By: Kate Fast B.Sc (Kin), B.Sc (PT), Dip. Sport (PT)

In 1999, there were 102,000 Anterior Cruciate Ligament (ACL) reconstructions in the United States (3), and over 130 in Saskatoon. According to the NCAA injury Surveillance System, the ACL is the most common severe ligamentous injury (3). Although there may be some debate as to whether ACL injuries are on the rise or that they are being identified more often, the number of athletes with known ACL injuries is clearly increasing. As a result, coaches and therapists are more likely to be involved in the rehabilitation of ACL injures, and in the management of ACL injured athletes as they return to sport.

Although the high number of reconstructions being performed may give the impression that the management and rehabilitation of this injury is routine, a careful review of the literature supports the opposite conclusion. In fact, the ACL injured knee should command the utmost care and attention given it’s natural history.

This is the first in a series of three articles regarding ACL injuries and rehabilitation. The purpose of this article is to provide some background in terms of function, mechanism of injury, and injury classification. The natural history and management of the complete (3rd degree) ACL tear will also be described. The next article will present some current rehabilitation approaches following ACL reconstruction. The last paper in the series is devoted to the rehabilitation of the female ACL injured athlete.

ACL Function, Mechanisms of Injury and Classification
The anterior cruciate ligament is one of a matched pair of ligaments in the middle of the knee. It’s function is to stop the tibia (calf) from slipping forward under the femur (thigh) during pivoting and twisting motions.

Men and women tend to injure their knee ligaments in different ways. Men frequently injure their ACL because of contact with another player or piece of equipment. The knee is struck from the outside, with the foot planted and the knee straight. On the other hand, women hurt their ACLs most often during non-contact events. Landing from a jump onto a straight knee, or rapid planting and cutting are the most common mechanisms of ACL injuries in women (8).

The classification of ACL injuries follows traditional definitions. A first-degree ligament injury is characterized by a minimal amount of microscopic disruption of the ACL fibers. The length of the ligament, and therefore it’s function, remains intact. A second degree ACL sprain causes a moderate amount of tearing of the fibers. As a result, the ligament lengthens, and consequently, the knee becomes slightly loose. A third degree ACL sprain is a complete disruption of fibers. Tearing the ligament fibers causes bleeding into the joint space, commonly know as a hemarthosis. Because the fibers are completely torn, the ACL does not function to prevent forward sliding of the tibia under the femur. The knee becomes unstable during pivoting and twisting motions.

Consequences of ACL Tears
ACL injuries occur when significant forces are transmitted through the joint, and such extreme forces can damage many structures within the knee, including other ligaments, menisci and the articular cartilage.
When the ACL tears, other ligaments sustain a 2nd or 3rd degree sprain in 20 to 40% of reported cases (3,4). Most often, the medical collateral ligament (MCL) and occasionally the posterior cruciate ligament (PCL) may be damaged. These ligaments work in concert with each other to limit the motion of the knee. While the ACL limits the slide of the tibia from under the femur, the MCL stops the knee from hinging sideways. Significant damage to two ligaments leads to abnormal movement in two directions, and thereby, a profoundly unstable knee.

Over 40% of 3rd degree ACL tears sustain significant meniscal injury (3). The menisci are two washer-like fibrous discs which function to maintain ideal alignment between the bones of the knee. They also bear and distribute body weight and forces through the joint. Unfortunately, there is a direct relationship between meniscal damage and the development of arthritis (7). It is because of this association that surgeons now spare and repair as much of the meniscus as possible.

In addition to multiple ligament and meniscal trauma, ACL deficient knees show significant signs of articular cartilage damage. Articular cartilage is the layer of cells which line the bony surfaces of the joint. It’s function is to decrease friction within the joint, and to absorb and distribute forces acting on the joint. A recent MRI study showed that knees with acute (newly acquired) 3rd degree tears had evidence of bone bruising in 80% of cases examined (2). Such bruising indicated significant trauma to the articular cartilage and underlying bone. Even more disconcerting is evidence that this kind of damage is still present on MRI 6 years after the injury (2). It appears that 3rd degree injuries to knee cause severe, permanent changes to the articular cartilage (2, 7). Impaction fractures, an even more severe form of bruising, have been reported in 23% of acute ACL tears (7). Such articular cartilage damage is of serious concern, as is it may predispose these joints to the development of arthritis.

Because of the high frequency of injuries to multiple injuries in conjunction with 2nd or 3rd degree ACL injury, the ACL deficient knee should be viewed as a syndrome as opposed to isolated ligament tear. These injuries have significant long-term consequences for the health and function of the knee joint.

The Natural History of the ACL Deficient Knee
Studies show that the majority of young athletes return to sport within one year of sustaining a 3rd degree ACL injury. Unfortunately, approximately half of these knees sustain a second significant giving-way injury within that one-year period and 44% of those knees experience multiple episodes (5). Every significant giving-way episode is associated with increased pain, swelling and a deterioration in functional knee scores.

There is evidence that this repeated trauma to the knee is harmful. As the length of time from injury increases, there may be increasing signs of arthosis in the x-rays. In one study, 46% of ACL deficient knees showed some signs of change on x-ray 2 and one half years following the initial injury (5).

Unfortunately, there are few indicators with which predict which ACL deficient knees will become reinjured. There is a relationship between damage to the secondary restrains, such as the MCL, PCL or menisci, and an increase in the risk of reinjury. The risk of re-injury is not strongly related to the level of sport, (i.e., recreational versus college athletics) or gender.

Prospective studies have lead to the “Rule of One-thirds”(1), regarding the long-term outcome of ACL deficient knees. Approximately one third of all ACL deficient athletes return to their previous level of play, and do not experience significant dysfunction. These are the “super-copers”, who’s hamstring muscles and secondary restraints have successfully compensated for the lack of ACL. These individuals report very few episodes of swelling or giving-way.

About one third of ACL deficient young athletes will not have giving way with activities of daily living, but will have instability with sports. These individuals tend to decrease their function to equal the capability of their knee and report only occasional swelling. They tend to intentionally avoid high-intensity, pivoting sports because they realize their knee is not reliable under those conditions. Some of these individuals do return to competitive sport using a brace to provide additional stability.

The remaining one-third of ACL deficient athletes are referred to as “knee abusers”. They have repeated episodes of giving way, but continue to participate at levels of sport beyond the knee’s capacity.

Management of Knees with ACL Deficiency
A specific rehabilitation plan for the acute 3rd degree ACL depends on the extent of collateral damage to other knee structures. There is usually a period of rest (crutch-walking, non-weightbearing), and frequently the knee is immobilized with a brace or splint. This is followed by a gradual reintroduction of weight-bearing. Finally, there is a period of controlled training to regain the strength and agility lost. The time frames and level of protection are usually defined by the orthopedic surgeon and are typically individualized to each athlete.

Although there has been an evolution in the care and rehabilitation of the ACL reconstructed knee, the approach to the ACL deficient knee has not changed significantly. A basic but thorough management of the ACL deficient knee was proposed 17 years ago by Noyes (Table 1). It comprises an 8 point plan, characterized by ongoing, careful evaluation and monitoring of knee function, and working in close association with an orthopedic surgeon.

Table 1
Management Program for Knees with Chronic Laxity of the Anterior Cruciate Ligament
1. Identify and correct strength, power and endurance deficits in all low-extremity muscle groups. Rehabilitate for agility and neuromuscular coordination prior to sports participation.
2. Establish a weekly maintenance-exercise program to prevent recurrence of muscle deficits. This is particularly stressed for those engaged in any sports activity.
3. On recommendation from the orthopedic surgeon, modification of substitution of specific types of activities or sports is required in the majority. Jumping, cutting, and sudden turning or twisting activities place the knee at great risk for reinjury and joint deterioration over time.
4. Counseling from the orthopedic surgeon concerning the risk of future arthritis when activities are pursued or continued despite symptoms, particularly with chronic swelling or recurrent giving-way. Such symptoms, even though subtle and not restriction a specific activity, may add to joint deterioration over time.
5. When necessary, recommendation for an appropriate knee brace to be used during recreational and sports activities.
6. Assessment of the functional knee disability after successful completion of the previous five steps .
7. Early utilization of arthroscopy for knees resistant to this program to define the true extent of joint deterioration for counseling purposes and potential treatment of meniscal or other problems that are preventing rehabilitation.
8. Periodic examination and close follow-up to detect subtle joint deterioration before it is too late to modify activities or intercede surgically when warranted.

Knowing the potential complications to which the ACL deficient knee may succumb, it is the responsibility of trainers, coaches and therapists to educate their athletes. Such knowledge is not meant to scare an athlete, but may be used to motivate them to comply with a prolonged and specific rehabilitation protocol. Adopting the management program suggested here may result in the development of more ‘super-copers’. It will certainly assist trainers and coaches in identifying athletes who’s knees are not coping, thereby making appropriate referral to an orthopedic surgeon more likely.


  1. Andersson., C, Odentsten, M and Gillquest, J.:Knee function after surgical and non-surgical treatment of acute rupture of the anterior cruciate ligament: A randomized study with a long-term follow-up period. Clinical Orthopedics and Related Research, 264: 256-263. 1991.
  2. Faber, KJ, Dill, JR, and Amendola, A. et al.: Occult Osteochondral lesions after anterior cruciate ligament rupture. American Journal of Sports Medicine, 27(4): 489-494. 1999.
  3. Noyes, FR. ACL-deficient knee: natural history risk analysis, non-operative treatment. Presentation at Advances on the Knee and Shoulder, May 29, 2000.
  4. Noyes, FR, and Barber-Westin, SD.: A comparison of results in acute and chronic anterior cruciate ligament ruptures of arthroscopically assisted autogenous patellar tendon reconstruction Journal of Sports Medicine, 25(4):460-466. 1997.
  5. Noyes, FR., Mooar, PA., Matthews, DS, and Butler, DL: the symptomatic anterior cruciate-deficient knee. Part I: the long-term functional disability in athletically active individuals. Journal of Bone and Joint Surgery 65A, No. 2.: 154-162. 1983.
  6. Noyes, FR., Mooar, LA., Morrman, CT and McGinniss, GH.: Partial Tears of the anterior cruciate ligament: progression to complete ligament deficiency. Journal of Bone and Joint Surgery 71-B, No. 5:825-833. 1989.
  7. Spindler, KP, Schils, JP., and Bergfeld, JA et al.: Prospective study of osseous, articular, and meniscal lesions in recent anterior cruciate ligament tears by magnetic resonance imaging and arthroscopy. American Journal of Sports Medicine, Vol. 21(4): 551-557. 1993.
  8. Wilk, KE., Arrigo, C., Andrews, JR., and Clancy, WG.: Rehabilitation after anterior cruciate ligament reconstruction in the female athlete. Journal of Athletic Training, 34(2): 177-192. 1999.