High Incidence of Pseudotumors After Hip Resurfacing Even in Low-Risk Patients; Results From Intensified MRI Screening Protocol
A Metal-on-Metal hip pseudotumor is a mass of tissue that can form and grow around a Metal-on-Metal (MoM) hip implant. Pseudotumors are an indicator of MoM disease, a type of implant failure. This study uses Metal-Artefact Reduction Sequence magnetic resonance imaging (MARS-MRI) scans to screen for pseudotumors in patients who have received Metal-on-Metal hip resurfacing arthroplasty (MoM HRA). MARS-MRI is a specific setting for MRI scans that makes it easier to see tissue formations in the presence of metal implants.
The authors attempt to figure out if physicians should adopt more rigorous screening methods, such as MARS-MRI scans, for MoM HRA patients when looking for pseudotumors because current screening methods may not always detect pseudotumors. Currently, physicians only use clinical outcomes and plain radiographs to screen for pseudotumors in patients who have received MoM HRA.
44 MRI scans were taken in total. The scans revealed that 15 of the 44 hips had pseudotumors. Of those 15, 6 had mild MoM disease, 8 had moderate MoM disease, and one had severe MoM disease. Additionally, 12 of the patients with pseudotumors showed no symptoms associated with pseudotumors or implant failure. Two patients were found to need revision surgeries after the MRI scans. Metal ion levels in the blood were normal in 80% of the patients, even though 34.1% of the patients had MoM disease. This is concerning because doctors often use metal ion blood tests to diagnose patients with MoM disease.
In conclusion, using regular testing methods (clinical follow-ups and plain radiographs) the authors did not find any pseudotumors in their patients. However, using the MARS-MRI scans, they were able to detect 15 pseudotumors, 12 of which were in patients with no other symptoms of MoM disease. Therefore, the authors conclude that current methods for detecting pseudotumors in MoM HRA patients are insufficient.
Over time, MoM implants can wear down and release metal debris into the blood and tissue of patients. This can lead to implant failure associated with many negative symptoms, including the formation of pseudotumors. Pseudotumors, an indicator of implant failure, can force patients to undergo revision surgery on their implants, which is difficult to perform and does not have a high success rate.
To measure the success rate of MoM implants and screen for pseudotumors, most past studies used clinical outcome scores and plain radiographs. These studies found that MoM implants had excellent success rates. However, more recent studies have included MARS-MRI images or ultrasounds in their pseudotumor detection techniques. These new studies with more rigorous screening methods have found a much higher incidence of pseudotumors in patients with MoM implants.
In light of these new findings, the authors suspect that many patients who think their implants have been successful might actually have pseudotumors that have gone undetected.
Patients and Methods
All of the patients in this study received the same implant: the Biomet ReCap hip resurfacing system. The researchers divided their patients into three groups: those with a high risk of developing a pseudotumor (11 hips), those with low risk of developing a pseudotumor (10 hips), and a control group (23 hips). Previous studies have determined what risk factors may make individuals more likely to develop pseudotumors.
The authors of the study performed enhanced screening in the form of MARS-MRI imaging. Regular methods of pseudotumor screening (clinical outcomes and plain radiographs) did not reveal any pseudotumors in a group of 289 HRA patients. However, once the authors took MARS-MRI scans of 44 of those patients, they detected 15 pseudotumors. This means that 34.1% of the patients screened developed pseudotumors, and they would not have been detected through the current screening methods. Pseudotumors were found in all three groups of patients, even those with low risk of developing MoM disease. Additionally, 12 of the 15 patients with pseudotumors showed no other symptoms of implant failure.
Pseudotumors were very prevalent among this group of patients with Biomet ReCap HRA implants. They occurred even in patients whose records show they were at low risk of implant failure. Pain was not a reliable indicator that a pseudotumor was present; in other words, even patients who experienced no pain could have had pseudotumors.
Patients with few symptoms of implant failure are reluctant to undergo revision surgery. However, some patients have been alarmed by the recent coverage of MoM implant failure in the media, and have asked for revision surgery regardless of whether they had symptoms.
Several studies suggest that pseudotumor occurrence in MoM HRA patients is relatively low, but these studies base their results only on plain radiograph screenings. In contrast, studies that use MARS-MRI technology or computer tomography (CT) scanning to screen for pseudotumors find high rates of pseudotumor occurrence, similar to the rate found in this study (34.1%). Pseudotumor occurrence is even higher in patients who have undergone MoM total hip replacement surgery (MoM THA) than it is in patients who have undergone MoM HRA.
This study found that clinical outcomes and radiographs are not effective at detecting pseudotumors after MoM HRA. Screening with MARS-MRI, CT scans, or ultrasounds on all patients might be the only way to discover the real risk of developing pseudotumors after MoM HRA.
The Biomet ReCap implant has been banned in the Netherlands due to its high risk of failure.
Walter van der Weegen, Jose M.H. Smolders, Thea Sijbesma, Henk J. Hoekstra, Koen Brakel and Job LC. van Susante, Hip Int. 2013; 23(3): 243-9
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