What is the ACL, and what happens when a surgeon rebuilds it?

Your ACL is one of the most important ligaments in your knee. When it tears, surgery can rebuild it. This page explains what the ACL does, how tears happen, and what reconstruction surgery involves — in plain language.

What is the ACL?

The ACL (anterior cruciate ligament) is a thick, rope-like band of tissue inside your knee. It connects the bottom of your thigh bone (femur) to the top of your shin bone (tibia).

The ACL has two main jobs:

• It stops the shin bone from sliding too far forward under the thigh bone.
• It limits how much your knee can rotate or twist.

You have two cruciate ligaments that cross each other inside the knee like an X. The one in front is the ACL. The one in back is the PCL (posterior cruciate ligament).

How does the ACL tear?

Most ACL tears happen without any contact. A sudden change of direction, a hard landing, or a twisting stop can put so much force on the ligament that it snaps. You may hear a pop, and the knee often swells quickly over the next few hours.

Sports that involve cutting, pivoting, or jumping — such as soccer, basketball, football, skiing, and gymnastics — put the most demand on the ACL. Young, active people who play these sports are at the highest risk.^[20]

Why doesn't the ACL heal on its own?

Unlike a broken bone, a torn ACL usually does not repair itself. The ACL lives inside the knee joint, bathed in joint fluid. That environment makes it hard for the torn ends to rejoin. For people who want to return to pivoting sports, surgery is usually needed to restore full stability.

Some people — especially those with low activity demands — can live well without surgery. Your surgeon will help you decide which path is right for you.

What is ACL reconstruction?

ACL reconstruction (ACLR) is the surgery to rebuild the torn ligament. The surgeon does not sew the old ligament back together. Instead, they replace it with a new piece of tissue called a graft.

The surgeon drills small tunnels through the thigh bone and shin bone. They thread the graft through these tunnels and fix it in place with screws or buttons. Over several months, the graft "ligamentizes" — it gradually takes on the structure and properties of a normal ligament.

Where does the graft come from?

There are a few options. A graft taken from your own body is called an autograft. Common sources are the hamstring tendons at the back of your thigh, the patellar tendon at the front of your knee, or the quadriceps tendon above your kneecap. A graft from a donor is called an allograft.

Each option has different pros and cons. The next page walks through all four options in detail.

What does recovery look like?

Recovery takes months, not weeks. Most people can walk without crutches within two to four weeks. Returning to sport takes nine months to a year for most active patients — and sometimes longer.

Rehabilitation (physio exercises) is essential. A well-done rehab program helps the new graft mature and restores the muscle strength and movement patterns that protect the knee.

What is the risk that the graft tears again?

For young, active people (under 25) who return to pivoting sports, the risk that the new ACL graft tears is meaningful — studies show about 1 in 9 hamstring grafts re-tear in this group.^[21] That is why surgeons sometimes consider adding an extra procedure — a Lateral Extra-articular Tenodesis (LET) — to give the knee more stability. You can read about that on the LET page.