Introduction: Analyses of the 3-D kinematics of TKA in vivo using the x-ray image matching techniques have been widely reported. However, the accuracy of those techniques has seemed not to be discussed enough. To demonstrate more accurate technique for those analyses, we developed the new calibration flame to detect the geometry of the x-ray source and more accurate image matching methods.

Materials and methods: A calibration flame was composed of four ball bearing markers. First, the optimal distributions of those ball markers were determined by computer simulations, and then, using the high-resolutional computed radiography (CR) of a metallic ball taken with the obtained optimal calibration flame, the resolving power of three degrees of freedom (DOF) translations were calculated. Next, the computer-synthesized projected images of the femoral component of TKA were calculated using the estimated x-ray source geometry and experimentally measured geometric data of the prosthesis when the full six DOF poses of the prosthesis were calculated. Matching the computer-synthesized images with the extracted and binalized 2-D CR images of the prosthesis was done automatically using computer in order to minimize the exclusive OR (XOR).

Results: The geometry of the x-ray source was estimated with accuracy of below 0.5 mm in computer simulations. The CR images of the prosthesis were matched with the computer-synthesized images until the XOR reached under one pixel and then, the accuracy of below 1.0 mm translations and 1.0 degrees rotation were recognized for the resolving power of six DOF poses of the prosthesis.

Discussion: The more accurate measurement of full six DOF poses is indispensable to estimate not only the 3-D kinematics but also the contact stresses or predicted polyethylene wear on TKA in vivo. The new calibration flame and the image matching technique we developed appear to be effective for analyses of TKA in vivo.