Susan M. Stover

COLLAGEN FIBER ORGANIZATION IS RELATED TO MECHANICAL PROPERTIES AND REMODELING IN EQUINE BONE. A COMPARSION OF TWO METHODS

We studied birefringence as an indicator of collagen fiber orientation in the diaphysis of the equine third metacarpal bone. We had previously shown that tissue from the lateral cortex of this bone is stronger monotonically, but less fatigue resistant, than tissue from the medial and dorsal regions. To learn whether collagen fiber orientation might play a role in this regional specialization, we tested three hypotheses using the same specimens: (1) collagen fiber orientation is regionally dependent; (2) remodeling changes collagen fiber orientation; (3) longitudinal collagen fibers correlate positively with modulus and monotonic bending strength and negatively with flexural fatigue life. Beams (N = 36) cut parallel to the long axes of six pairs of bones had been tested to determine elastic modulus (N = 36), and fatigue life (N= 24) or monotonic strength (N = 12) in four-point bending. Subsequently, histologic cross-sections were prepared, and porosity, active remodeling and past remodeling were quantified. Birefringence was measured as an indicator of transverse collagen orientation using plane-polarized light (PPL), and again using circularly polarized light (CPL). The CPL measurement was less variable than the PPL measurement. Both birefringence measures indicated that collagen was more longitudinally oriented in the lateral cortex than in the other two cortices. Longitudinally disposed collagen correlated with greater modulus and monotonic strength, but did not correlate with fatigue life. Remodeling was associated with more transverse collagen. Neither measure of birefringence was significantly correlated with porosity. It was concluded that, in the equine cannon bone, longitudinal collage fiber orientation is regionally variable, contributes to increased modulus and strength but not fatigue life, and is reduced by osteonal remodeling.

CALCIUM BUFFERING IS REQUIRED TO MAINTAIN BONE STIFFNESS IN SALINE SOLUTION

This work determined whether mineral dissolution due to prolonged testing or storage of bone specimens in normal saline would alter their elastic modulus. In one experiment, small pieces of equine third metacarpal bone were soaked in normal saline supplemented with varying amounts of CaCl2. Changing Ca ion concentrations in the bath were monitored and the equilibrium concentration was determined. In a second experiment, the elastic moduli of twenty 4 x 10 x 100 mm equine third metacarpal beams were determined non-destructively in four-point bending. Half the beams were then soaked for 10 days in normal saline, and the other half in saline buffered to the bone mineral equilibrium point with Ca ions. Modulus measurements were repeated at 6 and 10 days. The equilibrium Ca ion concentration for bone specimens was found to be 57.5 mg l-1. The modulus of bone specimens soaked in normal saline significantly diminished 2.4%, whereas the modulus of those soaked in calcium-buffered saline did not change significantly.

Changes in canine cortical and cancellous bone mechanical properties following immobilization and remobilization with exercise.

The purpose of this study was to assess cortical and cancellous bone responses to unilateral limb immobilization and, subsequently, to remobilization with exercise, in a young adult canine model. Right forelimbs of 14 1-2-year old mongrel dogs were immobilized in a non-weight-bearing position by a bandage for 16 weeks. Six control dogs were untreated. At 16 weeks, seven immobilized and three control dogs were euthanized. The remaining seven immobilized dogs began a recovery protocol consisting of 16 weeks of kennel confinement (without the right forelimb bandaged) followed by 16 weeks of treadmill exercise conducted three times per week. These seven dogs and three control dogs were euthanized at 48 weeks. Bone mineral density of the proximal radii was determined with dual-energy X-ray absorptiometry and humeral middiaphyseal cross-sectional areas were determined with computed tomography. Humeri were tested in cranio-caudal three-point bending to failure. Cancellous bone cores from the lateral humeral condyles had wet apparent density determined and were tested to failure in compression. Mechanical properties, bone density, and cross-sectional areas were compared between immobilized (right forelimb), contralateral weight bearing (left forelimb), and control forelimbs with Kruskal-Wallis and post hoc tests. At 16 weeks, bone mineral density, cortical load, yield, and stiffness as well as cancellous bone failure stress, yield stress, and modulus were significantly lower (p < 0.02) for immobilized limbs than control limbs. Immobilized limb cancellous bone mechanical properties were 28%-74% of control values, and cortical bone mechanical properties were 71%-98% of control values. After 32 weeks of remobilization, cortical and cancellous bone mechanical properties were not different from control values except that cortical bone failure stress and modulus were significantly higher (p < 0.01) between remobilized and control limbs. In summary, 16 weeks of forelimb immobilization was associated with significantly lower mechanical properties, and with greater differences in cancellous than cortical bone properties. Mechanical properties were not different from control values after 32 weeks of recovery that included 16 weeks of treadmill exercise.

Pathophysiology of Navicular Syndrome

Navicular syndrome is a degenerative disorder of the distal half of the flexor surface of the proximal sesamoid bone that is predisposed by faulty foot conformation. In horses that become symptomatic, the faulty conformation results in sustained application of nonphysiologic pressure by the deep digital flexor tendon against the flexor cortex of the bone. This force stimulates an intense bone remodeling response in order to attenuate the pressure. An unfortunate sequela of this response is active hyperemia and edema formation in the medullary cavity of the bone. The edema is organized by fibrous tissue resulting in venous entrapment, venous hypertension, vascular bone pain, and the onset of clinical signs.

Cumulative racing-speed exercise distance cluster as a risk factor for fatal musculoskeletal injury in Thoroughbred racehorses in California

Abstract Thoroughbred racehorses which suffered a fatal musculoskeletal injury (FMI) while racing or race training at a California racetrack during 9 months of 1991 were studied to determine the importance of intensive, high-speed exercise schedules prior to injury. Seventy-seven horses which sustained an FMI while racing and 45 horses which sustained an FMI while race training were successfully matched by race or timed workout session with one control horse and included in the analyses. Race and timed workout (racing-speed exercise) histories were obtained for the case and control horses. Two-month cumulative, racing-speed cutoff distances were calculated from the control horse sample by two methods. Median racing-speed exercise frequencies and distances of the control horses were used to estimate age-specific (2, 3, 4 and ≥ 5 years), 2-month cumulative, racing-speed distances (Method 1). For the second method, the last race or timed workout for each control horse occurring just prior to, or on the date of injury for the matched case horse was identified. Cumulative racing-speed distances 2 months prior to these exercise events were determined for each control horse and used to estimate median age-specific (2, 3, 4 and ≥ 5 years), 2-month cumulative racing-speed distances (Method 2). The cumulative cutoff distances estimated from both methods were used to classify each matched pair according to the presence or absence of a 2-month cumulative, racing-speed distance which exceeded the age-appropriate cutoff distance (exercise distance cluster) within 6 months prior to injury. Manlel-Haenszel matched-pair odds ratios and 95% confidence limits were calculated separately for the racing and race-training fatal injuries. The relative risk for racing FMI was significantly greater for those horses which ran 2-month, cumulative racing and timed workout distances in excess of the cutoff values determined with Method 1 (relative risk (RR) = 3.0, 95% confidence interval (CI) = 1.2, 7.6) and Method 2 (RR = 7.2, 95% CI = 2.6, 20.6). The relative risk for race-training FMI was significantly greater for those horses which ran 2-month, cumulative racing and timed workout distances in excess of the cutoff values determined with Method 2 (RR = 3.4, 95% CI =1.0, 13.2).

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 105 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.

Hoof accelerations and ground reaction forces of Thoroughbred racehorses measured on dirt, synthetic, and turf track surfaces

OBJECTIVE To compare hoof acceleration and ground reaction force (GRF) data among dirt, synthetic, and turf surfaces in Thoroughbred racehorses. ANIMALS 3 healthy Thoroughbred racehorses. PROCEDURES Forelimb hoof accelerations and GRFs were measured with an accelerometer and a dynamometric horseshoe during trot and canter on dirt, synthetic, and turf track surfaces at a racecourse. Maxima, minima, temporal components, and a measure of vibration were extracted from the data. Acceleration and GRF variables were compared statistically among surfaces. RESULTS The synthetic surface often had the lowest peak accelerations, mean vibration, and peak GRFs. Peak acceleration during hoof landing was significantly smaller for the synthetic surface (mean + or - SE, 28.5g + or - 2.9g) than for the turf surface (42.9g + or - 3.8g). Hoof vibrations during hoof landing for the synthetic surface were < 70% of those for the dirt and turf surfaces. Peak GRF for the synthetic surface (11.5 + or - 0.4 N/kg) was 83% and 71% of those for the dirt (13.8 + or - 0.3 N/kg) and turf surfaces (16.1 + or - 0.7 N/kg), respectively. CONCLUSIONS AND CLINICAL RELEVANCE The relatively low hoof accelerations, vibrations, and peak GRFs associated with the synthetic surface evaluated in the present study indicated that synthetic surfaces have potential for injury reduction in Thoroughbred racehorses. However, because of the unique material properties and different nature of individual dirt, synthetic, and turf racetrack surfaces, extending the results of this study to encompass all track surfaces should be done with caution.

Residual strength of equine bone is not reduced by intense fatigue loading: Implications for stress fracture

Fatigue or stress fractures are an important clinical problem in humans as well as racehorses. An important question in this context is, when a bone experiences fatigue damage during extreme use, how much is it weakened compared to its original state? Since there are very limited data on this question and stress fractures are common in racehorses, we sought to determine the effect of fatigue loading on the monotonic strength of equine cortical bone. Beams were machined from the dorsal, medial and lateral cortices of the third metacarpal bones of six thoroughbred racehorses. Beams from left and right bones were assigned to control and fatigue groups, respectively (N = 18 each). The fatigue group was cyclically loaded in three-point bending at 2 Hz for 100,000 cycles at 0-5000 microstrain while submerged in saline at 37 degrees C. These beams, as well as those in the control group, were then monotonically loaded to failure in three-point bending. The monotonic load-deflection curves were analyzed for differences using three-factor (fatigue loading, anatomic region, and horse) analysis of variance. The mean failure load was 3% less in the fatigue group, but this reduction was only marginally significant. Neither elastic modulus nor yield strength was significantly affected by the fatigue loading. The principal effects of fatigue loading were on post-yield behavior (yield being based on a 0.02% offset criterion). The work done and the load increase between yield and failure were both significantly reduced. All the variables except post-yield deflection were significantly affected by anatomic region. In summary, loading equivalent to a lifetime of racing does not significantly weaken equine cortical bone ex vivo. The clinical implication of this may be that the biological repair of fatigue damage can actually contribute to stress fracture if pressed too far.

Mechanical property and tissue mineral density differences among severely suppressed bone turnover (SSBT) patients, osteoporotic patients, and normal subjects

Pathogenesis of atypical fractures in patients on long term bisphosphonate therapy is poorly understood, and the type, the manner in which they occur and the fracture sites are quite different from the usual osteoporotic fractures. We hypothesized that the tissue-level mechanical properties and mean degree of mineralization of the iliac bone would differ among 1) patients with atypical fractures and severely suppressed bone turnover (SSBT) associated with long-term bisphosphonate therapy, 2) age-matched, treatment-naïve osteoporotic patients with vertebral fracture, 3) age-matched normals and 4) young normals. Large differences in tissue-level mechanical properties and/or mineralization among these groups could help explain the underlying mechanism(s) for the occurrence of typical osteoporotic and the atypical femoral shaft fractures. Elastic modulus, contact hardness, plastic deformation resistance, and tissue mineral densities of cortical and trabecular bone regions of 55 iliac bone biopsies--12 SSBT patients (SSBT; aged 49-77), 11 age-matched untreated osteoporotic patients with vertebral fracture (Osteoporotic), 12 age-matched subjects without bone fracture (Age-Matched Normal), and 20 younger subjects without bone fracture (Young Normal)--were measured using nanoindentation and quantitative backscattered electron microscopy. For cortical bone nanoindentation properties, only plastic deformation resistance was different among the groups (p<0.05), with greater resistance to plastic deformation in the SSBT group compared to all other groups. For trabecular bone, all nanoindentation properties and mineral density of the trabecular bone were different among the groups (p<0.05). The SSBT group had greater plastic deformation resistance and harder trabecular bone compared to the other three groups, stiffer bone compared to the Osteoporotic and Young Normal groups, and a trend of higher mineral density compared to the Age-Matched Normal and Osteoporotic groups. Lower heterogeneity of modulus and contact hardness for cortical bone of the SSBT and trabecular bone of the Osteoporotic fracture groups, respectively, compared to the non-fractured groups, may contribute to fracture susceptibility due to lowered ability to prevent crack propagation. We tentatively conclude that, in addition to extremely low bone formation rate, atypical fractures in SSBT and/or long-term bisphosphonate treatment may be associated with greater mean plastic deformation resistance properties and less heterogeneous elastic properties of the bone.

The California Postmortem Program: Leading the Way

The California Postmortem Program studies disease surveillance, discovers new causes of death, and develops new diagnostic methods for disease detection. It also informs directions for research focused on elucidating the etiopathogenesis of, and risk factors for, catastrophic fractures and other causes of death. Because most fatal injuries seem to be the acute culmination of a more chronic process, intervention and prevention of injuries is possible with enhanced knowledge of the etiopathogenesis of injuries and risk factors for injuries. These advances also would facilitate development of management strategies for injury prevention.