Evaluation of impingement behaviour in lumbar spinal disc arthroplasty.

INTRODUCTION: The objective of our in vitro study was to introduce a test method to evaluate impingement in lumbar spinal disc arthroplasty in terms of wear, contact pattern, metal ion concentration and particle release. MATERIAL AND METHOD: Impingement wear simulation was performed on a 6-station spinal wear simulator (Endolab, Germany) on a lumbar spinal disc system (activ L Aesculap AG, Germany) using four different protocols specific to impingement in flexion, in extension, in lateral bending and in combined flexion bending. Impingement contact stress is intentionally created by applying an angular displacement of +2° in addition to the intended range of motion in the impingement direction, whereas a bending moment of 8 Nm remains constant during the impingement phase (plateau). RESULTS: An average volumetric wear rate of 0.67 mm(3)/million cycles was measured by impingement under flexion, of 0.21 mm(3)/million cycles under extension, of 0.06 mm(3)/million cycles under lateral bending and of 1.44 mm(3)/million cycles under combined flexion bending. The particle size distribution of the cobalt-chromium wear particles released by impingement in flexion (anterior), extension (posterior), lateral bending (lateral) and combined flexion bending (antero-lateral) revealed that most of the detected cobalt-chromium particles were in a size range between 0.2 and 2 µm. CONCLUSION: The impingement wear simulation introduced here proved to be suitable to predict in vivo impingement behaviour in regard to contact pattern seen on retrieved devices of the activ L lumbar disc arthroplasty design in a pre-clinical test.

A new universal, standardized implant database for product identification: a unique tool for arthroplasty registries.

INTRODUCTION: Every joint registry aims to improve patient care by identifying implants that have an inferior performance. For this reason, each registry records the implant name that has been used in the individual patient. In most registries, a paper-based approach has been utilized for this purpose. However, in addition to being time-consuming, this approach does not account for the fact that failure patterns are not necessarily implant specific but can be associated with design features that are used in a number of implants. Therefore, we aimed to develop and evaluate an implant product library that allows both time saving barcode scanning on site in the hospital for the registration of the implant components and a detailed description of implant specifications. MATERIALS AND METHODS: A task force consisting of representatives of the German Arthroplasty Registry, industry, and computer specialists agreed on a solution that allows barcode scanning of implant components and that also uses a detailed standardized classification describing arthroplasty components. The manufacturers classified all their components that are sold in Germany according to this classification. The implant database was analyzed regarding the completeness of components by algorithms and real-time data. RESULTS: The implant library could be set up successfully. At this point, the implant database includes more than 38,000 items, of which all were classified by the manufacturers according to the predefined scheme. Using patient data from the German Arthroplasty Registry, several errors in the database were detected, all of which were corrected by the respective implant manufacturers. CONCLUSIONS: The implant library that was developed for the German Arthroplasty Registry allows not only on-site barcode scanning for the registration of the implant components but also its classification tree allows a sophisticated analysis regarding implant characteristics, regardless of brand or manufacturer. The database is maintained by the implant manufacturers, thereby allowing registries to focus their resources on other areas of research. The database might represent a possible global model, which might encourage harmonization between joint replacement registries enabling comparisons between joint replacement registries.

Biotribological evaluation of artificial disc arthroplasty devices: influence of loading and kinematic patterns during in vitro wear simulation.

Wear simulation is an essential pre-clinical method to predict the mid- and long-term clinical wear behavior of newly introduced devices for total disc arthroplasty. The main requirement of a suitable method for spinal wear simulation has to be the ability to distinguish between design concepts and allow for a direct comparison of predicate devices. The objective of our study was to investigate the influence of loading and kinematic patterns based on two different protocols for spinal wear simulation (ISO/FDIS 18192-1 (2006) and ASTM F2423-05). In vitro wear simulation was performed with six activ L lumbar artificial disc devices (Aesculap Tuttlingen, Germany). The applied kinematic pattern of movement was multidirectional for ISO (elliptic track) and unidirectional with a curvilinear shape for ASTM. Testing was done for 10 million cycles in the ISO loading mode and afterwards with the same specimens for 5 million cycles according to the ASTM protocol with a customized six-station servohydraulic spinal wear simulator (EndoLab Thansau, Germany). Gravimetrical and geometrical wear assessment, a slide track analysis correlated to an optical surface characterization, and an estimation of particle size and morphology were performed. The gravimetric wear rate for the first 10 million cycles was ISO(initial) = 2.7 +/- 0.3 mg/million cycles. During the ASTM test period (10-15 million cycles) a gravimetric wear rate of 0.14 +/- 0.06 mg/million cycles was estimated. The wear rates between the ISO and ASTM driven simulations differ substantially (approximately 20-fold) and statistical analysis demonstrates a significant difference (p < 0.001) between the test groups. The main explanation of divergency between ISO and ASTM driven wear simulations is the multidirectional pattern of movement described in the ISO document resulting in a cross-shear stress on the polyethylene material. Due to previous retrieval observations, it seems to be very unlikely that a lumbar artificial disc is loaded with a linear wear path.Testing according to ASTM F2423-05 with pure unidirectional motion does not reflect the kinematics of TDA patients' daily activities. Based on our findings it seems to be more reliable to predict the clinical wear behavior of an artificial disc replacement using the ISO/FDIS 18192-1 method.

Biotribology of a vitamin E-stabilized polyethylene for hip arthroplasty - Influence of artificial ageing and third-body particles on wear.

The objective of our study was to evaluate the influence of prolonged artificial ageing on oxidation resistance and the subsequent wear behaviour of vitamin E-stabilized, in comparison to standard and highly cross-linked remelted polyethylene (XLPE), and the degradation effect of third-body particles on highly cross-linked remelted polyethylene inlays in total hip arthroplasty. Hip wear simulation was performed with three different polyethylene inlay materials (standard: γ-irradiation 30 kGy, N2; highly cross-linked and remelted: γ-irradiation 75 kGy, EO; highly cross-linked and vitamin E (0.1%) blended: electron beam 80 kGy, EO) machined from GUR 1020 in articulation with ceramic and cobalt-chromium heads. All polyethylene inserts beneath the virgin references were subjected to prolonged artificial ageing (70°C, pure oxygen at 5 bar) with a duration of 2, 4, 5 or 6 weeks. In conclusion, after 2 weeks of artificial ageing, standard polyethylene shows substantially increased wear due to oxidative degradation, whereas highly cross-linked remelted polyethylene has a higher oxidation resistance. However, after enhanced artificial ageing for 5 weeks, remelted XLPE also starts oxidate, in correlation with increased wear. Vitamin E-stabilized polyethylene is effective in preventing oxidation after irradiation cross-linking even under prolonged artificial ageing for up to 6 weeks, resulting in a constant wear behaviour.

Knee wear simulation under conditions of highly demanding daily activities--influence on an unicompartmental fixed bearing knee design.

The objectives of our in vitro study were to evaluate a knee wear simulation based on patient daily activities in combination with artificial ageing of polyethylene inserts to create an optimised simulation of in vivo wear modes. A wear simulation was performed on fixed bearing unicompartmental knee arthroplasty (UKA) devices in a direct comparison of level walking (as given by the ISO 14243-1:2002(E) profiles) and in a customised test configuration based on activities for level walking (10%), stairs ascending (40%), stairs descending (40%), chair rising (8%) and deep squatting (2%). The cumulative gravimetric wear was estimated to be 15.3mg for level walking (ISO) and 69.6 mg for high demanding activities (HDA). The gravimetric wear rate of the ISO group was 3.0mg/million cycles, compared to 11.7 mg/million cycles for the HDA protocol. Level walking wear testing conditions (ISO) and artificial ageing alone is not sufficient to reproduce in vivo failure modes. After 3 million cycles all gliding surfaces of the HDA group developed in the tibio-femoral articulation markable areas of structural material fatigue and delamination. In conclusion a combination of artificial ageing to clinical relevant oxidation grades and a sequence of various high demanding daily patient activities is necessary to represent a revised in vitro behaviour of abrasive-adhesive wear and delamination in artificial knee replacements.

Definition and evaluation of testing scenarios for knee wear simulation under conditions of highly demanding daily activities.

The objective of our study was the definition of testing scenarios for knee wear simulation under various highly demanding daily activities of patients after total knee arthroplasty. This was mainly based on a review of published data on knee kinematics and kinetics followed by the evaluation of the accuracy and precision of a new experimental setup. We combined tibio-femoral load and kinematic data reported in the literature to develop deep squatting loading profiles for simulator input. A servo-hydraulic knee wear simulator was customised with a capability of a maximum flexion of 120°, a tibio-femoral load of 5000N, an anterior-posterior (AP) shear force of ±1000N and an internal-external (IE) rotational torque of ±50Nm to simulate highly demanding patient activities. During the evaluation of the newly configurated simulator the ability of the test machine to apply the required load and torque profiles and the flexion kinematics in a precise manner was examined by nominal-actual profile comparisons monitored periodically during subsequent knee wear simulation. For the flexion kinematics under displacement control a delayed actuator response of approximately 0.05s was inevitable due to the inertia of masses in movement of the coupled knee wear stations 1-3 during all applied activities. The axial load and IE torque is applied in an effective manner without substantial deviations between nominal and actual load and torque profiles. During the first third of the motion cycle a marked deviation between nominal and actual AP shear load profiles has to be noticed but without any expected measurable effect on the latter wear simulation due to the fact that the load values are well within the peak magnitude of the nominal load amplitude. In conclusion the described testing method will be an important tool to have more realistic knee wear simulations based on load conditions of the knee joint during activities of daily living.

Ultrasound-based registration of the pelvic coordinate system in the lateral position using symmetry for total hip replacement.

In total hip replacement, patient placement in the lateral position is preferred by many surgeons. However, it complicates registration of the so-called pelvic coordinate system that is the standard reference for surgeons to measure cup orientation. This coordinate system comprises the anterior pelvic plane and the mid-sagittal plane, and it is conventionally registered on the basis of bony anatomical landmarks including the left and the right anterior superior iliac spine (ASIS). Ultrasound has been suggested for transcutaneous palpation of the bone surface. The difficulty in registration of the pelvic coordinate system with the patient in the lateral position arises because the contralateral ASIS cannot be reached easily by a mechanical pointer and is not accessible by means of an ultrasound probe. Up to now, methods to compensate for these missing data have not been used in clinical routine. This paper describes a new ultrasound-based method that requires neither image segmentation nor statistical shape models and uses symmetry to approximate the position of the contralateral ASIS. A detailed analysis based on computed tomography data of 60 hips following a cadaver study is presented to show the ability of our method to reliably reconstruct the pelvic coordinate system. The median angles between ground truth planes and the "reconstructed" planes were <2°. By choosing a standard cup orientation w.r.t. the "reconstructed" planes, the median abduction and version angle errors were <2°, too.

Alternative bearing materials for intervertebral disc arthroplasty.

The objective of our study was to test alternative polymer-on-polymer articulations for cervical total disc arthroplasty with favourable biotribological properties and the benefit of radiolucency in comparison to the clinically well established metal-on-polyethylene coupling. In vitro wear simulation was performed according to ISO 18192-1:2008 (E) with the clinically introduced activ C cervical artificial disc (Aesculap AG Tuttlingen, Germany) made of UHMWPE/CoCr29Mo6 in a direct comparison to experimental disc articulations made of PEEK, CFR-PEEK and PEK. Each material combination was tested for 10 million cycles with a customised 6 station spinal wear simulator (EndoLab Thansau, Germany). Gravimetric and geometric wear assessment, optical surface characterisation and an estimation of particle size and morphology were performed. The gravimetric wear rate of the clinical reference polyethylene-on-cobalt-chromium was 1.0+/-0.1 mg/million cycles, compared to 1.4+/-0.4 mg/million cycles for PEEK, to 0.02+/-0.02 mg/million cycles for CFR-PEEK and 0.8+/-0.1 mg/million cycles for PEK. In conclusion, a number of different candidate materials for total cervical disc arthroplasty were compared using the same disc design. Whereas the polymer-on-polymer articulation of PEK showed no substantial benefit in comparison to polyethylene-on-cobalt-chromium and whereas natural PEEK tends towards pitting and delamination, the carbon fibre reinforced PEEK demonstrated an excellent wear behaviour with a reduction in order of a magnitude. Therefore, the CFR-PEEK based polymer-on-polymer articulations may be an alternative to polyethylene-on-metal and have a high potential for next generation disc replacements.

Biotribology of alternative bearing materials for unicompartmental knee arthroplasty.

The objective of our wear simulator study was to evaluate the suitability of two different carbon fibre-reinforced poly-ether-ether-ketone (CFR-PEEK) materials for fixed bearing unicompartmental knee articulations with low congruency. In vitro wear simulation was performed according to ISO 14243-1:2002 (E) with the clinically introduced Univation F fixed bearing unicompartmental knee design (Aesculap AG, Tuttlingen, Germany) made of UHMWPE/CoCr29Mo6 in a direct comparison to experimental gliding surfaces made of CFR-PEEK pitch and CFR-PEEK PAN. Gliding surfaces of each bearing material (n=6+2) were gamma-irradiated, artificially aged and tested for 5 million cycles with a customized four-station knee wear simulator (EndoLab, Thansau, Germany). Volumetric wear assessment, optical surface characterization and an estimation of particle size and morphology were performed. The volumetric wear rate of the reference PE1-6 was 8.6 +/- 2.17 mm(3) per million cycles, compared to 5.1+/-2.29 mm(3) per million cycles for PITCH1-6 and 5.2 +/- 6.92 mm(3) per million cycles for PAN1-6; these differences were not statistically significant. From our observations, we conclude that CFR-PEEK PAN is obviously unsuitable as a bearing material for fixed bearing knee articulations with low congruency, and CFR-PEEK pitch also cannot be recommended as it remains doubtful wether it reduces wear compared to polyethylene. In the fixed bearing unicompartmental knee arthroplasty examined, application threshold conditions for the biotribiological behaviour of CFR-PEEK bearing materials have been established. Further in vitro wear simulations are necessary to establish knee design criteria in order to take advantage of the biotribiological properties of CFR-PEEK pitch for its beneficial use to patients.