Canine Lumbar Spinal Internal Fixation Techniques A Comparative Biomechanical Study

The stability (flexural rigidity) and strength of five canine spinal internal fixation techniques were quantitated and compared to each other and to the rigidity and strength of the intact spine. The techniques were applied to isolated canine lumbar spines (L2–L5) on which a complete spinal injury was surgically simulated at L3–L4. The spine-implant preparations were subjected to four-point bending and tested once to failure in flexion. The bending moment vs. L3–L4 angular deformation curves were recorded; rigidity and load sustained at failure (10° angular deformation) were compared. The combination of dorsal spinous process plate and dorsolateral vertebral body plate was the most rigid and most strong of the techniques tested. The dorsolateral vertebral body plate was the most rigid and most strong of the individual techniques, followed by the dorsal spinous process plate and the polymethylmethacrylate-pin technique. Vertebral body crosspins provided the least strength and stability of any of the techniques tested.     

Spinal Fixation in Dogs Using Steinmann Pins and Methylmethacrylate

Steinmann pins and methylmethacrylate were used to stabilize 17 vertebral fractures or luxations and one unstable congenital spinal deformity in 18 dogs of a wide range of ages and body weights. Of 12 dogs available for follow-up examination (4–43 months), 10 were normal or only mildly ataxic, and two were ambulatory but severely ataxic. Five dogs died or were euthanized in the early postoperative period, but none of the deaths could be attributed to the technique. Uncommon complications associated with this fixation technique were pin migration and wound infection.     

In Vitro Biomechanical and Histological Assessment of Pilot Hole Diameter for Positive-Profile External Skeletal Fixation Pins in Canine Tibiae

This study was conducted to evaluate the effect of pilot hole (PH) diameter (0, 1.5, 2.0, 2.7, 3.1, 3.3, 3.5, and 3.7 mm) on the biomechanical and microstructural performance of positive-profile threaded external skeletal fixation pins (3.18 mm inner diameter, 3.97 mm outer diameter) using cadaveric canine tibiae. Eight pins per pilot hole diameter (four pins per bone) were used to assess differences in end-insertional torque and pin pull-out strength. Histological evaluation of eight bicortical pin tracts per pilot hole diameter was accomplished using computer-interfaced videomicroscopy on specimens processed using a bulk-staining technique. Compared with no predrill, use of 2.7 mm PH increased end-insertional torque and pull-out strength by 25% and 13.5%, respectively. No significant differences were observed in biomechanical variables for the PH diameter range of 2.0 to 3.1 mm. Compared with no predrill, use of a 3.1 mm PH increased thread area by 18%. Microfracturing around the threads decreased as PH diameter increased. Damage to the interface at the entry and exit sites of both near and far cortices also decreased as PH diameter increased. It was concluded that predrilling a PH whose diameter approximates, but does not exceed the inner diameter of the positive profile pin will not only improve initial pin stability compared with no predrilling, but it will also reduce microstructural damage that may lead to excessive bone resorption and premature pin loosening.     

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.     

A Biomechanical Comparison of Screw and Wire Fixation With and Without Polymethylmethacrylate Re-Enforcement for Acetabular Osteotomy Stabilization in Dogs

Objective— Compare the biomechanical characteristics of screw and wire fixation with and without polymethylmethacrylate (PMMA) re-enforcement for acetabular osteotomy stabilization in dogs. Animals— Pelves removed from 8 adult mixed breed dogs weighing between 25 and 30 kg. Procedure— The pubic symphysis of each pelvis was split and a central transverse acetabular osteotomy was performed. One hemipelvis from each dog was stabilized with the composite fixation (interfragmentary Kirschner wire, two screws and a figure-of-eight orthopedic wire with PMMA). The contralateral hemipelves was stabilized with an interfragmentary Kirschner wire, two screws, and a figure-of-eight orthopedic wire without PMMA. All hemipelves were tested in bending by using a materials testing machine at a cross head speed of 5 mm/min. An extensometer was placed on the dorsomedial surface of the hemipelves centered over acetabular osteotomy to record distraction of the osteotomy during loading. A load/deformation curve and a load/distraction curve was produced for each hemipelvis. The slope for the initial linear portion of the load/deformation curve and the load/distraction curve, yield load and maximum load sustained were compared between repair groups using a paired t-test with P < .05 considered significant. Results— The slope of the load/deformation curve was significantly greater (P= .001 ) for hemipelves stabilized with the composite fixation (mean ± SD: 69 ± 18 N/mm) compared with hemipelves stabilized without PMMA (mean ± SD: 39 ± 8 N/mm). There was no significant difference (P= .593 ) between repair groups in the slope of the load/distraction curves as measured on the extensometer. Yield load was significantly greater (P= .0002 ) for hemipelves stabilized with the composite fixation (mean ± SD: 184 ± 25 N) compared to hemipelves stabilized without PMMA (mean ± SD: 74 ± 12 N). Maximum load sustained was also significantly greater (P= .013 ) for hemipelves stabilized with the composite fixation (mean ± SD: 396 ± 71 N) compared to hemipelves stabilized without PMMA (mean ± SD: 265 ± 94 N). Failure of hemipelves stabilized with the composite fixation occurred primarily by ventrolateral bending of the cranial and caudal pelvic segments at the osteotomy site. Failure of hemipelves stabilized without PMMA occurred by ventrolateral bending of the cranial and caudal pelvic segments at the osteotomy site with pronounced concurrent ventrolateral rotation of the cranial pelvic segment. Conclusion— PMMA improves the mechanical characteristics of acetabular fracture fixation, at least in part by neutralization of rotational forces. The results of this study justify use of PMMA as a component of the composite fixation when repairing acetabular fractures.     

An In Vitro Comparison of Hollow Ground and Trocar Points on Threaded Positive-Profile External Skeletal Fixation Pins in Canine Cadaveric Bone

OBJECTIVE: To compare the microstructural damage created in bone by pins with lathe-cut and rolled-on threads, and to determine the peak tip temperature and damage created by positive-profile external fixator pins with either hollow ground (HG) or trocar (T) tips during insertion. STUDY DESIGN: An acute, in vitro biomechanical evaluation. SAMPLE POPULATION: Twenty-seven canine tibiae. METHODS: Lathe-cut thread design with T point (LT-T), rolled-on thread design with T point (RT-T), and rolled-on thread design with HG point (RT-HG) pins were evaluated. Twenty pins of each type were inserted under constant drilling pressure into 12 canine tibiae (12 diaphyseal and 8 metaphyseal sites per pin type). Peak pin tip temperature, drilling energy, end-insertional pin torque, and pullout force were measured for each pin. For the histologic study, five pins of each type were inserted into cortical and cancellous sites in 15 additional tibiae. Entry and exit damage, and thread quality were assessed from 100 micron histologic sections by using computer-interfaced videomicroscopy. RESULTS: T-tipped pins reached higher tip temperature in both diaphyseal and metaphyseal bone compared with HG-tipped pins. RT-T pins had higher pullout strength (diaphyseal) and end-insertional torque compared with other combinations. No differences in drilling energy or insertional bone damage was found between the three pin types (P < .05). CONCLUSIONS: T-tipped pins mechanically outperformed HG-tipped pins. Pin tip and thread design did not significantly influence the degree of insertional bone damage. CLINICAL RELEVANCE: T-tipped pins may provide the best compromise between thermal damage and interface friction for maximizing performance of threaded external fixator pins.     

Mechanical Evaluation of Two Canine Iliac Fracture Fixation SysteMS

Twenty-three canine pelves were tested bilaterally to determine the stiffness and strength of intact ilium and stabilized oblique iliac osteotomies that simulated a common clinical fracture. Fixation systems tested were three 4.0 mm cancellous screws inserted ventral to dorsal across the osteotomy site and one laterally placed five hole 3.5 mm dynamic compression plate. Specimens were mechanically tested to failure under torsional, axial, or axial plus bending loads. Lag screw fixation was stiffer and stronger than plate fixation in all testing modes. The differences were statistically significant (p less than .05) in the torsional and axial plus bending loading modes. Fatigue testing was performed on implanted specimens with low-level cyclic loading under axial plus bending loading conditions. Physiologic loading conditions failed to produce mechanical failure of either fixation system after 100,000 cycles.     

Results of Screw/Wire/Polymethylmethacrylate Composite Fixation for Acetabular Fracture Repair in 14 Dogs

Objective- To assess the clinical results in dogs with acetabular fractures stabilized using a screw-wire-polymethylmethacrylate (SWP) composite fixation.
Study Design- A retrospective study of client-owned dogs with acetabular fractures.
Animals- Fourteen dogs ranging in age from 4 to 95 months (mean, 34 ±25 months; median, 25 months) and body weight from 8 to 39 kg (mean, 25 ±6 kg; median, 27 kg).
Methods- Medical records and radiographs were retrospectively evaluated to determine location of the fracture, presence of preexisting degenerative joint disease, accuracy of fracture reduction and complications associated with surgery. Long-term results were evaluated by subjective assessment of lameness, elicitation of pain and/or crepitus on manipulation of the coxofemoral joint, measurements of pelvic limb circumference, coxofemoral joint goniometric measurements, and radiographic evaluation.
Results- Fracture reduction was considered anatomic in 13 dogs. At the time of the last follow-up evaluation (mean, 347 ±261 days; median, 380 days) 10 dogs were sound on the affected limb, three dogs had a subtle weight-bearing lameness of the affected limb, and the remaining dog had a consistent non-weight-bearing lameness of the affected limb. Mild (n = 10) or moderate (n = 1) degenerative changes of the affected coxofemoral joint attributed to the acetabular fracture and its repair were noted on the follow-up radiographs in 11 dogs. Limb circumference of the affected limb ranged from -8.2% to +10.8% (mean, -0.8 ±4.2%; median, -0.7%) of the contralateral limb.
Conclusions- The SWP composite fixation consistently maintained anatomic reduction, was associated with few complications, and yielded satisfactory clinical results.
Clinical Relevance- The SWP composite fixation technique would seem to be an acceptable means of stabilizing acetabular fractures in dogs.     

骨外固定器在一例犬小腿骨骨折中的应用

骨外固定是指在骨折两端,经皮穿放钢针再用连接杆及钢针固定夹将裸露在皮外的针端彼此连结起来,从而固定骨折断端的一种治疗方法。论文通过对1例小腿骨折的德牧犬进行X光检查、手术及术后护理,探讨骨外固定在小动物临床上的应用,进一步提高对骨折的治疗水平。     

Ex Vivo Biomechanical Comparison of Pin Fixation Techniques for Canine Distal Femoral Physeal Fractures

Objective— To compare the biomechanical properties of five intramedullary (IM) pin fixation techniques for Salter-Harris type I fractures of the distal femur in dogs.
Study Design— Randomized, one-way factorial design composed of five treatment groups: (1) single IM pin, (2) dynamic IM crossed pins, (3) paired convergent pins, (4) crossed pins, and (5) crossed polyglycolic acid (PGA) rods.
Sample Population— Forty pairs of cadaver canine femurs.
Materials— One femur of each pair was manually fractured and subsequently repaired; the contralateral intact femur served as its control. Each femur was loaded in torsion until failure occurred and load-deformation curves were generated.
Results— The crossed-pin technique sustained the greatest load to failure (116.8%) followed by the paired convergent pins (104.8%), dynamic IM pins (90.6%), single IM pin (72.1%), and crossed PGA rods (71.9%). Statistically significant differences in strength at failure were detected between the crossed-pin and single IM pin and the crossed-pin and crossed PGA rod techniques. All fixation techniques underwent greater deformation (1.5 times as much) and had a lower stiffness (66% to 75%) compared with the intact controls; however, there was no significant difference between techniques. Failure in the paired convergent and crossed-pin techniques occurred by fracture of the bone; failure in the other techniques occurred by distraction at the fracture site.
Conclusion— The rotational stability of any of the fixation techniques appears to be primarily determined by the ability to prevent distraction and maintain interdigitation of the physis.
Clinical Relevance— When choosing a particular fixation technique for repair of a distal femoral physeal fracture, consideration should be given to the technique's relative biomechanical merits.     

Holding Power of Threaded External Skeletal Fixation Pins in the Near and Far Cortices of Cadaveric Canine Tibiae

We compared the pin-bone interfaces at the near and far cortical penetration sites of positive-profile end-threaded external fixation pins in cadaveric canine tibiae. The holding power of the pins in each cortical surface was independently measured in 21 pin-bone sections. Scanning electron microscopy (SEM) was used to compare subjectively the microstructural appearance of the pin-bone interfaces at the near and far cortical penetration sites in eight pin-bone sections. The far cortical penetration site provided greater holding power than did the near cortical site. SEM evaluation suggested more bony microfractures and debris with less pin-bone interlock in the near cortical penetration sites than in the corresponding far cortical penetration sites. This study showed that after low-speed power insertion of positive-profile end-threaded pins in canine cadaveric tibiae, the near cortical penetration site contributes approximately 25% less to the overall holding power of the pin than does the far cortical penetration site.     

犬股骨远端髁间T型骨折内固定实验模型的建立

为创建犬股骨远端髁间T型骨折并同期行内固定术的实验动物模型,观察这种内固定术后的愈合情况。对8条成年犬通过截骨的方法制作动物模型,并分别于术后1d、1周、2周、4周、8周、12周对实验犬进行X线检查。除一条实验犬创口感染形成假关节外,其余的犬对手术耐受良好,X线检查显示,随着固定时间延长,骨折间隙逐渐变窄,骨痂从无到有,骨折线从开始模糊到消失。本实验模型中采用的内固定方法能很好的治愈犬股骨远端髁间T型骨折。     

Biomechanical Comparison of the Trocar Tip Point and the Hollow Ground Tip Point for Smooth External Skeletal Fixation Pins

Objective —To compare the insertional characteristics of external fixator pins with hollow ground (HG), modified HG, and trocar (T) points.
Study Design —An acute, in vitro biomechanical evaluation.
Sample Population—Thirteen radii from canine cadavers.
Methods —A total of 16 T-tipped and 16 HG-tipped pins were inserted into 8 canine radii. Ten pins of each modification of the HG tip (length of the cutting edge reduced by 0.127 mm and 0.254 mm, respectively) were inserted into another five radii. All pins were inserted with low-speed power drilling and 80 N drilling load. Differences between peak tip temperature, drilling energy, and pullout force were determined for each pin type at both diaphyseal and metaphyseal locations.
Results —HG-tipped pins showed a 40% lower tip temperature in diaphyseal bone, a 25% reduction in drilling energy in diaphyseal bone, and a reduction of pullout force in both diaphyseal (65%) and metaphyseal (50%) bone compared with T-tipped pins. HG 0.254-mm pins generated higher tip temperatures and had greater pullout than HG pins in diaphyseal bone.
Conclusions —The HG tip was a more efficient design; however, the reduction in pullout force suggests that, because a better hole was drilled, radial preload is reduced. Reduction of the cutting edge by 0.254 mm increased the pullout force but also increased the temperatures.
Clinical Relevance —Thermal and microstructural damage are reduced by the HG tip, but pin-bone interface stability is also compromised. The use of a tip with 0.254 mm reduction in the cutting edge may optimize the biological and mechanical factors at the pin-bone interface.     

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.