Computers have changed the world we live in, and have invaded every aspect of our lives. Joint replacement surgery is no exception to this. The use of computers in joint surgery is called Computer – Assisted Surgery (CAS) or Computer Navigation.
There are separate computer softwares available for hip and knee replacement, but at present navigation is used mainly in knee replacement. To understand how computers work for us in this surgery, let us first consider a practical example.
Most cars that we use today are equipped with a GPS tracking system. It works as follows: the GPS transmitter in the car sends a radio signal to four satellites in the sky. These radio signals travel along straight lines, and so the four satellies receive the signals from four different directions.
Theoretically, three satellites would be enough, but as the earth is not a perfect sphere, a fourth satellite is used. The satellies communicate with each other, and based on the different directional radio signals received, the software in the satellites’ computers pin-points the location of the car with a fair degree of accuracy. This information is fed back to the car, where it is integrated with maps stored in the on-board computer.
Thus, we can know our location accurately in real-time, and navigate our way on the surface of the earth, irrespective of whether we are in Mumbai, New York or Sydney.
A similar principle is used in computer navigation in knee surgery. The surgical instruments represent the car. The navigation camera replaces the satellite. The patient’s bony anatomy represents the roadmap.
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First, some reflective spheres are attached to pins which are inserted in the patient’s femur (femoral array) and tibia (tibial array); and to a hand-held device (hand-held array). The navigation computer is mounted on a stand, about 2 meters from the patient, on the opposite side of the surgeon. A transmitter in the computer beams infra-red rays onto the spheres.The rays are reflected back from the spheres and are captured by a special camera connected to the computer. This information is fed to the computer software. The femoral array moves with the femur and the tibial array moves with the tibia. The hand-held array is moved by the surgeon along the different parts of the patient’s knee, as per prompts from the interactive computer screen.
Thus, the computer gains information about the relative positions of the the different elements of the patient’s arthritic knee. Then the computer does some smart calculations and forms a digital image of the patient’s knee in virtual, 3-D space. This is displayed on the screen in the form of text data and animated images. Then the computer goes further and suggests to the surgeon the best possible way to make the bone cuts and place the implants. The software also allows for assessment of the ligament balance.
The surgeon uses the computer’s instructions and makes the bone cuts himself with the help of a fourth array (this is a bone–cutting tool, also mounted with the reflective spheres). The surgeon may tweak the computer software before actually doing the bone cuts. The interactive navigation screen provides visual confirmation at each step in the surgery. In other words, the surgeon can perform a virtual surgery on the computer before commiting himself to the actual bone cuts.
The computer navigation unit is compatible with implants from different manufacturers, with the appropriate software.
Different navigation systems from several manufacturers are available. The accuracy of most systems is within 0.5 mm and 0.50. One must note that even without CAS, conventional TKR surgery is also very accurate and reliable. The computer helps us by eliminating the factor of human error.
Specialized training and hands-on experience is mandatory to learn CAS. But once mastered, this is a powerful tool in the hands of the surgeon. Using the computer in knee replacement surgery converts the surgeon into a ‘systems-operator’ just like the modern-day airline pilot. And just like flying an aircraft, knee surgery is as much an art as it is a science.
