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Tiny mechanical wrist gives new dexterity to needlescopic surgery

25 July 2015

With the flick of a tiny mechanical wrist, a team of engineers and doctors hope to give needlescopic surgery a whole new degree of dexterity.

The mechanical wrist is less than 2mm in diameter: small enough to be used in needlescopic surgery (photo: MED Lab, Vanderbilt University)

Needlescopic surgery, which uses surgical instruments shrunk to the diameter of a sewing needle, is the ultimate form of minimally invasive surgery. The needle-sized incisions it requires are so small that they can be sealed with surgical tape and usually heal without leaving a scar.

Although it's been around since the 1990s, the technique, which is also called mini- or micro-laparoscopy, is so difficult that only a handful of surgeons around the world use it regularly. In addition, it has largely been limited to scraping away diseased tissue with sharp-edged rings called curettes or burning it away with tiny lasers or heated wires.

Now, a research team headed by Vanderbilt University's associate professor of Mechanical Engineering, Robert Webster has developed a surgical robot with steerable needles equipped with wrists that are less than 2 mm thick.

The new device is designed to provide needlescopic tools with a degree of dexterity that they have previously lacked. Not only will this allow surgeon-operators to perform a number of procedures such as precise resections and suturing that haven't been possible before, but it will also allow the use of needles in places that have been beyond their reach, such as the nose, throat, ears and brain.

For the past six years, Webster and his colleagues have been developing a surgical robot that uses 'steerable needles'. This is a system of telescoping tubes that are made out of nitinol, a 'memory metal' that retains it shape. Each tube has a different intrinsic curvature. By precisely rotating, extending and retracting the tubes, an operator can steer the tip in different directions, allowing it to follow a curving path through the body.

This design allows the needles to operate in areas of the body that manual endoscopic instruments, which are straight rods equipped with a variety of end effectors, can reach. However, its usefulness was limited by the fact that the needles didn't have a wrist.

The researchers made a number of unsuccessful attempts to build mechanical wrists that were small enough. "We kept trying to build the wrists out of a lot of small pieces, but we couldn't get them to work up to our standards," said Webster. Instead of combining these pieces, the team started with a tiny nitinol tube and began thinking about what they had to remove.

The tube is extremely rigid, but they discovered that if they cut a series of tiny slots down one side, the rigidity decreased substantially: enough, in fact, so they could get it to bend up to 90 degrees by pulling on a small wire that runs inside the tube attached at the tip. The wrist springs back to a straight position when tension on the wire is released.

Vanderbilt University applied for a provisional patent on the design in May.

Team members are keen to test the system by using it for transnasal surgery: operations to remove tumors in the pituitary gland and at the skull base that traditionally involve cutting large openings in a patient's skull and/or face. Studies have shown that using an endoscope to go through the nasal cavity is less traumatic, but the procedure is so difficult that only a handful of surgeons have mastered it.

By the end of the summer, the team hopes to have completed the control software and the interface that allows the surgeons to operate the device.

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