Introduction: MRI of the knee is routinely performed in supine position, without providing information about physiological weight bearing. Since erect knee MRIs can be performed in a double-donut MRI, we sought to compare supine and weight bearing knee MRIs of patients with and without osteoarthritis (OA).

Materials and Methods: On a 0.5T double-donut open MR 16 patients were studied in supine and erect: 10 with OA and 6 age matched asymptomatic patients.

The joint space, coronal and sagittal meniscal thickness, extrusion in 4 directions, meniscal angles, intermeniscal space and evaluation of menisci, ligaments and marrow were compared between positions in the OA and control groups. Correlation with medial knee pain was obtained.

Results: The average intermeniscal space was greater in OA and in the erect position. The joint space was narrower in OA especially in the medial compartment and in the supine position (p< 0.02). The mean meniscal thickness was similar in both positions and groups.

In OA the meniscal angle was larger when upright, without statistical significance.

The medial, lateral and anterior meniscal extrusion were greater in OA on vertical and supine, especially the anterior extrusion of medial meniscus which was significantly higher in OA (p= 0.0259, 0.0122, vertical, supine, respectively( and on vertical position (p= 0.0041).

Medial extrusion was higher in OA on both positions (p= 0.0228, 0.0184 vertical , supine).

Medial meniscal tears were seen in 6/10 OA and 2/6 controls. MCL grade 1 sprain pattern was seen in 4/10 OA, chronic ACL tear was seen in 5, and subchondral marrow edema in 8/10.

7/10 OA patients complained of knee pain which was predominantly medial.

Conclusions: Standing MRI shows relative widening of the joint, possibly due to the altered osseous alignment. Extrusion of the meniscus and intermeniscal space are more common in OA and under weight bearing. Medial meniscal extrusion correltaes with osteoarthritis and worsens under weight bearing. This may explain the common medial knee pain in osteoarthritis as seen in data from the orthopedic literature, as well as the clinical evaluation of our patients, and may be attributed to increased pressure on the capsule or MCL.