Textbook of Ilizarov Surgical Techniques: Bone Correction and Lengthening Vladimir Golyakhovsky, Victor H Frankel
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1Introductory Chapters2

Biomechanics of the Ilizarov External FixatorChapter 1

The Ilizarov multiplanar circular external fixator is a remarkable versatile device; it possesses some of the most optimal biomechanical characteristics for the correction, distraction and compression of the bone. Unlike the uni- and biplanar large-pin fixators it can be used to treat virtually any type of limb deformities. When the bone is loaded, the fixator maintains higher axial elasticity and stiffness to bending, in comparison to other conventional fixators.
Various pieces of this device permit assemblage of many different frames combinations, and adjustment or reassembly during the long course of the treatment.
When connected to the bone, the resultant frame of fixator replicates the cylindrical shape of the tubular cortical bone shell, with greater diameter. This design allows the fixator frame to accommodate high axial, torsion and compression loading.
With the fixator in place, the mechanical action of compression or distraction can be performed in many different directions; in any of the positions fixator maintains proper bone fragment stability, important for achievement of tissue growth and regeneration.
There are several biomechanical factors related to the increase of the apparatus stiffness assisting to the necessary stability of fixation. These are the extrinsic factors as distinguished from intrinsic factors of the local tissues pathological changes. The extrinsic factors must be considered in advance before application of the apparatus. For intelligent use of the Ilizarov fixator the general recommendations are as follows:
  1. Success of treatment depends on the frame proper composition. The main part of any type of frame is the ring with a flat surface and multiple holes. The number of the rings is most important factor for the frame stability – increased number brings about more stable construction. Simple two rings frame is recommended for the upper extremity. The optimal number of rings for lower extremity is four. The rings have to be allocated by two to each bony segment above and below fracture/nonunion. This design allows the frame to accommodate very high axial torsion, compression, shear, and combined torsion-compression loading. Biomechanical principles have shown that, for the strength and stiffness of frame, the distance between two neighboring rings must be not greater than that of the diameter of the ring.
  2. The positioning, orientation, and number of rings are critical factors for deformity correction. In some cases two half-rings can be connected to form the full ring. Greater stability of the frame can also be achieved through introduction of additional half-rings connected to the full ring. Two half rings connected parallel to each other can be introduced into frame as a “puller” or “pusher” device for correction of an angular deformity.4
  3. It is preferable that the rings of the frame are of the same size. The important criteria for the consideration of the rings size is a maximal diameter of the limb segment. Because the breadth of extremity varies at different levels, the space between the skin and the ring also will vary. As a rule K-wire span must be at least 3 cm to prevent possible ring pressure in case of the soft tissue swelling.
  4. Rings orientation must be aligned strictly perpendicular to bony fragments in a configuration with their inclination or angulation, with the corresponding holes of all rings along the same straight line, making rings connection with the threaded rods easy. In case of angular deformity the ring orientation establishes their positioning in relation to the long axis of each bone segment.
  5. The central rings must be in a close position to the fracture/nonunion site. The closer the ring is placed to the tip of free bone fragment end, the more stable this fragment is during all necessary bone movements – compression, distraction or correction. The feeble bone structure and soft tissues condition have to be taken into consideration. In case of osteoporotic changes the central rings have to be located farther from nonunion site.
  6. In many cases the two connected half-rings can be used instead of a full ring. They have the advantage of being easily reinstalled during the long course of treatment. The half-rings are mostly used for the upper and lower arm frames, and for the foot component of a leg frame. As the foot component the half-ring must be elongated by adding two connecting plates to each of its ends.
  7. If the ring is situated close to the joint it can prevent or severely restrict the joint motion. In such cases it is useful to incorporate into frame a five-eighths ring or a half-ring with curved end plates. Both of these ring modifications are not so strong as full ring but their three-point connection to the adjacent ring permits it to bear any necessary loading stresses.
  8. For use in the proximal part of the femur frame there is a special arch in two versions: 90- and 120-degree with slots. Assemblage of these components is facilitated by the introduction of two or three half-pins of 4 or 5 mm in diameter instead of wires.
  9. Ring (and arches) connections play a crucial part in the assembly of any type of the frame, they determine rational distribution of frame strength, balance arrangement of different structural loading and stability of the frame attachment to the bone fragments. They serve also as ring direction guides. Main type of ring connector is the threaded rods of different lengths but with the same pitch (i.e. the distance between two threads) which equals 1 mm. This signifies that one full turn of the nut corresponds to a change also equal to 1 mm. At least four rods must be used to connect two neighboring rings, affixed at equal distance both vertically and horizontally on the rings. Biomechanically, four threaded rods provide much greater protection against bending.
    The slotted cannulated rod has special functions in connecting rings. This rod may serve as a connector and at the same time may be used as pulling device.
  10. The graduated telescopic rods of four different lengths are the hollow aluminum cylindrical tubes with partially threaded shaft. They are significantly stiffer than threaded rods. One end of telescopic rod is fastened to a ring with a bolt, and a partially threaded shaft protruding from the cylinder attached to the next ring by the partially threaded rod. The smooth surface of the partially threaded rod provides greater stiffness to accommodate the stress of the tightened bolt connected to the ring.
  11. The graduated telescopic rod is much like the telescopic rod. It is a cylinder with one end coupled to a ring via a tightened bolt. It has a square head that is adjustable by hand, permitting easier and simpler adjustment by a quarter of mm for compression and distraction. It locks after each turn, has visual references on the square sides, and clicks audibly after every turn.
  12. Fastening of the frame is achieved with the nuts and bolts. Dr. Ilizarov consciously excluded anything sharp from his fixator system, in particular, the 5screws and screwdriver, to prevent any serious injury to the surrounding tissue.
    There are three types of hexagonal 10-mm nuts: a full or 6-mm thick nut, a three-quarter, or 5 mm thick nut, and a half, or 3 mm thick nut. One thread's pitch of all three nuts equals 1 mm; thus the full nut has six threads, the three-quarter nut has five threads, and the half nut has three threads. Depending on the number of threads, different nuts serve different purposes in the frame assembly. Inasmuch as one thread's pitch equals 1 mm, one full turn will produce 1 mm of movement on the rod, which recommended to average amount of distraction-compression in the lengthening method for 1 day. Thus the turn of the nut is used as a driving force in the technique.
    To facilitate a one-fourth turn, Richards Medical Company introduced a special quadragonal nut with 15 threads; its head marked with dots from 1 to 4, signifying increments of the full turn.
  13. Using bolts is the best and simplest way to tighten the parts of the frame. There are three types of connecting bolts with the standard hexagonal head. They differ from each other in leg length: 10-mm length, 16-mm length, and 30-mm length. The pitch of the thread in all three types is equal 1 mm. The 10-mm and 16-mm bolts are mostly used for connecting the post and plates. The 30-mm bolt is used to connect three or more parts.
    There are two types of bolts specially designed to fasten K-wires to the flat surface of the ring: the cannulated wire-fixation bolt and the slotted wire-fixation bolt.
  14. Introduction of Kirschner wires (K-wires) for the bone fixation is a key technique for the apparatus application. It determines both the positioning and stability of rings. Drilling the smooth K-wires of small diameter (1.5 mm and 1.8 mm) creates small hole in the soft tissue, compact bone and bone marrow. Introduced multidirectional and transfixed to the rings under tension, K-wires bring firm stability of the bone fragments. The entrance and exit points of the K-wire must be located at least 1.5 to 2.0 cm from the major blood vessels and nerves. Each wire drilled through the bone must be introduced correctly at the first attempt. The rule is, one wire, one hole.
  15. Multidirectional bone transfixation, called “transosseous osteosynthesis” by Dr. Ilizarov, is done by the K-wires, it brings maximum stability to the rings and to the bone fragments. This stability can also be reinforced by the offset K-wires connected to the ring by the supports, posts, and half-hinges. The number and configuration of the offset wires provide not only better ring stability but increases the stiffness of the whole frame.
  16. For the proper K-wires introduction the ring must be equally centered to the bone axis. This helps to achieve a wide angulation between two K-wires. The wide spread between the two crossings K-wires brings about more even distribution of the loading forces on the bone cortex along the plane of the ring. This spread has to be as much as possible close to 90/90 degrees.
  17. Introduction of the stopper wires with an olive shaped stoppers (a wire with a rounded bulge that becomes seated against the compact bone), adds to the bone and the frame stability. Use of the stopper wires is recommended for the fragment's deviation correction, technique of bone pulling, and interfragmentary compression, and at the sites of osteoporotic bone.
  18. For correction of hand deformities there are specially designed miniparts of fixator, one-fourth of size of the original parts. With the transfixation of the metacarpal bones the special light frame allows to achieve lengthening and correction of the fingers. The same miniparts can be used for the toes deformity treatment.
  19. Introduction of the hinges between two neighboring rings bring desirable gradual correction of angular deformities. A two-axis hinge consists of a single unit that permits rotation around two orthogonal axes. Used in combination with distraction-compression devices, the hinges help gradually to correct deformities, with simultaneous transformation of the bone and soft tissues. Subsequently, the hinges are used as pivotal (rotation) point components for 6necessary straightening. The position, orientation, and number of hinges are critical factors for deformity correction. The advantages of using hinges are: they constrain motion in a specific plane or planes; they provide a specific fulcrum for calculation and control of correction of the bone fragments angulation or displacement; they provide biologic adaptation of tissues to new desired positioning.
    Several important parameters must be considered in hinge positioning:
    1. Two rings to which hinges are attached must be strictly perpendicularly to bone fragments; they either start perpendicularly to end angulated, or vice versa.
    2. Two hinges located at the opposite sides of deformity usually required for stability.
    3. Two hinges rotation axis must be situated on the same horizontal level of the deformity apex.
    4. It is critically important that hinges of the same plane are oriented along the same plane as the deformity.
    5. Movement of the axis of the hinges rotation to concave or convex sides of deformity produces corresponding compression or distraction of the fragments.
    6. The positioning of the hinges can be used to achieve different types of deformity correction, such as opening wedge, distraction, compression, translation, and derotation.
  20. In some cases there are the big posttraumatic or postsurgical skin scars, the introduction of the hinges between two rings can help for skin stretching by the local tissue sprain. This helps for development of neovascularization of soft tissue and brings about the softening of a scarred area – an important additional effect of treatment during correction of the bone deformity.
Most of the parts of fixator have been developed by Dr. Ilizarov during his work in very harsh and poor conditions at the Siberian city of Kurgan, in 1950-60s. Some of them may look today like the old fashion design (for example, the simple threaded rod). But they did work well for the treatment of thousands patients. Starting in 1990s, the new modern design of many of the same parts have been developed (for example the gradual telescopic rod instead of the threaded rod), and new lighter material is used for their production. Esthetically these modern parts look more attractive and up to date but they carry out essentially the same function as the old ones.