Surgical technique
For all arthroplasty, the same cemented prosthesis (NexGen, Legacy, Posterior-Stabilized Prosthesis; Zimmer, Warsaw, Indiana) was used. Almost the total knee surgeries (92/100) were performed by the senior orthopedic surgeon (JWW) with extensive experience in the primary knee replacement using a mini-mid vastus approach under general anesthesia, and other surgeries (8/100) were performed by another orthopedic surgeon (SHY) first, then were finally checked by JWW. A pneumatic tourniquet was inflated to a pressure of 300 mmHg. All arthroplasties are performed in tibia first technique. The proximal tibia cut was done by conventional extramedullary instruments. The femoral preparation technique is depending on which group the patient is assigned in: (1) In the navigation group, the varus/valgus, extension/flexion, thickness of distal cut of femur was determined and done by pinless navigation system. After cutting, the instant information of resection level could be shown on the display screen. If the alignment was satisfying, the anterior/posterior femoral cut, chamfer cut and box resection were done by cutting jig. If a relevant displacement of the cutting jig of at least 1° varus/valgus was measured, correction cutting was performed and the result was digitized again. (2) In the convention of group, the femoral alignment was determined by intramedullary guide. The femoral alignment jig was set to 5°–7° valgus dependent on the preoperative radiograph. A bone plug taken from the resected femoral condyle was sealed into the entry hole of femoral medullary canal before prosthesis placement. A hemovac drain was placed before wound closure, and TXA 1 g was intravenously injected after closure of the joint capsule.