Whenever bone fractures occur, external forces produce continuous interfragmentary motion and a stabilization method is necessary. It is known that the mechanical conditions at the fracture site influence bone callus formation during healing process. To achieve primary (direct) bone healing, absolute stability at the fracture site is necessary. Perren’s concept of strain determines that relative deformation at the fracture gap depends on the original gap’s conformation. Simple fractures (without comminution) are considered high strain fractures since a small force applied to the fracture site results in great movement with deleterious effects on the healing process. The purpose of this study is to review the available literature regarding factors that influence the mechanics of high strain fractures in veterinary medicine, its treatment methods and implants available. Each fracture configuration requires special attention and critical care in choosing the osteosynthesis method and the type of stability required for consolidation to occur within the expected time. One must know the strain theory to become an orthopedic surgeon.

OBJECTIVE To compare the torsional mechanical properties of 2 external skeletal fixators (ESFs) placed with 2 intramedullary pin (IP) and transfixation pin (TP) size combinations in a model of raptor tibiotarsal bone fracture. SAMPLE 24 ESF-synthetic tibiotarsal bone model (polyoxymethylene) constructs. PROCEDURES Synthetic bone models were fabricated with an 8-mm (simulated fracture) gap. Four types of ESF-synthetic bone model constructs (6/group) were tested: a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs. Models were rotated in torsion (5°/s) to failure or the machine angle limit (80°). Mechanical variables at yield and at failure were determined from load deformation curves. Effects of overall construct type, connecting bar type, and IP and TP size combination on mechanical properties were assessed with mixed-model ANOVAs. RESULTS Both FESSA constructs had significantly greater median stiffness and median torque at yield than both acrylic bar constructs; FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had greatest stiffness of all tested constructs and lowest gap strain at yield. No FESSA constructs failed during testing; 7 of 12 acrylic bar constructs failed by fracture of the connecting bar at the interface with a TP. CONCLUSIONS AND CLINICAL RELEVANCE Although acrylic bar ESFs have been successfully used in avian patients, the FESSA constructs in this study were mechanically superior to acrylic bar constructs, with greatest benefit resulting from use with the larger TP configuration.

In this case report, treatment of the proximal tibial fracture of a calf with a linear external fixator (orthofix) system was presented. Oblique fracture was diagnosed proximal to left tibia regarding the clinical and radiological findings. Following the routine preparatory steps for the operation, the extremity was suspended and the fracture was reduced with orthofix from the lateral side of the tibia under general anesthesia. After the operation, it was observed that the calf could functionally use the related extremity from the first day. The consolidation was completed on the 41st day, and the fixator was removed on 47th day. In conclusion, it was considered that the proximal tibial fractures of calves could be successfully treated with a linear external fixator.

Objective-To compare variables for screw insertion, pushout strength, and failure modes for a headless tapered compression screw inserted in standard and oversize holes in a simulated lateral condylar fracture model. Sample Population-6 pairs of third metacarpal bones from horse cadavers. Procedure-Simulated lateral condylar fractures were created, reduced, and stabilized with a headless tapered compression screw by use of a standard or oversize hole. Torque, work, and time for drilling, tapping, and screw insertion were measured during site preparation and screw implantation. Axial load and displacement were measured during screw pushout. Effects of drill hole size on variables for screw insertion and screw pushout were assessed by use of Wilcoxon tests. Results-Drill time was 59% greater for oversize holes than for standard holes. Variables for tapping (mean maximum torque, total work, positive work, and time) were 42%, 70%, 73%, and 58% less, respectively, for oversize holes, compared with standard holes. Variables for screw pushout testing (mean yield load, failure load, failure displacement, and failure energy) were 40%, 40%, 47%, and 71% less, respectively, for oversize holes, compared with standard holes. Screws could not be completely inserted in 1 standard and 2 oversize holes. Conclusions and Clinical Relevance-Enlarging the diameter of the drill hole facilitated tapping but decreased overall holding strength of screws. Therefore, holes with a standard diameter are recommended for implantation of variable pitch screws whenever possible. During implantation, care should be taken to ensure that screw threads follow tapped bone threads.

A 3.5-mm cortical orthopedic screw was compared with a 4.0-mm cancellous screw for maximal load to failure in the pelvis of immature dogs. The pelvis from young cadavers (7 to 13 months old) was divided into hemipelves and used for testing of the 2 screw types. Two sites in each hemipelvis were used, mid-shaft of the ilium and mid-sacrum, including the wing of the ilium. The screws were extracted, and maximal load to failure and mode of failure were recorded. Maximal load to failure per millimeter of engaged thread was calculated. In either pelvic site, the 4.0-mm cancellous screw required a significantly (P < 0.05) higher pullout force per millimeter of engaged screw threads than did the 3.5-mm cortical bone screw.

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.

Bilateral, midshaft metacarpal osteotomies were performed in 11 sheep and bilateral, midshaft radial osteotomies were performed in 7 sheep. The lesions were repaired with bone plates. One of each pair of plates was luted with polymethylmethacrylate and all screws were tightened uniformly with a torque screwdriver. Sheep were allowed unrestricted exercise after surgery. At 8 weeks, 10 of 11 sheep with metacarpal osteotomies were sound and both osteotomies were healing. Seven were lame on the limb with the unluted plate during the first 3 weeks; 4 were never lame on either limb. The screws of the unluted plates were significantly (P < 0.01) looser at 8 weeks than those in the luted plates. All of the sheep with radial osteotomies were lame in the limb with the unluted plate. Four of 7 sheep had overt loosening of the unluted plates. One sheep only had mild screw loosening with continued alignment of the osteotomy. Two of 7 sheep fractured the radius with the luted plate; these 2 sheep were lame in the limb with the unluted plate and were using the limb with the luted plate vigorously. Excluding the 2 sheep with fractures, all had substantially more screw loosening in the unluted plate. Histologically, there were no discernible differences in the vascularity or porosity of the bone under the luted vs the unluted plates. The only adverse consequence of the luting technique was introduction of a small amount of polymethylmethacrylate into the osteotomy gap in 5 bones.

Objective: To compare the axial stiffness, maximum axial displacement, and ring deformation during axial loading of single complete and incomplete circular (ring) external skeletal fixator constructs. Sample: 32 groups of single ring constructs (5 constructs/group). Procedures: Single ring constructs assembled with 2 divergent 1.6-mm-diameter Kirschner wires were used to stabilize a 60-mm-long segment of 16-mm-diameter acetyl resin rod. Construct variables included ring type (complete or incomplete), ring diameter (50, 66, 84, or 118 mm), and fixation wire tension (0, 30, 60, or 90 kg). Axial loading was performed with a materials testing system. Construct secant stiffness and maximum displacement were calculated from the load-displacement curves generated for each construct. Ring deformation was calculated by comparing ring diameter during and after construct loading to ring diameter prior to testing. Results: Complete ring constructs had greater axial stiffness than did the 66-, 84-, and 118-mm-diameter incomplete ring constructs. As fixation wire tension increased, construct stiffness increased in the 66-, 84-, and 118-mm-diameter incomplete ring constructs. Maximum axial displacement decreased with increasing fixation wire tension, and complete ring constructs allowed less displacement than did incomplete ring constructs. Incomplete rings were deformed by wire tensioning and construct loading. Conclusions and Clinical Relevance: Mechanical performance of the 66-, 84-, and 118-mm-diameter incomplete ring constructs improved when wire tension was applied, but these constructs were not as stiff as and allowed greater displacement than did complete ring constructs of comparable diameter. For clinical practice, tensioning the wires placed on 84- and 118-mm-diameter incomplete rings to 60 kg is recommended.

Objective-To describe a novel interlocking nail (ILN) and locking system and compare the torsional properties of constructs implanted with the novel ILN or a standard 8-mm ILN (ILN8) by use of a gap-fracture model. Sample Population-8 synthetic specimens modeled from canine tibiae. Procedures-An hourglass-shaped ILN featuring a tapered locking mechanism was designed. A synthetic bone model was custom-made to represent canine tibiae with a 50-mm comminuted diaphyseal fracture. Specimens were repaired by use of a novel ILN or an ILN8 with screws. Specimens were loaded for torsional measurements. Construct compliance and angular deformation were compared. Results-Compliance of the ILN8 was significantly smaller than that of the novel ILN. Mean +/- SD maximum angular deformation of the ILN8 construct (23.12 +/- 0.65 degrees) was significantly greater, compared with that of the novel ILN construct (9.45 +/- 0.22 degrees). Mean construct slack for the ILN8 group was 15.15 +/- 0.63 degrees, whereas no slack was detected for the novel ILN construct. Mean angular deformation for the ILN8 construct once slack was overcome was significantly less, compared with that of the novel ILN construct. Conclusions and Clinical Relevance-Analysis of results of this study suggests that engineering of the locking mechanism enabled the novel hourglass-shaped ILN system to eliminate torsional instability associated with the use of current ILNs. Considering the potential deleterious effect of torsional deformation on bone healing, the novel ILN may represent a biomechanically more effective fixation method, compared with current ILNs, for the treatment of comminuted diaphyseal fractures.

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.