The first generation of metal-on-metal (MoM) hip arthroplasty implants made of stainless steel was introduced in 1951; however, early failure and risks associated with MoM articulations, including loosening, metallic debris, metal hypersensitivity, and carcinogenesis, encouraged surgeons to avoid the use of MoM hip replacements for several decades1-4. Although short to mid-term outcomes after metal-on-polyethylene (MoP) total hip arthroplasty were favorable, complications of polyethylene wear after long-term implantation included osteolysis and aseptic loosening in patients with MoP total hip replacements. Therefore, the second-generation MoM total hip replacements and resurfacings were reintroduced in the late 1990s. These MoM articulations were initially considered to generate low volumetric wear, resulting in a low rate of aseptic loosening, especially in young patients. However, high rates of early failure and an increasing rate of adverse reactions to metal debris (ARMD) were reported, so surgeons declined to implant MoM hip articulations1-4. Medical device alerts or clinical recommendations for the use of MoM hip arthroplasty were announced by several government or academic associations5-7. Although MoM hip replacements generally have been associated with worse outcomes than MoP total hip replacements, the prevalence of ARMD after MoM hip arthroplasty has varied among reports and with respect to implant types and has been associated with large-diameter MoM articulations1,4. The long-term results after MoM total hip arthroplasty with a small-diameter articulation and the association between ARMD and femoral head size remain unclear2.
In this Level-IV study, the authors present a cross-sectional investigation using metal artifact reduction sequence magnetic resonance imaging (MARS MRI) to explore the prevalence of and the factors related to ARMD in patients with a small-head MoM total hip replacement and a minimum follow-up of 10 years. The results showed that ARMD was found in 27 (41%) of 66 hips using MARS MRI and that cobalt (Co) ion levels positively correlated with the presence of ARMD. The high prevalence of ARMD after small-head MoM total hip arthroplasty suggests that ARMD also is common after MoM total hip arthroplasty with a small femoral head after long-term implantation, and most ARMD lesions after small-head MoM total hip arthroplasty were small and asymptomatic. Unfortunately, the current cross-sectional study did not show the natural history of ARMD and did not propose a strategy for follow-up to detect ARMD after small-head MoM total hip arthroplasty. Previous studies have proposed that high Co and/or chromium (Cr) ion levels (>5 μg/L) in peripheral blood serum should be used as a surrogate marker for excessive metal wear in patients with an MoM hip replacement made of Co-Cr alloy4. In the current study, the authors selected 107 patients who showed either Co or Cr blood ion levels of >1 μg/L for further MARS MRI and finally 53 patients (66 hips) were included. Although Co ion levels positively correlated with the presence of ARMD, 3 of 4 patients (5 hips) who underwent revision surgery showed both Co and Cr blood ion levels of <2 μg/L. These results suggest that Co and/or Cr blood ion levels themselves cannot be used as an indicator for failure of a MoM total hip replacement. Previous studies have demonstrated that ARMD may correlate with implant-related factors (head size, taper design, head-neck alloy coupling, and acetabular cup design with head-cup contact mismatch), surgical factors (acetabular inclination angle and acetabular version angle), and patient-related factors (sex, acetabular dysplasia, metal sensitivity, low body mass index, and activity level)4. The current study did not identify a correlation between clinical factors and ARMD.
The current study showed a favorable clinical outcome after small-head MoM total hip arthroplasty with a minimum follow-up of 10 years, even with a high prevalence of ARMD. Currently, ARMD is considered to be a phenomenon not only after MoM hip arthroplasty but also after MoP hip arthroplasty with a head-neck taper junction where fretting and/or corrosion may occur. Currently, most surgeons may not choose MoM hip arthroplasty implants because of high early failure rates and a high prevalence of ARMD, although the clinical outcome after total hip arthroplasty with a small-head MoM articulation remains inconclusive. Appropriate follow-up and treatment for ARMD in patients with small-head MoM hip arthroplasty implants should be further investigated, and identification of the precise mechanisms for ARMD after total hip arthroplasty with MoP as well as MoM articulations should be explored for the ideal design of the total hip arthroplasty implants in the future.
1. Lombardi AV Jr, Barrack RL, Berend KR, Cuckler JM, Jacobs JJ, Mont MA, Schmalzried TP. The Hip Society: algorithmic approach to diagnosis and management of metal-on-metal arthroplasty. J Bone Joint Surg Br. 2012 Nov;94(11)(Suppl A):14-8.
2. Hwang KT, Kim YH, Kim YS, Choi IY. Is second generation metal-on-metal primary total hip arthroplasty with a 28 mm head a worthy option?: A 12- to 18-year follow-up study. J Arthroplasty. 2013 Dec;28(10):1828-33. Epub 2013 Jul 25.
3. Sugano N, Iida H, Akiyama H, Takatori Y, Nagoya S, Hasegawa M, Kabata T, Hachiya Y, Yasunaga Y. Nationwide investigation into adverse tissue reactions to metal debris after metal-on-metal total hip arthroplasty in Japan. J Orthop Sci. 2014 Jan;19(1):85-9. Epub 2013 Dec 12.
4. Liow MHL, Kwon YM. Metal-on-metal total hip arthroplasty: risk factors for pseudotumours and clinical systematic evaluation. Int Orthop. 2017 May;41(5):885-92. Epub 2016 Oct 20.
5. Medical and Healthcare products Regulatory Agency (MHRA). Medical device alert (MDA/2012/036). All metal-on-metal (MoM) hip replacements. 25 June 2012. https://assets.publishing.service.gov.uk/media/5485abf6ed915d4c10000273/con155767.pdf
. Accessed 2017 June 20.
6. European Federation of National Associations of Orthopaedics and Traumatology. Consensus statement “Current evidence on the management of metal-on-metal bearings.” April 16, 2012. https://www.efort.org/wp-content/uploads/2013/10/2012_05_10_MoM_Consensus_statement1.pdf
. Accessed 2017 June 20.
7. U.S. Food & Drug Administration. Medical devices. Metal-on-metal hip implants; information for orthopaedic surgeons. http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/ImplantsandProsthetics/MetalonMetalHipImplants/ucm241667.htm
. Accessed 2017 June 20.