Resurfacing hip implants differ in macro- and microstructure. Manufacturing related parameters like clearance or carbon content influence the wear behaviour of these metal-on-metal bearings. The aim of this study was to analyse the main macro- and micro-structural differences of commercially available resurfacing hip implants.

Ten different commercially available resurfacing hip implant designs were included in this investigation:

- BHR^® (Smith& Nephew/MMT)

The heads and cups were measured in a coordinate measuring machine and radial clearance as well as sphericity deviation were calculated. Surface roughness measurements were carried out. The microstructures of the heads and cups were inspected using SEM and element analysis was performed using EDX to identify carbides and the alloy composition.

The mean radial clearance was found to be 85.53 μm. The range was from 49.47 μm (DePuy, ASR^®) to 120.93 μm (Biomet, ReCap^®). All implants showed a sphericity deviation of less than 10 μm. The highest sphericity deviation was found to be 7.3 μm (Corin Cormet^® head), while the lowest was 0.8 μm (Smith& Nephew BHR^® head). On average, the heads tended to have a higher sphericity deviation (4.1 μm, SD: 2.3 μm) compared to the cups (2.7 μm, SD: 1.4 μm). SEM revealed that most manufacturers use a high carbon alloy casting manufacturing process combined with heat treatment after casting (Corin Cormet^® and Wright Conserve^®: head and cup; DePuy ASR^®: cup; Eska BS^®: head).

Commercially available resurfacing hip implants differ in design and manufacturing parameters, including macro- and microstructure, which are critical in achieving low wear and ion release. This study was designed to aid in the understanding of clinical observations. Also, specific information is now available for surgeons choosing an implant designs.

Resurfacing hip implants differ in macro-and microstructure. Manufacturing related parameters like clearance or carbon content influence the wear behaviour of these metal-on-metal bearings. The aim of this study was to analyse the main macro- and micro-structural differences of commercially available resurfacing hip implants. Ten different commercially available resurfacing hip implant designs were included in this investigation:

BHR^® (Smith& Nephew/MMT) Durom® (Zimmer)

The heads and cups were measured in a coordinate measuring machine and radial clearance as well as sphericity deviation were calculated. Surface roughness measurements were carried out. The microstructures of the heads and cups were inspected using SEM and element analysis was performed using EDX to identify carbides and the alloy composition. The mean radial clearance was found to be 85.53μm. The range was from 49.47μm (DePuy, ASR®) to 120.93μm (Biomet, ReCap®). All implants showed a sphericity deviation of less than 10μm. The highest sphericity deviation was found to be 7.3μm (Corin Cormet® head), while the lowest was 0.8μm (Smith& Nephew BHR® head). On average, the heads tended to have a higher sphericity deviation (4.1μm, SD: 2.3μm) compared to the cups (2.7μm, SD: 1.4μm). SEM revealed that most manufacturers use a high carbon alloy casting manufacturing process combined with heat treatment after casting (Corin Cormet® and Wright Conserve®: head and cup; DePuy ASR®: cup; Eska BS®: head). Commercially available resurfacing hip implants differ in design and manufacturing parameters, including macro- and microstructure, which are critical in achieving low wear and ion release. This study was designed to aid in the understanding of clinical observations. Also, specific information is now available for surgeons choosing an implant designs.

For wear testing of knee implants, ISO 14243 is the most used testing protocol. In force control, this standard requires linear motion restraints for simulation of ligaments. The aim of this study was to investigate if a nonlinear, physiological motion restraint would influence the wear behaviour of the implants.

A wear study was performed on a highly conforming knee implant design. Three implants were tested forced controlled according to ISO 14243-1 on an AMTI knee simulator. Linear motions restrain of 30 N/mm for AP-translation and 0.6 Nm/° for IE-rotation were applied as required per ISO 14243-1. A second wear test was performed on the same implant design. Based on the data given by Kanamori et al. and Fukubayashi et al., a physiological, nonlinear ligament constraint model (sectioned ACL) was adopted and implemented in the simulation. The implants were pre-soaked and a soak controls was used. Wear was measured gravimetrically.

A mean gravimetric wear rate of 2.85 mg/10E6 cycles was found for the implants which were tested using a linear motion restraint as required per ISO 14243-1. Simulating a physiological, nonlinear motion restraint resulted in a 60% increase in gravimetric wear (mean gravimetric wear rate: 4.75 mg/10E6 cycles). As expected, the kinematics of the implants differed between wear tests. The mean AP-translation increased from 2.89 mm (linear motion restraint) to 4.82 mm (physiological motion restraint). A similar behaviour was observed for the IE-rotation. The IE-rotation increased from 4.09° (linear motion restraint) to 5.94° (physiological motion restraint).

The reaction of the ligaments is not linear in the human knee joint. This study showed that wear and kinematics change when simulating physiological ligament reactions. Wear increased by 60%, an effect which can likely be credited to fundamental differences in kinematics. The ACL is commonly sacrificed during surgery. Thus, more attention should be paid to ligament simulation when performing wear tests on knee implants.

The introduction of mobile bearings for unicompartimental knee implants resulted in heightened interest in this implant design in the field of orthopaedics. This study aims to determine the effect of the mobile and fixed design concepts on the wear progression in unicompartmental knee implants using a knee simulator.

An unicompartmental knee implant design, which is available in a fixed and mobile version, was tested using a knee simulator. For the wear test, the medial and lateral compartments were implemented in the simulator. To account for the physiologically higher medial load compared to the lateral compartment, a medially-biased load distribution was implemented. The wear test was performed force controlled according to ISO 14243. Wear was measured gravimetrically separately for the medial and lateral compartments. To evaluate implant kinematics, AP-translation and IE-rotation were measured during the simulation.

Gravimetric wear was higher medially than laterally for both designs. The mean wear rate of the medial mobile compartment was found to be 10.70 mg/10E6 cycles, whereas a mean wear rate of 6.05 mg/10E6cycles was found for the medial compartment of the fixed design. Lateral wear rates, which were about 50% lower than medial wear rates, were found to be 5.38 mg/10E6 cycles in the mobile design and 3.23 mg/10E6 cycles in the lateral design. Wear of the mobile design was higher compared to the fixed design, both medially and laterally. Surprisingly the kinematics of both designs were very similar. A low AP-translation of 2.7 mm in the mobile and 2.4 mm in the fixed designs was documented. High IE-rotations of 6.5° and 6.7° for the mobile and the fixed design, respectively, were observed.

In bicondylar bearing knee designs, reduced wear has been reported for mobile polyethylene inlays. This study showed that the wear behaviour of unicompartmental knee implants differs from bicondylar implants and that the introduction of the mobile concept may lead to increased wear.

In hip joint simulator studies, wear measurement is usually performed gravimetrically. This procedure is reliable for metal-on-polyethylene or ceramic-on-polyethylene bearings, in which relatively high amounts of abrasive wear particles are produced. With modern hard-on-hard bearings, volumetric wear decreases significantly up to 100 to 200-fold. The gravimetric method reaches its detection limit with metal-on-metal bearings and even more so with ceramic-on-ceramic bearings. This study establishes a new method of determining wear in hard-on-hard bearings by measuring the amount of worn particles/ions in the serum of hip simulators.

A wear study on three resurfacing hip implants (BHR^®, Smith& Nephew) was conducted using a hip joint simulator. Prior to the wear study, tests were performed to validate the detection power for high resolution-inductively coupled plasma-mass spectrometry (HR-ICP-MS). More importantly the system’s accuracy was compared to the gravimetric method, which is described in ISO 14243-2. The simulator was altered to run completely metal ion free. The ion concentration in the serum was measured every 100 000 cycles up to 1 500 000 cycles and subsequently in intervals of 500 000 cycles using HR-ICP-MS. The implants were neither removed from the simulator nor excessively cleaned during the course of the simulation. Serum was refreshed every 500 000 cycles. The serum samples were digested with purified nitric acid and hydrogen peroxide using a high pressure microwave autoclave in order to measure wear particles as well as dissolved ions. All steps were carried out under clean room conditions. Wear was calculated using the ion concentration and measured serum volume. Wear rates and transition from running-in to steady-state wear phases were calculated.

A detection power better than 0.028 μg/l for Co (cobalt), 0.017 μg/l for Cr (chromium) and 0.040 μg/l for Mo (molybdenum) was found for HR-ICP-MS. The validation of HR-ICP-MS showed good agreement between gravimetric data and measured ion concentrations. The tested implants showed similar wear behaviour. Implant wear resulted in high ion concentrations during the first 380 000 to 920 000 cycles. During this period, a mean wear rate of 6.96 mm3/10E6 cycles was determined. Subsequently, the wear rate significantly decreased to a mean wear rate of 0.37 mm3/10E6 cycles. Thus, a mean ratio between running-in and steady-state wear of 18.8 was found. The mean overall wear volume at the end of the simulation was 4.42 mm3.

This study showed that measuring the ion concentrations in the serum of hip simulators can be used to determine wear in metal-on-metal bearings. The main advantages of this new method are the ability to detect ultra-low wear rates and to precisely specify the duration of different wear phases. Because the implants do not have to be removed from the simulator and aggressive cleaning processes may be skipped, fluctuations in wear detection are extremely low. This in turn leads to a shorter duration of the simulation. Wear rates of the tested implants are low compared to polyethylene. Transferring the results to patient activity, wear would be the same during the first four to six months after implantation as in the next ten years. Minimizing the duration of running-in would be most effective in further reducing wear of metal-on-metal bearings.