Anterior Cruciate Ligament

Anterior Cruciate Ligament

All You Need to Know About ACL (Anterior Cruciate Ligament)

Anterior Cruciate Ligament (ACL) is a tissue that connects the femur (thigh bone) to tibia (shin bone). The ACL is one of the major ligaments of the knee. The tear or sprain that occurs in the ACL ligament is referred to as ACL injury.

What is the anatomy of the knee?

The four primary components of the knee are: bones, cartilage, ligaments and tendons. The femur, the tibia, and the patella form the bone structure of the knee joint. ACL(Anterior Cruciate Ligament) is one of the four main ligaments within the knee that connect the femur to the tibia. The fibrous connective tissue that connects bones to other bones is a ligament. The tibia (shin bone) is a long leg bone which is the leg’s weight bearing bone and it is the second largest bone after the femur in the body. 

A ligament is a tough bundle of fiber which connects two or more bones to each other around the joints. These are made of collagen fibers and are not visible on X-rays.The knee is a hinge joint held together by the ligaments of the Medial Collateral (MCL), Lateral Collateral (LCL), Anterior Cruciate (ACL) and Posterior Cruciate (PCL).

The ACL runs diagonally, In the center of the knee, preventing the tibia from sliding out in front of the femur, as well as providing the knee with rotational support. A covered layer of articular cartilage protects the weight-bearing surface of the knee. The medial meniscus and the lateral meniscus are on either side of the joint, between the cartilage surfaces of the femur and tibia. In order to minimize tension between the femur and tibia, the menisci serve as shock absorbers and work with the cartilage.

anatomy of the knee

What are the types of Ligament injuries?

Anterior cruciate ligament is one of the knee ligaments that is damaged most often. The frequency of ACL injury is higher in individuals participating in high-risk activities such as basketball, football, skiing, and soccer.

Around half of ACL injuries are associated with meniscus, articular cartilage, or other ligament damage. Furthermore, patients may have bone bruises under the surface of the cartilage. A magnetic resonance imaging (MRI) scan can detect these injuries and damage to the overlying articular cartilage.

Injured ligaments known as “sprains” are rated on a scale of severity.

  • Grade 1 Sprain: In a grade 1 sprain, the ligament is slightly affected. It could be slightly stretched, but can still help preserve the flexibility of the knee joint.
  • Grade 2 Sprain: A Grade 2 sprain extends the ligament to the point that it becomes loose. This is often referred to as a partial ligament tear.
  • Grade 3 Sprain: Most generally, this type of sprain is referred to as a full ligament tear. The ligament is usually broken into two parts and is unstable at the knee joint.

Ligament injuries

What are the causes of knee injury?

The injury to Anterior cruciate ligament can occur in several ways:

  • Changing direction rapidly
  • Slowing down while running
  • Direct contact or collision, such as a football tackle
  • Stopping suddenly
  • Landing from a jump incorrectly

Several research studies have shown that in certain sports, female athletes have a greater incidence of ACL injury than male athletes.

Physical examination and investigations for a knee injury.

Patients usually experience discomfort and swelling immediately after the injury making the knee feel unstable. They also have a considerable amount of knee swelling, a loss of full range of motion, pain or tenderness along the joint axis, and discomfort when walking within a few hours after a new ACL injury.

A physician may recommend X-ray to check for any potential fractures when a patient with an ACL injury is initially observed for evaluation in the clinic. In order to assess the ACL and to search for signs of damage to other knee ligaments, meniscus cartilage, or articular cartilage, the patient can also be advised for an MRI scan.

knee injury

An MRI of a complete ACL tear

In the image above, the fibers of the ACL have been disrupted and appear wavy (indicated by the red arrow in the image). In addition to performing special tests to detect meniscus tears and damage to other knee ligaments, the Lachman test will also be performed by the doctor to see if the ACL is intact. If the ACL is torn, the examiner will feel increased forward (upward or anterior) movement of the tibia with the femur and a smooth, mushy endpoint (because the ACL is torn) when this movement stops.

What happens naturally with an ACL injury?

ACL injury varies as per the level of activity, the degree of injury and symptoms of instability of each patient without surgical intervention. Some patients with partial ACL tears can continue to have signs of instability. Due to partial ACL tears, close clinical follow-up and a complete course of physical therapy help to identify those patients with unstable knees.

Without surgical intervention, complete ACL ruptures have a less favorable outcome. Some patients will be unable to engage in cutting or pivoting-type sports after a full ACL tear, while others have instability even for regular activities such as walking. There are a few unusual cases where patients are without any signs of instability and can participate in sports activities. This variability is due to the seriousness of the initial knee injury, as well as the physical demands of the patient.

About half of the ACL injuries occur in conjunction with injury to the meniscus, articular cartilage or other ligaments. Secondary damage can occur for patients who have repeated episodes of instability due to ACL injuries. When reassessed 10 or more years after the initial injury, a vast number of patients will have meniscus damage who have chronic dysfunction. Likewise, the incidence of articular cartilage lesions increases for patients who have ACL deficiency for 10-years or more.

What is the non-surgical treatment approach for ACL injury?

Progressive physical therapy and rehabilitation will help the knee return to a state similar to its pre-injury phase. The use of a hinged knee brace can help, however because of repeated instability episodes, many individuals who chose not to have surgery may experience secondary injury to the knee.

To cope with combined injuries, surgical care is usually recommended (ACL tears in combination with other injuries in the knee). For select patients, however, opting against surgery is fair.

Non surgical management of isolated ACL tears is likely to be successful or may be indicated in the below mentioned patient profiles:

  • with partial tears and no instability symptoms
  • who perform light manual work or live sedentary lifestyles
  • with complete tears and no symptoms of knee instability during low-demand sports, who are willing to give up high-demand sports
  • whose growth plates are still open (children) 

What is the surgical treatment for ACL injury?

The torn ACL is replaced by a substitute graft which is made of tendon. The various type of grafts used are:

  • Hamstring tendon autograft
  • Patellar tendon autograft 
  • Quadriceps tendon autograft
  • Allograft of patellar tendon, Achilles tendon, semitendinosus, gracilis, or posterior tibialis tendon

Patient considerations for ACL surgery.

Surgical treatment is recommended for active adult patients participating in sports or occupations that include pivoting, turning or hard-cutting, as well as intense physical labor. This involves older patients for whom ACL surgery has traditionally been removed from consideration. Activity and not the age, should decide whether to consider surgical intervention.

Early ACL repair poses a potential risk of growth plate injury in young children or teenagers with ACL tears, leading to bone growth issues. When the child is closer to skeletal maturity, the surgeon may postpone ACL surgery or the surgeon may change the technique of ACL surgery to reduce the risk of injury to the growth plate.

There is a high risk of experiencing secondary knee injury in a patient with a torn ACL and severe functional instability, so ACL reconstruction should be considered. ACL injuries combined with meniscal injury, articular cartilage, collateral ligaments, joint capsules, or a combination of the above are frequently seen. Often seen in football players and skiers, the “unhappy triad,” consists of injuries to the ACL, the MCL, and the medial meniscus.

Surgical care can be warranted in cases of mixed injuries and usually provides better results. In conjunction with the ACL reconstruction, up to half of meniscus tears can be repairable and can heal faster if the recovery is completed.

What are the graft choices for ACL injuries?

Hamstring Tendon Autograft: In creating the hamstring tendon autograft for ACL reconstruction, the semitendinosus hamstring tendon on the inner side of the knee is used. An additional tendon, the gracilis, is used by some surgeons and is attached below the knee in the same region. This produces a graft of a two or four-strand tendon. Proponents of the Hamstring graft say that compared to the patellar tendon autograft, there are less problems associated with graft harvesting, including:

  • Fewer problems with anterior knee pain or kneecap pain after surgery
  • Smaller incision
  • Less postoperative stiffness problems
  • Faster recovery

Hamstring Tendon Autograft

Hamstring tendon autograft prepared for ACL reconstruction

Patellar Tendon Autograft: The patellar tendon autograft uses the middle third of the patient’s patellar tendon, along with a bone plug from the shin and the kneecap. Occasionally referred to as the ‘gold standard’ for ACL reconstruction by some surgeons, it is also recommended for high-demand athletes and patients whose occupations do not entail a large amount of kneeling.

The rate of graft failure was lower in the patellar tendon community in studies comparing the outcomes of patellar tendon and hamstring autograft ACL reconstruction. In addition, in terms of postoperative tests for knee laxity (Lachman’s, anterior drawer and instrumented tests), most studies display similar or better results when this graft is compared to others. Patellar tendon autografts, however, have a greater frequency of symptoms and other concerns of postoperative patellofemoral pain (pain behind the kneecap).

The pitfalls of the patellar tendon autograft are:

  • Postoperative pain behind the kneecap
  • Slightly increased risk of postoperative stiffness
  • Low risk of a patella fracture
  • Pain with kneeling

Quadriceps Tendon Autograft: For patients who have already failed ACL reconstruction, the quadriceps tendon autograft is also used. It uses the middle third of the quadriceps tendon of the patient and a bone plug from the upper end of the knee cap. For taller and heavier patients, this creates a larger graft. Since only one side has a bone plug, the fixation is not as good as the patellar tendon graft. There is a high correlation with anterior knee pain following surgery and a low chance of a fracture of the patella. Patients may discover that the incision is not cosmetically attractive.

Allografts: The grafts taken from cadavers are allografts. These grafts are also used to restore or reconstruct more than one knee ligament for patients who have failed ACL reconstruction before and during surgery. The advantages of using allograft tissue include the reduction of discomfort caused by the patient receiving the graft, shortened surgical time, and smaller incisions. There is no donor site morbidity unlike autografts.With screws, the patellar tendon allograft facilitates good bony attachment in the tibial and femoral bone tunnels.

What is the surgical procedure for ACL surgery?

The patient is usually recommended for physical therapy prior to any surgical procedure. Patients who have a stiff, swollen knee that at the time of ACL surgery lacks a full range of motion may have serious difficulties restoring mobility after surgery. In order to recover the maximum range of motion, it typically takes three or more weeks from the time of injury. It is also advised to brace and allow certain ligament injuries to heal before ACL surgery.

The anaesthesia used for surgery is selected by the patient, the physician, and the anesthesiologist. To alleviate postoperative pain, patients may benefit from an anaesthetic block of the leg nerves.

Typically, the procedure starts with an examination of the patient’s knee while the patient is comfortable due to the anaesthesia effects. This final inspection is used to verify that the ACL is torn and also to check other knee ligaments for looseness that may need to be repaired or postoperatively treated during surgery.

The selected tendon is harvested (for an autograft) or thawed (for an allograft) if the physical examination clearly indicates the ACL is broken, and the graft is prepared for the patient to the right size. Passage of the graft of the patellar tendon into the knee tibial tube.

The whole procedure is done with the help of an arthroscope.The surgeon places an arthroscope into the joint after the graft has been prepared. To insert the arthroscope and instruments, tiny (one-centimeter) incisions called portals are made in the front of the knee and the surgeon examines the condition of the knee. Injuries to the meniscus and cartilage are trimmed or fixed and then the broken ACL stump is removed.

surgical procedure for ACL surgery

How to manage pain after ACL surgery?

The patient may feel some pain and pressure post surgery. This is a part of the natural healing process. The doctor and nurse will focus on the pain relief, which will help to heal more easily from surgery.

For short-term pain relief after surgery, drugs are also prescribed. Many forms of medications, including opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and local anaesthetics, are available to help treat pain. A combination of these drugs may be used by your doctor to enhance pain relief, as well as minimize the need for opioids. It is possible to use the drugs in injectable or oral form. Stop taking opioids as soon as the pain starts to improve. Speak to your doctor if within a few days of your surgery, your pain has not started to improve.

How Rehabilitation therapy is performed after ACL surgery?

An important aspect of successful ACL surgery is physical therapy, with workouts starting shortly after the surgery. Much of ACL reconstructive surgery’s effectiveness depends on the devotion of the patient to intensive physical therapy. Present physical therapy uses an accelerated path of recovery with advanced surgical procedures and improved graft fixation.

Postoperative Course: 

  • The wound is kept clean and dry in the first 10 to 14 days after surgery, and early focus is placed on regaining the ability to completely straighten the knee and restore control of the quadriceps.
  • The knee is frequently iced to minimize swelling and discomfort. 
  • Using a postoperative brace and using a machine to shift the knee across its range of motion can be determined by the surgeon. 
  • Weight-bearing status (use of crutches to hold some or all of the weight of the patient off the surgical leg) is often determined by the preference of the physician, as well as any injuries dealt with at the time of surgery.


The aim of ACL reconstruction recovery includes:

  • Minimizing knee swelling
  • Retaining knee cap stability to avoid issues with anterior knee pain
  • Regaining the full range of knee motion
  • Strengthening the muscles of the quadriceps and hamstring

When there is no pain or swelling, when the full range of movement of the knee has been achieved and when muscle strength, stamina and functional use of the leg has been completely restored, the patient can return to sports.

It is also important to regain the patient’s sense of balance and control of the leg through exercises designed to enhance neuromuscular control. Typically, this takes 4 to 6 months. After an effective ACL reconstruction, the use of a functional brace when returning to sports is preferably not necessary, although some patients may feel a greater sense of security by wearing one.

What are the complications involved after the surgery?

  • Infection: The occurrence of infection following reconstruction of the arthroscopic ACL is very poor. It’s basically a superficial infection that can be treated with oral antibiotics if it happens. It can progress to deep infection very rarely, in which case it requires more intensive care.
  • Numbness: It is not unusual to have temporary or permanent numbness in the outer part of the upper leg close to the incision.
  • Blood clot: While rare, a potentially life-threatening complication is a blood clot in the veins of the calf or thigh. A blood clot in the bloodstream may break off and travel to the lungs, causing a stroke, causing pulmonary embolism or to the brain.
  • Instability: Chronic instability due to reconstructed ligament tear or stretching or poor surgical technique is feasible.
  • Stiffness: Some patients have reported knee stiffness or loss of motion after surgery.
  • Extensor mechanism failure: Rupture of the patellar tendon (autograft of the patellar tendon) or patella fracture (autograft of the patellar tendon or quadriceps tendon) can occur due to weakening at the graft harvest site.
  • Growth plate injury: Early ACL repair poses a potential risk of growth plate injury in young children or teens with ACL tears, leading to issues with bone growth. Until the child is closer to reaching skeletal maturity, ACL surgery may be postponed. Alternatively, to decrease the likelihood of growth plate damage, the surgeon may be able to adjust the technique of ACL reconstruction.
  • Kneecap pain: Postoperative anterior knee pain after reconstruction of the ACL autograft patellar tendon is particularly common. In studies, the incidence of pain behind the kneecap varies significantly, while after patellar tendon autograft ACL reconstruction, the incidence of kneeling pain is always higher.
  • Viral transmission: Allografts are directly linked, despite thorough screening and handling, with the risk of viral transmission, including HIV and Hepatitis C. It is estimated that the risk of receiving a bone allograft from an HIV-infected donor is less than 1 in a million.




About Author –

Dr. Shashi Kanth G, Sr. Consultant Orthopedic Surgeon, Yashoda Hospitals, Hyderabad
He is specialized in arthroscopy, sports medicine, and orthopedics. His expertise includes Lower Limb Joint Replacement Surgery, Lower Limb Arthroscopy, Sports Injuries, Foot and Ankle Surgery, & Management of Complex Trauma.

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