Comparison of extracapsular pseudotumors seen in magnetic resonance imaging and in revision surgery of 167 failed metal-on-metal hip replacements

Abstract

Background and purpose: Magnetic resonance imaging (MRI) is important for detecting extracapsular pseudotumors, but there is little information on the accuracy of MRI and appropriate intervals for repeated imaging. We evaluated the sensitivity and specificity of MRI for detecting pseudotumors in 155 patients (167 hips) with metal-on-metal (MoM) hip arthroplasties that failed due to adverse reactions to metal debris (ARMD).

Methods: Preoperative MRIs were performed with two 1.5 T MRI scanners and graded by a senior musculoskeletal radiologist using a previously described MRI pseudotumor grading system. Revision findings were retrieved from surgical notes, and pseudotumors were retrospectively graded as fluid-filled, mixed-type, or solid.

Results: The sensitivity of MRI was 71% and the specificity was 87% for detecting extracapsular pseudotumors. The sensitivity was 88% (95% CI: 70-96) when MRI was performed less than 3 months before the revision surgery. Interestingly, when the time that elapsed between MRI and revision was more than 1 year, the sensitivity calculated was only 29% (95% CI: 14-56). Comparison between MRI and revision classifications gave moderate agreement (Cohen's kappa = 0.4).

Interpretation: A recent MRI predicts the presence of a pseudotumor well, but there is more discrepancy when the MRI examination is over a year old, most likely due to the formation of new pseudotumors. 1 year could be a justifiable limit for considering a new MRI if development of ARMD is suspected. MRI images over a year old should not be used in decision making or in planning of revision surgery for MoM hips.

Figure 1.

Images from a 70-year-old man who had undergone total hip arthroplasty of the right hip 3.4 years earlier. He had a tingling sensation in the trochanteric region and the replaced right hip made clacking sounds. Whole-blood metal ion levels were slightly elevated (cobalt 7.5 ppb and chromium 5.8 ppb; normal reference values are < 0.8 ppb for Co and Cr). Axial view of a thin-walled cystic pseudotumor in the greater trochanteric region (arrows) with fluid-like low signal intensity in T1 (panel A) and high in STIR (B). A thin-walled and fluid-filled pseudotumor with metal staining was encountered at revision surgery (C).

Figure 2.

Images from a 64-year-old woman who had undergone total hip arthroplasty of the right hip 4.7 years earlier. She had stiffness and exercise-related pain in the replaced right hip. Whole-blood cobalt was 6.9 ppb and chromium was 4.8 ppb (normal reference values are < 0.8 ppb for Co and Cr). A thick-walled pseudotumor with solid content was seen extending posterolaterally from the hip joint region on the right side. Variable signal intensity was seen in axial T1 (panel A). Synovial hypertrophy was best seen in coronal STIR view (B). A mixed-type pseudotumor with thick walls and partially solid contents was seen at revision surgery (C).

Figure 3.

Images from a 43-year-old woman who had undergone total hip arthroplasty of the left hip 2.5 years earlier. Her replaced hip made clacking sounds, and she also had intense pain in both the groin and in the trochanteric region during exercise—and even at rest. Whole-blood cobalt was 8.8 ppb and chromium was 3.1 ppb (normal reference values are < 0.8 ppb for Co and Cr). A. Axial T1 view of a thick-walled partly cystic large pseudotumor mass extending from the iliopsoas region to the posterolateral region. The posterolateral part of the pseudotumor appeared mostly solid with variable signal intensity in T1 (panel B) and STIR (C). A predominantly solid pseudotumor was encountered in revision surgery (D).