OBJECTIVE: Medial patellar luxation (MPL) is prevalent in small dogs; however, the causes vary, and few studies have reported tibial deformities. We aimed to conduct a detailed morphological investigation of internal tibial torsion and internal foot rotation and compare the results between MPL grades to better understand the pathogenesis of MPL. STUDY DESIGN: We performed a morphological study of the tibia and comparison with MPL grade, and established a new parameter, metatarsal orientation relative to the orientation of the tibial tuberosity, the proximal tibia metatarsal angle (PTMTA). The distal tibia metatarsal angle was also established, and tibial torsion angle (TTA), mechanical medial proximal tibial angle, medial distance of the tibial tuberosity/proximal tibial width, and crural rotation angle were compared among the grades of MPL. RESULTS: The PTMTA was significantly higher in grades 3 and 4 than in the normal group. TTA, medial distance of the tibial tuberosity/proximal tibial width, and mechanical medial proximal tibial angle were significantly higher in grade 4 than in the other grades, which is consistent with previous reports. A correlation was also observed between the PTMTA and the TTA. CONCLUSION: The PTMTA functioned as a parameter that included elements of the distal tibia metatarsal angle, TTA, and medial distance of the tibial tuberosity/proximal tibial width, and is considered clinically useful because it can be visually ascertained during palpation. Our results may play a major role in surgical decision-making in the treatment of MPL.