Coupling drives surgical advances

February 26, 2001

Jacksonville, FL -Surgeons increasingly use power tools when they get inside a patient's head. Engineers who design those tools wind up with the headache.

That's because powered surgical instruments for endoscopic ear, nose, and throat (ENT) surgeries have to satisfy clashing design goals. The instruments have to be compact and curved so that surgeons can guide them through the head's convoluted passages without "nicking" nearby arteries or nerves. At the same time, they have to pack enough power to remove not only soft tissue but also to grind away bone. To help resolve this classic clash between size and power, Medtronic Xomed engineer Ken Adams devised a small, strong, flexible coupling for the company's curved, rotating surgical tools.

Made from a flexible tube surrounded by a rigid lumen, the coupling transmits up to 10 in-oz of torque at 6,000 rpm from a motorized handpiece to a bur about five inches away and around a 40-degree bend. It also acts a bit like a vacuum-cleaner hose as the instrument sucks blood, saline, and bone chips through the coupling's hollow center.

The coupling's most inventive aspect is the construction of its inner tube. To give it enough flexibility to work within the bent outer lumen, Medtronic first incises the tube walls with a helical cut that occupies the final one-and-a-half inches from the bottom of the cutting tip. Two layers of stainless steel ribbon, wrapped in opposing directions, then go over the helical cut to give the tube enough axial strength and structure to withstand the torque. "Without the wraps, the tube is basically a glorified spring," Adams says, explaining that motor's twisting force helps stabilize the wraps by winding tight against one another and the base tube. Completing the coupling are a steel adapter to attach the cutting tip to the end of the base tube and a thermoplastic hub mechanism to join the coupling to the handpiece.

A better bur. The new coupling first turned up as part of the Rad Frontal Finesse Bur, a patent-pending tool for removing bone during delicate frontal sinus surgeries. The new coupling, which hits the market early this year, enabled a collection of improvements over earlier burs that used couplings made from reinforced polyurethane tubing.

Size reduction tops the list. According to Adams, the new coupling's slim base tube (0.087 inch OD), enabled a 20% reduction in the coupling's overall diameter compared to earlier couplings. That decrease shrinks the OD of the bur from 3.6 mm down to 3.0 mm. And given the confines of the frontal sinus cavities, size really matters. Smaller instruments block less of the surgeon's view through the endoscope and can be easier to maneuver. "Even a small reduction in size can be a huge benefit," says Dr. Donald Leopold, the University of Nebraska surgeon who helped develop the bur. He notes that frontal sinus "drillout" procedures bring the cutting tools within just one or two millimeters of the optic nerve, anterior ethmoid artery, and the brain itself. One slip and the bur would "wind up this soft tissue like spaghetti," he

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