Degenerative Disease Of The Cervical Spine

also have some bearing on the likelihood of subsidence or graft extrusion [25].

The modulus of elasticity of the grafting material can influence the likelihood of its compressing into the adjacent vertebral endplates. Cortical bone, as seen in the fibular strut graft, is relatively rigid compared to the endplates and underlying cancellous bone in the vertebral bodies. Theoretically, this substrate is more likely to subside than cortical cancellous allo-graft. The cross-sectional diameter of the bone graft is also relevant: the smaller the cross-sectional diameter, the greater the pressure per square unit of volume exerted by the bone graft on the endplates. Therefore, the larger the graft, the less likely it is to sink into the endplate. Ideally, a graft of at least two-thirds of the cross-sectional diameter of the disk space should be used. The positioning of the bone graft within the decompression channel may also have some bearing on its tendency to subside. Placement of the cortical portion of the graft anteriorly, in alignment with the anterior aspect of the vertebral bodies, probably reduces the likelihood of subsidence. When the graft is placed in this fashion, the vertical force exerted on it through

Fig. 31.8. Caspar cervical plate (Aesculap Incorporated, Center Valley, Pennsylvania, USA). A first-generation anterior cervical plate. The screws are not rigidly linked to the cervical plate and require bi-cortical purchase in order to optimize the fixation.

the superior vertebrae is transmitted through the cortical portion of the graft, which is in continuity with the cortex of the adjacent vertebrae. This configuration allows for a relatively non-compressible cortex/cortex construct.

The complication rate of insertion of a lengthy bone graft, involving three or more motion segments, is higher than that for shorter bone grafts. This is especially true when the bone graft extends down to the cervical thoracic junction, where the spine curvature is in transition from the cervical lordosis to kyphosis. The translational force across the inferior aspect of the bone graft at the cervical thoracic junction predisposes to anterior extrusion of the inferior aspect of the graft.

Anterior cervical plating technology has evolved significantly over the past 15 years. The Caspar plate (Aesculap Incorporated, Center Valley, Pennsylvania, USA) was the first type of anterior cervical plate to be used for a broad range of cervical problems (Fig. 31.8). The coupling between the screws which fixed the device to the vertebral bodies was semi-rigidly attached to the plate. In order to augment the stability characteristics of the fixation,

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