Use of Acrylic in Fixation of Fractures

James Tomlinson, DVM, Diplomate American College of Veterinary Surgeons

       The standard External fixator has limitations in its use because of transfixation pin size, weight of the device, and lack of adaptability of the device to certain fracture situations.  Acrylic external skeletal fixation was developed to treat fractures not amenable to the standard Ex Fix.  Most of the initial indications for use of acrylic external skeletal fixation involve small breeds of dogs, cats and birds. Advantages of use of acrylic external skeletal fixation are a strong but lightweight device, low cost, pin sizes suitable for small bones, and adaptability to varied fracture situations.  Acrylic external skeletal fixation was initially used by the author to stabilize distal radial-­ulnar fractures in small dogs.  Other fractures especially amenable to acrylic external skeletal fixation are mandibular and maxillary fractures, and wing and leg fractures in birds. However, other limb fractures can be repair if the acrylic column is large enough in diameter.  A number of recent articles have evaluated the strength of various size columns for use in larger animals.  In dogs over 10 kilograms, there probably is no major advantage of using acrylic external skeletal fixation over the standard Ex Fix apparatus for limb fractures other than less expense. 

Materials Needed

Most of the materials needed for application of an acrylic external skeletal fixation device are readily available and inexpensive.  Standard intramedullary pins or Ex Fix pins are used for the transfixation pins. The use of threaded pins will enhance the stability of the fixation device over time due to less pin loosening.  The appropriate size pin for the bone size must be used.   Kirschner wires (0.028, 0.035, 0.045, 0.062 inches in diameter) are commonly used for transfixation pins in smaller animals.  Acrylic fixation pins (threaded) are available from IMEX as are the larger diameter threaded pins.  The acrylic most commonly used is a form of methyl­methacrylate that is very strong but lightweight.  Four types of acrylic are available for creating the external connecting bar; Technovit (a hoof repair material); dental grade nonsterile methylmethacrylate(bought from a dental supply house); outdated surgical grade methylmethacrylate (get from a scrub nurse or orthopedic surgeon friend from a human hospital); and epoxy putty Oatey Epoxy Putty, Cleveland, OH).  Innovative Animal Products markets an acrylic Ex Fix system that comes with all the materials needed for application of this type of device.  The acrylic comes as a liquid and powder that must be mixed together (except the epoxy putty).  The acrylic should not be allowed to touch the patient's skin as it may burn the skin.  The time until the acrylic hardens is usually between 5-10 minutes.  The hardening time will vary with the type of acrylic used and the relative proportions of the liquid and powder components.  Any flexible tubing of the correct diameter can be used as a mold to contain the acrylic while it is hardening. Silastic Medical-Grade tubing (Dow Corning) comes in various diameters and is very flexible, and can be sterilized.  Tubing with an external diameter of 3/8 inch and internal diameter of 1/4 inch is the most common size used for small bones.  For larger dogs, larger diameter tubing is required.  Syringe cases and anesthesia machine gas hoses work well.  Research has shown that 3/4" and 2 cm diameter acrylic columns are stronger than a 3/16" metal bar.  The acrylic is injected into the tubing with a syringe.  A 2-ounce catheter syringe works well for injecting the acrylic because the syringe tip correctly fits the 1/4 inch internal diameter of the tubing and sufficient pressure can be generated by the syringe to fill the tube.  As an alternative to the use of tubing, the acrylic is allowed to become doughy and then molded by hand to bent over fixation pins.  Epoxy putty, in particular, is used free form with no tubing.  The epoxy putty is mixed and then formed around the pins in the shape and diameter needed.

General Application

The general principles of application of a standard external skeletal fixator apply to the application of an acrylic external skeletal fixation device.  The fracture can be aligned by either closed or open reduction.  Half pins inserted on one side of the limb are connected to form type I devices.  Full pins are connected on both sides of the limb to form a type II device.  The distal and proximal most pins are inserted first. If silastic tubing is used as a mold, the second pin (and subsequent pins) should be driven through the tubing before being driven into the bone.  Driving the pins through the tube before driving the pins in the limb necessitates that the tubing be sterile.  If possible, a minimum of three pins is inserted on either side of the fracture line.  The tubing should be positioned far enough from the skin to prevent skin erosion and to allow a pin cutter to be slipped under the bar for pin cutting.  A distance of 5-10 millimeters between the skin and acrylic bar is usually satisfactory.  Once the fracture is reduced and the pins are all inserted, the acrylic is mixed, loaded into the syringe and injected into the tubing.  The fracture must be held in reduction until the acrylic has hardened.  As an alternative to the use of the tubing, the pins can be bent over so that they overlap each other to form a bridgework to mold the acrylic around.  The acrylic is mixed up, stirred until it becomes doughy and molded over the pins.

Radial-Ulnar Fractures in Small Dogs

Fractures of the distal radius and ulna of small dogs are common and have a high incidence of delayed and nonunion healing.  Casting and pinning are not acceptable methods of stabilization of this fracture and often lead to delayed and nonunion healing complications. Plating of these fractures is the best method of repair and the author's first choice for treating these fractures, but most veterinarians do not have the equipment or the training necessary to use this technique.  Some fractures of the distal radius and ulna are too distal to allow proper plating techniques.  Economic considerations by the owner do not allow the use of plating in some cases.  Acrylic external skeletal fixation has proven to be a reliable method of repair even for the most distal of these fractures.  Reduction of fractures of the distal radius and ulna can be accomplished either closed or open.  A type II external skeletal fixator is applied to this fracture.  Because these fractures are so distal, it generally is possible to place only two K-wires in the distal fragment.  With the fracture held in reduction, the silastic tubing is placed over the K-wires on both sides of the fracture.  If the distal fragment is too short to allow the placement of two full pins, the distal most pin is placed as a full pin and a half pin is driven from both the medial and lateral sides of the limb. The proximal fragment pins are inserted as full pins.  Once all the K-wires are driven, the acrylic is mixed, injected and allowed to harden with the fracture held in reduction.  If an open reduction is performed, a cancellous bone graft is placed and the soft tissue closed prior to mixing the acrylic.

Mandibular Fractures

Unilateral and bilateral fractures of the mandible, whether simple or comminuted, can be treated with acrylic external skeletal fixation.  Advantages of use of external skeletal fixation for repair of mandibular fractures include no metal at the fracture line to potentiate infection and adaptability to varied fracture configurations.  Proper occlusion of the teeth is paramount in repair of mandibular fractures.  When driving the mandibular pins, care must be taken to miss the roots of the teeth.  Bilateral fractures of the mandible are particularly suited to this type of fracture fixation.

Maxillary Fractures

Unstable maxillary fractures which involve the maxilla, nasal bones, and hard palate are not common fractures in dogs but have presented great difficulty for providing stability until the advent of acrylic external skeletal fixation.  Any size dog can be treated with this type of fixation.  The one prerequisite for use of this type of fixation is that enough maxilla is present caudal to the fracture line to allow placement of the fixation pins.  As with mandibular fractures, the teeth must interdigitate properly to ensure normal mastication.

Other Applications

Acrylic external fixation can be applied for stabilization of any fracture type that is amenable to a standard Ex Fix.  As long as the proper size fixation pins and acrylic bar are used, a satisfactory rate of fracture healing will occur.  For long bone fractures in larger dogs, acrylic Ex Fix does not provide any advantages over the standard metal Ex Fix except of being less expensive to apply.

Cold Acrylic Repair

Another method of stabilizing mandibular or maxillary fractures using an acrylic is available to veterinarians.  This method entails the use of a "cold" acrylic that is molded around the teeth and intraoral wires to hold the fracture in reduction.  Three "cold" acrylics are available for use; Jet Repair Acrylic (Jet Repair Acrylic, Lang Dental Mfg Co., Wheeling, IL 60090), Maxi-Temp (Henry Schein, Port Washington, NY) and Pro Temp Garant (ESPE America, Inc, Norristown, PA).  Cold acrylics produce very little heat during the hardening process compared to the standard acrylics (methyl­methacrylate) that become very hot.  The cold acrylics can be applied in direct contact with the mucosa without burning the tissue.  Jet Repair Acrylic hardens in 6-8 minutes and is extremely strong for its weight and volume.  Another advantage to the use of a cold acrylic is that it will mold to various shapes.  The other two products harden in a similar time frame. These materials can be used for unilateral or bilateral fractures and there use is only limited by your imagination. 

The fracture is reduced and wired in place with either interdental or intra-oral wiring techniques.  Other fixation devices can also be used to stabilize the fracture.  Jet Repair Acrylic is supplied as a powder and a liquid catalyst.  The powder is applied to the affected area and the liquid dripped on it with an eye-dropper that is included in the kit.  A wax mold can be placed around the area of interest to contain the acrylic powder until the liquid is applied.  The author personally finds that the wax molds are not needed.  The acrylic is molded around the teeth and interdental or intraoral wires with a cotton-tipped swab.  The cotton-tipped swab is also used to smooth the edges so that sharp edges are not left that would cut or irritate the mucosa.  The acrylic is applied in layers until thick enough to support the fracture.  Multiple thin layers are applied to minimize the heat production from the acrylic.  The acrylic takes about 6-8 minutes to set up to a hard consistency.  Care must be taken to not apply an excessive amount so that the teeth can still interdigitate somewhat so that the mouth can close properly.  The teeth should be cleaned before applying the acrylic.  The acrylic will not directly bond to the teeth unless the teeth are acid etched with phosphoric acid gel (40%) (Etch Gel, Henry Schein Inc., Post Washington, NY  11050).  The phosphoric acid gel is left in contact with the teeth for 2-3 minutes and then rinsed off.  It is not necessary in all cases to acid etch the acrylic to the teeth.  If the acrylic is acid etched to the teeth, it may require a dental bur to remove it from the teeth.  Over time, the acrylic will naturally separate from the teeth.  The acrylic may be cracked off the teeth also.  If acid etching is not used, the acrylic can be removed by cracking it off the orthopedic wire and from around the teeth.  The Maxi Temp and Pro Temp Garant are dispensed from a syringe gun (that comes with the kit) and thus does not require mixing of the components.  These products are applied to the area of interest and molded around the wires and teeth in the same manner as the Jet acrylic.

The acrylic splint is left on until the fracture heals.  Food may collect around the wires and acrylic, so the owner should be instructed to flush the mouth on a regular basis to help prevent bad mouth odor and infection.  As with all oral fractures, a soft diet is fed until the fracture is healed.

Wiring of Mandibular Fractures

Wiring of mandibular fractures is a practical method of repair that can be done by practitioners.  Two basic types of wiring will be covered:  interfragmentary wiring and interdental wiring.  Indications for use of wiring techniques include simple unilateral fractures, "butterfly" fractures, some bilateral fractures, and an adjunctive means of fracture fixation.

Correct wire type and size are important.  Only monofilament wire should be used.  The following is a guideline for the size of wire to use:  less than 6 pounds - 22 gauge wire, 6 to 25 pounds - 20 gauge wire, and animals over 25 pounds - 18 gauge wire.  The wire must be adequately tightened.  Loose wires lead to bone resorption, infection and wire breakage besides not providing the stability needed for the bone to heal.  The ends of the wires are bent over to make sure that they do not lacerate the soft tissue or gingiva in the area.  The combination of interdental and interfragmentary wiring will give added stability to the fracture.

Interfragmentary Wiring

Two basic methods of interfragmentary wiring of mandibular fractures can be performed:  intraoral and extraoral wiring.  Drill the holes for the wires with either Kirschner wires (K-wires) or drill bits.  The important thing is to not wrap up the gingiva or soft tissue.  Drill the holes so that they miss the roots of the teeth.  The holes should not be closer than 5 mm from the fracture edge to lessen the chance for the bone breaking.  Place the wire as perpendicu­lar to the fracture line as possible.

Intraoral interfragmentary wiring is most suited for the rostral half of the mouth because of the ease of access and room to work.  For this technique, the fracture is manually reduced.  The wire runs from the buccal side to the lingual side and back to the buccal side of the mandible where it will be tightened.  The wire can be passed under the mucosa but difficulty may be encountered.  The wire, if left on top, will cut through the mucosa, but this does not seem to cause a significant problem.

Interdental Wiring

Interdental wiring involves securing the wire around the base of the teeth just below the gingiva to stabilize the fracture.  This procedure depends on the teeth, so they must be stable and of adequate height and shape to keep the wire from slipping off.  Generally, premolar tooth 1 and sometimes 2 are not good teeth to use.  At least one solid tooth must be bridged on either side of the fracture line.  The technique works well for unilateral simple fractures, especially in the caudal half of the mandible and for bilateral fractures when used in combination with extraoral wiring or some other means of fixation.  The secret to using this technique is to pass the wire right under the gingiva on the bone cranial and caudal to the appropriate teeth to give maximum holding power.  The wire can be inserted directly through the gingiva from lateral to medial if it is stiff enough so that it does not bend.  The author generally will take a 20 gauge hypodermic needle and make a path for the wire to follow.  This allows for more accurate placement of the wire.  The wire is placed from the buccal side to the lingual side and then back to the buccal side of the mandible.  On the lingual and buccal side, the wire is placed on top of the gingiva.  It is possible in some cases to slide the wire under the mucosa on the lingual side, but it generally is difficult to do.  Once the wire is in place, the fracture is reduced and the wire is tightened.