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
Treatment Goals
Although the indications and strategies vary, depending on the age of the patient and the deformity encountered, the goals of surgical treatment are essentially the same. The primary goals are to halt curve progression, relieve pain, and improve cosmesis and function. The surgical goals are to obtain a solid arthrodesis with a well-balanced three-dimensional spine.5 In adolescents, the focus should be on curve correction, improving cosmesis, and halting curve progression. As mentioned earlier, treatment of axial and/or radicular pain is much less of a concern.6 In adults, the focus is much less on curve correction but rather on balancing the spine and attaining solid arthrodesis and neurologic decompression if required.
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.
FIGURE 91-2 The end vertebrae are defined (blue lines) as the most rostral and caudal vertebrae in the curve.
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.
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.3,9–11 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.
Scheuermann Kyphosis
This type of kyphosis, which may occur solely in the thoracic or thoracolumbar spine, may present at any time between adolescence and adulthood. The cause is thought to involve asymmetrically higher ventral intradiscal pressures, which may lead to focal disc herniation of the end plates, generating Schmorl nodes. There may be injury to the growth plate, disproportional loss of ventral vertebral body height, irregularities of the end plates, and narrowing of the disc interspaces.11 Patients may present with pain, progression of kyphosis, or cosmetic concerns.
Principles of Deformity Correction
Cantilever Forces
This is probably the most common technique used for the correction of deformity. In general, a rod is bent to the desired contour for optimal alignment and is then connected sequentially to each pedicle screw or hook previously placed. The correction is greater with multiple points of fixation (i.e., multiple pedicle screws or hooks).13,14 As each screw is sequentially connected to the preformed rod, the spine begins to conform to the rod’s contoured design. The spine may be contoured in this manner in both the coronal and sagittal planes.
