Objective Ultrasound (US) imaging may play a fundamental role in the earlier detection and assessment of first metatarsophalangeal joint (MTPJ) osteoarthritis (OA) due to its ability to depict tissue-specific morphological changes before the point of irreversible structural damage. However, the role of US in supporting the diagnosis of OA in foot joints has not been clearly defined. The aims of the study were to develop a semiquantitative US atlas (the AUTUSI atlas) to grade the degree of osteoarthritic change in the first MTPJ and to evaluate the intra-examiner and inter-examiner reproducibility of using the atlas.

Design Ultrasound images were obtained from 57 participants (30 participants with radiographically confirmed first MTPJ OA). The AUTUSI atlas supports the examination of grading joint effusion, synovial hypertrophy, synovitis, osteophytes, joint space narrowing, and cartilage thickness. Six examiners used the atlas to independently grade 24 ultrasound images across two sessions. Intra-examiner and inter-examiner reproducibility were determined using percentage agreement and Gwet's AC2.

Results Observations using the AUTUSI atlas demonstrated almost perfect-to-perfect inter-examiner agreement (percentage agreement ranged from 96% to 100%, and Gwet's AC2 values ranged from 0.81 to 1.00) and moderate-to-perfect intra-examiner agreement (percentage agreement ranged from 67% to 100%, and Gwet's AC2 values ranged from 0.54 to 1.00).

Conclusion The AUTUSI atlas demonstrated excellent intra-examiner and inter-examiner reproducibility for evaluating first MTPJ joint effusion, synovial hypertrophy, synovitis, joint space narrowing, osteophytes, and cartilage thickness. The AUTUSI atlas affords an opportunity to detect prognostic markers of OA earlier in the disease cascade and has the potential to advance understanding of the pathological process of first MTPJ OA.