Circumferential Cervical Spinal Fusion

Pathophysiology

The chronic changes that are associated with cervical spondylitic myelopathy (CSM) are unique in the cellular and tissue derangement that occurs. Recent work by Karakimas et al. has been able to provide new mouse and rat models that provide a similar clinical picture that mimics the disease process in humans. Specifically, this has provided a window into the microvasculature changes that occur with both static and dynamic forces on the spinal cord.

A recent review by Nouri et al. outlines the key biological changes as: decreased spinal cord perfusion, endothelial cell dysfunction, loss of spinal cord blood barrier, glutamate toxicity, microglia and macrophage activation, apoptosis of neurons and oligodendrocytes, axonal degeneration, and gliosis. There is no doubt that this list is not inclusive.

The Rationale for Circumferential Surgery

It is perhaps useful to first review the goals of surgery on the cervical spine to best put each item in its proper context and how it may influence the choice to consider circumferential surgery for a given patient. These include decompression of the neural elements, restoration of spinal alignment, spinal stability, proper environment for fusion, and the health of the patient.

Certainly, one of the most obvious reasons for choosing bidirectional surgery of the cervical spine would be that the patient presents with obvious compression of both the ventral and dorsal surfaces of the spinal cord ( Fig. 140.1 ). If the patient is not overly frail, then one can make a rationale for this approach. There is some degree of inherent common sense to directly remove the compressive structure of the spinal cord. One variation might be surgery to treat ossification of the posterior longitudinal ligament. Patients with dramatic anterior compression are commonly treated by posterior decompression in order to avoid the significant pitfalls of anterior surgery.

Restoration of spinal alignment has recently received an increasing degree of attention as a potential crucial factor in the outcome of patients with CSM. Most would agree that circumferential surgery is indicated for patients that present in frank kyphosis ( Fig. 140.2 ). This may result from numerous factors including posterior migration of the cord, vascular perfusion, and longitudinal stresses on the cord. If a patient is left in a kyphotic position, then draping and potential anterior compression of the spinal cord are certainly possible. The classic example of this would be postlaminectomy kyphosis. Nottmeier et al. published results of cervical kyphotic deformity correction using an anterior–posterior reconstruction. Their patient population had a mean preoperative kyphosis of 18 degrees. They were able to achieve a mean correction of a sagittal angle of 22 degrees. This resulted in a mean postoperative sagittal angle of 4 degrees. There was no reported loss of deformity correction during the follow-up period and they reported a fusion rate of 97.5%.

Several studies have looked at the potential importance of postoperative posterior spinal cord migration. There is a concern that residual kyphosis may allow the spinal cord to be draped across the vertebral column, which may contribute to compromise of the spinal cord via vascular compromise or longitudinal stress. Lee et al. attempted to quantitatively assess preoperative factors that would predict this postoperative movement of the cord. Besides the commonly stated importance of preoperative sagittal balance, they also found significance in the relative degree of stenosis directly above the “laminectomized” level to be particularly important. However, the clinical importance of cord shift remains uncertain. Hatta et al. found no clinical correlation in myelopathy outcomes to cord migration.

There has also been some attention given to the potential effects of “longitudinal stress” to the spinal cord by Uchida et al. It is interesting to consider the potential similarities of this to the physiologic derangement that has been documented to occur with tethering of the spinal cord. ^,

History

Bidirectional surgery for the cervical spine was originally reserved for cases of clear violation of stability of both the anterior and posterior elements of the cervical spine. Most cases dealt with either trauma or neoplastic disease. Instrumentation of the spine was limited to wiring techniques ( Figs. 140.3 , 140.4 , and 140.5 ). Cases frequently required staging, and patients commonly required prolonged immobilization.

The evolution of surgical hardware has taken us from stainless steel wiring techniques to lateral mass plates and to polyaxially screw and rod fixation ( Figs. 140.6 and 140.7 ). This has made single-stage anterior-posterior reconstructive procedures more feasible and reasonable in a larger number of patients.

The surgical training for the treatment of complex spinal disorders has also rapidly expanded. As a result, anterior decompression involving segmental reconstructions and corpectomies has become increasingly routine for spinal surgeons.

Technique

Many variations can occur regarding the strategy on planning these cases. Typically, the authors prefer to initiate the approach from the anterior direction first. However, this can be influenced by such things as the need to remove posterior instrumentation. This description will be focused on the typical case of cervical degenerative disease requiring a circumferential decompression.