The da Vinci® system: technology and surgical analysis

Passive arms®

The slave part is a massive structure made of four arms. The kinematics of each single arm mostly use serial linkages and rely on the concept of the remote center of motion (RCM) [ ]. A passive proximal part, dedicated to set-up the RCM point of each arm, is built from four joints ( Figure 6.2 ). The first joint is prismatic and allows adjustment of height. This is followed by three planar revolute joints of the SCARA type, thus allowing easy passive manipulation. The prismatic joint is electrically actuated while the other joints only contain brakes with a push-button to release the brakes during the set-up process.

The bulky system base is sufficiently heavy to make the whole system immobile once the roller brakes are engaged, and the system is therefore not mechanically secured to the operating table. The latest versions of the system are motorized in order to assist the nurse, who may not have the necessary strength to set-up the system.

Actuated arms

The structure of these arms may be broken down into two sub-units: the manipulator arms with 3 active degrees of freedom (DOF) and the end-effector instrument, which is actively actuated with 3 further DOF, together with an optional grip function, depending the instrument used.

The manipulator active arm is the result of long research [ ] and combine serial linkages with a parallelogram parallel structure used for the RCM ( Figure 6.3 ).

Initially the RCM was built around rigid linkages but has evolved into a cable-driven remote control of motion in order to make each arm lighter and more compact ( Figures 6.4 and 6.5 ).

EndoWrist® instruments

These form the terminal part of the telemanipulator system and are probably the most imaginative contribution of the whole da Vinci® system to surgery. They also represent the most important part of the Intuitive Surgical Inc. patent portfolio with more than 200 related patents.

EndoWrist® includes a range of instruments with various mechanisms ( Figure 6.6 ). Our aim is limited to a kinematic analysis of forceps which are the most mechanically sophisticated item and the most important in transoral robotic surgery.

EndoWrist® forceps have been designed to enable the kinematic system to work within the body. This ingenious yet sophisticated concept provides exceptional dexterity which is unachievable with conventional instrumentation. Three DOF plus the forceps jaws opening-closing actuation are operated by tendon-driven mechanisms.

EndoWrist® instruments are available with 8 and 5 mm output diameters, a total length of 57 cm and a usable length of up to 38 cm [ ]. Eight millimeters instruments have cardan joints while 5 mm ones have a more sophisticated articulated structure [ ] formed from four serial joints ( Figure 6.7 ).

The 5 mm instruments are the most commonly used for transoral robotic surgery due to the limited space available in the oropharyngeal cavity.

This four vertebral joints tip structure allows the joint diameters to be reduced, at the cost of increased length: craniocaudal Φ and lateral rotations θ are each uncoupled in two alternating rotation stages, Φ ₁ , θ ₁ , Φ ₂ , θ ₂ , respectively. Cables and pulleys are used to control joint angles and grip function (forceps jaws opening-closure). The system use an agonist-antagonist control scheme which therefore requires two actuating cables by DOF. Subsequently, 6 ropes are required in order to actuate craniocaudal and lateral rotations and the forceps jaws. The path of these cables runs inside the instrument shaft with spacers to avoid cable conflict and the resulting friction. The 4 ^th DOF is the shaft rotation Ψ , which is operated directly inside the actuation box. Four capstans are each dedicated to a DOF, while tensioning pulleys ensure constant cable tensions which otherwise would deteriorate as a result of ambient temperature, wear, and repeated sterilizations.

Each capstan itself is mechanically connected to a rotating drum when the EndoWrist® instrument is fixed onto a manipulator arm. Another set of cables provides motion for these drums and consequently, actuation of the 4 DOF of the forceps.

Kinematically, this configuration is of the serial kind, allowing a large range of motions with a very limited end-effector footprint. Forceps full angulation, Φ and θ is ± 90°, while Ψ allows full 360° axial rotation, providing very comfortable reach. Payload capabilities are roughly of the same order of those which are usual in open surgery, although there is no accurate information on this feature.

The drawback of 5 mm articulated instrumentation is that biplanar vertebral continuum joints have a greater radius of curvature and therefore require more space to make them angulated than cardan-jointed 8 mm instruments.

Due to its complex and therefore expensive factoring process and the relative weakness of EndoWrist® architecture, the manufacturer has compromised in order to comply with these contradictory statements. The system is sold to be used 5 or 10 times [ ] (depending the instrument) before being discarded. It introduces a new concept in the history of medical devices: the middle-consumable instrument. An electronic chip with a thermistor which is incorporated into the control box memorizes each sterilization before destroying an identifier code, obliging the user to discard the instrument once its maximum number of cycles has been reached.