Complementary angles. A simplification of sagittal plane rotational assessment in cervical instability.

The use of complementary angle measurement improves the physician's ability to quantify the magnitude of the angle of rotation directly. By constructing a right triangle, with its base determined by the inferior end plate of the vertebral body, the angle of sagittal plane rotation can be determined readily. A perpendicular line drawn from the inferior end plate of the vertebra to the adjacent superior vertebra's inferior end plate will create an acute angle. This angle is complementary to the angle of sagittal plane rotation for this motion segment. The sum of two complementary angles, the acute angle described and the angle of sagittal plane rotation is equal to 90. Assessment of cervical spinal stability is crucial to the physician caring for traumatized patients. A checklist previously was developed by others to evaluate spinal stability; this relied heavily on the findings of routine lateral radiographs. Differences in the angle of sagittal plane rotational of greater than 11 between adjacent vertebra were equivalent to two points on this checklist, and five points or more suggested instability.