The study of rigid body dynamics in soft tissue is an essential part of orthopaedic imaging. Our focus is the foot and ankle structure, which consists of 28 bones surrounded by a variety of soft tissue such as 30 muscles, numerous ligaments, bursae and nerves. The importance of understanding the involved dynamics is evident by the frequent need to find functional joint replacements to fight diseases such as hindfoot arthritis. We study a simplified problem by constructing a phantom with two

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... es" submerged in silicone and a remotely controlled LEGO robot that moves one of the two bones. We perform motion registration by manipulating the bone segmented from an initial static scan and matching its digitally reconstructed radiographs to a sequence of scanned X-ray projections. The registration of the 3D volume to 2D projections requires the right combination of optimization and interpolation to achieve an optimal result. We test four different approaches with the help of the Flexible Algorithms for Image Registration (FAIR) toolbox.