3. HIP - Mobile as well as stable
• Strong bones
• Powerful muscles
• Strongest ligaments
• Depth of acetabulum , narrowing of mouth by acetabular
labrum
• Length and obliquity of neck of femur
• MOBILITY is due to the long neck which is narrower than the
diameter of the head
4. The Neck of Femur
• Angulated in relation to the shaft in 2 planes :
sagittal & coronal
• Neck Shaft angle
– 140 deg at birth
– 120-135 deg in adult
• Ante version
– Anteverted 40 deg at birth
– 12-15 deg in adults
5. Acetabular Direction
• long axis of acetabulum points
– forwards : 15-200
ante version
– 450 inferior inclination
ante version
6. Axis of lower limb
Mechanical axis line passes
between center of hip joint
and center of ankle joint.
Anatomic axis line is between
tip of greater trochanter to
center of knee joint.
Angle formed between these
two is around 70
7. Biomechanics- HIP
• First order lever
fulcrum (hip joint)
forces on either side of fulcrum
i.e, body weight & abductor tension
8. To maintain stable hip, torques produced by the body weight is
countered by abductor muscles pull.
Abductor force X lever arm1 = weight X leverarm2
Biomechanics
10. 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
maintains a level pelvis
Joint reaction force
-2W during SLR
- 3W in single leg stance
-5W in walking
-10W while running
Joint reaction force
11.
12.
13. Coupled forces:
Certain joints move in such a way that rotation
about one axis is accompanied by an
obligatory rotation about another axis & these
movements are coupled
Joint congruence – the proper fit of two articular
surfaces, necessary for joint motion
14. Instant centre of rotation:
• Point at which a joint rotates
• Normally lies on a line perpendicular to the
tangent of the joint surface at all points of
contact
15. Centre of gravity
• Wts. of the objects act through the centre of
gravity.
• In humans just anterior to S2
16. Forces across the hip joint in
two leg stance
• L.L 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)
17. Single leg stance - Right
• Rt. LL supports the body wt & also the Lt
LL’s 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.
i.e. Body wt acts eccentrically on the hip
and tends to tilt the pelvis in adduction -
---- balanced by the abductors
4/6 +1/6 =5/6
Typical levels for single leg stance
are 3W, corresponding to a level
ratio of 2.5.
18. Single leg stance - Right
4/6 +1/6 =5/6
• Rt. LL supports the body wt & also the Lt
LL’s i.e. 5/6th total body wt.
• Effective Centre of gravity shifts to the
non-supporting 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.
i.e. Body wt acts eccentrically on the hip
and tends to tilt the pelvis in adduction --
--- balanced by the abductors
Typical levels for single leg stance
are 3W, corresponding to a level
ratio of 2.5.
19. USE OF CANE / WALKING STICK
• It creates an additional force that keeps the pelvis level in the face
of gravity's tendency to adduct the hip during unilateral stance.
• decreases the moment arm between the center of gravity and
the femoral head(R)
• The cane's force must substitute for the hip abductors.
• Long distance from the centre of hip to contralateral hand
offers excellent mechanical advantage
21. Cane and Limp
• Both decrease the force exerted
by the body wt on the loaded
hip
• Cane: transmits part of the
body wt to the ground thereby
decreasing the muscular force
required for balancing
• Limping shortens the body lever
arm by shifting the centre of
gravity to the loaded hip
22. Stand on LEFT leg—if RIGHT hip
drops, then it's a + LEFT
Trendelenburg
The contralateral side drops
because the ipsilateral hip
abductors do not stabilize the
pelvis to prevent the droop.
TRENDELENBURG SIGN
24. Biomechanics in neck deformities :
Coxa valga
• Increased neck shaft angle
• GT is at lower level
• Shortened abductor lever arm
• Body wt arm remains same
• Increased joint forces in hip during one leg
stance
• Less muscle force required to keep pelvis
horizontal
26. Coxa Vara
• Decreased neck shaft angle
• GT is higher than normal
• Increased abductor lever arm
• Abductor muscle length is shortened
• Decreased joint forces across the hip
during one leg stance
• Higher muscle force is required to keep
pelvis horizontal
28. WITH WEIGHT GAIN
• Abductor muscular forces are to be increased to counteract
body wt
• Increased joint forces across the joint leading to increased
degeneration
• Rationale of decreasing body wt in OA – decrease in body wt
force & hence abductor force required to counter balance
decreasing joint reaction forces across that hip
29. Biomechanics of THR
Principle – to decrease joint reaction force
• 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.
30. Joint reaction forces are minimal if hip centre placed in
anatomical position
Adjustment of neck length is important as it has effect on both
medial offset & vertical offset
31. Offsets………
• Vertical Ht (offset)
Determined by the Base length
of the Prosthetic neck and
length gained by the head
33. IF……….
• Medial offset is inadequate shortens the moment arm
limp, increase bony impingement
• Excessive medial offset – dislocation, increases stress on stem
& cement
stress # or loosening
34. • In regular THR , the Femoral component must be inserted
in the same orientation as the femoral neck to achieve the
rotational stability .
• Modular component in which stem is rotated
independently of the metaphyseal portion
• Anatomical stems have a few degrees of ante version built
into the neck
35. HEAD DIAMETER
• Large diameter head compared to Small head
– Less prone for dislocation
– Range of motion is more
36. • Femoral components available with a fixed neck shaft angle -
135º
• Restoration of the neck in ante version - 10-15º
– Increased ante version anterior dislocation
– Increased retroversion posterior dislocation
• Cup placed in 150-200 of ante version and 450 of inclination
