Off‐resonance based assessment of metallic wear debris near total hip arthroplasty
Historically, the most common bearing surfaces used in THA are composed of an ultrahigh molecular weight polyethylene acetabular component articulating against a cobalt‐chromium (CoCr) alloy femoral head. Although this metal‐on‐polyethylene combination generally yields excellent results 4, polymeric debris from the articular surface can initiate a host‐dependent inflammatory reaction. These reactions often cause bone resorption (osteolysis) and subsequent implant loosening, which is a common cause for THA failure 5.
Metal‐on‐metal bearing THAs were introduced, in part, to allow the use of larger femoral heads that are associated with a lower incidence of dislocation. Metal‐on‐metal devices are composed primarily of CoCr alloy and have annual linear wear rates that are more than 20 times less than metal‐on‐polyethylene articulations and generate smaller 7 but significantly more 9 wear particles. Observation of substantial metallic debris on imaging in peri‐implant tissues can confirm the generation of wear debris at articular surfaces related to edge loading 11, misalignment 12, or electrochemical corrosion 7. Debris deposition into the surrounding soft tissue envelope, which is often described as “metallosis,” leads to tissue necrosis of important structures, such as hip abductors, resulting in patient morbidity with altered gait mechanics and pain. As few noninvasive methods exist to quantify metallosis, whole blood or serum ion levels are often used as an index of implant wear and tissue metal content 13. Serum ion levels may correlate with implant wear 12, but this methodology lacks spatial information about the sources of ion generation and has been shown to be less accurate than magnetic resonance imaging (MRI) in determining the extent of soft tissue pathology according to histologic samples from extracted tissues 15.
In this work, we present and utilize an imaging‐based mechanism for quantifying the severity of metal particle deposits in the immediate vicinity of THA. Though pockets of dense particulate debris (metallic or ultrahigh molecular weight polyethylene) can be crudely identified using magnitude MRI data, where they present as hypointense regions in proton‐density weighted acquisitions 16, the methods presented here offer a quantitative and specific assessment of metallic particle deposition. This approach leverages the magnetism of metallic debris products and the sensitivity of MRI to local changes in effective bulk material magnetization. The presented methods could further lend support to the use of MRI as a biomarker for tissue necrosis due to metallic corrosion after THA.