The Mechanics of External Fixation
External fixation has evolved from being used primarily as a last resort fixation method to becoming a main stream technique used to treat a myriad of bone and soft tissue pathologies. Techniques in limb reconstruction continue to advance largely as a result of the use of these external devices. A thorough understanding of the biomechanical principles of external fixation is useful for all orthopedic surgeons as most will have to occasionally mount a fixator throughout their career. In this review, various types of external fixators and their common clinical applications are described with a focus on unilateral and circular frames. The biomechanical principles that govern bony and fixator stability are reviewed as well as the recommended techniques for applying external fixators to maximize stability. Additionally, we have illustrated methods for managing patients while they are in the external frames to facilitate function and shorten treatment duration.
Key wordsIlizarov reconstructionexternal fixationbiomechanicstraumadeformitylimb lengthening
External fixation entails the use of percutaneously placed transosseus pins and/or wires secured to external scaffolding to provide support to a limb. In this way, a bone or joint can be stabilized in the setting of trauma or limb reconstruction. The technique of external fixation was popularized in the mid-20th century when Hoffman introduced a device that used Steinman pins and bars to stabilize long bone fractures. Charnley concomitantly impressed the orthopedic community when he introduced an external fixator for knee arthrodesis. With a simple compression frame, he was able to dramatically increase knee fusion rates and decrease consolidation time. Behrens  described three basic concepts that govern the safe and effective application of external frames for bony trauma: The pins and wires should avoid damage to vital structures, allow access to the area of injury, and should meet the mechanical demands of the patient and the injury. While the Western world was using external fixators sparingly, external fixation was becoming a mainstay of orthopedic treatment in Russia and later in Northern Italy. In Kurgan, Siberia, Professor Ilizarov found external frames to be invaluable for a myriad of applications including posttraumatic and congenital limb reconstruction, limb salvage, complex arthrodesis, management of osteomyelitis and bone defects, and deformity correction. Using a circular fixation design with simple and versatile components, he was able to develop a method for osteogenesis that relied on a percutaneous approach with minimal trauma to the limb, closed anatomic fracture reduction, and excellent bony stability that allowed early weight bearing. In today’s fast growing field of limb lengthening and limb reconstruction, external fixators are powerful tools that enable surgeons to carry out the Ilizarov method and provide solutions to many difficult musculoskeletal problems.
The mechanics of unilateral frames
The mechanics of circular frames
Comparison of Ilizarov and unilateral fixators
Many studies have been performed that compare the stability of circular and unilateral frames in various modes of loading. As frame stability directly affects osteogenesis, these studies have great clinical relevance. The optimal design for an external fixator is one that is rigid in torsion, bending, and shear but allows for axial movement [14, 15]. Paley et al  found the EBI and Orthofix (McKinney, TX, USA) monobody frames to be more rigid than the Ilizarov tibial frame, preventing axial motion at the osteotomy site. This raised concerns from the authors that these unilateral frames may be too stiff, causing stress shielding and delays in healing. With weight bearing monolateral frames experience cantilever bending delivering asymmetric compression to the fracture site. In contrast, the more even loading of the bone ends provided by the Ilizarov frame leads to uniform compression. This was thought to enhance healing and prevent malunion. In studying circular fixators, both Gasser et al  and Podolsky and Chao  noticed the nonlinearity of the load deformation curve exhibited by the Ilizarov frame in response axial loading that was not seen in the monolateral fixators. The Ilizarov frame demonstrated a load dependence of axial stiffness; when subjected to low loads, the frame had low axial stiffness; at higher loads the stiffness of the wires and frame increased significantly. This nonlinear behavior is reminiscent of the viscoelastic properties of biological materials, such as tendons and ligaments, and may be responsible for the promotion of fracture healing. The low frame rigidity seen at lesser loads allows more axial motion and is presumed to be useful for stimulation of fracture callus formation. The higher frame rigidity seen at increased loads is thought to protect the healing fracture tissues from excessive motion preventing pain and fibrous nonunion. This property may explain how the Ilizarov frame has been able to promote osteogenesis where other frames have failed. One must keep in mind that these studies were performed with the all-wire Ilizarov frame, which is not commonly used in the West, where hybrid fixation constructs are predominant. Some researchers have found that hybrid and all-wire frames exhibit similar properties [11, 17]. Others have reported increased stiffness of the hybrid frames in bending and torsional loading .
Provisional fracture stabilization
The rigid monobody fixators have been very useful for lengthenings and deformity correction procedures of the humerus and femur in particular. These fixators offer a several advantages over circular fixation in these locations. The humerus and femur are surrounded by a bulky soft tissue envelope requiring wires to pass through muscle compartments making them less safe to insert and more uncomfortable. The arm is close to the torso making the use of full rings impractical. The use of arches on the humerus provides improved patient comfort but is mechanically similar to unilateral designs. The thigh does not tolerate full rings well as the opposite thigh impinges. Furthermore, thigh rings are very uncomfortable as the patient cannot sit or lie down easily. Unilateral fixators provide stability and much improved patient comfort. Newer models allow for the simultaneous correction of deformity in multiple planes. (Fig. 7) When compared with ringed fixators, the monobody designs are less awkward. Their reduced bulk facilitates hygiene maintenance and accommodates the use of greater clothing options.
Until recently circular fixation had not been embraced in the United States, where most surgeons prefer the more familiar unilateral fixators. This is unfortunate because the Ilizarov fixator is quite versatile. It is a completely modular system that can be assembled on the patient in the operating room to provide optimal stability while respecting safe corridors and skin condition. The rings raise the extremity off of the bed both protecting the skin and providing elevation. The rings also provide circumferential access to the limb for placement of wires and half pins making multiplanar fixation simple. The circular nature prevents cantilever bending. Partial rings can be used to accommodate joint motion or wound access for the plastic surgeons. Additional rings can be attached distally or proximally by using smooth wires to provide temporary improved stability with plans for early removal in the office when healing is underway. Gradual corrections of deformity and shortening can be simultaneously addressed . For large bone defects, multilevel osteotomies can be made to reduce bone transport time. When a bone defect is accompanied by overlying soft tissue loss, simultaneous transport of bone and soft tissue has been successfully performed using the Ilizarov method . For large lengthenings, lengthening over a nail or lengthening and then nailing techniques reduce time in the frame dramatically. Major joints can be spanned with hinges to maintain stability and preserve motion. Hinges can also be used to gradually correct joint contractures.
Mechanics of wires and half pins
The mechanics of transfixion wires, including proper insertion and tensioning techniques, have been elucidated. Wire stability increases with increasing wire diameter and increasing tension placed across the wire. Wire frame stability is enhanced by using more wires per ring, placing wires on opposite sides of the ring, securing wires directly to the ring, and inserting wires in different planes including crossing from the top to the bottom of the ring [10, 17, 21, 22]. Ilizarov  taught that increasing the crossing angles of wires approaching 90° provided maximal stability. It was assumed that this was true in all modes of loading. Further testing has demonstrated that a 90° wire crossing angle provides maximal stiffness to axial loading, but a 30° crossing angle for medial-to-lateral wires provides superior resistance to medial bending forces [10, 23].
A thorough knowledge of the cross-sectional anatomy of the extremity is necessary to avoid neurovascular injury. If under general anesthesia, the patient should not receive paralytic agents as these will mask the important signs of flickering of the distal extremities when a motor nerve is inadvertently irritated by a wire. Spinal or epidural anesthesia will not mask the irritation of a motor nerve. Proper wire insertion demands the use of a low heat technique. It is important to minimize the heat generated during drilling of the wire through the bone in order to prevent bony and soft tissue necrosis, which will typically cause infection and loosening of the wire. To prevent thermal necrosis, the tourniquet should never be inflated prior to drilling as normal blood circulation will help cool passing wires. Saline-soaked sponges can be used to cool and direct the wire during drilling. If the bone is particularly hard, as is the case in diaphyseal wires, then frequent pauses will prevent heat buildup. The all-cortical wires should be avoided as this will generate heat rapidly. When the tip of the wire is through the soft tissue on the opposite side of the extremity, the wire should be pushed the rest of the way using a mallet to avoid winding up of soft tissue.
When inserting a wire near a joint, the joint should be placed in the end range of motion. For example, when placing a wire across the distal tibiofibular joint, and the wire is inserted from posterolateral to anteromedial, the foot should me maximally dorsiflexed prior to piercing the posterolateral skin with this wire and held in that position while drilling through the fibula and tibia to preserve ankle dorsiflexion. The foot should then be positioned in plantarflexion before the wire exits the anteromedial side, insuring uninhibited plantarflexion. Following these recommendations will improve patient comfort and ankle mobility. The same concept is used if passing through fascial compartments, although transcompartmental wires should be avoided as they are uncomfortable and may become infected from movement of the soft tissue at the wire site when activating the muscles .
Wire tensioning greatly enhances the rigidity of the wire and the stability of the frame . Wires are typically tensioned to 130 kg or until the ring begins to deflect whichever comes first. If an open ring or an arch is used, then less tension is utilized (70–100 kg). When older tensioners that do not have a load gauge in them are used, it is theoretically possible to cause plastic deformation of the wire. Tension beyond 155 kg will cause this stretching deformation of the K-wire .
Half pins (Shantz screws)
The mechanics of these pins follow the same trends as those of transfixion wires. As the core diameter of the pins increases, so does the rigidity. A 90° crossing angle of half pins is desirable for improved control in multiple planes. Knowledge of the anatomic safe zones and respect for the soft tissues apply. The principle of low heat generation during half pins’ insertion is again of paramount importance. All half pins should be bicortical and predrilled using a sharp drill bit with a tissue protection sleeve. In hard bone, frequent pauses are prudent, and the drill flutes may need to be wiped clean before passing through the far cortex to avoid thermal damage. Cannulated drills should be avoided in the diaphysis as they will cause thermal damage to the bone. Pins are inserted by hand.
Wires versus half pins
Whether to use a wire or a half pin has become a subject of debate between classic and modern Ilizarov surgeons. Decisions will be made based on surgeon preference, anatomic constraints, and mechanical principles. While all wire or all half pin frames can be correct if applied with sound mechanical principles, we prefer hybrid frames in which wires and half pins are used optimally. Transverse wires are useful in the metaphysis where they avoid muscle compartments and help establish proper ring orientation (reference wire). The reference wire helps with the application of a reference ring to a given bony segment. Wires are useful for tibia and fibula fixation in both the proximal and distal leg—a necessary part of any lengthening procedure. Stable fixation of small bone segments is made possible by using multiple wires. This is especially true in pediatric fractures with narrow metaphyseal segments between the fracture and the growth plate. This is also the case when fixing a short, periarticular bone segment. Multiple wires provide improved fixation in osteoporotic bone. Wires allow for axial motion at the fracture/osteotomy site producing a “trampoline effect” with weight bearing, which is thought to encourage osteogenesis. Because wires can be easily removed in the clinic, they can be used when only temporary fixation is needed. (For example, foot wires and a foot ring are used to temporarily span the ankle joint. The wires and ring can then be removed in the office with minimal discomfort.)
The use of half pins in lieu of some wires gained popularity in the West. Most external fixators used in the United States rely on half pins to provide bony fixation. Advantages of half pin fixation include familiarity in application, patient comfort, rigid fixation, and a low infection rate. Half pins are particularly useful in the diaphysis where large crossing angles can be achieved without invading the muscle compartments of the leg. In the metaphysis of the tibia, half pins can be inserted in a greater anterior to posterior orientation than could be achieved with wires. Half pins are very useful in the femur and humerus for anatomic reasons.
When deciding whether to use half pins or wires for fixation, the clinical scenario may call for one over the other. We have concerns about the use of half pins in patients with neuropathy as the pins have tendency to become loose and infected, or fatigued and break at the level of the cortex. On radiographs, we have observed large areas of bone resorption around the half pins, which are likely the result of uncontrolled weight bearing in patients who lack the protection of pain feedback. (Minimal weight bearing is recommended to these patients). In children we tend to use more wires close to the growth plate. In children with congenital pseudarthrosis of the tibia, we avoid half-pins because of their poor potential for remodeling larger holes from half-pins.
Frame stability and weight bearing
A key component for successful bone healing is early weight bearing and functional activity. Weight bearing provides axial loading to the fracture, nonunion, or osteotomy site that stimulates osteogenesis. Sarmiento et al’s  classic experiment in rat femur fractures treated with either early weight bearing or nonweight bearing showed that functional weight bearing accelerated fracture healing and significantly improved the strength of the fracture callus. Klein et al  found that early weight bearing in combination with an appropriate amount of motion at the fracture site results in accelerated healing rate with increased stiffness of fracture callus. Kenwright et al  found that in tibia fractures treated with a dynamic unilateral fixator, both clinical and mechanical healing were enhanced by the addition of axial micromotion at the fracture site. He added that although rigidly locked frames did allow for earlier full weight bearing, they also prevented axial interfragmentary motion and resulted in delayed healing. Ilizarov  reported that a lack of axial loading in the presence of normal blood supply and adequate bony stability will cause resorption at the bone gap site. He added that weight bearing alone without a sufficient blood supply will inhibit osteogenesis, and weight bearing with poor fixation will lead to resorption of the bone ends. Only the combination of a sufficient blood supply, bony stability, and axial loading will provide the necessary environment for osteogenesis.
The stability of the bone fragments and the rigidity and design of the fixator will determine how much weight bearing is possible. A stable frame will control the bone ends and make ambulation comfortable, which is the first step in getting the patient to walk. The geometry of the fracture or osteotomy gap will affect stability. Good bony contact at the gap site dramatically augments frame stability and the ability to bear weight [7, 10]. Likewise, in a lengthening model, the volume of regenerate will affect frame stability. A shorter and wider regenerate, such as occurs in a metaphyseal osteoplasty location, provides greater stability then a long diaphyseal regenerate. As the regenerate ossifies and unloads the pins, the patient will walk more comfortably.
In general, frames should initially be very stable. This will help with early fracture healing or deformity correction and allow early weight bearing. As healing progresses, the bone bears greater load and less stability is needed for comfort or bony healing. Frames can be gradually dynamized or minimized in the office by removing wires and rings.
Compression and distraction forces
The effect of compression or distraction of the frame across a fracture or osteotomy site is an increase in frame stability. The mechanics of this increase in frame stability is attributable to the effect of compression and distraction on the wires and soft tissues. Compression of the bone ends causes an increase in wire tension and wire rigidity that improves the stiffness of the entire frame. Good bony contact under compression also increases the intrinsic bony stability . Distraction of the bone ends generates significant resistance in the surrounding soft tissue envelope that imparts great tension onto the frame, dramatically increasing stability despite the presence of a bone gap .
Articulated external fixation
Fracture reduction techniques
Removal of external fixator
The first step of frame removal is preparing the bone for upcoming loss of external support. An important part of the bone healing process is the gradual addition of increasing load across the site of osteogenesis as the callus gains stability. This process of dynamization prevents stress shielding by the external fixator and results in greater callus formation. Dynamization is accomplished by destabilizing the frame. Unilateral fixators can be made more unstable by removing bars, sliding the bars further away from the bone, removing half pins, and releasing tension or compression from the system. Circular frames can be dynamized by removing wires, releasing tension from the wires, removing connecting rods between rings, removing whole rings from a ring block, releasing all tension or compression from the system. The main drawback of dynamization is early loss of frame stability leading to delayed union, refracture, or development of secondary deformity.
Timing of removal
Making the decision of when a fixator is ready to be removed is as much an art as a science. General convention is when three of four cortices demonstrate radiographic healing, the bone has enough intrinsic stability to remove the fixator. A more objective method to measure the bone’s ability to tolerate full, unsupported weight has been suggested. While patients were still in the frame, the load passing through the fixator was compared to the load through the extremity at multiple intervals in patients during the consolidation phase of Ilizarov distraction osteogenesis. A load/share ratio was created to calculate the percentage of the weight-bearing load that was being carried by the fixator. As healing progressed, the amount of load that the bone supported increased and the ratio decreased. The group found that when 10% of load passed through the fixator, the frame was ready to be removed . This study shows that more accurate methods are being developed to determine ideal timing for frame removal to reduce time in the frame while avoiding complications of premature removal. Some surgeons are so concerned about these complications that they will remove the fixator and retain the pins for an additional week. If there are signs of bony instability upon the return visit, the frame will be reapplied.
Pin site infection is a common complication of external fixation. In our practice, most infections respond well to aggressive local pin care and empiric oral antibiotics. If the infection does not resolve quickly, then culture-specific antibiotics are used. More advanced infections are aggressively treated with removal of the pin or wire. Cellulitis is treated with intravenous antibiotics. Rarely, operative intervention is needed for pin infections to replace an important pin or debride osteomyelitic bone. Historically, malunion has been a commonly reported complication for fracture treatment with fixators, but this has not been the case in our practice. Septic arthritis may complicate any patient being treated with fixators. Avoidance of intracapsular wires is important. A high index of suspicion should be maintained when using fixation around periarticular fractures, where intraarticular communication with the fracture site is present.
While malunion is a known complication of fracture treatment by external fixation, modern fracture management approaches have minimized this problem. When external fixators are used as a means of obtaining temporary bony stability, obtaining anatomic alignment is not of primary importance. When delayed definitive fixation is performed, typically with an internal implant, the alignment is addressed and malunion is uncommon. Definitive treatment with an external fixator typically involves the use of the newer generation reconstruction frames including the Ilizarov/TSF or a solid, adjustable monobody design. These frames provide the stability needed to prevent fracture fragment shifting and malunion. More importantly, these fixators provide postoperative adjustability. An impending malunion can be realigned postoperatively by using the fixator to correct deformity. This does not typically require additional surgery. By correcting all deformity before the bone heals, a malunion is avoided.
Deep vein thrombosis is a complication that accompanies any lower extremity surgery. Treatment is geared toward prevention. The mobility afforded to patients by external fixation helps with early rehabilitation and prevention of venous stasis decreasing the incidence of this complication. Anticoagulation is often a part of the postoperative protocol.
Although the fixator protects the bone that it spans, it creates a stress riser in the adjacent bone that is not within the frame. This situation can lead to fractures around the frame. Often, this occurs through the most proximal or most distal screw hole. Once the frame has been removed, stress fractures may occur through any of the screw holes before they have had a chance to remodel.
Stiffness of adjacent joints occurs if the joint has been spanned for some time and during lengthening procedures. The ankle joint tolerates this immobility far better than the knee joint. When trauma fixators are used to stabilize tibia fractures, strong consideration should be given to initially including the foot (spanning the ankle joint) to prevent the development of an early equinus contracture. Once the patient is more mobile and pain-free, the foot ring is removed, allowing for ankle motion and easier weight bearing.
The Ilizarov method has greatly contributed to the fields of trauma, limb lengthening, limb reconstruction, and deformity correction. Central to the success of this method is the Ilizarov circular frame both in its traditional and hybrid forms, whose mechanical properties have been demonstrated. Over the last decade the Taylor Spatial Frame has been introduced and embraced by the Trauma and Limb Lengthening communities. This frame has been used with great success for multiplanar deformity corrections, distraction osteogenesis, nonunion management, and fracture care. Although the TSF allows for increased motion while in its neutral mode (due to the universal joint struts-to-ring connections), when under tension or compression the frame clinically provides the stability needed for osteogenesis. Further biomechanical testing and clinical studies need to be carried out with the TSF looking at frame stability compared with the Ilizarov system.