Tiny 'wrist' to assist needle surgery

A tiny flexible wrist component for needle-sized surgical equipment could enable surgeons to perform operations in tiny spaces which would involve incisions so small that they could be sealed just with surgical tape, rather than sutures. Less than 2mm in diameter, the wrist, which would form part of the suite of equipment for a type of minimally invasive technique known as needlescopic surgery, has ben developed by engineers at Vanderbilt University in Nashvile, Tennessee.

Needlescopic surgery, also known as mico-laproscopy, uses instruments about the size of a sewing needle inserted through incisions that are typically 5-10mm long. It causes less pain to patients, reduces tissue damage and scarring, and speeds up recovery times; and is particularly useful for operations in the head and neck. But such tiny instruments are difficult to control.

The Vanderbilt team, led by mechanical engineer Robert Webster, is developing a surgical robot for needlescopic surgery, which a surgeon would operate remotely, like the Da Vinci robot which is now mainly used for abdominal operations such as prostate surgery. “The da Vinci uses a wire-and-pulley system that is extremely difficult to miniaturise any further, so it won’t work in smaller spaces like the head and neck,” said Webster. Instead of Da Vinci’s rigid rods tipped with pulley-operated instruments, Webster’s team is working on a robot whose ‘arms’ are steerable needles made from a memory metal called nitinol. These needles are curved, and nested inside each other in a telescoping configuration. By extending the telescoping sections and rotating them, the surgeon can move the tip of the needle to the site of surgery with great accuracy.

The current project addresses the problem of what happens when the surgeon gets the needle tip where it needs to go. Without a ‘wrist’ on the end of the needle, it’s hard for the the surgeon to cut or remove tissue, or repair damage. Most microlaproscopy uses sharp-edged rings or heated wires on the end of the needle to scrape or burn tissue away by moving the tip back and forth. “The smaller you can make surgical instruments the better…as long as you can maintain an adequate degree of dexterity,’ Webster explained.

Tiny mechanical wrists proved very difficult to make. “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. So rather than adding components together, Webster’s team focused on taking material away. “Instead of combining a bunch of pieces, we started with a tiny nitinol tube and began thinking about what we had to remove,” said team member Philip Sawney. Cutting minute slots down the side of the rigid nitinol tube made it more flexible; running a slender wire down the middle of the tube and attaching it to the tip beyond the slits gave a  mechanism to bend it, by just pulling the wire. This will bend the tip by up to 90°, and because of nitinol’s memory properties, it spring back when tension is released. Combined with a pincer on the end of the wrist, this could allow surgeons to make precise cuts and tissue removals and even microscopic internal suturing. “Adding the wrists to the steerable needles greatly expands the system’s usefulness,” said Duke Herrell, a professor urology surgery at Vanderbilt who is consulting on the project. “There are a myriad of potential applications in some really exciting areas such as endoscopic neurosurgery, operating within small lumens such as the ear, bronchus, urethra, etc. This would allow us to do surgeries that at present require much larger incisions and may even enable us to perform operations that are not feasible at present.”

The team is now working on the user interface and control software for the device, and hopes to test it first in transnasal surgery, which aims to remove tumours at the base of the skull by inserting instruments through the nose. “It should be useful for a number of other operations as well,” said Webster. “We think once we give this tool to surgeons they will find all kinds of applications we haven’t thought of.” Webster is hoping to find a commercial partner to develop the robot and take it through FDA approval, a process which he believes will take around five years.