Introduction

Cervical stabilization might be indicated for vertebral column instability resulting from trauma, anomalies, or pathologic conditions such as diskospondylitis. In these scenarios, orthopedic fixation principles are used to achieve functional alignment and apply rigid fixation. Stabilization has also become increasingly popular for the treatment of cervical spondylomyelopathy (CSM). For CSM, the goal of surgery is to eliminate motion at the affected vertebral articulation, thereby decreasing tissue responses secondary to instability such as ligamentous or annular hypertrophy as well as articular facet proliferation.

Distraction of the intervertebral disk space is beneficial for traction responsive lesions, such as soft tissue compression by thickened dorsal longitudinal ligament (DLL) or ligamentum flavum. While distraction per se may not lead to improvement of bony compression by facet proliferation, it may improve compression by proliferative soft tissues. The use of a disk spacer in a neutral or slightly distracted position maintains distraction and is beneficial for load sharing between the affected vertebrae, thereby improving implant longevity. Interbody fusion is desired to provide long‐term stability and prevent implant failure. This chapter will focus on ventral stabilization techniques of the cervical vertebral column used primarily to treat dogs with CSM. The principles presented here can be applied to many other conditions of the cervical spine requiring stabilization.

Preoperative Planning

Well positioned orthogonal radiographs of the cervical vertebral column are obtained to gain a general idea of vertebral body and disk space dimensions. To be used for measurements, radiographs must be calibrated with an appropriated positioned calibration tool (i.e. metal ball of known diameter). For plate fixation, radiographs help determine plate size and length and screw location. Knowledge of the latter is important to avoid screw placement in intervertebral disk spaces. Computed tomography provides excellent bony detail and is considered the most useful modality for preoperative planning of vertebral column stabilization. Vertebral body dimensions, in particular height, should be determined via CT at several locations to assist with selection of screw lengths. If an intervertebral spacer is used, endplate height and width and disk space depth need to be calculated to obtain an appropriate size spacer. Disk space dimensions should be obtained from unaffected sites (if available) to determine “normal” dimensions. While MRI is considered the gold‐standard imaging modality for spinal cord evaluation and invaluable for overall assessment of disease, the osseous detail it provides can be challenging for preoperative planning.

Anatomical Considerations

Cervical vertebral anatomy is challenging for rigid fixation due to limited compact bone stock and the proximity of important neurovascular structures (Figure 11.1). Dorsal approaches for stabilization of the cervical vertebral column in large dogs are uncommon. The vertebral body offers the most bone for fixation and is easily approached via a standard ventral approach. However, even this part of the vertebra offers on average only 6–12 mm in bone depth in most large dogs until the vertebral canal is breached. Due to the hour glass shape of the vertebral body, the greatest bone depth is near the vertebral endplate and the thinnest portion over the mid‐body. Intervertebral disk orientation is oblique in a craniodorsal to caudoventral direction; therefore, most bone will be purchased if implants are directed parallel to the endplates. The presence of the sternum and the limited exposure of the caudal cervical vertebral column can make implant placement parallel to the endplate in C6 and C7 challenging. The transverse processes offer additional fixation sites; however, bone is quite thin and care must be taken to avoid penetration into the transverse foramen (present C1–C6), which houses the vertebral artery. While some spinal locations (i.e. lumbar spine) offer additional bone purchase by also utilizing the vertebral pedicle, this is not recommended in the cervical vertebral column. Pedicle width is limited and not uniform throughout the individual cervical vertebra, making it challenging to be engaged with a pin or screw (Figure 11.1).

Implant Selection

Evidence in the veterinary literature strongly supports that bicortical implant placement in the cervical vertebral bodies is associated with a high risk of injury to important neurovascular structures such as spinal cord, nerve roots, or vertebral vasculature [1–3]. Therefore, bicortical vertebral body implants are not recommended for cervical fixation. Despite less bone purchase and concern for implant stiffness, monocortical screw fixation seems to compare favorably to bicortical pin fixation and eliminates the potential for major iatrogenic injury [3, 4]. Monocortical screw fixation has become the standard in current techniques publications [5–9]. The use of monocortical screws with Poly(methyl methacrylate) (PMMA) provides the most freedom in regard to screw placement; however, the presence of a large bulk of cement can interfere with adjacent soft tissues and also makes implant removal more time consuming. Plate fixation eliminates bulky implants, improves soft tissue closure and ease of implant removal if required. Depending on the surgeon’s comfort level with the technique and available inventory, locking plates with monocortical screws can be applied to the ventral vertebral bodies. In contrast to angle‐stable locking plates, polyaxial locking plates allow screw insertion within a given range of variable angles, which gives these plates some degree of flexibility with screw placement. Traditional nonlocking plates should be avoided as they are challenging to apply with appropriate contouring to produce excellent plate/bone contact and friction.

To date, most used veterinary plate systems are made of stainless steel, prohibiting the use of postoperative MRI. Titanium implants significantly reduce artifact development on MRI and are the preferred metal for vertebral instrumentation, regardless of these being titanium alloy screws in conjunction with PMMA or plate systems [10].

Positioning and Approach

The dog is placed in dorsal recumbency with thoracic limbs tied back caudally (Figure 11.2). The cervical vertebral column is supported with towels and a bean bag to level the spine horizontally in a neutral position. Placing a large roll of towels under the neck should be avoided as this overly extends the vertebral column and makes it more difficult to maintain the spine in alignment while traction is applied. Care must be taken to securely tie down the dog’s body to prevent displacement, especially if manual traction is applied for distraction. The dog’s right side should be placed toward the edge of the surgery table to improve ease of access for the main surgeon during a routine right‐sided ventral approach. Both proximal humeri are included in the sterile field to allow harvesting of an autologous cancellous bone graft (if desired). Bone graft should be obtained as late as possible during the procedure and stored in a blood‐soaked sponge until implantation to improve graft survival.

With the surgeon standing on the right side, a standard ventral midline or right paramedian approach to the cervical vertebral column is performed. With a paramedian approach, the insertion of the right sternocephalicus muscle can be partially tenotomized from the sternum to improve access to the caudal cervical spine. In some cases, it is necessary to split the manubrium sterni [11].

For a single site stabilization, the ventral vertebral bodies adjacent to the affected disk space are exposed but dissection can usually spare the neighboring disk spaces. If PMMA fixation is used, part of the longus colli musculature can be resected to make room for the cement. Otherwise, soft tissues are preserved.

Vertebral Distraction

Distraction of the intervertebral space can be achieved via manual traction or vertebral distractors. Good distraction greatly facilitates diskectomy and decompression of extruded material if present. For manual traction it is important to secure the patient’s body sufficiently to the surgical table without interfering with respiration or compromising circulation of blood flow to distal limbs. The pinnae and mandibular rami can be used as extra anchor points while gently pulling the dog’s head rostrally. Manual traction usually leads to sufficient disk space distraction to perform the diskectomy, remove extruded disk material from the vertebral canal, and place a disk spacer. It avoids placement of distractors or distractor pins that can interfere with access to the affected disk space(s) and possibly compromise bone needed for fixation. However, manual traction is labor intense and can often only be maintained for a few minutes without losing distraction. The person applying traction must do so in a slow and deliberate way and avoid sudden collapse of the vertebral articulation with possible injury to the spinal cord. A variety of human vertebral distractors are available with the most commonly used one in veterinary surgery being a Caspar‐style distractor (Figure 11.3). This instrument uses two fixation pins placed perpendicularly into adjacent vertebral bodies. These pins are then connected to the distractor and the disk space is carefully distracted. If properly placed and assembled, distraction is easily achieved and greatly facilitates diskectomy. However, over distraction is also easily achieved and must be avoided to protect articular facet integrity and prevent nerve root injury. Also, since most distraction and stabilization techniques require implant placement along the ventral aspect of the vertebral bodies, distractor pins can be in the way of and compromise valuable bone stock for instrumentation. A commonly used veterinary instrument to aid in disk space distraction is a modified Gelpi‐retractor, where the tips have been shortened to 2–3 mm length. The Gelpi‐retractor is placed into small holes that are drilled into the ventral vertebral bodies adjacent to the affected disk space. As with Caspar distractors, use of such retractor – while beneficial for disk space distraction – will occupy and potentially compromise valuable bone needed for fixation. Therefore, the benefits of a distractor need to be weighed against the potential disadvantages.

Diskectomy

Diskectomy is performed with the goal of ultimate arthrodesis between the affected vertebral bodies. Both endplates must be cleared of disk material to allow bone fusion as any remaining soft tissue will impede bony bridging. Sharp dissection of the ventral annulus fibrosus with an 11 blade will speed up disk removal. Lempert rongeurs are used to carefully remove the nucleus pulposus and as much of the lateral and dorsal annulus as possible. Distraction is required to allow rongeurs to be inserted sufficiently deep to remove enough disk. The goal is to remove approximately 90% of the disk, leaving a thin rim of annulus along the lateral and dorsal aspect (Figure 11.4). The remaining rim will prevent graft material and the intervertebral spacer from protruding into the vertebral canal or dislodging laterally. Aggressive dorsal and lateral annulus resection should be avoided to prevent inadvertent damage to the spinal cord as well as adjacent vasculature and nerve roots.

If a disk extrusion has occurred, the dorsal annulus must be partially removed to allow extraction of extruded material from the vertebral canal. Once as much disk material as possible has been removed by rongeurs, bone curettes are used to gently clear both vertebral endplates of remaining cartilaginous material. Careful use of a pneumatic drill can assist in clearing of the endplates; however, excessive remove of bone can weaken the endplates and lead to subsidence of the disk spacer (if used).

Intervertebral Spacer

Distraction of an intervertebral articulation ameliorates spinal cord compression by proliferated soft tissues and protruding dorsal annulus fibrosus. As there is no bone on bone contact between the affected vertebrae, stabilizing implants must carry most of the load during normal movement of the cervical vertebral column. The goal of an intervertebral spacer is to maintain the desired disk space depth and provide load sharing between the affected vertebral bodies, thereby increased implant longevity. Addition of an intervertebral spacer significantly increases construct stiffness [12].

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