serious issue. Recently, gas plasma (GP) sterilization has been used as a less residual radical sterilization method. In this study, we ask a question: the GP sterilization would affect to PMPC surface and/or PE substrate? Hence, we investigated surface chemical, wear, mechanical, physical and oxidation properties of GP sterilized PMPC-grafted highly cross-linked PE (CLPE). GP-sterilized CLPE and PMPC-grafted CLPE (CLPE (GP) and PMPC-CLPE (GP), respectively; GUR 1020 resin, 75 kGy irradiation), and 25 kGy-gamma-sterilized PMPC-grafted CLPE (PMPC-CLPE (g); GUR 1020 resin, 50 kGy irradiation) were evaluated. Surface property of PMPC layer was evaluated by X-ray photoelectron spectroscopy (XPS), fourier-transform infrared (FT-IR) spectroscopy, fluorescence microscope and cross-sectional transmission electron microscope (TEM) observations. Wettability and lubrication of the PMPC-CLPE surface were evaluated by static water contact angle measurement and ball-on-plate friction test, respectively. Wear properties of the acetabular cups were examined by using hip simulator in the combination with Co-Cr-Mo femoral heads. To evaluate the GP sterilization effect to the CLPE substrate, tensile test, izod impact test, small punch test, gel content, residual radical concentration and oxidation degradation were conducted. Oxidation degradation was evaluated as oxidation index by using a FT-IR spectroscopy. By the XPS and FT-IR measurements, phosphorus peak and P-O peak attributed to grafted PMPC were observed, respectively. Uniform PMPC layer (100–200 nm thick) was observed on both surfaces of PMPC-CLPE (g) and PMPC-CLPE (GP) [Fig. 1]. Water contact angle of CLPE (GP) was almost 100 degree, while those for PMPC-CLPE (g) and PMPC-CLPE (GP) decreased dramatically to almost 10 degree. Dynamic coefficient of friction of PMPC-CLPE (g) and PMPC-CLPE (GP) was lower than that for CLPE (GP). In the hip simulator wear test, PMPC-CLPE (g) and PMPC-CLPE (GP) cups showed significantly lower amount of wear than that of CLPE (GP) [Fig. 2]. The number of the wear particles was extremely less in PMPC-CLPE (g) and PMPC-CLPE (GP), though the size was not different of all cases. Water thin film might be formed at the grafted PMPC layer, which acted as significantly efficient lubricant. There was no difference in the mechanical and physical properties among three groups. Oxidation index for PMPC-CLPE (GP) after acceleration of aging was lower than that of PMPC-CLPE (g). The GP sterilization might affect only to the PMPC-grafted surface, whereas gamma irradiation affects also to the PE substrate. From these results, the PMPC-CLPE (GP) is expected to be one of the great bearing materials having not only high-wear resistance but also high-oxidation resistance, which could give further longevity of implantation

Introduction. Reverse total shoulder arthroplasty (RTSA) is a semi-constrained joint replacement with an articulating cobalt-chromium glenosphere and ultra-high molecular weight polyethylene (PE). Because of its limited load bearing, surgeons and implant manufacturers have not elicited the use of highly cross-linked PE in the shoulder, and to date have not considered excessive PE wear in the reverse shoulder a primary concern. As the number of shoulder procedures is expected to grow exponentially in the next decade, however, it is important to evaluate how new designs and bearing materials interact and to have an understanding of what is normal in well-functioning joint replacements. Currently, no in vivo investigation into RTSA PE wear has been conducted, with limited retrieval and simulation studies. In vitro and in silico studies demonstrate a large range in expected wear rates, from 14.3 mm. 3. /million cycles (MC) to 126 mm. 3. /MC, with no obvious relationship between wear rate and polyethylene diameter. The purpose of this study is to evaluate, for the first time, both volumetric and linear wear rates in reverse shoulder patients, with a minimum six-year follow-up using stereo radiographic techniques. Methods. To date, seven patients with a self-reported well-functioning Aequalis Reversed II (Wright Medical Group, Edina, MN, USA) RTSA implant system have been imaged (mean years from surgery = 7.0, range = 6.2 to 9). Using stereo radiographs, patients were imaged at the extents of their range of motion in internal and external rotation, lateral abduction, forward flexion, and with their arm at the side. Multiple arm positions were used to account for the multiple wear vectors associated with activities of daily living and the shoulder's six degrees of motion. Using proprietary software, the position and orientation of the polyethylene and glenosphere components were identified and their transformation matrices recorded. These transformation matrices were then applied to the CAD models of each component, respectively, and the apparent intersection of the glenosphere into the PE recorded. Using previously validated in-house software, volumetric and maximum linear wear depth measurements were obtained. Linear regression was used to identify wear rates. Results. The volumetric and linear wear rates for the 36 mm PE liners (n = 5) were 39 mm. 3. /y (r. 2. = 0.86, range = 24 to 42 mm. 3. /y) and 0.09 mm/y (r. 2. = 0.96, range = 0.08 to 0.11 mm/y), respectively. Only two patients with 42 mm PE liners were evaluated. For these, volumetric and linear wear rates were 110 mm. 3. /y (r. 2. = 0.81, range = 83 to 145 mm. 3. /y) and 0.17 mm/y (r. 2. = 0.99, range = 1.12 to 1.15 mm/y), respectively. Conclusion. For the first time, PE wear was evaluated in the reverse shoulder in vivo. More patients are required for conclusive statements, but preliminary results suggest first order volumetric and linear wear rates within those predicted by simulation studies. It is interesting to note the increased wear with larger PE size, likely due to the increased contact area between congruent faces and the potential for increased sliding distance during arm motion

Introduction. Wear of polyethylene tibial inserts has been cited as being responsible for up to 25% of revision surgeries, imposing a very significant cost burden on the health care system and increasing patient risk. Accurate measurement of material loss from retrieved knee bearings presents difficult challenges because gravimetric methods are not useful with retrievals and unworn reference dimensions are often unavailable. Geometry and the local anatomy restrict in vivo radiographic wear analysis, and no large-scale analyses have illuminated long-term comparative wear rates and their dependence on design and patient factors. Our study of a large retrieval archive of knee inserts indicates that abrasive/adhesive wear of polyethylene inserts, both on the articular surface and on the backside of modular knees is an important contributor to wear, generation of debris and integrity of locking geometry. The objective of the current study is to quantify wear performance of tibial inserts in a large archive of retrieved knees of different designs. By assessing wear in a large and diverse series, the goal is to discern the effect on wear performance of a number of different factors: patient factors that might help guide treatment, knee design factors and bearing material factors that may inform a surgeon's choice from among the array of arthroplasty device options. Methods. An IRB approved retrieval database was queried for TKA designs implanted between 1997 and 2017. 1385 devices from 5 TKA designs were evaluated. Damage was ranked according to Hood's method, oxidation was determined through FTIR, and wear was determined through direct measurement of retrieved inserts using a previously established protocol. Design features (e.g. materials, conformity, locking mechanisms, stabilization, etc.) and patient demographics (e.g. age, weight, BMI, etc.) were cataloged. Multivariate analysis was performed to isolate factors contributing to wear, oxidation, and damage. Results. Wear and oxidation were both found to scale with time in vivo in conventional and crosslinked polyethylene. Wear rate was also found to scale with time in vivo, but was not found to be a function of oxidation. Regression shows patient age and female sex to correlate negatively with wear rate. Polished trays, crosslinked polyethylene, and constrained knee designs are all correlated with decreased wear rates. Discussion. While this study indicates that loosening and infection are predominant causes for TKA revision, wear related failure remains common. We believe this to be the largest existing comparative study of modern TKA wear rates. Insert wear is shown to correlate with several patient factors. Wear performance also varies significantly between knee designs, polyethylene material choice and tray surface finish. When compared to a historical standard for knee wear rates, all designs evaluated in the current study exhibited significant improvements in wear rates. Retrieval analysis can provide insight into implant and patient related factors that contribute to knee wear, with the goal of improving patient outcomes and best matching design decisions to patient populations

In joint prostheses using ultra-high molecular weight polyethylene (UHMWPE) as bearing material, wear problems are not yet completely solved under severe conditions in various daily activities, although efficacious treatments such as crosslinking, addition of vitamin E and the grafting of phospholipid polymer improved the wear properties. In contrast, in healthy natural synovial joints possessing articular cartilage as biphasic bearing material lubricated with synovial fluid, minimal wear with extremely low friction has been maintained for a whole life. Therefore, the joint prosthesis with artificial hydrogel cartilage with similar properties to articular cartilage is expected to show superior tribological functions with very low friction and infinitesimal wear if the appropriate lubrication mechanism is actualized. In this study, the effectiveness of biphasic lubrication mechanism in hydrogel through significant load support by fluid phase is evaluated in finite element (FE) analysis for reciprocating motion. As biocompatible artificial hydrogel cartilage materials, two kinds of poly (vinyl alcohol) (PVA) hydrogels were prepared by the repeated freezing-thawing method and the cast-drying method, which are physically crosslinked with hydrogen bonding but different in structure and mechanical properties. To evaluate these time dependent behaviors of load-support ratio of fluid/solid phases and friction, two-dimensional biphasic FE analysis for cylindrical PVA hydrogel cartilage as 1.5 mm thick soft layer and radius of 5 mm was conducted under continuous loading of 0.2 N/mm by impermeable rigid plate in reciprocating motion in Fig. 1. The sliding speed is 4 mm/s for stroke of 8 mm at period of 4 s. A commercial package ABAQUS (6.8–4), which was appropriately evaluated for the biphasic FE analyses, was used in this study. The biphasic tissue was modeled by CPE4RP (four-node bilinear displacement and pore pressure, reduced integration with hour glass control) elements. The mechanical properties such as permeability, Young's modulus and Poisson ratio were estimated by curve fitting to stress relaxation behaviors in compression test. As indicated in Fig. 2, it is worth noting that the cast-drying PVA shows significant interstitial fluid pressurization compared with a repeated freezing-thawing PVA hydrogel at 292 s after start-up, where coefficient of friction for solid-to-solid was assumed as 0.2. Changes in friction for PVA hydrogels in reciprocating motion were estimated as shown in Fig. 3. In spite of high friction (0.2) for solid-to-solid, cast-drying PVA brought the gradual decreasing in friction, probably due to rising of load-support ratio by fluid phase from initial 74% to 80%. In human body, lubricating constituents in synovial fluids such as hyaluronic acid, proteins, glycoproteins and phospholipids can reduce the coefficient of friction for solid-to-solid. As suggested for low coefficient of friction for solid-to-solid as 0.01 in Fig. 3, rubbing friction is expected to be reduced to significantly low level. As described above, the effective biphasic lubrication can sustain low friction level and minimal wear in synergistic action with soft-elastohydrodynamic lubrication, hydration lubrication and boundary lubrication as a similar mechanism to natural cartilage in various daily activities. These results indicate the usefulness of artificial hydrogel cartilage for longer durability in joint prostheses for clinical application

As reverse total shoulder arthroplasty (RTSA) systems expand with longer durations in vivo, so does the concern and potential complications of wear, debris and osteolysis. Despite some other profound attempts, no wear testing method has stood out to compare implants across systems and labs. The main reasons may have been the diverse sources of forces and motions used in testing, widely different wear amounts which resulted and the general lack of dedicated shoulder simulators. To add a dedicated shoulder simulator to hip and knee simulators would burden the resources of any testing lab. In this study we propose a shoulder wear test method which addresses the above. Harnessing the wealth of force-motion data from telemetrized shoulder implants from the Bergman's group in Berlin, we synthesized their results to devise a wholistic multi-axes simulation regime for reverse shoulders. The alignment and motions of the humeral cup and the glenosphere were kept anatomically correct (relative to each other) and yielded a physiologically realistic wear-inducing articulation. However, we opted for a very unusual installation/orientation of the whole implant system to allow a twelve station AMTI (hip) simulator to be adapted for this study. The shoulder constructs were aligned with novel fixtures such that the machine's vertical compressive force mimicked the average forces of the shoulder found from the in vivo telemetry data in magnitude and nominal direction. Aligned thus, a patient with a shoulder installed would neither stand, nor lie down, but be oriented in a composite angle relative the simulator original axes. Each anatomic shoulder motion would be achieved by unique computed combinations of the three simulator motion actuators, none of which would be aligned anatomically for the shoulder on its own. The maximum ranges of cyclic shoulder motion achieved with the constraints of the simulator were 38°–79° of forward elevation repeated in two separate (15°and 45°) elevation planes. The change of elevation plane inherently involved abduction-adduction motion, and simultaneously also involved variation of internal-external rotation within a 57° range. Each elevation rise (twice per cycle) was also accompanied by a sinusoidally rising and falling compressive load in the range 50N–1700N. The test method was tested (!) by simulating for 2.5 million of the above (double-elevation) cycles and gravimetrically measuring wear of twelve 36 mm size RTSA systems. We compared six systems having vitamin E-infused highly cross-linked polyethylene bearings (100 kGy radiation) to six controls with a medium cross-linked polyethylene of half the radiation dose. Significant wear resulted for the control bearing material (average 17.9 ± 0.851 mg/MC) which was no less than many hips and knees. Multiply (and statistically significantly, p < 0.001) less average wear (3.42 ± 0.22 mg/MC) resulted for the highly cross linked bearings. The above demonstrated the effectiveness of the test method. Significant wear resulted under physiologically realistic cyclic motion and forces with strong discrimination between two systems whose bearing materials were known to be different in resilience to wear. Using novel fixtures and unusual orientation to utilize a standard commercially available joint simulator promises efficacy of the test method and utility across different labs

The objective of this study was to determine whether the bearing surface is a risk factor for revision after late dislocation in total hip arthroplasty (THA). Data from primary THAs were extracted from the New Zealand Joint Registry over a 13-year period. The mean age of patients was 68.9 years; 53.2% were female. The surgical approach used was posterior in 66% of THAs, lateral in 29% and anterior in 5%. There were 53,331 (65.1%) metal-on-polyethylene THAs, 14,093 (17.2%) ceramic-on-polyethylene, 8,177 (10.0%) ceramic-on-ceramic, 461 (0.5%) ceramic-on-metal, 5910, and (7.2%) metal-on-metal. The primary endpoint was late revision for dislocation, with ‘late’ defined as greater than one year post-operatively. 73,386 hips were available for analysis. The overall revision rate was 4.3% (3,130 THAs), 1.1% (836) were revised for dislocation. Only 0.65% (470) hips were revised for dislocation after the first post-operative year. The unadjusted hazard ratios (HR) showed significantly higher rates of revision for dislocation in ceramic-on-polyethylene (HR 2.48; p=0.001) and metal-on-polyethylene (HR 2.00; 95% p =0.007) compared to ceramic-on-ceramic. However, when adjusted for head size, age and surgical approach, only ceramic-on-polyethylene (HR 2.10; p=0.021) maintained a significantly higher rate of revision, whereas metal-on-polyethylene approached significance (HR 1.76; 95% p = 0.075). In New Zealand, dislocation is the most common reason for revision, ahead of aseptic loosening of the acetabular component. The relationships between bearing materials and risk of revision for late dislocation is controversial. However, in this study ceramic-on-ceramic shows lower risk rates for revision than other bearing surface combinations. Low wear and less debris, limited peri-articular inflammatory reaction and an healthy fibrotic pseudo-capsule are potential factors determining long-term stability of the hip joint