Osteoarthritis (OA) is one of the most prevalent diseases, with a global disease burden estimated at nearly 528 million affected persons in 2019 [1]. Imaging has played a critical role in OA research for the past several decades. The advent of computed tomography (CT) and magnetic resonance imaging (MRI) has helped our understanding of the interaction between different tissues in the osteoarthritic joint, with the bulk of research focused on the knee joint. Largely thanks to the application of MRI to large clinical studies, it is now accepted consensus to perceive OA as a “whole-organ” disorder involving multiple joint tissues, eventually leading to joint failure. Nonetheless, despite major advances in OA imaging research, much remains to be done. To date, no disease-modifying OA drug (DMOAD) has been approved by the regulatory agencies. This is also a result of the reliance on radiography as the main modality to be used in DMOAD trials. Radiographic joint space narrowing (JSN) is currently recommended by both the Federal Drug Administration (FDA) and European Medicines Agency (EMA) guidance documents as the imaging endpoint for DMOAD trials. However, radiographic changes occur late in the disease, are restricted to the assessment of bony features of OA, and do not correlate well with the patient’s symptoms [2]. Alternative imaging modalities and methodologies for the evaluation of structure in a clinical trial context to be used for eligibility screening and as outcome measures have been suggested [3].

Since the publication of one of the first papers using MRI and correlating bone and soft tissue changes in and around the osteoarthritic joints in 1987 [4], more than 7500 papers have been published using MRI in OA clinical practice and research such as epidemiological studies and clinical trials. Subsequently, we now have a much better understanding of the natural history and pathophysiology of OA compared to the time when the Kellgren and Lawrence system for severity grading of OA from radiographs—which is still in use today—was published. Several disease pheno- or endotypes have been suggested, some of which are based on the predominant structural manifestation of the disease [5]. From the imaging perspective, at least 5 phenotypes have been introduced: inflammatory, subchondral bone, cartilage-meniscus, atrophic, and hypertrophic OA [6]. MRI allowed correlation of pain with structural damage, early changes in OA, and most importantly, elucidated the risk factors for OA incidence and progression.

The last two decades have seen the introduction of several other imaging modalities/technologies with a focus on early OA and targeting OA tissue damage/change. Indeed, compositional MRI techniques were introduced to show early biochemical tissue changes before morphological changes happen with a focus on cartilage. CT is used for depicting early changes in bone shape and subchondral bone structure and helps understand the role of crystal disease in OA incidence and progression [7]. Molecular imaging using positron emission tomography (PET)-CT and PET-MRI offers a unique insight into bone and active synovial inflammation in OA [8].

In the last 5 years, OA imaging research has seen marked acceleration thanks to the rapid advances in imaging technologies coupled with the unique advances in artificial intelligence (AI) approaches. Applications of AI in OA are diverse and include disease stratification, risk assessment, tissue damage detection, localization, and segmentation [9].

This special issue focuses on all the imaging modalities, their pros and cons, and their contribution to the diagnosis, prognosis, and treatment of OA. We tried our best to include all joints and not only the knee which has been the focus of most of the imaging research for the last decades. Imaging-based conventional and new interventional OA treatments are also introduced in this special issue. Finally, the role of imaging in OA in clinical practice will be discussed, and an update on imaging biomarkers in OA will be presented.

We are honored and excited to present this comprehensive update on the imaging of OA which we think will benefit OA researchers, clinical practitioners, and trialists. We are thankful to all our colleagues and expert authors for their excellent contributions. We hope that skeletal radiology readers will find this special issue on the imaging of OA helpful, educational, and stimulating!