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Biomedical subjects

S M Stover

Publications and source records attributed to S M Stover.

At least 19 recordsLinked to original sources
PubMed2004

Do microcracks decrease or increase fatigue resistance in cortical bone?

Fatigue of cortical bone produces microcracks; it has been hypothesized that these cracks are analogous to those occurring in engineered composite materials and constitute a similar mechanism for fatigue resistance. However, the numbers of these linear microcracks increase substantially with age, suggesting that they contribute to increased fracture incidence among the elderly. To test these opposing hypotheses, we fatigued 20 beams of femoral cortical bone from elderly men and women in load-controlled four point bending having initial strain ranges of 3000 or 5000 microstrain. Loading was stopped at fracture or 10(6) cycles, whichever occurred first, and microcrack density and length were measured in the loaded region and in a control region that was not loaded. We studied the dependence of fatigue life and induced microdamage on initial microdamage, cortical region, subject gender and age, and several other variables. When the effect of modulus variability was controlled, longer fatigue life was associated with higher rather than lower initial crack density, particularly in the medial cortex. The increase in crack density following fatigue loading was greater in specimens from older individuals and those initially having longer microcracks. Crack density increased as much in specimens fatigued short of the failure point as in those that fractured, and microcracks were, on average, shorter in specimens with greater numbers of resorption spaces, a measure of remodeling rate.

Adult↗
PubMed2003

Equine cortical bone exhibits rising R-curve fracture mechanics.

Previous studies of the fracture properties of cortical bone have suggested that the fracture toughness increases with crack length, which is indicative of rising R-curve behavior. Based on this indirect evidence and the similarity of bone to ceramic matrix composites, we hypothesized that bone would exhibit rising R-curve behavior in the transverse orientation and that the characteristics of the R-curves would be regionally dependent within the cortex due to variations in bone microstructure and toughening mechanisms. To test these hypotheses, we conducted R-curve experiments on specimens from equine third metacarpal bones using standard fracture mechanics testing methods. Compact type specimens from the dorsal and lateral regions in the middle of the diaphysis were oriented for crack propagation transverse to the longitudinal axis of the bone. The test results demonstrate that equine cortical bone exhibits rising R-curve behavior during transverse crack propagation as hypothesized. Statistical analyses of the crack growth initiation toughness, K0, the peak toughness, Kpeak, and the crack extension at peak toughness, deltaa, revealed significant regional differences in these characteristics. Specifically, the lateral cortex displayed higher crack growth initiation and peak toughnesses. The dorsal cortex exhibited greater crack extension at the peak of crack growth resistance. Scanning electron microscopy revealed osteon pullout on fracture surfaces from the dorsal cortex and but not in the lateral cortex. Taken together, the significant differences in R-curves and the SEM fractography indicate that the fracture mechanisms acting in equine cortical bone are regionally dependent.

Animals↗
PubMed2003

Osteon pullout in the equine third metacarpal bone: effects of ex vivo fatigue.

An important concept in bone mechanics is that osteons influence mechanical properties in several ways, including contributing to toughness and fatigue strength by debonding from the interstitial matrix so as to "bridge" developing cracks. Observations of "pulled out" osteons on fracture surfaces are thought to be indicative of such behavior. We tested the hypothesis that osteon pullout varies with mode of loading (fatigue vs. monotonic), cortical region, elastic modulus, and fatigue life. Mid-diaphseal beams from the dorsal, medial, and lateral regions of the equine third metacarpal bone were fractured in four point bending by monotonic loading to failure under deflection control, with or without 10(5) cycles of previous fatigue loading producing 5000 microstrain (15-20% of the expected failure strain) on the first cycle; or sinusoidal fatigue loading to failure, under load or deflection control, with the initial cycle producing 10,000 microstrain (30-40% of the expected failure strain). Using scanning electron microscopy, percent fracture surface area exhibiting osteon pullout (%OP.Ar) was measured. Monotonically loaded specimens and the compression side of fatigue fracture surfaces exhibited no osteon pullout. In load-controlled fatigue, pullout was present on the tension side of fracture surfaces, was regionally dependent (occurring to a greater amount dorsally), and was correlated negatively with elastic modulus and positively with fatigue life. Regional variation in %OP.Ar was also significant for the pooled (load and deflection controlled) fatigue specimens. %OP.Ar was nearly significantly greater in deflection controlled fatigue specimens than in load-controlled specimens (p=0.059). The data suggest that tensile fatigue loading of cortical bone eventually introduces damage that results in osteonal debonding and pullout, which is also associated with increased fatigue life via mechanisms that are not yet clear.

Animals↗
PubMed2002

Compliance calibration for fracture testing of equine cortical bone.

An experimental compliance calibration method for measuring crack length in fracture toughness tests of cortical bone was developed. Calibration tests were conducted on twenty compact type fracture specimens machined from the mid-diaphysis of five pairs of equine third metacarpal bones. Specimens were oriented for crack propagation in a direction transverse to the longitudinal axis of the bone. Specimen compliance was determined from the load vs. crack opening displacement record over a range of crack lengths from 0.48 to 0.75 times the specimen width. The results demonstrate that the compliance calibration method developed for isotropic materials can be used to determine crack length in bone, which is transversely isotropic. However, specimens from lateral and dorsal regions exhibited significantly different compliance calibrations even after differences in elastic modulus were taken into account in the normalized compliance.

Animals↗
PubMed2002

Stiff and strong compressive properties are associated with brittle post-yield behavior in equine compact bone material.

Our hypothesis was that post-yield mechanical behavior of compact bone material in compression, defined as the stress, strain, or energy absorbed between 0.2% strain-offset and the point of maximum stress, is correlated with material density, modulus, strength, histomorphometric evidence of remodeling, and post-failure gross specimen morphology. Post-yield behavior of compact bone material from the third metacarpal bone of 10 horses, ages 5 months to 20 years, was investigated using single-load compression-to-failure. The post-yield stress, strain, and absorbed energy were compared with the compressive elastic modulus, yield stress, ash density. post-failure macroscopic appearance of the specimen, and histologic evidence of remodeling. High values of elastic modulus, yield stress, and ash density were associated with low values of post-yield mechanical properties (stress, strain, and absorbed energy). Macroscopic post-failure morphology was associated with post-yield mechanical behavior, in that specimens displaying fractures were associated with lower post-yield mechanical properties, and that those without evidence of frank fracture were associated with higher post-yield mechanical properties. Microscopic evidence of remodeling activity was associated with high post-yield mechanical properties, but not with gross post-failure morphology. There was an abrupt change from relatively high values to extremely low values of post-yield mechanical properties at intermediate levels of ash density. This feature may serve as a functional tipper limit to the maximization of bone material stiffness and strength.

Animals↗
PubMed2002

Serum fluoride concentrations, biochemical and histopathological changes associated with prolonged sevoflurane anaesthesia in horses.

The volatile anaesthetic sevoflurane is degraded to fluoride (F-) and a vinyl ether (Compound A), which have the potential to harm kidney and liver. Whether renal and hepatic injuries can occur in horses is unknown. Cardiopulmonary, biochemical and histopathological changes were studied in six healthy thoroughbred horses undergoing 18 h of low-flow sevoflurane anaesthesia. Serum F- concentrations were measured and clinical laboratory tests performed to assess hepatic and renal function before and during anaesthesia. Necropsy specimens of kidney and liver were harvested for microscopic examination and compared to pre-experimental needle biopsies. Cardiopulmonary parameters were maintained at clinically acceptable levels throughout anaesthesia. Immediately after initiation of sevoflurane inhalation, serum F- levels began to rise, reaching an ongoing 38-45 micromol 1(-1) plateau at 8 h of anaesthesia. Serum biochemical analysis revealed only mild increases in glucose and creatinine kinase and a decrease in total calcium. Beyond 10 h of anaesthesia mild, time-related changes in urine included increased volume, glucosuria and enzymuria. Histological examination revealed mild microscopic changes in the kidney involving mainly the distal tubule, but no remarkable alterations in liver tissue. These results indicate that horses can be maintained in a systemically healthy state during unusually prolonged sevoflurane anaesthesia with minimal risk of hepatocellular damage from this anaesthetic. Furthermore, changes in renal function and morphology observed after sevoflurane inhalation are judged minimal and appear to be clinically irrelevant; they may be the result of anaesthetic duration, physiological stressors, sevoflurane (or its degradation products) or other unkown factors associated with these animals and study conditions.

Anesthetics, Inhalation↗
PubMed2002

Microstructure and mineral content of femora in male turkeys with and without fractures.

Microscopic and radiographic observations, as well as ash, calcium, phosphorus, copper, manganese, and zinc concentrations, were evaluated from femora of 32- to 35-wk-old male turkeys from two different farms (A and B). Turkeys from Farm A were divided into birds with femoral fracture (AF) and without femoral fracture (ANF). Turkeys from Farm B did not have femoral fractures (BNF). In the mid-diaphyseal cortex of turkeys without femoral fractures, collagen fibers were evenly distributed parallel to the transverse plane of bone surface. In contrast, collagen fibers were disorganized and the cortex was osteoporotic in turkeys with femoral fractures. Each turkey with femoral fractures also had callus on the mediocaudal region of the femoral middiaphysis. The periosteal callus was interrupted at the insertion of the puboischiofemoralis muscle. Muscle attachment to the bone might have interfered with the formation of periosteal callus. The concentration of calcium was lower in the cortex of turkeys in group AF compared to turkeys in groups ANF and BNF. The focal nature of the callus, along with the low levels of calcium, is indicative of a local repair response with alteration in cortical structure and altered concentrations of minerals in the cortex. These changes in the bone and the routine handling of the birds might have predisposed the turkeys to complete fractures of the femur.

Animals↗
PubMed2001

Risk factors for and outcomes of noncatastrophic suspensory apparatus injury in Thoroughbred racehorses.

OBJECTIVE: To evaluate effects of toe grabs, exercise intensity, and distance traveled as risk factors for subclinical to mild suspensory apparatus injury (SMSAI) in Thoroughbred racehorses and to compare incidence of severe musculoskeletal injury (MSI) in horses with and without SMSAI. DESIGN: Nested case-control study. ANIMALS: 219 Thoroughbred racehorses racing or in race training. PROCEDURE: Racehorses were examined weekly for 90 days to determine incidence of suspensory ligament injury and monitor horseshoe characteristics. Every horse's exercise speeds and distances were recorded daily. Conditional logistic regression was used to compare exposure variables between incident case (n = 25) and selected control (125) horses. Survival analysis was used to compare time to MSI for horses with (n = 41) and without (76) SMSAI. RESULTS: The best-fitting logistic model for the data included age (< 5 vs > or = 5 years old), toe grab height the week of injury (none vs very low, low, regular, or Quarter Horse height), and weekly distance the week preceding injury (miles). Although the 95% confidence intervals for all odds ratios included 1, the odds for SMSAI appeared to increase with the presence of a toe grab, higher weekly distance, and age > or = 5 years. Horses that had SMSAI were significantly more likely to have a severe MSI or severe suspensory apparatus injury than were horses that did not. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that pre-existing SMSAI is associated with development of severe MSI and severe suspensory apparatus injury. Modifying training intensity and toe grab height for horses with SMSAI may decrease the incidence of severe MSI.

Age Factors↗
PubMed2001

In vitro experimental study of the effect of radial shortening and ulnar ostectomy on contact patterns in the elbow joint of dogs.

OBJECTIVE: To determine effects of incremental radial shortening and subsequent ulnar ostectomies on joint surface contact patterns in a canine elbow joint model. SAMPLE POPULATION: Paired forelimbs from 9 adult dogs. PROCEDURE: Joint casting was performed by placement of colored polymethylmethacrylate in the elbow joint cavity and loading in a materials testing system at physiologic angle and load. Joint casting was performed in unaltered specimens, after radial shortening, and after subsequent distal ulnar ostectomy, proximal ulnar ostectomy, and proximal ulnar ostectomy with intramedullary pinning. Computer-aided analysis of photographs of proximal radial and ulnar articular surfaces without joint casts was performed before and after each casting procedure. RESULTS: All increments of radial shortening changed the size and location of radial and ulnar contact areas. The radial contact area became smaller, the anconeal contact area disappeared, the medial coronoid contact area migrated craniolaterally, and the lateral projection of the coronoid process became a contact area. A proximal ulnar ostectomy stabilized with an intramedullary pin restored normal contact area size and location and restored continuity of the radial and coronoid contact areas across the radioulnar articulation in 6 of 10 specimens. A midshaft ulnar ostectomy, distal to the level of the radioulnar ligament, had no effect on contact patterns. A proximal ulnar ostectomy without stabilization resulted in varus deformity during loading. CONCLUSIONS AND CLINICAL RELEVANCE: Proximal radial shortening, which creates articular step incongruity, changes the location and size of the radioulnar contact areas. Dynamically stabilized ulnar ostectomies proximal to the radioulnar ligament restore contact patterns in vitro.

Animals↗
PubMed2001

Biomechanical analysis of suture anchors and suture materials used for toggle pin stabilization of hip joint luxation in dogs.

OBJECTIVE: To compare in vitro mechanical properties of toggle pins and toggle rods used as suture anchors and of 3 suture materials (50-lb monofilament polybutester, No. 5 braided polyester, and 5-mm woven polyester) commonly used as prosthetic ligaments in the repair of hip joint luxation in dogs. SAMPLE POPULATION: Femoropelvic specimens from the cadavers of 18 dogs. PROCEDURE: Suture anchors were compared by use of pullout tests. Suture materials were compared by use of monotonic and cyclic tensile tests; cyclic tensile tests were performed with the suture placed over the edge of an aluminum bar to simulate the edge of the femoral bone tunnel. In vitro mechanical properties of the ligament of the femoral head were determined by use of monotonic tensile tests, using bone-ligament-bone cadaveric specimens. The in vitro mechanical properties of the acetabulum-ligament-femur complex and of this complex following rupture of the ligament and stabilization with a toggle rod and 5-mm woven polyester were determined by use of compression tests that simulated weight-bearing. RESULTS: Mechanical properties of the toggle rod were not significantly different from those of the toggle pin. Woven polyester had the longest fatigue life in cyclic testing. Hip joints stabilized with a toggle rod and woven polyester had less than half the strength in vitro of intact joints. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that a toggle rod or toggle pin can be used for stabilization of hip joint luxations in dogs. Of the materials tested, braided polyester had the best in vitro mechanical properties.

Animals↗
PubMed2001

Overload arthrosis: strain patterns in the equine metacarpal condyle.

An overload arthrosis occurs consistently in the palmar region of the metacarpal condyle of the equine fetlock (metacarpophalangeal) joint characterized by subchondral bone sclerosis, devitalization and mechanical failure leading to collapse of the overlying articular cartilage. Samples were selected of joints with mild, moderate, and severe subchondral sclerosis, in which cartilage collapse had not yet occurred. An additional group that had severe sclerosis with focal rarefaction suggesting impending collapse was also studied (n=5/group). Parasagittal slices were milled to 2.0 mm thickness and subjected to palmar forces 50 to 200% of those applied by the sesamoid bone at angles corresponding to early, mid and late stance support phases of the gait cycle. From contact radiographs in the loaded and unloaded samples, strains were determined by recognizing displacements in the trabecular patterns using texture correlation analysis. Failure did not occur in any of the samples. Strains were generally proportional to the forces applied and greatest at midstance. Strain patterns varied between samples and with the different loading positions. With increased subchondral bone sclerosis there was greater shear strain in overlying trabeculae. Strain patterns were not consistently different within the sclerotic bone at the site of failure. Focally higher strains at the surface were sometimes related to the edge of the platen which was molded to mimic the sesamoid bone in vivo. These results indicate that sclerotic thickening of subchondral bone transmits stresses to overlying trabeculae. No consistent strain pattern was recognized where devitalization and mechanical failure occurs. Focally higher strains related to the edge of the opposing sesamoid bone may play a role.

Journal Article↗
PubMed2000

Morphometric and mechanical properties of femora in young adult male turkeys with and without femoral fractures.

Morphologic and torsional mechanical properties of femora were evaluated in three groups of turkeys to determine whether turkeys that incurred femoral fracture have different radiographic, morphologic, and torsional mechanical properties than do turkeys without femoral fracture, and if body weight was a factor differentiating affected and nonaffected turkeys. Nine turkeys with unilateral femoral fractures and nine turkeys without fractures from Farm A were compared with a group of nine age and genetically matched turkeys from Farm B. Turkeys from Farm A were 10 to 25% heavier (26.2+/-0.9 kg) and turkeys from Farm B were 10% lighter (18.2+/-0.9 kg) than expected breed standards. Femoral length, cortical thickness, medullary diameter, and diaphyseal curvature were measured from craniocaudal and lateromedial radiographs. One intact femur from each of five turkeys with a contralateral femoral fracture (Farm A), nine unaffected turkeys from Farm A, and nine turkeys from Farm B were subjected to failure in a torsion test. Geometric and mechanical results were compared. Turkeys from Farm A had a thicker cortex and a smaller medullary diameter than turkeys from Farm B. Turkeys from Farm A had significantly lower torsional stiffness and failure torque values compared with turkeys from Farm B, when differences in body weight were accounted for (analysis of covariance, P < 0.05). Mechanical failure torque calculated from geometric values suggests that organic or inorganic material arrangement and composition may be responsible, in part, for differences between groups. Femoral fractures may be associated with insufficient skeletal adaptation to heavy body weight.

Animals↗
PubMed2000

Comparison of the fit and geometry of reconstruction of femoral components of four cemented canine total hip replacement implants.

OBJECTIVE: To compare fit and geometry of reconstruction of femoral components of 4 canine cemented total hip replacement implants and determine which implants are most compatible with current principles of cemented arthroplasty. SAMPLE POPULATION: Paired femurs from 16 adult mixed-breed dogs. PROCEDURE: Femurs were prepared for femoral stem implantation of either the Bardet, BioMedtrix, Mathys, or Richards II implant. Mediolateral and craniocaudal radiographs were obtained with femoral components in situ. Cross-sectional analysis of implant fit was performed on transected cemented specimens. Computer-aided analyses of digitized images were performed. RESULTS: The Bardet and Richards II implants reconstructed the original femoral head position significantly better than the other 2 implants. None of the implants allowed neutralization of the implant axis in the sagittal plane or were routinely centralized in the femoral canal. The Bardet implant had the smallest minimum distal tip offset in the sagittal plane. Greatest tip to cortex distance was provided by the Richards II implant in the transverse plane and the Mathys implant in the sagittal plane. The thinnest cement mantle regions for all implants were in the central longitudinal third of the femoral stem. CONCLUSIONS AND CLINICAL RELEVANCE: The Bardet and BioMedtrix implants had stem design characteristics that were most compatible with principles of cemented stem fixation. None of the implants completely satisfied the theoretically optimal conditions of centralization and neutralization of the femoral stem. Innovative design modifications, therefore, may be needed if these conditions are important to the long-term success of canine total hip replacement.

Animals↗
PubMed2000

Limb loading activity of adult horses confined to box stalls in an equine hospital barn.

OBJECTIVE: To determine a range of limb loading activity for healthy adult horses confined to box stalls in an equine veterinary teaching hospital and determine the effects of hospital environmental factors on load rates and daily limb loading patterns. ANIMALS: 6 mature healthy horses of various ages, breeds, and sexes, and 1 horse with a repaired metatarsal fracture. PROCEDURE: Step monitors were placed on 2 limbs of adult horses confined to box stalls. Relocation steps and weight shifts were recorded, as loading events, for 24 hours. Influence of forelimb versus hind limb and environmental factors on load rate (loading events per hour) were assessed with repeated-measures ANOVA. RESULTS: Loading activity was greater for the forelimb than the hind limb and was greater during the day than the night. Loading activity differences were not associated with daytime environmental factors. CONCLUSIONS AND CLINICAL RELEVANCE: Horses with normal locomotor activity appear to have higher load rates for forelimbs compared with hind limbs and higher load rates during the day compared with night. Knowledge of influence of environmental factors and mechanical restraint on limb loading activity may be useful in management of horses with musculoskeletal disorders. This information may also be used for in vitro simulation of in vivo loading of limbs during cyclic biomechanical investigations.

Analysis of Variance↗
PubMed1999

The effects of testing methods on the flexural fatigue life of human cortical bone.

A flexural model of four-point bending fatigue that has been experimentally validated for human cortical bone under load control was used to determine how load and displacement control testing affects the fatigue behavior of human cortical bone in three-point and symmetric four-point bending. Under load control, it was predicted that three-point bending produced no significant differences in fatigue life when compared to four-point bending. However, three-point bending produced less stiffness loss with increasing cycles than four-point bending. In four-point bending, displacement control was predicted to produce about one and a half orders of magnitude greater fatigue life when compared to load control. This prediction agrees with experimental observations of equine cannon bone tested in load and displacement control (Gibson et al., 1998). Displacement controlled three-point bending was found to produce approximately a 25% greater fatigue life when compared to load control. The prediction of longer fatigue life under displacement control may have clinical relevance for the repair of damaged bone. The model can also be adapted to other geometric configurations, including modeling of whole long bones, and with appropriate fatigue data, other cortical bone types.

Bone and Bones↗
PubMed1999

Pathologic changes in the lumbosacral vertebrae and pelvis in Thoroughbred racehorses.

OBJECTIVE: To describe the prevalence, characteristics, and severity of soft-tissue and osseous lesions in the caudal portion of the thoracic and lumbosacral vertebral column and pelvis in Thoroughbred racehorses. ANIMALS: 36 Thoroughbred racehorses that died or were euthanatized at California racetracks between October 1993 and July 1994. PROCEDURE: Lumbosacral and pelvic specimens were collected and visually examined for soft-tissue and osseous lesions. RESULTS: Acute sacroiliac joint injury was observed in 2 specimens. Signs of chronic laxity or subluxation of the sacroiliac joint were not observed in any specimens. Impingement of the dorsal spinous processes and transverse processes was observed in 92 and 97% of specimens, respectively. Thoracolumbar articular processes had variable degrees of degenerative change in 97% of specimens. Degenerative changes were observed at lumbar intertransverse joints and sacroiliac articulations in all specimens. Some degenerative changes were widespread and severe. CONCLUSIONS: Numerous degenerative changes affected vertebral processes, intervertebral articulations, and sacroiliac joints in these Thoroughbred racehorses. CLINICAL RELEVANCE: Various types of vertebral and pelvic lesions need to be considered during clinical evaluation of the back and pelvis in horses. Undiagnosed vertebral or pelvic lesions could be an important contributor to poor performance and lameness in athletic horses.

Animals↗
PubMed1999

Effect of surgical technique and use of a rigid centralizing device on stem positioning and geometric reconstruction in the sagittal plane during total hip replacement in canine cadavers.

OBJECTIVE: To determine the effect of surgical technique and use of a rigid centralizing device on stem positioning and geometric reconstruction in the sagittal plane during total hip replacement in dogs. SAMPLE POPULATION: Bilateral femurs from 8 adult mixed-breed canine cadavers. PROCEDURE: Femurs were prepared for femoral stem implantation, using 4 variations in technique. Proximal femoral reconstruction and femoral stem positioning were evaluated on radiographs. RESULTS: Implants evaluated in this study accurately reconstructed displacement of the femoral head of the intact canine femur in the sagittal plane. Centralization of the distal aspect of the stem was optimized by use of an undersized femoral stem. Ostectomy at the level of the lesser trochanter resulted in the smallest diaphysis-to-implant angle. Anteversion and retroversion of implants significantly decreased the distance between the distal tip of the implant and the adjacent cortex, compared with normoversion. The centralizing device significantly increased the minimum distance between the distal tip of the implant and adjacent cortex but did not improve the odds of actually centralizing the tip of the implant. CONCLUSIONS AND CLINICAL RELEVANCE: Malpositioning of implants in the sagittal plane may be minimized through ostectomy at the lesser trochanter and use of an undersized implant positioned in normoversion. Use of a polymethylmethacrylate centralizing device will help eliminate contact between the implant tip and adjacent cortex. Implantation of an undersized femoral component, avoidance of substantial anteversion or retroversion, and use of a rigid centralizing device are recommended when using the prosthesis described-for total hip replacement of dogs.

Animals↗