The earliest descriptions of thoracic disc disease were published by Key in 1838 and by Middleton and Teacher in 1911. Early treated cases Were reviewed by Hawk in 1936, and this review was followed by several reports dealing with a sizable number of cases treated in the 1950s and 1960s. These cases were surgically treated by laminectomy with extradural and sometimes intradural removal of the protruding disc and occasionally only with decompression. Some good results were obtained, but in general the outlook for these patients was poor. When the results were critically reviewed, it was found that a large number of the patients were made worse or paraplegic by laminectomy, and this was particularly true with midline hard disc protrusions causing spinal cord compression with severe neurological deficits. The poor results with laminectomy have served as a stimulus for the development of alternative approaches to these difficult lesions. Several reviews of surgically treated series using more modem operative approaches have been published in the last few years. All demonstrate dramatic improvement in the surgical outcome.

Clinical Features

The incidence of clinically significant thoracic disc protrusion bas been estimated at 1 patient per year per 1,000,000 population. The true incidence is certainly higher, because methods of diagnosis (although they have improved greatly) are not foolproof and because sporadic cases are often not reported. Most thoracic disc protrusions occur in the lower thoracic spine; midthoracic protrusions are less common, and disc protrusions occur very infrequently in the upper thoracic spine. It is generally conceded that there is no overall typical clinical syndrome caused by protrusion of a thoracic intervertebral disc. If these disc protrusions are broken down into groups consisting of those that are mainly lateral or centrolateral and those that are central in the spinal canal certain clinical features predominate. Radicular-type pain is often seen with protrusions that are lateral or centrolateral, and signs of cord compression may or may not be present. Central protrusions are associated with a high incidence of spinal cord compression and long tract signs. Back pain is a common initial symptom. In a significant number of patients, the initial symptoms have been precipitated, or previous symptoms have been intensified, by a fall onto the buttocks or feet. Many patients complain of disagreeable paresthesias below the level of the lesion. These are usually bilateral, but in some cases they are confined to one side of the body.

Some weakness in one or both legs is commonly present. Occasionally the history is of long duration, and there may be spontaneous remissions and exacerbations of symptoms and signs. In severe cases, the lesion may progress rapidly to incomplete or total flaccid paraplegia.

Radiologic Features

Since these lesions may mimic many spinal diseases, including tumors and demyelinating processes, precise radiologic studies are of the utmost importance. Calcification within the thoracic disc space seen on plain x-ray films is a highly significant finding and has been reported in 30 to 70 percent of patients with symptomatic thoracic disc protrusion. Calcified material in the spinal canal is another important radiologic feature; it was present in 55 percent of the cases reported by McAllister and Sage.

At this time the most common initial diagnostic test for patients with suspected thoracic disc lesions is magnetic resonance imaging (MRI). MRI is an excellent screening test. MR imaging of the thoracic spine has shown the incidence of thoracic disc herniations to be much higher than previously suspected and has revealed increasing numbers of cases with multiple herniations. MRI studies have also demonstrated thoracic disc protrusions in asymptomatic patients and as incidental findings in other patients with significant spinal pathology.

Myelography with a water-soluble contrast agent, usually followed by computed tomography (CT). has been for several years a standard diagnostic imaging procedure for thoracic disc disease. This study gives more essential information for surgical planning (exact level, relationship of the protrusion to the spinal cord) than does MRI.

Operative Approaches and Techniques

Thoracic Laminectomy

Although some lateral thoracic disc protrusions may be adequately treated with meticulous technique through a wide laminectomy approach. this method must be considered risky. because 45 percent of all the thoracic disc patients treated by this approach before 1978 either deteriorated or had no benefit from the procedure. This method is particularly hazardous in patients with central thoracic disc protrusions located above the T10-11 level. When laminectomy is attempted for a lateral thoracic disc protrusion. it should be carried far laterally into the facet joint.

Patterson and Arbit have described an approach through the pedicle to a protruded thoracic disc. Through a midline incision, the facet joint and part of the pedicle of the vertebra caudal to the protruded disc are removed with an air drill. The intervertebral disc is entered. and disc material is removed from the center of the disc. After a cavity is created in the center. posterior disc material is evacuated. After the cord is decompressed anteriorly. a laminectomy is then carried out starting laterally and moving medially across the midline to the opposite side. If an intradural or intramedullary fragment is suspected, the dura can be opened and the anterior surface of the spinal cord and dura inspected.

Carson and colleagues have described a far lateral laminectomy approach. Through a midline incision, the spines and lamina of the vertebrae on either side of the affected disc are exposed.. Removal of the laminae is carried out as far as the lateral limits of the dural sleeve. A transverse skin incision is then made at the level of the disc, and the erector spinae muscle mass is divided laterally. The incision is deepened to allow an approach to the disc which lies only slightly posterior to the horizontal plane. The disc is then approached posterolaterally after the medial parts of the articular facets are removed.

Lateral Approaches

Because of dissatisfaction with the results of laminectomy. and at the suggestion of G. L. Alexander. Hulme used a "lateral costotransversectomy approach to thoracic disc protrusions." This procedure uses a paramedian incision along the outer border of the paravertebral muscle mass. If the protrusion is situated more on one side of the spinal canal than the other, it is approached from the side closer to the disc. The outer border of the paravertebral muscles is exposed, and the muscles are reflected medially to expose the posterior aspects of the ribs, medial to the angles, and the tips of the transverse processes. The correct level is identified. and portions (about 5 cm) of the ribs on each side of the affected disc are resected. The pleura is mobilized and reflected forward and laterally, and the remaining heads and necks of the ribs are removed. The intervertebral foramen is identified by tracing the intercostal nerve medially, and it is enlarged by removing portions of the pedicles until the dura is exposed . With a high-speed drill, bone is removed from the posterior aspects of the vertebral bodies beneath the disc protrusion. The disc itself is then delivered into the opening created by the bony removal.

This approach has the advantage of allowing a more lateral view of the dura and the posteriorly protruding disc than is afforded by a laminectomy or the lateral extensions of a laminectomy. As Hulme pointed out, one disadvantage is that the preoperative diagnosis must be accurate. because this exposure would be inappropriate for other kinds of intraspinal lesions such as neoplasms. It is also conceivable that one could interrupt a significant radicular artery supplying blood to the spinal cord, and great care must be taken to avoid this.

To gain a wider lateral exposure to the thoracic spine. Maiman and colleagues used a lateral extracavitary approach with good results. This approach had been developed by Capener for the treatment of tuberculous spondylitis. Dietze and Fessler have further modified it into a minilateral extracavitary approach.

Transthoracic Approach

The transthoracic approach was first used for the total removal of a midline thoracic disc protrusion causing spinal cord compression by Perot and Munro in 1964. Ransohoff and colleagues also reported favourable results with this technique. As with costotransversectomy, the use of this approach presupposes accurate imaging studies that demonstrate an anteriorly situated intervertebral disc protrusion.

For midthoracic disc protrusions, the preferred approach is through the right side. The heart and great vessels do not impede exposure of the lateral aspect of the spine at this level, and as significant thoracic spinal radicular arteries are unilateral and more often enter the spinal canal from the left side. they are less apt to be traumatized by the procedure. A standard thoracotomy is carried out with the patient positioned on the left side. The appropriate ribs can be located usually by counting down from the second rib. which is identified by palpating the insertion of the serratus anterior muscle. The posterior muscles are divided. and the ribs above and below the disc space are divided just lateral to their insertion on the transverse processes. The pleura is opened. the wound is held open with a rib spreader. and the lung is collapsed and packed off. In earliest cases, the heads and necks of the ribs adjacent to the appropriate interspace were removed. This is no longer done because of the possibility of injury to an important spinal radicular artery. Instead, it is now common practice to drill away the head of the rib overlying the disc and the intervertebral foramen.

It is important to recognize that the intercostal nerve enters the intervertebral foramen at its most cephalad and dorsal corner, and that the intervertebral disc lies at the most caudal and ventral corner of the intervertebral foramen. The caudal three-fourths of the intervertebral foramen and the dorsolateral surface of the intervertebral disc are completely covered by the head of the rib where it articulates with the adjacent vertebral bodies. The head of the rib and the radiate ligament are drilled away, with careful attention to the most cephalad aspect since it lies next to the point where the neurovascular bundle enters the cephalad end of the intervertebral foramen. Each intercostal artery gives off a dorsal branch that follows the intercostal nerve toward the intervertebral foramen by passing just anterior to the free medial edge of the superior costotransverse ligament. This dorsal branch gives off a spinal branch that enters the vertebral canal through the intervertebral foramen. A given spinal branch may or may not be of importance in supplying the spinal cord. Preoperative intercostal angiography could be carried out to identify important spinal radicular arteries, but it is probably not necessary. Even if the spinal radicular artery entering the foramen is important. it should be possible to spare it by careful removal of the head of the rib. In addition to removing the rib head. bone is removed from the adjacent vertebral bodies immediately ventral to the intervertebral foramen.

If the disc protrusion is lateral or centrolateral. it may be encountered before the dura is visualized. If the protrusion is exactly in the midline. then a cap of dura overlying it may be visualized first. Drilling is continued straight across the spine in front of the dural sac to create a trough below the disc protrusion. Bone is removed approximately two-thirds of the distance across the spine. Since the pedicle of the vertebral body caudal to the disc protrusion lies much closer to the disc space than the one cephalad to it, the cephalad edge of the pedicle forming the caudal edge of the intervertebral foramen will have to be removed. Before the ventrolateral dural surface is encountered, the lateral extension of the posterior longitudinal ligament must be excised. After the bony opening has been accomplished anterior to the dural tube. the protrusion may be carefully curetted ventrally and away from the spinal cord into the cavity created by the bony removal. The dissection is greatly facilitated by the use of loupes or the operating microscope. Rarely, the disc protrusion may be densely stuck to the dura, or it may have actually eroded through it and be adherent to the spinal cord. Because the angle of approach is anterolateral and not posterolateral, this situation can be visualized and dealt with. After the protrusion has been removed. posterior osteophytic projections of the adjacent vertebral bodies into the spinal canal may have to be drilled away. Bony fusion is not necessary. Repair of the pleura over the lateral aspect of the spine is also not required. The chest incision is closed in the usual fashion after a large-diameter drainage tube is brought out through a separate stab wound below the incision.

The main advantage of the transthoracic operation for a protruded thoracic intervertebral disc is that it allows the operator to visualize the anterior surface of the dura in a truly anterolateral direction rather than a posterolateral one. This greatly facilitates the removal of a midline hard bony disc where the dural sac and spinal cord are arched over the protrusion in a caplike fashion. The procedure is no more formidable than costotransversectomy, and the fact that a thoracotomy is required has not led to any increased postoperative morbidity.

Surgical Results and Complications

To date, the best results from the removal of thoracic intervertebral disc protrusions have been reported in patients with lateral or centrolateral protrusions and only radicular pain or very mild signs of myelopathy. These protrusions are most often located at lower thoracic levels. With the possible exception of very lateral protrusions, the standard laminectomy approach can no longer be regarded as the procedure of choice for these lesions. Even for laterally situated disc protrusions. the laminectomy should be carried well out to the side of the lesion. Significantly better results have been reported with the use of lateral or anterolateral approaches afforded by the transpedicular, lateral extracavitary. or transthoracic techniques. Regardless of the approach, those patients with severe preoperative deficits have, in general, had the poorest results. Marked preoperative deficits and a long duration of symptoms should not preclude operation, however, since it is well documented that adequate lateral or anterolateral decompression leads to striking recovery in some of these patients.

The complications and results of thoracic discectomy by transthoracic or lateral extracavitary operations for pain and/or myelopathy in three series have been tabulated by Dietze and Fessler. The three series contained a total of 91 cases. Pain resolved or improved in 67 to 94 percent of patients, and myelopathy improved in 71 to 97 percent. Bowel and/or bladder dysfunction improved in 60 to 100 percent. Complications in the 91 patients included superficial wound infection (3 percent), urinary tract infection (1 percent). atelectasis (4 percent). transient paraparesis (1 percent), postoperative seizure (1 percent), anaesthesia dolorosa (2 percent), compression fracture (1 percent), pleural tears (3 percent), and pneumonia (1 percent). There were no deaths.

Great progress has been made in the surgical management of these difficult lesions in the last 50 years. This improvement has been made possible by the introduction and perfection of anterolateral transthoracic and lateral extracavitary approaches in place of laminectomy.