The Knee

The information presented here is for educational purposes only.


This installment of The Med Cell looks at the knee. The knee is an important target in close combat. Injury can lead to loss of mobility and possibly even loss of the leg itself through disruption to blood supply. The amount of force required to cause ligamentous injury is minimal, and in fact can be brought about without external force such as in twisting the knee. The amount of external force that must be applied to dislocate the knee (a very serious injury) is probably around 80 – 100 pounds per square inch. Knowledge of knee anatomy can help predict which injuries are likely to result when a force such as a foot strike is directed at the knee from various angles.

In this installment diagnosis and treatment will not be examined due to limitations of space. In fact, knee injuries can be difficult to diagnose without proper imaging techniques e.g. MRI, and physical examination is of limited use in the acute phase due to pain and swelling, so it is inappropriate to discuss here.

Also, a future article on firearms related injuries will examine gunshot wounds to the knee including ‘knee capping’, but is outside the scope of the present article.


More than 1 million patients are seen annually in U.S. emergency departments with complaints of acute knee injuries. Knee injury is the most common cause of disability in relation to sporting injuries.

More specifically, knee dislocations account for about 1 in 300 parachuting related injuries in military jumpers. The figure is higher for civilian jumpers.


The knee is the largest joint in the body and is also the most complicated. It is a modified hinge joint, and contains synovial fluid.

The knee joint connects the two longest mechanical levers in the human body, the thigh and the lower leg. As a result, large forces act in and around the joint, contributing to its instability. Stability is mainly provided by the ligamentous structures, and these are detailed below.

Three bones articulate at the knee joint: the femur, tibia and patella (commonly referred to as the knee-cap). See X-rays 1 and 2 below. As such, the tibiofemoral and patellofemoral joints make up the larger complex knee joint

X-ray 1: View of the right knee front-on showing the femur, tibia and fibula
X-ray 1: View of the right knee front-on showing the femur, tibia and fibula

X-ray 2: Lateral view of the right knee showing the femur, tibia and patella
X-ray 2: Lateral view of the right knee showing the femur, tibia and patella

Note that in proper terms the fibula is NOT included in the knee joint as it forms a separate articulation with the tibia called the tibiofibular joint.

Individual Bones:

Femur (see diagram 1 below)
The lower end of the femur consists of a lateral and medial condyle (condyle means the rounded projection of bone forming an articulating surface. It comes from the Greek word for knuckle. The condyles are separated by the intercondylar fossa behind and are joined in front by a trochlear (or pulley-like) surface for the patella to glide over. The lateral condyle projects further forward than the medial condyle, which helps to stabilize the patella (otherwise the patella would be pulled side-ways by the vastus lateralis muscle). The fossa provides attachment for the cruciate ligaments.

Importantly, the condyles give rise to small projections called epicondyles, which serve as sites of origin for the collateral ligaments.

Diagram 1: Bony aspects of the right femur.
Diagram 1: Bony aspects of the right femur.

Tibia (see diagram 2 below)
The tibia has a large upper end, flaring out to accommodate the large condyles of the femur. The tibia itself has a medial and a lateral condyle. The superior surface of the tibia is called the tibial plateau.

Diagram 2: Bony features of the tibia.
Diagram 2: Bony features of the tibia.

The medial surface of the plateau is larger and more oval while the lateral surface is smaller and more circular. There is a grooved ridge that runs from front to back to form the lateral and medial intercondylar tubercles. These serve as landmarks for the attachments of the menisci and the cruciate ligaments. The tibia narrows below to form the shaft. At the upper end of the shaft is the tibial tuberosity to which is attached the quadriceps tendon via the patellar ligament.

Patella (see diagram 3 below)
A sesamoid bone is a bone which is embedded within a tendon. The patella is the largest sesamoid bone in the body. It is embedded within the quadriceps tendon which then continues on as the patellar ligament to be inserted into the tibial tuberosity. This is what doctors hit with a tendon hammer to elicit the knee jerk. The patella itself is roughly triangular in shape, easiest to picture as an upside down triangle, with the base being the uppermost edge, tapering to a pointed apex at the lower margin. It has two surfaces: the rough anterior surface which is covered by the fibres of the quadriceps tendon, and the posterior or articulating surface which is smooth and has a vertical ridge dividing the surface into medial and lateral facets. In extension of the knee the medial facet does not make contact with the medial femoral condyle (it only does so in flexion), whereas the lateral facet of the patella is always in contact with the lateral femoral condyle.

Diagram 3: Bony features of the patella.
Diagram 3: Bony features of the patella.

The patella is mobile from side to side. In the human, the femur is situated obliquely, like so:

This means that when the quadriceps contracts, it has a tendency to pull the patella laterally. This is prevented in 3 ways, the last being the most important.

  1. The bony forward projection of the lateral femoral condyle
  2. Fibrous extensions of the quadriceps called retinacula which connect the sides of the patella with the tibial condyles, and
  3. Most importantly, the vastus medialis muscle, the lower fibres of which counteract the pull of the quadriceps muscle.


  1. Increases the leverage of the quadriceps by increasing the angle at which the muscle acts. (Twice as much torque is needed to extend the knee the final 15° as to bring it from a fully flexed position to 15°)
  2. Protects the front of the knee joint

The patellofemoral joint is a saddle joint.

Articulating Surfaces

If we look at the articulating surfaces of the femur and tibia, it can be seen that they are asymmetrical. The medial articulating surface of the femur is narrower and more curved than the lateral articulating surface. As a result, the medial articulating surface of the tibia is more oval and elongated front to back, whereas the lateral articulating surface of the tibia is more circular. Why is this of any consequence? As we will see later, it has a direct bearing on the shape of the menisci, and ultimately on injury patterns.

Ligaments of the knee joint

Now we come to the most difficult section of the article. Because the articulating bony surfaces of the knee joint do not contact one-another over a large area, there is an inherent instability in the knee joint. This is overcome by the presence of a large number of strong and extensive ligaments.

The ligaments of the knee can be divided into two groups: extrinsic and intrinsic. For purposes of simplification, only the most commonly injured and well known ligaments are described. Other ligaments may be injured, but rarely in isolation, and in any case the direction of force does not change in order to elicit injury in one or more ligaments.

(The previous med cell article on the ankle defined the grades of ligament tears, so the reader is referred to that article for an understanding of the severity of ligament injury. It is enough to say that the extent of ligament damage rises with increasing application of force.)

Extrinsic Ligaments

Capsule of the knee joint

The knee joint is surrounded by a thick ligamentous sheath that is composed of the tendons of muscles that pass around the joint, or by extensions of these tendons. This sheath of ligaments attaches to the bony margins of the joint. It is not necessary to go into further detail here as this will confuse an already complex subject. Simply imagine the sheath as being like an ACE knee brace.

Patellar Ligament
Remembering that the quadriceps tendon inserts onto the patella, it then gives off a thin sheet of connective tissue which covers the front of the patella and passes into the patellar ligament which inserts onto the tibial tuberosity.

Diagram 4: Patellar ligament.
Diagram 4: Patellar ligament.

Collateral Ligaments (see diagram 5 below)

The knee has two collateral ligaments

  1. The medial collateral ligament on the medial or inner aspect of the knee
  2. The lateral collateral ligament on the lateral or outer aspect of the knee

Both ligaments are taut when the knee is in extension, and lax when the knee is in flexion. So they act to stabilize the knee in extension, especially rotational movements. The MCL runs down and forwards, while the LCL runs down and backwards.

Diagram 5: Collateral ligaments and menisci of the right knee.
Diagram 5: Collateral ligaments and menisci of the right knee.

Medial Collateral Ligament (MCL)
The medial collateral ligament is the broader of the two ligaments. It is attached above to the medial condyle of the femur and below to the medial condyle and medial surface of the body of the tibia. The MCL blends posteriorly with the medial meniscus. This (as will be discussed later) has implications on meniscal injuries. It also blends with the capsule of the knee.

Lateral Collateral Ligament (LCL)
The lateral collateral ligament is a strong rounded cord that is attached above to the lateral femoral condyle and below to the head of the fibula. Unlike the MCL it does not have any attachment to the capsule or the lateral meniscus.

Intrinsic Ligaments

Cruciate Ligaments (see diagram 6 below)

The cruciate ligaments cross one another like a cross or X. In fact, the word cruciate comes from the Latin word for cross (crucifix = cross)

There are two cruciate ligaments

  1. The anterior cruciate ligament (ACL)
  2. The posterior cruciate ligament (PCL – the stronger, thicker and shorter of the two)

Both are very strong, and keep the articular surfaces of the tibia and femur in contact while stabilizing the knee joint. Both ligaments arise from the intercondylar groove of the tibia to be inserted onto the inside of the femoral condyles, the difference being the anterior cruciate ligament running from front (anterior) to back, and the posterior cruciate ligament running from back (posterior) to front. They cross at right angles, like the letter X, the posterior cruciate ligament inserting onto the inside of the medial femoral condyle, while the anterior cruciate ligament inserts onto the inside of the lateral femoral condyle.

Diagram 6: Cruciate ligaments and menisci.
Diagram 6: Cruciate ligaments and menisci.

The posterior cruciate ligament prevents the tibia sliding to the rear while the anterior cruciate ligament prevents the tibia sliding to the front, especially in running and jumping movements. Importantly for our purposes, the anterior cruciate ligament prevents excessive hyperextension, so a blow to the front of the extended leg will rupture the anterior cruciate ligament.

Diagram 7: Cross-section through the knee showing the meniscus as a wedge shaped cartilage.
Diagram 7: Cross-section through the knee showing the meniscus as a wedge shaped cartilage.

Menisci (see diagrams 5, 6 and 7)

The menisci are crescent shaped cartilages (meniscus means crescent in Latin) that are attached to the tibial plateau. Their ends are called horns. They are wedge shaped, with the sharp edge of the wedge directed towards the midline of the tibial plateau, while the base blends laterally with the capsule of the knee.  See diagram 7. They act as shock absorbers, taking about 1/3rd  to one half of the loads transmitted during movement. They also function to widen the area over which force is transmitted.

There are two menisci:

  1. Medial meniscus.
  2. Lateral meniscus

Medial Meniscus
This is almost a semicircle. It is firmly attached to the capsule and the medial collateral ligament. Due to its elongated shape and the fact that it is attached to the medial collateral ligament means that it is far easier to rupture than the lateral meniscus as it is less mobile.

Lateral Meniscus
This is almost a complete circle. It is not attached to the lateral collateral ligament so it is more mobile and less prone to injury than the medial meniscus.


There are a dozen bursae which communicate with the knee joint in some fashion. They are fluid filled sacs which act to minimise friction when two structures slide over one-another. It is sufficient to note that when these become inflamed through over-use injuries or when they burst, there is resultant swelling and pain that can limit range of motion

Blood Supply

The blood supply to the knee comes from 5 branches of the popliteal artery (called genicular arteries). They form an anastomosis (interconnections between the arteries) around the knee.

It is worth pointing out the course of the popliteal artery as it has implications in armed combat. It is itself almost impossible to injure in unarmed combat, although the small genicular branches may become ruptured. However, as it is anchored firmly at either end of the popliteal fossa, traumatic knee dislocations can damage the artery and threaten the limb.

The popliteal artery is the continuation of the femoral artery. It courses behind the knee in the popliteal fossa. It arises one hand’s breadth above the knee, and becomes the anterior and posterior tibial arteries one hand’s breadth below the knee. It is a deep structure, being covered by tough fascia. The first part of the artery lies on the femur bone itself. This makes it difficult to injure unless the mechanism is a penetrating one such as a gunshot wound or a stabbing wound.

Note that the popliteal VEIN follows a parallel course to the artery, and is thus similarly susceptible to injury. (not shown in diagram).

Nerve Supply (see diagram above)

The sciatic nerve divides into the tibial nerve and the common peroneal nerve. The peroneal nerve supplies branches to the knee. It comes to lie on the neck of the fibula, and can actually be rolled under the fingers. As it lies so close to the skin (and on bone at that) it is particularly prone to injury, the result of which can be foot drop as it also supplies the muscles which dorsiflex the foot.

Popliteal fossa

The popliteal fossa is the diamond shaped hollow at the back of the knee. It is important because several important structures pass through here. The ones to remember here are the popliteal artery and vein, and the tibial and common peroneal nerves.

On either side it is easy to palpate the medial border (tendon of semitendinosus and semimembranosus) and lateral border (tendon of biceps femoris). These make up the tendons of the hamstring muscles. Severing these tendons will effectively cripple that limb – hence the expression ‘to be hamstrung’.

Photo 1: Popliteal fossa. On either side of the circle are the tendons of the hamstring muscles.
Photo 1: Popliteal fossa. On either side of the circle are the tendons of the hamstring muscles.

The knee has six degrees of freedom: flexion-extension, internal-external rotation, and abduction-adduction.

Flexing the knee is performed by the hamstring muscles and to a lesser degree by other smaller muscles such as gracilis and sartorius. Flexion is limited to 150 degrees, due to the soft tissues behind the knee compressing one-another.

Extension is achieved by contracting the quadriceps and is aided by tensor fascia latae. The knee in full extension is actually hyper-extended around 5 – 10 degrees.

There is limited external and internal rotation, and limited abduction and adduction which make the knee prone to injury if force is applied in these planes.

Two important terms when discussing knee mechanics and knee injuries need to be defined.

  1. Valgus means angled outward or away from the midline. A valgus force will cause the knee to angle outwards.
  2. Varus means to angle inwards or toward the midline. A varus force will cause the knee to angle inwards.

Tibial Plateau Fractures

The tibia is a triangular shaped bone, and although strong, is the most commonly fractured long bone. It takes 85% of the load in weight bearing. Tibial plateau fractures are fractures through the articular surface of the tibia. As with any articular fracture, they can lead to long term disability.

It is commonly referred to as a bumper fracture due one common mechanism of injury although these account for only 1 in 4 tibial plateau fractures.

The mechanism for fracturing the tibial plateau is either

  1. Force directed medially (varus) or laterally (valgus) e.g. leg stamp to the side of the knee.
  2. Compressive axial force (fall from a height)
  3. Combination of both the above

Essentially the femoral condyles exert force onto the medial and lateral portions of the tibial plateau, effectively ‘crushing’ them. The resulting injury is mostly either a split fracture or a depression fracture (see x-ray 3 below)

X-ray 3: Tibial plateau fracture of the left knee (lateral plateau).
X-ray 3: Tibial plateau fracture of the left knee (lateral plateau).

Lateral plateau

  • More common
  • 80 -85%
  • Not as strong as the medial plateau
  • Associated with MCL/ACL rupture

Medial Plateau

  • More force required
  • Can lead to popliteal artery injury, anterior tibial artery, peroneal nerve paralysis and PCL injury

Simply leg stamping the knee either laterally or medially can produce this fracture, though the medially directed force is more likely to injure. However, a laterally directed force i.e. kicking the inside of the knee, if sufficiently powerful will result in a much more serious and possibly limb threatening injury. The leg does NOT need to be fully extended in order to be injured in this way.

Fibula fractures

This was touched on in the previous ankle article. A laterally directed strike to the head of the fibula will result in a transverse fracture the majority of the time and can stretch the peroneal nerve causing paralysis and subsequent ‘foot drop’.

Patellar fractures

1 in 100 of all fractures

Result of

  • direct trauma
  • indirect trauma

Patellar fractures are often a combination of the two mechanisms. Direct trauma such as a strike to the knee, or falling onto a hard surface e.g. concrete, usually results in what is known as a stellate (‘star shaped’) fracture. This arises as the patella is forced onto the femoral condyles. Although comminuted, they are rarely displaced, the patellar fragments being embedded as they are in the tendon. Articular surfaces are damaged and this has serious implications for future function.

In contrast, indirect trauma e.g. eccentric contraction of the quadriceps usually results in a transverse fracture. This can be brought about by sudden flexing of the knee while the quadriceps is fully contracted as in jumping. How could this happen in a combat scenario. One hypothetical situation would be kicking the lower leg out from under the person while that leg is fully extended (attacking from the front to rear). A complete transverse fracture WILL result in displacement, and may lead to an inability to straighten the leg.

Having said the above, the x-ray below shows a transverse fracture and complete separation of the two fragments caused by a direct blow to the knee. This man was unable to extend his knee as a result of this fracture.

X-ray 4: Transverse fracture of the patella after a direct blow to the knee.
X-ray 4: Transverse fracture of the patella after a direct blow to the knee.

Patellar dislocations

These are almost always laterally displaced. The mechanism is forced rotation of the femur on a fixed and externally rotated tibia while the knee is flexed. Another potential scenario is stumbling with the knee partly flexed, or when suddenly changing direction while running. Patellar dislocations are unlikely to occur as a result of direct trauma.

Photo 2: Dislocated left patella.
Photo 2: Dislocated left patella.

Having dislocated the patella once, the risk of recurrent dislocations becomes greater.

Knee dislocation

Knee dislocations are true orthopedic emergencies as the potential for loss of the affected limb is high. This is usually due to disruption of blood supply to the lower limb via the popliteal artery. It is important that the combat practitioner realize that this is an entirely different entity to a patellar dislocation. If the injured popliteal artery is not repaired within the first 8 hours, there is a 9 in 10 chance the leg will have to be amputated.

A knee dislocation occurs when the femur and tibia articulation is disrupted. The literature describes 5 classes of knee dislocation, all of which are based on the position of the tibia in relation to the femur. For example, a posterior dislocation results in the femur sitting forward of the tibia.

The five types of knee dislocation are

  1. anterior (most common)
  2. posterior (2nd most common)
  3. lateral
  4. medial
  5. rotational

The direction of the applied force (e.g. foot strike) determines what type of dislocation will occur. In photo 3 below, the posteriorly directed kick would result in posterior dislocation.

Photo 3: an axe kick to the front of  the leg as demonstrated can result in posterior dislocation of the knee.
Photo 3: an axe kick to the front of the leg as demonstrated can result in posterior dislocation of the knee.

Almost all types of dislocation have rupture of both cruciate ligaments.
The collateral ligaments may or may not be disrupted to varying degrees.

Knee Dislocations: Associated Injuries

Popliteal artery disruption occurs in up to 60% of cases and is limb threatening.
Popliteal nerve injury occurs in 20 – 40% of knee dislocations, half of these being permanent.

Ligamentous injuries

Meniscal Injuries
Meniscal tears come about when a rotational force is applied to the loaded joint. The femoral and tibial condyles shear the meniscus. The medial meniscus is injured more commonly than the lateral meniscus (3 times as often) as it is less mobile. In both menisci, posterior tears are more common than anterior tears. The usual mechanism is ‘twisting’ the knee.

The symptoms include pain – located at the joint line, swelling, and ‘locking’ of the knee joint.

Any movement in which the lower leg is fixed and the femur/upper body rotated can cause a meniscal tear. Like wise, applying a rotatory force to the tibia while the subject is prone on the ground with knee flexed can cause meniscal tears. This injury is not as immediately debilitating as a dislocation or bony fracture. Note also that a meniscal tear cannot occur when the leg is fully extended.

Cruciate Ligament Injuries

The cruciate ligaments as mentioned previously are the most important stabilizing ligaments for anterior and posterior movements of the knee.

Anterior Cruciate Ligament Injury

The anterior cruciate ligament (or ACL) is the most commonly injured major ligament of the knee. In the U.S. more than 95 000 ruptured ACL’s are reported annually. 50% of ACL tears have associated meniscal injuries, the lateral meniscus being more commonly involved in acute trauma.

Mechanisms of anterior cruciate ligament tears are divided into contact and non-contact. Contact injuries may arise from a direct blow to the knee from any angle, usually when it is fixed, flexed and rotated. A high-low wrap combined with a foot strike to the knee will cause this. It is also possible to tear the ACL by causing excessive flexion or extension.

Non-contact ACL rupture occurs during ‘cutting’ moves e.g. when an athlete changes direction so that the body is still twisting while the foot is fixed in the new direction.

Usually there is a sudden onset of pain, immobility, and the knee will ‘give way’. 2/3rd of patients report hearing a ‘pop’

Posterior Cruciate Ligament Injury

Injuries to the posterior cruciate ligament are less common than those of the ACL, and are less likely to be the result of direct trauma, though direct trauma from just about any angle to the knee can cause PCL rupture if severe enough. The most likely mechanism is excessive force applied to the front of the knee at the level of the tibia. It usually requires 30 degrees of hyperextension to rupture the PCL, and a further 20 degrees will result in rupture of the popliteal artery.

Collateral Ligament Injuries

Collateral ligament injuries account for about 1 in 4 presentations to the emergency department with acute knee injury, so they are quite common. They are caused by excessive valgus or varus forces (MCL and LCL respectively), and can also be caused by excessive lateral rotation of the knee.

Medial Collateral Ligament Injury

As noted previously, the MCL functions to stabilize the knee joint by preventing excessive valgus, i.e. it prevents the knee ‘opening up’ on the medial side. The MCL is the most common isolated knee ligament injury, and is the injury most commonly associated with ACL injury.

Most common mechanism is excessive valgus force applied to the lateral aspect of the knee, e.g. when a rugby player gets clipped on the outside of the leg. This then opens the angle between the femur and tibia, stretching the MCL leading to tear or complete rupture.

In combat, a strike to the outside of the knee directed medially will cause MCL rupture. (See photo 4) Note that the knee does not need to be fully extended i.e. straight for injury to occur. With the knee partially flexed, it is still easy to rupture the MCL +/- the ACL. This has obvious implications in combat in that a strike to the knee from the outside will cause injury regardless of the degree of leg extension in the upright position.

Photo 4: Medially directed kick (exerting a valgus force), with disrupted red lines representing ruptured MCL.
Photo 4: Medially directed kick (exerting a valgus force), with disrupted red lines representing ruptured MCL.

The x-ray below demonstrates a medial femoral condyle fracture (which can be seen) with an associated MCL rupture (cannot be seen)

X-ray 5: Arrow points to avulsed bony fragment from the medial femoral condyle with certain MCL rupture.
X-ray 5: Arrow points to avulsed bony fragment from the medial femoral condyle with certain MCL rupture.

Lateral Collateral Ligament Injury

In a similar but directionally opposite manner, the LCL can be ruptured by excessive force applied to the medial aspect of the knee. The injury is less common than MCL injury for a couple of reasons.

  • The opposite leg shields the medial aspect of the knee in question.
  • The ligament is more mobile, so has a small amount of ‘give’.

Because greater force is required to rupture this ligament than the MCL, it is often accompanied by other injuries, especially common peroneal nerve injury, and rupture of the biceps femoris (hamstring) tendon.

In a clinch or wrap, for example, a kick to the inner aspect of the knee can cause LCL and associated injury.

The “Terrible Triad”

To complete this section on ligamentous injuries, a trio of injuries known as the terrible triad is worth mentioning. A lateral blow to the knee in slight flexion and rotation can cause rupture to the MCL, the ACL, and the medial meniscus. This is a serious injury requiring surgery.

The photo below shows such a strike, and the direction of force is the most destructive leading to maximal injury, both bony and ligamentous.

Photo 5: Force directed posteromedially. This will result in the terrible triad of injuries, in addition to probable bone fracture.
Photo 5: Force directed posteromedially. This will result in the ‘terrible triad’ of injuries, in addition to probable bone fracture.

Summing Up
In brief, the knee can be injured with any direction of applied force in either flexion or extension. A laterally directed force will injure structures on the lateral aspect of the knee, and vice versa for a medially directed strike. The same holds true for anterior and posterior strikes, although the anterior strike directed backwards is much more likely to cause injury. The amount of force need not be excessive, but it is wise to remember that the direction is critical if the maximum amount of damage is desired, that being posteromedially or as in the photo above.

Article written by Dr Stefan Eriksson