Association of Subcutaneous Fat Index, Paraspinal Muscle Characteristics, and Facet Morphology With Lumbar Facet Joint Degeneration

This is a prospective, non-interventional observational clinical study conducted in patients with chronic low back pain. Participants will be adults aged 18 to 75 years who present to the Algology outpatient clinic of Marmara University Pendik Training and Research Hospital and who have a clinical indication for lumbar MRI as part of routine patient care, independent of study participation.

No additional MRI examinations, radiation exposure, medication administration, injections, or invasive procedures will be performed for research purposes. Participation in the study will not alter the patients' routine diagnostic, treatment, or follow-up procedures.

Demographic and clinical characteristics, including age, sex, height, weight, body mass index, smoking status, diabetes mellitus, hypertension, duration and severity of low back pain, and the presence of leg or radicular pain, will be recorded. Pain severity will be assessed using the Numeric Rating Scale.

Lumbar MRI images obtained during routine clinical care will be evaluated for subcutaneous fat index, intervertebral disc degeneration, facet joint degeneration, facet joint angle and tropism, paraspinal muscle cross-sectional area and fat infiltration, and Modic changes. Subcutaneous fat index will be measured at the L1-L2 level. Intervertebral disc degeneration will be assessed using the Pfirrmann classification, while facet joint degeneration will be assessed using the Weishaupt classification. Facet tropism will be calculated as the absolute difference between the right and left facet joint angles. Paraspinal muscle measurements will be performed at the L3-L4, L4-L5, and L5-S1 levels for the multifidus and erector spinae muscles.

The primary objective is to evaluate the association between subcutaneous fat index and moderate-to-severe lumbar facet joint degeneration. Secondary objectives include evaluating the relationships between subcutaneous fat index and total disc degeneration, disc degeneration and facet joint degeneration, facet morphology and facet joint degeneration, paraspinal muscle characteristics and disc or facet joint degeneration, and subcutaneous fat index and pain severity. An exploratory mediation analysis will also assess the potential mediating role of disc degeneration in the relationship between subcutaneous fat index and facet joint degeneration.

Imaging assessments will be performed by an evaluator blinded to pain severity, body mass index, and other clinical findings. Inter-rater or intra-rater reliability will be assessed in a randomly selected subset of approximately 20% of the study sample.