Using a Mini or Small SK™ Fixator with the Linear Distraction/Compression Assembly

In recent years, IMEX™ has received a number of requests from veterinarians to develop a method and produce instrumentation to allow for true linear distraction and compression for application in distraction osteogenesis or acute fracture reduction. The most common request was for use with type II fixators for treatment of antebrachial growth deformities when significant bone lengthening was needed in addition to angular deformity correction.

Over the years, IMEX™ has produced a number of prototype distraction/compression devices for these uses. Early models were based on the KE- device. Because of the relative weakness of KE rods, these initial devices frequently failed to provide adequate stability during distraction. Use of larger rods addressed the stability issues but required custom clamps. With the advent of the SK™ ESF System by IMEX™, stronger rods are now standard as are SK™ clamps, which are compatible with the larger rods used for the distraction/compression unit. These technological advancements have renewed interest in a commercially available linear distraction/compression device. Our most recent version is now offered as a standard item, with the body of the distraction unit being the main component.

Postoperative photo of 4.5 month old  Weimaraner treated for femoral shortening. A small SK distraction device was utilized over an IM nail to lengthen the limb.

The body is available in mini SK™ or small SK™ sizes. One end of the body has standard right-hand threads, while the other end has left-hand threads that accept a special left-handed hybrid distraction rod. The end with the left-hand threads is etched with an “L” to remind the surgeon that a left-handed rod is required in that location. Left-handed rods are also clearly marked to avoid any confusion with standard hybrid rods, which are used in many other applications. For the small distractor/compressor, standard hybrid rods are available in 50mm, 75mm, 100mm, 150mm, 200mm and 250mm lengths. Left-handed hybrid distraction rods for the small SK™ distractor are available in 50mm, 100mm and 150mm lengths. Both right and left-hand rods have a thread pitch of 1.0mm resulting in 2.0mm of compression or distraction for each turn of the distraction body. An 8mm wrench is used to turn the small body. Overall length of the small distraction/compression body is 50mm and approximately 23 mm of compression or distraction can be obtained without repositioning rods.

The mini distractor/compressor features a single length choice of left and right-hand modified hybrid rods, which are cut to the final desired length. The standard mini hybrid rod can also function with the mini assembly. All rod components of the mini unit feature a 0.7mm thread pitch, resulting in 1.4mm of compression or distraction for each turn of the distraction body. A 7mm wrench is used with the mini body and mini SK™ ESF clamps. Overall length of the mini distraction/compression body is 23mm and approximately 6mm of compression or distraction can be obtained without repositioning rods. By selecting the appropriate rod lengths based on patient anatomy and distractor body location, a custom fit assembly is produced.

When using the assembly as a distraction device, both rods are threaded to the middle of the body. If compression is the desired use, both rod ends are started closer to their respective body end. Distraction or compression can be achieved by turning the body after fixator clamps and pins have been applied. The body is clearly etched to indicate the turn direction for either compression or distraction (“close” or “open”.) Increments and frequency of distraction are determined on a case-by-case basis. The surgeon must pay careful attention to the rods and body to avoid the rods being pushed out of the body during distraction. Small windows in the body allow visualization of rod positions.

Threaded portions of the rods are not locked into the holes; therefore, potential for rotational instability exists when used in a unilateral frame. In reported cases to date, unilateral frame lengthening applications have incorporated an IM rod. Torsional instability does not seem to be an issue with bilateral frames. Uses to date include: femoral and humeral lengthening over nail, radial lengthening with Type II or Type I-b ESF frame, acute fracture distraction, and creative incorporation into circular ESF frames. This device is not designed to replace circular ESF, but rather provide capabilities for linear distraction/compression for appropriate indications. Potential complications include all linear ESF frame complications plus potential instability, breakage of hybrid rods at threads and all complications typically associated with distraction osteogenesis.