Posterior Thoracic Microdiscectomy

Robert E. Isaacs, Mick J. Perez-Cruet, and Steven H. Cook

Abstract

Posterior thoracic microdiscectomy via the transfacet pedicle-sparing and transpedicular approaches provides safe and minimally invasive access to disc herniations within the thoracic spine. These approaches have been developed to treat both radiculopathy and myelopathy in a manner that allows complication avoidance and relief of symptoms. The minimally invasive nature of the procedure allows access to a noncalcified herniated disc without traversing the thoracic cavity and reduction in soft tissue disruption. Intraoperative anatomic landmarks are critical to successful discectomy, and these are discussed in terms of both visual and radiographic appearances. Cerebrospinal fluid leak, spinal cord injury, and pneumothorax are known complications of this procedure; however, maneuvers for avoidance as well as treatment are provided. Outcomes from posterior thoracic microdiscectomy have improved dramatically in recent years with improvements in imaging, instrumentation, and techniques. Mastery of the skills and tools described within this chapter will allow surgeons to manage thoracic disc herniations via a minimally invasive approach.

Keywords: thoracic herniated disc, microdiscectomy, transfacet pedicle sparing, transpedicular

23.1 Introduction

Thoracic disc herniation is a relatively rare condition with an inconclusive incidence ranging from 1/1,000 to 1/1,000,000 patients per year and occurs in a central or centrolateral location without gender predilection.¹^,²^,³ Theoretically, because of decreased flexion and increased stability of the costovertebral joints, the incidence of thoracic disc disease is low in comparison with disc pathology of the cervical and lumbar regions.⁴^,⁵ Furthermore, between 10 and 37% of such herniations are asymptomatic.²^,⁶^,⁷^,⁸^,⁹^,¹⁰ When symptomatic, the patient may present with a combination of pain and neurological symptoms. The most common reported symptom is pain, found in 76% of patients, followed by sensory impairment and paresis.³

In the thoracic region, the diameter of the spinal canal is less than in the cervical and lumbar region. In addition, vascular supply to the cord is limited in the thoracic region, making manipulation of the cord tenuous¹¹; thus, proper visualization of the disc herniation must be established to avoid excessive or even minor cord manipulation. Although traditional approaches have been successful to some degree in reaching thoracic disc pathology, each procedure requires a relatively large skin incision and/or extensive bone resection that may subsequently cause significant postoperative morbidity or lead to the need for instrumentation and fusion.¹²^,¹³^,¹⁴^,¹⁵

The numerous approaches developed to address thoracic disc herniation include: posterior (laminectomy),¹⁶ posterolateral (costotransversectomy),¹⁷^,¹⁸ transfacet pedicle sparing,¹⁹ transpedicular,²⁰ transversoarthropediculectomy,²¹ lateral (extracavitary),²² thoracotomy,¹⁷^,²³ transthoracic (transpleural and extrapleural),²⁴^,²⁵^,²⁶ transsternal,²⁷ and thoracoscopy.²⁸^,²⁹^,³⁰

The first posterior approach described for thoracic discectomy was by laminectomy via a posterior approach.¹⁶^,³¹ Outcomes from this approach were extremely poor, with the majority of patients failing to improve, with morbidity rates from 18 to 75% (including paraplegia) and in some cases mortality rates nearing 50%.¹⁶^,¹⁷^,³²^,³³^,³⁴^,³⁵^,³⁶ Therefore, this approach was abandoned and should be avoided unless significant spinal stenosis exists from hypertrophy of posterior thoracic column elements. Posterolateral approaches were subsequently developed to allow visualization of discs without a midline approach; however, this was balanced with the resulting tissue disruption, devascularization, and possible kyphosis.³⁷ With calcified and more centrally located discs, a lateral or anterior approach was desired as this allowed for direct visualization of surrounding anatomy without manipulation of the spinal cord, complete removal of calcified discs, and repair of the thecal sac in cases of dural erosion. However, these anterior approaches place the great vessels and thoracic cavity contents at risk along with the pain and healing concerns associated with the tissue disruption.¹⁴

Clinical studies have shown minimally invasive spine surgery to be associated with less disruption of normal tissue, less blood loss, and shortened procedural time when compared with traditional open approaches. This translates into patients experiencing reduced postoperative pain, hospitalization, and recovery time.¹²^,³⁸^,³⁹^,⁴⁰ In order to diminish the various potential complications and morbidities associated with more traditional surgical procedures used to treat a herniated thoracic disc, minimally invasive techniques such as the transfacet, pedicle-sparing and transpedicular approaches are becoming more frequently used. These two approaches will be discussed.

23.2 Indications and Contraindications

Because of the infrequency of clinically significant thoracic disc herniation as well as the potential risks to the thoracic spinal cord, operative intervention should be limited to those patients with debilitating symptoms. A clear indication to perform a posterior microdiscectomy is in patients with soft thoracic disc herniations causing myelopathy in order to improve neurologic function and prevent further injury to the spinal cord.⁴¹^,⁴²

Isolated thoracic radiculopathy from disc herniation is typically a sharp, lancinating pain that radiates unilaterally in a dermatomal distribution around the chest cavity and usually responds to conservative management such as medications, intercostal nerve injection, extension bracing, or activity modification.¹⁷ Although a rarer presentation, lateralized foraminal discs need to be considered when reviewing imaging. Severe and refractory radiculopathy not responsive to treatment for at least 6 months is another indication for surgery. It remains controversial whether patients with localized back pain and axial pain without myelopathy require surgery; most surgeons do not recommend thoracic discectomy for isolated back pain unless it is associated with neurologic deficit.³^,¹⁴^,³⁰^,⁴³^,⁴⁴

A more absolute contraindication to this technique is large calcified midline thoracic discs that may require an anterior approach to reduce manipulation of the spinal cord to achieve adequate decompression. For more compressible centrally located discs, the experience of the surgeon would be the determining factor for using this approach safely and effectively.

23.3 Preoperative Planning

The preoperative evaluation of the patient should include a thorough physical exam. This includes a neurological evaluation for signs of myelopathy or radiculopathy that will help guide surgical decision making. With thoracic disc herniations, myelopathy and radiculopathy can occur exclusively or concomitantly depending on extent of the disc herniation. A skin examination should note any prior incisions to assist in planning surgical approach.

Radiographic evaluation should be performed with thoracic spine X-rays to visualize alignment and curvature of the spine. Preoperative chest X-ray view allows for identification of ribs for intraoperative localization. Additionally a preoperative AP and lateral X-ray of the thoracic and lumbar spine allows for identification of uncommon anatomical variations (i.e., four or six lumbar vertebrae). Computed tomography (CT) is recommended in order to determine the degree of calcification of the herniated disc as this will affect management of centrally located discs. Magnetic resonance imaging (MRI) should be performed to accurately localize the level of the disc herniation and whether it is located in a lateral or central position. This imaging is also helpful in noting the degree of compression on neural elements and ruling out any intradural or posterior lesions that may be contributing to symptoms. Given the reliance on intraoperative imaging for a successful surgical outcome, localizing levels (T1 or sacrum) should be clearly identifiable on imaging by the surgeon. To avoid wrong level surgery, there has been increasing interest in preoperative-level marking with radiopaque materials.⁴⁵

23.4 Instrumentation

The instruments used are similar to those used to perform a microscopic lumbar discectomy with the system of tubular dilators.⁴⁰

Required Instrumentation for Thoracic Microdiscectomy

Imaging requirements:

• Fluoroscopy.

• Lead drape including thyroid shield for surgeon and operative personnel.

• Radiolucent table and frame that permits adequate anteroposterior and lateral fluoroscopic views of the spine.

Surgical instruments:

• Tubular dilator set.

• Long tapered high-speed drill.

• Microsurgical instruments (curettes, probes, etc.).

• Microscope (endoscope use has been performed).

23.5 Surgical Approach and Technique

The approach for the transfacet, pedicle-sparing and transpedicular approaches relies on intra-operative imaging to delineate the docking point for tubular retraction. The differences lie in starting point for dilator placement and amount of bony removal. Both use a small paramedian incision with a medial direction of approach.

Patient positioning and incision is similar for both the transfacet, pedicle-sparing and transpedicular approaches. After general endotracheal induction and placement of a Foley catheter, the patient is positioned prone on a radiolucent Wilson frame or Jackson table. All pressure points are padded adequately and the arms are placed above the patient unless an upper thoracic disc is being approached, making sure not to overextend at the shoulder beyond 90 degrees ( Fig. 23.1a). Somatosensory evoked potentials (SSEPs) are measured throughout the procedure, if desired. The fluoroscopic C-arm is positioned to allow for a lateral X-ray image of the operative area. ( Fig. 23.1b). The microscope is draped sterilely. The back is shaved, prepared, and draped in a routine surgical fashion.

Under fluoroscopic guidance, a spinal needle is used to locate the level of the herniated thoracic disc. The exact location of the thoracic disc herniation is confirmed by counting from the sacrum below and/or from the C7–T1 junction above. Alternatively, the ribs can be counted from T1 or T12. Preoperative chest X-ray is helpful in identifying any variations in the ribs (i.e., small 12th rib). Once the correct level is identified, a craniocaudal incision of approximately 2 cm is made 3 cm lateral to the midline. Preoperative axial MRI can be helpful in identifying how lateral the incision should be placed. The center of this incision should be in same axial plane of the inferior border (superior endplate of inferior vertebral body) of the disc being approached. Larger and more muscular individuals require a more lateral approach for adequate visualization of the foramen and safe removal of the thoracic disc herniation without cord manipulation. The more central the herniation, the more lateral the incision. An initial dilator is placed at the center of the pedicle caudal to the disc herniation for the transpedicular route and at the medial aspect of the transverse process (at junction with rib head) caudal to the thoracic disc herniation for the transfacet route. Next, a series of tubular muscle dilators are placed under fluoroscopic guidance ( Fig. 23.2a, b). Finally, a tubular retractor is then placed over the final muscle dilator and then affixed to a flexible arm secured firmly to the operative table ( Fig. 23.2,c). At this point, confirmation of level should be obtained via imaging and reference to anatomic localizer (T1 or sacrum).

23.5.1 Transfacet Pedicle-Sparing Approach

Adequate intraoperative imaging not only ensures that the operative level is identified, but can also be used if the surgical anatomy is distorted to help guide bone removal and delineate the disc space ( Fig. 23.2d). This can be facilitated by lateral and anteroposterior (AP) fluoroscopic images during the procedure. The lateral projectory of the tubular retractor through a transforaminal-type approach allows for extensive disc removal under the thoracic spinal cord and dura, enabling even midlinelocated thoracic disc herniations to be removed safely.⁴⁶