A Pressing Question - How Much Contact Pressure Is Too Much?

Size: px

Start display at page:

Download "A Pressing Question - How Much Contact Pressure Is Too Much?"

Hugo Neal
5 years ago
Views:

1 A Pressing Question - How Much Contact Pressure Is Too Much? 10th Applied Ergonomics Conference March 15, 2007 Tom Albin Auburn Engineers ENGINEERS

2 Support vs. Contact Pressure Providing support to a worker is generally seen as beneficial However, support implies contact pressure Contact pressure is an issue of concern ENGINEERS

3 Benefits of Forearm/Hand Support Well Established Rempel et al Showed benefit of a forearm support Marcus et al Lower incidence of MSDs when keyboard was at least 13 cm from edge of desk Berqvist et al Support decreased incidence of MSD symptoms ENGINEERS

4 Avoid Contact Pressure! OSHA NIOSH/CalOSHA ENGINEERS

5 Which of These Involves Suspect Levels of Contact Pressure? qsubmerging your hand in 14 deep in water qsleeping on a mattress. qsitting in a chair. qresting your arms on a desk ENGINEERS

6 Avoiding Contact Pressure ENGINEERS

7 What is Pressure? Pressure is a Force (such as weight) applied over an area Pressure = Force/Area ENGINEERS

8 Some Depictions of Contact Pressure ENGINEERS

9 Compression Effects on Tissue General Discomfort Skin and Muscle Breakdown, necrosis Nerves Changes in cell structure Changes in conduction Tendon Increased fibrocartilagenous content Increased ability to hold water ENGINEERS

10 Capillary Closing Pressure A primary mechanism of tissue injury Excessive externally applied pressure collapses capillaries Tissues become ischemic 30 mm Mercury (~ 0.5 psi) is rule-of-thumb capillary closing pressure However, capillary closing pressure is related to diastolic blood pressure (higher BP, higher closing pressure) ENGINEERS

11 External Pressure Greater Than Internal Pressure Collapses Capillaries Open Capillary Collapsed Capillary ENGINEERS

12 Upper Extremity Contact Pressure A complex picture ENGINEERS

13 Pressure to Hand Causes Discomfort Pressure Pain Threshold (PPT) Kilbom et al Pressure applied to the palm until maximum tolerable pressure is reached instantaneously ~ 750 (± 240) KPa [110 (±35)] psi for males ~ 440 (± 155) KPa [64 (± 22)] psi for females The tolerance for sustained pressure is lower ~ 104 KPa (15.1 psi) for males ~ 37 KPa ( 5.4 psi) for females ENGINEERS

14 Is Externally Applied Pressure Transmitted Undiminished to Internal Structures? ENGINEERS

15 Tourniquet Studies Hargens et al found that internal pressure is different than the applied pressure ENGINEERS

16 Carpal Tunnel Study Cobb et al measured Intracarpal Tunnel Pressure (ICTP) as a function of externally applied pressure Highest ICTP was about 135 mm Hg (2.6 psi) resultant from an applied pressure of about 500 kpa (72 psi or 3740 mm Hg) ENGINEERS

17 Pressure Effects (Hand/Wrist) Sustained Pressure Tolerance ~ 5.4 psi (External Pressure) Immediate Discomfort PPT (External Pressure ) Pins and Needles at mins. (~ 1.7 psi ICTP) Projected External Pressure needed for 0.5 psi ICTP Pressure (psi) ENGINEERS

18 Pressure Alone Does Not Account for the Observed Effects Submerging the hand in water to a depth of 14 inches exposes the area of the palm over the carpal tunnel to an external pressure of about 30 mm Hg Resting the forearm on a desk creates exposure to contact pressure ENGINEERS

19 Interaction of Exposure Time and Pressure ENGINEERS

20 Pressure and Exposure Time Lundborg et al applied sufficient external pressure to the palm to produce ICTPs of 90 mm Hg 60 mm Hg 30 mm Hg ENGINEERS

21 Lundborg s Results for the Carpal Tunnel (median nerve) 90 mm Hg Sensory response disappeared minutes after application of pressure Motor action potential disappeared minutes later 60 mm Hg Sensory response disappeared after minutes Motor action potential deteriorated, but present 30 mm Hg For 3 of 4 subjects, motor and sensory responses decreased during the first hour ENGINEERS

22 Rempel s Results for Carpal Tunnel (median nerve) Static Pressure 30 mm Hg applied directly to the nerve produces median nerve conduction changes and cell changes Fluctuating Pressure Continuously alternating 20, 35, 50 mm Hg at one second each for 5. 5 hours ~ same effect as 40 mm Hg static pressure for same period Note average is 35 mm Hg ENGINEERS

23 Prielapp - Pressure on Ulnar Nerve During Surgery Prielapp studied effect of pressure on ulnar nerve while in postures commonly seen during surgery 8 of 16 Subjects reported paresthesias at An average of 37 minutes (20-59 minutes) From an average applied pressure of 80 mm Hg ( mm Hg) ENGINEERS

24 Summary Externally applied pressure creates changes in exposed tissue Decreased blood flow Changes in cellular structure Pressure (30 mm Hg) applied to capillaries may collapse them, producing ischemic state Effects are an interaction between pressure magnitude and exposure time Internal Pressure not necessarily the same as external pressure ENGINEERS

25 Conclusions Support surfaces must be designed to Provide benefit of support Reduce exposure to contact pressure below threshold Threshold has time and pressure components However, threshold not well established Effect of fluctuating pressure not well studied More research is required. However, Contact pressures less than 30 mm Hg (0.5 psi) for exposure times less than mins. probably present low risk ENGINEERS

26 Questions? ENGINEERS

Muscular Skeletal Disorders

Alabama Department of REHABILITATION SERVICES "for Alabama's children and adults with disabilities" Muscular Skeletal Disorders Michael Papp, MSE Muscular Skeletal Disorders What are MSDs? Muscles Nerves