The objective was to determine the relationship between medial tibiofemoral joint space width (JSW) measured on fixed-flexion radiographs versus 3D JSW on low-dose, standing CT (SCT). Radiographic JSW values exceeded the minimal JSW on SCT by a mean of 2.0 mm. There were greater offsets and variability in the offsets between modalities closer to the tibial spine. In addition to avoiding bony overlap obscuring the joint, the current study supports a role for SCT in the evaluation of tibiofemoral OA.