Osteoarthritis (OA) is a prevalent, chronic, and disabling joint disease that imposes a significant global health burden. The foot is a target region for OA, but foot research is a novel and evolving discipline within the broader field of OA. The most commonly reported affected area in the foot is the first metatarsophalangeal joint (MTPJ). The current method of diagnosing OA using conventional radiography is reactive, detecting OA later in the disease process when irreversible structural damage has already occurred.

Ultrasound imaging (USI) represents an alternative for the diagnosis of OA with potentially inherent advantages, in its ability to detect tissue-specific morphological changes before pain and irreversible structural damage occur. USI can play a fundamental role in the early detection and assessment of foot OA. However, the role of USI for OA diagnosis in foot joints such as the first MTPJ is not clearly defined. The aim of the thesis was to (i) critically evaluate and summarise relevant studies that have used USI to examine foot OA, (ii) develop a USI acquisition procedure and grading system to examine OA features in the first MTPJ, and (iii) develop a USI atlas to grade the degree of osteoarthritic change in the first MTPJ and determine its reproducibility.

Chapter 1 provides an overview of the thesis with Chapter 2 introducing the basic concepts of OA and Chapter 3 introduces the role of imaging in the diagnosis of OA. Chapter 4 reports a systematic review showing the wide degree of variation in which OA features were assessed, how features were defined, and what grading system was applied. Chapter 5 reports a scoping review that showed limited implementation of consensus-based recommendations to guide the development and implementation of USI procedures to assess the first MTPJ. Chapter 6 reports a bibliometric analysis that revealed that MRI, CT, and USI studies continue to evolve in research in this field. Chapter 7 presents an international multispecialty Delphi study that identified 16 essential items that the USI acquisition procedure should encompass when examining the first MTPJ.

Chapter 8 describes the development of a USI acquisition procedure and grading system for examining OA features in the first MTPJ. The USI acquisition procedure and grading system were reliable in assessing first MTPJ OA features in participants with radiologically confirmed OA. Chapter 9 describes the development of a semiquantitative USI atlas (called the AUTUSI atlas) to grade the extent of osteoarthritic change at the first MTPJ and assess the intra- and inter-examiner reproducibility of using the AUTUSI atlas. The AUTUSI atlas demonstrated excellent intra- and inter-examiner reproducibility for evaluating first MTPJ joint effusion, synovial hypertrophy, synovitis, joint space narrowing, osteophytes, and cartilage thickness.

The research successfully developed a reliable procedure to assess both structural and inflammatory features specific to first MTPJ OA. The AUTUSI atlas offers the opportunity to identify prognostic inflammatory features earlier in the course of the disease, before potentially irreversible damage or disability occurs. The AUTUSI atlas may be the catalyst for the development of a USI classification criterion for defining and detecting early first MTPJ OA. Ultimately, the AUTUSI atlas will improve understanding of OA, provide capacity for earlier detection and standardisation of diagnosis, and provide a more sensitive method for classifying and grade the disease process.