Scoliosis Diagnosis: Cobb Angle, Risser Sign and Curve Types

Scoliosis is diagnosed by combining a clinical examination (shoulder height, rib prominence, waistline and a neurological assessment) with standing X-rays on which the Cobb angle is measured. A curve of 10 degrees or more confirms scoliosis. The Risser sign, read from the same films, shows how much growth remains and therefore how likely the curve is to progress.

Dr. Manoj Miglani, Principal Director Orthopaedics, Fortis Flt. Lt. Rajan Dhall Hospital, Vasant Kunj, New Delhi, India, treats this work-up as the foundation of every later decision in adolescent idiopathic scoliosis. The plan, whether observation, bracing or surgery, follows from four measurements made at this stage: the size of the curve, its level, its flexibility and the child's skeletal maturity. The complete guide to adolescent idiopathic scoliosis covers the condition's cause, prevalence and natural history.

The clinical examination

Most adolescents with scoliosis come to attention through screening or because a parent, teacher or friend has noticed something. In adolescent idiopathic scoliosis the deformity starts to develop at around 10 to 12 years of age, which is when it tends to be picked up.

What to look at

Dr. Miglani listed the visible components of the deformity:

• Shoulder height. An uneven shoulder line is the feature that catches the eye immediately, and it also shapes the surgical plan, because the level of the shoulders influences how far up the spine a fusion must extend.
• Rib prominence. Rotation of the vertebrae pushes the ribs backwards on one side, producing a rib hump.
• Waistline. The waist creases become asymmetric.

Each of these is a cosmetic marker, and together they feed into the decision between correcting the curve and observing it.

Structural or postural scoliosis?

Once a curve is suspected, the first question is whether it is a true structural deformity, with significant rotation, or a postural curve: a child who habitually stands in an awkward way can look scoliotic without having a fixed deformity. A postural curve corrects fully when the posture changes; a structural curve does not, and it carries the rotation that defines true scoliosis.

The neurological examination

A neurological assessment is part of every evaluation. Its purpose is to rule out spinal cord anomalies and other causes of scoliosis. Adolescent idiopathic scoliosis does not produce limb weakness, paraparesis or paraplegia. Dr. Miglani was clear that a neurological deficit changes the plan totally: the child needs an MRI to find the cause before anything else is done.

Documenting the deformity

Because adolescents are sensitive about their appearance, the examination calls for privacy and tact. Dr. Miglani also recommended regular photographic documentation as the ideal way to record whether the curve is progressing.

How scoliosis is diagnosed on X-ray

Dr. Miglani's standard set of radiographs has three components:

1. A standing coronal X-ray (taken from the front or back) to measure the curve.
2. A standing sagittal X-ray (taken from the side) to assess kyphosis and lordosis.
3. Left and right bending X-rays to show how flexible, or correctable, each curve is.

Together these films show the amount of deformity, the vertebral levels involved, the degree of rotation and the curve's flexibility. Combined with the child's age, those four facts drive the plan. Bending films matter most when surgery is being considered, because a curve that straightens substantially on bending behaves differently from a rigid one.

Classifying the curve

Classification serves two purposes in Dr. Miglani's practice: it documents the kind of curves being treated, and it lets clinicians communicate precisely with each other.

By location of the apex

The apex is the vertebra at the centre of the curve, where the deformity and the vertebral rotation are greatest. Dr. Miglani offered a visual: imagine a rod bent around its own axis. The centre of the bend is the most rotated point and, as the centre of a "C", it is also the most horizontal vertebra.

Curves are named by where the apex sits:

• Cervical
• Cervicothoracic
• Thoracic
• Thoracolumbar
• Lumbar

By number of structural curves

Depending on the rigidity of each curve, and particularly once curves pass 40 degrees, the clinician decides whether the pattern is:

• A single curve, such as a single thoracic (dorsal) or single lumbar curve.
• A double curve, such as a double thoracic or a double major curve.
• A triple curve, where three structural, significant curves are present. Triple curves are, in Dr. Miglani's words, the complex scoliosis.

The distinction between structural curves and compensatory ones matters for treatment. A compensatory (secondary) curve forms above and below the main curve to keep the head and trunk balanced over the pelvis; it is usually flexible and is not itself the target of correction.

How to measure the Cobb angle

The Cobb angle is the universal measure of curve size. Dr. Miglani described the method step by step:

1. Identify the upper end vertebra and the lower end vertebra of the curve.
2. Draw a line along the superior end plate of the upper end vertebra.
3. Draw a line along the inferior end plate of the lower end vertebra.
4. Measure the angle between the two lines.

If the lines would meet beyond the edge of the film, perpendiculars are drawn from each line and the angle between the perpendiculars is measured instead. The geometry is identical.

Finding the end vertebrae

The measurement is only as good as the choice of end vertebrae, and this is where Dr. Miglani focused. Every main curve has compensatory curves at each end running in the opposite direction: one starting at the upper end vertebra and running upwards, and one starting at the lower end vertebra and running downwards. The end vertebra is the point where the curve changes direction.

The disc spaces reveal that point. Within a curve, disc height is reduced on the concave side and increased on the convex side, so each disc is wedged. At the ends of the curve the disc spaces become parallel: the disc may be tilted, but its upper and lower margins are parallel from left to right. The level at which the disc space turns parallel is the level at which the primary curve ends and the secondary curve begins, or at which a right-sided curve turns into a left-sided one.

The Risser sign: how skeletal maturity is graded

Dr. Miglani singled out the Risser sign as another of the most important things to assess. It is an indirect measure of skeletal maturity, and in adolescent idiopathic scoliosis maturity determines how much the curve is likely to progress and when to intervene.

How the Risser grade is read

The Risser sign is graded from the iliac apophysis, the growth centre along the top of the pelvic bone (the iliac crest), which is visible on the same standing X-ray. The crest is divided into four parts, and the grade reflects how far the apophysis has ossified across it and whether it has fused:

In Dr. Miglani's framing, a child at grade 4 or 5 is more or less skeletally mature, and the deformity is assessed on that basis. A child at grade 0, 1 or 2 still has a great deal of growth ahead.

Using the Risser sign to predict progression

Dr. Miglani gave two contrasting cases:

• A curve of around 35 degrees in a child at Risser 4 or 5. The curve is unlikely to progress. If it is not cosmetically obvious, the child can be treated conservatively.
• A 12-year-old at Risser 2. The curve is highly likely to progress. The child needs strict observation, with monitoring every six months.

The stakes are practical. The more severe a deformity becomes, the harder it is to correct, the greater the morbidity of surgery and the higher the chance of complications. Knowing the Risser grade lets the clinician prognosticate about the degree of deformity and intervene before the curve becomes severe.

Menarche as a second clock

In girls, the menstrual history adds a second measure of maturity. Dr. Miglani noted that the maximum growth spurt occurs about one to one and a half years before menarche. A girl who has already reached menarche has passed her fastest growth, so less progression is expected; a 12-year-old after menarche with a 15 to 20 degree curve simply needs observation. The history therefore belongs in every assessment alongside the Risser grade.

From work-up to plan

At the end of the evaluation the clinician holds five pieces of information: the Cobb angle, the curve location and pattern, its flexibility on bending films, the Risser grade and, in girls, menarchal status. A sixth comes with time: how the curve behaves over successive visits. A child who suddenly decompensates and develops a rapidly worsening curve needs intervention, while a curve that has not changed over six months or a year can be checked less often.

How those findings translate into observation, bracing or surgery is the subject of the article on scoliosis treatment thresholds. Attending doctors' questions on how often to repeat X-rays at different ages are answered in the Live Discussion from this masterclass.