An in vitro biomechanical comparison of a prototype equine metacarpal dynamic compression plate fixation with double dynamic compression plate fixation of osteotomized equine third metacarpal bones

OBJECTIVES: To compare the monotonic biomechanical properties of a prototype equine third metacarpal dynamic compression plate (EM-DCP) fixation with a double broad dynamic compression plate (DCP) fixation to repair osteotomized equine third metacarpal (MC3) bones. STUDY DESIGN: In vitro biomechanical testing of paired cadaveric equine MC3 with a mid-diaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. POPULATION: Twelve pairs of adult equine cadaveric MC3 bones. METHODS: Twelve pairs of equine MC3 were divided into 3 test groups (4 pairs each) for (1) 4-point bending single cycle to failure testing, (2) 4-point bending cyclic fatigue testing, and (3) torsional testing. The EM-DCP (10-hole, 4.5 mm) was applied to the dorsal surface of one randomly selected bone from each pair. Two DCPs, 1 dorsally (10-hole, 4.5 mm broad) and 1 laterally (9-hole, 4.5 mm broad) were applied to the contralateral bone from each pair. All plates and screws were applied using standard AO/ASIF techniques to MC3 bones that had mid-diaphyseal osteotomies. Mean test variable values for each method were compared using a paired t-test within each group. Significance was set at P<.05. RESULTS: Mean 4-point bending yield load, yield bending moment, bending composite rigidity, failure load and failure bending moment of the EM-DCP fixation were significantly greater (P<.0001) than those of the double broad DCP fixation. Mean cycles to failure in 4-point bending of the EM-DCP fixation was significantly greater (P<.0008) than that of the double broad DCP fixation. Mean yield load, composite rigidity, and failure load in torsion of the EM-DCP fixation were significantly greater (P<.0035) than that of the double broad DCP fixation. CONCLUSION: The EM-DCP provides increased stability in both static overload testing and cyclic fatigue testing. CLINICAL RELEVANCE: Results of this in vitro study support the conclusion that the prototype EM-DCP fixation is biomechanically superior to the double broad DCP fixation for the stabilization of osteotomized equine MC3.     

An in vitro biomechanical comparison of a limited-contact dynamic compression plate fixation with a dynamic compression plate fixation of osteotomized equine third metacarpal bones

OBJECTIVES: To compare the monotonic biomechanical properties and fatigue life of a broad, limited contact, dynamic compression plate (LC-DCP) fixation with a broad, dynamic compression plate (DCP) fixation to repair osteotomized equine 3rd metacarpal (MC3) bones. STUDY DESIGN: In vitro biomechanical testing of paired cadaveric equine MC3 with a mid-diaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. ANIMAL POPULATION: Twelve pairs of adult equine cadaveric MC3 bones. METHODS: Twelve pairs of equine MC3 were divided into 3 test groups (4 pairs each) for (1) 4-point bending single cycle to failure testing, (2) 4-point bending cyclic fatigue testing, and (3) torsional single cycle to failure testing. An LC-DCP (8-hole, 4.5 mm) was applied to the dorsal surface of 1 randomly selected bone from each pair. One DCP (8-hole, 4.5 mm broad) was applied dorsally to the contralateral bone from each pair. All plates and screws were applied using standard AO/ASIF techniques to MC3 bones that had mid-diaphyseal osteotomies. Mean test variable values for each method were compared using a paired t-test within each group. Significance was set at P<.05. RESULTS: The mean 4-point bending yield load, yield bending moment, composite rigidity, failure load, and failure bending moment of LC-DCP fixation were significantly greater (P<.01) than those of broad DCP fixation. Mean cycles to failure for 4-point bending was significantly (P<.001) greater for broad DCP fixation compared with broad LC-DCP fixation. Mean yield load, mean composite rigidity, and mean failure load in torsion was significantly (P<.02) greater for broad LC-DCP fixation compared with broad DCP fixation. CONCLUSION: Broad LC-DCP offers increased stability in static overload testing, however, it offers significantly less stability in cyclic fatigue testing. CLINICAL RELEVANCE: The clinical relevance of the cyclic fatigue data supports the conclusion that the broad DCP fixation is biomechanically superior to the broad LC-DCP fixation in osteotomized equine MC3 bones despite the results of the static overload testing.     

In vitro biomechanical comparison of locking compression plate fixation and limited-contact dynamic compression plate fixation of osteotomized equine third metacarpal bones

Objective— To compare monotonic biomechanical properties and fatigue life of a broad locking compression plate (LCP) fixation with a broad limited contact dynamic compression plate (LC‐DCP) fixation to repair osteotomized equine third metacarpal (MC3) bones. Study Design— In vitro biomechanical testing of paired cadaveric equine MC3 with a mid‐diaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. Animal Population— Cadaveric adult equine MC3 bones (n=12 pairs). Methods— MC3 were divided into 3 groups (4 pairs each) for: (1) 4‐point bending single cycle to failure testing; (2) 4‐point bending cyclic fatigue testing; and (3) torsional single cycle to failure testing. The 8‐hole, 4.5 mm LCP was applied to the dorsal surface of 1 randomly selected bone from each pair. One 8‐hole, 4.5 mm LC‐DCP) was applied dorsally to the contralateral bone from each pair. All plates and screws were applied using standard ASIF techniques. All MC3 bones had mid‐diaphyseal osteotomies. Mean test variable values for each method were compared using a paired t‐test within each group. Significance was set at P<.05. Results— Mean yield load, yield bending moment, composite rigidity, failure load and failure bending moment, under 4‐point bending, single cycle to failure, of the LCP fixation were significantly greater than those of the LC‐DCP fixation. Mean cycles to failure for 4‐point bending was significantly greater for the LCP fixation compared with LC‐DCP fixation. Mean yield load, mean composite rigidity, and mean failure load under torsional testing, single cycle to failure was significantly greater for the broad LCP fixation compared with the LC‐DCP fixation. Conclusion— The 4.5 mm LCP was superior to the 4.5 mm LC‐DCP in resisting the static overload forces (palmarodorsal 4‐point bending and torsional) and in resisting cyclic fatigue under palmarodorsal 4‐point bending. Clinical Relevance— The results of this in vitro study may provide information to aid in the selection of a biological plate for the repair of equine long bone fractures.     

An in vitro biomechanical comparison between prototype tapered shaft cortical bone screws and AO cortical bone screws for an equine metacarpal dynamic compression plate fixation of osteotomized equine third metacarpal bones

OBJECTIVES: To compare biomechanical properties of a prototype 5.5 mm tapered shaft cortical screw (TSS) and 5.5 mm AO cortical screw for an equine third metacarpal dynamic compression plate (EM-DCP) fixation to repair osteotomized equine third metacarpal (MC3) bones. STUDY DESIGN: Paired in vitro biomechanical testing of cadaveric equine MC3 with a mid-diaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. ANIMAL POPULATION: Adult equine cadaveric MC3 bones (n=12 pairs). METHODS: Twelve pairs of equine MC3 were divided into 3 groups (4 pairs each) for (1) 4-point bending single cycle to failure testing, (2) 4-point bending cyclic fatigue testing, and (3) torsional single cycle to failure testing. An EM-DCP (10-hole, 4.5 mm) was applied to the dorsal surface of each, mid-diaphyseal osteotomized, MC3 pair. For each MC3 bone pair, 1 was randomly chosen to have the EM-DCP secured with four 5.5 mm TSS (2 screws proximal and distal to the osteotomy; TSS construct), two 5.5 mm AO cortical screws (most proximal and distal holes in the plate) and four 4.5 mm AO cortical screws in the remaining holes. The control construct (AO construct) had four 5.5 mm AO cortical screws to secure the EM-DCP in the 2 holes proximal and distal to the osteotomy in the contralateral bone from each pair. The remaining holes of the EM-DCP were filled with two 5.5 mm AO cortical screws (most proximal and distal holes in the plate) and four 4.5 mm AO cortical screws. All plates and screws were applied using standard AO/ASIF techniques. Mean test variable values for each method were compared using a paired t-test within each group. Significance was set at P<.05. RESULTS: Mean 4-point bending yield load, yield bending moment, bending composite rigidity, failure load and failure bending moment of the TSS construct were significantly greater (P<.00004 for yield and P<.00001 for failure loads) than those of the AO construct. Mean cycles to failure in 4-point bending of the TSS construct was significantly greater (P<.0002) than that of the AO construct. The mean yield load and composite rigidity in torsion of the TSS construct were significantly greater (P<.0039 and P<.00003, respectively) than that of the AO construct. CONCLUSION: The TSS construct provides increased stability in both static overload testing and cyclic fatigue testing. CLINICAL RELEVANCE: The results of this in vitro study support the conclusion that the EM-DCP fixation using the prototype 5.5 mm TSS is biomechanically superior to the EM-DCP fixation using 5.5 mm AO cortical screws for the stabilization of osteotomized equine MC3.     

An In Vitro Biomechanical Comparison of a 5.5 mm Locking Compression Plate Fixation with a 4.5 mm Locking Compression Plate Fixation of Osteotomized Equine Third Metacarpal Bones

Objectives: To compare the monotonic biomechanical properties and fatigue life of a 5.5‐mm‐broad locking compression plate (5.5 LCP) fixation with a 4.5‐mm‐broad locking compression plate (4.5 LCP) fixation to repair osteotomized equine 3rd metacarpal (MC3) bones. Study Design: In vitro biomechanical testing of paired cadaveric equine MC3 with a middiaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. Animal Population: Fifteen pairs of adult equine cadaveric MC3 bones. Methods: Fifteen pairs of equine MC3 were divided into 3 test groups (5 pairs each) for (1) 4‐point bending single cycle to failure testing, (2) 4‐point bending cyclic fatigue testing, and (3) torsional single cycle to failure testing. An 8‐hole, 5.5 LCP was applied to the dorsal surface of 1 randomly selected bone from each pair and an 8‐hole, 4.5 LCP was applied dorsally to the contralateral bone from each pair using a combination of cortical and locking screws. All plates and screws were applied using standard ASIF techniques. All MC3 bones had middiaphyseal osteotomies. Mean test variable values for each method were compared using a paired t‐test within each group with significance set at P<.05. Results: Mean yield load, yield bending moment, composite rigidity, failure load, and failure bending moment, under 4‐point bending, single cycle to failure, of the 5.5 LCP fixation were significantly greater than those of the 4.5 LCP fixation. Mean cycles to failure in 4‐point bending of the 5.5 LCP fixation (170,535±19,166) was significantly greater than that of the 4.5 LCP fixation (129,629±14,054). Mean yield load, mean composite rigidity, and mean failure load under torsional testing, single cycle to failure was significantly greater for the broad 5.5 LCP fixation compared with the 4.5 LCP fixation. In single cycle to failure under torsion, the mean±SD values for the 5.5 LCP and the 4.5 LCP fixation techniques, respectively, were: yield load, 151.4±19.6 and 97.6±12.1 N m; composite rigidity, 790.3±58.1 and 412.3±28.1 N m/rad; and failure load: 162.1±20.2 and 117.9±14.6 N m. Conclusion: The 5.5 LCP was superior to the 4.5 LCP in resisting static overload forces (palmarodorsal 4‐point bending and torsional) and in resisting cyclic fatigue under palmarodorsal 4‐point bending. Clinical Relevance: These in vitro study results may provide information to aid in selection of an LCP for repair of equine long bone fractures.     

An in vitro biomechanical comparison of a 5.5 mm limited-contact dynamic compression plate fixation with a 4.5 mm limited-contact dynamic compression plate fixation of osteotomized equine third metacarpal bones

Objectives— To compare monotonic biomechanical properties and fatigue life of a 5.5 mm broad limited‐contact dynamic compression plate (5.5‐LC‐DCP) fixation with a 4.5 mm broad LC‐DCP (4.5‐LC‐DCP) fixation to repair osteotomized equine third metacarpal (MC3) bones. Study Design— In vitro biomechanical testing of paired cadaveric equine MC3 with a mid‐diaphyseal osteotomy, stabilized by 1 of 2 methods for fracture fixation. Sample Population— Adult equine cadaveric MC3 bones (n=18 pair). Methods— MC3 were divided into 3 test groups (6 pairs each) for: (1) 4‐point bending single cycle to failure testing; (2) 4‐point bending cyclic fatigue testing; and (3) torsional single cycle to failure testing. The 8‐hole, 5.5 mm broad LC‐DCP (5.5‐LC‐DCP) was applied to the dorsal surface of 1 randomly selected bone from each pair. One 8‐hole, 4.5 mm broad LC‐DCP (4.5‐LC‐DCP) was applied dorsally to the contralateral bone from each pair. Plates and screws were applied using standard ASIF techniques. All MC3 bones had mid‐diaphyseal osteotomies. Mean test variable values for each method were compared using a paired t–test within each group. Significance was set at P<.05. Results— Mean yield load, yield bending moment, composite rigidity, failure load and failure bending moment under 4‐point bending, single cycle to failure, of the 5.5‐LC‐DCP fixation were significantly greater (P<.024) than those of the 4.5‐LC‐DCP fixation. Mean cycles to failure for 4‐point bending was significantly (P<.05) greater for the 4.5‐LC‐DCP fixation compared with the 5.5‐LC‐DCP fixation. Mean yield load, mean composite rigidity, and mean failure load in torsion for the 5.5‐LC‐DCP fixation was not significantly different (P>.05) than those with the 4.5‐LC‐DCP fixation. Conclusion— 5.5‐LC‐DCP fixation was superior to 4.5‐LC‐DCP fixation in resisting the static overload forces under palmarodorsal 4‐point bending. There was no significant difference between 5.5‐LC‐DCP fixation and 4.5‐LC‐DCP fixation in resisting static overload forces under torsion; however, the 5.5‐LC‐DCP offers significantly less stability (80% of that of the 4.5‐LC‐DCP) in cyclic fatigue testing. Clinical Relevance— The results of this in vitro study may provide information to aid in the selection of a biological plate for long bone fracture repair in horses.     

A Biomechanical Comparison of 7-Hole 3.5 mm Broad and 5-Hole 4.5 mm Narrow Dynamic Compression Plates

Seven-hole 3.5 mm broad and 5-hole 4.5 mm narrow dynamic compression plates were applied to paired canine cadaveric tibias in a stable fracture model. Paired tibias were tested to acute failure in rotation and four-point bending, and to fatigue failure in four-point bending. Resistance to screw pullout was measured for three 3.5 mm cortical screws and two 4.5 mm cortical screws inserted in the configurations of the bone plates. All plate-bone systems failed by fracture of the bone through a screw hole. The 3.5 mm plate-bone system was stronger in acute failure in rotation and in four-point bending. There was no difference in stiffness, and no difference in the number of cycles to failure in fatigue testing. Three 3.5 mm screws had greater resistance to pullout than two 4.5 mm screws. Results indicate that the 7-hole 3.5 mm broad dynamic compression plate has a biomechanical advantage over the 5-hole 4.5 mm narrow dynamic compression plate.     

A biomechanical comparison of double-plate and Y-plate fixation for comminuted equine second phalangeal fractures

OBJECTIVES: To compare the biomechanical properties, in full limb preparations, of intact second phalanx and a simulated comminuted second phalangeal fracture stabilized with either two bone plates or a custom Y-plate. STUDY DESIGN: In vitro biomechanical assessment of intact limbs and of paired limbs with a simulated second phalangeal fracture stabilized by one of two fixation methods. Animal Population-Thirteen pairs of equine cadaveric forelimbs. METHODS: A comminuted second phalangeal fracture was created in six paired cadaveric limbs. For each limb pair, the fracture was stabilized with two plates in one limb, and with a Y-plate in the contralateral limb. These limbs and seven pairs of intact limbs were subjected to axial compression in a single cycle until failure. Mechanical properties were compared with a mixed-model ANOVA and post hoc contrasts. Joint contact pressure, screw insertion torque, and final screw torque remaining after mechanical testing were also evaluated for constructs. RESULTS: No significant differences in mechanical testing variables were detected between construct types. However, the Y-Plate construct had significantly greater yield load, yield displacement and yield energy, and failure load and stiffness values than those for intact specimens, whereas the double-plate construct only had greater stiffness than intact specimens. There were no significant differences in joint contact pressures for both constructs. The final screw torque for proximal phalangeal screws was significantly greater for the Y-plate constructs than for double-plate constructs. CONCLUSIONS: The Y-plate was as effective as the double-plate technique for stabilization of simulated comminuted second phalangeal fractures in monotonically tested equine cadaveric forelimbs. CLINICAL RELEVANCE: This investigation supports evaluation of the Y-plate for repair of comminuted second phalangeal fractures in equine patients. Its specific design may facilitate repair of second phalangeal fractures, and may provide increased stability by allowing the proximal fragments of the second phalanx to be fixed with three screws placed through the plate.     

A Biomechanical Evaluation and Assessment of the Accuracy of Reduction of Two Methods of Acetabular Osteotomy Fixation in Dogs

Objective—To compare the accuracy of reduction, biomechanical characteristics, and mode of failure of two methods of acetabular osteotomy repair. Study Design—Acetabular osteotomies were created in 16 paired hemipelves and stabilized with a screw/wire/polymethylmethacrylate composite fixation technique (SWP) or a 2-mm veterinary acetabular plate (VAP). Eight intact hemipelves were used as controls. Sample Population—Twelve canine cadavers. Methods—Accuracy of osteotomy reduction was evaluated grossly and by measurement of articular incongruencies formed in polyvinylsiloxane impression casts. Acetabula were loaded in modified bending until failure using a universal testing machine. Data from load-deformation curves were used to determine the biomechanical characteristics of the repaired and intact acetabula. Mode of failure was evaluated grossly and radiographically. Results—Osteotomy reduction was superior in acetabula stabilized with SWP. Mean values ± standard deviation for load at failure and stiffness of the intact acetabula were 2,796 ± 152.9 N and 267.5 ±61.9 N/mm. Corresponding values for SWP and VAP were 1,192 ± 202.7 N and 136.3 ± 76.5 N/mm, and 1,100.5 ± 331.6 N and 110.0 ± 51.3 N/mm, respectively. The mean load at failure and stiffness of intact acetabula was significantly greater than acetabula stabilized with SWP or VAP. There was no significant difference between SWP and VAP for load at failure or stiffness. Failure of acetabula stabilized with SWP occurred by fracture of the polymethylmethacrylate and ventrolateral bending of the wires. Acetabula stabilized with VAP failed by ventrolateral twisting of the plate and bending of the caudal screws. Conclusions—SWP and VAP provide comparable rigidity, however, the SWP facilitates more accurate osteotomy reduction. Clinical Relevance—These findings support the use of the SWP technique as an alternative method of acetabular fracture repair.     

Failure mode and bending moment of canine pancarpal arthrodesis constructs stabilized with two different implant systems

Objective— To compare failure mode and bending moment of a canine pancarpal arthrodesis construct using either a 2.7 mm/3.5 mm hybrid dynamic compression plate (HDCP) or a 3.5 mm dynamic compression plate (DCP).
Study Design— Paired in vitro biomechanical testing of canine pancarpal arthrodesis constructs stabilized with either a 2.7/3.5 HDCP or 3.5 DCP.
Sample Population— Paired cadaveric canine antebrachii (n=5).
Methods— Pancarpal arthrodesis constructs were loaded to failure (point of maximum load) in 4-point bending using a materials-testing machine. Using this point of failure, bending moments were calculated from system variables for each construct and the 2 plating systems compared using a paired t-test. To examine the relationship between metacarpal diameter and screw diameter failure loads, linear regression was used and Pearson' correlation coefficient was calculated. Significance was set at P <.05.
Results— HDCP failed at higher loads than DCP for 9 of 10 constructs. The absolute difference in failure rates between the 2 plates was 0.552±0.182 N m, P =.0144 (95% confidence interval: −0.58 to 1.68). This is an 8.1% mean difference in bending strength. There was a significant linear correlation r=0.74 ( P -slope=.014) and 0.8 ( P -slope=.006) between metacarpal diameter and failure loads for the HDCP and 3.5 DCP, respectively.
Conclusion— There was a small but significant difference between bending moment at failure between 2.7/3.5 HDCP and 3.5 DCP constructs; however, the difference may not be clinically evident in all patients.
Clinical Relevance— The 2.7/3.5 HDCP has physical and mechanical properties making it a more desirable plate for pancarpal arthrodesis.     

The biomechanical properties of the feline femur

The purpose of this study was to determine the biomechanical properties of feline long bone by testing cadaver bone from mature cats in compression, three-point bending, notch sensitivity and screw pull-out strength. The determination of these properties is of clinical relevance with regard to the forces resulting in long bone fractures in cats as well as the behaviour and failure mode of surgical implants utilized for fracture stabilization and repair in the cat. Cadaveric cat femurs were tested in compression, three-point bending and in three-point bending after the addition of a 2.0 mm screw hole. Cortical screws, 2.7 mm in diameter, were inserted in cadaveric cat femur samples for screw pull-out testing. The mean maximum load to failure of mid diaphyseal feline femurs tested in compression was 4201+/-1218 N. Statistical analysis of the parameter of maximum load tested in compression revealed a statistical difference between sides (p=0.02), but not location (p=0.07), or location by side (p=0.12). The maximum strength of mid diaphyseal feline femurs tested in compression was 110.6+/-26.6 MPa. The modulus of elasticity of mid-diaphyseal cat femurs tested in compression was determined to be 5.004+/-0.970 GPa. The mean maximum load to failure of feline femurs tested in three-point bending was 443+/-98 N. The mean maximum load to failure of feline femurs tested in three-point bending after a 2.0 mm diameter hole was drilled in the mid-diaphyseal region of each sample through both cortices was 471+/-52 N. The mean maximum load required for screw pull-out of 2.7 mm cortical screws placed in feline femurs tested in tension was 886+/-221 N. This data should be suitable for investigating fracture biomechanics and the testing of orthopaedic constructs commonly used for fracture stabilization in the feline patient.     

A mechanical comparison of 4.5 mm narrow and 3.5 mm broad plating systems for stabilization of gapped fracture models

OBJECTIVE: To evaluate and compare the mechanical properties of 4.5 narrow and 3.5 broad plating systems using their respective cortical and cancellous screws in unstable, central, and eccentric gap fracture models. STUDY DESIGN: Mechanical evaluation and comparison of 2 dynamic compression plate (DCP) systems. SAMPLE POPULATION: Eighteen cortical and 30 cancellous gapped fracture models. METHODS: DCP (4.5 mm narrow, 3.5 mm broad) with their respective cortical screws were applied to cortical bone density polyurethane foam blocks to construct center gap cortical fracture models that were tested in gap closing monotonic 4-point bending. DCP (4.5 mm narrow, 3.5 mm broad) with their respective cancellous screws were applied to cancellous bone density polyurethane foam blocks to construct eccentric gap cancellous fracture models. The cancellous constructs were tested in monotonic gap opening and gap closing cantilever bending and in cyclic axial loading. Univariate and multivariate repeated measures ANOVA were used to compare the maximum loads at failure of the 4.5 mm constructs and 3.5 mm constructs. RESULTS: The 4.5 mm narrow plating system withstood significantly higher loads at failure than the 3.5 mm broad plating system in 4-point bending (P<.0001) and gap opening cantilever bending (P<.0001). The 4.5 mm system failed in gap closing cantilever bending by plastic deformation of the plate, whereas the 3.5 mm system failed by screw pullout. There was no difference between the 2 systems in cyclic axial loading. CONCLUSION: Results indicate that the 4.5 mm narrow plating system has a mechanical advantage over the 3.5 mm broad plating system for stabilization of gapped fracture models. CLINICAL RELEVANCE: The 4.5 mm narrow plating system may be mechanically advantageous compared with the 3.5 mm broad plating system for stabilizing unreconstructed comminuted long bone fractures in large dogs.     

An In Vitro Biomechanical Study of a Multiplanar Circular External Fixator Applied to Equine Third Metacarpal Bones

The biomechanical characteristics of a 4-ring circular multiplanar fixator applied to equine third metacarpal bones with a 5 mm mid-diaphyseal osteotomy gap were studied. Smooth Steinmann pins, either 1/8 inch, 3/16 inch, or 1/4 inch, were driven through pilot holes in the bone in a crossed configuration and full pin fashion and fastened to the fixator rings using cannulated fixation bolts. The third metacarpal bone fixator constructs were tested in three different modes (cranial-caudal four-point bending, axial compression, and torsion). Loads of 2,000 N were applied in bending and axial compression tests and a load of 50 N ± m was applied during testing in torsion. Fixator stiffness was determined by the slope of the load displacement curves. Three constructs for each pin size were tested in each mode. Comparisons between axial stiffness, bending stiffness, and torsional stiffness for each of the three different pin sizes were made using one-way analysis of variance. There was no visually apparent deformation or permanent damage to the fixator frame, and no third metacarpal bone failure in any of the tests. Plastic deformation occurred in the 1/8 inch pins during bending, compression, and torsion testing. The 3/16 inch and 1/4 inch pins elastically deformed in all testing modes. Mean (±SE) axial compressive stiffness for the 1/8 inch, 3/16 inch, and 1/4 inch pin fixator constructs was: 182 ± 16 N/mm, 397 ± 21 N/mm, and 566 ± 8.7 N/mm; bending stiffness was 106 ± 3.3 N/mm, 410 ± 21 N/mm, and 548 ± 12 N/mm; and torsional stiffness was 6.15 ± 0.82 N.m/degree, 7.14 ± 0.0 N±m/degree, and 11.9 ± 1.0 N.m/degree respectively. For statically applied loads our results would indicate that a 4-ring fixator using two 1/4 inch pins per ring may not be stiff enough for repair of an unstable third metacarpal bone fracture in a 450 kg horse.     

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.     

Metacarpal fractures associated with pancarpal arthrodesis in dogs

OBJECTIVE: To report the prevalence of third metacarpal bone fractures after dorsal plating for pancarpal arthrodeses in dogs and to identify predisposing factors. STUDY DESIGN: Retrospective clinical study. ANIMALS: Fifty-four client-owned dogs. METHODS: Pancarpal arthrodesis was performed using dorsally applied 2.7- or 3.5-mm bone plates. Medical records and radiographs were retrospectively evaluated to identify those dogs with metacarpal bone fractures after pancarpal arthrodesis and to determine the angle of arthrodesis, the percentage of the third metacarpal bone covered by the bone plate, and the percentage width of the bone occupied by the screw. Radiological evidence of arthrodesis at follow-up examination 6 weeks postoperatively was recorded. Long-term results were obtained by telephone follow-up with the owners. All lame dogs were evaluated clinically and radiographically. RESULTS: Metacarpal fractures occurred in 6 of 54 dogs. One of these dogs had a stress fracture of the third metacarpal 10 months after implant removal and was not included in the statistical analysis. Fractures occurred through the distal screw hole in four dogs and involved two metacarpal bones (III and IV) in two dogs. The median ratio of bone screw diameter-to-metacarpal bone diameter was the same for dogs with metacarpal bone fractures and those with no fractures (44%), and the median angles of arthrodesis were 8 degrees and 7 degrees. These values were not statistically significant. The percentage length of the metacarpal bone covered by the plate was 53% (no fracture) compared with 46% (fracture), and this difference was statistically significant (P = .035). CONCLUSIONS: Screw diameter was not implicated as a predisposing factor for metacarpal bone fracture in dogs undergoing pancarpal arthrodesis using a dorsally applied dynamic compression plate. The length of the metacarpal bone covered by the bone plate did affect the frequency of metacarpal fracture, with fewer fractures occurring when greater than 53% of the bone length was covered by the dynamic compression plate. CLINICAL RELEVANCE: When performing pancarpal arthrodesis with a dorsally applied bone plate, it is recommended that at least 50% of the length of the third metacarpal bone should be covered by the plate.     

Effects of clustered drill holes on the breaking strength of the equine third metacarpal bone

The breaking strength (stress at failure) of equine third metacarpal bones, with and without clustered drill holes, was determined in vitro. Paired ossa metacarpalia II-IV of 39 horses (n = 39) between 2 and 7 years old were tested in palmarodorsal 3-point bending. Four treatments were compared. Clustered 2.7- or 3.5-mm drill holes, in a 4- or 7-hole pattern, were made in the dorsal cortex of the distal diaphysis of the left third metacarpal bone. Undrilled right third metacarpi were used as controls. Bones with clustered drill holes failed by an oblique fracture through 1 or more drill holes, whereas undrilled bones failed with a middiaphyseal transverse fracture. Clustered drill holes acted as a stress concentrator and significantly (P less than 0.05) decreased the stress required for failure. However, differences in breaking strength between treatment groups were not significant (P greater than 0.05).     

A Biomechanical Comparison of Headless Tapered Variable Pitch and AO Cortical Bone Screws for Fixation of a Simulated Lateral Condylar Fracture in Equine Third Metacarpal Bones

OBJECTIVE: To compare drilling, tapping, and screw-insertion torque, force, and time for the 4.5-mm AO and 6.5-mm Acutrak Plus (AP) bone screws, and to compare the mechanical shear strength and stiffness of a simulated complete lateral condylar fracture of the equine third metacarpal bone (MC3) stabilized with either an AO or AP screw. STUDY DESIGN: In vitro biomechanical assessment of screw-insertion variables, and shear failure tests of a bone-screw-stabilized simulated lateral condylar fracture. SAMPLE POPULATION: Eight pairs of cadaveric equine MC3s METHODS: Metacarpi were placed in a fixture and centered on a biaxial load cell in a materials-testing system to measure torque, compressive force, and time for drilling, tapping, and screw insertion. Standardized simulated lateral condylar fractures were stabilized by either an AO or AP screw and tested in shear until failure. A paired t test was used to assess differences between screws, with significance set at P < .05. RESULTS: Insertion and mechanical shear testing variables were comparable for AO and AP insertion equipment and screws. CONCLUSION: The 6.5-mm tapered AP screw can be inserted in equine third metacarpal condyles and is mechanically comparable with the 4.5-mm AO screw for fixation of a simulated lateral condylar fracture. CLINICAL RELEVANCE: Considering the comparable mechanical behavior, the potential for less-persistent soft-tissue irritation with the headless design, and the ability to achieve interfragmentary compression by inserting the screw in one hole drilled perpendicular to the fracture plane, the 6.5-mm tapered AP screw may be an attractive alternative for repair of incomplete lateral condylar fractures in horses.     

Arthrodesis of the equine proximal interphalangeal joint: a mechanical comparison of 2 parallel 5.5 mm cortical screws and 3 parallel 5.5 mm cortical screws

OBJECTIVE: To compare the biomechanical characteristics and mode of failure of 2 techniques using parallel 5.5 mm screws for pastern joint arthrodesis in horses. STUDY DESIGN: Randomized block design, for horse (1-5), method of fixation (two 5.5 mm screws versus three 5.5 mm screws), side (right, left), and end (front, hind). Constructs were tested to failure in 3-point bending. SAMPLE POPULATION: Twenty limbs (5 cadavers). METHODS: A combined aiming device was used to facilitate screw placement. Two parallel 5.5 mm screws were inserted in lag fashion in 1 limb of a pair, and three 5.5 mm screws were inserted in the contralateral limb. Constructs were then tested in 3-point bending in a dorsal-to-palmar (plantar) direction using a materials testing machine at a loading rate of 19 mm/s. Maximal bending moment at failure and stiffness were obtained from bending moment-angular deformation curves. RESULTS: There was no significant difference between two and three 5.5 mm screw constructs for bending moment and stiffness (P<.05). All constructs ultimately failed by bone fracture or screw bending. For proximal interphalangeal (PIP) joint arthrodesis constructs loaded in 3-point bending, no significant effect of treatment, side, or end on maximal bending moment or stiffness was detected. CONCLUSIONS: Two 5.5 mm cortical screws inserted in parallel should provide a surgically simpler and equally strong PIP joint arthrodesis compared with three 5.5 mm cortical screws. CLINICAL RELEVANCE: Two 5.5 mm cortical screws inserted in parallel for PIP joint arthrodesis should perform similarly under conditions used in this study, as three 5.5 mm screws inserted in a similar manner, when loaded under bending.     

In vitro cyclic biomechanical properties of an interlocking equine tibial nail

OBJECTIVE: To determine cyclic biomechanical properties of gap osteotomized adult equine tibiae stabilized with an equine interlocking nail (EIN). STUDY DESIGN: In vitro experimental biomechanical investigation. SAMPLE POPULATION: Thirteen adult equine cadaveric tibiae. METHODS: Adult equine tibiae with transverse, midshaft, 1-cm gap osteotomies, stabilized with an equine interlocking nail, underwent cyclic biomechanical testing in vitro under axial compression, 4-point bending, and torsion. Different specimens were subjected to different load levels that represented estimated in vivo loads at 2 Hz for 740,000 cycles. Fatigue life and gap strain were calculated. RESULTS: Compression and bending, but not torsional, fatigue life were longer than time necessary for bone healing. Compressive, but not bending or torsional, gap strains were small enough to be compatible with fracture healing by primary bone formation. Gap strains for compressive, bending, and torsional loads were compatible with indirect, or secondary, bone formation. CONCLUSIONS: Further modification should be made to the equine interlocking nail to increase bending stiffness and torsional fatigue life. CLINICAL RELEVANCE: The stainless steel equine intramedullary interlocking nail is unlikely to provide appropriate long-term stability for fracture healing in adult equine tibiae without modifications in the nail design and material.