Basic Study for 3D Kinematic Measurement of Patella from Single-Plane Fluoroscopic Image Using Intensity-Based 2D/3D Registration

The 3D measurement of dynamic knee kinematics under in vivo conditions is highly valuable for understanding the effects of joint diseases, dysfunction and for evaluating the outcome of surgical procedures. For artificial knee implants, to achieve 3D measurement of the dynamic kinematics, 2D/3D registration techniques which use X-ray fluoroscopic images and computer-aided design model of the implants have been applied to clinical cases. These fluoroscopic techniques have also been applied for motion measurement in joints without implants in recent years, where 3D bone models created from CT or MRI images are utilized. In previous studies, however, the pose estimation accuracy for patella was not sufficient for analyzing 3D knee kinematics, particularly out-of-plane rotation error was relatively large because of small shape and poor geometrical feature of patella. Therefore, this study presents a method to determine 3D kinematics of patella using single-plane fluoroscopic image. The 3D pose of patella is estimated using an intensity-based 2D/3D registration technique, which uses a digitally reconstructed radiography (DRR) image created from 3D bone volume model. The 3D bone volume model for patella was created using CT scan data from a single subject. The 3D pose of the patella model is estimating by maximizing similarity measures between the DRR and fluoroscopic images iteratively with an optimization technique. In order to validate the pose estimation accuracy of patella including femur and tibia/fibura using the intensity-based 2D/3D registration, computer simulation test was performed. A set of synthetic silhouette images was created for each knee model in known typical orientations, and the test was carried out using three similarity measure methods. The result of computer simulation test showed that the root mean square errors were around 1.0 mm, 1.0° except for out-of-plane translation, and the reliability and feasibility of present method was demonstrated.