Text by Michael Frind. Colour diagrams by SMG Medical Graphics.

Seventeenth version: March 24, 2008.

References: Knee Ligament Rehabilitation, by Todd S. Ellenbecker (Churchill Livingstone, 2000), various websites and articles from peer-reviewed medical journals, and the classic sports-injury textbooks by Arnheim and Prentice, as well as by Anderson, Hall, and Martin. (Full-chapter excerpts from these texts are now present in the Knee Library.)

The knee, also known as the genual joint, is situated at the interface of the human body's two longest bones, the tibia and the femur. The joint, essential in nearly every activity of daily living as well as in many athletic endeavours, is the most vulnerable to severe injury of any in the body. No orthopedic injury causes the active person more grief; only CNS (brain and spine) injuries are more devastating overall. Prior to delving into the different injury types, it is worthwhile to review how the knee works and why it is so uniquely prone to catastrophic injuries. This document is intended to serve to provide a concise yet reasonably comprehensive overview of knee function, in order to enable users of the Knee Library to more fully comprehend complex knee problems as well as to obtain greater benefit from the advanced-level resources proffered on this site.

The following descriptions refer to the diagrams provided below. (Please scroll down to view the diagrams as required.) But first, here are some terms:

Ligament: strong band of connective tissue that connects one bone to another. Ligaments are very much alive: they contain blood vessels and are innervated. Their strength derives from their parallel-aligned collagen fibres. In simplest terms, ligaments handle tensile forces.

Tendon: strong band of connective tissue that connects a muscle group to a bone. A tendon's structure is similar to that of a ligament. (Like ligaments, tendons handle tensile forces only.)

Cartilage: in the context of orthopedics, this is generally a bearing surface. This is what the menisci (discussed subsequently) and articular cartilage are comprised of. Cartilage handles compression and shear forces.

Retinaculum: connective tissue, which in orthopedics helps keep a certain structure in place. (One example of retinacular tissue is that which helps keep the patella from moving side-to-side.)

Proximal: closest to the person's torso

Distal: furthest from the person's torso

Anterior: towards the front of the body (or simply the frontmost portion of the structure under discussion). Example: anterior cruciate ligament

Posterior: towards the rear of the body (or simply the rearmost portion of the structure under discussion). Example: posterior cruciate ligament

Medial: closest to the centreline of the body. Example: medial collateral ligament (discussed below; also see diagrams)

Lateral: furthest from the centreline of the body. Example: lateral collateral ligament (discussed below; also see diagrams)

Sagittal: acting from front-to-back or back-to-front (i.e. same as anterior-posterior or posterior-anterior). Memory aid: think of the mythical archer Sagittarius: the plane of his bow is a sagittal plane. (A sagittal plane is any plane parallel to the plane of Sagittarius's bow.)

Coronal (same as Frontal): if you lie on a bed on your back, then this is any plane running through your body that is parallel to the bed surface.

Transverse: This is any plane that cuts your body is the shortest way possible. If you stand in a pool in waist-deep water, the surface of the water is bisecting your body in a transverse plane.

Valgus: bent inwards, or inwards-directed forcing. Knockkneedness=genu valgum. (To remember this, think of the upper-case letter L superimposed on a picture of a knock-kneed person: the person's legs are widest at the feet, just like the letter L.)

Varus: bent outwards, or outwards-directed forcing. Bowleggedness=genu varum. (To remember this, think of the lower-case letter r being superimposed on a picture of a bow-legged person: the person's legs are widest at the knees...exactly where that superimposed letter r is widest.)

Bony anatomical aspects:

Femur: the thighbone. This happens to be the largest bone in the body, and it runs from the hip to the knee. At the knee, we can feel the condyles of the femur (medial and lateral condyles, i.e. the two large bony knobs that define the distal end of this bone). At the hip, we can feel the greater trochanter. Otherwise, the femur is completely encased by thick musculature.

Tibia: the shinbone. The proximal end of this bone forms the lower portion of the knee (also known as the tibial plateau). The front of the tibia is known as the tibial crest (discussed below), and it can easily be felt as a long bony ridge running down the front of the lower leg.

Fibula: the lower leg harbours two bones; parallel to the tibia is the fibula. The fibula is located on the outside (i.e. lateral) side of the lower leg, and is somewhat shorter than the tibia. Having two bones in the lower leg enables the foot to the swivelled side-to-side in the transverse plane. (This is similar to the forearm, which harbours the radius and ulna; these can cross each other, thereby enabling the person to turn his/her palm upwards.)

Tibial Tubercle: (also known as tibial tuberosity). This is the bony knob just below the patella (described subsequently). It serves as the attachment point for the patellar tendon.

Note: patellofemoral-pain (and also chondromalacia) problems can be very frustrating and debilitating, and they tend to be complex. They are NOT something which family physicians and general practitioners should be dealing with! Today, many family/general doctors, somehow secure in their belief that they know enough about the knee to qualify them to treat it (when reality, they know just enough to be dangerous), are injecting cortisone directly into the joint space to relieve pain and inflammation. Avoid this route! Intra-articular cortisone (corticosteroid, hydrocortisone, etc.) injections are very harmful to the knee, as they cause cumulative and permanent damage to bones and connective tissue...and such damage lasts for the rest of your life. Evidence for the harmfulness of such injections can be seen in the fact that the standard recommendation is no more than three such injections per joint (for the entire lifetime of the person).

Major ligament anatomical aspects (diagrams are provided at the end of this document):

Note: The knee's two cruciate ligaments, so-named because they cross each other, are located at the centre of the knee. (They are considered intra-articular ligaments, because they live directly within the joint space and hence are surrounded by the joint's lubricating synovial fluid.) The cruciate ligaments work together to govern the knee's natural roll-and-glide motion (in the sagittal plane), and thereby ensure that the loadings (compression and shear) and dealt with as safely as possible. (The knee's actual motion is more than simply flexion and extension. The joint does twist slightly [i.e. the tibia rotates inwards] as it extends; it also becomes effectively a bit "knock-kneed" as it reaches full extension. These motions are not important here.) The two cruciate ligaments stabilize the knee against sagittal-plane (anterior-posterior) shearing-type forces, and also against twisting. Loss of either of these ligaments translates into increased wear-and-tear on the articular cartilage (discussed later) and menisci. (The eventual result is painful and debilitating osteoarthritis.)

ACL: Anterior Cruciate Ligament. This ligament consists of two distinct bundles of fibres, and it connects the tibia to the femur. The ACL is located at the centre of knee, in front of the PCL. It is the most commonly injured (at least in terms of health-care-system costs) of all knee ligaments, due to the fact that it is essential in protecting the knee against twisting (primarily inwards rotation of the tibia). The ACL is also heavily involved in limiting how far the knee extends (i.e. protecting against injurious hyperextension), and it is responsible for controlling how far forwards the tibia slides with respect to the femur. Injury occurs most commonly via planting and twisting (i.e. as in cutting-type sports such as soccer and basketball), or by excessive extension (i.e. injurious hyperextension, as could occur during a collision in contact sport). Injury can also occur via a forwards force being applied to the tibia of a partly flexed knee, or via a sudden deceleration (e.g. landing from a long jump). The ACL can also be torn in conjunction with the MCL, usually during a combined inwards-forcing-and-twisting situation. If the ACL is only slightly partly torn, then the knee remains stable. However, complete or nearly complete tears are far more common; these are denoted by the knee giving way at the time of the injury. Other notable signs/symptoms of full/severe ACL tearing: loud "pop" or ripping noise (not always heard, but often felt) from the rapid tearing of the ACL, swelling of the knee (joint space fills with blood from the torn ligament), pain (e.g. "explosion" at time of incident, but subsides as the nerve endings of the torn ligament soon die off; a few ACL-tearing cases involve no pain at the initial injury incident), tenderness of the joint, muscle spasms around the joint (common with any joint injury), inability to bear weight on the knee, and subsequent stiffness. Note that ACL tearing is very often accompanied by meniscal tearing and/or articular-cartilage tearing. Additionally, ACL tearing can occur via excessive extension (injurious hyperextension) of the knee, as well as in cases where the tibia is forced forwards (this can occur in alpine skiing, and is known as the "boot-induced ACL tear"), or in certain types of high-impact collisions (in which case other structures of the knee are likely to be injured too).

An ACL injury is clinically diagnosed by pulling the tibia forwards with the knee partly flexed; these are the anterior drawer and Lachmann tests. Other ACL-integrity tests entail twisting the foot: for example, the pivot-shift test and Macintosh test, are also used. Female athletes are at 2-8 times the ACL-injury risk of their male counterparts; reasons for this discrepancy include proportionally weak hamstrings, a tendency to land jumps with insufficient knee flexion, a monthly hormonal cycle that includes ligament-weakening estrogen spikes, balance/co-ordination peculiarities, an ACL-guillotining narrow intercondylar notch, and other factors. The first two factors are the most important overall; fortunately, these can be rectified via training programs such as Cincinnati Sportsmetrics.

Torn ACLs can either be left as is (in which case the knee remains unstable forever, and the person must forever be careful to avoid twisting the joint and must avoid knee-demanding activities) or they can be reconstructed via tendon-grafting (a demanding surgery, but worthwhile given that it stops the cycle of cumulative, permanent damage that ensues from each and every giving-way incident of a chronically unstable knee). Twisting-type ACL injuries can be prevented by maintaining strong leg muscles (good strength plus good endurance), and by learning to pivot only on the front portion of the foot (instead of planting the entire shoe sole when changing direction). Functional knee braces are useful for protecting against sideways forcing and excessive extension, but because the soft tissues which surround the tibia and femur are easily sheared (to see this, grab your thigh and twist, even with the leg musculature tensed), it is difficult for a brace to protect a knee against injury via twisting.

Dealing with ACL tearing via surgical reconstruction is a complex topic in itself, and is not discussed here. In-depth discussions on the topic and other knee-related topics can be reviewed via searching the Bob's Board (also known as the Kneeboard) forum. Articles dealing with surgical options can be found here, here, and here. Articles dealing with post-surgery problems such as arthrofibrosis (i.e. scar tissue inside the joint, typically pursuant to surgery) can be found here. Articles dealing with knee bracing can be found here. Postings and discussions dealing with the use of knee bracing can be found in the Bob's Board Interim Archive in addition to the Bob's Board discussion forum. Previous postings are searchable here. With any of these topics, you are encouraged to post questions to the aforementioned discussion forum. (Said forum is the largest and oldest of its type in the world. It was founded in 1996, and is home to more than 200,000 threaded postings on a wide variety of knee topics.)

Note: If you have torn your ACL, keep in mind that other tensile structures might be damaged as well (in addition to damage to knee cartilage, which is discussed subsequently and which is distinguished biomechanically by the fact that it handles compression and shear forces). Problems with the posterolateral corner (which is known by the acronym PLC) are a major cause of ACL-graft failures. (The PLC-structure group includes the popliteus tendon, the arcuate ligament complex, the LCL, and the lateral capsular ligaments. The posterolateral-corner structures play an important role in rotation control.) Also, note that if a knee is allowed to remain ACL-deficient for extended periods of time, then stretching-out of the PLC tends to occur. So, a chronically ACL-deficient knee will typically be much looser than one in which the ACL has been torn only a few weeks ago. (This explains why allograft-type ACL reconstructions [also known as cadaver grafts] are not recommend in any knee which has been ACL-deficient for more than three months; also relevant here is that allografts take much longer to revascularize than autografts.)

PCL: Posterior Cruciate Ligament. Connects tibia to femur; located at centre of knee, behind ACL. The PCL happens to be stronger than the ACL, and is not injured as often as the ACL. The most common cause of PCL injury is a rearwards impact-type force to the knee bent at 90 degrees, with said force being applied to the tibial tuberosity (i.e. just below the kneecap). (Injury is diagnosed by forcing the tibia backwards; this is the posterior sag test.) Like the ACL, the PCL has a multifascicular (multi-bundle) structure. Although the PCL does have some ability to self-heal (as Dr. Shelbourne has noted), the problem is that this healing is biomechanically poor, and a self-healed PCL will be structurally very different from a normal PCL. This is something to watch out for, because on an MRI scan, a torn-and-self-healed PCL might appear to have continuity...but its torn-and-scarred-together fibres will probably be very much overlength and badly dysfunctional in practice (even though a quasi-normal posterior-sag-test result might be obtained). If the patient is lucky, a so-healed torn PCL might provide satisfactory serviceability (provided the patient avoids knee-demanding activities). Otherwise, proper tendon-graft reconstruction (dual-bundle format) would be indicated.

MCL: Medial Collateral Ligament (also known as tibial collateral ligament, or TCL): located on the medial side of the knee, the MCL serves to protect the knee against inwards-acting (valgus-directed) forcing. Because it is surrounded by connective tissue (the retinaculum), it can (if not too severely torn) scar over and thereby "self-heal". (The MCL is the only major knee ligament capable of this. Note that for this "self-healing" to be of value, the knee must be protected against valgus-type forcing during the scarring-over period.) The MCL connects the tibia to the femur.

LCL: Lateral Collateral Ligament (also known as fibular collateral ligament, or FCL): Protects knee against outwards (varus-directed) forcing. Connects fibula to femur. This ligament is rarely injured in isolation because varus-directed forcing occurs rarely. But one often-overlooked injury involves the aforementioned PLC (posterolateral corner), of which the LCL is part.

(Several small and very minor ligaments, for example the meniscofemoral ligaments, are not discussed here due to the need for conciseness in this synopsis document.)

The term knee dislocation is defined as tearing the ACL and PCL, along with at least one of the collateral ligaments (MCL or LCL). This is an extremely severe injury, and it tends to occur only in very-high-kinetic-energy situations (e.g. extreme sports). Restoration of normal knee function after such an injury is exceedingly difficult, and recovery is usually frustratingly incomplete, although with competent surgical treatment and diligent rehabilitation, satisfactory results can be obtained. Please be careful not to confuse total knee dislocation with patellar (kneecap) dislocation, a much less severe injury. (Dislocating the kneecap is painful. However, once the patella has been reduced so that it is returned to its groove, the acute pain usually subsides. To prevent future such dislocations, the retinacular structures responsible for patellar tracking must be reconstucted. This is a much simpler surgery than reconstructing the cruciate ligaments inside the knee.)

Major muscles of the knee (Note: all of these muscles are actually biarticulate: they operate more than one joint, but each group operates one joint more than another. Muscles are never operated entirely in isolation: your body automatically operates other muscles too, in order to keep the joint that you don't want to move stationary.)

Quadriceps group: responsible for extending the knee. These thigh muscles comprise the largest, most powerful group of muscles in the entire body. Essential in postural maintenance. (Minor role of quadriceps: to flex the hip.) The patellar tendon can easily be felt, just below (i.e. distal to) the patella. (The quadriceps, technically known as the quadriceps femoris group, comprises the vastus lateralis, vastus intermedius, rectus femoris, and vastus medialis. The vastus medialis is particularly important with regards to patellar tracking. One additional anterior thigh muscle, the sartorius, is not considered part of the quadriceps group.)

Hamstring group: responsible for flexing the knee. These muscles are important in protecting the ACL, because they counteract anterior-drawer-type forcing. (Minor role of hamstrings: to extend the hip.) You can easily feel the two sets of hamstring tendons with your hand, when sitting in a chair. The hamstrings also have some role in limiting how far the knee extends. Unfortunately, the anatomical aspects of how the hamstrings connect (technically known as origin and insertion points) engender a problem here: at full extension (which is exactly the point when the knee is most vulnerable to injury by excessive extension, also known as injurious hyperextension), the hamstrings are biomechanically least able to do anything of value, given their mechanical disadvantage. The hamstrings are best able to flex the knee when the knee is already flexed to 90 degrees, which is the angle obtained when sitting normally in a chair. (The hamstring group, defined as the posterior thigh muscles, comprises the biceps femoris on the lateral side of the leg, and the semitendinosus and semimembranosus on the medial side. The medial-thigh muscles, the gracilis, pectineus, and adductor longus/brevis/magnus, are technically not part of the hamstring group. However, a popular ACL graft is harvested from the semitendinosus and gracilis tendons; for simplicity, this type of graft is known as the hamstring autograft.)

Gastrocnemius: this muscle's primary responsibility is for extending the ankle. (To feel it work, stand on tiptoe, in which case the gastrocnemius works in conjunction with the soleus muscle underneath.) But the gastrocnemius also helps in flexing the knee.

Articular cartilage and menisci:

Menisci: these are two crescent-shaped pieces of tough cartilage that reside between the articular-cartilage-coated ends of the femur and tibia at the knee. (They appear edge-on in Figures 3, 4, and 5.) They serve to cushion the joint against impact-type loadings (some people compare them to shock absorbers in this regard), and distribute compressive and shear loadings across the vulnerable articular-cartilage surfaces. Also, they assist in joint lubrication and cartilage nutrition, and they contribute to overall joint stability. Clearly, the menisci are extremely important in their role of protecting the weight-bearing surfaces of the tibia and femur.

Meniscal tears very commonly occur in conjunction with ACL tearing; the medial one is more often torn than the lateral one. (One reason for this has to do with the stresses on the medial compartment being greater than in the lateral compartment.) In the past, surgeons would often completely remove torn menisci. Such surgery, known as menisectomy, was a guarantee of premature osteoarthritis. Although surgeons know better today, it is interesting to note that the importance of the menisci is something that was, from the viewpoint of biomechanics (which is more a physical science than a biological one), entirely obvious long before the first surgeon took a scalpel to the knee.

Today, when confronted with a torn meniscus, most surgeons simply recommend to trim away the torn portions. However, many meniscal tears are in fact amenable to repair via careful cross-suturing. (Much ground-breaking research in this regard was done by Dr. Frank Noyes; some medical-journal articles on this topic can be found here in the Knee Library.) Regrettably, because (in many countries, including the United States) the health-insurance billing codes for meniscal repair are the same as the billing codes for partial menisectomy, and also because meniscal repair takes several hours and menisectomy takes minutes, most surgeons have an obvious financial incentive to recommend the partial menisectomy.

Note, too, that a loss of even a small portion of a meniscus can engender huge increases in compressive and shear loadings on the articular-cartilage surfaces. So, it means absolutely nothing if a surgeon says "don't worry, I'm only trimming away a quarter of your meniscus"...when in fact, even though 75% of the meniscus is left, the removal of the damaged portion (which, because it was injured, is probably what was biomechanically the most essential anyway) might easily bring a quadrupling of compressive and shear stresses on the articular cartilage. So, the surgeon's sweeping "don't worry" statements are nothing but empty reassurances. (And, of course, the patient is the one who will be dealing with the pain of premature osteoarthritis, not the surgeon! Furthermore, if a surgeon says he/she will remove a certain percentage of your meniscus, how is this percentage being measured...on a surface-area basis or on a volume basis? The menisci are shaped as crescents: thin at the centre of the knee, and thick at the edges; this is what enables them to cup the ends of the femur nicely.)

Remember that each and every time an ACL-deficient knee is allowed to give out, further damage occurs to both the meniscal and articular cartilage. Such damage invariably brings aftereffects that are both cumulative and permanent, and which increase the spectre of premature osteoarthritis...and which complicates future attempts at surgical repair.

Articular cartilage: a thin layer of high-quality, ultra-slippery hyaline cartilage which covers the ends of bones which slide against each other at the joint. The cartilage can easily be damaged by various types of trauma, or it can be worn away as a consequence of missing or damaged menisci. Perhaps the most common articular-cartilage injury scenario entails ACL tearing as well, and so it is not surprising that over 85% of ACL injuries are accompanied by what is known as bone-bruising. Bone-bruising is simply a smashing of the articular cartilage (and the underlying microtrabecular bone structure). Although bone-bruising generally does heal on its own, the healing is biomechanically suboptimal, as the native hyaline cartilage is replaced by inherently poor-quality fibrocartilage. The bone-bruising issue is one reason why a knee that has any history that includes full ACL tearing will likely be forever predisposed to becoming prematurely ostoarthritic; however, avoidance of further giving-way incidents (via having the ACL reconstructed, avoidance of knee-twisting activities, or a combination of measures) is helpful in limiting the damage.

The human knee’s extreme vulnerability to ligament injuries is a function of biomechanics:

1. The femur and tibia are each more than twice as long as the wrench used for changing a tire on a passenger car. This makes for some very huge wrenching forces (also known as torques or moments). These routinely translate into astronomical tensile forces on the ligaments. (Consider, too, how the fact that humans walk on two long legs makes for huge demands on the knees. Having four legs, as all small mammals do, enables easy acceleration/deceleration and direction-changing; moreover, having short legs results in one's centre of gravity being close to the ground and therefore also makes manoeuvering easier. This is dramatically illustrated if you try to chase a squirrel, an animal whose straight-line running distance is only a fraction of that of humans. The squirrel will easily escape via repeated high-speed direction changes, even if it has no tree to climb. Meanwhile, dogs and cats are notable in their superb abilities not only to run at very high speeds, but also to start and stop quickly and change direction almost instantly. So, when humans evolved from quadrupedal to bipedal, we gained the ability to use our hands for all sorts of purposes, but our knees became much more highly stressed in the process. With our enormously long legs as well as the fact that we only have two such legs, we humans are biomechanically unique, albeit we do share bipedalism with our avian friends.)

2. The dynamic loadings (caused by acceleration/deceleration) on the knee far exceed the static loadings. A person with a 170-pound torso-and-upper-legs-section exerts 170 pounds of force on the tibial plateau regions of the knees when standing soldier-style. But when this person walks (and then runs and jumps), the forces can be increased by up to 20 times! (Think of what happens when you jump up and down on a bathroom scale...in fact, the mechanism of the scale will likely be damaged.)

3. The knee has virtually zero native bony stability. Yes, at full extension, the femoral condyles and the tibial plateau share the maximum possible flat surface area (and this is helped by a slight twisting of the knee, known as the screw-home mechanism; this locks the knee at extension for tireless standing), but this can hardly be termed true native bony stability. (In contrast, the hip has significant native bony stability; the forces exerted by a standing person actually serve to make the hip stronger, because they press the ball into the cup-shaped acetabulum.)

4. Granted, the leg muscles (chiefly hamstrings and quadriceps) are essential in helping keep the knee stable. But in situations where these muscles are not activated soon enough (e.g. an unexpected impact in any type of team sport, or a high-speed injury such as could occur in alpine skiing), the ligaments likely would end up being largely on their own...and therefore horrifyingly vulnerable to injury. (Of course, the leg muscles would be somewhat tensed for postural maintenance in any case. But keep in mind that the orientation of the leg muscles is not always optimal with regards to dealing with forcing that the knee may be subjected to. An example of this was noted earlier, with regards to the hamstrings.)

Kinetic Energy versus Speed

Remember, too, that kinetic energy increases with speed squared. This simple relationship of physics is arguably the most important physical relationship in all our lives, and yet few people seem to consider how profoundly it influences not only the realm of knee injuries, but almost every aspect of our daily living. The ever-increasing popularity of "extreme sports" (e.g. downhill mountain biking, stunt-biking, stunt-skateboarding, motocross racing, steep-slope alpine skiing, wakeboarding), all of which involve extreme risk and all of which are rife with reports of serious knee injuries, implies that there is a shortage of people who appreciate the meaning of the speed-versus-kinetic-energy relationship.

If we double the speed, we instantly quadruple the kinetic energy...and so any accident or injury will be four times as severe. Similarly, if we increase the speed by a factor of five (which can easily be obtained by changing from walking or jogging to bicycling or skiing), then the kinetic energy increases by a factor of twenty-five! If you feel drawn to extreme sports, think of this fundamental physical relationship the next time you are considering barrelling down a near-vertical triple-black-diamond alpine-ski run, or the next time you are contemplating riding a mountain bike at high speed down an obstacle-strewn convoluted pathway through a forest. (The nonlinear relationship between kinetic energy and speed also explains why speeding is often listed as a factor in fatal car accidents. Consider, too, that the Swiss, long-time mountain inhabitants, typically prefer to ski on relatively gentle slopes, and they ride their mountain bikes on gravel roads instead of trails.)

So, the faster you are moving, the more distance you require in order to safely slow down (and this distance also increases with the square of speed). This raises an additional concern germane to high-speed movement: the reduced reaction time. Remember that it takes about a second from the time you see something to the time you physically react to it. Therefore, the faster you are moving, the further you travel during this one-second time-lag period. This, too, applies in all sports, but it is especially relevant anytime high speeds are involved. (Veteran alpine skiers with ACL injuries often describe the speed with which their knee injuries occurred: by the time they realized what was happening, it was already too late. Basketball, volleyball, and soccer players can also relate to the suddenness with which knee injuries occur. This does not mean that these sports should be avoided. All it means that that one should be aware of the risk, and therefore take measures to reduce the risk.)

With regards to speed and knee-injury risk, note that the direction of movement also comes into play. Technically we consider velocity, which is speed but with the additional attribute of direction. In physics, this direction-dependent quantity is termed a vector. In terms of knee injuries, we see how the ACL can be torn as a result of a simple direction change: the person's speed could remain the same, but he/she could plant a foot and pivot suddenly. The moving athlete, with all that kinetic energy, must now suddenly change his/her course of travel (trajectory) within a very short period of time -- a situation which is a very common cause of ACL tearing as well as other knee injuries. Finally, acceleration/deceleration manoeuvres can also be hard on the knee's cruciate ligaments. Acceleration (and also negative acceleration, hence deceleration) is closely related to kinetic energy, given that acceleration is simply defined as the rate of change of velocity.

A few often-overlooked aspects of knee injuries...

Keep in mind, too, that injury to any structure of the knee is in itself evidence of the importance of that structure! So, if you have torn your ACL (or any other knee ligament) in alpine skiing or basketball or soccer, then you have very clear evidence that the ligament is overwhelmingly important to safe pursuit of such activities. Conversely, if you elect to leave a fully torn ACL (or other ligament) in such a condition, then it is prudent to discontinue partaking in such activities.

With regards to knee-ligament injuries in particular, note that although it is possible to compensate for ligament deficiencies through the use of strong leg musculature, when muscle fatigue sets in (or if the muscles are not activated in time, for example during an unanticipated event such as a contact-sport collision), the compensation capabilities of the muscles are greatly reduced. This means during such situations, the sloppiness and looseness of a ligament-deficient knee will manifest themselves much more obviously...and also, ancillary damage (such as posterolateral-corner stretching and erosion of the rearmost edges of the menisci in an ACL-deficient knee, or erosion of the frontmost edges of the menisci in a PCL-deficient knee) will become a correspondingly greater concern. (A functional knee brace is helpful in some respects, and such braces are ideal for protecting against sideways forcing and hyperextension. But there are limits to what braces can do; plant-and-twist injuries fall outside the protective capabilities of knee bracing. No brace can replace a missing ligament.)

Finally, please remember that the knee ligaments are more than just strands of tensile collagen fibres optimally oriented in helical-parallel fashion. They are very much alive, and contain blood vessels as well as nerve endings. The nerve endings serve to keep the brain informed as to the goings-on in the ligaments; this continual feedback process is known as "proprioception" (formerly "kinesthetic sense"), and is essential to optimal muscle-activation timing. In the case of the commonly injured ACL, the nerve endings not only perform a proprioceptive function, but they are also responsible for reflexively triggering the hamstring muscles. Tensing the hamstring muscles flexes the knee, but also counteracts anterior-drawer forcing. This provides protection against ACL tearing via two modes (excessive extension and forwards tibial shifting), although twisting (generally the most common cause of ACL tearing) remains problematic.

Tearing the ACL, because it results in severing of the nerve endings, wipes out the aforementioned neurological functioning, although there is encouraging evidence (discussed further in various articles in the Knee Library) that the nerve endings eventually regrow in a reconstructed ACL. (A recontructed ligament uses tissue borrowed from a tendon elsewhere to form a sort of scaffolding. The graft tissue dies after transplantation, because it is not connected to any blood supply. However, it serves as a scaffolding upon which the body grows a new ligament.) Because the nerve endings take at least several years to regrow (and then there is still the question of whether they actually reconnect in a meaningful way), this means that a ligament-reconstructed (any ligament reconstruction, not just the ACL) knee will remain at increased vulnerability to re-injury for several years post-operative. (However, the risk of re-injury is still far less than it would be had the unstable knee been left unreconstructed. And, because the devastating cycle of further instability-caused damage is halted, the knee can be expected to last longer.) There are many exercises that are useful in helping regain lost proprioception. The use of external devices worn on the knee (e.g. braces, sleeves) seems to cause some useful surrogate proprioception; research in this area remains ongoing. (Again, articles on this topic can be found in the Knee Library as well as elsewhere.) A person recovering from knee surgery will also find that the musculature of the affected leg will prone to protracted weakness (one cause of this is the use of a blood-stanching tourniquet during the surgery); therefore, single-leg exercises will be indicated.

Bob's ACL WWWBoard is home to in-depth discussions on all aspects of knee-ligament injuries (not just to the ACL but also the PCL, MCL, and LCL), as well as on other types of knee injuries (patellofemoral pain/chondromalacia, osteoarthritis, meniscal tearing, articular-cartilage damage including bone-bruising). The forum, founded in 1996 by software developer and dual-ACL-injury veteran Bob Willmot, is the largest of its type in the world. The site also includes the Knee Library, a unique and continually growing on-line resource that proffers free access to high-quality, full-text, peer-reviewed medical-journal articles and other reading material.

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Please note that all of the diagrams below show the right knee. (Left knee is mirror image.)

Figure 1: Flexion of the knee. (Diagram by SMG Medical Graphics)

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Figure 2: Extension of the knee. (Diagram by SMG Medical Graphics)

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Figure 3: Perspective view of knee. Extension mechanism (patellar tendon, patella, quadriceps muscle group with associated tendons) is present. (Note: The patellar tendon could really be considered to be part of the quadriceps-group tendon, if we consider the patella as simply a small bone that is embedded in the quadriceps-group tendon. But anatomists have decided that the patellar tendon, the short section which connects the patella to the tibia at the bony prominence known as the tibial tuberosity, is a separate tendon. The patellar tendon is thus the only exception to the rule that tendons connect muscles to bones. In spite of this consensus, some people prefer to term the structure "patellar ligament".) (Diagram by SMG Medical Graphics)

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Figure 4: Anterior (front) view of knee. Note that the extensor mechanism has been removed, save for the patella. (Diagram by SMG Medical Graphics)

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Figure 5: Close-up of knee, again from front, showing cruciate ligaments and the menisci. (Diagram by SMG Medical Graphics)

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Figure 6: Frontal view of extended knee, with quadriceps tendon cut and patella reflected distally. (Diagram scanned from Arnheim and Prentice, and modified by Michael Frind in order to improve suitability for overhead projection purposes.)

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Figure 7: Colour frontal view of partly flexed right knee with all major components labelled. If you look closely, you can see the helical-parallel alignment of the fibres of the ACL. The patellar tendon has been cut, and the patella has been reflected proximally. The tibial tuberosity (where the patellar tendon attaches) is also shown. (Diagram by SMG Medical Graphics)

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Figure 8: Right knee, viewed from front but at an angle.

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Need more detailed descriptions of knee injuries and related aspects? Click here to access the textbook chapters section!

Anderson-Hall-Martin-Ch15-2000.pdf. Chapter 15: Knee Conditions. From the classic text Sports Injury Management, Second Edition, by Anderson, Hall, and Martin, 2000. (File Size: Approximately 12 MB.)

Arnheim-Prentice-Ch19-1997.pdf. Chapter 19: The Knee and Related Structures. From the classic text Principles of Athletic Training, Ninth Edition, by Arnheim and Prentice, 1997. (File size: Approximately 9 MB.)

For even more detailed, in-depth reading material, including everything from advanced-level peer-reviewed medical-journal articles to rehabilitation resources, click here to view the Knee Library's subsections.

Click here for a simple example of static-equilibrium analysis of forces on a joint.

Click here for calculations showing anterior-drawer-counteraction forcing being exerted by a functional knee brace on an ACL-injury-history knee. (Contrary to the marketing by a certain knee-brace maker based in Vista, California, such anterior-drawer-counteraction forcing can be exerted by nearly any hard-shell, dual-upright-type knee brace. Additional comments are provided in the document.)

Click here for an annotated diagram showing anterior-drawer-counteraction forcing being exerted by a functional knee brace on an ACL-injury-history knee. (Contrary to the marketing by a certain knee-brace maker based in Vista, California, such anterior-drawer-counteraction forcing can be exerted by nearly any hard-shell, dual-upright-type knee brace.)

Click here for a diagram showing posterior-drawer-counteraction forcing being exerted by a functional knee brace on a PCL-injury-history knee. (Contrary to the marketing by a certain knee-brace maker based in Vista, California, such anterior-drawer-counteraction forcing can be exerted by nearly any hard-shell, dual-upright-type knee brace.)

Click here for a diagram showing how a functional knee brace (with an adjustable extension-limitation device built into the hinges) protects the knee against hyperextension.

Click here for a diagram showing how an osteoarthritis (OA) knee brace works for a knee with lateral OA.

Click here for a diagram showing how an osteoarthritis (OA) knee brace works for a knee with medial OA.

To view MRI and X-ray images from normal, injured, and reconstructed knees, please click here.

If you encounter anatomical terms which are not covered herein, or if you desire more detailed definitions, a good quick-reference resource is Wheeless' Textbook of Orthopaedics, which is made freely available by Duke University Medical Center and Data Trace Publishing.

Detailed information on knee anatomy, physiology and injuries can be found by reading the full-text articles provided in this Knee Library. (It is also possible to find abstracts-only articles on websites such as National Library of Medicine and PubMed. If you come across abstracts of articles for which you want the full-text article, e-mail Michael Frind and I will dig them up for you.) Writings on knee-related topics by Michael Frind can also be found by searching Interim Archive of Bob's Board, the the Bob's Board (Kneeboard) discussion forum, and by using said forum's integrated search feature (be sure to set the search-engine time frame for more than the default three days).

Click here to return to the Main Entrance Page of the Knee Library.

Looking for the Main Index Page of Bob's ACL WWWBoard? Click here!

To find recent postings on Bob's ACL WWWBoard, use the Search Engine.

To find older postings on Bob's ACL WWWBoard, use the On-Line Archive.