Updated: 6/9/2021

Hip Biomechanics

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Questions
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Evidence
9
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Cases
1
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  • Joint Biomechanics Definitions
    • 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
    • Coupled forces
      • when two movements and associated forces are coupled
    • Joint congruence
      • relates to fit of two articular surfaces
      • high congruence increases joint contact area
    • Instant center of rotation
      • point about which a joint rotates
      • often changes during rotation due to joint translation
      • center of gravity of human is just anterior to S2
    • Friction and lubrication
      • not a function of contact area
      • lubrication decreases friction
      • examples
        • coefficient of friction of human joints is .002 to .04
        • TJA (metal on PE) is .05 to .15
  • Free Body Analysis Definitions
    • Newtons laws
      • first law
        • if there is no net force on an object, its velocity remains constant
      • second law
        • force equals mass multiplied by acceleration
        • F=ma
      • third law
        • when a first body exerts a force on a second body, the second body exerts a force that is equal in magnitude and opposite in direction on the first body
        • F2=-F1
    • Force
      • definition
        • a push or pull on an object resulting from the object's interaction with another object
      • equation
        • force = mass x acceleration, F=ma
        • 1 Newton = force required to give 1 kg mass an acceleration of 1 m/s2
    • Vector
      • definition
        • a quantity that contains both direction and magnitude
        • scalar quantities do not have direction
      • forces and velocity can be broken down into vectors
    • Moment (torque)
      • definition
        • the tendency of a force to rotate a body around an axis
      • equation
        • moment (torque) = force(perpendicular) X distance
    • Work
      • definition
        • when a force acts upon an object to create displacement
      • equation
        • work = force (vector parallel to displacement) x distance
    • Energy
      • definition
        • ability of an object to perform work
      • classification
        • potential energy
          • equation
            • U (potential energy) = mass x gravity x height
        • kinetic energy
          • equation
            • KE (kinetic energy) = 1/2mv2
  • Hip Free Body Analysis
    • Free body analysis diagram
      • W = gravitational force
        • weight of the body minus weight of ipsilateral extremity (or 5/6 body weight)
      • M = abductor muscle force
      • R = joint reaction force
        • can reach 3 to 6 times body weight
    • Solving for joint reaction force (R)
      • step 1: calculate My
        • principle
          • sum of all moments equals 0
          • in this case, the moments are created by My and W
        • equation
        • (A x My) + (B x W) = 0
          • assume A = 5cm and B = 12.5cm (this information will be given to you)
          • My = 2.5W
      • step 2: calculate Ry
        • Ry = My + W
        • Ry = 2.5W + W
        • Ry = 3.5W
      • step 3: calculate R
        • R = Ry / (cos 30°)
          • R = 3.5W / (cos 30°)
          • R = ~4W
  • Clinical Implications
    • Actions that decrease joint reaction force include
      • increase in ratio of A/B (shift center of rotation medially)
        • acetabular side
          • moving acetabular component medial, inferior, and anterior
        • femoral side
          • increasing offset of femoral component
          • long stem prosthesis
          • lateralization of greater trochanter
            • by using increased offset neck/prosthesis
          • varus neck-shaft angulation
            • increases shear across joint
        • patient's gait
          • shifting body weight over affected hip
            • this results in Trendelenburg gait
          • cane in contralateral hand
            • reduces abductor muscle pull and decreases the moment arm between the center of gravity and the femoral head
          • carrying load in ipsilateral hand
            • produces additional downward moment on same side of rotational point
    • Actions that increase joint reaction force include
      • valgus neck-shaft angulation
        • decreases shear across joint
Questions (5)

(OBQ11.221) A cane held in the contralateral hand reduces joint reactive forces through the affected hip approximately 50% by which of the following mechanisms?

QID: 3644
1

Reducing hip abductor muscle pull

73%

(3403/4675)

2

Increasing hip flexor muscle pull

1%

(41/4675)

3

Moving the center of rotation for the femoroacetabular joint

21%

(979/4675)

4

Increasing joint congruence at the femoroacetabular joint

1%

(50/4675)

5

Moving the center of gravity posterior to the second sacral vertebra

4%

(165/4675)

L 3 C

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(OBQ09.172) Figure A represents a free body diagram of the hip of a patient standing on the right leg. The forces and distances are labeled on the diagram and the resulting hip joint force (J) = 1800N. What is the resultant value for J when the acetabular component is medialized given the new distances shown in Figure B?

QID: 2985
FIGURES:
1

1000N

5%

(171/3130)

2

1200N

64%

(2002/3130)

3

1800N

8%

(237/3130)

4

2200N

10%

(316/3130)

5

3600N

11%

(349/3130)

L 3 C

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(OBQ05.76) Patients display a Trendelenburg gait to compensate for weakness in which of the following muscle groups?

QID: 962
1

hip adductors

1%

(18/1570)

2

hip abductors

97%

(1519/1570)

3

hip flexors

1%

(11/1570)

4

knee extensors

0%

(2/1570)

5

hip extensors

0%

(7/1570)

L 1 D

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Evidence (9)
VIDEOS & PODCASTS (2)
CASES (1)
EXPERT COMMENTS (33)
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