Introduction & aim: Pelvic morphology in the sagittal plane is usually described by the pelvic incidence (PI), pelvic tilt, and sacral slope. However, the ischium is not included in this assessment. Yet, the position and orientation of the ischium relative to the SI-joint is essential for enabling the typical human fully upright way of ambulation with fully extended hips and knees, carrying the weight of the upper body constantly positioned above the pelvis. This provided an evolutionary advantage, and is believed to be the decisive step forward in human evolution. This so called “pelvic lordosis (PL)” has never been quantified in the normal asymptomatic population. In addition, the significance of the PL in relation to the sagittal spinopelvic alignment is investigated by correlation analysis between the PL and the PI.

Materials and methods: PL was defined as the angle between the axis of the ischium, and the line connecting the midpoint of the sacral endplate to the hip axis when the pelvis is projected in the sagittal plane. Using in-house developed software, the PL and PI were measured semi-automatically from three-dimensional CT scan data of the pelvis in a total of 348 normal subjects.

Results: The mean PL was 25° (range, 9-46) with an SD of 6.7°. No statistical difference was observed between the genders. A weak but significant correlation between the PL and age was only found in males (r=0.14, P=0.02) but not in females. A strong correlation was observed between the PL and PI in both genders (r=0.51, P=0.00).

Discussion & conclusion: A relatively wide range of values for the PL was observed, illustrating a relatively wide natural variation in the normal population. The PL was strongly positively correlated with the PI. A well-developed PL may increase the pelvis's ability to retroflex, and thus might be an important factor in maintaining congruent spinal balance. Therefore the role of the PL in spinal pathology warrants further investigation.