A comparison of the diagnostic accuracy of MARS MRI and ultrasound of the painful metal-on-metal hip arthroplasty

Background and purpose

Metal artifact reduction sequence (MARS) MRI and ultrasound scanning (USS) can both be used to detect pseudotumors, abductor muscle atrophy, and tendinous pathology in patients with painful metal-on-metal (MOM) hip arthroplasty. We wanted to determine the diagnostic test characteristics of USS using MARS MRI as a reference for detection of pseudotumors and muscle atrophy.

Patients and methods

We performed a prospective cohort study to compare MARS MRI and USS findings in 19 consecutive patients with unilateral MOM hips. Protocolized USS was performed by consultant musculoskeletal radiologists who were blinded regarding clinical details. Reports were independently compared with MARS MRI, the imaging gold standard, to calculate predictive values.

Results

The prevalence of pseudotumors on MARS MRI was 68% (95% CI: 43–87) and on USS it was 53% (CI: 29–76). The sensitivity of USS in detecting pseudotumors was 69% (CI 39–91) and the specificity was 83% (CI: 36–97). The sensitivity of detection of abductor muscle atrophy was 47% (CI: 24–71). In addition, joint effusion was detected in 10 cases by USS and none were seen by MARS MRI.

Interpretation

We found a poor agreement between USS and MARS MRI. USS was inferior to MARS MRI for detection of pseudotumors and muscle atrophy, but it was superior for detection of joint effusion and tendinous pathologies. MARS MRI is more advantageous than USS for practical reasons, including preoperative planning and longitudinal comparison.Approximately 1.5 million metal-on-metal (MOM) hip arthroplasties have been implanted worldwide since 1996 (FDA 2012). A high early failure rate for these prostheses has recently been demonstrated (Smith et al. 2012). The pattern of failure appears to differ from other hip arthroplasty types, with adverse reactions to wear-related metal debris being a prominent feature (Pandit et al. 2008, Kwon et al. 2011). These solid or cystic lesions have been termed pseudotumors (Pandit et al. 2008). A wide spectrum of adverse tissue reactions have been described. Histologically, using their pseudocapsules, these have been termed aseptic lymphocytic vasculitis-associated lesions (ALVALs) (Willert et al. 2005).Health regulatory guidelines recommend investigation with cross-sectional imaging, using either metal artifact reduction sequence (MARS) MRI or ultrasound scanning (USS), to evaluate the periprosthetic soft tissues. A number of advantages and disadvantages have been reported (Sabah et al. 2011, Liddle et al. 2013). The high-resolution capability of USS allows detailed imaging of solid or cystic extra-articular lesions and also detection of muscle atrophy (Sofka et al. 2004), joint effusions, gluteal tendon avulsion, and iliopsoas or trochanteric bursitis (Long et al. 2012). USS is also commonly used during guided anesthetic injection or fluid aspiration.

Table 1.

A comparison of the advantages and disadvantages of MARS MRI and ultrasound imaging of metal-on-metal hips

	Ultrasound	MARS MRI
1) Clinical evaluation	No metal artifact produced. Operator-dependent; requires an experienced musculoskeletal sonographer. Must be reported at the time of scanning.	Not operator dependant. Can be reported later. Images can be sent off-site for specialist opinion. Useful during preoperative planning for revision arthroplasty (Hart et al. 2012).
a) Pseudotumors	Excellent at visualizing extra-articular fluid collections (including within the iliopsoas and trochanteric bursa). Can differentiate easily between solid and fluid composition. Deep posterior lesions and small lesions can often be missed (Nishii et al. 2012).	High sensitivity for the detection of solid and cystic soft tissue lesions including both small lesions and posterior lesions (Hart et al. 2012, Liddle et al. 2013).
b) Muscles	Dual-view function can be used to simultaneously compare muscles on contralateral sides. Currently not validated to assess muscle atrophy of the hip rotator cuff.	T1-weighted images excellent for assessment of the degree of muscle atrophy (Bal and Lowe 2008, Sabah et al. 2011). Complete cross-sectional imaging allows easy comparison with the contralateral side. Images can be accurately compared over time.
c) Other pathology	Sensitive for joint effusion diagnosis (Foldes et al. 1992). Can visualize the iliopsoas and gluteal tendons in detail. Can be used to detect tendon avulsion of hip abductor muscles (Garcia et al. 2010). Dynamic imaging is possible. Hands-on examination can help localize pathology.	Sensitive modality for the assessment of gluteal tendon avulsion. Other pathology can be identified, including metastatic disease, fractures, and muscle and bone marrow edema. Metal artifact may obscure effusions and tendons directly adjacent to the implant.
2) Patient acceptability	Safe, with no major contraindications (can be used on patients with cardiac pacemaker implants). No problems with claustrophobia. Non-invasive. Invasive when used for guided fluid aspiration or injection.	Enclosed space often unacceptable to patients with claustrophobia. Contraindicated in patients with incompatible metallic implants (e.g. a cardiac pacemaker).
3) General	Relatively low costs. Compact equipment requires minimal space. Often readily accessible in smaller healthcare trusts.	Relatively expensive. The bulky equipment requires a relatively large space. May not be accessible in smaller healthcare trusts.

Open in a separate windowThere is debate as to whether USS or MARS MRI should be used as the initial imaging modality for detection of pseudotumors around MOM hips. There have not been any published studies directly comparing the diagnostic performance of the 2 modalities, and guidelines leave the use of either investigation at the discretion of the surgeon.We determined the sensitivity, specificity, and predictive values of USS using MARS MRI as a reference for the detection of pseudotumors and muscle atrophy.