Unicompartmental knee arthroplasty (UKA) is an effective treatment for end-stage anteromedial osteoarthritis (AMOA). The key to UKA is the flexion-extension gap balance, which is closely related to postoperative complications such as bearing dislocation, bearing wear, and arthritis progression. The traditional gap balance assessment is performed by indirectly sensing the tension of the medial collateral ligament by a gap gauge. It relies on the surgeon's feel and experience, which is imprecise and difficult for beginners. To accurately assess the flexion-extension gap balance of UKA, we developed a wireless sensor combination consisting of a metal base, a pressure sensor, and a cushion block. After osteotomy, the insertion of a wireless sensor combination allows the real-time measurement of intra-articular pressure. It accurately quantifies the flexion-extension gap balance parameters to guide further femur grinding and tibia osteotomy, to improve the accuracy of gap balance. We conducted an in vitro experiment with the wireless sensor combination. the results showed that there was a difference of 11.3 N after applying the traditional method of flexion-extension gap balance performed by an experienced expert.