Congruent Spinopelvic Alignment on Standing Lateral Radiographs of Adult Volunteers

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Abstract
Radiographs of 75 healthy volunteers were measured to decide parameters and ranges for “congruent” sagittal spinopelvic alignments using the pelvic radius technique. A subset of 30 of the volunteers subsequently had a second radiograph to assess for changes in the repeated measurements. Measurement of spinal alignment is important. Radiographic parameters for “congruent” spinopelvic balance over the hips and changes in sagittal spinal alignments over time have not been defined. Measurement techniques for spinal alignments and to quantitate pelvic morphology need to be standardized. The 75 volunteers (44 men/31 women, mean age 39 years, range, 20 to 63 years) had 36-inch standing lateral radiographs of the thoracolumbar spine and pelvis taken that included both hips. Thirty volunteers (19 men/11 women) had a second radiograph taken 5 to 6 years later. Radiographic measurements were made using the pelvic radius technique. This required locating a midpoint between the approximate centers of both femoral heads to establish a pelvic hip axis. A line between the hip axis and the posterior superior corner of S1 for the pelvic radius was drawn and measured for length. Angles were measured from the pelvic radius to tangents along the vertebral endplates on the 105 films with an electronic digital readout device. These angles included PR–S1 for pelvic morphology and PR–T12 for total lumbopelvic lordosis. A pelvic angle was measured from a vertical line through the hip axis to the pelvic radius. This angle gave the sagittal alignment for the pelvis over the hips. Longitudinal measurements between radiographs were compared for minimum and maximum change. Significant statistical correlations for the measurements were carefully studied to determine potentially important clinical relationships. In addition, thoracic kyphosis/lumbar lordosis ratios were assessed. The most constant measurement with the least change on the repeated radiographs was that for pelvic morphology (PR–S1 angle) followed by length of the pelvic radius, pelvic alignment over the hips (pelvic angle), and total lumbopelvic (PR–T12) and lumbosacral (T12–S1) lordosis. Other longitudinal measurements, including those for thoracic kyphosis and spinal balance by a plumbline, showed greater change. Measurements for pelvic morphology by the pelvic radius technique were correlative with standing total lumbosacral lordosis, regional lumbopelvic lordosis, pelvic alignment, pelvic radius length, and gender (P ≤ 0.006 for each). The correlations between total and regional lumbopelvic lordosis and pelvic alignment measurements were even higher(P < 0.0001). Of possible clinical importance was the finding that standard measurements for lordosis were dependent on individual pelvic morphology quantitated by the pelvic radius technique. In all of the sagittally balanced subjects studied, “congruent” spinopelvic alignment on all 105 standing lateral radiographs could be defined by four parameters using the pelvic radius technique: total lumbopelvic lordosis (PR–T12), incorporating complementary angles for lumbosacral lordosis (T12–S1), and pelvic morphology (PR–S1 angle) that summarily were always between −69° to −116° (±3°); centered pelvic alignment over the hips, as determined by the pelvic angle, that was always between −3° to −32° (±2°); compensated spinal balance, with a sagittal plumbline from the center of the T4 body always posterior to the hip axis as well as the center of the L4 vertebral body; and a concordant T4–T12 kyphosis/PR–T12 lordosis ratio that was always negative and between 0.15 to 0.75.