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Abstract
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Introduction
Ceramic-on-metal (CoM) bearings show significantly lower particle wear compared to metal-on-metal (MoM) in laboratory settings. FDA approved CoM bearings in 2011. More recent in vivo studies found serum ion levels (Co and Cr) lower in CoM compared to MoM bearings three years after implantation, but an increase after 5 years. The reason for increased wear rates remains debated. Nevertheless, only few case reports exist describing implant loosening due to metallosis for CoM bearings.
Case report
We present the case of a 50 year old female patient who after 18 previous surgeries due to bilateral high hip dislocations received a total hip prosthesis in 2010 with a ceramic-on-metal bearing. For the past 2 years she suffered of constant hip pain and was only able to ambulate with crutches. Radiographs showed an eccentric position of the head in the cup with position of implants in a normal range. Revision surgery was performed using a stepped trochanteric flip osteotomy as approach. Intraoperatively black liquid filled the joint and massive metallosis stained bone and surrounding tissue. The anterior bony half of the acetabulum was missing, leaving half of the metal back hanging free with the psoas tendon rubbing over the metal. After removal of all implants, a thorough debridement was performed. The acetabular defect was reconstructed with a shell formed from a femoral allograft head held in place with an acetabular reinforcement ring with hook. The new bearing is ceramic-on-polyethylene, the stem uncemented.
One year postoperatively the patient has little to no pain and is able to ambulate without assistive devices for short distances.
Discussion
Eccentric wear of the liner as well as metallic marks on the ceramic head were visible after removal. We therefore hypothesize that despite acceptable position of the implants edge loading was the main cause for the metallic wear. The age of the bearing may be of less importance.
Conclusion
CoM bearings are still FDA approved to this day, but have never been widely used in Switzerland. Due to the negative experience with this case without a clear cause being identifiable, we recommend monitoring of all these patients especially as recent findings show a possible increase in serum ion levels (chromium) after five years. Therefore, regular controls of serum ion levels and in case of elevated values or clinical symptoms MRI imaging to rule out pseudotumor formation seems mandatory to us.
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