A Mechanical Evaluation of the Resistance of Various Interfragmentary Wire Configurations to Torsion

Fourteen interfragmentary orthopedic wire configurations were tested in torsion using a transverse fracture polyvinylchloride pipe model. These models included single and double Kirschner pins with and without orthopedic wire added to the configuration. The orthopedic wire was applied in either an encircling, figure-of-eight (skewer pin), or cruciate pattern. Double Kirschner pins were applied in a mono- or biplanar fashion. An external fixator model was also tested. Stiffness, yield load, safe load, and energy of absorption were measured and calculated for each model. Orthopedic wire added to any configuration increased stiffness. All single pin configurations with orthopedic wire and the external fixator had the highest stiffness. Two Kirschner pins had a higher torsional yield load and safe load than single pin configurations with or without orthopedic wire. The external fixator model had the highest torsional yield load, safe load and energy of absorption of all configurations tested. However, the external fixator was only significantly different in safe load from the 90° biplanar configurations with wire and the cross pin configuration with encircling wire. The 90° biplanar configurations with wire and the cross pin configuration with encircling wire were equally as effective as the external fixator model in yield load and energy of absorption.     

Type I external skeletal fixation of radial fractures in microchiropterans.

An adult male big brown bat (Eptesicus fuscus) and an adult female hoary bat (Lasiurus cinereus) were presented with open transverse middiaphyseal left radial fractures. Initial repair was attempted by intramedullary pinning. When the fractures did not heal, intramedullary pins were removed and type I external skeletal fixators were placed. The fractures healed, and the big brown bat regained normal flight but the hoary bat did not.     

Stifle joint luxation in the cat: treatment using transarticular external skeletal fixation

Extra-articular suturing techniques and transarticular external skeletal fixators were used to repair traumatic luxation of the stifle joint in four cats. Rupture of the cranial cruciate, caudal cruciate and medial collateral ligaments, together with injury to one or both menisci, were the most common injuries observed. The method of stifle repair was successful in all cases, but serious complications occurred when cats with transarticular external fixators were not kept confined indoors. Complications consisted of pin loosening and disruption of the fixator, or fractures through proximal or distal pins. Transarticular external skeletal fixation was considered to be a simple and effective method of maintaining short-term joint stability to allow healing of injured soft tissue structures. The apparatus facilitated early weightbearing and, on removal, allowed for the return of near-normal stifle function. Careful pin insertion and owner compliance in enforcing confinement are essential in minimising complications associated with immobilising the stifle joint using transarticular external skeletal fixation.     

External fixation of the lumbar spine in a canine model

OBJECTIVE: To compare the mechanical properties of two types of external skeletal fixation of the lumbar spine with polymethylmethacrylate (PMMA)/Steinmann pin fixation in a canine unstable spine model. STUDY DESIGN: Cadaver study. SAMPLE POPULATION: Lumbar spines of 17 mature large-breed dogs. METHODS: Spine stiffness (N-m/deg) in flexion, extension, and rotation under physiological loading conditions and spine strength (N-m) in flexion were determined. Spines were destabilized at L3-L4, instrumented and retested. Fixation techniques included four-pin PMMA (PMMA4), eight-pin PMMA (PMMA8), eight-pin biplanar type I external skeletal fixator (ESF) (SK), and eight-pin spinal arch ESF (ARCHES). RESULTS: All fixation groups were as stiff as intact spines in extension and rotation and were significantly stiffer in flexion. In flexion, both PMMA8 and ARCHES were significantly stiffer than SK, and PMMA8 was significantly stiffer than PMMA4. In rotation, PMMA8 and ARCHES were significantly stiffer than SK, and in flexion to failure, PMMA8 and ARCHES were significantly stiffer than PMMA4. CONCLUSIONS: External skeletal spinal fixation (ESSF) has mechanical properties comparable to more commonly used PMMA/pin internal fixation techniques. CLINICAL RELEVANCE: External fixation of the canine spine has several potential advantages over internal fixation including minimal dissection for pin placement, the ability to span affected vertebrae with placement of implants distant from the site of injury, postoperative adjustability, and complete removal of implants after healing. This study supports the biomechanical stability ESSF of the canine lumbar spine. Further studies are indicated to evaluate zones of consistently safe and secure placement of pins and clinical efficacy.     

Biomechanical comparison of a circular external skeletal fixator construct to pin and tension band wire fixation for the stabilization of olecranon osteotomies in dogs: a cadaveric study

OBJECTIVE: To compare anatomic reduction and the biomechanical properties of a circular external skeletal fixator (CESF) construct to pin and tension band wire (PTBW) fixation for the stabilization of olecranon osteotomies in dogs. STUDY DESIGN: Cadaveric study. ANIMALS: Forelimbs from 12 skeletally mature mixed-breed dogs, weighing 23 to 28 kg. METHODS: An olecranon osteotomy was stabilized with either a CESF construct or PTBW fixation. A single distractive load to failure was applied to each specimen through the triceps tendon. Osteotomy reduction and biomechanical properties were compared between fixation groups. RESULTS: Reduction was not significantly different (gap: P =.171; malalignment: P =.558) between fixation groups. Osteotomies stabilized with the CESF had greater stiffness (P <.0001) and maximum load sustained (P <.0001) compared to PTBW fixation. There was no significant difference for yield load (P =.318) or for load at 1 mm of axial displacement (P =.997) between fixation groups. Failure of fixation occurred by bending of the intramedullary Steinmann pin and the fixation wires in the CESF specimens and by untwisting of the tension band wire knot with pullout and bending of the Kirschner wires in the PTBW specimens. CONCLUSIONS: Specimens stabilized with the CESF construct had similar reduction and yield load, greater stiffness and maximum load sustained, and less elastic deformation than specimens stabilized with PTBW fixation. CLINICAL RELEVANCE: The CESF construct may provide a biomechanically favorable alternative to PTBW fixation for stabilization of olecranon osteotomies in dogs, and its application warrants clinical investigation.     

Influence of an interdental full pin on stability of an acrylic external fixator for rostral mandibular fractures in dogs

OBJECTIVE: To determine total stiffness and gap stiffness of an external fixation system in a canine mandibular fracture gap model incorporating a full interdental pin as the only point of rostral fixation in a bilateral type-I external fixator. SAMPLE POPULATION: 10 canine mandibles. PROCEDURE: Bilateral mandibular ostectomies were performed between premolars 3 and 4. A type-I external fixator incorporating a full interdental pin was placed to stabilize a 0.5-cm fracture gap. Four pin configurations (intact mandibular bodies with fixator; ostectomized mandibular bodies and complete fixator; ostectomized mandibular bodies with caudal pins of rostral fragment cut; ostectomized mandibular bodies with all pins of rostral fragment cut) were tested in dorsoventral bending 5 times on each mandible. The full interdental pin remained intact in all configurations. Total stiffness and gap stiffness were determined for each configuration on a materials testing machine. RESULTS: Total stiffness of intact mandibles was significantly greater than that of ostectomized mandibles, regardless of external fixator configuration. However, total stiffness and gap stiffness were not significantly different among different external fixator configurations applied to ostectomized mandibles. CONCLUSION AND CLINICAL RELEVANCE: External fixator configurations with only the full interdental pin engaging the rostral fragment were as stiff as configurations that had 2 or 4 additional pins in the rostral fragment for the applied loads. External fixators for rostral mandibular fractures may be rigidly secured with rostral fragment implants applied extracortically, avoiding iatrogenic trauma to teeth and tooth roots.     

External skeletal fixation for stabilisation of comminuted humeral fractures in cats

Thirteen feline humeral fractures were stabilised using external skeletal fixation alone or in combination with supplementary devices. Eleven of the 13 fractures healed. Pins placed in the distal humerus should be angled to avoid the supracondylar foramen and radial nerve. An intramedullary pin in combination with external skeletal fixation is indicated for mildly comminuted reconstructable fractures. For severely comminuted humeral fractures, closed application of an external skeletal fixator may be preferable to open reduction and internal fixation to provide healing and is recommended as an alternative to amputation.     

A biomechanical comparison of external skeletal fixation and plating for the stabilization of ilial osteotomies in dogs

This in vitro study compares the biomechanical properties of two methods of ilial fracture repair in dogs. Ten pelves were harvested from skeletally mature mixed breed dogs weighing 20-27 kg and bilateral oblique ilial body osteotomies were created. One hemipelvis from each dog was stabilized with a 2.7 mm plate and screws and the contralateral hemipelvis was stabilized with a five pin linear external fixator construct. Each hemipelvis was mounted at an angle of 30 degrees to an actuator platform, such that the acetabulum was centrally loaded by a steel sphere attached to the load cell of a servohydraulic materials testing machine. The construct was loaded at a constant rate of 20 mm/min. A load/displacement curve was generated for each hemipelvis by plotting the sustained load against the actuator movement. The stiffness, yield load and failure load for each hemipelvis were determined from the load/displacement curve. Bending stiffness was defined as the slope of the load/displacement curve from 100 N to yield load. The mode of failure was determined by observations made during testing and gross inspection of each specimen. The mean construct stiffness, yield load and failure load were compared between stabilization groups using a Student's paired t-test with statistical significance set at p<0.05. Nine out of 10 of the hemipelves that were stabilized by plates and screws failed catastrophically by fracture through the caudal screw holes and nine out of 10 of the hemipelves that were stabilized using an external fixator failed by fracture of the ischium in the region supported by the mounting roller, propagating through the most caudal ischial pin. There was not any significant difference (P=0.22) in bending stiffness between stabilization techniques, but yield (1467 N vs 2620 N; P=0.04) and failure (1918 N vs 2687 N; P=0.002) loads were significantly greater for hemipelves stabilized with external fixators.     

Circular external skeletal fixation stabilization of antebrachial and crural fractures in 25 dogs

Fracture stabilization using circular external skeletal fixation was evaluated in 14 dogs with antebrachial fractures and 11 dogs with crural fractures. Most dogs were consistently weight bearing on the stabilized limb by 3 days following surgery. Although all dogs developed minor wire/pin tract inflammation and eight dogs developed major wire/pin tract inflammation, postoperative lameness was not consistently associated with wire/pin tract complications. Fractures in 23 dogs achieved radiographic union (mean +/- standard deviation [SD], 61 +/- 21 days; median, 57 days) without additional surgery; two dogs required restabilization of their fractures with linear fixators. Twenty of the 21 owners that could be contacted felt their dog had no (n=15) or only a mild (n=5) intermittent lameness at the time of final, long-term (mean +/- SD, 37 +/- 17 months; median, 42 months) assessment.     

Instrumentation for external fixation.

External skeletal fixation is a very useful technique for managing many orthopedic problems in veterinary practice. The Kirschner apparatus has been the most widely used fixator for many years in veterinary orthopedics because of its versatility, simplicity, and economy in use. The medium-sized device has the widest indications and is easiest to begin with. The small size can be acquired later for use on cats and small dogs. The new "raised thread" fixation pin designs improve bone-pin integrity and can be used in combination with nonthreaded pins to decrease the incidence of postoperative complications and for economy. Acrylic-pin external fixators are particularly useful for treatment of mandibular fractures and transarticular application since they allow nonlinear placement of fixation pins in highly contoured bones. A commercial system, currently being developed, will have all the equipment and materials necessary for their application in a convenient kit. Circular fixators (Ilizarov design) use thin K wires placed under tension to replace rigid fixation pins. Their unique adjustability characteristics make them useful in the treatment of limb deformity and shortening. Many other human fixators can be used for veterinary application if the basic principles of fixators are followed. Instrumentation required for external fixator application include a pin driver, pin cutter, and wrenches. Although surgical versions of these materials are available, less expensive alternatives are available using gas sterilization instead of autoclaving. The use of selected orthopedic instrumentation such as a periosteal elevator, bone clamps, and curets will facilitate fracture management.     

Use of a hybrid external skeletal fixator for repair of a periarticular tibial fracture in a Patagonian cavy

An 8-week-old female Patagonian cavy was examined because of acute right hind limb lameness; radiography revealed a moderately displaced, comminuted fracture of the proximal third of the tibia. The fracture was stabilized with a hybrid external skeletal fixator. Two Kirschner wires were placed in the main proximal fragment, parallel to the tibial plateau and at right angles to each other. These wires were connected to a partial circular external fixator ring. Three half pins were placed in the distal fragment, and a straight connecting rod positioned on the medial side of the limb was connected to these pins and the fixator ring. A second connecting rod was positioned on the craniomedial side of the limb and was connected to the fixator ring and a fourth half pin in the distal fragment. The fracture healed without complications, and the fixator was removed 3 weeks after surgery. Hybrid external skeletal fixators combine the benefits of circular and linear external skeletal fixation methods, enabling rigid fixation of periarticular long bone fractures without adversely affecting mobility of the adjacent joint.     

Internal Skeletal Fixation Using a Kirschner Apparatus for Stabilization of Fracture/Luxations of the Lumbosacral Joint in Six Dogs: A Modification of the Transilial Pin Technique

A technique using a modification of the transilial pin technique for stabilization of fracture/ luxations of the lumbosacral joint was performed in six dogs. This technique used an internal skeletal fixator composed of two transilial pins secured with two double fixator clamps. Collapse (shortening) of the seventh lumbar vertebra (L7) was observed in five dogs without compromise of the vertebral canal. Kirschner wires placed across the articular facets as supplemental fixation devices migrated to the subcutaneous tissues in two dogs and were easily removed. Migration of the internal fixators was not observed during healing; vertebral canal dimensions were maintained in all cases, and the internal fixators were not removed after resolution of fracture healing. All fractures were healed within 6 to 12 weeks of surgery without evidence of pain, neurologic impairment, or long-term complications.     

Effect of a supplemental plate on the stiffness of a type I external fixator

OBJECTIVE: To assess the effect of a supplemental plate on the stiffness of a six-pin unilateral external skeletal fixator. STUDY DESIGN: Mechanical testing performed on models. METHODS: Wooden (birch) dowels were used to create five models of a fracture. A commercially available external fixation system was applied to the model with a uniform unilateral six-pin fixator design. The models were mechanically tested with and without a supplemental plate attached to the 2 clamps adjacent to the fracture gap. Testing was conducted in axial loading, medial to lateral bending, and cranial to caudal bending. RESULTS: Results showed a 4.42-fold increase in stiffness in axial load, a 4.23-fold increase in stiffness in medial to lateral bending, and a 1.94-fold increase in stiffness in cranial to caudal bending with the addition of the plate. CONCLUSIONS: The addition of a supplemental plate increases the mechanical stiffness of unilateral fixators. This was especially true in axial load and medial to lateral bending. CLINICAL RELEVANCE: A supplemental plate can be used with unilateral fixators to increase stiffness of the fixator. Conversely, the plate can be removed to decrease stiffness without the removal of fixation pins.     

Evaluation of strength at the acrylic-pin interface for variably treated external skeletal fixator pins

Objective: To report pullout force to failure at the acrylic–pin interface for variably treated 3.2 mm external skeletal fixator pins. Study Design: In vitro biomechanical evaluation. Sample Population: 3.2 mm external skeletal fixator pins in polymethylmethacrylate bars. Methods: 3.2 mm external skeletal fixator pins were used for each of 5 treatment groups: polished, unpolished, 3 notched, 5 notched, and machine knurled. Each pin was seated into a 2‐cm‐diameter acrylic connecting bar and tested in pullout force to failure. Each group consisted of 6 pins. The force required to remove the pins from the acrylic bar was measured and compared between groups. Results: Significant differences between treatment groups were determined (P<.05). Within a construct group failure mode was consistent. Fracture of the acrylic bar was only seen with knurled pin ends. Conclusions: When using 2 cm acrylic bars in external skeletal fixation (ESF), a knurled pin shaft or a pin surface with 5 notches should be considered to improve the overall stability of the ESF construct.     

Comparison of Two Methods of Long Bone Fracture Repair in Rabbits

Two different methods of rabbit femoral fracture repair were evaluated: (1) stainless-steel surgical plate and bone screws; and (2) placement of an intramedullary pin and an external skeletal fixator device. On average, bones repaired with the bone plate method withstood 35.1 lb/47.6 N (range, 14.4-63.0 lb/19.5-85.4 N) of compressive and bending forces before failure occurred. Bones repaired with intramedullary pin and external skeletal fixator device method withstood an average of 67.7 lb/91.8 N (range, 48.7-94.8 lb/66.0-128.5 N) of compressive and bending forces before failure, but the bone was more likely to shatter during implant application. Normal rabbit femurs placed in the control group were able to withstand an average of 148.4 lb/201.2 N (range, 100.0-192.0 lb/135.6-260.3 N). The fragility of rabbit bones made testing of any implant viability problematic. This study demonstrates advantages and disadvantages to each method of fixation in rabbits and compares important differences in application of fracture repair implants with those of other domestic species.     

Nonlinear stiffness profiles of external fixators constructed with composite rods

OBJECTIVE: To determine if composite connecting rods confer nonlinear stiffness characteristics on unilateral and bilateral external skeletal fixators (ESF) in cranial-caudal bending and axial loading. STUDY DESIGN: Mechanical testing performed on models. SAMPLE POPULATION: Six models of 6-pin ESF constructs composed of birch dowels, a commercial ESF system, and composite connecting rods. METHODS: Unilateral and bilateral ESF configurations were assembled using either specially designed composite titanium and silicone (composite group) or solid titanium (solid group) connecting rods. Mechanical testing was performed in axial loading and 4-point cranial-caudal bending. Stiffness was determined at a low and high-load range, and was considered increasing and nonlinear if the stiffness at high loads was greater than at low loads. RESULTS: The stiffness of the solid group was linear in all testing modes and configurations. Bilateral composite fixators had a nonlinear increasing stiffness in axial loading and cranial-caudal bending. Unilateral composite fixators had a nonlinear increasing stiffness in axial loading, but not cranial-caudal bending. Solid connecting rods conferred a higher stiffness in all testing modes and configurations. CONCLUSIONS: Composite connecting rods resulted in nonlinear increasing axial and bending stiffness in bilateral fixators, and in axial load in unilateral fixators. CLINICAL RELEVANCE: Conventional ESF can be constructed so that the stiffness increases as load increases. This provides the surgeon with additional options to control the local mechanical environment of a healing fracture, which may be used to enhance fracture healing.     

Comminuted long bone fractures in cats caused by pneumatic gun shot and repaired using the external fixator "tie-in" technique

Five cats with femoral fractures caused by pneumatic gun shot wounds were treated using intramedullary pin/external skeletal fixation "tie-in" (IM/ESF tie-in) technique between 1997 and 2005. Clinical and radiographical examinations were used to assess bone healing, limb function recovery and complications. Clinical results as well as fracture healing were excellent in all five cats. The study showed that the IM/ESF tie-in technique can be used in cats for the treatment of comminuted femoral fractures caused by pneumatic bullets.     

Epoxy Putty for Free-Form External Skeletal Fixators

Objective —To evaluate the suitability of epoxy putty for use as a connecting beam material in a free-form external skeletal fixator.
Design —Mechanical evaluation of beams and the pin-material interface of commonly used methacrylates and the proposed epoxy putty.
Procedure —The apparent modulus, bending strength, and toughness of 10 beams of three methacrylates (Technovit, APEF System, Bone Cement) and three epoxy putties (Oatey Epoxy Putty, All-Metals PowerPoxy, and Plumber's PowerPoxy) were determined in three-point bending. The shear strength of smooth and roughened-shaft pins embedded in the three methacrylates and the Oatey Epoxy Putty was determined by pull-out testing.
Results —The epoxy putties had similar strength, greater apparent modulus, and reduced toughness when compared with the methacrylates. The shear strength of the smooth pin interface with the Oatey Epoxy putty was greater than that with the methacrylates. The interface with roughened pins was much stronger than that with smooth pins for all materials tested.
Clinical Relevance —Epoxy putty is a suitable material for free-form external fixators. It is easy to handle, inexpensive, and has suitable setting times and mechanical properties.     

Mechanical testing of a steel-reinforced epoxy resin bar and clamp for external skeletal fixation of long-bone fractures in cats

AIMS: To provide veterinarians with confidence when using a commercially available epoxy resin in external skeletal fixators (ESF), testing was conducted to determine exothermia during curing of the epoxy resin compared to polymethylmethacrylate (PMMA), the hardness of the epoxy resin as a bar over 16 weeks, and the strength of the epoxy resin bar compared with metal clamps in similarly constructed Type 1a ESF constructs simulating the repair of feline long bone fractures.

METHODS: Exothermia of the epoxy resin during curing was tested against PMMA with surface temperatures recorded over the first 15 minutes of curing, using four samples of each product. The hardness of 90 identical epoxy resin bars was tested by subjecting them to cyclic loads (1,000 cycles of 20.5?N, every 7 days) over a 16-week period and impact testing 10 bars every 2 weeks. Ten bars that were not subjected to cyclic loads were impact tested at 0 weeks and another 10 at 16 weeks. Strength of the epoxy resin product, as a bar and clamp composite, was tested against metal SK and Kirschner-Ehmer (KE) clamps and bars in Type 1a, tied-in intramedullary pin, ESF constructs with either 90° or 75° pin placement, subjected to compressive and bending loads to 75?N.

RESULTS: The maximum temperature during curing of the epoxy resin (min 39.8, max 43.0)°C was less than the PMMA (min 85.2, max 98.5)°C (p<0.001). There was no change in hardness of the epoxy resin bars over the 16 weeks of cyclic loading (p=0.58). There were no differences between the median strength of the epoxy resin, SK or KE ESF constructs in compression or bending when tested to 75?N (p>0.05). Stiffness of constructs with 75° pin placement was greater for SK than epoxy resin constructs in compression (p=0.046), and was greater for KE than epoxy resin constructs in bending (p=0.033).

CONCLUSIONS: The epoxy resin tested was found to be less exothermic than PMMA; bars made from the epoxy resin showed durability over an expected fracture healing timeframe and had mechanical strength characteristics comparable to metal bar and clamp ESF constructs.

CLINICAL RELEVANCE: The epoxy resin ESF construct tested in this study can be considered a suitable replacement for SK or KE ESF constructs in the treatment of feline long-bone fractures, in terms of mechanical strength.     

Transarticular application of external skeletal fixation.

External skeletal fixators provide a useful alternative to external coaptation techniques for immobilization of joints in selected patients. This is especially true when the orthopedic injury involves an open wound requiring daily treatment. Devices such as the K-E splint, the Rudy external fixator boot, and acrylic frame fixators are economical and effective for transarticular fixation in small animal patients. Clinical indications for these techniques have included fixation of tibial or radial fractures with a short distal fragment; arthrodesis of the elbow, carpus, stifle, or tarsus; protection of Achilles tendon repairs; protection of collateral ligament repairs of the tarsocrural joint; and protection when multiple ligaments of the stifle joint have been reconstructed. Use of contoured rods has facilitated transarticular application of type II Kirschner-Ehmer splints. Use of acrylic frames and the Rudy boot technique have extended safe use of transarticular external fixation to extremely small patients not accommodated by the Kirschner system alone. An understanding of the advantages and disadvantages inherent in each of these techniques is critical to proper selection of the best method for a given patient. Attention to the basic principles of pin selection, pin insertion, and frame design are needed for successful execution of the chosen technique.