Utility of the L1 pelvic angle (L1PA) for assessment of sagittal alignment of the lumbar spine following short segment fusion: Comparison to segmental measures.

Objective: The Lumbar Pelvic Angle (L1PA) was introduced as a convenient intraoperative and postoperative measure of sagittal alignment. However, the utility of L1PA for short segment lumbar fusions remains incompletely understood. This study investigates the relationship between L1PA and segmental parameters of spinal alignment for short segment lumbar fusions. Methods: This retrospective analysis was conducted on adult patients with degenerative conditions undergoing primary spinal fusion surgery between L4 and S1. Patients with <5° change in L1PA from pre- to postoperative measurements were stratified into 2 groups: those (1) achieving >35° of regional lordosis at L4-S1 postoperatively and (2) those with >5% change in Lumbar Distribution Index (LDI). Spinopelvic parameters were compared between preoperative and postoperative measurements. Results: The study cohort included 539 patients, of which 360 (66.8%) had an L1PA change less than <5° pre- to postoperatively. Patients in both Group 1 and 2 had significant increases in the L5-S1 segmental angle (both p<.001) without a significant change at the L4-L5 segment. In Group 2, there was a significant increase in PI-LL mismatch (p<.001). In both groups there was a significant increase in L4-S1 lordosis, a relaxation in L1-L4 lordosis (both p<.001), and no significant change in total LL. Conclusions: This investigation assessed L1PA in over 500 patients undergoing lumbar fusion between L4 and S1 and found that despite value to the measure, important changes to spinal shape and segmental alignment may occur even in the face of Δ L1PA <5°. As such, L1PA serves as a potentially useful adjunct to clinical assessment and radiographic measurement in lumbar degenerative fusion but should not serve as a stand-alone measurement.