A biomechanical comparison of equine third metacarpal condylar bone fragment compression and screw pushout strength between headless tapered variable pitch and AO cortical bone screws

OBJECTIVES: To compare bone fragment compression and the mechanical pushout strength and stiffness of 6.5-mm Acutrak Plus (AP) and 4.5-mm AO cortical (AO) bone screws after stabilization of a simulated equine third metacarpal (MC3) bone complete lateral condylar fracture. STUDY DESIGN: In vitro biomechanical paired study of screw insertion variables, bone fragment compression, and screw pushout tests using a bone screw stabilized simulated lateral condylar fracture model. SAMPLE POPULATION: Six 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. Fragment compression was measured with a pressure-sensing device placed between the simulated fracture fragments during screw insertion for fragment stabilization. Subsequently, screws were pushed out of the stabilized bone fragments in a single cycle to failure. A paired t test was used to assess differences between site preparation, screw insertion, fragment compression, and screw pushout variables, with significance set at P <.05. RESULTS: Measured drilling variables were comparable for AO and AP specimens. However, the AP tap had significantly greater insertion torque and force. Mean maximum screw insertion torque was significantly greater for AO screws. For fragment compression, AP screws generated 65% and 44% of the compressive pressure and force, respectively, of AO screws. AP screws tended to have higher overall pushout strength. Pushout stiffness was similar between both screw types. CONCLUSION: The 6.5-mm tapered AP screw generated less interfragmentary compressive pressure and force but had similar pushout stiffness. Evaluation of failure patterns demonstrated that AP screws had greater pushout strength compared with 4.5-mm AO screws for fixation of a simulated complete lateral condylar fracture. CLINICAL RELEVANCE: The 6.5-mm tapered AP screw should provide ample holding strength but would provide less interfragmentary compression than 4.5-mm AO screws for repair of complete lateral condylar fractures in horses.     

Comparison of insertion time and pullout strength between self-tapping and non-self-tapping AO 4.5-mm cortical bone screws in adult equine third metacarpal bone

OBJECTIVE: To compare screw insertion characteristics and pullout mechanical properties between self-tapping (ST) and non-self-tapping (NST) AO 4.5-mm cortical bone screws in adult equine third metacarpal bone (MC3). STUDY DESIGN: In vitro biomechanical experiment. ANIMALS OR SAMPLE POPULATION: Seven pairs of adult equine MC3. METHODS: Bicortical holes were drilled transversely in proximal metaphyseal, diaphyseal, and distal metaphyseal locations of paired MC3. NST screws were inserted in pre-tapped holes in 3 sites of one bone pair, and ST screws were inserted in non-tapped holes of contralateral MC3. Tapping and screw insertion times and maximum torques were measured. Screw pullout mechanical properties were determined. RESULTS: Screw insertion time was longer for ST screws. Total time for tapping and insertion (total insertion time) was over twice as long for NST screws. Statistically significant differences were not observed between screws for any pullout mechanical property. From pullout tests, diaphyseal locations had significantly stiffer and stronger structure than metaphyseal locations. Pullout failure more commonly occurred because of screw breakage than bone failure. Bone failure and bone comminution were more commonly associated with ST screws. Bone failure sites had pullout failure loads that were 90% of screw failure sites. CONCLUSIONS: NST and ST 4.5-mm-diameter cortical bone screws have similar pullout mechanical properties from adult equine MC3. ST screws require less than half the total insertion time of NST screws. CLINICAL RELEVANCE: Use of ST 4.5-mm-diameter cortical bone screws should be considered for repair of adult equine MC3 fractures; however, bone failures at screw sites should be monitored.     

A Comparison of the Synthes 4.5-mm Cannulated Screw and the Synthes 4.5-mm Standard Cortex Screw Systems in Equine Bone

Objective —To determine risk of failure of the Synthes 4.5-mm cannulated screw system instrumentation in equine bone and to compare its application with the Synthes 4.5-mm standard cortex screw system.
Study Design —The maximum insertion torque of the cannulated and standard cortex screw systems were compared with the ultimate torsional strengths of the equipment. Pullout strength and ultimate tensile load of cannulated and standard cortex screws were also determined.
Sample Population—Paired equine cadaver third metacarpal and third carpal bones.
Methods —Maximum insertion torque and ultimate torsional strengths were determined by using an axial-torsional, servohydraulic materials testing system and a hand-held torquometer. Pullout tests were performed by using a servohydraulic materials testing system.
Results —Maximum insertion torque of all cannulated instrumentation was less than ultimate torsional strength at all locations ( P < .05). Maximum insertion torques of cannulated taps and screws were greater than for standard taps and screws in the third carpal bone ( P < .002). Pullout strength of the cannulated screws was less than the standard cortex screws at all sites ( P < .001). Cannulated screws broke before bone failure in all but one bone specimen. Conclusions—The risk of cannulated instrument or screw failure during insertion into bone is theoretically low. The relatively low pullout strength of the cannulated screws implies that the interfragmentary compression achievable is likely to be less than with standard cortex screws. Clinical Relevance—The relatively low pullout strength of the cannulated screw suggests that its risk of failure during fracture repair is greater than with the standard cortex screw.     

Effect of Ethylene Oxide Sterilization and Storage Conditions on Canine Cortical Bone Harvested for Banking

Two hundred seventeen cortical bones were harvested cleanly and prepared for banking for 2 months by using one of three types of packing materials, two ethylene oxide (EO) concentrations and procedures, and two storage temperatures. Bone subjected to the various treatments was compared to freshly harvested cortical bone and bone tested immediately after sterilization. Parameters evaluated were handling characteristics during preparation for and placement of a 3.5 mm cortex screw, and the percentage of weight lost as water when the bones were dried in an oven at 100 degrees C for 72 hours. Methods of sterilization, packaging material, and temperature of storage affected the handling characteristics and dehydration of the bone specimens. Twelve per cent EO at an elevated temperature and pressure, paper packaging, and room temperature storage appeared to cause the most significant changes. The use of 84% EO at room temperature and pressure, polyethylene wrapping material, and storage at -20 degrees C appeared to protect bone from dehydration. There was an increase in cracking and splitting of the bones as the percent of water loss decreased (indicating dehydration at the time of testing). Dehydration due to sterilization and storage processes may lead to difficulty in preparing bone for insertion of a bone screw and subsequently jeopardizing the stability of a repair in which such alloimplants are used.     

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.     

An in vitro biomechanical comparison of the insertion variables and pullout mechanical properties of ao 6.5-mm standard cancellous and 7.3-mm self-tapping, cannulated bone screws in foal femoral bone

OBJECTIVE: To compare screw insertion variables and pullout mechanical properties between AO 6.5-mm cancellous and 7.3-mm cannulated bone screws in foal femoral bone. STUDY DESIGN: A paired, in vitro mechanical study. SAMPLE POPULATION: Seven pairs of femora from immature (1-7 months) foals. METHODS: The 6.5 cancellous and 7.3-mm cannulated screws were inserted at standardized proximal and distal metaphyseal, and mid-diaphyseal locations. Insertion torque, force, and time to drill, tap (6.5-mm cancellous), guide wire insertion (7.3-mm cannulated), and screw insertion were measured. Screw pullout properties (yield and failure load, displacement, and energy, and stiffness) were determined from mechanical tests. The effects of screw type and location on insertion variables and pullout properties were assessed with repeated measures ANOVA. Pairwise comparisons were examined with post hoc contrasts. Significance was set at P<.05 for all comparisons. RESULTS: Insertion torques for the 7.3-mm cannulated screws were significantly greater than for the 6.5-mm tap, but significantly lower than for the 6.5-mm cancellous screws. Total screw insertion times were similar. Pullout properties of both screws were similar at each femoral location. The holding power of both screws was significantly greater in the mid-diaphysis than in either metaphyseal location. Pullout failure occurred by bone shearing at the bone-screw interface in all specimens. CONCLUSIONS: The 6.5-mm cancellous and 7.3-mm cannulated screws vary in insertion properties, but have similar pullout properties in the mid-diaphysis, proximal, and distal metaphysis of foal femora. Both screw types have greater holding power at the mid-diaphyseal location compared with metaphyseal locations. Based on overall similar holding powers of 6.5-mm cancellous and 7.3-mm cannulated screws, it is unlikely that increasing the screw diameter beyond 6.5 mm will provide increased holding power in foal femoral bone. CLINICAL RELEVANCE: Use of the 7.3-mm cannulated screw should be considered for foal femoral fracture repair when greater accuracy is needed, or when bone threads for the 6.5-mm cancellous screw have been stripped.     

In vitro evaluation of screws and suture anchors in metaphyseal bone of the canine tibia

OBJECTIVE: To compare ease of insertion, load to failure, and mode of failure of cortical and cancellous screws, BoneBiter, IMEX, and TwinFix suture anchors in canine metaphyseal tibial bone. STUDY DESIGN: Experimental biomechanical study. ANIMALS: Canine cadaveric tibias. METHODS: One investigator inserted all anchors and subjectively evaluated ease of placement. Anchor systems were loaded to failure along axis of insertion with audio-video recording to determine failure mode. RESULTS: BoneBiter was the most difficult anchor to insert successfully. Mean+/-SD loads to failure were cancellous screw (711+/-193 N), IMEX 4.7 mm 18 g wire (661+/-163 N), IMEX 4.0 mm 18 g wire (661+/-165 N), cortical screw (635+/-184 N), BoneBiter #5 Kevlar suture (393+/- 109 N), and TwinFix 5.0 mm #2 polyester (267+/-73 N). No significant differences were noted among the cortical screw, cancellous screw, IMEX 4.7 and 4.0 mm, all of which were significantly (P<.001) greater than BoneBiter and TwinFix . Failure modes were pullout of bone, suture-wire breakage, eyelet breakage, or no failure to 1000 N: screws (18,0,0,2), IMEX (18,1,1,0), BoneBiter (2,8,0,0), and TwinFix (0,10,0,0). CONCLUSIONS: Fixation devices were user friendly, with the exception of BoneBiter. Mode of failure is dependent on suture material and anchor design. Cortical and cancellous screws, and IMEX anchors with 18 g wire have significantly greater load to failure compared with BoneBiter and TwinFix suture anchors. CLINICAL RELEVANCE: Based on load to failure, ease of use, design characteristics, and cost, IMEX anchors may have advantages over other comparable soft tissue fixation devices.     

Effect of laser shock peening on fatigue life and surface characteristics of stainless steel cortical bone screws

OBJECTIVE: To investigate the effect of laser shock peening on the fatigue life and surface characteristics of 3.5-mm-diameter cortical bone screws. SAMPLE POPULATION: 32 stainless steel, 3.5-mm-diameter cortical bone screws. PROCEDURE: Screws were randomly assigned to an untreated control group or 2 power-density treatment groups, 6 gigawatts (GW)/cm2 and 8.5 GW/cm2, for laser shock peening. Number of cycles to failure and findings on scanning electron microscopy-assisted morphometric evaluation, including the mode of failure, surface debris, surface damage, and thread deformation, were compared between control and treated screws. RESULTS: The 6 GW/cm2 treated screws had a significant (11%) improvement in fatigue life, compared with untreated control screws. The 8.5 GW/cm2 treated screws had a significant (20%) decrease in fatigue life, compared with control screws. A mild but significant increase in thread deformation was evident in all treated screws, compared with control screws. The 8.5 GW/cm2 treated screws had significantly more surface irregularities (elevations and pits), compared with control or 6 GW/cm2 treated screws. CONCLUSION AND CLINICAL RELEVANCE: A modest positive increase in fatigue strength was produced by this design of laser shock peening on the midshaft of cortical bone screws. High laser shock peening power densities were detrimental, decreasing screw fatigue strength probably resulting from structural damage. Greater fatigue life of cortical bone screws can be generated with laser shock peening and could reduce screw breakage as a cause of implant failure; however, future studies will be necessary to address biocompatibility, alternative cleaning techniques, alterations in screw strength and pullout characteristics, and effects on susceptibility to corrosion.     

Holding Power of Orthopedic Screws in Equine Third Metacarpal and Metatarsal Bones: Part II. Adult Horse Bone

Comparison was made of the holding power of 5.5 and 4.5 mm cortical orthopedic screws inserted into third metacarpal and metatarsal cadaver bones from 3- and 8-year-old horses. The tensile strength of these screws was tested mechanically. In nine comparative trials of these screws, 5.5 mm screws pulled out of bone in five trials at an average of 116.0 kg tensile force and broke in four trials at an average of 1383.2 kg. A 4.5 mm screw pulled out of bone at 834.5 kg in one trial, and screws broke at an average of 849.2 kg in eight trials. The larger 5.5 mm screw required a significantly greater (p = 0.022) pullout force than the mean force at 4.5 mm screw breakage. Fixation failure was due to screw breakage or bone shear, with 5.5 mm screws occasionally creating bone fragmentation during pullout. The average tensile breaking strengths of the 5.5 mm screws (1391.4 kg) and 4.5 mm screws (832.7 kg) determined mechanically were similar to forces at screw breakage during pullout testing in bone. Since the 5.5 mm screws have greater holding power and tensile strength than 4.5 mm screws, the use of the 5.5 mm screw in fracture repair in adult horses is recommended.     

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.     

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.     

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.     

Biomechanical Comparison of the Herbert and AO Cortical Bone Screws for Compression of an Equine Third Carpal Bone Dorsal Plane Slab Osteotomy

Objective—To assess feasibility of insertion of 4.5-mm Herbert cannulated bone screws (HS) using fluoroscopic guidance and compare the mechanical shear strength of these HS and 4.5-mm AO cortical bone screws (AO) for fixation of dorsal plane slab osteotomies in equine cadaver third carpal bones (C3).
Animals or Sample Population—Eight equine cadavers.
Methods—Bone mineral composition and density of contralateral C3 were confirmed to be equivalent using dual-energy x-ray absorptiometry. A standard 10-mm C3 slab osteotomy was reduced using HS or AO instrumentation under fluoroscopic guidance. Specimens were loaded in shear until failure, using a materials testing apparatus.
Results—HS and AO instrumentation allowed accurate reconstruction of the osteotomy, but there was difficulty encountered seating the HS proximal self-tapping threads. There was no significant difference in maximal load to failure, stiffness, or mode of failure of constructs created with the HS and AO screws.
Conclusions —Use of 4.5-mm HS for repair of C3 radial facet, dorsal plane slab fractures may result in a mechanically comparable fixation to a repair using a 4.5-mm AO. Equine dorsal C3 may be too dense, however, to allow placement of the proximal self-tapping threads of the HS without potentially excessive application of torque to the screw itself.
Clinical Relevance —Dorsal plane, radial facet slab fractures of the equine C3 are a significant clinical problem. Accurate reconstruction and stabilization are necessary for return to athletic function.     

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.     

In Vitro Pullout Strength of Screws Inserted in Adult Equine Third Metacarpal Bone After Overdrilling a 4.5-mm Threaded Insertion Hole

Objective—To determine and compare the in vitro pullout strength of 5.5-mm cortical versus 6.5-mm cancellous bone screws inserted in the diaphysis and metaphysis of adult equine third metacarpal (MCIII) bones, in threaded 4.5-mm cortical bone screw insertion holes that were then overdrilled with a 4.5-mm drill bit to provide information relevant to the selection of a replacement screw if a 4.5-mm cortical screw is stripped. Study Design—In vitro pullout tests of 5.5-mm cortical and 6.5-mm cancellous screws in equine MCIII bones. Sample Population—Two independent cadaver studies each consisting of 14 adult equine MCIII bones. Methods—Two 4.5-mm cortical screws were placed either in the middiaphysis (study 1) or distal metaphysis (study 2) of MCIII bones. The holes were then overdrilled with a 4.5-mm drill bit and had either a 5.5-mm cortical or a 6.5-mm cancellous screw inserted; screw pullout tests were performed at a rate of 0.04 mm/second until screw or bone failure occurred. Results—In diaphyseal bone, the screws failed in all tests. Tensile breaking strength for 5.5-mm cortical screws (997.5 ± 49.3 kg) and 6.5-mm cancellous screws (931.6 ± 19.5 kg) was not significantly different. In metaphyseal bone, the bone failed in all tests. The holding power for 6.5-mm cancellous screws (39.1 ± 4.9 kg/mm) was significantly greater than 5.5-mm cortical screws (23.5 ± 3.5 kg/mm) in the metaphysis. There was no difference in the tensile breaking strength of screws in the diaphysis between proximal and distal screw holes; however, the holding power was significantly greater in the distal, compared with the proximal, metaphyseal holes. Conclusions—Although tensile breaking strength was not different between 5.5-mm cortical and 6.5-mm cancellous screws in middiaphyseal cortical bone, holding power of 6.5-mm cancellous screws was greater than 5.5-mm cortical screws in metaphyseal bone of adult horses. Clinical Relevance—If a 4.5-mm cortical bone screw strips in MCIII diaphyseal bone of adult horses, either a 5.5-mm cortical or 6.5-mm cancellous screw, however, would have equivalent pullout strengths. A 6.5-mm cancellous screw, however, would provide greater holding power than a 5.5-mm cortical screw in metaphyseal bone.     

Effect of diameter of the drill hole on torque of screw insertion and pushout strength for headless tapered compression screws in simulated fractures of the lateral condyle of the equine third metacarpal bone

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.     

Mechanical performance of a screw-type veterinary suture anchor subjected to single load to failure and cyclic loads

OBJECTIVE: To characterize the mechanical performance of a veterinary bone anchor under static and cyclic loads. STUDY DESIGN: Mechanical testing study. ANIMALS: Cadaveric canine humeri. METHODS: Humeri (6 pairs) were collected from skeletally mature dogs (mean [+/-SD] age, 17.2+/-2.1 months; weight, 20.8+/-1.5 kg). Bone anchors were inserted in the proximal metaphysis using nylon, and were longitudinally extracted. For the opposite humerus, anchors were subjected to longitudinal cyclic load (50% of the load at failure of their pair) for 1200 cycles then longitudinally loaded to failure. Anchors were then installed in a similar and adjacent area of these 2(nd) humeri with nylon and cyclically tested perpendicular to the axis of anchor insertion (100% of the longitudinal holding power of their pair) for 1200 cycles, then perpendicularly loaded to failure. Paired t-tests were used to compare holding power before and after longitudinal cyclic testing. RESULTS: Longitudinal holding power of the screw-type anchor in the proximal humerus was 385+/-30 N. Anchor pullout was the only mode of failure. Anchors in the paired humeri did not fail after 1200 cycles of 50% longitudinal loading, and post-cycle holding strength was not different (335+/-87 N; P=.32). Perpendicularly loaded anchors did not fail after 1200 cycles of 100% of opposite longitudinal holding strength, and had post-cycle perpendicular holding strengths of 514+/-72 N. Suture breakage was the mode of failure. CLINICAL RELEVANCE: Bone anchor holding strength is dependent on orientation of suture load. Screw-type bone anchor holding strength was not affected by longitudinal cyclic loading, and holding strengths of approximately 385 N can be expected in metaphyseal bone of large-breed mature dogs. Perpendicularly loaded anchors have higher failure loads, and holding strength of approximately 514 N can be expected in metaphyseal bone of the proximal humerus.     

Effect of bone diameter and eccentric loading on fatigue life of cortical screws used with interlocking nails

OBJECTIVE: To test the effects of bone diameter and eccentric loading on fatigue life of 2.7-mm-diameter cortical bone screws used for locking a 6-mm-diameter interlocking nail. SAMPLE POPULATION: Eighteen 2.7-mm-diameter cortical bone screws. PROCEDURE: A simulated bone model with aluminum tubing and a 6-mm-diameter interlocking nail was used to load screws in cyclic 3-point bending. Group 1 included 6 screws that were centrally loaded within 19-mm-diameter aluminum tubing. Group 2 included 6 screws that were centrally loaded within 31.8-mm-diameter aluminum tubing. Group 3 included 6 screws that were eccentrically loaded (5.5 mm from center) within 31.8-mm-diameter aluminum tubing. The number of cycles until screw failure and the mode of failure were recorded. RESULTS: An increase in the diameter of the aluminum tubing from 19 to 31.8 mm resulted in a significant decrease in the number of cycles to failure (mean +/- SD, 761,215 +/- 239,853 to 16,941 +/- 2,829 cycles, respectively). Within 31.8-mm tubing, the number of cycles of failure of eccentrically loaded screws (43,068 +/- 14,073 cycles) was significantly greater than that of centrally loaded screws (16,941 +/- 2,829 cycles). CONCLUSIONS AND CLINICAL RELEVANCE: Within a bone, locking screws are subjected to different loading conditions depending on location (diaphyseal vs metaphyseal). The fatigue life of a locking screw centrally loaded in the metaphyseal region of bone may be shorter than in the diaphysis. Eccentric loading of the locking screw in the metaphysis may help to improve its fatigue life.     

Evaluation of Canine Cortical Bone Graft Remodeling

A stable cortical bone fracture model was developed to evaluate the remodeling rate of cortical bone grafts. Samples of cortical bone were harvested with a trephine and press fit into predrilled holes in the femoral diaphyses of four live dogs. The percentages of new bone, unremodeled graft bone, porosity, forming bone surface area, and resorbing bone surface area were determined morphometrically and compared in cortical autografts, cortical allografts sterilized with 84% ethylene oxide (EO), and allografts sterilized with 12% EO. The host-graft interfaces healed without formation of fibrous tissue or cartilage, indicating a stable fracture surface. The amount of new bone formed in cortical autografts and allografts sterilized with 84% EO was significantly greater than the amount of new bone in allografts sterilized with 12% EO. There was no significant difference between the amounts of new bone formed in the allografts sterilized with 84% EO and the cortical autografts. No significant differences were detected in percentages of porosity or bone surface areas.