Medical manufacturers find that simple can be innovative.

Success of medical equipment and implants is not a luxury, but rather a necessity. Lives of patients are at stake--and it almost always comes down to how well the fasteners function. Critical or sensitive devices such as lasers, scanners, knee joints, and implantable pacemakers as well as instruments such as stethoscopes must be reliable upon delivery ... and beyond.

There is little room for the $15 billion per year spent by 25 of the largest U.S. manufacturers on warranty claims--processing those claims eat up 2.5 percent to 4.5 percent of revenues for companies across all industries. Customer satisfaction and warranty problems associated with fastener failure must be prevented or quickly and economically handled.

Fasteners such as C-clips and specialized retaining rings can prove too cumbersome and costly, due to the unique nature of medical equipment applications. Because repetitive loads, shock, and vibrational loosening must be decisively managed, traditional fasteners susceptible to self-loosening rotational movement, stripping, and shearing are not always appropriate. Testing, in fact, has found that the first two threads of traditional fasteners can carry as much as 80 percent of the load, permitting stripping or shearing. As a result, the subsequent male threads "float" within the female threads.

With a variety of new technologies, medical equipment manufacturers are successfully attacking these problems. Spiralock designed a device created by re-engineering the standard (female) internal thread form and adding a unique "wedge-ramp" at the root of the thread. Unlike traditional fasteners that use a 60-degree thread, the device has a 30-degree wedge ramp cut at the root of the female thread. This unidirectional internal thread form mates with standard 60-degree male thread fasteners.

The wedge ramp allows the bolt to spin freely relative to the female threads until clamp load is applied. Under clamp load, the crests of the threads of standard male bolts are drawn tightly against this wedge ramp. This eliminates not only the sideways motion that causes vibrational loosening, but it also distributes the threaded joint's load throughout all engaged threads. The load percentage on the first engaged thread is significantly lower than traditional thread forms, which further reduce possible bolt failure and improves product performance.

Research at both the Massachusetts Institute of Technology and the University of Michigan confirm that the load percentage carried by each engaged thread using a Spiralock tap is much more uniform than with conventional 60 degree thread forms.

Lasers in line

Sometimes, the most innovative solutions are also the simplest.

Consider the case of Santa Clara, CA-based Lumenis Ltd., a global developer of laser and light-based devices for medical, aesthetic, ophthalmic, dental, and veterinary applications. Lumenis wanted to rule out the possibility of laser misalignment during rough shipping or handling. To safeguard its precision critical laser alignments, the company rejected standard thread forms and selected a self-locking fastener designed to resist loosening even under loads and vibrations strong enough to break fasteners.

"We wanted to increase the design's robustness to better withstand the shock and vibration of shipping as well as carting from room to room in the field," explains Peter Hines, Lumenis R&D engineer for laser design. "The self-locking design alleviates concerns about misalignment. With its shock and vibration resistance, we're preventing screws from backing out and preserving critical alignment for enhanced reliability."

Hines ruled out chemical adhesives because of the tendency to emit gaseous materials, which can collect on sensitive optics and degrade their performance. He was also less than satisfied with traditional techniques such as locking washers, which adds weight and complexity due to their less than desired locking ability. He decided on the self-locking Spiralock.

Knowing the knee

Warsaw, IN-based Biomet, Inc developed an artificial knee implant that closely fits the individual patient's anatomy, creating a wide range of motion and superior joint functioning compared to previously available implants. By splitting the tibial component of the implant into two pieces fastened with a single screw, surgeons can select each component's size based on the patient's specific anatomy.

However, success of this process is highly dependent on the threaded fastener that must guarantee repetitive loading without loosening--since the knee implant is designed to function for the rest of the patient's life without failure and the human knee is subject to 1,000,000 or more loading cycles per year. Extensive fatigue tests showed that snap-fit fasteners tended to work loose under extensive loading. But the Spiralock thread form did not fail, regardless of the length and severity of tests.

A modular tibial system consists of a titanium alloy tray with fins to resist rotational forces, a polyethylene bearing, and a titanium modular tibial stem that attaches to the tibial tray for improved stability. A standard titanium screw attaches to a Spiralock tapped hole in the stem to integrate the tray, polyethylene bearing, and stem into a single unit using a Morse taper fit between the tray and stem. After several years of in-body application, not a single screw fastened with the Spiralock thread form has come loose.

Replicating anatomy

Biomet's elbow replacement device joins a polyethylene bearing of an ulna component with two cobalt chrome articulating surfaces on the humeral component using the Spiralock thread. C-clips, specialized retaining rings, and other fasteners proved too cumbersome and costly for the design. "We had to find a way to lock the articulation together in a compact form without violating the elbow condyles," says Matt Brzozowski, Biomet extremities product manager. "To replicate anatomy as closely as possible, we not only have to keep the threads secure, but also want to provide a simpler approach to the articulating components should additional surgery be necessary."

For vibration control, loading, and shear strength, Spiralock threads were chosen in conjunction with titanium screws. The screws survived a shear test to approximately 2,700lb, at which point they broke, but remained locked in the thread form.

If revision surgery is necessary, the humeral stem may remain in place and surgeons don't have to take the component out, because the way the threads work allows the surgeon to access them from the posterior side of the elbow. For patients, that means less time in the operating room, Brzozowski says.

Though many implants are designed for permanent function without revision, some ultimately require revision due to unavoidable wear over time or the temporary nature of the implant itself. In these cases, the implant must function flawlessly while inside the patient, while keeping the healing, revision, or removal process as simple as possible. Spiralock Corp., www.rsleads.com/ 606tp-158