Pseudotumor frequency from metal-on-metal total hip arthroscopy

Pseudotumors, the occurrence of cystic and/or solid masses in periprosthetic tissue, are a well-known complication of metal-on-metal total hip arthroscopy (MoM THA). The imaging examination of choice to identify these pseudotumors is a magnetic resonance (MR) scan with metal artifact-reducing sequences.MR imaging grading systems have been developed to classify the severity of pseudotumors. In a study evaluating the validity of three commonly used pseudotumor grading systems, researchers from the Netherlands determined that grading outcomes were not consistent with each other and that interobserver reliability was limited.

More importantly, this study, published online in Skeletal Radiology, also revealed that 43% of 201 patients who were not categorized as either as being at high- or low-risk of developing pseudotumors were diagnosed with them.¹ This unexpected finding suggests that unless categorized as “low risk,” every patient is at substantial risk for developing a pseudotumor and that radiologists should be aware of this.

A total of 240 patients with an uncemented MoMTHA who had a metal-artifact-reducing sequence MR scan (MARS-MRI) between February 2008 and January 2011 at Leiden University Medical Centre and Meander Medical Centre in Amersfoort were included in the retrospective study. The imaging exam had been ordered if osteolysis had been identified on a radiograph, if patients were experiencing pain, and/or if patients had elevated serum metal ion levels above 5 μg/l. The authors categorized the patients into one of three groups (high risk, low risk, control).

Two musculoskeletal radiologists, one highly experienced and the other in training, applied three classification systems (Anderson², Hauptfleisch³, and Matthies⁴) to each set of exams. A total of 106 pseudotumors were identified by consensus. Their prevalence as determined by the MARS-MRI scans was 59% in the high-risk group of 34 patients, 0% in the low risk group (5 patients), and 43% in the control group (201 patients). The authors stated that serum cobalt values were increased in the high-risk group compared to the other two groups, and for their patient cohort the Matthies classification system was the most reliable.

Nearly half (49%) of the patients diagnosed with pseudotumors were asymptomatic. Lead author Christiaan Smeekes, MD, of the Department of Orthopaedics of Leiden University Medical Centre, reported that no differences were identified in control group patients who had a pseudotumor and those who did not. There also were no significant differences in the type of pseudotumor, pain and gender.

“MARS-MRI is one of the tools that aids in clinical decision-making regarding revision MoM THA, but whether or not a pseudotumor grading system should be used, or if so, which one, is still under debate,” they concluded. “When interpreting this exam, radiologists need to remember that asymptomatic, all but low-risk patients have a likelihood of having developed a pseudotumor.”

REFERENCES

1. Smeekes C, Schouten BJM, Nix M, et al. Pseudotumor in metal-on-metal hip arthroplasty: a comparison study of three grading systems with MRI. Skeletal Radiol. Published online February 1, 2018. doi.org/10.1007/s00256-018-2873-0.
2. Anderson H, Toms AP, Cahir JG, et al. Grading the severity of soft tissue changes associated with metal-on-metal hip replacements: reliability of an MR grading system. Skeletal Radiol. 2011 40;3:303-307.
3. Hauptfleisch J, Pandit H, Grammatopoulos G, et al. A MRI classification of periprosthetic soft tissue masses (pseudotumours) associated with metal-on-metal resurfacing hip arthroplasty. Skeletal Radiol. 2012 41;2:149-155.
4. Matthies AK, Skinner JA, Osmani H, et al. Pseudotumors are common in well-positioned low-wearing metal-on-metal hips. Clin Orthop Relat Res. 2012 470;7:1895-1906.