Page 206 - Zoo Animal Learning and Training
P. 206

Chapter 25: Vertebral Fracture and Luxation Repair  211

               approaches allows familiarization of location and dimension of the   In  small‐breed dogs, reduction can sometimes be achieved with
               canal in relation to implants at the opened site, they should only be   thumb forceps alone.
               performed if compression is deemed sufficiently significant.
                                                                  Monocortical Screw and PMMA Fixation
                                                                  The ventral aspect of the vertebral bodies is utilized for monocortical
               Cervical Injuries                                  screw fixation with PMMA. Since bone near the vertebral endplates
               Cervical fractures or luxations generally require substantial force as   has the largest dimensions, screw position should be as cranial or
               the surrounding musculature provides relative protection during   caudal in relation to the vertebral body as possible. Care must be
               trauma. Unstable vertebral body fractures are the most common   taken not to perforate the vertebral endplate and the screw should be
               indication for cervical stabilization.             oriented parallel to the caudoventral/craniodorsally angled disc
                                                                  space. Medium‐ to large‐breed dogs can usually accommodate 3.5‐
               Anatomical Considerations                          mm cortical screws, while 2.0‐mm or 2.7‐mm screws are more
               The vertebral bodies offer the most substantial amount of bone and   appropriate for smaller dogs and cats. The use of titanium screws
               can easily be exposed via a ventral approach. Pedicle dimensions are   allows for improved postoperative advanced imaging, especially
               small and the presence of the transverse foramen, which houses the   MRI. Nonself‐tapping screws are preferred to avoid inadvertent per­
               vertebral artery, makes safe transpedicular implant passage chal­  foration of the transcortex into the vertebral canal. When drilling, a
               lenging. Articular processes extend more laterally than dorsal and   drill stop is advantageous to help prevent penetration into the verte­
               are in close proximity to the exiting nerve roots via the interverte­  bral canal. Screw‐on drill stops (e.g., Positive Drill Stop by Animal
               bral foramina. More challenging muscular coverage and the lack of   Orthopedics) are positioned along the drill bit based on preoperative
               easily palpable landmarks hinders lateral approaches. Dorsal access   measurements of the height of the vertebral body. Alternatively, drill­
               to the vertebral canal requires an approach through what is often­  ing can commence without a stop, relying on the change in drill bit
               times massive dorsal cervical musculature with the potential for   pressure to indicate penetration of the cis‐cortex into the narrow
               increased postoperative morbidity.                 medullary space of the vertebral body. An appropriate size depth
                                                                  gauge or blunt straight probe can be used to carefully determine
               Positioning and Approach                           integrity of the trans‐cortex. Two to four screws can be placed into
               Careful positioning of the anesthetized dog in dorsal recumbency   each vertebral body as necessary depending on the size of the bone
               with the cervical spine in relative extension helps with alignment   and the degree of instability. Screw length must be sufficient to allow
               and reduction of some fractures/luxations. A standard ventral mid­  about 10–15 mm of screw protruding that will be incorporated into
               line or paramedian approach will provide access to the ventral   the PMMA. If screws are placed parallel to one another near an end­
               aspect of the vertebral column. For caudal cervical trauma, partial   plate, they may be angled slightly toward the midline to prevent inter­
               myotomy at the insertion of the sternocephalicus muscle from the   ference  of  implants  during  drilling  and  placement  of  the  second
               sternum may provide a somewhat improved access. Bony land­  screw. Transverse processes offer an additional location for screw
               marks for a ventral approach include the wings of C1, the increased   placement (Figure 25.4). While these processes do not provide a large
               spacing between the ventral processes of C2 and C3 compared with   amount of bony purchase, bicortical screw placement is possible,
               other vertebrae, the prominent transverse processes of C6, and the
               first pair of ribs. Surgical stabilization generally requires instrumen­
               tation of vertebrae adjacent to the site of trauma; therefore, the
               approach often exposes most of the cervical vertebral bodies.

               Implant Selection
               Bicortical pin fixation with smooth or positive‐profile end‐threaded
               fixation pins has long been the standard for cervical vertebral fixa­
               tion. Unfortunately, it carries an unacceptably high risk of injury to
               neurovascular structures as pins, even at recommended insertion
               angles (30–40°) and landmarks (ventral midline), may violate the
               vertebral canal, transverse foramen, or intervertebral foramen.
               Therefore, bicortical cervical implants are no longer recommended.
               Monocortical implants significantly decrease the risk of such injury.
               While either cortical or cancellous screws can be utilized for mono­
               cortical fixation, cortical screws may offer improved resistance to
               shear forces due to their larger core diameter. Biomechanical stud­
               ies are supporting the use of monocortical screw constructs in the
               canine cervical spine [2,3].

               Reduction
               Gentle manipulation for reduction of the unstable cervical vertebral
               column can be achieved in several ways. Point‐to‐point reduction   Figure 25.4  Monocortical vertebral body and bicortical transverse process
                                                                  screw fixation with PMMA in the cervical spine. Note how the transverse
               forceps can be placed carefully around the mid‐vertebral body to   process screws avoid the transverse foramen. Vertebral body screws can be
               manipulate the vertebra. Screws or small positive‐profile end‐  placed anywhere along the ventral aspect to accommodate the injury,
               threaded pins can used for manipulation if they are inserted in an   affording a high degree of freedom in application. For fixation of two adja­
               area that does not jeopardize bony purchase for surgical fixation.   cent vertebrae, 20 g PMMA is usually sufficient.
   201   202   203   204   205   206   207   208   209   210   211