The bone just below the knee or stifle joint is the tibia.  The bone above is the femur. The top of the tibia is the tibial plateau. The patellar tendon attaches the knee cap (patella) to the tibial tuberosity.

During weight bearing, the femur slides down the tibial plateau. The ACL stops this downward slope, therefore, is under constant tension during weight bearing. This biomechanical problem results in ACL rupture and is directly related to the angle between the tibial plateau slope and the patellar tendon. A more perpendicular angle results in a more stable joint.

The TTA involves an osteotomy in the non-weight bearing portion of the tibia, just behind the tibial tuberosity. The tibial tuberosity is advanced to achieve a perpendicular relationship between the tibial plateau slope and patellar tendon. This relationship results in a stable joint.