Spinal Deformity and Correction: The Fundamentals

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Chapter 91 Spinal Deformity and Correction

The Fundamentals

Spinal deformity is a three-dimensional alteration of spinal alignment in both the coronal and sagittal planes. Scoliosis, strictly speaking, is a curvature greater than 10 degrees in the coronal (frontal) plane, as determined by measuring the Cobb angle (Fig. 91-1). There are many causes, and the condition may occur throughout life from infancy through adulthood. The type of deformity and patient age at presentation have a significant impact on the ultimate treatment of the condition. These topics will be discussed separately throughout this book and are beyond the scope of this chapter. Surgical treatment of these conditions varies depending on the age of the patient and the degree of the deformity. Adults with spinal deformity often seek surgical consultation for pain associated with the deformity, spinal imbalance, or neurologic signs/symptoms. Children and adolescents are most often sent for surgical evaluation for cosmetic concerns and risk of curve progression. Pain and/or neurologic compromise are rare in this patient cohort. For the purposes of this chapter, the authors will focus on the principles of spinal deformity and not the causes or specific treatments. These principles may be applied to most conditions affecting spinal alignment.

The treatment of spinal deformity is not new. Hippocrates attempted traction scoliosis, and Pare used an iron corset in the 16th century in an attempt to correct a similar deformity.1 Not until the development of Paul Harrington’s rod system in the 1960s did the surgical correction of spinal deformity take off.2 For the first time, Harrington’s distraction rod systems, then compression rod systems, permitted correction and improved arthrodesis of the deformed spine. This development truly revolutionized treatment of these dynamic and complex conditions. Early strategies for fixation focused largely on coronal plane correction. Often excellent results were obtained, but these resulted in flattening of the sagittal plane. Unfortunately, this often led to the development of decompensation in the sagittal plane and subsequent pain. This has been referred to as flatback deformity.3 The next phase in development was the application of hooks, then screws, in the thoracolumbar spine. The use of hooks and screw fixation of the spine permitted a greater control over the spine and corrective maneuvers. Greater degrees of correction could be obtained, fixation was improved, and shorter constructs could be used. Powerful control of individual segments of the spine with three-dimensional correction that was not limited by postoperative bracing became the basis for present-day deformity correction systems.4

General Terms

A unique set of terms may be applied to spinal deformity and should be reviewed briefly. Spinal deformity involves a curvature and obligatory rotation (coronal plane) in either the coronal and/or sagittal planes. Curves in either plane are measured end vertebra to end vertebra. The end vertebra is the most cephalad and most caudal vertebra of a curve (Fig. 91-2). Lines extended along the end plates of the vertebral bodies that are part of the curve in question all converge toward a central point within the concavity of a curve. Lines extended along the end plates of vertebral bodies not involved become divergent. The most rostral or caudal vertebral body visualized is the end vertebra. The neutral vertebra is that vertebra between curves that demonstrates the least rotation. Both pedicles should be relatively symmetrical. The stable vertebra is the vertebra that is bisected by the center sacral vertical line. This line is determined by first drawing a line connecting the most rostral point on each of the iliac crests. A perpendicular drawn from the midpoint of the S1 vertebra superiorly defines this line. Surgeons often use the stable or neutral vertebra when deciding where to end a construct/fusion.

Spinal balance is critical for optimal biomechanics. This is determined using a plumb line. In the coronal plane, the line is drawn from the center of the C7 vertebral body to the sacrum. This line should fall within 2.5 cm of the center of the sacrum.7 Deviation greater than this provides evidence of coronal decompensation. This may be determined using 36-inch scoliosis radiographs or directly on the patient, estimating the location of C7 and using the gluteal cleft as the midsacrum. In the sagittal plane, the line should extend from the center of the C7 vertebra and the dorsal aspect of the L5-S1 disc space.8 On the patient, a plumb line may be drawn from the external meatus of the ear, and the line should fall along the greater trochanter when the spine is balanced.

Spinal curves may be classified as structural or compensatory nonstructural. A structural curve is usually the larger or major curve of the deformity and is closely related to the inciting pathology responsible for the deformity. On bending radiographs, structural curves maintain a significant curve magnitude, generally greater than 25 degrees in the coronal plane. Compensatory curves are countercurves that allow the spine to “compensate” for the structural curve in an attempt to maintain balance. On bending radiographs, compensatory curves are less than 25 degrees, smaller in magnitude than structural curves. Compensatory curves are flexible. Curves are also measured in the sagittal plane. Normal measurements of thoracic kyphosis and lumbar lordosis have been determined. Deviations from these “normal” values may be defined as hyperkyphosis, hypokyphosis, or lordosis. When planning surgical deformity correction, most often the structural curve is instrumented, whereas instrumentation of the compensatory nonstructural curve(s) is avoided or selectively limited. Sagittal curves must be accounted for as well. Hypokyphosis should be addressed in the thoracic spine, even if the Cobb angle of the coronal curve may not be of a significant magnitude.

Causes of Spinal Deformity

Idiopathic Scoliosis

By far the most common type of spinal deformity is idiopathic scoliosis. As the name implies, the cause for the condition is unknown; however, significant evidence suggests genetic influences.3,911 These curves present in adolescence and have a risk of progression during growth of the spine. Surgery is not always required. A discussion of surgical indications is beyond the scope of this chapter and is related to curve magnitude, location, and maturity status of the patient’s spine.

Degenerative Deformities

As the spine progresses down the degenerative cascade as defined by Kirkaldy-Willis12 and does so asymmetrically, deformity may, and often does, occur. Classically, this involves the lumbar spine. Curvature, lateral listhesis, and rotation are usually seen. Patients may present with lumbar axial pain, radiculopathy, and/or neurogenic claudication. Treatment is dependent on the presenting symptomatology. Radicular pain may only require foraminotomy, whereas decompensation and axial mechanical pain may require deformity correction and stabilization.