# An Anatomical-Based Subject-Specific Model of In-Vivo Knee Joint 3D Kinematics From Medical Imaging
## 2 Methods  
### Data Acquisition  
A healthy subject (male, 66 years old, 169 cm, 58 kg) with no history of knee joint diseases volunteered for the study. The present study was approved by the ethical committee at the Istituto Ortopedico Rizzoli, and the volunteer gave his informed consent.  
Computed tomography (CT) (Brilliance CT 16-slice system, Philips Healthcare, Best, Netherlands) and magnetic resonance imaging (MRI) (1.5 Tesla MRI; SIGNA EXCITE HDxt, GE Healthcare, Chicago, IL, USA) scans were performed. With the subject supine, local CT scans of the right knee (\(220\times 220\) mm, \(512\times 512\) pixels, 2 mm slice thickness) were taken in the transverse plane. MRI scans of the same knee were collected in both the sagittal, coronal and axial planes (\(220\times 220\) mm, \(512\times 512\) pixels, 0.8 mm slice thickness) using Sagitttal Cube PD, Coronal Cube PD, Axial PD Fast Spin Eco sequences, respectively. To define the anatomical reference systems for the bone models, local CT scans of the right ankle and hip joints (\(220\times 220\) mm, \(512\times 512\) pixels, 5 mm slice thickness) in the transverse plane and a CT scout view of the whole lower limb at full extension (\(1120\times 500\) mm, \(1128\times 512\) pixels) in the frontal plane were taken as well.  
The experimental natural knee motion was reconstructed using single plane 3D video-fluoroscopy (3 DV) by means of a digital remote-controlled diagnostic radiological device with flat-panel detector (CAT Medical Systems, Monterotondo, Italy). According to an established protocol [29], the reconstruction procedure starts with the collection of a series of digital video-fluoroscopic images of the knee joint in the sagittal plane: frequency was set at 15 frame/s and the exposure parameters were set initially at 50 mA and 60 kV and then adjusted automatically by device-based automatic exposure control. The \(430\times 430\) field of view (FOV) was set to record the proximal half of the tibia and the distal half of the femur, whereas the 3D geometry of the fluoroscopy device (principal distance and principal point), as well as the definition of the reference frame of the detection plane, were derived from images of calibration objects [30], these being manufactured via numerical control with known specific geometrical features. The volunteer was asked to perform a knee flexion-extension movement starting at full extension. The task was performed with the volunteer supine, the right lower limb lying on a smooth plane, by pulling the leg towards the chest up to full flexion with the aid of a lace tied around the thigh, and then by letting the leg extend back to full extension. The whole movement lasted about 4 s. A wedge-shaped support was placed behind the subject's back to prevent thigh muscles contraction during the test. The heel of the subject was placed on a polytetrafluoroethylene disk allowed to slide smoothly on the plane to reduce friction. Two acquisitions of a complete knee motion cycle were performed, since the FOV of the fluoroscope did not allow the motion to be recorded in one take. The first acquisition recorded the range full extension, \(80^{\circ}\) and back to full extension, while the second recorded the range from \(80^{\circ}\) to full flexion and back to \(80^{\circ}\).