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Registration of pre-operative CT datasets to intra-operative 3D freehand ultrasound has been of high interest for computer assisted orthopedic surgery. Feature-based registration relies on an accurate detection of the bone surface in the B-mode ultrasound images. In this work we present a fully automatic bone detection approach for US. The pre-operative CT is utilized to create a patient-specific bone model for our joint detection-registration framework. The model provides a geometric constraint for accurate and robust detection. Simultaneously to the detection, our method yields a close estimate of the rigid transformation from US to CT, which can be used as an initialization for further refinement through sophisticated intensity-/feature-based registration methods. We evaluated our approach on datasets of the human femur acquired in a cadaver study and demonstrate a mean bone detection error of below 0.4 mm.