Imagine trying to be quiet while entering a church or synagogue service during a silent moment only to have your new hip replacement squeak loud enough to be heard by all. Or picture yourself walking your daughter down the aisle for her wedding. With every step, that hip replacement creaks like a rusty barn door. Anyone with this odd complication can’t help but ask, Doc, what is causing this new hip to squeak like that?
An investigation at the Thomas Jefferson University Hospital in Philadelphia, Pennsylvania might just have an answer to that question. They divided patients receiving a ceramic-on-ceramic implant into two groups based on implant design and then compared the results.
Group one got an implant that had a special coating on the stem made of a titanium-aluminum-vanadium alloy. The stem was shaped with a C-taper neck and had a wide, thick midsection. The stem portion of a hip replacement fits down inside the long shaft of the femur (thigh bone). Group two was given an implant with a stem made of a different combination of metals: titanium-molybdenum-zirconium-iron alloy. The design was a V-shape instead of a C-shape and the midsection wasn’t as thick as in group one.
The ball-shaped head of the femur was made of ceramic. It fit inside a metal cup (socket replacement) that was lined with ceramic making the implant a ceramic-on-ceramic joint. Many patients prefer a ceramic-on-ceramic implant because the ceramic wears well and glides smoothly. And fewer patients have problems with inflammation like those who have implants that are made of plastic.
So what causes the squeak that’s so loud anyone standing nearby can hear it? No one knows just yet! Sometimes metal-on-metal implants squeak or make some other obvious noise but the noise seems to disappear after awhile. Not so with the ceramic-on-ceramic implants. And squeaking isn’t the only problem. Some patients end up with clicking. No one included in this study had clicking, just squeaking.
Everyone in the study was operated on by the same surgeon. The surgeon was careful to make note of the size of the ceramic head in case that was contributing to the problem. X-rays were taken after the implant was put in place to verify the position and alignment of the new joint. Patients in the two groups were matched so they were equal in age, sex, height, weight, and body mass index (BMI).
Only four of the 135 hips in group one reported squeaking (that’s only about two per cent). On the other hand, 18 per cent of the patients in group two developed squeaking. There was no predicting when the squeaking might begin. Some patients developed the noise within the first month while others didn’t notice it until many years later. Replacing the ceramic cup with a plastic liner eliminated the problem in all cases.
Crunching the numbers (statistical analysis) showed that patients with the thinner V-shaped neck and titanium-molybdenum-zirconium-iron stem were seven times more likely to develop a squeak. This stem is more flexible with a lower frequency of resonance. Vibrations created by the ceramic-on-ceramic movement are amplified (made louder) when there’s a lower frequency. And evidently, the oscillations can be amplified enough to generate a sound that can be heard.
The authors conclude by saying patients don’t have to give up the good quality of motion provided by ceramic-on-ceramic hip replacements. Surgeons just have to avoid using implants with the V-40 neck and choose the stiffer, C-taper stem instead. They should also make sure the materials are not made of the titanium-molybdenum-zirconium-iron alloy.