PURPOSE: To verify the effectiveness of detecting medial meniscus posterior root tears (MMPRTs) using weight-bearing posterior-anterior (PA) radiographs. MATERIALS AND METHODS: Twenty-three patients were diagnosed with an MMPRT using magnetic resonance imaging (Group A), with 23 matched individuals forming the control group (Group B). The distance between medial tibial eminence and the lateral edge of the medial femoral condyle (MTE–MFC distance) and medial joint space (MJS) width were measured on weight-bearing PA radiographs, with the knee flexed at 45° (Rosenberg view). Absolute medial meniscus extrusion (MME) was measured on magnetic resonance images. RESULTS: The MTE–MFC distance was greater and the MJS width was smaller in Group A than Group B (7.7 ± 1.7mm versus 6.0 ± 1.24mm and 3.2 ± 0.8mm versus 4.5 ± 0.7 mm, respectively; P < 0.05). The MTE–MFC distance and MJS width correlated with MME (r = 0.603 and 0.579, respectively; P < 0.05), and the extent of MME was greater in Group A than Group B (4.1 ± 1.1mm versus 1.8 ± 1.5 mm, respectively; P < 0.05). CONCLUSIONS MMPRTs increase the MTE–MFC distance and decrease the MJS width, with these measurements correlating to the MME. Therefore, measurement of the MTE–MFC distance and MJS width on the Rosenberg view could be a useful preliminary method for the diagnosis of an MMPRT.LEVEL OF EVIDENCE: IV