17: Anterior Thoracolumbar Spinal Fusion via Open Approach for Idiopathic Scoliosis

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Procedure 17 Anterior Thoracolumbar Spinal Fusion via Open Approach for Idiopathic Scoliosis

Indications

To halt progression of spinal curvature

To restore spinal alignment and balance

To preserve caudal motion segments (typically one to two segments) when compared with posterior spinal fusion (PSF)

To prevent the crankshaft phenomenon in skeletally immature patients (Risser 0, open triradiate cartilage)

To allow for a thoracoscopic approach in selected cases

Appropriate candidates typically include Lenke type 1 (single structural thoracic) and Lenke type 5 (single structural thoracolumbar/lumbar) curves (Figure 17-1, A and B).

FIGURE 17-1, A-B

Indications Pitfalls

• Anterior surgery tends to be “kyphogenic.” This may not be appropriate in curves with thoracic kyphosis greater than 40 degrees or in cases of thoracolumbar (“junctional”) kyphosis or lumbar hypolordosis.

• Patients weighing greater than 60 kg, with curves greater than 75 degrees, with or without hyperkyphosis, are not candidates for single-rod thoracic open or thoracoscopic fusion.

Indications Controversies

• Does sparing a spinal motion segment significantly benefit the patient?

• Are there more risks/complications with anterior surgery?

• Is there better correction of curvature with anterior versus posterior surgery?

Examination/Imaging

Clinical assessment of curve location, coronal and sagittal balance, trunk rotation, shoulder asymmetry, pelvic obliquity, integrity of the neuraxis, and any other associated anomalies

Full-length standing posteroanterior, lateral, and right and left supine bending radiographs to assist in curve classification, assessment of curve flexibility, and selection of fusion levels

Magnetic resonance imaging in selected cases (e.g., neurologic signs or symptoms, early- or juvenile-onset scoliosis, rapid curve progression, unusual curve pattern)

Treatment Options

• Posterior spinal fusion with dual-rod instrumentation

• Anterior spinal fusion with single- or dual-rod instrumentation, by open or thoracoscopic technique

Positioning

The surgical approach is always from the convex side of the curvature in the lateral decubitus position.

Positioning Pearls

• Copious padding of all bony and soft tissue prominences, such as the axilla (“axillary roll”) and lateral knee, is required to prevent a compressive neuropathy or pressure necrosis of the upper or lower extremity or trunk.

• Electrophysiologic monitoring of the upper extremities will allow early detection of compressive neuropathies, prompting immediate repositioning and/or repadding of the upper extremity.

• A flat radiolucent table and beanbag positioner are utilized.

• When utilized, unobstructed fluoroscopic access is verified before preparation and draping.

• Neurophysiologic monitoring of spinal cord function, using both somatosensory evoked potential and transcranial motor evoked potential monitoring, is recommended to optimize patient safety.

Portals/Exposures

Thoracic

Standard thoracotomy approach gives access to vertebral levels T2 to approximately L1.

Expanded access can be achieved with the use of a double thoracotomy if absolutely necessary.

Thoracolumbar

A tenth rib thoracoabdominal approach allows the greatest exposure and versatility for fusions crossing the thoracolumbar junction (Figure 17-2).

FIGURE 17-2

Lumbar

An anterior retroperitoneal flank approach gives sufficient access to vertebral levels.

Portals/Exposures Pearls

• Thoracic (open technique)

• Single-lung ventilation is not required but does allow for better visualization during the procedure.

• Standard thoracotomy is utilized, typically choosing the interspace corresponding to the apex of the deformity.

• Constructs spanning more than seven levels may require a double thoracotomy.

• To limit incision size and chest wall dissection, disk excision and screw implantation can be done through small percutaneous accessory incisions at the more cephalad or caudad levels.

• Thoracolumbar

• Split the costal cartilage at the tip of the tenth rib, maintaining the attachments to the diaphragm (cephalad cartilage tip) and abdominal musculature (caudad cartilage tip).

• Use temporary stay sutures to mark the diaphragm as it is incised, for later reapproximation.

• Reapproximate the split tenth rib costal cartilage to initiate closure.

• Beware of the thinning musculature and superficial location of the peritoneum medially near the rectus sheath.

• Dissection should proceed medial to the psoas muscle.

• The genitofemoral nerve lies directly on the psoas muscle.

• The iliolumbar vein consistently requires ligation when working at the L4-5 level.

• Preserve paraspinous sympathetic fibers that do not interfere with the dissection.

Procedure: Thoracolumbar Spine Fusion via an Open Approach Using Single-Rod Instrumentation

Step 1: Anterior Release and Diskectomy

Once adequate spinal exposure has been obtained, transection of the anterior longitudinal ligament and complete diskectomy is performed to, or including, the posterior longitudinal ligament.

Step 1 Pearls

• Release of the anterior longitudinal ligament and complete diskectomy allows for maximal curve flexibility and deformity correction.

• A malleable ribbon retractor placed around to the far side of the disk space affords protection to the vascular structures.

• For severe, rigid curvatures that have developed wedging of the vertebral bodies, vertebral osteotomy may be required to gain maximum correction.

Step 2: Placement of the Anterior Vertebral Body Screws

The entry point of the vertebral body screw is preferentially the junction of the pedicular origin and the vertebral body, crossing the center of the vertebral body and directed perpendicularly across to the far side. At the cephalad and caudad end vertebrae a staple can be impacted to prevent pullout or plow-through of the screws. At intervening levels, a washer can be placed to help distribute load.

The entry hole is made first with a sharp awl. A straight pedicle probe is then directed toward the surgeon’s finger on the far side of the body. The desired screw length is then measured. A tap is then placed, followed by screw placement. Image guidance is optional, but can assure screw placement central in the body and parallel to the vertebral end plates.

Step 2 Pearls

• Adequate exposure should allow easy access to the far (concave) side of the vertebral body for finger palpation of the probe, tap, and blunt tip of the vertebral body screws.

• Bicortical screw purchase should be obtained, with no greater than 2 mm of screw-tip protrusion.

• Instrumentation must include the entire Cobb angle.

Step 2 Pitfalls

• The surgeon must be cognizant of the location of the posterior vertebral wall and the spinal canal near the base of the pedicle, to avoid entering the canal with the vertebral body screw.

Step 3: End-Plate Ablation

Complete ablation of the cartilaginous vertebral body end plates, down to a raw, bleeding bony surface, is required for bony fusion (Figure 17-3).

FIGURE 17-3

Step 4: Placement of Anterior Interbody Structural Supports

Anterior interbody structural supports are important for overall strength of the construct and for setting the desired lordosis in the lumbar spine. When using single-rod instrumentation, a structural support is placed at each level in the lumbar spine, starting from the apex and moving cephalad and caudad. The graft should be impacted toward the concave side and positioned to lie flush with the anterior aspect of the vertebral body when setting the lordosis. Upon placement of the structural grafts, the spinal curvature is usually fully corrected and the lumbar lordosis is “set” before rod placement.

In the case of single structural thoracic (Lenke 1) curves fused to the thoracolumbar junction (T12 or L1), structural grafts are used one or two levels above the last instrumented vertebra to prevent junctional kyphosis.

Step 4 Pearls

• Any morselized rib autograft or harvested iliac crest graft should be placed around the interbody supports. In the case of a femoral ring allograft, the hollow center of the allograft can be filled with autograft.

Step 4 Pitfalls

• Compression anteriorly is a kyphosing maneuver. Be careful not to compress the disk spaces excessively when in the lumbar spine or at the thoracolumbar junction.

Step 5: Rod Placement

The rod is appropriately contoured to the desired sagittal and coronal plane configuration and introduced into the screws. The rod is reduced to the screws and captured sequentially.

Some degree of rod rotation or screw rotation may be necessary to “fine-tune” the correction. This should not require much force at this stage.

Some final compression of the screws across the disk spaces to firmly compress the grafts can be performed at this stage if necessary.

Step 6: Placement of Chest Tube and Wound Closure

A standard chest tube is placed and withdrawn from a small separate stab incision.

Wound closure must allow for meticulous reapproximation of the diaphragm (see Portals/Exposures Pearls).

The chest tube is maintained on wall suction until the first or second postoperative day. It is removed upon resolution of the pneumothorax/hemothorax and when output decreases to less than 75 to 100 mL over a 12-hour period.

Procedure: Thoracolumbar Spine Fusion via an Open Approach Using Dual-Rod Instrumentation

Step 1: Anterior Release and Diskectomy

This is performed in the same manner as described in step 1 for single-rod instrumentation.

Step 2: Placement of the Anterior Vertebral Body Screws

With systems using a dual-rod construct, there is typically a two-holed tined vertebral staple that is implanted first, which then receives the vertebral body screws (Figure 17-4). When positioning these devices, care should be taken not to allow the anterior screw to be too close to the anterior aspect of the vertebral body, because vertebral body fracture can occur. The proper staple is selected by identifying the size that maximizes the coverage of the lateral aspect of the vertebral body without violating the adjacent disk space.

Anteriorly placed devices may also incur more kyphosis, which is undesirable in the lumbar spine. The posterior screw should be placed in a position as posterior as possible, without allowing for canal intrusion.

FIGURE 17-4

Step 3: End-Plate Ablation

This is performed in the same manner as described in step 3 for single-rod instrumentation.

Step 4: Placement of Anterior Interbody Supports

This is performed in the same manner as described in step 4 for single-rod instrumentation.

Step 5: Rod Placement

The rods are appropriately contoured to the desired sagittal and coronal plane configuration.

The posteriormost rod is reduced to the screws first and captured sequentially.

Some degree of rod rotation may be necessary to “fine tune” the correction, which has already largely occurred from the meticulous diskectomy and structural graft placement.

The anterior screws can be used to further derotate the vertebrae at this point, if necessary. Some final compression of the posterior screws across the disk spaces to firmly compress the grafts before final tightening can be performed at this stage. The anteriormost rod is then placed and captured at this point, essentially in situ.

Some spinal implant systems will use a cross connector, which can be placed at this point (Figure 17-5).

FIGURE 17-5

Step 6: Placement of Chest Tube and Wound Closure

This is performed in the same manner as described in step 6 for single-rod instrumentation.

Postoperative Care and Expected Outcomes

The patient is mobilized beginning on the day following surgery. If single-rod instrumentation is utilized, a spinal orthosis is fashioned and worn for 3 to 4 months postoperatively. If dual-rod instrumentation is utilized, no brace is employed.

Activity modifications are necessary for at least the first 6 months postoperatively, with a return to competitive sports at 12 months or when clear evidence of fusion is seen radiographically.

Radiographs are taken to assess the implant stability, maintenance of correction and maturation of the fusion at 1, 3, 6, and 12 months postoperatively (Figures 17-6, A and B, and 17-7). Yearly radiographs are obtained thereafter until definitive fusion is seen.

FIGURE 17-6, A-B

FIGURE 17-7

Postoperative Controversies

• When using a single-rod system with rod diameter greater than 5 mm and multilevel structural interbody support, bracing may not be necessary in the early postoperative phase.

Fricka KB, Mahar AT, Newton PO. Biomechanical analysis of anterior scoliosis instrumentation: differences between single and dual rod systems with and without structural interbody support. Spine. 2002;27:702-706.

In bovine specimens, dual-rod constructs were stiffer in torsion and flexion-extension loading than in single-rod systems. Lateral bending stiffness was similar for both constructs. When structural interbody support (SIS) was added, stiffness in flexion increased significantly in single-rod constructs, approaching that of dual-rod constructs.

Lowe TG, Alongi PR, Smith DA, et al. Anterior single-rod instrumentation for thoracolumbar adolescent idiopathic scoliosis with and without the use of structural interbody support. Spine. 2003;28:2221-2232.

Forty-one patients with adolescent idiopathic scoliosis underwent anterior spinal fusion using a single-rod (6.0- or 6.5-mm) construct. SIS was used in 21 patients, and packed morselized autograft alone was used in 20 patients. There were no rod or screw failures and no obvious pseudarthroses at 3-year follow-up. Results were similar for both groups regarding curve correction and restoration of sagittal balance, and the Scoliosis Research Society Outcomes Instrument. The outcomes from use of SIS versus morselized autograft did not appear to be significantly different when these large-diameter single rods were used.

Lowe TG, Enguidanos ST, Smith DA, et al. Single-rod versus dual-rod anterior instrumentation for idiopathic scoliosis: a biomechanical study. Spine. 2005;30:311-317.

In human cadaveric specimens, SIS appeared to contribute the most to construct stiffness in flexion, whether single- or dual-rod constructs were used. In lateral bending, stiffness of single- and dual-rod constructs with and without SIS was equivalent. In torsion, single- and dual-rod instrumentation and SIS contributed to global stiffness. Transverse rod connectors in dual-rod constructs only contributed to stiffness in torsion. In bovine specimens, dual rods were stiffer than single-rod constructs, with SIS playing only a minor role.

Polly DWJr, Cunningham BW, Kuklo TR, et al. Anterior thoracic scoliosis constructs: effect of rod diameter and intervertebral cages on multi-segmental construct stability. Spine J. 2003;3:213-219.

In bovine specimens, single-rod constructs utilizing a 4- and 5-mm rod were tested with a seven-level interbody cage construct and compared with constructs using only one (apical disk), two (end disks), and three (apical and end disks) levels. Intervertebral cages at every level significantly improved construct stiffness when compared with increasing rod diameter alone. When structural supports were not used, axial compression created the greatest strain.

Potter BK, Kuklo TR, Lenke LG. Radiographic outcomes of anterior spinal fusion versus posterior spinal fusion with thoracic pedicle screws for treatment of Lenke type I adolescent idiopathic scoliosis curves. Spine. 2005;30:1859-1866.

This retrospective review compared curve correction and derotation among 40 curve-matched cohorts of Lenke type 1 curves treated by spinal fusion performed anteriorly with single-rod instrumentation versus posteriorly with thoracic pedicle screw (PSF/TPS) constructs. Anterior surgery allowed for an average of one less vertebral level fused. However, the PSF/TPS group demonstrated greater correction of the main thoracic curve and greater spontaneous correction of the uninstrumented thoracolumbar-lumbar curve, and improved correction of thoracic torsion and rotation.

Rhee JM, Bridwell KH, Won DS, et al. Sagittal plane analysis of adolescent idiopathic scoliosis: the effect of anterior versus posterior instrumentation. Spine. 2002;27:2350-2356.

This retrospective study evaluated the postoperative sagittal profile of 110 consecutive patients with adolescent idiopathic scoliosis. Sixty patients underwent posterior dual-rod instrumented fusion, and 50 patients underwent anterior instrumented fusion using a single-rod construct. At a follow-up of 32 months, the proximal junctional (kyphosis) measurement (measured between the proximalmost instrumented vertebra and the segment two levels cephalad) increased most in the posterior group; thoracic kyphosis (T5-12) increased most in the anterior group, and lumbar lordosis was enhanced with either approach. No significant change in the distal junctional measurement (measured between the distal instrumented vertebra and the segment two levels caudal) occurred in either group. The authors conclude that each approach affects the sagittal profile differently, albeit to a small degree. When properly performed, both approaches can give an acceptable sagittal profile.

Smith JA, Deviren V, Berven S, Bradford DS. Does instrumented anterior scoliosis surgery lead to kyphosis, pseudarthrosis or inadequate correction in adults? Spine. 2002;27:529-534.

This retrospective review of 14 consecutive adult patients with scoliosis treated by anterior spinal fusion using a single-rod (6-mm) construct demonstrated no cases of pseudarthrosis, progressive kyphosis, or instrumentation failure. Average correction of the Cobb angle was 66%, and the thoracolumbar sagittal plane alignment was maintained or improved in every patient. The patients scored satisfactorily on the Scoliosis Research Society Outcomes Instrument in the areas of satisfaction, pain, self-image, function, and mental health.

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