Posterior 'tension band' stabilization may be achieved with sublaminar wires, screws and hooks, secured to posterior metal rods or rectangles. Hook-and-rod constructs are best suited to the thoracic spine, whereas pedicle screw-and-rod systems are favoured in the lower thoracic and lumbar spine. Rod systems may be classified as distraction, segmental or pedicle screw systems.
1. Distraction systems (Harrington). This system relies on three-point bending for mechanical fixation. Hooks secure the posterior rods to three vertebrae above and three below the injured segment. This system was very popular and is still used for thoracic injuries. The device is unsuitable for distraction injuries.
2. Segmental fixation systems (Cotrel-Dubousset or Luque). These systems rely on the distribution of forces over multiple vertebral levels, with attachment of hooks, wires or screws. Compression and distraction may be used at different segmental levels within the same construct. The Cotrel-Dubousset system is widely used for trauma. The Luque system was principally developed for deformity associated with paralytic disorders.
3. Pedicle screws. Inserted through the pedicles into the vertebral body, pedicle screws give very secure fixation. The screws (modified Schanz screw) are linked via clamps to plates or rods. Rod systems (AO, Colarado, Olerud, Vermont) are versatile and may extend over a few or many vertebral levels, allowing segmental compression or distraction. These systems are commonly used in the trauma setting to stabilize unstable fractures at the thoracolumbar junction and in the lumbar spine.
Screw entry point and direction of insertion is critical for safe use, and two-plane fluoroscopic guidance is essential. The entry point is best identified at the junction of the transverse process and the outer facet. An awl is used to carefully create a guide hole down the pedicle into the vertebral body. As the antero-medial direction of the pedicle varies with vertebral level (LI - 10°, L5 - 25°) from the saggital plane, it is best to start laterally and aim medially. A pedicle 'feeler' and depth gauge are used to ensure that the pedicle walls have not been penetrated and the correct length screw is used. Complications include pedicle fracture, malposition, facet impingement, implant failure, nerve root and even cord injury. Computer-assisted techniques may help reduce complications.
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