Posterior Exposure of the Thoracic and Lumbar Spine

 

 

 

DEFINITION

Scoliosis is a three-dimensional deformity of the spine and rib cage.

The hallmark of scoliotic spines is curvature in the coronal plane along with abnormal curvature in the sagittal plane (eg, lordoscoliosis in adolescent idiopathic scoliosis) as well as abnormal vertebral rotation in the transverse plane.

A Cobb angle measurement of greater than 10 degrees distinguishes minor spinal asymmetry from true scoliosis.

The posterior approach to the thoracic and lumbar spine takes advantage of the segmental innervation of the posterior spinal musculature to obtain an internervous and intermuscular plane to provide access to the posterior elements of the spine.

The posterior approach is the most commonly used route for spinal fusion and instrumentation in the scoliotic spine.

 

 

ANATOMY

 

Surface landmarks in the prone position

 

 

The vertebra prominens (C7) is typically the most prominent bony structure palpated at the base of the neck.

 

 

The superior angle of the scapula is at the level of the T3 spinous process. The scapular spine is at the level of the T4 spinous process.

 

The inferior angle of the scapula is at the level of the T7 spinous process.

 

With the patient in the prone position, the iliac crests are palpated with the fingers and the thumbs brought together at the midline, where they typically overlie the L4-L5 interspace.

 

The posterior superior iliac spines are at the level of the L5-S1 interspace.

 

Posterior spinal musculature is divided into superficial and deep layers. The superficial layer, also known as the erector spinae, is composed of the iliocostalis, longissimus, and sacrospinalis muscles. The deep layer consists of the short rotators (multifidus and rotatores) as well as the intertransversarii and interspinous muscles (FIG 1A,B).

 

Segmental innervation of spinal musculature

 

 

 

Provided by the dorsal rami of the thoracolumbar nerve roots Segmental blood supply

 

The posterior intercostal arteries branch from the aorta and subsequently send a dorsal branch posteriorly

to the spinal musculature. On its way past the neural foramina, the spinal artery branches off and is sent through the foramina. The spinal artery then divides into anterior and posterior radicular branches within the spinal canal, ultimately supplying the anterior and posterior spinal arteries. Care should be taken to cauterize the branches that lie adjacent to the lateral aspect of the facet (FIG 1C).

 

In the scoliotic spine, there is rotation of the vertebral bodies in the transverse plane with the spinous processes rotating toward the concavity of the curve.

 

In the scoliotic spine, the pedicles on the concave side are shorter and have a smaller diameter.5

 

In scoliosis, the dural sac hugs the concavity of the spinal canal2 and the aorta is posterolateral to its normal position.8

PATHOGENESIS

 

 

Idiopathic Congenital

 

Failure of formation or segmentation of vertebral precursors leading to asymmetric vertebral growth with subsequent abnormal curvature

 

Neuromuscular

 

 

Variety of etiologies, such as cerebral palsy, muscular dystrophy, polio, spinal muscular atrophy, and myelomeningocele

 

Related to an inability to provide muscular support to the spinal column

 

NATURAL HISTORY

Idiopathic

 

Infantile (0 to 3 years of age)

 

 

 

Less than 1% of all cases of idiopathic scoliosis More common in boys

 

 

Left thoracic curves predominate Most resolve spontaneously

 

Juvenile (3 to 10 years of age)

 

 

 

Eight percent to 16% of all cases of idiopathic scoliosis More even female-male ratio

 

Bracing may correct some curves.

 

Curves of more than 30 degrees usually progress to surgery.

 

Adolescent (10 to 18 years of age)

 

 

Most common form of idiopathic scoliosis

 

Etiology and pathogenesis are not well understood.

 

Family history is positive in 30% of cases but does not predict curve magnitude or progression.

 

More common in girls. The female-male ratio is 1.4:1 for curves 11 to 20 degrees and increases to 5:1 for curves greater than 20 degrees.

 

Curves have the greatest chance of progression in the period of peak growth velocity leading up to skeletal maturity (prior to menses in females), after which the potential decreases significantly.1

 

Scoliotic curves measuring less than 20 degrees are at lower risk for progression.

 

 

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FIG 1 • A,B. Cross-sections of paraspinal musculature. C. Overview at the level of the lumbar spine. The segmental artery courses posteriorly, adjacent to the vertebral body toward the posterior spinal musculature. On passing the neural foramen, the vessel sends a branch through the neural foramen to supply the spinal cord. The vessel continues toward the posterior spinal musculature arising between the transverse processes during the surgical approach where it is prone to bleed.

 

 

Scoliotic curves measuring greater than 50 degrees are at higher risk for further progression during adult life (with a percentage of these progressing at a rate of about 1 degree per year).9

 

There are no significant differences in the prevalence of back pain between adults with scoliotic spines and the general population.7, 10

 

Scoliotic curves measuring greater than 100 degrees have an increased prevalence of cardiopulmonary compromise (eg, cor pulmonale, restrictive lung disease).6

Congenital

 

Severity of deformity related to type and location of anomaly

 

Highest chance of curve progression with unilateral unsegmented bar with contralateral hemivertebrae (nearly 100%), followed by a lone unilateral unsegmented bar, double convex hemivertebrae, single convex

hemivertebrae, and finally the block vertebrae3

 

Neuromuscular

 

 

Most curves are progressive and are more difficult to manage nonoperatively. Curves can cause pelvic obliquity and sitting problems in nonambulatory individuals.

PATIENT HISTORY AND PHYSICAL FINDINGS

 

Complete history, including age at onset, timing of growth spurts, menses, presence of pain, family history of scoliosis, nerve, or muscle diseases

 

A complete examination is important to obtain a diagnosis because certain etiologies can predispose the patient to increased operative risk (eg, cardiac abnormalities in patients with Marfan syndrome).

 

The skin is inspected for café-au-lait spots, the axilla for freckling, and the lumbosacral area for sinus tracts, hairy patches, or dimples. Axillary freckling and multiple café-aulait spots are associated with neurofibromatosis. Sinus tracts,

 

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hairy patches, or dimples in the lumbosacral area are associated with intraspinal anomaly.

 

The Adams forward bending test detects curvatures by physical examination. Abnormalities in vertebral rotation become apparent as an asymmetric rib hump, prominence, or fullness, leading to possible identification of patients at risk for having scoliosis.

 

Any shoulder or scapular asymmetry is noted. It is important to point out to parents that this is not always corrected by surgery.

 

Pelvic obliquity can indicate a possible leg length discrepancy that can mimic a lumbar scoliosis.

 

Trunk shift and sagittal profile are noted; these indicate coronal balance and sagittal balance, respectively.

 

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

Plain radiographs, including standing posteroanterior (PA) and lateral views of the entire spine, should be obtained to determine the degree of scoliosis, to identify any skeletal abnormalities (eg, hemivertebra), and to evaluate overall alignment.

 

 

PA views are obtained to decrease exposure of sensitive breast tissue to ionizing radiation in girls.

 

Side-bending supine views of the thoracolumbar spine are useful to determine the flexibility of the primary and secondary curves. This information is useful during preoperative planning in choosing fusion levels and determining the approach.

 

Risser staging system for gauging skeletal maturity (FIG 2)

 

 

Ossification of the iliac apophysis proceeds along the iliac crest from the anterior superior iliac spine to the

posterior superior iliac spine. When ossification is complete, fusion of the apophysis to the iliac crest occurs.

 

 

 

Risser 0 = no ossification Risser 1 = 25% excursion

 

Risser 2 = 50% excursion

 

Risser 3 = 75% excursion

 

Risser 4 = 100% excursion

 

Risser 5 = fusion of iliac apophysis to the iliac crest

 

In girls, the end of spinal growth corresponds to Risser stage 4.

 

In boys, spinal growth can occur after Risser stage 4 and is less well defined.

 

Magnetic resonance imaging (MRI) should be obtained for patients with an onset before age 10 years; left thoracic curves, kyphoscoliotic curves, or rapidly progressive curves; patients with moderate to severe back pain; patients with congenital or neuromuscular scoliosis; and patients with abnormal findings on the physical examination.

 

 

 

 

FIG 2 • Risser staging system. Ossification proceeds from the anterior superior iliac spine to the posterior superior iliac spine.

 

DIFFERENTIAL DIAGNOSIS

Scoliosis Idiopathic Congenital

Neuromuscular

Limb length discrepancy Osteoid osteoma

 

Sprengel deformity

 

 

NONOPERATIVE MANAGEMENT

 

Observation for progression for curves of 0 to 20 degrees. Patients are followed with serial clinical and radiographic examinations.

 

Bracing for progressive curves of 20 to 40 degrees if the patient is skeletally immature. Braces cannot correct curves; their purpose is to prevent curve progression.

 

SURGICAL MANAGEMENT

Preoperative Planning

 

Preoperative radiographs with supine side-bending films are obtained to determine fusion levels according to the Lenke criteria.

 

Cobb angles (FIG 3A)

 

 

 

Quantitates the degree of curvature Method

 

Determine apex of curve to be measured.

 

Select the most tilted vertebra above the apex of the curve and draw a line along the top of the vertebral endplate.

 

Select the most tilted vertebra below the apex of the curve and draw a line along the bottom of the vertebral endplate.

 

 

Drop perpendicular lines to these previous two lines. The angle subtended by the two lines is the Cobb angle.

 

The intraobserver intrinsic error in Cobb angle measurements is about 5%; interobserver validity is about 7%.4

 

Center sacral vertical line (FIG 3B)

 

 

Used to help determine distal extent of fusion

 

 

The vertebral body most closely bisected by the center sacral line is the stable vertebra. Fusion is usually extended to the stable vertebra or the one immediately cephalad.

Positioning

 

Patient is intubated in the supine position on the stretcher.

 

Neurologic monitoring leads are placed cranially, on the intercostal and abdominal musculature, and on all four extremities.

 

Multiple large-bore intravenous access is obtained for fluid management, and an arterial line is placed for intraoperative blood pressure monitoring.

 

The patient is transferred to the prone position on a well-padded operating room table such as a Jackson frame (Mizuho Osi, Orthopaedic Systems, Union City, CA).

 

Care should be given to the degree of hip flexion-extension, as this can affect the amount of lordosis in the lumbar spine.

 

Bolsters underneath the chest and anterior superior iliac spines prevent abdominal compression and allow epidural venous return, thus decreasing epidural bleeding.

 

All bony prominences are well padded, including medial elbows, knees, pretibial areas, and ankles.

 

 

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FIG 3 • A. Cobb angle is a quantitative measure of the coronal curvature. Lines parallel to the vertebral end-plates of the end vertebrae are drawn. Perpendiculars to these two lines are drawn. The angle subtended is the Cobb angle. B. Center sacral vertical line (CSVL) is the vertical line in a PA radiograph that passes through the center of the sacrum, identified by suitable landmarks, preferably on the first sacral segment (SRS terminology). The vertebra most closely bisected by the CSVL is considered the stable vertebra.

 

 

Care is taken to avoid abduction and forward flexion past 90 degrees at the shoulder and flexion past 90 degrees at the elbow (FIG 4).

 

Skin is shaved if necessary.

 

Clear adhesive surgical drapes (3M Steri-Drape towel drapes) are placed around the perimeter of the surgical site, extending from the hairline to the top of the gluteal crease (regardless of levels to be fused, the entire spine should be draped).

 

 

 

FIG 4 • Patient positioning. All bony prominences are well padded. Note the positioning of the upper extremities.

 

TECHNIQUES

• Incision

   

  A Bovie electrocautery cord is centered over the back using the vertebra prominens and the gluteal crease as landmarks, and the line for the straight midline back incision is marked based on the extent of the intended fusion (TECH FIG 1A).

   

  The skin is sharply incised with a scalpel, and electrocautery is then used to dissect through the subcutaneous fat until the thoracolumbar fascia is reached.

   

  Weitlaner retractors are placed.

   

  The spinous processes are identified via palpation (TECH FIG 1B).

   

  Electrocautery is used to split the apophyses along the tips of the spinous processes. A clamp can be used to isolate each spinous process sequentially (TECH FIG 1C-E).

   

   

  These incisions are connected (TECH FIG 1F). This proceeds throughout the extent of the incision.

   

  Care is taken at the cephalad and caudal aspects of the dissection to preserve the interspinous ligaments.

   

   

   

  TECH FIG 1 • A. Bovie electrocautery cord centered over back using vertebra prominens and gluteal crease as cephalad and caudal landmarks. B. Mixture of local anesthetic and epinephrine being injected along the course of incision. (continued)

   

   

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  TECH FIG 1 • (continued) C. Spinous process identified via palpation after dissection down to the thoracodorsal fascia. D. Splitting of the spinous process apophysis with electrocautery. E. A clamp can be used to define the spinous process. F. Dissection at three adjacent spinous processes are connected.

• Subperiosteal Dissection

   

  Dissection then proceeds subperiosteally.

   

  In skeletally immature individuals, the apophyses of the spinous processes are further dissected with a Cobb elevator. It is often helpful to peel subperiosteally with one Cobb elevator on each spinous process/lamina to put the soft tissues on tension while the exposure is advanced with electrocautery (TECH FIG 2A,B).

   

  Electrocautery is used to advance the dissection deep along the spinous processes until the laminae are reached and the retractors are repositioned (TECH FIG 2C).

   

   

   

  TECH FIG 2 • A. Further dissection of spinous process apophysis with a Cobb elevator. B. Placing a Cobb elevator on each spinous process/lamina and putting the soft tissues on tension allows the exposure to be advanced with electrocautery. (continued)

   

   

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  TECH FIG 2 • (continued) C. Dissection is advanced down the laminae out to the tips of the transverse processes.

   

   

  In the thoracic spine, dissection proceeds until the tip of the transverse process is fully exposed.

   

  In the lumbar spine, dissection proceeds until the facet joint, pars interarticularis, and transverse process are exposed.

   

  Once the exposure is completed, it is vital that one check to be sure that the appropriate levels have actually been exposed.

   

  By convention, we typically place a pedicle marker in what we believe to be the left T12 pedicle and check a C-arm image to confirm that the correct levels have been exposed. It is important to compare the presence/size of the T12 rib on the C-arm image to the preoperative radiographs to reduce the risk of making a mistake. If T12 is not going to be included within the fusion segments, a different level can be chosen for the marker.

   

  At this point, the spine is instrumented and the correction is performed (see Chap. 85).

• Closure

 

Before closure, the wound is assessed for any frank bleeding vessels.

 

All exposed areas of bone are decorticated using a burr in order to facilitate fusion.

 

The wound is now irrigated with 3 L of normal saline via low-pressure cystoscopic tubing.

 

The spine is bone grafted using a mixture of locally harvested autograft and cancellous allograft.

 

At our institution, vancomycin powder is mixed with the bone graft for all patients. In certain neuromuscular patients, gentamicin can be used as well.

 

 

Fascia is closed with a running braided absorbable suture (no. 1 Vicryl). The goal is a watertight closure. A medium Hemovac drain is placed superficial to the fascia exiting at the inferior extent of the wound.

 

Subcutaneous layers are closed with running braided absorbable suture (no. 0 and 2-0 Vicryl). The goal is to decrease wound tension.

 

The subcuticular layer of skin is closed with a running single filament absorbable suture (4-0 Monocryl).

The goal is cosmetic closure.

Skin closure is reinforced with 1-inch Steri-Strips.

A silver-impregnated dressing is used (Mepilex Border, Mölnlycke Health Care, Göteborg, Sweden).

 

 

Decreasing

blood loss

• Subperiosteal dissection

• Bovie electrocautery on high power

• Traction with self-retaining retractors

• Topical thrombin (FloSeal)

• Mean arterial pressure is 60 to 70 mm Hg during exposure but is increased to more than 70 mm Hg during correction to maintain spinal cord perfusion.

Determining ▪ Marker placed in what is believed to be left T12 pedicle and a C-arm image is

vertebral level taken to confirm that the appropriate levels have been exposed. It is important intraoperatively to compare the presence/size of the T12 rib on the C-arm image to the

preoperative radiographs to reduce the risk of making a mistake.

Exposure

• Neuromuscular blockade during exposure allows for ease of retraction of

paralyzed spinal musculature.

PEARLS AND PITFALLS

 

 

 

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POSTOPERATIVE CARE

 

 

 

No postoperative immobilization is required with modern pedicle screw fixation. Postoperative restrictions include limitations with lifting, bending, and twisting. Intravenous antibiotics are used based on local protocol.

 

 

Neurovascular checks are made every 2 hours for the first 8 hours, then every 8 hours. Patients are out of bed on postoperative day 1.

 

 

The Foley catheter and Hemovac are typically removed on postoperative day 2. Diet is advanced as tolerated.

 

Patient-controlled analgesia is used for appropriate patients. Continuous narcotic infusion with demand for the first 24 hours is followed by demand only for the next 24 hours, followed by oral pain medications when tolerating diet.

 

A 3-4 day hospital course is typical.

 

 

Routine follow-up is done at 6 weeks and 6 months and at 1, 2, and 5 years. Activity is increased based on the degree of fusion.

OUTCOMES

With meticulous attention to detail with regard to instrumentation and fusion techniques, excellent outcomes in terms of correction and fusion of the scoliotic spine can be expected.

Long-term outcomes are variable and depend on the underlying diagnosis and the extent of retained spinal mobility.

 

 

COMPLICATIONS

Early or late infection: less than 5% Wound dehiscence

Hematoma Instrumentation failure

Pseudarthrosis: 1% to 12% depending on type of fusion and underlying diagnosis Neurologic injury

Spinal cord injuries: consider initiating steroid protocol Nerve root injuries

Wrong-level surgery

 

 

ACKNOWLEDGMENT

 

The authors would like to acknowledge James T. Guille and Reginald S. Fayssoux for their contributions to the previous edition.

 

REFERENCES

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6. Pehrsson K, Bake B, Larsson S, et al. Lung function in adult idiopathic scoliosis. Thorax 1991;46:474-478.

    

    

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8. Sucato DJ, Duchene C. The position of the aorta relative to the spine: a comparison of patients with and without idiopathic scoliosis. J Bone Joint Surg Am 2003;85-A(8):1461-1469.

    

    

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