Author:

Keywords:

biomechanics, continuum mechanics, finite element analysis, damage, robotic surgery, cardiovascular tissue, soft tissue biomechanics

Abstract:

Surgery is shifting towards less invasive techniques, which unfortunately come at the cost of increased complexity. At the same time, surgeons have welcomed robotic tools into the surgical theatre, of which telesurgery is a prominent example. Again, this comes at a cost, namely a decreased or even lost sense of haptic feedback. Fortunately, the increased complexity and loss of haptic feedback can be (partly) compensated by an increased level of 'intelligent' information flow to the surgeon, a track followed here. This PhD introduces the concept of interpreting intra-operatively acquired haptic and visual information by means of biomechanical models, thereby obtaining information on the mechanical loading of the manipulated tissue. The aim is to provide the surgeon with 'intelligent' information feedback in the form of safety limits on tissue loading, thereby minimizing unnecessary intra-operative trauma.Part I of the PhD thesis conceptually outlines a fundamental research track and an applied research track. In the former, an experimental and computational framework is described to define damage thresholds for soft tissues. In the latter, the steps required to actually implement these thresholds as active constraints into a telesurgical setting are formulated. In the second part of the thesis, the concepts of the fundamental research track are applied to the specific case of arterial clamping. In vivo experiments are performed to clamp and subsequently quantitatively evaluate the induced damage to the contractile capability of the artery, which is related to the integrity of the smooth muscle cells and the endothelial cells. An experimental quantitative relationship is thereby found between mechanical load and different kinds of damage. To allow a general interpretation of this quantitative relation, the entire experimental process is simulated numerically, using the finite element method. First, the clamping process is simulated using the Holzapfel-material model, thereby evaluating the effect of residual strains, mechanical property variation and clamp design. The clear inhomogeneous distribution of the stress proves the necessity and potential of accurate finite element modelling for damage threshold identification and for `smart' instrument design.Finally, because the Holzapfel-material model is suited only for the physiological loading regime and does not capture contractility, a new material model is developed. Besides the typical nonlinear behaviour of arterial tissue, this model also captures the active contraction of the smooth muscle cells as well as the degradation of the tissue in response to mechanical overload. The material model is used in a second finite element simulation in which also the damage quantification method is simulated. A comparison with the performed experiments enables a fitting of the parameters of the new material model.The research performed on the specific case of arterial clamping has demonstrated the potential of the general frameworkto define damage thresholds for soft tissues. The approach was interdisciplinary, and aimed at bridging the gap between the biomedical empirical reality and engineering design and computation. Hence, the foundations were laid for a broad range of new research tracks in the area of biomechanics, all directed towards enhancing surgical quality by increasing intra-operative safety.