This issue of The Journal of Bone and Joint Surgery includes a report on two patients with metal-on-metal total hip replacements who developed systemic neurological and cardiac complications associated with an elevation in serum cobalt levels. Causality is suggested by the fact that the systemic symptoms improved in at least one of the two patients following revision surgery. The report is unusual because of the rarity of the occurrence of metal-induced systemic complications in patients with total hip replacement and the fact that the author was one of the patients. As millions of patients worldwide have undergone total hip replacement, these cases represent rare events indeed.
Metal-on-metal bearing surfaces in total hip arthroplasty have been in use for over fifty years. The earliest iterations of these devices lost favor in the 1970s and were replaced by metal-on-ultrahigh molecular weight polyethylene implants. As the indications for total hip replacement were extended to younger, more active patient populations with higher activity demands, clinically important problems such as osteolysis and aseptic loosening related to bearing surface performance were reported with increasing frequency. So-called alternative bearing materials were developed with the goal of reducing wear debris and decreasing osteolysis, aseptic loosening, and other wear-related failures. Metal-on-metal bearings generally demonstrated reduced volumetric wear in comparison with metal-on-polyethylene bearings in hip simulator and implant-retrieval studies. Metal-on-metal bearings allow the use of thinner acetabular components and larger femoral heads, which have been associated with a lower rate of postoperative hip dislocation. Thinner acetabular components also facilitate hip resurfacing.
As is the case with metal-on-polyethylene bearings, ceramic-on-polyethylene bearings, and ceramic-on-ceramic bearings, there are potential risks associated with metal-on-metal bearings that accompany the potential clinical benefits. One such risk associated with metal-on-metal bearings is the potential for adverse biological responses to elevated levels of cobalt and chromium in local and remote tissues. This risk is not unique to metal-on-metal bearings as other metal devices with accelerated damage due to wear or corrosion can lead to elevated levels of metal in local and remote tissues. In fact, four of the six reported cases to date that have demonstrated chromium and cobalt-associated neurological and/or cardiac complications had bearing couples other than metal-on-metal. In orthopaedic surgery, metal devices are used for joint replacements, fracture fixation, spine fusion, and tumor reconstruction and therefore are essential for the orthopaedic surgeon. However, if metal release is extensive as a result of wear or corrosion, adverse local and systemic biological responses can ensue. At the present time, there is no generally accepted threshold beyond which serum or blood concentrations of cobalt and chromium are known to be toxic in patients with a metal implant. As there are known risks of revision surgery, undertaking such a procedure on the basis of the blood or serum metal level alone and in the absence of symptoms is problematic. The value of these case reports would have been enhanced if there had been additional clinical, histological, and radiographic detail demonstrating implant orientation (particularly in Case 2, in which this information is quite limited) and if there had been additional detail regarding the methodology of the metal measurement in serum. The latter is particularly critical given the known variability in these measurements, which makes comparison of the metal concentrations in different published series difficult. Nonetheless, there are important lessons that can be learned from these two cases: (1) routine periodic surveillance by an orthopaedic surgeon is recommended for patients with total joint replacements to identify and treat, in a timely fashion, any prosthetic-related complications; (2) surveillance should include both an evaluation of the replaced joint and a discussion of the overall health of the patient; (3) determination of concentrations of metal in the blood or serum may be helpful for the evaluation of patients with local pain and/or otherwise unexplained systemic symptoms and should be used in concert with other clinical and radiographic information; (4) the orthopaedic surgeon should be cognizant of these rare systemic complications of metal devices undergoing accelerated wear and corrosion; and (5) expeditious revision surgery should be considered for a symptomatic patient with evidence of accelerated wear or corrosion who demonstrates local and/or systemic complications associated with the device.