<p>The interphalangeal (IP) joint of the thumb are formed by the articulation between the proximal phalanges head (PPH) and the distal phalanges base (DPB). The measurement of the contact behavior, i.e., distribution and magnitude of the joint contact area with motion is necessary to understand the effects of loading on articular cartilage. The purpose of this study is to determine the in vivo contact behavior of the IP joint of thumb using the MRI. Five normal right thumbs from 5 healthy subjects were examined. Subjects were scanned with their dominant hand in three positions. Contact area was determined by measuring the length of contact between the PPH and DPB in each slice, multiplying this length by slice thickness of 1-mm, and summing these values. We found that the average cartilage contact area attains a minimum value in the neutral position. We also found that the distal phalanx of thumb IP joint was caused internal rotation on the distal phalanx with flexion.</p>