Richard M. Jerram, BVSc, DACVS
Veterinary Specialist Group @ Unitec, Mt. Albert, Auckland, New Zealand
Spinal surgery has some specific considerations with respect to local anatomy and potential complications. There are a number of surgical procedures described in veterinary surgical textbooks; however, some of the tricks that can help the surgeon tend to be omitted. Unfortunately, as there is very little to no evidence-based information on the ideal technique to perform in a particular area of the spine much of the surgical descriptions available are based on opinion and experience. Spinal surgery is complex and should not be undertaken lightly as the development of technique-related complications can be devastating.
As previously stated, accurate preoperative neurologic examination and diagnostic testing combined with careful surgical planning are imperative in attempting to achieve a satisfactory surgical outcome. As the consequences of postoperative infection can be devastating in spinal surgery patients, all spinal surgery should be performed under strict aseptic conditions. This should include sterile surgical preparation of the surgical site, draping of the entire animal and surgical table, placement of an adhesive plastic sheeting on the exposed skin, appropriate surgical attire for the surgical team including, mask, sterile gown, and gloves. If spinal stabilization with orthopaedic implants is anticipated or if the surgical procedure is expected to take longer than 90 minutes, intravenous prophylactic antibiotic therapy (a penicillin-derivative or a cephalosporin) should be administered. The operating room should be isolated from the remainder of the hospital and positive-pressure air-conditioning is preferred. A high-quality surgical suction unit and electrocautery including bipolar hand-pieces are absolutely essential for spinal surgery. A surgical assistant to provide retraction and flush/suction is also very helpful.
Some specialized surgical equipment/instrumentation is necessary to improve the efficiency of spinal surgery. For the past two years I have been using a set of operating magnification loupes with a mounted headlight in all cases of neurosurgery. This has made a huge difference to my ability to be able to more carefully perform all of the procedures that are indicated. Magnification loupes are commercially available and can be manufactured specifically for the individual surgeon with a variety of magnification distances. I use a 3.5 X magnification lens set with a belt-clipped halogen headlight. As the anatomy of the spine is complex, having an anatomical specimen available for assessment in the preoperative planning or even during surgery can also be very helpful. There are some instruments that I regard as essential for spinal surgery; these include Gelpi self-retaining retractors, Freer periosteal elevators, fine bone rongueurs, nerve root retractors, small probes, and a pneumatic neurosurgical drill with multiple sizes of burs. As well as instrumentation, the other pieces of equipment that are particularly useful in spinal surgery including sterile bone wax, absorbable hemostatic agents such as Gelfoam or Surgicel, and blood vessel ligating clips for hemostasis. Sterile polymethylmethacrylate bone cement and a full set of orthopaedic surgical equipment need to be prepared and readily available if spinal stabilization is expected or possible.
Surgical Tips for the Cervical Spine
The ventral slot technique is the most commonly performed surgical procedure of the cervical spine, this procedure cannot be performed without a fine neurosurgical bur instrument. A variety of different sized burs are recommended. The surgical approach is generally made by splitting between the sternothyroideus muscles but the approach can also be made paramedian to this muscle group. The recurrent and para-recurrent laryngeal nerves need to be protected during dissection. I typically use a moistened surgical sponge with an Army-Navy retractor to retract the trachea. Drop-angled Gelphi retractors are very helpful for retraction of the longus colli musculature. Several sizes of Fraser flexion tips are also useful to enable suction without minimizing visualization of the deeper aspects of the slot. The ventral slot should be no more than one third of the width of the vertebra and one third of its length, with the disk space at its centre once the vertebral canal is reached. The vertebral venous sinuses can be avoided by keeping the site as central as possible. The dorsal longitudinal ligament in the floor of the slot can be elevated using a dural hook and incised using a number 11 scalpel blade. When ventral stabilization of the cervical spine is performed, pins or screws should be directed at a dorsolateral angle of approximately 35° to avoid engaging the vertebral canal. A steeper angle of approximately 45° from vertical may be preferable in the C7 vertebra. To provide distraction prior to stabilization a long-handled Gelphi retractor with the tips cut off can be used. A small slot is drilled into the ventral aspect of the adjacent vertebra to allow insertion of the "cut tip" retractor. More recently, the use of the locking plate technology for ventral cervical spinal stabilization has been described. This may be more beneficial for stabilization as the screws are only required to be monocortical and the plate can be positioned close to but not necessarily in contact with the underlying bone.
Fenestration of the cervical discs ventrally requires elevation of the longuscolli muscles at each of the intervertebral disk spaces and a small windows is cut in the ventral aspect of the annulus fibrosus to allow removal of the nucleus pulposus.
The surgical approach for cervical dorsal laminectomy requires maintaining the dissection as close to the midline as possible. Both monopolar and bipolar cautery are recommended for hemostasis. The dorsal lamina is removed using a neurosurgical bur, however, care must be taken during this procedure as the bone thickness varies along regions of the lamina.
Several techniques have been described for stabilization of the atlantoaxial joint spaces. I prefer the ventral approach and place pins across the articular surface as well as monocortical screws in the ventral aspect of C1 and the ventral aspect of C2 followed by placement of a polymethylmethacrylate bridge. The articular cartilage between C1 and C2 is removed as much as possible using a neurosurgical bur or a number 11 scalpel blade and autogenous cancellous bone graft obtained from the proximal humerus is packed into these joint spaces prior to placement of the surgical implants. A small hook probe can be placed in the alar notch on the cranial aspect of the wing of the atlas to provide a sighting point for trans-articular pin direction.
Surgical Tips for the Thoracolumbar Spine
The hemilaminectomy appears to be the most commonly performed decompressive procedure performed in the thoracolumbar spine. Some surgeons will modify the hemilaminectomy by preserving the articular facet (pediculectomy, mini-hemilaminectomy) but, in my experience, these procedures provide decreased exposure of the spinal cord and possibly create the inability to remove all of the offending disk material. Most surgeons will approach the thoracolumbar vertebral column via a dorsal approach. In this approach, the multifidus musculature is elevated from the midline and the tendinous attachments of the multifidus muscle to the caudolateral aspect of each mammillary process of the vertebrae are released with scissors. Gelphi or Weitlander retractors are used to maintain exposure. The tendon of the longissimus lumborum muscle can be excised from the accessory process on adjacent vertebrae to improve exposure of the vertebral column. Care must be taken when transecting the longissimus tendon to avoid damaging the spinal nerve and associated vasculature that lie just ventral to the tendon. The accessory process can also be used to identify the ventral aspect of the spinal canal. Visualization of the presence of the 13th* rib is used to provide confirmation of the appropriate disk space(s) for the hemilaminectomy procedure. The hemilaminectomy is performed using a neurosurgical bur typically using smaller sizes as the laminectomy site becomes deeper. The depth of the laminectomy site is evaluated by initially drilling through the white external cortical bone then the red cancellous bone followed by the thinner white inner cortical bone.
If fenestration is performed during the hemilaminectomy, the spinal nerve should be retracted to provide visualization of the lateral aspect of the annulus fibrosus. A number 11 scalpel blade is used to make a window in the annulus fibrosus to remove the nucleus pulposus.
Some controversy surrounds the use of durotomy to provide additional spinal cord decompression and as dogs with focal myelomalacia may still recover neurologic function, the use of durotomy to infer prognosis is probably not justified.
Stabilization of the thoracolumbar region of the spine is performed when traumatic fracture/luxation is present or gross instability is present at the time of spinal cord decompression. Pins or screws and polymethylmethacrylate bone cement have been used to provide stabilization, however, the use of locking plate stabilization systems is probably a simpler method of stabilization. The use of external fixation devices using arch bars from circular external fixation equipment has also been described. The major disadvantage of this is the labor-intensive postoperative management and the risk of premature removal.
To reduce the possibility of a laminectomy scar developing, the use of a fat graft placed over the hemilaminectomy site has been recommended. A section of 3–5 mm thick subcutaneous fat larger than the laminectomy defect is placed over the laminectomy site but it is somewhat unclear if this technique is completely justified.
Surgical Tips for the Lumbosacral Spine
The lumbosacral region of the spine is almost exclusively approached using a dorsal approach with bilateral elevation of the sacrocaudalis muscles from the dorsal spinous processes and the dorsal lamina of the vertebrae. Typically, dorsal laminectomy is combined with discectomy and, more recently, distraction and stabilization of the L7–S1 intervertebral disk space is recommended. The dog is typically positioned with the pelvic limbs in a squatting position to provide a more normal standing position of the lumbosacral space. The dorsal spinous process of the L7 vertebra can be identified as it is typically shorter than the more cranial dorsal spinous process of the L6 vertebra. The laminectomy is performed on the midline and begins by the removal of the majority of the caudal aspect of the dorsal spinous process of the L7 vertebra. The laminectomy is then extended caudally to the mid-sacrum. The bone of the caudal aspect of L7 is typically thicker than that of the sacrum so care is taken during drilling to ensure that the cortical bone is visible along the entire laminectomy site prior to exposure of the caudal equina. The ligamentum flavum between L7 and S1 is often hypertrophied and sharp excision of this will be necessary to complete the exposure of the cauda equina. A Kerrison rongueur can be used to remove the lateral aspects of the laminectomy site without damage to the underlying nerve roots. If foraminotomy is performed by extending the laminectomy laterally dorsal to the L7 nerve root then care is taken to avoid removing too much bone laterally on the caudal aspect of L7 otherwise postoperative fracture of the caudal articular facet may occur. Complete removal of the articular facet (facetectomy) may be considered to completely expose the L7 nerve root.
The dorsal annulus fibrosis of the L7–S1 intervertebral disk is usually evident bulging into the vertebral canal. The nerve roots are retracted to one side and can be maintained in this position using 25 gauge needles inserted into the cranial and caudal aspects of the disk. The protruding annulus fibrosus is resected using a number 11 scalpel blade and a small alligator rongueur or curette is used to remove degenerative disk material. The nerve roots are then retracted in the opposite direction to remove the remainder of the dorsal aspect of the disk.
If instability is suspected following the laminectomy or if there appears to be subjective instability of the articular facet joints then stabilization techniques should be performed. I use a towel clamp on the remnants of the dorsal spinous process on the L7 vertebra and on the sacrum to determine whether significant instability of the articular facets is present and I also evaluate the relative positions of the articular facets for evidence of overlapping of the articular surfaces. Several techniques for stabilization of this region have been described, I prefer to use the trans-facet screw technique or locking plates (String-of-Pearls). Distraction is performed using a laminectomy spreader prior to placement of the chosen stabilizing implants. Corticocancellous bone graft can be obtained from the adjacent ilial wings and packed around the articular facet joints to improve postoperative healing.
Postoperative Care and Physical Rehabilitation
Quality postoperative management is a critical determinant in the successful recovery of dogs following spinal surgery. In the perioperative period, both preemptive and postoperative pain relief can improve recovery and avoid the psychological impact of pain (e.g., anxiety, sleep deprivation). Opioids are the drugs of choice preoperatively and nonsteroidal anti-inflammatory medication can be used 48 to 72 hours after surgery. Early physical therapy (cold- and hot-packing the surgical wound) can also provide some analgesic effects and decreased the amount of local inflammation.
Bladder management begins immediately after surgery. Regular manual bladder expression should be performed until the patient has regained the ability to urinate voluntarily. Intermittent or indwelling catheterization may be necessary in patients that are difficult to express manually. More long-term management of urination may require the use of drugs that reduce hypertonicity of the urethral sphincter (phenoxybenzamine) or enhance contractility of the detrusor muscle (bethanechol). Care must be taken to monitor patients for the development of urinary tract infections. Management of long-term recumbency requires further physical therapy including massage, passive and active exercise, hydrotherapy, and dedicated physiotherapy.
Clinical conditions such as acute intervertebral disk extrusion in dogs are seen frequently in veterinary practice and surgical treatment of these conditions is often justified. Having appropriate surgical facilities and equipment as well as a sound understanding of neurosurgical techniques is critical to help in obtaining a satisfactory neurologic outcome. There are many techniques and alternatives described for spinal surgery and it is important that each surgeon develops their own surgical skills and techniques based on a thorough review of the literature and the anecdotal information from experienced veterinary neurosurgeons.