Knee 2

• Joint Kinematics

  • Primary angular (or rotary) motion of the tibiofemoral joint
  • Translatory motons of the tibia on the femur
• Primary angular (or rotary) motion of the tibiofemoral joint

  • Flexion/extension
  • Medial/lateral (internal/external) rotation
  • Varus/valgus (adduction/abduction)
• Translatory motons of the tibia on the femur

  • Anteroposterior translation
  • Medial and lateral translations
• Flexion/Extension


  1. Axis is a horizontal line passing through the femoral epicondyles

  2. Initiation of knee flexion occurs primarily as posterior rolling of the femoral condyles on the tibia that moves the contact of the femoral condyles posteriorly on the tibial plateau

  3. As flexion continues, the rolling of the femoral condyles is accompanied by a simultaneous anterior glide that is just sufficient to create a nearly pure spin of the femur on the posterior tibia with little linear displacement of the femoral condyles after 25° of flexion

  4. Extension occurs initially as an anterior rolling of the femoral condyles on the tibial plateau, displacing the femoral condyles back to a neutral position on the tibial plateau

  5. After the initial forward rolling, the femoral condyles glide posteriorly just enough to continue extension of the femur as an almost pure spin of the femoral condyles on the tibial plateau
• When the tibia is flexing on a fixed femur, the tibia both rolls and glides posteriorly on the relatively fixed femoral condyles
• Extension of the tibia on a fixed femur incorporates an anterior roll and glide of the tibial plateau on the fixed femur
• Passive range of knee flexion

  Generally considered to be 130° to 140°
• Deep knee squatting


  • Knee flexion may reach as much as 160° as the hip and knee are both flexed and the body weight is superimposed on the joint
• Normal gait on level ground

  Requires approximately 60° to 70° of knee flexion
• Ascending stairs


  Requires about 80° of knee flexion
• Sitting down into and arising from a chair


  Requires 90° of knee flexion
• Knee joint extension (or hyperextension)

  Up to 5° is considered within normal limits
• Excessive knee hyperextension


  Beyond 5° of hyperextension, is termed genu recurvatum
• Many of the muscles acting at the knee are two-joint muscles crossing not only the knee but also the hip or ankle. Therefore, the hip and/or ankle joint position can influence the knee joint's ROM
• Passive insufficiency of the rectus femoris could limit knee flexion to 120° or less if the hip joint is simultaneously extended
• Rotation of the knee joint

  Angular motions that describe the motion (or relative motion) of the tibia on the femur
• Longitudinal axis


  Runs through or close to the medial tibial intercondylar tubercle
• Medial condyle


  • Acts as the pivot point while the lateral condyles move through a greater arc of motion
• Center of rotation


  • Located within the medial tibial plateau, the contact forces are focused on a smaller area on the medial condyle whereas the contact forces are distributed over a larger surface area on the lateral tibial condyle
• Menisci will distort in the direction of movement of the corresponding femoral condyle and, therefore, maintain their relationship to the femoral condyles
• Tibia medially rotates (femur laterally rotates on the tibia)

  1. Medial meniscus will distort anteriorly on the tibial condyle to remain beneath the relatively anteriorly moving medial femoral condyle

  2. Lateral meniscus will distort posteriorly to remain beneath the posteriorly moving lateral femoral condyle
• Axial rotation


  Permitted by articular incongruence and ligamentous laxity
• Range of knee joint rotation

  Depends on the flexion/extension position of the knee
• Knee in full extension

  • Ligaments are taut, the tibial tubercles are lodged in the intercondylar notch, and the menisci are tightly interposed between the articulating surfaces; consequently, very little axial rotation is possible
• Knee flexed toward 90°

  • Capsular and ligamentous laxity increase, the tibial tubercles are no longer in the intercondylar notch, and the condyles of the tibia and femur are free to move on each other
• Maximum range of axial rotation

  Available at 90° of knee flexion
• Magnitude of axial rotation


  Diminishes as the knee approaches both full extension and full flexion
• At 90° of knee flexion

  Total medial/lateral rotation available is approximately 35°, with the range for lateral rotation being slightly greater (0° to 20°) than the range for medial rotation (0° to 15°)
• Frontal plane motion at the knee

  Although minimal, does exist and can contribute to normal functioning of the tibiofemoral joint
• Range of abduction (valgus)/adduction (varus)

  Typically only 8° at full extension, and 13° with 20° of knee flexion
• Tibiofemoral motions


  • Flexion and extension do not occur as pure sagittal plane motions but include frontal plane components termed "coupled motions"
  • Medial femoral condyle lies slightly distal to the lateral femoral condyle, which results in a physiological valgus angle in the extended knee
  • With knee flexion around the obliquely oriented axis, the tibia moves from a position oriented slightly lateral to the femur to a position slightly medial to the femur in full flexion; that is, the foot approaches the midline of the body with knee flexion
• Flexion


  Coupled to a varus motion
• Extension


  Coupled with valgus motion
• Automatic or Locking Mechanism of the Knee

  1. Obligatory lateral rotation of the tibia that accompanies the final stages of knee extension that is not voluntary or produced by muscular forces

  2. Last 30° of non-weightbearing knee extension (30" to 0"), the shorter lateral tibial plateau/femoral condyle pair completes its rolling-gliding motion before the longer medial articular surfaces do

  3. As extension continues, the longer medial plateau continues to roll and to glide anteriorly after the lateral side of the plateau has halted; results in lateral rotation of the tibia on the femur, with the motion most evident in the final 5° of extension

  4. Bringing the knee joint into its close-packed or locked position. The tibial tubercles have now become lodged in the intercondylar notch, the menisci are tightly interposed between the tibial and femoral condyles, and the ligaments are taut; automatic rotation is also known as the locking or screw home mechanism of the knee
• Initiating knee flexion from full extension

  1. The knee must first be "unlocked"; that is, the laterally rotated tibia cannot simply flex but must medially rotate concomitantly as flexion is initiated

  2. Flexion force will automatically result in medial rotation of the tibia because the longer medial side will move before the shorter lateral compartment

  3. In weight-bearing, the freely moving femur medially rotates on the relatively fixed tibia during the last 30° of extension Unlocking, consequently, is brought about by lateral rotation of the femur on the tibia before flexion can proceed
• Muscles of the Knee Complex

  • Each of the muscles that flex and extend the knee has a moment arm that is capable of generating both frontal and transverse plane motions
• Knee Flexor Group


  • Semimembranosus
  • Semitendinosus
  • Biceps femoris (long and short heads)
  • Sartorius
  • Gracilis
  • Popliteus
  • Gastrocnemius (plus the plantaris muscle?)
• Knee Flexor Group


  • Exception of the short head of the biceps femoris and the popliteus, all of the knee flexors are two-joint muscles
  • Five of the flexors (the popliteus, gracilis, sartorius, semimembranosus, and semitendinosus muscles) have the potential to medially rotate the tibia on a fixed femur, whereas the biceps femoris has a moment arm capable of laterally rotating the tibia
  • Lateral muscles (biceps femoris, lateral head of the gastrocnemius, and the popliteus) are capable of producing valgus moments at the knee, whereas those on the medial side of the joint (semimembranosus, semitendinosus, medial head of the gastrocnemius, sartorius, and gracilis) can generate varus moments
• Hamstring muscles


  • Semitendinosus
  • Semimembranosus
  • Long and short heads of the biceps femoris
• Hamstring muscles


  • Semitendinosus muscle attaches distally to the anteromedial aspect of the tibia, semimembranosus muscle inserts posteromedially on the tibia, both heads of the biceps femoris muscle attach distally to the head of the fibula, with a slip to the lateral tibia
  • Greater hamstring force is produced with the hip flexed because the hamstrings are lengthened across the hip
  • Two-joint hamstrings are required to contract with the hip extended and the knee flexed to 90° or more, the hamstrings are shortened across both the hip and the knee. The hamstrings produce less force as knee flexion
  • In non-weightbearing activities, the hamstrings generate a posterior shearing force of the tibia on the femur that increases as knee flexion increases peaking between 75 and 90° of knee flexion. This posterior shear or posterior translational force can reduce strain on the anterior cruciate ligament, although conceivably it increases the strain on the posterior cruciate ligament