Pressure Formula
Force/Area
Why is pressure important?
- Tells us tissue integrity and possible causes of pain
How does pressure lead to skin breakdown?
- 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.
Shear can cause occlusion of ____.
How does this occur?
- capillaries
- shearing of the layers compresses the vessel
Properties of a good sensor
- Sensor Size
- Sensor Height
- Sensor drift, hysteresis, temperature drift
- Calibration
Properties of a force transducer and pressure sensors; what is same, what is different?
Types of sensors for pressure
- Air
- Resistive (FSR: Force resisting resistor)
- Capacitance
This is a ____ sensor.
Give an example of one that is commonly use in PT
- 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.
Resistive Sensor
- 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
This is a ____ resistor
force sensing
This is a ____ sensor.
- 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)
Why do we often measure bony geometry with sensors?
- They often have the highest pressure and leads to ulcers or injury
Why is the size of the measurement for sensors important?
- Too large of an area can’t measure extreme pressure at smaller pressure points
- Large sensors lose peak pressures of small areas.
Why is sensor size important?
- 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!
Sensor Height
- 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.
Sensor Hysteresis
- 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
Calibration Schematic
- 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
Types of Measurement
- Use of discrete sensors (bony landmarks/prior knowledge of high pressure areas)
- Matrix of individual sensors (Entire area, compare one region to the next)
How does a sensor matrix work?
- 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
How do you calculate contact area from sensors? Solve this problem
- 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
What are the most reliable pressure distibution measurements?
- Timing and spaital (step and stride length)
- Contact/No contact; Pressure switches
Least Reliable GAITRite Measures
- 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)
In general, we want to use ____ for pressure platforms
smaller sensor size (higher resolution)
Practical Considerations for pressure distribution
- Gait Speed: Timing or metronome
- In shoe: footwear or orthoses
- Seating: Time, surface, body position
