Torn Anterior Cruciate Ligament (ACL) of the Knee

Perry McLimore, MD., JD.


Human Knee Joint. Anatomy of the Knee. Menisci of the knee. Medial meniscus. Lateral meniscus. Meniscus tear and surgery
Human Knee Joint. Anatomy of the Knee. Menisci of the knee. Medial meniscus. Lateral meniscus. Meniscus tear and surgery


Four bones make up the knee joint: the femur (thigh bone), tibia (shin bone), fibula (shin bone), and patella (knee cap). The fibula plays very little role in the knee joint’s function.

The femur is the longest and strongest bone in the body. It starts at the hip (proximal) and ends at the knee (distal). There are muscles of the thigh surrounding the femur. The quadriceps (four muscles) are located anterior and connect to the patella. Posterior to the femur are the hamstring muscles and connects to the knee. At the distal end (knee) of the femur are two enlarged areas called the medial and lateral condyles. These articulate with the tibia and patella.

The tibia is distal to the femur and medial to the fibula. The proximal or knee end of this bone have medial and lateral condyles and a tibial tuberosity. Such structures articulate with the condyles of the femur. The fibula articulates with the tibia on its lateral surface and is not directly involved in weight bearing. The patella is flat with a proximal curved surfaced and tapers distally. The back of the patella articulates with the femur. The tendon of the quadriceps muscles encompasses the patella.


There are four major ligaments composing the knee. Outside the joint space are the medial and lateral collateral ligaments. Inside the joint space are the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). Both cruciate ligaments are attached to the femur and tibia and prevent these two bone from sliding back and forth on each other. All these ligaments are made of dense connective tissue and are very strong.


Two cartilage structures are in the knee joint space. Both are oval and act as shock absorbers for the knee.


The knee is a synovial joint. Its joint space is covered with a smooth surface of hyaline cartilage. The cartilage keeps the bones from rubbing together. Synovial fluid is secreted from the blood that bathes the joint space and nourishes the cartilage.







The ACL attaches to the anterior intercondylar area of the tibia and passes superior, posterior, and lateral to the lateral condyle of the femur. It keeps the knee joint from hyperextending and is crucial for the stability of the joint. The ACL consists of two bundles of fibers: the anteromedial bundle and the posterolateral bundle.


The ACL is the most common ligament injured in the knee. It can be partially torn or completely torn. Sometimes the ligament is intact, but a piece of the tibia or femur bone is avulsed off. Sudden hyperextension with a pivot motion is the usual mechanism causing a torn ACL. A tear can occur with or without contact. Women are more likely to tear their ACL due to female anatomy and reduced muscle mass that protects the knee.


Most people fell or hear a pop in the knee and sudden pain. In a short period, the knee will swell due to blood entering the joint from the torn ligament. The knee will feel unstable and walking will be difficult and painful.


The first step in diagnosing a torn ACL is a physical exam looking for instability in the knee. The examiner may perform a Lachman test. While lying flat, the examiner bends the knee a little (20 degrees). The thigh is stabilized with one hand while the shin is pushed forward or anterior with the other hand. There will be laxity compared to the unaffected knee if the ACL is torn.

Possibly an anterior drawer test may be utilized during the examination. The patient is lying on his back, and the knee is bent so the foot is flat on the table or bed. The examiner will grasp the knee with his thumbs on the front (anterior) surface of the knee. His fingers will be positioned behind the knee joint. The tibia (shin) is pulled forward to test for laxity in the joint.

The third test possibly done is the pivot-shift test. The patient is lying on his back and flexing his hip about 30 degrees. The lower leg and ankle will be grasped by the examiner allowing the knee to extend or straighten. The examiner will get hold of the lower part of the knee (tibia). As the knee is flexed (bent), the examiner twists the knee checking for anterior subluxation or instability.

X-rays may be done looking for any fractures in the bones. An MRI will be ordered if there is any question of joint instability or possible ACL tear. The MRI will clearly reveal any ACL injury, along with any other associated knee injuries like a torn meniscus.


If the MRI reveals an avulsed part of a bone and the ligament is intact, a repair can be done. The surgeon will go in and reattach the avulsed bone to the tibia or the femur. This is called repair. If the MRI shows a torn ACL, there can be no repair. The torn ACL must be reconstructed. Not everyone with a torn ACL requires surgery. If the tear in minimal than physical therapy rehabilitation may all that is needed. If the patient is not in athletics or does not depend on his or her knee on the job, subsequently surgery may be declined if the patient does not mind some instability in the knee. If the ACL is completely torn, then not having reconstructive surgery is associated with a high risk of further knee damage.

For reconstruction, a graft is needed to replace the torn ACL. A piece of the patellar tendon or hamstring tendon (autograft) is used. Sometimes a cadaver graft is employed. This is an allograft. There are advantages and disadvantages to each type of graft. The surgeon may prefer one over the others or may possess more experience with a particular graft.


ACL reconstruction is usually done by arthroscopic means. The surgery involves two or three incisions around the knee joint allowing insertion of a scope, as well as instruments. Rarely, a large incision is made (open reconstruction) along the knee. During arthroscopy, a scope is inserted into one of the incisions so the orthopedic surgeon can better visualize the damage. Normal saline is infused into the joint space to open the surgical field so the instruments can easily be manipulated. The body will resorb the saline.

The surgeon will drill holes in the femur and tibia where the graft will be attached. A graft from the patellar tendon will include small pieces of bone from the patella and tibia. Theoretically, using bone on the graft speeds and strengthens healing because the tendon has parent bone attached to it. If the graft is taken from the hamstring tendon, no bone is utilized.

The graft is pulled through the two holes drilled in the femur and tibia. The graft ends are then secured with biodegradable screws. The incisions are closed with stitches, staples, or special tape.1 Around 95 percent of the time, the body will grow blood vessels to the tendon graft, which will strengthen the fresh tissue. In theory, the new, living ACL graft should last a lifetime.


Most ACL reconstructions are done with a single tendon graft. In double bundle surgery, two grafts are used mimicking the two bundles of fibers in the ACL. This requires four holes being drilled into the femur and tibia, and an additional incision. There may be some advantages to double bundle surgery, but it is too early to tell. Some orthopedic surgeons have not performed the double bundle procedure and are, thus, inexperienced at it.2


The patient is sent home with crutches and a knee brace, along with pain and anti-inflammatory medications. A continuous passive motion machine may be prescribed. The CPM is used four to six hours a day to flex the knee from zero to 40 degrees.


Physical therapy rehabilitation is critical and will be done for four to nine months. For the first two or three weeks, the goal is to increase the range of motion of the knee. Excessive flexing (bending) is dissuaded since the graft is not entirely healed. Flexion of the knee about 90 degrees is expected. From three to six weeks, a full range of motion is pushed along with strengthening exercises, like stationary bicycles. For the remaining months, the joint undergoes strengthening exercises while maintaining normal range of motion. A brace is utilized during rehabilitation and, in some professional athletes, after rehabilitation.


Such a decision can be difficult. Reconstructive surgery requires much rehabilitation time and effort. Unfortunately, there is no magic number or percentage tear that can guide whether surgery is necessary. It depends on the stability of the joint and the activities of the patient. If the patient is engaged in athletics or has a job that depends on using the knee, surgery is generally recommended. If not and the patient can tolerate a little joint instability, then surgery may be declined. Again, it is a case to case decision.


ACL reconstructive surgery is generally safe. The usual complications can occur such as infection, numbness, blood clot in the leg, and typical risks of anesthesia. When a patellar tendon is utilized, grating or crepitus of the kneecap can occur. Furthermore, seen in patellar grafts, is pain upon kneeling. Overall, ACL reconstructive surgery is 90 to 95 percent successful. Repeat injury is the most common “complication.” Repeat reconstruction is more problematic and less successful. One needs to be careful.


  1. Honkamp N, et al. “Anterior Cruciate Ligament Injuries in Adults.” JC. DeLee et al Eds. DeLee and Drez’s Orthopedic Sports Medicine: Principles and Practice, 3rd edition, vol. 2, pp.1644-1676. Saunders Elsevier, Philadelphia, 2010.
  2. Zantop T, et al. “The Role of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament in Anterior Tibial Translation and Internal Rotation.” Am J Sports Med 35: 223-227, 2007.