let's fix up the joint
The medical electrode passes the test for reliability and resisting stress.
More than ever these days, people are hard at
play in the strenuous games that shape muscles and stress joints. And,
while sports injuries may have increased over the past several years,
technology has been providing tools to patch the damage.
When joint injuries are serious enough to require surgery, for instance,
they are often treated with minimally invasive techniques. Through incisions
no larger than keyholes, a surgeon inserts very small instruments, including
an arthroscope, which has a video camera connected to a monitor so the
doctor can see to operate. A smaller invasion reduces pain, scarring,
and healing time.
Gyrus Medical Ltd. of Cardiff, Wales, developed its VAPR bipolar radio
frequency system for work like that. The system, which is marketed by
Mitek Worldwide of Norwood, Mass., consists of a generator and a selection
of instruments, or electrodes, for different treatments.
The generator creates radio frequency energy waves, of 500 kilohertz,
which are dispersed at the instrument's tip, generating heat that can
vaporize soft tissue, coagulate blood, or modify ligaments.
To give surgeons greater control over this last procedure, thermal modification,
Gyrus developed a specialized thermal control electrode. In thermal modification,
the surgeon wants to heat connective tissue only to a temperature that
will cause it to contract and tighten a joint. According to Greg Belloli,
team leader for VAPR research and development at Mitek, all the electrodes
can perform thermal modification, but the new one keeps surgeons informed
about operating temperatures.
Gyrus's
electrode was put to thermal analysis using Algor software.
The job of studying the design's thermal stresses fell to Gyrus's senior
materials analyst, Mike Hagland. As he put it, "Since the instrument
is inserted into a patient's body for tissue modification or repair during
surgery, evaluating its performance and obtaining accurate results is
critical."
Hagland began with a three-dimensional model of the tip assembly of the
thermal control instrument in Solid Edge, the computer-aided design software
from EDS of Plano, Texas. The model was specified as a stainless steel
tube and tip, biocompatible adhesive layers, and a polymer insulator.
The materials were chosen because engineers expected them to withstand
high operating temperatures.
Hagland's job was to put the expectation to a test, before the company
invested money in building a working prototype. He translated the CAD
file into a finite element analysis model by using the InCAD program,
which is part of software from Algor Inc. of Pittsburgh. Hagland used
the Algor software to study the model through steady-state heat transfer
and linear static stress analyses. He specified custom material properties
for the adhesive and insulator parts of the assembly and used standard
properties from the built-in material library for the stainless steel
parts.
A temperature loading of 65°C was specified for the steady-state
heat transfer analysis, which simulated the operating temperature at the
required 20-watt power setting needed to provide proper shrinkage of a
ligament during shoulder surgery. Convection parameters were applied using
a built-in convection calculator to simulate saline at the tip and air
around the tube.
Temperatures from the steady-state heat transfer analysis were then used
as input to a linear static stress analysis to determine the thermal stresses.
The FEA results revealed that the thermal stresses encountered during
the procedure were well within acceptable limits for each component of
the instrument.
Gyrus performed laboratory tests later on a prototype and confirmed the
FEA results. "Prototype testing was done with a silicone material
in a saline solution, which mimicked the operating environment,"
Hagland said. "Correlation between the test data and the FEA results
provided confidence in the design."
The thermal control instrument is currently being used by surgeons worldwide
as part of the VAPR system.
This article was prepared by staff writers in collaboration
with outside contributors.