External skeletal fixation (ESF) is an extremely versatile, affordable, and adaptable method of fracture fixation. Indications for ESF include long bone fractures, mandibular fractures, spinal fractures, nonunions and delayed unions, stabilisation of osteotomies, arthrodesis, and temporary joint immobilization. Stiffness of the ESF is affected by frame configuration, connecting bars, and pin factors. External skeletal fixation frames utilizing acrylic connecting bars offer increased versatility. Potential complications associated with ESF can be minimized by following basic principles when applying the fixator. External skeletal fixation is the only fracture stabilisation technique that allows for stage disassembly in order to improve bone healing.
This paper will address the basic principles of ESF, especially acrylic ESF.
External skeletal fixation (ESF) was first utilized in the late 1800s but lost popularity during and after WW2 due to the unacceptably high complication rates. These complications included pin loosening, pin tract sepsis, osteomyelitis, and non union. Use of ESF gained popularity in the 1970s again and is widely used in veterinary orthopaedics today. Complications have been reduced, mainly by improved pin design and pin insertion techniques.
Advantages of ESF:
Ability to apply with minimal trauma/closed reduction
Does not require specialized equipment when compared to plate/screws
Components of an ESF system:
Transfixation pins (smooth vs. threaded; cortical vs. cancellous; negative profile vs. positive profile; full vs. half)
Connecting bars (stainless steel, titanium, acrylic)
Pin gripping clamps
Principles of External Skeletal Fixation
Important factors when using ESF are frame stiffness and pin-bone interface. Pin-bone interface is very important to prevent premature loosening of the ESF. The main factors influencing pin-bone interface include pin design and pin application technique. Frame stiffness is affected by frame configuration/design, number of transfixation pins, pin design, pin placement, type of clamp and connecting bar, diameter of connecting bar, and the fracture configuration.
One can see from the above that all these factors, with the exception of fracture configuration, are controlled by the surgeon. It is important that the chosen frame has adequate stiffness for the specific fracture. A simple transverse fracture in an 8-month-old dog will not require a frame as stiff as a highly comminuted fracture in an old dog.
External skeletal fixation frames may be classified as unilateral or bilateral and uniplanar or biplanar. External skeletal fixation frames are commonly classified as type 1, 2, or 3 frames.
Type 1a = unilateral, uniplanar
Type 1b = unilateral, biplanar
Type 2 = bilateral, uniplanar
Type 3 = bilateral, biplanar
All configurations may be used on the distal limb (distal to elbow & stifle). Proximal to the elbow and stifle, one is usually restricted to a type 1a frame due to the presence of the body wall on the medial aspect of the limb, which prevents placement of a medial connecting bar. For distal humerus and femur fractures, an enhanced biplanar hybrid may be utilized (i.e., the distal pin is a full pin and a second diagonal connecting bar connects the medial end of the distal pin with the lateral pins proximally). This type of frame results in increased stiffness.
A "tie-in" configuration refers to the combination of a type 1a frame with an intramedullary pin. The proximal end of the intramedullary pin is left long, protruding through the skin and is connected to the connecting bar (i.e., "tied-in" to the ESF). This configuration is applicable to humerus and femur fractures and increases frame stiffness.
Connecting bars may be manufactured from stainless steel, titanium or acrylic. Acrylic connecting bars offer several advantages:
Ability to contour bar to desired shape, angle depending on fracture, body part or joint
Pins do not all have to be placed in the same plane
Very useful for mandibular fractures
Allows correct angle when using transarticular frames
May use polymethylmethacrylate (PMMA) or dental acrylic
Equivalent strength compared to stainless steel bars
Research has shown that an acrylic bar with a 0.75-inch diameter has equivalent strength to a 4.8-mm stainless steel bar. It also revealed that the acrylic/fixation pin junction is stiffer and able to withstand greater loads when compared to a connecting bar/clamp/fixation pin junction (Kirchner clamp). When the fixation pins are notched the pin/acrylic bar junction is further strengthened.
Acrylic bars are best applied by mixing to a pourable consistency and then pouring into plastic tubing that has been placed over the ends of the fixation pins. The setting process of the acrylic is an exothermic reaction. Care must be taken when the acrylic is setting that the pins and soft tissue are kept cool by flushing with saline and keeping the connecting bar 1 cm from the soft tissue. Heat conducted along the pins may result in thermal necrosis to the bone and contribute to premature pin loosening.
Pins are classified according to their placement (i.e., pins penetrating both the near and the far cortex, but not the far skin, are termed half pins; pins penetrating the near and far cortex and the far skin are known as full pins). Pins are also classified as smooth or threaded. Threaded pins are further classified as positive or negative thread and as cortical or cancellous thread. Negative-profile thread pins have the thread cut into the pin (i.e., the shank of the pin becomes narrower) and are therefore theoretically weaker). Positive-profile pins have the thread on the shank (i.e., there is no narrowing of the pin) and these pins are considered stronger.
The stiffness of an ESF frame is affected by many factors. Frame configuration affects stiffness with a type 3 frame being the stiffest. Four pins per fragment give optimal stiffness. Larger-diameter transfixation pins improve frame stiffness (up to 30% of bone diameter). Large connecting bars placed closer to the skin improve stiffness. Spacing transfixation pins (i.e., close to fracture and far from fracture) improved stiffness. Fracture configuration will also affect construct stiffness (i.e., if the fracture can be reduced and can share the load it will be a stiffer construct when compared with a non-reducible comminuted fracture).
Pin Insertion Technique
Pin insertion technique is critical to maintenance of the pin-bone interface. Any mechanical or thermal damage caused during pin insertion has the potential to decrease the longevity of the pin-bone interface and lead to premature pin loosing as well as increased pin tract discharge.
Ideally, pins should be placed with a low-speed power drill (< 150 rpm). Placement with a hand chuck should be avoided due to the risk of mechanical damage caused by "wobble." Threaded pins, especially positive-profile pins should be inserted by pre-drilling a hole with a slightly smaller diameter than the pin to be placed.
Ideally, four pins per fragment should be placed with a pin close to the fracture (1 cm) and another as far as possible from the fracture (close to the joint). Pins should have a diameter of no greater than 30% of the bone diameter (ideal 25–30%).
Soft Tissue and ESF
Pins should ideally be placed through safe corridors avoiding large muscle masses and neurovascular structures. A releasing or stable incision should be made in the skin and a tunnel bluntly dissected to the bone to allow pre-drilling and pin placement. When full pins are used, a stab incision should be made where the pins exit the skin on the opposite side of the limb.
Postoperatively, there is differing opinion as to the care of the ESF and soft tissues. Some people advocate bandaging of the ESF. This potentially reduced movement between the skin and the pins, thereby reducing inflammation and pin tract infection. Pin tracts/skin wounds may be cleaned as necessary dependent of the amount of discharge present.
Staged disassembly refers to the conversion of a rigid frame to a less rigid frame once fracture healing has begun. This results in a phenomenon known as dynamization (i.e., free axial loading but resistance to bending and torsion). This results in micromotion at the fracture site. Controlled micromotion enhances fracture healing. There appears to be an ideal time during healing for staged disassembly and dynamization. When performed at 4 weeks postoperatively, it resulted in increased callus formation and reduced strength; when performed at 12 weeks postoperatively, it appeared to have no effect on fracture healing; but when performed at 6 weeks postoperatively, it resulted in enhanced healing and increased strength. Staged disassembly is normally achieved by reducing the number of transfixation pins or converting a type 2 frame to a type 1 frame
Pin tract discharge and sepsis
Premature pin loosening
Skin necrosis/pressure necrosis for connecting bar