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Principles of External Skeletal Fixation

Numerous methods of fracture fixation are available to the veterinary surgeon.

External skeletal fixation is an effective method of fracture repair, which has experienced a resurgence in popularity in the last few years. Several types of external skeletal fixation devices are commonly utilized, including the Kirshner-Ehmer apparatus and the Synthes and Hoffman external fixators. Many configurations and various modifications have been described for the application of external fixators. A review of apparatus design has resulted in a classification of external skeletal fixators into three types, each possessing separate attributes and indications for use. Type 1 for unilateral splints uses fixation pins, which are inserted through both bone cortices but penetrate only on the skin surface. Type 2 or bilateral splints use fixation pins, which are inserted through both cortices and both skin surfaces. Type 3 or biplanar splints are a combination of unilateral and bilateral splintage employed in a three-dimensional configuration.

An external skeletal fixator (ESF) can be used as the primary method of fracture fixation or can be used to enhance the stability provided by another primary fixation modality. External fixators may be used in a variety of clinical situations including simple fractures, open or compound fractures, delayed and non-unions, highly comminuted fractures, fractures in which there is extensive soft tissue damage, and infected fractures. They are also frequently recommended in cases requiring transarticular stabilization and for stabilization of corrective osteotomies. When used properly, application of an ESF results in a number of advantages over other techniques of fracture repair, including minimal disruption of soft tissues attachments to bone and minimal disturbance of the blood supply to the bone. When used in difficult fracture situations involving open or compound wounds, osteomylitis and/or extensive soft tissue injury, the contaminated fracture sites are not disturbed by the presence of the fixation device and dissemination of the contamination is minimized.

As with any surgical technique, the use of an ESF is not without some disadvantages or potential complications. Disadvantages of an ESF include delayed healing under certain condition, ideal reduction is not always possible, the fixation may fail in cases of osteoporosis, and the ESF may catch on an object thereby ruining the fixation. Complications associated with the use of an ESF include pin tract drainage and infection, pin loosening, pin breakage, iatrogenic fractures, damage to vessels and nerves, and disturbed muscle function due to pin placement. The disadvantages and complications must be taken into consideration when deciding whether to use external fixation. The majority of the disadvantages and complications associated with an ESF can be alleviated if the important principles of application are followed carefully.

Historically, the major limitation for the use of external skeletal fixation has been its ability to adequately stabilize fracture fragments until healing has occurred. It is absolutely essential to maintain the holding power of the pins the bone and the stiffness of the fixation pins if rigid immobilization is to be maintained. Maximum stress of the fixation pins occurs at the pin-bone interface. Stress transfer from bone to metal and, over time, stress concentration at these sites can eventually lead to pin loosening, drainage, infection, or breakage. Information gathered from numerous studies and clinical experience indicates that stiffness, bone holding power, and clinical performance of an ESF is dependent upon numerous factors including configuration, diameter and number of connecting bars, pin diameter, number of pins, type of pin, angle and location of pins in cortical bone, length of pins from the connecting bars to the bone, method of pin insertion, and inherent stability at the fracture site. Each of these factors must be critically assessed by the surgeon to decrease the likelihood of pin loosening and loss of fixator stiffness and associated morbidity.

The method of fixation pin insertion used should avoid generating mechanical damage and bone necrosis. High speed power insertion of pins results in thermal necrosis of bone, while insertion by a hang drill results in excessive mechanical damage. Both techniques are associated with a decreased force required for axial extraction of the fixation pins. Current recommendations include predrilling with a smaller drill bit and either low speed power or hand chuck insertion of fixation pins to decrease the incidence of mechanical or thermal necrosis and subsequent premature pin loosening.

The type of pin used influences greatly the stability of the pin-bone interface, as well as fixators stiffness. While nonthreaded pins exhibit decreased bone holding power, they are stiffer, stronger, and more resistant to bending and breaking than threaded pins. A recent study indicated that single cortex partially threaded pins compare favorable to pins with threads engaging both cortices with regard to holding power. In addition, these pins provided more resistance to bending at the pin-bone interface than fully threaded pins. Essentially, the single cortex partially threaded pin combines the increased holding power of threaded pins with the stiffness of the nonthreaded pins. The use of partially threaded pins, either exclusively or in combination with nonthreaded pins, should be considered in clinical cases where prolonged external skeletal fixation is required. Other studies have indicated that morbidity decreased significantly with the exclusive use of threaded pins or a combination of threaded and smooth pins as compared to the exclusive use of the smooth pins. Prolonged stability of the pin-bone interface was considered to be the reason.

Numerous recommendations have been made with regard to the angle and location of fixation pin placement in cortical bone. Information gathered from many studies indicates that an angle of approximately 70 degrees to the long axis of the bone and inward (central) angling of the pins improves fixation stiffness. It may also reduce pin loosening, because nonparallel pins will tend to restrict the motion of their neighbor. The appropriate number of pins per fragment has not been objectively determined; however, a minimum of 2 and perhaps 3 or 4 pins per fragment should be used as increasing the number of fixation pins per fragment reduces the incidence of premature loosening. Each pin should be inserted through a separate stab incision in intact skin and avoid penetration of large muscles masses. This practice will help alleviate problems with incision or wound management, decrease the incidence of pin tract infection and make incision closure easier.

Bone-connecting bar distance should be minimized while avoiding interference with the skin. Doubling the bone-clamp distance reduces the fixator stiffness by 25%. Increasing the diameter of the fixation pins or the diameter and number of the connecting bars will increase fixator stiffness. The configuration of the fixators will also affect fixator stiffness, with Type 3, or biplanar splints, being the strongest configuration.

In conclusion, since fractures vary widely in type, stability, the condition of the soft tissues, and activity and size of the patient, it is obvious that no one configuration is best suited for all fractures. Providing that the proper principles of application are followed, external skeletal fixation can provide the stable fixation necessary for fracture healing and good to excellent post-operative limb function. The information presented should hopefully enable the surgeon in choosing the best ESF design for the fracture type under consideration.

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