Meccanica (2011) 46: 207–220 DOI 10.1007/s11012-010-9405-x PARALLEL MANIPULATORS A novel 3D parallel mechanism for the passive motion simulation of the patella-femur-tibia complex Nicola Sancisi · Vincenzo Parenti-Castelli Received: 24 September 2009 / Accepted: 26 November 2010 / Published online: 30 December 2010 © Springer Science+Business Media B.V. 2010 Abstract Parallel mechanisms have been exploited for the kinematic modelling of the passive motion, i.e. the motion under virtually unloaded conditions, of the patella-femur-tibia human joint. In particular, a new mechanism is devised in this paper: a 3D model of the patella-femur relative motion is presented which, com- bined with a previous simplified model of the femur- tibia relative motion, provides a suitable tool for the design of knee prostheses. Although less accurate than a previously presented model of the patella-femur- tibia joint, the new mechanism still replicates passive knee motion quite well and is simpler from a mechani- cal point of view. Experimental results validate the ef- ficiency of the proposed model. Keywords Knee model · Patella · Passive motion · Parallel mechanism · Spherical wrist 1 Introduction Great attention has been devoted to devise kinematic and dynamic models of diarthrodial joints for the sci- entific as well as technical significance these models have in the orthopaedic and rehabilitation fields. Mod- els make it possible to replicate the joint motion and N. Sancisi · V. Parenti-Castelli ( ) DIEM—Department of Mechanical Engineering, University of Bologna, 40136 Bologna, Italy e-mail: vincenzo.parenti@unibo.it are a powerful tool on one hand to investigate the role of the main anatomical structures and on the other hand to plan surgical operations and to develop pros- thesis design. Moreover, fundamental characteristics such as the joint functionality and stability under vari- ous loading conditions may also be efficiently investi- gated by models. Kinematic and dynamic models of human joints have been proposed in the literature [3, 7]. Particu- lar attention has been focussed on the human knee joint for the great significance it has in human loco- motion. In particular, kinematic knee models for pas- sive motion simulation, i.e. for simulation of motion under virtually unloaded conditions, revealed their im- portance and effectiveness for a deeper understanding of the role played by the main anatomical structures such as ligaments and articular surfaces in joint mo- tion and stability [5]. They also allow the definition of more feasible boundary conditions for kinetostatic and dynamic models, thus enhancing the performances of these more complex representations of the joint. A large number of planar models of the knee pas- sive motion have been proposed in the literature, while 3D models have been presented only recently. Consis- tently with a vast collection of experimental observa- tions, the relative motion of the femur and tibia has been found to be a complex 3D motion with one de- gree of freedom (DoF), and equivalent spatial parallel mechanisms have been exploited to successfully sim- ulate this motion [10, 11, 13, 14].