Polyethylene wear debris produced in a knee simulator model: Effect of crosslinking and counterface material

NANOMECHANICAL CHARACTERIZATION OF ZIRCONIA THIN FILMS DEPOSITED ON UHMWPE BY PULSED PLASMA DEPOSITION

Plastic deformation and wear of the ultra-high molecular weight polyethylene (UHMWPE) insert have been pointed out as major issues relating to the long-term stability of an orthopaedic implant. The deposition of protective hard, tough and well-adhered zirconia ( ZrO 2) thin films directly on the surface of the UHMWPE component via the Pulsed Plasma Deposition (PPD) technique has been already demonstrated to be a feasible way to approach this problem. In the current study, the tribo-mechanical properties of ZrO 2-coated UHMWPE with respect to pristine UHMWPE were investigated in detail. Specifically, strength to local plastic deformation, indentation work portioning and creep behavior were evaluated through nanoindentation and micro-scratch tests. Further, preliminary wear data (i.e., rate and volume) were obtained by tribology tests mating coated and pristine UHMWPE with an alumina ball under dry conditions. The results of the mechanical tests evidenced a strong reduction of plastic deformation under both normal and tangential local loads and a drop of the 80% of the creep phenomenon for coated UHMWPE compared to pristine UHMWPE. Despite tribological tests showed similar wear data for coated and pristine UHMWPE, a different wear mechanism was detected between the two groups. The reported results supported the possibility to pursue this novel approach of depositing ZrO 2 thin film to protect the UHWMPE insert and enhance the long-term stability of the orthopaedic implants.

Displacement or Force Control Knee Simulators? Variations in Kinematics and in Wear

The problems associated with prosthetic failure and revision surgery still constitute the main clinical problem of prosthetic surgery. The objective of wear evaluation is to determine the wear rate and its dependence on the test conditions. To obtain realistic results, a wear test can be performed to reproduce in vivo working conditions and compare the wear characteristics of various total knee prostheses designs. At the state of the art, two simulation concepts are available and defined in ISO 14243 standards series. In both these guidelines, level walking is the sole activity of daily living that is represented for testing. With so many variables and so many sources of error and the sensitivity of the output to these errors, can the motion determined in the simulator be representative of the in vivo motion? This article goes beyond the current status of these knee simulations comparing literature results.

Wear Debris Characterization and Corresponding Biological Response: Artificial Hip and Knee Joints

Wear debris, of deferent sizes, shapes and quantities, generated in artificial hip and knees is largely confined to the bone and joint interface. This debris interacts with periprosthetic tissue and may cause aseptic loosening. The purpose of this review is to summarize and collate findings of the recent demonstrations on debris characterization and their biological response that influences the occurrence in implant migration. A systematic review of peer-reviewed literature is performed, based on inclusion and exclusion criteria addressing mainly debris isolation, characterization, and biologic responses. Results show that debris characterization largely depends on their appropriate and accurate isolation protocol. The particles are found to be non-uniform in size and non-homogeneously distributed into the periprosthetic tissues. In addition, the sizes, shapes, and volumes of the particles are influenced by the types of joints, bearing geometry, material combination, and lubricant. Phagocytosis of wear debris is size dependent; high doses of submicron-sized particles induce significant level of secretion of bone resorbing factors. However, articles on wear debris from engineered surfaces (patterned and coated) are lacking. The findings suggest considering debris morphology as an important parameter to evaluate joint simulator and newly developed implant materials.

Characterization of polyethylene wear particle: The impact of methodology

Due to the prevalence of problems caused by wear particles, the reduced durability of total joint replacements is well documented. The characterization of wear debris enables the size and morphology of these wear particles to be measured and provides an assessment of the biological response in vivo. However, the impact of different methodologies of particle analysis is not yet clear. Hence, the aim of this investigation was to analyze the influence of different particle characterization methods performed by three research centers within the scope of a "round robin test". To obtain knowledge about possible pitfalls, single steps of the particle characterization process (storage, pore size of the filter, coating durations by gold sputtering and scanning electron microscopy (SEM) magnification) were analyzed. The round robin test showed significant differences between the research groups, especially for the morphology of the particles. The SEM magnification was identified as having the greatest influence on the size and shape of the particles, followed by the storage conditions of the wear particle containing lubricant. Gold sputter coating and filter pore size also exhibit significant effects. However, even though they are statistically significant, it should be emphasized that the differences are small. In conclusion, particle characterization is a complex analytical method with a multiplicity of influencing factors. It becomes apparent that a comparison of wear particle results between different research groups is challenging.

Highly crosslinked polyethylene does not reduce the wear in total knee arthroplasty: in vivo study of particles in synovial fluid

The aim was to assess if the reduction in polyethylene wear with highly crosslinked polyethylene suggested by studies with knee simulators is confirmed in patients with a knee arthroplasty. The use of a conventional or a highly crosslinked polyethylene was randomly assigned intraoperatively. Twelve months after surgery a knee arthrocentesis was performed and the synovial fluid of 17 patients in each group was studied analysing the number, size and shape of the polyethylene particles by scanning electron microscope. We found no significant differences in the concentration, size or morphology of polyethylene particles between groups. The great variability in the number of particles between individuals suggests that in vivo polyethylene wear depends on many factors and probably the type of polyethylene is not the most significant.

[Polyethylene particles in synovial fluid after knee arthroplasty with a conventional or highly cross-linked polyethylene. Preliminary study]

AIM OF THE STUDY

In recent years cross-linked polyethylenes have been developed in an attempt to reduce the wear, as has been demonstrated in knee simulators. The aim is to assess, by counting particles of polyethylene in synovial fluid, whether the reduction in wear is confirmed in patients with a highly crosslinked polyethylene prosthesis.

MATERIAL AND METHODS

A prospective randomised study was designed. During the implantation of a knee prosthesis, one group of patients was assigned the use of a conventional polyethylene (group A), and the other group a highly crosslinked polyethylene (X3, Stryker Orthopaedics) (group B). At 12 months after surgery a knee arthrocentesis was performed, and the number of polyethylene particles was counted in a scanning electron microscopy. Fourteen samples from each group were studied.

RESULTS

Both groups were comparable in all study variables. We found no significant differences in the concentration of polyethylene particles/ml (1.49 ± 0.85 million in group A vs 1.42 ± 0.91 million in group B, P=.60) or the total number of isolated particles. We found no differences either in size or morphology of particles between both groups.

DISCUSSION AND CONCLUSIONS

Although several in vitro studies in vitro using different types of highly crosslinked polyethylene found a significant reduction, we did not find that that wear was reduced in the knees of these patients. The great variability in the number of particles between individuals suggests that polyethylene wear in vivo depends on many factors, so perhaps the type of polyethylene is not the most significant factor.

The use of highly cross-linked polyethylene in total knee arthroplasty

Polyethylene wear, with resultant particle-induced osteolysis, is a cause of late failure of total knee arthroplasty. The causes of both wear and osteolysis are multifactorial; still, improvements in the polyethylene liner have been investigated. Available highly cross-linked polyethylene tibial liners and patellar prostheses differ greatly in the amount and method of irradiation, thermal treatments, and sterilization techniques they undergo. Several varieties of highly cross-linked polyethylene reduce the gravimetric and volumetric wear of tibial liners in knee simulator studies. However, reduced fracture toughness and the generation of smaller and possibly more reactive particles also have been reported with some varieties of polyethylene. Clinical studies of the use of highly cross-linked polyethylene in total knee arthroplasty are limited. Two nonrandomized trials of highly cross-linked polyethylene in total knee arthroplasty have reported a nonsignificant decrease in radiolucent lines at 2 and 5 years, respectively. The risks of using highly cross-linked polyethylene include fracture of the liner or of a posterior-stabilized tibial post, liner dislodgement or locking mechanism disruption, and possibly more osteolysis. Highly cross-linked polyethylene tibial liners may be considered for younger, more active patients. However, until additional clinical results are available, a cautious approach is warranted to the widespread use of highly cross-linked polyethylene in total knee arthroplasty.

Wear analysis of unicondylar mobile bearing and fixed bearing knee systems: a knee simulator study

Unicondylar knee arthroplasty is an attractive alternative to total knee arthroplasty for selected patients with osteoarthritis. Mobile bearing knee designs have been developed to improve knee kinematics, lower contact stresses and reduced wear of ultra-high molecular weight polyethylene compared with fixed bearing designs. This study compared in vitro wear behavior of fixed and mobile unicondylar bearing designs. Analysis was performed using a force-controlled AMTI knee simulator according to ISO 14243-1:2002(E). The wear volume of the implants was determined gravimetrically. Optical surface characterization and an estimation of wear particle size and morphology were performed. Implant kinematic data for both designs were determined. The wear rates averaged 10.7 ± 0.59 mg per 10(6) cycles for the medial and 5.38 ± 0.63 mg per 10(6) cycles for the lateral components of the mobile bearings, compared with 7.51 ± 0.29 mg per 10(6) cycles and 3.04 ± 0.35 mg per 10(6) cycles for the fixed bearings. The mobile bearings therefore exhibited higher wear rates (P<0.01) compared with the fixed bearings. The tibial polyethylene inserts of the mobile bearings showed pronounced backside wear at the inferior surface. The kinematics of both designs was similar. However, anterior-posterior translation was lower in the mobile bearings. The wear particles were mainly elongated and small in size for both designs (P=0.462). This study shows that wear may play an important role in unicondylar mobile bearing knee designs. Advantages of unicondylar mobile designs compared with fixed bearing designs, which have been proposed in terms of wear behavior and improved kinematics, could not be confirmed.