North Haven, CT —Spinal fixation implants can be a real pain in the neck—not just for the patients, but for the surgeons too.
Used to stabilize surgically fused vertebrae, the implants consist of a collection of titanium bone screws and reinforcing rods. When assembling these medical Erector Sets, surgeons once had to twist in as many as two dozen troublesome threaded fasteners. Now, engineers at Tyco Healthcare's Surgical Dynamics subsidiary have developed two threadless fasteners that promise faster, stronger connections between the implant components.
The first of these patent-pending fasteners replaces the set screws that attach the spinal implant's longitudinal rods to bone screws installed in the vertebrae. With a rotating locking cap that engages a slot in the bone screw and a saddle-like base that sits atop the rod, the new fastener combines the best of press fit and threaded connections. "The nice thing about threads is that everybody knows how to use them," notes David Nichols, the company's director of research and development. Like a screw, this new press fit fastener does twist to lock. But it needs far less twisting than ordinary threaded fasteners, thanks to a locking feature machined in the rotating cap. This feature, a helical dovetail that tapers 0.044 inch over a 180°arc, locks with a single 90°turn to create a press fit between the bone screw, the rod, and the locking cap.
The resulting connection offers both holding power and easy installation. According to Dennis Sullivan, engineering project manager, the press fit provides two to three times more resistance to torsional slip than comparable set screws. It also installs with the same relatively low torque (70-90 in-lb) as the threaded fasteners it replaces. Getting the torque right—by optimizing the taper and putting an anodized coating on all mating surfaces—was no trivial matter. As Sullivan explains, too much torque increases the chance of a surgical slip during installation, while too little plays into a medical misconception about fasteners. "A lot of surgeons wrongly believe it's 'easy in, easy out'," he says.
Surgical Dynamics' second fastening innovation can be found on the telescoping cross-connectors that relate to the longitudinal rods like rungs to a ladder. In previous implants, these transverse rods have attached with set screws, while a separate locking screw held the telescoping sections in place. The new connectors do away with both threaded connections. Each cross connector now ends in a set of cam-driven, crescent-shaped jaws that fit over the longitudinal rod. Turning the cam by 90°forces the jaws to come together, grasping the longitudinal rod with about 100 lbs of force. Surgical Dynamics engineers also devised a collet mechanism to lock the telescoping sections in place without a screw.
Threading problems. Developed for the company's new Spiral Radius 90D system, both of the new fasteners address common threading problems. "Threads do a good job in many applications," says Nichols. "But they have fundamental problems in the operating room." For one, the spinal implant needs to be assembled through a rather gory incision. "With all the soft tissue, blood, and other fluids, it can be difficult to catch the first thread," Nichols explains. For another, soft titanium screws installed in this blood-soaked environment can too easily suffer from cross threading. "Without any threads, our fasteners obviously can't cross thread," he says.
And then, there's the problem of inconsistent tightening. Some surgeons take a white knuckle approach and over-tighten while less forceful practitioners may under-tighten. "It's difficult to know just how tight the screw is," Nichols says. The helical-dovetail fastener solves the problem with machined mechanical stops in both the installation tool and in the fastener's locking cap. The fastener also sports a visual indicator: lasermarked alignment lines in the cap and bone screw. Together these features ensure that the surgeon twists no more or no less than the ninety degrees required for a complete lock. The connector rod's cam arrangement, too, inherently eliminates the possibility of over-tightening.
Though the ease-of-use shortcomings of threads don't necessarily compromise patient safety, Nichols notes that they can prolong a patient's time under anesthesia as surgeons fiddle with screws. Each threaded connection requires five or six complete revolutions of the surgeon's "screwdriver," compared with a single 90°turn for the Surgical Dynamics fasteners. From observations in the operating room, Nichols estimates that the new fasteners go in at least ten times faster than a threaded fastener—potentially shaving precious minutes off the surgery time.
Threaded fasteners face a more serious problem in Nichols' view. Given the inherent tolerance-fit limitations of threads, traditional set screws can work their way loose when confronted with continuous micromotion of the spine. The Surgical Dynamics fasteners seem to resist this kind of loosening. In tests conducted for the company, the implant components have shown no loosening after five million motion cycles with loads up to 80 lb. "All threaded fasteners have a propensity to loosen that our fasteners don't have," Nichols concludes.
David Nichols, Surgical Dynamics, Tel: (203) 492-7438 ; Fax: (203) 492-7334; E-mail: david.nichols@.ussurg.com.