Risks of ACL Surgery and Ligament Reconstruction

Anterior cruciate ligament (ACL) tears are a common knee injury that often requires surgery to reconstruct the torn ligament. Although ACL reconstruction surgeries have an 80% to 90% success rate, some patients may struggle with the decision to have it because of the complications that may follow.

Learn about some of the complications of ACL surgery and what you can do to ensure the best possible outcome if you experience them.

A man with a large scar on his knee
Yenwen Lu / Getty Images

Anterior Knee Pain

The most common complication of ACL surgery is pain around the kneecap. This complication is more likely in patients who have surgery performed with a patellar tendon graft, as these patients have bone removed from the kneecap as part of the procedure.

These patients can even have complications such as patellar fracture and patellar tendon tear, although these are very uncommon. However, patients having hamstring grafts or donor grafts can also have symptoms of anterior knee pain.

Anterior knee pain is thought to result from altered joint mechanics and can often be overcome with physical therapy. It's important for athletes to adhere to postoperative rehab protocols to ensure their knee mechanics improve.

Stiffness (Arthrofibrosis)

Stiffness in the knee joint is common after ACL surgery. Fortunately, most patients with stiffness can address this complication with aggressive rehabilitation.

However, in some cases, a ball of scar tissue forms in the front of the knee even despite rehabilitation, and it can cause an inability to fully straighten the knee. This ball of scar tissue is known as a cyclops lesion, and arthroscopic surgery is often needed to clean out the scar tissue.

The most critical step to avoid stiffness is to get the knee moving quickly after ACL surgery through rehabilitation exercises. In the past, doctors used specialized machines to bend the knee, called CPM machines. However, these machines have not been shown to lead to long-term improvement.

If stiffness is slow to improve after ACL surgery and rehabilitation, research shows that arthroscopic surgery to remove scar tissue in the knee from the injury is generally effective in increasing mobility.


As the ACL is a major stabilizer of the knee, an ACL tear can cause the knee to be unstable. But even after reconstruction surgery, instability of the knee may still be an issue. Typically, this happens due to a surgical error or an undiscovered injury that occurred in or around the knee at the time of the ACL tear.

However, in some cases, post-surgical instability is present even after successful surgery and rehabilitation. Doctors and researchers are still finding the answers to why this may occur, but current research has suggested some possible explanations.

One study that followed up with 131 patients with knee instability after ACL reconstruction surgery found that either an accompanying grade 2 medial collateral ligament (MCL) injury or a delay in surgery for more than 12 weeks may be risk factors.

A sensation of the knee giving out is a common complaint of those dealing with knee instability.

However, studies are revealing possible ways to prevent and improve instability after reconstruction surgery.

For example, clinical studies have shown that some surgical methods, such as using two smaller grafts (double-bundle reconstruction) to reconstruct the ACL instead of one (single-bundle reconstruction), may decrease the chance of instability after surgery. And in one case study, an ACL revision surgery improved stability in a patient who had instability after a successful ACL procedure.

Graft Stretching

When an ACL is reconstructed, the ACL is replaced by an autograft or allograft. An autograft is a transfer of bone or tissue from one part of a patient's body to another. An allograft is a transfer of bone or tissue from a donor or cadaver. Sometimes, the graft used to replace a torn ACL is stretched, potentially leading to other complications like instability or graft failure.

Poor surgical technique is a common cause of graft stretching, and ACL revision surgery is often needed. In ACL revision surgery, other parts of the knee, such as the meniscus, may need to be repaired in addition to replacing the prior graft.

Ganglion Cyst Formation

A rare complication from ACL reconstruction surgery is the formation of a cyst within or around the graft. These cysts are usually harmless and can be routinely removed, but can sometimes cause symptoms like:

  • Knee pain
  • Clicking of the knee
  • Reduced knee extension
  • Stiffness

Although the causes of these cysts aren't clear, studies suggest that bodily reactions to specific surgical screws and the incorrect placement of a graft may be possible factors.

Growth Plate Injury

For adolescent patients, it's possible that the growth plates of the knee can be damaged during ACL reconstruction surgery, which can lead to bone growth problems. Because of the risk, some doctors may elect to wait until the child grows older to perform the surgery.

However, methods are emerging that lessen the risk for growth plate injury, such as using a retractable drill to bore smaller holes for the placement of the graft.


Bleeding at the incision site of an ACL surgery is common but can be a concern if the blood is excessive enough that it soaks through wound dressings and does not stop after pressure is applied to the area.

The reason for bleeding after ACL surgery varies and is usually caused by an underlying issue. For example, in some cases, injury to an artery during surgery may cause bleeding, and surgically repairing the artery will stop the bleeding from continuing.

Disease Transmission

Infection is a rare complication but can be serious when it occurs. When the infection is inside the knee joint, there is a concern about the ACL graft becoming infected. If the infection is serious and does not get better with antibiotics, the graft may need to be surgically cleaned or removed to eliminate the infection.

To prevent infection, surgeons typically follow specific protocols for an ACL surgery, such as using new and sterilized surgical equipment for each procedure and ensuring that the patient receiving surgery is properly covered.

Blood Clots

Following ACL surgery, blood clots can form in the thigh or calf, with the potential to break off in the bloodstream and travel to other parts of the body, such as the lungs or brain. The reason for blood clots that develop after ACL surgery is unknown, but research suggests that factors can include:

  • Personal history of blood clotting
  • Age (over 30 years)
  • Hypertension
  • Prolonged operating times

Before ACL surgery, a doctor may prescribe blood thinners for those who have a family history of blood clots or who have conditions such as diabetes and obesity that make them more susceptible to blood clots. Other methods of prevention include:

  • Encouraging movement after surgery
  • Compression stockings
  • Using CPM machines

Re-Rupture of the ACL Graft

Repeat rupture of the ACL graft is uncommon but sometimes occurs. Whenever there is a re-rupture of the graft, a surgeon should carefully assess for possible technical failures of the first surgery.

Possible problems that can lead to the re-tear of the ACL include suboptimal positioning of the graft, improper tension on the graft, or failure of the fixation of the graft.

ACL grafts are very strong. In fact, autograft tissue (tissue from one's own patellar tendon or hamstring tendon) is stronger than the ACL. Allograft tissue (donor tissue), on the other hand, is not as strong, and re-tear rates are known to be higher with this type of graft. That is why competitive athletes often choose to use their own tissue, even though rehab may be more difficult.

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By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.