Hip biomechanics

PRESENTER : DR. SUDHEER KUMARPOST GRADUATE IN ORTHOPAEDICS

NARAYANA MEDICAL COLLEGE

BIOMECHANICS OF HIP

INTRODUCTION

BIOMECHANICS – Science that deals with the study of forces (internal or external ) acting on the living body

HIP - Mobile as well as stable• Strong bones

• Powerful muscles

• Strongest ligaments

• Depth of acetabulum , narrowing of mouth by acetabular labrum

• Length and obliquity of neck of femur

• MOBILITY is due to the long neck which is narrower than the diameter of the head

The Neck of Femur • Angulated in 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

Acetabular Direction • long axis of acetabulum points – forwards : 15-200

ante version

– 450 inferior inclinationante version

Axis of lower limb Mechanical axis line passes

between center of hip joint and center of ankle joint.

Anatomic axis line is between tip of greater trochanter to center of knee joint.

Angle formed between these two is around 70

Biomechanics- HIP• First order lever

fulcrum (hip joint) forces on either side of fulcrum i.e, body weight & abductor tension

To maintain stable hip, torques produced by the body weight is countered by abductor muscles pull.

Abductor force X lever arm1 = weight X leverarm2

Biomechanics

Biomechanics

• Forces acting across hip joint

Body weight Abductor muscles

force 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 momentarms of the body weight and abductor tension

maintains a level pelvis

Joint reaction force -2W during SLR - 3W in single leg stance -5W in walking -10W while running

Joint reaction force

Coupled forces:

Certain joints move in such a way that rotation about one axis is accompanied by an obligatory rotation about another axis & these movements are coupled

Joint congruence – the proper fit of two articular surfaces, necessary for joint motion

Instant centre 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

Centre of gravity• Wts. of the objects act through the centre of

gravity.• In humans just anterior to S2

Forces across the hip joint in two leg stance

• L.L constitute 2/6 (1/6 + 1/6), and U.L & trunk constitute 4/6 the total body wt

• Little or no muscular forces required to maintain equilibrium in 2 leg stance

• Body wt is equally distributed across both hips

• Each hip carries 1/3rd body weight

– (4/6 = 2/3 = 1/3 + 1/3)

Single leg stance - Right• Rt. LL supports the body wt & also the Lt

LL’s i.e. 5/6th total body wt.

• Effective Centre of gravity shifts to the non-supportive leg (L) & produces downward force to tilt pelvis

• Rt .abductors must exert a downward counter balancing force with right hip joint acting as a fulcrum.

i.e. Body wt acts eccentrically on the hip and tends to tilt the pelvis in adduction ----- balanced by the abductors

4/6 +1/6 =5/6

Typical levels for single leg stance are 3W, corresponding to a level ratio of 2.5.

Single leg stance - Right

4/6 +1/6 =5/6

• Rt. LL supports the body wt & also the Lt LL’s i.e. 5/6th total body wt.

• Effective Centre of gravity shifts to the non-supporting leg(L) & produces downward force to tilt pelvis

• Rt. abductors must exert a downward counter balancing force with right hip joint acting as a fulcrum.

i.e. Body wt acts eccentrically on the hip and tends to tilt the pelvis in adduction ----- balanced by the abductors

Typical levels for single leg stance are 3W, corresponding to a level ratio of 2.5.

USE OF CANE / WALKING STICK • It creates an additional force that keeps the pelvis level in the face

of gravity's tendency to adduct the hip during unilateral stance.

• decreases the moment arm between the center of gravity and the femoral head(R)

• The cane's force must substitute for the hip abductors.

• Long distance from the centre of hip to contralateral hand offers excellent mechanical advantage

USE OF CANE / WALKING STICK

Cane and Limp • Both decrease the force

exerted by the body wt on the loaded hip

• Cane: transmits part of the body wt to the ground thereby decreasing the muscular force required for balancing

• Limping shortens the body lever arm by shifting the centre of gravity to the loaded hip

Stand on LEFT leg—if RIGHT hip drops, then it's a + LEFT

Trendelenburg

The contralateral side drops because the ipsilateral hip

abductors do not stabilize the pelvis to prevent the droop.

TRENDELENBURG SIGN

normal

affected

1 2

Biomechanics in neck deformities :

Coxa valga• Increased neck shaft angle

• GT is at lower level

• Shortened abductor lever arm

• Body wt arm remains same

• Increased joint forces in hip during one leg stance

• Less muscle force required to keep pelvis horizontal

Coxa valga

Resultant force R is more than a normal hip

Coxa Vara • Decreased neck shaft angle

• GT is higher than normal

• Increased abductor lever arm

• Abductor muscle length is shortened

• Decreased joint forces across the hip during one leg stance

• Higher muscle force is required to keep pelvis horizontal

Coxa Vara

Resultant force R is less than a normal hip

WITH WEIGHT GAIN

• Abductor muscular forces are to be increased to counteract body wt

• Increased joint forces across the joint leading to increased degeneration

• Rationale of decreasing body wt in OA – decrease in body wt force & hence abductor force required to counter balance

decreasing joint reaction forces across that hip

Biomechanics of THR Principle – to decrease joint reaction force

• Centralization of femoral head by deepening of Acetabulum - decreases body wt lever arm

• Increase in neck length and Lateral reattachment of trochanter - lengthens abductor lever arm

• This decreases abductor force, hence joint reaction force, & so the wear of the implants.

Joint reaction forces are minimal if hip centre placed in anatomical position

Adjustment of neck length is important as it has effect on both medial offset & vertical offset

Offsets………

• Vertical Ht (offset) Determined by the Base length of the Prosthetic neck and length gained by the head

• Horizontal Offset

(Medial offset) center of the head to the axis of the stem

IF……….• Medial offset is inadequate shortens the moment arm

limp, increase bony impingement

• Excessive medial offset – dislocation, increases stress on stem & cement stress # or loosening

• In regular THR , the Femoral component must be inserted in the same orientation as the femoral neck to achieve the rotational stability .

• Modular component in which stem is rotated independently of the metaphyseal portion

• Anatomical stems have a few degrees of ante version built into the neck

HEAD DIAMETER

• Large diameter head compared to Small head

– Less prone for dislocation – Range of motion is more

• Femoral components available with a fixed neck shaft angle -135º

• Restoration of the neck in ante version - 10-15º

– Increased ante version anterior dislocation

– Increased retroversion posterior dislocation

• Cup placed in 150-200 of ante version and 450 of inclination

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