Chapter 56

Posterior Exposure of the Thoracic and Lumbar Spine

James T. Guille and Reginald S. Fayssoux

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 spine 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 intermuscu-lar 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 L4–5 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 intertransver-sarii 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.

  1408

• 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)

  • 8% 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.

• 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 one degree per year).9

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

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

Trapezius Longissimus Iliocostalis

Rib Lung

Intercostal

artery and nerve

Segmental artery

Posterior primary rami

Spinalis Semispinalis

Multifidus

Rotators

Posterior primary rami

Segmental lumbar artery

Cauda equina

Lumbodorsal fascia

Multifidus

Longissimus

Iliocostalis

Pedicle

Quadratus lumborum

Psoas

A

Segmental artery

Posterior spinal arteries

Aorta Esophagus

Segmental

B artery Aorta

Vertebral body

Posterior radicular artery

Anterior radicular artery

Posterior intercostal artery

C

Congenital

Thoracic (decending) aorta

Anterior spinal artery

Anterior segmental medullary artery

Spinal branch

Dorsal branch of posterior intercostal artery

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.

• The skin is inspected for café-au-lait spots, the axilla for

• 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).

freckling, and the lumbosacral area for sinus tracts, hairy patches, or dimples. Axillary freckling and multiple café-au-lait spots are associated with neurofibromatosis. Sinus tracts, 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 asymmetrical 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

2

3

4

5

1

Right

Left

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

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 apophyses 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.

• 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.

  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.

       

      90

      Most tilted vertebra above apex

       

      Apex

      90

      Most tilted vertebra below apex

      B

       

       

      Cobb angle

       

      A

       

      FIG 3 • A. Cobb angle is a quantitative measure of the coronal curvature.

      CSVL

       

       

      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.

       

      • 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 (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.

         

         

         

         

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

         

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

    • 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 excessively hairy.

    • 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).

    • If a wake-up test is going to be used by the surgical team, a clear plastic C-arm cover or equivalent clear drape is laid over the exposed feet for visualization during the test.

    • A disposable plastic ruler used for measuring the pedicle probe for pedicle depth is placed caudal to the field on the buttocks and covered with a clear Tegaderm dressing.

       

       

      INCISION

      • A Bovie electrocautery cord is centered over the back using the vertebra prominens and the gluteal crease, and the line for the straight midline back incision is marked (TECH FIG 1A).

      • Bupivacaine (0.25%) with 1:200,000 epinephrine may be injected along the course of the incision for local anesthesia and hemostasis (TECH FIG 1B).

• The skin is sharply incised with a no. 10 blade scalpel and electrocautery is then used to dissect through the subcutaneous fat until the thoracolumbar fascia is reached.

  TECHNIQUES

   

• Weitlaner retractors are placed.

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

   

   

   

   

   

   

   

  A B

   

  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 anes-thetic and epinephrine being injected along the course of incision. C. Spinous process identified via palpation after

  C dissection down to the thoracodorsal fascia. (continued)

   

   

   

   

   

   

   

  TECHNIQUES

   

  D E

   

  TECH FIG 1 • (continued) D. Splitting of the spinous process apophysis with electrocautery. E. Dissection at three adjacent spinous processes. F. Dissection at three adjacent spinous processes

  F connected.

   

  • Electrocautery is used to dissect through the thoracolumbar fascia at the tips of adjacent spinous processes (TECH FIG 1D,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 leave the interspinous ligaments intact.

   

   

  SUBPERIOSTEAL DISSECTION

  • Dissection then proceeds subperiosteally.

  • In skeletally immature individuals, the apophyses of the spinous processes are further dissected with a Cobb elevator (TECH FIG 2A).

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

     

• 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.

• At this point the spine is instrumented.

   

   

   

   

   

  A B

   

  TECH FIG 2 • A. Further dissection of spinous process apophysis with a Cobb elevator. B. Dissection is advanced down to the laminae.

   

   

   

  CLOSURE

  • At our institution, before closure the wound is assessed for any frank bleeding vessels and bone graft is placed. Drains are placed for wounds considered at risk for hematoma formation.

  • Fascia is closed with figure 8 braided absorbable suture (no. 1 Vicryl). The goal is a watertight closure.

  • Subcutaneous layers are closed with interrupted braided

   

  absorbable suture (no. 0 and 2-0 Vicryl). The goal is to decrease wound tension.

  TECHNIQUES

   

• Skin is closed with a running single filament absorbable (3-0 Monocryl). The goal is cosmetic closure.

• Skin closure is reinforced with 1-inch Steri-Strips and surgical adhesive (Mastisol Liquid Adhesive).

• Sterile compression dressings are applied to decrease the risk of postoperative hematoma.

   

  PEARLS AND PITFALLS

  Decreasing blood loss

   

  Determining vertebral level intraoperatively

   

  Exposure

  • Injection of skin with bupivacaine and epinephrine mixture

  • 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

  • First pedicle screw is placed in area known to be within planned levels. Fluoroscopic image intensification can then be used to determine level. This eliminates confusion associated with determination of level on radiograph taken with a hemostat on the spinous process.

  • Neuromuscular blockade during exposure allows for ease of retraction of paralyzed spinal musculature.

   

   

  POSTOPERATIVE CARE

  • No postoperative immobilization is required with multiseg-mental constructs.

  • Postoperative restrictions include limitations with lifting, bending, and twisting.

  • It is important to maintain mean arterial blood pressure above 70 mm Hg overnight and hemoglobin above 10 g/dL to maintain spinal cord perfusion.

  • Intravenous antibiotics are maintained for 48 hours post-operatively.

  • 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 is 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 4-day hospital course is typical.

  • Routine follow-up is done at 1, 3, 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 straightening 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 method of diagnosis

  • Neurologic injury

    • Spinal cord injuries: consider initiating steroid protocol

    • Nerve root injuries

  • Wrong-level surgery

REFERENCES

1. Dimeglio A. Growth in pediatric orthopaedics. J Pediatr Orthop 2001;21:549–555.

2. Liljenqvist UR, Allkemper T, Hackenberg L, et al. Analysis of vertebral morphology in idiopathic scoliosis with use of magnetic resonance imaging and multiplanar reconstruction. J Bone Joint Surg Am 2002;84A:359–368.

3. McMaster MJ, Ohtsuka K. The natural history of congenital scoliosis: a study of two hundred and fifty-one patients. J Bone Joint Surg Am 1982;64:1128–1147.

4. Morrissy RT, Goldsmith GS, Hall EC, et al. Measurement of the Cobb angle on radiographs of patients who have scoliosis: evaluation of intrinsic error. J Bone Joint Surg Am 1990;72A:320–327.

5. Parent S, Labelle H, Skalli W, et al. Thoracic pedicle morphometry in vertebrae from scoliotic spines. Spine 2004;29:239–248.

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

7. Ramirez N, Johnston CE, Browne RH. The prevalence of back pain in children who have idiopathic scoliosis. J Bone Joint Surg Am 1997;79A:364–368.

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;85A:1461–1469.

9. Weinstein SL, Ponseti IV. Curve progression in idiopathic scoliosis. J Bone Joint Surg Am 1981;65A:447–455.

10. Weinstein SL, Zavala DC, Ponseti IV. Idiopathic scoliosis: long-term follow-up and prognosis in untreated patients. J Bone Joint Surg Am 1981;63A:702–712.