INTRODUCTION

Several surgical approaches have been developed to remove thoracic vertebral lesions from the anterior to posterolateral direction. The choice of surgical approach is decided depending on the disease entity, level of the lesion, laterality, multiplicity, presence of instability, and the necessity of reconstruction. The posterior approach provides less extensive exposure of the vertebral body than the anterior approach, but the involved anatomy is familiar to the spinal surgeons and it can be applied to patients with anesthetic risk.¹

Advantages of the posterior approach over the anterior approach are as follows ^2,³:

1 Earlier identification of the spinal cord.

2 The ability to treat disease in the posterior elements.

3 The ability to simultaneously stabilize the spinal column with posterior instrumentation devices.

4 Imbalances in the sagittal plane (listhesis), coronal plane (scoliosis), and axial plane (rotation) can be more easily corrected with the posterior approach.

5 Avoidance of thoracotomy.

With increasing bone removal, from the transpedicular approach to the costotransversectomy, and finally to the lateral extra-cavitary/lateral parascapular approach, the operative trajectory becomes more lateral to better visualize the affected vertebrae.⁴ In this chapter, overall review of the posterior or posterolateral approach to thoracic spine will be presented with the pertinent anatomical assessment.

MUSCULAR ANATOMY

The most superficial muscle of the dorsal spine is the trapezius muscle (Fig. 26-1). The trapezius originates along the external occipital protuberance and each spinous process from C1 to T12. The insertion of the trapezius is the lateral third of the clavicle, the acromion and the scapular spine. This muscle provides the stabilization and abduction of the shoulder. Immediately deep to the trapezius muscle on the upper thoracic level lie the rhomboid major, rhomboid minor, and levator scapulae muscles (see Fig. 26-1). The rhomboid muscles originate from the spinous processes of the cervical and thoracic spine and insert to the ventral edge of the scapula.⁵ The levator scapulae muscle connects the scapula to the upper cervical vertebrae.

Fig. 26-1 Superficial layer muscles: trapezius, rhomboids, levator scapulae muscles, and latissimus dorsi muscle.

The serratus posterior superior muscle is another muscle that fixes the cervicothoracic junction area spinous processes to the lateral part of rib cage (Fig. 26-2). In the exposure of the cervicothoracic junction, the spinous process insertions of these muscles are taken down as a single group for lateral retraction.⁶ As these muscles are taken down, the scapula is released from its attachments to the spinous processes and rotates anterolaterally out of the operation field.

Fig. 26-2 Deep layer muscles: erector spinae muscle and transversospinalis muscle.

On the lower portion of the back, the latissimus dorsi muscle spans over the body. It originates from the spinous processes of the six lower thoracic vertebrae, lumbar and sacral vertebrae, and ilium, inserting onto the humerus (see Fig. 26-1).

In the deeper part of the back, two large groups of muscles are located: the erector spinae muscles (sacrospinalis muscle) and the transversospinalis muscles.

The erector spinae muscles are a group of muscles running from the sacrum and iliac crest to the ribs or transverse process of the vertebrae. They have three separated groups: iliocostalis (lateral), longissimus (middle), spinalis (medial) (see Fig. 26-2). The iliocostalis muscle is inserted into the angles of the ribs and into the cervical transverse processes from C4 through C6. The longissimus thoracis muscles are inserted into the thoracic transverse processes and nearby parts of the ribs between T2 and T12. The spinalis muscle is largely aponeurotic and extends from the upper lumbar to the lower cervical spinous processes.

The transversospinalis group of muscle passes obliquely cephalad from the transverse processes to the spinous processes immediately deep to the erector spinae muscle. These muscles fall into three layers. The most superficial layer, the semispinalis muscle, arises from the tips of the transverse process and inserts to the tips of the spinous processes. The semispinalis capitis passes from the upper six thoracic transverse processes and lower four cervical articular processes to the occipital bone between the superior and inferior nuchal lines. The semispinalis cervicis muscle also starts from the upper thoracic and lower cervical transverse processes and attaches to spinous processes of C2 through C5.

The semispinalis thoracis muscle runs from the transverse processes of the lower six thoracic vertebrae onto the spinous processes of the upper thoracic and last two cervical vertebrae. The intermediate layer, the multifidus, arises from the sacrum, posterior sacroiliac ligament, accessory processes of the lumbar spine, and the articular processes of the thoracic spine and inserts to the spinous processes of the vertebrae up to C2 vertebra. The deepest muscles of this group, the rotators, are small muscles that bridge from the transverse processes to the lamina of the vertebra directly above.