Total joint replacement is a successful clinical intervention. However, aseptic loosening due to wear related particulate debris is still one of the most frequent reasons for late revision of total joint replacement. This lecture gives an overview about the application of methods to study wear and friction in total joint replacements (e.g. hip, knee, shoulder). This involves complex joint simulation conditions as well as analytical assessments. Regarding joint simulation the focus will be on ligament stabilized joints. New approaches will be shown and discussed. Furthermore, analytical methods to study the release of wear products in term of solid particles and soluble complexes like metal ions will be presented.
The CoCrMo large bearings had shown a high failure rate, because of metal ion and particle release. Alumina matrix composite (AMC) ball heads have shown to mitigate such phenomena. The aim of this study was to investigate the leaching properties of AMC clinically as well as experimentally. Two patient groups were compared: a control group (n=15) without any implant (Controls) and 15 Patients with unilateral treatment with Biolox delta ceramic-on-ceramic (CoC). Whole-blood samples of Controls and Patients (after 3 and 12 months from treatment with CoC) were measured by means of trace element analysis using a HR-ICPMS. The leaching behaviour of BIOLOX delta was also analysed in-vitro: five Biolox delta heads and five CoCrMo heads were immersed in serum for seven days at 37°C. Aluminium, cobalt, chromium and strontium were detected based on HR-ICPMS.Background
Methods
In total hip arthroplasty the femoral head is connected to the stem based on a taper connection (Fig. 1). Implant manufacturers proclaim that the modular tapers are not standardized and can vary from manufacturer to manufacturer. The combination of different implant components from different manufacturers (Mix & Match) is not permitted. However, in case of revision surgery, where the stem is still well fixed, the surgeon may decide to use a femoral head of a different manufacturer (Mix & Match). This decision may be related to a limited availability of a manufacturer-identical head or the manufacturer can't be identified. In this study, different taper combinations were experimentally investigated to assess the effect of Mix & Match on taper strength. Hip stems and metal heads (n=3) of four different implant manufacturers (Biomet, DePuy, Smith&Nephew, Zimmer) were used. Firstly, torque-off tests similar to the ISO draft (ISO/DIS 7206-13) were performed without mixing the components. Subsequently, the stems were combined with metal heads from other manufacturers. All taper connections were impacted with 3 kN and then an increasing torque was applied until the head disconnected using a material testing machine (MTS Mini Bionix), (Fig. 2). The maximal torque off value was used as a measure for the taper strength.Introduction
Methods
Taper corrosion and fretting has been identified to be a major problem in total hip replacement during the past years. Taper design and manufacturing are not been standardised, and therefore it can be assumed that the tapers vary among different implant manufacturers. This can lead to variable contact situations and stresses in the taper junction depending on the combination. It can be assumed that the taper strength will influence the occurrence and magnitude of micromotions which are known to influence corrosion. Therefore, the aim of this study was to assess the influence of the taper angle clearance on the taper connection strength. For the investigation stem dummys with different taper angles were used that were manufactured from titanium alloy. The stem dummys were combined with ceramic heads with identically taper angles. Out of this, there were seven groups ranging from distal contact through full contact up to proximal contact. Three samples were used in each group and five repetitive measurements per samples were performed. All taper connections were impacted with different forces (1 kN, 3 kN, 6 kN and 10 kN) and afterwards an increasing torque was applied until the head disconnected. The maximal torque off value was used as a measure for the taper strength.Introduction
Material & Methods
Total shoulder arthroplasty (TSA) has become a successful treatment option for degenerative shoulder disease. With the increasing incidence in primary TSA procedures during the last decades, strategies to improve implant longevity become more relevant. Implant failure is mainly associated with mechanical or biological causes. Chronic inflammation as a response to wear particle exposure is regarded as a main biological mechanism leading to implant failure. Metal ions released by fretting and corrosion at modular taper connections of orthopedic implants can cause cell-mediated hypersensitivity reactions and might lead to aseptic loosening. Modularity is also commonly used in total shoulder replacement. However, little is known about metal ion exposure in patients following TSA. The objective of this study was to determine in-vivo blood metal ion levels in patients after TSA and to compare blood metal ion levels to control subjects without metal implants. A total of 19 patients with anatomical total shoulder prosthesis (TSA group) and 20 patients with reverse total shoulder prosthesis (RSA group) who underwent unilateral total shoulder replacement at our hospital between March 2011 and December 2014 with no other metal implant or history of environmental metal ion exposure were recruited for analysis of blood metal ion concentrations of cobalt (Co), chromium (Cr) and titanium (Ti) at a mean follow-up period of 2.3 years (0.7–4.3). For comparison of metal ion concentrations blood samples were obtained in a healthy control group of 23 subjects without metal implants. Ethical approval and informed consent of each patient were obtained for this study.Purpose
Methods
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. Three groups, each with 10 Balb / c mice were generated. Group A: injection of a 50 µl metal ion suspension at a concentration of 200 µg / l in the left knee. Group B: injection of a 50 µl 0,1 vol% metal particle suspension into the left knee joint. Group C (control group): injection of a 50 µl of 0,1 vol% PBS-suspension in the left knee. Incubation for 7 days, followed by euthanasia of the animals by intracardiac pentobarbital. The left and right knee, the lungs, kidneys, liver and spleen were removed. Histologic paraffin sections in 2 microns thickness were made, followed by HE (overview staining) and Movat (Pentachrom staining) staining. The histologic analysis was a done by a light microscopic evaluation of the subdivided visual fields at 200× magnification.Introduction
Material and Methods
Wear plays a key role in the clinical outcome of total hip replacements (THR). In addition, increased frictional moment can stress the implant interfaces which may lead to high torsional loadings in the intermodular taper junction (fretting) and cup loosening and to the development of noise (squeaking). Against the background of larger head diameters (increased range of motion and decreased risk of dislocation), the friction induced by the joint articulation is of particular interest. As of now, the investigation of friction with the use of relevant joint kinematics and loadings are limited to numerical studies. Experimental approaches use simplified models which do not take into consideration complex activities. Thus, with the aim of this study is the identification of articular frictional moments that consider critical A standard hip simulator (Minibionix 852 with 4 DOF Hip setup, MTS, Eden Prairie, USA) was modified in order to allow for high-precision friction measurements during head-insert articulation in all 6 DOF (MC2.5D-500, AMTI, Boston, USA). Disturbing systemic effects have been minimized by using quasi frictionless aerostatic lateral force compensation (Eitzenberger, Wessobrunn, Germany) and cross talk compensation. Beside the standard protocoll for Introduction
Materials and Methods
This study was performed to investigate the failure mechanism of one specific hip arthroplasty cup design that has shown a high clinical failure rate. The aim of this study was to identify general design problems of this polyethylene inlay. 55 consecutive retrievals of a cementless screw ring (Mecron) were collected. In any case a 32 mm ceramic head was used. All implants failed due to aseptic loosening. The follow-up of the implants was 3 to 16 years. We recorded backside wear, fatigue of the polyethylene at the flanges on the outer rim and at the cup opening (32 mm inner diameter). To assess the deformation of the inlay, the smallest and the median diameter of the cup opening were measured using a 3 dimensional coordinate measuring machine (Multisensor, Mahr, Germany).Introduction
Material and Methods
The numbers of anatomic total shoulder joint replacements (ATSR) is increasing during the past years with encouraging clinical results. However, the survivorship of ATSR is lower as compared to total knee and hip replacements. Although the reasons for revision surgery are multifactorial, wear-associated problems like loosening are well-known causes for long-term failure of ATSR. Furthermore there is lack of valid experimental wear tests for ATSR. Therefore the purpose of this study was to define experimental wear testing parameters for ATSR and to perform a wear study comparing ceramic and metallic humeral heads. Kinetic and kinematic data were adopted from in-vivo loading measurements of the shoulder joint ( The studied ATSR (TuronTM, DJO Surgical, USA) resulted in a polyethylene wear rate of 62.75 ± 1.60 mg/106 cycles in combination with metallic heads. The ceramic heads significantly reduced the wear rate by 26.7 % to 45.99 ± 1.31 mg/106 (p<0.01). The wear scars dimensions were in good agreement to clinical retrievals. This study is the first that experimentally studied the wear behavior of ATSR based on clinical and biomechanical data under load controlled conditions. In term of wear the analyzed ATSR could clearly benefit from ceramic humeral heads. However, in comparison to experimental wear studies of total knee and hip replacements the wear rate of the studied ATSR was relatively high. Therefore further research may focus on optimized wear conditions of ATSR and the hereby described method may serve as a tool to evaluate a wear optimization process.
The optimal bearing for hip arthroplasty is still a matter of debate. in younger and more active patients ceramic-on-polyethylene (CoP) bearings are frequently chosen over metal-on-polyethylene (MoP) bearings to reduce wear and increase biocompatibility. However, the fracture risk of ceramic heads is higher than that of metal heads. This can cause serious issue, as ceramic fractures pose a serious complication often necessitating major revision surgery – a complication more frequently seen in ceramic-on-ceramic bearings. To date, there are no long-term data (> 20 years of follow-up) reporting fracture rates of the ceramic femoral heads in CoP bearings. We retrospectively evaluated the clinical and radiographic results of 348 cementless THAs treated with 2nd generation Biolox® Al2O3 Ceramic-on-Polyethylene (CoP) bearings, which had been consecutively implanted between January 1985 and December 1989. At implantation the mean patient age was 57 years. The cohort was subsequently followed for a minimum of 20 years. At the final follow-up 111 patients had died, and 5 were lost to follow-up (Fig. 1). A Kaplan-Meier survivorship analysis was used to estimate the cumulative incidence of ceramic head fractures over the long-term.Introduction
Patients and Methods
Taper corrosion has been identified to be major problem in total hip replacement during the past years. Patients may suffer from adverse local tissue reactions (ALTR) due to corrosion products that are released from modular taper connection. So far, the mechanism that leads to taper corrosion in taper connections is not fully understood. Some retrieval studies tried to correlate implant related design parameters to the incidence and the severeness of taper corrosion. For example Kocagöz et al.[1] have not seen an influence of the taper clearance to taper corrosion. Hothi et al.[2] showed that shorter and rougher tapers increase taper corrosion. One limitation of retrieval studies may be that the analysed tapers are used and may have been altered during in vivo service. Beside the effect of taper corrosion many surgeons are not aware that the tapers may vary among different manufactors. With our study we want to provide taper related data that may be used for comparison and correlation (e.g. retrieval studies). Therefore we aimed to assess and compare geometric and topographic design parameters of currently available hip stem tapers from different manufacturers. For comparison well established cementless hip stems made of titanium alloy were choosen. All of them have a 12/14-taper. The analysed implants are shown in Fig. 1. As geometrical parameters the taper angle, the opening taper diameter and the taper length were measured using a coordinate measuring machine (CMM) (MarVision MS 222, Mahr, Göttingen, Germany; accuracy: ±2.3 µm). Several topographical parameters (e.g. Ra, Rz, etc.) were determined using a tactil roughness measurement instrument (Perthometer M2, Mahr, Göttingen, Germany; accuracy: 12 nm). Three independent samples of each taper were measured five times.Introduction
Material
Temporary use of antibiotic-impregnated polymethylmethacrylate (PMMA) bone cement spacers in two-stage revisions is considered to be standard of care for patients with a chronic infection of a joint replacement. Spacers should be wear resistant and load-bearing to avoid prolonged immobilisation of the patient and to reduce morbidity. Most cement spacers contain barium sulphate or zirconium dioxide as radio-opaque substrate. Both are quite hard materials that may negatively influence the wear behaviour of the spacer. Calcium carbonate is another radio-opaque substrate with lower hardness potentially increasing the wear resistance of the spacer materials. The purpose of the study was to compare a prototype PMMA knee spacer (calcium carbonate loaded) with a commercially available spacer (containing barium sulphate) regarding the wear performance and particle release in a knee wear simulator. Spacer K (TECRES, Italy) was used as barium sulphate (10%) containing spacer material. A prototype material (Heraeaus Medical, Germany) with 15% calcium carbonate was compared. Both were gentamicin impregnated, ready-made for clinical application (preformed) and consist of a tibial and a femoral component. Force-controlled simulation was carried out on an AMTI knee simulator. The test parameters were in accordance to ISO 14243–1 with a 50% reduced axial force (partial weight bearing). Tests were carried out at 37 °C in closed chambers filled with calf serum. Tests were run for 500,000 cycles at a frequency of 1 Hz. For wear analysis, gravimetric wear measurements according to ISO 14243–2 and wear particle analysis according to ASTM F1877–05 were performed.Introduction
Material and Methods
Modular knee implants are used to manage large bone defects in revision total knee arthroplasty. These implants are confronted with varying fixation characteristics, changes in load transfer or stiffen the bone. In spite of their current clinical use, the influence of modularity on the biomechanical implant-bone behavior (e.g. implant fixation, flexibility, etc.) still is inadequately investigated. Aim of this study is to analyze, if the modularity of a tibial implant could change the biomechanical implant fixation behavior and the implant-bone flexibility. Nine different stem and sleeve combinations of the clinically used tibial revision system Sigma TC3 (DePuy) were compared, each implanted standardized with INTRODUCTION
MATERIAL & METHODS
Knee wear simulator studies are performed to evaluate wear behavior of implants. Simulation of the human gait cycle is often carried out continuously, without considering resting periods as they are part of patient’s daily live. In addition to dynamic activities like walking, daily activities also consist of static periods like standing, sitting or lying. During the day dynamic activities alternate continuously with static periods and most of the day is spent in passive periods, where no joint motion occurs. Such resting periods have not yet been considered in prosthetic knee wear tests. Implementing resting periods may cause an increase in friction and thus increased wear of the implant. The aim of the current study was to determine if the implementation of resting periods would increase polyethylene (PE) wear in total knee replacement (TKR). Two wear studies were conducted using a force controlled AMTI knee simulator on a conventional bicondylar TKR. For the first study, simulation was carried out continuously according to ISO 14243-1. For the second test, four active gait cycles according to ISO 14243-1 were followed by one resting period cycle. In both tests 5x10E6 active load cycles at a frequency of 1 Hz (resulting in additional 1.25x10E6 pause cycles for the second test) were applied. Wear was measured gravimetrically and wear scars were documented photographically. The mean wear rates measured 2.85 ± 0.27 mg/10E6 cycles for the ISO test without considering resting periods and 2.27 ± 0.23 mg/10E6 cycles for the test with resting periods implemented. There was no significant difference (p=0.22) in wear rate between both tests. The inserts showed similar wear scars in both tests and no relevant differences in dimension and localization on the surface. Therefore the wear behavior after the two tests was similar. Since wear is one of the most limiting factors for implant longevity, proper preclinical wear studies are essential. Based on the results of this experimental wear study, a continuous simulation without additional resting periods seems to be valid in wear simulation of TKR.
Tension wires seem to dominate revision surgery and studies only refer to the advantages of cable wires in general. This in-vitro study analyzed the infiuence of both, tension and cable wires on primary stability of cementless revision stems. We aimed to examine the effectivity of wiring a femoral osteotomy, differences achieved with each method, and whether one wire has advantages regarding the fixation concepts of revision stems (meta- and diaphyseal).
Resurfacing hip implants differ in macro- and microstructure. Manufacturing related parameters like clearance or carbon content influence the wear behaviour of these metal-on-metal bearings. The aim of this study was to analyse the main macro- and micro-structural differences of commercially available resurfacing hip implants. Ten different commercially available resurfacing hip implant designs were included in this investigation:
- BHR® (Smith&
Nephew/MMT) - Durom® (Zimmer) - Conserve Plus® (Wright Medical) - Cormet® (Corin) - Icon® (IO) - ReCap® (Biomet) - Adept® (Finsbury) - ASR® (DePuy) - BS® (Eska) - Accis® (Implantcast). The heads and cups were measured in a coordinate measuring machine and radial clearance as well as sphericity deviation were calculated. Surface roughness measurements were carried out. The microstructures of the heads and cups were inspected using SEM and element analysis was performed using EDX to identify carbides and the alloy composition. The mean radial clearance was found to be 85.53 μm. The range was from 49.47 μm (DePuy, ASR®) to 120.93 μm (Biomet, ReCap®). All implants showed a sphericity deviation of less than 10 μm. The highest sphericity deviation was found to be 7.3 μm (Corin Cormet® head), while the lowest was 0.8 μm (Smith&
Nephew BHR® head). On average, the heads tended to have a higher sphericity deviation (4.1 μm, SD: 2.3 μm) compared to the cups (2.7 μm, SD: 1.4 μm). SEM revealed that most manufacturers use a high carbon alloy casting manufacturing process combined with heat treatment after casting (Corin Cormet® and Wright Conserve®: head and cup; DePuy ASR®: cup; Eska BS®: head). Commercially available resurfacing hip implants differ in design and manufacturing parameters, including macro- and microstructure, which are critical in achieving low wear and ion release. This study was designed to aid in the understanding of clinical observations. Also, specific information is now available for surgeons choosing an implant designs.
For wear testing of knee implants, ISO 14243 is the most used testing protocol. In force control, this standard requires linear motion restraints for simulation of ligaments. The aim of this study was to investigate if a nonlinear, physiological motion restraint would influence the wear behaviour of the implants. A wear study was performed on a highly conforming knee implant design. Three implants were tested forced controlled according to ISO 14243-1 on an AMTI knee simulator. Linear motions restrain of 30 N/mm for AP-translation and 0.6 Nm/° for IE-rotation were applied as required per ISO 14243-1. A second wear test was performed on the same implant design. Based on the data given by Kanamori et al. and Fukubayashi et al., a physiological, nonlinear ligament constraint model (sectioned ACL) was adopted and implemented in the simulation. The implants were pre-soaked and a soak controls was used. Wear was measured gravimetrically. A mean gravimetric wear rate of 2.85 mg/10E6 cycles was found for the implants which were tested using a linear motion restraint as required per ISO 14243-1. Simulating a physiological, nonlinear motion restraint resulted in a 60% increase in gravimetric wear (mean gravimetric wear rate: 4.75 mg/10E6 cycles). As expected, the kinematics of the implants differed between wear tests. The mean AP-translation increased from 2.89 mm (linear motion restraint) to 4.82 mm (physiological motion restraint). A similar behaviour was observed for the IE-rotation. The IE-rotation increased from 4.09° (linear motion restraint) to 5.94° (physiological motion restraint). The reaction of the ligaments is not linear in the human knee joint. This study showed that wear and kinematics change when simulating physiological ligament reactions. Wear increased by 60%, an effect which can likely be credited to fundamental differences in kinematics. The ACL is commonly sacrificed during surgery. Thus, more attention should be paid to ligament simulation when performing wear tests on knee implants.
The introduction of mobile bearings for unicompartimental knee implants resulted in heightened interest in this implant design in the field of orthopaedics. This study aims to determine the effect of the mobile and fixed design concepts on the wear progression in unicompartmental knee implants using a knee simulator. An unicompartmental knee implant design, which is available in a fixed and mobile version, was tested using a knee simulator. For the wear test, the medial and lateral compartments were implemented in the simulator. To account for the physiologically higher medial load compared to the lateral compartment, a medially-biased load distribution was implemented. The wear test was performed force controlled according to ISO 14243. Wear was measured gravimetrically separately for the medial and lateral compartments. To evaluate implant kinematics, AP-translation and IE-rotation were measured during the simulation. Gravimetric wear was higher medially than laterally for both designs. The mean wear rate of the medial mobile compartment was found to be 10.70 mg/10E6 cycles, whereas a mean wear rate of 6.05 mg/10E6cycles was found for the medial compartment of the fixed design. Lateral wear rates, which were about 50% lower than medial wear rates, were found to be 5.38 mg/10E6 cycles in the mobile design and 3.23 mg/10E6 cycles in the lateral design. Wear of the mobile design was higher compared to the fixed design, both medially and laterally. Surprisingly the kinematics of both designs were very similar. A low AP-translation of 2.7 mm in the mobile and 2.4 mm in the fixed designs was documented. High IE-rotations of 6.5° and 6.7° for the mobile and the fixed design, respectively, were observed. In bicondylar bearing knee designs, reduced wear has been reported for mobile polyethylene inlays. This study showed that the wear behaviour of unicompartmental knee implants differs from bicondylar implants and that the introduction of the mobile concept may lead to increased wear.
In hip joint simulator studies, wear measurement is usually performed gravimetrically. This procedure is reliable for metal-on-polyethylene or ceramic-on-polyethylene bearings, in which relatively high amounts of abrasive wear particles are produced. With modern hard-on-hard bearings, volumetric wear decreases significantly up to 100 to 200-fold. The gravimetric method reaches its detection limit with metal-on-metal bearings and even more so with ceramic-on-ceramic bearings. This study establishes a new method of determining wear in hard-on-hard bearings by measuring the amount of worn particles/ions in the serum of hip simulators. A wear study on three resurfacing hip implants (BHR®, Smith&
Nephew) was conducted using a hip joint simulator. Prior to the wear study, tests were performed to validate the detection power for high resolution-inductively coupled plasma-mass spectrometry (HR-ICP-MS). More importantly the system’s accuracy was compared to the gravimetric method, which is described in ISO 14243-2. The simulator was altered to run completely metal ion free. The ion concentration in the serum was measured every 100 000 cycles up to 1 500 000 cycles and subsequently in intervals of 500 000 cycles using HR-ICP-MS. The implants were neither removed from the simulator nor excessively cleaned during the course of the simulation. Serum was refreshed every 500 000 cycles. The serum samples were digested with purified nitric acid and hydrogen peroxide using a high pressure microwave autoclave in order to measure wear particles as well as dissolved ions. All steps were carried out under clean room conditions. Wear was calculated using the ion concentration and measured serum volume. Wear rates and transition from running-in to steady-state wear phases were calculated. A detection power better than 0.028 μg/l for Co (cobalt), 0.017 μg/l for Cr (chromium) and 0.040 μg/l for Mo (molybdenum) was found for HR-ICP-MS. The validation of HR-ICP-MS showed good agreement between gravimetric data and measured ion concentrations. The tested implants showed similar wear behaviour. Implant wear resulted in high ion concentrations during the first 380 000 to 920 000 cycles. During this period, a mean wear rate of 6.96 mm3/10E6 cycles was determined. Subsequently, the wear rate significantly decreased to a mean wear rate of 0.37 mm3/10E6 cycles. Thus, a mean ratio between running-in and steady-state wear of 18.8 was found. The mean overall wear volume at the end of the simulation was 4.42 mm3. This study showed that measuring the ion concentrations in the serum of hip simulators can be used to determine wear in metal-on-metal bearings. The main advantages of this new method are the ability to detect ultra-low wear rates and to precisely specify the duration of different wear phases. Because the implants do not have to be removed from the simulator and aggressive cleaning processes may be skipped, fluctuations in wear detection are extremely low. This in turn leads to a shorter duration of the simulation. Wear rates of the tested implants are low compared to polyethylene. Transferring the results to patient activity, wear would be the same during the first four to six months after implantation as in the next ten years. Minimizing the duration of running-in would be most effective in further reducing wear of metal-on-metal bearings.
Element analysis was performed by EDX (Oxford D. 7060) to identify carbides and the alloy composition. Element distribution maps were taken to separate the single elements.
All implants showed a sphericity deviation less than 10μm. On average the heads tended to have a higher spherical deviation of 4.1μm (SD: 2.3μm) compared to the cups 2.7μm (SD: 1.4μm). Based on the SEM and EDX inspection the manufacturing process, heat treatment and carbide distribution could be clarified.
The characteristically unspheric formations of the heads may be due to the cooling process after manufacturing the implant and there is also a relation between the wall thickness of the implant and the unspheric formations. With decreasing wall thickness the implant cools faster locally. Additionally a cup with a thin wall may deform under loading condition and a very tight clearance could be detrimental. This study will help to understand clinical observations. It still has to be proven that these biomechanical factors influence the clinical performance of hip resurfacing implants.
