Hip joint biomechanics

1. Biomechanics of hip joint
Presenter : Dr. Renjith R John
Moderators : Dr. Surendra Kamath
Dr. Rajendra

2. • Biomechanics is the science that examines forces acting
upon and within a biological structure and the effects
produced by those forces

3. Introduction
• Coxofemoral joint – articulation of
acetabulum and head of femur
• Ball and socket- diarthrodial joint
• 1° function - Support weight of H.A.T
Static posture Dynamic postures
(walking, running,
stair climbing)

4. Bony Structure
• Acetabulum
3 separate ossification centres :
- Ilium (40%),
- Ischium (40%)
- Pubis (20%)
- laterally, downwards & forwards
Oriented 15°- 20° anteriorly (anteversion)
&
45° inferiorly (inclination) (AI)
Angle of
inclination

5. Acetabular labrum
- Rim of fibrocartilage - circumferentially around
- augments depth
- joint stability
- contact area , reduces contact stress
- sealed cavity for joint lubrication
maintains negative intra-articular pressure

6. Angulations of femur
Neck angulated in 2 planes neck w.r.t shaft
1) Neck shaft angle : between the shaft and neck
125 ° to 135° in adults
Pathological - coxa valga
Decrease - coxa vara
Leads to altered biomechanics & gait
abnormalities

7. 2) Femoral anteversion /angle of
femoral torsion
• Degree of fwd projection of femoral
neck from the coronal plane of the
femoral shaft
Best viewed - top to bottom
In adults 10-15°

9. Femoral anteversion : >20 degrees
• Restricted ER and increased IR
• Reduces joint stability as femoral
articular surface is more exposed
anteriorly
• Line of hip abd lies posteriorly,
the moment arm

11. Articular congruence
Measure of how well/close the joints articulate
a. Neutral position : articular cartilage -anterior
head exposed
b. Maximum articular contact
FABER

12. Movements
• 3 degrees of freedom
- flexion/ extension - saggital plane
- abduction/ adduction- coronal
- medial/ lateral rotation- transverse

13. ROM
• Flexion : 110- 120°
• Extension : 10 -15°
• Abduction : 30 - 40°
• Adduction : 25 - 30°
• Internal Rotation : 25 - 30°
• External rotation : 35 - 45°
Ref : S.Das clinical surgery

14. Muscles around hip

16. AXIS
• Mech axis : centre of hip joint and centre
of ankle joint
• Anatomical axis : tip of GT to centre
of knee joint.
Angle between – roughly 7 degrees
Centre of gravity of human body :
just anterior to S2 vertebra

17. Forces transmitted across hip joint
- A.k.a Balancing moments across hip joint
Moment – measure of tendency to cause a body
to rotate about a given specific fixed point or axis.
M = F x D (from which it acts)

18. Balancing the weight moment
Bilateral stance
• BW - transmitted through SI
joints & pelvis to right and left
femoral heads equally
•Joint axis of each hip lies at equal
distance from line of gravity of
H.A.T

19. In bilateral stance :-
• 2 opposing gravitational moments of
equal magnitude balance each other
• Hence pelvis maintained in equilibrium
in frontal plane without active support
from abductor muscles

20. Joint reaction force
Force generated within a joint in response to forces
acting on joint
- Primarily compressive force in the hip joint by which
abductors maintain level pelvis
•Balance - moment arms of body weight & abd.
tension
•Maintains - level pelvis
o

22. • Lower limbs – 2/6th of total BW ( 1/6 + 1/6)
Upper limbs and trunk – 4/6th of total BW
divided equally among both hips
each hip 2/6th or 1/3rd BW

23. Single-leg stance

24. Gait video

25. One-leg stance on Rt Lower limb , left LL lifted
- wt of left LL added to BW , COG displaced to left
- assuming abd moment arm (B-O) is 1/3rd of BW
moment arm from head to COG(OC)
- force of abductors = BW (pelvis stabilized)
(k = 5/6 BW)
Net JRF = Abductor force + Body weight
3K + K = 4K ( 3.3 TBW)

26. Cane and limp
• Both force exerted by BW on loaded hip
• Biomech studies : 15% cane- force (3K- 1.5K)
• Cane transmits part of BW , producing counter
force , reducing abd. force required for
balancing the pelvis and hence the net JRF on
hip.
Reduced BW moment arm
-
-
-
-
-
-
-
-

27. Coxa Vara
- GT at a higher level
- Increases the abd moment (OM, GT moves laterally)
- Decreases JRF at joint , F x d = BW (constant)
- Joint stability by increasing joint coverage
-But…abductor muscle length , hence efficiency
(fatigued easily due to shorter resting length)
o

28. Coxa Valga
GT lower than normal
- abductor moment arm (OM, GT moves medially)
- JRF, ( ) work done - abductor to keep pelvis
balanced ( F x d = BW , constant)
- Joint stability
o
M

29. Applied biomechanics in THA
a) Sir John Charnley b) Low friction torque arthroplasty c) 25 year follow-up d) Modular THA prosthesis
1962

30. Principle – Reduce net Joint reaction forces
wear and tear
1) Medialization of acetabulum
( B1-X < B-X)
( Deepening acetabulum reduces BW moment arm)

31. 2) Increased neck length / offset
&
3) Lateralised re-attatchment
of the GT
• Lengthens abductor moment arm

32. Decreased BW moment arm and increased abductor lever arm
Joint reaction forces
wear and tear of implants

33. Functional outcome of THA
Component design Soft tissue function
Component positioning Soft tissue positioning

34. Ultimate goal of biomechanically sound stable hip joint
Careful attention to restoration of normal COR of femoral head
This location determined by 3 factors :
a) Medial / horizontal offset
b) Vertical offset / vertical height
c) Version of femoral neck / anterior offset

35. Medial/ horizontal offset : Centre of head to axis of distal
part of stem
Verical offset / height : determined primarily by base
length of prostheric neck + length gained by the
modular head used
acc to depth implant is inserted
vertical height altered

36. Inadequate medial offset
shorter abductor moment arm
limping Increased JRF Bony
Impingement
Dislocation

37. Excessive medial offset
Biomechanically- Increased stress on cement and stem
loosening stress fracture
Ideally JRF lowest as possible

38. Component orientation
• Acetabular orientation
Anteversion : 15 degrees
Inclination : 40–45 degrees

39. • Femoral stem position
Antiversion : 10 – 15 degrees
• Combined Antiversion : 30 - 35 degrees
(acetabulum + femur)

40. References
• Campbell’s operative orthopaedics 14th Ed. Vol 1
• Biomechanics of hip arthroplasty – Michael M Morlock
• Anatomy and Biomechanics of the hip – Damiene P. Briene et al
• Turek’s Orthoapedics : Principle and their applications – 7th Ed. Vol 1
• Manual on clinical Surgery - S. DAS , 9th Ed

41. THANK
YOU