Fat infiltration in the vastus medialis implicates joint structural abnormalities in early-stage symptomatic knee osteoarthritis

Objective Early knee osteoarthritis (KOA) presents as minor structural abnormalities in joint tissues, such as cartilage and subchondral bone, that cannot be assessed radiographically. Identification of a sensitive and convenient marker for early disease has the potential to enhance patient outcomes. This study determined 1) whether fat infiltration in muscle (i.e., muscle quality), as measured by ultrasound, is associated with structural abnormalities seen in early KOA and 2) which quadriceps muscles are appropriate as a novel marker for early KOA. Methods Participants with early symptomatic KOA (Kellgren Lawrence grade 1-2) underwent ultrasound assessment to measure the echo intensity of the vastus medialis and rectus femoris. The echo intensity corrected for ultrasound wave attenuation caused by subcutaneous fat was then calculated (i.e., corrected echo intensity). Structural abnormalities were assessed using the whole-organ magnetic resonance score (WORMS). A generalized linear mixed model was used to assess the relationship between the corrected echo intensity and WORMS score. Results Forty-nine participants (ages: 44-78 years, 65.3% women) with 52 knees were included. After adjustment for covariates, increased corrected echo intensity (i.e., poor muscle quality) in the vastus medialis muscle was significantly associated with greater structural abnormalities, including disrupted cartilage integrity in the medial tibiofemoral joint. The association was not significant in the rectus femoris muscle. Conclusion Individuals with poor muscle quality in the vastus medialis displayed compromised joint integrity. This study suggests that fat infiltration in vastus medialis assessed by ultrasound is an indicator of early symptomatic KOA. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any funding. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee/IRB of Medical Corporation Nagomi gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.