External Fixation in Orthopedics

In the presence of adequate stability and vascularity, bone fractures heal with histologically normal tissue, unlike injuries to most tissues, which typically develop fibrous scar tissue. Historically, physicians have sought methods to treat extremity fractures to achieve stability, including casts and internal fixation. Rigid casts made from plaster of Paris or synthetic plastics are noninvasive and inexpensive and can be used for the treatment of most stable fractures. However, casts do not provide adequate immobilization for grossly unstable fractures, and inadequate immobilization may result in nonunion, delayed union, or displacement of fracture segments. Also, casts immobilize muscles and joints adjacent to the fracture, which can result in weakness and fibrosis of muscles, stiffness of joints, and contractures. Because of these potential problems, surgical treatment with internal and external fixation is often indicated for unstable fractures.

Intramedullary nails and plate and screw combinations are examples of internal fixation. Internal fixation allows the surgeon to align and stabilize the bone under direct vision or, more recently, under radiographic control using minimally invasive techniques. However, there are disadvantages to internal fixation, including surgical dissection for introduction of the fixation device; insertion of a foreign body and the additional difficulty of eliminating bacterial contamination in open fractures; development of osteoporosis of the bone directly under the plate due to stress shielding; and in some cases the need for supplementary cast immobilization and avoidance of full function.

External fixation is a minimally invasive technique that obviates many of the disadvantages of casts and internal fixation. It is accomplished by placing a scaffold-like frame with attached wires, pins, or both outside of the extremity. The wires and pins have small diameters and transfix the bone.

External fixation allows for acute or gradual fracture reduction. If gradual reduction is indicated, the transfixed bone can be slowly realigned with frequent small adjustments to the external apparatus. Joint stiffness and muscle weakness are reduced because the stable, rigid fixator allows for full functioning of the extremity while the fracture is healing. External fixation can also be used for reconstructive orthopedics: the treatment of deformities,¹ bone loss or defects,² chronic infections,³ contractures,⁴ limb length discrepancy,⁵ and nonunions.⁶ In all cases, the external fixator remains in place until healing occurs and is then removed, leaving no foreign material in the extremity.

External fixators are classified by their design as either monolateral, multiplanar, or hybrid (Figure 1). A monolateral fixator is a bar or rail with attached pins that transfix the bone (Figure 1, A). It is applied to the lateral aspect of the femur and to the medial aspect of the tibia in order to avoid transfixion of neurovascular structures and minimize transfixion of muscle. Multiplanar external fixators consist of rings or sections of rings (arcs), with attached pins, wires, or both that are used to secure fixation of the bone (Figure 1, B). The choice of wires vs pins is determined by the proximity of the implants to neurovascular structures and muscle. A frame consisting of both rings (multiplanar) and bars (monolateral) is called a hybrid system.

External fixators can also be classified by their function as either stationary or moving. Stationary fixators are used for the acute stabilization of fractures. Their placement and the alignment of the bony fragments remains unchanged from application to removal. Moving fixators are used in reconstructive orthopedics and, more recently, in the gradual reduction of acute extremity fractures. They can change configuration for gradual correction. Both monolateral and multiplanar fixators can function as either stationary or moving fixators.

Although external fixation can be used to treat almost any acute extremity fracture, it is frequently the preferred treatment for juxta-articular fractures, open fractures, and fractures with bone loss. Fractures near joints (juxta-articular fractures) are often comminuted, with the fracture lines extending into the joint. Examples include fractures of the distal radius, proximal tibial plateau, and distal tibial pilon. Because of their proximity to a joint and because the fragments are often small, it may be impossible to stabilize them with a nail or plate and screws. Computed tomography can be used to define the fracture pattern. Using these images, the surgeon can then determine the ideal placement of the wire implants to achieve reduction and maintain stability during healing.⁷

Open fractures of the extremities present a particular challenge: they require soft tissue coverage and elimination of bacterial contamination. It may be difficult to eliminate infection of an internal fixation implant in an open fracture because the properties of the receptor surface of the device encourage bacterial production of an overlying glycocalyx biofilm on the device. This disadvantage is obviated if the external fixator is placed distal and proximal to the open fracture. The fixator provides stability at the fracture site without introducing implants into the contaminated area. The wound can then be sterilized using debridement, the application of antibiotic-impregnated polymethylmethacrylate beads, and soft tissue coverage.

In cases of polytrauma and severe soft tissue injury, rapid external fixation allows timely treatment of the more emergent injuries. If there is a compartment syndrome or laceration of blood vessels or nerves, fractures can be stabilized until compartment pressure is relieved and neurovascular structures can be repaired. Once this is accomplished, anatomic alignment can be restored.

Open long bone fractures associated with significant bone loss are an indication for external fixation. Two techniques are used for the treatment of bone loss: bone transport^{2 ,6} and acute shortening with subsequent gradual lengthening.⁸ With bone transport, the original distance between the proximal and distal segments is restored and stabilized with the external fixator (Figure 1, A). A corticotomy (cracking of the bone) is performed on the uninjured metaphysis of one or both segments to create a transport segment. The transport segment is gradually moved into the gap area at 0.25 to 1 mm/d, generating a cylinder of new bone. External fixation with bone transport preserves bone length and alignment, allows maximum function, avoids kinking of soft tissues, and avoids the need for fibular shortening in the treatment of tibial fractures. The other method used for treatment of fractures associated with bone loss is acute shortening followed by subsequent gradual lengthening. At the time of shortening, a corticotomy is simultaneously performed at an uninjured metaphyseal site using the external fixator as a lengthening device, which allows fracture reduction under direct vision through the soft tissue defect.

External fixation offers options for the treatment of nonunions, which are classified as hypertrophic or atrophic based on the radiographic appearance. Hypertrophic nonunions are characterized by transverse bone growth at the fracture without new bone growth across the fracture; the radiographic appearance has been compared with 2 apposing elephants' feet. Hypertrophic nonunions have sufficient vascularity and osteoblast cells for bone growth but lack stability for the ossification of the fibrous and cartilaginous tissue spanning the fracture site. These nonunions, with or without deformity, will heal if distraction stress is applied longitudinally to the apposing bone segments via an external fixator. The fixator gives stability to the nonunion; the longitudinal traction converts fibrous tissue to lamellar bone.

Atrophic nonunions lack adequate stability and vascularity at the fracture site. The radiographic appearance of the bone ends may resemble licked candy sticks or a ball in socket.Atrophic nonunions are treated with bone transport, similar to open fractures with bone loss. The nonunion is resected to create a segmental defect. Then, a corticotomy is performed on a healthy metaphyseal area to serve as a site for new bone growth as the defect is gradually filled by advancement of the transport segment. Resection and transport can also be used to treat chronic osteomyelitis, with or without fracture nonunion. External fixation has also been used for correction of angular deformities, in limb-sparing treatment of bone malignancies,⁹ and for limb lengthening in short stature and dwarfism.¹⁰

The external fixation technique was improved by the Soviet physician Gavriil Ilizarov and first used in the United States in 1986.¹¹ Compared with previous lengthening methods, this technique increased the potential for bone lengthening and allowed simultaneous lengthening and correction of deformities. The Ilizarov method is the technique for generation of both bone and soft tissue through slow distraction divided into multiple small increments by using an external fixator. Although the mechanical characteristics of other fixator systems differ from the Ilizarov fixator, the same principles of multiple small increments of lengthening per day, preservation of the blood supply to the bone through minimally invasive corticotomies, and full function of the extremity are followed to achieve similar results independent of the brand of fixator. The surgeon performs a corticotomy after the fixator has been applied parallel to the axis of the bone. The frame is lengthened in small increments (typically 0.25 mm) 2 to 4 times daily. These adjustments are made by the patient or a family member after hospital discharge until the desired length is achieved. Normal lamellar bone forms as tension is gradually applied to the organizing hematoma that forms at the corticotomy site. Lengthening of 15% to 20% of a bone's original length is usually feasible. By the use of repeat lengthenings and lengthening at 2 levels of the same bone simultaneously, greater lengthenings are achieved.

Postoperative infection of the implant site still occurs despite implant hygiene protocols. Many different protocols are used; there are no studies to document which protocol is best.¹² Internal fixation used simultaneously with external fixation or used subsequent to external fixator removal can reinforce bone treated with external fixation. This combination of fixation methods has decreased the incidence of refracture and decreased the time a patient must wear an external fixator.^{13 - 14} Acceleration of bone healing in external fixators with the use of low-intensity ultrasound,¹⁵ pulsed electromagnetic fields,¹⁶ and bone growth factors¹⁷ has been documented in animal studies but human applications have not been reported.

As the indications continue to increase, methods for resolving inherent challenges become even more critical. However, external fixation already offers an excellent option for the treatment of many traumatic, developmental, infectious, and neoplastic conditions, providing good results with minimal hospitalization and interruption of employment or education.