Surgical Techniques in the Management of Thoracic Disc Herniations

Important Concepts

• Thoracic disc herniations, although uncommon, are encountered by spine surgeons.

• Laminectomy is an unacceptable treatment for thoracic disc herniations.

• Posterior techniques include the transpedicular, Stillerman transfacet pedicle sparing, transcostovertebral, costotransversectomy, and lateral extracavitary.

• Posterior approaches are generally favored in cases of more lateral, noncalcified, extradural disc herniations.

• Anterior approaches include the transthoracic, retropleural, and transsternal.

• Anterior techniques offer better ventral exposure for discs that are centrally located, calcified, and/or intradural.

• Complication rates for the common posterior and anterior procedures are similar.

Thoracic disc herniations account for a minority of the disc herniations evaluated by a spine surgeon. This reflects the relative immobility of the thoracic spine as compared with the cervical and lumbar regions and thus the low incidence of degenerative changes. Contemporary series suggest that thoracic herniated discs represent less than 1% of all symptomatic discs and, accordingly, represent less than 1% of all disc operations. This establishes an incidence of symptomatic patient per 1 million individuals per year.

The majority of thoracic herniated discs are asymptomatic. As many as 37% of patients harbor an asymptomatic herniated thoracic disc as defined on randomly sampled magnetic resonance imaging (MRI) scans. More conservative estimates propose that this number lies in the range of 10% to 15%. ^{, ,}

The majority of symptomatic discs are found in the lower third of the thoracic region, with most being found between the T8 and T11 levels. ^, Herniated discs in the upper third of the thoracic spine are rare. Herniated discs in the thoracic spine commonly occur centrally within the canal (77% to 94%) and are often calcified (22% to 65%). ^, A small but very important 6% to 7% of discs prove to be intradural.

Symptomatic thoracic herniated discs manifest with a wide array of findings. Stillerman and colleagues presented an exhaustive review of their personal findings in 71 patients with thoracic disc disease as well as a meta-analysis of 13 series encompassing 247 patients ( Table 141.1 ). Both of these surveys demonstrated that sensory changes were seen in more than 60% of cases. Pain was a finding in greater than half of the patients but was actually more likely to be axial than the typically presumed radicular pattern. Motor weakness and spasticity or hyperreflexia were present in 55% to 58% of patients. As with many cervicothoracic lesions, bowel and bladder dysfunction was one of the least-common symptoms, appearing in only 24% to 35% of patients.

The radiographic identification of these lesions is obviously most commonly performed with MRI. MRI allows visualization of the disc herniation and the surrounding neural elements ( Fig. 141.1 ). However, computed tomography/myelography remains a viable imaging technique, with excellent resolution of the affected region, despite the inconvenience and invasiveness of the procedure. Plain computed tomography scanning also serves a supportive role because it often assists MRI in evaluating whether a given disc is calcified and offers a better analysis of the bony anatomy ( Fig. 141.2 ).

Selecting patients for surgery is much like selection for degenerative processes afflicting the remainder of the spine. That is, the indications for surgery are by no means objectified and established but rather are physician defined. As with most cervical lesions, myelopathy with or without bowel or bladder involvement is a nearly absolute operative indication. Surgical treatment of thoracic herniated discs for radiculopathy, back pain, or sensory changes is much more difficult to uniformly define. The natural history of thoracic disc herniations is not completely understood. Brown and colleagues followed 40 patients with symptomatic thoracic disc herniations and determined that 77% returned to work symptom free without surgical intervention. Therefore, an argument can be made to manage patients who do not have significant neurologic insult (weakness, spasticity) in a fashion similar to those with degenerative cervical and lumbar conditions. These nonoperative measures can include a combination of rest, physical therapy, oral antiinflammatory medication (steroidal and/or nonsteroidal), and/or steroid injections.

If a patient has myelopathy or nonmyelopathic symptoms not amenable to conservative measures, surgical intervention may be necessary. At this point, a surgeon must decide on an operative approach. Again, a wide variety of techniques may be used for thoracic disc herniations, each method possessing its own advantages and compromises.

One of the reasons for the development of this variety of procedures is the surgical constraint of the thoracic spine anatomy. In this region, the spinal cord lies within the relatively narrow thoracic spinal canal and the thecal sac cannot be manipulated as freely as within the lumbar region. Therefore, the operative approach needs to minimize manipulation of the dura and spinal cord. Although this feature is similar to the cervical region, the anterior approaches to the thoracic region (excluding the very superior thoracic spine) are more involved than standard anterior cervical procedures. Thus the ideal procedure would afford the surgeon a ventral view of the region while maintaining the more straightforward technical aspect of a posterior approach. As a result of these requirements, more-aggressive posterior procedures have been developed in attempts to gain greater “anterior” perspectives and circumvent the need for more-involved anterior thoracic exposures.

Considerations for an operative approach for herniated thoracic discs include the following:

• Level of the herniation

• Mediolateral localization of the herniation (location with respect to canal and cord and the foramen and root and the resultant symptoms)

• Complicating factors of the disc itself (intradural, calcification)

• Presence of multiple disc herniations

• Health of the patient

• Experience of the surgeon

Although not inclusive, the following guidelines can be made in terms of operative approaches.

Posterior approaches can be employed at any level. Their utility depends largely on the laterality of the disc; that is, the more lateral the disc, the more accessible and successful a posterior approach might prove. The one caveat is that a standard posterior laminectomy is not an adequate approach for any thoracic disc herniation. Laminectomies for thoracic disc herniations have been repeatedly shown to carry unacceptable rates of morbidity. Excepting a truly foraminal herniated disc or a case of isolated stenosis from thoracic spondylosis (and not a disc herniation), the laminectomy should not be used. Rather, standard posterior approaches include the transpedicular, Stillerman transfacet pedicle sparing, transcostovertebral, costotransversectomy, and lateral extracavitary. In that order, these procedures offer gradually increasing exposures toward the midline of the canal despite the incursion of increasingly larger and more destructive incisions and dissections. Unfortunately, no posterior approach allows a true ventral view of the spinal canal and dura. Therefore, features favoring a posterior approach include more laterally located discs, soft discs without evidence of calcification, and extradural lesions. Often these approaches are better tolerated by the patient than anterior approaches, making them more attractive in patients with comorbidities, especially in regard to pulmonary disease. Last, the anatomy encountered by the surgeon in these procedures is very familiar, and mastery of even the more aggressive posterior procedures is usually quickly attained by most surgeons.

Anterior approaches (transsternal and transmanubrial, transthoracic, anterolateral, and retropleural) can be used selectively at varying levels. The transmanubrial and transsternal approach is best suited for superior thoracic lesions above T4 (below this level, the aortic arch and associated veins become an obstacle). Below T4, anterior approaches are either transthoracic or retropleural. All the anterior approaches afford the surgeon a more direct view of the ventral dura and disc herniation. This allows greater ease in managing calcified or intradural herniations. Central discs are also more directly accessed through these approaches. The downside of these operations is usually a more substantial operative event for the patient, requiring a greater period of recovery. They can be contraindicated in patients with significant comorbidities. Multiple disc herniations need to be within one or two levels of one another to be accessed ventrally. The surgical approach and anatomy are somewhat foreign to most neurosurgeons and require focused training and repetitive procedures to maintain proficiency. This can be somewhat overcome by cardiothoracic surgeons aiding in the exposure of these regions, but the neurosurgeon must still manage the bony dissection, which is not as familiar to many surgeons as the anatomy encountered in typical posterior spinal procedures.

Intraoperative Localization

Although radiographic localization is often taken for granted in the cervical and lumbar regions, it poses a significant hurdle in the thoracic spine. Lower thoracic levels can often be imaged with standard lateral radiographs. However, in some larger patients and certainly in mid- to upper thoracic regions, lateral radiographs are very difficult to interpret and have no reference with which to localize appropriate levels.

First, a surgeon needs to identify the level of a herniated disc using the same method by which the preoperative level is identified. That is, if the preoperative investigations identify the affected thoracic level counting down from C1, then the surgeon should not necessarily use the sacrum as a reference point intraoperatively, because a lumbarized sacral segment could obviously cause an error in localization. Rather, the surgeon can employ an intraoperative anteroposterior (AP) radiograph and count down from the first thoracic ribs, or the sacrum can be used to count upward, if this has been confirmed as an accurate means of reference. A preoperative localization can be easily performed by taping a radiopaque marker to the patient and obtaining a standard AP radiograph ( Fig. 141.3 ).

Lateral radiographs are of limited utility in the thoracic spine, and AP radiographs must often be used to adequately visualize spinal elements and levels. The surgeon has the option to prepare the operating table for the insertion of AP radiograph cassettes or to use a radiolucent operating table with intraoperative fluoroscopy. Either option is effective, but exclusively lateral thoracic radiographs can prove to be very difficult to interpret.

Description of Individual Procedures

Posterior Procedures

Transpedicular

The transpedicular approach is perhaps the most commonly considered procedure for a thoracic disc herniation. Its origins date to the initial description of Patterson and Arbit in 1978, which represent a modification of Carson’s 1971 technique. The transpedicular approach allows the surgeon relatively straightforward access into the most lateral region of the spinal canal ventral to the spinal cord ( Fig. 141.4 ). The patient is placed in a prone position on gel rolls or a Wilson frame, and a vertical midline incision is centered over the level of interest. Sharp dissection is carried deeply to the thoracodorsal fascia. A unilateral exposure can be performed to minimize trauma to the contralateral paravertebral musculature. Once through the thoracodorsal fascia, a subperiosteal dissection is performed with Cobb elevators until the lateral facets are exposed on the affected side.

Viewing the facet joints, the surgeon needs to then develop an idea as to the location of the pedicle and the associated nerve root. The thoracic pedicle to be drilled is centered 1 to 2 mm beneath the edge of the inferior facet of the superior vertebra ( Fig. 141.5 ). For example, for a T7-8 disc herniation the surgeon will aim to drill the T8 pedicle, which is centered just below the inferior edge of the T7 inferior facet.

By using a combination of the pneumatic drill and Kerrison rongeurs (generally no larger than 2 mm), a lateral laminotomy is performed to visualize the lateral edge of the thecal sac. This visualization allows the surgeon to safely proceed with further bone resection. The removal of bone is then extended laterally to the medial facet joint and ventral pedicle. Once the dorsal aspect of the facet is drilled adequately to visualize the exiting nerve root, the axis of the pedicle can be discerned. At this point, the interior of the pedicle is drilled, allowing the superior and medial cortex to act as a protective barrier against the nerve root and thecal sac, respectively. Often this drilling is undertaken with the aid of the operating microscope. Once the drilling has been performed to the depth of the disc space or vertebral body, a curet may be used to fracture and remove the remaining cortical shell of the pedicle, being careful to fracture the remnant pedicle away from the dura.

The increased lateral exposure obtained by removing the superomedial pedicle allows the surgeon to view a portion of the lateral disc space and spinal canal tangentially under the nerve root and thecal sac. The lateral aspect of the herniated disc is then incised and disc material is removed from centrally within the disc space. This allows the surgeon to displace more-medial herniated tissue into the disc space (the force being directed away from the spinal cord) using a down-biting curet. Once this is completed, the disc fragments can be removed safely without disturbing the dura.

Closure resembles that of posterior lumbar and cervical procedures.

Transfacet Pedicle Sparing

This technique shares many similarities with the transpedicular approach but avoids the resection of the pedicle. Devised by Stillerman and colleagues, the technique employs a setup similar to that used in the transpedicular technique. The patient is positioned prone. Stillerman and colleagues recommend the use of AP fluoroscopy to verify the proper level during the exposure. A linear midline incision is used, and the standard dissection and subperiosteal exposure is performed so as to expose the ipsilateral facet joint.

At this point, the proper disc level and overlying facet joint are verified with fluoroscopy. As with the transpedicular procedure, a pneumatic drill is used to penetrate the medial aspect of the facet joint. Unlike the previous procedure, the inferiorly lying pedicle is not entered with the drill. Once the surgeon reaches neural foramen, blunt dissection may be performed to visualize the disc space lying at the inferior aspect of the foramen ( Fig. 141.6 ). The exiting nerve root is usually found in the extreme superior aspect of the foramen and therefore is often not encountered (except in the higher thoracic spine). Once the annulus is identified, the disc removal proceeds in a fashion similar to that of any of the other posterior procedures.

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