July 24, 2013
My take on this is that it is not an unexpected response to the generally higher than desired failure rate of MOM total hips. None of the doctors I know will primarily recommend a MOM THR anymore. A few short years ago these were very popular, mostly due to the fact that dislocations continue to plague THR, representing the single biggest reason for revision THR in the USA. Since dislocation goes down with bigger heads, and the biggest heads of all are with MOM hips, your average hip surgeon went for the biggest heads to avoid dislocation and used a MOM THR.
Unfortunately, as happens in technology development, there was an unanticipated problem. The junction between the head and stem of a THR (a “Morse taper”), is a cold weld designed back in the days of 28 and 32mm heads. Placing a very large MOM head, perhaps 50-58mm, exerted so much torque compared to the original 28 and 32mm heads that it overcame the design limits of the Morse taper, and micromotion with fretting and corrosion developed at this junction. This is not metal debris from the bearing (the ball in the socket), it’s from the junction between the head and the neck of the MOM THR. Research has shown 10 times as much metal debris emanating from this junction as from the adjacent MOM bearing, where all the attention was directed!
Many MOM THR’s were done around the world, and actually Smith and Nephew’s did better than most, if not all, of the others. Nevertheless, it was not as good as it needed to be, so they recalled it. Now they recommend it only for use in femoral side revisions of their BHR, so long as ther are no concerns with the socket. It is worth noting that this is the only FDA-approved use of their big head MOM THR already.
So, it’s a problem with MOM THR’s that have modular junctions. In the 60’s, the MOM THR’s that McMinn sought to replicate as a resurfacing in the 90’s, did not have modularity, so there was no Morse taper. The metal levels in these are more like resurfacing. Low and safe. Now, what about resurfacing? Here, there is no modularity, no Morse taper, no concerns about fretting and corrosion. It’s just the bearing. Here, bigger is better. And the amazing thing is that registry data confirms the hypothesis: the failure rate of MOM THR’s is worst in the bigger diameters (more torque, more fretting and corrosion at the taper junction), yet the reverse is true in resurfacing, where the bigger diameters do the best (better lubrication, less fussy socket positioning, less edge loading, more likely a male). MOM THR and MOM resurfacing are totally different animals, and while I do not recommend MOM THR, I continue to recommend resurfacing as the best option in young healthy people, of adequate size, using a well-designed device, and done correctly.
Best regards, Peter Brooks MD, FRCS(C)