Characterizing the Nonlinear Mechanical Response of Liver to Surgical Manipulation
Computer-aided medical technologies such as simulators for surgical training, planning, and assessment, are currently limited by the inability to realistically portray the behavior of the involved tissues. The goal of this work is to accurately characterize the mechanical behavior of liver under large deformations typical of surgical manipulation.
Three steps are required for this characterization. First, the effect of testing conditions on the sought-after behavior is identified. A study that evaluated the effects of perfusion on the viscoelastic response of liver resulted in the development of an ex vivo perfusion system that nearly approximates the in vivo condition. Second, a mathematical model is derived that is capable of capturing the liver’s nonlinear, viscoelastic response based on the physical make-up of the tissue. Third, using the perfusion apparatus, indentation tests are performed to identify and validate the models parameters by solving the inverse problem using an iterative approach.
Researchers: Amy Kerdok
Image-Based Mechanical Characterization of Soft Tissue using Three-Dimensional Ultrasound
There are two surgical simulations and virtual surgical environments that aim at improving the quality of medical personnel training, reducing training costs, and eliminating the need for animal subjects. Accurate mechanical models of tissue and whole organ behavior are crucial for the successful implementation of these technologies. Historically, the focus of biomechanics research has been on hard tissues (bone, teeth, etc.) and load-bearing soft tissues, such as muscle and cartilage, while the mechanics of non-load-bearing soft tissues has received much less attention. Our ultimate goal, in collaboration with the Center for Integration of Medicine and Innovative Technology (CIMIT) Simulation Group, is the development of whole-organ mechanical models for real-time implementation in surgical simulators. Our initial focus is on modeling the slow deformation of frequently manipulated and highly homogeneous organs, including the liver, spleen, and kidney.
Formulation of a robust mechanical model, along with tissue-specific and pathology-specific model parameters, is key to accurate modeling of soft tissue mechanical response. The specific goal of this project is the development of an in vivo imaging technique for mechanical characterization (parameter identification) of abdominal organs and soft tissues.
Researchers: Petr Jordan
Modeling Myocardial Injury During Trans-atrial Intracardiac Procedures
Trans-atrial intracardiac procedures performed with robotic assistance (MV Annuloplasty and ASD Closure) can cause significant myocardial injury in areas around port sites. Understanding the relationship between mechanical loading and myocyte injury may improve the safety of these procedures and may permit the development of direct trans-atrial surgical procedures in the future.
As an initial approach to limiting this type of intraoperative myocardial injury, a model of the atrial wall was used to study the effects of port/instrument positioning and loading on myocyte injury. The relevant boundary conditions and geometry of the problem are shown in the graphic on the right. This illustration shows the region of risk between and around trans-atrial ports. This problem was then meshed into a 5300-element quadrilateral planar mesh (FEMAP 8.0) with the myocardial material properties based on the strain energy function by Smaill & Hunter for both the fiber and cross-fiber directions. The validity of this definition was tested by stretching a single element biaxially and comparing these results to those published by Smaill & Hunter. Then, using the Galerkin Finite Element Method 50 Fiber-Direction and 40 Cross-Fiber Direction port position/displacement combinations (ABAQUS Standard 5.8.1) were solved.
Researchers: Jeremy W. Cannon and Pedro J. del Nido
Mechanics of the Human Fingerpad
We investigated the dynamic response of the human fingerpad in vivo to a compressive load. A flat probe indented the fingerpad at a constant velocity, then held a constant position. The resulting force (0 – 2 N) increased rapidly with indentation, then relaxed during the hold phase. A quasi-linear viscoelastic model successfully explained the experimental data. The instantaneous elastic response increased exponentially with position, and the reduced relaxation function included three decaying exponentials (with time constants of approximately 4 msec, 70 msec and 4 sec) plus a constant. The model was confirmed with data from sinusoidal displacement trajectories.
To find the dynamic pressure distribution across the fingerpad, a tactile array sensor was mounted on the probe, and the finger was indented with the same ramp-and-hold and sinusoidal displacement trajectories. The local pressure variation exhibited nonlinear stiffness (exponential with displacement) and significant temporal relaxation, as with the total force. The shape of the contact pressure distribution could plausibly be described by an inverted paraboloid. A model based on the contact of a rigid plane (the probe) and a linear viscoelastic sphere (the fingerpad), modified to include a nonlinear modulus of elasticity, can account for the principal features of the distributed pressure response.
Researchers: Dianne T.V. Pawluk
