anatomy,biomechanics

1. Biomechanics of Hip
Dr. K.PRASHANTH KUMAR
Resident, Dept. of Orthopaedic surgery,
HINDU RAO HOSPITAL,DELHI
1

2. “Biomechanics is the science that examines
forces acting upon and within a biological
structure and effects produced by such forces.”
- Jim Hay
2

3. HIP- Mobile as well as stable
• Strongbones
• Powerful muscles
• Strongestligaments
• Depth of acetabulum , narrowing of mouth by acetabular
labrum
• Lengthand obliquity of neckoffemur
• MOBILITYisdue to the long neckwhich isnarrower than
the diameter of thehead

4. Biomechanics of Hip
4
Biomechanics of
THA

5. • To perform the procedure properly
• To manage the problems that may arise during and after surgery
successfully.
• To select the components intelligently.
• To counsel patients concerning their physical activities.
the WHY?
5

6. Bony anatomy
6
✤ Ball and socket
synovial joint
✤ Acetabulum
✤ Acetabular
labrum
✤ Femoral head
✤ Femoral neck

7. TheNeckof Femur
• Angulatedin 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

8. Neck of femur
(a) Normal femoral neck angle,
(b) a decreased femoral neck angle (coxa vara)
(c) an increased femoral neck angle (coxa valga)
Angulated in relation to the shaft in two planes - sagittal(neck shaft angle)
and coronal(ante-version).
8

9. Ante-version
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• Angle between the neck and shaft in the
coronal plane(viewed from above)
• Axis of the neck and the trans-condylar
axis
• 15-20 degrees anterior to coronal plane

10. Acetabular version
Anteverted(forward) 15 degree
Abducted(laterally) 45 degree
10

11. Muscles
11

13. Hip Biomechanics
13

14. Centre of gravity
14
In humans- just anterior to S2

15.  Hip joint extension through posterior tilting of the pelvis
 Hip flexion through anterior tilting of the pelvis

17. It has 3 axis and three degrees
of freedom.
Hip motion takes place in all
three planes:
Saggital (flexion-extension)
Frontal(abduction-adduction)
Transverse (internal-
external rotation)

18. 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
• Maintains a level pelvis
18

19. 19

20. Biomechanics-HIP
• First orderlever
fulcrum (hipjoint)
forceson either sideof fulcrum
i.e, body weight & abductor tension

21. body weight isTomaintain stablehip, torques produced by the
countered by abductor muscles pull.
Abductor force X lever arm1 =weight X leverarm2
Biomechanics

22. Bi-pedal stance
✤ Body weight is equally
distributed across both
hips
✤ Each hip supports
4/6th or 1/3rd the BW
✤ Little or no muscle
force required to
maintain equilibrium
BW
R
22
R

23. Single leg stance
Effective centre of gravity
moves distally and away
from the supporting leg
since the non supporting
leg is now calculated as
part of the body mass
acting upon the weight-
bearing hip

24. The force of the abductor muscles also creates a moment
around the centre of the femoral head;
however this moment arm is considerably shorter than the
effective lever arm of body weight. Therefore the combined
force of the abductors must be a multiple of body weight.
The magnitude of the forces depends critically on the lever
arm ratio, which is that ratio between the body weight
moment arm and the abductor muscle moment arm (a:b).
Typical levels for single leg stance are three times
bodyweight, corresponding to a level ratio of 2.5.

25. Anything that increases the lever arm ratio
also increases the abductor muscle force
required for gait and consequently the force
on the head of the femur.
Short femoral necks
Wide pelvis
This tendency means that women have
larger hip forces than men.

27. Joint Reaction Force
• Forces generated within a joint in response to
external forces (both intrinsic and extrinsic).
Can reach 3 to 6 times body weight & is primarily
due to contraction of muscles crossing the hip.
Twice during SLRT
3 times in single leg stance
5 times in walking
Upto 10 times while running
Reduced to half upon using a cane

28. Coxa Valga
✤ GT is lower than
normal
✤ Reduced
abductor lever
arm
✤ Increased joint
reaction force
28

29. Coxa Vara
29
✤ GT is higher than
normal
✤ Increased abductor
lever arm
✤ Decreased joint
reaction force
✤ But…abductor
inefficiency

30. Compensatory Lateral Lean of the Trunk
Gravitational torque at the pelvis is the product of body
weight and the distance that the LoG lies from the hip
joint axis (MA).
If there is a need to reduce the torque of gravity in
unilateral stance and if body weight cannot be reduced, the
MA of the gravitational force can be reduced by laterally
leaning the trunk over the pelvis toward the side of pain or
weakness when in unilateral stance on the painful limb.

31. Although leaning toward the side of pain might appear
counterintuitive [Contrary to what common sense would suggest],
the compensatory lateral lean of the trunk toward the
painful stance limb will swing the LoG closer to the hip
joint,
thereby reducing the gravitational MA.

32. Because the weight of HATLL must pass through the
weight-bearing hip joint regardless of trunk position,
leaning toward the painful or weak supporting hip does not
increase the joint compression caused by body weight.
However, it does reduce the gravitational torque. If there
is a smaller gravitational adduction torque, there will be a
proportional reduction in the need for an abductor
countertorque

33. Although it is theoretically possible:
To laterally lean the trunk enough to bring the LoG
through the supporting hip (reducing the torque to
zero) or
To the opposite side of the supporting hip (reversing
the direction of the gravitational torque),
these are relatively extreme motions that require high
energy expenditure and would result in excessive wear
and tear on the lumbar spine.
 More energy efficient and less structurally stressful
compensations can still yield dramatic reductions in the
hip abductor force

34. Whether a lateral trunk lean is due to muscular
weakness or pain,a lateral lean of the trunk during
walking still uses more energy than ordinarily required
for single-limb support and may result in stress
changes within the lumbar spine if used over an
extended time period.
Use of a cane or some other assistive device offers a
realistic alternative to the person with hip pain or
weakness.

35. Use of a Cane Ipsilaterally
Pushing downward on a cane held in the hand on the side
of pain or weakness should reduce the superimposed body
weight by the amount of downward thrust;
that is, some of the weight of HATLL would follow the
arm to the cane, rather than arriving on the sacrum and
the weight-bearing hip joint.

36. Inman et al. suggested that it is realistic to expect that
someone can push down on a cane with approximately
15% of his body weight.
The proportion of body weight that passes through the
cane will not pass through the hip joint and will not create
an abduction torque around the supporting hip joint.

37. Although a cane used ipsilaterally provides some benefits in
energy expenditure and structural stress reduction, it is not as
effective in reducing hip joint compression as the undesirable
lateral lean of the trunk.
 Moving the cane to the opposite hand produces
substantially different and better results.

38. Use of a Cane Contralaterally
 When the cane is moved to the side opposite the painful
or weak hip joint, the reduction in HATLL is the same as it
is when the cane is used on the same side as the painful hip
joint;
that is, the superimposed body weight passing through
the weight-bearing hip joint is reduced by approximately
15% of body weight.

39. However, the cane is now substantially farther from the
painful supporting hip joint than it would be if the cane is
used on the same side;
that is, in addition to relieving some of the superimposed
body weight, the cane is now in a position to assist the
abductor muscles in providing a countertorque to the
torque of gravity.

40. A classic description of the
benefit of using a cane in the
hand opposite to the hip
impairment presumes that the
downward force on the cane acts
through the full distance
between the hand and the stance
(impaired) hip joint

41. Stand on LEFTleg—ifRIGHThip
drops, then it's a + LEFT
Trendelenburg
The contralateral side drops
because the ipsilateral hip
abductors do not stabilizethe
pelvis to prevent thedroop.
TRENDELENBURGSIGN

42. normal
affected
1 2

43. Antalgic Gait
Acute synovitis, septic
arthritis
1. Decreased duration of
stance phase of the
affected limb
2. There is a lack of weight
shift laterally over the
stance limb
3. Decrease in stance phase
in affected side will result
in a decrease in swing
phase of sound limb
4. Person sways on the same
side
 body weight moment arm
is shortened
 abductor lever arm
remains same
 decreased joint reaction
force
Abductor force to maintain
equilibrium is reduced

44. Weight Gain
Body weight and lever arm
increases.
Abductor muscular forces are
to be increased to counteract
body weight.
Increased joint forces across
the joint leading to increased
degeneration

45. Osteoarthritis of Hip
Contact area is decreased, secondary to
deformity & loss of sphericity of femoral head.
This concentrates compressive forces across the
hip joint to smaller area resulting in increased
pressure or load.
In hip disease patient decreases this load by
tilting towards the affected side in stance phase
 decreasing partial body weight lever arm

46. For most painful hip joints, however, the reductions in
compression generally required are greater than can be
realistically achieved through weight loss.
The solution must be in a reduction of abductor muscle
force requirements.

47. Principles of Osteotomy
Osteotomies improve hip function
Increasing contact area / congruency
Improve coverage of head
Moving normal articular cartilage into weight bearing zone
Restore biomechanical advantage / Decreasing joint reactive forces
Stimulating cartilage repair

48. Biomechanical effects in Varus
osteotomy
Medial displacement of of femoral shaft-to
maintain the mechanical axis of leg
Lengthens the abductor muscle lever arm
Reduces the joint reaction forces
Increases weight bearing surface
Bending stresses in femoral neck and shaft
increases

49. Biomechanics like effects of
valgus osteotomy
Adducted position restores improved weight bearing area to the
diseased joint
Shortening of abductor lever arm
Lengthening of limb
So,lateral transposition of abductors,tenotomy of psoas and
adductor tendon shortening of femur may be required for optimal
biomechanical reconstruction

51. History of biomechanics inTHA
“First bonelaw”
JULIUS WOLFFE -1870
“Form follows function” 51

52. Julius Wolff, pioneered the mother of all bone laws stating
that bone adapts to the loads it is being exposed to.
 Wolff based his concept of the functional form of bone on
the similarity between the inner structure of the proximal
femur and the lines of internal stress observed in the
Fairbairn steam crane
Wolff's theory has a direct application to the design of
THA.
The femoral stems that bypass the proximal femur and
transfer loads directly to the cortical bone at the distal end of
the prosthesis will cause stress shielding .
This process gradually results in bone resorption of the
bypassed proximal femur and cortical thickening of the
loaded distal cortex

53. Stress shielding in a left uncemented femoral implant. Note the distal cortical
thickening around the canal filling stem and resorption in the metaphyseal Gruen zones
1 and 7

54. Static bio-mechanical model
54

55. Pauwels performed extensive research on the biomechanical
impact of a varus and valgus configuration of the proximal femur.
 He acknowledged the influence of the neck-shaft angle on the
reaction force of the hip and thereby the magnitude of stress on
the femoral head.
The theoretical reaction force is up to 25% lower in coxa vara
compared with the average hip, whereas in coxa valga, it is 25%
higher.
The change in magnitude of the reaction force is caused by the
change in the length of the abductor lever arm.
 As the neck-shaft angle increases, the abductor lever arm
decreases, thereby requiring a higher abductor force to balance
the BW.

56. Low friction torquearthroplasty
56

57. Charnley’s Concept
Shorten the lever arm of the body weight by deepening
the acetabulum (centralization of the femoral head)
Lengthen the lever arm of the abductor mechanism by
reattaching the osteotomized greater trochanter laterally.
Thus the moment produced by the body weight is
decreased, and the counterbalancing force that the abductor
mechanism must exert also is decreased.

58. Dynamic biomechanics
58

59. Applied biomechanics in THA
Principle- To decrease joint reaction force
Centralisation of the femoral head by deepening of acetabulum
- decreases BW lever arm
59

60. Increase in neck length and lateral reattachment of trochanter
- lengthens abductor lever arm
60

61. Decreased BW lever arm Lengthened abductor lever arm
Reduced wear of implants
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62. Torsional forces
62

63. Four important variables determine the stability of
total hip arthroplasty-
1. Component design
2. Component position
3. Soft tissue positioning (restoration of offset)
4. Soft tissue function
63

64. Offset
64
Medial or horizontal offset
Centre of the head to the axis of the
stem.
Vertical offset(height)
Determined by the base length of
the prosthetic neck and length
gained by head.
The depth the implant is inserted
into the femoral canal alters the
vertical height.

65. IF…….
Medial offset inadequate
Moment arm shortened
Limp
Increased bony
impingement
Dislocation
Raised JRF
65

66. IF…….
Excessive medial offset
Increased stress on
cement & stem
Loosening Stress #
Dislocation
66

67. ✤ Adjustment of neck length is important as it has effect
on both medial and horizontal offset
67
✤ Joint reaction forces are minimal if hip centre placed in
anatomical position.

69. ✤ Principle of medialization has given way to preserving subchondral
bone in the pelvis and to deepening the acetabulum only as muchas
necessary to obtain bony coverage for thecup.
69
✤ Most total hip procedures are now done without osteotomy of the
greater trochanter, the abductor lever arm is altered only relative tothe
offset of the head to the stem.
✤ These compromises in the original biomechanical principles of total hip
arthroplasty have evolved to obtain beneficial tradeoffs of a biologic
nature; to preserve pelvic bone, especially subchondral bone; and to
avoid problems related to reattachment of the greatertrochanter.

70. Range of Motion
Heavily influenced by prosthesis design
70

71. 71

72. 72

73. DI
A
HEAD
METER
• Large diameter head compared toSmal head
– Lessprone fordislocation
– Rangeof motion ismore

74. Head size Dislocation rates
22mm Upto 18 %
28mm 0.6-3%
32mm 0.5%
38mm 0.0%
Size is not the only parameter for dislocation tho…
Implant position and soft tissue tension is equally
important

75. • Femoralcomponents availablewith afixed neckshaft angle-
135º
• Restorationof the neckin ante version- 10-15º
– Increasedante version  anteriordislocation
– Increased retroversion  posteriordislocation
• Cupplacedin 150-200of ante version and 450of inclination

76. Component orientation
76
..”probably the most important biomechanical aspect for
the tribological and functional success of a THA procedure.”
-M.M. Morlock et al
Biomechanics of Hip arthroplasty

77. Acetabular position
Anteversion
5-25 deg
Abduction
30-50 deg
77

78. (a) Measurement of the AO and FO. (b) Cup abduction angle measured as the angle
between the inter teardrop line and the major axis of the cup projection.60 (c)
Anterosuperior weightbearing area in a cup with 45° of abduction and 15° anteversion,
(d) reduced anterosuperior weightbearing area when cup is placed in 60° of abduction,
(e) reduced anterosuperior weightbearing area with increased cup anteversion compared
to image (c) of the same figure, green arrows representing SA and LA of the cup.
Anteversion angle = asin (SA/LA) *180/pi. AO: Acetabular offset, FO: Femoral offset, SA:
Short axis, LA: Long axis

79. Femoral stem position
Anteversion
10-15 deg
Combined version(acetabulum + femur)
37 deg
79

80. Impact of femoral version on the “functional” femoral offset. As the femoral
anteversion increases, the femoral offset decreases resulting in higher hip
joint reaction forces. (a) 35° of femoral anteversion, (b) 10° of physiological
anteversion, (c) 10° of retroversion

81. Conclusion
The study of biomechanics in THA started out with unraveling
the general biomechanics of the hip joint and the impact of joint
replacement.
Over the years, the mathematical insights have proven to be
indispensable in both design and surgical techniques.
The nearby future holds promising answers in individualized
evaluation of total hip surgery which will hopefully translate into
improved functional restoration and longevity in THA

82. Take Home Message
An increase in the ratio of Abductor lever arm to
Body weight arm, decreases the joint reaction force.
Moving the acetabular component as far medial and
inferior.
Lateralization of Greater trochanter
Shifting body weight or leaning over affected hip
Varus neck-shaft Angulation
Cane in contralateral hand

83. Thank you…