knee biomechanics

2. 
The knee is a mechanism of three joints and
Four bones - the femur, tibia, patella and
fibula

Interact in separate joints - the tibiofemoral &
patellofemoral

The function of these joints is to allow certain
movements, restrict others, and to provide
load transfer through the lower limb.

3. Tibiofemoral joint
rotations
translations
screw home mechanism
 Axial & rotational alignment of knee
 Patello femoral joint
 Joint forces


4. 
Rotations
› Flexion/extension-0 to
1350
› varus valgus - 6-8o in
extension
› Int/ext rotation - 25 – 300
in flexion

Translations
› AP 5 - 10mm
› comp/dist 2 - 5mm
› medio-lateral 1-2mm

6. 
flexion axis varies in a helical fashion in a normal
knee, with an average of 2 mm of posterior
translation of the medial femoral condyle on the tibia
during flexion compared with 21 mm of translation
of the lateral femoral condyle.

Relevance :posterior rollback
› as the knee flexes, the instant center of rotation on
the femur moves posteriorly

7. 
flexion axis as varying in a helical fashion

8. allows for increased knee flexion by avoiding impingement

9. 


the external rotation of the tibia on
the femur during extension and
internal rotation of the tibia during
knee flexion.
cause
› medial tibial plateau articular
surface is longer than lateral
tibial plateau(Medially based
pivoting of the knee.)
relevance
› "locks" knee decreasing the
work performed by the
quadriceps while standing

10. 
mechanical axis of the lower limb is defined as the
line drawn on a standing long leg antero posterior
radiograph from the center of the femoral head to
the center of the talar dome

anatomical axes of the femur and the tibia form a
valgus angle of 6 2 degrees.

the tibial articular surface is in approximately 3 0of
varus with respect to the mechanical axis, and the
femoral articular surface is in 90 of valgus.

12. "sliding" articulation
› patella moves 7cm caudally
during full flexion
 maximum contact between
femur and patella is at 45
degrees of flexion


The primary function of the
patella is to increase the lever
arm of the extensor mechanism
around the knee, improving
the efficiency of quadriceps
contraction.

13. 
The quadriceps and patellar tendons insert anteriorly
on the patella, with the thickness of the patella
displacing their respective force vectors away from the
center of rotation of the knee .

This displacement or lengthening of the extensor lever
arm changes throughout the arc of knee motion.

the extensor lever arm is greatest at 20 degrees of
flexion, and the quadriceps force required for knee
extension increases significantly in the last 20 degrees
of extension

14. 
The length of the lever arm varies as a function
of the geometry of the trochlea, the varying
patellofemoral contact areas, and the varying
center of rotation of the knee.

15. passive restraints to lateral
subluxation
› medial patellofemoral
ligament
 primary passive restraint to
lateral translation in 20
degrees of flexion
 60% of total restraining force
› medial patellomeniscal
ligament
 13% of total restraining force
› lateral retinaculum
 10% of total restraining force
 dynamic restraint
› quadriceps muscles


16. 

The angle between the extended
anatomical axis of the femur &
the line between the center of the
patella & the tibial tubercle
normal Q angle
› in flexion
 males
 13 degrees
 females
 18 degrees
› in extension
 8 degrees

17. 
Limbs with larger Q angles have a greater tendency
for lateral patellar subluxation.

Because the patella does not contact the trochlea in
early flexion, lateral subluxation of the patella in
this range is resisted primarily by the vastus
medialis obliquus fibers.

18. Position
 Standing on both feet  Swing phase
 u/l stance phase
–
 Jogging
–
force acting on joint
equal to body wt
1/2 x b.wt
2-4 x b.wt
6x b.wt

19. 
Walking
› 0.3 x body weight

Ascending Stairs
› 2.5 x body weight

Descending Stairs
› 3.5 x body weight

Squatting
› 7 x body weight

20. Prevent anterior tibial displacement on femur
 Secondarily, prevents hyperextension, varus &
valgus stresses
 Least stress on ACL between 30-60 degrees of
flexion

Anteromedial bundle tight
in flexion & extension
Posterior lateral bundle
tight only in extension

21. Primary stabilizer of the knee against posterior
movement of the tibia on the femur
 resists rotation, esp.internal rotation of tibia on
femur

Two bundles
 Anterolateral, taut in flexion
 Posteromedial, taut in extension