A torn ACL is fixed surgically in one of two ways: reconstruction, where the damaged ligament is replaced with a tissue graft, or primary repair, where the torn ends are stitched back together and reinforced. Reconstruction has been the standard approach for decades and remains the most common procedure. Primary repair is a newer option that works only for certain tear patterns.
Repair vs. Reconstruction
The distinction matters because these are fundamentally different surgeries. In a reconstruction, the torn ACL is removed entirely and replaced with a new piece of tendon tissue. In a primary repair, the surgeon preserves your original ligament by reattaching it, often reinforcing it with a strong suture that acts as an internal brace while the tissue heals.
Primary repair is only possible when the ligament tears near its upper attachment point on the femur (the thighbone). These proximal tears preserve enough of the ligament’s length and tissue quality for reattachment. Tears that happen in the middle of the ligament or near its lower attachment on the shinbone leave compromised tissue that won’t heal reliably, making reconstruction the better choice. Most ACL tears fall into the categories that require reconstruction.
When repair is an option, the potential advantages are meaningful: no need to harvest tendon tissue from elsewhere in your body, less pain, and a potentially faster return to function. Early clinical data shows similar outcomes between repair and reconstruction at two years, though there are signs the revision rate after repair may be higher (14% vs. 6% in one trial, though the study was too small to confirm this statistically). Younger, highly active patients and competitive athletes are generally still directed toward reconstruction, since the long-term data for repair in those populations doesn’t exist yet.
How Reconstruction Works Step by Step
ACL reconstruction is performed arthroscopically, meaning the surgeon works through a few small incisions rather than opening up the entire knee. An arthroscope (a tiny camera) is inserted into one incision, and surgical instruments go through others. Saline solution is pumped into the joint to expand the space and give the surgeon room to work, with the camera feeding video to a monitor.
Before addressing the ACL itself, the surgeon inspects the rest of the knee. The meniscus (the rubbery cartilage cushions on either side of the joint) and the smooth cartilage covering the bone surfaces are examined for damage. If tears or lesions are found, they’re repaired during the same procedure.
Next, the graft is prepared. If your own tissue is being used, the surgeon harvests it through a separate small incision. Then the remnants of the torn ACL are removed, and precise tunnels are drilled into the thighbone and shinbone at the exact spots where the original ligament attached. The graft is threaded through these tunnels so it follows the same path as the native ACL, then secured on both ends with screws, buttons, or other fixation devices. Over the following months, bone grows into the graft and it becomes a living part of your knee.
Choosing a Graft
The graft is the replacement tissue that becomes your new ACL. Most surgeons use an autograft, meaning tissue taken from your own body. The three main options each come with tradeoffs.
- Patellar tendon: A strip of tendon from the front of your knee, taken with small bone plugs on each end. This graft has the longest track record of success and has historically been the go-to choice for competitive athletes in cutting and pivoting sports. The downside is pain with kneeling, which can be permanent, along with a larger incision, some numbness on the outer front of the knee, and a small risk of kneecap fracture.
- Hamstring tendon: Tendons harvested from the back of your thigh. Recovery from the harvest site is generally easier than with patellar tendon, but hamstring grafts are more prone to stretching out or re-tearing, especially in female athletes. Failure rates in young patients range from 13 to 23%. Lingering hamstring weakness is common.
- Quadriceps tendon: A strip from the large tendon above your kneecap. This graft is nearly twice as thick as a patellar tendon graft, making it stronger and stiffer. It’s harvested through a smaller incision, causes less kneeling pain than a patellar tendon graft, and carries a lower fracture risk. Reinjury rates are reported around 2 to 3%, making it an increasingly popular choice.
A fourth option is an allograft, which is donor tissue from a cadaver. This eliminates harvest-site pain entirely but comes with significantly higher failure rates, particularly in younger patients. In people 19 and under, allograft failure rates average around 25%, compared to roughly 8.5% for patellar tendon and 16.6% for hamstring autografts. Allografts are nearly four times more likely to fail than autografts in this age group. For adults over 30 who are less physically demanding on their knees, allografts can be a reasonable option.
Why Age Affects Outcomes
Younger patients face higher failure rates across all graft types. Studies of pediatric and adolescent patients report failure rates between 6% and 25%, while adult failure rates generally fall between 1% and 8%. This isn’t because surgery is done differently in young people. It’s because younger athletes tend to return to higher-risk sports at higher intensity levels, putting more stress on the graft. For children whose bones are still growing, surgeons may use special techniques that avoid drilling through growth plates, such as using the body’s own iliotibial band tissue on the outside of the thigh.
Preparing for Surgery
Going into surgery with a stronger, more mobile knee leads to a better recovery afterward. Most surgeons recommend a period of prehabilitation, or “pre-hab,” before scheduling the procedure. The goals are straightforward: get your knee fully straight, bend it to at least 120 degrees, walk with a normal gait, bring swelling down, and build up your quadriceps strength. This preparation also helps mentally, giving you a sense of control and familiarity with the types of exercises you’ll be doing after surgery.
Rushing into surgery too quickly can actually backfire. Having reconstruction within four weeks of the initial injury is a risk factor for developing excessive scar tissue in the joint afterward. Giving the knee time to calm down, reducing inflammation and swelling before operating, sets the stage for a smoother recovery.
Recovery Timeline
Rehabilitation after ACL reconstruction follows a structured progression, and the full process takes the better part of a year.
In the first two weeks, the focus is on reducing swelling, regaining full knee extension (the ability to straighten your leg completely), and activating your quadriceps enough to perform a straight leg raise without your knee sagging. Most patients with patellar tendon or quadriceps grafts can bear weight as tolerated from the start, while hamstring and allograft patients often stay on crutches with partial weight-bearing for up to six weeks.
By weeks three through five, you should be close to matching the bending range of your other knee and have full extension equal to the uninjured side. Weeks six through eight target achieving full, equal range of motion in both directions. Through weeks nine to twelve, the emphasis shifts to maintaining that motion while progressively building strength.
Plyometric and agility work, such as jumping drills, ladder drills, and forward and backward running, begins around three to five months. Hard cutting, pivoting, and sport-specific multidirectional drills are introduced around seven to nine months. These aren’t arbitrary timelines. They reflect how long the graft needs to mature and integrate with surrounding bone.
Clearance to Return to Sport
Getting back to full competition requires meeting specific, measurable benchmarks, not just feeling ready. The commonly accepted minimum criteria include being at least nine months post-surgery, achieving 90% or greater quadriceps strength symmetry compared to the other leg (measured on a specialized machine called an isokinetic dynamometer), scoring 90% or higher on a battery of single-leg hop tests, having full range of motion, no swelling, and no pain or instability. Psychological readiness also matters: a score of 80% or higher on a validated confidence questionnaire is part of the standard clearance criteria.
Each of these thresholds exists because research has linked premature return to sport with significantly higher re-tear rates. Passing all criteria doesn’t eliminate risk, but it substantially reduces it.
Possible Complications
The most talked-about risk is graft failure, but other complications are worth knowing about. Arthrofibrosis, or excessive scar tissue formation inside the knee, can develop when the body’s healing response doesn’t shut off normally. This leads to stiffness and restricted range of motion in both bending and straightening. Risk factors include operating too soon after injury, having significant swelling before or after surgery, and suboptimal tunnel placement during the procedure.
A specific type of scar tissue growth called a cyclops lesion (named for its appearance under a microscope) shows up on imaging in up to 25% of reconstructed knees at six months. Most cause no problems. Symptomatic cases produce a loss of full extension, along with catching or snapping sensations during walking or running. Only about 2% of cyclops lesions require a follow-up procedure to remove them.
Harvest-site issues are the other common concern. Depending on the graft type, you may deal with anterior knee pain, kneeling discomfort, residual numbness, or muscle weakness at the donor site. These effects range from temporary nuisances to long-term realities, which is why graft selection is one of the most important conversations to have before surgery.