Spinal Deformity

Classification and Definitions

Many different classification systems have been described for spinal deformities. Over the years, some of these classifications have fallen out of favor while new descriptive terms have been applied. To accurately diagnose a spinal deformity, it is important to understand the definitions of the terms used.

Spinal deformities are described in relation to the position of the deformity within the spine, the characteristics of the curve pattern, the size of the curve, the etiology of the deformity, and the age at onset. Deformity of the spine is described in relation to the three anatomic planes of the body, which are the coronal (frontal) plane, the sagittal (lateral) plane, and the horizontal (transverse or axial) plane. The spine is usually straight in the frontal plane; if it is not, then coronal asymmetry exists, which is termed scoliosis . This rather simplistic view of scoliosis is a result of the plain radiograph being a two-dimensional projection of a three-dimensional structure. CT and MRI have demonstrated that far from being a simple coronal-plane deformity, scoliosis results in deformity in all three planes. Nevertheless, if the spine is not straight on a frontal radiograph, then scoliosis exists.

In the sagittal plane, there are normal physiologic curves: a lordosis in the cervical and lumbar regions and a thoracic and sacral kyphosis.

The terms structural and nonstructural curves are used to differentiate a potentially progressive curve from a less important nonprogressive one. The term structural is applied to those curves in which there is a fundamental problem of growth and is most commonly seen in the idiopathic type of scoliosis. The key feature of this is deformity in all three planes, and this is recognized on the frontal radiograph by rotation of the spinous processes toward the concavity of the curve, indicating transverse plane deformity ( Fig. 105-1 ). It is apparent that if the spinous processes are lying in the concavity of the curve, they must take a shorter route than the vertebral bodies, leading to a relative lordosis (see eFig. 105-1 ). Indeed, the growth disturbance in idiopathic scoliosis is a relative overgrowth of the anterior spine, which results in a buckling and deformity in all three planes.

One of the basic principles of anterior corrective surgery is to prevent continued anterior growth by removing the intervertebral disks and end plates and fusing the spine between the vertebral bodies.

The term kyphoscoliosis is therefore inaccurate because this is actually a lordoscoliosis . The apparent clinical kyphosis is a result of the rib hump that occurs due to the transverse rotation.

Axial CT is helpful to determine to what extent the ribs contribute to the deformity and select which patients would benefit from costoplasty, which is a surgical removal of the ribs at the site of the rib hump ( eFig. 105-2 ).

The term nonstructural scoliosis is applied to nonprogressive curves in which the scoliosis is a secondary feature and not the consequence of an intrinsic growth problem. The most common cause of this is pelvic-tilt scoliosis secondary to leg-length inequality (see eFig. 105-3 ). The key features are a lumbar scoliosis with the sacrum at the bottom of the curve and the spinous processes not rotated into the curve, indicating the lack of a transverse deformity. The next most common cause of a nonstructural scoliosis is an irritating focus within the spine. A painful scoliosis is a strong clinical indicator of an irritating focus because idiopathic scoliosis is not a painful condition. Therefore, the presence of pain with scoliosis always warrants further investigation. MR sequences that are sensitive to edema may localize the problem, although MRI can easily miss a small osteoid osteoma because sections performed in the standard anatomic sagittal and axial planes will cut obliquely through a spine with a scoliotic deformity ( Fig. 105-2 ). In many cases, bone scintigraphy is the most sensitive method of isolating the lesion and allowing cross-sectional imaging to target the area of abnormality.

Even if the radiologic features suggest an idiopathic scoliosis, the presence of pain warrants further investigation. Idiopathic scoliosis is a relatively common condition in adolescent girls, and another cause for the pain should be sought because the scoliosis may be an incidental finding ( eFig. 105-4 ).

Curve Characteristics

By convention, a curve is described as right or left sided according to the side of convexity of the curve. The end vertebrae are those vertebrae at the top and bottom of the curve that are maximally tilted into the curve. The standard method of measurement of the curve is the Cobb angle (see Appendix 1 at ExpertConsult).

A line is drawn along the upper end plate of the upper end vertebra and along the lower end plate of the lower end vertebra ( eFig. 105-5 ). The angle where these lines intersect is the Cobb angle. Protractors with a freely hanging needle are available that allow measurement of the angle without actually marking the film. The method for measuring the Cobb angle has been simplified now by the widespread use of digital radiology, with angle measurement tools a standard feature of PACS software. Although the Cobb angle is a crude measurement of curve severity that records only the coronal plane deformity, it is an easy and reproducible method of recording curve progression.

The vertebra in the middle of the curve is termed the apical vertebra . This is the vertebra whose spinous process will be maximally rotated into the curve. The position of this apical vertebra determines the curve type. If the apex lies between T2 and T11, it is defined as a thoracic curve. A thoracic curve is the most common pattern and usually has an apex at T8 or T9. If the apex is at T12 or L1, it is defined as a thoracolumbar curve. A curve apical below L1 is termed a lumbar curve, and at C7 or T1 it is a cervicothoracic curve. The major curve is the one with the largest Cobb angle.

A single major curve will have compensatory curves above and below it correcting the vertical alignment of the spine. In this case the spine is said to be in balance. Occasionally, the whole spine will list toward the convexity of the major curve, if the compensatory curves progress beyond that which makes the spine vertical. This is an important observation to make if surgery is being considered, because correction of the major curve may throw the patient further out of balance, resulting in tilting of the shoulders. A common curve pattern is a thoracic curve with a thoracolumbar or lumbar curve below.

Etiology, Clinical Presentation, and Imaging Techniques

Spinal deformity can further be defined in terms of the etiology ( Box 105-1 ). The most common group are the idiopathic deformities, which comprise idiopathic scoliosis and idiopathic kyphosis, more commonly known as Scheuermann disease.