BIOMECHANICS OF THE KNEE

Axes of motion

JOINT

AXIS

MOTION

CLOSE-PACKED POSITION

tibio-femoral

lateral
longitudinal

flexion/extension
tibial rotation

extension

patello-femoral

ANATOMY

interactive MRI images of the knee by:

Hochman, M., & Rundle, D. (1995). University of Pennsylvania Medical Center Radiology Interactive. Retrieved 9- 8-99 from the World Wide Web: http://www.rad.upenn.edu/rundle/
Knee/kneeMRICONT.html

the knee in American literature

lateral view of knee joint

LIGAMENTS OF THE KNEE

  1. medial collateral ligament (MCL)
  2. lateral collateral ligament (LCL)
  3. anterior cruciate ligament (ACL)
  4. posterior cruciate ligament (PCL)

collateral ligaments

MCL
LCL
PCL
ACL

RESIST/LIMIT EXTENSION

MCL

RESISTS VALGUS STRESS/FORCE

LCL

RESISTS VARUS STRESS/FORCE
TIBIO-FEMORAL ARTHROKINEMATICS

Viewed in the sagittal plane, the femur's articulating surface is convex while the tibia's in concave. We can predict arthrokinematics based on the rules of concavity and convexity:

DURING KNEE EXTENSIONDURING KNEE FLEXION

OPEN CHAIN

CLOSED CHAIN

OPEN CHAIN

CLOSED CHAIN

TIBIA GLIDES ANTERIORLY ON FEMUR

FEMUR GLIDES POSTERIORLY ON TIBIA

TIBIA GLIDES POSTERIORLY ON FEMUR

FEMUR GLIDES ANTERIORLY ON TIBIA
from 20o knee flexion to full extension from full knee extension to 20o flexion

Tibia rotates externally

Femur rotates internally on stable tibia

Tibia rotates internally

Femur rotates externally on stable tibia

THE "SCREW-HOME" MECHANISM

Rotation between the tibia and femur occurs automatically between full extension (0o) and 20o of knee flexion. These figures illustrate the top of the right tibial plateau as we look down on it during knee motion.

top of tibial plateau top of tibial plateau top of tibial plateau
DURING KNEE EXTENSION, the tibia glides anteriorly on the femur. During the last 20 degrees of knee extension, anterior tibial glide persists on the tibia's medial condyle because its articular surface is longer in that dimension than the lateral condyle's. Prolonged anterior glide on the medial side produces external tibial rotation, the "screw-home" mechanism.
THE SCREW-HOME MECHANISM REVERSES DURING KNEE FLEXION
top of tibial plateau top of tibial plateau top of tibial plateau
When the knee begins to flex from a position of full extension, posterior tibial glide begins first on the longer medial condyle. Between 0 deg. extension and 20 deg. of flexion, posterior glide on the medial side produces relative tibial internal rotation, a reversal of the screw-home mechanism.

Remember: the shapes of the tibial and femoral surfaces do not cause these movements. Forces produce movements, including arthrokinematic movements. What forces produce roll and glide between the tibia and femur?

Last updated 9-8-00 ©Dave Thompson PT
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