Pelvic incidence variation among individuals: functional influence versus genetic determinism

Abstract

Pelvic incidence has become one of the most important sagittal parameters in spinal surgery. Despite its great importance, pelvic incidence can vary from 33° to 85° in the normal population. The reasons for this great variability in pelvic incidence remain unexplored. The objective of this article is to present some possible interpretations for the great variability in pelvic incidence under both normal and pathological conditions and to further understand the determinants of pelvic incidence from the perspective of the functional requirements for bipedalism and genetic backgrounds via a literature review. We postulate that both pelvic incidence and pelvic morphology may be genetically predetermined, and a great variability in pelvic incidence may already exist even before birth. This great variability may also serve as a further reminder that the sagittal profile, bipedal locomotion mode, and genetic background of every individual are unique and specific, and clinicians should avoid making universally applying broad generalizations of pelvic incidence. Although PI is an important parameter and there are many theories behind its variability, we still do not have clear mechanistic answers.

Change in Pelvic Incidence Associated With Sacroiliac Joint Dysfunction: A Case Report

“…The schematic diagram in Figure 2 depicts geometries for parameter measurements commonly utilized in clinical settings. PI is defined as the angle subtended by a line perpendicular to the midpoint of the sacral endplate and a line from this point to the axis of the femoral head [ 21 ]. In geometry, “incidence” refers to a binary relation between different types of object in space [ 4 ].…”

“…PI = SS + PT. As the sacrum is the entire weight of the spine to rest on, PI is the angle correlating with lumbar lordosis through SS and consequent spine degeneration [ 21 , 24 ]. Although PI has been considered as a static parameter in health individuals, it can be seen that when the sacral inclination changes in destabilized conditions, PI changes [ 4 ].…”

Imaging Methodology for Hip Preservation: Techniques, Parameters, and Thresholds

“…Spinopelvic parameters (SPPs) can be measured using lateral lumbosacral radiographs, 135,136 pelvic CT images, 137 and pelvic MRI. 53,138 Very few studies have addressed differences between measurements on distinct imaging modalities; Moon et al 139 found an increase in SS and a decrease in PT and PI (SS increased by 3.5 degrees, PT decreased by 6.7 degrees, and PI decreased by 3.2 degrees) when comparing XR and CT measurements, which might be associated with positional and methodological changes. Variability in standing to sitting position was described for PT, 140 and difference in measurements pertaining to XR (standing) and CT (decubitus) modalities were studied for all spinopelvic parameters.…”

Spinopelvic alignment predicts disc calcification, displacement, and Modic changes: Evidence of an evolutionary etiology for clinically‐relevant spinal phenotypes

“…PI is a “fixed” parameter, akin to one's genetic make‐up, at skeletal maturity and may vary between individuals . PI is essentially an individual's genetic blue‐print.…”

“…The three main defining spinopelvic parameters are pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS) (Figure 1). 18 PI is a "fixed" parameter, akin to one's genetic make-up, at skeletal maturity 19 and may vary between individuals. 14…”