Holding power was determined for various orthopedic screws in bones of calves. Holding power was defined as maximal tensile force required to remove a screw divided by thickness of bone engaged by the screw (kN/mm). Comparative pull-out tests were performed, using pairs of large metacarpal or metatarsal bones from calves aged 3 to 14 days. Comparisons were made of the holding power of 6.5-mm fully threaded cancellous screws and 5.5-mm cortical screws in the proximal and distal metaphyses, and of 4.5-mm and 5.5-mm cortical screws in the diaphysis. Sixteen repetitions of each comparative trial were performed. There was no statistically significant difference in the holding power of 4.5- and 5.5-mm cortical screws in the diaphysis. There was no significant difference in the holding power of 5.5-mm cortical and 6.5-mm fully threaded cancellous screws in the proximal metaphysis. In the distal metaphysis, 6.5-nu-n fully threaded cancellous screws had significantly (P < 0.001) greater holding power than did 5.5-mm cortical screws. There was no significant difference between the mean holding power of 5.5-mm cortical screws in the proximal metaphysis and 5.5-mm cortical screws in the distal metaphysis. There was significantly (P < 0.01) greater mean holding power of 6.5-mm cortical, fully threaded cancellous screws in the distal metaphysis, compared with the proximal metaphysis.

The effect of 5 fracture fixation methods on bone mineral density (BMD) measurement of femurs, using dual-energy X-ray absorptiometry (DXA) was determined in a canine model. Six regions of interest were measured, including the entire femur, the diaphysis of the femur, and small regions centered over the middiaphysis of the bone (lateral middiaphyseal cortex, medial middiaphyseal cortex, middiaphyseal medullary canal, and total middiaphysis). Eight unpaired femurs were collected and scanned by use of DXA before (5 separate scans/femur) and after (5 separate scans/femur) fixation by use of 1 of 5 fixation methods. These fixation methods included: intramedullary (IM) nail: IM nail and cerclage wires; IM nail and external skeletal fixation.; locked IM nail; and a dynamic compression plate (DCP). All implants were made of stainless steel. The IM nail fixation devices caused significant decreases in the DXA measurement of BMD in the small regions of interest, compared with femurs without fixation devices (mean decrease, 37.3%; P < 0.05). The locked nail caused similar, but larger, decreases in the DXA measurement of BMD, compared with the IM nail fixation methods (P < 0.05). Plate fixation caused a small, but significant (P < 0.05), decrease (2.8%) in the DXA measurement of BMD in the large regions of interest, but when all regions were averaged, it did not cause significant change in this measurement, compared with femurs without fixation devices. Plate fixation caused a large change in the DXA measurement of BMD in 1 region only in the lateral cortical bone under the plate where the DXA measurement of BMD was increased 13.3% over that in femurs without fixation devices (P < 0.05). In femurs without fixation devices, the precision error ranged from 0.5% for large regions of interest to 2.4% for small regions of interest. None of the fixation methods altered the precision error of large regions of interest (P > 0.05). In contrast, the precision errors of the small regions of interest were increased by the IM fixation methods and the locked IM nail, When all regions were combined, IM fixation methods caused significant (P < 0.05) increase in the precision error, compared with femurs without fixation devices (mean increase, 157%; range, 121 to 193%). Plate fixation did not change the precision error of any region of interest, compared with femurs without fixation devices (P > 0.05; power = 0.8 at delta = 64%).

Axial stability of equine oblique proxidmal phalangeal osteotonies with application of the standard short limb cast or 1 of 3 configurations of transfixation casts was determined in vitro. Transfixation cast methods included use of parallel pins, divergent pins, or parallel pins incorporating a metal walking bar. Displacement at the osteotomy was recorded for each limb at 4,448 N. Standard short limb casts provided significantly (P = 0.0002) less axial stability than did any form of transfixation cast. Significant differences were not found between the 3 transfixation casts.

Breaking strength (torque at failure) of equine third metacarpal bones, with transfixation pins placed parallel in the frontal plane and 30 degrees divergent from the frontal plane, was determined in vitro. Two transfixation pins were placed through the distal metaphysis, using a jig designed to drill the holes in the assigned configuration. Paired metacarpal bones II through IV from 12 horses were tested in torsion. The torsional moment of the force applied through the transfixation pins at failure was compared for each limb. Metacarpal bones with divergent pins were significantly (P = 0.030) stronger, compared with those with parallel pins. Metacarpal bones with parallel pins failed with longitudinal oblique fractures through a proxidmal bone-pin interface, whereas those with divergent pins failed with more comminuted fractures through multiple bone-pin interfaces.

As more sophisticated research is performed to refine fracture fixation techniques for horses, it is important that normal values for the geometric properties of the bones of the appendicular skeleton be determined and that suitable controls be available. We evaluated the geometric properties of total bone width, cortical bone width, and medullary canal/trabecular bone width measured from 2 radiographic projections of equine long bones (humerus, radius, third metacarpal bone, femur, tibia, and third metatarsal bone) obtained from a general population of horses. Measurements were performed on slices separated by intervals equal to 5% of the bone's length. Slices were then grouped into 5 regions: proximal epiphysis, proximal part of the metaphysis, diaphysis, distal part of the metaphysis, and distal epiphysis. Results validated use of the contralateral bone as a control for assessing experimental models or clinical cases. Of 858 homotypic slice comparisons between left and right bones, significant (P less than or equal to 0.05) differences were detected in 31 (3.6%) of the comparisons. Of 168 homotypic region comparisons, significant differences were observed in 3 (1.8%) of the comparisons. The greatest variation between left and right bones was observed in metaphyseal regions, areas with bony protuberances, and regions with prominent bone superimposition. At a power of 0.8 for the statistical tests performed in this study, the mean homotypic variation of bones in each region is < 5.8% for the proximal epiphysis, 11.3% for the proximal part of the metaphysis, 6.8% for the diaphysis, 12.2% for the distal part of the metaphysis, and 5.2% for the distal epiphysis.