Wear testing of total hip replacements under severe conditions

A Comparison of Wear Patterns on Retrieved and Simulator-Tested Total Knee Replacements

Aseptic implant loosening is the most common reason for revision surgery after total knee replacement. This is associated with adverse biological reactions to wear debris from the articulating implant components. To predict the amount of wear debris generated in situ, standard wear testing of total knee replacement (TKR) is carried out before its clinical use. However, wear data reported on retrievals of total knee replacement (TKR) revealed significant discrepancies compared with standard wear simulator studies. Therefore, the aim of the present study was to compare the wear patterns on identical posterior-cruciate-retaining TKR designs by analyzing retrieved and experimentally tested implants. The identification and classification of wear patterns were performed using 21 retrieved ultra-high-molecular-weight-polyethylene (UHMW-PE) inserts and four sets of inserts of identical design and material tested in a knee wear simulator. These four sets had undergone different worst-case conditions and a standard test in a wear simulator according to ISO 14243-1. Macroscopic and microscopic examinations of the polyethylene inserts were performed, including the determination of seven modes of wear that correspond to specific wear patterns, the calculation of wear areas, and the classification of the damage over the whole articulating area. Retrieved and standard wear simulator-tested UHMW-PE inserts showed significant differences in wear area and patterns. The total wear areas and the damage score were significantly larger on the retrievals (52.3% versus 23.9%, 32.7 versus 22.7). Furthermore, the range of wear patterns found on the retrievals was not reproducible in the simulator-tested inserts. However, good correspondence was found with the simulator-tested polyethylene inserts under worst-case conditions (third body wear), i.e., deep wear areas could be replicated according to the in vivo situation compared with other wear test scenarios. Based on the findings presented here, standard simulator testing can be used to directly compare different TKR designs but is limited in the prediction of their in situ wear. Preclinical wear testing may be adjusted by worst-case conditions to improve the prediction of in situ performance of total knee implants in the future.

Influence of Accelerated Aging on the Wear Behavior of Cross-Linked Polyethylene Liners—A Hip Simulator Study

Sequential cross-linked and annealed ultra-high-molecular-weight polyethylene (SX-PE) is known as a low-wear articulating partner, especially for total hip endoprostheses. Aging of polymeric materials, irrespective of if induced by shelf or in vivo life, can degrade their tribological and mechanical properties. However, changes in wear behavior of aged SX-PE liners have not been not quantified so far. An accelerated aging procedure, to simulate shelf and in vivo aging, was performed on thin SX-PE liners after five million load cycles using a simulator (“worn-aged”) as well as on new SX-PE liners (“new-aged”). A subsequent hip simulator test was performed with both thin SX-PE liner sets in combination with large-diameter ceramic femoral head, representing a combination known as advantageous for treatment after revision because of dislocation. Oxidation indices were measured on the liners after each step of the procedure. SX-PE liners after accelerated aging show bedding-in phases during simulator test, which was a characteristic only known from clinical investigations. Hence, the wear rates of the “new-aged” ((1.71 ± 0.49) mg/million cycles) and of the “worn-aged” ((9.32 ± 0.09) mg/million cycles) SX-PE were increased in the first period compared to new unaged SX-PE liners. Subsequently, the wear rates decreased for “new-aged” and “worn-aged” inserts to (0.44 ± 0.48) mg/million cycles and (2.72 ± 0.05) mg/million cycles, respectively. In conclusion, the results show promising effects of accelerated aging on SX-PE liners in simulator testing and for potential long-term use in clinical applications.

Rheologic Behavior of Bovine Calf Serum

Recent studies have illuminated the rheological behavior of synovial fluid and the role of protein and hyaluronan (HA). However, with respect to artificial joint replacement in standardized wear simulations, bovine serum is used as fluid test medium. Little is known about the rheological characteristics of bovine serum, which are needed for precise tribological investigations. The steady shear viscosity η of bovine calf serum is determined for protein concentrations used in standardized wear simulations depending on shear rate γ˙ and temperature T. Additionally, the density of the serum is determined for both protein concentrations. The results show shear thinning behavior of bovine calf serum with a nearly Newtonian behavior in the range of high shear rates. Within the range of high shear rates, mean viscosities of η = 0.82-0.88 mPa·s were found for protein concentrations of 20 g/L and mean viscosities of η = 0.88-0.94 mPa·s for 30 g/L, decreasing with temperature. Densities of 1.004-1.005 g/cm³ and 1.007-1.008 g/cm³ were found for 20 and 30 g/L protein concentrations, respectively.

Current Preclinical Testing of New Hip Arthroplasty Technologies Does Not Reflect Real-World Loadings: Capturing Patient-Specific and Activity-Related Variation in Hip Contact Forces

BACKGROUND

Total hip arthroplasty (THA) implants are routinely tested for their tribological performance through regulatory preclinical wear testing (eg, ISO-14242). The standardized loading conditions defined in these tests consist of simplified waveforms, which do not specifically represent in vivo loads in different groups of patients. The aim of this study is to investigate, through musculoskeletal modeling, patient-specific and activity-related variation in hip contact forces (HCFs) in a large cohort of THA patients during common activities of daily living (ADLs).

METHODS

A total of 132 THA patients participated in a motion-capture analysis while performing different ADLs, including walk, fast walk, stair ascent, and descent (locomotor); sit to stand, stand to sit, squat, and lunge (nonlocomotor). HCFs were then calculated using the AnyBody Modeling System and qualitatively compared across all activities. The influence of gender on HCFs was analyzed through statistical parametric mapping analysis.

RESULTS

Systematic differences were found in HCF magnitudes and individual components in both locomotor and nonlocomotor ADLs. The qualitative analysis of the ADLs revealed a large range and a large variability in forces experienced at the hip during different activities. Significant differences in the 3-dimensional loading patterns were observed between males and females across most activities.

CONCLUSION

THA patients present a large variability in the forces experienced at the hip joint during their daily life. The interpatient variation might partially explain the heterogeneity observed in implant survival rates. A more extensive preclinical implant testing standard under clinically relevant loading conditions has been advocated to better predict and avoid clinical wear problems.

Patient characteristics affect hip contact forces during gait

OBJECTIVE

To examine hip contact force (HCF), calculated through multibody modelling, in a large total hip replacement (THR) cohort stratified by patient characteristics such as body mass index (BMI), age and function.

METHOD

132 THR patients undertook one motion capture session of gait analysis at a self-selected walking speed. HCFs were then calculated using the AnyBody Modelling System. Patients were stratified into three BMI groups, five age groups, and finally three functional groups determined by their self-selected gait speed. By means of statistical parametric mapping (SPM), statistical analyses of the 1-dimensional time series were performed to separately evaluate the influence of age, BMI and functionality on HCF.

RESULTS

The mean predicted HCFs were comparable to HCFs measured with instrumented prostheses reported in the literature. The SPM analysis revealed a statistically significant positive linear correlation between BMI and HCF, indicating that obese patients are more likely to experience higher HCF during most of the stance phase, while a statistically significant negative correlation with age was found only during the late swing-phase. Patients with higher functional ability exhibited significantly increased peak HCF, while patients with lower functional ability demonstrated lower HCFs overall and a pathological flattening of the typical double hump force profile.

CONCLUSION

HCFs experienced at the bearing surface are highly dependent on patient characteristics. BMI and functional ability were determined to have the biggest influence on contact forces. Current preclinical testing standards do not reflect this.

A novel approach to determine primary stability of acetabular press-fit cups

Today hip cups are used in a large variety of design variants and in increasing numbers of units. Their development is steadily progressing. In addition to conventional manufacturing methods for hip cups, additive methods, in particular, play an increasingly important role as development progresses. The present paper describes a modified cup model developed based on a commercially available press-fit cup (Allofit 54/JJ). The press-fit cup was designed in two variants and manufactured using selective laser melting (SLM). Variant 1 (Ti) was modeled on the Allofit cup using an adapted process technology. Variant 2 (Ti-S) was provided with a porous load bearing structure on its surface. In addition to the typical (complete) geometry, both variants were also manufactured and tested in a reduced shape where only the press-fit area was formed. To assess the primary stability of the press-fit cups in the artificial bone cavity, pull-out and lever-out tests were carried out. Exact fit conditions and two-millimeter press-fit were investigated. The closed-cell PU foam used as an artificial bone cavity was mechanically characterized to exclude any influence on the results of the investigation. The pull-out forces of the Ti-variant (complete-526 N, reduced-468 N) and the Ti-S variant (complete-548 N, reduced-526 N) as well as the lever-out moments of the Ti-variant (complete-10 Nm, reduced-9.8 Nm) and the Ti-S variant (complete-9 Nm, reduced-7.9 N) show no significant differences in the results between complete and reduced cups. The results show that the use of reduced cups in a press-fit design is possible within the scope of development work.

Surface modifications and oxidative degradation in MPC-grafted highly cross-linked polyethylene liners retrieved from short-term total hip arthroplasty

Highly cross-linked polyethylene (HXLPE) hip liners grafted with 2-methacryloyloxyethyl phosphorylcholine (MPC) on their bearing surfaces have recently been commercialized as components of a new generation of artificial hip joints, while improvements in wear resistance and biocompatibility were reported based on in vitro studies. The present study aimed at evaluating the surface modification and oxidative degradation in short-term retrieved MPC-grafted liners by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) with attenuated total reflection (ATR) equipment and Raman spectroscopy. In none of 3 samples of retrieved MPC-grafted liners, detectable MPC graft remained on the bearing surfaces although 2 samples yet contained remains of MPC polymer in their rim zone. These results revealed that the MPC polymer might have quickly disappeared from the bearing surface under in vivo loading, which is more severe than the in vitro one. Furthermore, a detectable oxidation index (OI) value (>0.1) was not only observed in any zone of any sample investigated, but also in the rim zones of Samples 1 and 2, which surprisingly experienced the most remarkable increase in OI value detected in this study. We thus confirmed that: (i) annealing of HXLPE cannot completely remove free radicals; (ii) the MPC graft has no beneficial effect in protecting HXLPE against oxidation and wear; and, (iii) lipid absorption occurred even in the rim zone where the MPC layer remained. Based on these evidences we consider that the declaimed advanced MPC technology is not a suitable one to elongate the in vivo lifetime of hip joints.

STATEMENT OF SIGNIFICANCE

Several studies reported that highly crosslinked polyethylene (HXLPE) have resulted in reduced wear in total hip arthroplasty. Beyond those studies, HXLPE hip liners grafted with 2-methacryloyloxyethyl phosphorylcholine (MPC) on their bearing surface were extensively studied in vitro and then commercialized as a new generation of artificial hip joints. The present study reports for the first time results about the evaluation of surface modification and oxidative degradation in retrieved the MPC grafted liners. The findings of this investigation clearly show that the MPC layer has been peeled off on the bearing surface of the liner main wear zone although the MPC layer remained on the surface of the rim zones. Furthermore, we assessed the microstructural modifications and the oxidation drifts that occurred in vivo in the hip joints despite the presence of the MPC layer.