The femoro-sacral posterior angle: an anatomical sagittal pelvic parameter usable with dome-shaped sacrum

Abstract

The sagittal pelvic morphology modulates the individual alignment of the spine. Anatomical angular parameters were described as follows: the "Pelvic Incidence" (PI) and the Jackson's angle "Pelvic Lordosis" (PR-S1). Significant chains of relationships were expressed connecting these angles with pelvic and spinal positional parameters. This allows an individual assessment of the harmony of the sagittal spinal balance. But in case of spondylolysis with high-grade listhesis, the upper plate of the sacrum shows a dome-shaped deformity. The previous anatomical parameters are therefore imprecise. Indeed, the anterior part of the sacrum being inaccurate, an exact assessment of these angles becomes impossible. Therefore, we propose a new angular parameter named "Femoro-Sacral Posterior Angle" (FSPA): the angle between the posterior wall of the first sacral vertebra, always well definite, and the line connecting the posterior part of the sacral plate to the femoral axis. The validation of this parameter was performed and compared with the classical published parameters. It showed good inter-observer reliability, even with dome-shaped sacral plate. In spite of lower correlation with the positional parameters than those observed with PI or PR-S1, the FSPA appeared to be reliable and precise for an exact evaluation of the sagittal spino-pelvic balance is case of spondylo-listhesis with dome-shaped sacral endplate.

Fig. 1

Dome-shaped sacrum. The inaccurate visualization of the anterior part of the superior plate of S1 does not allow an exact appraisal of its orientation. The measures of the parameter using this plate are imprecise (“pelvic incidence” varying from 70° to 86°)

Fig. 2

The studied parameters. The parameters described by Duval–Beaupère: pelvic incidence (PI), sacral slope (SS), pelvic tilting (PT), complementary angle of the pelvic incidence, described by During and Itoi (DA), lumbar lordosis (LL). The parameters described by Jackson: pelvic lordosis or Jackson’s angle (PR-S1), pelvic angle (PA), sacral inclination (SI), sacral table angle of S1 (STA), femoro-sacral posterior angle (FSPA)