Page 172 - Zoo Animal Learning and Training
P. 172

Chapter 19: Cervical Distraction and Stabilization  175

                                                                  space. Monocortical drilling and careful tapping (unless self‐tap-
                                                                  ping screws are used) commences as with regular screw placement.
                                                                  Measurement for screw length is performed prior to tapping and
                                                                  must be done through the locking plate, keeping in mind that
                                                                  many of these plates will sit proud on the bone and screws will
                                                                  traverse open space  before  engaging bone. The  plate  should  be
                                                                  held as close to the bone with digital pressure while maintaining it
                                                                  in the desired position during screw application. Accurate place-
                                                                  ment of the first screw is paramount, as this single screw, once
                                                                  locked, will determine the path for all subsequent screws.
                                                                  Other Techniques
                                                                  As an alternative or augmentation to monocortical screw/PMMA
                                                                  fixation of the vertebral bodies, bicortical screws can be placed in
                                                                  the prominent transverse processes of the cervical vertebral column
                                                                  (Figure 19.11). For medium to large dogs, 3.5‐mm cortical screws


























               Figure 19.10  Synthes locking compression plate applied to C5–C6 vertebral
               bodies. Because of the locking mechanism, the plate does not require con-
               touring and can sit proud on the bone. Screws engage the vertebral bodies in
               a monocortical fashion. Because of the dimensions of the plate, usually only
               two screws can be placed per vertebral body. Source: Reproduced with the
               permission of The Ohio State University.

                 biomechanically (Locking Compression Plate® from Synthes) [2,3,5].
               Locking plates rigidly couple screws to the plate via a variety of locking
               mechanisms. To appropriately lock, most mechanisms require the
               screw to be inserted at a fixed angle trajectory, which dictates screw
               orientation (Figure 19.10). There are several important advantages
               of locking plates over traditional plates. Because screws are locked,
               plates do not need to be contoured to the undulating bone of the
               ventral vertebral bodies but can be laid on the bony surface with a
               potential  offset of  several  millimeters. As  any  contouring  would
               change the fixed angle screw trajectory, contouring should be
               avoided unless the change of screw orientation is desired. The sec-
               ond benefit is that locking plates can be used with monocortical
               screws because rigid fixation does not rely on friction between the
               implant and the bone. This greatly reduces potential damage to spi-
               nal cord, nerve roots, and vessels by bicortical implants.
                 Technical challenges with application of locking plates arise
               from the predetermined screw location based on hole position   Figure  19.11  Bicortical transverse process screw and reinforcement bar
                                                                  fixation [7]. A Steinmann pin has been contoured to fit around the screws
               within the plate and the fixed screw trajectory. The dimensions of   and is secured to the screws with cerclage wire to act as a reinforcement bar.
               most plates allow two screws per vertebral body but care must be   The screws and bar are then incorporated into PMMA. Source: Reproduced
               taken not to inadvertently violate the slanted intervertebral disc   with the  permission of The Ohio State University.
   167   168   169   170   171   172   173   174   175   176   177