Anterior Cruciate Ligament Tears
The anterior cruciate ligament (ACL) is located deep within the center of the knee. It stabilizes the knee during extreme movements like those that can occur during sports. Tears of the ACL destabilize the knee, often resulting in further episodes of instability (giving way) and can also predispose other structures of the knee to injury. Unfortunately, a fully torn ACL has poor potential for healing because of its position deep inside the knee. There, the ACL is exposed to a “bath” of joint fluid and a relatively limited blood supply. The torn portion of the ligament often displaces (moves away) from its attachment site making it difficult to reattach to the bone.
To restore stability to the knee after a tear, ACL reconstruction surgery is often indicated. (“Reconstruction” indicates that the ligament is rebuilt, as opposed to “repaired,” which means the tendon is simply sewn back together or to the bone.)
Left photo: A normal ACL viewed arthroscopically. Right photo: A torn ACL; the torn end is pointing towards the camera.
Reconstructed ACL.
ACL tears are unfortunately fairly common, affecting about one out of every three thousand persons each year in the United States. In some parts of the population, particularly teenage females, the incidence of ACL tears has risen the last few years. This is felt to be due to several factors including: 1) relatively greater weakness in females versus males of the quadriceps and hip external rotator muscles, 2) different muscle activation patterns (essentially muscle coordination) in females versus their male counterparts, and 3) an increase in the number of females participating in organized sports over the last couple of decades along with a proportionate rise in the number of injuries occurring in females.
Some patients can “cope” with an ACL tear quite well (perhaps up to 30% of all patients with an ACL tear fall into this category). It is difficult to predict in advance who will cope well without an ACL reconstruction and who will have further instability. Therefore, several factors will factor into the decision of whether to proceed with surgery of not. There are no firm guidelines regarding age with respect to ACL reconstruction. The patient’s desired activity level and the absence or presence of instability of the knee following an injury are commonly used to decide whether to reconstruct the tear or to modify one’s activities (i.e. “live with it”). Many people wishing to continue with cutting and pivoting sports such as tennis, basketball, soccer, skiing, golf, et cetera will decide to proceed with a reconstruction.
Another reason to consider an ACL reconstruction is to prevent further damage to some of the other structures within the knee. Studies suggest that there is about a 1% chance per month (in the presence of an ACL tear) of further injury to other structures of the knee such as the meniscus or the articular cartilage. A successful ACL reconstruction can decrease the risk of damage to other structures within the knee to near that of an uninjured knee.
Arthritis is often seen in the knee following an ACL injury, whether or not a reconstruction is performed. The exact cause for this is not currently known; a likely explanation is that damage to the articular cartilage that covers the ends of the bones in the knee as well as its underlying bone occurs at the time of the ACL tear. The damaged bone or cartilage never regain their normal characteristics and eventually arthritis sets in. It used to be thought that ACL reconstruction would prevent progression to arthritis; unfortunately this does not appear to be true based on studies of arthritis following ACL surgery.
There are several different tissues used to reconstruct a torn ACL. The most commonly used are donor tissues (also known as allografts), hamstring tendons, and bone-patellar tendon-bone (BPTB). No major scientific differences between these tissues have been found in studies comparing their use in ACL reconstructions. Analysis of several studies (meta analyses) indicates that hamstrings (and allograft) reconstructions likely result in less front-of-the-knee pain than BPTB reconstructions. There is also limited evidence that BPTB reconstructions may be more stable than the other types. Typically, several patient related factors are considered in the decision of which type of tissue to use. Examples include the patient’s sports requirements, activity level, need to kneel or work on the knees, and presence of prior surgery or injury to the knee. Also, the surgeon’s preference for one type of tissue over another also factors into the decision, with different doctors possibly choosing different tissues for the same patient.
Following an ACL reconstruction, studies show that the risk of a re-tear of the repaired ACL is about 6%. Surprisingly, the risk of a tear to the ACL of the other, non-injured knee is about twice that at 11.8%.
There has been a lot of discussion recently about “double bundle” ACL reconstruction surgery. In this surgery, two separate drill holes or “tunnels” are placed in both the femur and the tibia in an attempt to reconstruct the two “bundles” of the normal ACL. Claims are made that a double bundle technique provides a better outcome or function after ACL surgery. However, this technique has not been proven to provide better clinical outcomes (measures of how well patients’ knees function after the surgery) in scientific studies to date (click here for the only meta-analysis* so far regarding double bundle surgeries: Meta-analysis of double bundle versus single bundle reconstructions ). The prevailing sentiment from these clinical reviews is that longer-term studies will be necessary to determine whether there is a benefit to double bundle versus single bundle reconstructions (another link: A review of Clinical Results ).
* Meta-analysis (according to the Merriam-Webster dictionary): a quantitative statistical analysis of several separate but similar experiments or studies in order to test the pooled data for statistical significance.
December 23, 2009 *Updated 4/19/2011
William Silver, MD
Triangle Orthopedic Associates
Raleigh, NC
See the following for more information on ACL tears:
American Association of Orthopaedic Surgeons - ACL Injuries (opens in new window)
American Orthopaedic Society for Sports Medicine - The Injured ACL (opens in new window)