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Updated: Jan 16 2022

Hip Biomechanics

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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
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