When Is Revision Surgery Indicated for a Patient with a Painful Metal-on-Metal THA?
Dr. Richard McCalden answers ICJR’s questions about diagnosing hip pain in patients with a metal-on-metal implant, findings that indicate the implant is the cause of the pain, and considerations for performing a revision procedure in these patients.
ICJR: What is the differential diagnosis for total hip arthroplasty (THA) patients with metal-on-metal (MoM) bearings who present with pain?
Richard W. McCalden, MD, MPhil (Edin.), FRCSC: Unfortunately, a significant number of MoM THAs were performed worldwide over the past 2 decades. Not all MoM implant designs have been problematic; however, a significant number of MoM THAs have failed and have required revision surgery.
Although the suspicion must remain high for the metal bearing (or trunnion) as the underlying cause for pain following MoM THA, the differential diagnosis must include a number of other possibilities for the pain:
- Adverse local tissue reaction (ALTR), with or without pseudotumor (related to metal debris from the bearing surface and/or tribo-corrosion from the trunnion junction)
- Periprosthetic joint infection (PJI); always a consideration with any painful THA
- Aseptic loosening of any of the implants
- Psoas-tendonitis (more common with large ball articulations)
- Greater trochanter bursitis
- Occult inguinal hernia
- Referred back pain
ICJR: What is your approach to evaluating these patients to determine the cause of the pain?
Dr. McCalden: As with any THA patient presenting with pain, sorting out a MoM THA requires a systematic approach to determine the underlying cause. This includes:
- History and physical
- Plain radiographs (including serial films, if available)
- Infection work-up (ie, bloodwork with or without aspiration)
- Serum metal ion analysis (serum cobalt and chromium)
- Cross-sectional imaging (ultrasound and/or MARS MRI)
History and Physical
- This is still an important part of the evaluation of a patient with hip pain and a MoM bearing.
- Obtain the operative note to identify the exact implant that was used. Is it a problematic design?
- Ask the patient about the location and onset of pain. Was the joint always painful, or is this a new onset of pain?
- Look for evidence of abductor dysfunction.
- Look for evidence of a mass/swelling, which is often associated with pseudotumor.
- Examine the spine, abdomen, and groin for other local causes of pain, such as a hernia or psoas tendinitis.
- Plain radiographs are helpful in the diagnosis, but keep in mind that they can appear normal in many patients.
- Identify the prosthesis. Is it an “at-risk” or recalled device?
- Is there evidence of bone loss or osteolysis? Are there any other changes from the index radiographs?
- Check the cup position. Is it excessively vertical or anteverted/retroverted, which can lead to edge loading?
- Is there evidence of a soft-tissue mass?
- Obtain an erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level, as well as an alpha-defensin test, if available. Consider joint aspiration if the ESR and CRP results are abnormal.
- Obtain a complete blood cell count of the hip aspirate, but keep in mind that the cell count and neutrophil percentage may be difficult to interpret: They may be elevated in patients with metallosis who do not have an infection. Consider a manual cell count to verify the results. However, there are no clear data to guide surgeons in the setting of metallosis.
- Final culture results are important to confirm or refute infection.
Metal Ion Analysis
- The cobalt (Co) and chromium (Cr) levels will almost certainly be elevated (ie, abnormal).
- Unfortunately, there is no good evidence to guide surgeons as to what level of “abnormal” is acceptable in patients with MoM implants.
- In general, the lower the values, the better (ie, less than 1 to 2 ug/L or ppb).
- Don’t confuse units of measure (ie, ug/L and nmol/L) when analyzing Co and Cr levels. The standard in the literature is ug/L.
- Concern with values greater than 7 ug/L is arbitrary, although values higher than 10 ug/L are very abnormal.
- Perhaps as important as the absolute values is the pattern; in other words, the ratio of Co to Cr levels.
- An increase in both Co and Cr represents articular metal wear. A Co level 3 to 4 times higher than the Cr level is highly suggestive of trunnion corrosion, which is common in patients with MoM THAs. 
- The important point is that elevated metal ion levels alone are not a reason to perform a revision procedure.
- Ultrasound is a good inexpensive, sensitive screening test. If the results are normal, the patient is unlikely to have a pseudotumor. 
- MARS MRI is the gold standard but is more expensive and depends on the MRI machine, the protocol, and the radiologist’s interpretation.
- MARS MRI findings can be abnormal even in patient with a well-functioning THA, although MARS is excellent at differentiating fluid collection (cystic) from a solid mass pseudotumor. 
ICJR: What findings indicate that the MoM bearing is the cause of pain post-THA?
Dr. McCalden: Determining if the MoM bearing is the cause of pain following THA can be difficult. The “red flags” to suggest that the MoM bearing (or trunnion) is likely the cause include:
- New mass/swelling around hip area
- Implant with a known poor track record and a high revision rate due to metallosis/pseudotumor, especially MoM THA systems with a taper adaptor sleeve for head/trunnion assembly
- Poorly positioned cup and/or evidence of periprosthetic osteolysis
- Negative infection work-up
- Elevated Co and/or Cr levels, as discussed above: elevation above 3-4 ug/L is concerning, elevation above 10 ug/L is alarming, and a marked increase of Co compared with Cr (Co 3 to 4 times higher) is strongly suggestive of trunnion corrosion
- Significant fluid collection or solid mass around the hip joint seen on ultrasound or MARS MRI
ICJR: What are your indications for a revision procedure?
Dr. McCalden: Determining when revision surgery is indicated involves careful review of the patient’s history and physical examination, laboratory tests, and imaging. The decision to operate involves consideration of a combination of factors including:
- Does the patient have a previously well-functioning hip that is now causing pain?
- Does the implant have a poor track record?
- Is there a mass/swelling?
- Is the implant (ie, cup) mal-positioned?
- Are the metal ion levels elevated?
- Is the cross-sectional imaging abnormal?
The patient doesn’t need to have all of the above to warrant surgery. In addition, if the hip is not painful, close surveillance rather than revision surgery may be appropriate, depending on the metal ion levels and/or imaging results.
Patients with a MoM THA may be very anxious and may want to undergo revision surgery, even if surgeon does not feel the procedure is warranted. It’s important to discuss all factors with the patient, including comparing the risks of revision surgery – which are not insignificant – with the benefits of the procedure.
Perhaps as important as knowing when to operate is knowing when not to operate on a patient with a MoM THA:
- The patient is asymptomatic and has a well-functioning THA.
- The implant has a good track record.
- The implant is well positioned and the radiographs are normal.
- The metal ion levels are low (less than 1 to 2 ug/L).
- The cross-sectional imaging is normal.
Close surveillance is acceptable in these patients. 
ICJR: What are the principles of revision when treating a patient with a painful MoM THA?
Dr. McCalden: The key principles for revision in these patients include the following:
- Complete the infection work-up, including intraoperative cultures and frozen section.
- Debride the pseudotumor, taking precautions around neurovascular structures and hip abductors.
- Remove the MoM bearing and convert to a cross-linked polyethylene-on-ceramic/oxidized zirconia bearing.
- Revise the mal-positioned cup, if necessary.
- Revise an all-metal monoblock cup, although in some cases it can be converted to a dual-mobility construct.
- Consider a dual-mobility construct or constrained liner in a patient with poor abductors.
- If there is evidence of trunnionosis, clean the taper and replace it with a ceramic head (and Ti taper adaptor sleeve) or an oxidized zirconium head.
- The stem rarely needs to be removed, unless the taper is grossly damaged.
- Beware of a high risk of postoperative complications, including infection, instability, and poor function. 
Richard W. McCalden, MD, MPhil (Edin.), FRCSC, is a Professor of Surgery at Western University, London, Ontario, Canada. He is also Fellowship Director in the Adult Reconstruction Unit at Western University.
Disclosures: Dr. McCalden has no disclosures relevant to this article.
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