Early failure of tibial implants following unicompartmental knee arthroplasty might result from a decrease in bone mineral density in the medial tibial plateau. This study quantitatively assessed this bone mineral density decrease caused by tibial overresection during medial unicompartmental knee arthroplasty, using Hounsfield units, and investigated factors influencing significant reductions in Hounsfield units in 40 knees (34 patients) before surgery. A three-dimensional tibial model was reconstructed from computed tomography scans, and mean Hounsfield units of the central, anterior, anteromedial, medial, posteromedial, and posterior regions of the medial tibial plateau were extracted in 2-mm-thick layers at depth of 4, 6, and 8 mm beneath the articular surface. Differences in Hounsfield units among the layers and the correlation between Hounsfield units at the central region and hip-knee-ankle angle (+valgus) were analyzed. The lowest Hounsfield units were observed in the posterior region for all layers, with a distal decrease in all regions. The most significant decrease was from the 4-mm layer (353.6 ± 118.1) to 6-mm layer (223.1 ± 94.4) in the central region (p < 0.001). The Hounsfield units at the central region were negatively correlated with the hip-knee-ankle angle in the 6-mm (r = −0.34;​ p = 0.029) and 8-mm (r = −0.33;​ p = 0.038) layers. Even a 2-mm tibial overresection during medial unicompartmental knee arthroplasty caused a significant decrease in Hounsfield units. Altogether, the posterior tibial cortex should be covered with the tibial component and tibial overresection should be avoided, particularly in cases with mild varus knees.