This paper sheds light on the management of patients who have a recalled dual-taper modular total hip replacement in place. The authors report on 83 patients divided into 2 groups: symptomatic (23) and asymptomatic (60). A pseudotumor was identified with metal artifact reduction sequence magnetic resonance imaging (MARS-MRI) in 30 (36%) of the 83 patients. Nine (15%) of the 60 asymptomatic patients and 21 (91%) of the 23 symptomatic patients had a pseudotumor. Patients with a pseudotumor had a significantly higher cobalt level (8.0 μg/L) compared with those with no pseudotumor (2.0 μg/L). On the basis of this finding, the authors recommend MARS-MRI for all patients with elevated cobalt levels, even in the absence of hip symptoms.
If the goal is to minimize cost associated with MRI scans, then the authors’ recommendation of obtaining them only when the metal-ion level is elevated is sound. If the goal is to minimize any irreversible soft-tissue damage caused by pseudotumors, then it should be noted that some pseudotumors may be missed unless the cobalt threshold is quite low (for example, 2 to 4 μg/L). Since there was at least 1 patient with a pseudotumor who had a cobalt level of <4.0 μg/L, a reasonable threshold for the cobalt level detected on MARS-MRI may be >2.0 μg/L.
The natural history of pseudotumors is not known. A 36% prevalence in <2 years is remarkable. Experience tells us that more often than not these pseudotumors increase in size. The damage to the hip abductor musculature can be irreparable, leading to a poor functional result for the patient no matter how meticulous the femoral component removal. Also, radiographs are not particularly helpful in this situation. The taper corrosion seen with this implant rarely produces osteolysis (at least prior to 5 years), and these stems are typically well ingrown and can be very difficult to remove. In addition to abductor dehiscence, trochanteric fracture, dislocation, infection, and revision, component subsidence can occur1.
The authors offer as a point of reference the prevalence of asymptomatic pseudotumors associated with metal-on-metal (MoM) total hip replacement (31% to 65%) and surface replacement arthroplasty (SRA) (27% to 68%). However, the comparison can be misleading. While MoM and SRA revisions do occur, the rate is not as high as the 28% revision rate at 2 years reported by Meftah et al.2 and the 86% revision rate at 3 to 5 years reported by Bernstein et al.3 with this particular hip stem. If all of the 23 symptomatic patients in this report underwent a revision, the revision rate would be 28% after a mean of 22 months of follow-up.
The authors state “…MARS-MRI is indicated for patients with elevated metal-ion levels. A longitudinal study is required to determine whether asymptomatic patients with a taper-corrosion-related pseudotumor will become symptomatic with time.” It may be more helpful to obtain an MRI for all patients at least once in the 1 to 2-year postoperative period so that the 15% of asymptomatic patients who have a pseudotumor can be identified and counseled regarding revision surgery. Another indication for MRI may be a slightly elevated cobalt level (>2.0 and <4.0 μg/L) to make sure that the patient can be safely observed for another year.
When should revision surgery be recommended? Currently, no “revision threshold” has been agreed on for metal-ion levels or for pseudotumor size. Ideally the revision should be performed before abductor muscle damage occurs. On the basis of this report and others4, and the nearly universal failure of this implant, the decision for revision surgery can be based on several factors: pain and elevated/increasing cobalt levels as well as MRI findings.
Revision of this particular femoral stem may be recommended for patients with (1) mechanical hip symptoms, (2) elevated cobalt levels, or (3) a pseudotumor seen on MRI. Close follow-up can be recommended in the absence of hip symptoms, a cobalt level of <4.0 μg/L, and no pseudotumor seen on MRI scanning. Yearly evaluations, including a history, physical examination, and measurement of metal-ion levels, should be performed. An MRI is not necessarily required for all revision cases (for example, if the patient has pain and an elevated cobalt level) but likely will be helpful in planning the surgical procedure.
↵* The author indicated that no external funding was received for any aspect of this work. On the Disclosure of Potential Conflicts of Interest form, which is provided with the online version of the article, the author checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work.
- Copyright © 2016 by The Journal of Bone and Joint Surgery, Incorporated