Fig 1.
Full-body model and anatomical parameters.
The AnyBody full-body musculoskeletal model in standing position (A) and sagittal views depicting sagittal vertical axis (SVA) (B), Roussouly lumbar typology (RT) (C), sacral slope (SS), and pelvic incidence (PI) (D). Muscles, arms, and pelvis are not shown in the sagittal views.
Fig 2.
Modeling of the lumbar typologies.
Modeling of the alignment of the lumbar vertebrae (from L1 to L5) and intervertebral joints (depicted as red spheres) in the four Roussouly types (RT) [2]. Curve apex and sacral slope (SS) identifying the different RTs are reported.
Table 1.
Anatomical parameters and reference rotations of the lumbar vertebrae.
Fig 3.
Sacrum segment orientation, intersegmental load, and PI changes.
Panel (A): default orientation of the sacrum segment and mesh surfaces of the sacrum and lower lumbar vertebrae. The sacrum local reference system (x',y') with reference nodes is given in yellow and the global reference system (x,y) in black. The sacral slope (SS) is obtained by manually measuring the endplate inclination from the view. Panel (B): example of predicted intersegmental loads at levels L4L5 and L5S1 (FL4L5 and FL5S1), and corresponding axial and anterior directions. The configuration is RT1, with SS 25°, PI 40°, and SVAmed. Panel (C): changes in the PI angles in the assigned RT and SS.
Fig 4.
Box (A): axial compression force at level L4L5 (), computed in relation to changes in sacral slope (SS) and pelvic incidence (PI) in RT1. The results for the three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and backward and frontward imbalanced alignments (SVAback and SVAfront) are presented as green, blue, and red surfaces, respectively. The two boxes (B and C) on the right present the results in the SS and PI perspectives, respectively.
Fig 5.
Axial compression force (, upper row) and anterior shear (
, lower row) computed in relation to changes in sacral slope (SS) and pelvic incidence (PI) in the four lumbar typologies (RT1, RT2, RT3, RT4). The results for the three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and the backward and frontward imbalanced alignments (SVAback and SVAfront) are presented as green, blue, and red surfaces, respectively.
Fig 6.
Axial compression force (, upper row) and anterior shear (
, lower row) computed in relation to changes in sacral slope (SS) and pelvic incidence (PI) in the four lumbar typologies (RT1, RT2, RT3, RT4). The results for the three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and the backward and frontward imbalanced alignments (SVAback and SVAfront) are presented as green, blue, and red surfaces, respectively.
Table 2.
Predicted intersegmental forces.
Table 3.
Predicted muscle forces.
Fig 7.
Axial compression force (, upper plot) and anterior shear (
, lower plot) at the L4L5 level, computed at minimum and maximum sacral slope (SSmin and SSmax) for the four lumbar typologies (RT1, RT2, RT3, RT4). SSmin and SSmax were, respectively, 25° and 35° for both RT1 and RT2, 35° and 45° for RT3, and 45° and 55° for RT4 (Table 1). The results correspond to the central pelvic incidence (PI) values for the RT (40° in RT1 and RT2, 50° in RT3, 60° in RT4, see Table 1). The three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and the backward and frontward imbalanced alignments (SVAback and SVAfront) are presented as green, blue, and red bars, respectively. The lumbar lordosis (LL) is reported as well.
Fig 8.
Axial compression force (, upper plot) and anterior shear (
, lower plot) at the L5S1 level computed at the minimum and maximum sacral slope values (SSmin and SSmax) in the four lumbar typologies (RT1, RT2, RT3, RT4). SSmin and SSmax are, respectively, 25° and 35° for both RT1 and RT2, 35° and 45° for RT3, 45° and 55° for RT4 (Table 1). The results correspond to the central pelvic incidence (PI) values for the RT (40° in RT1 and RT2, 50° in RT3, 60° in RT4, see Table 1). The three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and the backward and frontward imbalanced alignments (SVAback and SVAfront) are presented as green, blue, and red bars, respectively. The lumbar lordosis (LL) is reported as well.
Fig 9.
Muscle forces in relation to SS.
The muscle forces of the multifidus (FMF, upper plot), erector spinae (FES, central plot), and rectus abdominis (FRA, lower plot), computed at the minimum and maximum sacral slope values (SSmin and SSmax) in the four lumbar typologies (RT1, RT2, RT3, RT4). SSmin and SSmax are, respectively, 25° and 35° for both RT1 and RT2, 35° and 45° for RT3, and 45° and 55° for RT4 (Table 1). The results are taken in correspondence to the central pelvic incidence (PI) values for each RT (40° in RT1 and RT2, 50° in RT3, 60° in RT4, Table 1). The three sagittal vertical axis (SVA) conditions interpreting the balanced global alignment (SVAmed) and the backward and frontward imbalanced alignment (SVAback and SVAfront), are presented as green, blue and red, bars, respectively. The lumbar lordosis (LL) is reported as well.