Multidirectional Motion Research Articles

A Major Step Towards Efficient Sample Preparation with Bead-Beating

A Major Step Towards Efficient Sample Preparation with Bead-Beating

The influence of wear paths produced by hip replacement patients during normal walking on wear rates

Variation in wear paths is known to greatly affect wear rates in vitro, with multidirectional paths producing much greater wear than unidirectional paths. This study investigated the relationship between multidirectional motion at the hip joint, as measured by aspect ratio, sliding distance, and wear rate for 164 hip replacements. Kinematic input from three-dimensional gait analysis was used to determine the wear paths. Activity cycles were determined for a subgroup of 100 patients using a pedometer study, and the relationship between annual sliding distance and wear rate was analyzed. Poor correlations were found between both aspect ratio and sliding distance and wear rate for the larger group and between annual sliding distance and wear rate for the subgroup. However, patients who experienced a wear rate <0.08 mm/year showed a strong positive correlation between the combination of sliding distance, activity levels, and aspect ratio and wear rate (adjusted r(2) = 55.4%). This group may represent those patients who experience conditions that most closely match those that prevail in simulator and laboratory tests. Although the shape of wear paths, their sliding distance, and the number of articulation cycles at the hip joint affect wear rates in simulator studies, this relationship was not seen in this clinical study. Other factors such as lubrication, loading conditions and roughness of the femoral head may influence the wear rate.

Guidance of visual direction by topographical vibrotactile cues on the torso

Vibration on localised areas of skin can be used to signal spatial orientation, multi-directional motion and also to guide arm and hand movements. This study investigated the possibility that vibration at loci on the skin might also be used to cue gaze direction. Eight subjects made eye or (head + eye) gaze saccades in the dark cued by vibration stimulation at discrete loci spaced on a horizontal contour across the chest. Saccade and gaze amplitudes, latencies, and directions were analysed. In the first experiment, performed without training, subjects could only use vibration cues to direct their gaze in cardinal directions and gross quadrature. There was a high variability in the relationship between locus on the trunk and gaze direction in space, both within and between subjects. Saccade latencies ranged from 377 to 433 ms and were related to the loci of vibration; the further from the body midline the quicker the response. Since the association of skin loci with gaze direction did not appear intuitive a sub-group of four subjects were retested after intensive training with feedback until they attained criterion on midline identical with 0 degrees and 15 cm (to right/left of midline) identical with 45 degrees gaze shifts right and left. Training gave a moderate improvement in directional specificity of gaze to a particular locus on the skin. Gaze direction was linearly rescaled with respect to skin loci but variability and saccade latencies remained high. The uncertainty in the relationship between vibration locus and gaze direction and the prolonged latencies of responses indicate circuitous neuronal processing. There appears to be no pre-existing stimulus-response compatibility mapping between loci on the skin and gaze direction. Vibrotactile cues on the skin of the trunk only serve a gross indication of visual direction in space.

Seismic Evaluation and Retrofit of Asymmetric Multi-Story Wood-Frame Building

The presented research focuses on large-scale seismic testing under multi-directional ground motion of a three-story high, wood-frame residential building representing late 1960's California construction. Earthquake lateral resistance is provided by plywood shear walls around the perimeter of the building with an open front in the first story for tuck-under parking. Accordingly, the as-built structural configuration is asymmetric in plan and discontinuous in elevation with tendency to twist about a vertical axis and to form a weak story mechanism. The test results confirm this tendency. They also reveal the sensitivity of the response to multi-direction ground motion. Asymmetric damage patterns are induced by the multi-component motions in the walls oriented perpendicular to the open front for the as-built test structure, with or without finish materials. However, the observed damage remained noncritical as far as structural integrity is concerned even for ground accelerations exceeding 120% of that recorded during Northridge earthquake. This is viewed as a consequence of the better construction of the test building compared to actual construction. Investigated retrofit includes adding a welded moment resisting steel frame around of the garage opening and strengthening the diaphragm to header beam connections. The study indicates that the retrofit significantly reduced the maximum story drift in the open front. Moreover, the finish material and the retrofit greatly reduce the maximum rotation of the building about the vertical axis.

Biopolymer wear screening rig validated to ASTM F732-00 and against clinical data

The ASTM standard F732-00, 'standard test method for wear testing of polymeric materials used in total joint prostheses' offers several pieces of data against which a test rig for such biopolymers can be validated. These conditions include a linear wear in the range from 0˙07 to 0˙2 mm/million cycles for ultrahigh molecular weight polyethylene (UHMWPE), and a wear rate ranking of poly tetra fluoro ethylene (PTFE)>>polyacetal≥UHMWPE. Using a modified pin on plate wear test rig, these three biopolymers were tested under a 40 N load against polished stainless steel plates while using dilute bovine serum heated to 37°C as the lubricant. When subject to multidirectional motion, mean wear factors of 37×10–6 mm3 Nm–1 for PTFE, 3˙8×10–6 mm3 Nm–1 for polyacetal and 1˙1×10–6 mm3 Nm–1 for UHMWPE were obtained. Therefore the wear ranking recommended by the ASTM standard was achieved. When the corrected mean weight loss of the UHMWPE test pins was converted to a linear wear rate, a value of 0˙12 mm/million cycles was obtained. Therefore the ASTM recommended target for linear wear rate was met. When the wear factors from the test materials were compared with clinical wear factors of 37×10–6 mm3 Nm–1 for PTFE, 3˙5×10–6 mm3 Nm–1 for polyacetal and 0˙95– 1˙45×10– 6 mm3 Nm–1 for UHMWPE, good correspondence between in vivo and in vitro wear factors was obtained, further validating the modified pin on plate wear test rig.

Development of a hip wear simulation rig including micro-separation

Currently, hip joint simulation is an important tool in both testing total hip prosthesis and developing new biomaterials with enhanced tribological properties. During total hip arthroplasty (THA), the ligament of the femoral head (LHF) and the cartilage of the acetabulum pelvic cup are often disrupted or degenerated and are surgically removed. Recent studies have reported that one consequence of the absence of LHF and cartilage dissection on THA patients is the micro-separation between the femoral head and the acetabulum in the artificial hip joint during normal gait. In the present work, a new hip joint simulator was developed, integrating mechanical devices and electronic control. This hip simulator provides a dynamic load close to human body conditions during walking and jogging. Also, micro-separation is included and multidirectional motions with the following amplitudes: flexion-extension (FE) ±23°, abduction-adduction (AA) ±23° and internal-external (IER) ±8°. This machine allows flexibility to use different path loads and different micro-separations amplitudes (1–4 mm) through a pneumatic muscle adapted to a springs system. Using this system is possible to measure the combined effect of micro-separation (micro-impact between the contact surfaces), multidirectional motion and the Paul load peak in a similar lubricated system, allowing more accurate replication of in vivo conditions.

A multi-directional in vitro investigation into friction, damage and wear of innovative chondroplasty materials against articular cartilage

The wear of the biomaterial/cartilage interface is vital for the development of innovative chondroplasty therapies. The aim of this study was to investigate potential chondroplasty biomaterials when sliding against natural articular cartilage under uniaxial reciprocating and multi-directional rotation/reciprocating motions. Three biphasic hydrogels were compared to articular cartilage (negative control) and stainless steel (positive control). Friction was measured by means of a simple geometry friction and wear simulator. All tests were completed in 25% bovine serum at 20 degrees C. Mechanical alterations to the surface structure were quantified using surface topography. Articular cartilage produced a constant friction value of 0.05 (confidence interval=0.015) with and without rotation. Stainless steel against articular cartilage produced an increase in friction over time resulting in a peak value of 0.7 (confidence interval=0.02) without rotation, increasing to 0.88 (confidence interval=0.03) with rotation. All biphasic hydrogels produced peak friction values lower than the positive control and demonstrated no difference between uni- and multi-directional motion. Degradation of the opposing cartilage surface showed a significant difference between the positive and negative controls, with the greater cartilage damage when sliding against stainless steel under uni-directional motion. The lower friction and reduction of opposing cartilage surface degradation with the potential chondroplasty biomaterials can be attributed to their biphasic properties. This study illustrated the importance of biphasic properties within the tribology of cartilage substitution materials and future work will focus on the optimisation of biphasic properties such that materials more closely mimic natural cartilage.

Effects of Simple and Complex Motion Patterns on Gene Expression of Chondrocytes Seeded in 3D Scaffolds

This study investigated the effect of unidirectional and multidirectional motion patterns on gene expression and molecule release of chondrocyte-seeded 3D scaffolds. Resorbable porous polyurethane scaffolds were seeded with bovine articular chondrocytes and exposed to dynamic compression, applied with a ceramic hip ball, alone (group 1), with superimposed rotation of the scaffold around its cylindrical axis (group 2), oscillation of the ball over the scaffold surface (group 3), or oscillation of ball and scaffold in phase difference (group 4). Compared with group 1, the proteoglycan 4 (PRG4) and cartilage oligomeric matrix protein (COMP) mRNA expression levels were markedly increased by ball oscillation (groups 3 and 4). Furthermore, the collagen type II mRNA expression was enhanced in the groups 3 and 4, while the aggrecan and tissue inhibitor of metalloproteinase-3 (TIMP-3) mRNA expression levels were upregulated by multidirectional articular motion (group 4). Ball oscillation (groups 3 and 4) also increased the release of PRG4, COMP, and hyaluronan (HA) into the culture media. This indicates that the applied stimuli can contribute to the maintenance of the chondrocytic phenotype of the cells. The mechanical effects causing cell stimulation by applied surface motion might be related to fluid film buildup and/or frictional shear at the scaffold-ball interface. It is suggested that the oscillating ball drags the fluid into the joint space, thereby causing biophysical effects similar to those of fluid flow.

In vitro investigation into the effects of uni and multi-directional motion on the friction, damage and wear of innovative chondroplasty materials against articular cartilage

In vitro investigation into the effects of uni and multi-directional motion on the friction, damage and wear of innovative chondroplasty materials against articular cartilage

The Quantification of Physiologically Relevant Cross-Shear Wear Phenomena on Orthopaedic Bearing Materials Using the MAX-Shear Wear Testing System

Background: The occurrence of multi-directional sliding motion between total knee replacement bearing surfaces is theorized to be a primary wear and failure mechanism of ultra-high molecular weight poly(ethylene) (UHMWPE). To better quantify the tribologic mechanisms of this cross-shear wear, the MAX-Shear wear-testing system was developed to evaluate candidate biomaterials under controlled conditions of cross-shear wear. Method of approach: A computer controlled traveling x-y stage under a 3 degree-of-freedom statically loaded pin is used to implement the complex multi-directional motion pathways observed during TKR wear simulation. A MHz collection of dynamic x-y friction was available on all six environmentally controlled stations. The functionality of this testing platform was proven in a 100,000 cycle, 11.6 MPa, wear test using 15.0 mm diameter polished stainless steel spheres against flat GUR4150 UHMWPE. A five-pointed star wear pattern was used to incorporate the physiologically relevant cross-shear sliding conditions of stop/start, 50mm∕s entraining velocity and five crossing angles of 72°. Using normalized volumetric reconstruction of the resulting surface damage, a direct quantitative relationship between linear and cross-shear surface damage intensity was obtained. Results: Cross-shear surface damage volume loss was found to be 2.94 (±0.88) times that associated with linear sliding under identical tribologic conditions. SEM analysis of linear wear damage showed consistent fibril orientation along the direction of sliding while cross-shear wear damage showed multi-directional fibril orientations and increased surface roughness. Significant increases in discrete crossing-point friction coefficients were recorded throughout testing. Conclusions: This scientific approach to quantifying the tribologic effects of cross-shear provides fundamental wear mechanism data that are critical in evaluating potential biomaterials for use as in vivo bearings. Relevant multi-axis, cross-shear wear testing is necessary to provide quantifiable measures of complex biomaterials wear phenomena.

The effect of counterface roughness on the wear of UHMWPE for rectangular wear paths

Ultra-high molecular weight polyethylene (UHMWPE) is used worldwide as a bearing material in total joint replacement prostheses. Multi-directional motion has been identified as a major factor affecting the wear rate of UHMWPE in total hip replacement prostheses. The trajectory of relative motion between a femoral head and an acetabular cup takes a general quasi-elliptical or rectangular shape during the patient's gait cycle. Differences in motion pattern can affect the in vivo wear rates of UHMWPE cups in patients when all other factors are equal. In a previous study that utilized smooth Co–Cr counterfaces, we compared wear factors, k, for UHMWPE articulated in a series of rectangular wear paths (width = A, length = B) with systematically increasing aspect ratios ( B/ A) and linear tracking ( A = 0), all with identical path lengths per cycle. The results showed that the wear factor significantly decreased in the rectangular wear path with the highest aspect ratio and in linear tracking. The goal of our current study was to quantify the effect of a roughened counterface on the cross-path wear of UHMWPE. UHMWPE pins were articulated against both smooth and rough Co–Cr disks in diluted calf serum using a multi-directional wear tester under physiological loading conditions. Five different rectangular wear path geometries and linear tracking, all with identical path lengths per cycle, were employed for each wear test. Gravimetric weight loss was converted into volumetric wear rates and wear factors, k. The results showed that roughened counterfaces produced a larger increase in the wear factor in rectangular wear paths with higher aspect ratios. The ratio of k rough/ k smooth decreased monotonically as a function of increasing width of rectangles, normalized by total path length, or A/( A + B).

Wear, Debris, and Biologic Activity of Cross-linked Polyethylene in the Knee

Cross-linked polyethylene currently is being introduced in knee prostheses. The wear rates, wear debris, and biologic reactivity of non cross-linked, moderately cross-linked, and highly cross-linked polyethylene have been compared in multidirectional wear tests and knee simulators. Multidirectional pin-on-plate wear studies of noncross-linked, moderately cross-linked (5 Mrad), and highly cross-linked (10 Mrad) polyethylene showed a 75% reduction in wear with the highly cross-linked material under kinematics found in the hip, but only a 33% reduction under wear in kinematics representative of the knee. In knee simulator studies, with the fixed-bearing press-fit, condylar Sigma cruciate-retaining knee under high kinematic input conditions, the wear of 5 Mrad moderately cross-linked polyethylene was 13 +/- 4 mm per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 +/- 6 mm/1 million cycles). For the low-contact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 +/- 1 mm per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 +/- 2 mm/1 million cycles). The wear debris isolated from the fixed-bearing knees showed the moderately cross-linked material had a larger percentage volume of particles smaller than 1 mum in size, compared with GVF GUR 1020 polyethylene. Direct cell culture studies of wear debris generated in sterile wear simulators using multidirectional motion showed a increase (p < 0.05) in tumor necrosis factor-alpha levels and reactivity for GUR 1050 cross-linked polyethylene debris compared with an equivalent volume of noncross-linked GUR 1050 polyethylene. The use of cross-linked polyethylene in the knee reduces the volumetric wear rate. However, the clinical significance of reduced fracture toughness, elevated wear in abrasive conditions, and the elevated tumor necrosis factor-alpha release from smaller more reactive particles warrant further investigation.

Motion-onset VEPs to translating, radial, rotating and spiral stimuli

Motion-onset related visual evoked potentials (M-VEPs) were recorded as a result of the three basic (translating, radial and rotating) and one complex (spiral) motion stimulations in five subjects. Low contrast, retinotopically scaled patterns evoked potentials with major motion-onset specific negativity N160 with maximum in the parieto-temporal region. All multidirectional motion stimuli elicited the motion-onset response of significantly higher amplitude and shorter latency compared to the translating (unidirectional) motion. The rotation-onset evoked potentials had significantly shorter latencies than the rest of explored stimuli. The most stable responses with the largest N160 amplitude were recorded to the radial motion. After masking of the central 20 degrees of the visual field these motion-onset VEPs were acquired without statistically significant amplitude drop. The efficiency and usefulness of the radial stimulus is presented in two clinical cases.

The influence of design, materials and kinematics on the in vitro wear of total knee replacements

Debris-induced osteolysis due to surface wear of ultra high molecular weight polyethylene (UHMWPE) bearings is a potential long-term failure mechanism of total knee replacements (TKR). This study investigated the effect of prosthesis design, kinematics and bearing material on the wear of UHMWPE bearings using a physiological knee simulator. The use of a curved fixed bearing design with stabilised polyethylene bearings reduced wear in comparison to more flat-on-flat components which were sterilised by gamma irradiation in air. Medium levels of crosslinking further improved the wear resistance of fixed bearing TKR due to resistance to strain softening when subjected to multidirectional motion at the femoral–insert articulating interface. Backside motion was shown to be a contributing factor to the overall rate of UHMWPE wear in fixed bearing components. Wear of fixed bearing prostheses was reduced significantly when anterior–posterior displacement and internal–external rotation kinematics were reduced due to decreased cross shear on the articulating surface and a reduction in AP displacement. Rotating platform mobile bearing prostheses exhibited reduced wear rates in comparison to fixed bearing components in these simulator studies due to redistribution of knee motion to two articulating interfaces with more linear motions at each interface. This was observed in two rotating platform designs with different UHMWPE bearing materials. In knee simulator studies, wear of TKR bearings was dependent on kinematics at the articulating surfaces and the prosthesis design, as well as the type of material.

Quantification of the effect of cross-path motion on the wear rate of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is used worldwide as a bearing material in orthopaedic implants. Multi-directional motion or “cross-shear” motion has been identified as one of the most significant factors affecting the wear rate of UHMWPE in total hip joint replacement prostheses. The loci or trajectory of motion at the point of contact between a femoral head and an acetabular cup takes a general quasi-elliptical or rectangular shape during a gait cycle. Due to variations in gait patterns, some patients have either more elongated or square motion patterns than others. It is postulated that differences in motion pattern affect the in vivo wear rates of UHMWPE cups in patients where other factors such as age, weight and body proportion are similar. In this study, UHMWPE pins were articulated against cobalt chromium disks in diluted calf serum using an OrthoPOD™ multi-directional wear tester under physiological loading conditions. Five different rectangular wear path geometries and linear tracking, all with identical path lengths per cycle, were tested. Gravimetric weight loss was converted into both volumetric wear data and wear factor values, k, in order to determine the effects of motion pattern on the wear rate of UHMWPE. The results supported the hypothesis that wear rate is dependent upon the wear path geometry that is in turn dependent upon gait cycle.

MULTI-DIRECTIONAL DUAL COMB-DRIVE ACTUATOR FOR OPTICAL APPLICATIONS

A novel dual comb-drive actuator based on SOI wafer that is capable of generating multi-directional motions is developed in the paper. In order to obtain multi-directional motions, a new type of electrostatic vertical comb-drive actuator with dual-layer of electrodes is designed. Here, each comb finger of the comb-drive actuator is consisted of upper and lower conducting electrodes. By applying voltages between different upper and lower electrodes with appropriate spring designs, we are able to produce multi-directional actuating forces, which can result in vertical, torsion or horizontal motions. The present fabricated dual comb-drive actuator is capable of moving 165nm vertically with 24V-applied voltage. The dynamic characteristics of the first and the second resonant frequency of the device are 10.5 KHz and 23 KHz respectively. Experimental results indicated that the measured data agreed well with simulation results using the ANSYS and Coventor Ware.

Slide track analysis of eight contemporary hip simulator designs

In an earlier paper, the authors presented a new method of computation of slide tracks in the relative motion between femoral head and acetabular cup of total hip prostheses. For the first time, computed tracks were verified experimentally and with an alternative method of computation. Besides being an efficient way to illustrate hip kinematics, the shapes of the slide tracks are known to be of fundamental importance regarding the wear behaviour of prostheses. The verified method was now applied to eight contemporary hip simulator designs. The use of correct motion waveforms and an Euler sequence of rotations in each case was again found to be essential. Considerable differences were found between the simulators. For instance, the shapes of the tracks drawn by the resultant contact force included a circle, ellipse, irregular oval, leaf, twig, and straight line. Computation of tracks correctly for the most widely used hip simulator, known as biaxial, was made possible by the insight that the device is actually three-axial. Slide track patterns have now been computed for virtually all contemporary hip simulators, and both for the heads and for the cups. This comparative analysis forms a valuable basis for studies on the relationship between the type of multidirectional motion and wear. These studies can produce useful information for the design of joint simulators, and improve the understanding of wear phenomena in prosthetic joints.

Slide track analysis of the relative motion between femoral head and acetabular cup in walking and in hip simulators

Joint simulators are important tools in wear studies of prosthetic joint materials. The type of motion in a joint simulator is crucial with respect to the wear produced. It is widely accepted that only multidirectional motion yields realistic wear for polyethylene acetabular cups. Multidirectionality, however, is a wide concept. The type of multidirectional motion varies considerably between simulators, which may explain the large differences in observed wear rates. At present, little is known about the relationship between the type of multidirectional motion and wear. One illustrative way to compare the motions of various hip simulators is to compute tracks made on the counterface by selected points of the surface of the femoral head and acetabular cup due to the cyclic relative motion. A new computation method, based on Euler angles, was developed, and used to compute slide tracks for the three-axis motion of the hip joint in walking, and for two hip simulators, the HUT-3 and the biaxial rocking motion. The slide track patterns resulting from the gait waveforms were found to be similar to those produced by the HUT-3 simulator. This paper is the first to include a verification of the computed simulator tracks. The tracks were verified in the two simulators using sharp pins, embedded in acetabular cups, engraving distinct grooves onto the femoral heads. The engravings were identical to the computed tracks. The results clearly differed from earlier computations by another research group. This study is intended to start a thorough investigation of the relationship between the type of multidirectional motion and wear.

A comparison of the wear of cross-linked polyethylene against itself under reciprocating and multi-directional motion with different lubricants

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of ‘pin-on-plate’ wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40N rubbing against XLPE plates. All the XLPE came from the same batch.In all tests, pin wear was much less than plate wear. Under reciprocation only, the least wear was found when bovine serum was the lubricant (k=0.6×10−6mm3/Nm) and the maximum wear was when distilled water was the lubricant (k=5.8×10−6mm3/Nm). When multi-directional motion was applied to the test pins, increased wear occurred under lubrication with bovine serum (k=3.4×10−6mm3/Nm). Surprisingly, wear decreased when distilled water was used (k=0.7×10−6mm3/Nm), yet wear factors remained similar in the ‘dry’ test (k=0.7×10−6mm3/Nm). The dry tests had remarkably low wear.

A new pin-on-disk wear testing method for simulating wear of polyethylene on cobalt-chrome alloy in total hip arthroplasty

Abstract: Hip simulator studies show that the wear of ultra-high molecular weight polyethylene against a cobalt alloy head depends on the wear path, especially the combination of a predominantly linear wear direction on which is superimposed motions in different directions. We postulated that multidirectional motion was necessary to generate realistic wear rates in pin-on-disk testing. To assess this hypothesis, a new pin-on-disk tester was developed, capable of unidirectional and bidirectional motion. Unidirectional motion produced no detectable wear. The rectangular motion produced wear rates, surface morphologies, and wear particles consistent with human acetabular specimens. The results for 1 Hz and 2 Hz were similar.