A recent review article on 'Current progress in patient-specific modeling' in Briefings in Bioinformatics contains the statement summarizing the results of our previous study 'On the unimportance of constitutive models in computing brain deformation for image-guided surgery' published in Biomechanics and Modeling in Mechanobiology as confirmation of adequacy of linear elastic model for such computation. The purpose of this Letter to the Editor is to clarify this statement by informing the Readers of Briefings in Bioinformatics that our study indicates the following: (i) a simple linear elastic constitutive model for the brain tissue is sufficient when used with an appropriate finite deformation solution (i.e. geometrically non-linear analysis); and (ii) Linear analysis approach that assumes infinitesimally small brain deformations leads to unrealistic results.

This paper explores the significance of the inherent strain rate-dependence of many biological materials. Such characteristics are pertinent to computational modelling of these materials for the purposes of surgical robotics and simulation. A pair of 2-dimensional finite element models of the human brain/ventricle system are developed and used to analyse the brain structural disease, hydrocephalus. The models are geometrically identical, and both incorporate a linear biphasic material model for the brain parenchyma. Model 1 is assigned a Young modulus value, E/sub 0/ = 3156 Pa, based on the instantaneous elastic response derived from a hyperviscoelastic model. Model 2 accounts for the extremely slow loading characteristic of the disease and a revised modulus value of E/sub /spl infin// = 584.4 Pa is assigned. This significant variation in stiffness generates similar differences in the simulation results. The conclusion is that consideration of the intrinsic strain rate-dependence of organic materials is vital to the satisfactory modelling of such materials, and hence significant to surgical robot performance.

This contribution presents the experimental verification of the newly developed Hamilton-based dynamic model of DELTA direct drive parallel robot by measurements of DELTA robot's motor torques. The strain gage technology was applied. The experiment is in author's opinion the first ever for robot DELTA. The experimental results validate the modelling of robot DELTA as a system of rigid bodies, connected by frictionless joints. The results obtained from the model based on the direct application of Hamilton's principle in extended space, taking into account the differences between real and desired accelerations, agreed with measurements to at least 5%. The motor torque measuring system may be in future used in force control of DELTA robot.