Femoral derotation osteotomy (FDO) is an established surgical procedure to recover physiological gait of patients with torsional bony malalignment. Yet in many cases, therapeutic outcome after functional rehabilitation is well behind expectations. Our investigations have revealed methodical problems of the procedure leading to significant measurement bias and poorly reproducible surgical results. Targeting to improve outcome quality, we developed a prototype system to quantify anatomically relevant parameters in FDO using a bone phantom. Using electromagnetic tracking and geometric reconstruction, we achieved accurate and precise measurement of anatomically relevant parameters. Considerations for intraoperative application promise a successful implementation into a CAS system.