Pressure Sores

A healthy person can develop a pressure sore in 6 to 12 hours. While 63 percent of patients who sit for unlimited periods of time develop pressure sores, only 7 percent who sit for maximum periods of 2 hours develop pressure sores. Although there are several intrinsic (within the body) risk factors for pressure sores, there is only one extrinsic (coming from outside the body) risk factor for pressure sores: Pressure. Cushions can be used to reduce external pressure. Studies show it is best to change posture frequently and use a medium density, open-cell foam cushion that envelops the buttocks to minimize external pressure.

A Healthy Person can Develop Pressure Sores in 6 – 12 Hours

“A healthy individual can develop pressure sores in six to twelve hours if left undisturbed in the same position, (Hargast, T., 1979; Staggs, K., 1983; Torrence, C., 1981).” (2)

Stand Up at Least Every 2 Hours

In a randomized clinical trial of 57 orthopedic patients Gebhardt and Bliss (1994) found that (1):

  • 63 percent of patients who sat for an unlimited period of time developed pressure ulcers (pressure sores); 
  • 7 percent of patents who sat for maximum periods of 2 hours developed pressure ulcers (pressure sores).

Limiting Sitting Time and Other Preventative Measures for Pressure Sores

    • Limiting sitting time is only one starting point at preventing pressure ulcers (pressure sores). 

Prevention may be directed toward:

    • frequent posture changing; 
    • the use of pressure-reducing cushions; and 
    • sitting in appropriate postures. (1)  

External Pressure is 1/2 Internal Pressure

Externally measured pressure under the butt bones is only one half of the internal pressure.   Given the pressures that are prevalent, Staarink (1995) found it amazing that more people do not get pressure ulcers (1).

Several Intrinsic Factors for Pressure Sores

The authors include the following findings:  “A number of intrinsic (within the body) factors contribute to the development of pressure sores such as:

      • immobility,
      • poor circulation,
      • malnutrition,
      • and elderly skin

Only 1 Extrinsic Factor for Pressure Sores

“The only extrinsic (coming from outside the body) factor is:

      • pressure.”

6 Pressure­-Relieving Devices Tested:

In a Study done by The American Journal of Nursing, 1987,“Sitting Easy:  How Six Pressure-Relieving Devices Stack Up” 6 Pressure-Relieving Devices were Tested (2):

    1. air doughnut pillow;
    2. water donut  pillow;
    3. Eggcrate cushion;
    4. Spencegel pad;
    5. Sheepskin;
    6. Cotton-filled disposable pillow.

Air Donut was Least Effective 

“Of the six different pressure-relieving devices studied, the air donut was least effective in reducing inter surface pressures.  In addition, subjects reported it to be quite uncomfortable to sit on.”

People Should Have More Than 1 Pressure-Relieving Device

All the other devices reduced inter-surface pressure about equally . . . having more than one pressure relieving device to choose from allows selection based on individual patient comfort.

Stress Distribution in a Seated Buttock

A study was done to determine the best foam to use to minimize risk factors for the development of pressure sores. 

The Study titled, “Model Experiments to Study the Stress Distributions in a Seated Buttock,” was published in the J. Biomechanics, Vol. 15, No. 7, 1982. (3) 

Sitting May Lead to Pressure Sore Formation 

“Mechanical stress states that develop in the buttock during sitting may exceed tissue tolerance and lead to decubitus ulcer (pressure sore) formation in susceptible patients.

Risk Factors May be Reduced by using Cushions 

“The danger of this complication can be reduced by using suitable cushions to minimize stress magnitudes and gradients within soft tissues.”

5 Materials Selected for Initial Tests

“Although many cushion materials are in current commercial use, the following five representative materials were selected for these initial tests:

    1. Gel;
    2. Medium density foam;
    3. Soft foam;
    4. Stiff foam;
    5. Viscoelastic ’T-Foam.’”

Medium Density Open Cell Foam – Best

Medium density open-cell foam reduces pressure best.  

Medium density foam reduces pressure better than soft open-cell foam only because the soft open-cell foam tends to “Bottom Out” and cause pressure transmission from the surface below. 

Increasing Compressive Stress Listed 

“In order of increasing maximum compressive stress generated in the buttock model, the material samples of equal thickness can be ranked as follows:

    1. Medium density foam;
    2. Soft foam;
    3. Gel;
    4. Viscoelastic foam;
    5. Stiff foam.”

Enveloping Foam Reduces Sitting Pressure

“The enveloping property of a seat cushion is a measure of its tendency to wrap around the object it supports, (i.e., in the present case, the buttock model).  

Enveloping Foam Provides Large Contact Area

A good enveloping cushion provides a large contact area and a uniform stress distribution, (Chow, 1974; Cochran and Palmieri, 1979).’


  1. Applied Nursing Research, Vol. 12, No. 3, August 1999, pp. 136-142, “Sitting Posture and Prevention of Pressure Ulcers,” written by Tom Defloor, MScN, N.N.; and Maria H.F. Grypdonck, Ph.D., RN, Nursing Sciences, University of Gent, Belgium.
  2. American Journal of Nursing, 1987,“Sitting Easy:  How Six Pressure-Relieving Devices Stack Up”:   written by Robin Chagares, R.N., M.A., M.S.N.; and Bettie S. Jackson, RN, Ed.D., F.A.A.N., Montefiore Medical Center, N.Y., U.S.A.
  3. J. Biomechanics, Vol. 15, No. 7, 1982, “Model Experiments to Study the Stress Distributions in a Seated Buttock,” Narender P. Reddy, Himanshu Patel, George Van B. Cochran, Biomechanics Research Unit, Helen Hayes Hospital; and John B. Brunski, Center for Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, N.Y., U.S.A.

Leave a Reply

Your email address will not be published. Required fields are marked *