In this case-control study, cases were chronic LBP patients aged 30-50 in whom a lumbar MRI had been prescribed. Controls were subjects aged 30-50 in whom a cranial MRI had been prescribed for headache, who did not suffer from LBP and had no history of clinically relevant LBP. Two hundred and forty cases and 64 controls were recruited consecutively in Radiology services across six cities in Spain. Imaging findings and subjects' characteristics were gathered through previously validated instruments. Radiologists who interpreted MRIs were blinded to subjects' characteristics. A multivariate logistic regression model was developed to assess the association of Modic changes with LBP, adjusting for sex, age, body-mass index, lifetime exposure to smoking, physical activity, and disk degeneration.

Inclusion Criteria:

• For all subjects: age between 35 and 50 years.
• For subjects with LBP: having a diagnosis of chronic low back pain (of 3 or more months' duration) as the reason for an MRI.
• For subjects without LBP: subjects with a cranial MRI (described as normal) for cephalalgia, and who had neither current LBP nor history of LBP.

Exclusion criteria were: history of spine surgery, current pregnancy, scoliosis with >15º curvature, vertebral fractures, "red flags" for potential underlying systemic diseases (oncologic disease during the previous 5 years, constitutional symptoms -unexplained weight loss, fever, chills-, history of intravenous drug use, or immunocompromised host), or signs suggesting cauda equine syndrome (relevant or progressive paresia, loss of sphincter control or saddle anesthesia).

Cases: chronic LBP patients aged 30-50 in whom a lumbar MRI had been prescribed. Controls: subjects aged 30-50 in whom a cranial MRI had been prescribed for headache, who did not suffer from LBP and had no history of clinically relevant LBP.