Biomech

7/15/99

Knee Joint

• Longest lever system
• No muscles surrounding the joint to stabilized
• Unstable joint
• Largest surface area joint
• Thickest cartilage in body is the back of the patella—more stress
• 4 joints
• medial tibial femoral
• lateral tibial femoral
• tibial fibula
• patella femoral—highest stressed joint

• ligaments—ACL, PCL, medial collateral, lateral collateral, meniscus (guides the motion of the femur, no direct support)
• test the good leg first to establish baseline
• ACL—anterior drawer test—keeps tibia from going forward
• Grade 1, 2, or 3 (complete tear)

• Sag sign—both legs should be 90° --PCL will be further down—should be done first to avoid false readings
• Locheman's test--30° bend, anterior medial pull—more sensitive test (95%)
• Collateral—extended, slightly bent, fully bent—valgus and vargus
• McMurray's test—posterior lateral tears, supine

Patella

• Increase momentum arm of quads—mechanical advantage 33%
• Protective mechanism for ligaments
• Help quad tendon do would not rub on femur—these muscles receive a lot of stress
• Sesamoid bone

Meniscus

• Stress distribution in a normal knee and in a knee w/the menisci removed. Removal of the menisci increases the mangnitude of stresses on the cartilage of ht tibial plateau and changes the size and location of the tibiofemoral contact area. W/the menisci intact the contact area encompasses nearly the entrie surface of the tibial plateau. W/the menisci removed the contact area is limited to the center of the tibial plateau
• Helps w/nutrition of cartilage
• Spreads out the force
• Helps guide the movement of the femur
• Lubricates

ACL

• Do you need to repair it?
• Loaded w/mechanoreceptors
• Primary proprioceptive input for lower extremity
• Contributes to the reflexes

Forces by Quads

• Going up and down stairs
• When we bend the knee, has to overcome body weight
• The more you bend, the longer moment arm, pushes body weight further back
• As bend, legs have to work harder
• At 90° , lose ½ the surface area therefore doubling the force
• Curve of the patella is different in everyone
• Flattened—dislocation
• W/of condylar notch—ACL tears

Forces at the Knee

• Where and why does it occur
• Where the muscles that cross the knee fire
• Right before toe off