Chapter 4: Risk Factors
In this chapter
- Risk factors for friction blisters on the feet
- Blisters and temperature
RISK FACTORS FOR FRICTION BLISTERS ON THE FEET
The four requirements for blister-causing shear are listed below. Factors contributing to these conditions are quite obvious risk factors for blister development.
- Thick and immobile skin
- A high coefficient of friction (friction and pressure)
- Moving bone
And research has identified other factors that seem to correlate to blister development too - factors that from the outside, seem unlikely to have any bearing. Factors like gender, ethnicity, tobacco use and fitness. There are conflicting results with some and the research base is not exactly substantial for many. So it’s not clear how relevant some are as risk factors.
Please note: A risk factor does not necessarily imply causation. “Risk factors or determinants are correlational and not necessarily causal, because correlation does not prove causation.”¹²² “A famous slogan in statistics is that correlation does not imply causation. We know that there is a statistical correlation between eating ice cream and drowning incidents, for instance, but ice cream consumption does not cause drowning. Where any two factors A and B are correlated, there are four possibilities:
- A is a cause of B
- B is a cause of A
- the correlation is pure coincidence
- as in the ice cream case, A and B are connected by a common cause. Increased ice cream consumption and drowning rates both have a common cause in warm summer weather.”¹¹⁰
Figure 14: Risk factors for foot friction blisters identified in the literature
BLISTERS AND TEMPERATURE
Despite popular belief, blisters are not a burn. Here's why:
- Thermal burns that produce blisters are second degree burns, and second degree burns go deeper - into the dermis. Friction blisters are more superficial, existing within the epidermis. And friction blisters do not resemble thermal burns either clinically or histologically.² ⁶ ⁷ ³⁷
- Research has found that rubbing causes only moderate increases in skin temperatures, to between 41 degrees and 50 degrees Celsius which are insufficient to cause a burn.¹ ⁷ ¹¹ ¹² ³³ ³⁷ ¹⁰² ¹⁰⁹ ¹¹⁹
The relevance of heat to blister development: In almost every discussion you’ll read on blisters, heat is always mentioned as a causative factor. Although not directly a cause (not a burn), the involvement of temperature is multi-factorial.¹ ⁷ ¹¹ ¹² ³⁷ ¹⁰²
a) The act of rubbing produces heat – The act of rubbing any two surfaces together causes the production of heat and it's no different for the skin.¹⁷ ¹⁰² Li¹⁰² demonstrated that rubbing causes a moderate increase in skin temperature, and that this warmer area of skin increases sweat production, which increases skin friction. This perspiration/friction mechanism [rubbing - heat - more sweat - higher friction - blister] is in contrast to the burn mechanism [rubbing - heat - burn - blister] and this distinction should be recognised.² ⁷ ¹¹ ¹² ³⁷ ¹⁰² ¹⁰⁹ Besides, if heat transfer from the skin surface to the stratum spinosum is a major factor in blister development, one would expect the thicker corneum of the soles and palms to afford a level of blister protection. But a thick corneum is one of the requisites for blister formation.² ¹¹
b) Ambient and skin temperature – Higher ambient temperatures cause more perspiration.¹ And as you’d expect, increased walking speed causes higher in-shoe temperatures⁶¹ ⁶⁸ which results in increased sweating.⁸³ This perspiration provides a ‘moist’ environment, and a moist environment (neither very wet nor very dry) is known to increase friction coefficients¹ ¹¹ ²⁰ which increases blister incidence³ ²⁴ by way of shear.¹² Although blisters occur more readily in hot temperatures, blisters still occur in cold climates.⁷ ²⁸ Griffin⁷ produced experimental blisters on chilled, warmed and “normal” temperature skin and noted:
- blisters formed quicker when initial skin temperature was higher
- blisters took longer to form on chilled skin (14 degrees) compared to normal (30 degrees) and warmed skin (46 degrees)
- when the initial skin temperature was low, the skin temperature at the time of blistering was also lower
c) The inflammatory process – Hashmi¹⁰⁹ measured temperature changes during and after blister formation. They specifically aimed to minimise heat from rubbing, instead focusing on temperatures after blister formation. They demonstrated temperature increases due to inflammation, indicating heat is a result of the injury.³⁷ ¹⁰⁹
Overall, heat causes sweating which increases skin friction, leading to a higher likelihood of blisters. And heat is a result of the blister injury.
Figure 15: Hashmi demonstrated an increase in temperature for several hours after blister formation indicating the inflammatory response (Ref 109)
- Risk factors for foot blisters that have been identified in the literature are presented.
- Among others, there is conflicting evidence regarding gender, age, body-mass index and fitness as risk factors for blisters.
- Blisters are not a burn. But heat increases perspiration which increases skin friction and makes blister more likely to form.