Originally Published EMDM October 2005
Technology News
Hanging by a Thread: Proprietary Fastener Design Improves Implant Performance |
An internal thread design from Spiralock withstands severe loading and shock that may cause traditional fasteners to fail. |
A knee implant designed to fit individual patient anatomies allows for a wide range of motion and improved joint functionality compared with previous models. Developed by Biomet Inc. (Warsaw, IN, USA), the replacement tibia is divided into two modular units that are fastened with a screw. The successful performance of the device relies partly on a threaded fastener supplied by Spiralock (Market Harborough, Leics, UK).
Traditional fasteners can be susceptible to self-loosening rotational movement, according to Spiralock. Furthermore, the company says testing has shown that the first two threads of traditional fasteners can carry as much as 80% of the load, leading to stripping and shearing. Self-locking fasteners resist loosening, even under loads and vibrations strong enough to break the fastener.
A 30°-wedge ramp cut at the root of the female thread is responsible for the product’s robust performance. (Traditional fasteners use a 60° thread.) Under clamp load, the crests of the threads in the male bolt are drawn tightly against the wedge ramp. This design eliminates sideways motion that may cause vibrational loosening and distributes the threaded joint’s load throughout all of the engaged threads. The load percentage on the initial thread is significantly lower than in traditional fasteners, reducing potential bolt failure and improving product performance.
Biomet’s modular tibia system comprises a titanium alloy tray, a polyethylene bearing, and a titanium stem. A standard titanium screw attaches the tray to the stem by means of a Spiralock-tapped hole. During several years of in vivo use, Spiralock fasteners have never come loose, according to the company.
Biomet also uses a Spiralock thread in its elbow replacement device. The thread joins a polyethylene bearing with two articulating surfaces on the humeral component. “We had to find a way to lock together the articulation without violating the elbow condyles,” says extremities product manager Matt Brzozowski.
“To replicate the anatomy as closely as possible, we not only had to keep the threads secure but provide a simpler approach to the articulating components should additional surgery be necessary.” The firm chose Spiralock threads and titanium screws, which survived shear tests attaining 2700 lb. While the screws broke under this extreme load, they remained locked within the threaded form.
Spiralock products are used in a range of other medical devices including heart pumps and MRI equipment. They are also being considered for use in implantable cardiovascular products and orthopaedic surgical instruments.
Copyright ©2005 European Medical Device Manufacturer
