Technical concept
The physiological rolling-gliding motion of the human knee joint is represented in many typical prosthesis designs by motion occurring between the femur component and the polyethylene tibia plateau. The normalised gait cycle (ISO 14243-1) accordingly permits the following types of motion: gliding, rolling with slip, and rolling between the femur component and tibia plateau, depending on the range of motion of the knee prostheses design [5, 6].
Our aim was to induce these three different types of motion (pure rolling, pure gliding, rolling with slip) on simplified specimen geometries. A concept was adopted in which a base-plate was horizontally moved relative to a rotating cylindrical counter-body under axial loading (Figure. 1). In this configuration, the planar base-plate represents the tibia plateau, the so-called "inlay" of a typical knee prosthesis, and the cylindrical counter-body corresponds to one femoral condyle (Figure. 2).
The "Rolling-Gliding Wear Simulator" created based on this concept consists of a horizontally oscillating stage onto which the base-plate is mounted, as well as a cylindrical counter-body which is statically vertically loaded (Figures. 1, 2 and 3). The counter-body is freely pivoted and, a rolling motion is hence induced by the driven base-plate. The rotational range of motion of the counter-body is limited by two adjustable stoppers: when the counter-body hits the first stopper whilst the stage continues in motion, a transition from rolling to gliding occurs. After a short gliding segment, the base-plate changes direction and initiates the second half of one motion cycle. At this point the cylindrical counter-body starts to roll again until hitting the second stopper inducing a second transition from rolling to gliding. The cyclical motion of the base-plate thus induces sequential cycles of rolling-gliding motion. In this manner, two different areas of wear are produced on the base-plate: a pure rolling segment in the middle of the base-plate, and two symmetrical areas of overlapping rolling and gliding adjacent to the pure rolling area. Small transition phases of rolling with slip occur between the pure rolling and overlapping rolling and gliding segments.
The material samples can be removed from the device for gravimetric wear measurement according to ASTM F1715 and ISO 14243-2, or alternatively for analysis of the wear surface profile using topological methods such as scanning electron microscopy or others [7, 8].
Mechanical Configuration
Material specimens are held in place in their two respective mounting points by a keyway and rectangular key design, with the addition of a small clamping mechanism to prevent lateral motion or dropping out of the cylindrical counter-body (Figures. 1, 4). This arrangement facilitates easy installation and removal. A platform with static weights (Figure. 3) provides constant vertical load between the counter-body and the base-plate. The platform is attached to a column which allows vertical motion only, and allows for compensation of irregularities within the articulation of the cylindrical counter-body on the base-plate.
An adjustable eccentric driven by a servo motor (Heidolph Elektro GmbH, Kehlheim, Germany, HeiDrive, Q125-0257 and D271.65) supplies the horizontal motion of the translation stage. A fluid tray, equipped with a heat-exchanger, is mounted on the translation-stage (Figures. 1, 3 and 4) into which the mounting point of the base-plate is integrated. The tray can freely tip along the translational axis to compensate for slight misalignments between the cylindrical body and the plate. This generates a uniform contact loading. A thin perspex lid covers most of the tray during testing (Figure. 4), and a peristaltic pump (not shown in figures) is calibrated to add distilled water automatically to compensate for evaporation of the test medium. Long-term-testing for well over 48 hours can thus be performed without user intervention. A second reference fluid tray (not shown in figures), also equipped with a heat-exchanger, is placed beside the test set up to use for material reference to facilitate the testing of materials which absorb liquid.
An electronic control module and user interface generated using LabView (Laboratory Virtual Instrument Engineering Workbench, Version 6.8, National Instruments, Austin, TX, United States) are used to manage the testing device. The software allows the user to adjust the number of testing cycles, the rotation speed of the servo motor and the amount of distilled-water added per hour. The software records the input data coming from sensors which read the linear and rotational movements, and the relative amounts of rolling, rolling and gliding, and pure gliding are graphically displayed (Figure. 5 top). A hardware counter (independent of the PC and software) provides redundancy for the prevention of data-loss in the case of a software failure.
Testing Protocol
The wear tests are conducted under the application of a constant compressive axial load of 714 N (700 N load bars + 14 N dead load of structure) to the cylindrical counter-body. Since a single condyle is being modelled in this wear test, 714 N reflect about a half of an average normal load during stance phase of gait (0% - 66% of gait cycle) [5]. The geometry of the cylindrical counter-body is 10 mm × 21 mm (w × h) with a radius of 32 mm. The base-plate must be a minimum of 15 mm high, 15 mm wide and 30 mm long. The length of the wear track covers 15 mm, which is similar to the length of contact area on the medial compartment of the tibia during flexion [9]. To achieve a rolling-gliding, roll, rolling-gliding ratio of 1:1:1 (5 mm : 5 mm : 5 mm), the adjustable stoppers were set to allow a counter-body rotation of 17.9° (± 8.95°) which causes a rolling track distance of 10 mm using a cylindrical counter-body with a radius of 32 mm. The calculated outcome of this is a 5 mm pure rolling track and two 5 mm superimposed rolling and gliding tracks occurring on the base-plate. This correspond a rolling-gliding and rolling ratio of 2:1, which correlates with the early stages of flexion in human knee [10].
Tribological Pairings
Altogether eight tribological pairings were tested, consisting of three different materials: Ultra High Molecular Weight Polyethylene (UHMWPE, Aesculap), Cobalt Chrome (CoCr, Aesculap), and a zirconia toughened alumina bioceramic material (BIOLOX®
delta, CeramTec AG). The CoCr and PE specimens were tested as provided by the manufacturer; the ceramic specimens were tested as provided, or finished by the authors. The materials were tribologically paired as follows: a UHMWPE plate against a polished CoCr counter-body (TP1), a UHMWPE plate against a ceramic counter-body (TP2), and a ceramic plate against a ceramic counter-body (TP3. and TP4). Furthermore, three ceramic-ceramic tribological pairings with surfaces finished to different degree of roughness (Ra) were tested: ground to Ra = 825 nm (TP5), polished to Ra = 382 nm (TP6), and highly polished to Ra = 52 nm (TP7). Finally, another ceramic-ceramic pairing was carried out with convex and concave shaped surfaces of the base-plate and the counter-body (TP8), whereby the base-plate had a convex radius of 36.48 mm and the counter-body a concave radius of 37.45 mm, which represents a difference in radius of 0.97 mm. For every test series performed, a reference material sample was included, which was unloaded.
A total number of three million cycles was applied and wear was measured after 100,000, 500,000 and subsequently every millionth cycle. The wear simulator was run at 1 Hz with a mixture of 25% ± 2% calf serum and distilled water maintained 37°C ± 2°C as a test-medium, and wear was measured gravimetrically according to ASTM F1715, ISO 14243-2 and ASTM F2025 [7, 8, 11]. To avoid biological reactions and degradation of the testing medium, 0.2% (weight fraction, ASTM F1715-00) sodium azide was added as a biocide. When testing the UHMWPE, a material which absorbs liquid, the reference sample of the same size is placed into the second reference fluid tray as an unloaded soak control [7, 11]. The gravimetrical wear was measured using a scale with an accuracy of ± 0.1 mg (ACCULAB ALC - 110.4, Sartorius Group). The gravimetrical measurement protocol includes three steps: sample preparation, weighing, and wear calculation. To prepare the testing and reference samples, they were cleaned according to a standardized protocol. To calculate the wear, the measurement results were adjusted using the reference samples and a volumetric wear rate was calculated using the density of the material. Scanning electron microscopy was used to examine the surface and wear marks on the samples.