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FEA UNDERPINS ADVANCED TREATMENT OF SPORTS INJURIES

Gyrus Medical, Ltd. Chose ALGOR for Analysis of an Arthroscopic Surgical Instrument

The VAPR system consists of a generator and various styles of surgical instruments including the TC (temperature control) instrument, which was developed by Gyrus Medical, Ltd.

Sports-related injuries are a common cause of damaged ligaments and tendons in shoulders, knees, wrists, elbows and ankles and can usually be repaired through arthroscopic surgery, a procedure for visualizing, diagnosing and treating joint problems. Arthroscopic surgery significantly reduces the disturbance and traumatization to the joint as compared to conventional surgery, thus minimizing the amount of invasion, discomfort, scarring and recovery time. Through an incision no larger than a keyhole, a surgeon inserts an instrument called an arthroscope, which provides a display of the joint on a video monitor during the operation.

Instruments used to perform arthroscopic surgery include specialized, computer-controlled systems such as those designed and manufactured by Gyrus Medical, Ltd. Gyrus is a 150 million pound ($233 million USD) medical technology company based in Cardiff, Wales, United Kingdom and in the United States in Minneapolis (Gyrus Medical, Inc.) and Memphis (Gyrus ENT).

Recently, Gyrus developed a TC (temperature control) instrument as a component of the VAPR Bipolar Radiofrequency System held by Mitek Products (www.vapr.com), a division of ETHICON, Inc., a Johnson & Johnson company. The challenge that Gyrus faced was to design and manufacture an instrument that would perform with required precision and accuracy, even at the expected high operating temperatures.

Gyrus used Solid Edge computer-aided design (CAD) software to model the tip assembly of the TC instrument and ALGOR finite element analysis (FEA) software to verify its performance under thermal stress. ALGOR's InCAD technology provided direct data exchange of the tip assembly with ALGOR, streamlining the modeling and analysis process, which helped Gyrus to successfully deliver the final TC instrument design on schedule.

Making Surgery Quicker and Safer for Patients
An arthroscopic display shows a surgical instrument being used to treat shoulder instability through the thermal modification of soft tissue.

The VAPR system consists of a generator and the instrument required for the type of arthroscopic surgery to be performed. In the case of the TC instrument, the generator creates radiofrequency (RF) energy waveforms at a frequency of more than 500,000 oscillations per second, which are dispersed at the instrument's tip, generating heat in the process. During a procedure called thermal modification, the surgeon uses the TC instrument to apply heat to the connective tissue of a joint, which causes the connective tissue to contract and thereby tighten the joint.

For example, consider a patient who has a shoulder joint that is unstable and keeps popping out of place resulting in an inability to lift the arm normally. The function of the shoulder joint is impaired because the capsular ligament surrounding the shoulder joint is stretched and loose. Thermal modification surgery can then be used to tighten the capsular ligament and restore normal function to the shoulder joint.

Gyrus needed to design a TC instrument that would provide rapid and precise soft tissue thermal modification while withstanding stresses due to thermal expansion. One key aspect was how the instrument behaved at operating temperatures. Heat can cause parts of the instrument to expand, which results in thermal stress. However, thermal expansion must not affect the instrument's ability to allow the surgeon to set a specific temperature for the procedure being performed and maintain and control that temperature for accurate thermal modification of the tissue.

Meeting the Design Challenge with CAD and FEA
Gyrus Medical, Ltd. created a 3-D model of the TC instrument tip assembly in Solid Edge. The tip was the area of interest because it is inserted inside the patient's body during surgery to repair a damaged joint. An exploded view shows the components of the tip assembly.

According to Gyrus Senior Materials Analyst Mike Hagland, "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 created a three-dimensional (3-D) model of the TC instrument tip assembly using Solid Edge. The model consisted of a stainless steel tube, bio-compatible adhesive layers, a polymer insulator material and a stainless steel tip. These parts have material properties that enable them to withstand high operating temperatures and thermal stresses. Hagland then captured the complete assembly model through direct data exchange with ALGOR.

In ALGOR, Hagland studied 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 65C was specified for the steady-state heat transfer analysis, which simulated the operating temperature at the required 20W power setting necessary to provide proper capsular shrinkage 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.

Laboratory tests performed at Gyrus' on-site testing facility confirmed the FEA results. "Prototype testing was done with a silicone material in a saline solution, which mimicked the operating environment," explained Hagland. "Correlation between the test data and the FEA results provided confidence in the design. We used ALGOR analysis and practical testing for a fast, progressive method of concurrent design."

Gyrus Medical, Ltd. used ALGOR's InCAD technology to provide direct data exchange of the TC instrument tip assembly from Solid Edge with ALGOR software for finite element analysis. First, steady-state heat transfer analysis was used to determine temperature results at the tip (left). Then, a multiphysics automation tool (center) was used to input the temperature results as loads in a linear static stress analysis (right).
Mike Hagland, Gyrus Senior Materials Analyst, modeled and analyzed the TC instrument tip assembly using Solid Edge CAD software and ALGOR FEA software.

The TC instrument is currently being used by surgeons worldwide as part of the VAPR system. According to Hagland, "ALGOR's software made a valuable contribution in helping us to deliver the product on schedule. The direct data exchange between Solid Edge and ALGOR analysis software and the ability to quickly verify the precision and accuracy of the TC instrument design helped us get the product to market faster."



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