Basingstoke Sports Council

Serving Basingstoke's sporting community for over 25 years.

ACL Injuries

One of the most common problems involving the knee joint is an anterior cruciate ligament tear. The anterior cruciate ligament (also called the ACL) is one of four ligaments that are critical to the stability of the knee joint. A ligament is made of tough fibrous material and functions to control excessive motion by limiting joint mobility. Of the four major ligaments of the knee, the ACL injury is the most common knee ligament injury.

What is the normal function of the ACL?

The anterior cruciate ligament is the primary restraint to forward motion of the shin bone (tibia). The anatomy of the knee joint is critical to understanding this relationship. Essentially, the femur (thigh bone) sits on top of the tibia (shin bone), and the knee joint allows movement at the junction of these bones. Without ligaments to stabilize the knee, the joint would be unstable and prone to dislocation. The ACL prevents the tibia from sliding too far forward. The ACL also contributes stability to other movements at the joint including the angulation and rotation at the knee joint. The ACL performs these functions by attaching to the femur on one end, and to the tibia on the other. The other major ligaments of the knee are the posterior cruciate ligament (PCL), and the medial and lateral collateral ligaments (MCL and LCL, respectively).

Anatomy of the Knee

The anterior cruciate ligament (ACL) is one of four main ligaments of the knee. It is located inside the knee and runs from the thigh bone (femur) to the shin bone (tibia).  Its functions are to keep the tibia (shin bone) from sliding forward on the knee and to stabilize the knee when it twists (rotation). So in straight-ahead sports like jogging, swimming, and biking, there is little stress to the ACL. But in the sports that involve cutting, planting and changing direction (football, soccer, basketball, skiing, gymnastics, field hockey, ice hockey, wrestling, lacrosse, rugby, tennis) the ACL plays a vital role.


ACL injuries occur when an athlete rapidly decelerates, followed by a sharp or sudden change in direction (cutting). ACL failure has been linked to heavy or stiff-legged landing; as well as twisting or turning the knee while landing, especially when the knee is in the valgus (knock-knee) position.

Women in sports such as football (soccer), basketball, tennis and volleyball are significantly more prone to ACL injuries than men. The discrepancy has been attributed to differences between the sexes in anatomy, general muscular strength, reaction time of muscle contraction and coordination, and training techniques. A recent study suggests hormone-induced changes in muscle tension associated with menstrual cycles may also be an important factor). Women have a relatively wider pelvis, requiring the femur to angle toward the knees.The majority of ACL injuries occur in athletes landing flat on their heels. The latter directs the forces directly up the tibia into the knee, while the straight-knee position places the lateral femoral condyle on the back-slanted portion of the tibia. The resultant forward slide of the tibia relative to the femur is restrained primarily by the now-vulnerable ACL.

Causes of Injury

The reasons why this is the case are not easily explained. It is helpful to divide these causes into two groups:
1. Mechanisms of Injury
2. Anatomical and Hormonal Factors

Mechanisms of Injury

Most ACL injuries are non-contact related: running and cutting sharply in a more erect posture than in men, landing from a jump without bending the knees sufficiently, and playing on surfaces with a high coefficient of friction. The level of skill and experience of the athlete is always of significance. Of course, contact injuries like a direct blow to the knee from the rear or side can cause a tear in the ACL. The tear usually occurs in the mid-portion of the ligament which is the weakest part.

"Commonly, the athlete runs, suddenly stops, and then turns, thereby causing a deceleration of the lower limb, a forced hyperextension of the knee, or a forced tibial rotation...." "Other mechanisms include an internal rotary force applied to a femur on a fixed weight-bearing tibia, an external rotation force with a valgus [outward] force, or a straight anterior force applied to the back of the leg, forcing the tibia forward relative to the femur."

Noncontact injury
An audible pop often accompanies this injury, which often occurs while changing direction, cutting, or landing from a jump (usually a hyperextension/pivot combination). Within a few hours, a large hemarthrosis develops. Patients usually are unable to return to play, secondary to pain, swelling, and instability or giving way of the knee.

Contact and high-energy traumatic injuries
These injuries often are associated with other ligamentous and meniscal injuries. The classic "terrible triad" involves a valgus stress to the knee with resultant injury to the ACL, Medial Collateral Ligament (MCL), and medial meniscus.

Injury Prevention: The Good News
Multiple studies have shown that a proper training program can be very effective at decreasing the rate of ACL injury.

"A training program developed at the University of Vermont Medical School designed to prevent ACL injuries in skiers demonstrated a 69% decrease in injuries among ski patrol personnel and instructors who received the training compared with those who did not. Another prevention program developed in Cincinnati demonstrated that a six week program of training could reduce the risks of knee ligament injuries in female athletes. One of the important aspects of the training program is to train these athletes to rely more on hamstring muscles than quadriceps, thereby protecting the knee ligaments. A controlled study of women athletes who did not participate in the training program compared with those who did indicated a five times higher incidence of knee injury than in male athletes. Females who participated in the training had injury levels equal to or only one or two times higher than males."


Symptoms of an ACL injury include hearing a sudden popping sound, swelling, and instability of the knee (i.e., a "wobbly" feeling). Continued athletic activity on a knee with an ACL injury can have devastating consequences, resulting in massive cartilage damage, leading to an increased risk of developing osteoarthritis later in life.


Research has shown that the incidence of non-contact ACL injury can be reduced anywhere from 20% to 80% by engaging in regular neuromuscular training that is designed to enhance proprioception, balance, proper movement patterns and muscle strength.

1. Proper leg muscle strength and flexibility training as well as core training.

2. Proper neuromuscular (balance and speed) training. A proper training program would include squats, lunges, stiff-leg deadlifts, hamstring curls, and different types of jumping movements. These jumping movements would progress from: slow jumps landing on two legs in one plane of motion, slow jumps landing on two legs in multiple planes of motion, slow jumps landing on one leg in one plane of motion, slow jumps landing on one leg in multiple planes of motion and faster jumps landing on one or two legs in multiple planes of motion
3. Proper coaching on jumping and landing and avoiding any straight knee landing.
4. Proper footwear and orthotics if necessary.

What happens when there is an ACL injury?

When an ACL injury occurs, the knee becomes less stable. The ACL injury is a problem because this instability can make sudden, pivoting movements difficult, and it may make the knee more prone to developing arthritis and cartilage tears.

Why are ACL tears such a big problem?

When the knee is unstable, patients often complain of a sensation that the knee will 'give out' from under them. When this giving way sensation is because of an ACL injury, the knee joint is sliding too much. This can be a problem because each episode of instability (the 'giving way' sensation) can cause damage to the knee cartilage. Therefore an ACL injury makes patients more prone to developing arthritis and meniscus tears. Athletes often have particular difficulty once they have sustained an ACL injury.Many sports require a functioning ACL to perform common maneuvers such as cutting, pivoting, and sudden turns. These high demand sports sports include, but are not limited to:  Football Basketball -  Skiing -  Gymnastics -  Hockey (Ice and Field) -  Wrestling -  Lacrosse -  Rugby Patients may be able to function in their normal daily activities without a normal ACL, but these high-demand sports may prove difficult. Therefore, athletes are often faced with the decision to undergo surgery in order to return to their previous level of competition.

What factors contribute to ACL injuries?

ACL injuries can strike anyone, but there are certainly some individuals more prone to ACL injury. Sports listed above can cause high forces to be placed on the ACL. Participants in these sports are especially prone to ACL injury. Another factor that contributes to ACL injuries is the gender of the patient. In college basketball, women players may be up to 8 times more likely than their male counterparts to sustain an ACL injury. There are different theories as to why women are especially susceptible to ACL injury.

What are the symptoms of an ACL tear?

An ACL tear most often occurs during sporting activities when an athlete suddenly pivots causing excessive rotational forces on the ligament. Other mechanisms that can cause an ACL tear include severe trauma and work injuries. Individuals who experience ACL tears usually describe a feeling of the joint giving out, or buckling--patients also often say they hear a "pop."

Signs you may have sustained an ACL tear: Sudden giving way of the knee, Hearing a 'pop' at the time of injury, Sudden swelling of the knee joint or Pain in the knee when walking

What should I do if I think I have an ACL tear?
Patients who think they have an ACL tear should be evaluated by their doctor. An ACL tear may be difficult to diagnose immediately after the injury because of associated pain and swelling. There may also be muscle spasm that contributes to making the knee difficult to examine. Therefore, it may not be possible to conclusively determine the presence of an ACL tear soon after the injury. If an ACL tear is suspected, you will return for follow-up evaluation with your orthopedic surgeon.

How does my doctor make the diagnosis of an ACL tear?
The presence of an ACL tear is suspected whenever a patient has an injury to their knee. In the orthopedists office, knee instability can be assessed by specific manoeuvres performed by your physician. These manoeuvres test the function of the ligament to determine if an ACL tear is present. The most commonly used tests to determine the presence of an ACL tear include:

Lachman Test
The Lachman test is performed to evaluate abnormal forward movement of the tibia (shin bone). By pulling the shin bone forward, your surgeon can feel for an ACL tear. If there is an ACL tear, the shin bone will move too far forward.

Pivot Shift Manoeuvre
The pivot shift is difficult to perform in the office, it is usually more helpful in the operating room with a patient under anesthesia. The pivot shift manoeuvre detects abnormal motion of the knee joint when there is an ACL tear present.

What other tests are needed to diagnose an ACL tear?

A complete examination of the knee is also necessary to determine if other injuries may have occurred that could be causing your symptoms. Your physician will also evaluate x-rays of the knee to assess for any possible fractures, and a MRI may be ordered to evaluate for ligament or cartilage damage. However, MRI studies may not be needed to diagnose an ACL tear. In fact, the physical examination and history are just as good as a MRI in diagnosing an ACL tear! Many patients are concerned when their doctor does not order a MRI. While the MRI may be necessary in some cases, it is not necessary to diagnose most ACL tears.

Will I need an operation for an ACL tear?

The decision as to whether or not to operate on an ACL tear is dependent on several factors. Some patients who experience ACL tears are able to resume normal daily activities without surgical reconstruction of this ligament. There are some important factors to consider in making the decision as to whether or not operative treatment of an ACL tear is needed. These factors include the age of the patient, the activity level of the patient (both recreational and occupational), the expectations of the patient, the ability and willingness of the patient to participate in post-operative rehabilitation, the degree of instability of the joint, and any other associated injuries to the knee (e.g. other ligamentous or meniscal problems). 

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