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 says.
As much as a small size helps, it's not enough. The instruments have to have the right shape to reach cutting sites within the sinus cavity. Adams reports that the wrapped steel coupling can accommodate bends up to 100 degrees without kinking, while reinforced tubing could only handle 60 degrees. And this extra bit of design freedom allows clinical needs rather than mechanical constraints to dictate the instrument's contours. For the Radial Frontal Finesse Bur, which has to reach to the back of the frontal sinus cavities, Leopold specified what he calls a "very tricky" shape that puts a 40-degree bend close to the distal end of the instrument. "From a power transmission standpoint, it would have been much easier to put the bend further away from the distal end," notes Leopold, who also has an engineering background.
Reinforced tubing also offers plenty of flexibility around a bend, but it suffered from kinks. "It would buckle around bends, which would restrict the flow through the tube," Adams recalls. A reinforcing spring within the bend did prevent some kinks, but it also became a "choke point for bone chips," Adams adds. In spite of its 18% smaller inside diameter, the steel tube eliminates that choke point for a higher flow than reinforced tubing. Adams made good use of that flow to add an aspiration port at the end of the bur, rather than the proximal location found on earlier designs.
Durability and strength also set the new coupling apart from its plastic predecessor. Adams reports that the bent portion of reinforced tubing or its induction-heated adhesive joints would sometimes disintegrate without making it through even a single surgery. Surgeons would then have to take the time to put on a second bur. "Plastic was just not strong enough to handle both the bend and the torque," Adams says. The metal coupling, however, doesn't lack torsional strength. Though it only sees about 10 in-oz of torque from the handpiece motor, the coupling has stood up to 7.0 in-lb in tests for a 438% improvement over the reinforced tubing, Adams reports. Adams believes it also has more axial strength, which it needs to resist pushing and pulling motions as the surgeon drills through bone in the sinus cavity.
The steel coupling's contributions to the new bur promise to make an unpleasant surgery a little easier for both the surgeon and the patient. Its size and optimized shape extend the range of powered instruments. "We're able to reach places we couldn't reach before," Leopold says. And the coupling's resistance to clogging and its centrally located aspiration port save time in the operating room. Leopold explains that the new port location directs fluid and debris away from the endoscope lens rather than toward it as older designs did. Fewer lens cleanings and clogs help the new bur shave something other than bone—time. Leopold estimates that the bur cuts 45 minutes from a three hour frontal sinus drillout procedure.
Though developed for the Radial Frontal Finesse Bur early last year, Adams has since adapted the wrapped coupling to a variety of ENT instruments by tweaking the thickness of the ribbon, the geometry of the laser cut, and the bend's contour and location. With the addition of stronger materials for the hub mechanism, the steel coupling has even been modified for a new line of high-speed (12,000 rpm) instruments. "We're now in the process of replacing reinforced tubing on everything we make," Adams says.
Ken Adams Medtronic
Xomed, Tel: (904) 296-9600; Fax: (904) 281-1057;
E-mail: firstname.lastname@example.org .