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Biomedical subjects

W C Hayes

Publications and source records attributed to W C Hayes.

At least 181 records · Page 10Linked to original sources
PubMed1976

Postyield behavior of subchondral trabecular bone.

Cylindrical specimens of bovine subchondral trabecular bone were tested to uniaxial compressive strain levels of 75% to study energy absorption during pore collapse. Stress-strain curves were characterized by macroscopic yield at about 8% strain followed by a significant horizontal pore collapse regime. Energy absorption occurred largely in this postyield regime. Yield strength and energy absorption capacity were found to increase linearly with specimen apparent density. Microstructural analysis of the deformed specimens verfied that the mechanism for energy absorption was primarily fracture and buckling of trabeculae. The results suggest that during fracture, the collapse of trabecular bone (and the consequent absorption of energy) serves to attenuate stresses transmitted through the skeleton and thus protect vital structures such as the brain.

Animals↗
PubMed1976

The fracture mechanics of fatigue crack propagation in compact bone.

The purpose of this investigation was to apply the techniques of fracture mechanics to a study of fatigue crack propagation in compact bone. Small cracks parallel to the long axis of the bone were initiated in standardized specimens of bovine bone. Crack growth was achieved by cyclically loading these specimens. The rate of crack growth was determined from measurements of crack length versus cycles of loading. The stress intensity factor at the tip of the crack was calculated from knowledge of the applied load, the crack length, and the specimen geometry. A strong correlation was found between the experimentally determined crack growth rate and the applied stress intensity. The relationship takes the form of a power law similar to that for other materials. Visual observation and scanning electron microscopy revealed that crack propagation occurred by initiation of subcritical cracks ahead of the main crack.

Animals↗
PubMedDate not supplied

Distribution of contact force during impact to the hip.

Hip fracture is a common, costly, and debilitating injury occurring primarily in the elderly. Commonly viewed as a consequence of osteoporosis, it is less often appreciated that > 90% of hip fractures are caused by falls, and that fracture risk is governed not only by bone fragility, but also by the mechanics of the fall. Our goal is to develop experimental and mathematical models that describe the dynamics of impact to the hip during a fall, and explain the factors that influence hip contact force and fracture risk during a fall. In the current study, we used "pelvis release experiments" to test the hypothesis that, during a fall on the hip, two pathways exist for energy absorption and force generation at contact: a compressive load path directly in line with the hip, and a flexural load path due to deformation of muscles and ligaments peripheral to the hip. We also explored whether trunk position or muscle contraction influence the body's impact response and the magnitude of force applied to the hip during a fall. Our results suggest that only 15% of total impact force is distributed to structures peripheral to the hip and that peak forces directly applied to the hip are well within the fracture range of the elderly femur. We also found that impacting with the trunk upright significantly increases peak force applied to the hip, whereas muscle contraction has little effect. These results should have application in the development of fracture risk indices that incorporate both fall severity and bone fragility, and the design of interventions such as hip pads and energy-absorbing floors that attempt to reduce fracture risk by decreasing in-line stiffness and hip contact force during a fall.

Accidental Falls↗
PubMedDate not supplied

Compressive fatigue behavior of bovine trabecular bone.

We studied the fatigue behavior of bovine trabecular bone specimens under stress control using a sinusoidal uniaxial compressive load profile with a frequency of 2 Hz. The stress range was determined from the corresponding initial global platen-to-platen strain range, where the maximum initial strain was between 0.8 and 2.1% and the minimum strain was 0.6%. The local strain distribution was measured on the same type of specimen by affixing glass spheres and photographing them in the unloaded and loaded positions using multiple exposures. The number of cycles to failure (defined as a 5% decrease in secant modulus) was strongly correlated with the initial global maximum strain (r2 = 0.78) and ranged from 20 cycles at 2.1% strain to 400,000 cycles at 0.8% strain. All of the fatigue specimens showed a region of transverse failure approximately 1 mm from the end of the specimen. Microscopic examination of the failure zones revealed two failure modes: a straight transverse brittle-like fracture through the trabeculae, most often found in trabeculae transverse to the loading direction, and buckling-like failure, common in oblique trabeculae, involving bending and splitting. The local strain increased towards the ends of the specimens to a value 2-4 times that in the middle. Modulus degradation with the number of cycles was distinctively different for high-cycle and low-cycle fatigue, suggesting the possibility that both creep and damage accumulation contribute to fatigue failure of trabecular bone.

Animals↗
PubMedDate not supplied

Theoretical analysis of the experimental artifact in trabecular bone compressive modulus.

A theoretical analysis was performed to characterize potential experimental artifacts in conventional compression testing of trabecular bone, where strains are based on the relative displacements of the two loading platens. We assumed that the total experimental artifact for modulus was the sum of a damage and friction artifact and derived equations to describe these artifacts. The two unknown constants in these equations were found using a combination of data derived from linear finite element analyses and in vitro uniaxial compression tests. Subsequent finite element analyses allowed estimation of the artifacts for a wide range of specimens (cube, 1:4-3:1 aspect ratio cylinders). If friction is completely eliminated at the specimen-platen interface, the Young's modulus of a 5 mm sized (1:1 aspect ratio dimension) specimen which has a damage artifact due to machining may be underestimated by at least 45% regardless of specimen geometry; otherwise, the platens modulus may vary from less than 30 to over 175% of the Young's modulus, depending upon the specimen geometry and Poisson's ratio of the bone. Increasing the specimen size reduces the artifact only slightly. Since Poisson's ratio can be large for trabecular bone and is rarely known a priori, the precision of the conventional compression test will, therefore, be poor unless friction is completely eliminated at the specimen-platen interface. However, without friction at the interface, the platens modulus will always underestimate Young's modulus, thereby reducing the accuracy of this test. There was also evidence that the strength may be affected by these artifacts.(ABSTRACT TRUNCATED AT 250 WORDS)

Artifacts↗
PubMedDate not supplied

Direct and computed tomography thickness measurements of the human, lumbar vertebral shell and endplate.

Although the trabecular bone of the human vertebral body has been well characterized, the thin "cortical" shell and endplate that surround the trabecular centrum have not. In addition, the accuracy of estimating the thickness of the shell and endplate using computed tomography (CT) has not been evaluated directly. To address these issues, we measured the thickness of the vertebral shell and endplate in the mid sagittal plane of 16 human L1 vertebral bodies using direct and CT based methods. Specimens were assigned to four equal sized groups based on age (middle-aged, mean age = 49 years; old, mean age = 84) and gender. We investigated the dependence of the shell and endplate thicknesses on age, gender, and anatomic region. Our findings indicate that the shell and endplate in vertebrae over age 45 are porous and often irregular, with an average thickness of approximately 0.35 mm. However, when measured from CT images, the vertebral shell and endplate appear significantly thicker, indicating that measurements based on clinical CT scans overestimate the thickness by a factor of at least two. In addition, our data indicated that, in the midsagittal plane, the anterior shell is thicker than the posterior shell or either endplate. Although these data indicated that thickness did not depend on age or gender, these particular findings are inconclusive given the small and heterogeneous sample we examined. We conclude that the so-called cortical shell and endplate of the vertebral body are thin (less than one-half of a millimeter) and porous, and perhaps are better thought of as thin membranes of fused trabeculae than as true cortices.

Adult↗
PubMedDate not supplied

Ethylene glycol monomethyl ether II. Reproductive and dominant lethal studies in rats.

Groups of male and female Sprague-Dawley (CD) rats were exposed to 0, 30, 100, or 300 ppm ethylene glycol monomethyl ether (EGME) vapor 6 hours/day, 5 days/week for 13 weeks. The 0 and 30 ppm groups each contained 30 rats/sex and the 100 and 300 ppm groups each had 20 rats/sex. Following the exposure period, males were bred to unexposed females to evaluate reproductive capability and dominant lethality. Additional matings of control and 300 ppm exposed males were performed during the post-exposure period in order to evaluate the recovery of fertility. Exposed females were bred with unexposed males to assess reproductive parameters. Results of the present study indicate a potential for inhaled EGME to completely suppress fertility in male rats at the 300 ppm level. Fertility of these rats was partially restored at 13 weeks post-exposure. Body weights of animals in the 300 ppm group were reduced as a result of the exposures. No dominant lethal effect or impaired fertility was observed in male rats exposed to 30 or 100 ppm EGME. Treatment-related pathologic alterations were observed only in male rats at the 300 ppm level and induced decreased testicular size and atrophic seminiferous tubules. Female rats tolerated up to 300 ppm EGME without any adverse reproductive effects. Based on these results, it was concluded that the no-adverse effect level of EGME for fertility and reproduction was 100 ppm in rats.

Animals↗
PubMedDate not supplied

[Biomechanical studies of bisegmental lumbosacral stabilization using the internal fixator or PMMA-simulated fusion].

The implantation of fixation devices increases the primary stability after surgery significantly. Little is known about the comparison of flexibility of instrumented and posterolateral fusions or the lumbosacral distractions spondylodesis (LSDS). The purpose of the study was to compare the flexibility of instrumented and posterolateral fusions or LSDS after simulation with polymethylmethacrylate (PMMA). Dynamic testing from 0-16 Nm in flexion/compression and 0-900 N in compression was performed on 14 lumbosacral spines. The spines were tested intact, after laminectomy, instrumentation with an internal fixateur L5-S1, and simulation of posterolateral fusion L4-S1 or LSDS and thereafter a pseudoarthrosis was simulated. The laminectomy resulted in a significant increase of flexibility and posterior distraction in flexion and compression. The stabilization with an internal fixateur or PMMA simulation for posterolateral fusion or LSDS resulted in a significant decrease of the flexibility. Creation of a unilateral pseudoarthrosis did not effect the flexibility. The instrumented fusion as well as the PMMA simulated bony posterolateral fusion and LSDS resulted in this model in a significant stabilization of the laminectomy. After unilateral fusion the flexibility was not significantly increased.

Aged↗
PubMedDate not supplied

Critically sized osteo-periosteal femoral defects: a dog model.

A 21-mm defect was created in 1 femoral diaphysis each of 15 dogs. Periosteum as well as a cylinder of bone was removed, and the defect was stabilized with a bone plate. Twelve of the defects were filled with an equal volume of autogenous cancellous bone harvested from the ipsilateral ilium. Three defects were left untreated. Cranial to caudal radiographs were taken postoperatively and every 4 weeks for 16 weeks. The radiographs were evaluated for healing using two ordinal scales. At 16 weeks, the dogs were euthanized and the femurs harvested for biomechanical testing and histologic evaluation. Both operated and contralateral not operated femurs were mechanically tested to failure in torsion, and load at failure and stiffness were calculated. All dogs tolerated the procedure well, and were using the operated limb within 1 or 2 days postoperatively. There were no complications noted during the 16 weeks of the study. Unfilled defects did not heal and became atrophic nonunions. The defects filled with autogenous cancellous bone healed in a consistent pattern of consolidation, incorporation, and remodeling, with uniform increases of both ordinal scales used. The femoral cortex opposite the bone plate demonstrated most mature remodeling, evident both radiographically as well as histologically. Unoperated femurs failed at 13.61 +/- 3.88 N-m and grafted femurs failed at 2.96 +/- 1.3 N-m, which was 23% of the measurement of the unoperated femur. Relative stiffness of the unoperated femurs was 5974 +/- 4316 N-m2/radian, and grafted femurs had a relative stiffness of 642 +/- 561 N-m2/radian, which was 10.4% of the measurement of unoperated femur. This model proved to be a critically sized defect, which when left unfilled resulted in an atrophic nonunion, and when filled with cancellous bone resulted in a consistent healing pattern.

Animals↗
PubMedDate not supplied

Mechanical properties of trabecular bone from the proximal femur: a quantitative CT study.

We have investigated the relationships between trabecular bone compressive strength and elastic modulus and the directly measured apparent density and noninvasively measured CT equivalent mineral density for 49 cylindrical specimens harvested from fresh human proximal femora. Compressive strength demonstrated a high positive correlation with both densities, being proportional to the apparent density raised to the 1.8 power (R2 = 0.93) and equivalent mineral density to the 1.5 power (R2 = 0.89). Similarly, the compressive modulus demonstrated a high correlation with both density measures, being proportional to the apparent density raised to the 1.4 power (R2 = 0.91) and CT equivalent mineral density to the 1.2 power (R2 = 0.90). Though variations in architecture and bone marrow fat were observed to influence trabecular properties, the data presented here demonstrate that apparent density, compressive strength, and elastic modulus can be determined accurately using single energy quantitative CT. We expect that the use of these noninvasive data will result in improved estimates of that component of hip fracture risk that is attributable to bone strength.

Biomechanical Phenomena↗