The coronary artery bypass graft (CABG) surgery is a commonly performed procedure worldwide. A device that would enable surgeons to perform the operation on a beating heart would make this procedure less invasive, thus reducing the post-operative complications.

The occlusion of a coronary artery prevents perfusion of the heart tissue and it may result in necrosis of the tissue and an infarct with a severe and life threatening damage to the myocardium [April96]. The purpose of the CABG operation is to provide blood flow through the coronary circulation by attaching a blood vessel (graft) from some other part of the body to a coronary artery. The sewing of the bypass graft to the coronary arteries entails very little detrimental physiologic impact to the patien t and could, if there were no other issues, be considered a minor procedure in terms of patient recovery [Treat97]. What makes the operation fairly invasive and detrimental to the patient in the short run is the need to open the chest cavity ("sternotomy ") to obtain access to the heart and also to place the patient's entire circulation on a cardio-pulmonary bypass machine. Although the negative effects, such as the post-operative pain, of the sternotomy are real, they are greatly overshadowed by the neg ative effects of the cardiopulmonary bypass machine. Placing a patient on the bypass machine is quite destructive to the circulating blood elements such as red cells and platelets and produces profound and sometimes life-threatening post-operative compli cations including bleeding and cerebrovascular strokes [Treas97]. The reason for placing the patient on the bypass machine is to keep the heart still while the surgeon performs the sewing of the graft onto the coronary arteries that are fairly small and require optimum conditions to work on [Borst97]. Although cardiac surgeons are attempting to sew grafts onto the beating heart, this procedure is obviously difficult and is at the outer limits of human surgical skill.

Surgical operations on the beating heart would be much easier to perform if the surgeon had a steady view of the heart surface. A video camera can provide a stabilized image of the heart. The idea behind this CABG surgery device is to move the video ca mera in concert with the heart and provide the surgeon with a view free of cardiac motion. The camera and custom-designed surgical instruments are mounted on a robotic manipulator. The camera motion is controlled by a heart motion-tracking algorithm. This provides the surgeon with a stationary picture of the heart on the video camera’s monitor. The surgeon can then control the surgical instruments telerobotically and perform the surgery more easily.

This thesis describes a manipulator which will be used to move the camera and the surgical tools with high velocities and high accuracy. A parallel link manipulator called the Stewart Platform is proposed because of its advantages over serial link mani pulators. The sections that follow review the current state of the art, describe the proposed mechanism, analyze its kinematic properties, and discuss design tradeoffs in building the mechanism. Simulation results are also presented for a number of design s.

Minimally invasive coronary artery bypass (CABG) surgery is a relatively new technique with increasing numbers of operations performed every year. Research is directed towards operation on the beating heart without need for cardiopulmonary bypass and heart access through small incisions. Most minimally invasive CABG procedures are limited to the cases of a single-vessel disease. The heart can then be accessed through a minithoracotomy, allowing anastamosis of the left internal mammary arte ry (LIMA) to left anterior descending coronary artery (LAD) [Mishra97]. The heart surface is stabilized using commercially available systems, for example the Access Platform and Stabilizer (CardioThoracic Systems, Inc., Cupertino, CA) [Cremer97] or by a s uction device the Octopus Tissue Stabilizer (Medtronic, Inc., Minneapolis, MN) [Mack97a]. The motion of a marker on the unrestrained heart surface covers an area of 15mm x 15mm. If the Octopus device is used to restrain the movements of the heart then the marker displacement is reduced to about 1mm x 1mm [Borst97]. The animal trials of a totally endoscopic CABG (E-CABG) are performed, using a stereo camera to obtain a three-dimensional video image. The heart is stabilized and a three-dimensional video hea d-mounted display is used for visualization of the operative field [Mack97a]. According to Mack, the procedures can be made more user friendly by means of robotic virtual immobilization of the heart surface. A high-speed tele-operated surgical robot would be placed in the chest cavity through ports and sit there in a "cardiostationary orbit", tracking a fixed point on the heart while the surgeon would be viewing a "virtually immobile" surgical field. Alternatives to the visual imaging are laser speckle surface imaging, three-dimensional confocal laser microscopy and mechanical spectroscopic imaging. Utilizing some of these technologies, only the structures of interest could be imaged creating virtually bloodless field for the performan ce of the procedure.

Robots are widely used for orthopedic surgery and stereotactic neurosurgery. They offer high precision positioning of the surgical instruments and better stability of the bone machining tools. Some examples of such robots are given in [Troccaz97]. ROBODOC is an active six Degree-Of-Freedom (6-DOF) robotic system for femoral bone machining in hip surgery, and AESOP (Computer Motion Inc) is a SCARA robot for laparoscopy with six DOF. The AESOP system can be controlled by the surgeon’s voice. Four of its six DOF are active and two passive, which gives it some safety features. Some of the robot assisted surgical tools are configured as synergistic devices and utilize the force feedback. These include the Passive Arm with Dynamic Constra ints (PADyC, TIMC Laboratory) and the knee surgery Active Constraint ROBOT (ACROBOT, Imperial College of London). ACROBOT employs backdrivable motors so that both the user and the motors together actuate the tool. These areas were, until recently, dominat ed by serial link manipulators as it can be seen from other overviews of medical robots [Burdea96], [Khoda96]. [Lavallee96] describes a system for stereotactic neurosurgery in clinical use since 1989. The system uses a robotic arm with 6-DOF with repeatab ility of ±0.2mm. Commercially available robots like PUMA-560 [Santos95] and IBM Scara 7565 [Rovetta96] are often used. All these systems tried to integrate visual imaging with the three-dimensional data obtained by CT or MRI, but the surgeon was responsib le for the control of the robotic tool. [Taylor96] presents a telerobotic system for laparoscopic surgery with intelligent trajectory and motion control, where the surgeon just points to the structure of interest and an algorithm handles the trajectory ge neration.

Recently, Stewart Platform (SP) type manipulators have been used for robotic assisted surgery. Stewart Platform with Fixed Actuators has been designed for ophthalmic surgery by Grace [Grace93]. An SP manipulator is being tested for image guided orthope dic surgery. After the registration of the manipulator position, a tool is mounted on the manipulator platform and the bone is machined. The problem of small workspace of the parallel link mechanisms is solved by adding special adapters that can reach the region of interest while the robot is moving within its working envelope. The robot workspace is 65x65x65 mm³ if the angle between the base and the platform is less than 15° and the robot maximal velocity is 10mm/s [Brandt97].

Tele-robotic systems have also been proposed to solve problems in micro-surgery and remote surgery. The idea of using robots in microsurgery is to scale down a surgeon’s motion by a master-slave manipulator. A cable driven 6-DOF manipulator for microsu rgery called RAMS has been developed at JPL [Schener95]. An experimental master-slave 6-DOF scaling teleoperation system has been developed at the University of British Columbia, Canada. The manipulator is positioned near the region of interest by a large r 6-DOF robot. Both master and slave are magnetically levitated manipulators, the motion of the master is scaled-down to smaller motions of the slave and the scaled-up force feedback is applied to the master, in proportion to the force sensed by the slave end-effector [Salcu97]. Mitsuishi et al. implemented a master-slave tele-robotic system for micro blood vessel suturing with two small robotic arms. The surgeon can see on a monitor a magnified image from a small CCD microscope. The display monitor move s, tracking the motion of the operator’s face while the CCD microscope rotates correspondingly about the focal point of the microscope. The operator controls two rotational-force-feedback-free master-slave manipulators (only translational forces feedback) [Mitsu97]. Tele-robotic principles are applied also at larger distances: a robot in Italy was remotely controlled by a surgeon in the United States performing an operation on a model with a pig’s organs. The two laboratories 10,000 km far from each other were connected via a satellite link [Rovetta96].

A series of robotic systems for neurosurgery have been developed as a result of the ET project. Through its ten-year development, the systems went through a number of modifications, most of which involved different devices for track ing the position of an object in three-dimensional space [Reinhardt96]. The system was used to superimpose coordinates of the surgical instrument with the three-dimensional images recorded by a CT, so that the surgeon could accurately operate the tissues in the brain not easily identified by a human. The first version of the system employed a mechanical digitizer to determine the tool position and achieved an accuracy of ±3mm. The next version used the same principle to resolve the tool position, but a h eavier and more robust digitizer arm yielded an accuracy of ±2mm. In the next stage of the project, ultrasonic spatial localization device Sonic Digitizer GP8-3D (Science Accessories Corp., Stratford, Conn.) could determine the positions of ultrasound tra nsmitters in space, sampling the distances between the transmitters at the rate of 24kHz. The surgical instrument was now hand held, with the transmitters attached to it. The accuracy of the system was ±1mm. Three CCD line-scan cameras manufactured by Pix sys Inc. (Boulder, CO) were used in the following version of the system to detect positions of LEDs with the measuring accuracy of ±1mm.

Tracking of an object can be performed using ultrasound. Several piezzo-electric transducers can be attached to the mobile object and to some known points in space. Ultrasonic systems determine the position of the object by measuring the distances betw een the transducers. These systems can acquire the distances very fast, but the computation of the object position is very time consuming. Sonometrics Inc. system has been used to determine the shape of the heart surface but the actual position computatio n is done off-line [Dickstein96]. In order to speed-up the position estimation, the transducers can be arranged on the vertices of a cube [Alusi97].

Another system, often used for tracking the spatial position, is Optotrak manufactured by Northern Digital, Inc. It is an optical system with active IR beacons that are flashing one at a time, while their position is sensed by three line-scan CCD camer as, with an absolute accuracy better than ±0.1mm [Cutting96], [Taylor94]. Instead of active IR beacons that are connected to the electronic device with wires, some optical tracking systems use passive markers illuminated by an IR source BrainLAB (BrainLAB GmbH, Germany, VISLAN (Guy's Hosp., London, U.K.). If the markers are to be occluded during the surgical procedure, an electromagnetic system can be used (Polhemus Inc, Ascension Technologies Inc.), but large errors can occur if a ferromagnetic material is brought near the measuring system.

Taylor implants three pins into the bone and uses an off-the-shelf video camera to determine the position of the bone. An object can be tracked using a video camera by detecting four co-planar markers on the object, utilizing the principles of uncalibr ated stereo. Sometimes an object to be tracked does not have a planar surface that can be used to attach the markers, for example a human leg. Uenohara and Kanade attach several markers on the object surface, record the images of the moving object and the n choose only four of those markers that appear to be closely co-planar. Later, they use these four markers for tracking the object in space [Uenoh95]. Marker detection is simplified if an image of the region of interest is analyzed and a histogram of the colors in the image is created. [Wei97] describes color coding for tracking of laparoscopic instruments based on the green color that does not appear in a laparoscopic image.