Nicola Sancisi

A one-degree-of-freedom spherical mechanism for human knee joint modelling:

In-depth comprehension of human knee kinematics is necessary in prosthesis and orthosis design and in surgical planning but requires complex mathematical models. Models based on one-degree-of-freedom equivalent mechanisms have replicated well the passive relative motion between the femur and tibia, i.e. the knee joint motion in virtually unloaded conditions. In these mechanisms, fibres within the anterior and posterior cruciate and medial collateral ligaments were taken as isometric and anatomical articulating surfaces as rigid. A new one-degree-of-freedom mechanism is analysed in the present study, which includes isometric fibres within the two cruciates and a spherical pair at the pivot point of the nearly spherical motion as measured for this joint. Bounded optimization was applied to the mechanism to refine parameter first estimates from experimental measurements on four lower-limb specimens and to best-fit the experimental motion of these knees. Relevant results from computer simulations were compared with those from one previous equivalent mechanism, which proved to be very accurate in a former investigation. The spherical mechanism represented knee motion with good accuracy, despite its simple structure. With respect to the previous more complex mechanism, the less satisfactory results in terms of replication of natural motion were counterbalanced by a reduction of computational costs, by an improvement in numerical stability of the mathematical model, and by a reduction of the overall mechanical complexity of the mechanism. These advantages can make the new mechanism preferable to the previous ones in certain applications, such as the design of prostheses, orthoses, and exoskeletons, and musculoskeletal modelling of the lower limb.

Synthesis of Spatial Mechanisms to Model Human Joints

The role played by diarthrodial joint models in surgery, pre-surgical planning and prosthesis design has been widely recognized. This chapter presents a procedure for the modelling of the diarthrodial human joints. The procedure features three main sequential steps, each of them leading to the kinematic, kinetostatic and dynamic models of the joint respectively. In particular, the chapter focuses on the first model, which can replicate the joint passive motion, i.e. the joint motion under virtually unloaded conditions. This model proves to be of great relevance for a deeper understanding of the joint anatomical structures and is the basic step for the next two kinematic and dynamic models. The first model is represented by a spatial mechanism called equivalent mechanism. Special emphasis is devoted to the synthesis of the mechanism. Examples of knee, ankle, and lower limb modelling are reported that prove the potential of the procedure.

Helical axis calculation based on Burmester theory: experimental comparison with traditional techniques for human tibiotalar joint motion

In prosthetics and orthotics design, it is sometimes necessary to approximate the multiaxial motion of several human joints to a simple rotation about a single fixed axis. A new technique for the calculation of this axis is proposed, originally based on Burmester’s theory. This was compared with traditional approaches based on the mean and finite helical axes. The three techniques were assessed by relevant optimal axis estimation in inxa0vitro measurements of tibiotalar joint motion. A standard jig and radiostereometry were used in two anatomical specimens to obtain accurate measurements of joint flexion. The performance of each technique was determined by comparing the motion based on the resulting axis with the experimental data. Random noise with magnitude typically similar to that of the skin motion was also added to the measured motion. All three techniques performed well in identifying a single rotation axis for tibiotalar joint motion. Burmester’s theory provides an additional method for human joint motion analysis, which is particularly robust when experimental data are considerably error affected.

A Procedure to Analyse and Compare the Sensitivity to Geometrical Parameter Variations of One-DOF Mechanisms

Sensitivity analysis to geometrical parameter variations aims at assessing how small changes in a mechanism parameters, for instance due to tolerances and clearance, can alter its nominal motion. This issue is very important for the characterization of a mechanism and has been widely addressed in the literature, but it needs further investigations. The high number of parameters characterizing the problem and their mutual influence indeed make it difficult to assess the sensitivity from a global point of view. In this paper, a procedure is presented to characterize and evaluate the sensitivity of one-degree-of-freedom mechanisms. As an example, such a procedure is applied on two mechanisms which are the basis to design knee prosthetic devices. In this context, sensitivity analysis is important to understand how the motion generated by a prosthetic device can be influenced by small variations of its geometry or by small misalignments during the installation phase on the patient.Copyright

Validation of a one degree-of-freedom spherical model for kinematics analysis of the human ankle joint

Background During passive motion, the human tibiotalar (ankle) joint behaves as a single degree-of-freedom (1DOF) system [1,2]. In these conditions, fibres within the ligaments remain nearly isometric throughout the flexion arc and articular surfaces nearly rigid. Relevant theoretical models are showing that the ligaments and the articular surfaces act together as mechanisms to control the passive joint kinematics [3-5]. Kinematic measurements and corresponding model predictions also

On the Role of Passive Structures in the Knee Loaded Motion

The role of the passive structures of the human knee in loaded and unloaded motion has been deeply investigated in the literature. However, what makes its comprehension difficult is the inherent redundancy of the anatomical structures that constrain the motion itself. This paper, based on simulation and experimental data, provides some inferences on the role of the constraint redundancy of this complex anatomical joint. The results suggest that the knee behaves prevalently like a one-degree-of-freedom isostatic system when moderate external loads are applied, continuously constrained by 5 articular structures recruited according to the external loads and to the system configuration.

Simultaneous Identification of the Human Tibio-Talar and Talo-Calcaneal Joint Rotation Axes by the Burmester Theory

In several applications, such as the design and setting of prostheses, orthoses and exoskeletons, and the multibody modelling of the lower limb, it is sometimes necessary to approximate the spatial motion of human joints to a rotation about a fixed axis. The identification of these axes can be particularly difficult at the ankle joint, where two different articulations are observed, namely the tibio-talar joint (connecting the tibia and talus) and the talo-calcaneal joint (connecting the talus and calcaneus). Thus, the ankle requires two distinct axes to be identified in order to correctly describe the joint motion. A new technique is proposed here for the identification of the tibio-talar and talo-calcaneal rotation axes. The technique is based on a particular use of the Burmester theory and exhibits several advantages: the talus motion is not required, and only the tibia-calcaneus motion is needed; the identification of both rotation axes is simultaneous and thus the identification accuracy of both axes is independent; the method is not based on optimization techniques and thus it does not require the definition of an objective function; it is robust with respect to experimental inaccuracies; it makes it possible to obtain the talus motion, even if it could not be measured during the experimental session. The technique reveals particularly useful during in vivo measurements based on skin markers or other non-invasive measures, since the talus motion cannot be obtained from skin measurements. An application example is shown by means of experimental data measured on an ankle specimen.© 2013 ASME