PubMed Health⌕ Search

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

Source of the slippage in the universal joints of the Hoffmann external fixator.

An investigation was conducted to determine what improvements in the resistance to slippage could be obtained in selected interfaces (rod/clip torsional, cheek/bowl and cheek/clip) of the Hoffmann external fixator. The modification involved changing the standard wing-nut clamp for a bolt with a thread of 7 mm and a 1 mm pitch and placing an FAG 28-303 thrust-bearing (needle roller and cage assembly) between the bolt and the cheek. The results showed a significant improvement in the slippage values of all interfaces; increases of approximately six times were obtained at all torque values of the wing-nut clamp or fastener tested. Such improvements would markedly increase the reliability of external fixation systems and thus reduce the incidence of loss-of-reduction of fracture due to slippage of the universal joint.

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↗
PubMed1989

Stability and anchorage considerations for cementless tibial components.

The ability of several idealized tibial anchorage systems to resist distractive relative motion between the substrate and the component when subjected to eccentric loads was assessed using mechanical tests employing a urethane foam as a bone substitute and finite element models. Eccentric loads had to be placed within 30-35% of the edge of the tibial plate for separation to occur between an unanchored plate and the substrate. The finite element model predicted that a 90%/10% medial/lateral load distribution was required to maintain compressive vertical stresses beneath the unanchored plate. An anchorage system with four pegs each 10 mm in diameter and 20 mm long and an anchorage system with four cancellous bone screws each 6.5 mm in diameter were loaded eccentrically, and both systems prevented distraction of greater than 76 microns when mounted on urethane foam. Shorter pegs (10 mm) or a single central peg allowed distractions of 150 microns or greater to occur. These data are relevant to the design and testing of tibial components for total knee arthroplasty.

Biomechanical Phenomena↗
PubMed1989

In vitro analysis of proximal femoral strains using PCA femoral implants and a hip-abductor muscle simulator.

The strains produced in the proximal femur by noncemented and cemented PCA femoral implants have been compared to each other and to the strains in the same intact femur. The effect on the strain pattern of a hip-abductor muscle simulator was also tested. Nine embalmed femora were tested; two were used for development of the protocol, one was covered with a reflective photoelastic coating, and six were instrumented with eight strain gauge rosettes on each femur. For a given body weight on the photoelastically coated intact bone, the abductor-simulator increased the mean shear strains on both the medial (132%) and lateral (153%) aspects, with standard deviations of 13% and 20%, respectively; however, no strain-concentrated areas were observed. With an abductor simulator on the strain-gauged femurs, calcar shear strains were significantly reduced (P less than .01), from those on the intact bone, by both noncemented and cemented implants. These reductions were 74.2% and 91.8%, respectively, with no significant difference between the two techniques. Reductions in shear strain were noted at midstem on the medial side for the noncemented (20.5%) and cemented (35.9%) implants; however, only the cemented implants produced significantly less (P less than .05) strain than the intact bone at this location. At midstem on the medial aspect of the femur, there was a significant difference (P less than .05) between the data for the cemented and noncemented implants. Analysis of variance identified no other regions of significant change.

Biomechanical Phenomena↗
PubMed1989

Stiffness of bone underlying the tibial plateaus of osteoarthritic and normal knees.

The mechanical properties of normal cancellous bone in the proximal tibia have been reported extensively in previous studies in terms of support of a total knee arthroplasty (TKA); yet, little is known about these mechanical properties in the osteoarthritic (OA) state. Fifteen normal and 28 OA tibial plateaus, obtained from autopsy or TKA, were mechanically tested using an indentor technique to assess the variation of stiffness patterns. The medial:lateral stiffness ratio calculated for the normal plateaus was significantly different from the ratios computed for specimens with medial compartment OA and lateral compartment OA; however, the ratio was unchanged in tricompartment OA. These data should receive greater consideration in the design of TKA, which has traditionally been designed on the mechanical properties of normal tibiae.

Adult↗
PubMed1989

Analysis of the pull-out strength of screws and pegs used to secure tibial components following total knee arthroplasty.

The purpose of this study was to compare the pull-out forces of 3.8-mm and 6.5-mm diameter cancellous screws and a round peg 10 mm in diameter, implanted in bovine vertebral cancellous bone and two different grades of polyurethane. The relationships between the pull-out forces and the properties of the material into which they were inserted were also examined. For the same depth of insertion, the pull-out force of a 6.5-mm diameter screw was two times larger than that of a 3.8-mm diameter screw. Despite its smaller diameter, a 6.5-mm diameter screw demonstrated a pull-out force 13.6% greater than that of an optimally press-fitted 10-mm diameter round peg when both were inserted to the same depth; however, optimal compression for the peg was strongly dependent upon the size of predrilling and the modulus of elasticity of the material into which the peg was inserted. The pull-out force of a cancellous screw has a linear relationship with the shear strength of the material into which it is inserted. The pull-out force of a peg exhibits a linear relationship with the modulus of elasticity of the material into which it is inserted.

Animals↗
PubMed1988

In-vivo comparison of four absorbable sutures: Vicryl, Dexon Plus, Maxon and PDS.

Absorbable sutures are initially equal or superior to nonabsorbable sutures in terms of tensile strength but are absorbed at variable rates by the action of hydrolysis. This study demonstrated that the in-vivo half-life tensile strength of the braided absorbable sutures polyglycolic acid (Dexon Plus) and polyglactin 910 (Vicryl) is 2 weeks, whereas those of the monofilament absorbable sutures polyglyconate (Maxon) and polydioxanone (PDS) are 3 and 6 weeks respectively. The addition of a single hitch or six knots reduced the in-vitro tensile strength by 30% to 35%. Polyglyconate (Maxon) suture demonstrated the best in-vitro knot security.

Animals↗
PubMed1988

External skeletal fixation: choosing a system based on biomechanical stability.

Due to the increased popularity of external fixators for treating long-bone fractures, many devices are being introduced to the market. The choice of a particular fixation device depends on the anticipated loading conditions or the demands the fixator might encounter during the healing process. This study compares the biomechanical stability (rigidity, yield-load, failure-load) and load to produce 1 mm of fracture-gap displacement of various half-frames of five systems tested in axial compression, torsion, and both anterior-posterior and medial-lateral bending; the slippage tolerances of various interfaces of the universal joints or clamps were also analyzed. The frames were mounted on acrylic rods, with a midshaft transverse saw-cut, displaced by 10 mm, and set to standardized dimensions and tightened at set torques. In terms of stability, the Brooker and Hoffman systems are, in general, less stable than the RxFx, AO, and Orthofix fixators. The single half-frames of all systems, except the Orthofix, were particularly weak, and the double and stacked half-frames of each system were more stable.

Biomechanical Phenomena↗
PubMed1987

Cancellous bone support for patellar resurfacing.

The purpose of this study was to determine the compressive resistance of the cancellous bone in nine fresh specimens of human patellae. Standardized radiographic assessment and subsequent osteotomies were performed on each specimen. Grid patterns based on patellar anatomy enabled topographic compression on a material test system (MTS) Model 835 Bionix Universal Testing Machine (MTS Corporation, Minneapolis, MN) using a 4-mm diameter indentor. It was demonstrated that sacrifice of the patellar subchondral bone removes with it the strong cancellous bone support and topographic strength patterns of the cancellous bone on the surface of a patellar osteotomy does not have a consistent pattern in relation to the intracondylar eminence. Consequently, placement of fixation pegs is not critical with respect to strength of bony support.

Adult↗