Dr. Steven Tower, MD: The Dangers of Metal-on-Metal Hip Implants

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Metal-on-metal hip prostheses, called MoM from the English Metal on Metal, are a particular class of prosthesis characterized by the coupling of two metal joint components, one femoral and the other acetabular. There are two types:

total prostheses (THR from the English Total Hip Replacement) consisting of a metal head, a metal stem and a metal acetabulum (monobloc or with a metal insert);
resurfacing prostheses (HRA from the English Hip Resurfacing Arthroplasty) in which the femoral head is trimmed and covered with a metal cap, while a metal cup without an insert is implanted at the level of the acetabulum.

MoM prostheses have been placed on the international market since the late 1990s with the aim of offering a number of advantages, compared to traditional implants, benefits that can be briefly summarized in the points:

  • less wear of mechanical surfaces compared to systems with conventional coupling (
  • metal-polyethylene; ceramic-ceramic; ceramic-polyethylene; ceramic-metal);
  • less possibility of implant mobilization;
  • less chance of prosthetic breakage;

in surface MoM prostheses (HRA), greater anatomical and functional preservation of the hip joint which makes them particularly suitable for use in younger and more active subjects.
In view of these advantages, however, it must be observed that in this type of prosthesis wear of the articular surfaces can occur which leads to the production and accumulation of metal ions (metallosis from cobalt and chromium) and debris (debris) at the level of the peri-prosthetic tissues. . These phenomena may be at the origin, in some patients, of local inflammatory reactions which in turn cause the appearance of symptoms affecting the hip which can result in implant failure and the need for a surgical revision of the same. In addition, the local release and absorption into the circulation of the aforementioned metal ions may be the cause in some patients of adverse reactions affecting various target systems / organs, such as:

  • general hypersensitivity reactions (skin rashes);
  • neurological symptoms also affecting the sensory organs (auditory or visual alterations);
  • alterations in the psychological state;
  • cardiomyopathy;
  • changes in kidney function;
  • thyroid dysfunction.

In consideration of the foregoing, MoM-type hip prostheses have recently been the subject of growing interest from the international scientific community and the national and supranational health authorities responsible for the supervision and surveillance of medical devices, in order to review and re-discuss them. ” use in terms of efficacy and safety for patients with carriers and candidates for the implant.

The SCENIHR, a body established in 2004 to provide the European Commission with unequivocal scientific opinions on safety and on the global assessment of the risks associated with the use of new technologies, questioned on the assessment of the risks associated with the use of MoM prostheses, recommended with an opinion of 25 September 2014 the clinical – instrumental – laboratory follow-up, according to locally defined protocols, of all subjects with MoM, identifying the categories most at risk.

Furthermore, the SCENIHR highlighted the need for medium and long-term studies in order to obtain conclusive evidence on the toxicity of metals in subjects with MoM and in general on the safety of these prostheses for patients. The Ministry of Health, for the benefit of the various stakeholders, has translated the abstract of the opinion into Italian.

The Ministry of Health is at the forefront in monitoring the MoM problem, through continuous vigilance and surveillance and active participation in international scientific discussions.

The Ministry promotes the establishment of the Register of implantable prostheses, including orthopedic prostheses. To this end, it supports the implementation of the National Register of Arthroplasty (RIAP) coordinated by the Istituto Superiore di Sanità, to create a complete data collection that allows information and subsequent processing as accurate and valid from a scientific point of view to be obtained.

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