Chapter 12

Knee Joint Complex

Learning Outcomes: Students will be able to:

  1. Describe the musculoskeletal anatomy of the knee joint complex and associated connective tissues that support these joints.
  2. Identify the functional design of the knee and patellofemoral joint and the osteokinematic and arthrokinematic movements possible at the joints.
  3. Describe the origin, insertion, actions, and nerve innervation of the muscles that act on the knee joint.
  4. Identify how to strengthen and stretch each of the muscles or muscle groups that cross and act on the knee joint.
  5. Describe anatomical deviations of the knee or the segments that make up the knee joint.
  6. Describe mechanisms of injury for selected knee pathologies and potential long term consequences.

The knee joint complex consists of the Tibiofemoral joint (ginglymus/hinge joint) and the Patellofemoral joint (Arthrodial/gliding joint). The primary joint is the Tibiofemoral joint which is the articulation between the medial and lateral condyles of the femur and the medial and lateral plateaus of the tibia.

Closed Pack Position: Fully extended                  Open Pack Position: Flexed 25º

The Fibrous Joint Capsule of the knee joint extends from the distal femur to the proximal tibia. As such, when there is knee trauma the pattern of swelling can aid in determining what structures are damaged.

Effusion: Intra-capsular tissue damage: Swelling will involve the entire knee and extend above the knee cap.

Edema: Extra-capsular tissue damage: Swelling will be more localized.

The capsule of the knee joint is somewhat lax allowing for significant range of motion. Therefore, it is reinforced by many ligaments, muscles, and fascia. They are:

  • Anterior capsule: distal quadriceps tendon, patella and its infrapatellar ligament, and the expansion of the quadriceps muscles
  • Posterior capsule: oblique popliteal ligament, arcuate popliteal ligament, and the popliteus, gastrocnemius, and hamstring muscles
  • Lateral capsule: lateral collateral ligament, iliotibial band, and vastas lateralis muscle
  • Medial capsule: medial collateral ligament, the pes anserine muscle group, and the vastus medialis muscle

Joints and Connective Tissues of the Knee


Tibiofemoral (ginglymus or ginglymotrochoidial)

Patellofemoral (arthrodial)


Tibial (Medial) Collateral Ligament: A flat broad band ligament that attaches from the medial epicondyle of the femur to the proximal medial condyle of the tibia. (also has a medial meniscus attachment) This ligament becomes taut during knee extension, protecting against a valgus stress to the knee.

Fibular (Lateral) Collateral Ligament: A cord-like ligament that attaches from the lateral femoral epicondyle to the styloid process of the head of the fibula. This ligament also becomes taut during knee extension, protecting against a varus stress to the knee.

 Figure 2       

Joints and Connective Tissues of the Knee cont . . . .

Anterior Cruciate Ligament: Connects the anterior aspect of the intercondylar eminence of the tibia; runs superiorly, laterally, and posteriorly to attach to the medial aspect of the lateral femoral condyle. This ligament protects against anterior displacement of the tibia on the femur, and conversely, posterior displacement of the femur on the tibia. It also tightens on knee extension limiting hyperextension, and also extreme knee flexion. Provides rotary stability.

Posterior Cruciate Ligament: Connects the posterior aspect of the intercondylar fossa of the tibia; runs superiorly, medially, and anteriorly to attach to the anterolateral aspect of the medial condyle of the femur. This ligament protects against posterior displacement of the tibia on the femur, and conversely, anterior displacement of the femur on the tibia. It also tightens on knee flexion. Provides rotary stability.

Oblique Popliteal Ligament: Attaches posteriorly from the lateral femoral condyle to the distal tendon of the semitendinosus muscle. Reinforces the posterior knee joint capsule and resists full knee extension.

Medial Meniscus (C shaped): Located on top of the medial tibial plateau. Has attachments to the MCL and capsule. Because of this, its movement is restricted and is more susceptible to injury.

Lateral Meniscus (circular shaped): Located on top of the lateral tibial plateau. Has attachments to the arcuate popliteal ligament and popliteus muscle. Able to move more than medial meniscus and is less susceptible to injury.


Both menisci are thicker on their periphery compared to the internal edge of the meniscus and as such serve to deepen the joint to enhance stability, congruency, and to absorb shock. They carry 70% of the weight bearing load on the tibia. Only the outer 1/3 has vascularization and therefore is very poor at healing.

The open ends are named “horns” and are secured to the tibia via horn ligaments. Coronary ligaments attach the periphery of the menisci to the tibial condyles. The Transverse ligament attaches the anterior horns of the two menisci to each other.

The menisci are somewhat mobile with the greatest mobility occurring in the lateral meniscus. They move posteriorly during flexion and anterior during extension. They tend to follow the femoral condyles during rotation.


Figure 3

NOTE: ACL injury is often a non-contact event where the foot is planted, a valgus stress is applied at the knee while cutting combined with a later rotation of the knee. Hip adduction and internal rotation often accompanies this injury.

Figure 4

Knee Joint Movements and Range of Motion

Flexion: 135° ,                                              

Extension: 0°

Rotation (with knee flexed 30° or more):

  • 15° internal rotation
  • 30° external rotation

Flexion: decreasing the angle between the femur and lower leg, characterized by the heel moving towards buttocks.

Extension: increasing the angle between the femur and lower leg, characterized by heel moving away from buttocks.

Internal Rotation: rotary movement of the lower leg medially toward the midline.

External Rotation: rotary movement of the lower leg laterally away from the midline

    Figure 5        

Screw Home Mechanism: During the terminal degrees of knee extension, the tibia must externally rotate approximately 10º. Purposes: 1) align the femoral condyles with two knee joint menisci, and 2) to situate the foot/ankle so push off occurs at the 1st MTP joint for greater power. The combination of knee extension and external rotation results in tightening of both cruciate ligaments, which locks the knee for greater stability.

  • Knee external rotation must occur if the femur is fixed
  • Hip internal rotation must occur if the tibia is fixed

Note: The screw home mechanism does not need muscle contraction to occur during knee extension. However, “unlocking” the knee joint to perform knee flexion requires muscular contraction and is primarily accomplished by the popliteus muscle.

  • If the femur is fixed, the popliteus internally rotates the knee as knee flexion begins
  • If the tibia is fixed, the popliteus externally rotates the femur as knee flexion begins


Figure 6

Accessory Structures of the Knee

Patellar Fat Pad and Bursae

  1. Shock absorption
  2. Joint lubrication

Knee Joint Complex Major Bursae

  1. Suprapatellar
  2. Prepatellar
  3. Superficial Infrapatellar
  4. Deep Infrapatellar
  5. Pes Anserines
  6. Semimembranosus (Baker’s Cyst)


Figure 7                                                     Figure 8

Angulations and Deviations at the Knee Joint

Genu Varum (Bow Legged) Medial deviation of distal tibia

Potential Causes:

  • Coxa Valga
  • Weight bearing on a metabolically weak bone (Rickets, Paget’s Disease)
  • Bone irregularities
  • Lead or fluoride poisoning

Treatment: Braces, casts, special shoes

Possible complications: Early knee arthritis

Figure 9


Genu Valgum (Knock Knees) Lateral deviation of  distal tibia

Potential Causes:

  • Coxa Vara
  • Injury to shin bone or shallow medial femoral condyle
  • Overweight and obesity
  • Rickets

Treatment: Bracing, but usually not treated

Possible complications: Early knee arthritis

Figure 10


Genu Recurvatum (Backward bow of legs)

Potential Causes:

  • Weak ligamentous structures
  • Weak hamstrings
  • Tight quadriceps
  • Equinus

 Figure 11  


Q Angle (quadriceps angle)

The angle formed by the intersection of:

  1. Quadriceps line of pull – ASIS to center of patella
  2. Patellar tendon line of pull – Center of patella to center of tibial tuberosity
  1. Normally <10° for men, <15° for women
  2. Q Angles that are >20° are associated with an increased risk for ACL injury, chondromalacia patella, and patella dislocation/subluxation.

Figure 12


Patellofemoral Joint (arthrodial)

The patellofemoral joint is formed by the articulation between the posterior surface of the patella and the intercondylar groove of the femur. The lateral surface of the patella is shallower than the medial surface of the patella. This is one of the reasons that patellar dislocations occur laterally. Movement of the patella in this groove is referred to as “tracking” and is a nonaxial gliding movement. If the proper sliding of the patella up and down in the intercondylar groove is altered, “improper tracking” will occur and can contribute to a breakdown of the patellar articulating cartilage surface and lead to a condition known as patellofemoral pain syndrome. Contributing factors to this condition include: Weak quadriceps, patella Baja, Q- Angle >20º.


The patella acts as an anatomical pulley, changing the line of pull and increasing the leverage and force that the quadriceps muscle group exerts on its insertion point at the tibial tuberosity. Without the mechanical advantage provided by the presence of the patella the quadriceps muscle group would lose approximately 30-50% of its torque. In addition, the patella’s dense, tough articular cartilage on its articulating surface helps protect against friction that would occur if the tendon itself had to slide across the distal surface of the femur, and is designed to properly tract or glide smoothly in the intercondylar tract.

Patellar Length to Patellar Tendon Length Ration

The proper ratio is 1:1 (patella length equal to length of the patellar tendon).

Patella Alta: Ratio >1:1 (patellar tendon longer)

  • Camel or Hump Sign (patella & fat pad)
  • Increases risk for patellar subluxation

Figure 15 

Patella Baja: Ratio <1:1 (patellar tendon shorter)

  • Increases risk for chondromalacia patella


Figure 16

Femoral vs. Tibial Torsion

  1. Types of torsion. “Distal part is the reference”

              Internal Tibial, External Tibial, Internal Femoral, External Femoral

  1. Testing procedure

      Step #1: Determine that a torsion is present while subject is

                     standing (patella and feet do not line up)

      Step #2: Have subject sit on table with feet dangling free.

  • Femoral torsion: Feet point straight ahead
  • Tibial torsion:         

            -Feet point out = external tibial torsion

            -Feet point in = internal tibial torsion

Note: Combinations: Most common is external tibial-internal femoral torsion

Figure 17

Muscles of the Knee (Those not presented in the Pelvis and Hip section)

Vastus Lateralis




Nerve Innervation:

Vastus Intermedius




Nerve Innervation:

Vastus Medialis




Nerve Innervation:





Nerve Innervation:


Figure 18

Figure 19

A Few Knee Joint Review Questions

  1. What structures are involved in forming the Q-angle?
  2. What is the pes anserine?
  3. Describe the MOI for most ACL injuries/tears.
  4. Describe the screw home mechanism motion specific to tibia movement on femur and femur movement on tibia.
  5. What is the direction of the arthrokinematic glide/slide at the knee during the downward movement of a squat?
  6. How would you personally position (supine or prone) a patient to stretch their rectus femoris muscle? Explain why

you chose the position you did.

  1. Can the principle of neutralization take place in the knee joint? If so, explain.
  2. What class lever system is working at the knee during seated knee extension?

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