Reliability of Semi-Automated Volumetric Quantification of Knee Effusion Using MRI After Acl Injury.

Background: Effusion is a frequent finding after anterior cruciate ligament (ACL) injury and reconstruction, but the reliability of its quantification on routine [non-contrast] magnetic resonance imaging (MRI) is not established. This study sought to evaluate the intra-rater and inter-rater reliability of volumetric quantitative measurements (VQM) of knee effusion volumes (EV) using axial proton density-weighted, fat-suppressed (PD-FS) sequences after ACL injury and reconstruction.

Methods: Twenty deidentified MRIs (10 ACL-injured knees and 10 uninjured knees) were randomly selected from a prospective observational cohort of participants aged 15-35 years. Two independent raters analyzed EV using a custom MATLAB protocol. Intra- and inter-rater reliability was assessed using intraclass correlation coefficients (ICC_3,1) with minimal detectable changes calculated. Measurement agreement was assessed using Bland-Altman plots.

Results: Mean EV were higher in the injured compared to uninjured knees (11.88±8.14 mL vs 2.03±0.76 mL). Volumetric effusion measurements demonstrated excellent intra-rater (ICC_3,1=0.993, 95% CI: 0.981-0.997, p<0.001) and inter-rater reliability (ICC_3,1=0.974, 95% CI: 0.907-0.991, p<0.001). The minimal detectable change was 0.145 mL for intra-rater measurements and 0.504 mL for inter-rater measurements. Bland-Altman plots revealed no systematic bias between raters, although variability increased with larger volumes (>8 mL).

Conclusion: VQM using standard axial PD-FS MRI sequences demonstrate excellent reliability across raters. Restricting segmentation to the anterior compartment aligns with clinical assessment (e.g., sweep tests), though it does not capture whole-joint eflusion. This semi-automated approach may provide a relevant biomarker for monitoring EV in ACL research and rehabilitation trials.

Level of evidence: III.