Review Question - QID 3592

Pelvic Incidence (Angle X) = Pelvic Tilt (Angle Z) + Sacral Slope (Angle Y) as shown in Answer 1.

Of the spinopelvic radiographic measurement, Pelvic Incidence (PI) correlates strongest with isthmic spondylolisthesis grade. PI is the angle subtended by an initial line from the center of the femoral head to the midpoint of the sacral endplate and a second line perpendicular to the center of the sacral endplate. PI is relatively constant during childhood (~47 degrees), increases during adolescence, and remains constant in adulthood (~57 degrees). Unlike many other parameters of pelvic morphology, PI is not affected by changes in posture. A low PI indicates low shear forces at the lumbosacral junction and less lumbar lordosis.

Hanson et al reviewed 40 patients with spondylolisthesis, comparing sagittal alignment, sacral inclination, slip angle and Pelvic Incidence. They concluded that PI was significantly higher in patents with spondylolisthesis and correlated significantly with Meyerding grade.

Labelle et al reviewed the radiographic measurements in L5/S1 spondylolisthesis. They determined that pelvic shape, best defined by PI, determines the position of the sacral endplate which in turn is increased in patients with spondylolisthesis.

Illustration A shows the Pelvic Incidence on an illustration.

Incorrect Answers:
Answer 2: Angle X - Angle Y = Pelvic Tilt (Angle Z)
Answer 3: Angle X - Angle Z = Sacral Slope (Angle Y)
Answer 4: Angle Z = Pelvic Tilt
Answer 5: Angle Y = Sacral Slope