MR imaging of the type II lunate bone: frequency, extent, and associated findings.

OBJECTIVE: Type II lunate bones have an "extra" facet that articulates with the hamate bone, which frequently leads to arthritis. Because the only prior studies, to our knowledge, on this common and clinically significant variant involved dissection of cadavers, we performed an MR imaging study of the type II lunate. MATERIALS AND METHODS: We retrospectively reviewed MR images obtained at 1.5 T of 186 wrists for frequency of type II lunates, size of the extra facet, amount of hamatolunate apposition, and presence of hamate subchondral edema. Of the 186 wrists, 28 also had correlation with findings on wrist arthroscopy. RESULTS: One hundred seven wrists (57.5%) had type II lunates with an average extra facet size of 4.6 mm (range, 1.2-12.0 mm). Apposition (articulation) of the extra lunate facet with the hamate averaged 77.4% (range, 0-100%). Hamate edema was seen in only nine wrists (4.8%), all of which had type II lunates. Arthroscopic evidence of focal hamate chondromalacia and MR imaging evidence of marrow edema were seen in six of these nine wrists; marrow edema was seen only in wrists with frank cartilage changes on arthroscopy. CONCLUSION: An extra facet (type II lunate) was seen in approximately half of 186 wrists. On MR imaging, type II lunates usually have an innocuous appearance. A large percentage of patients with type II lunates also have chondromalacia, which often is occult on MR imaging. When visible, such chondromalacia is the cause of marrow edema similar to that seen in patients with carpal fractures.