3-DIMENSIONAL DETERMINATION OF FEMORAL-TIBIAL CONTACT POSITIONS UNDERIN-VIVO CONDITIONS USING FLUOROSCOPY

Objective. A method has been developed to accurately measure three-dim ensional (3-D) femoral-tibial contact positions of artificial knee imp lants in vivo from X-ray fluoroscopy images using interactive 3-D comp uter vision algorithms. Design. A computerized graphical (CAD) model o f an implant component is displayed as an overlay on the original X-ra y image. An image matching algorithm matches the silhouette of the imp lant component against a library of images, in order to estimate the p osition and orientation (pose) of the component. The operator further adjusts the pose of the graphical model to improve the accuracy of the match. Background. Previous methods for in vivo measurement of joint kinematics make only indirect measurements of joint kinematics, requir e invasive procedures such as markers or pins, or make simplifying ass umptions about imaging geometry which can reduce the accuracy of the r esulting measurements. Methods. Fluoroscopic videos are taken of impla nted knees in subjects performing weight-bearing motion. Images from t he videos are digitized and stored on a computer workstation. Using co mputerized model matching, the relative pose of the two knee implant c omponents can be determined in each image. The resulting information c an be used to determine where the two components are contacting, the a rea of the contact region, liftoff angle, and other kinematic data. Re sults, Accuracy tests done on simulated imagery and in vitro real imag ery show that the pose estimation method is accurate to less than 0.5 mm of error (RMS) for translations parallel to the image plane. Orient ation error is less than or equal to 0.35 degrees about any axis. Erro rs are larger for translations perpendicular to the image plane (up to 2.25 mm). In a clinical study, the method was used to measure in vivo contact points, and characterize the kinematic patterns of two differ ent knee implant designs. Conclusions. The ability to accurately measu re knee kinematics in vivo is critical for the understanding of the be havior of knee implant designs and the ultimate development of new, lo nger lasting implants.