Pelvic incidence: a fixed value or can you change it?

Area of Science:

• Spine surgery and biomechanics
• Radiographic assessment of spinal parameters
• Pelvic and spinal alignment

Background:

• Spinal surgeons focus on pelvic parameters like pelvic tilt, sacral slope, and pelvic incidence (PI) for adult spinal deformity.
• Pelvic incidence (PI) is traditionally considered a fixed measurement, unlike pelvic tilt and sacral slope.
• The assumption of PI's fixed nature has not been validated in healthy, asymptomatic populations.

Purpose of the Study:

• To investigate variations in pelvic incidence (PI) across three distinct pelvic positions.
• To compare PI measurements in resting, maximal anterior pelvic rotation, and maximal posterior pelvic rotation.
• To evaluate the dynamic nature of PI in functional activities.

Main Methods:

• Prospective study involving 50 healthy, asymptomatic individuals.
• Radiographic measurements of PI were taken in resting, maximal anterior, and maximal posterior pelvic tilt positions.
• Measurements were independently assessed by an orthopedic spine surgeon and a musculoskeletal radiologist.

Main Results:

• Pelvic incidence (PI) values changed in 88% of subjects during maximal anterior pelvic rotation (mean change 2.9°).
• PI values changed in 80% of subjects during maximal posterior pelvic rotation (mean change 2.82°).
• Significant changes (≥3°) in PI were observed in 46% and 54% of subjects for anterior and posterior rotation, respectively.

Conclusions:

• Pelvic incidence (PI) is not a fixed parameter in a significant percentage of healthy individuals.
• The study suggests potential functional motion at the sacroiliac joint and challenges the assumption of PI's immutability.
• Findings may have implications for surgical planning and understanding posture's effect on PI.