Actuated and sensorized surgical instruments

In recent years minimally invasive surgery (MIS) has been established more and more. Access to the operation site is gained through small incisions in the patient’s skin using long, slender instruments. Since direct manual access to the operation site is restricted, surgeons have to train new operation techniques and learn to overcome the loss of haptic and tactile information.

Since manipulation is constrained by the fixed point of incision, traditional MIS instruments have limited degrees of freedom (DoF) restricting movement of the instrument tip, thereby hindering the surgeon’s dexterity. New forms of therapy overcoming these drawbacks are achievable only by the application of novel technical devices – in our opinion especially by medical robotics. Today’s commercially available medical robotic systems provide full dexterity inside the patient’s body, however information about the true tissue manipulation forces is currently not available to the surgeon.

We are developing novel instruments with additional degrees of freedom at the distal end – to retain full dexterity – and integrated force torque sensors (FTS). Using these 6-DoF sensors, true manipulation forces acting on the surrounding tissue can be acquired. The use of force feedback input devices together with advanced control algorithms enables the generation of realistic contact impressions and their presentation to the surgeon. Objective target of this research is a system that provides the surgeon with crucial kinaesthetic information, facilitating, e.g., the recognition of uncharacteristic tissue stiffness or the optimal amount of tension being applied to suture material. This will alleviate one of the main drawbacks of MIS compared to open surgery and greatly decrease incidents of tissue damage and failure of suturing material.

Presently we are working on a sterilizable minimally invasive gripper (10mm in diameter, which is an average size in MIS) using a cardanic joint to provide additional degrees of freedom. The cardanic joint allows for twisting of the forceps around its longitudinal axis while keeping the shaft stationary, which improves manipulability for common surgical primitives, e.g. knot tying. A Stewart platform based FTS and an independent gripping force sensor detect manipulation reactions. The instrument is tendon-driven and includes a propulsion unit. The complete system is self-contained and is provided only with electrical power and data by the medical robot.

In the propulsion unit tendon forces and positions are measured. Assuming known tendon compliance, the tendon force data greatly improves controllability and positioning accuracy of the gripper, while – compared to the manipulation forces – providing plausibility checks and error detection.

Autoclaving of sensors, motors and other thermo sensitive components can be largely avoided by dividing the propulsion unit into two parts. The part without patient contact containing motors and electronics is mist sterilisable, the one with patient contact is steam sterilisable and consists of cable and pulley mechanisms.

The geometry of the forceps branches can easily be varied, creating needle holders or clip applicators, while the propulsion unit remains unchanged. The system therefore is very adaptable.