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Published 12/1/2008

One more reason for squeaky hips?

I am writing in response to the article “Squeaky hips make media, medical noise,” in the October 2008 AAOS Now. A very important explanation of squeaky hips and for that matter, premature wear in all total joints, which I have not seen addressed in any of the orthopaedic literature, is … the actual manufacturing of the implant.

No matter how well engineered these devices are, the current manufacturing tolerances in the construction of these devices leave much to be desired. Many implants sold under the name of renowned orthopaedic implant companies are not manufactured by them. These companies may design and market these devices, but the actual manufacturing of the devices is done by original equipment manufacturers, who function as jobbers for the companies.

In my interactions with these companies, I became aware that orthopaedic devices [are manufactured] to meet regular industry standards, which are plus or minus 1 mil (one thousandth of an inch). An implant 44 mm in diameter could be 44 mm plus or minus 1 mil. The same tolerance would apply to an acetabular component that is nominally 44 mm in diameter.

When an orthopaedic surgeon selects random acetabular and femoral components, even of the same size, the probability exists of significant incongruence between the components that could affect long-term function. The effect would be failure of total contact of the implants; ring or point contact would result in excess pressure on the contact points. The result may be noises from the implants, excessive wear, premature failure, and excess production of wear products.

The same issues apply to the manufacture of polyethylene and ceramic components, which are harder to fabricate to close tolerance than metal implants. Wide manufacturing tolerances also exist in sphericity of implants …

To my knowledge, no machine is capable of doing the final polish on the articular surfaces of metal orthopaedic implants; [this] is done manually by skilled humans. This polishing removes material in a nonuniform way and can also affect the congruence between an acetabular cup and a femoral head …

[H]igher levels of precision in manufacturing are likely to improve long term survival of total joints with less chance of noise, lower levels of wear, and less creation of wear debris. Demanding a higher level of manufacturing tolerances would result in less wear, consistently longer life, and less noise in total joints.

Michael Dolin, MD
Rockville Centre, N.Y.

The AAOS Biomedical Engineering Committee responds:
Current total hip prostheses with hard-on-hard bearings are typically manufactured with a ball diameter approximately 50 to 150 microns smaller than the cup diameter. This intentional clearance promotes entrainment of fluid between the bearing surfaces during motion to minimize friction and wear and to avoid the binding that would occur, as Dr. Dolin describes, if the ball diameter exceeded the cup diameter.

To eliminate the potential for binding, the tolerance (the allowable deviation from the specified diameter) typically is on the order of microns (an order of magnitude smaller than the clearance). These tolerances include the effects of polishing; automated machinery used to polish the components stay well within the allowed tolerance on the sphericity.

Laboratory wear simulations and examination of clinically retrieved implants show that modern metal-on-metal hips typically undergo a “wearing-in” process during the first year of use, increasing the conformity and the degree of polish in the ball-cup contact zone. This tends to improve the degree of fluid film separation, reducing the rate of wear by an order of magnitude after the wearing-in process is complete.

Wearing in also likely occurs with ceramic-ceramic bearings, albeit to a lesser extent. Only one manufacturer holds the FDA PMA approval and produces ceramic bearings for all major orthopaedic device companies. Outsourcing of other ortho­paedic devices does occur, with quality assurance measures to en­sure that tolerances are being upheld.

Although higher tolerances may reduce the likelihood of a noise-producing ceramic bearing, other factors contribute to this phenomenon. The Committee is unaware of any evidence to date to suggest that squeaking is caused by the diameter of the ball exceeding that of the cup, or that it would be alleviated by manufacturing the bearings to a closer tolerance. No tribological evidence exists that higher tolerances would improve the bearings friction and wear characteristics or long-term clinical performance. Inappropriate cup position, neck-to-rim impingement, shell deformation during implantation, insufficient joint tensioning leading to mircoseparation and edge loading, and other implant-specific design characteristics have all been implicated. These individual factors, or a combination of factors, may result in a noise-producing bearing. We suggest that orthopaedic surgeons give careful consideration to design characteristics when selecting an implant system for use in their patients and stress the importance of sound surgical technique.