1. Hip joint anatomy and biomechanics
Dr. Shirish Karki
MS Ortho, Resident
NAMS

2. Overview
• Enormous volume of literature concerns the
anatomy and biomechanics of the hip
• Little of it is specifically organized from the
perspective of total hip arthroplasty
• The section on anatomy - encountered at hip
arthroplasty from the perspective of the
surgeon

3. Few terms and facts
• Statics
• Dynamics
– Kinematics- study of motion
– Kinetics- action of forces on bodies to
their resulting action
– Kinesiology- study of human motion

4. Few terms and facts
Kinematics
• Flexion- 120 degrees
• Extension 30
• Abduction 50
• Adduction 30
• External rotation 45
• Internal rotation 45

5. Few terms and facts
Kinetics
• Joint reaction force- 3 times in single leg
stance
• 5 times in walking
• Twice during SLR
• Upto 10 times while running

6. Few terms and facts
This equation plays a paramount role
in determining the modality of
management in hip disorders

7. Few terms and facts

8. • Basically tubular strt with bows and twists
• The anterior bow is well recognized and adapted
into prosthesis
• The posterior bow of the proximal femur is just as
constant as the midportion anterior bow
• This bow is constant and the radius of curvature
does not seem to change dramatically with the
size of the femur
Osteology

9. The Neck-Shaft Angle
• The center of the femoral head is extended
medially and proximally by the femoral neck
so that the center of the femoral head is at
the level of the tip of the trochanter.
• The effect of the overhanging head and neck
is to lateralize the abductors, which attach to
the greater trochanter, from the center of
rotation (center of the femoral head).

10. The Neck-Shaft Angle
• Reducing this level arm (coxa valga) increases total load across
the hip, and coxa vara reduces it to the extent it increases the
lever arm. (Coxa vara with a short neck would have a negative
affect.)
• This increases the torque generated by the abductors and
reduces the overall force necessary to balance the pelvis
during single leg stance.

11. Femoral Anteversion
• The coronal plane of the femur is generally
referenced to the posterior distal femoral
condyles.
• When oriented in this plane, it can be seen
that the proximal femur, including the femoral
head and neck, are rotated anteriorly.
• This is commonly referred to as femoral head-
neck anteversion.
• Normal angle – 10-15o anteriorly

12. Femoral Anteversion

13. Distribution of Cancellous
Bone in the Proximal Femur
• It appears to be a characteristic of the articulating ends
of long bones that the broad ends, covered with
articular cartilage, are supported principally by
cancellous bone and a very rudimentary cortex in the
form of a subchondral plate.
• The forces applied to the articular surfaces are carried
by the cancellous bone out to the cortex.
• The orientation of the trabecular pattern may be
significantly disturbed in the diseased hip, the overall
distribution of cancellous bone is still the same.

14. Cross-Sectional Analysis

15. Cross-Sectional Analysis

16. Acetabulum
• The acetabulum is formed by the confluence
of the ilium, pubis, and ischium at the
triradiate cartilage
• The normal bony acetabulum is slightly less
than a hemisphere, but its functional
dimensions are extended by the tough
fibrocartilaginous labrum

17. Orientation of the acetabulum
controversy in the literature concerning the
orientation of the acetabulum
• Getz, Steindler, and von Lanz all give figures for acetabular
anteversion of around 4O° (38- 42°)
• They made their measurements with the pelvic brim
approximately horizontal, where the acetabula are obviously
facing forwards. However, in the erect position, the anterior-
superior iliac spines and pubic symphysis are in the same
plane and the acetabulae are not as obviously anteverted

18. Orientation of the acetabulum
• McKibbin measured 30 each adult male and female
pelvises, oriented this position, and recorded an average
anteversion of 14° (5-19°) for men and 19° (10-24°) for
women
• When the pelvis is flexed, as it is in sitting, the forward facing
of the acetabulum is accentuated.
• In the erect position, the anterior-superior iliac spines and the
symphysis pubis lie in the coronal plane and the acetabulum
opening is directed approximately 45° laterally and 15°
forward.
• Flexion of the pelvis on the lumbar spine increases the
apparent anteversion without greatly changing the apparent
lateral opening, whereas lumbar lordosis does the opposite

20. Biomechanics of the Hip
• When the weight of the body is being borne
on both legs, the center of gravity is centered
between the two hips and its force is exerted
equally on both hips
• The weight of the body minus the weight of
both legs is supported equally on the femoral
heads, and the resultant vectors are vertical

21. Double leg stance

22. Double leg stance
• When the hips are viewed in the sagittal plane and if the
center of gravity is directly over the centers of the femoral
heads, no muscular forces are required to maintain the
equilibrium position, although minimal muscle forces will
be necessary to maintain balance.
• If the upper body is leaned slightly posteriorly so that the
center of gravity comes to lie posterior to the centers of the
femoral heads, the anterior hip capsule will become
tight, so that stability will be produced by the Y ligament of
Bigelow.
• Therefore, in symmetrical standing on both lower
extremities, the compressive forces acting on each femoral
head represent approximately one-third of body weight

23. Single leg stance
• In a single leg stance, the effective center 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.
• Since the pillar of support is eccentric to the line of
action of the center of gravity, body weight will exert a
turning motion around the center of the femoral head.
This turning motion must be offset by the combined
abductor forces inserted into the lateral femur.
• In the erect position, this muscle group includes the
upper fibers of the gluteus maximus, the tensor fascia
lata, the gluteus medius and minimus, and the
pyriformis and obturator internus.

24. Single leg stance
M
C
O
B
• M= Line of action
• BO= effective lever arm
of the resultant force
• OC= effective lever arm
of body weight acting
through the center of
gravity
• R= Resultant force
K
R

25. Single leg stance
• The combined resultant vector of the abductor group
can be represented by the line of action M in Figure.
• Since the effective lever arm of this resultant force (BO)
is considerably shorter than the effective lever arm of
body weight acting through the center of gravity
(OC), the combined force of the abductors must be a
multiple of body weight.
• The vectors of force K and force M produces a resultant
compressive load on the femoral head that is oriented
approximately 16° obliquely, laterally, and distally. The
orientation of this resultant vector is exactly parallel to
the orientation of the trabecular pattern in the femoral
head and neck

26. Practical aspect
• The effect of this combined loading of body
weight and the abductor muscle response
required for equilibrium results in the loading of
the femoral head to approximately 4 times body
weight during the single leg stance phase of gait
• This means that in normal walking the hip is
subjected to wide swings of compressive loading
from one-third of body weight in the double
support phase of gait to 4 times body weight
during the single leg support phase

27. Practical aspect
• The factors influencing both the magnitude
and the direction of the compressive forces
acting on the femoral head are
1) the position of the center of gravity
2) the abductor lever arm, which is a
function of the neck-shaft angle and
3) the magnitude of body weight.

28. Practical aspect
• Shortening of the abductor lever arm through
coxa valga or excessive femoral anteversion will
result in increased abductor demand and
therefore increased joint loading
• If the lever arm is so shortened that the muscles
are overpowered, then either a gluteus minus
lurch (the center of gravity is brought laterally
over the supporting hip) or a pelvic tilt
(Trendelenburg gait) will occur

29. Limping
• Since the loading of the hip in
the single leg stance phase of
gait is a multiple of body
weight, increases in body
weight will have a particularly
deleterious effect on the total
compressive forces applied to
the joint.
• The effective loading of the
joint can be significantly
reduced by bringing the
center of gravity closer to the
center of the femoral head

30. Limping
• Sideways limping requires
acceleration of the body
mass laterally, its
deceleration during the
stance phase of gait, and
then its acceleration back
to the midline or even to
the other side as the
single leg stance phase
changes to the opposite
extremity. This requires
considerable energy
consumption.
•Another effect of sideways
limping is that the resultant
vector becomes more vertical
because the center of gravity
is acting in a more vertical
direction, and therefore the
bending moment the femoral
neck is increased

31. Cane walking
• Use of walking stick in the
opposite hand.
• Since some of its force is
transferred to the walking
stick through the hand, the
effective load of body weight
is thus reduced in two ways:
1) the effective load of body
weight is reduced; 2) since
the turning moment around
the femoral head is
reduced, the abductor
demand is also reduced