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.     

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.     

Holding Power of Orthopedic Screws in Equine Third Metacarpal and Metatarsal Bones: Part I. Foal Bone

The holding power of orthopedic screws in the third metacarpal and metatarsal cadaver bones of foals that were aged from 1 to 14 days was tested. Comparative trials between screws inserted at the same site in contralateral bones from the same foal were performed to compare the holding power of 5.5 mm cortical and 6.5 mm cancellous screws in the metaphysis, and the holding power of 5.5 and 4.5 mm cortical screws in the diaphysis. A MTS servohydraulic tensile testing machine was used to perform screw pullouts at a displacement rate of 19 mm/sec. There was no significant difference between maximum holding power of 5.5 mm cortical screws and 6.5 mm cancellous screws in the metaphysis when expressed as kg per mm of bone width at the screw insertion site (p = 0.097) or as kg per mm of screw thread engaged in the bone (p = 0.17). There was no significant difference in holding power of 5.5 and 6.5 mm screws in the proximal versus distal metaphysis (p = 0.10). The 5.5 mm screws had significantly greater holding power than the 4.5 mm screws in the diaphysis (p = 0.0097). Fixation failure at screw pullout was always due to bone shear. In internal fixation in foal bone, the 5.5 mm screws may be a suitable alternative to 6.5 mm screws in the metaphysis. Use of 5.5 mm rather than 4.5 mm screws is recommended in the diaphysis because of greater holding power.     

Surgical Management of Proximal Splint Bone Fractures in the Horse

Fractures of Metacarpal and Metatarsal II and IV (the splint bones) were treated in 283 horses over an 11 year period. In 21 cases the proximal portion of the fractured bone was stabilized with metallic implants. One or more cortical bone screws were used in 11 horses, and bone plates were applied in 11 horses. One horse received both treatments. Complications of screw fixation included bone failure, implant failure, radiographic lucency around the screws, and proliferative new bone at the ostectomy site. Only two of the horses treated with screw fixation returned to their intended use. Complications of plate fixation included partial fixation failure (backing out of screws), wound drainage, and proliferative bony response around the plate. Six of the 11 horses treated by plate fixation returned to their intended use. The authors recommend consideration of plate fixation techniques for repair of fractures in the proximal third of the splint bone.     

Biomechanical Study of Canine Spinal Fracture Fixation Using Pins or Bone Screws With Polymethylmethacrylate

Five configurations of pins or screws interconnected with polymethylmethacrylate (PMMA) were applied to isolated canine lumbar spines (L2 to L5) in which a complete fracture-luxation had been produced at L3 to L4. Twenty-five repaired spines and five intact control spines were subjected to four-point bending and tested once to failure in ventral flexion. The purpose of this study was to determine the effects of pin number, pin angle, and use of 3.5-mm cortical bone screws instead of smooth 3.2-mm diameter pins on rigidity and ultimate strength of spinal fractures repaired by the implant-PMMA fixation technique. Bending moment versus the angular deformation curves were recorded. Rigidity, bending moment at 10° angular deformation, moment at failure, and deformation at failure of each type of fixation were compared using analysis of variance. Spinal segments stabilized with eight pin-PMMA fixation had significantly greater rigidity and strength at failure than four pin-PMMA fixations ( P < .05). Furthermore, spinal segments stabilized with eight pins angled away from the fracture failed at significantly greater bending moment than those with eight pins angled toward the fracture ( P < .05). However, for four-pin fixation, greater strength was achieved by angling pins in the bone toward the fracture site ( P < .05). Screw-PMMA fixations failed by screw bending and were less rigid and weaker at failure than the corresponding configuration of pin-PMMA fixation ( P < .05).     

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.     

Absence of bactericidal effect of focused shock waves on an in-vitro biofilm model of an implant

The objective of this study was to evaluate the bactericidal effect of shock waves (SWs) on gram-negative or gram-positive monocultured biofilms grown on an orthopedic implant in vitro. Cortical bone screws were individually cultured with Escherichia coli or Staphylococcus epidermidis to produce a biofilm. In each run of 8 screws, 6 screws were treated with shock waves and then sonicated to disrupt the biofilm. One screw was sonicated only and one was not shock waved or sonicated before sampling for plate count dilutions. Post-treatment serial dilutions and plate counts were done on an aliquot from the vial containing each screw to obtain the number of colony-forming units (CFUs). Shock waves were at a constant energy of 0.15 mJ/mm². Pulse number and screw orientation were varied. A linear mixed-effects model was used with “treatment” as a fixed effect and “run” as a random effect. Pairwise comparisons of treatments were performed with Tukey-Cramer’s adjustment for P-values. Sonicated plate counts were greater than nonsonicated counts for each run. When all sonicated screws were compared to all nonsonicated screws, the counts were significantly increased (P = 0.0091). For each paired comparison between sonicated and shock wave treatment, the only significant difference was in the S. epidermidis biofilm treated at 2000 pulses in a horizontal position, which increased the post-treatment count (P = 0.0445). No bactericidal effects were seen on monocultured biofilms on cortical bone screws treated with shock waves.     

Comparison of locking and conventional screws for maintenance of tibial plateau positioning and biomechanical stability after locking tibial plateau leveling osteotomy plate fixation

Objective— To compare locking screws with conventional screws inserted in the tibial plateau fragment for reduction and stability of the construct after tibial plateau leveling osteotomy (TPLO), using a locking TPLO plate.
Study Design— Experimental biomechanical study.
Animals— Cadaveric canine pelvic limbs (n=8 pairs).
Methods— TPLO was stabilized with either conventional cortical screws or locking screws in a compressed osteotomy model. Titanium pins inserted into the tibial plateau and proximal metaphysis were used to track bone fragment location by computed tomography (CT) imaging. CT imaging was performed after osteotomy reduction, after plate stabilization, and after 30,000 cycles of axial compression testing. After 30,000 cycles, cyclic loading was continued with monotonically increasing peak-load until failure.
Results— The magnitude of rotation about the sawing axis was significantly greater for the conventional screw group because of plate application ( P =.009). Translational movement of the tibial plateau fragment toward the plate was significantly greater for the conventional screw group ( P =.006). There were no significant differences between groups in stiffness or number of cycles to failure.
Conclusion— Maintenance of tibial plateau position was significantly superior for the locking screw group during plate application; however, screw type had no effect on fixation stability under cyclic loading.
Clinical Relevance— These results suggest that conventional screws and careful contouring of the TPLO plate can provide comparable mechanical stability to fixation with locking screws in the tibial plateau under load-sharing conditions, but potentially at the expense of osteotomy reduction.     

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.     

Pantarsal arthrodesis with a customised medial or lateral bone plate in 13 dogs

Pantarsal arthrodesis was performed in 12 dogs using a customised medial bone plate, and in one using a lateral bone plate. The dogs' ages ranged from 14 months to 144 months (median 30 months) and their bodyweights ranged from 7 to 66 kg (median 32 kg). Before the surgery they had been lame on a pelvic limb for between one and 16 months (median eight months). In eight of the dogs the fixation of the plate was augmented with a calcaneotibial positional screw, and in one of them with a talocrural lag screw. A cranial half cast was applied to 12 of the dogs and an external skeletal fixator to the other for six to eight weeks. Five complications were recorded in four of the dogs. Between 29 and 156 weeks postoperatively the clinical outcome was graded as excellent in six dogs, good in six dogs and fair in the other.     

Increased sacral screw purchase minimizes screw loosening in canine triple pelvic osteotomy

OBJECTIVE: To determine if screw loosening in triple pelvic osteotomies (TPO) is minimized when screws cranial to the ilial osteotomy had maximal sacral purchase. STUDY DESIGN: Prospective study. ANIMALS: Forty-six dogs with decreased acetabular coverage of the femoral head and minimal degenerative joint changes. METHODS: TPOs were performed where screws cranial to the ilial osteotomy were inserted to maximally engage sacral bone. Data collected were: use of ilial and ischial cerclage wire, screw length, ventrodorsal radiographic sacral width (most caudal aspect), pelvic canal diameter, and sacral penetration of the 3 cranial screws. On all subsequent radiographs, changes in screw position, pelvic canal diameter, and sacral purchase were noted. RESULTS: For 69 TPOs, 414 screws were used. Mean radiographic cranial screw length was 34.54 mm. Combined sacral depth of all 3 screws was 93.3% of sacral width. All osteotomies healed uneventfully. Twenty-four screws (6%) loosened with 12 being in the most cranial positions. Use of ischial or ilial cerclage wires did not statistically influence screw loosening. Pelvic diameter decreased by a mean of 7.79% from postoperative radiographs to the last radiographic recheck. CONCLUSIONS: By sufficiently engaging the sacrum with screws cranial to the ilial osteotomy, implant failures can be avoided and screw loosening minimized when a 6-hole TPO plate is used. CLINICAL RELEVANCE: To minimize screw-loosening in TPO, screws inserted cranial to the ilial osteotomy should be inserted to maximum sacral depth without penetrating the vertebral canal.     

Comparison of a 3-hole 4.5-mm Dynamic Compression Plate and a 7-hole 5.5-mm Y Locking Compression Plate for Arthrodesis of the Proximal Interphalangeal Joint in Horses—an Ex Vivo Biomechanical Study

The objective of this study was to compare the biomechanical properties in a single cycle axial loading test and the types of failures in two constructs (a 3-hole 4.5-mm dynamic compression plate (DCP) and 7-hole 5.5-mm Y locking compression plate (Y-LCP)) in equine proximal interphalangeal joint (PIJ) arthrodesis. One limb in each pair was randomly assigned to PIJ arthrodesis using a 3-hole 4.5-mm DCP combined with two transarticular 5.5-mm cortical screws, whereas the contralateral limb was submitted to PIJ arthrodesis using a 7-hole Y-shaped 5.0-mm LCP in conjunction with one transarticular 4.5-mm cortical screw inserted through the central plate hole. Cortical screws were inserted in lag fashion. Constructs were submitted to a single axial load cycle to failure. Construct stiffness, load, and deformation were analyzed. Dynamic compression plate and Y-LCP arthrodesis constructs did not differ significantly and were equally resistant to axial loading under the conditions studied (DCP and Y-LCP group stiffness, 5685.22 N/mm and 6591.10 N/mm, respectively). Arthrodesis of the PIJ using a DCP and two transarticular 5.5-mm cortical screws or a Y-LCP yielded biomechanically equivalent outcomes under the test conditions considered. However, Y-LCP provides less impact in the palmar/plantar bone. Application of Y-LCP with unicortical screws has equivalent biomechanical characteristics of DCP and may be a safe option for PIJ arthrodesis, where potential trauma secondary to applying bicortical screws in the palmar/plantar aspect of the pastern can be avoided.     

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.     

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.     

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.     

Evaluation of ilial screw loosening after triple pelvic osteotomy in dogs: 227 cases (1991-1999)

OBJECTIVE: To investigate factors influencing screw loosening after triple pelvic osteotomy (TPO) and ischial wire stabilization of the acetabular segment. DESIGN: Retrospective case series. Animals-227 dogs with congenital hip dysplasia or subluxated hip joints. PROCEDURES: Medical records and radiographs of 227 dogs that underwent 332 TPO procedures were evaluated, and data pertaining to screw type, plate position, sacral screw engagement, use of ischial interfragmentary wires, and pelvic alignment were assessed for associations with screw loosening. RESULTS: Complications developed in 96 of the 332 (29%) procedures. Cancellous screws without sacral engagement were associated with the lowest frequency (6%) of loosening, compared with cancellous and cortical screws engaging the sacrum and cortical screws that did not engage the sacrum. Frequency of screw loosening increased when cortical or cancellous screws engaged the sacrum and when cortical screws were used. In dogs that had surgery bilaterally, the first limb on which TPO was performed had a higher frequency of screw loosening than the second limb. Pelvic alignment loss was greatest (5.4 degrees ) when the 3 most cranial screws were loosened. Loss of pelvic alignment was significantly different between dogs that underwent surgery and had complications and those that underwent surgery and did not have complications in association with loosening of 1, 2, and 3 screws. CONCLUSIONS AND CLINICAL RELEVANCE: TPO screw loosening was multifactorial and related to stability of the affected ilium, screw type, and screw position. Placing cancellous screws that do not engage the sacrum in pelvic osteotomy plate positions 1 through 3 may decrease the number of screws that loosen.     

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.     

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.     

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.     

Mechanical Comparison of 3.5 mm Broad Dynamic Compression Plate,Broad Limited‐Contact Dynamic Compression Plate,and Narrow Locking Compression Plate Systems Using Interfragmentary Gap Models

Objectives— To compare (1) pullout properties between 3.5 mm cortical and locking screws, and (2) mechanical properties and gap displacements between the 3.5 mm broad limited‐contact dynamic compression plate (LC‐DCP), broad dynamic compression plate (DCP), and narrow locking compression plate (LCP), during axial loading of plate‐stabilized diaphyseal fragments with an interfragmentary gap. Study Design— In vitro mechanical testing of implanted polyurethane foam (PUF) hollow cylinders that simulated compact or osteopenic diaphyseal bone. Sample Population— (1) Five cortical and locking screws and (2) 4 PUF‐plate constructs for each plate type; using high‐ and low‐density (0.8 and 0.32 g/cm³) cylinders. Methods— (1) Screws were completely extracted at 5 mm/min. (2) Plated constructs were axially compressed at 300 N/s for 10 cycles from 5 to 355 N to determine gap displacement during physiologic loading, followed by single cycle increasing load to failure. Results— Pullout properties were not different between screw types. All plate constructs had yield loads over 3 times trotting loads. Gap closure occurred with LC‐DCP and DCP constructs, but not LCP constructs. LCP construct properties were most similar to LC‐DCP and DCP construct properties in the low‐density model. Conclusion— All plate systems sustained physiologic limb loads. Only LCP constructs maintained some gap integrity, although LC‐DCP and DCP screws were placed in neutral position. Clinical Relevance— The LCP system is more likely than LC‐DCP and DCP systems, with neutrally positioned screws, to maintain a planned interfragmentary gap, although gap strains range from 0% to 15% across the 2 mm gap during a trot load.