In Computer Assisted Orthopaedic Surgery (CAOS), surgeons have to acquire some anatomical landmarks as
inputs to the system. To do so, they use manual pointers that are localized in the Operating Room (OR) space
using an infrared camera. When the needed landmark is not reachable through an opening, it is palpated directly
on skin and there is a loss of precision that can vary from several millimeters to centimeters depending on the
thickness of soft tissues. In this paper, we propose a new framework based on three main steps to register the
bone surface and extract automatically anatomical landmarks with an ultrasound probe. This framework is based
on an oriented gradient calculation, a simulated-compound and a contour closure using a graph representation.
The oriented gradient allows extracting a set of pixels that probably belong to the bone surface. The simulatedcompound
step allows using ultrasound images properties to define a set of small segments which may belong
to the bone surface, and the graph representation allows eliminating false positive detection among remaining
segments. The proposed method has been validated on a database of 230 ultrasound images of anterior femoral
condyles (on the knee). The average computation time is 0.11 sec per image, and average errors are: 0.54 mm
for the bone surface extraction, 0.31 mm for the condylar line, and 1.4 mm for the trochlea middle.