Pressure Distribution

Question 1

Pressure Formula

Answer 1

Force/Area

Question 2

Why is pressure important?

Answer 2

• Tells us tissue integrity and possible causes of pain

Question 3

How does pressure lead to skin breakdown?

Answer 3

• Pressure creates stress (Shear and Tensile)
• Tensile: Surgace between bone and tissue
• Shear: Between layers of tissue (Ex: Blister)
• We are only able to measure the surface pressure which can relate to compressive stress. Internally may be worse.

Question 4

Shear can cause occlusion of ____.

How does this occur?

Answer 4

• capillaries
• shearing of the layers compresses the vessel

Question 5

Properties of a good sensor

Answer 5

• Sensor Size
• Sensor Height
• Sensor drift, hysteresis, temperature drift
• Calibration

Question 6

Properties of a force transducer and pressure sensors; what is same, what is different?

Question 7

Types of sensors for pressure

Answer 7

• Air
• Resistive (FSR: Force resisting resistor)
• Capacitance

Question 8

This is a ____ sensor.

Give an example of one that is commonly use in PT

Answer 8

• Pneumtic Cuff

Examples
* BP cuff; pump it up and you get a pressure value
* Cranio-cervical Flexion Test; Apply force to the bladder by chin tuck.

Question 9

Resistive Sensor

Answer 9

• Circuit; apply more or less force, the resistance changes
• More accurate with air sensors
• More expensive because it requrie large amount of power; therefore often not portable

Question 10

This is a ____ resistor

Answer 10

force sensing

Question 11

This is a ____ sensor.

Answer 11

• Sandwich with foam in the middle that is compressed (Elastomear). As the size of foam changes size, so does the capacitance.
• Similar to a burger, doesn’t want to spring back as normal.
• Medium pressure range of standing and walking
• Insoles for high pressure of running
• Low pressure for mats or seats (conforms to surface well)

Question 12

Why do we often measure bony geometry with sensors?

Answer 12

• They often have the highest pressure and leads to ulcers or injury

Question 13

Why is the size of the measurement for sensors important?

Answer 13

• Too large of an area can’t measure extreme pressure at smaller pressure points
• Large sensors lose peak pressures of small areas.

Question 14

Why is sensor size important?

Answer 14

• It is important because the more sensors in the same area results in a higher resolution to specifically tell us where high pressures are at
• Can’t compare new data to old data necessarily because they didn’t have as great resolution back then!

Question 15

Sensor Height

Answer 15

• We want short sensors!
• Kelvins Law: Don’t want to alter movement.
• Devices deform both body 1 and 2. Therefore, the smaller it is the less it will warp the bodies.

Question 16

Sensor Hysteresis

Answer 16

• Pressure sensors generally have higher hysteresis aka loading rate than force transducers
• Example: Memory Foam
• Need to understand hysteresis as we will need to apply our understanding of the movement that is taking place as the decreasing load may be altered and incorrect. Ex: Gait; first half is correct, second half is skewed due to slow hysteresis

Question 17

Calibration Schematic

Answer 17

• Sensors in a device that give a known pressure through a rubber membrane to calibrate machine.
• Each sensor cell has a unique calibration curve given applied pressure
• Thus, each insole, platform, or mat is different.
• Ex: Low pressure linear, at upper ranges curves out and is now not linear and may not form back to original state

Question 18

Types of Measurement

Answer 18

• Use of discrete sensors (bony landmarks/prior knowledge of high pressure areas)
• Matrix of individual sensors (Entire area, compare one region to the next)

Question 19

How does a sensor matrix work?

Answer 19

• Sensors at all locations
• Reads everything on one cell then moves on to read the next, etc.
• Cannot sample fast
• We want less sensor cell as otherwise may only capture extremely slow movements

Question 20

How do you calculate contact area from sensors? Solve this problem

Answer 20

• F = PxA
• Need to calculate force for every sensor
• F = (0 x 2) + (20 x 2) + (30 x 2) + (20 x 2) + (40 x 2) + (5 x 2) = 230N

Question 21

What are the most reliable pressure distibution measurements?

Answer 21

• Timing and spaital (step and stride length)
• Contact/No contact; Pressure switches

Question 22

Least Reliable GAITRite Measures

Answer 22

• Software identify the shape of foot (If contact area is not similar to foot will not register properly)
• Dependent on good recognition of foot shape to create step width and toe out angle
• Poorest Measurement (Pressure related; Peak pressure, pressure-time)

Question 23

In general, we want to use ____ for pressure platforms

Answer 23

smaller sensor size (higher resolution)

Question 24

Practical Considerations for pressure distribution

Answer 24

• Gait Speed: Timing or metronome
• In shoe: footwear or orthoses
• Seating: Time, surface, body position

Question 25

Maximum Pressure Picture (MPP)

Answer 25

• Artificial picture of the foot with pressure values
• Can determine peak pressure, total contact area, and peak force

Question 26

Variables for Pressure Measurements

Answer 26

• Total vs Regional Values
• Peak Pressure
• Pressure Time Impulse
• Peak Force
• Force Time Impulse
• Contact Area
• Timing Measurements

Impulse is not as accurate