Objective:To explore the effects of posterior malleolar fracture fixation on the rotational stability of the ankle joint.Methods:A total of 20 fresh cadaveric specimens of lower limbs were anatomized to measure the area of attachment of the posterior inferior tibiofibular ligament and transverse ligament complex to the posterior surface of the tibia. One healthy volunteer was selected to construct a finite element model for the intact tibiofibular and ankle joints and finite element models for posterior malleolar fracture with different posterior projection areas. A load of 600 N was vertically applied to the inferior calcaneus along the tibial mechanical axis. The contact area and maximum Von Mises stress of the distal tibial articular surface were analyzed to verify the validity of the model for the intact tibiofibular and ankle joints. In the finite element models for the posterior malleolar fracture (S, 1/2S, 1/4S, 1/8S and 1/16S model groups, with S standing for the complete projection area of the ligament complex on the posterior surface of the tibia), the width increase in the tibiofibular clear space was measured when a vertical load of 600 N and external rotation load of 5 N·m were applied to the ankle joint after the reduction and fixation of posterior malleolar fracture. The cutoff value of the posterior projection area of posterior malleolar fracture that could maintain the rotational stability of the ankle joint was assessed.Results:The measurement results of the cadaveric specimens showed that the area of attachment of the posterior inferior tibiofibular ligament and transverse ligament complex to the posterior surface of the tibia was relatively large. It was attached to the posterolateral area of the distal tibia with the highest point located at (45.2±5.6)mm from the articular surface. With the increase in the distance from the joint line, the width of the posterior attachment area of the ligament complex was decreased. Results of the finite element analysis showed that in the finite element model for the intact tibiofibular and ankle joints, the tibiotalar joint contact area was 324.02 mm 2 and the maximum Von Mises stress was 4.495 1 MPa with a vertical load of 600 N. In the finite element models for the posterior malleolar fracture, the distal tibiofibular clear spaces of the S, 1/2S, 1/4S and 1/8S model groups increased by less than 2 mm following loading, while it was increased by 3.445 8 mm in the 1/16S model group. The cutoff value of the posterior tibial projection area that could maintain the rotational stability of the ankle joint was 1/8S. Conclusions:The attachment area of the posterior inferior tibiofibular ligament and transverse ligament complex to the posterior surface of the tibia is large. Both the axial stability and rotational stability of the ankle joint should be considered in the treatment selection for posterior malleolar fracture. Simple posterior malleolar fixation is recommended to restore the rotational stability and axial stability of the ankle joint when tibiofibular syndesmosis is unstable and the cutoff value is larger than or equal to 1/8, while tibiofibular syndesmosis screws must be implanted when tibiofibular syndesmosis is unstable and the cutoff value is less than 1/8.