Methods

Retrieved total ankle replacements (n=11) from consecutive revision surgeries carried out at Chapel Allerton Hospital, Leeds between August 2012 and January 2014, were collected for study at the University of Leeds, under an NRES approved procedure (09/H1307/60).

The bearing surfaces of the samples were visually inspected for evidence of damage and wear. The bearing surfaces between the tibial component and the flat surface of the polyethylene insert were then examined using a scanning white light interferometer (NP Flex, Bruker, USA). It was not possible to characterise the talar bearing surface or the inferior polyethylene surface at this stage through interferometry due to the curvature of the surface. The components were aligned and five sections on each of the surfaces measured. These sections represented; anterior-medial, anterior-lateral, posterior-medial, posterior lateral and central regions of the bearing surfaces. 3D roughness values were recorded, and the mean 3D surface roughness compared between implants. Measurements were taken on the medial and lateral aspects of the bearing surfaces to investigate whether damage was location specific. A coefficient of determination was calculated to assess the relationship between implantation time and surface roughness.

Methods

The wear of GUR1020 UHMWPE pins articulating against PEEK plates (R_a ∼0.06µm) was compared to highly polished cobalt chrome plates (R_a <0.01µm) in a 6-station multi-axial pin-on-plate rig using kinematics to replicate those in total knee arthroplasty. Tests were carried out at either ∼20°C or ∼36°C and wear was investigated under varying concentrations of bovine serum (0, 25 or 90%). Studies were carried out for 1 Million cycles with wear of the UHMWPE pins assessed gravimetrically using unloaded soak controls to compensate for moisture uptake.

Statistical analysis was carried out using ANOVA with significance taken at p<0.05.

Methods

The following materials were studied: unfilled PEEK (OPTIMA, Invibio) and CFR-PEEK (MOTIS, Invibio) against either high carbon (HC) CoCr or Biolox Delta ceramic plates. The comparative control material was a moderately cross-linked PE (Marathon, DePuy Synthes).

A simple geometry wear study was undertaken. A rotational motion of ±30° across a sliding distance of ±28 mm (cross shear of 0.087), and contact pressures of 1.6 or 4 MPa were applied. The lubricant was 25% (v/v) bovine serum and the wear test was conducted for 1 million cycles at 1 Hz. Wear was assessed gravimetrically. A validated soak control method was used to adjust for serum absorption-induced mass changes during the wear test. Surface profilometry was assessed pre and post wear test.

Methods

Third body particles of polymerised GMV bone cement and the bone void fillers; OsteoSet (with tobramycin), Stimulan and Stimulan+ (with vancomycin and tobramycin) (provided by Biocomposites Ltd.) were trapped between an UHMWPE pin and a highly polished cobalt chrome plate. A load of 120N was applied to the pin and using an Instron materials testing machine, the plate was pulled beneath the pin to recreate third body damage [1]. The resulting surface topography of the plate was analysed using white light interferometry (Bruker NPFLEX). Pin on plate wear tests of GUR 1020 UHMWPE pins were carried out against the plates perpendicular to the direction of damage for 500,000 cycles in 25% bovine serum using a 6-station multi-axial reciprocating rig under conditions to replicate the kinematics in total knee replacement. Wear of the pins was determined by gravimetric analysis and results were compared to negative (highly polished) control plates and positive controls scratched with a diamond stylus (lip height 2µm). Statistical analysis was carried out using one-way ANOVA with significance taken at p<0.05.

Materials and Methods

Ceramic-on-ceramic bearings where the ceramic liner was inserted into a titanium alloy cup (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes, UK) were tested on the six-station Leeds II hip simulator. The first test was run with the cups inclined at an angle equivalent, clinically, to 45° (n=6) and the second test was run with the cups inclined at an angle equivalent, clinically, to 65° (n=6). A standard gait cycle was run. A fixed surgical translational mal-positioning of 4mm between the centres of rotations of the head and the cup in the medial/lateral axis was applied on all stations. Both tests ran for three million cycles each. The lubricant used was 25% new-born calf serum. Wear was assessed gravimetrically using a microbalance (XP205, Mettler Toledo, UK) and geometrically using a coordinate measuring machine (CMM, Legex 322, Mitutoyo, UK). Statistical analysis was done using one way ANOVA with significance taken at p<0.05.

Methods

Two novel materials were studied as an alternative to polyethylene as the tibial bearing surface in a TKR configuration using a Cobalt chrome femoral bearing in current clinical use. Six right Sigma CR fixed bearing TKRs (DePuy Synthes, UK) were paired with either PEEK or CFR-PEEK custom-made flat inserts (Invibio, UK) in a Prosim knee simulator (Simulator Solutions, UK). The tibial inserts were 14mm thick, to give an equivalent thickness to existing insert designs. A flat geometry was selected as this has previously been shown to yield low wear in polyethylene bearings [3]. The tests were conducted under High Kinematics, with anterior-posterior and internal-external displacement control [4]. Tests were conducted for three million cycles, lubricated with 25% bovine serum, with wear assessed gravimetrically at 1 and 3Mc.

Images of the wear scars were recorded at completion of the study. Visual inspection of the tibial inserts was used to identify regions of wear damage for SEM analysis (EVO MA15-Smart SEM, Zeiss, Germany)

Introduction

Wear of total knee replacement (TKR) continues to be a significant factor in the clinical performance of the implants. Historically, failure due to delamination and fatigue directed implant design towards more conforming implants to reduce contact stress. However, the new generations of more oxidatively-stable polyethylene have improved the long-term mechanical properties of the material, and therefore allowed more flexibility in the bearing design. The purpose of this study was to investigate the effect of insert conformity and material on the wear performance of a fixed bearing total knee replacement through experimental simulation.

Introduction

Larger head size in total hip replacement theoretically provides increased range of motion and enhanced stability. However, there are potential clinical concerns regarding increased frictional torques with large diameter metal-on-metal bearings causing loosening of the acetabular cups and corrosion at the taper. The aim of this study was to determine the frictional torques of large diameter BIOLOX® delta ceramic-on-ceramic bearings.

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).

Materials and Methods

The components used in this study were zirconia-toughened-alumina ceramic-on-ceramic bearings (36 mm) inserted into titanium alloy acetabular cups (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes Joint Reconstruction, Leeds, UK). Six couples were tested for two million cycles under 0.5 mm dynamic microseparation conditions on the Leeds II hip joint simulator as described by Nevelos et al² and Stewart et al³ (Figure 1). Ten bearing couples were tested for two million cycles under microseparation conditions achieved in two different ways on the ProSim pneumatic hip joint simulator (SimSol, Stockport, UK). Two conditions were tested; condition (1)- the femoral head was left to completely separate (the vertical motion was controlled at 1 mm) causing it to contact the inferior rim of the acetabular cup before edge loading on the superior rim at heel strike (n = 5) and condition (2)- springs were placed below the plate holding the femoral head to control the tilt of the head laterally towards the rim of the acetabular cup as the negative pressure was applied (n = 5; Figure 1). Wear was assessed gravimetrically every million cycles using a microbalance (Mettler AT201, UK). Three-dimensional reconstructions of the wear area on the heads were obtained using a coordinate measuring machine (Legex 322, Mitutoyo, UK) and SR3D software (Tribosol, UK).

Methods

Six right Sigma CR fixed bearing TKRs (DePuy Synthes, Leeds, UK) with curved polyethylene inserts (GVF, GUR1020 UHMWPE) were tested in Prosim knee simulator (Simulator Solutions, UK). The femoral bearing was set up with the centre of rotation (CoR) on either on the distal radius of the implant (Distal CoR), as indicated by the device design, or according to the ISO specification (ISO CoR; ISO14243-3). The tests were conducted under ‘High Kinematics’ (2). It was necessary to reverse the direction of the anterior-posterior displacement for the tests conducted with the ISO centre of rotation to maintain the contact region within the insert surface (Reverse High Kinematics). Tests were conducted for three million cycles, lubricated with 25% bovine serum, with wear assessed gravimetrically.

The computational wear model for the TKR was based on the contact area and an independent experimentally determined non-dimensional wear coefficient, previously validated against the experimental data (3).

Materials and Methods

The Leeds/Prosim knee simulator was used to investigate the wear of two types of commercially available patellae. The PFC Sigma cobalt chrome femoral component was coupled with 2 types of patellae buttons: round and oval dome. The UHMWPE was the same for the both types – GUR1020 GVF (gamma irradiated in vacuum and foiled packed). 25% bovine serum was used as the lubricant. The test were carried out at three conditions – high medial lateral (ML) rotations (<4°) and uncontrolled ML displacement (<4 mm), low ML rotation (<1°) and uncontrolled ML displacement (<4 mm); the physiological gait cycle; and low ML rotation and controlled ML displacement (<1.5 mm). In this abstract the two designs were tested in physiological gait condition (Figure 1). Patella ML displacement and tilt were passively controlled and measured after every 300,000 cycles. A ligament resisting force equivalent to 10 N4 was applied on the lateral side of the patella to avoid patella slip.

Five samples of each design were tested for 3 million cycles at a cycle rate of 1 Hz. The wear volume was obtained gravimetrically every million cycles and presented with 95% confidence limits. Statistical significance was taken at p<0.05.

Materials and Methods

The wear of a new TKR the Attune knee was investigated using a physiological six station Prosim knee wear simulator (Simulator Solutions, UK). Six mid-size Attune fixed bearing cruciate retaining TKRs (DePuy Inc, Warsaw, USA) were tested for a period of 6 million cycles. The inserts were manufactured from AOX™, a compression moulded GUR1020 polyethylene incorporating Covernox™ solid anti-oxidant. The AOX polymer was irradiated to 8M Rad, to give a moderately cross-linked material.

High and intermediate kinematics, under anterior-posterior displacement control were used for this study (McEwen et al 2005). The maximum femoral axial loading was 2600N, with flexion-extension of 0 to 58°, an anterior-posterior displacement of 0–10 mm for high kinematics and 0–5 mm for intermediate, and an internal-external rotation of ±5°. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (v/v) sodium azide solution in deionised water, as an antibacterial agent, and was changed approximately every 0.33 Mc. Wear was assessed gravimetrically and moisture uptake was assessed using unloaded soak controls. Volumetric wear was calculated using a density of 0.94 mg/mm³, and compared with a previous study examining the Sigma XLK TKR design which uses moderately crosslinked polyethylene which is in current clinical use (Brockett et al 2012).

METHODS

Ceramic-on-metal (n=3) and metal-on-metal (n=3) 36mm hip prostheses (supplied by DePuy International Ltd, UK) were tested in the Leeds Physiological Anatomical Hip Joint Simulator. Liners were mounted to provide a clinical angle of 45o, and stems positioned anatomically. A simplified gait cycle and microseparation was applied as previously described [5] for two million cycles in 25% new born calf serum. Gravimetric analysis was completed every million cycles and wear volumes calculated.

Methodology

Five 36mm diameter Biolox Delta heads were paired with extruded CFR-PEEK cups and tested in a hip wear simulator (Simulator Solutions, UK) for 10 million cycles (Mc). Tests were performed in a Prosim hip simulator, which applied a twin peak loading cycle, with a peak load of 3kN. Flexion-extension of −15 to +30 degrees was applied to the head and internal-external rotation of +/− 10 degrees was applied to the cup, components were mounted anatomically. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide. Wear was assessed gravimetrically at several intervals adjusted for moisture uptake using loaded and unloaded soak controls.

MATERIALS AND METHODS

The wear of size 28mm and 36mm ceramic-on-ceramic bearings (BIOLOX® Delta, CeramTec, Germany) was determined under different in vitro conditions using the Leeds II hip simulator. For each size bearing, two clinical cup inclination angles were considered, 55° (n=3) and 65° (n=3) for the 28mm bearing and 45° (n=3) and 65° (n=3) for the 36mm bearing. The first two (28mm study) or three (36mm study) million cycles ran under standard gait conditions and a subsequent three million cycles ran under microseparation conditions. A standard gait cycle included a twin peak load (300N–3000N), extension/flexion (−15°/+30°) and internal/external rotation (±10°). Microseparation³ was achieved by applying a 0.4–0.5mm medial displacement to the cup relative to the head during the swing phase of the standard gait cycle resulting in edge loading at heel strike. The lubricant was 25% (v/v) new-born calf serum which was changed approximately every 333,000 cycles. The wear volume was ascertained through gravimetric analysis every million cycles. One way ANOVA was performed (significance: p<0.05), and 95% confidence limits were calculated.

Method

Five tests using a Paul type walking cycle (ISO 14242) were carried out on two ProSim hip simulators:

28mm MoM, standard walking, cup inclination 45°, (n = 5)

All samples had matched clearances, measured using a CMM (Prismo Navigator, Zeiss, Germany). Wear was measured gravimetrically (Sartorius ME235S: 0.01mg).

MATERIALS AND METHODS

The wear of size 28mm and 36mm MoM THRs bearings was determined under different in vitro conditions using the Leeds II hip simulator. For each size bearing, two clinical cup inclination angles were considered, 45° (n=3) and 65° (n=3). The first three million cycles were run under standard gait conditions and subsequently three million cycles were run under microseparation conditions. Standard gait cycles included a twin peak load (300N–3000N), extension/flexion (−15°/+30°) and internal/external rotation (±10°). Microseparation⁴ was achieved by applying a 0.4–0.5mm medial displacement to the cup relative to the head during the swing phase of the standard gait cycle resulting in edge loading at heel strike. The lubricant was 25% (v/v) new-born calf serum. The wear volume was determined through gravimetric analysis every million cycles. One way ANOVA was performed (significance: p<0.05), and 95% confidence limits were calculated.