Michele Conconi

A New Assessment of Singularities of Parallel Kinematic Chains

This paper presents a novel assessment of singularities of general parallel kinematic chains. Hierarchical levels in which different critical phenomena originate are recognized. At each level, the causes of singular events are identified and interpreted, and on their basis, a comprehensive taxonomy is proposed. First, the unactuated kinematic chain is studied. The concepts of leg and passive-constraint singularities are described, and stationary and increased instantaneous mobility configurations are identified. Then, a set of motorized joints is chosen. The effects of first-level singularities on the actuated chain are investigated, and further phenomena are identified, such as redundancy singularities and active-constraint singularities. The notions of reaction and action spaces are originally discussed. The consequences on the ability of the actuated chain to effectively govern its local and global freedoms are analyzed, and the complex interactions between the various singular events are studied. The instantaneous redundancy of actuators, which occur when these work either against each other or against joint constraints, is also evaluated. Finally, when the input-output mechanism is considered, the events described at the previous stages are interpreted within the perspective of the machines desired use.

Is early osteoarthritis associated with differences in joint congruence

Previous studies suggest that osteoarthritis (OA) is related to abnormal or excessive articular contact stress. The peak pressure resulting from an applied load is determined by many factors, among which is shape and relative position and orientation of the articulating surfaces or, referring to a more common nomenclature, joint congruence. It has been hypothesized that anatomical differences may be among the causes of OA. Individuals with less congruent joints would likely develop higher peak pressure and thus would be more exposed to the risk of OA onset. The aim of this work was to determine if the congruence of the first carpometacarpal (CMC) joint differs with the early onset of OA or with sex, as the female population has a higher incidence of OA. 59 without and 38 with early OA were CT-scanned with their dominant or arthritic hand in a neutral configuration. The proposed measure of joint congruence is both shape and size dependent. The correlation of joint congruence with pathology and sex was analyzed both before and after normalization for joint size. We found a significant correlation between joint congruence and sex due to the sex-related differences in size. The observed correlation disappeared after normalization. Although joint congruence increased with size, it did not correlate significantly with the onset of early OA. Differences in joint congruence in this population may not be a primary cause of OA onset or predisposition, at least for the CMC joint.

Joint kinematics from functional adaptation: A validation on the tibio-talar articulation.

Biologic tissues respond to the biomechanical conditions to which they are exposed by modifying their architecture. Experimental evidence from the literature suggests that the aim of this process is the mechanical optimization of the tissues (functional adaptation). In particular, this process must produce articular surfaces that, in physiological working conditions, optimize the contact load distribution or, equivalently, maximize the joint congruence. It is thus possible to identify the space of adapted joint configurations (or adapted space of motion) starting solely from knowledge of the shape of the articular surfaces, by determining the envelope of the maximum congruence configurations. The aim of this work was to validate this hypothesis by testing its application on 10 human ankle joints. Digitalizations of articular surfaces were acquired in 10 in-vitro experimental sessions, together with the natural passive tibio-talar motion, which may be considered as representative of the adapted space of motion. This latter was predicted numerically by optimizing the joint congruence. The highest mean absolute errors between each component of predicted and experimental motion were 2.07° and 2.29 mm respectively for the three rotations and translations. The present kinematic model replicated the experimentally observed motion well, providing a reliable subject-specific representation of the joint motion starting solely from articulating surface shapes.

A sound and efficient measure of joint congruence

In the medical world, the term “congruence” is used to describe by visual inspection how the articular surfaces mate each other, evaluating the joint capability to distribute an applied load from a purely geometrical perspective. Congruence is commonly employed for assessing articular physiology and for the comparison between normal and pathological states. A measure of it would thus represent a valuable clinical tool. Several approaches for the quantification of joint congruence have been proposed in the biomechanical literature, differing on how the articular contact is modeled. This makes it difficult to compare different measures. In particular, in previous articles a congruence measure has been presented which proved to be efficient and suitable for the clinical practice, but it was still empirically defined. This article aims at providing a sound theoretical support to this congruence measure by means of the Winkler elastic foundation contact model which, with respect to others, has the advantage to hold also for highly conforming surfaces as most of the human articulations are. First, the geometrical relation between the applied load and the resulting peak of pressure is analytically derived from the elastic foundation contact model, providing a theoretically sound approach to the definition of a congruence measure. Then, the capability of congruence measure to capture the same geometrical relation is shown. Finally, the reliability of congruence measure is discussed.

Joint Kinematics from Functional Adaptation: An Application to the Human Ankle

The aim of this paper is to exploit the concept of functional adaptation to model the motion of human joints and to present an application to the human tibio-talar articulation. With respect to previous works, a new algorithm is presented here that improves the model outcomes and numerical stability, also reducing the computational cost. Moreover, a refined measure for joint congruence is proposed, which requires only the knowledge of the articular surface shapes. This measure is hypothesized to be proportional to the joints ability to withstand an applied load. Biological tissues tend to achieve the necessary mechanical resistance with the smallest amount of material (functional adaptation). Conversely, adapted tissues employ their material optimally, maximizing their mechanical resistance. It follows that, as a result of the functional adaptation process, an adapted joint will move along the envelope of maximum resistance and thus maximum congruence configurations. This envelope defines a spatial trajectory along which the functional adaptation requirements are satisfied and it may thus be called functionally adapted trajectory. The functionally adapted trajectory obtained by simulations is compared with in vitro measured one. Preliminary results provided strong support to the theoretical model prediction.

Subject-Specific Model of Knee Natural Motion: A Non-invasive Approach

The capability to model human joint motion is a fundamental step towards the definition of effective treatments and medical devices, with an increasing request to adapt the devised models to the specificity of each subject. We present a new approach for the definition of subject-specific models of the knee natural motion. The approach is the result of a combination of two different techniques and exploits the advantages of both. It relays upon non invasive measurements based on which a kinematic model of the natural motion is built, suitable to be extended to the definition of static and dynamic models. Comparison of the model outcomes with in vitro measurements performed on one specimen shows promising results supporting the proposed approach.

A New Bone Fixation Device for Human Joint Test Rig Machine

In vitro experimental tests are of great importance in human joint biomechanics. These tests allow the study of the kinetostatic and dynamic behaviour of human joints: a limb specimen is connected to a rig, controlled loads are applied, and the joint displacements are measured. Fixation of the bone to the rig is a typical problem that raises in these applications. A new fixation device is presented in this paper that features several interesting and innovative characteristics: it consists of a passive parallel mechanism that allows adjustment of the pose of the bone with respect to the rig with six degrees of freedom; if needed, the bone can be removed from the rig and repositioned exactly at the same pose during the experimental tests; all six degrees of freedom of the fixation device can be locked by acting only on two screws, thus simplifying the fixation operation; the parallel structure of the mechanism guarantees a high fixation stiffness. The complete design is presented and discussed, together with the results from the workspace and stiffness analyses of the mechanism, that are particularly important for the considered applications.

Functional Modeling of Human Joints: A Feasibility Study for the Knee

Biological tissues are plastic with respect to the mechanical environment to which they are exposed. This makes them able to modify their architecture and inner structure in order to respond to different loading conditions with the smallest biological effort (functional adaptation). As a result, tissues can optimally adapt their structures to the task they have to perform.Based on these concepts, a kinetic model of the ankle joint has been recently developed. The tibio-talar relative motion was obtained by imposing the congruence maximization as a biological optimum throughout the entire flexion range.The aim of this work is to investigate the applicability of the proposed approach to the knee and to evaluate the weight of the meniscal contribution to the global femoro-tibial congruence.Copyright

Optimized Kinematical Positioning and Guidance of a Serial Robot for Motion Simulation

Described in this paper is an approach for generating control inputs for a serial robot motion simulator such that prescribed linear acceleration at the end-effector are accomplished as close as possible. The method takes into account the workspace limits, and uses the internal interpolation cycle of the robot controller to generate the corresponding trajectories. The PI parameters of the internal interpolation are identified from test measurements. The path planning algorithm uses the damped least squares method together with a refinement based on optimization for navigating the robot along the user-prescribed accelerations under avoidance of singularities. The approach is demonstrated for the Kuka robot roboCoaster. It is shown that the desired accelerations can be generated accurately and with high repeatability, making the approach suitable for generic simulation tasks.