Cervical Spine

Indications

Ventral fenestration for cervical disc disease is advocated in animals demonstrating pain, stiffness of the neck, or foreleg paresis. This technique is effective when degenerating discs protrude and cause nerve fiber and rootlet disorders, which account for most cervical disc problems. This procedure accomplishes intervertebral disc decompression by opening the ventral annular fibers for removal of the nucleus pulposus.

Cervical fenestration is not effective if foreleg paralysis or tetraplegia results from the presence of disc material within the spinal canal. These circumstances indicate a decompressive procedure.

Surgical Technique

The animal is placed in dorsal recumbency with a sandbag under the neck to produce dorsal flexion of the cervical spine, facilitating exposure. A ventral midline skin incision is made from the larynx to the thoracic inlet. The paired bellies of the sternohyoid muscle are separated (Figure 47-1), and the trachea is displaced laterally and is held with a self-retaining retractor. Blunt dissection of the deep fascia reveals the V-shaped longus colli muscle (Figure 47-2), which lies on the midline. Locating this muscle is essential to ensure midline identification. The ventral tubercles of the first and second cervical vertebrae is located at the level of the wings of the atlas (Figure 47-3A) for orientation. Because a disc is not present at this interspace, it is not fenestrated. The remaining ventral tubercles are midline projections that are directed caudally from the caudal ventral aspect of the vertebrae and provide the insertion site for the two bellies of the longus colli muscle (Figure 47-3B). The ventral entrance to the intervertebral space is covered by these bellies and their tendinous attachment. The muscle attachment is snipped with scissors, exposing the ventral longitudinal ligament. A No. 10 scalpel blade is used to cut the longitudinal ligament and ventral annular fibers (Figure 47-4A). A tartar scraper (SCLB Miltex Tartar Scraper, Victor Medical, Irvine, CA) (Figure 47-5) is used to fenestrate the disc (See figure 47-4B). All readily accessible cervical discs (C2-3, C3-4, C4-5, C5-6) are fenestrated.

The self-retaining retractor is removed, and the sternohyoid muscle bellies are sutured with a 3-0 polydioxanone (PDS, Ethicon, Somerville, NJ). The skin is then closed.
 

Postoperative Care

Antibiotics are given for 5 to 10 days postoperatively. Corticosteroids (dexamethasone, 1 mg/lb body weight) are administered intramuscularly once or possibly twice each week. Buffered aspirin is given for 7 to 10 days if pain persists. After 10 to 14 days, complete remission of signs is expected.
 

The ventral approach to the cervical spine allows for direct access to the vertebral bodies and intervertebral disks. The ventral slot procedure requires minimal dissection through normal tissue planes and minimal disruption of normal anatomic structures. Minimal manipulation of the spinal cord is necessary, and recovery is usually rapid with few complications.^1-5

Patient Position

The patient is placed in dorsal recumbency with the forelimbs secured caudally. The cervical spine should be supported by placing a vacuum positioner or rolled towel beneath the neck. Excessive dorsiflexion (hyperextension) should be avoided. The head can be secured by placing one inch tape on the rostral third of the mandible and securing it to the sides of the table. Gentle traction can thus be applied to the cervical spine resulting in distraction of the intervertebral disk spaces and enhanced access to the spinal canal (Figure 47-6).
 

Approach to the Cervical Vertebrae and Intervertebral Disks^1-5

A cutaneous incision is made from the larynx to the manubrium. The paired muscle bellies of the sternocephalicus muscles are sharply separated. The paired sternohyoideus muscles are sharply separated on the midline exposing the trachea. The thyroid ima, a single unpaired blood vessel, lies between the left and right sternohyoideus muscles. If the branches of the thyroid ima are ligated and transected on the right, then this vessel can be reflected with the left sternohyoideus muscle.

Blunt dissection along the right side of the trachea allows retraction of the trachea to the left and retraction of the right carotid sheath to the right. Care should be taken to identify and protect the right recurrent laryngeal nerve. The endotracheal tube must be of sufficient length to avoid collapse of the trachea during retraction. The esophagus should also be retracted to the left exposing the longus colli muscle. An esophageal stethoscope or soft rubber tube placed in the esophagus will enhance palpation of the esophagus during retraction. Care should be used when retracting these tissues. This retraction can be aided with the use of 4x4 surgical sponges or laparotomy pads if the patient is a large breed canine. These tissues can be retracted digitally and held in place by paired self-retaining retractors that are placed cranial and caudal to the affected interverbral disk spaces once the paired longus colli muscles have been identified and bluntly dissected along the median raphe.

The tendons of insertion of the longus colli muscles are transected from their origins on the caudoventral midline aspect of the affected cervical vertebral bodies, thus exposing the underlying intervertebral disk. This can be done at each disk space intended for surgery. The location of the intended intervertebral disk can be determined by palpating the large, prominent transverse processes of C6. The C5-6 intervertebral disk lies on the midline at the cranial aspect of the C6 transverse processes. Palpating along the midline, the large ventral prominence of the caudal aspect of each vertebral body and the origin of the tendon of the longus colli muscle can be palpated. The transverse processes of C1 can also be used as a point of reference. The ventral process of C1 is particularly prominent and sharp ventrally; this also can or may be palpated. (Sharp/Wheeler). There is no intervertebral disk at C1-2.

Technique^2,3

Further elevation of the longus colli muscle with a periosteal elevator should be performed in preparation for the ventral slot. The retractors can be repositioned between the muscle bellies of the longus colli muscles. The prominence of the point of origin of the longus colli muscle on the caudoventral midline aspect of the cervical vertebral body can be removed with rongeurs and the intervertebral disk fenestrated.

Using a #11 or # 15 blade to fenestrate, a defect is made in the ventral anulus fibrosus. Starting on the midline of the cranial aspect of the vertebral body caudal to the disk, with the cutting edge of the blade directed towards the surgeon, the blade is gently advanced until the disk is reached. Alternatively, a hypodermic needle can be used to localize the intervertebral disk space. With the blade directed in a slightly cranial direction, the blade is inserted to the level of the dorsal anulus fibrosus, against and parallel to the vertebral end plate. This distance can be estimated from the lateral radiographic view of the cervical spine. The blade is advanced to no more than 1/2 the width of the intervertebral disk. The cutting edge of the blade is then directed cranially and advanced up to the caudal end plate of the cranial vertebra. The blade is then directed to left lateral and advanced to no further than 1/2 the width of the disk space. Again, the blade is angled cranially such that it is against and parallel to the caudal end plate of the cranial vertebral body. The blade is directed caudally and advanced up until the cranial end plate of the caudal vertebral body is reached. The blade is then directed and advanced towards the midline to complete the rectangular shaped excision (window) (Figure 47-7). This portion of excised ventral anulus fibrosus can then be removed with rongeurs and the nucleus pulposus gently removed with curettes or dental scraper (Figure 47-8). Care must be taken such that additional disk material is not forced dorsally into the spinal canal. This can be accomphished by directing the curette or dental scraper in a direction parallel to the plane of the patient’s body instead of ventrodorsally.
 

Following fenestration, a high speed 4-5 mm carbide burr is used to create a slot in the vertebral bodies cranial and caudal to the intervertebral disk. Overheating of the burr can be prevented with saline lavage. We prefer a long, narrow slot for removal of herniated disk material. The slot should be no wider than 1/3 the vertebral body width and no longer than 1/3 the vertebral body length (Figure 47-9). Since the disk space angles craniodorsally, the caudal aspect of the slot can begin at the endplate of the caudal vertebral body (Figure 47-10). The slot can be deepened to the level of the cortical bone of the ventral spinal canal. The depth of the defect can be determined by identifying the difference in bone density of the cortical and cancellous bone.
 

Cortical bone is white and hard whereas the cancellous bone is red to purple in color. The inner cortical bone tends to have a pearl color and can appear transparent as it becomes thinner with drilling. Once the burr has penetrated the inner cortical layer, a small bone curette can be used to enlarge the slot. The remaining dorsal anulus fibrosis and dorsal longitudinal ligament can be removed with rongeurs, forceps, curettes or hemostats. Small instruments such as ophthalmic spatulas, loop curettes, fine curved forceps and suction can be used to retrieve herniated disk material from the canal (Figure 47-11). Disk material on the midline should be removed first and then laterally extruded disk material can be removed to avoid damage to the venous sinus.

Damage to the venous sinus results in excessive hemorrhage and obstruction of visualization of the spinal cord. This can be controlled with suction and hemostatics, such as Gelfoam^a or Surgicel.^b Suction can be used to evacuate the blood. A small piece of precut Gelfoam^a, presoaked in saline, can be placed at the site of the hemorrhage. Cottonoid^c or sponge is placed over the Gelfoam^a to prevent inadvertent aspiration of the hemostatic. Suction of the overlying sponge or Cottonoid^c is performed until hemorrhage has stopped. The sponge or Cottonoid^c can then be removed. The hemostatic can be removed after 5 minutes and disk material retrieval can be resumed. In some cases, a small remnant of either Gelfoam or Surgicel can be left at the site to aid in hemostasis. Monofilament absorbable suture material such as 3-0 or 2-0 PDS is used to close the sternohyoideus and sternocephalicus muscles. Closure of subcutaneous tissues and skin is routine.
 

^a Gelfoam: Upjohn Co., Kalamazoo, MI 
^b Surgicel: Johnson and Johnson, Arlington, TX 
^c Cottonoid: Codman and Scurtleff, Randolf, MA

Post-operative Management

Analgesics such as opiods or NSAIDS should be continued for 24-48 hours postoperatively. Corticosteroids are not indicated in the post-operative period. A thoracic harness should be used instead of a neck collar.

Post-operative management of cervical decompressive slot patients is generally divided into ambulatory or non-ambulatory convalescence. Patients with an ambulatory status post-operatively are generally managed in the following manner: cage confinement, brief exercise 2 to 3 times a day for 2 to 3 weeks, and home on restricted exercise and/or passive range of motion exercises 2 to 3 times a day. Non-ambulatory patients are managed in the following manner: elevated padded cage rack or waterbed, turned every 2 to 4 hours to prevent decubital ulcers and passive lung congestion or pneumonia, bladder expressions to 3 times a day, passive range of motion exercises at least 2 to 3 times a day, and frequent hydrotherapy until return to an ambulatory status is achieved. Non-ambulatory patients will require support to be held in a sternal position to eat and drink and to avoid aspiration pneumonia.

Crate or pen confinement is recommended for 6 to 8 weeks with gradual return to normal activity to follow.

Neurologic recovery is generally very rapid. Neck pain usually subsides within 24 to 48 hours. Tetraparetic patients may begin to show improvement within days, as well. Owners, however, should be counseled as to the unpredictabilty of spinal surgery and its complications to include delayed return to function and recurrent neck pain.

References

1. Piermattei DL. An atlas of surgical approaches to the bones and joints of the dog and cat. 3rd ed. WB Saunders, 1993; 54-59.
2. Swaim SF. Ventral decompression of the cervical spinal cord in the dog. JAVMA 1974; 164, 491-495.
3. Seim HB and Prata RG. Ventral decompression for the treatment of cervical disk disease in the dog: a review of 54 cases. J Am Anim Hosp Assoc 1982; 18, 233-240.
4. Sharp NJ and Wheeler SJ: Cerviacl Disc Disease. In Small Animal Spinal Disorders. Philadephia; Elsevier, 2005. 96-105.
5. Fry TR, Johnson AL, Toombs J. Surigical treatment of cervical disc herniations in ambulatory dogs. Ventral decompression vs. fenestration in111 cases (1980-1988). Progress in Veterinary Neurology 1991;2, 165-173.
    

Introduction

There are two separate philosophical approaches to the surgical treatment of caudal cervical spondylomyelopathy (CCSM) in large breed dogs, direct decompression versus decompression by distraction and stabilization. In general, patients with malformation/malarticulation or static compressive lesions benefit from direct decompressive surgical techniques whereas patients with dynamic compressive lesions such as cervical vertebral instability (CVI) require distraction and stabilization. As witnessed from a review of the literature on the subject, no one technique for repair of dynamic lesions is considered the gold standard. Repair of these compressions is contingent upon the patient’s demeanor, general overall health, surgeon experience, and owner expectations for recovery.

Treatment by Direct Decompression using an Inverted Cone Modified Ventral Slot¹

The inverted cone modified ventral slot is a direct decompressive technique for the removal of hypertrophied dorsal anulus fibrosus associated with cervical vertebral instability. This technique is most useful in patients with a static lesion, unchanged by distraction. The hypertrophied dorsal anulus fibrosus can be difficult to remove from the canal using the classic ventral decompressive slot technique.^1,2,3,4 This technique or a combination of this technique with the classic approach may have merit in allowing better retrieval of anulus from the canal. The slot resembles an inverted cone wherein the base of the cone is at the ventral spinal canal.¹ (Figure 47-12).
 

Technique

The approach to the affected intervertebral disk space is the same as described for ventral cervical slot. Using a high speed bur, the slot is created from the caudal aspect of the intervertebral disk to involve the caudal 1/4 of the cranial vertebral body. The width of the slot is limited to 1/5 the width of the vertebral body. The slot is enlarged as it is deepened by moving the bur in a sweeping motion laterally, creating an elliptical slot. The slot is carried to the level of the inner cortical layer while preserving the dorsal anulus fibrosus. The dorsal anulus fibrosus can then be retracted back into the slot and excised (Figures 47-13 and 47-14). The inner cortical bone layer is removed with the high speed bur and additional anulus and dorsal longitudinal ligament can be excised (Figure 47-15). Closure is routine.
 

Treatment by Distraction and Stabilization Utilizing Pins or Screws and Polymethyl Methacrylate^5,6

Distraction and stabilization utilizing Steinmann pins or bone screws and polymethyl methacrylate has been described. Advantages of this technique include: adequate spinal cord decompression without entering the spinal canal, reduced risk of iatrogenic cord trauma and bleeding, as well as improvement in the percent, rate and duration of recovery as compared to other techniques. In addition, a neck brace is not required. This technique is used most commonly for dynamic lesions that involve both the annulus fibrosus and the dorsal longitudinal ligament.

Technique⁵

A ventral approach, as described for the ventral decompressive slot, is performed to expose the vertebral bodies and intervertebral spaces cranial and caudal to the affected intervertebral space. The patient is positioned in dorsal recumbency such that the cervical spine is distracted, as described for the ventral slot. The affected intervertebral space is then pulled into additional linear traction by one of two techniques. A Gelpi retractor, modified by blunting the tips, can be used as a vertebral retractor. A defect is created in the vertebral bodies cranial and caudal to the affected vertebral bodies with a high-speed surgical bur. The defects are created just large enough to accept the tips of the modified Gelpi retractor. The retractor is engaged, and the affected intervertebral space spread an additional 2 to 3 mm (Figure 47-16). This technique of vertebral spreading may have merit over insertion of the tips of the Gelpi retractor into fenestrated disk spaces. Fenestration of the intervertebral disks may predispose them to degenerative changes and collapse.⁷ Distraction results in decompression of the spinal cord.^8,9
 

A ventral slot is performed at the affected intervertebral space, however the slot is wider and shorter than a classic ventral decompressive slot. The depth is carried only to the level of the inner cortical bone layer. The spinal canal is not entered. The width of the slot should be no more than 1/2 the width of the vertebral body. The length of the slot is determined by the thickness of the vertebral endplates. Once the cortical endplate on each vertebral body has been removed, burring should cease. Autogenous cancellous bone is harvested from the heads of the humeri and placed into the distracted slot. Two 7/64 or 1/8 inch Steinmann pins are inserted into the ventral surface of the vertebral body cranial to the affected intervertebral space and two similar size pins are inserted into the vertebral body caudal to the affected intervertebral space. The pins are inserted on the ventral midline of the vertebral body and directed 30-35 degrees dorsolaterally to avoid entering the spinal canal. It is important that two cortices are engaged by each pin. The pins are cut leaving approximately 1.5 to 2 cm exposed. The exposed portion of each pin is notched with pin cutters allowing the bone cement to grip and prevent pin migration. Bone screws 3.5 to 4.0 mm in diameter long enough to engage both cortices may be used instead of Steinmann pins. Sterile polymethyl methacrylate powder is mixed with liquid monomer until it reaches a doughy consistency and can be handled without sticking to the surgeon’s gloves. The cement is then meticulously molded around each pin (Figure 47-17). Irrigation with sterile saline solution for 5-10 minutes dissipates the heat of polymerization. The vertebral spreaders are removed once the cement has hardened. Closure of the longus colli muscle is performed cranial and caudal to the cement mass. The remainder of the closure is routine. Postoperative care includes strict confinement for 4-6 weeks.
 

Treatment by Distraction and Stabilization utilizing a Polymethyl Methacrylate Plug¹¹

Another technique utilizing an intervertebral plug of polymethyl methacrylate to accomplish distraction and stabilization has been described.¹¹ There is no apparent advantage in rate of recovery and overall success rate as compared to distraction and stabilization using pins and polymethyl methacrylate, however risk of implant failure or iatrogenic spinal cord trauma from improperly placed pins is less with this technique. In addition, this technique can be performed at multiple disk spaces if necessary.¹¹

Technique¹¹

A ventral approach, as described for ventral decompression, is performed to expose the vertebral bodies and intervertebral spaces cranial and caudal to the affected intervertebral space(s). The affected intervertebral space is then pulled into additional linear traction as previously described in the pins and polymethyl methacrylate technique. The original authors of the PMP technique have successfully used other vertebral spreaders as well. The affected disk material is removed to the level of the dorsal anulus fibrosus. Troughs are cut into the endplates using a high speed drill and a 2 to 4 mm bur to anchor the PMP. These anchor troughs should be made approximately 5 to 10 mm in lateral width, 4 mm in depth and 4 mm in dorso-ventral height (Figure 47-18). An angled attachment will allow better access to the caudal vertebral endplate. One gram of sterile cefazolin powder can be mixed with the sterile polymethyl methacrylate. The polymethyl methacrylate powder is mixed with liquid monomer until it reaches a liquid consistency and can be infused into the intervertebral disk space to the level of the ventral aspect of the vertebral bodies and gently packed digitally. Irrigation with sterile saline solution for 5 to 10 minutes dissipates the heat of polymerization. The vertebral spreaders are removed once the cement has hardened. Autogenous cancellous bone is harvested from the heads of the humeri and placed ventral to the vertebral bodies and PMP to stimulate osseous fusion (Figure 47-19). Closure of the longus colli muscle is performed over the cancellous bone graft. The remainder of the closure is routine. A neck brace may be used post-operatively to limit excessive movement, but may not be required (Dixon). The Synthes Locking Plate (Syncage-C intervertebral implant and cervical spine locking plate) may prove to be a viable option in the future. The Synthes locking plate has been discussed in the treatment of dynamic lesions to include single lesions (as repaired with the cement plug), or as a rescue technique after a failed ventral slot decompression or with multiple lesions. To date, the use of the Locking plates on multiple lesions is still undergoing further evaluation.¹⁸ Preliminary results with this technique are encouraging. A swivel ring in the plate hole means that the screws may be inserted at any angle within a range of +/- 20 degrees and the screw holes lock in the plate via a unique locking mechanism. This device (the Syncage) is designed to maintain distraction. It stays within the intervertebral space and is packed with cancellous bone. This device adds strength and bridge greater than one space.¹⁸ The utility of this device for multiple lesions in the canine is yet to be elucidated. Limitations of this technique can include cost of the implants and the lack of case numbers that support further use and feasibility of this technique.
 

Treatment by Direct Decompression using a Continuous Dorsal Laminectomy¹²

Continuous dorsal laminectomy is a decompressive technique. This technique is most useful in patients with multiple lesions and dorsal lesions. Although this technique does not address the underlying pathophysiologies associated with CCSM, relief of spinal cord compression is achieved. Dorsal laminectomy is advocated for single or multiple, dorsal, traction non-responsive (static) lesion(s).¹⁸ The major disadvantage of this procedure is the significant, short-term morbidity with deterioration in neurological status, which can be substantial in the giant breed dogs who most likely require this technique. The most common lesions associated with the use of this technique are bulbous articular facets, ligamentum flavum hypertrophy or a combination of both.

Approach¹³

With the patient in sternal recumbency the front feet are secured cranially and the head and neck elevated from the surgical table. Tape placed over the muzzle and thorax help secure the neck. A midline incision is made in the skin over the dorsal processes of the cervical spine from the poll of the cranium to T3. After the subcutaneous fascia and aponeurosis of the platysma muscle are incised, an incision is made through the median fibrous raphe. The origins of the splenius and serratus dorsalis muscles can be incised from the raphe and reflected to expose the nuchal ligament, dorsal spinous processes of the thoracic vertebrae and the long spinal muscles. These muscles are separated from the midline and reflected form the dorsal spinous processes to expose the dorsal laminae.

Technique¹²

After exposure of the cervical vertebrae, the dorsal spinous processes of the affected vertebrae are removed with rongeurs and the dorsal lamina is carefully removed using a high speed surgical bur. The length of the laminectomy may be from 3/4 the length of each vertebrae up to a continuous laminectomy extending from C4 to C7. The width of the laminectomy is limited by the medial aspect of the articular facets of the cranial vertebra. The initial depth of the laminectomy defect is to the periosteum of the inner cortical layer of the laminae. Following penetration into the spinal canal, the remaining laminae and ligamentum flava are gently excised and removed en bloc (Figure 47-20). Kerrison rongeurs can be quite useful for this procedure. If needed, resection of the lateral aspects of the vertebral arches can be continued to the level of the ventral vertebral veins using rongeurs. It is important to preserve the articular facets. Hypertrophied joint capsule and ligamentum flavum is resected to achieve decompression of the spinal cord. Transarticular hemicerclage wires or lag screws may need to be placed through the facets for additional stability. If stabilization is required, an appropriate sized hole is drilled through the articular facet. Removal of the articular cartilage is achieved using a high speed surgical bur. An 18-gauge stainless steel wire is placed through the hole and twist tightened or, alternatively, the hole is tapped and a lag screw placed. Cancellous bone is placed around the joint to promote arthrodesis.^{14,15,16,17,18} An autogenous fat graft placed over the laminectomy site will prevent the formation of a fibrous laminectomy membrane with subsequent stricture and spinal cord compression. Paraspinal muscles and fascia are approximated and the remaining closure is routine. A cervical bandage or brace is generally required.

Post-operative Management of CCSM Patients

Analgesics may be necessary for 24 to 48 hours postoperatively. Corticosteroids are not indicated in the postoperative period and may be contraindicated. Non-steroidal antiinflammatories and oral opiods can be used for post-operative pain management. A cervical bandage of rolled cotton and stretch gauze can be placed postoperatively to prevent excessive head and neck movements. This bandage can remain in place for 3 weeks. If warranted and tolerated, a neck brace constructed of fiberglass cast material or a heat moldable splint material, incorporating the cervical and cranial aspect of the thoracic spine may limit movement, thereby promoting fusion. Handles built into the brace may allow for better assistance when rising and walking. A thoracic harness should be used instead of a neck collar. Post-operative management of CCSM patients is generally divided into ambulatory or non-ambulatory convalescence. Patients with an ambulatory status post-operatively are generally managed in the following manner: cage confinement, brief exercise 2 to 3 times a day for 2 to 3 weeks, and home on restricted exercise and/ or passive range of motion exercises. Non-ambulatory patients are managed in the following manner: elevated padded cage rack or waterbed, passive range of motion exercises and turned every 2 to 4 hours, bladder expressions 4 to 5 times a day, serial neurologic evaluations and frequent hydrotherapy (swimming with support) until return to an ambulatory status is achieved. Care most be taken especially in the giant breeds to monitor for and prevent aspiration pneumonia secondary to recumbancy and poor lower esophageal sphincter tone after anesthesia. Patients who are recumbent should be held up in sternal recumbancy when offered food and water to prevent the occurrence of aspiration. Crate or pen confinement is recommended for 6 to 8 weeks with gradual return to normal activity to follow.

Neurologic recovery is generally very rapid, but exceptions do occur. Neck pain usually subsides within 24 to-48 hours. Tetraparetic patients should begin to show improvement within days, as well. Any neurologic improvement within 3 weeks of surgery is encouraging. The neurologic status 6 weeks postoperatively is a good indication of ultimate neurologic recovery, however patients may show improvement in function up to 6 months postoperatively.^4,5 Serial neurologic exams and compliant owners are essential in the follow-up of these patients. Iatrogenic spinal cord trauma, post-operaative compressive hemorrhage, irreversible demyelination and myelomalacia or agenesis of the affected spinal cord limits the success of surgical techniques used to treat CCSM.

References

1. Goring RL, Beale BS, Faulkner RF. The inverted cone decompression technique: A surgical treatment for cervical vertebral instability “Wobbler Syndrome” in Doberman pinschers. Part 1. J Am Anim Hosp Assoc 1991; 27: 403-409.
2. Chambers JN, Betts CW. Caudal cervical spondylopathy in the dog: a review of 20 clinical cases and the literature. J Am Anim Hosp Assoc 1977; 13: 571-576.
3. Chambers JN, Oliver JE, Bjorling DE. Update on ventral decompression for caudal cervical disk herniation in Doberman pinschers. J Am Anim Hosp Assoc 1986; 22: 775-778.
4. Bruecker KA, Seim HB, Withrow SJ. Clinical evaluation of three surgical methods for treatment of caudal cervical spondylomyelopathy of dogs. Vet Surg 1989; 18: 197-203.
5. Bruecker KA, Seim HB, Blass CE. Caudal cervical spondylomyelopathy: decompression by linear traction and stabilization with Steinmann pins and polymethyl methacrylate. J Am Anim Hosp Assoc 1989; 25: 677-683.
6. Ellison, GW, Seim HB, Clemmons RM. Distracted cervical spinal fusion for management of caudal cervical spondylomyelopathy in large breed-dogs. J Am Vet Med Assoc 1988; 193: 447-453.
7. Lincoln JD, Pettit GD. Evaluation of fenestration for treatment of degenerative disk disease in the caudal cervical region of large dogs. Vet Surgery 1985; 14: 240-246.
8. Seim HB, Withrow SJ. Pathophysiology and diagnosis of caudal cervical spondylomyelopathy with emphasis on the Doberman pinscher. J Am Anim Hosp Assoc 1982; 18: 241-251.
9. Seim HB, Bruecker KA. Caudal Cervical Spondylomyelopathy (Wobbler Syndrome). In, (ed) Bojrab, Disease Mechanisms in Small Animal Surgery, 2nd ed. Lea and Febiger. 1993: 979-983.
10. Walker TL. Use of Harrington Rods in Caudal Cervical Spondylomyelopathy. In, (ed)Bojrab, Current Techniques in Small Animal Surgery, 3rd ed. Lea and Febiger. 1989: 584-586.
11. Dixon BC, Tomlinson JL, Kraus KH. Modified distraction-stabilization technique using an interbody polymethyl methacrylate plug in dogs with caudal cervical spondylomyelopathy. J Am Vet Med Assoc 1996; 208: 61-68.
12. Lyman, R. Continuous dorsal laminectomy for the treatment of Doberman pinschers with caudal cervical vertebral instability and malformation. Abstracts, 5th Annual Meeting of the American Animal Hospital Association 1987: 303-308.
13. Piermattei DL. An atlas of surgical approaches to the bones and joints of the dog and cat. 3rd ed. WB Saunders, 1993; 60-69.
14. Walker TL, Tomlinson JL, Sorjonen DC, Kornegay JN. Diseases of the spinal column. In, (ed) Slatter, Textbook of Small Animal Surgery. WB Saunders, 1985; 1367-1391.
15. Trotter EJ, deLahunta A, Geary JC, Brasmer, TH. Caudal cervical vertebral malformation-malarticulation in Great Danes and Doberman Pinschers. J Am Vet Med Assoc 1976; 10: 917-930.
16. Dueland R, Furneaux RW, Kaye MM. Spinal fusion and dorsal laminectomy for midcervical spondylolisthesis in a dog. J Am Vet Med Assoc 1973; 162: 366-369.
17. Hurov LI. Treatment of cervical vertebral instability in the dog. J Am Vet Med Assoc 1979; 175: 278-285.
18. Sharp N, Wheeler S. Cervical Spondylomyelopathy. Small Animal Spinal Disorders. Second edition. Elsevier. 2005. 211-246.
     

This topic is written based on the available literature through 2010 and does not cover the most current literature on this topic.

Two categories of surgical techniques have been described. Both dorsal and ventral approaches aim to stabilize the atlantoaxial joint in the normal position; however, only ventral approaches allow for complete fusion of the involved cervical vertebrae and permit excision of the dens if necessary.

Ventral Approach

Atlantoaxial instability can be resolved permanently by fusing the two vertebrae in anatomic alignment, a procedure that is easier from a ventral approach. This approach also allows access to the dens if removal is indicated because of fracture or severe dorsal displacement. With the dog in dorsal recumbency, the head and neck should be extended and supported by padding under the cervical area (Figure 47-21A). The surgical approach is made through a ventral midline incision extending from the larynx to the manubrium, followed by separation of the paired sternothyroid muscles. The trachea, esophagus, and carotid sheath are bluntly dissected to allow lateralization. The paired hypaxial muscles ventral to the atlas and axis then are separated carefully on the midline and are lateralized with self-retaining retractors.

The joint capsule of the atlantoaxial articulation should be identified and opened with a No. 11 Bard-Parker blade (Figure 47-21B). In chronic cases, the joint capsule may be thickened and may contain increased volumes of joint fluid. The joint may be reduced to normal position by retraction with small, pointed reduction forceps on the caudal body of the axis. If the dens is fractured or ununited, it should be removed through an incision through the membrane between the two articulations. The ligaments attached to the apex of the odontoid process are exposed through a ventral opening in the fascia covering the foramen magnum. The dens may be removed after careful severance of these apical and alar ligaments. Removal of the dens should not be necessary if it is united to the body of C2 and, if accurate, stable realignment can be accomplished.

Arthrodesis of CI and C2 is optimized by removal of the articular cartilage from the joint spaces and placement of a cancellous bone graft obtained from the proximal humerus. Access to the joints may be increased by gentle caudal retraction of C2 with reduction forceps, and the cartilage may be removed with rongeurs or an air drill. Because of the architecture and location of the joints, it is unrealistic to expect removal of all the articular cartilage; removal of the ventral 75% from all four articular surfaces is probably adequate. The bone graft is packed into the joint spaces after adequate removal of cartilage and lavage of the surgical site.

Ventral stabilization of the atlantoaxial joint may be achieved using pins alone, pins and polymethylmethacrylate, lag screws, or bone plates. A power drill is necessary for accurate placement of pins and screws. If pins alone are to be used, two small Steinmann pins or large Kirschner wires are driven from the center of the axis across the atlantoaxial joint and are seated in the atlas just medial to the alar notch (Figure 47-21C and D).¹ The point of each pin must be kept as ventral as possible to avoid penetrating the dorsal surface of the thin wings of the atlas. The length of the pins is premeasured from the point of entry into the axis to the palpable medial aspect of the alar notches on the atlas. When both pins are seated, they are cut off close to the body of the axis. The protruding ends are crimped and bent to prevent cranial migration of the pins into the occipital condyles.

The addition of polymethylmethacrylate to the stabilization technique may increase the odds of successful arthrodesis by enhancing stability and may reduce the risk of pin migration (KS Schultz, Waldron DR, unpublished data). Pins are first placed into the atlas (Figure 47-22A). This placement is facilitated by gentle dorsiflexion of the atlantoaxial joint that allows visualization of the spinal canal. Kirschner wires or small threaded pins are directed perpendicular to the long axis of the spine from ventral to dorsal into each of the pedicles of the atlas. The atlantoaxial joint is then reduced, and pins are placed across the joints as described for pin stabilization alone. One or two pins are then placed into the caudal body of the axis (Figure 47-22B). All pins are cut short and are bent, leaving enough pin length to engage a small mass of polymethylmethacrylate (Figure 47-23). Antibiotic powder should be added to the cement, and cool saline flush should be applied during polymerization of the cement to dissipate heat.

The surgical approach and preparation of the atlantoaxial joints are identical for stabilization with lag screws.² In small dogs, 1.5-mm cortical screws are placed across each of the joints in a lag fashion. This technique may be facilitated by use of a cannulated drill and screw system. In either case, placement of the screws is in a direction similar to that of the transarticular pins. Ventral application of bone plates has also been described; however, the size of most patients may limit the practical application of this technique.³
 

Postoperative radiographs should be obtained after stabilization with any of the ventral techniques to demonstrate reduction of the atlantoaxial joint and accurate placement of implants. Neck braces should be maintained if possible for several weeks, and initial cage rest is strictly enforced. Radiographs may be obtained 8 weeks postoperatively to evaluate maintenance of reduction and progression of arthrodesis.

Complications of ventral stabilization techniques include implant migration and loosening.⁴ The result may be subsequent instability and recurrence of neurologic signs. Placement of pins or screws within the vertebral canal may also worsen the neurologic signs. Tracheal necrosis has been reported with the ventral approach; therefore, gentle dissection and attention to preservation of the delicate blood supply of the region are indicated. As with any surgical implantation of polymethylmethacrylate, concern exists for thermal injury and infection.

Dorsal Approach

The dorsal arch of the atlas is secured to the dorsal spine of the axis with heavy suture material, orthopedic wire, or grafts of the nuchal ligament. Descriptions of these techniques are available in the third edition of this text. Although ventral techniques are more difficult, we recommend them because of their lower failure rate.⁴ Complications of dorsal techniques include instability resulting from breakage of the suture, wire, or graft and fracture of the axis or atlas. Wire stabilization may fail because of cycling, and the addition of polymethylmethacrylate to the wire technique has been recommended to alleviate this complication. Fracture of the axis may be due either to inappropriate placement of the holes or to the remaining motion of the joint, which places excessive forces on the stabilization technique.

Medical management including cervical splinting has been successful in selective cases; however, surgical therapy is recommended for patients demonstrating significant neurologic signs that have no other contraindications for anesthesia or surgery.⁵ Ventral techniques are technically challenging, but because of the higher failure rates of dorsal techniques, the routine use of dorsal procedures should be avoided.⁴

References

1. Sorjonen DC, Shires PK. Atlantoaxial instability: a ventral surgical technique for decompression, fixation, and fusion. Vet Surg 1981;10:22-29.
2. Denny HR, Gibbs C, Waterman A. Atlanto-axial subluxation in the dog: a review of thirty cases and an evaluation of treatment by lag screw fixation. J Small Anim Pract 1988;29:37-47.
3. Thomas WB, Sorjonen DC, Simpson ST. Surgical management of atlantoaxial subluxation in 23 dogs. Vet Surg 1991;20:409-412.
4. McCarthy RJ, Lewis DD, Hosgood G. Atlantoaxial subluxation in dogs. Compend Contin Educ Pract Vet 1995;17:215-226.
5. Gilmore DR. Nonsurgical management of four cases of atlantoaxial subluxation in the dog. J Am Anim Hosp Assoc 1984; 20:93-96.
    

General Considerations

When considering treatment options for a patient with a spinal fracture, luxation or subluxation, several factors should be considered; 1) results of the neurologic examination, 2) is the fracture pathologic or traumatic, and 3) is the fracture stable or unstable.

The neurologic examination is critical in determining prognosis. If the patient has lost all sensory and motor function caudal to the lesion, the prognosis is unfavorable and treatment is generally supportive. Surgery in this situation may be indicated for prognostic purposes only (ie. exploratory laminectomy). If deep pain perception is still present, the prognosis is guarded to favorable (depending on the degree of neurologic dysfunction and the timing of the event or injury) and surgical decompression and stabilization is performed with curative intent.

Patients with pathologic fractures have an underlying localized or generalized disorder. Examples of this would include a solitary plasma cell tumor, multiple myeloma or other classifications of paraneoplastic or infectious disorders. The cause of the underlying disorder must be determined and therapy instituted prior to or concurrent with spinal fracture/luxation repair.

Physical examination findings and radiographic assessment may be helpful in determining the inherent stability of the fracture/ luxation.¹ In small animal patients, traumatic disruption of the spinal column can be divided into dorsal compartment injuries, ventral compartment injuries, or combined compartment injuries. Combined compartment injuries are more devastating and more common than injuries isolated to one compartment. The majority of spinal injuries are flexional injuries, but occasionally hyperex- tension or direct compression injuries may occur. Rotation is a common concurrent force associated with these injuries.

Fractures may be classified as stable or unstable by the radiographic appearance and by the force causing the injury. Forces resulting in damage to the dorsal compartment generally result in an unstable injury. Examples include laminar or pedicle fracture, dorsal spinous process fracture, articular process fracture, and supraspinous/interspinous ligament rupture.²

If surgery is deemed necessary, it is important to select a technique that will not further destabilize the spine. Herniated disk material or osseous fragments within the spinal canal may be anticipated in flexion or bursting type injuries. Concussive and contusive forces can cause spinal cord swelling even without evidence of an extradural mass. Extradural hematoma formation can be quite extensive and can be delineated on MRI or CT scan imaging. Infolding of the ligamentum flavum during hyperextension injuries may also result in spinal cord injury.

Generally, stable fractures in patients with good voluntary motor movements to the limbs are successfully managed by conservative means, including the use of analgesics, non-steroidal anti-inflammatory agents, body splints, and strict cage confinement.^3,4 Serial neurologic examinations are performed (twice daily) to determine the response to treatment.

Surgical management is indicated 1) if the fracture/luxation is considered unstable, 2) if the patient presents nonambulatory paraparetic or tetraparetic with no voluntary motor movements, or 3) if with conservative therapy, the patient remains unacceptably static or deteriorates neurologically.

Several factors must be considered when selecting a stabilizing technique: 1) location of the fracture/luxation (cervical, thoracic, lumbar, sacral), 2) presence of a compressive lesion within the spinal canal (ie. osseous fragment, disk material, hematoma), 3) size of the patient, 4) age of the patient, 5) equipment available, 6) experience of the surgeon, and 7) physical and emotional capability of the owner to provide postoperative nursing care.

Surgical Techniques

The two objectives of any surgical technique used to repair spinal fracture/luxation are decompression and stabilization. Many techniques have been successfully used to stabilize spinal fracture/luxation in small animals. In the following discussion, techniques commonly used to repair fractures and luxations of the spine will be described as they are indicated in various regions of the vertebral column. These chapters will be divided into surgical treatment of cervical spinal fractures, luxations and subluxations and surgical management of thoracolumbar, lumbar and lumbosacral fractures, luxations and subluxations. The above discussion regarding prognosis and patient selection in cases of spinal trauma applies to both categories of injury and disease.

Fractures of the Cervical Spine

Cervical spinal fractures are uncommon.^4,5,6 Most fractures of the cervical spine involve C1 (axis), particularly the dens and/or body.⁶ In fact, the most frequent anatomic location of cervical fracture/luxation is the cranial cervical region with 80% occurring at C1-2. Because the cervical region has the largest ratio of vertebral canal to spinal cord diameter, conservative management consisting of external support and cage rest in unstable and/or displaced fractures may carry a more favorable prognosis than elsewhere in the spine.⁷ Mortality rates can be as high as 35 to 40% with surgery.⁷ Severe intraoperative hemorrhage may also occur with C2 fractures and reduction can be challenging.^5,7,8 External splints, though cumbersome, may be made from various materials. Surgery is best reserved for those animals that 1) are tetraplegic or have poor ventilatory function 2) show neurologic deterioration despite proper confinement or external fixation, and 3) remain painful beyond the initial 48 to 72 hour period following injury.⁵

Fractures of the dorsal spine of the axis should be approached dorsally and stabilized with orthopedic wire to reestablish the continuity of displaced fragments. A decompressive hemilaminectomy can be performed if fragments of bone are present in the spinal canal, or if a displaced body fracture cannot be reduced. Atlantoaxial subluxation can be repaired from a dorsal approach utilizing either a double or single wiring or suturing technique.⁷

C1-C2 body fractures/luxations, traumatic cervical disk extrusions, and atlantoaxial subluxation can be approached ventrally. Ventral cross-pin techniques may be used for stabilization of atlantoaxial subluxation¹⁰ (Figure 47-24).

Fractures and luxations rarely occur from C3 to C7 however, a predisposition to luxations at C5-C6 may exist.^6,11 Fracture/ luxations of C3-C7 may be approached dorsally or ventrally. Dorsal techniques include articular facet wiring or screwing, dorsal spinous process plating and multiple Steinmann pins and polymethyl methacrylate (described in detail under Fractures of the T-L and Lumbar Spine). Ventral techniques include pins and polymethyl methacrylate and ventral body plating (plastic [footnote a] or metal [footnote b]). One advantage to the ventral approach is that a ventral slot can be performed if disk fragments have extruded into the spinal canal.
 

The use of pins (or screws)and polymethyl methacrylate should be considered for cervical spinal fractures involving the vertebral bodies of C2-C7. The ventral aspect of the involved vertebrae is exposed.¹² Once the fracture is reduced, a minimum of two trocar tip pins should be placed in the cranial fragment and a minimum of two pins should be placed in the caudal fragment. Alternatively, the fractured vertebral body can be bridged by insertion of pins into the vertebrae cranial and caudal to the fracture. It is important to engage two cortices with each pin. The pins are inserted on the ventral midline of the vertebral body and directed 30 to 35 degrees dorsolateral to avoid entering the spinal canal. In addition, the pins can be angled cranially and caudally to enhance stability of the implant. The pins are cut leaving 1 to 1.5 cm exposed. The exposed pins can be notched with pin cutters and covered with sterile polymethyl methacrylate¹³ (Figure 47-25). Preferentially, specific purpose acrylic pins with threaded trocar tips and knurled shaft allows good bone anchor and foothold for the PMMA. The heat of polymerization is dissipated with 5 to 10 minutes of cool saline irrigation. A neck brace may be used for 4 to 6 weeks postoperatively. The limiting factor of this technique is the purchasing ability of the pins in small fragments. Screws can also be used as described above.^14,15,16 The main disadvantage of the above described techniques can be failure if used to span more than one intervertebral space especially in cases where the vertebral body is shattered or collapsed. In these cases, at least 3 implants should be placed on either side of the fracture and Steinman pins used to reinforce the cement. In some reports^2,11 if dorsal stability is required (ie especially after facet luxation), screws can be placed transarticularly.⁵ Reduction of cervical fracture/luxations can be facilitated by gently distracting the affected vertebral bodies. Fenestration of the adjacent intervertebral disks or slots drilled into the vertebral bodies cranial and caudal to the fracture/luxation can be created to accommodate a vertebral distractor. A Gelpi retractor, modified by blunting the tips, is a useful vertebral distractor.

Stabilization with ventral locking plates and screws has become another method of repair and will be of interest in the future once the technique is perfected and the price of the spinal plates becomes less cost prohibitive.

After cervical spinal stabilization, complications can include Horners syndrome (which can be permanent or transient) and respiratory depression and/or distress secondary to diaphragmatic paresis or paralysis. This occurs in the aftermath of hemorrhage into or contusion to the phrenic nerve nuclei located at the C4-6 cervical spinal cord segments.
 

References

1. Feeney DA and Oliver JE. Blunt spinal trauma in the dog and cat: neurologic, radiologic and therapeutic correlations. J Am Anim Hosp Assoc 1980;16:664-668.
2. Swaim SF. Biomechanics of cranial fractures, spinal fractures, and luxations, in (ed) Bojrab, Pathophysiology in Small Animal Surgery. 1981:774-778.
3. Carberry CA, Flanders JA, Dietze AE, et al. Nonsurgical management of thoracic and lumbar spinal fractures and fracture/luxations in the dog and cat: a review of 17 cases. J Am Anim Hosp Assoc 1989;25:43-54.
4. Feeney DA and Oliver JE. Blunt spinal trauma in the dog and cat: insight into radiographic lesions. J Am Anim Hosp Assoc 1980;16:885-890.
5. Sharp NJ and Wheeler SJ: Trauma. In Small Animal Spinal Disorders. Philadelphia; Elsevier, 2005, 282-305.
6. Stone EA, Betts CW, Chambers JN. Cervical fractures in the dog: a literature and case review. J Am Anim Hosp Assoc 1979;15:463-471.
7. Hawthorne JC, et al. cervical vertebral fractures in 56 dogs: a retrospective study. JAAHA, 35, 135-146.
8. Boudrieau RJ. Distraction-stabilization using the Scoville-haverfield self-retianing laminectomy retractors for repair of 2nd cervical vertebral fractures in 3 dogs. Vet and Comp Orthopaedics and Traumatology 10, 71.
9. Oliver JE and Lewis RE. Lesions of the atlas and axis in dogs. J Am Anim Hosp Assoc 1973;9:304-313.
10. Sorjonen DC and Shires PK. Atlantoaxial instability: A ventral surgical technique for decompression, fixation, and fusion. Vet Surgery 1981;10:22-29.
11. Basinger RR, Bjorling DE, Chambers JN. Cervical spinal luxation in two dogs with entrapment of the cranial articular process of C6 over the caudal articular process of C5. J Am Vet Med Assoc 1986;188:865-867.
12. Piermattei DL. An atlas of surgical approaches to the bones and joints of the dog and cat. 3rd ed. WB Saunders, 1993;45-89.
13. Blass CE, Waldron DR, van Ee RT. Cervical stabilization in three dogs using steinmann pins and methylmethacrylate. J Am Anim Hosp Assoc 1988; 24:61-68.
14. Rouse GP and Miller JI. The use of methyl methacrylate for spinal stabilization. J Am Anim Hosp Assoc 1975;11:418-425.
15. Rouse GP. Cervical Spinal Stabilization with polymethylmethacrylate. Vet Surg 8.1979.1.
16. Schulz KS, et al. Application of ventral pins and polymethylmethacrylate for management of atlantoaxial instability: results in 9 dogs. Vet Surg 26.317-325.