Metal ions

[John C]

Along the lines of the importance of looking at long range data with MoM bearings, there are studies out there that do look at the long term effects of MoM THRs on the health of patients. Though this is obviously not the same as HR, it does give us a look at the long range health effects from MoM bearings. I apologize for not having the links to these handy, but there are long term studies out there if you look. The ones that I read focused on major health issues such as cancer, and the bottom line from the ones that I looked at are that there were no long term health issues, and some even showed lower cancer rates for people with MoM prosthesis.
My point here is that when trying to get a handle on the long term effects of MoM bearings, we may not need to restrict our research to HRs with their shorter term results, but it may make sense to include long term data from MoM THRs.

[Dayton96]

Do I understand the issue correctly, in that the hazard of MoM particles applies equally to HR and THR?

Mac

[JMS]

If the elevated ion levels were a result of simple metal shedding from normal joint articulation, or a by-product of surgery when the heavy use of force necessary for correct placement might have led to some fragmentation, then indeed the risks from MOM resurfs and total hips could be expected to be the same.

However, elevated metal ions in resurfacings appear to be highly correlated with edge loading, which is rare in THRs. Of course, if the two components of a MOM THR were mis-aligned and edge-loading, they would shed metal in exactly the same way as an HR MOM. The difference is that such mis-alignments are rare in THRs.  In part this is because the THR is an easier surgery -- having removed the femoral head, the surgeon has a wider field of access for accurate placement.  But in addition, and more significantly, the removal of the top part of the femur includes the “shoulder” just below the femoral neck. This protrusion is heavily implicated as a cause of possible impingement, which prevents the components coming together smoothly and leads to edge loading.

[John C]

Good points JMS. The thought behind my post was that for those that want to look at research on the long term effects of metal ions produced by well functioning hip prostheses, it would make sense to look at the longer term studies of MoM THRs, in addition to HR studies.

Dayton96; As we have seen with the ASR and the Durom products, there can be a huge variation in problematic ion issues, even from brand to brand, whether they be THRs or HRs. For example, I believe that large head MoM THRs that used the problematic ASR or Durom cups had similar ion problems to the HRs that used those cups. However, I would think that if we were to look at well functioning devices for the purpose of researching the long term effects of normal ion production, it would be reasonable to include studies of large head MoM THRs along with studies of HRs, since the articulating portions of the components are nearly identical. It is worth noting that most THRs do have the possible additional issue of metal ion production from the junction where the head sits on top of the stem.