Ion release in ceramic bearings for total hip replacement: Results from an in vitro and an in vivo study.

INTRODUCTION: Literature on the potential release of trace elements following implantation of Zirconia-platelet toughened alumina (ZPTA) ceramic components is scant. The present study therefore analysed the in vitro and in vivo potential release of ions from ZPTA bearings. MATERIAL AND METHODS: An in vitro and in vivo study was conducted. The in vitro study compared leaching in bovine serum from two groups: ZPTA ceramic heads and Co-28Cr-6Mo alloy heads, both 28-mm diameter. A third group without implant served as reference group. An in vivo clinical study compared trace elements in the whole blood of patients with 36-mm diameter ZPTA ceramic-on-ceramic articulation after three and 12 months. A cohort of subjects without any prosthesis was used as control group. The release of ions was determined by high resolution-inductively coupled plasma-mass spectrometry. RESULTS: In the in vitro experiment, significant differences (p ≤ 0.01) in trace element release for chromium, cobalt and molybdenum were found, with increased levels of ion release in the Co-28Cr-6Mo metal group. The very low detection limit for yttrium allowed detection of a small yttrium release from the ZPTA heads, which was not confirmed by the in vivo study. No significant difference between the groups was found for strontium, aluminium, and zirconium. In the in vivo study, no relevant differences in ion levels between the reference group without any implant and the study group were found at the three and 12-month follow-up. CONCLUSION: This study supports that ZPTA ceramic articulation components are safe in terms of ion release, and may be an excellent alternative to bearings based on Co-28Cr-6Mo alloys.

Three-dimensional friction measurement during hip simulation.

OBJECTIVES: Wear of total hip replacements has been the focus of many studies. However, frictional effects, such as high loading on intramodular connections or the interface to the bone, as well as friction associated squeaking have recently increased interest about the amount of friction that is generated during daily activities. The aim of this study was thus to establish and validate a three-dimensional friction setup under standardized conditions. MATERIALS AND METHODS: A standard hip simulator was modified to allow for high precision measurements of small frictional effects in the hip during three-dimensional hip articulation. The setup was verified by an ideal hydrostatic bearing and validated with a static-load physical pendulum and an extension-flexion rotation with a dynamic load profile. Additionally, a pendulum model was proposed for screening measurement of frictional effects based on the damping behavior of the angular oscillation without the need for any force/moment transducer. Finally, three-dimensional friction measurements have been realized for ceramic-on-polyethylene bearings of three different sizes (28, 36 and 40 mm). RESULTS: A precision of less than 0.2 Nm during three-dimensional friction measurements was reported, while increased frictional torque (resultant as well as taper torque) was measured for larger head diameters. These effects have been confirmed by simple pendulum tests and the theoretical model. A comparison with current literature about friction measurements is presented. CONCLUSIONS: This investigation of friction is able to provide more information about a field that has been dominated by the reduction of wear. It should be considered in future pre-clinical testing protocols given by international organizations of standardization.

Wear Performance of Calcium Carbonate-Containing Knee Spacers.

Articulating spacers should be wear-resistant and load-bearing to avoid prolonged immobilization of the patient and to reduce morbidity. However, due to the articulation of both components, a release of cement wear particles is to be expected. The aim of this study was to investigate the wear performance of a new spacer cement that contains calcium carbonate as a radio-opaque substance, in comparison to an established barium sulphate-containing spacer material, and also to characterize the amount, morphology, and size distributions of the released cement particles in detail. Force-controlled simulation was carried out on an AMTI knee simulator. The test parameters were in accordance with the standard ISO 14243-1 with a 50% reduced axial force. Tests were run for 500,000 cycles at a frequency of 1 Hz. For wear analysis, photographic documentation of the wear scars, gravimetric wear measurements and wear particle analysis were performed. The barium sulphate spacer material showed a total articular wear of 375.53 ± 161.22 mg. For the calcium carbonate-containing cement, reduced articular wear of 136.32 ± 37.58 mg was determined. Isolated cement wear particles of the barium sulphate-containing cement had a diameter of 0.429 ± 0.224 µm and were significantly larger compared to the calcium carbonate-containing cement (0.380 ± 0.216 µm, p = 0.02). The calcium carbonate-containing cement showed better wear performance in terms of gravimetric wear and particle release. Thus, calcium carbonate seems to be a promising material as a radio-opaque substrate in cement spacers.

Wear testing of total hip replacements under severe conditions.

Controlled wear testing of total hip replacements in hip joint simulators is a well-established and powerful method, giving an extensive prediction of the long-term clinical performance. To understand the wear behavior of a bearing and its limits under in vivo conditions, testing scenarios should be designed as physiologically as possible. Currently, the ISO standard protocol 14242 is the most common preclinical testing procedure for total hip replacements, based on a simplified gait cycle for normal walking conditions. However, in recent years, wear patterns have increasingly been observed on retrievals that cannot be replicated by the current standard. The purpose of this study is to review the severe testing conditions that enable the generation of clinically relevant wear rates and phenomena. These conditions include changes in loading and activity, third-body wear, surface topography, edge wear and the role of aging of the bearing materials.

Experimental testing of total knee replacements with UHMW-PE inserts: impact of severe wear test conditions.

Aseptic implant loosening due to inflammatory reactions to wear debris is the main reason for the revision of total knee replacements (TKR). Hence, the decrease in polyethylene wear particle generation from the articulating surfaces is aimed at improving implant design and material. For preclinical testing of new TKR systems standardized wear tests are required. However, these wear tests do not reproduce the entire in vivo situation, since the pattern and amount of wear and subsequent implant failure are underestimated. Therefore, daily activity, kinematics, implant aging and position, third-body-wear and surface properties have to be considered to estimate the wear of implant components in vivo. Hence, severe test conditions are in demand for a better reproduction of the in vivo situation of TKR. In the present article an overview of different experimental wear test scenarios considering clinically relevant polyethylene wear situations using severe test conditions is presented.

Wear in total knee arthroplasty--just a question of polyethylene?: Metal ion release in total knee arthroplasty.

PURPOSE: Biological reactions against wear particles are a common cause for revision in total knee arthroplasty. To date, wear has mainly been attributed to polyethylene. However, the implants have large metallic surfaces that also could potentially lead to metal wear products (metal ions and debris). The aim of this study was to determine the local release of cobalt, chromium, molybdenum and titanium in total knee arthroplasty during a standard knee wear test. METHODS: Four moderately conforming fixed-bearing implants were subjected to physiological loadings and motions for 5×10(6) walking cycles in a knee wear simulator. Polyethylene wear was determined gravimetrically and the release of metallic wear products was measured using high resolution-inductively coupled plasma-mass spectrometry. RESULTS: A polyethylene wear rate of 7.28 ± 0.27 mg/10(6) cycles was determined and the cumulative mass of released metals measured 1.63 ± 0.28 mg for cobalt, 0.47 ± 0.06 mg for chromium, 0.42 ± 0.06 mg for molybdenum and 1.28 ± 0.14 mg for titanium. CONCLUSION: For other metallic implants such as metal-on-metal total hip arthroplasty, the metal wear products can interact with the immune system, potentially leading to immunotoxic effects. In this study about 12 % by weight of the wear products were metallic, and these particles and ions may become clinically relevant for patients sensitive to these materials in particular. Non-metallic materials (e.g. ceramics or suitable coatings) may be considered for an alternative treatment for those patients.