## 1 Introduction  
Accurate and noninvasive measurement of the three-dimensional (3D) kinematics of human limbs is fundamental for exploring their biomechanical characteristics [1]. With quantified data of rigid-body kinematics and cartilage contact arthrokinematics of the tibofemoral joint, both subject- and task-specific biomechanics and pathomechanics can be explored further [2; 3], leading to a more objective and precise evaluation of functional performance in normally healthy [4; 5; 6], injured [7], and treated knee joints [8; 9]. Direct measurement of skeletal motion has also been increasingly employed for the assessment of other motion capture instruments affected by soft tissue artifacts [10; 11; 12]. To gain insight into the internal structures of the joint, medical imaging facilities are typically employed to acquire cross-sectional slices or projection images of the target organ. Among these, X-ray fluoroscopy [13], four-dimensional computed tomography (4D CT) [14], and dynamic magnetic resonance (MR) image technologies [15; 16] enable the recording of continuous two-dimensional (2D) or 3D images of joint motion at a limited temporal or spatial resolution. The registration of volumetric bone models obtained from high-resolution CT or MR images to a series of continuous images of dynamic joint movement, which is commonly called model-based tracking, can take advantage of the functionalities of different image modalities, yielding continuous six-degree-of-freedom (6-dof) data of skeletal and joint motion. Model-based tracking can be conducted fully based on MR images by registering MR-derived bone models to each frame of a relatively lower-resolution set of dynamic MR image slices [17; 18; 19] or volumetric images [20]. However, closed-bore MR image scanners restrict subjects from performing limited motions and are also not suitable for patients with metal prostheses. The application of standard X-ray fluoroscopy or newly developed mobile fluoroscopic imaging systems enable more dynamic kinematic measurements during functional tasks [21; 22]. By numerically registering the subject-specific model of the prosthesis or bones to the single-plane fluoroscopic image, model-based tracking routines reproduce 6-dof kinematics of natural knee [23; 24] and replacement joints [25; 26]. While experimental evaluations showed that the methods enabled the determination of accurate planar translations and rotations, the out-of-plane motion components were found to be substantially less accurate [27; 28]. Introducing an additional synchronous X-ray unit yields a framework of stereo X-ray imaging [29], which has been widely demonstrated to be useful for determining 3D tibofemoral kinematics in submillimeter and subdegree accuracy with biplane 3D to 2D image registration [30; 31; 32]. Synchronous biplane X-ray imaging of dynamic knee motion is currently achieved by applying custom-made high-speed stereo radiography [33; 34] or integrating two C-arm fluoroscopies [35; 36], but it is infeasible in clinical biplane X-ray imaging systems [37]. To minimize cross-scatter contamination [38], the clinical imaging system is normally operated with an asynchronous X-ray pulse and acquisition scheme between two X-ray units, generating interleaved biplane fluoroscopic images with a constant temporal offset. The interleaved images have adverse effects on the biplane model-based tracking of the joint motion, however, leading to erroneous skeletal kinematics estimation [39; 40]. To address the limitations of the interleaved biplane images, new model-based tracking methods were introduced with strategies related to kinematic interpolation [39; 40]. The method termed motion component partition and interpolation (MCPI) [40] was assessed for its accuracy in determining tibofemoral kinematics in our previous study. While the MCPI improved translation accuracy over single-plane model-based tracking, rotational errors were not significantly improved [40].