By Brett Greenky, MD
Metal on Metal (MoM) hip replacement and resurfacing implants have been prominent in the news over the last several years. MoM articulations have been attractive to joint replacement engineers because of the potential advantages of decreased wear when compared to metal on plastic hip (MoP) articulations. The McKee-Farrar MoM implant of the 1970’s showed initial promise, but in the end resulted in much higher rates of failure compared to plastic and metal. New advances in metallurgy (the science that deals with the physical and chemical behavior of metallic elements) have rekindled interest in MoM designs. Laboratory simulations of implant wear have shown dramatic reductions in the overall volume of wear material in MoM hips compared to MoP, however, MoM articulations create many more wear particles (although smaller in size) when compared with MoP. It still remains unclear if MoM will be superior to MoP for most patients. Two implants have gained much attention in the lay media over the last 5 years, one in a positive and the other in a negative manner. Birmingham Hip Resurfacing is emerging as a successful procedure in certain patient populations while the industry “recall” of a MoM Hip Replacement system is controlling overall MoM enthusiasm. (**see implant photos at end of narrative**)
The Birmingham Hip Resurfacing was approved for use in the USA in 2006 based on an excellent track record largely from one center in Birmingham, England. There, Dr. McMinn the implant’s surgeon-developer gained a cohort of high functioning patients with greater than 8 years of success using his second version of the implant. The first version of this implant developed failures at the 6-8 year mark after surgery. Subsequent modifications of the manufacturing process resulted in preservation of high levels of Carbides in the Cobalt Chrome implants has now resulted in the new version having much better success.
Hip resurfacing’s unique attraction when compared to traditional Hip Replacement is that the femoral head is largely preserved. The femoral head (the uppermost part of the thigh bone) is reshaped and resurfaced in the procedure rather than removed and replaced with a manmade head in traditional hip replacement. Theoretically, the femoral head bone that is preserved during the resurfacing procedure allows more options at a later time if additional surgery is ever needed. It also permits the leg bones to accept weight in a more natural way than with the stem version of replacement. The rationale is that the less invasive resurfacing operation with its bone preservation will allow the operation to be performed on younger patients who will enjoy higher activity levels and if revision is ever needed, the surgery will be easier because the femoral head was preserved during the initial procedure. The future will reveal whether these claims will be proven true.
In the United States, we do not have a National Joint Replacement Registry. A registry is a data bank which holds the records for all joint replacement surgeries performed and tracks the results. The database allows healthcare professionals to detect implant designs or surgical techniques that are underperforming at an early date! Such registries have established and functional for decades in Australia, the United Kingdom and Sweden. The registries in these countries have documented an excellent track record for the Birmingham device! When applied to the correct patient and when the surgery is performed perfectly, the results are as good as any of the traditional hip replacement devices. It is also true that patients, young men in particular, enjoy very high functional activities with the implant. Whether the success in these patients is completely due to the Birmingham procedure itself, or if it should be attributed to “selection bias” remains to be seen. Selection bias is a process in which the patients most likely to have high functional activities (irrespective of the type of operation performed) are getting resurfacings rather than the more invasive replacements, and thus making the resurfacing group look better rather than from actually from the superiority of the procedure or the implants used.
There have been other resurfacing issues illustrated both by the foreign registries, as well as our own experience here in the United States. The resurfacing operation is somewhat larger and technically more difficult to perform than replacement. There is a steep learning curve for the surgeon. This is not an operation for the occasional hip surgeon, but rather for the Joint Replacement Specialist. Postoperative fracture of the femoral neck (the flattened section of bone connecting the femoral head with the long part of the femur) occurs 1% of the time in the hands of the seasoned specialist, and much more frequently with the occasional hip surgeon. Wear rates (the speed of material change due to wear) at the ball-socket interface are related to the alignment of the implants more in MoM implants than when compared with MoP interfaces. Orthopedic experts suggest that at least 30 cases are critical for the surgeon to gain the necessary surgical experience. Both foreign registries and surgeon experience in the US have shown a much higher complication rate with resurfacing compared with hip replacement. Experienced surgeons do have lower complication rates as do hospitals performing high volumes of these procedures. The data also shows that the resurfacing operation is not for most women. The complications are much higher in women even when under the care of the experienced surgeon. The risk of needing a second operation within 3 years of a resurfacing procedure is 3 times higher for women than men. This phenomenon has been noted in the registries maintained in Australia, England and Sweden. Here in the US, the same problems for women have been noted and reported by researchers at Rush University Medical Center in Chicago. The etiology of the gender difference is not completely known. Lower bone density in women (and older men) certainly increases the risk of the femoral neck fracture complication. Smaller size hip sockets are also associated with higher rates of problems, and certainly women in general have smaller socket sizes.
Overall, the Birmingham Resurfacing surgery does have excellent results in the correct patient population. Increasingly apparent is that the target population for this procedure is men below the age of 60 who want to continue with higher impact activities after the surgery. Women candidates need bone density testing and consultation with an experienced surgeon to make sure they understand the increased risks.
Further Reading:
1) Della Valle, et al. Hip Resurfacing Surgery Report, Rush University Medical
Center, Nov. 2008.
2) Meier, Barry. For Women, Red Flags About a Hip Device, the New York Times,
11/8/08.
3) www.revolutionhealth.com/blogs/jamesherndonmd
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| Birmingham Hip Implant |
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Metal on Metal Total Hip Implant |
I am 63 I had resurfacing 2 yrs ago. So far no problems. I can run and shoot hoops again. Time will tell though.
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