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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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