Squeaky Hips

Most Hip replacements move silently. However some small ball size ceramic on ceramic hips squeak. That is they emit a noise whilst walking, stair climbing or indeed rising from a low seat.

Modern big ball ceramics (36mm and bigger) do not, in my experience, squeak.

The reason for squeak is a lack of lubrication between the ball and socket. Specifically the lubrication fluid is squeezed out from between the ball and socket to leave surfaces of the ball and socket to rub directly against each other. The surfaces rub, the ceramic vibrates and thus emits a squeak.

Artifical hips have the same lubrication as the natural hip, ie synovial fluid produced from the lining of the hip joint. Interestingly even if the lining is removed at surgery it regrows or reforms.

Squeak is due to sliding of two hard surfaces on each other due to lack of lubrication, ie it is grinding of one surface on another. Traditional metal on plastic (polyethylene) hips undoubtably did rub but the direct contact did not lead to a squeak, probably because the plastic did not vibrate like the ceramic can.

There are 3 issues contributing to lack of lubrication between the ball and the socket:

1. Ball size. The lubrication (imagine a thin film of fluid between the ball and socket) is better with big balls (much better from enineering formulas pertaining to the diameter of the ball: simply there is further for the fluid to be squeezed out from). Thus most if not all of the squeaks are related to due the smaller and standard 28mm rather than the 'big ball' 36mm or bigger.

2. Cup inclination (and anterversion), cups that have been technically inserted at a vertical angle, or relatively so, may squeak. Again because of poor lubrication; termed edge loading.

3. Body mass. High body mass will compress the socket on to the ball squeezing out the layer of lubrication and may lead to squeak.

In summary a large diameter ceramic is less likely to squeak. Component position is important to facilitate lubrication to avoid edge loading.

Ceramic on ceramic remains an excellent wear resistant couple. It remains the preference of Orthopaedic Surgeons in terms of long term safety and low revision rates due to its very low wear rate.
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