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Biomedical subjects

J B Finlay

Publications and source records attributed to J B Finlay.

At least 19 recordsLinked to original sources
PubMed1992

Hoffmann half-frame external fixation rigidity and its relationship to universal joint slippage.

Hoffmann half-frame external fixation device configurations often fail under minimal loads secondary to joint slippage. In these experiments improved universal joints that were developed in an earlier study were tested on Hoffmann half-frame assemblies. The rigidity of selected half-frame configurations was tested in four modes (axial compression, torsion, medial-lateral and anterior-posterior four-point bending). These results were compared to those of an earlier, similar study looking at the standard Hoffmann half-frame. No changes in overall rigidity were noted, but significant increases in yield loads and loads to frame failure were achieved. Such improvements will increase the reliability and usefulness of the Hoffmann device to the orthopaedic community.

Equipment Design↗
PubMed1992

Evaluation of linear finite-element analysis models' assumptions for external fixation devices.

Linear finite-element models (FEMs) have enjoyed an increased use in orthopaedic research, including the use for modeling external fixation devices. These fixator FEMs depend on a number of basic assumptions concerning the overall fixation frame stability and the components' rigidity. Among the more important ones are: (i) rigid fixation at both ends of the pin and sidebar; (ii) that the sidebar can be treated essentially as a rigid entity, with all bending occurring in the bone pins; and (iii) that the system can be treated as linearly elastic. Prior work done by the authors questions some of these assumptions. Thus, this study sought an empirical evaluation of the validity of some of these a priori assumptions. A Hoffmann single half-frame was tested in its standard form and then according to a stepwise protocol wherein the frame was welded to eliminate any possible points of instability. These tests looked at the stability and rigidity in various modes (axial compression, torsion, and medial-lateral and anterior-posterior four-point bending). The basic assumptions concerning the frame stability, frame rigidity and the frame's response to loads were found to be erroneous. Component failure was common under minimal loads and statistically significant differences (p less than 0.05) of up to 75% were noted in frame rigidity among the various frame forms tested. Thus, considerable caution must be exercised when employing the FEM technique for evaluating the fixator properties.

Biomechanical Phenomena↗
PubMed1992

Universal joint slippage as a cause of Hoffmann half-frame external fixator failure [corrected].

Slippage of the universal joints of external fixation devices is known to occur but its significance or incidence is often overlooked. In this study, controlled experiments were used to determine the relationship between joint slippage and the maximum loads a single half-frame could bear for the Hofmann device. The experiments showed that: (a) the joints slipped at minimal loads and (b) frame failure, i.e. loss of initial alignment of the frame components, was determined by joint slippage. The importance of the control of slippage cannot be overstated; the orthopaedic community must educate itself and its patients and guard against the problem in order to avoid complications secondary to slippage.

Biomechanical Phenomena↗
PubMed1991

An evaluation of three loading configurations for the in vitro testing of femoral strains in total hip arthroplasty.

Despite recent advances in total hip arthroplasty, proximal femoral resorption and osteopenia remain problems. To analyze the proximal strain effects of three different loading conditions, strains produced in intact and postarthroplasty femora have been compared. Ten adult cadaveric femora of similar size, shape, and rigidity were tested. Ten strain-gauge rosettes were positioned on each femur. To simulate the "single leg support" phase of gait, fixtures were developed to load the femora under three different configuration: the VS (vertical shaft) configuration, with a vertically orientated femur having rotational freedom proximally and distally; the ITB (iliotibial band) configuration of a femur with rotational freedom positioned 11 degrees from the vertical, with a strain-gauge adjustable metallic simulation of the lateral muscles of the thigh; and the ABD (abductor) configuration of a femur with rotational freedom positioned 11 degrees from the vertical, with a strain-gauge adjustable metallic simulation of the abductor muscles. Each femur was loaded less than or equal to 600 N through the medial point, located at one sixth of the transcondylar distance. Strain patterns and magnitudes produced by the three loading configurations were quite different in both the intact and postarthroplasty femora. Both the ITB and the ABD configurations resulted in greater proximal medial compression and lateral tension than did the VS configuration. The magnitudes of the proximal strains were significantly greater in the ABD configuration (p less than 0.05). Postarthroplasty femora showed similar proximal results. It is proposed that meaningful strain data for the physiologically loaded femur can be obtained only with simulations that include the forces produced by the iliotibial band. To overcome the indeterminate nature and biological variation in these forces, the studies have to consider a range of forces.

Femur↗
PubMed1991

Deformation of the cement mantle of tibial components following total knee arthroplasty: a laboratory study.

An in vitro model has been developed to measure in-plane strains of the cement mantle, sandwiched between the tibial component and the underlying cancellous bone following total knee arthroplasty. Maximal in-plane strains occurred in the cement mantle below the contact points between the femoral and tibial components. These strains were significantly reduced by increasing the thickness of the polyethylene and even more impressively by metal backing. Eccentric loading, by as little as 5 degrees, increased the strains in the loaded compartment by 26 per cent and decreased those in the unloaded compartment by 62 per cent. The addition of torsion to axial loading did not significantly alter the principal direct strains or the principal shear strains. Although surface-covering tibial components have been advocated, continuous support of the cortical rim did not appear to be important in reducing cement mantle strains. While other studies have emphasized the critical stresses that may occur in the polyethylene tibial components of total knee implants, this study highlights the potential for localized cement fatigue with improperly sized components or with eccentric loading.

Biocompatible Materials↗
PubMed1991

Joint slippage in the Hoffmann external fixator. No effect of loading rate in bench experiments.

For tibial fractures, half-frames, such as the Hoffmann fixation device, sometimes fail when subjected to weight-bearing loads. Because the joints of the Hoffmann system are known to slip, which could lead to frame failure, three interfaces of the standard Hoffmann joint were tested at different clamp torques and different rates of load application. No difference in mean slippage values was noted for any interface at similar clamp torques. Joint slippage and any subsequent frame failure are thus not related to rate of load application, but to the magnitude of the load alone.

Equipment Failure↗
PubMed1990

Applying tribological principles to improve the performance of the Hoffmann external fixator's universal joint.

As part of a continuing investigation of sources of slippage in the universal joints of external fixation devices, a study was conducted to determine if selected interfaces of the Hoffmann external fixator (rod/clip torsional, cheek/bowl and clip/cheek) could be improved by moving the point of contact between the wing-nut clamp and cheek closer to the centre of the wing-nut clamp. It was felt that the movement of the point of contact would reduce the magnitude of the frictional torque resisting the tightening of the wing-nut clamp. The point of contact was changed by the addition of a Belleville washer between the interface of the wing-nut clamp and the cheek. Increased slippage torques of approximately 100 per cent were noted in all interfaces at low values of tightening torque (6 and 8 N m) of the wing-nut clamp and improvements of not less than 50 per cent were obtained at higher tightening torques (10 and 12 N m) on the wing-nut clamp.

Biomechanical Phenomena↗
PubMed1978

Impact characteristics of articular cartilage.

A rigid l m high stainless steel drop tower employing linear bearings has been used to study the impact characteristics of human articular cartilage. Instrumentation included a specially designed optoelctronic position transducer, piezoelectric force transducer, and high speed storage oscilloscope. Forty-eight 9 mm diameter samples of living articular cartilage and subchondral bone from the tibial plateaus of 4 human donors have been impacted at strains from 10 to 50%, and strain rates of 500s-1 and 1000 s-1. The integral bone/cartilage samples were mounted in polymethyl methacrylate for testing. Bone and cement impact characteristics have been studied separately. Stress, strain, and energy absorption data have been assembled for all the samples. Chondrocyte viability subsequent to impact has been investigated with the use of tritum labeled proline and autoradiography. Viability has been studied in relation to both the mechanical data and structural damage.

Autoradiography↗
PubMed1977

Survival of articular cartilage after controlled impact.

Survival characteristics of forty-three specimens of living human bone and articular cartilage from the knees of eight renal-transplant donors were studied, using a drop-tower device. Autoradiography and light and scanning electron microscopy revealed no evidence of chondrocyte death or structural damage until stress levels of twenty-five newtons per square millimeter were reached, corresponding to strains on the order of 20 to 30 per cent and involving energy absorption of one millijoule per cubic millimeter. The data for strain rates of 500 and 1000 s-1 suggest that impact loads sufficient to fracture a femoral shaft of an automobile occupant are nearly sufficient to cause chondrocyte death and fissuring in the articular cartilage of either the knee or the hip if the load-bearing areas measure less than 500 square millimeters.

Autoradiography↗
PubMed1976

Scanning electron microscopy of normal human scar tissue and keloids.

The fibrous architecture of 5 normal human scars between 1 week and 1 year old and 4 keloid scars has been studied by scanning electron microscopy. In normal wounds, significant changes in fibril and fibre orientations and mode of aggregation take place as the scars mature, indicating that remodelling carries on for many months. There are also major differences between the edge and centre of the wound. These changes can be related to the stresses placed on the fibroblasts which have responded by laying down collagen aligned in such a way as to resist these forces. It is suggested that keloid fibroblasts may lack this ability to respond appropriately to orientational stress.

Adult↗
PubMedDate not supplied

Microstructural organization of human and bovine cruciate ligaments.

Human and bovine anterior cruciate ligaments studied with the SEM demonstrate fascicular bundle and connective tissue sheath components. In the human, collagen fibers of the connective tissue sheath, although approximately the same diameter as the fascicular fibers, have an orientation suggestive of a binding rather than a tensile function. While both elements have tensile properties, it is hypothesized that the contribution from the sheath component is minimal and, consequently, detailed studies of the tensile properties of ligaments will produce meaningful data when related to the appropriate cross-sectional areas. In direct contrast to the human counterpart, the SEM appearance of bovine cruciate ligaments is tendon-like, characterized by very dense subfasciculi surrounded by very thin connective tissue sheaths.

Animals↗
PubMedDate not supplied

Controlled voltages for electroconvulsive therapy.

A description is given for the design of an isolated output, servo-controlled voltage, electroconvulsive therapy (ECT) machine for the safe, controlled application of 60-Hz alternating current (ac) signals between 0 and 230 V rms for controlled periods ranging from 0.1 to 2 sec. Added safety is provided by current limiting in the range of 360 to 770 mA rms for patient impedances ranging from 500 to 50 omega. The use of this equipment has permitted a detailed description of the impedances characteristic of unilateral ECT.

Electric Conductivity↗