Three sacral parameters (Fig. 10-14) are measured: pelvic incidence (PI), sacral slope (SS), and pelvic tilt (PT). Pelvic incidence (PI) is a fixed anatomic parameter unique to the individual. Sacral slope (SS) and pelvic tilt (PT) are variable parameters. The relationship among the parameters determine the overall alignment of the sacropelvic unit according to the formula PI = PT 1 SS.
• Pelvic incidence (PI) is the angle defined by a line perpendicular to the sacral end plate line at its midpoint and a line connecting this point to the femoral rotational axis.
• Pelvic tilt (PT) is defined by a vertical reference line and a line from the midpoint of the sacral endplate to the femoral rotational axis.
• Sacral slope (SS) is the angle defined by a line along the sacral end plate line and a horizontal reference line.
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