Fatty infiltration (FI) of the rotator cuff muscles represents chronic deterioration in muscle quality that can result from tendon injury, disuse, ageing and nerve injury. Accurate imaging assessment of FI is important for evaluating muscle degeneration, informing rehabilitation planning and enabling valid comparisons across clinical and research settings. A range of approaches has been used to assess FI, including semi-qualitative grading systems and quantitative measurements applied across imaging modalities such as CT, MRI, ultrasound and advanced techniques like Dixon MRI. However, the reproducibility of FI grading or quantification at the level of image interpretation is reported to vary widely, which limits confidence in diagnosis, prognosis and outcome comparisons.
This protocol sets out a systematic review and meta-analysis specifically intended to evaluate the reliability of imaging-based FI assessment in rotator cuff disorders at the level of image interpretation.
The primary objective is to evaluate the inter-rater and intra-rater reliability of image-based assessment of fatty infiltration in rotator cuff disorders. Reliability will be examined for both semi-qualitative grading schemes and quantitative measurements and will be analyzed by imaging modality where data permit.
The review will follow the Preferred Reporting Items for Systematic Review and Meta-Analyses Protocols (PRISMA-P) guidelines. The protocol has been preregistered in PROSPERO with registration number CRD420251242564. Two reviewers will independently perform screening, data extraction and quality assessment. Methodological quality of reliability studies will be assessed using the Quality Appraisal of Reliability Studies (QAREL) checklist. The COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) framework will guide interpretation of reliability-related measurement properties.
Comprehensive searches will be conducted in the following electronic databases from inception to the search date: MEDLINE, Embase, CINAHL, Web of Science and the Cochrane Library. The intent is to capture studies that report on the reliability of image interpretation for fatty infiltration assessment in rotator cuff disorders across CT, MRI, ultrasound and advanced MRI methods such as Dixon sequences.
Two reviewers will independently screen titles and abstracts and then full texts to determine eligibility. Data extraction will also be performed in duplicate, with disagreements resolved by consensus or a third reviewer when required. Extracted data will include study characteristics, imaging modality, FI assessment method (semi-qualitative grading or quantitative measurement), details of raters and timing for intra-rater assessment, and reported reliability statistics (for example intraclass correlation coefficients, kappa statistics or other measures of agreement) when available.
If important study details or numerical results are not reported in the primary articles, the protocol indicates that those details will be noted as not reported in the source.
Study quality and risk of bias for reliability assessments will be evaluated using the QAREL checklist. The COSMIN framework will be used to interpret the measurement properties related to reliability, helping to contextualize the methodological quality findings and guide synthesis decisions.
Primary outcomes are the inter-rater and intra-rater reliability estimates that reflect the consistency of image-based FI grading or quantification. Where sufficient and sufficiently homogeneous data exist, pooled reliability estimates will be calculated separately by imaging modality (for example MRI, CT and ultrasound) and by type of reliability (inter-rater and intra-rater).
If quantitative pooling is not feasible because of limited numbers of studies or substantial heterogeneity in methods, modalities or reported statistics, findings will be synthesised narratively. Summary tables will present study-level results, methodological quality assessments and any notable differences in reliability across grading systems, measurement approaches or imaging techniques.
When meta-analysis is appropriate, pooled estimates will be calculated using a restricted maximum likelihood random-effects model. Analyses will be conducted in R, version 4.3.2, using the metafor package. The protocol specifies separate pooling strategies by imaging modality and by reliability type to reflect important sources of clinical and methodological heterogeneity.
If the source studies report different metrics of agreement (for example different forms of intraclass correlation or kappa statistics), the review will follow appropriate statistical approaches for pooling or will present results in a narrative format if pooling is not statistically or clinically appropriate.
Ethical approval is not required for this review because it will use only published data. The review is registered in PROSPERO (CRD420251242564). Results will be disseminated through a peer-reviewed publication and conference presentations with the aim of informing both research methodology and clinical practice in musculoskeletal imaging.