Planes and Deformities

Spinal deformities are described according to the three planes of the body ( Fig. 2.1 ). In the coronal (frontal) plane the spine should be straight, and any lateral curvature is referred to as scoliosis. In the sagittal (lateral)plane, once a child has achieved head control and started walking, there are four natural spinal curvatures, two convex anteriorly (lordoses), in the cervical and lumbar regions, respectively, and two curvatures convex posteriorly (kyphoses), in the thoracic and sacral regions, respectively ( Fig. 2.2 ). Only when these lateral curvatures are excessive or reduced do they assume clinical significance.

Nonstructural and Structural Scolioses

To differentiate between relatively unimportant scolioses and important ones, the terms nonstructural and structural are applied, the former comprising purely lateral curvatures, whereas the latter have a rotational component in the transverse plane. Nonstructural curvatures are nonprogressive and secondary to some other problem (e.g., a pelvic tilt secondary to leg-length inequality or muscle spasm in association with back pain) ( Fig. 2.3 ). Structural scolioses are important and often progressive. The rotational component is clearly seen on X-ray films, with the spinous processes rotating toward the curve concavity ( Fig. 2.4 ), and is seen clinically by the presence of a rib or loin hump as the attached ribs and transverse processes rotate with the spine ( Fig. 2.5 ). Structural curves are primary or intrinsic to the spine itself.

Named Vertebrae

The apical vertebra or vertebrae are those at the center of the scoliotic curve ( Fig. 2.6 ). These are the most rotated vertebrae in the transverse plane. The upper and lower neutral vertebrae are the first nonrotated vertebrae above and below the scoliotic curve. The end-vertebrae are those maximally tilted above and below the apex of the curve. By convention, the extent of a scoliotic curve is described from neutral vertebra to neutral vertebra (e.g., T4 to L1), whereas the end-vertebrae are the reference points for measurement of the magnitude of the curve.

Curve Size

The usual method of measuring curve size is that of Cobb ( Fig. 2.6 ).2 In this method, lines are drawn along the upper endplate of the upper end-vertebra and the lower endplate of the lower end-vertebra, with the angle subtended by these lines being the Cobb angle. The case of large curves these lines intersect on the X-ray film, whereas for smaller curves perpendiculars have to be dropped for the measurement to be made. However, use of the Oxford Orthopaedic Engineering Centre Cobbometer obviates these problems and reduces the measurement error to much less than half of what it would otherwise be ( Fig. 2.7 ).3

It should be remembered that these measurements are made on an anteroposterior (AP) radiograph of the patient, and that the vertebrae within the curve are rotated out of the frontal plane of the patient, the apical vertebra being the most rotated. Accordingly, bigger curves are progressively rotated further away from the plane of the patient and thus, for example, a curve of 60 degrees is much more than twice as big as a curve of 30 degrees because the 60 degree curve is seen less en face ( Fig. 2.8 ). For this reason Stagnara favored taking radiographs of the scoliotic spine with respect to the amount of apical rotation, and he called these plan d’election views.5 If, for instance, the apical vertebra is rotated 30 degrees from the frontal plane of the patient, the patient or X-ray beam is turned 30 degrees from the frontal plane, so that a true AP plan d’election view is obtained. Necessarily, the size of the scoliotic curve on the plan d’election AP view is larger than on the AP view of the patient. So as to understand this point more clearly it is useful, for example, to examine a coat hanger in different planes of projection ( Fig. 2.9 ). When the coat hanger is at right angles to the plane of projection (Stagnara’s AP plan d’election view), the angle measures 60 degrees. If the coat hanger is rotated 90 degrees from this, then no angle is subtended, or the angle is 0 degrees and represents the true lateral projection. As the coat hanger is rotated from this true lateral projection to the true AP projection, the angle increases, so that the angle subtended halfway between the true lateral and AP planes is 30 degrees.

For both the coat hanger and the patient, the Cobb angle changes magnitude simply in terms of the plane of projection.

Although publications about scoliotic curve size before and after treatment compare mean Cobb angles, these are clearly not arithmetic data, because a curve correction from 60 degrees to 30 degrees as the result of treatment represents much more than a 50% improvement.

Vertebral Rotation

The extent to which the apical vertebra is rotated from the frontal plane in a patient with scoliosis can be measured with Perdriolle’s protractor6 ( Fig. 2.10 ) or by the method of Nash and Moe,7 which measures the displacement of the convex pedicle from the convex side of the vertebral body. With these techniques the amount of vertebral rotation does relate linearly to the size of the spinal deformity, but they are much less popular measurements than measurement of the Cobb angle. Moreover, obvious limitations to these other techniques are the inability to determine pedicular landmarks after instrumentation.

Another index of rotation, in addition to the position of the pedicles, is the angular appearance of the ribs on each side of the scoliotic curve. This is of particular significance for idiopathic deformities of early onset (infantile idiopathic scoliosis), which are common in the United Kingdom but much less prevalent in the United States. Fortunately, the majority of such deformities resolve, but some progress and can cause cardiopulmonary dysfunction. It is therefore critically important to identify those that need immediate treatment and those that can be monitored.

Thirty years ago, Min Mehta studied a large number of infants with scoliosis, and one of the important measures he used was the rib-vertebra angle difference (RVAD) ( Fig. 2.11 ).8 In this procedure the angle that the neck of the rib makes with the vertical axis of the apical vertebra (the rib-vertebra angle [RVA]) is measured on each side of the vertebra, and if the difference (the RVAD) is 20 degrees or more, there is a strong likelihood of progression of scoliosis. Therefore, this radiographic measure is particularly important in addition to the clinical assessment of such infants.