Computer-assisted robotic surgical systems have made total endoscopic cardiac surgery a reality. However, current robotic surgical systems are limited by the lack of tactile or haptic feedback critical to performing complex, delicate surgical tasks.⁴ Clinical successes with robot-assisted minimally invasive cardiac surgery have lagged behind those achieved with robot-assisted laparoscopic general surgery due, in large part, to the lack of feedback. This deficiency with current robotic systems is a significant handicap in performing the technically more intricate and delicate surgical tasks inherent in cardiac surgery. For example, suturing a coronary arterial anastomosis with fine polypropylene suture is a highly dexterous task, in which the surgeon typically uses his or her sense of touch to puncture tissue with a fine needle, pull the suture through, and tie and tighten knots. In our own observations of experienced and talented cardiac surgeons training with the da Vinci surgical system, fine polypropylene sutures are often broken, and delicate tissues torn, due to the application of excessive forces conventionally attenuated with haptic feedback. The consequences of such surgical errors or delays in cardiac surgery (e.g., coronary microvascular or great vessel trauma, prolonged cardiopulmonary bypass) present much greater potentials for irreversible injury, excessive hemorrhage, or even death for the patient.

Computer-augmented, robot-assisted surgery has enhanced the ability of surgeons to perform minimally invasive procedures by placing a digital interface between the surgeon and the instrument tips. The digital interface enhances the surgeon’s ability to perform minimally invasive cardiac surgery in several ways. First, by filtering high-frequency signals, surgical tremor is eliminated. Second, the digital interface allows for motion scaling in which easy-to-perform macroscopic movements at the surgeon console are scaled down to a microscopic scale inside the patient, enhancing dexterity. Finally, the computer interface permits the accurate translation of the surgeon’s hand motions to an endoscopic wrist placed within the chest cavity, conferring much higher degrees of freedom than traditional manually actuated endoscopic instruments.

The da Vinci system has been used successfully to perform endoscopic coronary artery bypass grafting^5,6 and mitral valve repairs.⁷ These early operations have been performed in carefully selected patients by a select few surgeons who were intimately involved with the development of these systems in the laboratory and the operating room. Despite these early procedural successes, improvements and adjunct technologies are needed before widespread use of the current systems can occur.⁸ For instance, the lack of tactile feedback has led to prolonged operative times and difficulty in performing some forcesensitive surgical tasks required in cardiac operations.⁴ Experienced robotic and endoscopic surgeons use visual cues (e.g., tissue deformation) to approximate how much force is being applied: however, this type of sensory substitution is applicable only if the surgeon has a good internal model of tissue consistency to correlate applied forces and tissue deflection.

Ideally, haptic feedback in robotic surgical systems would be relayed directly to the surgeon’s control actuators, rendering a true tactile feedback. This capability, however, will require the application of force-sensors on the endoscopic instrumentation. Our laboratories are actively pursuing this goal. As an interim alternative, we have employed sensory substitution to display haptic information via a visual display. Previous work in using audio feedback of force levels has been a success in microsurgery.⁹ However, additional noise, added to the sounds already associated with cardiac operations (e.g., cardiopulmonary bypass pump, hemodynamic/pulse oximetery monitors), may prove undesirable. A visual force-feedback display in the operative field display may prove to be more ergonomic and have already found application. Image-guided virtual environments in the form of advanced video game systems and surgical simulators utilize surface image deformation, sometimes in conjunction with haptic instrumentation, to provide realtime simulations of operative procedures to include spine surgery, cholecystectomy, and hysteroscopy, and colonoscopy.^10,11 Future developments in this area includes visco-elastic modeling of the surface image deformations as well as higher fidelity force computations and system validation. In these virtual simulators, the image itself serves as a sensory substitution modality. The impetus of these early experiments was to apply the virtual environmental concept of visual force feedback to the real world surgical task of suture knot tying.

Our results demonstrate significantly greater and more consistent tensions applied to suture materials, without breakage, during robotic knot tying enhanced with haptic feedback compared to knots tied without feedback. During the execution of these experiments, several interesting observations were noted. Surgeons broke the finer polypropylene sutures on several occasions both with and without sensory substitution. Interestingly, surgeons were able to partially compensate for the lack of haptic feedback by visually observing the deformation of tissue when the knot was pulled away from the tissue. We also noted that different users employed different tying techniques; some surgeons used equal force with both hands, while others favored increased control with one hand.