AMSTERDAM, The Netherlands – New, high-resolution forms of magnetic resonance imaging (hrMRI) and positron emission tomography (hrPET) can be used to visualize the bone, cartilage, ligaments, and soft tissues of the hand, and studies reveal that changes in ligaments are the first sign of incipient hand osteoarthritis (OA), according to Ai Lyn Tan, MD. Dr. Tan, who is in the academic unit of musculoskeletal disease at the University of Leeds, UK, discussed her work with high-resolution hrMRI and hrPET at the 2006 EULAR meeting.¹

Dr. Tan said that hrMRI uses custom-built scanners with a 1.5 Tesla magnet and a microsurgery MRI coil. The result is a scan with 80– 100 μm resolution, fine enough to visualize many of the structures of the hand. Dr. Tan tested this device in a comparison study of 39 OA and 17 non-OA patients.

"Finger and hand joints in the OA subjects showed loss of cartilage, evidence of subchondral bone formation, bone edema, synovial enhancement, formation of osteophytes, and bone erosion. Every joint structure showed comparable abnormalities in both early and chronic OA, although the abnormalities were milder in early OA," Dr. Tan said. She found that Heberden's or Bouchard's node formation occurred at sites where soft tissue "bulged through the capsule between the dorsal tendons and collateral ligaments." Bone edema was often focal and subchondral, especially in areas of cartilage loss.

Overall, hand joints in normal subjects showed two layers of uniform cartilage. In OA joints, the layers were uneven, collateral ligaments were often disrupted, and soft tissue was enhanced on the scan, indicating the presence of inflammation. Dr. Tan said that the collateral ligament was often severely disrupted in more advanced OA but might show only a slight thickening in early OA. Similar thickening also appears as a normal aspect of hand structure in normal older subjects, she said. In OA hands it was often accompanied by proximal enthesis of the collateral ligament where it presses against bone and is compressed during normal hand movements.

Clinically normal finger joints in OA hands often showed thickening and enhancement of ligaments, a condition Dr. Tan labeled "latent OA."

"The hitherto unappreciated ligament abnormalities in OA appear to be the primary event in OA, as [evidenced] by the abnormality preceding cartilage or other structural damage, and also often being the only abnormality in latent OA and in the older normal subjects," Dr. Tan said.

The hrMRI technique provides remarkably clear images of small joints in the hand, as well as identifies sites of soft tissue and bone inflammation, but it does not provide information about the physiologic activity of the involved tissues. For that, Dr. Tan turned to the new hrPET. "This technique shows the chemical functioning of organs and tissues in vivo and can detect changes even before anatomical change occurs," she said.

The usefulness of PET scanning in studies of the hand has been limited because of the low level of resolution. Dr. Tan solved this problem by using a special scanner developed for use in whole-body studies of small animals such as mice. The hrPET scan is done using the bone-seeking isotope 18-fluoride (18F), which has a short half-life and is deposited at sites of increased bone turnover or remodeling. In essence, hrPET permits studies of bone metabolism in relation to signs and symptoms of OA in the hand.

Dr. Tan reported a study comparing hands in 21 OA patients (630 joints) with those in nine normal subjects (270 joints). The involved tissues were dramatically apparent in the hrPET scans, and 18F uptake was enhanced at sites of pain in the hand joints.

"More interestingly, the bones of OA joints frequently had small, intense foci of 18F uptake corresponding to sites of ligament attachments and osteophyte formation, complementing the hrMRI data and reinforcing the importance of the role of ligaments in OA," Dr. Tan concluded. She also showed examples of combined hrMRI and hrPET scans. "This is a novel way of assessing regional bone metabolism and inflammation in small hand joints," she said.

Reference

1. Tan A. High-resolution PET and high-resolution MRI assessment of hand osteoarthritis. Presented at: EULAR 2006 Meeting; June 21–24, 2006; Amsterdam, the Netherlands. Abstract SP0060.