Use of IMEX SK-Circular External Fixator Hybrid Constructs for Fracture Stabilization in Dogs and Cats

Introduction

Comminuted long-bone fractures with a short segment of intact bone adjacent to a joint can be difficult to stabilize. Consideration must be given to the length and conformation of the fracture segments when selecting a type of fixation. Bone plates are less amenable for use on fractures with very short proximal or distal segments, particularly if the fracture cannot be reconstructed, and contouring plates near certain joints can be difficult.¹ The Kirschner-Ehmer splint has been the most commonly utilized external skeletal fixation system in dogs and cats since its introduction in 1947.² Although this system is widely used, limitations exist relating to its design. Specifically, these limitations included a relatively weak connecting system; an inability to add or remove individual, centrally located fixation clamps without disassembling the connecting system; the inability to place positive profile fixation pins³ directly through fixation clamps; the lack of a convenient system for predrilling pilot holes for fixation pins; an inability to consistently and accurately place multiple, full-pin splintage fixation pins in a single longitudinal plane; and impaired visualization of osseous structures on postoperative radiographs.

Several new external skeletal fixation systems have recently been developed to address many of the deficiencies of the Kirschner-Ehmer apparatus.^4–6 One of these systems is the IMEX SK external skeletal fixation system.^a In the SK system, the diameter of the connecting rod has been substantially increased to improve the biomechanical characteristics of the fixator frame. SK connecting rods are manufactured from materials (i.e., carbon fiber, titanium, or aluminum) that are lighter than stainless steel; thus, the increased diameter rods do not result in excessively heavy constructs. Carbon fiber and aluminum rods have the additional advantage of being radiolucent. The SK system also features redesigned fixation clamps that have a two-piece clamp body, which can be readily assembled and disassembled. This allows fixation clamps to be added or removed from a construct at any time. The SK clamp bolt features a retained clamp washer that secures the fixation pin. The clamp washer has a multi-toothed surface that interdigitates with the clamp body, eliminating unwanted fixation pin/clamp slippage on the connecting rod. This also allows stable fixation, irrespective of fixation pin angle. The fixation clamp washer has a meniscus (i.e., a semicircular cut-out section) that engages the shank of the fixation pin and allows the use of fixation pins of varying diameter. The diameter of the hole in the fixation bolt can accommodate direct placement of appropriate-diameter, positive-profile, partially threaded fixation pins.⁵

Another recent innovation in external skeletal fixation is the use of circular external skeletal fixators in dogs and cats.^6–10 Circular external skeletal fixators were developed by the Russian orthopedic surgeon, Dr. G.A. Ilizarov. These modular systems have been used to stabilize fractures and arthrodeses, perform bone lengthening and transport, and correct limb deformities.⁶⁸¹⁰ Circular external skeletal fixator frames consist of a series of full or partial or both external rings that are interconnected by threaded rods. Circular external skeletal fixators classically use small-diameter wires, rather than pins, as fixation elements. These wires are typically tensioned to improve their stiffness characteristics. The tensioned wires adequately resist bending, shear, and torsional displacement of the secured bone segments while allowing axial micromotion at the osteotomy or fracture site. These biomechanical characteristics are considered to be important in creating an environment conducive to osteogenesis.¹¹¹² An additional advantage of using fixation wires is the ability to obtain multiple points of fixation in a very short segment of bone.

IMEX recently began producing 6.3-mm diameter titanium hybrid rods that have a smooth shaft and 6-mm diameter threads at one end. The nonthreaded portion of the rod accommodates small SK clamps, while the threaded portion of the rod can be secured to ring components of the IMEX circular external skeletal fixation system to construct SK-circular fixator hybrid frames. This paper describes the use of IMEX SK-circular fixator hybrid frames for the stabilization of long-bone fractures with short distal segments of bone in three dogs and three cats.

Materials and Methods

The medical records and radiographs of all dogs and cats managed with an IMEX SK-circular external fixator hybrid construct between December 1998 and March 2000, at the University of Florida’s Veterinary Medical Teaching Hospital, were reviewed. Information regarding signalment, weight, history, fracture description, surgical repair, and outcome was recorded.

Results

Fracture repair was performed in three dogs (case nos. 3, 4, 6) and three cats (case nos. 1, 2, 5). Details regarding the animals’ signalment and history, fracture description, complications, time to radiographic union, and outcome are listed in Table 1. Age ranged from 10 months to 9 years (mean, 4.7 years; median, 3.75 years). Body weights ranged from 1.9 to 15 kg (mean, 6.7 kg; median, 5.4 kg). All of the fractures were closed and involved the distal diaphysis, metaphysis, or both. The proximal diaphysis was also involved in the two segmental femur fractures. Five of the six fractures were comminuted. In one of the tibial fractures (case no. 2), the fibula was intact and some inherent stability was assessed to be present prior to surgery. The cat with the femur fracture (case no. 1) also had ipsilateral pubic fractures and a contralateral stifle injury that included partial tears of the cranial cruciate ligament, lateral collateral ligament, and caudal horn of the medial meniscus.

For all but one of the procedures, intraoperative fluoroscopic evaluation of implant placement was performed. An open approach was used to anatomically reconstruct the two femoral fractures and one of the tibial fractures. An IM Steinmann pin was placed in normograde fashion in the two femoral fractures. In both fractures, the proximal end of the pin was left protruding through the skin and was “tied in” to the lateral frame. In case nos. 3 and 6, an allogenous cancellous bone graft^d was used. A closed reduction technique was performed in three animals.

Descriptions of the hybrid constructs are listed in Table 2. Partial (two 5/8, three stretch) rings were used in five constructs, and a complete ring was used in one construct. A centrally threaded full pin was used as the sole ring fixation element in the humeral fracture and one of the femoral fractures. Three constructs consisted of a titanium primary connecting rod, a shorter titanium secondary connecting rod, and a carbon fiber tertiary rod that bridged the primary and secondary rods cranially. A hinge assembly was used in two cases to connect the secondary rod to the ring. In one of these cases, the secondary connecting bar was connected to the primary rod with a double clamp, and in the other to the most proximal half pin with a single clamp. Secondary and tertiary connecting rods were not used in the final animal until the revisional surgery. In two of the animals (case nos. 3, 4), in addition to the half pins placed with the primary connecting rod, one half pin was placed from the secondary or tertiary rod.

Three of the animals (case nos. 1, 5, 6) experienced complications that required a second surgical procedure. In case no. 1, a previously undiagnosed femoral neck fracture was observed on postoperative radiographs. In addition, placement of the two half pins in the middle segment of the femur created a new, simple oblique, mid-diaphyseal fracture. This fracture caused caudal displacement of a large butterfly bone fragment and loss of pin purchase. The cat was anesthetized 2 days later, and a surgical approach was performed through the initial skin incision to repair the two fractures. The femoral neck fracture was stabilized with a Magic Pin^e and a diverging Kirschner wire. After removing the two half pins from the mid-diaphysis, the fracture was repaired with multiple cerclage wires and a single half pin was placed in the proximal diaphysis.

The fracture repair of case no. 5 was found to be unstable 12 days after surgery due to inadequate bone purchase of one of the Kirschner wires in the distal fracture segment, resulting in some loss of the initial fracture reduction. It was also believed that the distal bone segment had translocated along the fixation wires. Revision involved removal of the malpositioned Kirschner wire and placement of two opposing olive (i.e., stopper) wires as ring fixation elements.

For the first 12 days following surgery, case no. 6 did not bear any weight on the operated limb. Early radiographic evaluation on day 12 did not reveal any evidence of implant failure or complications. At the 1-month recheck evaluation, the most proximal half pin was found to be broken at the thread-smooth shaft interface. The exposed portion of the broken pin was removed, but the pin was not replaced. On the 8-week evaluation, the dog was still not weight-bearing, and bone lucency was observed around the most proximal remaining half pin. This pin was loose and was removed. Three weeks later, the fracture was assessed as a nonunion with little chance of healing. Resorption was noted around the IM pin, and instability was detected in the proximal component of the segmental fracture. Seventy days after the initial fracture repair, a surgical revision was performed. This involved placing two (cranial and lateral) diverging, partially threaded IMEX pins^f in the proximal fracture segment, replacing the IM pin with a larger Steinmann pin, and replacing the two ring fixation wires with two opposing olive wires. Also, an additional connecting rod and hinge assembly was added to further increase stability across the cranial aspect of the construct. Approximately 2 months after the revisional surgery, radiographic union of the proximal fracture was observed.

In case no. 3, the owner noted 7 days after surgery that the distal aspect of the secondary rod was rubbing on the dorsum of the paw, and the proximal portion of the primary rod was rubbing on the medial aspect of the thigh. Impingement was corrected by moving the secondary rod to a more lateral location and by cutting the proximal aspect of the primary connecting rod to a shorter length.

Excessive pin tract drainage was noted during one recheck evaluation for three of the cases. Two of these cases had loose implants at the time the observation was made, and the discharge resolved following revisional surgery. The other case was suspected by the referring veterinarian to have an infection at the skin-pin interfaces and was treated with antibiotics prior to recheck evaluation. There was no evidence of pin tract infection or significant drainage when that animal was later evaluated at the authors’ hospital.

Time to radiographic union ranged from 64 to 137 days (mean, 90 days; median, 84 days). The fixators were removed at the time radiographic union was observed in all cases except for case no. 6, in which the fixator was left on for an additional 30 days. The functional outcome was considered excellent in all six cases. All animals were able to bear weight on the operated limb at the time of discharge, and all were observed to be fully weight-bearing at the 1-and 2-month postoperative evaluations, with the exception of case no. 6, which did not begin to bear weight until 2 months after the revisional surgery.

Discussion

SK-circular hybrid fixators proved useful for successfully managing long-bone fractures with short distal segments. In all cases in this report, external fixation was chosen because the length of the distal fracture segments was considered too short for effective bone plate application. Application of ring components was particularly advantageous in these cases, as they allow for multiple points of fixation in extremely small fracture segments. All three tibial fractures were oblique, comminuted, and extended distally to within 1.0 cm of the tarsocrural joint. Despite the proximity to the joint, enough length was present in the distal bone segment to accommodate three fixation wires and a 50-mm ring in each case. Also, bicortical bone purchase, which can be difficult in sections of a fracture segment in which the cylinder is not complete (e.g., with oblique or comminuted fractures), is more feasible with ring fixation wires than with screws and pins used with bone plates and Kirschner-Ehmer splints, respectively. The probability of driving a wire through a bone segment and engaging two cortices is high, because fixation wires can be driven across a bone at various angles within a given transverse plane.

Three or four rings are generally used to stabilize fractures with traditional circular external skeletal fixators.⁶⁸ The limited anatomical working space in the smaller sized animals in the authors’ case series would have made the use of three or four rings difficult. Application of the hybrid constructs was not hindered by the limited amount of working space. Diverging the fixation wires makes translocation of a bone segment less likely and improves bending resistance of the construct.¹⁵ Optimally, two fixation wires placed on a ring should cross each other at 90° angles; however, anatomical constraints usually prevent maximal wire divergency. Olive wires can be used to improve shear stiffness¹⁵¹⁶ and prevent translocation of bone along the wires, particularly if angle of divergency between the wires is small.⁹¹²¹⁷ Translocation of the distal bone segment in case no. 5 was attributed to an insufficient angle of divergency between Kirschner wires and could have been prevented by the use of olive wires.

Hybrid constructs may utilize both small-diameter wires and larger diameter pins as fixation elements. Thus, hybrids can combine the circular fixator advantages of being able to place multiple fixation elements in a short segment of bone, with the advantages of the linear systems, such as the use of half pins in areas of bone covered by significant muscle mass (e.g., the proximal tibia). Overall, the presence of fixation wires and pins did not interfere with limb function, as usage during the convalescent period was very good.

The selection of a single full pin or multiple fixation wires in these cases was somewhat arbitrary, and both techniques yielded successful outcomes. Previous studies have shown that controlled axial micromotion stimulates callus formation in the fracture site.¹²¹⁸ Although this has been shown to be beneficial in fracture gap models, the authors speculated that axial micromotion in anatomically reconstructed fractures could be detrimental to fracture healing by causing a high amount of strain. For one of the femur fractures repaired, this theory was the basis for deciding to use a full pin as the ring fixation element, as it was expected that the rigidity of the full pin would prevent axial micromotion. The clinical significance and validity of this theory, however, must be questioned in the population studied, as one of the anatomically reconstructed fractures in which wires were used as ring fixation elements had a very short healing time (66 days).

Application of type II and III fixators, which provide greater stability than type I fixators,¹⁹ is limited to fractures distal to the elbow and stifle. Modified type I fixator configurations, in which a full-pin splintage pin is placed through a distal fracture segment, have been described to manage comminuted supracondylar humeral and femoral fractures.^1120–22 A bent Steinmann pin and Kirschner-Ehmer connecting rods and clamps or an acrylic column can be used to interconnect the medial portion of the transarticular pin with the fixator frame. Utilization of hybrid constructs in this present case series simplified the assembly of these complex frames.

Rings in the IMEX system are manufactured from a high-strength, tempered aluminum alloy which imparts strength to the supporting elements without making the fixator too heavy for use in cats and dogs.⁶ The circular design offers a convenient way to provide bilateral support and great flexibility in the placement of the primary and secondary titanium connecting rods. Placing the secondary connecting rods slightly cranial (rather than directly medial or lateral) prevented contact of the connecting bars with the animal during joint motion. The spherical washers and nuts allowed adjustment of the angular position of the titanium hybrid rod with respect to the ring component, while securely fixing the rod to the ring.

The SK clamps are versatile, allowing for a variety of hybrid fixator configurations. Double connecting clamps were used to interconnect the titanium and carbon fiber rods. The ability of the SK double connecting clamp to join two connecting rods and the ability of the SK single clamp to accommodate a variety of pin sizes, allows IM pins to be “tied in” to hybrid fixators easily and quickly.

Because the SK carbon fiber rods are lightweight and radiolucent, their use as tertiary connecting rods increases stiffness of the construct without adding substantial weight to the construct or obscuring radiographic visualization of the fracture. Their application is simpler than bending stainless steel connecting rods or using polymethylmethacrylate. SK-circular hybrids, unlike the acrylic fixators, can be easily and rapidly disassembled and reassembled, allowing for postoperative modifications or adjustments (as were necessitated in case nos. 3 and 6). Angular hinge assemblies were used to connect the tertiary connecting rods directly to the rings in two dogs, because double clamps were not available at that time. The titanium connecting rods were screwed directly into the hinge assemblies, and the hex broach fitting on the hinges was tightened to lock the hinges.

Although complete rings are biomechanically superior to partial rings, the use of complete rings near high-motion joints is usually not possible due to anatomical constraints. Various shapes of supporting elements of the circular system, such as the 5/8 and stretch rings, allow its use in close proximity to high-motion joints. In the authors’ experience, 5/8 rings may be used successfully in distal tibial and humeral fractures, while the stretch ring appears to be a better choice for fractures of the distal femur.

Fluoroscopy was particularly helpful for confirming proper placement of the initial fixation wire, which dictated the alignment of the ring on the limb. Driving the ring fixation elements through the distal fracture segment parallel to the transverse plane of the adjacent joint was more complicated in those patients in which a closed approach was used, because alignment of the bone had not yet been restored. Fluoroscopy was also helpful for confirming proper implant placement and assessing fracture reduction in these cases.

In the three cases that required surgical revision, complications had occurred during placement of the half pins (case nos. 1, 6) or fixation wires (case no. 5). The fracture created by placement of the half pins in the diaphyseal segment of case no. 1 was believed to result from deflection of the half pin by the IM pin, as no preexisting fracture fissure was appreciated radiographically or intraoperatively. Case no. 5 was repaired via a closed approach, which may have contributed to the already difficult task of obtaining adequate bone purchase in a short distal bone segment. The need for revisional surgery in case no. 6 resulted from implant failure and loosening 70 days after the initial surgical procedure. This occurred because the femur of the 1.9-kg dog was too small to accommodate the 2.4-mm, end-threaded SCAT half pins used in the other cases. Instead, Magic Pins, which are partially threaded Kirschner wires available in small (e.g., 1.2 mm) diameters, were used as half pins. As these pins have a considerable change in diameter at the junction of the threaded and nonthreaded portions, this area of the pin is very susceptible to failure during axial loading of the limb. Despite these complications, the functional outcome in all six cases was excellent, and the median time to fixator removal of 12 weeks compares favorably with previous reports.²³²⁴

In the authors’ experience, both the open and closed approaches produced successful outcomes, providing excellent ultimate limb function. The authors did have considerable experience working with circular external fixators prior to this case series; however, the technique is fairly easy to learn and can be easily combined with the linear IMEX external skeletal fixation system. Although three animals experienced complications, all fractures obtained union, and in only one case was the healing considered delayed. The authors expect that their complication rate will decrease with this and subsequent experience with the technique. The successful outcomes of these cases illustrate the utility of SK-circular hybrid constructs for the management of complex long-bone fractures with short distal segments, particularly in smaller dogs and cats.