Hybrid external skeletal fixation (HESF) is rapidly becoming recognized as a very valuable tool for repair of fractures with a short distal or proximal segment. Short, juxta-articular fracture segments are often difficult to adequately purchase with a proper number of external fixation pins or bone screws, necessitating transarticular ESF frames or external coaptation of tenuous bone plate repairs. HESF offers a simple alternative for these repairs by using two to three small diameter wires for stabilization of small juxta-articular fracture segments while the primary bone segment is stabilized with traditional linear ESF components and pins. In contrast, true circular external skeletal fixation (CESF) frames consist of multiple external rings interconnected by multiple threaded rods. CESF frames classically utilize these small diameter fixation wires in all locations instead of half-pins or full-pins typically seen with linear fixators. The small diameter of these fixation wires is what makes them an attractive alternative to pins or screws for stabilization of small fracture fragments.

 

In order to compensate for small diameter, small fixation wires are often tensioned or “stretched” across the ring element to increase their mechanical performance. When using small rings on small patients, tensioned wires are not necessary; however, wires are always tensioned when using larger rings. Tension is applied to each wire by use of the IMEX® Dyna™ Wire Tensioner. In order to share loading equally, it is important that each wire is tensioned to approximately the same level. Many feel it is important to tension two wires on opposite ring surfaces simultaneously in order to avoid ring warping that might result in wire tension disparity. When loaded, fixation wires behave in a non-linear fashion demonstrating low stiffness early, but increasing in stiffness as loading increases. Non-linear stiffness allows a theoretical, controlled micromotion that is considered a key component of the Ilizarov CESF method. Professor Ilizarov believed that controlled micromotion enhanced fracture healing and insisted that wires be exclusively utilized instead of ESF pins for this reason.

When one combines elements of traditional linear ESF frames and circular ESF frames, the resulting frame is called a “hybrid frame.” Ilizarov considered the addition of even a single half-pin to a CESF frame to yield a hybrid fixator. However, for the purpose of this article, HESF frames will be considered as constructs containing at least one ring or partial ring, combined with one or more special ESF connecting rods (hybrid connecting rods). Most often, hybrid frames consist of a single ring with wires securing a small bony fragment coupled with what appears to be a Type I-a or Type I-b ESF frame. This “simple” hybrid frame is inexpensive and straightforward to construct compared to more complex CESF frames.

The hybrid rod is the element most commonly used to link the linear and circular ESF frame components. SK® hybrid rods are SK® connecting rods that have been modified to include a short threaded end that couples to CESF components. Since the linear SK® ESF device is available in three sizes, SK® hybrid rods are also available in three sizes. The mini SK® hybrid rod features a 4mm threaded end with a 3.2mm rod shaft. It is available in one length and is easily cut to the appropriate length needed. The small SK® hybrid rod features a 6mm threaded end with a 6.3mm shaft and is the most popular hybrid rod. The large SK® hybrid rod also features a 6mm threaded section, since this is the standard thread for the IMEX® circular external skeletal fixation device, while including a 9.5mm rod diameter. Since most hybrid frames are applied with multiple bars or struts to support the ring, there is not often a strength-need to utilize the large SK® hybrid rods. However, if patient size dictates pins larger than can be used with the small SK® clamp, or if a long unsupported rod length is necessary, then the large hybrid rod and large SK® ESF clamp are appropriate. Since they are difficult to cut, small and large hybrid rods are available in a variety of lengths in 50mm increments.

It is beyond the scope of this article to function as a text for clinical application of hybrid ESF frames. Courses that include HESF frame construction and management are recommended before utilizing HESF clinically. However, representative case examples are provided to highlight frame constructs and demonstrate cases where HESF frames were deemed attractive.

  • Grade II Open Fracture of Distal Radius and Ulna
    Pre-operative radiograph of a 5-year, female spayed Labrador Retriever presented with a grade II open fracture of distal radius and ulna. 

  • Post-operative radiograph demonstrating Type I-b SK® hybrid ESF frame with 4 fixation wires stabilizing distal radial fragment and 6 Interface® half-pins stabilizing proximal radial fragment. Use of flat washers, slotted washers, posts or other methods to place more than two wires per ring is a common technique used to increase stability and decrease patient morbidity when applying hybrid ESF frames. 

  • Intra-operative imaging assisted placement of initial fixation wire and associated ring. 

  • Hybrid ESF frame was applied in a closed manner using hanging limb prep. Frame was removed at 8 weeks.

  • Open Juxta-articular Fracture of Distal Tibia
    Pre-operative radiograph of a 4-year, male feline presented with an open juxta-articular fracture of distal tibia. 

  • Post-operative radiograph showing Type I-a hybrid ESF frame.

  • Comminuted Proximal Tibial Fracture
    Adult Patagonian Cavy presented with comminuted proximal tibial fracture that was treated using a hybrid ESF frame to stabilize short proximal tibial fragment. Fracture healed without complications.

  • Distal Humeral Fracture
    A pre-operative radiograph of an 8-year, Labrador Retriever with a very distal humeral fracture. 

  • Post-operative radiograph showing fracture stabilization with an SK® hybrid ESF frame. Instead of small diameter fixation wires distally, a Centerface® full-pin with an Interface® half-pin were placed using the ring as a platform. Use of traditional fixation pins in this fashion is often termed “linearization” of a hybrid frame. 

  •  Close-up photo shows tie-in of intramedullary pin and additional struts to support the ring. 

  • Post-operative photo showing patient and frame. This frame mimics a two-frame modified Type I-b linear frame which is often utilized on challenging fractures of the distal humerus and femur, but simplifies use of medial half-pins distally.

  • Pes Varus with Distal Apex of Deformity
    An 11-month, male Dachshund with history or progressive deviation of distal tibia resulting in grade 2/4 lameness. Radiographs supported diagnosis of pes varus with very distal apex of deformity.

  • Post-operative view of hybrid CESF frame stabilizing opening wedge tibial osteotomy

In summary, simple hybrid frames are practical alternatives for repair of juxta-articular fractures and selected growth deformities. Although the SK® ESF device is a superior stand-alone linear external fixator due to it’s modular design, it is readily combined with IMEX® CESF components for the construction of hybrid frames. Customers currently using the SK® ESF device can purchase a hybrid starter kit to provide the necessary components for construction of hybrid frames. Veterinarians that are not using the SK® ESF device should consider doing so for many reasons, including the ability to simply and accurately create a hybrid ESF frame when this is needed.