Introduction: The two main methodologies described for the assessment of spinal sagittal alignment are the pelvic radius (PR) technique and that based on measures of the Pelvic Incidence (PI) and Spino-Sacral Angle (SSA). Both methods stress the fundamental relationship between the anatomical position and orientation of the sacrum within the pelvis and the spinal curves above. The aim of the current study was to assess the strengths and potential weaknesses of the PR technique. The PR technique uses measures based on a line (the PR), drawn between the hip axis and the posterior corner of the S1 endplate. The angle formed between the PR line and the sacral endplate, PRS1, is a developmental measure of sacropelvic morphology. Geometrically, PI and PRS1 are approximately complementary angles and both reflect reciprocal alterations in pelvic tilt (for PI) or angulation (for PRS1) and the slope of the S1 endplate. The angle formed between PR and T12, the PR-T12, reflects a combined measure of pelvic morphology and lumbar lordosis. It appears to be a useful measure, which provides a simple and rapid assessment of lumbopelvic sagittal balance, but only in the presence of a congruent thoracic curvature.
Materials and methods: After reviewing the literature, published measures made using the PR technique were compared to measures taken from a substantial patient population (479 adult patients).
Conclusions: Errors can occur using the PR technique if the PRT12 is viewed in isolation from the thoracic kyphosis. We found the ratio of the thoracic kyphosis to lumbar lordosis (T4-T12/T12-S1) to be a useful predictor of congruent sagittal alignment, which may alert the clinician to situations where use of the PR-T12 in isolation may be misleading.