The Two Distinct Functional Lordoses of the Lumbar Spine

Retrospective radiographic study of a multicentric prospective database. This study aimed to analyze the relationships between lumbar lordosis (LL) proximal and distal arcs and spinopelvic parameters, and to determine how their distribution varies with pelvic incidence (PI). LL is a key component of sagittal spinal alignment, yet its definition and assessment remain debated. Recent evidence suggests LL is not a uniform curvature but rather a complex structure, with proximal and distal arcs that may exhibit distinct biomechanical roles. This study included 642 healthy volunteers (mean age: 37.6&#xb1;16.3) with full-body stereoradiographs, without spinal deformities. Maximum LL was assessed (LLmax), measured between the thoracolumbar inflection vertebra upper endplate and the sacral plateau. Correlations between proximal lordosis (LLprox, measured between the thoracolumbar inflection vertebra upper endplate and the lumbar apex upper endplate), distal lordosis (LLdist, measured between the lumbar apex upper endplate and the sacral plateau), and spinopelvic parameters were sought. Subjects were stratified into five PI groups to assess variation in lordosis distribution with PI in terms of angle and the number of vertebrae included in the curvature. LLdist strongly correlated with pelvic parameters, particularly sacral slope (SS, r =-0.83, P <0.001) and PI ( r =-0.54, P <0.001), but not with thoracic alignment. LLprox correlated with both pelvic and thoracic parameters, including SS ( r =-0.31, P <0.001) and thoracic kyphosis ( r =-0.35, P <0.001). As PI increased, the contribution of LLdist to LLmax rose from 55&#xb1;12% to 66&#xb1;8% ( P <0.001), while the number of vertebrae in the distal arc increased from 1.52&#xb1;0.50 to 2.32&#xb1;0.59 ( P <0.001). Consequently, LLprox's proportional contribution to LLmax decreased, despite a stable vertebral extent. LL comprises two functionally distinct arcs: a pelvic-driven distal arc and a proximal arc influenced by overlying alignment and pelvic parameters. As PI increases, the LL distal arc becomes preponderant. These findings advocate for a segmental approach in clinical assessment and surgical planning, emphasizing the need to consider both arcs independently. Level III.