# In-Vivo Quantification of Knee Deep-Flexion in Physiological Loading Condition trough Dynamic MRI
## 3 Results
The registration algorithm proposed here is general and, thus, extendable to other DMR scanners, reasonably resulting in a higher accuracy. The registration approach proved to be almost insensitive to the initial pose provided by the operator. In the few cases in which the algorithm did not converge on the reference motion, the results were evidently wrong and not physiological, thus allowing the easy identification of possible errors. Finally, the experimental procedure shows a good repeatability, allowing longitudinal investigations. The reconstructed knee kinematics agree well with previous measurements [19,20]. In particular, the tibia internal rotation and the femur roll back associated with flexion are easily observable for all the volunteers. The comparison between dynamic and static evaluation of knee kinematics shows differences smaller than what was reported in the  
Figure 8: **(a)** Comparison between knee kinematics reconstructed from three repetition of dynamic (blue) and static (black) MRI for one volunteer. The Abduction/adduction (AA), internal/external rotation (IE), anterior/posterior (AP), proximal/distal (PD), and medial/lateral (ML) translations are plotted. **(b)** Comparison among the kinematics reconstructed from DMRI without (blue) and with (red) perturbations. In red, the 1421 cases (97.6%) not affected by perturbation of the initial pose are plotted. literature [7]. It is worth noting that static scans were collected in weight-bearing conditions and that dynamic scans were recorded at slow speed, possibly reducing the differences, although part of them could be ascribed to the different measuring conditions. In general, however, it is possible to observe a reduction in the maximum flexion reached during dynamic measurements, which is possibly associated with adduction of the pelvis during static scans. It is worth noting that the maximum flexion value was dictated by the possible stroke of the hydraulic piston in the rig, which was kept above a safety value for this investigation. Future tests will extend the maximum achievable flexion. The clinical application of DMRI could provide complementary information to what is obtainable with traditional MRI. The latter provides very accurate yet static images of the joint structures; on the other side, DMRI allows for the observation of the interaction of the elements that participate in an articulation during its function, providing a new level of knowledge. For example, laxity tests could be performed in DMRI, making it possible to directly observe the load response of injured ligaments. In general, a quantification of the relative bone motion makes it possible to find the measure of quantities not directly observable in-vivo. For example, the amplitude and location of articular contact areas during knee flexion can be reconstructed from the tibo-femoral kinematics, providing data that may help to better understand the etiology and development of pathologies such as osteoarthritis. Aside from the direct clinical applications, this procedure for the in-vivo quantification of knee joint kinematics has very interesting biomechanical applications. Indeed, the possibility to measure the individual joint kinematics non-invasively and in-vivo will help in the definition and validation of patient-specific joint and musculoskeletal models [21; 22; 23; 24; 25]. The work has limitations. Acquisitions on the two DMRI planes used for 2D/3D registration were performed in a series. It is thus possible that the two acquired motions differ, introducing some errors in the reconstructed motion. The same kind of approximation is, however, done with traditional cine-MRI, where a cyclic motion is reconstructed from successive scans taken at different times in subsequent cycles. Only three knees were analyzed in this investigation. A wider study will establish the performance of the proposed procedure. Future work will test the presented algorithm on data from a 1.5 T MR scanner. Other anatomical compartments will be also investigated. Conceptualization, M.C., N.S.