External skeletal fixators (ESF) are commonly used in small animal orthopaedics for primary repair or as secondary supportive apparatus of fractures, luxations, and ligamentous and tendon injuries. In addition, they can be used for realignment following treatment of long bone growth deformities to assist with long bone lengthening procedures such as distraction osteogenesis.

Table 1. Indications and contraindications of ESF placement - adapted from the BSAVA Manual of Small Animal Fracture Repair & Management¹

Fixators themselves consist of pins that are placed percutaneously and that cross the bone being connected externally using clamps and connecting bars, rings, or a combination. They can be placed in a variety of different ways, most commonly in a linear fashion, although circular and hybrid frame configurations also exist.

Advantages of ESF placement over some of the more traditional methods of fracture repair include the ability to repair fractures in a minimally invasive manner, avoiding damaging the fracture callus and encouraging biologic osteosynthesis. They allow access to the skin, allowing treatment of external wounds and dressing changes, so are particularly useful for treatment of open fractures and shearing injuries. They are well tolerated in small animal patients and do not require a surgical approach for removal. The frames can be adjusted after placement if necessary and may allow alignment changes in the early stages of healing.

The components of linear external skeletal fixators include transfixing pins or Kirschner wires, connecting bars and clamps, whilst circular and hybrid frames use rings and threaded bars in addition to the transfixing components.

Linear ESF Frames

There are 4 basic frame types used in small animals - type 1a and b, type II, and type III (illustrated in the lecture). As frame configuration progresses from type Ia–Ib–II–III, so does frame stiffness under torsion and axial loads, although type Ib frames resist shear forces under compression better than type II.²

Type I/Unilateral - Most widely used frame, can be used on all long bones.

One plane - Useful for primary repair in fractures likely to heal quickly. Also used for auxiliary fixation - in conjunction with a lag screw or IM pin. Mandibular fractures.

Two plane - More robust, with increased torsional stability.

Type II/Bilateral one plane - Uses full pins so can only be used on distal limbs. Commonly used for treatment of radial and tibial fractures.

Type III/Bilateral two plane - Most complex and strongest design. Used for highly unstable fractures or when time to fracture healing is likely to be prolonged. Useful in comminuted tibial fractures but can be used on the radius.

Circular/Hybrid Frames

Ring fixators use K-wires as fixation pins, the stiffness being achieved by tensioning them as they are attached to the circular ring frame. Using threaded connecting bars allows tension or compression to be applied to the frames, making these frames especially suited to corrective osteotomies or arthrodesis. Due to the small fixation pins, rings may be used in juxta-articular fractures where placement of normal pins is impossible.³ In these cases, they can be combined with linear frames (hybrid).

Principles of ESF Placement

There are fifteen critical principles, determined by experimental investigations and clinical research that should be followed when placing an ESF.⁴

 Use aseptic technique

 Use ideal bone surface location for insertion of the pins

 Radius - craniomedial

 Tibia - medial

 Humerus - craniolateral

 Femur - lateral

 Use the most suitable frame configuration

 Auxiliary fixation should be used when indicated

 The fracture should be reduced and maintained in reduction during application of the frame

 Insert pins through soft tissue in a manner that does not distort the tissue

 Pin-drilling technique is critical

 Insert pins through both cortices of the bone

 Insert smooth and negative profile pins at an angle of 70 degrees to the long axis of the bone

 Insert all related pin fixation clusters in the same plane

 Insert pins in the proper location of the bone fragment

 Insert two to four pins in each major bone fragment

 Choose optimal-size fixation pins

 Place connecting rods an optimal distance away from the skin

 Use a bone graft for significant cortical defects

The weakest point of any frame is the pin-bone interface where stress concentrates during loading. This is most commonly seen in unilateral frames at the cis-cortex of insertion. Over time this can lead to bone resorption and cartilage production, leading to loosening of the pin. This reduces the stability afforded to the frame and causes soft tissue and periosteal irritation.

There are a number of techniques that can be used to minimize this phenomenon.

 Placing the pins using low-speed power into slightly smaller pre-drilled holes. Manual placement allows wobble when placing pins, and high-speed drilling can cause heat necrosis of the bone, leading to bone resorption and subsequent loosening.⁵

 Using threaded pins. Pins can be positive profile (threads rolled onto the pin during manufacture), or negative profile (threads cut into the pin). The negative profile pins have a stress riser at the thread-pin interface and are therefore weaker than positive profile pins.⁶ If negative profile pins are used, then the whole of the threaded section must be placed within the medullary canal of the bone.

Pin size is also an important consideration. Pins should not exceed 20–25% of the bone diameter to prevent iatrogenic fracture during placement.⁷

Bone Healing

As with all fractures, bone healing depends on a variety of biologic and mechanical influences acting on the site itself. More rigid frames reduce movement at the fracture site, encouraging rapid union through primary bone healing, whilst less rigid frames encourage callus formation. The type of injury also determines speed of healing. In cases of severely traumatic fractures with damage to the vascular supply and surrounding soft tissues, ESF placement allows the surgeon to choose where to place transfixing pins in a minimally invasive manner, avoiding the damaged tissue and preserving the biologic potential of the fracture site.

Aftercare

Immediately following surgery, the pin-skin interface wounds should be dressed, and compressive sponges should be placed between the connecting bars and the skin to reduce swelling. Pin tracts should be kept clean and free from discharge - daily dressing changes may be performed initially with gentle cleaning of the tracts with dilute chlorhexidine or povidone-iodine. The clamps and any sharp ends from the transfixation pins should be dressed to protect the patient and owners from getting scratched by the frame. The client should be instructed to check the pin tracts daily and clean them as necessary. If they notice excessive discharge or swelling around them, veterinary advice should be sought. A veterinary check should be performed every two weeks whilst the frame is in place to ensure no frame slippage, loosening, and to evaluate pins - broken pins can commonly be missed by owners when inspecting the frame.

Frame removal should take place following evidence of healing, although this can be challenging when some of the more complex frames are placed, due to metal overlying the fracture site. Transarticular frames for stabilisation following ligament/tendon repair or stifle stabilisation surgery should be removed at six weeks due to the risk of contracture developing and cartilage damage.

When removing the frame, sedation is usually required. Full transfixation pins should be cut at one cortex, allowing only a short piece of pin to be pulled back through the bone. Positive profile pins are often more challenging to remove, and patients may need to be heavily sedated or fully anaesthetised to allow removal. The skin should be clipped and scrubbed prior to removal, and pin tracts cleaned after removal of the frame. Patients often are unwilling to weight bear for 24 hours following removal. Provision of anti-inflammatory medication for a couple of days is often warranted. A further 3- to 4-week period of restricted exercise should be followed after frame removal.

Complications

The most common complication seen following ESF placement is pin-tract drainage. This is most commonly seen when the pin is placed through deeper soft tissues or when the pins are loose. Pin-tract discharge prevents granulation of the sites, maintaining an open wound that is susceptible to contamination from skin commensals.⁸ Regular pin-tract cleaning by the owner and application of dressings may resolve the issue if there is not significant loss of function. If, however, the patient has loss of function, removal of loose pins or appropriate antibacterial therapy may be indicated. Increasing the skin incision around the pin's tract may also reduce skin movement and inflammation.

Sequestra are small pieces of bone that separate from their vascular supply and may develop when pins are placed using excessively high speed or under excessive pressure. This is a rare complication and is managed by removal of the pin and associated sequestrum. Iatrogenic fracture during pin placement is usually avoided by ensuring appropriate pin sizes are used and by avoiding placement in fissures or too close to the fracture line. If fracture does occur, pin replacement in intact bone is performed.

References

1.  Miller A. Principles of fracture surgery. In: BSAVA Manual of Small Animal Fracture Repair & Management. 2nd ed. Gloucester: BSAVA; 2006:59–85.

2.  Egger EL. Static strength evaluation of six external skeletal fixation configurations. Vet Surg. 1983;12:130.

3.  Lewis DD, Lanz OI, Welch RD. Biomechanics of circular external skeletal fixation. Vet Surg. 1998;27:454.

4.  Piermattei DL, Flo GL, DeCamp CE. Fractures: classification, diagnosis and treatment. In: Brinker, Piermattei and Flo's Handbook of Small Animal Orthopedics and Fracture Repair. 5th ed. St Louis, MO: Saunders Elsevier; 2016:24–152.

5.  Egger EL, Powers BE, Blass C, Histand MB. Effect of fixation pin insertion on the bone-pin interface. Vet Surg. 1986;15:246.

6.  Anderson MA, Mann FA, Kinden DA, et al. A comparison of non-threaded, enhanced threaded, and Ellis fixation pins used in type I external skeletal fixators in dogs. Vet Surg. 1993;22:482.

7.  Martinez SA, DeCamp CE. External skeletal fixation. In: Veterinary Surgery: Small Animal. St. Louis, MO: Saunders; 2012:608–627.

8.  Aron DN, Dewey CW. Application and postoperative management of external skeletal fixators. Vet Clin North Am Small Anim Pract. 1992;22:69–98.