EXTERNAL SKELETAL FIXATION
0195-5616/92 $0.00 + .20
INSTRUMENTATION FOR EXTERNAL FIXATION Erick L. Egger, DVM
Instrumentation associated with the use of external fixation can be divided into two groups: fixation devices and the equipment needed or useful for fixator application. Throughout this article, generic, descriptive nomenclature is used except where reference is made to a specific commercial product. The source of these products and their approximate cost are given in Table 1. EXTERNAL FIXATION DEVICES
Many external fixation devices have been designed and manufactured over the last 90 years.!3 This discussion, however, concentrates on those devices and implants currently being used for small animal veterinary application. Kirschner Apparatus
In North America, the Kirschner apparatus (see Table 1) is the most commonly used fixator in veterinary practice. It is based on the Roger Anderson splint, which was designed in 1934 for treatment of distal extremity fractures in humans.! In 1946, Dr. E. A. Ehmer modified the size and material of the design slightly, had it manufactured by the Kirschner machine shop, and introduced it as the Kirschner-Ehmer Spliht.!2 Over the years, the fixator evolved into the Kirschner apparaFrom the Department of Clinical Sciences, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Fort CoIlins, Colorado
VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 1 • JANUARY 1992
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Table 1. SOURCES OF INSTRUMENTATION AND EQUIPMENT Material
Approximate Catalog Cost
Kirschner Medical (Orthopaedic Division, 10 W. Aylesbury Rd., Timonium, MD 21093) Single connecting clamps $12-18 Double connecting clamps $24-30 Connecting rods $2-8 Steinmann pins $3-7 Kirschner wires $3 Ellis pins $5-6 Power drill adapter and linen shroud $130 Drill guide $55 Bar wrenches $10 Socket wrench $40 Large pin cutter $350 Hand bending irons $50 Kern bone clamp $145-180 $50 Curets Gauthier Medical, Inc. (3105 N.w. 22nd SI. , Rochester, MN 55901)
Enhanced thread half-pin or full-pin Acrylic-pin external fixation system (APEF) Medium materials kit (for 2 fixators) Small materials kit (for 2 fixators) Application kit (for both small and medium APEF construction)
$11 $150 (estimate) $140 (estimate) $150 (estimate)
IMEX Veterinary, Inc. (1227 Market SI., Longview, TX 75604)
IMEX interface (half-pin) IMEX centerface (full-pin)
$6-8 $8-11
Jorgensen Laboratories (1450 N. Van Buren Ave., Loveland, CO 80538)
Technovit (methyl methacrylate) 750 g Circular fixator (llizarov-like) Kern bone clamp Pointed fracture reduction forceps
$32 $1200 $118 $84
Dr. Antonio Ferretti (V. Petrarca 36, 20015, Parabiago, Italy)
Dural aluminum rings
Custom made
Synthes (1690 Russell Rd., PO Box 1766, Paoli, PA 19301)
Small external fixation set Drill guide Plate bending press Plate bending irons Serrated fragment reduction forceps
$3680 $55-90 $756 $180 $240-280
EBI Medical Systems, Inc. (PO Box 682, 300 Fairfield, NJ 07007) Orthofix, fixator and application kit
$2000-4000
H. W. Andersen Products, Inc. (221 South SI. , Oyster Bay, NY 11771)
Anprolene sterilizer (model AN-72) cost of gas per cycle
$600 $5
Makita Electric Works, LTD. (Anjo, Aichi, Japan) Makita rechargeable drills
$55-90
Animal Clinic Products (3505 McGehee Rd., Montgomery, AL 36111)
Extend-a-chuck (drill extension) and shroud
$135
INSTRUMENTATION FOR EXTERNAL FIXATION
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Figure 1. Small (A), medium (8), and large (C) size Kirschner apparatus.
tus, a device that is currently produced in four sizes. Three of the sizes are useful for small animal application (Fig. 1). The small device is used on cats and small dogs weighing less than 10 to 12 kg and depending on bone size. The medium-sized device is used on most medium-sized and large dogs. The large device is based on Kirschner's human tibial frame using large threaded fixation pins and can be used on giant breed dogs. The "heart" of the Kirschner apparatus is the single connecting clamp that is used to affix the fixation pins to the connecting rods. The single connecting clamp consists of a U piece (through which the connecting rod passes), bolt (through which the fixation pin passes), and nut (Fig. 2). All nuts, bolts, and the Us, of the small and medium-
Figure 2. Exploded single connecting clamp: (A) "U", (8) bolt, (C) nut.
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sized clamps are made of stainless steel, whereas the Us of the large clamps are anodized aluminum. Tightening the nut compresses the fixation pin to the U and squeezes it around the connecting rod, thus locking both simultaneously. This design allows the use of fixation pins the same diameter size as or smaller than the hole in the bolt (%4" for small, Va" for medium, 3/16" for large). Consequently a wide variety of fixation pin diameters can be used with just two or three sizes of clamps. Fixation pin diameter selection is determined by the patient's size (generally a pin diameter of about 20% of the bone diameter is recommended). For example, in fixing a tibial fracture of a I8-Kg dog, %2" fixation pins through medium clamps might be appropriate. The U design, however, requires a specific connection rod diameter for each clamp size (Vs" for small, 3116" for medium, 7/16" for large). Stainless steel connecting clamps are quite durable and can routinely be reused many times. Individual clamp replacement parts can also be ordered. Double connecting clamps are used to attach connecting rod to rod
Figure 3. Double connecting clamp (arrows) used to attach two connecting rods, used in pairs to create a highly adjustable frame.
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(Fig. 3). Using two double connecting clamps together allows universal adjustment of fixator alignment. They are, therefore, used in construction of fixator configurations that allow manipulation of reduction after the entire device has been placed. Double connecting clamps, however, are relatively expensive and heavy, and their mechanical stability is limited. 9 Consequently their use is generally avoided or limited to rapidly healing, stable situations. The small and medium Kirschner connecting rods are made of standard stainless steel stock, whereas the large rods are aluminum. Steinmann pins can be used as connecting rod, but this is actually more expensive, since the process of grinding trocar tips on the pins is a major part of their cost. Recently there has been research in fixation pin design. 20, 21 In general, Steinmann pins cut in half are used for non threaded fixation pins. Kirschner wires can likewise be used on very small patients. Although partially threaded (thread is cut into the pin shaft) Steinmann pins provide greater resistance to pin pull out,4 the stress collector effect at the threaded-non threaded junction has often been the location of pin bending or breakage. One solution is threading the pin only a short distance (about three times the diameter) from the tip so the pin will thread only into the far cortex, thus protecting the weak stress collector in the medullary canal (Fig. 4).4 Kirschner (see Table 1) is marketing this design as the Ellis pin. Some complications with the Ellis pins, however, have still been reported. 19 Fixation pins with threads extending out from a constant diameter shaft are not significantly weakened and have been used in human fixators for many years. 13 These human products, however, are usually too large and
Figure 4. Standard partially threaded fixation pin (A) and Ellis fixation pin (8).
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Figure 5. Improved "raised thread" profile pins: (A) medium half pin, and (8) small, (C) medium, and (0) large full (centrally threaded) pins.
their cost prohibitive for routine veterinary usage. Recently two companies have begun producing both half and full pin designs (Fig. 5) in veterinary sizes at a reasonable cost. Gauthier Medical, Inc. (see Table 1) markets the Enhanced Thread Half and Full Pin designs, and IMEX Veterinary, Inc. (see Table 1) markets the 1M EX Interface (half-pin) and IMEX Centerface (full-pin) designs. Mechanical and in vivo studies of these pins are currently under way. Clinical experience has suggested using a combination of nonthreaded and threaded pins to minimize the postoperative complications of pin loosening and pin tract drainage while maintaining good fixation pin strength at a reasonable cost. 3 Kirschner sells their products as complete sets or individual components. The sets provide only enough clamps for four pins per device, however. The author, therefore, suggests buying individual components starting with about 12 single clamps. This will be enough to construct one complex or two simple frames. Based on technical ease, use of the medium-sized apparatus is indicated most often in veterinary practice. Practitioners can expand their inventory to include the small apparatus for cats and small dogs as they become more comfortable with application of the system. Use of the large apparatus is infrequent because the improved fixation pin thread designs are now available for the medium-sized apparatus, and very rigid configurations that used medium-sized components have evolved. 7, 9 Additionally the large clamps and connecting rods are made of aluminum and are not very durable, often requiring replacement after two or three uses. The Kirschner apparatus is not inexpensive, but it is relatively economical for use by veterinarians because a minimal inventory of component and specialized application equipment is necessary. Economy for the client comes from the reusability of the connecting clamps
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and most rods. To encourage return of these components, we charge the clients full replacement cost at the time of patient dismissal, then allow a 75% refund on return of reusable components. This leaves us with a 25% user fee and encourages owner compliance with the recheck schedule. The refund is usually offset by the cost of recheck radiographs, thus requiring no additional cash transaction for the owner or practice. Acrylic-Pin Splints
Acrylic-pin splints are external fixators in which the connecting clamps and rods have been replaced by acrylic columns. They are particularly useful for mandibular fractures and trans articular application because the acrylic connecting columns are easily contoured to the shape of the body (Fig. 6). Commercial kits for the application of acrylicpin splints in human orthopedics have been popular for many years because of their ability to provide maximum support of difficult fractures with minimal cost and ease of application. 13 The use of these human devices for small animal fracture treatment has been reported,14 but authors increasingly describe the use of simple "homemade" splints as
Figure 6. Acrylic·pin fixators applied to bilateral manibular fractures (A) and tarsal arthro· desis (8).
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Figure 7. Acrylic pin external fixation (APEF) System kit creates fixators comparable to small (A) or medium (8) Kirschner frames.
being more economical and appropriate in size for veterinary use. 22 Homemade splints, however, require procurement of diverse supplies and equipment that may be difficult to obtain, may be messy to apply, and may not yield optimal results. Consequently Gauthier Medical, Inc. is introducing the "Acrylic-pin external fixation" (APEF) system (Fig. 7). The APEF system consists of three kits. The medium materials kit contains the special fixation pins, prepackaged acrylic, and sterilized acrylic column molding tubes required for construction of two modified type II fixators comparable in size and strength to the medium Kirschner apparatus. The small materials kit contains similar materials comparable in size to the small Kirschner apparatus. The application kit contains a reusable temporary alignment frame used in the construction of both sizes of fixators. Individual APEF components are also available. The APEF system is usually applied with a two-stage technique (Fig. 8). In the first (sterile) stage, special fixation pins (a combination of enhanced thread pins for optimal bone-pin integrity and nonthreaded pins for economy) are inserted in the bone fragments and the acrylic column molding tubes are "impaled" on the pin ends. The tubes are positioned parallel to the limb about V2 to 1 inch from the skin. A temporary alignment frame (from the application kit) consisting of mechanical clamps and rods is then added. The fracture is reduced by either closed manipulation or via a limited open approach, and the
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temporary frame is locked. The surgical incision is closed, and any open wounds are bandaged. Radiologic evaluation may be employed at this time to confirm adequate fracture reduction. In the second (nonsterile) stage, one end of each molding tube is plugged. The acrylic is then mixed, introduced into the open end of each tube, and allowed to cure (6 to 10 minutes). Finally the temporary alignment frame and excess pin length is removed. Readjustment of a completed APEF (Fig. 9) requires removing a short segment of the acrylic column with a hack saw or cast cutter. The plastic molding tube is peeled back from each end and several holes drilled in the remaining acrylic to provide a base for the patch. A small amount of new acrylic is mixed and molded by hand to fill the gap and overlap the existing column ends. The fracture is then manipulated into correct reduction and held while the acrylic cures. APEF removal (Fig. 10) is achieved by cutting each fixation pin between the skin and acrylic column. Each pin is then removed using a hand chuck or pliers. Homemade acrylic-pin splints are similarly constructed using polymethyl methacrylate, which is available from Jorgensen Laboratories (see Table 1) as hoof repair acrylic. The acrylic can be poured into molds made of thin-walled tubing or molded by hand around the pins once the acrylic cures to dough stage. Mechanical testing suggests a column diameter of %" provides superior fixation stability to a medium Kirschner apparatus. 25
Figure 8. Two-stage application of APEF.'A, Special fixation pins are implanted in the bone fragments. B, Acrylic column molding tubes are impaled on the ends of the tubes and positioned about .5 to 1 inch from the skin parallel to the bone. Illustration continued on following page
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Figure 8 (Continued). C, A temporary alignment frame is added to the end of the pins outside the molding tubes. 0, The fracture is reduced and the alignment frame locked. Approach incisions are closed. Fracture reduction may be radiographically confirmed if closed reduction was undertaken. E, One end of each molding tube is plugged.
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Figure 8 (Continued). F, Acrylic is mixed in prepackaged mixing and dispensing pouch . G, One end of mixing pouch is cut off, and acrylic is squeezed into molding tubes. H, After the acrylic cures, the temporary alignment frame is removed and excess pin length is cut off.
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Figure 9. Readjustment of completed APEF. A. A short segment of column over the fracture is removed with a hack saw. B, Plastic molding tube is peeled back from acrylic ends, and several holes are drilled to provide a good base. C, New acrylic is mixed and hand-molded overlapping the column ends and filling the gap. The fracture is manipulated and held in reduction until the acrylic cures.
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Figure 10. APEF removal. A. Individual pins are cut between the column and skin. B. Pin tips are individually removed.
Circular Fixators
Circular (ring) fixators use small-diameter, flexible Kirschner wires instead of rigid fixation pins. 13 These wires are driven through the bone fragments at perpendicular angles and attached under tension to rigid rings. The rings are then connected together with adjustable longitudinal rods. Circular fixators were developed by Ilizarov in the 1940s and have been used extensively in Eastern Europe. 13 They have only recently been introduced to North America, however. Although the device was originally designed for fracture fixation, its complex application will probably limit its use for this purpose. Circular fixators probably have more potential use for progressive limb lengthening and deformity correction in the treattnent of premature physeal closures. Reports of veterinary application are beginning to appear in the literatureY Jorgensen Laboratories is marketing a sophisticated circular fixator
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Figure 11. Circular fixator being marketed by Jorgensen Laborato· ries. A, Appearance of device on forelimb, B, contents of kit. (Cour· tesy of Jorgensen Laboratories, Loveland, CO.)
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designed for small animal application (Fig. 11). We have had limited experience treating premature radial physeal closures using a homemade device (Fig. 12). The apparatus was constructed of dural aluminum rings (supplied by Dr. Ferretti of Italy [see Table 1]), W' staiI1less steel stock and bolts (hardware store variety) for the connecting rods and fixation wire connectors, and 0.062 Kirschner wires for fixation pins. Human External Fixation Systems
Many external fixation systems designed for human application can be used for selected veterinary applications, 18 particularly those models designed for forearm fractures. Some of these fixators (see Table 1) (Fig. 13) offer elegant features such as improved adjustability, innovative tapered fixation pin design, and even the capacity to allow dynamic axial fracture compression to stimulate fracture healing. 8, 10 Unfortunately the inherent complexity and versatility generally result in prohibitive retail cost. Such devices are usually considered disposable and not reused on human patients; therefore contacting a local orthopedic surgeon or hospital may result in an abundant supply of "free" equipment. Modifying the principles of external fixator application presented elsewhere in this issue to the particular fixator should allow its use with excellent results.
,.
Figure 12. Homemade circular fix- • ator using custom made dural aluminum rings, threaded bar stock, and Kirschner wires.
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Figure 13. Currently popular human external fixators: A, ASIF small external fixator, B, Orthofix dynamic axial fixator.
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APPLICATION INSTRUMENTATION
One of the major economic advantages of using external fixation for fracture management is the minimal amount of equipment specifically needed for its application. Indeed fixators can be applied with little more than general surgical instruments, a means of driving pins and a pin cutter. A limited amount of selected orthopedic instrumentation, however, will greatly simplify and s}3eed the process. Fixation Instrumentation
Some means for inserting fixation pins into bone is required. For many years, only hand chuck placement was recommended because of concern about thermal necrosis associated with power insertion. 6 Pin chuck or hand drill placement into mature canine cortical bone is difficult, however. These types of placement commonly cause wobbling, which results in oversized holes and poor bone purchase by the fixation pins. More recent research has shown that low-speed power insertion (150 rpm) will actually improve resistance to acute pin pullout and is not deleterious to long-term pin purchase in canine diaphyseal bone.11, 15 A standard %" variable speed corded drill can be used if ethylene oxide sterilization is available, H. W. Andersen Products, Inc. (see Table 1) produces the relatively inexpensive Anoprolene line of gas sterilizers. Alternatively many community hospitals will gas sterilize equipment for a nominal charge. Rechargeable electric drills (hardware store variety) have the advantages of lacking the cord and being limited to slower speeds. The drills made by Makita (see Table 1) are generally more expensive but appear to be the most durable. Rechargeable drills can be gas sterilized or used with an autoclavable shroud and extended chuck (Fig. 14) available from Animal Clinic Products (see Table 1) or Kirschner. Many autoclavable nitrogen-driven drills designed for surgical use are available. They tend to be expensive, however, and often have inadequate torque for pin insertion except at high speed. For extremely dense bone such as the olecranon or calcaneus, predrilling a pilot hole before pin insertion may be advantageous. 2 A twist bit slightly smaller in diameter ('l'64" for Vs" fixation pins) is used to cut the pilot hole. This results in a slight radial pre loading of the bone when the pin is inserted. 16 Hand chuck or low-speed power is then used to insert the pin. Standard drill guides available from Kirschner or Synthes are useful for protecting soft tissues from the twist bit during the drilling process (Fig. 15). A tightening wrench is necessary for all mechanical fixators. The Kirschner apparatus requires %" for small, 5/16" for medium, and 'i'16" for large diameter wrenches for tightening single and double connecting clamps. It is convenient to have both open-ended bar and socket wrenches available for access and speed of use (Fig. 16). Chrome-plated models are available from Kirschner that can be autoclaved, although inexpensive steel wrenches can be used if gas sterilized to avoid rusting.
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Figure 14. Autoclavable extended chuck and shroud (A) allows the use of inexpensive rechargeable electric drills. Nonsterile drill is dropped into sterile shroud by assistant (8). Sterile chuck extension is screwed into drill through shroud aperture (C).
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Figure 15. Drill guides can be used to protect soft tissues when using a twist bit to predrill fixation pin pilot holes.
Figure 16. Open-end bar wrench and socket wrench used to tighten nuts of Kirschner clamps.
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A large pin cutter will be needed to remove excess fixation pin length and shorten connecting rods . Although chrome-plated, autoclavable, surgical models are available (Kirschner), inexpensive large bolt cutters can be used if gas sterilized. Recently the transarticular application of external fixation has been described for periarticular fracture treatment, arthrodesis, and extended support of tendon and ligament repairs .5 Bending the connecting rods will simplify the device, allow the use of more fixation pins, and permit joint stabilization at a normal walking angle (Fig. 17). 24 In extremely unstable distal femoral fractures, an additional connecting rod can be arched around the front of the limb from a proximal half pin to a distally placed full pin (Fig. 18). The resulting triangular frame appears to be more resistant to axial compressive forces and has little quadriceps muscle impingement. A plate bending press (Synthes)
Figure 17. Contoured connecting rods simplifies transarticular application of the Kirschner apparatus.
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Figure 18. Attaching a lateral proximal half-pin to the medial end of a distal full pin with a curved connecting rod appears to increase resistance to axial compressive forces.
or hand-held plate bending irons (Kirschner) simplify the bending process (Fig. 19). Alternatively two Jacob hand chucks can also be used. General Orthopedic Instrumentation and Equipment
A sharp periosteal elevator is extremely useful for developing fracture exposure and levering fracture fragments during open approaches (Fig. 20). Bone holding forceps, such as Kerns (Kirschner and Jorgense~), are used to grasp and manipulate the major fragments. Serrated and pointed reduction forceps (Synthes and Jorgensen) are used to manipulate and hold the fracture in reduction (Fig. 21). Spoon type bone curettes (Kirschner) are useful for collecting autogenous cancellous bone grafts after creating a cortical defect with a slightly larger Steinmann pin (Fig. 22). Ancillary fragment fixation can be accomplished, where appropriate, with cerclage wires, interfragmentary lag screws, or multiple divergent Kirschner wires placed in cross pin
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Figure 19. Bending a connecting rod with a pair of plate·bending irons.
Figure 20. Periosteal elevators useful for developing exposure and levering fracture fragments into reduction during open approaches.
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B Figure 21. Bone clamps useful for fragment manipulation and temporary stabilization. A, Baby Kerns, B, serrated reduction forceps, C, point-reduction forceps.
Figure 22. Spoon-type currettes used for collecting autogenous cancellous bone grafts. A no. % for small patients, B no. 5 for medium and large.
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fashion. The latter requires only a small pin cutter and results in the least tissue damage during application. SUMMARY
External skeletal fixation is a very useful technique for managing many orthopedic problems in veterinary pr~ctice. 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. References 1. Anderson R: AI) automatic method of treatment for fractures of the tibia and fibula.
Surg Gynecol Obstet 58:639, 1934 2. Aron DN, Toombs JP: Updated principles of external skeletal fixation. Comp Cont Ed Pract Vet 6:845, 1984 3. Aron DN, Toombs JP, Hollingsworth SC: Primary treatment of severe fractures by external skeletal fixation: Threaded pins compared with smooth pins. J Am Anim Hosp Assoc 22:659, 1986 4. Bennett RA, Egger EL, Histand MB, et al: Comparison of the strength and holding power of 4 pin designs for use with half-pin (type I) external skeletal fixation . Vet Surg 16:207, 1987 5. Bjorling DE, Toombs JP: Transarticular application of the Kirschner-Ehmer splint. Vet Surg 11:34, 1982 6. Brinker WO, Flo GL: Principles and application of external skeletal fixation . Vet Clin North Am Small Anim Pract 2:197, 1975 7. Brinker WO, Verstraete MC, Soutas-Little RW: Stiffness studies on various configurations and types of external fixators . J Am Anim Hosp Assoc 21:801, 1985
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8. De Bastiani G, Aldegheri R, Brivio LR: The treatment of fractures with a dynamic axial fixator. J Bone Joint Surg (Br) 66:538, 1984 9. Egger EL: Static strength evaluation of six external skeletal fixation configurations. Vet Surg 12:130, 1986 10. Egger EL, Gottsauner-Wolf F, Aro HT, et al: Effect of dynamization after one week on healing of a delayed union model. Trans Orthop Res Soc 16:142, 1991 11. Egger EL, Histand MB, Blass CB, et al: Effect of fixation pin insertion on the bonepin interface. Vet Surg 15:246, 1986 12. Ehmer EA: Bone pinning in fractures of small animals. I Am Vet Med Assoc 110:14, 1946 13. Green SA: History of external fixation. In: Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 59 14. Greenwood KM, Creagh GB: Biphase external skeletal splint fixation of mandibular fractures in dogs. Vet Surg 9:128, 1980 15. Gumbs J, Brinker WO: Comparison of acute and chronic pull out resistance of pins used for external splint fixation . Trans Vet Orthop Soc 13:18, 1986 16. Hyldahl C, Pearson S, Tepic S, et al: Induction and prevention of pin loosening in external fixators in the sheep tibia. Orthop Trans 12:378, 1988 17. Latte Y: Treatment of radius curvus by Ilizarov apparatus. Trans Vet Orthop Soc 16:10, 1989 18. Olds R, Green SA: Hoffmann's external fixation for arthrodesis and infected non unions in the dog. J Am Anim Assoc 19:705, 1983 19. Palmer RH, Aron DN: EIlis pin complications in seven dogs. Vet Surg 19:440, 1990 20. Prinz H, Blomer A, Echterhoff M: Pin-track infection. In Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 149 21. Shearer J, Egan J: Computerized analysis of pin geometry. In Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 129 22. Staumbaugh JE, Nunmaker DM: External skeletal fixation of comminuted maxillary fractures in dogs. Vet Surg 2:72, 1982 23. Tomlinson JL, Constantinescu GM: Acrylic external skeletal fixation of fractures. Comp Cont Ed Pract Vet 13:235, 1991 24. Toombs JP, Aron DN, Basinger RB: Angled connecting bars for transarticular application of Kirschner-Ehmer external fixation splints. J Am Anim Hosp Assoc 25:213, 1989 25. Willer RL, Egger EL, Histand MB: A comparison of stainless steel versus acrylic for the connecting bar of external skeletal fixators. J Am Anim Hosp Assoc Oct-Nov, 1991
Address reprint requests to Erick L. Egger, DVM Veterinary Teaching Hospital Colorado State University College of Veterinary Medicine and Biomedical Sciences Fort Collins, CO 80523
0195-5616/92 $0.00 + .20
INSTRUMENTATION FOR EXTERNAL FIXATION Erick L. Egger, DVM
Instrumentation associated with the use of external fixation can be divided into two groups: fixation devices and the equipment needed or useful for fixator application. Throughout this article, generic, descriptive nomenclature is used except where reference is made to a specific commercial product. The source of these products and their approximate cost are given in Table 1. EXTERNAL FIXATION DEVICES
Many external fixation devices have been designed and manufactured over the last 90 years.!3 This discussion, however, concentrates on those devices and implants currently being used for small animal veterinary application. Kirschner Apparatus
In North America, the Kirschner apparatus (see Table 1) is the most commonly used fixator in veterinary practice. It is based on the Roger Anderson splint, which was designed in 1934 for treatment of distal extremity fractures in humans.! In 1946, Dr. E. A. Ehmer modified the size and material of the design slightly, had it manufactured by the Kirschner machine shop, and introduced it as the Kirschner-Ehmer Spliht.!2 Over the years, the fixator evolved into the Kirschner apparaFrom the Department of Clinical Sciences, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Fort CoIlins, Colorado
VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 1 • JANUARY 1992
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Table 1. SOURCES OF INSTRUMENTATION AND EQUIPMENT Material
Approximate Catalog Cost
Kirschner Medical (Orthopaedic Division, 10 W. Aylesbury Rd., Timonium, MD 21093) Single connecting clamps $12-18 Double connecting clamps $24-30 Connecting rods $2-8 Steinmann pins $3-7 Kirschner wires $3 Ellis pins $5-6 Power drill adapter and linen shroud $130 Drill guide $55 Bar wrenches $10 Socket wrench $40 Large pin cutter $350 Hand bending irons $50 Kern bone clamp $145-180 $50 Curets Gauthier Medical, Inc. (3105 N.w. 22nd SI. , Rochester, MN 55901)
Enhanced thread half-pin or full-pin Acrylic-pin external fixation system (APEF) Medium materials kit (for 2 fixators) Small materials kit (for 2 fixators) Application kit (for both small and medium APEF construction)
$11 $150 (estimate) $140 (estimate) $150 (estimate)
IMEX Veterinary, Inc. (1227 Market SI., Longview, TX 75604)
IMEX interface (half-pin) IMEX centerface (full-pin)
$6-8 $8-11
Jorgensen Laboratories (1450 N. Van Buren Ave., Loveland, CO 80538)
Technovit (methyl methacrylate) 750 g Circular fixator (llizarov-like) Kern bone clamp Pointed fracture reduction forceps
$32 $1200 $118 $84
Dr. Antonio Ferretti (V. Petrarca 36, 20015, Parabiago, Italy)
Dural aluminum rings
Custom made
Synthes (1690 Russell Rd., PO Box 1766, Paoli, PA 19301)
Small external fixation set Drill guide Plate bending press Plate bending irons Serrated fragment reduction forceps
$3680 $55-90 $756 $180 $240-280
EBI Medical Systems, Inc. (PO Box 682, 300 Fairfield, NJ 07007) Orthofix, fixator and application kit
$2000-4000
H. W. Andersen Products, Inc. (221 South SI. , Oyster Bay, NY 11771)
Anprolene sterilizer (model AN-72) cost of gas per cycle
$600 $5
Makita Electric Works, LTD. (Anjo, Aichi, Japan) Makita rechargeable drills
$55-90
Animal Clinic Products (3505 McGehee Rd., Montgomery, AL 36111)
Extend-a-chuck (drill extension) and shroud
$135
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Figure 1. Small (A), medium (8), and large (C) size Kirschner apparatus.
tus, a device that is currently produced in four sizes. Three of the sizes are useful for small animal application (Fig. 1). The small device is used on cats and small dogs weighing less than 10 to 12 kg and depending on bone size. The medium-sized device is used on most medium-sized and large dogs. The large device is based on Kirschner's human tibial frame using large threaded fixation pins and can be used on giant breed dogs. The "heart" of the Kirschner apparatus is the single connecting clamp that is used to affix the fixation pins to the connecting rods. The single connecting clamp consists of a U piece (through which the connecting rod passes), bolt (through which the fixation pin passes), and nut (Fig. 2). All nuts, bolts, and the Us, of the small and medium-
Figure 2. Exploded single connecting clamp: (A) "U", (8) bolt, (C) nut.
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sized clamps are made of stainless steel, whereas the Us of the large clamps are anodized aluminum. Tightening the nut compresses the fixation pin to the U and squeezes it around the connecting rod, thus locking both simultaneously. This design allows the use of fixation pins the same diameter size as or smaller than the hole in the bolt (%4" for small, Va" for medium, 3/16" for large). Consequently a wide variety of fixation pin diameters can be used with just two or three sizes of clamps. Fixation pin diameter selection is determined by the patient's size (generally a pin diameter of about 20% of the bone diameter is recommended). For example, in fixing a tibial fracture of a I8-Kg dog, %2" fixation pins through medium clamps might be appropriate. The U design, however, requires a specific connection rod diameter for each clamp size (Vs" for small, 3116" for medium, 7/16" for large). Stainless steel connecting clamps are quite durable and can routinely be reused many times. Individual clamp replacement parts can also be ordered. Double connecting clamps are used to attach connecting rod to rod
Figure 3. Double connecting clamp (arrows) used to attach two connecting rods, used in pairs to create a highly adjustable frame.
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(Fig. 3). Using two double connecting clamps together allows universal adjustment of fixator alignment. They are, therefore, used in construction of fixator configurations that allow manipulation of reduction after the entire device has been placed. Double connecting clamps, however, are relatively expensive and heavy, and their mechanical stability is limited. 9 Consequently their use is generally avoided or limited to rapidly healing, stable situations. The small and medium Kirschner connecting rods are made of standard stainless steel stock, whereas the large rods are aluminum. Steinmann pins can be used as connecting rod, but this is actually more expensive, since the process of grinding trocar tips on the pins is a major part of their cost. Recently there has been research in fixation pin design. 20, 21 In general, Steinmann pins cut in half are used for non threaded fixation pins. Kirschner wires can likewise be used on very small patients. Although partially threaded (thread is cut into the pin shaft) Steinmann pins provide greater resistance to pin pull out,4 the stress collector effect at the threaded-non threaded junction has often been the location of pin bending or breakage. One solution is threading the pin only a short distance (about three times the diameter) from the tip so the pin will thread only into the far cortex, thus protecting the weak stress collector in the medullary canal (Fig. 4).4 Kirschner (see Table 1) is marketing this design as the Ellis pin. Some complications with the Ellis pins, however, have still been reported. 19 Fixation pins with threads extending out from a constant diameter shaft are not significantly weakened and have been used in human fixators for many years. 13 These human products, however, are usually too large and
Figure 4. Standard partially threaded fixation pin (A) and Ellis fixation pin (8).
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Figure 5. Improved "raised thread" profile pins: (A) medium half pin, and (8) small, (C) medium, and (0) large full (centrally threaded) pins.
their cost prohibitive for routine veterinary usage. Recently two companies have begun producing both half and full pin designs (Fig. 5) in veterinary sizes at a reasonable cost. Gauthier Medical, Inc. (see Table 1) markets the Enhanced Thread Half and Full Pin designs, and IMEX Veterinary, Inc. (see Table 1) markets the 1M EX Interface (half-pin) and IMEX Centerface (full-pin) designs. Mechanical and in vivo studies of these pins are currently under way. Clinical experience has suggested using a combination of nonthreaded and threaded pins to minimize the postoperative complications of pin loosening and pin tract drainage while maintaining good fixation pin strength at a reasonable cost. 3 Kirschner sells their products as complete sets or individual components. The sets provide only enough clamps for four pins per device, however. The author, therefore, suggests buying individual components starting with about 12 single clamps. This will be enough to construct one complex or two simple frames. Based on technical ease, use of the medium-sized apparatus is indicated most often in veterinary practice. Practitioners can expand their inventory to include the small apparatus for cats and small dogs as they become more comfortable with application of the system. Use of the large apparatus is infrequent because the improved fixation pin thread designs are now available for the medium-sized apparatus, and very rigid configurations that used medium-sized components have evolved. 7, 9 Additionally the large clamps and connecting rods are made of aluminum and are not very durable, often requiring replacement after two or three uses. The Kirschner apparatus is not inexpensive, but it is relatively economical for use by veterinarians because a minimal inventory of component and specialized application equipment is necessary. Economy for the client comes from the reusability of the connecting clamps
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and most rods. To encourage return of these components, we charge the clients full replacement cost at the time of patient dismissal, then allow a 75% refund on return of reusable components. This leaves us with a 25% user fee and encourages owner compliance with the recheck schedule. The refund is usually offset by the cost of recheck radiographs, thus requiring no additional cash transaction for the owner or practice. Acrylic-Pin Splints
Acrylic-pin splints are external fixators in which the connecting clamps and rods have been replaced by acrylic columns. They are particularly useful for mandibular fractures and trans articular application because the acrylic connecting columns are easily contoured to the shape of the body (Fig. 6). Commercial kits for the application of acrylicpin splints in human orthopedics have been popular for many years because of their ability to provide maximum support of difficult fractures with minimal cost and ease of application. 13 The use of these human devices for small animal fracture treatment has been reported,14 but authors increasingly describe the use of simple "homemade" splints as
Figure 6. Acrylic·pin fixators applied to bilateral manibular fractures (A) and tarsal arthro· desis (8).
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Figure 7. Acrylic pin external fixation (APEF) System kit creates fixators comparable to small (A) or medium (8) Kirschner frames.
being more economical and appropriate in size for veterinary use. 22 Homemade splints, however, require procurement of diverse supplies and equipment that may be difficult to obtain, may be messy to apply, and may not yield optimal results. Consequently Gauthier Medical, Inc. is introducing the "Acrylic-pin external fixation" (APEF) system (Fig. 7). The APEF system consists of three kits. The medium materials kit contains the special fixation pins, prepackaged acrylic, and sterilized acrylic column molding tubes required for construction of two modified type II fixators comparable in size and strength to the medium Kirschner apparatus. The small materials kit contains similar materials comparable in size to the small Kirschner apparatus. The application kit contains a reusable temporary alignment frame used in the construction of both sizes of fixators. Individual APEF components are also available. The APEF system is usually applied with a two-stage technique (Fig. 8). In the first (sterile) stage, special fixation pins (a combination of enhanced thread pins for optimal bone-pin integrity and nonthreaded pins for economy) are inserted in the bone fragments and the acrylic column molding tubes are "impaled" on the pin ends. The tubes are positioned parallel to the limb about V2 to 1 inch from the skin. A temporary alignment frame (from the application kit) consisting of mechanical clamps and rods is then added. The fracture is reduced by either closed manipulation or via a limited open approach, and the
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temporary frame is locked. The surgical incision is closed, and any open wounds are bandaged. Radiologic evaluation may be employed at this time to confirm adequate fracture reduction. In the second (nonsterile) stage, one end of each molding tube is plugged. The acrylic is then mixed, introduced into the open end of each tube, and allowed to cure (6 to 10 minutes). Finally the temporary alignment frame and excess pin length is removed. Readjustment of a completed APEF (Fig. 9) requires removing a short segment of the acrylic column with a hack saw or cast cutter. The plastic molding tube is peeled back from each end and several holes drilled in the remaining acrylic to provide a base for the patch. A small amount of new acrylic is mixed and molded by hand to fill the gap and overlap the existing column ends. The fracture is then manipulated into correct reduction and held while the acrylic cures. APEF removal (Fig. 10) is achieved by cutting each fixation pin between the skin and acrylic column. Each pin is then removed using a hand chuck or pliers. Homemade acrylic-pin splints are similarly constructed using polymethyl methacrylate, which is available from Jorgensen Laboratories (see Table 1) as hoof repair acrylic. The acrylic can be poured into molds made of thin-walled tubing or molded by hand around the pins once the acrylic cures to dough stage. Mechanical testing suggests a column diameter of %" provides superior fixation stability to a medium Kirschner apparatus. 25
Figure 8. Two-stage application of APEF.'A, Special fixation pins are implanted in the bone fragments. B, Acrylic column molding tubes are impaled on the ends of the tubes and positioned about .5 to 1 inch from the skin parallel to the bone. Illustration continued on following page
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Figure 8 (Continued). C, A temporary alignment frame is added to the end of the pins outside the molding tubes. 0, The fracture is reduced and the alignment frame locked. Approach incisions are closed. Fracture reduction may be radiographically confirmed if closed reduction was undertaken. E, One end of each molding tube is plugged.
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Figure 8 (Continued). F, Acrylic is mixed in prepackaged mixing and dispensing pouch . G, One end of mixing pouch is cut off, and acrylic is squeezed into molding tubes. H, After the acrylic cures, the temporary alignment frame is removed and excess pin length is cut off.
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Figure 9. Readjustment of completed APEF. A. A short segment of column over the fracture is removed with a hack saw. B, Plastic molding tube is peeled back from acrylic ends, and several holes are drilled to provide a good base. C, New acrylic is mixed and hand-molded overlapping the column ends and filling the gap. The fracture is manipulated and held in reduction until the acrylic cures.
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Figure 10. APEF removal. A. Individual pins are cut between the column and skin. B. Pin tips are individually removed.
Circular Fixators
Circular (ring) fixators use small-diameter, flexible Kirschner wires instead of rigid fixation pins. 13 These wires are driven through the bone fragments at perpendicular angles and attached under tension to rigid rings. The rings are then connected together with adjustable longitudinal rods. Circular fixators were developed by Ilizarov in the 1940s and have been used extensively in Eastern Europe. 13 They have only recently been introduced to North America, however. Although the device was originally designed for fracture fixation, its complex application will probably limit its use for this purpose. Circular fixators probably have more potential use for progressive limb lengthening and deformity correction in the treattnent of premature physeal closures. Reports of veterinary application are beginning to appear in the literatureY Jorgensen Laboratories is marketing a sophisticated circular fixator
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Figure 11. Circular fixator being marketed by Jorgensen Laborato· ries. A, Appearance of device on forelimb, B, contents of kit. (Cour· tesy of Jorgensen Laboratories, Loveland, CO.)
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designed for small animal application (Fig. 11). We have had limited experience treating premature radial physeal closures using a homemade device (Fig. 12). The apparatus was constructed of dural aluminum rings (supplied by Dr. Ferretti of Italy [see Table 1]), W' staiI1less steel stock and bolts (hardware store variety) for the connecting rods and fixation wire connectors, and 0.062 Kirschner wires for fixation pins. Human External Fixation Systems
Many external fixation systems designed for human application can be used for selected veterinary applications, 18 particularly those models designed for forearm fractures. Some of these fixators (see Table 1) (Fig. 13) offer elegant features such as improved adjustability, innovative tapered fixation pin design, and even the capacity to allow dynamic axial fracture compression to stimulate fracture healing. 8, 10 Unfortunately the inherent complexity and versatility generally result in prohibitive retail cost. Such devices are usually considered disposable and not reused on human patients; therefore contacting a local orthopedic surgeon or hospital may result in an abundant supply of "free" equipment. Modifying the principles of external fixator application presented elsewhere in this issue to the particular fixator should allow its use with excellent results.
,.
Figure 12. Homemade circular fix- • ator using custom made dural aluminum rings, threaded bar stock, and Kirschner wires.
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Figure 13. Currently popular human external fixators: A, ASIF small external fixator, B, Orthofix dynamic axial fixator.
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APPLICATION INSTRUMENTATION
One of the major economic advantages of using external fixation for fracture management is the minimal amount of equipment specifically needed for its application. Indeed fixators can be applied with little more than general surgical instruments, a means of driving pins and a pin cutter. A limited amount of selected orthopedic instrumentation, however, will greatly simplify and s}3eed the process. Fixation Instrumentation
Some means for inserting fixation pins into bone is required. For many years, only hand chuck placement was recommended because of concern about thermal necrosis associated with power insertion. 6 Pin chuck or hand drill placement into mature canine cortical bone is difficult, however. These types of placement commonly cause wobbling, which results in oversized holes and poor bone purchase by the fixation pins. More recent research has shown that low-speed power insertion (150 rpm) will actually improve resistance to acute pin pullout and is not deleterious to long-term pin purchase in canine diaphyseal bone.11, 15 A standard %" variable speed corded drill can be used if ethylene oxide sterilization is available, H. W. Andersen Products, Inc. (see Table 1) produces the relatively inexpensive Anoprolene line of gas sterilizers. Alternatively many community hospitals will gas sterilize equipment for a nominal charge. Rechargeable electric drills (hardware store variety) have the advantages of lacking the cord and being limited to slower speeds. The drills made by Makita (see Table 1) are generally more expensive but appear to be the most durable. Rechargeable drills can be gas sterilized or used with an autoclavable shroud and extended chuck (Fig. 14) available from Animal Clinic Products (see Table 1) or Kirschner. Many autoclavable nitrogen-driven drills designed for surgical use are available. They tend to be expensive, however, and often have inadequate torque for pin insertion except at high speed. For extremely dense bone such as the olecranon or calcaneus, predrilling a pilot hole before pin insertion may be advantageous. 2 A twist bit slightly smaller in diameter ('l'64" for Vs" fixation pins) is used to cut the pilot hole. This results in a slight radial pre loading of the bone when the pin is inserted. 16 Hand chuck or low-speed power is then used to insert the pin. Standard drill guides available from Kirschner or Synthes are useful for protecting soft tissues from the twist bit during the drilling process (Fig. 15). A tightening wrench is necessary for all mechanical fixators. The Kirschner apparatus requires %" for small, 5/16" for medium, and 'i'16" for large diameter wrenches for tightening single and double connecting clamps. It is convenient to have both open-ended bar and socket wrenches available for access and speed of use (Fig. 16). Chrome-plated models are available from Kirschner that can be autoclaved, although inexpensive steel wrenches can be used if gas sterilized to avoid rusting.
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Figure 14. Autoclavable extended chuck and shroud (A) allows the use of inexpensive rechargeable electric drills. Nonsterile drill is dropped into sterile shroud by assistant (8). Sterile chuck extension is screwed into drill through shroud aperture (C).
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Figure 15. Drill guides can be used to protect soft tissues when using a twist bit to predrill fixation pin pilot holes.
Figure 16. Open-end bar wrench and socket wrench used to tighten nuts of Kirschner clamps.
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A large pin cutter will be needed to remove excess fixation pin length and shorten connecting rods . Although chrome-plated, autoclavable, surgical models are available (Kirschner), inexpensive large bolt cutters can be used if gas sterilized. Recently the transarticular application of external fixation has been described for periarticular fracture treatment, arthrodesis, and extended support of tendon and ligament repairs .5 Bending the connecting rods will simplify the device, allow the use of more fixation pins, and permit joint stabilization at a normal walking angle (Fig. 17). 24 In extremely unstable distal femoral fractures, an additional connecting rod can be arched around the front of the limb from a proximal half pin to a distally placed full pin (Fig. 18). The resulting triangular frame appears to be more resistant to axial compressive forces and has little quadriceps muscle impingement. A plate bending press (Synthes)
Figure 17. Contoured connecting rods simplifies transarticular application of the Kirschner apparatus.
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Figure 18. Attaching a lateral proximal half-pin to the medial end of a distal full pin with a curved connecting rod appears to increase resistance to axial compressive forces.
or hand-held plate bending irons (Kirschner) simplify the bending process (Fig. 19). Alternatively two Jacob hand chucks can also be used. General Orthopedic Instrumentation and Equipment
A sharp periosteal elevator is extremely useful for developing fracture exposure and levering fracture fragments during open approaches (Fig. 20). Bone holding forceps, such as Kerns (Kirschner and Jorgense~), are used to grasp and manipulate the major fragments. Serrated and pointed reduction forceps (Synthes and Jorgensen) are used to manipulate and hold the fracture in reduction (Fig. 21). Spoon type bone curettes (Kirschner) are useful for collecting autogenous cancellous bone grafts after creating a cortical defect with a slightly larger Steinmann pin (Fig. 22). Ancillary fragment fixation can be accomplished, where appropriate, with cerclage wires, interfragmentary lag screws, or multiple divergent Kirschner wires placed in cross pin
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Figure 19. Bending a connecting rod with a pair of plate·bending irons.
Figure 20. Periosteal elevators useful for developing exposure and levering fracture fragments into reduction during open approaches.
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B Figure 21. Bone clamps useful for fragment manipulation and temporary stabilization. A, Baby Kerns, B, serrated reduction forceps, C, point-reduction forceps.
Figure 22. Spoon-type currettes used for collecting autogenous cancellous bone grafts. A no. % for small patients, B no. 5 for medium and large.
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fashion. The latter requires only a small pin cutter and results in the least tissue damage during application. SUMMARY
External skeletal fixation is a very useful technique for managing many orthopedic problems in veterinary pr~ctice. 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. References 1. Anderson R: AI) automatic method of treatment for fractures of the tibia and fibula.
Surg Gynecol Obstet 58:639, 1934 2. Aron DN, Toombs JP: Updated principles of external skeletal fixation. Comp Cont Ed Pract Vet 6:845, 1984 3. Aron DN, Toombs JP, Hollingsworth SC: Primary treatment of severe fractures by external skeletal fixation: Threaded pins compared with smooth pins. J Am Anim Hosp Assoc 22:659, 1986 4. Bennett RA, Egger EL, Histand MB, et al: Comparison of the strength and holding power of 4 pin designs for use with half-pin (type I) external skeletal fixation . Vet Surg 16:207, 1987 5. Bjorling DE, Toombs JP: Transarticular application of the Kirschner-Ehmer splint. Vet Surg 11:34, 1982 6. Brinker WO, Flo GL: Principles and application of external skeletal fixation . Vet Clin North Am Small Anim Pract 2:197, 1975 7. Brinker WO, Verstraete MC, Soutas-Little RW: Stiffness studies on various configurations and types of external fixators . J Am Anim Hosp Assoc 21:801, 1985
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8. De Bastiani G, Aldegheri R, Brivio LR: The treatment of fractures with a dynamic axial fixator. J Bone Joint Surg (Br) 66:538, 1984 9. Egger EL: Static strength evaluation of six external skeletal fixation configurations. Vet Surg 12:130, 1986 10. Egger EL, Gottsauner-Wolf F, Aro HT, et al: Effect of dynamization after one week on healing of a delayed union model. Trans Orthop Res Soc 16:142, 1991 11. Egger EL, Histand MB, Blass CB, et al: Effect of fixation pin insertion on the bonepin interface. Vet Surg 15:246, 1986 12. Ehmer EA: Bone pinning in fractures of small animals. I Am Vet Med Assoc 110:14, 1946 13. Green SA: History of external fixation. In: Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 59 14. Greenwood KM, Creagh GB: Biphase external skeletal splint fixation of mandibular fractures in dogs. Vet Surg 9:128, 1980 15. Gumbs J, Brinker WO: Comparison of acute and chronic pull out resistance of pins used for external splint fixation . Trans Vet Orthop Soc 13:18, 1986 16. Hyldahl C, Pearson S, Tepic S, et al: Induction and prevention of pin loosening in external fixators in the sheep tibia. Orthop Trans 12:378, 1988 17. Latte Y: Treatment of radius curvus by Ilizarov apparatus. Trans Vet Orthop Soc 16:10, 1989 18. Olds R, Green SA: Hoffmann's external fixation for arthrodesis and infected non unions in the dog. J Am Anim Assoc 19:705, 1983 19. Palmer RH, Aron DN: EIlis pin complications in seven dogs. Vet Surg 19:440, 1990 20. Prinz H, Blomer A, Echterhoff M: Pin-track infection. In Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 149 21. Shearer J, Egan J: Computerized analysis of pin geometry. In Coombs R, Green SA, Sarmiento A (eds): External Fixation and Functional Bracing. London, Orthotext, 1989, p 129 22. Staumbaugh JE, Nunmaker DM: External skeletal fixation of comminuted maxillary fractures in dogs. Vet Surg 2:72, 1982 23. Tomlinson JL, Constantinescu GM: Acrylic external skeletal fixation of fractures. Comp Cont Ed Pract Vet 13:235, 1991 24. Toombs JP, Aron DN, Basinger RB: Angled connecting bars for transarticular application of Kirschner-Ehmer external fixation splints. J Am Anim Hosp Assoc 25:213, 1989 25. Willer RL, Egger EL, Histand MB: A comparison of stainless steel versus acrylic for the connecting bar of external skeletal fixators. J Am Anim Hosp Assoc Oct-Nov, 1991
Address reprint requests to Erick L. Egger, DVM Veterinary Teaching Hospital Colorado State University College of Veterinary Medicine and Biomedical Sciences Fort Collins, CO 80523
