Introduction

It is well-known that wear debris generated by metal-on-metal hip replacements leads to aseptic loosening. This process starts in the local tissue where an inflammatory reaction is induced, followed by an periprosthetic osteolysis. MOM bearings generate particles as well as ions. The influence of both in human bodies is still the subject of debate. For instance hypersensitivity and high blood metal ion levels are under discussion for systemic reactions or pseudotumors around the hip replacement as a local reaction. The exact biopathologic mechanism is still unknown. The aim of this study was to investigate the impact of local injected metal ions and metal particles.

Improvements in arthroplasty design and materials led to superior lifetime of the implants. Nevertheless, aseptic loosening due to particulate debris is still one of the most frequent late reasons for revision of hip and knee replacements. The complex process of inflammation and osteolysis due to wear particles is not understood in detail so far. A cellular and receptor mediated response to wear particles results in a release of pro-inflammatory cytokines and induces an inflammatory reaction causing periprosthetic osteolysis. The overall cellular response is influenced by particle volume as well as characteristics. But there is still a lack of data concerning all signalling pathways that are involved. To answer some open questions appropriate in vivo models are shown closing the loop between wear simulation, particle analysis, generation of sterile particles and biological evaluation. Beyond that, new aspects of particle effects and deposits in retrieved human tissue are given.

The biological reaction in metallosis and pseudotumor generation after metal on metal total hip arthroplasty or corroding metal implants remains unsettled. Clinically, still lethal cases appear with massive bone loss and metal ions are suspected to be responsible for this inflammatory reaction, solid metal wear particles instead are usually not observed in the common literature. The aim of this study was to compare the biological reactions of metal ions and metal wear particles in a murine in vivo model. Metal ions (CoCr), metal particles (CoCr), polyethylene particles (UHMWPE) and phosphate buffered saline (PBS) were injected into the left knee joint of female BALB/c mice. 7 days after injection, the microcirculation was observed using intravital fluorescence microscopy, followed by euthanasia of the animals. After the assessment of the knee diameter, the knees underwent histological evaluations of the synovial layer. Throughout all recorded data, CoCr particles caused higher inflammatory reactions compared to metal ions and UHMWPE particles. The mice treated with the solid particles showed enlarged knee diameters, more intensive leukocyte–endothelial cell interactions and an elevated functional capillary density. Pseudotumor-like tissue formations in the synovial layer of the mice were only seen after the exposition to solid CoCr particles. Even if the focus of several national guidelines concerning metallosis and pseudotumor generation is on metal ions, the present data reveal that solid CoCr particles have the strongest inflammatory activity compared with metal ions and UHMWPE particles in vivo.

Introduction

Metal on metal bearings are used especially in hip resurfacing. On the one hand, small bone preserving implants can be used. On the other hand recent studies found a variety of local and systemic side effects, for instance the appearance of pseudotumors, that are explained by pathologic biological reaction of the metal wear debris. The detailed mechanisms are still not understood until now. Thus it was the aim of this study to investigate the local reaction of metal wear particles and metal ions in a murine model. The hypothesis was that mainly metal ions provoke adverse histopathological reactions in vivo.

Knee arthroplasty with a rotating hinge knee (RHK) prosthesis has become an important clinical treatment option for knee revisions and primary patients with severe varus or valgus deformities and instable ligaments. The rotational axle constraints the anterior-posterior shear and varus-valgus moments, but currently used polyethylene bushings may fail in the mid-term due to insufficient creep and wear resistance of the material. Due to that carbon-fibre-reinforced (CFR) PEEK as an alternativ bushing material with enhanced creep, wear and fatigue behaviour has been introduced in a RHK design [Grupp 2011, Giurea 2014]. The objective of our study was to compare results from the pre-clinical biotribological characterisation to ex vivo findings on a series of retrieved implants.

In vitro wear simulation according to ISO 14243-1 was performed on rotating hinge knee devices (EnduRo® Aesculap, Germany) made out of cobalt-chromium and of a ZrN multi-layer ceramic coating for 5 million cycles. The mobile gliding surfaces were made out of polyethylene (GUR 1020, β-irradiated 30 ± 2 kGy). For the bushings of the rotational and flexion axles and the flanges a new bearing material based on CFR-PEEK with 30% PAN fiber content was used.

Analysis of 12 retrieved EnduRo^® RHK systems in cobalt-chromium and ZrN multi-layer in regard to

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loosening torques in comparison with initial fastening torques

with a focus on the four CFR-PEEK components integrated in the EnduRo^® RHK design.

For the rotating hinge knee design with flexion bushing and flanges out of CFR-PEEK the volumetric wear rates were 2.3 ± 0.48 mm³/million cycles (cobalt-chromium) and 0.21 ± 0.02 mm³/million cycles (ZrN multi-layer), a 10.9-fold reduction (p = 0.0016). The UHMWPE and CFR-PEEK particles were comparable in size and morphology and predominantly in submicron size [5]. The biological response to representative sub-micron sized CFR-PEEK particles has been demonstrated in vivo based on the leucoyte-endothelian-cell interactions in the synovia of a murine intra-articular knee model by Utzschneider 2010. Schwiesau 2013 extracted the frequency of daily activities in hip and knee replacement patients from literature and estimated an average of 1.76 million gait cycles per year. Thus, the 5 million cycles of in vitro wear testing reflect a mean in vivo service life of 2.9 years, which fits to the time in vivo of 12–60 months of the retrieved RHK devices. The in vitro surface articulation pattern of the wear simulation tests are comparable to findings on retrieved CFR-PEEK components for both types of articulations – cobalt-chromium and ZrN multi-layer coating.

For the rotating hinge knee design the findings on retrieved implants demonstrate the high suitability of CFR-PEEK as a biomaterial for highly loaded bearings, such as RHK bushings and flanges in articulation to cobalt-chromium and to a ZrN multi-layer coating.

Introduction

The complex process of inflammation and osteolysis due to wear particles still is not understood in detail. So far, Ultra-high-molecular-weight-polyethylene (UHMWPE) is the bearing material of choice in knee arthroplasty and revision knee arthroplasty, but there is a growing demand for alternative bearing materials with improved wear properties. Lately, increasing interest developed in the use of natural and carbon-fiber-reinforced-poly-ether-ether-ketones (CFR-PEEK).

While there is a lack of data concerning the effects of CFR-PEEK particles on human tissue, the effects of such wear debris in vitro and in animal studies is controversially discussed.

The aim of this study was to analyze human tissue containing CFR-PEEK as well as UHMWPE wear debris.

The authors hypothesized no difference between the used biomaterials because of similar size parameters of the wear particles in a prior knee simulator study of this implant.

Introduction

The complex cellular mechanisms of the aseptic loosening of total joint arthroplasties still remain not completely understood in detail. Especially the role of adherent endotoxins in this process remains unclear, as lipopolysaccharides (LPS) are known to be very potent modulators of the cell response on wear particle debris. Contributing factors on the LPS affinity of used orthopedic biomaterials as their surface roughness have to be investigated. The aim of this study was to evaluate the affinity of LPS on the surface roughness of different biomaterials in vitro. The hypothesis of the study was that rough surfaces bind more LPS than smooth surfaces.

Introduction

Minimally invasive implanted unicompartmental knee arthroplasty (UKA) leads to excellent functional results. Due to the reduced intraoperative visibility it is difficult to remove extruded bone cement particles, as well as bone particles generated through the sawing. These loose third body particles are frequently found in minimally invasive implanted UKA.

The aim of this study was to analyse the influence of bone and cement particles on the wear rate of unicompartmental knee prostheses in vitro.

Periprosthetic osteolysis depends on the biological activity of wear particles, but there is little known about the distribution of polyethylene wear particles (PE) in the surrounding joint tissue. The purpose of this study was to examine the localisation of wear particles of six different PEs, including four crosslinked polyethylenes (XPE), as well as their biological activity in the murine knee.

Introduction

Unicompartmental knee arthroplasty (UKA) in patients with isolated medial osteoarthritis of the knee is nowadays a standard procedure with good results, especially with the minimally-invasive approach. However, the survival rate of the unicompartmental knee prostheses is inferior to that of total knee prostheses. Therefore, further studying of UKA is still necessary. In most mobile bearing designs the femoral component has a spherical surface and therefore its positioning is not crucial. The role of the tibial slope in UKA has not been investigated so far. The manufacturers recommend tibial slopes with values between 10° positive slope and 5° negative slope. Most surgeons try to reconstruct the anatomical slope with a high failure by measuring the slope on x-rays. The aim of this study was to investigate the influence of the tibial slope on the wear rate of a medial UKA.

Objectives: The aim of this study was a clinical and radiological evaluation of 68 shoulders operated with the Delta reverse-ball-and-socket total shoulder prosthesis by the senior author with a mean follow-up of 42 months.

Methods: This is a retrospective study in one consecutive series of 68 shoulders, operated by the senior author, which were clinically assessed using the Constant score for pain, Constant Shoulder Score, Oxford Shoulder Score, UCLA Shoulder rating scale, DASH Score, Rowe Score for Instability and Oxford Instability Score. Radiological evaluation was graded by the classification according to Nerot et al. and complications were analysed according to Goslings and Gouma. Patients were evaluated before surgery and at a mean clinical follow-up of 42 months.

Results: There was a significant improvement in all clinical and stability scores. On the average, the Constant score for pain increased from 4.62 to 11.08 points (p< 0.05); the Constant Shoulder Score from 32.65 to 60.31 (p> 0.05); the Oxford Shoulder Score increased from 32.65 to 60.31 (p< 0.05) and the UCLA Shoulder rating scale increased from 15.08 to 27.42 (p< 0.05). The evaluation of stability showed an increase from 49.42 to 80.19 points in the Rowe Score for Instability and from 22.04 to 37.62 in the Oxford Instability score (p< 0.05). According to the Nerot classification, 65 percent of patients were graded as “0”, 20 percent as “1”, 3 percent as “2”, 6 percent as “3” and 6 percent as “4”. Eight complications occurred in terms of a nerve lesion which was graded according to Goslings and Gouma as “1” once, loosening of the humeral stem which was graded as “2” three times and loosening or fracture of the glenoid component which was graded as “2” in five times. At mean follow-up of 42 months, one patient of this series had died of decrepitude which was graded as “4” and one patient was lost of follow-up.

Conclusions: We summarize, that there were significant advantages identified in terms of the Constant score for pain, all clinical scores and the instability scores. Radiological analyses showed 85 percent of patients without or with a small notch only. On the other hand, the rate of complications should be taken into account. We conclude that shoulder arthroplasty with the Delta prosthesis shows significant benefits in terms of less shoulder pain, a higher stability and a gain of range of motion but on the other hand, we emphasize that this treatment remains a salvage procedure in the elderly only.

Crosslinked polyethylene (XPE) was developed to reduce wear in hip and knee arthroplasty. Periprosthetic osteolysis depends on many factors including biological activity of wear particles. This study examines the relative inflammatory effect of different crosslinked polyethylenes compared to ultra-high-molecular-weight-polyethylene (UHMWPE) particles in vivo.

Materials and Methods: Wear particles of 3 XPE- (1 sequential irradiated/annealed; 2 remelted inserts) and 1 UHMWPE-insert were isolated from a knee joint simulator (20nm-nucleopore-filter;acid digestion method;ISO). Particles were analysed by scanning electron microscopy (n=66000). For all groups the particles were smooth, granular, irregular and less fibrillar. More than 85% of the particles were submicron. After removal of endotoxin the particles were suspended in a phosphate buffered saline solution (0.1% vol/vol (particle volume/PBS volume)). Endotoxin levels were controlled using standardised endotoxin detection tests (Lonza) in all samples.

40 female Balb/c mice were randomly assigned to one of five treatment groups (according to the national guidelines of animal protection laws): control (n=8); XPE1 (95 kGy E-beam, remelted; n=8); XPE2 (65 kGy E-beam, remelted; n=8), XPE3 (3x30 kGy Gamma, annealed and sequential irradiated; n=8) and UHMWPE particles (n=8). 50 μl of the particle suspension were injected into the murine left knee under sterile conditions. The leukocyte–endothelial cell interactions and the synovial microcirculation were performed by intra-vital fluorescence microscopy one week after particle injection to assess the inflammatory reaction to the particles (by measuring the rolling fraction of leukocytes, the adherent cells and the functional capillary density (FCD)). Data analysis was performed using a computer-assisted microcirculation analysis system (Cap-Image).

For the statistical analysis the Kruskal-Wallis test was used to determine differences within the groups, followed by an all pairwise multiple comparison procedure with a Bonferoni correction. The level of significance was set at p< 0.05.

Results: The fraction of the rolling leukocytes, adherent cells and FCD increased significantly (p< 0.05) in all bio-materials compared to control group. However, there was no significant difference between the UHMWPE and the XPE particle groups (p> 0.05).

Conclusion: Our data suggest that crosslinked polyethylene wear particles do not lead to a higher inflammatory reaction in vivo compared to UHMWPE particles.

Osteolsyis is one of the main reasons for revision of total joint replacements. The osteolytic reaction is influenced by dose, size (particles < 1μm are believed to be biologically more active) and shape of wear particles, so that low wear rates and biologically less active particles are required.

In addition, in the knee a range of design and kinematic variables have to be considered as they can markedly influence wear regardless of the type of polyethylene used. Furthermore, UHMWPE (ultra-high-molecular-weight-polyethylene) fatigue occurs more frequently in the knee joint than in the hip joint due to its changing tribocontact areas combined with high weight bearing. This is why crosslinked polyethylene (XPE) is still controversially discussed for use in total knee prostheses.

Question: Is XPE appropriate for both fixed- and mobile-bearing knee prostheses? Are XPE-particles different from UHMWPE-particles?

Method: In a knee-joint-simulator (Stallforth-Ungeth-uem) 4 XPE (1 sequential irradiated/annealed, 3 different remelted, fixed- and mobile-bearing-types) and 2 UHMWPE-inserts (fixed- and mobile-bearing) were tested (ISO). The gravimetric wear rates (mg/year) were measured (5mill. cycles), the wear mechanism was analysed by scanning electron microscope (SEM). Furthermore, 100.000 particles were analysed by SEM (20nm-nucleo-pore-filter;acid digestion method;ISO) in size and shape.

Results: All the inserts showed traces of abrasion, scratching and wear polishing. XPEs produced lower wear rates (range 0,6–4,3mg/year;p< 0,05) than UHM-WPEs (range 8,4–8,5mg/year) in fixed- as well as mobile-bearing knee prostheses without fatigue reactions. The sequentially irradiated and annealed insert showed the lowest wear rate (0,6mg/year;p< 0,05) overall. For all the groups the particles were smooth, granular, irregular and less fibrillar and more than 85% of the particles were submicron. The particle size was independent of the radiation dose. Fixed-bearings produced a larger amount of particles < 1μm (93,2–96,3%;p< 0,05) than mobile-bearings (85,5–89,5%).

Conclusion: All crosslinked tibial inserts, fixed- as well as mobile-bearings, showed statistically significant (p< 0.05) reduced wear rates without any fatigue reactions. The fixed-bearing sequential irradiated and annealed insert had the lowest wear rate (p< 0.05). XPEs (fixed- and mobile-bearings) and UHMWPEs have similar wear particles in shape. The particle size was independent of the radiation dose, but most of the particles are in the biologically more active range of size (fixed> mobile-bearing). Due to this further investigation with in-vivo-models is required.

Periprosthetic osteolysis, caused in a chronic inflammatory adverse reaction to wear particles in the surrounding tissues, is one of the major reasons for revision arthroplasty so that articulating surfaces with low wear rates are required. Compared with conventional ultra high molecular weight polyethylene (UHMWPE), highly crosslinked polyethylene (HXLPE) shows a reduced wear rate in a hip simulator. The crosslinking process which is achieved by gamma or electronic radiation, followed by heat treatment either above the melting point (remelting) or below (annealing), reduces the mechanical properties of UHMWPE, particularly its fatigue strength. UHMWPE fatigue occurs more frequently in the knee than in the hip due to its higher contact stresses. This is why HXLPE is still controversially discussed for use in total knee prostheses. We have examined the wear behaviour of different HXLPEs [one cruciate-retaining (CR; sequential irradiation and annealing), one ultra-congruent (remelting), one CR (remelting)], compared with conventional UHMWPE in a knee simulator (Stallforth-Ungethuem). In the fixed bearing knee recommended from the manufacturer the wear rates [gravimetric (mg/year); volumetric (mm³/year)] were determined according to the ISO standard and the wear mechanism was analysed by means of a scanning electron microscope.

All insert showed signs of abrasion, scratching and wear polishing, but no traces of fatigue reactions. All HXLPEs produced lower (p< 0.05) wear rates (0.47–3.3 mg/year; 0.5–3.5 mm3/year) than the UHMWPE (8.1–9.1 mg/year; 8.6–9.7 mm3/year), the inserts of HXLPE manufactured by sequential irradiation and annealing showed the lowest wear rates (p< 0.05) overall.

Due to the reduced wear rates without any fatigue symptoms, we conclude that HXLPE is suitable for total knee prostheses and a monitored clinical investigation can be recommended. HXLPE manufactured by sequential irradiation and annealing seems to produce still lower wear rates than those manufactured by remelting, at least when used in total knee prostheses.