**6. Choice of implant**

There are theoretical concerns when choosing the right implant. For those who choose anatomical and kinematic alignment, the implant tends to reproduce the knee's anatomy and the surgeon should preserve at the most ligaments and the natural inclination of the native's articular line. In such cases, the PCL should be preserved whenever it's possible, since it is a varus/valgus stabilizer, and it can absorb stress.

Anterior cruciate ligament (ACL) plays a role with PCL during knee flexion and extension. As the knee flexes, the femur slides back in tibia (rollback) until a point where the ACL is completely strengthened. As the knee extends, the femur slides forward until a point that the PCL resists this movement. For arthrosis of one compartment, when ALC and PCL are intact, a meniscal-bearing design could be used (e.g., Oxford® unicompartmental prosthesis). As arthrosis progresses, ACL becomes insufficient. When PCL is not sacrificed, the movement of rollback occurs and theoretically, the tibial baseplate should be flat. When PCL is sacrificed, the tibial baseplate should be concave, containing forward and backward motion. However, some of the newer implants now allow PCL sacrifice or retention, regardless the shape of tibial baseplate.

Mobile-bearing designs have increased the sagittal plane conformity which helps to control anteroposterior translation. The increased coronal plane conformity typically presented in mobile-bearing TKA also increases the contact area and lessens contact stresses. These advantages tend to reduce the rate of polyethylene wear. Polyethylene is self-aligned with the femoral component. It reduces the cross-shear stresses and facilitates central patellar tracking. In a fixed-bearing TKA, if the tibial component is left in internal rotation, it moves tibial tuberosity laterally, enhancing the risk of patellar subluxation. Besides, mobile-bearing designs also contribute to diminish the incidence of lateral releasing. Clinically, the fixed-bearing and mobile-bearing TKA systems have performed similarly in outcome studies. These authors favor the use of mobile-bearing designs, especially for younger and higher-demand patients with longer life expectancies.
