Neurosurgery

bi-cortical screw purchase was required. Obtaining bi-cortical screw purchase, incorporating the posterior cortex of the vertebrae, increased the risk of spinal cord injury, with encroachment upon the spinal canal by the screw. Despite the bi-cortical screw purchase, there remained a percentage of failure with these plates, especially in cases where the bone quality was poor or a long plate was required for a lengthy anterior fusion.

Based on limitations of the Caspar plate, newer generations of plates have been devised (Fig. 31.9). Although the conventional plates carry similar biomechanical characteristics, most of the plates, except for the DOC plate (Johnson and Johnson/DePuy AcroMed Corp., Cleveland, Ohio, USA), incorporate a rigid coupling of the screw to the plate. Various devices to achieve this locking of the screw to the plate have been devised, including internal screws to expand the head and/or shaft of the screw, cam-locking mechanisms and interference-type screws. Rigid coupling has decreased the failure rate of plates and reduced the likelihood of subsidence; however, complications from plating have not been eliminated. Screw back-out is very uncommon in the newer-generation plates. Screw fracture or interosseous migration can result if the forces promoting subsidence overcome the resistance provided by the graft and the fixation device (Fig. 31.10). Newer-generation dynamic plates (e.g. DOC Plate) have been designed to allow for some degree of settling of the graft, without putting direct stress upon the bone/metal interface of the screws (Fig. 31.11).

The decision to proceed with an anterior/posterior (a combined fusion) procedure is determined by a number of factors, including the

Fig. 31.9. CLSP plate (Synthes Incorporated, Paoli, Pennsylvania, USA). Lateral view of a CLSP plate (a). The heads of the screws are locked to the plate with an expanding screw, placed co-axially into the inner perforation of the bone screws. An AP X-ray (b) of the cervical spine following an interbody fusion supplemented with an anterior cervical plate.

Fig. 31.9. CLSP plate (Synthes Incorporated, Paoli, Pennsylvania, USA). Lateral view of a CLSP plate (a). The heads of the screws are locked to the plate with an expanding screw, placed co-axially into the inner perforation of the bone screws. An AP X-ray (b) of the cervical spine following an interbody fusion supplemented with an anterior cervical plate.

Fig. 31.10. A post-operative lateral cervical spine X-ray approximately 1 year following a two-level diskectomy and interbody fusion at C5-C6 and C6-C7. There is clear lucency across the interspace at C6-C7. The inferior screws and the plate have toggled as the disk space of C6-C7 settled, with resorption of the graft.

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