## 4 Discussion  
A new model-based tracking scheme was developed to accommodate interleaved biplane fluoroscopy for measuring tibofemoral kinematics. It was primarily accomplished by synthesizing an intermediate image with a frame interpolation approach to integrate asynchronous information provided by the clinical biplane fluoroscopy to yield a pseudosynchronous pair of biplane images. In previously reported techniques with the same objective, the approaches have normally relied on the data generated from the interpolation of 3D skeletal kinematics [39; 40], which must be obtained from an additional image registration procedure based on a single-plane fluoroscopic image. This therefore led to two image analysis steps for an entire kinematics reconstruction process. Successful single-plane image registration must be ensured to generate precise kinematic interpolation, which subsequently affects the accuracy of the measured bone poses. In contrast, the present approach requires only a single image analysis step. In combination with template registration for initial pose estimation, fully automated image registration for a complete image set was achieved. Directly comparing the present accuracy values to those reported with different biplane fluoroscopy-based model-based tracking approaches was difficult, as the imaging system used, experimental conditions of the motion tasks, and the reported error metrics were different (Table 2). Nonetheless, a comparison of the primary error metrics calculated for the determination of tibofemoral kinematics, such as bias, precision, and Rms error (Table 2), was made. It appeared that the present MIBFT with a clinical biplane X-ray imaging system was capable of producing submillimeter and subdegree accuracy in terms of the compared error metrics and was well within the values generated by techniques based on custom-made imaging systems, which supports the applicability of the clinical asynchronous X-ray imaging system for precise functional examinations of the knee. On the same dataset, we further implemented and evaluated a single-plane MBT [24] and the relevant MCPI algorithm specifically devised for clinical asynchronous biplane fluoroscopy [40]. The present MIBFT was shown to not only be capable of addressing the out-of-plane translation problem in single-plane MBT and reducing errors in out-of-plane rotation, as shown in MCPI [40], but further reduce errors in other relatively accurate motion components (Figure 9). Among the 6-dof reconstructed tibofemoral kinematics, the internal/external rotation was less accurate than the other motion components (Table 2) and was the only one that showed an LoA beyond \(1^{\circ}\) (Figure 8). Similar results were also found in previous studies [31; 32] (Table 2) conducted on the synchronous biplane imaging system. This is reasonable, as the nature of the cylindrical shape of both the femur and tibia leads to a less sensitive, and thus less accurate, model rotation around the long axis during image registration [32]. On the other hand, the specimen leg was hung on a frame in the experimental setup used in the present study, which caused the long axes of the bones to be most near-parallel to the y-axis of the fluoroscopic coordinate system (Figure 1C), even during motion tasks. The less accurate y-axis rotation of the bone (Table 1), as a result of being an out-of-plane motion component from the perspective of both detectors [40], would also propagate the errors to the long-axis rotation of the bones and the description of the tibofemoral internal/external rotation. Template registration on video fluoroscopy has been applied for the analysis of the planar displacement of vertebrae during motion in three-parameter space [13; 61; 62; 63]. While the pose parameters may be accurately reproduced through a semirigid template [61] or with an exhaustive search strategy to ensure a global optimum [13; 63], increased computational effort and run time are inevitable.