The anterior cruciate ligament (ACL) is one of the main stabilizers of the knee. The media is full of reports of injuries to athletes’ ACLs and their subsequent surgeries. Despite this, few people have a comfortable understanding of the actual injury and possible treatments available. It used to be that injuries to this ligament meant the end of an athletic career, but technological advances have been made such that more players have been able to resume their sports without significant limitations. Due to the complexity and the confusion surrounding ACL injuries, the topic will be divided into two articles. This first will discuss the anatomy, types of injury, and an explanation of why many athletes receive surgery nowadays. The next article will then cover the techniques and specifics of ACL surgery.

Anatomy of the Knee

The anatomy of the knee can easily be broken down into several components. The knee consists of two bones that are held together by an arrangement of four ligaments which permit a stable range of motion to occur. Ligaments are specialized bundles of collagen (like a rope) that connect one bone to another. In the knee, the ligaments along the outside are called the collaterals (see figure). These collateral ligaments are on both the inner aspect (medial collateral ligament or MCL) and along the outer aspect (lateral collateral ligament or LCL). There are also two large ligaments that cross within the knee termed cruciates (from the Latin term meaning crossing). These are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL is much more commonly injured than the PCL.

The ACL functions to prevent the tibia from sliding forward under the femur and prevents a form of rotation (or pivot). This knee ligament is important for athletic maneuvers that require sudden stopping, twisting, or change in direction. A knee with a torn ACL is like a car with bad brakes. If the motion is simply straight ahead with simple bending of the knee without much stopping or turning (i.e. biking or rowing), then the knee may perform relatively well. If however, the sport requires rapid changes in direction (i.e. football, soccer, basketball), then the knee may be unstable and give way. Some people with a torn ACL may be unstable enough to give way with even simple daily activities besides sports.

Causes of ACL Injuries

The injury to an ACL can occur in many different situations. The player’s knee may be entangled or twisted during a collision with another player. However, the injury is frequently non-contact when the athlete simply twists or rotates the knee when the foot is planted on the ground. Examples of this include a running back cutting before even being hit or a basketball player coming down with a rebound and twisting to avoid another player. When injured, the player may feel or hear a “pop.” Generally, the immediate discomfort is great enough to prevent him or her from continuing to play. Within a few hours, the knee swells and the pain and stiffness increase. Much of the swelling and pain will resolve in several days and the injured athlete will begin to feel better. Some people may then start to walk normally and wonder if they even need to see a doctor. It is important though not to return to sports after a major injury until examined by a physician.


The diagnosis is usually evident by a orthopedic physical exam. An orthopedic doctor is one who specializes in treatment of bone and joint injuries. X-rays are necessary to ensure that there are no accompanying fractures of the bones. Occasionally, an MRI will be ordered to evaluate possible associated injuries, such as the meniscus. Once all of the injuries have been identified, decisions must be made regarding treatment.

Unfortunately, a torn ACL does not generally heal itself enough to make the knee fully stable. Partial or incomplete injures to the ligament are rare and most ACL injuries are complete tears. The treatment of a player with an injured ACL is based on the specifics of that individual. Some of the things considered are age, associated injuries, activity level, and expectations. Age is especially important in young patients (usually those under 15 years old) when the growth plates are still open in the knee. These are responsible for growth of the bones and care must be taken to avoid any injury to them. Associated injuries, such as a torn meniscus, may influence the need for surgery and the timing of any surgery planned. As mentioned, some sports place higher demands on the knee than others. These sports may be difficult to perform well without some form of surgery.

Prevention of ACL tears remains an important goal of sports medicine physicians, but there are not any concrete means to achieve this yet. Knee braces have not been shown to prevent these injuries. Recently, a great deal of attention has been paid to the ACL tears in females. Certain studies have indicated that girls may have a higher rate of injury than boys in certain sports, especially soccer and basketball. There are currently many theories as to why this may be occurring, but to date none of them have been proven. As such, there are no specific methods prevent these injuries in girls. Conditioning and good technique remain the most sound advice.

Treatment Options

There are essentially two main forms of treatment for a torn ACL. One is conservative or non-operative. This involves rehabilitation to restore motion and strength to the knee. The strengthening includes the muscle on the front of the thigh (quads) and those on the back of the thigh (hamstrings). Once these goals are met, then participation in sports may gradually be progressed. Some physicians may include the use of a specific knee brace. The brace may help with some function of the knee, but it generally cannot fully restore stability as it is difficult for anything attached with straps over thick muscle (like the thigh and calf) to fully control all motion in the knee. This choice of treatment could be reconsidered if the knee continues to give way, swell, or lock.

A knee without an ACL may give way under stress. This giving way may not only be bothersome, but the rotation and grinding of the knee can cause further injury. The giving can lead to damage of the meniscal cartilage or the articular cartilage (the smooth covering of the joint). In fact, a large percentage of people with torn ACLs who continue to play sports will further injure their knee. It is for this reason that many athletes opt for surgery before trying to return to sports.

Many different approaches have been used to surgically treat an ACL injured knee. The next article will continue discussion on the ACL and cover the actual types of surgery performed.