Simulating Energy-Tissue Interactions for Improved Patient Outcomes

By Casey Ladtkow and Arlen Ward, COVIDIEN EBD

The first commercial electrosurgical generator is credited to Dr. William T. Bovie, who developed the instrument in 1920. Eventually, with advances in technology, solid-state generators replaced the original spark gap and vacuum tube models. In the early 1970s two companies, Valleylab and Electro Medical Systems (EMS), introduced the first solid-state electrosurgical generators, thereby establishing the modern era of isolated outputs, complex waveforms, increased safety, and more. Currently, electrosurgery is used in more than 90% of all surgeries performed in the United States.

Covidien’s ForceTriad™ energy platform and a few of the associated electrosurgical devices that use it as an energy source.

Electrosurgery is the application of high-frequency electric current to biological tissue as a means to cut through the tissue (vaporize) and/or stop bleeding (coagulate). These types of surgeries are performed using an electrosurgical generator and a hand piece that includes one or several electrodes. During electrosurgery procedures, current that passes though the tissue is converted to heat. The amount of heat generated determines if the tissue is vaporized or if coagulation occurs. This degree of control during surgery, as well as electrosurgery’s precise cuts with limited blood loss, makes electrosurgical devices preferred over many alternative methods.

A leader in the energy-based medical treatment systems industry is Covidien Energy-based Devices (EbD, formerly Valleylab). Their systems include electrosurgical generators, accessories, argon enhanced electrosurgery systems, patient return electrodes, electrosurgical generators, and laparoscopic instruments.

With a goal toward advancing research and technology development for electrosurgery, vessel sealing, and tumor ablation, engineers at Covidien EbD employ computer simulations to describe the interaction between highly coupled physical effects where the application of energy changes tissue properties. According to Arlen Ward, a senior R&D Engineer at Covidien EbD, the simulations are used for understanding these complex interactions, demonstrating these effects to others both inside and outside the company, reducing prototyping costs, and investigating tighter energy control and subtle tissue effects. To this end, he and his colleagues have been using COMSOL Multiphysics.

Optimizing the Energy Source

Mr. Ward explained how the simulation of energy-tissue interactions is a key part of the research performed at Covidien EbD to aid product development. “COMSOL has been applied to specific problems as a flexible tool by individuals within various research and development projects. As the benefits of using multiphysics simulations have become apparent, other groups have expressed a desire to incorporate simulation in their projects.” For example, Mr. Ward and his colleague at Covidien EbD, Senior R&D Engineer Casey Ladtkow, have been using COMSOL for about four years. While their projects are vastly different, they come together to share solutions to issues they have with the model itself.

Casey Ladtkow (left) and Arlen Ward demonstrate an electrosurgery simulation.

“COMSOL has been applied to specific problems as a flexible tool by individuals within various research and development projects. As the benefits of using multiphysics simulations have become apparent, other groups have expressed a desire to incorporate simulation in their projects.”

Electrosurgical generators produce a variety of electrical waveforms. As waveforms change, so do the corresponding tissue effects. Tissue heating rates determine whether one waveform cuts tissue and another stops bleeding. These are the two primary surgical effects that Mr. Ward is interested in — the ability to cut through tissue and the ability to control bleeding. “Knowing how much energy is used to vaporize the tissue (providing the cutting effect) and how much energy remains to provide hemostaisis (bleeding control) is key. You need to have a threshold amount of thermal margin to create the homeostasis, but you don’t want an excess of heating because you don’t want to damage the tissue unnecessarily.”

Figure 1. 2D axisymmetric COMSOL model of the cutting rate and thermal effects of a TURP loop electrode.

As the manufacturer of the energy source used in electrosurgery, Covidien EbD provides a platform that accepts a range of specialty surgery instruments, including those made by other manufacturers, and it is important that they understand the interactions of all types of devices to make sure the energy is applied in an appropriate way. Recently, Mr. Ward has been using COMSOL to investigate how energy delivered from one of Covidien’s products, the ForceTriad™ energy platform, works with other loop-shaped electrode devices used for removing prostate tissue during a surgical procedure called TURP (transurethral resection of the prostate). There are many small capillaries in the prostate and Mr. Ward uses COMSOL to model how much energy is required to cut through the prostate while providing enough extra energy to stop the bleeding (Figure 1). “We enter the prostate tissue properties, the geometries of the electrodes, and look at the way that we are applying energy in terms of electrical voltages or currents, and make sure we are putting in enough energy to vaporize the cut at that speed and that we are getting enough thermal margin to provide homeostasis, but also optimizing it to the correct amount,” he said. “The useful part is to be able to start with simple models — like coupling the electrical and thermal properties — and then once you’re satisfi ed that those are working together well, being able to add complexity to those models.”

According to Mr. Ward, his primary challenge in modeling is the amount of available information. “As soon as we have to start defining the work in terms of differential equations and defining the interactions within simulations, it becomes apparent that we often need more information — whether it’s tissue properties or perhaps discovering the physical mechanism that is driving the tissue effect. These are questions that have to be answered before you have a realistic model. We have had to iterate through that a number of times, but every time we do we gain more understanding and therefore make the next time we apply the model much easier.”

Modeling Tumor Ablation

Figure 2. 3D COMSOL model of the impact of a large blood vessel on the ablation size and shape for a microwave tumor treatment.

Mr. Ladtkow is heading another project employing COMSOL. He is examining Covidien EbD’s tumor ablation line to review the differences between radio frequency (RF) and microwave (MW) instruments close to blood vessels.

Called the vessel effect model (Figure 2), Mr. Ladtkow uses three different tools to evaluate the issue: bench modeling with static tissue, preclinical testing with in vivo tissue, and modeling. These three tools are used to support each other in order to verify the findings. “The thing about the bench model and the in vivo model is that it’s really time consuming, you don’t get a lot of repeatability, and you don’t get a lot of control over tissue properties. What the modeling brings is a measure of repeatability where you can test a wide variety of variables and scenarios quickly,” he said.

The main challenge in designing RF and MW ablation products is tuning the energy delivery algorithm to a wide range of tissue conditions, explained Mr. Ladtkow. “Tissue is just so variable and it’s so inhomogeneous, so you get results from tests where you can’t really interpret what’s going on because of the noise. I really think the opportunity for COMSOL is to overcome the noise you get from doing those experiments. I think it’s larger than what you would find in other industries.”

The Practical Advantages

While Mr. Ward and Mr. Ladtkow utilization of COMSOL varies greatly, they both agree that it offers exceptional compatibility with other software programs. For example, they both use COMSOL in conjunction with MATLAB®. Mr. Ladtkow mentioned how COMSOL has allowed for the simulation of a wide range of tissue conditions. “I think there is a lot of power to be able to use some of the toolboxes that come with MATLAB, like the genetic algorithms and additional search and optimization algorithms where you can couple it with COMSOL and you can optimize parameters in your model using some pretty sophisticated optimization algorithms. I think that is a really powerful thing to be able to do,” he said. While the primary output of the work done in COMSOL has been educational, key customer concerns have been addressed, thereby allowing Covidien EbD’s devices to be used with more confidence and improved patient outcomes. “The end result of a better understanding of the relationship between the application of energy and the desired surgical effect and the ability to clearly demonstrate that relationship through images and animations increases everyone’s understanding and comfort with new technologies and how they are best used in a surgical environment,” asserted Mr. Ward.