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BIOMECHANICS OF HIP
By : Dr . Belehalli Pavan
INTRO AND HISTORY
• Pauwel ( 1935 ) – First described the
mechanical aspects of functioning of a hip joint.
• Delineated the direction and magnitude of forces
across the hip in normal and pathological
conditions.
BASIC DEFINITIONS
• Biomechanics – Science of action of forces
internal or external on the living body.
• Dynamics – Study of motion of bodies and
forces that produce the motion. Studied under 3
a) Kinematics – Study of motion in terms of
displacement velocity and acceleration without
ref to the cause of motion.
DYNAMICS (contd)
b) Kinetics – Relates to action of forces on
bodies to their resultant action.
c) Kinesiology – Study of human movement or
motion.
Def related to Biomechanics
• FORCE - Push or pull causing int or ext effects
- Can be split into their components in x and y
axes.
- Resultant force – Single force equivalent to a
system of forces ; Calculated by adding all the
forces acting at a point.
- Equilibriant force – Force equal in magnitude
and opposite in direction to Resultant force.
Def (contd)
• Moment (M)Rotational effect of a
equlas Force
perpendicular
distance from a
specified point
(moment arm) M = F
*d
Def ( contd )
• Free Body
Diagram
( FBD ) – Skeleton of a
body or portion
thereof isolated from
all other bodies
showing all forces
acting on it.
Defn ( Contd )
• Degree of freedom – Described based on
rotation and translation in x , y and z axes.
• Joint reaction force (R) – Forces generated
within a joint in response to external forces.
• Instant center 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 .
Hip Biomechanics
• Forces acting across a hip joint can be measured
either
▫ Directly with implanted strain-gauged
endoprosthesis
▫ Mathematical model calculations – 2D static
analysis
FBD ANALYSIS
A
B
• R in the hip can reach
three to six times body
weight (BW) and is
primarily due to
contraction of muscles
crossing the hip. This can
be demonstrate with a
FBD.
• If A=5 and B=12.5, using
standard FBD analysis,
Joint Reaction Force R =
4W.
Analysis (Cntd)
• By the fig we can see that
the R can be decreased
by
• 1. Medialisation of
Acetabulum
• 2. Long neck Prosthesis
• 3. Lateralization of
Greater Trochanter.
FORCES IN A TWO LEG STANCE
• BW- equally distributed
across both hips.
• Lower limbs – 2/6th of
BW
• Upper limbs and trunk –
4/6th
• Therefore each hip bares
1/3rd of BW
• Direction of action of BW
– directly perpendicular
to ground.
SINGLE LEG STANCE
• Forces on head of femur
= 2 to 3 times BW.
• Rt LL supports BW +
Weight of Left lower
extremity ,
i.e. 4/6th + 1/6th = 5/6th
• Rt Abductors exert
downward counter
balancing force.
• Center of Gravity shifts to
left.
SINGLE LEG STANCE
• Resultant force originates at an angle of 165 – 170 deg
from vertical axis.
• Resultant force = Force by abductors * Distance from
• This should be equal to partial BW * Distance from
• If abductor lever arm is 5 cm
• BW arm = 10 cm
• Then, abd force * 5 cm = HATL * 10 cm
SINGLE LEG STANCE
• By calculation abd force
comes to 2*BW = 140 kg
• Joint load = HATL + abd
force; 70+140=210 kg i.e.
3 times the BW.
Bio Mechanics In Neck Deformities
Coxa Vara
•
•
•
•
Decreased neck shaft angle
Increased abd lever arm
Abd muscle length shortened
Decreased joint forces across
during one leg stance
Coxa Valga
•
•
•
•
Increased neck shaft angle
Abd arm is shortened
BW remains same
Increased joint forces in hip
during one leg stance
• Valgus Osteotomy may
move GT laterally
improving the strength of
contraction but joint
forces may increase
Biomechanics
• Improving abductor
function will decrease
joint reactive forces
Bio Mechanics of Walking Stick
• Cane in Contralateral
hand decreases JRF
• Long moment arm makes
so effective
• 15% BW to cane reduces
joint contact forces by
50%
• Decreases the body
turning movement and in
turn R by exerting a force
on a cane in the opposite
direction.
Bio Mechanics of CDH
• GT is closer to the fulcrum
• Force acts on the upper border of actabulum
instead of being distributed over the whole
• Excessive pressure exerted over a small area
gives rise to degeneration
Bio Mechanics of Trendlenberg Gait
• Abd muscle force is in-adequate. Due to deformities that
- Require increased force to maintain equilibrium
- Or reduce the effectiveness of muscle contraction
• By an adequate movement during single leg stance the
centre of gravity can be shifted over the hip joint so that
an unstable equilibrium exists and no abd muscle force
is required.
Bio Mechanics of OA
• Contact area decreased due to deformity and loss of
sphericity
• Concentrates the compressive forces on to a smaller area
• Patient decreases this load by tilting towards the affected
side in stance phase thereby decreasing partial BW lever
arm
• Thus the rationale of decreasing BW in OA patients to
decrease the abd force to counter balance and ultimately
decreasing R
Bio Mechanics of THR
• Centralization of femoral head by deepening the
acetabulum decreases the BW lever arm
• Lateral attachment of GT lengthens the abd lever arm
• Lengths of the lever arms are surgically changed to
approach a ratio of 1:1 which reduces total hip load by
30%
• Abd lever arm can be increased by increasing the medial
offset of the femoral component or lateral or distal
reattachment of GT.
FEMORAL OFFSET?!!
• The femoral offset is the
distance from the centre of
rotation of the hip joint to
the line of action of the femur
• The BW of the person will
create a clockwise (negative)
centre of the hip. This torque
must be balanced by an equal
but opposite
counterclockwise (positive)
moment produced by the
abductor muscle force.
Offset (Cntd)
• The moment arm of the abductor muscle is
directly related to the magnitude of the femoral
offset. As the offset increases in length the force
required by the abductor muscles to balance the
BW moment would reduce, thereby increasing
the efficiency of the abductors.
Bio Mechanics of THR (Cntd)
• R is minimal if hip centre was placed in
anatomical position
• Adjustment of neck length is important as it has
effect on medial and vertical offset.( (N) 25 – 50
mm)
• Excessive medial offset results in stress fracture,
loosening
• Inadequate medial offset results in limp,
impingement and dislocation
Last but not the least!!
“The importance of Bio Mechanics as the
foundation for understanding the normal and
pathalogical states of different joints and the
treatment options involved cannot be under
estimated.”
REFERRENCES
Campbell’s Operative Orthopaedic Surgery
Textbook of orthopedics and traumaG.S.Kulkarni 4:2888
Mercer’s Orthopaedic Surgery 789-791
Current Orthopaedics (2006) 20, 23–31
Basic biomechanics of human joints: Hips, knees
and the spine T.D. Stewarta,, R.M. Halla,b
QUESTIONS???
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