  The anatomical axis of long bones is an important reference line for guiding
fracture reduction and assisting in the correct placement of guide pins,
screws, and implants in orthopedics and trauma surgery. This study investigates
an automatic approach for detection of such axes on X-ray images based on the
segmentation contour of the bone. For this purpose, we use the medically
established two-line method and translate it into a learning-based approach.
The proposed method is evaluated on 38 clinical test images of the femoral and
tibial bone and achieves a median angulation error of 0.19{\deg} and 0.33{\deg}
respectively. An inter-rater study with three trauma surgery experts confirms
reliability of the method and recommends further clinical application.
