'Squeaky' Hip Implants

To the acoustical signature of the orthopedic implant detected by a security sweep, add the distinctive "hip squeak." The sound is associated with implants made of a material known as alumina ceramic-on-ceramic — and is audible to the person with the implant and those nearby.

While the hip squeak was previously known to experts, its precise cause has been a mystery. The Mayo Clinic Biomechanics Laboratory report released today sheds light on the potential causes of the squeak, thus guiding means of eliminating it.

After 11,000 cycles of tests in a mechanical simulator that reproduced the flexion and extension motions of the hip, Mayo Clinic investigators concluded that:

• Squeaking occurred when the film fluid between the two moving surfaces was disrupted.
• Disruption could be caused by the presence of particles that originate from wear and tear, or from imperfect alignment or positioning of implant surfaces.
• Squeaking occurred especially quickly under highest pressure on the artificial joint.
• Once squeaking started, it didn't stop — and was constant at all frequencies tested.

According to Robert Trousdale, M.D., the lead Mayo investigator: "Adding a small amount of lubricant solved the problem in the lab. Our research is helpful because it can be applied to devising a solution to the squeak problem. Most likely that will consist of improving the design of implants so they have less chance of material transfer and disruption, which appears to be an important aspect in the basis of the squeak."

Mayo Clinic orthopedic surgeons presented this research at the annual meeting of the American Academy of Orthopedic Surgeons (AAOS) in Las Vegas, Feb. 25-March 1.