Students in biomechanics and graphic design to study strategies for successful computer-assisted orthopaedic surgery

Brussels 24 May 2000At Health Care 2000, biomechanics and graphic design student Geert Van Ham presented a survey of the scientific research, conducted by Dr. Ing. Jos Van der Sloten and his student team at the Catholic University of Leuven, in order to catalogue and promote the use of surgical robots for orthopaedic interventions. The computer-assisted orthopaedic applications enable the surgeon to carefully plan the operation in detail, whereas during the actual procedure, movement of the surgical tools can actively be limited. Another major advantage is the automatic and fast robot reaction to sensor signalling. Also, there are no constraints in implant design. Excellent news for the patient because this unexpected freedom allows to develop alternative types of artificial limbs which far better match the patient's anatomy.

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Orthopaedic surgery requires the utmost precision to enable an accurate positioning with regard to the bone as to stabilise its structure. The surgeon's strategy consists in attentively studying the image scans in 2D or 3D during the pre-operative planning, to adapt the model to the patient and to feed the measurement data to the robot. The act of registration forms the critical link between planning and performance, as Mr. Van Ham explained. The bone structure has to be immobilised. Two types of robots are currently being used to execute the pre-operative plan: application specific devices for hip or knee surgery which are intrinsically safe, and general purpose robots that are programmed to perform a medical task. The latter still require more built-in safety.

The robot has to hold the virtual body part into the right position to match the patient's bone structure. The active computer systems perform this task independently whereas the semi-active ones only start acting on command of the surgeon who is supervising the entire operation. Mr. Van Ham presented a few examples of currently existing surgical robots to the audience. One of them is ROBODOC, designed by Integrated Surgical Systems for femoral drill operations. The clinical results are spectacular since no complications have been registered after more than 4000 interventions. Starting from a CT-scan, the surgeon produces a 3D-model of the femoral structure during the pre-operative planning. In the registration phase, the femoral structure is being immobilised with regard to the robot base.

In the traditional approach, an extra operation often was needed because the patients started suffering from the dislocation of ill-fixed screws. Today, the surgeon supervises the general purpose robot, equipped with safety sensors, and is able to intervene at any stage. Robot application even can be extended to other orthopaedic interventions, such as revision procedures, according to Mr. Van Ham. CASPAR, developed by the German company Ortomaquet, is a system similar to ROBODOC. Instead of applying the conventional method to surgically implant pins into the bone, the CASPAR system is able to perform registration by means of surface geometry. The ACROBOT, designed by Brian Davies at the Imperial College of London, uses its four degrees of freedom to perform an interactive orientation of the hip prosthesis components.

ACROBOT also provides haptic feedback control to avoid contacts occurring outside the safe regions. Unfortunately, the system cannot be extended for use in other surgical interventions. At the Catholic University of Leuven, the students of Dr. Ing. Van der Sloten are working on their own project relating to the pre-operative planning of knee and hip surgery with robot-assistance. In the first case, frontal and sagittal X-ray images are made from the tibia to speed up the procedure. However, no 3D-visualisation is possible like in the slower method with CT-scans. After a screw has been introduced, a cylinder is rotated until the screw points to the front. A general purpose robot with force feedback is applied, allowing the surgeon to keep the movement under control without leaving the safety zone.

In turn, the hip surgery is pre-planned by producing a 3D model of the pelvis based on a patient CT-scan. Here, the anatomic structure is somewhat more complicated that that of the knee. The robot measures all reference points in order to fit them into the CT-image. The registration is performed during the actual operation, with a continuous feedback to the surgeon. The reference points have to be well spread by a general purpose robot with force feedback. The robot is able to limit the movement to the spherical cavity. Drilling forces are measured on-line to offer additional feedback to the surgeon with regard to the fixation technique by means of screws. The temperature rise is directly related to the quality of the bone. Therefore, temperatures during drilling are carefully measured because beyond 45°C, the bone degrades.

Future research will concentrate on an optimised biomechanical planning of the surgical intervention, as Mr. Van Ham pointed out. Especially, the stage of registration will get every attention. As for the robot control, the research team will strive for a perfect synergy between the surgeon and the robot. In addition, the performance of application specific computers will be compared against the general purpose robot outcomes. Major issues equally constitute the different tension parameters as well as the heating effects which have to be controlled by defining a minimum rise of temperature. The students of the Department of Biomechanics and Graphic Design thus hope to contribute to a safer and patient-friendly application of orthopaedic surgical robots within the operating theatre.


Leslie Versweyveld

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