Posterior Approach to the Thoracic and Lumbar Spines for Scoliosis

The posterior approach to the thoracic and lumbar spines is the approach used most frequently for the surgical treatment of scoliosis.29–33 The approach is safe, avoiding vital structures, and allows direct approach to the posterior aspect of the vertebral bodies in an internervous plane. This approach is used for the following:

1.   Scoliosis surgery (see the section regarding rib resection and theposterior approach to the iliac crest for bone graft)

2.   Posterior spine fusions (extensive and limited; see the section regardingthe posterior approach to the iliac crest for bone graft)

3.   Removal of tumors of the posterior aspect of the vertebrae

4.   Open biopsy

5.   Stabilization of fractured vertebrae (see the section regarding theposterior approach to the iliac crest for bone graft)

Position of the Patient

Place the patient prone on the operating table, with bolsters along each side so that the anterior chest wall clears the table and the chest can expand. The bolsters should be long enough to reach and support the anterior superior iliac spine so that the anterior abdominal wall clears the table as well; this allows emptying of the nonvalvular vertebral venous plexus into the vena cava, reducing operative bleeding (Fig. 6-101).

Landmarks and Incision

Landmarks

The gluteal cleft and the C7-T1 spinous processes mark the midline. The beginning of the gluteal cleft should be draped with a clear plastic drape so that it still can be seen. The spinous processes of C7 and T1 are the largest spinous processes in the lower cervical and upper thoracic spines. They offer a guide to the location and level of the incision if the spinous processes are counted down from C7.

Figure 6-101 The position of the patient on the operating table for the posterior approach to the thoracic and lumbar spines. Place the bolsters so that the anterior abdominal wall clears the table; this allows emptying of the vertebral venous plexus to the vena cava.

Incision

Make a straight midline incision above the thoracic and lumbar spines that require surgery. Use the spinous processes of C7 and T1 and the gluteal cleft as guides. A scalpel drawn along a straightedge between these two points leaves an exact midline incision (Fig. 6-102). (Frequently, the spinous processes are rotated away from the midline in association with scoliosis; nevertheless, for cosmetic reasons, the incision should be placed along the midline.)

Internervous Plane

The paraspinal muscles are innervated segmentally by the posterior primary rami of the individual nerve roots in the thoracic and lumbar spines. Because the incision is in the midline, it is truly internervous; the nerves do not cross the midline.

Superficial Surgical Dissection

Palpate the individual spinous processes. Determine whether they have deviated from the midline as they rotate in scoliosis. Continue dissecting down to the middle of the spinous processes and move the muscle origins to either side of the surface. In children, split the spinous process apophyses longitudinally and dissect them to each side of the processes with a Cobb elevator (Fig. 6-103).

Deep Surgical Dissection

Remove the paraspinal muscles from the spinous processes and partially from the laminae by subperiosteal dissection (Fig. 6-104). In the thoracic area, work in a distal to proximal direction, in the direction of the muscle fibers along the spinous process. After the paraspinal muscles have been stripped from the spinous processes and laminae, keep the dissection open with self-retaining retractors (Fig. 6-105).

Now, still using the Cobb instruments, remove the short rotators from the base of the spinous processes to the leading edges of the laminae. Then, strip the muscles from the rest of the laminae laterally, onto the transverse processes (Figs. 6-106 and 6-107).

Dangers

The posterior primary rami emerge posteriorly from between the transverse processes, close to the facet joints. Because of the significant overlap of innervation in the paraspinal muscles, loss of an individual posterior primary ramus is not harmful (see Figs. 6-106B and 6-111).

Segmental vessels coming directly off the aorta appear between the transverse processes and supply the paraspinal muscles. They bleed when muscles are stripped from the transverse processes and must be cauterized. The posterior primary rami are close to these vessels (see Figs. 6-106B and 6-111).

How to Enlarge the Approach

Local Measures

To widen the exposure, use self-retaining retractors and carry the dissection out onto the tips of the transverse processes. If the area being worked in is tight, extend the incision one vertebra higher or lower, whichever is appropriate.

Figure 6-102 Make a straight midline incision over the thoracic and lumbar spines that require surgery.

Figure 6-103 Dissect down onto the middle of the spinous processes. In children, split the spinous apophyses longitudinally and dissect them to either side with a Cobb elevator (inset).

Extensile Measures

This incision can be extended. It may be used to dissect the entire spine, from the cervical area to the coccyx. Because no nerves cross the midline of the body, the nerves that segmentally supply the paraspinal muscles remain safe.

Special Points

To determine a precise anatomic location, identify the 12th (last) rib and dissect one level distal to it to locate the transverse process of L1. Note that the last rib is mobile, a floating rib without sternal attachment, whereas the transverse process of L1 is quite rigid and firm, and does not yield to pressure. The rib also is longer and more tubular than the transverse process (see Fig. 6-100). After the last rib has been found, identify the nearby facet joints. The descending facet joint of T12 is a lumbar facet joint, set in the sagittal plane, whereas the ascending facet joint at the upper end of T12 is a thoracic facet joint, set in a frontal plane (see Fig. 6-110). Identifying the direction of facets, the last rib, and the first lumbar transverse process provides a precise anatomic location. The only alternative is to place markers in the spinous processes in the lumbar area and to obtain a radiograph, or to carry the dissection distally and identify the sacrum.

The musculature in the lumbar area may be stripped at each vertebral level, either in a proximal to distal direction or in a distal to proximal direction. A half-inch osteotome may be used in conjunction with the Cobb elevator to strip the facet joint capsules from the ascending and descending facets, and to continue the dissection laterally onto the transverse processes (see Fig. 6-106B).

Figure 6-104 Remove the paraspinal muscles from the spinous processes and partially from the laminae by subperiosteal dissection.

Figure 6-105 In the thoracic area, work from distal to proximal, in the direction of the muscle fibers along the spinous processes. With the use of Cobb elevators, remove the short rotators from the base of the spinous processes to the leading edges of the laminae. Then, strip the muscles from the rest of the laminae laterally onto the transverse processes.

Figure 6-106 A: In the lumbar area, strip the paraspinal muscles from proximal to distal. Remove the joint capsule from medial to lateral. After crossing the mamillary process on the tip of the ascending facet, dissect laterally and caudally onto the transverse process. Be prepared to cauterize the segmental vessels that appear between the transverse processes. B: Note that the transverse process is further anterior and distal than the mamillary process.

Figure 6-107 After you have stripped the paraspinal muscles from the spinous processes, laminae, and transverse processes, keep the dissection open with selfretaining retractors.