ACL surgery is a procedure that replaces or repairs a torn anterior cruciate ligament, one of the key stabilizing ligaments inside your knee. The operation is almost always done arthroscopically, meaning the surgeon works through small incisions using a tiny camera rather than opening the knee fully. Most people who tear their ACL and want to return to an active lifestyle will need this surgery, since the ligament has very limited ability to heal on its own.
Why the ACL Matters
The ACL runs diagonally through the center of your knee, connecting the thighbone to the shinbone. Its main job is preventing the shinbone from sliding forward and controlling rotational movements. When it tears, the knee can feel unstable, especially during cutting, pivoting, or sudden direction changes. Some people describe the knee “giving way” during everyday activities like walking down stairs or stepping off a curb.
Who Needs Surgery
Not every torn ACL requires reconstruction. The decision depends on your activity level, the severity of the tear, and how unstable your knee feels. Surgery is most strongly recommended when there is significant rotational instability (graded on a clinical test called the pivot shift), when you play sports involving cutting or pivoting movements, or when the injury is a revision of a previous ACL repair. Being younger than 25, having general joint hyperlaxity, or having a specific type of small fracture near the knee (called a Segond fracture) also tilts the decision toward surgery.
People who are less active and don’t experience instability during daily life can sometimes manage with physical therapy alone. But for athletes and active individuals, reconstruction is the standard path back to full function.
How the Procedure Works
The surgeon makes two small portals (incisions about the size of a buttonhole) on either side of the kneecap. A camera goes through one portal, and instruments go through the other. After inspecting the torn ligament and checking for any additional damage to the cartilage or meniscus, the surgeon prepares the knee for the new graft.
The torn ACL cannot simply be stitched back together in most cases. Instead, it is removed and replaced with a tendon graft that serves as scaffolding for a new ligament to grow into. The surgeon drills a tunnel through the shinbone and another through the thighbone, threads the graft through both tunnels so it sits in the same position as the original ACL, and then secures it with small fixation devices (usually buttons or screws that stay in permanently). The graft gradually incorporates into the bone over several months as your body grows tissue into it.
The entire operation typically takes one to two hours. Most people go home the same day.
Graft Choices
The graft can come from your own body (autograft) or from a donor (allograft). Three autograft options are most common: a strip of the patellar tendon (the tendon below your kneecap), one or two hamstring tendons from the back of the thigh, or a section of the quadriceps tendon above the kneecap. Clinical trials comparing all three show similar outcomes in knee function, revision rates, and patient satisfaction at one year, so the choice often comes down to surgeon preference and your specific anatomy.
Donor tissue is an option that avoids a second surgical site on your own knee, but it comes with a trade-off. In patients 19 and younger, allografts fail at roughly 25.5%, compared to about 8.5% for patellar tendon autografts and 16.6% for hamstring autografts. Donor grafts are nearly four times more likely to fail than a patient’s own tissue in this younger age group. For older, less active patients, the gap narrows and allografts become more reasonable.
A Newer Option: ACL Repair With BEAR
A more recent approach called bridge-enhanced ACL restoration (BEAR) attempts to heal the torn ligament rather than replace it. The surgeon places a small collagen sponge between the torn ends of the ACL, which holds the body’s natural blood clot in place and encourages the ligament to bridge the gap and heal itself. The potential advantage is preserving the nerve fibers in the original ligament, which play a role in balance and joint awareness, something a traditional graft cannot fully replicate.
Early clinical data shows functional outcomes and patient-reported scores comparable to traditional reconstruction, with the added benefit of better hamstring strength preservation. To be a candidate, you need at least half the length of your torn ACL still attached to the shinbone on MRI, and the surgery generally needs to happen within 50 days of the injury. This technique is FDA-cleared but still relatively new, so long-term data beyond a few years is limited.
Preparing for Surgery
If there is a gap between your injury and your surgery date, that time is valuable. A structured “prehab” program of four to six weeks focusing on quadriceps strength, range of motion, and balance can meaningfully improve your outcomes after surgery. One key benchmark: if you can build your quadriceps strength to 80% of your uninjured leg before the operation, you minimize your risk of persistent strength differences for up to two years afterward.
Prehab also improves early range of motion after surgery. Patients who do it regain motion faster at the three- and six-week marks compared to those who skip it. Going into surgery with a stronger, less swollen knee gives you a head start on the long rehabilitation process ahead.
Recovery Timeline
Recovery follows a predictable arc, though the pace varies by person.
In the first two weeks, the focus is on reducing swelling and regaining full knee extension (the ability to straighten your leg completely). Extension is prioritized over bending at this stage because losing it early can become a stubborn problem later. Most people are off crutches within seven to ten days.
From weeks two through six, you continue working on extension while gradually increasing knee flexion to about 90 degrees. Quadriceps activation exercises ramp up. By six weeks, most people are walking normally.
Between six weeks and four months, rehabilitation shifts toward strengthening, balance training, and building endurance. Jogging on a flat surface typically begins around months three to four, assuming adequate strength and no swelling.
From four to six months, the intensity increases with sport-specific drills, agility work, and progressive loading. The graft is still maturing during this period, so the exercises are carefully controlled.
Return to competitive sport generally falls in the nine- to twelve-month window. Professional ice hockey players tend to return earlier, averaging about 7.8 months, but for most people the recommended threshold is nine months at the earliest, with twelve months being more conservative and, for many surgeons, preferred.
Success Rates and Risks
The surgery is effective for the vast majority of people. Between 81% and 92% of patients return to some form of sport after reconstruction. The more meaningful number, though, is that only 55% to 79% return to their exact pre-injury level of competition. The gap is partly physical and partly psychological; fear of reinjury keeps some athletes from pushing back to their previous intensity.
About 3% of reconstructed ACLs fail and require revision surgery each year. The most common complication is arthrofibrosis, a buildup of scar tissue that limits knee motion, which occurs in 4% to 38% of cases depending on how it is defined and measured. Mild stiffness that responds to physical therapy is on the lower end; cases requiring additional procedures to break up scar tissue are rarer. Long-term, people who tear their ACL have an elevated risk of developing knee arthritis over the following decades, though surgery aims to improve knee stability and reduce further cartilage damage that accelerates that process.
What Shapes Your Outcome
The single biggest factor in a good result is rehabilitation. The surgery itself takes a couple of hours, but the rehab takes the better part of a year, and compliance matters enormously. Hitting strength benchmarks before returning to sport, rather than relying on a calendar date alone, is associated with lower reinjury rates. Your age, the condition of your meniscus, whether you had prehab, and your graft choice all play roles, but none of them outweigh the work you put in during the months after surgery.