Effectiveness of Metal-on-Metal Hip Implants
Each type of implant has its own benefits and risks. Overall, metal-on-metal (MoM) hip implants have been shown to provide high implant survivorship (no need to remove implant and put another in its place) in certain patient populations. In addition, the larger head size of MoM total hip implants is intended to reduce the risk of dislocation, which is a common adverse event associated with total hip replacements.
Metal-on-Metal Total Hip Replacement Systems
In 2011, the FDA launched the International Consortium of Orthopedic Registries (ICOR), an international collaborative effort that aims to address differences in orthopaedic registries around the world and allow for collaboration to harmonize and utilize the data available from these registries. In June 2012, the FDA's Orthopaedic and Rehabilitation Devices Advisory Panel discussed data from Orthopaedic device registries and other sources. Data from the Australian1 and United Kingdom2 Orthopaedic device registries, both of which contain the largest number of patients and the longest length of follow-up time, indicate that approximately 95 percent of patients with any kind of total hip replacement have not undergone revision surgery (a surgical procedure where your implant is removed and another is put in its place) for seven years after the initial implant.
These Orthopaedic device registries note differences in revision rates by primary diagnosis, sex and age. These registries also noted that the use of cement and the type of bearing surfaces had an impact on revision rates. More than 85 percent of patients with MoM total hip replacements from the U.K registry and more than 92 percent of patients with MoM total hip replacements from the Australian registry did not have a revision for seven years after the initial implant.
Registry data shows that MoM hip patients with larger head sizes (36 mm or larger) had more revisions than those with smaller head sizes.
When comparing outcomes of different bearing surfaces, limitations in any registry data make it difficult to draw definitive conclusions. To obtain additional data to address safety concerns related to MoM total hip replacement systems, in 2011, the FDA required manufacturers of MoM total hip replacement systems marketed in the U.S. at the time to conduct postmarket surveillance ("522") studies. Interim results of the 522 studies are provided in the FDA Activities.
Metal-on-Metal Resurfacing Hip Systems
Hip resurfacing is intended to be a bone preserving procedure on the femoral side, since failure of a resurfacing implant on the femoral side typically allows for revision to a conventional primary femoral component. Hip resurfacing implants are only available with metal-on-metal (MoM) bearings in the U.S.
Potential advantages of total hip resurfacing over total hip replacement is an area of controversy among orthopaedic surgeons1. These include ease of revision, a decreased risk of hip dislocation, more normal gait, and an increased incidence of return to high-demand activities2.
These potential advantages are outweighed by the increased risks for some categories of patients. These risks include increased risk of hip fracture and adverse reaction to metal debris. Pandit et al. noted pseudotumors in a case series of 17 female patients (20 hips) who underwent surface replacement arthroplasty4. Similarly, smaller component sizes have been associated with increased revision rates. However, in a recent study of the Birmingham Hip Resurfacing (BHR), an estimated rate of survival of 97.2% and 93.8% at 5 and 10 years was demonstrated2. When this group limited the use of BHR to their current inclusion criteria of active males <60 years of age with a primary diagnosis of osteoarthritis and anatomy conducive to a femoral head component of 48 mm diameter, the survival rate was 98.1% and 96.8% at 5 and 10 years, respectively2.
The manufacturers of MoM resurfacing hip systems are still conducting long-term post-approval studies assessing device survivorship and metal ion levels; the data will be made available after completion of all of the studies.
1 AAOS OrthoInfo. Hip Resurfacing. Accessed at: https://orthoinfo.aaos.org/en/treatment/hip-resurfacing/ accessed on: 11/23/18.
4 Pandit H, Glyn-Jones S, McLardy-Smith P, Gundle R, Whitwell D, Gibbons CL, Ostlere S, Athanasou N, Gill HS, Murray DW. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br. 2008 Jul;90(7):847-51