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 pri or studies, to our knowledge, on this common and clinically significant var iant involved dissection of cadavers, we performed an MR imaging study of t he 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 ede ma. Of the 186 wrists, 28 also had correlation with findings on wrist arthr oscopy. RESULTS. One hundred seven wrists (57.5%) had type II lunates with an avera ge extra facet size of 4.6 mm (range, 1.2-12.0 mm). Apposition (articulatio n) 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 I I lunates. Arthroscopic evidence of focal hamate chondromalacia and MR imag ing 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 app earance. A large percentage of patients with type II lunates also have chon dromalacia, which often is occult on MR imaging. When visible, such chondro malacia is the cause of marrow edema similar to that seen in patients with carpal fractures.