Anterior and Posterior Treatment for Thoracolumbar and Lumbar Scoliosis

Overview

Large and rigid scoliotic deformities can be treated with many methods that are constantly evolving. Preoperative traction has been reported by many authors for the treatment of rigid curves that would be otherwise treated with anterior release followed by a posterior fusion. The advantages of traction are gradual correction of a spinal deformity that may not tolerate acute correction in the operating room (OR) without a risk of neurologic injury. The disadvantage is the long hospital stay, the associated expense, and reported complications.

Intraoperative traction has also been shown to help reduce the larger curves into more manageable curves on the operating table. Intraoperative traction should only be done with neuromonitoring, because changes may be seen even before the incision is made. Lewis describes decreasing the amount of traction placed if neuromonitoring changes occur without permanent sequelae. Halo-femoral traction has been described, which also exposes the patient to a small risk of a distal femoral pin placement.

Intraoperative temporary rod distraction has also been described, which is used to correct large curves in stages to give the curve some time to stretch in between the stages. Neurologic changes can also occur in such procedures, and it is mandatory to have neurologic monitoring. The use of multiple-stage surgery has the drawbacks of increasing the chance of infection, because of going through the same incision repeatedly, and nutritional depletion.

Anterior release for treating rigid deformities has a long history, beginning with anterior débridement for tuberculosis infections. Anterior release has been identified to be helpful in achieving greater correction and higher fusion rates. Some advantages include that the disk space provides a very favorable environment for fusion owing to a large area of bleeding bone that provides an ideal bed. The distance the fusion has to occur across is relatively small compared with the distance between the transverse processes. In addition, the bone graft is under compression.

On the other hand, some disadvantages apply when performing an anterior approach in an adult patient. Morbidity is associated with the approach, because cutting the abdominal musculature and sometimes the chest wall may be necessary, depending on the location of the release needed. The musculature frequently stretches, giving at times a rather unsightly, protuberant look to the abdomen. The stretch can result from a combination of cutting the muscle and denervation of the muscle from damaging the eleventh thoracic nerve in thoracoabdominal approaches.

Anterior Approaches

Thoracic Curves

Anterior release in the thoracic spine requires a simple thoracotomy. The rib can be removed during the exposure and can be used as bone graft for the anterior or posterior part of the procedure. The level of thoracotomy can be determined by the rib that best covers the apex of the scoliotic curve. It is generally the rib level above the last level of diskectomy.

Once the thoracotomy is done, the pleura is cut over the convexity of the curve. The segmental vessels are then ligated to expose the disk spaces, and the disk is cut with a scalpel or electrocautery. A thin rongeur is then used to remove the annulus and nucleus pulposus. After a large portion of the annulus is removed, the cartilaginous end plate is removed from the bony end plate with a sharp Cobb elevator. A large pituitary or similar rongeur is used to debulk remnants of cartilaginous end plate and annulus. The posterior annulus is then removed by a sharp curette from the posterior longitudinal ligament (PLL). The PLL has fibers oriented vertically that become visible once the posterior annulus is removed. A distractor in the disk space is very helpful while removing the posterior annulus with a curette, because it shows the edge of the posterior annulus and the bony end plate very clearly. Care should be taken not to damage the bony end plate to avoid bony bleeding that might make the removal of the posterior annulus more difficult.

Thoracolumbar Spine

The thoracolumbar spine can be approached via the thoracoabdominal approach. This approach usually involves cutting through the diaphragm. The superior end of the psoas muscle must be stripped off the spine to expose the disk spaces in the upper lumbar spine.

Lumbar Spine

The lumbar approach is reserved for exposing the lumbar spine only, especially the lower lumbar spine. Anterior diskectomy in the lower lumbar spine is used to obtain correction of the fractional lumbar curve. The diskectomy allows sagittal plane correction of the flat lumbar spine, which is a deformity present in most idiopathic and degenerative lumbar curves.

The psoas muscle must be elevated from the vertebral bodies and the disk spaces. The psoas muscle is carfully retracted posteriorly to avoid injury to the lumbar plexus. The vascular structures are at greater risk during the lumbar approach. Generally, the iliolumbar vein must be ligated to mobilize the great vessels for adequate exposure of the lower lumbar spine segments.

Posterior Approach

The posterior approach is the simplest and most universal approach to the spine. The surgeon must expose the spinous processes, laminae, facet joints, and transverse processes, and an important part of the posterior exposure is identifation of the pars interarticularis; it leads to the pedicle entry point in the lumbar spine and prevents the surgeon from removing too much bone and destabilizing the lumbar segment. It is important to clean the soft tissue from the posterior elements using a curette or a sharp Cobb elevator. This tissue removal aids in identifying landmarks as well as preparing for a posterior element fusion. The hypertrophic facet joints also need resection to identify the pars and the transverse process in order to place the spinal instrumentation. An osteotome is very useful for removing the large hypertrophic facets.

Lumbar Decompression

Lumbar decompression is often needed in treatment of degenerative scoliosis and adult idiopathic scoliosis with degenerative changes in the lumbar spine. The most common levels with spinal stenosis are L3–L4 and L4–L5. The laminectomy is done by removing the spinous processes first; the lamina is then thinned with a high-speed burr, and the thinned lamina is resected with a Kerrison rongeur. The foraminal decompression is performed using Kerrison rongeurs to remove the ligamentum flavum and the superior facet from the lower spinal segment. The laminar resection is done so that the lateral lamina, pars interarticularis, and partial facet joint are preserved.

Posterior Release

Posterior release is an important part of any spinal deformity surgery, and it was described by Shufflebarger. The release includes removal of the interspinous ligament and ligamentum flavum with partial facet resection and removal of the facet capsule ( Fig. 60-1 ). A distractor is used between the lamina to help with the release, especially the lateral part of the canal through the facet joints. Once the posterior release is performed, it is much easier to manipulate the spine into realignment. The posterior release is often referred to as a Smith-Peterson osteotomy (SPO) or a Ponte osteotomy. The SPO was first described for a posteriorly fused spine. The fusion is removed between the pedicles, but the disks have to be mobile to achieve correction. The Ponte osteotomy was described for treatment of kyphosis associated with Scheurmann disease, and it was used in the thoracic spine. In the Ponte osteotomy, the facets are resected, and the kyphosis is reduced via compression across the resected facet joints. The reasoning was that the posterior column is abnormally lengthened, and shortening it reduces it to the normal curvature.

Posterior Instrumentation

Posterior instrumentation of the spine is performed with placement of hooks, wires, and screws. In recent years, more pedicle screws have been used than any other implant; but hooks are still useful, such as when pedicle screws cannot be placed or at the proximal end of the construct. The wires can be used on the concavity of the curve, where the pedicles are usually small and deformed; use of pedicle screws has improved the ability to correct spinal deformity ( Fig. 60-2 ). The use of offset connectors allows placement of the pedicle screws as needed without lining them up. The offset connectors allow the variable distance to the rod of the screw to be easily manageable. The rod therefore can be mostly bent in the sagittal plane without having it bent in the coronal plane to accommodate all the screws.

The thoracic screws are placed by first identifying the superior facet. The junction of the facet and the transverse process is identified, and a burr is used to remove the cortex; then a pedicle probe is placed, curving away from the midline, until it passes the 20-mm mark. The pedicle probe is then placed medially and advanced. The ball-tip probe is used to palpate the bony walls of the pedicle and vertebral body before placing the screw. The landmarks and trajectories of the screw placement have been described by Polly and other authors.

The lumbar pedicle screws are placed by first identifying the transverse process pars and the facet joint. The transverse process is then decorticated. The junction of the pars, facet joint, and transverse process defines the entry point. The pedicle probe is then used to cannulate the pedicle, facing the curve of the probe medially. The curve can be faced medially, because the pedicles are angulated quite medially in the lower lumbar spine, from L3 down to the sacrum. The pedicle screws are placed after the ball-tip probe is used to palpate the pedicle walls for any perforations.

The hooks can be placed in several positions in the spine: supralaminar, infralaminar, facet, and transverse process hooks may be used. Supralaminar hooks are placed by removing the ligamentum flavum from the superior lamina. A small (2-mm) Kerrison rongeur is then used to square the lamina for resting the throat of the hook. Usually, a small blade is placed in the supralaminar position, because the lamina is angling down toward the spinal cord. Thus the space between the spinal cord and the lamina is smaller. Additionally, only one supralaminar hooks is placed per level, because two may crowd the midline and create stenosis from the two side-by-side hooks. Facet hooks are placed by removing the inferior facet in a square pattern and then placing the hook with a short throat and a wide blade in the facet joint. This position is used for the thoracic level, only because of the anatomy of the facet joint. Infralaminar hooks are used mostly in the lumbar spine to augment pedicle screw fixation or when the pedicle anatomy does not permit the safe use of a screw. The hook starter is used to elevate the ligamentum flavum from the inferior portion of the lamina prior to placing the hook. The throat of the hook is usually larger, in the 9.0- to 11.5-mm range. The blade of the hook is also wide. The transverse process hook is still widely used, because it is used at the proximal portion of pedicle screw constructs. The use of the transverse process hook allows keeping the soft tissue from undergoing extensive dissection and keeps the facet joint at the end of the construct from getting disrupted, which is likely to happen when defining the anatomy and placing the pedicle screw.

Iliac screws are extremely useful in providing a strong base for long constructs. Ending a long construct at the sacrum only is prone to failure from splay, pullout, and pedicle fracture. The lumbosacral and thoracolumbar junction have been found to have a higher nonunion rate, although a combination of sacral screws and iliac screws has improved sacropelvic fixation for long constructs. Iliac screws are connected using a rodded connector to the open polyaxial screw. The screw should be at least 7 mm in diameter and 80 mm in length and are usually placed by making a separate fascial incision over the posterior superior iliac spine (PSIS). The outside of the iliac wing is exposed, and the PSIS is removed with a rongeur. A probe is placed through the channel of bone, roughly 1 cm superior to the greater sciatic notch, then the screw is placed.

The sequence of reduction techniques is dependent on the spinal deformity. In adult idiopathic scoliosis, the usual thoracic curves are to the right, and the lumbar curve is to the left. The concave rod is placed first, which is on the left side. The left side also is the convex side for the lumbar curve, therefore the curve is pushed down anteriorly while placing the left rod. This maneuver reduces the lumbar curve and improves lumbar lordosis, because the apex of the lumbar curve is in relative kyphosis. The right rod is then placed. The thoracic spine is generally compressed, and the individual lumbar segments are distracted on the concave side of the lumbar spine. The offset connectors are helpful in attaching the screws to the rod while achieving correction of the deformity ( Fig. 60-3 ).