This document presents a problem solving steps for calculating the resultant hip joint force (J) given new positions of the acetabulum. When the acetabulum is moved medially, the problem asks to calculate J. The document lists the given information, assumptions made, and outlines the steps to set up and solve the static equilibrium equations to find J. These steps include: drawing a free body diagram of the forces on the hip; selecting a point to take moments about; writing the moment equilibrium equations; and solving the equations to find the unknown force J.
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3. Summarize the problem…
What is J when the acetabulum is moved
medially on a patient standing on their right leg
given the information on the next slide?
4. Collect Information
• Non-medial
• Distance A = 50 mm =
0.05 m
• Distance B = 100 mm =
0.1 m
• J = Resultant Hip Joint
Force = 1800 N
• AT = Abductor Tension =
1200 N
• FBW = Body weight at
pubis = 5/6 x BW = 5/6 x
720 N = 600 N
• Medial
• Distance A = 50 mm =
0.05 m
• Distance B = 50 mm =
0.05m
• J = Resultant Hip Joint
Force = UNKNOWN
• AT = Abductor Tension =
UNKNOWN
• FBW = Body weight at
pubis = 5/6 x BW = 5/6 x
720 N = 600 N
5. Assumptions?
• Assumptions for this problem include:
• 1) This material exhibits ideal behavior for the models
used. This is termed a LEHI material.
• 2) 2D problem: this simplifies calculation of forces
• 3) Ignore all other forces: just focusing on the hip.
• 4) Hip in static equilibrium: allows for calculations to be
set to zero.
• 5) Forces are perpendicular to moment arm: this
eliminates the need for force calculations regarding
different angles, although some problems might
contain angles this one does not.
6. Isolate the area to be analyzed before drawing the free body diagram
18. Looking Back
Eliminating Answers…
M = r x F
M = r x F
Fc
J
M = r x F
Red – Medial hip body weight moment
Green – Medial and nonmedial hip moment from J
Dark Blue – Nonmedial hip moment from body weight
Fc