Hip, Pelvis, Biomechanics

1. BIOMECHANICS OF HIP &
PELVIS
Dr. Chhavi SinghTomar
Asst. Prof./Vice –Principal
NIMSCollege of Physiotherapy &
OccupationalTherapy
Nims University

3. INTRODUCTION
BIOMECHANICS – Science that deals
with the study of forces (internal or
external ) acting on the living body

4. 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

5. 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

6. Acetabular Direction
• long axis of acetabulum
points
– forwards :
15-200
ante
version
– 450 inferior
inclination
ante
version

7. 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

8. Biomechanics- HIP
• First order
lever
Fulcrum (hip joint)
forces on either side of fulcrum
i.e, body weight & abductor tension

9. To maintain stable hip, torques produced by the
body weight is countered by abductor muscles pull.
Abductor forceX lever arm1= weight X leverarm2
Biomechanics

10. Biomechanics
• Forces acting across
hip joint
 Body weight
Abductor muscles
force
 Joint reaction force

11. 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

14. 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

15. 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

16. Centre of gravity :
• Wts. of the objects act through the
centre of gravity.
• In humans  just anterior to S2

17. 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)

18. Single leg stance - Right
• Rt. LL supports the body wt & also the Lt LL’si.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.
Typical levels for single leg
stance are 3W, corresponding
to a level ratio of 2.5.
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

19. Single leg stance - Right
4/6 +1/6
=5/6
• Rt. LL supports the body wt & also the Lt LL’si.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.

20. 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. USE OF CANE / WALKING STICK

22. 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

23. 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. normal
affecte
d
1 2

25. 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
valga
Resultant force R is
more than a normal
hip

27. 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. COXA VARA
Resultant force R is less
than a normal
hip

29. 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

30. 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.

31. 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

32. Offsets………
• Vertical Ht (offset)
Determined by the Base
length of the Prosthetic
neck and length gained by
the head

33. • Horizontal Offset
(Medial offset) center of the
head to the axis of the stem

34. 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

35. • 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

36. HEAD DIAMETER
• Large diameter head compared to Small head
– Less prone for dislocation
– Range of motion is more

37. • 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

38.  THANKYOU