Fast marker based C-Arm pose estimation.

To estimate the pose of a C-Arm during interventions therapy we have developed a small sized X-Ray Target including a special set of beads with known locations in 3D space. Since the patient needs to remain in the X-Ray path for all feasible poses of the C-Arm during the intervention, we cannot construct a single marker which is entirely visible in all images. Therefore finding 2D-3D point correspondences is a non-trivial task. The marker pattern has to be chosen in a way such that its projection onto the image plane is unique in a minimal-sized window for all relevant poses of the C-Arm. We use a two dimensional adaption of a linear feedback shift register (LFSR) to generate a two-dimensional pattern with unique sub-patterns in a certain window range. Thereby uniqueness is not achieved by placing unique 2D sub patterns side by side but by the code property itself. The code is designed in a way that any sub window of a minimal size guarantees uniqueness and that even occlusions from medical instruments can be handled. Experiments showed that we were able to estimate the C-Arm's pose from a single image within one second with a precision below one millimeter and one degree.

A stereophotographic study of ankle joint contact area.

The purpose of this study was to measure the ankle joint contact area under physiological load magnitudes using a stereophotography technique that allows accurate analysis of the entire joint surface without disrupting the joint during loading. Ten cadaveric foot and ankle specimens were loaded to 1000 N in neutral, and 20 degrees dorsiflexion, supination, pronation, and plantarflexion. Photo targets rigidly fixed to each of the bones were imaged in the loaded joint position using a high-resolution stereophotography system. After testing, each ankle was disarticulated and the joint surfaces imaged relative to the photo targets. The photo targets were then used to spatially register the joint surfaces into the loaded joint position; the overlap of the surfaces was used to determine the joint contact area. The mean talo-tibia contact area was greatest in dorsiflexion 7.34 +/- 1.69 cm(2) and was significantly larger than in plantar flexion (p < 0.05), which showed the smallest joint contact area 4.39 +/- 1.41 cm(2). Considering talo-fibula, the maximum contact area was measured in dorsiflexion, 2.02 +/- 0.78 cm(2), and the minimum contact area occurred in pronation, 0.77 +/- 0.49 cm(2), respectively (p < 0.05). The reported stereophotography technique allows measurement of the joint contact area without disrupting the joint during loading. The contact area is larger than previously reported, as the entire joint surface was analyzed. Joint contact extends over both the talar dome and the talar shoulders where osteochondritis dissecans lesions commonly occur.