Strain Gauge Placement Research Articles

Evaluation of factors affecting bonding rate of calcium phosphate ceramic coatings for in vivo strain gauge attachment.

The aim of this study was to compare the bone-bonding rates of eight calcium phosphate ceramic (CPC) coatings attached to strain gauges, alone and in conjunction with an OP1 device (Creative BioMolecules, Hopkinton, MA) and autologous concentrated pericyte cells. These coatings were studied to develop faster bone bonding to long-term in vivo strain sensors. Characterization of the CPC powders using electron microscopy and X-ray diffraction showed that they had shapes ranging from spherical to rocklike and properties ranging from highly crystalline to amorphous. CPC coated gauges were placed on the femora of young male dogs during aseptic surgery and were initially held in place using resorbable sutures. Test groups were euthanized after 3, 9, and 12 weeks. Both femora of the dogs were explanted and cantilever loaded. Response of the implanted hydroxyapatite (HA) coated gauges were compared to the response of bench-top glued sets of gauges (controls) attached to the contralateral femur and reported as a percentage of the control values. One CPC coating type showed an average response of 30% of controls after 3 weeks, four showed average responses higher than 75% after 9 weeks, and three showed averages higher than 82% after 12 weeks in vivo. Amorphous CPC coatings bonded more quickly than crystalline ones and particle shape had less effect than crystal structure on bonding rates. When either OP1 or autologous concentrated pericyte cells were placed on selected CPC coated gauge surfaces, the CPC5 coated gauges bonded best after 3 weeks with a response of 59%. After the same time period in vivo, CPC3 and CPC7 provided responses of 40 and 16%, respectively. Comparison of a soluble calcium-coated CPC with an uncoated one that had identical crystal structure and similar particle shape indicated that the calcium coating slowed bone bonding substantially in the young dog model. Optical microscopy of stained undecalcified bone sections and backscattered electron imaging indicated bone formation at all bone-HA interfaces and an increase in the number of areas of bone remodeling adjacent to the gauge at all time periods. Gross bone remodeling due to strain gauge placement was only observed near the distalmost cell-seeded strain gauges. Selection of the type of coating and enhancement system can accelerate bone bonding to strain sensors but must be tailored to the bone of the model in which it is being used. Augmentation of CPC coatings with cells or OP1 resulted in variable enhancement of the bonding rate and depended on the CPC and the enhancement system.

Modelling an orthopaedic knee prosthesis as a layered elastic system part 1: Experimental and theoretical analyses

The purpose of this investigation was to produce a simplified model of the plastic tibial plateau of a typical unicondylar knee prosthesis that would allow the parametric study of contact stresses experienced by the plastic component during relatively severe loading conditions. This involved the design, production and testing of a three-dimensional axisymmetric embedded strain gauge model of the tibial plateau and the application of a suitable theoretical analysis. The principal feature of the strain gauge model was the possibility of varying the thickness during the experimental procedure while keeping the maximum number of embedded gauges active. The Hertzian contact theory was used as a basis for the prediction of integration errors associated with placing strain gauges in locations subject to large strain gradients. A theoretical analysis that took the layered nature of the contact model into account was carried out which provided full field data for comparison with Hertzian and experimental results. Good agreement was obtained between theoretical and experimental values along the model axis, while at off-axis locations theoretical results based on the layered analysis compared reasonably with embedded strain gauge data. Very slight discrepancies between the experimental and idealized boundary conditions present in the initial stages of testing resulted in significant differences between embedded strain gauge and theoretical data.

Biomechanical analysis of facet and graft loading in a Smith-Robinson type cervical spine model.

This study determined the effect of change in graft height on the forces across a Smith-Robinson graft as well as across the posterior elements of the same motion segment. The study utilizes a strain gauge technique for the measurement of facet joint loading and a subminiature load cell for the measurement of graft loads. A number of cases of Smith-Robinson procedures have had some form of collapse of the interspace and graft material after surgery. Some patients with collapse of the graft go on to have prolonged sclerotomal-type pain or pseudarthrosis. The appropriate amount of distraction is not well defined in the literature and may affect the outcome. Cervical spines (C5-C6) were instrumented by placing strain gauges bilaterally on the pedicles of C6 (to measure the forces across the posterior elements). A miniature load cell with matching metallic shims was used to measure the force across the graft site and to distract the segment. Forces across the posterior elements and the graft site were measured, during flexion loading, and compared as the disc space was distracted. The ratio of posterior element load to graft load with increasing disc space distraction significantly decreased from 1.06 +/- 0.65 (1.4 mm distraction) to 0.30 +/- .13 (4.6 mm distraction) (P < .03). The posterior element load decreased significantly after the same distraction, from 46.1 +/- 22.0 to 18.7 +/- 9.7 N/Nm (P < .05). In spondylotic specimens (4-5 mm disc heights) disc space distraction in excess of 3.0 mm from preoperative height caused a significant decrease in both the ratio of posterior element to graft loading and posterior element loads. These findings may help explain recent clinical reports of a limit of effective disc space distraction.

Hydroxyapatite-coated strain gauges for long-term in vivo bone strain measurements.

The aim of this study was to examine the bonding process between hydroxyapatite-coated strain gauges and bone in order to continue development of a long term in vivo strain measurement device. Two types of commercially available hydroxyapatite (HA) particles were applied to the sensing surface of uniaxial strain gauges using a polysulfone solution as an adhesive. Characterization by scanning electron microscopy and x-ray diffraction (XRD) was used to determine materials property differences between the two powders. Interfacial strengths between the HA coatings and the strain gauges were tested and found comparable to interfacial strength obtained by a plasma sprayed HA coating on the surface of a titanium implant. Gauges were surgically placed on the periosteal surface of greyhound femora. Three groups of dogs were implanted with gauges for periods of 3, 6, and 12 weeks using cyanoacrylate, resorbable sutures, and cable ties to initially hold the gauge against the surface of the bone. Following euthanasia, the femora of the dogs were explanted and subjected to cantilever loading. Response of the implanted HA-coated gauges were compared to a control set that had been freshly glued onto the contralateral femur. Full response, that is, 100% of the strain measurement with respect to the control, was obtained after 12 weeks in vivo. Attachment of HA-coated gauges with circumferential suture showed bonding, while HA-coated gauges attached with cyanoacrylate did not bond to bone. After mechanical testing, femora were embedded in polymethylmethacrylate, cut, ground, and polished. Sections were stained using mineralized bone stain (MIBS) and optical microscopy was performed using transmitted and fluorescent light to allow analysis of remodeling occurring in the region of the strain gauges. Bone formation occurred at the HA surface of sutured gauges, and a fibrous tissue layer developed between the bone and HA coating when the tissue adhesive was used to initially bond the gauge. Fluorescence microscopy indicated an increase in the number of areas of bone remodeling adjacent to the gauge but a normal rate of remodeling of 0.93 +/- 0.07 mum/day was observed. No gross bone remodeling due to strain gauge placement was observed. Backscattered electron imaging (BSE) indicated new bone apposition at all time periods.

Characterization of in vivo strain in the rat tibia during external application of a four-point bending load

This paper describes a technique for characterizing strains and stresses induced in vivo in the rat tibia during application of an external four-point bending load. An external load was applied through the muscle and soft tissue with a four-point bending device, to induce strain in a 11 mm section of the right tibiae of ten adult female Sprague-Dawley rats. Induced strains were measured in vivo on the lateral surface of the tibia. Inter-rat difference, leg positioning and strain gage placement were evaluated as sources of variability of applied strains. Beam bending theory was used to predict externally induced in vivo strains. Finite element analysis was used to quantify the magnitude of shear stresses induced by this type of loading. There was a linear relationship between applied load and induced in vivo strains. In vivo strains induced by external loading were linearly correlated ( R 2=0.87) with the strains calculated using beam bending theory. The finite element analysis predicted shear stresses at less than 10% of the longitudinal stresses resulting from four-point bending. Strains predicted along the tibia by finite element analysis and beam bending theory were well-correlated. Inter-rat variability due to tibial size and shape difference was the most important source of variation in induced strain (CV=21.6%). Leg positioning was less important (CV=9.5%).

Sliding-mode controller design of a single-link flexible manipulator under gravity

This is follow-up research of the regulator design for a single-link flexible manipulator using the sliding-mode technique (Yeung and Chen 1989). In this paper, we have extended the controller design for the regulation problem to that for the tracking problem when the single-link manipulator is moving on a horizontal plane. In addition, by placing strain gauges along the flexible arm (Hastings and Book 1985) we can substitute strain measurements for the need to estimate modal functions, which is a step close to implementational reality. Extension of the controller design is also carried out to the regulation of a flexible arm under gravity. A PID compensation is needed to compensate for the steady-state offset due to gravity. The controller design presented is simple and requires no inertia matrix inversion, no modal estimation and no payload forecast. Simulation studies demonstrate the robustness of the control against input disturbance and payload variations

4 K fracture toughness investigations of 316 LN stainless steel plate and forging materials

This report deals with the 4 K elasto-plastic fracture toughness measurements of different cryogenic 316 LN stainless steel materials. Compact tension type specimens with different front shapes and thicknesses between 25 and 15 mm were investigated. The test results reveal that a simplified specimen configuration could be applied, which is considered to be equivalent to that of usual recommended specimen type and thickness. In addition, besides the load-displacement record, the bending of the specimens was detected by placing strain gauges on top and on the back face of the specimens. The significance of the unstable crack advance in elasto-plastic regime was pointed out. The onset of the recorded instabilities are capable of characterizing materials' engineering fracture toughness level. Based on these experimental results a simplified method was proposed to determine the critical load at the crack initiation.