Wear of historical polyethylenes in hip prostheses. Biomechanical success and a biological failure

Dysregulated expression of antioxidant enzymes in polyethylene particle-induced periprosthetic inflammation and osteolysis

Small wear particles (0.1–10 μm) in total joint replacement are generally considered as the major causative agent leading to periprosthetic inflammation and osteolysis. However, little is known about the roles of larger wear particles (10–100 μm) in periprosthetic inflammation and osteolysis. Additionally, although ample studies demonstrated that increased oxidative stress is critically involved in particle-induced inflammation and osteolysis, detailed changes in antioxidant enzymes expression in the disease development remain largely unclear. Herein, we used a rat knee prosthesis model to assess effects of polyethylene (PE) particles (20–60 μm) on the levels of oxidative stress markers such as malondialdehyde (MDA) and total antioxidant capacity (TAC) in blood plasma, and on the expression profiles of antioxidant enzymes in knee joint tissues. In combination with a forced-exercise intervention for all surgical rats, we found that the rat groups treated with both artificial joint and PE particles exhibited higher MDA levels and lower TAC levels, together with lower levels of physical activity and higher levels of inflammatory markers, than the sham group and the groups receiving artificial joint or PE particles alone at weeks 20–24 post-operatively. Dose-response relationships between the exposure to PE particles and the induction of oxidative stress and inflammation were also observed in the artificial joint/PE groups. Under such conditions, we unexpectedly found that most of antioxidant enzymes displayed pronounced up-regulation, with concomitant induction of inflammatory and osteoclast-inducing factors (including IL-1β, NF-κB and RANKL), in the artificial joint/PE groups as compared to the sham, artificial joint only, or PE only group. Only a few antioxidant enzymes including SOD2 and GPx2 showed down-regulation. Collectively, our findings demonstrate that implantation of artificial joint along with large PE particles synergistically trigger the induction of oxidative stress; however, down-regulation of many antioxidant enzymes may not necessarily occur during the disease development.

Recovery of low volumes of wear debris from rat stifle joint tissues using a novel particle isolation method

Less than optimal particle isolation techniques have impeded analysis of orthopaedic wear debris in vivo. The purpose of this research was to develop and test an improved method for particle isolation from tissue. A volume of 0.018 mm³ of clinically relevant CoCrMo, Ti-6Al-4V or Si₃N₄ particles was injected into rat stifle joints for seven days of in vivo exposure. Following sacrifice, particles were located within tissues using histology. The particles were recovered by enzymatic digestion of periarticular tissue with papain and proteinase K, followed by ultracentrifugation using a sodium polytungstate density gradient. Particles were recovered from all samples, observed using SEM and the particle composition was verified using EDX, which demonstrated that all isolated particles were free from contamination. Particle size, aspect ratio and circularity were measured using image analysis software. There were no significant changes to the measured parameters of CoCrMo or Si₃N₄ particles before and after the recovery process (KS tests, p > 0.05). Titanium particles were too few before and after isolation to analyse statistically, though size and morphologies were similar. Overall the method demonstrated a significant improvement to current particle isolation methods from tissue in terms of sensitivity and efficacy at removal of protein, and has the potential to be used for the isolation of ultra-low wearing total joint replacement materials from periprosthetic tissues.

STATEMENT OF SIGNIFICANCE

This research presents a novel method for the isolation of wear particles from tissue. Methodology outlined in this work would be a valuable resource for future researchers wishing to isolate particles from tissues, either as part of preclinical testing, or from explants from patients for diagnostic purposes. It is increasingly recognised that analysis of wear particles is critical to evaluating the safety of an orthopaedic device.

Metal ions as inflammatory initiators of osteolysis

Osteolysis and aseptic loosening currently contribute 75 % of implant failures. Furthermore, with over four million joint replacements projected to be performed in the United States annually, osteolysis and aseptic loosening may continue to pose a significant morbidity. This paper reviews the osteolysis cascade leading to osteoclast activation and bone resorption at the biochemical level. Additionally, the metal ion release mechanism from metallic implants is elucidated. Even though metal ions are not the predominating initiator of osteolysis, they do increase the concentration of key inflammatory cytokines that stimulate osteoclasts and prove to be a contributor to osteolysis and aseptic loosening. Osteolysis is a competitive mechanism among a number of biological reactions, which includes debris release, macrophage and osteoclast activation, an inflammatory response as well as metal ion release. Pharmacological therapy for component loosening has also been reviewed. A non-surgical treatment of osteolysis has not been found in the literature and thus may become an area of future research. Even though this research is warranted, comprehensively understanding the immune response to orthopedic implants and their metallic ions, and thus, creating improved prostheses appears to be the most cost-effective approach to decrease the morbidity related to osteolysis and to design implants with greater longevity. The ionic forms, cytokines, toxicity, gene expression, biological effects, and hypersensitivity responses of metallic elements from metal implants are summarized as well.

(iv) Enhancing the safety and reliability of joint replacement implants

A new Stratified Approach For Enhanced Reliability (SAFER) pre-clinical simulation testing of joint prostheses is presented in this article. The aim of this approach is preclinical systematic testing of wear performance in the much wider envelope of conditions found clinically rather than relying only on the standard testing conditions that are currently used. The approach includes variations in surgical delivery, variations in kinematics, variations in the patient population and degradation of the biomaterial properties. Clinical experience of existing prostheses has been used to validate the new in vitro methods.

Larger diameter bearings reduce wear in metal-on-metal hip implants

Metal-on-metal hip arthroplasty has the longest clinical history of all total arthroplasties. We asked whether large diameter femoral heads would result in less wear than those with small diameters. We also asked if there is a threshold diameter that ensures good wear behavior. We tested three batches of cast high-carbon cobalt-chromium-molybdenum hip implants (28 mm, 36 mm, and 54 mm diameters) in a hip simulator for 5 million cycles. We used bovine serum as lubricant and weighed the samples at regular intervals during testing. The 28-mm configuration had almost twice the wear of the 54-mm configuration, but we observed no difference between the 36-mm and the 54-mm configurations. The similarity in the wear performances of the larger configurations supports the presence of a threshold diameter that ensures good wear behavior.

Finite Element Analysis of the Long-Term Fixation Strength of Cemented Ceramic Cups

Clinical studies have shown that adequate fixation of ceramic cups using bone cement is difficult to achieve. As the cement-ceramic bond strength is low, a satisfactory fixation strength requires a cup design that allows mechanical interlocking, although such a design will probably promote cement cracking and therefore cup loosening in the long term. An investigation has been carried out to establish whether a cemented ceramic cup can be designed in such a way that both a satisfactory initial fixation strength is obtained and cement cracking is reduced to levels found around PE cups functioning well in vivo. By means of finite element analysis, the fatigue loading of three geometrically different cemented acetabular cups, with ceramic and PE material properties, has been simulated, and the severity of the crack patterns produced in the cement has been analysed. Furthermore, the fixation strength has been analysed by simulating a pull-out test prior to and after fatigue testing. All ceramic cups produced much larger amounts of cement damage during fatigue testing than any PE cup, caused by stress concentrations in the cement that were attributable to the high stiffness of the ceramic. Even a completely smooth ceramic cup produced more damage than a sharp-grooved PE cup. Owing to the excessive cement cracking, the fixation strength of the ceramic cups dropped after fatigue loading. It is concluded that cemented ceramic cups have an increased risk of long-term mechanical failure by comparison with PE cups, and that a ceramic cup design that combines sufficient fixation strength with low cement failure may be difficult to achieve.

Treatment of the young active patient with osteoarthritis of the hip

We compared the five- to seven-year clinical and radiological results of the metal-on-metal Birmingham hip resurfacing with a hybrid total hip arthroplasty in two groups of 54 hips, matched for gender, age, body mass index and activity level.

Function was excellent in both groups, as measured by the Oxford hip score, but the Birmingham hip resurfacings had higher University of California at Los Angeles activity scores and better EuroQol quality of life scores. The total hip arthroplasties had a revision or intention-to-revise rate of 8%, and the Birmingham hip resurfacings of 6%. Both groups demonstrated impending failure on surrogate end-points. Of the total hip arthroplasties, 12% had polyethylene wear and osteolysis under observation, and 8% of Birmingham hip resurfacings showed migration of the femoral component. Polyethylene wear was present in 48% of the hybrid hips without osteolysis. Of the femoral components in the Birmingham hip resurfacing group which had not migrated, 66% had radiological changes of unknown significance.