A torn ACL is almost always fixed through a surgical reconstruction, where the damaged ligament is replaced with a tendon graft rather than stitched back together. The surgery is performed arthroscopically through small incisions around the knee, and most people return to full activity within 9 to 12 months. Here’s what actually happens during the procedure, what choices you’ll face, and what recovery looks like.
Repair vs. Reconstruction
The terms “ACL repair” and “ACL reconstruction” sound interchangeable, but they describe two different surgeries. A true repair reattaches the torn ligament to the bone. A reconstruction removes the damaged ligament entirely and replaces it with a substitute tendon, called a graft. Reconstruction is the standard approach for the vast majority of ACL tears.
Repair is only viable for a narrow set of injuries: acute tears where the ligament pulled cleanly away from the femur (the thighbone), typically operated on within 21 days. Ideal candidates are generally over 25, have moderate activity demands, and show a specific tear pattern on MRI. For most people, especially younger athletes with mid-substance tears (where the ligament ripped in the middle), reconstruction is the better-supported option.
When surgery is needed for an acute isolated ACL tear, the American Academy of Orthopaedic Surgeons strongly recommends early reconstruction because the risk of additional cartilage and meniscal damage starts to increase within about three months of injury.
What Happens During Reconstruction
ACL reconstruction is an arthroscopic procedure, meaning the surgeon works through two or three small incisions (called portals) rather than opening the knee. A tiny camera inserted through one portal gives a magnified view of the joint on a monitor. The surgeon first performs a diagnostic look through the entire knee to check for additional damage to the cartilage or meniscus, treating any issues found along the way. Fatty tissue at the front of the joint is cleared to give a clear view of where the original ACL attached.
The core of the operation involves drilling tunnels into the shinbone (tibia) and thighbone (femur) at the precise spots where the native ACL once attached. In newer “all-inside” techniques, these are half-tunnels (sockets) drilled from inside the joint outward, which preserves more bone. The graft is then threaded through these tunnels so it follows the same path as the original ligament. It’s secured on both ends with small fixation devices, most commonly either a bioabsorbable screw that wedges the graft snugly inside the tunnel, or a small button-like device on the outer surface of the bone that suspends the graft in place. Some surgeons use cross-pins that pass through the bone perpendicular to the tunnel to lock the graft. Over the following months, the bone grows into and around the graft, integrating it permanently.
The entire procedure typically takes about one to two hours. It’s done under general anesthesia or a regional nerve block, and most patients go home the same day.
Choosing a Graft
The graft is the replacement tissue that becomes your new ACL. You’ll typically choose between using your own tissue (autograft) or donor tissue from a cadaver (allograft), and there are real trade-offs worth understanding.
Patellar Tendon Autograft
This graft takes the middle third of your patellar tendon (the band running from your kneecap to your shinbone) along with small bone plugs on each end. It’s the most commonly used autograft because the bone plugs heal firmly into the tunnels, providing strong early fixation. The trade-off is harvest-site soreness: anterior knee pain and discomfort when kneeling are the most frequent complaints, and some patients experience lingering numbness around the incision from a small nerve that’s hard to avoid. More serious but rare complications include patellar fracture or weakening of the remaining tendon.
Hamstring Tendon Autograft
This uses one or two tendons from the inner side of the knee, folded over to create a thick, rope-like graft. Hamstring grafts cause less kneeling pain and anterior knee soreness than patellar tendon grafts. The hamstring tendons also have a documented ability to regenerate into tendon-like tissue after harvesting. In large reviews, the best available evidence suggests hamstring grafts are superior at preventing anterior knee pain, though patellar tendon grafts may offer a slight edge in measured stability. Graft failure rates are low for both: about 2.8% for patellar tendon and 2.84% for hamstring in a large meta-analysis of over 47,000 patients, though hamstring grafts fail at a slightly higher rate overall.
Donor Tissue (Allograft)
Allograft eliminates the pain and recovery issues of a harvest site entirely. For adults in their late 20s and older, studies show no meaningful difference in failure rates or outcomes between allograft and autograft. The picture changes dramatically for younger patients. In people 19 and under, allografts fail at nearly four times the rate of autografts, with pooled failure rates of 25.5% for allograft compared to 8.5% for patellar tendon and 16.6% for hamstring autografts. For this reason, most surgeons strongly favor autografts in younger, more active patients.
The BEAR Procedure
A newer option called Bridge-Enhanced ACL Restoration (BEAR) sits between a traditional repair and a full reconstruction. Instead of replacing the ligament, the surgeon places a small sponge-like scaffold made from bovine collagen between the two torn ends of the ACL. The scaffold is soaked in the patient’s own blood before implantation, which helps it mold to the gap and retain a natural clot rich in growth factors. This creates a bridge that encourages the native ligament to heal across the gap.
Early clinical trials show functional outcomes comparable to reconstruction in the short term. However, across three clinical trials totaling 123 BEAR patients, the re-tear rate was 15%, which is notably higher than the 2% to 3% graft failure rates seen with standard autograft reconstruction. Post-market data from real-world use has been more encouraging, with zero re-tears reported, though the average follow-up is still under a year. The longest follow-up data available for BEAR patients is only two years, so it’s still too early to know how the results hold up over a decade or more.
What Recovery Looks Like
Rehabilitation after ACL reconstruction is a long, structured process, and the quality of your rehab matters as much as the surgery itself. The standard timeline for returning to sport is 9 to 12 months, with six months considered the floor for an accelerated return. Pediatric protocols sometimes allow return as early as 7.5 months.
In the first weeks, the focus is on controlling swelling, restoring full range of motion (especially getting the knee completely straight), and reactivating the quadriceps muscle, which shuts down quickly after surgery. You’ll bear weight as tolerated almost immediately, progressing from crutches to walking over the first few weeks. Early exercises include partial squats, step-ups, and other movements where the foot stays planted on the ground.
By around three to four months, you’ll progress to more demanding strengthening work. At six months, strength and joint awareness are still measurably impaired compared to your uninjured leg, which is why the 9-to-12-month window exists. Sport-specific drills like jumping rope, cutting, and pivoting movements are introduced as strength and balance approach your pre-injury baseline. Patients who actually practice these sport-specific activities during rehab have higher odds of returning to their pre-injury level of play than those who skip them.
Professional athletes in basketball, soccer, football, and rugby generally return to competition within the standard 9-to-12-month window. Recreational skiers follow a similar timeline. Return-to-sport decisions are increasingly based on meeting functional milestones (strength tests, hop tests, movement quality) rather than simply counting months on the calendar.
Risks and Complications
The most common complication after ACL reconstruction is arthrofibrosis, an excessive buildup of scar tissue inside the joint that limits range of motion. It occurs in 2% to 35% of cases, a wide range that reflects differences in surgical technique, rehab aggressiveness, and patient factors. Women develop it more often than men. Mild cases respond to intensive physical therapy, while severe cases may require a follow-up procedure to break up the scar tissue.
Graft failure is relatively uncommon in adults, with studies generally reporting rates between 1% and 8%. Younger patients face a higher risk, with failure rates ranging from 6% to 25% across studies of pediatric and adolescent patients. Re-injury can happen to the reconstructed knee or, just as commonly, to the opposite knee’s intact ACL, particularly in young athletes returning to cutting and pivoting sports. Infection, blood clots, and nerve injury around the harvest site are possible but infrequent.