Sacral slope, pelvic tilt and pelvic incidence are pelvic parameters used in radiological assessment of the lumbosacral spine.
With evolution came upright posture which brought the required changes in the spine and pelvis.
The pelvis became broadened and vertical. There were changes in spinal curvature and muscular structures supporting the spine were substantially modified.
The pelvis forms the bond between the trunk and the lower limbs. Through femoral heads, pelvis transfers load to the lower limbs.
The sacral plateau is the point of transfer of load from the trunk to the pelvis.
Different Pelvic Parameters
Sagittal pelvic thickness (SPT)
The line joining center of the femoral head to center of the sacral plateau.
The angle formed by the tangent to the sacral slope and sagittal pelvic thickness line.
Pelvic Incidence Angle
A perpendicular is drawn to the tangent to the sacral slope. The angle formed by this perpendicular with a line depicting sagittal pelvic thickness is called the incidence angle.
The mean value of this angle of incidence is 55° ± 10 degrees.
The incidence angle has a direct bearing on the balance of the spine, which rests on the sacral plateau.
The sacral slope is the value of the angle between the superior plate of S1 and a horizontal line.
- Measurement of Sacral Slope
Thus a vertical sacrum is described by a low value, a horizontal sacrum measures a high value.
Sum of sacral slope and pelvic tilt gives pelvic incidence.
The pelvic tilt is the angle that pelvis forms with vertical. It is measured as the angle between the sagittal pelvic thickness line and a vertical line through femoral head.
Pelvic tilt indicates the spatial orientation of the pelvis, which changes according to the position.
The mean pelvic tilt angle, in the standing position, is 13 ± 6 degrees.
With an increase in pelvic tilt, the sacral plateau becomes increasingly horizontal, while the body of the sacrum becomes vertical.
Significance of the Measurements
Pelvic incidence determines the relative position of the sacral plate in relation to the femoral heads. The average value is around 52 degrees and the range is 35-85 degrees.
A low incidence signifies a relatively narrow and vertical pelvis. A pelvis with high angle of incidence is a large horizontal pelvis.
The position of the pelvis in turn affects the curvature of lumbar spine.
Greater the incidence, the greater the sacral slope and the higher the degree of lumbar lordosis.
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