Use of multisite quantitative ultrasonography for noninvasive assessment of bone in horses

OBJECTIVE: To evaluate the usefulness of multisite quantitative ultrasonography for noninvasive assessment of bone in horses. SAMPLE POPULATION: 12 healthy horses and both forelimbs from 8 clinically normal horses. PROCEDURE: For in vivo measurements, various regions of interest (ROI) were examined on the third metacarpal bone, radius, and tibia. Precision error for speed of sound (SOS) measurements was obtained by measuring each ROI of 4 horses 10 times with probe repositioning. Additionally, 3 operators measured each aspect of the third metacarpal bone of 6 horses 5 times each. For ex vivo measurements, third metacarpal bones were examined at 9 ROI, and SOS measurements were performed before and after soft tissue removal. One ROI of a single forelimb was subjected to 96 ex vivo measurements with 3 different contact media. RESULTS: The lateral aspect of the third metacarpal bone had significantly higher SOS values than the dorsal and medial aspect of the third metacarpal bone. No difference was obtained between SOS values of the lateral and medial aspect of the radius. The tibia had significantly higher SOS values than the lateral aspect of the radius and the dorsal and medial aspect of the third metacarpal bone. Intraoperator coefficients of variation ranged from 0.62 to 3.15%, and interoperator coefficients of variation ranged from 0.78 to 2.70%. Values of SOS were highest when silicone oil was used as the contact medium. CONCLUSIONS AND CLINICAL RELEVANCE: Speed of sound measurements obtained by quantitative ultrasonography in axial transmission mode can be used to precisely measure superficial cortical bone properties of third metacarpal bone, radius, and tibia in horses.     

Surgical treatment of dorsal cortical fractures of the third metacarpal bone in thoroughbred racehorses: 53 cases (1985-1989).

Between January 1985 and May 1989, 53 Thoroughbred horses (mean age 3.2 years) were surgically treated for dorsal cortical fractures of the third metacarpal bone (MC III). All horses were treated with cortical drilling through the fracture line (osteostixis). Diagnosis of the fractures was confirmed by xeroradiography. Lifetime racing records were obtained for all horses. Forty-seven horses returned to racing after surgery (89%). The mean time between surgery and the first race was 6.8 months. Horses had a mean of 10.9 starts before surgery and 16.1 starts after surgery. The mean earnings per start before surgery was $6,459 and after surgery was $5,685. Of the 47 horses that raced after surgery, 70% raced at the same class or improved. Complications related to surgery were seen in 10 horses. Two horses had a second fracture of MC III at the same site, and were again treated by osteostixis, after which both horses returned to competition. Fractured drill bits were left in the MC III of 4 horses. One of these horses had catastrophic failure of MC III. Two horses developed subcutaneous infections and 2 horses had catastrophic failure of MC III in the surgically treated limb. Osteostixis appears to be an effective treatment for returning horses affected with dorsal cortical fractures to racing.     

Pathophysiology and diagnosis of third carpal bone disease in horses: a review

Third carpal bone (C3) disease is a significant cause of lameness in Standardbred and Thoroughbred horses. The bone density of C3 increases as a result of exercise, reducing the compliance of the bone and predisposing it to injury. Currently, the most widely used method of diagnosis is subjective radiography using the tangential view. Radiographically, increases in bone mineral density (BMD) appear as sclerosis but it is not known at what point increases in sclerosis indicate the onset of disease or increased risk of C3 fracture. A quantitative assessment of the BMD of C3 in horses would improve understanding of the changes that occur within this bone and guide athletic management, as it is thought that BMD changes precede articular cartilage damage. Methods of non-invasive bone-mineral analysis used for the detection of osteoporosis in humans include single photon absorptiometry (SPA), dual x-ray absorptiometry (DXA), computed tomography (CT), radioabsorptiometry (RA), quantitative ultrasonography (QU) and magnetic resonance imaging (MRI). To date, DXA and RA are the most commonly used methods of quantitative non-invasive bone-mineral analysis in horses. The cost of equipment and difficulties in performing DXA in live animals preclude the routine use of this technique for diagnostic purposes. RA may become clinically applicable to C3 analysis in horses, but small variations in x-ray beam angle when taking the tangential view significantly affect results, making this technique clinically inapplicable at this time. Currently, methods of quantitative non-invasive bone-mineral analysis of C3 in horses are not suited to clinical application.     

The quantitative assessment of photodensity of the third carpal bone in the horse

AIM: To determine whether variation in x-ray-beam angle significantly affected photodensity of the third carpal bone (C3) in the horse using the tangential radiographic view, and indirectly determine whether radioabsorptiometry (RA) could be used to assess differences in bone mineral density (BMD) of C3 between exercised and non-exercised horses. METHODS: The left distal carpal row was isolated post-mortem from 2-year-old Thoroughbred horses that had been either exercised (n=7) for up to 14 weeks using a standard training regimen for racehorses, or not exercised (n=7). The effect of variation in x-ray-beam angle on photodensity was determined on these isolated carpal bones in vitro. Image analysis was used to assess photodensity (compared to a known thickness of aluminium) of four regions of interest (ROI) in C3 and one ROI in the fourth carpal bone (C4) of 14 isolated distal rows of carpal bones of the horse. The isolated carpal bones were placed fl at on a x-ray cassette and radiographed at 90 degrees (i.e. with the x-ray beam perpendicular to the cassette). The x-ray-beam angle was varied in the dorsal sagittal plane by 5 degrees increments to a total of 15 degrees from 90 degrees and from a base angle of 60 degrees (the x-ray-beam angle at which the tangential view is taken in clinical cases). RESULTS: Variation in beam angle of <10 degrees from 90 degrees significantly affected photodensity, and photodensity was significantly affected when the angle was varied <5 degrees from 60 degrees . When taken at an x-ray-beam angle of 60 degrees , the abaxial aspect of the radial facet of C3 had a consistently higher photodensity than the rest of C3 and C4. The photodensity of the third and fourth carpal bones was higher in exercised horses than in non-exercised horses. CONCLUSION: As variation in x-ray-beam angle significantly affected photodensity, RA using the tangential view is not considered clinically applicable for assessing BMD of C3 and the accuracy of subjective assessment of BMD of C3 using the tangential view in horses is questionable.     

Stress reactions and stress fractures of the proximal palmar aspect of the third metacarpal bone in horses: 58 cases (1980-1990).

The case records, radiographs, and nuclear bone scans of 58 horses with stress reactions or stress fractures of the proximal palmar aspect of the third metacarpal bone (MC3) were reviewed. There were 47 Standardbreds, 4 Quarter Horses, 3 Thoroughbreds, 2 Arabians, 1 Oldenburg, and 1 Pony of America. Fifty-six of the horses were racehorses or performance horses. The mean and median ages of affected horses were 4 and 3 years, respectively. Lameness ranged from mild to severe. Physical findings were usually subtle and included signs of pain on deep palpation of the proximal palmar aspect of MC3 and slight effusion of the middle carpal joint in some cases. Lameness was commonly improved by high palmar and palmar metacarpal nerve blocks or anesthesia of the middle carpal joint. Fifty-three horses had higher than normal radiopharmaceutical uptake in the proximal palmar aspect of MC3 in the left or right limb. The other 5 horses had higher than normal radiopharmaceutical uptake bilaterally. Fifty-six of the 63 limbs with abnormal bone scans also had abnormal radiographs. Treatment consisted of a variable period of rest (1 to 6 months). Healing was best assessed by follow-up bone scans. Of the 45 horses for which follow-up information was adequate, 29 (64%) returned to their previous level of performance.     

Effects of treadmill exercise on cortical bone in the third metacarpus of young horses.

The effects of exercise and relative inactivity on cortical bone were compared in young horses. Two groups were used; one was given a 14-week programme of exercise (n = 6) and the other kept as unexercised controls (n = 6). The first nine weeks of exercise involved trotting and cantering (2 to 4 km d-1 at speeds up to 12 m s-1) on a treadmill set at an incline of 3 degrees. Over the next five weeks the horses were trained at near maximal speeds (that is, up to 14.5 m s-1) with no incline of the treadmill. At the end of the programme marked differences in cortical porosity and distribution of subperiosteal osteogenesis at the mid-shaft of the third metacarpal bone were found between the groups. Histomorphometrical examination of the dorsal cortex showed minimal bone remodelling in the exercised horses, but extensive modelling as evidenced by the large amount of subperiosteal bone formation. In contrast, the unexercised horses had significantly more bone remodelling and less formation of subperiosteal bone. The histomorphometric and microradiographic findings provided an explanation for changes in the non-invasive bone measurements that occurred during training. Bone mineral content of the mid-metacarpus was found to increase more in the exercised than the unexercised horses despite a lower overall growth in bodyweight. In those horses that completed the full training programme, ultrasound speed increased significantly by the end of the training programme. It remained unchanged in the horse that did not complete the full exercise programme and decreased slightly in the unexercised horses. The difference in ultrasound speed between the groups was considered to reflect differences in intracortical bone porosity, endosteal bone formation and alterations in skin thickness. The stiffness of cortical bone increased significantly in the exercised horses but remained unaltered in the unexercised horses.     

Biomechanical investigation of the association between suspensory ligament injury and lateral condylar fracture in thoroughbred racehorses

Objective-To determine whether partial transection of the medial branch of the suspensory ligament (MBSL) alters equine third metacarpal bone (MC3) condylar surface strains and forelimb, distal joint angles in a manner consistent with promotion of lateral condylar fracture. Study Design-In vitro biomechanical experiment. Sample Population-Right forelimbs from 7 Thoroughbred horse cadavers. Methods-Lateral and medial MC3 condylar, dorsal and abaxial, bone surface strains and distal joint angles were measured both before and after partial transection of the MBSL during in vitro axial limb compression. Dorsal, principal bone strains and abaxial, uniaxial, and proximodistal strains were compared before and after MBSL partial transection at 1,400-, 3,000-, and 5,600-N loads. Results-Bone strains increased in all locations with increasing axial load. All lateral condylar bone strains were significantly higher, and abaxial surface medial condylar bone strain was significantly lower, after partial transection of the MBSL. Respective distal joints became more flexed or extended as axial load increased but were not significantly different after partial transection of the MBSL. Conclusions-Partial transection of the MBSL increases in vitro MC3 lateral condylar bone surface strains. Clinical Relevance-Loss of integrity of the medial branch of the suspensory ligament could increase the risk for lateral condylar fracture in Thoroughbred horses by amplifying bone strain in the lateral condyle.     

Third metacarpal bone laterality asymmetry and midshaft dimensions in Thoroughbred racehorses

OBJECTIVE: The aims of this study were to test whether longer third metacarpal (MC3) bones had thicker dorsal cortices in a group of racehorses that were exercising at similar maximum speeds, and to establish if horses with larger differences in length between their right and left MC3 bones showed larger differences in the dorsal cortical thickness between the two limbs. DESIGN: An observational study. PROCEDURE: Forty Thoroughbred racehorses aged between 2 and 6 years and in training at racing speed at two racing stables were used. Two sets of radiographs of each left and right metacarpus of each horse were measured for bone length and dorsal cortical width according to standardised methods. RESULTS: The dorsal cortex thickness showed a linear relationship with bone length for the range of lengths between 25 and 30 cm for both the right MC3 (R2 = 0.30, P = 0.0003) and the left (R2 = 0.23, P = 0.002). The longer bones had thicker dorsal cortices. When results from the two limbs were combined to test if the difference in length between the right and left MC3 in an individual horse was associated with a thicker dorsal cortex in the longer MC3 there was no consistent relationship (R2 = 0.008, P = 0.58). CONCLUSION: In this sample of racehorses longer MC3 bones were likely to have been exposed to a greater dorsopalmar bending moment at the mid shaft that was reflected in a thicker dorsal cortex. The lack of a relationship between midshaft thickness and bone length within individual horses suggests that direct mechanical effects of conformation and environment were less important than the individual's level of skill (or the degree of laterality in their movements) developed before their exposure to fast exercise. It is likely that racehorses with longer right MC3 bones were more able to control the loading of the right MC3 than the left during fast exercise.     

Proximal interphalangeal joint arthrodesis in 34 horses using two parallel 5.5-mm cortical bone screws

OBJECTIVE: To report clinical experience with arthrodesis of the proximal interphalangeal joint in horses using two parallel 5.5-mm cortical bone screws placed in lag fashion. STUDY DESIGN: Retrospective, clinical study. ANIMALS: Thirty-four horses, aged 1 to 19 years. METHODS: Medical records for all horses admitted (1991-1997) for pastern arthrodesis using two 5.5-mm ASIF cortical bone screws, in parallel orientation, and placed in lag fashion by use of a combined aiming device to facilitate accuracy were reviewed. Signalment, lameness diagnosis, duration of lameness, limb(s) involved, and outcome were recorded. Criteria for successful outcome were determined as return to previous level of function or future intended athletic use. RESULTS: Thirty-nine proximal interphalangeal joint arthrodeses were performed on 34 horses. One horse was euthanatized in the recovery room and was excluded from data analysis. Successful outcome occurred in 85% of frontlimbs and 89% of hindlimbs. Failure occurred in 5 joints; 1 horse had lameness directly associated with surgery, whereas 4 horses had unrelated lameness. CONCLUSION AND CLINICAL RELEVANCE: Age, breed, and initial disease did not affect outcome. Arthrodesis of the proximal interphalangeal joint by use of two 5.5-mm ASIF cortical bone screws, in parallel orientation, placed in lag fashion by use of a combined aiming device, resulted in sound use of the limb in >85% of the joints with shortened postoperative coaptation.     

Musculoskeletal responses of 2-year-old Thoroughbred horses to early training. 7. Bone and articular cartilage response in the carpus

AIM: To describe features of the morphology of the carpus, quantify the thickness of hyaline and calcified cartilage, and to describe the morphology and density of subchondral bone in the third carpal bone (C3) of young Thoroughbred horses in early training. METHODS: C3 of seven 2-year-old horses in training and seven untrained horses matched for age, sex and breed were assessed by gross appearance, computed tomography, fine-structure radiography, image analysis of high-resolution photographs, and histology. RESULTS: Macroscopic lesions in cartilage were few and mild, and not significantly different between groups. High bone mineral density (BMD), in some cases typical of cortical bone, was confined to the dorsal load path, and was significantly higher in trained than in untrained horses (p<0.01). In the most dorsoproximal aspect of the radial articular facet, apparently outside the dorsal load path, the BMD in both trained and untrained horses was significantly less than in other regions of interest (ROIs). Adaptive increase in density was associated with thickening of the (junctions of) trabeculae oriented proximo-distally. Hyaline cartilage was thicker (p<0.001) in the concavity of the radial articular facet than dorsal or palmar to it, and was thicker in the trained than untrained group (p=0.007). No such differences were detected in the thickness of articular calcified cartilage (ACC). CONCLUSIONS: The rapid response of bone in C3 to relatively small amounts of high-speed exercise was confirmed. A previously unreported increase in thickness of hyaline cartilage was evident, perhaps indicating that this tissue may be more responsive than hitherto thought, at least to particular types of exercise at particular times. These changes occurred with little evidence of abnormality, and thus appeared to be adaptive to the exercise regimen. The model developed should be used for further definition of the exercise stimulus required to produce adaptive, protective changes in sites susceptible to athletic injury. CLINICAL RELEVANCE: The data will serve as reference for use in subsequent imaging studies in which sophisticated aids such as magnetic resonance imaging (MRI) may be used to predict carpal lesions.     

Nuclear scintigraphic evaluation of third metacarpal and metatarsal bone fractures in three horses

Nuclear scintigraphy was used to evaluate healing of third metacarpal bone (MC III) fractures in 2 horses (horses 1 and 2) and a third metatarsal bone fracture in 1 horse (horse 3) after stabilization of each fracture with 2 broad dynamic compression plates. In horse 1, the fracture had uniform uptake of 99mTc methylene diphosphonate on days 1, 15, and 30 after surgery. The fracture healed, and the horse was discharged from the clinic on day 52. In horse 2, a 6-cm photopenic region (ie, area of low radioactivity) was seen over the diaphysis of MC III on day 3. The region persisted and became more distinct by day 32. The diaphysis of MC III sequestered, and horse 2 was euthanatized on day 44. In horse 3, vascularity was seen bridging the fracture on day 5, with a 3-cm photopenic region over the dorsal diaphysis of the third metatarsal bone. By days 18 and 32, uptake of 99mTc methylene diphosphonate in the region had increased, indicating vascularization of the site. the fracture healed, and horse 3 was discharged from the clinic on day 47. Our findings indicated that serial nuclear scintigraphy can be used to evaluate fracture vascularization after surgery in horses.     

1Alpha-hydroxyvitamin D3 prevents the decrease of bone mineral density in lactating beagles

We assessed the change of bone mineral density (BMD) in lactating beagles with dual energy X-ray absorptiometry (DXA) and the preventive effect of 1alpha-hydroxyvitamin D3 (1alpha(OH)D3) on the BMD. Beagles, two to five years old, were used for detecting the time course change of BMD. Since the coefficient of variation (CV(%)) on detecting lumber vertebral (L2-L4) and tibial BMD by DXA was about 0.5%, DXA was useful to detect the change of BMD in beagles. There was a marked decrease in vertebral BMD during lactational period in the control group. The BMD levels after weaning were found to reverse to the initial level at mating. The same tendency was observed in tibial BMD as vertebral BMD, though the BMD changes were not marked. Beagles were administered at a dose of 0.1 microg/kg of 1alpha(OH)D3 three times in a week, and it was found to suppress the decrease in vertebral BMD during the breast feeding period. Also, the administration of 1alpha(OH)D3 promoted the prevention of decreased BMD during lactation both in vertebrae and tibiae. Significant effects of 1alpha(OH)D3 administration on tibial BMD were not observed. No adverse effects, such as hypercalcemia and hypercalciuria, were observed during the experimental period. Therefore, DXA was useful for detecting the changes of BMD in lactating beagles and the change of BMD was marked in lumber vertebrae, which are rich in trabecular bone. The preventive effect of 1alpha(OH)D3 on the decrease of BMD during the lactation period was observed in beagles.     

Musculoskeletal responses of 2-year-old Thoroughbred horses to early training. 7. Bone and articular cartilage response in the carpus

AIM: To describe features of the morphology of the carpus, quantify the thickness of hyaline and calcified cartilage, and to describe the morphology and density of subchondral bone in the third carpal bone (C3) of young Thoroughbred horses in early training.

METHODS: C3 of seven 2-year-old horses in training and seven untrained horses matched for age, sex and breed were assessed by gross appearance, computed tomography, fine-structure radiography, image analysis of high-resolution photographs, and histology.

RESULTS: Macroscopic lesions in cartilage were few and mild, and not significantly different between groups. High bone mineral density (BMD), in some cases typical of cortical bone, was confined to the dorsal load path, and was significantly higher in trained than in untrained horses (p<0.01). In the most dorsoproximal aspect of the radial articular facet, apparently outside the dorsal load path, the BMD in both trained and untrained horses was significantly less than in other regions of interest (ROIs). Adaptive increase in density was associated with thickening of the (junctions of) trabeculae oriented proximo-distally. Hyaline cartilage was thicker (p<0.001) in the concavity of the radial articular facet than dorsal or palmar to it, and was thicker in the trained than untrained group (p=0.007). No such differences were detected in the thickness of articular calcified cartilage (ACC).

CONCLUSIONS: The rapid response of bone in C3 to relatively small amounts of high-speed exercise was confirmed. A previously unreported increase in thickness of hyaline cartilage was evident, perhaps indicating that this tissue may be more responsive than hitherto thought, at least to particular types of exercise at particular times. These changes occurred with little evidence of abnormality, and thus appeared to be adaptive to the exercise regimen. The model developed should be used for further definition of the exercise stimulus required to produce adaptive, protective changes in sites susceptible to athletic injury.

CLINICAL RELEVANCE: The data will serve as reference for use in subsequent imaging studies in which sophisticated aids such as magnetic resonance imaging (MRI) may be used to predict carpal lesions.     

Evaluation of dual energy x-ray absorptiometry for in situ measurement of bone mineral density of equine metacarpi

OBJECTIVE: To determine the accuracy and precision of dual energy x-ray absorptiometry (DEXA) for measuring bone mineral density in horses in situ. Sample Population-12 randomly selected forelimbs from 12 horses. PROCEDURE: Metacarpi were scanned in 2 planes and DEXA measurements obtained for 6 regions of interest (ROI). Each ROI was isolated and bone density measured by Archimedes' principle. Linear regression analysis was used to determine the correlation between the 2 measurements at each ROI. An additional metacarpus was measured 10 times to determine the coefficient of variation for both techniques. RESULTS: Dual energy x-ray absorptiometry and bone density were significantly associated at multiple ROI. The addition of age, weight, and soft tissue or bone thickness improved these associations. Repeated measurements had a low coefficient of variation. CONCLUSIONS AND CLINICAL RELEVANCE: Dual energy x-ray absorptiometry can be used to accurately and precisely measure the bone density in the equine metacarpus. Dual energy x-ray absorptiometry appears suitable for serial in vivo measurement of bone density of the equine metacarpus. Dual energy x-ray absorptiometry may be used for studies to evaluate the effects of diet or drugs on bone density or density changes from bone remodeling that develop prior to stress fractures.     

Axial compression generated by cortical and cancellous lag screws in the equine distal phalanx

Lag screw fixation using single 4.5 mm cortical bone screws is a recommended technique for repair of mid-sagittal plane fractures of the distal phalanx in adult horses. However, implant infection and technical difficulties in obtaining adequate interfragmentary compression have made this surgical procedure somewhat controversial. We hypothesized that use of larger diameter screws would result in increased axial compression and improved stability of this fracture.Paired distal phalanges from the forelimbs of 10 adult horses were collected at necropsy and divided in half in the midsagittal plane. Using a randomized block study design, four types of bone screws (4.5 mm cortical, 5.5 mm cortical, 6.5 mm cancellous pre-tapped, and 6.5 mm cancellous non-tapped) were inserted to a depth of 15 mm. During screw insertion, the axial force generated under the screw head was measured with a load washer containing a piezoelectric force transducer, while torque of insertion was recorded with a torsional testing machine. The 6.5mm screw inserted after pre-tapping generated significantly greater axial force (2781 N) than the 4.5 mm (1522 N), 5.5 mm (2073 N) or 6.5 mm non-tapped (2295 N) screws. The relationship between maximal applied torque and axial force generated was linear for each screw type. Each unit of torque applied during insertion of cortical screws resulted in a greater increase in axial compression, as compared to cancellous screws. These data suggest that use of larger diameter screws would result in improved interfragmentary compression of distal phalangeal fractures.     

Effects of phenylbutazone on bone activity and formation in horses

OBJECTIVE: To determine the effects of phenylbutazone (PBZ) on bone activity and bone formation in horses. ANIMALS: 12 healthy 1- to 2-year-old horses. PROCEDURES: Biopsy was performed to obtain unicortical bone specimens from 1 tibia on day 0 and from the contralateral tibia on day 14. Fluorochromic markers were administered IV 2 days prior to and on days 0, 10, 15, and 25 after biopsy was performed. Six horses received PBZ (4.4 mg/kg of body weight, PO, q 12 h) and 6 horses were used as controls. All horses were euthanatized on day 30 and tissues from biopsy sites, with adjacent cortical bone, were collected. Osteonal density and activity, mineral apposition rate (MAR), and percentage of mineralized tissue filling the biopsy-induced defects in cortical bone were assessed. Serum samples from all horses were analyzed for bone-specific alkaline phosphatase activity and concentration of PBZ. RESULTS: MAR was significantly decreased in horses treated with PBZ. Regional acceleratory phenomenon was observed in cortical bone in both groups but was significantly decreased in horses treated with PBZ. Osteonal activity was similar at all time points in all horses. In control horses, percentage of mineralized tissue filling the cortical defects was significantly greater in defects present for 30 days, compared with defects present for 14 days. Differences in percentage of mineralized tissue were not detected in horses treated with PBZ. CONCLUSIONS AND CLINICAL RELEVANCE: PBZ decreased MAR in cortical bone and appeared to decrease healing rate of cortical defects in horses.     

In vitro comparison of transfixation and standard full-limb casts for prevention of displacement of a mid-diaphyseal third metacarpal osteotomy site in horses

OBJECTIVE: To compare transfixation and standard full-limb casts for prevention of in vitro displacement of a mid-diaphyseal third metacarpal osteotomy site in horses. SAMPLE POPULATION: 6 forelimbs from 6 horses euthanatized for reasons not related to the musculoskeletal system. PROCEDURE: A 30 degrees osteotomy was performed in the mid-diaphysis of the third metacarpal bone. Two 4.5-mm cortical bone screws were placed across the osteotomy site to maintain alignment during casting. Two 6.35-mm Steinmann pins were placed from a lateral-to-medial direction in the distal aspect of the radius. A full-limb cast that incorporated the pins was applied. An extensometer was positioned in the osteotomy site through a window placed in the dorsal aspect of the cast, and after removal of the screws, displacement was recorded while the limb was axially loaded to 5,340 N (1,200 lb). Pins were removed, and the standard full-limb cast was tested in a similar fashion. RESULTS: The transfixation cast significantly reduced displacement across the osteotomy site at 445 N (100 lb), 1,112 N (250 lb), 2,224 N (500 lb), and 4,448 N (1,000 lb), compared with the standard cast. CONCLUSION AND CLINICAL RELEVANCE: A full-limb transfixation cast provides significantly greater resistance than a standard full-limb cast against axial collapse of a mid-diaphyseal third metacarpal osteotomy site when the bone is placed under axial compression. Placement of full-limb transfixation casts should be considered for the management of unstable fractures of the third metacarpal bone in horses.     

Dual-energy X-ray absorptiometry measurement of bone-mineral density in the distal aspect of the limbs in racing Greyhounds

OBJECTIVE: To determine those bones in the distal aspect of the limbs of Greyhounds with fatigue fractures that have the greatest left-to-right differences in bone-mineral density (BMD). SAMPLE POPULATION: Limbs obtained from 20 Greyhounds. PROCEDURE: Dual-energy x-ray absorptiometry (DXA) of the distal aspect of each limb and isolated bones from 10 dogs with a fracture of the central tarsal bone (CTB) of the right pelvic limb was performed. High-resolution scanning was performed on excised bones, and BMD measurements of CTB also were obtained from limbs of dogs without a CTB fracture. RESULTS: The BMD of the accessory carpal bone and calcaneus was not significantly different from the BMD of those bones in the contralateral limb. Although BMD of the CTB of the entire right pelvic limb and isolated bones from dogs with a CTB fracture was lower, compared with values for the entire left pelvic limb, values for isolated CTB from dogs without a CTB fracture were not significantly different. Metacarpal or metatarsal and thoracic or pelvic limb significantly affected BMD for measurements of the entire limb and isolated bones. Left-to-right differences in BMD were greatest for metacarpal 5. CONCLUSIONS AND CLINICAL RELEVANCE: Asymmetric adaptive remodeling of metacarpal 5 can be detected by DXA. The potentially confounding effects of CTB fracture and unknown racing history made it difficult to interpret BMD changes in the CTB of these specimens. Densitometry could be developed as an in vivo assessment for risk of fractures in racing Greyhounds.     

Short-duration exercise and confinement alters bone mineral content and shape in weanling horses

The hypothesis that short-duration exercise may ameliorate the decrease in bone mass observed with confinement was investigated with 18 quarter horses (nine colts and nine fillies) weaned at 4 mo of age and placed into box stalls. After a 5-wk adjustment period, individuals were grouped by age and weight, and then divided randomly into three treatment groups: 1) group housed; 2) confined with no exercise; and 3) confined with exercise. The confined and exercised groups were housed in 3.7 m x 3.7 m box stalls for the 56-d duration of the trial. The exercised group was sprinted 82 m/d, 5 d/wk, in a fenced grass alleyway. The weanlings were led down an alleyway, turned loose in a small pen, and then released and allowed to run back down the alley. The group horses were housed together in a 992-m2 drylot with free access to exercise. On d 0, 28, and 56, dorsopalmar and lateromedial radiographs of the left third metacarpal bone were taken to estimate changes in bone mineral content and cortical widths. Mean values of medial, lateral, and total radiographic bone aluminum equivalence increased over time (P < 0.05), whereas dorsal and palmar radiographic bone aluminum equivalence did not change significantly. Dorsal, medial, and total radiographic bone aluminum equivalence tended (P = 0.09) to differ by a treatment x day interaction, with values increasing over time only in the exercised group. Normalized medial and total radiographic bone aluminum equivalence tended (P < 0.1) to differ (P < 0.01) with treatment, with exercised horses having greater bone aluminum equivalence than confined horses. Dorsopalmar cortical width in exercised horses was greater than on d 56 (treatment x day; P = 0.07). The dorsopalmar medullary cavity decreased in exercised vs. group-housed horses (P = 0.027), whereas dorsal and medial cortical width tended to increase only in the exercised horses (treatment x day; P < 0.01). This study indicated that a short-duration exercise protocol might be effective in improving bone mass and therefore skeletal strength in horses.