External Skeletal Fixation (ESF) in Small Animals

External Skeletal Fixation (ESF) is a method of immobilizing long-bone fractures, as well as fractures of the mandible, by inserting transcutaneous pins into the bone fragments, which are then connected by external bars or rods.

Components of an ESF System

An ESF system typically consists of several key components:

• Fixation Pins: These are stainless steel pins that penetrate both the first and second cortices of the bone.

    ◦ Types: Pins can be smooth, partially threaded, or fully threaded. Positive-thread-profile pins, with threads built up to a larger diameter than the shaft, are available for both cortical and cancellous bone insertion and are often preferred for their improved immediate stability and reduced risk of early pin loosening and pullout. Negative-thread-profile pins have threads cut from the pin's stock.

    ◦ Configuration: Half pins penetrate one skin surface and two bone cortices, used in Type I frames. Full pins penetrate two skin surfaces and two bone cortices, forming the basis of Type II frames.

    ◦ Number and Placement: A minimum of two pins per major bone fragment is required for stability, but three or more are often indicated. Widening the spacing between pins to place them close to the bone ends and fracture line stiffens the construct. Angling smooth pins 70 degrees relative to the long axis of the bone stiffens the frame and helps prevent accidental dislodgement. This angulation is less critical with positive-thread-profile pins.

• Connecting Bars: These rods connect the fixation pin clusters to provide stability.

    ◦ Material and Stiffness: Typically solid stainless steel, but larger-diameter carbon fiber, titanium, or aluminum rods are available (e.g., IMEX-SK system) for increased stiffness without significant weight. Carbon fiber rods are also radiolucent, which aids in radiographic evaluation of healing.

    ◦ Contouring: Conforming the connecting bar close to the bone increases frame stiffness by decreasing the working length of the fixation pin. Acrylic materials can also be molded as connecting bars, particularly for bones like the mandible and maxilla, offering flexibility in pin placement.

• Clamps: These connect the fixation pins to the connecting bars.

    ◦ Types: Single clamps grip one pin and one bar, rotatable in two axes. Double clamps connect two bars and are integral to multiplane frames, though they are less stiff than single clamps.

    ◦ Design Improvements: Newer designs like the Secur-U clamp (Securos) and SK clamp (IMEX Veterinary) offer improvements such as attachment between previously installed clamps and acceptance of enhanced-thread pins. Secur-U clamps have been shown to be more resistant to pin-clamp slippage and provide greater rigidity than traditional Kirschner-Ehmer clamps.

Frame Configurations

ESF frames are classified by their configuration, which indicates their stiffness:

• Type I/Unilateral: The most widely used, applicable to all long bones.

    ◦ IA/One Plane: Uses single or double clamps with one or two connecting bars.

    ◦ IB/Two Plane: Consists of two Type IA single-clamp frames applied at an angle (60-90 degrees) to each other, increasing torsional rigidity.

• Type II/Bilateral One Plane: Uses full pins and is applicable only to the lower limbs (distal to elbow or stifle).

• Type III/Bilateral Two Plane: The strongest and most complex/costly design, used for extreme instability. Seldom used today due to being cumbersome and excessively rigid.

• Ring Fixator: Uses small-diameter, flexible Kirschner wires under tension, primarily for corrective osteotomy, limb lengthening, fracture repair, and arthrodesis. Also used in very distal or proximal tibial fractures where bone stock is limited.

• Hybrid Systems: Combine linear and circular components, useful when bone stock is limited or soft tissue injury restricts pin placement.

Biomechanical Considerations

Frame stiffness generally increases from Type IA to IB to Type II to Type III. All frames are stiffest in their plane of application. Increasing the number of fixation pins increases the pin-bone interface, reducing bone resorption and pin loosening. Stiffness of the connecting bar is crucial for Type I unilateral splints, and strategies to increase it include adding a second bar or conforming the bar close to the bone. Acrylic connecting bars also offer good mechanical characteristics and allow more freedom in pin placement.

Indications and Uses

ESF is highly versatile and applicable in various orthopedic situations:

• Fracture Types: Stable and unstable fractures, open fractures, gunshot fractures, and various diaphyseal fractures (Types B and C).

• Specific Conditions: Osteotomies, delayed unions, nonunions, arthrodesis of certain joints, and stabilization after ligament or tendon reconstruction. It is particularly adaptable to infected fractures and severely traumatized cases.

• Minimally Invasive Approaches: ESF lends itself to closed reduction or minimal open approach (biological osteosynthesis), preserving soft tissues and blood supply.

Advantages of ESF

• Ease of Application: Relatively straightforward to apply.

• Versatility: Can be used with both open or closed reduction methods.

• Minimal Approach: Minimizes surgical approach size.

• Wound Access: Pins can be inserted away from open wounds, allowing easy access for dressing changes.

• Compatibility: Compatible with other internal fixation devices.

• Tolerance: Well tolerated by both dogs and cats.

• Removability: Can usually be removed without general anesthesia.

• Cost: Generally reasonable cost.

• Infection Management: Especially useful for infected fractures.

Application Fundamentals

• Aseptic Technique: Essential for patient preparation, operating room, equipment, surgeon, and postoperative care.

• Pin Location: Pins should be inserted where they penetrate skin directly into bone, minimizing soft tissue irritation and maximizing pin stiffness. Preferred locations: medial for tibia, craniomedial/medial for radius, craniolateral for humerus, lateral for femur.

• Fracture Reduction: The fracture should be reduced and maintained in reduction during splint application to minimize tissue irritation and discomfort. Load sharing between bone and fixator is desirable to reduce stress on the fixator.

• Connecting Rod Distance: Optimal distance from skin is typically 3/8 to 1/2 inch (10-13 mm) to allow for postoperative swelling.

• Auxiliary Fixation: Lag screws, IM pins, Kirschner wires, and cerclage wires can be used to aid reduction and enhance stability, especially with less stiff frames.

Specific Applications

• Humerus: Applicable to all diaphyseal fractures and distal nonarticular Type A fractures. Unilateral Type I single-bar or double-bar fixators are used. Can be combined with IM pins for rotational and compressive shear forces. Distal pin may be transcondylar.

• Radius/Ulna: Particularly useful due to limited surrounding soft tissue. All linear configurations (Type I, II, III), circular, and hybrid fixators can be used. Type I single bar is often adequate and simplest. Closed reduction is more feasible than for humerus/femur.

• Femur: External skeletal fixators are limited to relatively weak Type IA frames. Often combined with IM pins to increase stability and maintain length. Preferred for smaller patients or when medial cortical continuity can be restored. Cats tolerate femoral ESF well, unlike dogs where it can lead to delayed healing and stifle joint tie-down.

• Tibia/Fibula: Applicable to almost all tibial shaft fractures, including delayed unions and nonunions. Type I splints are almost always applied medially due to absence of muscle coverage. Type IB biplanar frames are useful for proximal and distal fractures with limited bone stock.

• Joints (Arthrodesis/Shearing Injuries): Used to immobilize joints, particularly with open wounds, offering stability and allowing wound care. Common for tarsocrural, pantarsal, and proximal intertarsal arthrodesis.

• Mandible/Maxilla: Useful for nonunion, multiple, bilateral, or unstable fractures, especially with bone loss. Acrylic bars can be substituted for conventional clamps, simplifying application and allowing varied pin angles.

• Growing Animals: Fundamentals are similar to adults, but healing is rapid, less stiffness is required, and 2/2 pins are usually sufficient. Pins should not bridge growth plates or penetrate both bones of a paired system.

• Vertebral Column: Can be applied via open or closed (fluoroscopic guidance) approaches. Pins are typically placed bilaterally, spanning two vertebrae cranial and caudal to the injury, connected by carbon fiber arches. A main benefit is implant removal without a second large surgery.

Postoperative Care

• Bandaging: A compressive (light Robert-Jones) bandage is often applied post-surgery to protect incisions and minimize swelling, especially for radius/ulna and tibia fractures. For humeral and femoral fractures, this may not be possible, and a protective cover for the fixator is applied immediately.

• Wound Care: Open wounds and pin sites are covered with sterile, nonadherent dressings and padding. Frequent bandage changes (every 1-3 days) may be needed for open or severely injured fractures until granulation tissue forms.

• Activity Restriction: Exercise is limited during the healing period. Passive Range of Motion (ROM) exercises are encouraged as soon as tolerated to prevent joint stiffness, especially in the elbow and stifle.

• Dynamization: Staged disassembly of the fixator (e.g., removing bars or pins) can be done after early healing (around 6 weeks) to increase axial compressive loading and stimulate callus hypertrophy and remodeling.

• Implant Removal: Fixators are removed once clinical union is confirmed radiographically. Loose pins are removed. Pin sites are cleaned and covered with sterile gauze for 48-72 hours, but not sutured.

Complications

• Pin Track Drainage: The most common complication, often due to excessive skin/soft tissue movement or loose pins. Careful pin placement and soft tissue immobilization can minimize this.

• Pin Loosening: Caused by soft tissue interference, fracture instability, overly flexible frames, or insufficient pins. Leads to drainage and potential infection. Loose pins should be removed, and if stability is compromised, additional pins inserted.

• Infection: Often associated with pin tract drainage and can lead to delayed union or nonunion.

• Overfixation: Using excessively rigid frames can lead to delayed or nonunions due to "stress shielding" (where the implant carries too much load, preventing bone healing stimulation). It can also interfere with muscle function and recovery.

• Thermal Bone Necrosis: Can occur during pin insertion if drills are run too fast without adequate cooling, leading to ring sequestra.

• Soft Tissue Impingement: Can cause discomfort and restrict joint range of motion, particularly in the femur.