The developed USI acquisition procedure and grading system are reliable in evaluating first MTPJ OA features in participants with radiologically confirmed OA. Data revealed the assessment of joint effusion, synovial hypertrophy, synovitis, joint space narrowing, osteophytes, and cartilage thickness had good to excellent intra- and inter-examiner reliability. Poor intra-examiner reliability was only reported for cartilage thickness when assessed as a continuous measure. Absolute agreements were excellent for osteophytes and joint space narrowing.
It is well understood that inflammation is an important driver of the disease and contributes to the structural progression of OA (14, 25, 26). Despite this, within the foot clinicians and researchers are currently confined to radiographic grading or grading originally designed from populations with RA. The distinct difference of inflammation experienced in OA compared to RA and the inability of conventional radiography to detect inflammatory features provides significant limitations. Therefore, the development of our USI grading system is a fundamental step in determining the role of inflammation for first MTPJ OA and the prognostic value for structural progression. It is a pivotal finding that our developed USI procedure and grading system reported good to excellent intra and inter-examiner reliability for all inflammatory OA features. Particularly given the marked variations across studies in terms of how synovitis, synovial hypertrophy and joint effusion are defined and categorised as USI features (12). Consequently, it is essential to discriminate the existing disparities among various entities of synovial pathology that serve as indications of inflammation (12). In line with the Outcome Measures in Rheumatology (OMERACT) hand OA study, we scored greyscale inflammatory abnormalities for synovial hypertrophy and joint effusion separately. Synovitis was examined as a separate entity by power Doppler signal (flow signal detected within synovial hypertrophy was considered a sign of synovitis) (27, 28). Due to the marked variation in prevalence between greyscale and Doppler detected inflammatory features demonstrated in hand OA (27), only including greyscale features indicative of inflammation may result in OA being underestimated. A recent study that used magnetic resonance imaging to examine first MTPJ OA, was limited by the fact that they included effusion and synovitis as a combined proxy measure for synovitis, termed “effusion-synovitis” (29). Given the prognostic value of inflammatory features and the sensitivity USI possesses in detecting subclinical inflammatory change (2, 3), the inclusion of multiple inflammatory features that can be reliably quantified as separate entities, may be more helpful in elucidating the role of inflammation in OA.
The poor intra-examiner reliability for examining cartilage as a continuous measure may be attributed to scoring cartilage based on a single thickness measurement. It may be that as cartilage may not be uniform across the entire joint surface, a singular measurement of thickness may not provide an accurate representation of cartilage damage across the whole joint surface. Therefore, the ability to consistently examine the exact same part of cartilage, between sessions, will influence the reliability of this measure. The technique we employed requires examiners to measure a vertical line with consistent perpendicular alignment between cartilage borders at a subjective location. Small deviations in the location and orientation can result in thickness differences and measurement variance between sessions and/or examiners (30). This finding is consistent with previous OA and RA studies, which have reported difficulties when examining cartilage damage (23, 29). Our poor intra examiner reliability results may also be attributable to practical difficulties associated with the scanning of cartilage. To obtain the exact same ultrasound image, the beam angle and location used in session one would need to be precisely replicated in session two. This may not have occurred for all repeat scans which may have influenced the cartilage measurement (31, 32). Conversely the excellent inter-examiner reliability for cartilage thickness as continuous measure between session one and two may be explained by the fact that the radiologist graded already acquired images of cartilage thickness.
A previous attempt to develop semiquantitative 0–3 grading for cartilage in hand OA found moderate intra-reader and only fair inter-reader agreement (33). Even supporting definitions could not help to sufficiently discriminate between intermediary grades. A recent study on cartilage in RA patients simplified the scoring to a 0–2 scale and reported excellent intrareader and moderate interreader reliability in the metacarpophalangeal joint (34). Therefore, to mitigate issues with mid-range subjective grading we opted for a 0–2 semiquantitative scoring system based on the morphological integrity of the superficial interface of the cartilage and the cartilage thickness. To aid in visualisation of cartilage thickness, the acquisition procedure was modified to include plantarflexion of the first MTPJ. Consequently, the degree of plantarflexion achieved between sessions may have varied, this would undoubtedly have influenced cartilage thickness measures.
Variable intra- and inter-examiner reliability of USI has been reported in the literature (35, 36). Given the general perception that ultrasonography is a highly operator-dependent technique (35, 37), our results are encouraging and represent an important step in support of further application of USI to assess other foot joints. The results of this study will inform the methodological development of an USI atlas for grading the degree of osteoarthritic change at the first MTPJ. It is expected that the accompaniment of an illustrated manual (i.e., USI atlas), that clearly presents the USI procedure, features and grades will improve the consistency of interpretation and grading and improve the reliability when examining both structural and inflammatory change in first MTPJ OA.
This study must be viewed in the context of possible limitations. First, the USI procedure developed included only a dorsal scan of the first MTPJ. USI offers a multiplanar technique and as the medial and/or plantar aspect of the first MTPJ was not examined, the developed procedure may underestimate the prevalence or severity of features.
Second, the poor reliability reported for examining cartilage as a continuous measure could be mitigated by segmenting the entire cross-sectional area of the articular cartilage using ImageJ software (38). With this technique, the average cartilage thickness within standardised regions can be calculated. However, this technique requires additional expertise and overall time to complete the segmentation, which may limit the translation of cartilage thickness assessment to a clinical setting. Third, investigator bias may have occurred as the same radiologist reported on radiographic screening and graded the acquired ultrasound images. All participants were randomly assigned an alphanumerical code upon entry into the study to minimise this risk of bias. Finally, despite efforts to proactively recruit an ethnically diverse population that represents the broader New Zealand population, no Pacific peoples were included. Pacific peoples suffer from significant and longstanding health inequalities and poorer health outcomes compared to other New Zealanders. Therefore, the collection of accurate ethnic OA data is needed to better understand what factors contribute to these inequalities and to provide the capacity to measure progress.