Intraoperative 3D reconstruction of the PMMA plug for computer-assisted revision of total hip arthroplasty based on 2D X-ray images

Aim: Computer-assisted cement removal out of the femoral medullary canal requires the exact definition of the plug's shape within a 3D coordination system. Aiming at a sufficiently precise reconstruction based on segmentation of just a few 2D X-ray images a special mathematic procedure is needed for automatic surface interpolation. The geometric specification of the resulting virtual model should take into account the characteristic geometry of the navigated tools in order to achieve the best possible removal. Method: Studies were performed on anatomic specimens of entire human femora, that underwent cemented THA before being cut every 5 mm. The cross-sections were scanned at the high resolution of 600dpi. Segmentation of the cement was performed with the help of a virtual deformable template and was both used for simulation of X-ray projections from various points of view and for validation of the reconstructed 3D model. By this means systematic errors such as those possible during X-ray acquisition, tracking or segmentation could be avoided and the precision of the procedure could be measured exclusively. Results: With increasing number of X-rays the distance from the reconstructed 3D model to the original could continuously be reduced. Using only two x-rays a maximum error was measured with 6.5 mm, wheras 5 pictures taken from different angles showed to be enough to ensure an error below I mm in the distal part of the femur. By the use of 6 or more pictures no significant improvement could be attained. Conclusion: The innovative procedure is essential for future 2D image-based fluoroscopic navigation of PMMA removal and bears the options of computer-controlled and robotic material working, respectively.