Anterior cruciate ligament (ACL) injury is one of the most common injuries in athletes, accounting for 50% of all knee ligament injuries. It is caused by impacts to the knee joint, abrupt and sudden changes of direction, among others. A common treatment is ACL reconstruction surgery, for which there are different variations to be performed. One of them is the replacement of the ACL with a section of the patellar tendon taken from the same affected knee of the patient. This results in a change in the mechanical properties of the reconstructed ACL where it is not possible to quantitatively measure changes in intra-articular stresses. For this reason, in this project a 3D model of the knee joint has been made by means of diagnostic image segmentation in order to allow prediction of possible changes in the stability of the knee. The model presents the main bone components and stabilising soft tissues such as ligaments and menisci. To obtain a model of the surgery, the width of the quadriceps tendon is reduced and the mechanical properties of the ACL are changed to those of the tendon used. Simulations of body weight application in standing, in a flexion movement of the joint and in reaction to heel strike during human gait are performed. The results obtained are validated and compared with results reported in the literature. The results obtained show that in most cases this surgery causes an increase in the stresses exerted on the ACL. Thus, this model is an alternative as a decision tool to evaluate the possible biomechanical changes of the knee before performing this ACL reconstruction surgery.