Sacral SCP for P-L or P-R is located:

• A. Medial PSIS
• B. Inferior PSIS
• C. Acetabular ridge
• D. Halfway between PSIS and S2

Answer: (D)

The point to place the sacral SCP for P-L or P-R sits at the midpoint between the posterior superior iliac spine (PSIS) and the S2 vertebra. This location sits on the posterior sacral surface near the base of the sacrum, near the axis around which the sacrum tilts (nutation/counternutation). Placing the contact here provides a stable, central line of drive that effectively engages the sacral base and produces the intended rotational correction without misdirecting force.

Choosing a point at the medial or inferior PSIS would move the contact away from the sacral base and toward surrounding structures, reducing effectiveness and altering the vector of thrust. The acetabular ridge is part of the pelvic bone near the hip socket, not on the sacrum, so it isn’t aligned with the sacral motion axis and wouldn’t deliver the desired correction. The midpoint between PSIS and S2 uniquely aligns with the sacral base and its motion axis, making it the best site for this adjustment.