Abstract
The all-inside technique for bone tunnel drilling during ligament reconstruction procedures (ACL, PCL etc.) is gaining popularity as a bone preserving, less invasive technique with the potential for more rapid recovery. To preserve the advantages of closed-socket tunnels, it is essential not to violate the cortex during retro-drilling. The risk of cortical breach is higher with the tibial tunnel compared to the femoral one due to the obliquity of the tunnel relative to the cortex. Our purpose is to introduce a trigonometric formula, which allows the surgeon to calculate the safe tibial tunnel drilling length during all-inside ligament reconstruction and explain its proof.