5.1 Magnetic resonance imaging is ideally suited to image MOM hip arthroplasty due to its superior soft tissue contrast, multiplanar capabilities and lack of ionizing radiation. MR imaging is the most accurate imaging modality for the assessment of peri-prosthetic osteolysis and wear-induced synovitis (19, 20).

5.2 Before scanning a patient with a specific implant, the MR practitioner shall confirm that the device is MR Conditional and that the scan protocol to be used satisfies the conditions for safe scanning for the specific implant.

5.3 This guide can be used to identify the following adverse events.

5.3.1 Osteolysis-Magnetic resonance imaging is superior to conventional radiographs and computer tomography (CT) in the assessment of peri-prosthetic osteolysis and has been shown to be the most accurate method to locate and quantify the extent of peri-prosthetic osteolysis (19, 21). On MR imaging, osteolysis appears as well marginated intraosseous intermediate to slightly increased signal intensity lesions that contrast with the high signal intensity of the intramedullary fat. A characteristic line of low signal intensity surrounds the area of focal marrow replacement, distinguishing the appearance of osteolysis from tumoral replacement of bone or infection (22).

FIG. 4 Coronal (left) and Axial (right) FSE Images of a Left MOM Hip Arthroplasty

Note 1: There is focal osteolysis (white arrows) in the greater trochanter, which manifests as well-demarcated intermediate signal intensity, similar to that of skeletal muscle, replacing the normal high signal intensity fatty marrow. Images courtesy of Dr. Hollis Potter.

5.3.2 Component Loosening-While the data are preliminary, MR imaging can identify circumferential bone resorption that may indicate component loosening. Loosening may result from osteolysis, circumferential fibrous membrane formation or poor osseous integration of a non-cemented component. On MR imaging, component loosening typically manifests as circumferential increased signal intensity at the metallic-bone or cement-bone interface on fat-suppressed techniques (20). The finding of circumferential fibrous membrane formation or osteolysis also indicates potential loosening; this is in contrast to a well-fixed component, with high signal intensity fatty marrow directly opposed to the implant interface.

5.3.3 Wear-Induced Synovitis-Magnetic resonance imaging is the most useful imaging modality to assess the intracapsular burden of wear-induced synovitis surrounding MOM arthroplasty (23). Preliminary data indicate that the signal characteristics of the synovial response on MR imaging correlate with the type of wear-induced synovitis demonstrated on histology at revision surgery (24). Low signal intensity debris is suggestive of metallic debris on histology. Mixed intermediate and low signal debris correlate with the presence of mixed polymeric (polyethylene and/or polymethyl methacrylate) and metallic debris at histology. Magnetic resonance imaging can demonstrate decompression of synovitis or fluid into adjacent bursae, such as the iliopsoas or trochanteric bursa, which can present as soft tissue masses or with secondary nerve compression. On occasion, wear-induced synovitis can result in a chronic indolent pattern of erosion of the surrounding bone, even in the absence of focal osteolytic lesions (6).

FIG. 5 Axial (left) and Coronal (right) FSE Images of a Left MOM Hip Arthroplasty

Note 1: Wear-induced synovitis decompresses into the abductor musculature where there is low signal intensity debris (arrow), consistent with metallic debris. Images courtesy of Dr. Hollis Potter.

5.3.4 Infection-In the setting of infection, the synovium often demonstrates a hyperintense, lamellated appearance with adjacent extracapsular soft tissue edema. These appearances help to distinguish the synovial pattern of infection from wear-induced synovitis, although aspiration is still required for definitive diagnosis (22). The presence of a soft tissue collection, draining sinus or osteomyelitis further supports the diagnosis of infection on MR imaging.

FIG. 6 Axial FSE (left) and Inversion Recovery (right) Images of a Right MOM Hip Athroplasty

Note 1: There is a lamellated synovitis (black arrow) with adjacent extracapsular soft tissue edema (white arrow). Infection was confirmed at subsequent aspiration. Images courtesy of Dr. Hollis Potter.

5.3.5 Adverse Local Tissue Response-Adverse local tissue reactions can manifest as synovitis, bursitis, osteolysis and cystic or solid masses adjacent to the arthroplasty, which may be termed pseudotumors (19, 20). ALTR can also include the histopathologic feature of aseptic lymphocytic vasculitis-associated lesions (ALVAL), which can be confirmed at histology. A relatively common appearance of joints with ALVAL is expansion of the capsule with homogenous high signal fluid interspersed with intermediate signal intensity foci. More recent studies suggest that maximum synovial thickness and the presence of more solid synovial deposits highly correlate with tissue damage at revision surgery and necrosis at histologic inspection (15).

FIG. 7 Axial FSE Image in a Right MOM Hip Arthroplasty

Note 1: Fig. 7 demonstrates a large collection of fluid in the trochanteric bursa (arrow), which communicates with the hip joint via a dehiscence in the posterior pseudocapsule (not shown in these images). The fluid is high signal with fine intermediate signal intensity debris. A high ALVAL score was confirmed on histology at revision surgery. Images courtesy of Dr. Hollis Potter.

FIG. 8 Axial FSE Image in a Right MOM Hip Resurfacing Arthroplasty

Note 1: Fig. 8 demonstrates expansion of the pseudocapsule with fluid signal intensity decompressing into the trochanteric bursa. The pseudocapsule is thickened and of intermediate signal intensity (black arrows). There is additional solid extracapsular disease anteriorly (white arrow). At revision surgery, a mixed picture of ALVAL and metallosis was seen.

5.3.6 Modular Taper Associated ALTR-MRI can accurately describe ALTR attributed to tribocorrosion in modular femoral neck total hip arthroplasty. MRI characteristics, particularly maximal synovial thickness and synovitis volume, can predict histologic severity (22, 23). In addition, intra-capsular ALTR around either resurfacing MOM arthroplasty or around the trunnion in MOM THA may be obscured if 3D-MSI techniques are not utilized due to the susceptibility artifact. High-bandwidth FSE or FSE with view-angle tilt are not sufficient.

Note 1: Modular taper ALTR may occur in non-metal-on-metal implants as well as in metal-on-metal arthroplasty.