a new external skeletal fixation device that allows immediate full weightbearing application in the...

Veterinary Surgery, 15, 5, 345-355, 1986

A New External Skeletal Fixation Device* That Allows Immediate Full Weigh tbeari ng

Application in the Horse

D. M. NUNAMAKER, VMD, DiplomateACVS, D. W. RICHARDSON, DVM, DiplomateACVS, D. M. BUTTERWECK, BSE, M. T. PROVOST, BSCSE, and R. D. SIGAFOOS

The design and development of a new external skeletal fixator (patent pending) for use as a full weight supporting device in the treatment of fractures and joint instabilities in the horse are described. The application of the device incorporating the foot at the base of a specially designed frame allows for transfixation of the bone only above the injury. Full weightbearing is accomplished immediately following device application. In vitro testing of the device using a cadaver specimen substantiated theoretical design parameters. Fifteen clinical cases documented the use of this device.

LTHOUGH VARIOUS EXTERNAL skeletal fixation A techniques have been presented in the literature for use in large animals, external skeletal fixation has not become as widely used as in small animals or in

Repeated failure related to inadequate fixators has kept external skeletal fixation from be- coming a practical method for treating fractures in the horse.

We designed and developed a new external skeletal fixator for use in horses. This report describes this new device and its application. Indications for its use are outlined based on the clinical experience to date.

Design Criteria

High strength, compact form, light weight, ease of application, versatility, biocompatibility, and low cost are the most desirable qualities for an external fixator for use in the horse. Any design represents a series of compromises, and the success of that design is related to the number and magnitude of those compromises.

The major weaknesses of available devices for use in the equine patient relate to insufficient strength, high cost, and lack of versatility. Most readily avail-

able systems on the market today make use of transfixation pins above and below the fracture site. Since many of the fractures in the horse are distal to the carpus, this mode of treatment would be difficult because the bones of the distal extremity are short and purchase in these bones by pins below the fracture site might be impossible. Failures of available devices are related to breakage of the pin, pin-clamp, or failure of the pin-clamp interface.j The pins available have too small a diameter to allow immediate weightbearing and could easily undergo plastic deformation, catastrophic failure, or fatigue failure.6 None of the available systems are recommended for immediate full weightbearing, and disclaimers are provided by the manufacturers. Most designs available have been sized for use in humans and are not meant to immobi- lize limbs of large, generally uncooperative animals.

Strength of the fixator is intricately related to the diameter of the transfixation pins, the distance from the outer cortex of the bone to the connecting side bars, and the strength and stiffness of those sidebars

* US patent #4, 604, 9%.

~

From the Comparative Orthopaedic Biomechanics Laboratory, University of Pennsylvania, School of Veterinary Medicine, Depart-

This study was supported in part by Synthes AG Switzerland #6-29836. Reprint requests: D. M. Nunamaker, VMD, University of Pennsylvania, School of Veterinary Medicine, New Bolton'Center, 382 West

ment of Clinical Studies, New Bolton Center, Kennett Square, PA.

Street Road, Kennett Square, PA 19348.

345

346 NEW EXTERNAL SKELETAL FIXATION DEVICE

and connector^.^,^ Deflection of the transfixation pins is proportional to the cube of the distance between the bone and the sidebars. The stiffness of the pin is proportional to the fourth power of its diameter. These nonlinear relationships between pin diameter and sidebar to bone distance are most important in the design and application of any fixator subjected to weightbearing forces. The larger the pin diameter, the lower the stresses that are generated in the pin as well as at the bone-pin interface. The size of the pin holes drilled into the bone affects the ability of the bone to with- stand the stresses of weightbearing. The bone-pin interface is important when loading the frame during weightbearing and is involved with device failures related to pin loosening, pin tract infections, and bone failure (fracture). The biological and mechanical factors relating to this bone-pin interface are poorly un- derstood. Bone remodeling presumingly due to the cyclic loading of this interface often results in bone resorption and loosening of the fixation.

The pin diameters used in the development of this external skeletal fixation device were chosen based on the following factors: 1) calculations of the pin outer fiber stresses under assumed bending loads and the yield strength for stainless steel (410 megapascals [MPa]; tensile strength, 570 MPa), as well as the endurance level of the metal (30% assumed), i.e., 120 MPa;6 2) measured strains of loaded pins both in vitro and in vivo; and 3) estimates based on reports in the literature of bone hole diameters that could sustain the loads of weightbearing without breaking the bone itself. 7-g

The present thread pitch and fluting profiles of the transfixation pins were chosen after extensive testing in cadaveric bone to optimize thread cutting into bone without noticeable temperature elevations of the pins. These studies examined the use of self-threading and self-tapping pins placed into predrilled holes. A self- tapping pin has cutting flutes in the pin to cut the thread profiles within the bone. A self-threading pin has no cutting threads, and the thread is made by pressing the thread profile of the pin into the bone as it is inserted. Insertion of a self-threading pin into a predrilled equine third metacarpal bone generated so much heat that the area of the pin involved was too hot to hold between one’s fingers. No temperature eleva- tion of the pin was detected by this crude thermo- couple when a self-tapping pin of the same dimension was inserted into the same size hole. Thread quality in the bone hole was evaluated using magnification of cut surfaces.

A special frame with foot support was designed to take advantage of the attachment of a horse’s foot to a shoe which is incorporated into the external fixator

(Fig. 1) . The foot plate of this frame provides the means of attachment of the distal portion of the extremity to the frame without using transfixation pins in the most distal bones. The external fixator may also be used in a conventional manner with transfixing pins above and below the fracture or with pins only above the fracture and attached to the foot support shoe. When using the foot support device, additional smaller percutaneous pins or screws may be used to control fragments while incorporating them into the frame. These smaller devices, relieved of major weight supporting duties, function only to control movement of the fragments and do not interact with weight transfer within the frame.

Composite polyurethane steel-reinforced members are used as sidebars for the frame. These rigid links allow for a completely free initial manipulation of the fracture fragments and do not impede pin placement in any plane. Since the sidebars are constructed at the time of surgery using a two part filled polyurethane, there are no stress concentrations placed on any individual transfixation pin and absolutely no motion is allowed at the pin-sidebar junction. The strength of the sidebars is increased by the steel reinforcing bars and larger diameters of the tube to be filled with the polyurethane material. The size and shape of the sidebar are important to the structural rigidity of the frame. The filled polyurethane is available as a two part system which, when mixed, will solidify within about 5 minutes.* This allows for rapid construction of the sidebars. Flexible rubber tubingt is used to form the constraining mold for the polyurethane system as it is poured. The tubing used on an adult horse has an approximate inside diameter of 5.4 cm and a wall thick- ness of 1.27 cm. This soft rubber tubing is also valu- able as padding to protect the animal from the frame itself after the core has cured.

The transfixation pins are manufactured from 3 16 L stainless steel and threaded across 60% of their length’ from one end (Fig. 2). Pins used in the metacarpus or1 metatarsus are 25 cm long, 9.6 mm in diameter, and fluted at their threaded end so that they are self-tapping when used in bone. The pins are threaded by turning on a lathe so that each end of the pin has a different core diameter, hence a different stiffness. The diameter of the nonthreaded end is 9.6 mm and the core diameter of the threaded end is 8.6 mm. The thread therefore is made up from a core diameter of 8.6 mm and an outside thread diameter of 9.6 mm. The

* Isocast Browncast System, Isocast Systems, 8899 S.E. Jannsen

t Armstrong Armaflex tubing, Wilmington Supply Company, Road, Clackamas, OR 97015.

ZOO0 Maryland Ave, Wilmington, DE.

NUNAMAKER, RICHARDSON, BUTTERWECK, PROVOST, AND SIGAFOOS 347

W Fig. 1. Frontal view (A) and side view (8) of the external skeletal fixator applied to the forelimb of a horse. C. This frontal section through the limb and device shows the transfixation pins penetrating the third metacarpal bone and their anchorage in the sidebars. The metal bar running the length of the sidebars transfers the load to the ground through the use of a footplate and shoe.

thread pitch is 1.5 mm. Larger pitches yield larger and deeper threads but smaller core diameters, hence weaker pins. The thread is cut using a rounded tool, which reduces the notching and weakening effect of the normal V shaped thread (Fig. 3). The nonthreaded end of the pin is roughened on a grinder for a distance of several centimeters from its end to allow better ad- hesion of the polyurethane at the time of sidebar attachment.

The frame is now made of 1.27 cm diameter tubular steel with a cast aluminum base plate to conserve weight. Previous devices were made using steel rod of the same dimension (Fig. 4). The aluminum base is rounded on its bottom surface to reduce contact area with the ground and is angled 15 to 25 degrees from the ground surface at the hoof interface in order to elevate the heel of the hoof. This base plate will accept a bar shoe that can be screwed onto it. The final foot position is determined prior to fabrication of the device and depends on the injury involved. Any position may be considered, but moderate extension of the phalan- geal joints seems to be the most natural and non- distracting position for most fractures and other injuries. The bar shoe is made individually for each foot, and drilled and threaded to allow for attachment of the shoe to the frame.

Fig. 2. This cross-sectional view shows the partially threaded pin inserted into the bone and captured on both ends by incor- poration into the sidebars. The distance between the sidebars and the limb can be varied but should be as small as possible with consideration given for soft tissue swelling.

Application of the Device

The external fixator can be applied with or without the foot support. The description of its application here will be limited to use with the foot support and transfixation pinning of the third metacarpus as would be used for treatment of a severely comminuted first phalanx fracture. When using the foot support system, the device is built around a bar shoe. The shoe is made and nailed to the hoof with the animal under general anesthesia prior to surgical preparation. Implantation of at least three transfixation pins is accomplished using aseptic technique. The pins are placed above the fracture in the third metacarpal bone and are spaced about 5 cm apart. It is important that the drill holes go through the center of the bone, including the marrow cavity. Failure to drill the holes centrally may lead to fracture of the bone through the pin site at the time of weightbearing. Pin holes are hand drilled to 8.73 mm following the initiation of a pilot hole using power equipment and a 4.5 mm drill bit. The pins are threaded into these holes using a large Jacobs Chuck. Minimal internal fixation can be applied to the fracture at this time if nece~sa ry .~ Closed reduction is used in

Fig. 3. This threaded segment of the pin shows the thread profile obtained by using a rounded cutting tool to minimize the notching (stress concentrating) effect at the base of the thread.


Fig. 4. The frame for the external skeletal fixator is made of steel with an aluminum baseplate. The bar shoe is sized individually, nailed to the horse's hoof, and then attached to the baseplate.

most cases to avoid further disruption of the blood supply in badly comminuted fractures or because the severity of comminution does not allow for internal fixation. Following the placement of the pins or clo- sure of any surgical incision, the wound and pin sites are bandaged lightly with a sterile dressing.

At this time, the rubber tubing is applied to the transfixation pins by puncture of one wall of the tubing. It is important to perform this carefully since a good seal is needed between the pins and the rubber tubing to prevent leaks from developing when the polyurethane sidebars are poured. The tubing should be long enough to incorporate the frame below the junction of the two vertical bars distally and should extend above the upper pin at least 20 cm to allow for easy pouring of the polyurethane. The distance from the tubing wall to the skin should be about 2.5 cm. The vertical bars of the frame are placed within the tubing from the distal end, and the frame is slid up into position. The vertical bars may pass on either side of the transfixation pins. The vertical supports should be par- allel to the long axis of the third metacarpal bone when

the animal is in a weightbearing position. Once in position, the bar shoe is attached to the frame with screws and bolts. The ends of the rubber tubing are then tied tightly at their distal ends to prevent leakage from oc- curring during the filling of the rubber tubing with the polyurethane. One 9.6 mm steel rod the length of the finished sidebar is placed into each tube to act as an additional reinforcing member. If no internal fixation is used, it is important at this time to align the axis of the limb to reduce the fracture. The two-part filled polyurethane sidebar material, each part having been previously resuspended, is mixed in equal parts and poured into the sidebar tubing. It is best to do one sidebar at a time, allowing it to cure ( 5 minutes) before proceeding to the other sidebar. The proximal tubing can be cut off with a hacksaw about 5 cm above the upper pin. The frame is covered with a light wrap to keep the area clean, and the animal is allowed to re- cover from anesthesia.

When using the device without the foot support, i.e., with pins above and below the fracture site, the sidebars should extend at least 5 cm beyond the pins both above and below the fracture so that the sidebars will not break when subjected to weightbearing. It is also necessary to use reinforcing rods in the sidebar tubes. This is accomplished by placing the rods into the tubes before pouring the polyurethane.

Strain Measurements

In vitro and in vivo strain gage measurements were made correlating weightbearing with strain in the transfixation pins. The in vitro studies were carried out using a fresh equine cadaver forelimb. The limb was mounted in a universal testing machinef as previously reportedlo after the application of the external skeletal fixator using the foot support shoe (Fig. 5 ) . The load was transmitted axially through the specimen and the external skeletal fixation device. Three transfixation pins were placed through the third metacarpal bone, and the hoof was attached to the frame as previously described. The pins used were threaded only centrally and had diameters of 8.6 and 9.6 mm at either end. Unidirectional foil strain gages§ were used to measure strain on the pins as well as the bone. Gages were mounted using methyl cyanoacrylate". Signal conditioning was done with a Vishay 2100 system de-

$ Model I33 1 Servohydraulic closed-loop materials testing sys-

$ Micromeasurements CEA- 125UW-350, Measurements Group

" M bond 200, Measurements Group Inc., Raleigh, NC.

tem, Instron Corporation, Canton, MA 02021.

Inc., Raleigh, NC.


Fig. 5. This in vitro test specimen shows the strain gage in- strumentation applied to the bone and pins. Loads transmitted across this specimen allow determination of relative strains in the bone and the device.

vice 1. Data were recorded with a Honeywell Model 1858 Visicorder**.

Strain gages were placed on the proximal surface of each of the transfixation pins as they emerged from the bone and on the dorsal surface of the metacarpus in four locations. These four gages were located proximal and distal to the transfixation pins, as well as between them (Fig. 5 ) . Strain readings were accomplished at force levels of 4750 N, 5000 N, 9750 N, 16125 N, and 17250 N using loading rates of lm/min.

The stress on each pin was computed from measured strain for different loads using the relationship:

Stress = Youngs modulus x Strain

A linear regression of the load and stress paired data was then performed computing the regression equation coefficients (slopes and intercepts), correlation

ll Measurements Group Vishay Intertechnology Inc., Raleigh,

** Honeywell Test Instrument Division, Box 5227, Denver, CO NC.

80217.

coefficients, and significance tests (p < 0.05). All calculations were accomplished using UNIXSTAT, an on-line statistical analysis package?? (Fig. 6).

Results

Plastic deformation of the transfixation pins in this specimen occurred at a force of 17250 N (load equiv- alent = 1758.4 kg). Strains measured at this load were not included in the regression analysis because the linear stress-strain relationship no longer applied. As- suming the yield stress for this stainless steel to be 410 MPa, the force at which this stress occurs was calculated for pin 1 (proximal pin) from the corresponding regression equation as 19052 N.6 Lines were generated from all three regression equations using this load as the maximum. Pin stresses calculated from this strain data were compared to the theoretical yield stress of the material as discussed above (Fig. 6). Forces of 17250 N approached the calculated and sur- passed the observed yield strengths of the transfixation pins on the side of the smaller diameter (8.6 mm). Bending occurred at the junction of the bone and pin, as well as at the sidebar and pin. Experimental results were within 6% of calculations.

Bone strain measured in vitro was seen to decrease gradually below the first transfixation pin to the point that it was not recordable below the third transfixation pin except at overload failure of the device (Table 1). The strain gage mounted on the bone between the first and second pin recorded higher strains than the gage proximal to the first pin.

In vivo measurements of pin stresses were carried out in one clinical case while the animal was standing on all four feet, standing on three feet with the opposite forelimb flexed off the ground, and while the animal was walking on the external skeletal fixator. Only the smaller diameter of the top transfixation pin was monitored during this procedure. Stresses calculated from the measured strains in this experiment never exceeded 92 MPa.

Clinical Experience with the External Fixator

Fifteen horses have been treated with the external skeletal fixator using the foot supporting device (Table 2). Ten of the 15 horses were treated primarily with the device, while five animals were treated secondarily after failure of another treatment modality. Eight horses had comminuted open fractures or open injuries, while two additional cases treated secondarily had open septic injuries. All animals were adult horses

tt Gary Pearlman, Bell Labs, Murray Hill, NJ.


Stress (MPa)

I I I I I 1 I I I 1 I I I I 1 I I I I I 1 I

0 2000 4000 BOO0 W O O iO000 i2000 i4000 S72SO 10052

Force (Newt onsl

P i n 1: -0160~ + 105.1716: Ra.9196

---------- Pin 2: .0147x + 71.9735: R-.9173

---- Pin 3: , 0 1 2 0 ~ + 67.9040: Rm.9465

Fig. 6. This graph relates pin stresses to increasing force on the external skeletal fixation device. Pin 1 is the most proximal pin, pin 2 is the middle pin, and pin 3 is the most distal pin in the frame. The proximal pin receives the greatest stresses while supporting weight.

except one yearling Standardbred with an open fracture of the third metatarsus. Five closed injuries (3 P1 fractures, 1 P2 fracture, and 1 internal fixation failure of a traumatic disruption of the suspensory apparatus) were treated. The open fractures involved the first phalanx (4 horses), the metatarsus (1 horse), the meta-

carpus (1 horse), and traumatic disruption of the suspensory apparatus (2 horses).

Following application of the external skeletal fixation device, all animals were able to bear weight on the device and could easily walk back to their stalls. The subjective evaluation of animal comfort was the most impressive feature of the initial treatment of most animals with this external skeletal fixator (Fig. 7).

Radiographic evidence of pin loosening occurred at various intervals. Most animals were stable in their frames for 8 weeks or more. Gradual increasing lame- ness, when it occurred, was thought to be associated (N)

4750 279 346 159 0 with pin loosening. The distal end of the radius was 9750 308 362 190 o especially prone to pin loosening. Here, resorption 16125 360 560 235 0 around the pins could be seen radiographically within

several weeks. The metacarpus was slower to develop 17250 584 901 334

TABLE 1. Bone Strain Microstrain*

Force Above Above Above Below Pin 1 Pin 2 Pin 3 Pin

77

Surface strains on the bone around the pins are shown with l-adiOl~CenCies around the pins. Ring sequestra were seen only with the use of self-threading pins. Pins that corresponding loads applied to the frame in vitro.

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Fig. 7. Case 4 at the time the animal was returning for follow- up radiographs (9 weeks postoperatively).

were loose seemed to be associated with increased drainage from the pin site.

Ten of the 15 horses progressed to the point that their frames were removed during the course of treatment. Two horses (cases 10 and 12) broke their third metacarpal bones through the most proximal pin site while wearing the device and were killed; three other horses (cases 1, 2 and 8) with open infected fractures were killed at the request of their owners, while still in their frames, because of increasingly bleak prognoses. Two horses subsequently fractured their third metacarpal bone through a pin hole following implant removal. One of these animals (case 9) was killed and one (case 11) underwent three internal fixations before bony union of the fracture occurred. A complete ring sequestrum formed in this latter case at the level of the middle pin hole. The sequestrum had the effect of dou- bling the diameter of the pin hole and was removed at the time of pin removal. Fracture of the limb occurred during recovery from anesthesia while the animal was in a long leg cast. Following bone union of the third metacarpal fracture, this horse fractured its first phalanx transversely while in its stall and was killed 21 months following its original injury. None of the animals treated in this series developed draining pin tracts when the pins were removed. All wounds from the pin holes healed spontaneously.

Seven animals (cases 4, 5, 6, 9, 1 1 , 14, and 15) healed their primary injuries as demonstrated clini- cally and/or radiographically (Fig. 8). One horse (case 4) slipped on the ice while being ridden 9 months

after its fracture and reportedly fractured the third metacarpal bone; necropsy was not performed. Two horses (cases 5 , 14) survived without complications to go home, and one horse (case 15) developed laminitis of the opposite forelimb but healed his fracture and is now a breeding stallion. Joint sepsis related to the original open injuries accounted for the demise of one horse (case 6). The outcome of cases 9 and 11 have already been described.

Discussion

A new external skeletal fixation device that is useful for immediate full weightbearing should be helpful in treating some catastrophic injuries in the adult horse. Design and sophistication of the device have evolved with the experience of clinical failure and success. The initial comfort of the patient makes the device attrac- tive, especially where full weightbearing is thought to be essential for the survival of the animal.

The challenge of immediate unprotected weightbearing has not been addressed before in the orthopedic literature regarding an external skeletal fixator. Most devices are designed for non-weightbearing or only partial weightbearing, and weightbearing is added only as the fracture heals. This is not possible in large uncooperative animals. The strength of any system must be compatible with that species’ bone for which it is to be used. Drill holes of up to about 30% of the diameter of the bone can be used without catastrophic failure of the bone in the dog.7 Experiments are in progress in the horse to determine if the same guide- lines apply in this species. Some recent work has sug- gested that large holes reduce strength not just by increasing stress concentrations of the bone but by loss of bone material itself thereby decreasing the cross sectional area of the remaining bone, hence its strength.8 This work8 contradicts previous work7 and seems to point out the necessity of optimizing hole size for the use of any external fixator in every species.

The results of in vitro loading of the pins in the frame showed that the yield strength of that portion of the transfixation pin with an 8.6 mm diameter was only exceeded by forces larger than 16125 N. Stress levels reached 341 MPa. The other end of the pin with the 9.6 mm diameter did not approach the yield stress of the material and generated only 39.6 MPa. Therefore, the 1 mm enlargement of pin diameter decreased the stresses of the pin undergoing the same load to a level that insured a safety factor of three regarding the pre- sumed endurance level. Thus, it can be seen that only very small differences in pin diameters are needed to preserve the device once the maximum loads are known. If, however, the small increase in hole size is catastrophic to the integrity of the bone involved, then


Fig. 8. A. Preoperative radiograph of case 11 shows a comminuted fracture of the first phalanx. The 20-month postoperative films (8 and C) show fracture healing as well as fusion of the metacarpophalangeal joint and proximal interphalangeal joint. The two screws in the first phalanx were placed through stab incisions at the time of removal of the external skeletal fixation device. The distal end of the plate on the third metacarpal bone is also seen.

this pin size increase is not warranted. Different pin diameters may be necessary for different diameter bones within the same species, or even within the same bone depending on the diameter of the bone at the level chosen for pin placement. The modifications to the pin design that have taken place because of this study have strengthened the pin considerably. Using a fully threaded pin throughout the smaller core diameter and a rounded cutting tool to prevent stress concentrating effects within the threads have improved the load bearing capacity of the pin without enlarging the pin hole.

The fact that two out of 15 animals broke their bones through a pin hole during treatment with the external fixator shows the close association between hole size and bone strength. Part of that problem may have been related to the large contact area between the frame and the ground in those animals. This resulted in large torsional forces being transmitted into the bone when the animal was turning while weightbearing in the frame. The recent frame designs have decreased this contact area by convexly rounding the bottom plate at the area of ground contact hence decreasing this torsional moment. No further problems associated with bone fracture with the pin in place have occurred since this change has been incorporated (3 cases).

Two other horses fractured their third metacarpal bone through one pin hole soon after the device was

removed. One of these animals had been placed in a cast extending to the proximal metacarpal bone. The top of the cast was at the approximate level of the pin hole, and fracture occurred at the time of recovery from anesthesia. The other horse fractured its third metacarpal bone through the middle pin hole. This was associated with a large ring sequestrum associated with a self-threading pin of early design that generated a great amount of heat at the time of insertion. Con- trary to a report in the human literature," predrilling the holes in the bone was not an effective method of eliminating heat generation in equine cortical bone when using a self-threading pin. The use of a self-tapping threaded pin in a predrilled hole has resulted in no ring sequestra formation in four subsequent clinical cases.

One horse fractured the third metacarpal bone 7 months following removal of the transfixation pins. It is unknown whether this animal fractured the bone through a pin hole. The literature indicates that holes in dog bone do not represent significant stress concen- trators after a relatively short time (8 weeks) following implant removaLS Since the diameters of these pin holes are relatively large in relation to the diameter of the bone, grafting of the pin holes with autologous cancellous bone may be indicated at the time of pin removal.12 This, combined with proper external support for an additional 6 to 8 weeks, may help to elimi-


nate the problem of fracture through a pin hole. Pres- ently, all external skeletal fixators are being removed while the patient is standing and bone grafting is not used. This eliminates the problems of getting up after general anesthesia. No animals have fractured their limbs following removal of the device in this manner (five horses).

Since the pins were rigidly mounted in the frame, it was difficult to establish if or when the pins became loose at the bone interface. In most cases, this could only be determined at the time of device removal when the pins were separated from the frame. Loose pins could be removed by gentle traction while the stable pins had to be unscrewed from the bone, sometimes with a Jacobs chuck. Ring sequestra only occurred when a special pin thread design (self-threading) was used. Further development of the pin to a self-tapping thread seems to have eliminated this phenomenon. The pin bone interface represents one of the biggest challenges regarding the long-term success of any external skeletal fixation technique that uses weightbearing forces. The use of threaded pins seems to pro- long this interface in the horse. Thread pitch and depth may be important parameters affecting the interfacial strain relationships at the pin-bone junction. Optimiz- ation of these parameters could be instrumental in pre- venting bone resorption or in reducing its rate.

The gradual loosening of the device at the pin bone interface occurs at about the time the device can be removed in many cases. Since external support can be used for an additional time period following removal of the device, healing of the injury can progress during this phase of treatment.

It was interesting to note that the bone strain recorded in the in vitro study below the proximal fixation pin was greater than the bone strain recorded above the pin. This occurred at all loading levels but was only done in one specimen. It is of importance since the external skeletal fixator is designed to relieve the load (strain) of weightbearing from the injury. The bone strains recorded below the second pin and distal to the third pin seem to demonstrate the desired effect. The interaction of load bearing and the effects of stress concentrations caused by the top two transfixation pin holes may account for the increased strain seen around the first transfixation pin. The stress concentrating effects of the holes may be considerable. The strains recorded below the second pin would be lower, since some load is being transferred out of the bone and into the frame. Below the most distal pin, most of the load would be taken by the frame sidebars, bypassing the injury and contacting the ground through the base plate.

The in vivo strain gaging of the smaller end of the

proximal transfixation pin showed that the strains on the most highly stressed pin of the external skeletal fixator were below the endurance level of that pin. This means that the device should be able to undergo prolonged cyclic loading without fatigue failure of the pins. Clinical case #13 demonstrated the effect of prolonged cyclic weightbearing by being in the device for 66 days without significant weightbearing on the opposite leg. This animal suffered the effects of severe laminitis in the right hind foot, which resulted in the loss of the entire hoof, sole, and frog. This horse would only bear weight on its left hind leg in the frame for the duration of its application.

Since the technique was still under development during this study, the indications for treatment using this external fixator were based on the severity of the condition and the extremely poor prognoses afforded by other treatment methods. Initially, only severe comminuted fractures, grade three open fractures, or infected fractures and infected joints were included in the case material. Five of the cases were treated with the external fixator only after other treatment regi- mens failed. Indications for treatment have broadened somewhat since the study started, and now the device is used more frequently for comminuted closed fractures. The indications for use of this device relate to the attributes of the device itself: it allows complete immobilization of unstable comminuted fractures, ready access to local wounds or soft tissue injury, and the ability to deal with severely traumatized soft tissues without further tissue disruption via surgical intervention. Present clinical experience is insufficient to advocate the use of this device where other methods produce acceptable results.

The case material in this study shows the feasibility of using an external skeletal fixation device in the horse that allows immediate full unrestricted weightbearing. Complications with the use of this system continue to exist, but improvements in the technique and the device have overcome many of the problems. The case material in this article shows the possibilities for treatment of severe injuries in the horse. The results of the 15 cases presented here must be tempered with the knowledge that these cases represent an initial attempt at the use of a new fixation device. The external skeletal fixator has evolved through use in clinical situations as well as on the test stand. The desperate need of an individual clinical situation often demanded the use of an inadequately tested device. The successful resolution of the complex injuries and complications of orthopedic trauma in horses cannot be related to a single device or treatment method. Many factors are involved. It is hoped that the addi- tion of a new external skeletal fixator will provide a


useful technique in the treatment of these injuries. The device as presently used has changed shape, size, du- rability, strength, and weight from early models. The device will now support approximately three times the horse’s body weight before failure will occur.

References

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2. Bergevin J, Memtt F, Pickering L, Schoenberg R. External fixation device for lower limb fractures in horses. 23rd Annual Proceedings of the AAEP, Vancouver, B.C., 1977;219-22.

3. Verschooten F, DeMoor A, Desmet P, Steenhaut M. Surgical treatment of tibial fractures in cattle. Vet Rec 1972;90:24.

4. Hamilton GF, Tulleners EP. Transfixation of proximal tibia1 fractures in calves. J Am Vet Med Assoc 1980;8:725-7.

5. Seligson D, Pope MH. Concepts in external fixation. New York: Grune & Stratton, 1982.

6. Oberg E, Jones FD, Horton HL. Strength of materials. Schu- bert PB, ed. Machinery’s handbook. New York: Industrial Press, Inc., 1980.

7. Bechtol CO. Instructional Course Lectures. AAOS 1952;9:

8. McBroom RJ, Hayes WC. Strength reduction and fracture risk of cortical defects in the diaphysis of long bones. Proc Orthop Res SOC 1984;9320.

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11. Matthews LS, Green CA, Goldstein SA. The thermal effects of skeletal fixation-pin insertion in bone. J Bone Joint Surg 1984;66A: 1077-83.

12. Schenk RK. Fracture repair-Overview. H. Czitober, ed. Pro- ceedings of the Ninth European Symposium on Calcified Tis- sues XIII-XXII. Baden, FRG: Facta Publication, 1973: 113- 22.

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a new external skeletal fixation device that allows immediate full weightbearing application in the...

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