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CT, MRI and Ultrasound
Previously, CT and MRI were rarely used for patients with arthroplasties due to metallic artifact.
Multislice CT has markedly decreased this artifact, and can visualize bone immediately adjacent to hardware. It can be used to evaluate alignment abnormalities, bone graft, heterotopic ossification, osteolysis, and adjacent soft-tissue structures. Reformatted images in various planes can be generated with minimal artifact as well.
Multislice CT is particularly valuable in evaluation of rotational malalignment of the femoral and tibial components.
Axial and coronal CT demonstrating cement herniation into the anterior muscle compartment through defect of the medial cortex of the tibia.
MRI can also be used in patients with hardware by applying sequences that minimize metallic artifact. These sequences include T1 and STIR, employing long echo-train lengths. Gradient echo and fat saturation techniques are avoided when hardware is present. Periprosthetic soft tissues, including fluid collection, collateral ligaments, popliteus, patellar and quadriceps tendons can be evaluated.26
Mass in patient with modular endoprosthesis placed after resection of femur for osteosarcoma.
Correlative axial T1 and STIR images demonstrate similar findings.
Correlative coronal reformatted CT image and coronal T1 weighted image clearly demonstrate mass adjacent to modular endoprosthesis (arrows).
Coronal reformatted CT
14-gauge core needle biopsy performed under CT guidance with patient in prone position demonstrated recurrent osteosarcoma.
Ultrasound can also be used to evaluate periprosthetic soft tissues. It is particularly useful for localizing periarticular fluid collections and guiding aspiration and drainage.
Composite ultrasound image with hypoechoic pes anserine bursa (B) present medial to total knee replacement. F = femoral component, P = polyethylene tray, TC = tibial component, T = tibia.
Same patient as above with 18-gauge needle in bursa for fluid aspiration and culture (arrow points to needle tip). Pus was aspirated and cultures grew staphylococcus.