Assembly in Action: Thread Forms Critical to Implant Success
August 20, 2007
For all their cutting-edge design and materials, today’s medical implants still require simple fasteners to function correctly.
For all their cutting-edge design and materials, today’s medical implants still require simple fasteners to function correctly. In fact, fasteners serve a critical function in making sure, say, a hip implant functions as it should.
Recently, fastener performance was a major concern for implant manufacturer Biomet Inc. (Warsaw, IN), as it was developing a new artificial knee. Central to the implant’s success is a tibial, or shin, component made up of two pieces fastened with a single screw. This modular arrangement makes it possible for a physician to customize each knee for a patient’s specific anatomy.
Obviously, this single screw is extremely important to the success of the product. Knee implant surgery is intended to be permanent, so the implant assembly must function for the rest of the patient’s life, without failure. Retightening the implant in a second or third operation is not an option, despite that fact that studies have shown that the human knee is subject to 1,000,000 or more loading cycles per year. Unfortunately, the human body generally rejects add-on locking components, such as adhesives, rings and patches, making them unacceptable for implant use.
To solve the problem, Biomet decided to employ a “wedge ramp” thread form, as developed by fastener manufacturer Spiralock Corp. (Madison Heights, MI).
At the heart of this technology is a 30-degree ramp cut at the root of the female thread. Under clamp load, the crests of the threads on a standard bolt are drawn tightly against the wedge ramp, eliminating the sideways motion that causes loosening under vibration. The ramp also distributes joint load more uniformly throughout a fastener’s threads than with conventional 60-degree thread forms. Biomet’s own tests confirmed that Spiralock did not fail, regardless of the length and severity of the tests.
The modular tibial portion of the implant now includes a titanium alloy “tray,” a polyethylene bearing and a titanium modular tibial stem, which 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.
During testing, the titanium screws survived a shear test to approximately 2,700 pounds, at which point the screws broke, but remained locked in the thread form. To date, after several years of service in the field, not a single screw fastened with the Spiralock thread form has come loose.
For more on locking fasteners, visit www.spiralock.com or call 800-521-2688.