3. • BIOMECHANICS – Science that deals with the
study of forces (internal or external ) acting on
the living body
4. • Hip is a first order lever with fulcrum (hip
joint) located between the load and the effort
(body wt and abductor tension)
5. • To maintain stable
hip, torques produced
by the body weight is
countered by
abductor muscles
pull.
6. • Forces acting across hip joint
– Body weight
– Abductor muscles force
– Joint reaction force
• Weight X Weight arm = Abductor pull X Abductor arm
7. Joint reaction force
• Defined as force generated within a joint in
response to forces acting on the joint
• In the hip, it is the result of the need to balance
the moment arms of the body weight and
abductor tension
• Joint reaction force
– 2W during SLR
– 3W in single leg stance
– 5W in walking
– 10W while running
8. Two leg stance
• Lower limbs constitute 2/6 (1/6 + 1/6), and
U.L & trunk constitute 4/6 the total body wt
• Little or no muscular forces required to
maintain equilibrium in 2 leg stance
• Body wt is equally distributed across both hips
• Each hip carries 1/3rd body weight
– (4/6 = 2/3 = 1/3 + 1/3)
9. Single leg stance
• Rt lower limb supports the body wt & also the
Lt lower limb’s weight i.e. 5/6th total body wt.
• Effective Centre of gravity shifts to the non-
supportive leg (L) & produces downward force
to tilt pelvis
• Rt abductors must exert a downward counter
balancing force with right hip joint acting as a
fulcrum
10. Coxa vara and Coxa valga
• Coxa vara the abductor arm is increased and
hence lesser force is needed to keep the pelvis
level
• Coxa valga the abductor arm is decreased and
hence higher force is needed to keep the
pelvis level
11. • Use of a cane on the contralateral side
transfers some amount of weight and hence
reduces the amount of abductor force needed
12. Biomechanics of THR
• Centralization of femoral head by deepening
of Acetabulum - decreases body wt lever arm
• Increase in neck length and Lateral
reattachment of trochanter - lengthens
abductor lever arm
• This decreases abductor force, hence joint
reaction force, & so the wear of the implants.
13. • If weight arm is equal to the abductor arm
joint reaction force reduces by 30%
15. • Much more complex
than simple flexion and
extension.
• Knee motion during gait
occurs in flexion and
extension, abduction and
adduction, and rotation
around the long axis of
the limb
16. • Knee flexion, which
occurs around a varying
transverse axis is a
function of the articular
geometry of the knee and
the ligamentous
restraints.
• Flexion axis as varying in a
helical fashion (J)
17. • Dennis et al. described the flexion axis as
varying 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.
19. Femoral roll back
• Allows for increased knee flexion by avoiding
impingement
20. Screw home mechanism
• 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.
• Relevance
– "locks" knee decreasing the work performed by
the quadriceps while standing
21.
22. • In kinematic studies of the knee during
selected activities of daily living, normal gait
required 67 degrees of flexion during the
swing phase, 83 degrees for stair climbing, 90
degrees for descending stairs, and 93 degrees
to rise from a chair.
23. Patello femoral joint
• “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
mechanism
• Extensor lever arm is greatest at 20 degrees of
flexion
24.
25. Joint reaction force of tibiofemoral joint
• 1W when standing on both feet
• 2Wwhen standing on one foot
• 4W during stance phase
• 1/2W during swing phase
• 6W during Jogging
26. Joint reaction force of patellofemoral joint
• 1/3W during walking
• 2.5W during Ascending Stairs
• 3.5W during Descending Stairs
• 7W during Squatting
