Biomechanics is the science of forces acting on biological structures and the effects of these forces. This document discusses the biomechanics of the normal hip joint and total hip arthroplasty (THA). It describes the anatomy of the hip joint and muscles as well as normal and abnormal biomechanics. Factors like femoral neck angle, acetabular version, and center of gravity are discussed. Principles of THA like medialization, increasing neck length, and restoring offset are aimed at decreasing joint reaction forces. Component position, design, and soft tissue function and positioning are important for stability after THA. Range of motion is influenced by prosthesis design.

1. Biomechanics in Total
Hip Arthroplasty
Dr. Jose Austine
Resident, Dept. of Orthopaedic surgery,
Kasturba Medical College, Mangalore
Moderators
Dr. Surendra Umesh Kamath
Dr. Sharan Mallya
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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. Biomechanics of Hip
Biomechanics of
THA
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4. • 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?
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5. ✤ Anatomy of hip joint
✤ Biomechanics of Hip- Normal & Abnormal
✤ History of biomechanics in THA
✤ Bio-mechanical considerations in THA
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6. “Hips don’t lie”
• Strong bones
• Powerful muscles
• Strongest ligaments
• Tremendous degree of
forces acting around
• Mobile as well as stable
• “Self closed mechanism”
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7. Bony anatomy
✤ Ball and socket
synovial joint
✤ Acetabulum
✤ Acetabular
labrum
✤ Femoral head
✤ Femoral neck
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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).
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9. Ante-version
• 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
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10. Acetabular version
Anteverted(forward) 15 degree
Abducted(laterally) 45 degree
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11. Muscles
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12. Hip Biomechanics
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13. Centre of gravity
In humans- just anterior to S2
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14. 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
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15. 15

16. ✤ First order lever
✤ While standing 1/3rd of the body weight passes through both hips
✤ In swing phase 4 times the weight passes through the hip
✤ Forces acting on the hip
- Body weight
- Abductor muscle force
- Joint reaction force
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17. • Fulcrum
• Socket
• Muscle
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18. 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 R
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19. Single leg stance- Right limb
RM
19

20. Body wt vector- K
Abductor force vector- M
JRF vector- R
BW lever arm- h’
Ab lever arm-h
h’=3h
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21. 21

22. Coxa Valga
✤ GT is lower than
normal
✤ Reduced
abductor lever
arm
✤ Increased joint
reaction force
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23. Coxa Vara
✤ GT is higher than
normal
✤ Increased abductor
lever arm
✤ Decreased joint
reaction force
✤ But…abductor
inefficiency
23

24. Cane
&
Limp
✤ Both decrease the force exerted by the BW on the loaded hip
✤ Cane transmits part of BW to the ground and also provides a counter acting
force thereby decreasing the muscular force required for balancing
✤ Limping shortens the body lever arm by shifting the centre of gravity to the
loaded hip
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25. History of biomechanics in THA
“First bone law”
“Form follows function” 25

26. Static bio-mechanical model
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27. Low friction torque arthroplasty
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28. Dynamic biomechanics
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29. Applied biomechanics in THA
Principle- To decrease joint reaction force
Centralisation of the femoral head by deepening of acetabulum
- decreases BW lever arm
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30. Increase in neck length and lateral reattachment of trochanter
- lengthens abductor lever arm
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31. Decreased BW lever arm Lengthened abductor lever arm
Reduced wear of implants
31

32. Torsional forces
32

33. 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
33

34. Offset
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.
34

35. IF……..
Medial offset inadequate
Moment arm shortened
Limp
Increased bony
impingement
Dislocation
Raised JRF
35

36. IF……..
Excessive medial offset
Increased stress on
cement & stem
Loosening Stress #
Dislocation
36

37. ✤ Adjustment of neck length is important as it has effect
on both medial and horizontal offset
✤ Joint reaction forces are minimal if hip centre placed in
anatomical position.
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38. 38

39. ✤ Principle of medialization has given way to preserving subchondral
bone in the pelvis and to deepening the acetabulum only as much as
necessary to obtain bony coverage for the cup.
✤ Most total hip procedures are now done without osteotomy of the
greater trochanter, the abductor lever arm is altered only relative to the
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 greater trochanter.
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40. Range of Motion
Heavily influenced by prosthesis design
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41. 41

42. 42

43. Component orientation
..”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
43

44. Acetabular position
Anteversion
5-25 deg
Abduction
30-50 deg
44

45. Femoral stem position
Anteversion
10-15 deg
Combined version(acetabulum + femur)
37 deg
45

46. • To perform the procedure properly
• To manage the problems that may arise during and after surgery succ
• To select the components intelligently.
• To counsel patients concerning their physical activities.
46

47. 47
References
✤ Campbell’s operative orthopaedics 13th Ed Vol 1
✤ Biomechanical considerations of total hip replacement- Michael P
Kowaleski
✤ Anatomy and Biomechanics of the hip- Damien P Bryne et al- The open
sports medicine journal, 2010
✤ Biomechanics of hip arthroplasty- Michael M Morlock
✤ The history of biomechanics in total hip arthroplasty- Jan Van Houcke et
al- Indian journal of orthopaedics

48. THANK YOU
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