Expect the unexpected during metal-on-metal hip revisions
Early problem detection may lead to better outcomes after metal-on-metal implant revision.
A popular bearing surface from the 1990s to the mid-2000s, published literature estimated more than 1 million metal-on-metal total hip arthroplasties were performed since the design’s introduction, with 80% remaining in situ. However, metal-on-metal hip implant use decreased 10 years after its peak due to revisions related to adverse reaction to metal debris, infection and aseptic loosening.
“For the most part, people have abandoned the use of [MOM] metal-on-metal total hip replacements in favor of other potential bearings, most notably the use of crosslinked polyethylenes,” Douglas E. Padgett, MD, associate surgeon-in-chief at Hospital for Special Surgery, told Orthopedics Today. “What you have, though, are legions of patients who have undergone these procedures from the mid-2000s ... and even to this day, continue to receive some of these metal-on-metal [implants in the form of hip resurfacing].”
Although mechanisms of less volumetric wear in newer generations of MOM THA implants were proposed to lower failure rates, Young-Min Kwon, MD, PhD, director of Massachusetts General Hospital Center for Metal and Metal and Tribocorrosion Treatment and Evaluation, Harvard Medical School, noted national joint registries have reported a two- to three-fold higher failure rate of THA with MOM bearings compared with non-MOM bearings. An average revision rate for MOM procedures is about 12% at 10 years, with revision rates as high as 40% for certain implants, he said.
Sources who spoke with Orthopedics Today noted patients with a MOM hip implant may present with several different symptoms that indicate a need for revision.
“Usually these patients present with pain similar to any other failure mode,” Javad Parvizi, MD, FRCS, professor of orthopedic surgery at Rothman Orthopedic Institute at Thomas Jefferson University, said. “Occasionally, these patients have swelling in the region of the hip joint and sometimes that swelling is so severe that it presses on one or two of the veins or the lymphatics around the hip that then leads to swelling of the entire extremity.”
Kwon noted revision THA should be considered in patients with a progression in their symptoms, imaging abnormality and rising metal ion levels, in cases when patients are being monitored with surveillance.
“Revision surgery is also recommended for symptomatic recalled implants with elevated metal levels and abnormal cross-sectional imaging,” Kwon said.
Reasons for revision
Although MOM implants can need to be revised for the same reasons as non-metal implants, such as loosening and incorrect positioning, Michael R. Dayton, MD, vice chair of education and professor of orthopedics at University of Colorado School of Medicine, said MOM implants may need to be revised for additional reasons. These may include excessive metal ion production or possible allergy to metal.
“The metal ions produced by these metal implants are biocompatible,” Dayton said. “However, the ion production generated by the presence of the implant is generally eliminated or excreted from the body through the renal system. So, if somebody has relatively poor renal function, which may be age-related or just due to renal disease, then they cannot correspondingly eliminate those metal ions that are generated in their body. Thereby, a buildup of these ion levels may occur.”
Local toxicity caused by a hip implant may lead to destruction of the soft tissues and abductor weakness, Parvizi said.
“[Patients] start to limp and they start to have weakness related to all the other muscles around the hip joint,” he told Orthopedics Today.
Patients may also experience mechanical symptoms, such as a grinding or catching sensation in their hip, according to Thomas K. Fehring, MD, professor and chief of adult reconstruction at Atrium Musculoskeletal Institute. He noted component positioning and component types may also lead to complications.
“If the cup is not put in properly, it can lead to an increased problem, and certain components are more prone to adverse tissue reaction than others,” Fehring said.
However, Fehring noted patients may experience complications despite not having any symptoms. Therefore, surgeons should monitor all patients with MOM implants every 2 years, he said.
According to Parvizi, surgeons may have a low revision threshold, especially among patients with high cobalt chromium levels or female patients with a small, vertically positioned cup.
“In these circumstances, the likelihood of metal-on-metal failure causing severe soft tissue destruction is high, so, in those patients sometimes surgeons, for lack of a better word, prophylactically would remove the metal-on-metal [prosthesis],” Parvizi said.
Early detection of these symptoms after primary THA is paramount to a better outcome after revision surgery, according to Padgett. He said some patients with pain and a progressive limp may be experiencing a complex mechanical and chemical reaction at the interface between the junctional site, which can lead to corrosion.
“The corrosion appears to be initiated by the mechanical features of having some degree of micromotion and then there is an electrochemical reaction that starts to occur,” Padgett said. “You start to get the formation of different oxides and some other byproducts of this degradative process,” he said.
Some of the byproducts can lead to an aggressive inflammatory response, which then may result in the destruction of local soft tissue, including muscle, tendon and sometimes bone. This local tissue destructive process is often responsible for the increased rates of postoperative complications, including persistent pain, limp or dislocation, he said.
“The major issue with metal-on-metal failure relates to the soft tissues, unfortunately, because the ... destructive process results in terrible damage to the abductor mechanism,” Parvizi, an Orthopedics Today Editorial Board Member, said. “Even after revision of a metal-on-metal, the patient may still have a compromised functional outcome because the muscles are not functioning properly.”
Proper component position
Increased metal ion levels may be caused by improper component positioning, which Fehring noted can lead to edge loading. The risk can be reduced by placing the socket in a flat position instead of vertical, he said.
“What we are relying on in a metal-on-metal hip is lubrication of the joint,” Fehring told Orthopedics Today. “If the implant is put in properly, there is usually good boundary lubrication giving a little buffer between two pieces of metal. Some of those implants are performing beautifully. But, if it is put in incorrectly and there is edge loading, then you get metal scraping directly on the metal and the ion level is going to go up.”
Proper positioning and fixation of the cup and stem to the bone also affects stability of the articulation and dictates whether the revision is successful, Dayton said.
“[The] most successful surgery is going to result from ensuring that all the soft tissue is intact and that the head replacement and the liner can be well fixed or very stable,” he said.
A major challenge in revision surgery for patients with a MOM implant is often there is a need to remove all well-fixed implants which are responsible for the problem. The risk for iatrogenic fracture can occur when a MOM implant is not already loose at the time of revision, according to Padgett. Compounding the problem is that biological reaction to MOM implants can lead to delayed or failure of ingrowth of the implant when uncemented fixation is used, as well as an increase in the risk of infection after revision, he said.
“It is not entirely clear why, but the risk of infection after doing this revision operation is higher than in revision patients [who] have had prior standard total hips that are typically some form of metal against polyethylene or ceramic against polyethylene,” Padgett said.
Revision implant options
The indication for revision surgery plays a role in the type of implant a surgeon chooses to use to revise a primary MOM implant, according to Dayton.
“Obviously, an indication for treatment with sepsis or infection is going to suggest a completely different approach than somebody who just has a local soft tissue reaction,” Dayton told Orthopedics Today. “Without any tissue or bone disease, the possibility of just removing the metal aspects of the hip replacement and hip resurfacing without having to redo the whole hip is a less involved surgery than something where everything has to be removed.”
When bony fixation after removal of a MOM acetabular component is difficult to obtain due to ion-induced bone necrosis, surgeons should use a porous metal implant with multiple screws to help ensure osseous integration, Fehring said.
If the patient’s symptoms are attributed to the MOM implant, Padgett noted the implant should be replaced with one that has no cobalt and chromium and said he favors the use of femoral head implants manufactured with ceramic materials.
Dayton said, in such instances, surgeons can use a conventional metal head with polyethylene liner or a ceramic head with a polyethylene liner.
“The use of either one of those solutions are both prevalent right now and the ability to use either one of those options of surgery is predicated on whether an existing implant has options for ceramic heads, which most do. Otherwise, the surgeon will have to use a metal head,” he said.
A monoblock MOM implant in ideal position that lacks sharp edges may be revised with a dual-mobility polyethylene without extracting the acetabular component, Fehring noted.
“However, if you put a piece of plastic on something that is in a bad position or has sharp edges, it is just going to grind away some of the plastic, potentially causing particle-induced osteolysis,” Fehring said.
Although the dual-mobility cup can help with stability issues and has a low rate of corrosion, Padgett noted surgeons need to be wary of using a dual-mobility cup with a titanium alloy porous shell that typically has a liner made out of cobalt and chromium.
“So, now you reintroduced cobalt and chromium against a titanium interface in a modular setting that may or may not be another source of problems related to having cobalt chromium in the system,” Padgett said.
Acetabulum, femur considerations
According to sources for this article, the complexity of the revision surgery also depends on how involved of a revision is needed for the acetabulum and femur.
Kwon said surgeons most often perform acetabular revision due to malposition and loosening.
“For patients with high risk of postoperative dislocation due to significant soft tissue necrosis, a constrained liner or dual-mobility THA construct may need to be used,” he said.
Dayton noted revision of the acetabulum may be as simple as changing a liner or as complex as changing the entire acetabular shell or cup.
“A complete redo of a cup may be related to poor positioning or movement of the cup or loosening of the cup,” he said. “It may be necessary because of instability and improper positioning and poor performance of the hip, ie, it cannot stay in the socket.”
On the femoral side, indications for revision include dual-taper stem with cobalt-chromium modular neck, femoral component loosening and fracture, Kwon said.
A femoral component only revision requires a well-fixed and well-oriented acetabular component. For well-fixed cementless femoral stems, removal may require an extended trochanteric osteotomy, which he said is “an important tool in the armamentarium of the arthroplasty surgeon who performs revision hip surgery.”
“Although widely recognized as the workhorse procedure for revision hip surgery, [extended trochanteric osteotomy] ETO requires extended incisions ... and protected postoperative weight-bearing,” Kwon told Orthopedics Today. “Removal of well-fixed femoral stems without an ETO, such as with the ‘top-out’ technique, provides the potential advantages of eliminating risks of delayed bony union and potentially reducing time to ambulatory rehabilitation.”
He noted, however, “top-out” technique is also associated with the risk of intraoperative fracture.
There are still concerns about whether a new femoral head can be safely implanted onto a retained stem long-term when evidence of taper corrosion exists. Although there is limited consensus on the severity of taper damage that requires revision, Kwon said several studies suggest retaining a well-fixed femoral stem if trunnion corrosion is macroscopically mild.
“The taper must be cleaned and then examined. The revision femoral head must be securely mated with the existing taper as abnormal movement and increased stress at the head-neck junction increases the risk for failure,” Kwon said. “If the severity of corrosion compromises the mechanical integrity of the stem trunnion, stem revision is recommended.”
Before revision surgery, Kwon recommends careful preoperative planning with a review of prior operative notes as part of the patient’s history to help with component identification.
“If the implants are not specified in the operative report, implant records or the surgical log should be obtained to correctly identify the make and model of the components,” Kwon said. “The goals of treatment are to remove the source of metal-on-metal articulation and to restore hip stability.”
Regardless of whether a patient is being seen for a complaint related to his or her implant, Padgett said he establishes a baseline of overall function in patients who had a prior hip replacement or hip resurfacing with a MOM implant. For patients who experience pain, surgeons should examine the patient’s muscle power and other potential causes for the pain outside of the hip, he said.
“All patients will get standard radiographic series whether they have symptoms or not, and then I am a big advocate of establishing a baseline for how the joint is performing and whether or not there is any local body reaction to the presence of this metal-on-metal bearing,” Padgett said.
He said he will establish a baseline of laboratory markers for inflammation and detection of serum metal-ion levels in the blood, as well as a baseline of the tissue environment with MRI.
MRI is the most sensitive indicator of adverse local tissue reaction. Therefore, identifying extensive soft tissue or bone damage with MRI during the pre-revision work-up allows surgeons to have all potential components on-hand prior to surgery, Dayton said.
“I think the use of a MRI study certainly can illustrate the extent of a local soft tissue reaction, but a CT scan may be warranted, as well, to further elaborate any potential bone loss present so that the implants can be arranged and the surgeon will not be left without the necessary parts,” he said.
Surgeons can collect samples intraoperatively to ensure patients do not have an infection, Parvizi said.
Dayton noted the best preoperative test to date for patients undergoing MOM THA revisions has been a blood test to quantify corresponding cobalt and chromium levels.
“Generally, the observation intraoperatively is that there are multiple areas of tissue staining that accompany the metal-on-metal hips that would generally confirm the diagnosis, but pathologic and tissue pathology analysis is certainly an option,” Dayton said.
Considering situations that may arise during revision of MOM implants, Fehring said it is important to teach residents and fellows “to be comfortable with the unexpected.”
“It is my estimation that if [a surgeon does] not do a lot of revision work, [it is] probably best having the patients being revised by someone who does a lot of revision work, who is used to the unexpected and prepares for the unexpected,” Fehring said. – by Casey Tingle
- Colacchio ND, et al. J Arthroplasty. 2020; doi:10.1016/j.arth.2019.09.028.
- Crawford DA, et al. J Arthroplasty. 2019;doi:10.1016/j.arth.2019.04.019.
- Kwon YM, et al. J Arthroplasty. 2016;doi:10.1016/j.arth.2016.05.046.
- Law JI, et al. J Arthroplasty. 2020;doi:10.1016/j.arth.2020.01.011.
- For more information:
- Michael R. Dayton, MD, can be reached at 1635 Aurora Ct., Anschutz Outpatient Pavilion, 4th Floor, Aurora, CO 80045; email: firstname.lastname@example.org.
- Thomas K. Fehring, MD, can be reached at 250 N. Caswell Road, Suite 200A, Charlotte, NC 28207; email: email@example.com.
- Young-Min Kwon, MD, PhD, can be reached at 55 Fruit St., Boston, MA 02114; email: firstname.lastname@example.org.
- Douglas E. Padgett, MD, can be reached at 535 East 70th St., 3rd Floor, New York, NY 10021; email: email@example.com.
- Javad Parvizi, MD, FRCS, can be reached at Rothman Orthopaedic Institute at Thomas Jefferson University, Sheridan Building, Suite 1000, 125 S. 9th St., Philadelphia, PA 19107; email: firstname.lastname@example.org.
Disclosures: Fehring reports he is a consultant for and receives royalties from DePuy Johnson & Johnson. Kwon reports he receives institutional research funding as the principal investigator from Stryker, Zimmer Biomet, Smith & Nephew and Corentec. Dayton, Padgett and Parvizi report no relevant financial disclosures.
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