Patient-specific instrumentation (PSI) has been introduced in total knee arthroplasty (TKA) as a new technology for improving accuracy in restoration of the alignment and biomechanics of the lower limb. PSI in TKA refers to custom-made cutting jigs manufactured according to the patient's anatomic configuration of distal femur and proximal tibia based on preoperative magnetic resonance imaging (MRI) or computed tomography (CT) scans. MRI- and CT-based PSI systems are available from various manufacturers for preoperative planning. MRI offers precise visualization of articular cartilage without the risk of radiation exposure, but it is expensive and requires long scan times. In contrast, CT enables accurate identification of the contour of the femur and tibia at short scan times, but it does not provide information on the cartilage thickness and carries the risk of radiation exposure. As a result, there is a possibility of some discrepancy between the thickness of bone resection proposed by MRI- or CT-based PSI system and the actual thickness of bone cut. Although PSI has been the focus of study in many recent researches, there has been no clinical study comparing MRI-based and CT-based PSI systems in preoperative planning. Therefore the investigators questioned whether the MRI-based PSI that reflects the cartilage layer would provide more precision in TKA than the CT-based PSI. The purpose of this study was to investigate the accuracy of MRI-based PSI and CT-based PSI in predicting bone resection thickness in TKA.