WOODVILLE, South Australia, and BOSTON, Massachusetts—Changes in synovitis correlate with changes in knee pain in patients with knee osteoarthritis (OA) but not with cartilage loss, researchers from the Boston Osteoarthritis of the Knee Study (BOKS) report in Annals of the Rheumatic Diseases.1

"Our study demonstrates that change in synovitis over time was correlated modestly with changes in knee pain, but not at all with loss of cartilage. The correlation was direct—an increase in synovitis was associated with worsening pain and a decrease with less severe pain."—Catherine L. Hill, MD.
"Treatment of pain in knee OA needs to consider treatment of synovitis," writes lead investigator Catherine L. Hill, MD. Dr. Hill, with the rheumatology unit at The Queen Elizabeth Hospital, in South Australia collaborated with doctors at Boston University School of Medicine, in Massachusetts, among other research team members.

MRI scored synovitis at baseline, 15, and 30 months

All 270 subjects in the study had symptomatic knee OA according to ACR criteria and were BOKS participants. Patients had measurements at baseline and at 15- and 30-month follow-up that included knee magnetic resonance imaging (MRI), body mass index, and pain assessment (0-100 visual analogue scale, VAS). The investigators scored synovitis at the infrapatellar fat pad and suprapatellar and intercondylar regions using a semiquantitative (0-3) scale; scores at each site were added to give a 0-9 summary synovitis score.

Dr. Hill reports that baseline synovitis did not correlate with the baseline pain score but that changes in the summary synovitis score did correlate with the change in pain intensity (R = .09, P = .0003).

"An increase of one unit in summary synovitis score resulted in a 3.15 mm increase in VAS pain score (0-100 scale)," Dr. Hill writes. The strongest correlation with pain change was for synovitis in the infrapatellar fat pad. Interestingly, pain change did not correlate with changes in effusion.
"Despite cartilage loss occurring in over 50% of knees, synovitis was not associated with cartilage loss in either tibiofemoral or patellofemoral compartment."

Where does it hurt?

Pain is one of the hallmarks of knee OA but its source remains illusive. Cartilage has no pain fibers, but nociceptive fibers have been identified in the periosteum, subchondral bone, fat pad, capsule, and synovium, Dr. Hill points out. Inflammation and pro-inflammatory cytokines are also suspected of playing a part in OA knee pain, and the synovial thickening seen on MRI correlates with synovial inflammation on biopsy.

The study determined the association between baseline and longitudinal changes in MRI synovitis and the experience of knee pain. The investigators found that most knees had evidence of synovitis at baseline and that about 40% had changes in synovitis over time. Higher baseline synovitis scores were associated with slightly higher baseline pain scores. The more significant correlation was between changes in pain and changes in synovitis score. "This relation was strongest for change in synovitis in infrapatellar and intercondylar fat pads, in which there was a 5.7 mm and 4.9 mm increase in pain, respectively, per unit increase in synovitis," the study showed.

"Our study demonstrates that change in synovitis over time was correlated modestly with changes in knee pain, but not at all with loss of cartilage. The correlation was direct—an increase in synovitis was associated with worsening pain and a decrease with less severe pain," Dr. Hill concluded.

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Reference

1. Hill CL, Hunter DJ, Niu J, et al. Synovitis detected on magnetic resonance imaging and its relation to pain and cartilage loss in knee osteoarthritis. Ann Rheum Dis. 2007 May 9; [Epub ahead of print].