Reply to the Letter to the Editor: Is There a Cardiotoxicity Associated With Metallic Head Hip Prostheses? A Cohort Study in the French National Health Insurance Databases

To the Editor,

We thank Drs. Matharu and Whitehouse for their interest in our research [²] and for their valuable contribution. Although we appreciate their comments, we would like to make the following clarifications.

As a reminder, the main objective of our study was to assess the risk of dilated cardiomyopathy or heart failure specifically associated with metallic versus nonmetallic head THA using data from the French national health insurance databases.

In their first comment, Drs. Matharu and Whitehouse argue that because THA stems contain metal, not accounting for stem material in these THAs may have caused substantial residual confounding. We agree that all of the THA metal-containing components should be considered in order to properly quantify a potential role of THA metal overall. In our study, because information on stem material was not available and therefore could not be accounted for, the magnitude of the overall association between THA metal and the risk of dilated cardiomyopathy or heart failure may have been underestimated. However, since all THA stems are made of metal (most commonly cobalt chrome or titanium), and since there is no particular reason to believe that one head material (ceramic or metal) is more likely to be used with one of those stem materials, it seems unlikely to us that stem material would cause residual confounding.

Another critique relates to the lack of a control group of individuals without THA. We agree that such a control group would be of interest to assess the cardiovascular risks associated with THA. However, it turns out that osteoarthritis is itself a cardiovascular risk factor [¹] independent of the presence or absence of a THA, and so estimating the proper role of THA characteristics in explaining an increased risk in patients with versus without THA would require accounting for detailed information on individual osteoarthritis diagnosis and severity. Unfortunately, such information is not available in the French national health insurance databases. In our study, we compared the risk between patients with metallic versus nonmetallic head THAs. Regarding osteoarthritis diagnosis and severity, the two groups were a priori comparable after adjusting for major other cardiovascular risk factors. Such a design was the most appropriate to adequately answer the question of a potential role of metallic versus nonmetallic head THA in increasing the risk of dilated cardiomyopathy or heart failure.

Lastly, we agree with Drs. Matharu and Whitehouse’s point stating that, in addition to our study focusing on morbidity outcomes, further studies considering cause-specific mortality would be of interest to assess the cardiovascular risks of metallic head THAs in more detail.