ACL reconstruction is a surgery that replaces a torn anterior cruciate ligament in your knee with a piece of tendon, either from your own body or from a donor. The ACL is a band of tissue connecting your thighbone to your shinbone, and it keeps your knee stable during pivoting, cutting, and landing. When it tears completely, the knee often feels unstable and gives way during activity. More than 85% of elite athletes return to play after the procedure, with a graft failure rate of about 7%, making it one of the more reliable orthopedic surgeries performed today.
Why the Surgery Is Recommended
Not every ACL tear requires surgery. The decision depends on your activity level, how unstable your knee feels, and what you want to get back to doing. Reconstruction is most strongly indicated when your knee shows significant rotational instability on physical exam, when you play sports that involve cutting or pivoting, or when you’ve already tried bracing and physical therapy without improvement. Younger patients under 25, those with high-grade instability, and athletes in demanding sports are the most common candidates.
If you’re older, less active, or willing to modify your activities, non-surgical management with physical therapy can work well. The ligament itself won’t heal, but strengthening the muscles around the knee can compensate enough for daily life and straight-line activities like cycling or swimming.
Choosing the Graft
The replacement tissue has to come from somewhere, and this is one of the biggest decisions you’ll make with your surgeon. The three main options are autografts (from your own body), allografts (from a donor), and each comes with trade-offs.
- Hamstring tendon autograft: The most popular choice among surgeons today. Tendons are harvested from the back of your thigh. Recovery at the harvest site is generally easier than with patellar tendon grafts, though some hamstring weakness can persist.
- Patellar tendon autograft: Taken from the middle third of the tendon below your kneecap. Long considered the gold standard for its strong bone-to-bone healing, but it carries a higher risk of kneecap pain, tendon shortening, and difficulty kneeling after surgery.
- Quadriceps tendon autograft: Harvested from the tendon above the kneecap. Gaining popularity as a middle ground, with a large, strong graft and fewer kneecap issues than the patellar tendon option.
- Allograft (donor tissue): Eliminates harvest site pain entirely, which means less initial soreness and a potentially faster early recovery. However, that quicker early recovery can be deceptive. Patients with allografts sometimes return to high activity before the graft has fully incorporated, which may explain the slightly higher (though not statistically significant) re-tear rates seen in studies. Allografts also show more tunnel widening over time compared to autografts.
For young, active patients planning to return to competitive sports, most surgeons lean toward autografts. Allografts are used more often in older, less active patients or in revision surgeries where the original graft failed.
How the Surgery Works
ACL reconstruction is performed arthroscopically, meaning the surgeon works through small incisions using a camera and specialized instruments rather than opening the knee. The procedure typically takes one to two hours under general anesthesia and is done as an outpatient surgery, so you go home the same day.
The basic sequence starts with harvesting the graft if an autograft is being used. For a hamstring graft, a small incision is made a few centimeters below the joint line on the inner side of the shin, and the tendon is pulled free with a specialized stripper. The graft is then cleaned and prepared on a workstation, often soaked in an antibiotic solution.
Next, the surgeon removes the remnants of the torn ACL and drills tunnels into the thighbone and shinbone at the attachment points where the original ligament sat. The prepared graft is threaded through these tunnels and positioned to mimic the angle and tension of the native ACL. It’s secured on both ends using small fixation devices, often a button-like anchor on the thighbone side and a screw or similar device on the shinbone side. The graft is then tensioned and the knee is taken through its range of motion to confirm stability before closing the incisions.
Prehabilitation Before Surgery
What you do in the weeks before surgery significantly affects how well you recover afterward. If there’s a delay between your injury and the operation, four to six weeks of “prehab” focused on regaining range of motion, strengthening your quadriceps, and restoring balance can improve your outcomes for months or even years post-surgery.
The key benchmark is reaching 80% quadriceps strength in the injured leg compared to the healthy side before going into surgery. Patients who hit that target minimize their chance of persistent strength differences for up to two years afterward. Prehab programs typically include quad-strengthening exercises, mini-squats, straight leg raises, hamstring stretches, and balance work. Some research suggests the benefits of a solid prehab program can last up to six years, with improvements in strength, range of motion, function, and overall knee scores compared to patients who skipped it.
Recovery and Rehabilitation Timeline
Rehab after ACL reconstruction is a long process, and rushing it is one of the biggest predictors of re-injury. The average time to return to sport for professional athletes is about 292 days, or just under 10 months, and recreational athletes often take 9 to 12 months. Returning at six months is no longer considered standard.
Weeks 0 to 2
Weight bearing starts immediately, typically with crutches and a brace locked straight. The goals are controlling swelling, restoring full knee extension (getting the leg completely straight), and beginning gentle quad activation. You’ll advance off crutches once you can bear weight on the surgical leg without limping.
Weeks 2 to 6
Crutches are usually dropped during this phase once your gait is normal. Rehab focuses on gradually increasing your bending range of motion, continuing quad strengthening, and beginning exercises like stationary cycling and pool work. The graft is at its weakest during this period as it undergoes a biological remodeling process.
Weeks 6 to 14
Strengthening progresses to more demanding exercises like step-downs, leg press, and balance training. No running or sport-specific activity yet. Backward treadmill walking may be introduced. The milestone to move forward is pain-free performance of an 8-inch step-down with good control.
Weeks 14 to 22
Forward running on a treadmill begins once you pass the step-down test and your surgeon clears you. Running is introduced gradually, and any pain or swelling means pulling back. Agility drills and sport-specific movements start appearing toward the end of this phase.
Weeks 22 and Beyond
Return to sport requires meeting specific benchmarks, not just reaching a date on the calendar. The standard criteria include at least 90% limb symmetry on strength testing, at least 90% limb symmetry on hop tests, acceptable movement quality during cutting and landing, and psychological readiness to perform sport-specific movements without apprehension. Many athletes need 9 to 12 months to clear all these benchmarks.
Risks and Complications
ACL reconstruction is generally safe, but no surgery is risk-free. The most common complication is arthrofibrosis, an excessive buildup of scar tissue that limits knee motion. This occurs in roughly 3% to 8% of cases and sometimes requires a follow-up procedure to break up the scar tissue. Infection rates are low, in the range of 1% to 3%. Cyclops lesions, small nodules of tissue that form in front of the graft and block full extension, occur in a small percentage of patients.
Graft failure is the concern that worries most patients. For first-time reconstructions, the failure rate is around 7% in athletic populations. Revision surgeries (redoing a failed reconstruction) have higher failure rates, ranging from 11% to 22% depending on the technique, and outcomes are generally less predictable than with the initial surgery.
Long-Term Joint Health
One reality that often gets underemphasized in pre-surgery conversations is the long-term risk of arthritis. Around 57% of patients develop some degree of osteoarthritis in the reconstructed knee within 14 years, compared to about 18% in the opposite, uninjured knee. Moderate to severe arthritic changes show up in about 20% of ACL-injured knees within 10 years, compared to roughly 5% in uninjured knees.
This isn’t a reason to avoid surgery. Knees with ACL injuries that are not reconstructed actually have a higher relative risk of developing arthritis than those that undergo surgery. The initial injury to the cartilage and joint surface at the time of the tear is likely the primary driver of long-term degeneration, and reconstruction helps limit further damage from instability. But it’s important to understand that reconstruction restores stability, not the original biology of the joint.
ACL Surgery in Children and Teens
ACL tears in young athletes with open growth plates require special consideration. Standard adult techniques drill tunnels directly through the growth plates, which risks causing growth disturbances or limb-length differences. Surgeons use modified approaches depending on how much growing the child has left to do.
Physeal-sparing techniques avoid the growth plates entirely by routing the graft around them. These approaches produce the best knee stability, with only 0.22 mm of residual looseness compared to nearly 2 mm with techniques that cross the growth plates. Partial transphyseal techniques drill through one growth plate (usually the tibial side) while sparing the other. Complete transphyseal techniques, which are essentially the adult approach, are used in adolescents who are close to skeletal maturity. All three approaches produce similar functional outcomes and patient satisfaction scores, but the physeal-sparing method is generally preferred in younger children to ensure the best stability while protecting growth.
A Newer Alternative: Bridge-Enhanced ACL Repair
A procedure called bridge-enhanced ACL restoration (BEAR) takes a fundamentally different approach. Instead of replacing the torn ligament with a new graft, it places a collagen scaffold soaked in the patient’s own blood between the torn ends of the native ACL. The scaffold holds a blood clot in place, which releases growth factors that help the original ligament heal itself.
The potential advantage is preserving the nerve fibers in the native ACL that provide proprioception, your knee’s sense of position and movement, which a tendon graft can never fully replicate. Early clinical trial results show similar functional outcomes to traditional reconstruction, with the BEAR group showing faster early recovery, higher psychological readiness to return to sport at 6 months, and significantly better hamstring strength (since no hamstring graft is harvested). By one to two years out, the differences between the two approaches level off. The procedure is FDA-approved but still relatively new, and long-term data beyond a few years is limited.