Tech Briefs

Simulating an Arthroscopic Surgical Instrument

CAD and finite element analysis software help verify the design of a temperature control electrode.

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. Through an incision no larger than a keyhole, a surgeon inserts an endoscope to see inside the joint on a video monitor during the operation. 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.

Gyrus Medical, Ltd. (Minneapolis, MN), a medical technology company, developed a temperature control (TC) instrument as a component of the VAPR™ Bipolar Radiofrequency System held by Mitek Products, a division of ETHICON, a Johnson & Johnson company.

The TC electrode typically is used to treat joint instability through the thermal modification of soft tissue. During surgery, the VAPR system's radiofrequency (RF) technology creates rapid, predictable, and controlled temperatures at the working tip of the electrode. Heat from the TC electrode is applied to soft tissue such as capsular ligaments, which causes coagulation and contraction of collagen, a protein found in connective tissue. In this manner, a loose ligament can be tightened. Following post-operative recuperation, thermal surgery can restore joint stability, function and comfort.

Figure 1: The VAPR System consists of a generator and various styles of instruments, including the temperature control (TC) electrode developed by Gyrus Medical, Ltd.

The challenge faced by Gyrus was to design and manufacture an arthroscopic surgical electrode that would perform with required precision and accuracy, even at high temperatures. Heat can cause parts of the electrode to expand, which results in thermal stress. 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 rapid and accurate thermal modification of tissue.

Gyrus used Solid Edge computer-aided design (CAD) software from EDS to model the electrode tip assembly, and ALGOR finite element analysis (FEA) software to verify its performance under thermal stress. A 3D solid model of the TC electrode tip assembly was created using Solid Edge. The model consisted of a stainless steel tube, biocompatible adhesive layers, a polymer insulator material, and a stainless steel tip. The complete CAD assembly model was captured directly with ALGOR's InCAD technology.

Figure 2: Steady-State Heat Transfer Analysis was used to determine temperature results at the tip (left). Then, the temperature results were used as input to a linear static stress analysis (right).

In ALGOR, both 3D and axisymmetric 2-D versions of the model were studied using steady-state heat transfer and linear static stress analyses. A temperature loading of 65°C was specified for the steady-state heat transfer analysis, which simulated the performance condition at a 20W power setting to give the required 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 used as input to a linear static stress analysis to determine the thermal stresses. The FEA results revealed that the thermal stresses 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.

This work was done by Mike Hagland of Gyrus Medical, Ltd. For more information, visit For information on using ALGOR's finite element analysis products, contact ALGOR, Inc., 150 Beta Drive, Pittsburgh, PA 15238; Tel. 1-800-48-ALGOR;