Pcl balancing in knee replacement

A data processing method carried out by a data processing apparatus and comprising:

• (i) determining a femoral attachment position of a posterior cruciate ligament to a femur of a knee of a patient;
• (ii) determining a tibial attachment position of the posterior cruciate ligament to a tibia of the knee of the patient;
• (iii) determining a displacement of the tibia relative to the femur along an anterior-posterior axis of the tibia of the patient for each of a plurality of knee angles while the tibia is moved posteriorly with respect to the femur;
• (iv) determining a maximum length of the posterior cruciate ligament, the maximum length being the distance between the femoral attachment position and the tibial attachment position for each of the plurality of knee angles using the determined displacement, the femoral attachment position and the tibial attachment position;
• (v) determining the position of the femur and the tibia of the knee of the patient arising from a planned femoral implant position and a planned tibial implant position for each of the plurality of knee angles;
• (vi) determining a required length of the posterior cruciate ligament as the distance between the femoral attachment position and the tibial attachment position for each of the plurality of knee angles using the determined positions of the femur and the tibia, the femoral attachment position and the tibial attachment position;
• and
• (vii) outputting a comparison of the required length of the posterior cruciate ligament and the maximum length of the posterior cruciate ligament as a function of knee angle.