Biomedical subjects

W C Hayes

Publications and source records attributed to W C Hayes.

At least 109 records · Page 6Linked to original sources

PubMed1992

Failure of growth hormone to alter the biomechanics of fracture-healing in a rabbit model.

Standardized tibial osteotomies were created and stabilized with external fixation in twenty-seven skeletally mature rabbits. Fourteen animals were treated with a daily injection of human growth hormone (150 micrograms per kilogram of body weight), and thirteen received a daily injection of saline solution. Serial non-destructive biomechanical tests, radiography, and determinations of the levels of serum insulin-like growth-factor I were performed for all of the animals. Destructive strength-testing of the sites of osteotomy was performed at four, six, or eight weeks. Twenty-five of the twenty-seven osteotomies healed uneventfully. There were no significant differences in the serial biomechanical measurements at the sites of the healing osteotomies, in the radiographic measurements, or in the ultimate strength of the sites of the osteotomy between the two groups. The mean level of serum insulin-like growth-factor I increased 33 per cent relative to the preoperative baseline level in the group that received growth hormone and increased 10 per cent in the control group. This difference was not statistically significant. There was no significant correlation between the biomechanical properties at the sites of the osteotomies and the levels of serum insulin-like growth-factor I. Administration of growth hormone had no measurable effect on fracture-healing in this model of normal animals. It remains to be determined whether injection of growth hormone might affect healing when there is a state of deficiency of endogenous growth hormone or when there is a non-union of a fracture.

Analysis of Variance↗

PubMed1991

Biomechanical evaluation of a biodegradable composite as an adjunct to internal fixation of proximal femur fractures.

Internal fixation of comminuted unstable fractures of the severely osteoporotic proximal femur is sometimes supplemented with polymethyl-methacrylate (PMMA). We here report an in vitro biomechanical evaluation of a biodegradable particulate composite that might be used for similar purposes. The composite includes a matrix phase consisting of a hydrolyzable prepolymer [polypropylene fumarate (PPF)] cross-linked with methacrylate monomer, and a particulate phase consisting of tricalcium phosphate and calcium carbonate. We implanted dynamic hip screws in 22 cadaveric proximal femora and measured the yield load for an oblique force applied to the femoral head. The hip screws were then reinforced with either PMMA or the PPF composite and tested again. On the basis of analysis of variance, the average increases in yield load for PMMA and PPF reinforcement of 1,750 and 1,130 N were statistically significant (p less than 0.00005), suggesting that both materials enhance congruence between implant and bone and thereby increase the projected load-bearing area of the implant. The increase in yield force with PMMA was slightly higher than the increase with PPF (p less than 0.05), but both values after reinforcement were close (3,790 +/- 561 N for PMMA vs. 3,240 +/- 669 N for PPF). If we can demonstrate that appropriate rates of degradation, bony ingrowth, and static and fatigue properties can be achieved in vivo with this system, our data suggest that this PPF composite may have potential as an adjunct to the internal fixation of unstable fractures of the osteoporotic hip.

Aged↗

PubMed1991

Role of interfragmentary strain in fracture healing: ovine model of a healing osteotomy.

It has been hypothesized that the histological pattern of fracture healing is controlled at least in part by the local mechanical strains in the interfragmentary region. To test this "interfragmentary strain hypothesis," we applied cyclic bending deformations to tibial osteotomies in 11 sheep. An instrumented flexible plate spanning a 1-mm osteotomy gap was deformed to create a gradient of tissue elongation from 10% under the plate to 100% at the opposite cortex. The cyclic deformations were applied three times per minute, 24 h per day, for 1-5 weeks. However, as a result of tissue differentiation, the bone-plate complex increased in stiffness with healing time, resulting in a marked reduction of the gap deformation at approximately 4 weeks. Fracture healing was evaluated using vascular injection of India ink and conventional histology. A nonlinear three-dimensional finite element model of the interfragmentary tissue at the initial stage of healing was used to predict the complex tissue strains. The ingrowth of vascularized soft tissue into the interfragmentary gap, as well as the subsequent differentiation of this tissue, occurred earlier and to a greater degree in regions of lower strain. In contrast, the proliferation of callus tissue was greatest at the periosteal and endosteal surfaces of the cortex opposite the plate. Direct comparison of the finite element predictions with the histology demonstrated that the spatial distribution of bone resorption at the fracture fragment ends directly corresponded to the locations of elevated tissue strain and stress. However, there was no consistent numerical relationship between the magnitude of these local peak strains and the corresponding volume of cortical bone resorption over the bone cross section.

Animals↗

PubMed1991

Correlations between photon absorption properties and failure load of the distal radius in vitro.

The bone mineral content (BMC) and cross-sectional properties of cadaveric radii were assessed by single photon absorptiometry (SPA). A new multiple-angle scanning technique was used in conjunction with SPA to measure the area and moments of inertia of the radial cross section in intact forearms. The radii of the same forearms were then broken in a failure test to simulate a fall on the outstretched arm. Colles' fracture was produced in 16 of 18 radii tested. The BMC divided by the bone width, which is called bone mineral density (BMD) in clinical applications, did not correlate with the load at failure in cadaveric forearms. The BMC alone did correlate significantly with failure load (r2 = 0.62), and the cross-sectional properties gave the best correlation with failure load in a sequential multiple regression (r2 = 0.80). We conclude that if SPA is to be used in clinical studies to predict risk of radial fracture or of fractures at remote skeletal sites, then BMC should be used rather than BMD. An even better indicator of radial bone strength than BMC would be a biomechanical parameter derived from the cross-sectional area and moments of inertia, which can be obtained from multiple-angle SPA.

Absorptiometry, Photon↗

PubMed1991

Physical and mechanical properties of calf lumbosacral trabecular bone.

The physical and mechanical properties of calf lumbar and sacral trabecular bone were determined and compared with those of human trabecular bone. The mean tissue density (1.66 +/- 0.12 g cm-3), equivalent mineral density (169 +/- 36 mg cm-3), apparent density (453 +/- 89 mg cm-3), ash density (194 +/- 59 mg cm-3), ash content (0.6 +/- 0.05%), compressive strength (7.1 +/- 3.0 MPa) and compressive modulus (173 +/- 97 MPa) of calf trabecular bone are similar to those of young human. There were moderate, positive linear correlations between apparent density and equivalent mineral density, ash density, and compressive strength; and between compressive strength and equivalent mineral density (R2 ranging from 0.35 to 0.48, p less than 0.001). Apparent density, ash density, and equivalent mineral density did not differ significantly in different regions. In contrast to humans, the compressive strength increased from posterior, near the facet, to the anterior vertebral body. These comparisons of physical and mechanical properties, as well as anatomical comparisons by others, indicate that the calf spine is a good model of the young non-osteoporotic human spine and thus useful for the testing of spinal instrumentation.

Animals↗

PubMed1991

Mechanical properties of metaphyseal bone in the proximal femur.

We used a three-point bending test to investigate the structural behavior of 123 rectangular flat plate specimens harvested from the metaphyseal shell of the cervical and intertrochanteric regions of five fresh/frozen human proximal femora. For comparison purposes, 36 specimens of similar geometry were also fabricated from bone of the femoral diaphysis. All specimens were oriented in either the local longitudinal or transverse directions. The mean longitudinal elastic modulus was 9650 +/- 2410 (SD) MPa and demonstrated a 24% decrease from that measured for the diaphysis (12500 +/- 2140 MPa) using the same testing technique. However, the transverse elastic moduli did not differ significantly between the proximal (5470 +/- 1720 MPa) and diaphyseal (5990 +/- 1520 MPa) specimens. The globally averaged values for the ultimate tensile strengths of the metaphyseal shell were 101 +/- 26 MPa in the longitudinal and 50 +/- 12 MPa in the transverse directions. These compared with diaphyseal values of 128 +/- 16 MPa and 47 +/- 12 MPa, respectively. While these differences were largely due to the reduced density of the proximal specimens, a slight decrease in transverse anisotropy for the proximal specimens was also noted by comparing the ratio of longitudinal to transverse moduli (1.76) and tensile strength (2.02) to the diaphyseal values (2.09 and 2.71, respectively). Use of these data should lead to improved performance of analytical models for the proximal femur, and thus help focus increased attention on the structural contribution of trabecular bone to the strength and rigidity of the proximal femur.

Acrylic Resins↗

PubMed1991

Variations of stiffness and strength along the human cervical spine.

The load-displacement response and strength of the mid (C2-C5) and lower (C5-T1) cervical regions were determined for combinations of sagittal loads, in vitro. In unpaired t-test comparisons, the mid cervical region was significantly stiffer in compression and extension than the lower region. In tests to failure, failure in six out of seven mid cervical specimens resulted from flexion alone, while combined compression-flexion was required to fail five of the eight lower cervical specimens. Post-test dissections revealed no regional differences in the pattern of failure. In addition to sagittal tests, the load-displacement responses of three-vertebrae cervical specimens were measured with the upper body axially rotated with respect to the lower body. The effect of this pre-torsion was to diminish the zone of low slope near zero load for axial, shear, and flexion motion. Three of the four axially rotated specimens failed in flexion without added compression. These controlled load-displacement measurements of cervical spine specimens describe for the first time the continuous flexion-compression response up to failure, and suggest that consideration of the biomechanics of three apparently distinct mobile regions of the cervical spine (C1-C2, C2-C5, C5-T1) may facilitate the interpretation of hazardous conditions and the diagnosis of injury. These data also provide basic information for the in vitro investigation of passive cervical spine protection such as helmets and head-rests, suggesting that the head should be kept in a non-rotated position to reduce risk of injury.

Biomechanical Phenomena↗

PubMed1991

Compressive behavior of human bone-cement composites.

Current surgical practice in the implantation of cemented total joint arthroplasties generally creates a zone of variable thickness in which polymethylmethacrylate (PMMA) is intermixed with trabecular bone. The authors' objectives in these experiments were to characterize the compressive mechanical properties of this bone-cement composite material. They found that the mechanical properties of bone-cement composite specimens, fabricated under in vitro conditions that would promote nearly complete cement filling, are closer to the properties of trabecular bone than to those of cement. For both low-viscosity cement (LVC) and PMMA specimens, with the cement introduced by either hand-packing or pressurized injection at periods of 2 and 7 minutes, the compressive strengths ranged from 29 MPa to 50 MPa and the compressive moduli from 539 MPa to 1,210 MPa. Cement volume fractions achieved using different filling methods ranged from 76% to 87%. In contrast to previous studies of bone-cement composites using high-density bovine bone, neither mechanical properties nor filling parameters correlated significantly with bone porosity measured prior to filling. The authors expect that the mechanical properties of bone-cement regions created at surgery under less than these ideal in vitro filling conditions will only approach their values as an upper limit. Thus, bone-cement composites created in situ at surgery will also exhibit mechanical properties well below previously assumed values.

Aged↗

PubMed1991

Local demineralization as a model for bone strength reductions in lytic transcortical metastatic lesions.

The structural consequences of bone density changes associated with lytic metastatic lesions were investigated using an experimental model of regular, lytic metastatic lesions in bone. Circular holes were drilled in the mid-diaphyseal cortex of paired adult canine femora. The region around the defect was demineralized in one bone of each pair with 0.8 N HCl. Specimens were tested to failure in four-point bending. Defect size was determined from conventional planar radiographs as the maximum apparent defect diameter divided by the periosteal diameter. Demineralization resulted in irregular defect geometries, which increased the maximum defect dimension 33% to 57% with respect to the original drill hole diameter. Demineralization resulted in additional strength reductions beyond those expected from the original drill hole alone. Despite the irregular demineralization patterns observed, strength reductions were in close agreement with those predicted from data for regular, nondemineralized holes (r2 = 0.93). The results demonstrate that irregular diaphyseal defect borders may not require more complex fracture risk predictors than can be determined from analytic and experimental studies of regular defect geometries. Our results also demonstrate that errors of over 100% can occur when measuring diaphyseal defect size from radiographs that are not optimally aligned with respect to the defect.

Animals↗

PubMed1991

Three-dimensional load displacement properties of posterior lumbar fixation.

Pedicular fixation devices for the posterior treatment of segmental spinal instability are thought to offer enhanced stabilization compared with sublaminar wire systems, while avoiding the immobilization of multiple normal motion segments. We compared the performance of three dissimilar stabilization systems: the Hartshill rectangle, the Acromed/Steffee interpedicular screw and plate, and the Synthes/Dick fixateur interne. Human cadaveric lumbosacral specimens were first tested intact, then after a laminectomy and a facetectomy at the L3/L4 level, and finally after the fixation devices were sequentially attached. Constructs spanning two to four vertebral levels were compared for stabilization of the resected lumbar spine segments. When tested in compression, the Acromed/Steffee system with pedicular screws at L2-L5 allowed significantly less intersegmental distraction than the Synthes/Dick construct with screws at L2 and L5 only, and less than the intact and the destabilized uninstrumented spine. When sagittally rotated, the Acromed/Steffee construct with screws at L2-L5, or at L2 and L5, allowed significantly less distraction than the intact or destabilized segments, and the construct with screws at L2 through L5 allowed less distraction than the Synthes/Dick constructs with screws at L3/L4 or L2/L5. With the exception of the Acromed/Steffee system with screws at four levels, there were no significant differences in distraction allowed between the Synthes/Dick and Acromed/Steffee constructs, or between the multisegment and single segment constructs. There were no significant differences in stiffness across levels L3/L4 with the various implants. Results indicate that the use of posterior spine constructs significantly augment the stability of posterior segmental defects. Pedicular fixation immediately cephalad and caudad to the defect provided stable fixation in this application.

Bone Plates↗

PubMed1991

Importance of bone mineral density in instrumented spine fusions.

The effect of equivalent mineral density on pedicular screw fixation strength was investigated. The equivalent mineral density of human vertebral bodies was correlated highly with the pullout force of Kluger screws (r2 = 0.61, P less than 0.02). A moderate to high correlation existed between density and vertical force (r2 = 0.42 for Kluger screws, r2 = 0.55 for Steffee screws, P less than 0.02). In calf vertebral bodies of higher density (146 +/- 14 mg/cc), the forces were significantly higher than in the human vertebral bodies (P less than 0.05). Human lumbosacral spines were instrumented with three different fixators: Steffee plates, AO fixateur interne, and Kluger fixateur interne. Of five specimens with a mean density of 88 +/- 11 mg/cc, one screw loosened. More than one screw loosened in six specimens with a mean density of 63 +/- 12 mg/cc, and no screw loosened in four specimens with a mean density of 114 +/- 38 mg/cc. Measurement of equivalent mineral density correlates with the fixation strength of the intrapedicular screws in vitro and should be considered in patients with signs of osteopenia before using pedicular screws for spinal fusions. It is also concluded that calf spines are a good model for testing implants because they tend to focus failure processes in the implant rather than in the implant-bone interface.

Aged↗

PubMed1991

Fracture prediction for the proximal femur using finite element models: Part I--Linear analysis.

Over 90 percent of the more than 250,000 hip fractures that occur annually in the United States are the result of falls from standing height. Despite this, the stresses associated with femoral fracture from a fall have not been investigated previously. Our objectives were to use three-dimensional finite element models of the proximal femur (with geometries and material properties based directly on quantitative computed tomography) to compare predicted stress distributions for one-legged stance and for a fall to the lateral greater trochanter. We also wished to test the correspondence between model predictions and in vitro strain gage data and failure loads for cadaveric femora subjected to these loading conditions. An additional goal was to use the model predictions to compare the sensitivity of several imaging sites in the proximal femur which are used for the in vivo prediction of hip fracture risk. In this first of two parts, linear finite element models of two unpaired human cadaveric femora were generated. In Part II, the models were extended to include nonlinear material properties for the cortical and trabecular bone. While there was poor correspondence between strain gage data and model predictions, there was excellent agreement between the in vitro failure data and the linear model, especially using a von Mises effective strain failure criterion. Both the onset of structural yielding (within 22 and 4 percent) and the load at fracture (within 8 and 5 percent) were predicted accurately for the two femora tested. For the simulation of one-legged stance, the peak stresses occurred in the primary compressive trabeculae of the subcapital region.(ABSTRACT TRUNCATED AT 250 WORDS)

Accidental Falls↗

PubMed1991

Fracture prediction for the proximal femur using finite element models: Part II--Nonlinear analysis.

In Part I we reported the results of linear finite element models of the proximal femur generated using geometric and constitutive data collected with quantitative computed tomography. These models demonstrated excellent agreement with in vitro studies when used to predict ultimate failure loads. In Part II, we report our extension of those finite element models to include nonlinear behavior of the trabecular and cortical bone. A highly nonlinear material law, originally designed for representing concrete, was used for trabecular bone, while a bilinear material law was used for cortical bone. We found excellent agreement between the model predictions and in vitro fracture data for both the onset of bone yielding and bone fracture. For bone yielding, the model predictions were within 2 percent for a load which simulated one-legged stance and 1 percent for a load which simulated a fall. For bone fracture, the model predictions were within 1 percent and 17 percent, respectively. The models also demonstrated different fracture mechanisms for the two different loading configurations. For one-legged stance, failure within the primary compressive trabeculae at the subcapital region occurred first, leading to load transfer and, ultimately, failure of the surrounding cortical shell. However, for a fall, failure of the cortical and trabecular bone occurred simultaneously within the intertrochanteric region. These results support our previous findings that the strength of the subcapital region is primarily due to trabecular bone whereas the strength of the intertrochanteric region is primarily due to cortical bone.

Accidental Falls↗

PubMed1991

Prediction of femoral impact forces in falls on the hip.

A major determinant of the risk of hip fracture in a fall from standing height is the force applied to the femur at impact. This force is determined by the impact velocity of the hip and the effective mass, stiffness, and damping of the body at the moment of contact. We have developed a simple experiment (the pelvis release experiment) to measure the effective stiffness and damping of the body when a step change in force is applied to the lateral aspect of the hip. Results from pelvis release experiments with 14 human subjects suggest that both increased soft tissue thickness over the hip and impacting the ground in a relaxed state can decrease the effective stiffness of the body, and subsequently reduce peak impact forces. Comparison between our fall impact force predictions and in-vitro measures of femoral fracture strength suggest that any fall from standing height producing direct, lateral impact on the greater trochanter can fracture the elderly hip.

Accidental Falls↗

PubMed1991

Biomechanics of fracture risk prediction of the hip and spine by quantitative computed tomography.

In this review, we have made use of some simple engineering concepts to summarize current efforts relating QCT measures to bone density and strength. From a variety of in vitro experiments on cadaveric vertebrae and femora, it is evident that both apparent and ash densities are strong linear functions of QCT measures, with coefficients of determination ranging from 0.49 to 0.90 and relative errors from 44.9% to 7.1%. QCT data also can be used (with somewhat less confidence) to determine the compressive modulus (R2s from 0.36 to 0.68, relative errors from 45.0% to 35.5%) and compressive strength (R2s from 0.58 to 0.70, relative errors from 56.5% to 39.9%) of trabecular bone from the proximal femur and vertebral body. In cortical bone, material properties are only correlated weakly with QCT measures. Experiments designed to relate QCT data to failure loads for the proximal femur and vertebral body have been remarkably successful. Coefficients of determination have ranged from 0.32 to 0.93, with relative errors from 31.1% to 13.9%. However, when the in vitro failure loads determined in these experiments are compared against available estimates of in vivo loads on the spine and hip, it is apparent, at least in the elderly, that in vivo loads are relatively close to those that cause fracture in vitro. To assess the possibility of developing QCT-based clinical predictors of fracture risk for individual patients, we have introduced the concept factor of risk often used in engineering design to account for uncertainties in estimates of service loads and component strength. The factor of risk for a particular loading condition is defined as the ratio of expected service loads to the known failure loads. To extend this concept to densitometric fracture risk prediction in vivo, it is important to recognize that densitometric data must not only be used to predict the ultimate load carrying capacity of the region of interest, but that this ultimate load must then be compared to the forces expected in vivo under comparable loading conditions. One difficulty with this approach is that little is known about the in vivo forces that are associated with atraumatic age-related fractures of the hip and vertebrae and even less about the forces applied to the hip and spine during traumatic events such as falls. However, from available estimates of in vivo loads during bending and lifting, it is apparent that in the elderly, factors of risk for the spine can easily approach 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1990

Effects of retinacular release and tibial tubercle elevation in patellofemoral degenerative joint disease.

Release of the patellar retinaculum and tibial tubercle elevation have both been advocated for the treatment of patellofemoral degeneration. Questions remain, however, regarding the magnitude and predictability of such effects in diseased joints. Using cadaver knee joints exhibiting a range of patellofemoral cartilage degeneration, we investigated the effects on joint contact pressures on release of the patellar retinaculum, followed by tibial tubercle elevations of 1.25 and 2.5 cm. Retinacular release failed to alter the joint-loading parameters significantly. Tibial tubercle elevation reduced the patello-femoral joint contact area and contact force, but failed to cause a consistent change in contact pressure. Tibial tubercle elevation also caused a migration of the joint contact area superolaterally on the retropatellar surface. This migration occurred in conjunction with ventral tilting of the inferior pole of the patella as the tubercle was elevated, suggesting that significant changes in joint kinematics may result from this procedure.

Aged↗

PubMed1990

Structural consequences of transcortical holes in long bones loaded in torsion.

Finite element models were used to predict the structural consequences of transcortical holes through long bones loaded in torsion. Several parameters were investigated including hole size, anelastic behavior of the bone, cortical wall thickness, cortical wall symmetry, curvature along the bone's long axis and the axial length of the defect. Finite element model predictions of percent intact bone strength were compared to experimental data for sheep femora with transcortical drill holes loaded to failure in torsion. Hole size was expressed as hole diameter divided by the outer bone diameter. Linear finite element model predictions were in conservative agreement with the experimental data for large hole sizes. A transcortical hole with a diameter 50% of the outer bone diameter reduced the torsional strength by 60%. However, the linear models predict a 40% drop in strength for small holes whereas in vitro data suggest that small holes have no significant effect on strength. Models which represent non-linear anelastic behavior in bone overpredicted torsional strengths. Asymmetric cortical wall thickness and long bone bowing have minor effects, while the length of an elongated defect strongly influences the torsional strength. Strength reductions are greatest for bones with thin cortical walls.

Animals↗

PubMed1990

Compressive strength of autologous and allogenous bone grafts for thoracolumbar and cervical spine fusion.

The selection of the bone graft type for stabilization of spinal fusion depends on availability, the clinical situation, and the desired mechanical stability. The authors determined the potential immediate postoperative compressive strength of various types of bone grafts under axial compression on a material testing machine. The fibular strut graft (5,070 +/- 3,250 N, mean +/- standard deviation [SD]) was significantly stronger (P less than 0.05) than the anterior (1,150 +/- 487 N) and posterior (667 +/- 311 N) iliac crest grafts, and the rib grafts (452 +/- 192 N). Hydroxyapatite grafts with a pore size of 200 mu were significantly stronger (P less than 0.05) than those with a pore size of 500 mu (1,420 +/- 480 N versus 338 +/- 78 N). Ethylenoxide sterilization had no significant effect on the immediate compressive strength. Bicortical and tricortical Bailey-Badgley and Cloward bone grafts also were compared. Results showed that all cervical graft types may be sufficiently strong to support sizable loads.

Aged↗