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172 Section III: Spinal Procedures

annulus resection should be avoided to prevent inadvertent damage depth is estimated (traction may aid). Based on these measure-
to the spinal cord as well as adjacent vasculature and nerve roots. ments, a cortical allograft is ordered. Block segments are generally
If a disc extrusion has occurred, the dorsal annulus must be par- 2–5 cm long and several rings can be made from one length of bone
tially removed to allow extraction of extruded material from the for the same patient. Most large‐breed dogs undergoing cervical
vertebral canal. Once as much disc material as possible has been distraction and stabilization have similar disc space dimensions,
removed by rongeurs, bone curettes are used to gently clear both and if caseload is high enough several cortical ring allografts can be
vertebral endplates of remaining cartilaginous material. Careful use stored in hospital. At time of surgery, the allograft is thawed to
of a pneumatic drill can assist in clearing of the endplates; however, room temperature in warm sterile saline. A sagittal saw is used to
excessive removal of bone can weaken the endplates and lead to cut a cortical ring of sufficient depth (Figure 19.5). It is recom-
subsidence of the disc spacer (if used). mended to cut the ring slightly larger and do final adjustments by
removing more bone with a bone rasp to avoid a ring that is of
Intervertebral Spacer insufficient size. Fresh cancellous autograph is obtained from the
Distraction of an intervertebral articulation ameliorates spinal cord proximal humerus just prior to placement of the disc spacer
compression by proliferated soft tissues and protruding dorsal (Figure 19.6). The ring allograph is packed tightly with this fresh
annulus fibrosus. As there is no bone on bone contact between the graft. The disc space is distracted and the ring allograph is inserted
affected vertebrae, vertebral implants must carry most of the load along the endplate orientation. Care must be taken to insert the ring
during normal movement of the cervical vertebral column. The sufficiently dorsal. The concavity in the endplate of the cranial ver-
goal of an intervertebral spacer is to maintain the desired disc space tebral body can act as a trap for the ring, preventing its full insertion,
depth and provide load sharing between the affected vertebral bod- and leading to poor fit and inappropriate distraction of the disc
ies, thereby increased implant longevity. space. If allograft ring dimensions fit with disc space dimensions
Intervertebral spacers can range from simple cement plugs to and the discectomy has been performed appropriately, the ring allo-
carbon fiber reinforced polymer cages, cortical rings, and metal graft should be flush with the ventral aspect of the vertebral bodies.
cages. While solid materials such as cement plugs will maintain dis- Any remaining cancellous graft is placed around the ventral aspect
traction, they will not allow ingrowth of bone and fusion within the of the disc space and fixation of the affected vertebrae commences.
disc space. Cages or cortical ring allografts provide distraction
while allowing placement of a fresh cancellous autogenous bone Indication for Additional Decompression
graft in the middle of the cage/ring to encourage bone fusion [3,5]. The two main indications for decompressive procedures in addition
In the case of a cortical allograft, the ring will be resorbed and to cervical stabilization are compression by extruded nuclear mate-
replaced with autogenous bone over time. Cortical allografts are rial or severe dorsolateral compression by articular processes. In
commercially available in different length and diameters (Bergman case of disc extrusion, removal of sequestered nuclear material can
block; Veterinary Transplant Services, Inc., Kent, WA). A round or usually be achieved through the disc space after discectomy. If
slightly oval section of bone is preferred (femur or distal tibia). needed, a narrow ventral slot can be performed; however, any
Patient‐specific dimensions of a normal‐appearing cervical disc breach in vertebral endplate may compromise stability of a disc
space are measured. If all intervertebral discs are affected, the width spacer if used. If no spacer is inserted, the ventral slot space can be
and height of the vertebral endplates are obtained and disc space filled with cancellous bone graft prior to closure to aid bony fusion.

Figure 19.5 (A) A sagittal saw is used to cut a cortical allograft segment into a ring of appropriate depth to be used as an intervertebral spacer. (B) Several
cortical rings have been cut and two are packed with fresh cancellous bone graft. One of the rings has been modified for improved fit by carefully cutting
part the cortex. However, it is preferred to order a cortical segment that fits the patient‐specific disc space dimensions rather than compromising the struc-
ture of the ring.