Articular muscle of the knee: a muscle seldom recognized on MR imaging.

Objective: We studied the MR imaging characteristics of the articular muscle of the knee, an important but seldom recognized structure of the distal thigh and knee.
Subjects and Methods: We used a 1.5-T system to prospectively evaluate the articular muscle of the knee in five healthy volunteers (10 knees); then we retrospectively studied the knee examinations of 24 patients (30 knees). One reader made duplicate readings to prospectively assess the sagittal proton density-weighted and axial T1-weighted images of the healthy volunteers. Three independent readers retrospectively evaluated the sagittal proton density-weighted images of the 24 patients.
Results: The articular muscle of the knee was identified in 100% of prospective studies (10/10) and in 83% of retrospective studies (25/30). For the prospective study group, measurements (mean +/- SEM) were as follows: number of bundles, 2.4 +/- 0.68 (range, 1-4); length of uppermost bundle, 47.2 +/- 4.06 mm; width of origin, 8.6 +/- 0.79 mm; angle of origin, 11.4 degree +/- 1.29 degree; and cross-sectional bundle width, 19.8 +/- 2.05 mm2.
Conclusion: The articular muscle of the knee can usually be identified by MR imaging as an independent entity, separate from the great (vastus) muscle group.