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Torn anterior cruciate ligament injuries


  • The ACL acts as a stabilizer for the knee, preventing it from twisting. Injury to the ACL will cause the knee to lose its stability. Subsequent symptoms are dropping of the knee and unsteadiness of the knee during rapid changes of direction.
  • Proper treatment should be initiated after an ACL injury. For people who require effective knees for work, not receiving treatment can lead to knee instability, repetitive knee injury, and even danger.
  • Proper rehabilitation of the knee and muscles plays an essential role after ACL surgery. It prevents re-injury to the ACL and can return knee function to pre-injury levels.


The anterior cruciate ligament (ACL) is one of the ligaments within the knee joint. Looking into the joint in a coronal (frontal) plane, the ACL stays in front of the posterior cruciate ligament (PCL), with the two ligaments appearing as a cross. The ACL acts as a stabilizer for the knee, preventing it from twisting. Injury to the ACL will cause the knee to lose its stability. Subsequent symptoms are dropping of the knee and unsteadiness (giving way) of the knee during rapid changes of direction while walking or running, and while walking on a floor that has rough surface. An ACL injury can also cause knee pain due to inflammation. Incorrect treatment can lead to future injuries to the knee, such as meniscal tear or cartilage lesion. A repetition of these injuries can lead to an early osteoarthritis.

ACL Tear Causes and Risk Factors

1. Contact or Impact Injury

An ACL tear is usually associated with other ligament injuries around the knee joint. It usually occurs as a result of playing sports such as football and rugby, and often involves a contact injury. Other mechanisms causing an ACL tear are impact injuries, such as an awkward landing in gymnastics, and other accidents such as a fall from height or motor vehicle accident.

2. Non-Contact Injury

An ACL tear in a non-contact injury is associated with the fatigue and subsequent imbalance of the muscles surrounding the front and the back of the knee joint. The quadriceps muscle contracts more strongly than the hamstring during stepping, jumping, or twisting of the knee. This consequently leads to backward motion (hyperextension) and excessive twisting of the knee, resulting in an ACL rupture. Variation of individual physical anatomy is another factor in ACL tears. In females, there is a higher degree of knock-knee (valgus knee) than in males. This can cause the knee to twist inward, resulting in an ACL rupture.

ACL Tear Symptoms

  • Pain is usually present within about 15-20 minutes after an ACL tear. A patient with this injury can usually recall the incident that caused the injury, such as an event while playing football or basketball.
  • The knee usually becomes swollen within 2 hours after an ACL tear, a result of bleeding within the knee joint cavity (hemarthrosis).
  • A patient with an ACL tear usually cannot walk normally, due to knee pain and swelling, for approximately 2-3 weeks. Walking will get better over time.
  • After the pain subsides, the patient may have a feeling of instability in the knee.
  • The patient will not be able to perform a rapid turn of direction (zigzag movement) because of the anxious feeling caused by unsteadiness in the knee.

Signs That You Should See a Doctor

Minor injuries such as muscle strain or contusion will normally get better within 1-2 weeks. However, if the pain persists longer than 2-3 weeks, especially with knee swelling, it is recommended that the patient consult a Sports Orthopedic Surgeon or Orthopedic Surgeon for proper evaluation and treatment.

ACL Tear Diagnosis

  • Diagnosis should be performed by an experienced doctor with a specialty in knee injuries. The diagnosis will be made mainly by taking a patient’s medical history and conducting a physical examination.
  • Other structures will be examined to find associated injuries. There could be a meniscal tear, cartilage lesion, or other injuries in ligaments surrounding the knee. The patient will also be examined to see if it is a new injury or chronic repetitive injury. Nearby joints, namely the hip and ankle, will also be examined. A full evaluation is necessary for clinical assessment and treatment planning.
  • Plain radiographs are important for detecting a bone fracture within the knee joint and evaluating the alignment of bones in the knee joint.
  • Magnetic resonance imaging (MRI) is pain free and provides detailed information of the injured knee. However, it will be used only if it is necessary for the evaluation of the severity and location of the torn ACL. The MRI can also help diagnose other associated injured structures such as meniscus, cartilage, collateral ligament, and posterior cruciate ligament (PCL).

ACL Tear Treatment

The choice of treatment for an ACL tear involves many factors including age, job, lifestyle, and most importantly the patient’s expectation of the result.

2 Ways of Treating an ACL Tear

1. Non-operative Treatment

Non-operative treatment is suitable for patients who have a single isolated ACL injury and no other associated injury within the knee joint. These patients should be able to adapt their lifestyles to prevent rapid twisting of the knee. This type of treatment is also recommended for elderly patients or for patients who lead sedentary lifestyles.

2. Operative Treatment

Operative treatment is chosen for patients who have an ACL injury with associated injuries such as cartilage lesion, avulsion fracture, meniscal tear, or multiligamentous injuries.

Surgery is also recommended for patients whose activities or career involve rapid twisting of the knee, such as young adults that regularly play sports (football, basketball, badminton, volleyball, or tennis), athletes, soldiers, policemen, or anyone whose job involves climbing.

Surgery may also be suggested for patients who still have symptoms of an ACL tear after a period of conservative treatment.

In recent practice, operative treatment for ACL tears is performed via arthroscopy to avoid injury to muscles and nearby tissue, and to lead to a quick recovery. Arthroscopic surgery also allows for better examination and repair of the meniscus than open surgery.

There are many surgical techniques for treating an ACL injury. The surgeon will choose the best option for each patient. Surgical options are ACL reconstruction (single or double bundle anatomic reconstruction) and ACL repair.

1. ACL Reconstruction

ACL reconstruction in the past involved the belief that the length and tension of the tendon graft would remain the same while the knee moved. The landmark they believed to have the greatest isometry was at the highest and closest position to the posterior border of the intercondylar notch. This surgical technique was called “over the top.” However, later, it was found that this technique provided for a low ultimate load during knee twisting, and so its use has become less popular.

ACL reconstruction has been widely performed in an anatomic reconstruction fashion, which tries to find the best anatomic femoral footprint to obtain the most natural ACL. According to physical anatomy, this footprint (femoral footprint posterolateral bundle ACL) is located just behind the lateral intercondylar ridge and usually in front of the lateral bifurcate ridge.

Plain radiographs of post-operative ACL reconstruction of the right knee using the concept of isometry. Notice the femoral tunnel that stayed high over the top position.

A drawing showing the border of the ACL on the lateral femoral condyle which are lateral intercondylar ridge (blue line) and lateral bifurcate ridge (green line).

2. Anatomic Double Bundle ACL Reconstruction

The ACL consists of the anteromedial and posterolateral bundles, named according to where they connect with the tibia. The anteromedial bundle is larger in size and its main role is restraining forward and backward movement of the knee (anteroposterior translation). The bundle will be tense in all degrees of knee movement and will be the most tense when the knee is bent (knee flexion) at 45-60 degrees. The posterolateral bundle restrains the knee during twisting and tenses most when the knee is fully stretched out (full extension). These two bundles work together, each becoming tense at different angles of knee movement.

3. Anatomic Single Bundle ACL Reconstruction

Anatomic single bundle ACL reconstruction has been well known in this field. It gives good results among ordinary people and even in the athlete population. Factors in performing this surgery include the graft size, graft type (bone patella or hamstring) and graft orientation. This information will be advised by the surgeon.

Even though biomechanical studies show the double bundle has better stability during knee twisting than the single bundle, clinical trial studies found the single bundle to have similar results as the double bundle in function and clinical use. Single bundle ACL reconstruction also helps reduce the operative time, minimizes the use of surgical equipment, reduces the overall expense for the treatment and, most essentially, decreases the risk of complications from the surgery.

4. ACL Repair

In the past, ACL repair had been the first-line treatment for ACL Surgery. Until the 1980s, only one third of all ACL repairs was successful. When ACL reconstruction was invented it was found to give good clinical results, and so it has been popular since.

Nevertheless, using the patient’s own ligament in ACL repair has advantages, including the preservation of the nerve in the ACL and faster recovery, leading to more quickly regaining knee motion. The patient also does not have to sacrifice other tendons and their functions for the sake of an ACL graft. Moreover, if the ACL repair fails or the ACL ruptures from a new accident, a revision ACL reconstruction can still be performed. The results of the revision ACL reconstruction will be as good as those of the first time ACL reconstruction because the knee joint is not bruised much from the ACL repair.

The important issue in ACL repair is choosing which pattern of the ACL tear is appropriate for ACL repair. The surgeon will give advice to the patient.

ACL Surgical Treatment Preferred by the Author

In choosing an operative treatment for an ACL tear to give the best results, the author will consider many factors when making the decision. As no individual is exactly the same, no single fixed formula is best for everyone, no matter if the surgery is going to be ACL reconstruction or ACL repair.

ACL reconstruction will be planned according to the aforementioned technique, graft type, patient’s career, and associated injuries.

Most of the time, the author performs single bundle ACL reconstruction and prefers the posterolateral bundle, but will preserve the residual stump of the ACL for adequate coverage to the native ligament (remnant-preserving ACL reconstruction). However, if the ACL is torn from the femoral insertion with no tear along the ACL substance, an ACL repair will be considered for the benefits described above.

An arthroscopic view of the left knee. The black and green lines are the borders of the ACL graft insertion added to the ACL remnant.

Different patterns of ACL tears

Normal ACL right knee

Normal ACL right left knee

(Anatomic Single bundle ACL reconstruction)
 (ACL reconstruction by augmentation preserve remnant)

(ACL repair)

Rehabilitation after ACL Operative Treatment

Surgery may be important in treating an ACL injury, but proper rehabilitation of knee joint and muscles after the surgery is also essential. Benefits include:

  • Helps prevent re-injury and decreases the risk of a recurrent tear to the ACL
  • Helps improve the capacity of knee function and can return knee function to pre-injury levels
  • Helps improve proprioception of knee
  • Helps the important muscles to gain balance and thus can decrease the risk of re-injury of the ACL and other structures in the knee cavity.

Proper treatment should be initiated after an ACL injury, especially for certain groups of people. These include athletes that require zigzag movements, running or jumping, and people who need perfect knee function for their career, such as policemen, soldiers, and anyone who requires climbing. If an ACL injury is left untreated, knee instability could lead to repetitive trauma or, in certain careers, become dangerous to one’s life. Nevertheless, people with ACL injuries suffering knee instability might experience dropping of the knee. This could lead to further tearing of the ACL, meniscal tear, or fracture. Repetition of injury to these structures can ultimately result in post-traumatic osteoarthritis and a lower quality of life.

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Associate Professor Cholawish Chanlalit, M.D. Summary: Orthopedics Orthopedics