Introduction: The clinically significant threshold above which a scoliotic curve could be abnormal remains arbitrary. Data on normal adolescent and adult back shape are scarce. However clinical decision making based on subjective, visual criteria influences management. We aimed to produce measurable values of normal back shape, against which deformity could be defined. Method: 48 volunteering young adults perceiving themselves as “normal” participated in the study. All have been cleared previously by school screening. Their age (18–28 years old) precluded curve deterioration, but was close enough to adolescent measurements at the end of growth. Back shape was assessed with the ISIS system.

Results: A minority of 8% showed no curve, with 54% a single curve and the rest a double one. Right spinal asymmetry (77%) was more frequent than the left (52%). Mean values and 95% confidence intervals were 16.1° (14.0°–18.2°) for upper spinal asymmetry, 13.4° (10.1°–16.6°) for lower lateral asymmetry, 24.9 mm (20.6 mm–29.2 mm) for thoracic kyphosis and 14.9 mm (12.5 mm–17.2 mm) for lumbar lordosis. Increasing upper lateral asymmetry correlated with decreasing thoracic kyphosis (p< 0.01). Maximum skin surface angle correlated positively with only upper lateral asymmetry (p< 0.001).

Conclusion: Normal spines comprise of lateral asymmetries, where straight is the exception more than the rule. Scoliosis seems to be an exacerbation of this lateral asymmetry beyond 18° for the upper and 16° for the lower spinal asymmetry. Hypokyphosis is related to upper lateral spinal asymmetry. Skin surface angle is a very good indicator of only the upper lateral asymmetry.