External fixation devices

Introduction

Bone possesses the remarkable ability to heal without scarring. The repair processes involved are dependent on the biomechanical stability of the fracture, which is directly influenced by the choice of fixation method. Anatomical reduction and internal fixation, which creates absolute stability, results in primary fracture healing - the direct cortical healing of two fractured ends of bone without an intermittent cartilaginous stage. Methods such as external fixation or cast-immobilization, which allow micro-movements, result in secondary fracture healing. This process can be separated into different stages, although in reality these form a continuum and blend into one another by varying degrees.

  • Haematoma formation: Bleeding from blood vessels supplying the soft tissue, periosteum and medulla results in the formation of a haematoma.
  • Inflammatory phase: Osteocyte necrosis, as well as platelet and mast cell activation sees the development of an inflammatory state. Granulation tissue is formed under conditions of high strain, such as in mobile fractures, which osteoblasts are unable to tolerate. Osteoclasts resorb dead tissue.
  • Repair: Granulation tissue reduces the strain at the fracture site. Micro-movements stimulate formation of a cartilaginous "soft callus", which is eventually changed to bone via endochondrial ossification to form a "hard callus".
  • Remodelling: Excess callus is removed and woven "osteoid" bone is remodelled to mature lamella bone. Further bone is laid down and collagen is realigned in areas of increased stress (according to Wolff's law).

Fracture healing requires a rich blood supply, thus fractures compromising blood flow to the fracture site, such as those of the neck of femur and scaphoid, can be significantly complicated. Infection is also a cause of delayed or non-union due to the redirection of inflammatory and cellular activity away from bone healing.

Treatment of fractures

Initial fracture management should include adequate pain relief and control of blood loss. Definitive management requires reduction of the fracture to restore normal anatomy, followed by immobilisation to aid pain relief, prevent re-displacement, and avoid shearing movements which interfere with union.

  • External splints: This is the most common method of fixation in which a cast is used to hold the fracture. A layer of padding is placed underneath to prevent sticking and allow room for swelling. Continued swelling, however, can lead to ischaemia and compartment syndrome. Other disadvantages include disuse muscle atrophy and joint stiffness (due to the need for joint immobilisation), and prevention of access to the fracture site.
  • Continuous traction: In oblique or spiral fractures the elastic pull of the muscle tends to draw the distal fragment proximally, which is difficult to counteract with a cast. Applying a small weight along the axis of the bone, using either adhesive strapping or a pin distal to the fracture site, counteracts muscle contraction and acts like a splint. However, continuous traction confines the patient to bed, increasing the risk of pressure sores, chest infection, disuse osteoporosis and renal stones.
  • Internal fixation: This can be achieved by various methods involving metal plates, screws, pins and wires. It is indicated where splintage or traction are ineffective, open reduction has been required, or to restore mobility to the patient quickly. Closed methods of fixation are preferred due to the risk of neurovascular injury and infection.
  • External fixation: Pins or screws are inserted into the fragments of bone and fixed rigidly to an external device, providing fracture fixation by the principle of splinting. This is mainly indicated in the management of open fractures, poytrauma, or infected fractures where use of casting or internal fixation would be inappropriate. It maintains access to the skin overlying the fracture site, although there is infection risk associated with the pin track.

External fixation methods

There are three main types of external fixator: standard uniplanar, ring and hybrid . All are made up of Steinman pins, Schanz screws or Kirschner wires placed percutaneously into the bone either side of the fracture site. These are connected via clamps to external fixation rods or rings (made of stainless steel or carbon fibre).

  • Standard uniplanar external fixators: Screws are inserted through the near cortex, medullary canal, and into the far cortex of the bone (aiming to avoid penetration of the muscle compartment). These are then connected to an external rod. Their most common use is in the temporary fixation of fractures in the acute setting due to simplicity of application, relative safety and maintenance of accessibility to underlying tissue. They can be used for most long bone fractures, allowing the limb to remain functional, avoiding complication yet providing stability.
  • Ring fixators: Thin wires pass through the bone and are held under tension by an external frame. Wire-ring fixators are most commonly applied in deformity correction because of their ability to apply compression, as well as control distraction and orientation in three dimensions. Varieties include full/closed rings, which provide most stability, or partial/open rings and arches, which are helpful near joints where a full ring would inhibit function. Ring fixators offer the additional benefit of raising the limb off the bed, thus protecting the skin and providing elevation.
  • Hybrid fixators: The utilisation of a combination of standard uniplanar and ring fixation is most commonly seen in the treatment of proximal and distal tibial fractures that reside close to the joint.

When external fixation is utilised it is important to ensure adequate care of pin-track sites with daily cleansing and disinfecting to prevent infection. The integrity of the fixation can be monitored with radiography .

Biomechanical considerations

The aim of external fixation is to improve the stability of a fracture in terms of resistance to forces brought about by exercise, which primarily include passive bending forces and those applied by muscle activity. Passive bending forces act in a perpendicular fashion to the screws and the axis of the bone, and are equal to the product of the weight of the extremity distal to the fracture and the distance between the centre of gravity of this distal fragment and the fracture site. Other forces involved are those along the axis and rotational forces, however these are minor in comparison.

The stability brought about by external fixation is affected by several factors. Some such factors, notably bone strength and the modulus of elasticity of the fixation materials, are beyond the control of the surgeon. Geometrical positioning of the screws and the bone-to-bar distance, however, are modifiable and therefore allow influence over stiffness of the frame, as demonstrated by FIG 2. In this example looking at unilateral fixation of a midline long bone fracture, stiffness (force divided by extension) is shown to be improved by widening the pin spread about the fracture site. Group 2, whose outer screws were positioned a further 3cm from the central fracture site relative to group 1, had a stiffness of approximately 508N/mm compared with 162N/mm achieved by Group 1.

Minimisation of the bone-to-bar distance also increases stiffness. This is a result of the effect that this change has on screw pliancy. Pliancy, which is defined as the deflection in millimetres at a load of 1kp, is directly related to screw length, thus increasing the bone-to-bar distance increases pliancy of the screw, reducing stiffness and overall stability of the fixation unit. This is demonstrated by comparison of groups 1 and 3 in FIG 2, with group 3, whose bone-to-bar distance was greatest, displaying lower stiffness (~96N/mm vs. 162N/mm, respectively).

An additional feature not addressed in this experiment is increased screw diameter. Larger diameter screws have a higher resistance to bending forces due to the cross-sectional moment of inertia increasing with the fourth power of its radius.

Summary

The main indication for external fixation is in the treatment of fractures to the extremities that involve severe damage to bone and soft tissue. Their simplicity of application and relative safety whilst maintaining access to the soft tissues makes them ideal as a temporary fixation method. Maximum stability can be achieved with minimal intervention by taking advantage of the laws of mechanics and placing pins in a wide symmetrical spread about the fracture site, and maintaining a short bone-to-bar distance.

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