An innovative medical development company in Miami is using a self-locking fastener to dramatically extend the life of an elbow implant.
"We aimed to fill a gap in the market by providing the ability to orient the joint to its natural position during surgery, and then locking it in place for the rest of the patient’s life," says Tom Norman, vice president of engineering at Skeletal Dynamics. "We considered traditional fastener thread forms, but lacked confidence that they would stay in place for the up to a million load cycles an active patient could subject them to in their lifetime. Instead, we selected an innovative self-locking fastener called Spiralock."
Norman told Design News in an interview that the company's focus is innovative products for the body's upper extremities.
Traditional prosthetic radial head designs followed one of two approaches that have drawbacks. According to Norman, the traditional fixed monoblock could not be aligned to the patient’s anatomy, which tended to wear away natural tissue such as cartilage. In another approach, a bi-polar radial head was an attempt to align with the patient’s native anatomy. It was able to rotate in a polyethylene sheath, but would not remain in the correct position because it would not lock.
Testing has found that the first two threads of traditional fasteners can carry as much as 80 percent of the load, permitting stripping or shearing, while subsequent male threads “float” within the female threads.
The Spiralock self-locking fastener has a 30-degree “wedge” ramp cut at the root of the female thread. Traditional fasteners use a 60-degree thread. Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against the wedge ramp.
Surgeons can tighten a milled interrupted thread to lock a radial head into the patient's anatomy.
The ramp cut not only eliminates sideways motion that causes vibrational loosening, but also distributes the threaded joint’s load throughout all engaged threads, a claim supported by an MIT research study.
The load percentage on the first engaged thread is significantly lower than traditional thread forms, which further reduces possible bolt failure, and improves product performance.
In the medical field, because of their vibration-resistant, reliable self-locking features, innovative fasteners are used to hold various components together in implants, artificial limbs, heart pumps, and MRI machines. They are now being considered for cardiovascular devices such as pacemakers and implantable defibrillators, as well as for dental and orthopedic surgical instruments and CT scanning applications.
When Skeletal Dynamics’ Align Radial Head System is surgically installed in a patient, proprietary instrumentation allows alignment of the radial head as it would be in the patient’s native anatomy. Once the surgeon orients the device in this natural position, the surgeon tightens the set screw in a Spiralock milled interrupted thread made of cobalt chrome, against a long titanium stem designed for three-point fixation, to lock the device in the correct position.
“While fasteners used in upper extremities are frequently tested to 100,000 cycles, third-party testing showed that the Spiralock self-locking fastener used on the Align Radial Head successfully survived load and fatigue testing to one million cycles without loosening or backing out,” says Norman. “That gave us the design confidence we needed, and helped with our FDA approval.”
Early in the design process, when Skeletal Dynamics considered the manufacturability of the self-locking thread, Spiralock built prototype tooling, then customized tools to cut the thread form, and finally worked closely with a contract manufacturer to scale up production.