A number of complications from casting, padding, and the use of plaster bandages have been described including deformity, skin injuries, rashes, compartment syndrome, and burns . The mechanism of these injuries include: improperly and irregularly applied padding that leads to pressure sores beneath the cast, inadequate padding material at the ends of the cast leading to sharp edges and skin irritation, aggressive cast molding that leads to pressure sores beneath the cast, inadequate casting material leading to cast breakdown and loss of control of the unstable fracture, tight application of casting material or failure to allow for underlying injury swelling leading to compartment syndrome, and hot dip water leading to elevated setting temperatures and skin burns [2, 8].
The purpose of this study was to evaluate various factors and their effect on ultimate temperature beneath various casting materials and techniques. In a study of thermal injuries to the skin, Williamson  assessed the effect of elevated temperatures on the skin and the risk of 1st, 2nd, and 3rd degree burns relative to time exposure (Figure 1). While this study did not use casting as their model, their study showed that maintaining temperatures of over 49 degrees for an extended period of time risked 1st degree burns if the exposure was longer than 2–3 minutes, 2nd degree burns if the exposure was longer than 8 minutes, and 3rd degree burns if the exposure was longer than 12 minutes. This study was our basis of defining temperatures beneath a cast of greater than 49 degrees Celsius for an extended period of time as dangerous.
A number of authors have noted the importance of monitoring dip water temperature and its effect on level of the temperature achieved by the exothermic reaction [1, 5, 9, 10]. Indeed, Lavallette et al. [5, 6] demonstrated a direct effect with dip water temperature, the length of time the plaster is kept in the dip water, and the risk of burns. Our studies confirm the findings of Lavallette that dip water temperature can play a key role in the ultimate temperature beneath the cast. Kaplan  showed that temperature elevations could be related to the plaster being dipped too briefly and the water being squeezed too aggressively out of the plaster. The water itself helps to release the heat, and if there is not enough, the plaster gets hotter. In this study, we attempted to control this factor by maintaining a strict regimen of time in dip water, allowing bubbles to exude, and gentle squeezing the water out prior to application. In addition, in this study we attempted to maintain uniformity by molding the material for a defined amount for in each test sample. Regarding fiberglass cast material, Selesnick and Griffiths  recommended using only cool dip water to reduce the chance of burns. In this study, we used both the 32 and 39 degree temperature dip water for plaster and fiberglass to allow direct comparisons of the materials. Regarding the effect of dip water temperature, this study confirms a direct relationship with increasing dip water temperature from 32 to 39 degrees Celsius and the ultimate peak temperature beneath both plaster and fiberglass casts. The comparison of plaster material revealed an increased in peak temperature of 2 degrees and the comparison of 3 M fiberglass material revealed an increase of 3 degrees related to the higher dip water temperature. It is possible that even greater dip water temperatures could increase the ultimate temperature beneath the cast. Admittedly, this is hypothesis that was not confirmed within our range of constructs.
Dirty dip water and ambient humidity have also been implicated as contributing to temperatures beneath maturing casts. Lavalette [5, 6] and Ganaway  proposed that plaster residue in the dip water might also play a role in elevating cast temperature and broadening the time-temperature curve; i.e., maintaining the peak temperature for a longer period. In our study this factor was controlled by maintaining fresh dip water for each test. In the orthopaedist's office or emergency room that is doing a lot of casting, this factor may need to be accounted for to minimize the time that the temperatures beneath a cast are elevated. Ganaway  felt that ambient humidity also played a role in the ultimate cast temperature; therefore, in this study ambient humidity was controlled to within 1%.
Additional factors play significant roles on the ultimate temperature beneath a cast and were controlled variables in this study. They include fast versus slow setting plasters, cast thickness, different brands of material, and the thickness and type of cast padding. Ganaway  felt that cast padding played little role in effecting the temperature beneath a cast. Our initial hypothesis was that thicker padding would be protective of the underlying temperature. In contrast what we found was that while increased cast padding had little effect on the fiberglass casts, it had a significant effect of elevated temperatures when additional layers of Webril were applied beneath 20 layers of extra-fast setting plaster. This was exactly opposite of what we had hypothesized. This effect may be explained by increased insulation trapping the heat beneath. The Procel bubblewrap offered little variation compared to Webril when placed beneath a fiberglass cast. Cast padding likely plays a greater role to protect the skin against pressure points than its effect on temperature.
The assessment of temperature beneath prefabricated splints along with its comparison to other forms of casting has not been previously reported. We found that the prefabricated fiberglass splints correlated with reduced temperatures beneath the splint material. This was likely secondary to the absence of circumferential splint material that would trap the heat beneath the material which, in turn, allowed the heat to defervesce laterally and more quickly. This finding would clearly support the premise that these prefabricated splints are safer, relative to thermal injury, than circumferential casting techniques.
Regarding the effect of various plaster materials, our findings agree with those of Ganaway and Hunter  which revealed that faster setting plasters have earlier and higher peak temperatures. Comparing different brands of fiberglass (Tests 17 and 18) revealed differences in peak temperatures but not onset of peak temperatures between brands. Neither was noted to achieve dangerous levels of temperature with dip water temperature of 39 degrees Celsius.
Ultimate cast temperature is related to the amount of plaster, its surface area, and the external environment's ability to let plaster lose heat . In this study, we maintained the surface area constant with a standard diameter PVC tube. We then evaluated the effect of varying thickness of cast materials, padding, and external applied material (a pillow). Both Lavalette and Ganaway [4–6] felt that the thickness of plaster played a significant role in peak temperature. Both also agreed that poor cast ventilation (such as an externally applied pillow), would lead to increased peak cast temperature. In our study we found that for all cast materials, plaster or fiberglass, increased thickness led to increased temperatures beneath the cast. However, we found only one construct in which the temperatures achieved and the duration of that intensity fulfilled criteria deemed to be dangerous by Williamson et al. . When the thickest construct of extra-fast plaster (20 layers with 1 layer of Webril) dipped in 39 degree Celsius water was allowed to lie on a pillow through its maturation, temperatures exceeded 50 degrees Celsius for over 20 minutes. Using Williamson's work , this would translate to a 3rd degree burn if applied on a human extremity. In Test #9 the temperature beneath the a twenty layer thick extra fast setting plaster dipped in 32 degree water (not on a pillow) peaked at 46.5 degrees Celsius and over 40 degrees for 10 minutes. Indeed when 20 layers of the normal setting cast material was dipped in warmer water (tests 26–27), the peak temperatures achieved 44 degrees and were maintained over 40 degrees for at least 25 minutes. While these did not meet the minimum criteria of exceeding 49 degrees, the thermal exposure over 40 degrees for an extended period of time raises concern.
A potential criticism of this study is our selection of a polyvinyl (PVC) tube model instead of a glass cylinder filled with water as suggested by Lavellette . Previous authors have suggested that internal diffusion of heat by the fluid or by the blood in the human model may serve to defervesce the temperature more quickly and avoid dangerous temperature levels. We don't disagree that this may play a role. Our model allowed the PVC tube to equilibrate to 32 degrees C before each new test and used the hollow, air filled PVC to serve as our diffuser. In addition and unlike Lavellette's study, the size of tubing was selected to mimic the average size of an adult calf or upper arm. This allowed a consistent surface area of casting material. In addition in this study, we did not specifically compare the absolute temperatures achieved by Lavellette or others but rather the effect and trend of altering variables within our model. Our only absolute temperature measurement comparison was performed using the Williamson study  regarding what temperatures are necessary to cause thermal injuries to skin. Indeed a number of our constructs raised concern, especially when allowing casts to mature while lying on a pillow. Perhaps a follow-up study placing our thermometer below casts placed in-vivo on volunteers would confirm the absolute temperatures that we report in vitro to be consistent with those seen in vivo.
In summary, a number of studies have evaluated the exothermic reaction that occurs during casting and have looked at the effect of a number of variables on the temperature beneath the cast. Unlike the few studies available on this topic, this study is unique that it included modern materials of fiberglass, prefabricated fiberglass splints, synthetic Procel padding in comparison to the classic plaster and cotton Webril padding. We can conclude the following:
Extra fast setting plaster achieves peak temperatures quicker and higher than slower setting plasters.
Increased thickness of casting materials (both plaster and fiberglass) are related to increased temperatures beneath the cast.
Dip water temperature is directly related to the peak temperature beneath the cast.
Brand of fiberglass did not play a significant role in the brands we studied.
Prefabricated splints do not achieve the same temperature levels when compared to circumferential casts and, therefore, from a thermal perspective, may be safer.
Thickness and type of cast padding did not play a significant role regarding ultimate temperatures beneath the cast in this study. At the thicker levels of padding, it may actually serve as an insulator entrapping additional heat.
The greatest risk of thermal injury occurs when a thick cast using warm dip water is allowed to mature while resting on a pillow.