UNIT VII – The Knee


 At the completion of this unit the student will be able to:

      1.                  Identify the key bony structures of the knee joint.

2.                  Identify the articulations that comprise the knee joint.

3.                  Identify the key ligaments, capsule, and menisci about the knee.

4.                  Name the anatomical motions of the knee.

5.                  Name the key muscles that perform the motions about the knee, including the anatomical attachments.

6.                  Describe the role of the patella to knee function.

7.                  Describe the interrelationship of the static and dynamic knee stabilizers and their roles in stability of the knee joint.

8.                  Integrate knowledge of the knee to important lower extremity issues for the dancer.


Bony Structures of the Knee

The knee joint is the center joint that makes up the lower extremity kinetic chain. It is midway between the pelvis/hip complex and the foot/ankle.
The knee joint is comprised of three bones: the femur, the tibia, and the patella. The distal end of the femur articulates with the tibia in the lower leg and the patella or knee cap. The tibia below does not articulate with the patella. The distal end of the femur and the proximal aspect of the tibia splay out to provide a more stable base, like the column of a building.  The fibula, though not part of the knee joint proper, also plays a role in muscles attachment and overall range of motion at the ankle/knee complex.
The Distal Femur
As you recall from the previous unit on the hip, the main length of the femur is called the shaft. The distal end that forms part of the knee articulation consists of several important  bony landmarks, including the lateral and medial epicondyles. These are the more proximal aspects of the bony protrusions.
 Just distal to the epicondyles are the lateral and medial condyles of the femur. These condyles articulate with the tibia below.

Between the condyles is the patellar surface that articulates with the patella or knee cap. These landmarks are important attachments for ligaments and muscles which will be discussed later in this unit.

On the posterior aspect of the distal femur is the intercondylar fossa.  This is a large posterior opening between the two condyles.
The Tibia and Fibula
The tibia and fibula comprise the two bones of the lower leg. The tibia is the larger of the two bones and articulates with the femur to form the primary component of the knee joint. The tibia is wide at the proximal and distal ends to increase stability of  the knee and ankle.
The proximal tibia has lateral and medial condyles that fit against the structures of the same name on the distal femur above. The medial condyle is larger than the lateral condyle. The intercondylar eminence is a bony ridge that separates the two condyles and fits into the intercondylar fossa of the femur during flexion.(See picture below)
There is a large tibial tuberosity on the anterior surface of the proximal tibia. This tuberosity is an important attachment for muscle. Also on the lateral surface is the interosseus border for attachment with the interosseus membrane that connects the tibia and fibula along the length of their shaft. The anterior shaft of the tibia is also called the “shin” and has a sharp ridge.  The distal end of the tibia and fibula articulate to form the ankle joints and will be discussed in Unit VIII.
The Patella
© The patella is a sesamoid bone that develops within the tendon of the quadriceps muscle. Sesamoid bones are round or oval nodules that develop in certain tendons. They were originally called sesamoid bones because of their resemblance to sesame seeds. They are commonly found where tendons cross the end of long bones (as in the knee) and serve to protect the tendon from excessive wear. 
The sesamoid bone, the patella, also changes the angle of the tendon as it passes to its attachment, thus providing better mechanical advantage for the muscle.  The broad superior surface of the patella is called the base and the more pointed inferior edge is called the apex. The patella is shaped in a triangular pattern if viewed from the side. The posterior surfaces that contact the patellar surface of the femur are the bottom two angles of the triangle and are called the articular facets. These facets are covered with cartilage.

Other Important Anatomical Knee Structures

Menisci of the Knee

Another important anatomical component of the knee are the menisci. These structures are also called knee “cartilages”. They are ‘C’ shaped intra-articular discs that are made of fibrocartilage. The menisci are thin pads made of strong, fibrous tissue that rest between the condyles of the femur and the condyles of the tibia. They act as shock absorbers between the bony surfaces.

The menisci are slightly mobile and aid in spreading synovial fluid during movements of the knee. There is a lateral and a medial meniscus in each knee. These menisci also attach to the intercondylar eminence of the tibia, as well as to portions of ligament and muscle around the knee joint. The menisci provide a greater surface for articulation with the femoral condyles and promote better distribution of forces at the knee and provide knee stability. 
Without the menisci for shock absorption there is difficulty with activities such as running due to the wear and tear of bone-on-bone as the femur contacts the tibia. Also, there can be some loss of knee stability if a menisci is removed completely.


The Knee Joint Capsule

The capsule of the knee joint attaches just outside the articular surfaces of the three bones that make up the knee joint. The patella is contained within the anterior capsule.  There is a single synovial cavity that encapsulates the femur, patella and tibia and synovial fluid circulates within the capsule. 

The capsule has folds, especially in the anterior aspect and thus allow full knee flexion motion.  The posterior aspect of the capsule is thicker and stronger and provides for stability in the standing position. The knee joint has two fairly flat surface bones that rest on one another as the primary articulating surfaces. Therefore, the joint is unstable in its bony configuration. The knee relies heavily on other static stabilizers such as the capsule and the ligaments, as well as the dynamic stabilizers, the muscles, for stability at the joint. 

Let's take a look at the static stabilizers, the ligaments, now. There are 4 important ligaments in the knee area.

Ligaments of the Knee

The Cruciate Ligaments
There are two cruciate ligaments and two collateral ligaments of the knee. The cruciates or “crossed” ligaments are located in the intercondylar fossa of the femur. They are named according to where they attach below on the tibia. The anterior cruciate ligament is attached to the anterior intercondylar area of the tibia. It runs posteriorly and superiorly and laterally to attach to the medial aspect of the lateral femoral condyle.
The posterior cruciate ligament, which is stronger than its anterior counterpart, attaches to the posterior intercondylar area of the tibia. It runs anteriorly, superiorly, and medially to attach to the lateral surface of the medial femoral condyle. The anterior cruciate resists anterior displacement  or shifting of the tibia on the femur. The posterior cruciate resists posterior displacement or shifting of the tibia on the femur. The two ligaments together form an “X” inside the knee joint capsule.

If a cruciate is damaged the knee can become quite sloppy and unstable. The cruciates, especially the anterior, are often injured when the foot is planted and the individual pivots above, thus stressing the ligaments too far. Once torn, the individual will usually have the sensation that the knee is slipping or going out from under them.


The Collateral Ligaments
The collateral ligaments are located on the sides of the knee joint and serve to reinforce the knee joint capsule. The medial collateral ligament runs from the medial epicondyle of the femur to the medial condyle and upper medial shaft of the tibia. The medial collateral ligament functions to stabilize the joint and prevent it from opening on the medial (inside aspect of the knee). The lateral collateral ligament runs from the lateral epicondyle of the femur to the head of the fibula. This ligament prevents the joint from opening on the lateral (outer) side of the knee.
If either one of these ligaments is damaged or ruptured the knee will be unstable side-to-side, depending on which side was damaged. The medial collateral ligament is thicker than the lateral collateral ligament. This is due to the typical angle of the knee from the hip. This angle places more of a stress on the medial aspect of the knee joint, thus the need for further stabilization on that side of the joint. The collateral ligaments also serve to resist hyperextension of the knee and provide some stabilization for medial and lateral rotation of the tibia on the femur.


Motions of the Knee Joint  

The knee joint is primarily a hinge joint. It has two major motions – flexion and extension. There is, however, some medial and lateral rotation of the knee, especially when it is in a flexed position.

Extension - This occurs when the leg is straightened. There is 0 degrees of bend in the knee between the lower leg and the thigh. In this position all of the ligaments are taut and  the joint can be stabilized with out a lot of muscular activity, i.e. balancing on one foot. 
Knee extension can be limited if the hip is flexed and the hamstrings are tight. Due to muscle shortening in the back of the leg, the person is not able to get the knee straight when the hamstring is stretched.  It is also easy to produce hyperextension of the knee if the individual has a lot of flexibility at the joint.(See picture of dancer above)
Relevance to the Dancer- Because dancers tend to be very flexible, knee hyperextension is very common. In this situation the dancer lets the knee bow back beyond the normal extension range. The proximal aspect of the tibia is actually behind the distal end of the femur. This position puts a great deal of stress on the posterior capsule and the ligaments of the knee. It also moves the good axis of alignment so that the knee joint rests behind that line for the leg. It is not functional, or desirable, and can result in injury or chronic pain in the knee.
Flexion – This occurs when the tibia swing into a bent position and moves toward the back of the thigh.  Normal active knee flexion is about 135 degrees. It is limited by contact of the muscles in the back of the calf and the leg. When the knee is bent passively there is additional available motion. There is also less knee flexion when the hip is not bent, due to the muscle attachment of the rectus femoris that crosses both joints.  
Medial and Lateral Rotation of the Knee – When the knee is extended there is not much rotation at the knee joint because the ligaments are taut and hold the femur stable on the tibia.  When the knee is flexed the ligaments are relaxed. There is some normally occurring knee medial and lateral rotation of the tibia below the femur during normal walking. This is due to the shape of the knee joint condyles. Medial rotation is when the tibial tuberosity moves medially (or inwardly) while lateral rotation is when the tibial tuberosity moves laterally (or outwardly). 
Relevance to the Dancer - Remember this is the tibia below the femur that is rotating.  It is important to distinguish this motion from hip medial and lateral rotation that occurs above at the hip joint.  It is very important that dancers maintain outward rotation by controlling it from the top of the femur with the lateral rotators, not the knee joint.  “Twisting the tibia” by forcing outward rotation from the feet will cause injury to the knee joint.


Muscles of the Knee

The muscles that move the knee joint will be discussed relative to the motions they perform at the knee. 

Knee Extensors

The primary knee extensor is the quadriceps femoris. It is a very large muscle group comprised of 4 muscles. This muscle group as a whole  inserts into a single quadriceps tendon at the knee. The tendon inserts on and surrounds the patella bone, then continues as the patellar tendon to insert on the tibial tubersosity on the tibia bone. Collectively, the four muscles acting together extend or straighten the knee.

The quadriceps femoris is the strongest muscle in the body. The four component muscles of the quadriceps femoris are the: vastus lateralis, vastus medialis, vastus intermedius (beneath the central rectus femoris), and the rectus femoris.

Vastus Intermedius – This is the deepest muscle of the four. It arises from the upper two thirds of the anterior femoral shaft. It passes along the anterior aspect of the femur in line with the bone and joins the patellar tendon below.  ©


Vastus Lateralis and Medialis – These two muscles arise from either side of the linea aspera of the femur, just inferior to the borders of the greater trochanter on the posterior femoral shaft. They wrap around the sides of the femur and meet anteriorly, superificial to the vastus intermedius. They essentially encircle the intermedius from both sides and then join to pass below to insert into the patellar tendon as well.


Rectus Femoris – This is an important two joint muscle in the lower leg. It arises from the anterior inferior iliac spine and the groove just above the rim of the acetabulum on the ilium above. This muscle, therefore, crosses the hip joint and acts on the hip joint for motion. The vastus muscles just discussed do not cross the hip, only the knee joint, and thus have no function at the hip above. The rectus femoris passes superficial to the vasti muscles to insert on the common patellar tendon that attaches at the tibial tuberosity below.

Since the rectus femoris crosses both joints it  has actions at both the hip and knee joints. When the pelvis is fixed, the rectus femoris flexes the hip and extends the knee (example is walking). It can also act to pull the pelvis forward from its superior aspect (anteversion) if the femur below is fixed.

The Role of the Patella in Knee Function

The patella bone is imbedded in the patellar tendon of the quadriceps femoris. It serves 2 primary purposes.
  • It serves as a shock absorber to protect the knee when falling forward onto the knee. 
  • It also increases the angle for pull of the quadriceps tendon since it sticks out anteriorly. This increased angle increases the force in which the quadriceps can contract. 

The patella tracks in the groove between the femoral condyles like a rope in a pully.(See picture bottom left) 

The patella is not stable laterally because it follows the shaft of the femur bone. Since there is an angular pull on the patella it is at more risk to dislocate laterally. There are a lot of stresses and strains on the patella and it is at risk for arthritis and inflammatory conditions. More information on its alignment in the lower leg will be discussed at the end of this unit.

Relevance to the Dancer- The rectus femoris is a very strong, superficial muscle and can often be substituted as a hip flexor, due to its dual function at the hip and knee. Dancers are often challenged to utilize the iliopsoas muscle group more effectively for hip flexion, rather than the rectus femoris. The iliopsoas provides more stability for the center of the trunk and pelvis. Use of the rectus as a primary hip flexor results in strong contraction of the quadriceps muscle in the front of the leg. This is not efficient nor functional for the dancer.


Other Secondary Knee Extensors

Although the quadriceps femoris is the primary knee extensor, there is some action of knee extension provided by the superficial component of the gluteus maximus and from the tensor fascia latae which have already been discussed in the previous unit.


Knee Flexors

Semimembranosus – (far left) This is one of the hamstring muscles located on the medial aspect of the posterior thigh. It arises from the ischial tuberosity and passes down the back of the thigh to insert on the posteromedial aspect of the tibial condyle. It is a wider muscle belly than its companion muscle, the semitendinosus.

Semitendinosus – (immediate left) This muscle is the other hamstring muscle located on the medial aspect of the thigh. It also arises from the ischial tuberosity. It inserts to the superomedial tibial shaft on the posterior knee below. Its insertion is distal and medial  to the insertion of the semimembranosus.

© This picture to the left is a posterior view of both the semimembranosus and the semitendinosus.  They are powerful knee flexors. They also help to maintain the position of the pelvis (they attach to the "sitz bones) on a standing straight leg.  


The hamstrings play a role in extension of the leg (especially from a flexed position). They also serve as medial and lateral rotators at the knee that will be discussed later in this unit. 
Biceps Femoris –  This is the hamstring muscle group located laterally on the posterior thigh. It is comprised of two heads.  The long head arises from the ischial tuberosity. The short head arises from the posterior femoral shaft more distally. The two heads merge inferiorly in the posterior thigh area and insert via a common tendon to the head of the fibula. These hamstring muscles, like the rectus femoris in the front of the thigh, cross both the hip and the knee joints, and thus act at both joints.  ©
Relevance to the dancer:  Tight hamstrings can be stretched effectively by hanging over, hands to the floor. Allow gravity to lengthen the spine and elongate the back of the legs.  If it is difficult to maintain extended knees, then soften or bend the knees until the stretching process allows the muscle to lengthen. Maintain the alignment of hip to knee to ankle as seen to the left.


Secondary Knee Flexors

There are two other muscle groups that contribute to knee flexion: the popliteus and the gastrocnemius.
Popliteus – The popliteus is a small muscle that originates from the lateral aspect of the lateral femoral condyle and passes diagonally to insert on a triangular area of the posteromedial tibial shaft below. It flexes and medially rotates the knee.
Gastrocnemius – This muscle originates from two heads on the posterior/inferior femur, just above the condyles. It crosses the knee joint posteriorly and attaches on the calcaneus (heel bone) below. The gastrocnemius is primarily a muscle that functions at the ankle joint. However, due to its attachment on the femur, it also serves as a flexor and medial  rotator of the knee.
As previously noted, the primary motions at the knee are flexion and extension. During knee flexion, however, there is some medial and lateral rotation of the knee. Several muscle group contribute to these motions.


Medial and Lateral Rotators of the Knee

The sartorius, the medial hamstrings – semimembranosus and semitendinosus, the gracilis (an adductor of the hip), and the popliteus all contribute to medial rotation at the knee. 

The tensor fascia latae of the lateral leg, the superficial component of the gluteus maximus, and the lateral hamstring – the biceps femoris, contribute to lateral rotation of the knee.


Important Biomechanics of the Knee and Leg

  • As you can see from the discussion of muscles both at the hip and at the knee, there are many muscles that cross both joints and thus have multiple functions for lower body movement. This is also true for the ankle that will be discussed in Unit VIII. Since the primary function of the lower body is weight bearing for walking and locomotion through  space, there is integration of all the structures in order for this to happen efficiently.
  • There are several important considerations at the knee that relate to this issue. The first is the relationship of the static and dynamic stabilizers around the knee joint. As has been mentioned, the knee joint is not inherently stable from its bony structures. It must rely on the other structures to provide that stability. The ligaments, capsule, and menisci are considered the “static” stabilizers that provide holding stability at the joint. The muscles that function about the knee are the “dynamic” stabilizers that actively contract to provide both motion and muscular stability at the knee.
  • For optimal knee function it is important for the individual to have good integrity of the static stabilizers and good strength in the dynamic stabilizers. Unfortunately, there is a high risk of injury to the static stabilizers. These are usually injured when the foot is planted and the individual twists or pivots the body above. When this occurs there can be damage to the menisci, ligaments, or the capsule. When this occurs, significant instability in the knee results. It then becomes the job of the dynamic stabilizers (the muscles) to take up the slack to stabilize the knee.
Relevance to the Dancer It is very important for dancers and athletes to have strong lower extremity muscles to protect the static stabilizers. Maintaining good alignment of the knee under the trunk and over the foot with activities will also minimize the risk of knee injury.


The Q Angle of the Knee
Another important biomechanical consideration at the knee is the Q angle. The Q angle is the angular measurement between the line of the thigh as is passes down from the hip to the knee and the angle of the lower leg passing down through the midline of the tibia.  As you recall from the discussion on the hip joint, the head of the femur is angulated upwardly to articulate with the acetabulum. This angle places the angle of the thigh as it passes to the knee joint at an inward angle, not a straight line toward the floor.
This angle is more marked in females because they have a wider pelvis for childbirth. The normal angle for men is 10-13 degrees and for women 13-18 degrees. If this angle is larger it can place the individual at risk for patellar instability and inflammatory conditions around the knee. This can be important for the female dancer if she has a large Q angle. It is a bony configuration that cannot be changed. It is important to recognize that this may place her at risk for knee conditions, and she should be especially careful with good lower leg mechanics when dancing. Maintaining strong leg muscles will also be helpful. 
The knee is an important component for walking. This aspect will be discussed in detail after the next unit on the ankle. It is important to understand the anatomy structures for the hip, knee and ankle before tackling the biomechanics of walking and locomotion. 

This concludes Unit VII. Return to Blackboard to complete Quiz #3 for Unit VI & VII.

After completing Quiz #3 you will take the Mid-Term Exam, covering Units I-VII, before proceeding to Unit VIII. See the syllabus schedule.