Posterior Exposure of the Thoracic and Lumbar Spine



Posterior Exposure of the Thoracic and Lumbar Spine


Wudbhav N. Sankar

John M. Flynn





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.







    FIG 1A,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

Jul 22, 2016 | Posted by in ORTHOPEDIC | Comments Off on Posterior Exposure of the Thoracic and Lumbar Spine

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