Pelvic index: A new pelvic parameter for assessing sagittal spinal alignment

Justice U Achonu; Kenny Ling; Rohit Bhan; Alexander Garcia; David E Komatsu; Nicholas A Pallotta

doi:10.1016/j.xnsj.2023.100274

Pelvic index: A new pelvic parameter for assessing sagittal spinal alignment

N Am Spine Soc J. 2023 Sep 7:16:100274. doi: 10.1016/j.xnsj.2023.100274. eCollection 2023 Dec.

Authors

Justice U Achonu¹, Kenny Ling¹, Rohit Bhan¹, Alexander Garcia¹, David E Komatsu¹, Nicholas A Pallotta²

Affiliations

¹ Department of Orthopaedics and Rehabilitation, Stony Brook University, HSC-T18, Room 085, Stony Brook, NY 11794-8181, United States.
² Department of Orthopedic Surgery, Washington University School of Medicine, Campus Box 8233660, South Euclid Avenue, Saint Louis, MO 63110, United States.

Abstract

Background: The incidence of correctional surgery for adult spinal deformity (ASD) has increased significantly over the past 2 decades. Pelvic incidence, an angular measurement, is the gold standard pelvic parameter and is used to classify spinal shapes into Roussouly types. Current literature states that restoration of the spine to its original Roussouly classification optimizes outcomes. We propose a new pelvic parameter, pelvic index, as a length measurement to complement pelvic incidence in more accurately characterizing Roussouly types.

Methods: This study is a retrospective evaluation of sagittal spinal radiographs of 208 patients who were assessed by a single fellowship trained orthopedic spine surgeon between January and December 2020. Measurements included pelvic incidence, sacroacetabular distance, and L5 vertebral height. Pelvic index was calculated as the ratio of sacroacetabular distance to L5 height. Each spine was also classified into one of the Roussouly types: 1, 2, 3 anteverted pelvis (AP), 3, or 4. The 2 pelvic parameters were compared between groups to assess their ability to differentiate between Roussouly types.

Results: Of the 208 patients included, 103 (49.5%) were female and 105 (50.5%) were male. The mean pelvic incidence was 54.9 ± 12.3° and the mean pelvic index was 3.99 ± 0.38. The difference in mean pelvic index was statistically significant between types 1 and 2 (0.15; p=.046) and between types 1 and 3 AP (0.19; p=.029). It was not statistically significant between types 3 and 4 (0.05; p=.251). However, in terms of pelvic incidence, the mean difference was statistically significant only between types 3 and 4 (10.4; p<.001).

Conclusions: Pelvic index is the ratio of the sacroacetabular distance to the height of the L5 vertebra. In conjunction with pelvic incidence, pelvic index can help to distinguish between Roussouly types 1 and 2 and between types 1 and 3 AP, the low-pelvic incidence types.

Keywords: Adult spinal deformity; Pelvic incidence; Pelvic index; Pelvic parameter; Reconstructive surgery; Roussouly; Scoliosis.