Tears of the cruciate ligaments are significant injuries. 

 

How does one tear the anterior cruciate ligament (ACL)?

The ACL can be torn when the tibia is stressed suddenly in relation to the femur. Usually the ligament is injured without contact with another player, eg running and cutting to the same side, landing on one foot while the body is still twisting (non-contact injury), or in situations where -

Image showing non-contact stress

  • the body is forward flexed
  • the hip is abducted ('legs apart')
  • the knee is internally or externally rotated with valgus ('stressed from the outer side')
  • the foot is pronated('rolled over')

The ACL may also be injured when cutting to the same side as in this image.

Less frequently, the ACL is injured in a contact injury, one where the knee is forcibly hyper-extended, and the other where the knee is forcibly hyper-flexed.


Video Analysis of Anterior Cruciate Ligament (ACL) Injuries. Carlson VR, Sheehan FT and Boden BP. JBJS Rev. 2016 Nov 29; 4(11): e5.

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How does one tear the posterior cruciate ligament (PCL)?

PCL tears occur much less frequently than those of the ACL. In general, the disability of a PCL tear is less than with an ACL tear.

Situations in which the PCL can tear include -

  • excessive hyperflexion (forced bending), eg falling onto the shin with a bent knee and foot pointed
  • dashboard injury in a car - where the knee is bent to a right angle and a sudden force drives the tibia backwards

Both cruciates are necessary for full stability of the knee.  If the tibia is pulled forwards the ACL takes the strain. If the tibia is pulled backwards the PCL takes the strain.


Posterior Cruciate Ligament (PCL) Knee Injuries. Raj MA and Varacallo M. StatPearls Publishing; 2019 Jan.

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Early indicators of a cruciate tear

A tear of the cruciate ligament can be suspected if most of these features are present:

a definable injury or event

Patients with a torn cruciate generally recall the moment that the cruciate ligament gave out. Although the injury may be a contact or a non-contact injury, it  usually involves a twisting of the femur bone and tibia bone in relation to one another.

a loud 'pop' at the time of injury

As the cruciate snaps apart during the injury, very often the patient hears a loud 'pop' which may even be audible to others nearby.

unable to continue with activity

If it is the anterior cruciate ligament that is torn, usually the patient is unable to go on with the activity. Sometimes it is possible to continue if only the PCL is torn.

immediate knee swelling

Because the cruciate ligament has blood vessels in it, usually the knee swells up rapidly with blood (a haemarthrosis). This may become tense and painful and may need aspiration (sucking out with a syringe and needle).

knee instability

The instability may not be immediately apparent as the person is likely to be helped to a position of rest, but this tends to become a key symptom as soon as the patient tries to walk again.

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Ongoing symptoms of a torn cruciate ligament

Once the early pain and swelling have subsided, after a week or so after a cruciate injury, then it becomes more easy to evaluate the situation. There may simply be a feeling that the knee cannot be trusted. This may be especially so during twisting activities. Frank instability may be indicative that more than one cruciate is damaged, or that another structure is damaged in addition to a cruciate ligament. Although walking in a straight line may be quite easy, a torn cruciate may allow the knee to suddenly give way and collapse during a twisting motion.

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Symptoms of damage to other structures after a cruciate tear

One of the problems of evaluating a cruciate tear is that other stabilising structures may also be torn in the original injury, or the laxity due to the cruciate tear may secondarily lead to damage of other stabilising structures.

damage to the meniscus

Associated damage to a meniscus may add the symptoms of joint line tenderness, or locking.

damage to the joint surface

Damage to the joint surface may add the symptoms of achy knees and noises in the knee.

damage to the posterolateral corner

Damage to the structures of the posterolateral corner may add the feeling of the knee thrusting outwards with walking, and there may be other disturbances of gait.

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What should you do if you think your cruciate is torn?

A cruciate ligament tear is not a medical emergency. Although the injury is a serious one, the early mangement is relatively uncomplicated:

If pain and swelling are not too bad

If the skin is not damaged in the injury, and pain and swelling are not too severe, most clinicians will recommend the RICE (or PRICE) regime for the first few days of almost any knee injury, ie Protection, Rest, Icing, Compression, Elevation.

If pain and swelling are severe

If the pain and swelling are severe, then it is possible that there is tense blood within the knee cavity (haemarthrosis) and a visit to the Accident & Emergency unit is recommended. The doctor there may chose to aspirate the blood (suck it out with a syringe and needle), a procedure that can give considerable relief. The joint should be held still in an immobiliser brace until the specialist is seen.

When the knee is painful and swollen it is difficult for any doctor to fully examine and evaluate the knee. Once the injury has settled down, maybe after a week or so, then it is much easier to examine. Most surgeons will not operate anyway when the knee is still inflamed.

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Why do women get more cruciate tears than men?

As women increasingly enter competitive and demanding sports, surgeons are seeing more and more women with cruciate injuries. Researchers are identifying a number of reasons for the high incidence of cruciate injury in this population group, and found that women:

have a narrower notch in the femur

It is speculated that the narrower intercondylar notch (the notch between the two rounded ends of the femur bone) in women may contribute to cruciate damage.

have a relationship between cruciate injury and their ovulation cycle

Research has demonstrated that women who injure their cruciate ligament are more likely to do so mid-cycle.

have relatively weak hamstrings compared to men

The strength of the quadriceps is stronger compared to the hamstrings muscle in women, while men have more balanced muscles.

have a less flexible stance and stiffer landing during sporting activities

Women tend to keep their trunks more upright than men during sporting activities, and tend to use the squatting position less. Women tend to absorb the impact of landing poorly in comparison to men.

With specific training the incidence of cruciate damage in women approaches that in men.


Risk factors associated with non-contact anterior cruciate ligament injury: a systematic review. Pfeifer CE, Beattie PF, Sacko RS and Hand A. Int J Sports Phys Ther. 2018 Aug; 13(4): 575–587.

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The problems of cruciate tears in children

Cruciate ligament tears in children used to be very uncommon, but they are becoming more common as children participate more in demanding sports.

The issue of such injuries in children is that their bones are still growing, with special regions towards the ends of long bones ('growth plates' or 'open physes') having very actively-dividing cells. When a child becomes an adult, the cells in the physes stop dividing and are said to 'close'.

Surgeons are likely to be reluctant to do a cruciate ligament reconstruction on a child before the physes close, leaving the child vulnerable to ongoing knee instability and secondary damage to the meniscus or the joint cartilage surfaces. The reason for the reluctance is that any tunnels drilled through the bone to accommodate the graft are likely to go through the physis, and this may arrest growth or cause bany deformity. So the decision of whether to operate or not is a difficult one, and different surgeons will have different opinions on this. It is best to consult a surgeon who has a special interest in working with children.

Before surgery, surgeons will check where the injury fits on the Sherman System and how the child's physical maturity ranks on Tanner staging.


Pediatric ACL injuries: evaluation and management. Mall NA and Paletta GA. Curr Rev Musculoskelet Med. 2013 Jun; 6(2): 132–140.

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