With active baby boomers living and playing longer, traditional artificial implants, their components, and the fasteners that hold them together must be built to last longer than ever before. Skeletal Dynamics recognized this need and developed the Align Radial Head System, an artificial elbow joint designed to restore the natural function of the native radial head.

“We aimed to fill a gap in the market by providing the ability to orient the joint to its natural position during surgery and them locking it in place for the rest of the patient’s life,” said Tom Norman, Skeletal Dynamics’ vice-president of engineering. “We considered traditional fastener thread forms but lacked confidence that they would stay in place for up to one million load cycles an active patient could subject them to in their lifetime. Instead, we selected a self-locking fastener call Spiralock.”

Because of repetitive loads and shock, loosening must be decisively handled for implant use. Traditional fasteners which are susceptible to self-loosening rotational movement, stripping, and shearing are not always appropriate. Tests showed that the first two threads of traditional fasteners can carry as much as 80% of the load, which can strip or shear, while subsequent male threads “float” within the female threads.

Spiralock fasteners hold components together in implants, artificial limbs, heart pumps, and MRI machines.

To solve this problem, the Spiralock fastener has a 30º “wedge” ramp cut at the root of the female thread (traditional fasteners use a 60º thread). Under clamp loads, the crests of the threads on any standard male bolt are drawn tightly against the wedge ramp. This design eliminates sideways motion and distributes the threaded joint’s load throughout all engaged threads so it cannot loosen. The load on the first engaged thread is significantly lower than traditional thread forms, which reduces bolt failures and improves product performance.

When the Align Radial Head System is surgically installed, special instruments help align the radial head as it would be in a patient’s anatomy. Once the surgeon orients the device in this natural position, she tightens the set screw in a Spiralock thread with a 3-point fix to lock the device into 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 1,000,000 cycles without loosening or backing out,” says Norman. “That gave us the design confidence we needed and helped with our FDA approval.”

Skeletal Dynamics
www.skeletaldynamics.com

Spiralock Corp.
www.spiralock.com

::Design World::