Experimental Model
Acylindrical limb (forearm) and an L-shaped limb (leg and foot) were
constructed with polyvinylchloride pipe. Three diameters of the cylindrical
limb—standard (48 mm), small (27 mm), and large (89 mm)—were
tested. The L-shaped limb was constructed with 48-mm-diameter
polyvinylchloride pipe. The exterior of each limb model was layered with a
polyurethane-coated carbon fiber heating element (Thermion; Thermion Systems
International, Stratford, Connecticut) consisting of a fine mesh of conductive
carbon fibers that, when supplied with power, produce a uniform heat
distribution; a constant temperature of 33°C to 35°C (human skin
temperature) was maintained.
Type-T wire thermocouples, adhesive and insulated types (Physitemp
Instruments, Clifton, New Jersey, and Omega Engineering, Stamford,
Connecticut), were then used to monitor temperatures at selected points on the
artificial limb models (under the casts) and on the exterior of the casts. The
manufacturer (Omega Engineering) reports the maximum range of standard limit
of error on type-T thermocouples to be 0.75% or 1°C, whichever is greater.
A four-channel thermocouple data-logger (MadgeTech, Warner, New Hampshire),
which has a high accuracy (± 0.5°C) and resolution (0.1°C), was
chosen for this study. Data points were acquired every ten seconds.
Thermochromic liquid crystal thermometers (Telatemp, Fullerton, California,
and Thermographic Measurements, Flintshire, United Kingdom) with a temperature
range of 30°C to 60°C were also placed on the external surface of the
cast at various points. On selected runs, readings from the external liquid
crystal thermometer were obtained at roughly sixty-second intervals.
Model Validation
Our experimental limb model was tested against upper-limb casts worn by one
of us (A.D.H.). This cast was applied in extension, centered on the elbow, and
extended 6 in (15.2 cm) proximally and distally, and its dimensions
(circumference, ~24 cm; diameter, 76.4 mm) were between those of the
standard and large upper-limb models. Adhesive T-type thermocouples (Omega
Engineering) were applied to the skin and to the exterior of the casts with
the same data-logging system. On the basis of our in vitro data, two separate
attempts were made to apply a cast of twelve-ply thickness with use of
50°C dip water and cotton padding, but as a result of increasing
temperature and discomfort these runs were aborted prematurely. Triplicate
runs were then conducted with twelve-ply casts applied with use of lower
dip-water temperatures of 37°C and 22°C to 24°C. The average
ambient temperature (and standard deviation) during the reported human-limb
runs was 24.3°C ± 0.7°C. Internal temperatures were measured in
the middle of the casts at the medial epicondyle and the anterior antecubital
fossa. External temperatures were measured over this area with a thermocouple
probe and an adhesive liquid crystal thermometer (Telatemp).
Experimental Design
Several variables were tested in this study, including limb diameter, limb
shape (cylindrical and L-shaped), plaster thickness, cast type (plaster,
fiberglass with a waterproof liner, and composite [plaster overwrapped with
fiberglass]), dip-water temperature, and placement of the cast on a pillow
during curing. We evaluated different methods to avoid thermal injury,
including the application of ice and isopropyl alcohol to curing casts. In
addition, we attempted to determine whether the risk of thermal injury in the
concavity of an L-shaped cast was decreased by placement of a circumferential
overwrapping splint on the convexity of the limb, thus decreasing the
thickness of the cast material in the concavity. Each variable was tested a
minimum of three times with use of fresh dip water for each trial; the average
ambient temperature for each trial was 22.4°C ± 2.1°C.
Plaster casts were applied over three or four layers of cotton padding, and
we tested cast thicknesses of six, twelve, and twenty-four-ply. Four-inch
(10.2-cm) plaster bandages (Specialist Extra Fast Setting Plaster; BSN
Medical, Charlotte, North Carolina) were used in each plaster-cast experiment,
and dip-water temperatures of 22°C to 24°C and 50°C were tested.
The effects of limb diameter were assessed by testing the large
(89-mm-diameter), standard (48-mm-diameter), and small (27-mm-diameter) limbs
with a twelve-ply plaster cast applied with 50°C dip water. The internal
and external cast temperatures were measured at the convexity and concavity of
twelve-ply plaster casts applied with 50°C dip water to the L-shaped
limbs. Similar methods were used for the splint studies, which involved use of
twelve layers of the same plaster and cotton padding and overwrapping the
splint with cotton and an elastic wrap. In these studies, the splint was
purposely made too long so that one end could be folded back on itself. In the
splint experiments, the dip water was maintained at 37°C. Four runs were
conducted for the splint experiments because of data-collection problems
during one run.
The effects of placing a plaster cast on a pillow were assessed by removing
the limb from the stand after application of the cast (twelve-ply plaster,
50°C dip water) and placing it on a standard hospital pillow (a polyester
and vinyl shell with polyester fill) covered in a pillow case (50%/50%
cotton/polyester blend). Internal and external cast temperatures were
monitored on the side facing the pillow, and the internal cast temperature was
measured on the side facing the ambient air (top).
The risk of thermal injury with use of synthetic casting tape (Scotchcast
Plus; 3M Health Care, St. Paul, Minnesota) and either a normal cotton or a
Gore-Tex liner (W.L. Gore, Flagstaff, Arizona) was assessed. These casts were
applied with use of six-ply casting tape dipped in 50°C water. The effects
of a composite cast were tested by immediately overwrapping a twelve-ply
plaster cast with three-ply fiberglass, with both dipped in 50°C
water.
Intervention testing was performed only on plaster casts (twelve-ply,
dipped in 50°C water). These tests included placement of standard reusable
ice packs filled with ice between the pillow and the cast's surface. We also
tested the effect of fully saturating the cast with isopropyl alcohol when the
internal temperature reached a level between 43°C and 46°C.
Additionally, we tested whether the temperature rises in the concavity of an
L-shaped limb would be ameliorated by applying a convex splint of six-ply
plaster to the convexity prior to circumferential wrapping with six-ply
plaster.
Data Analysis
From selected variables, peak temperatures and times were determined from
the temperature graphs and were compared among the internal thermocouple, the
external thermocouple, and the liquid crystal thermometer (when used). We also
compared lag time and differences in peak temperature.
Several authors have developed methods for evaluating potential thermal
injury as a result of thermal conductance. In their classic studies, Henriques
and Moritz described time-temperature relationships based on porcine and human
experimental data and mathematical
calculations8,10.
More recently, Suzuki et al. found similar results in experiments on
rats9. Williamson
and Scholtz studied human subjects to determine blister formation as a
relationship of time and
temperature12, and
Lavalette et al.2
used the results of Williamson and Scholtz to generate a mathematical basis
for blister formation in order to study the thermal effects of cast
application. We used the classic work of Henriques and Moritz as well as that
of Lavalette et al. to qualitatively analyze the risk of thermal injury. Each
set of authors provided an equation or data for a reference line, which was
plotted on a log time-versus-temperature graph to represent the threshold at
which thermal injury can be predicted to occur. Henriques and Moritz equated
this injury with transepidermal necrosis, and Lavalette et al. equated it with
the degree of burn. If the experimental data clearly crossed the reference
lines, the limb was considered to be at risk for injury. We initially plotted
our experimental data on the graphs of Lavalette et al. and subsequently also
evaluated many of these variables on graphs based on the equations and data of
Henriques10. For
clarity, only the reference lines presented by Henriques and Moritz are shown
in this article. (Graphic representation of the other methods of burn
evaluation can be found in the Appendix.) Statistical differences in the risk
of burns between variables were then determined with use of the Fisher exact
text. The significance of differences in peak temperatures was determined with
use of the Student t test. P values of <0.05 were considered
significant.
Model Validation
In our tests of the casts on the human subject (one of the authors), we
documented slight differences between the skin and the exteriors of the casts.
The average maximum temperature differential (the difference between the
average maximum skin-surface temperature and the average maximum cast-exterior
temperature) for twelve-ply casts applied with use of dip-water temperatures
of 37°C and 22°C to 24°C was —0.5°C and 1.2°C,
respectively (see Appendix). The curve patterns for the experimental model
were similar to those for the human arm (see Appendix).
Experimental Testing
We found that casts consisting of only fiberglass material did not generate
enough heat to induce thermal injury regardless of the dip-water temperatures
or the type of cast lining tested. Thus, the remaining data pertain only to
plaster material.
Monitoring of the internal and external cast temperatures during
application of the plaster showed the external temperature of the cast to be
an average (and standard deviation) of 2.7°C ± 1.9°C cooler
than the internal temperature. The maximum difference in the temperatures was
7.0°C. The external temperature of the thicker (twenty-four-ply) casts
peaked at an average of 84 ± 42 seconds prior to the peak in the
internal temperature. Shorter lag times were found with the thinner casts. The
difference between the internal and external cast temperatures of the
twenty-four-ply plaster casts (4.9°C ± 1.3°C) was significantly
larger than that of the thinner casts (1.5°C ± 1°C) (p <
0.05). Use of the liquid crystal thermometers generated similar results,
measuring an average difference of 2.7°C ± 3.4°C
(Fig. 1).
With regard to the plaster casts, we found that maintaining the dip-water
temperature at <24°C would likely prevent thermal injury regardless of
the plaster cast thickness. Similarly, application of plaster that was thinner
than twelve-ply appeared safe with use of multiple dip-water temperatures.
However, a risk of injury was found in one run with use of 50°C dip water
and twelve-ply plaster. Increasing the dip-water temperature to 50°C and
the cast thickness to twenty-four-ply consistently yielded temperatures that
were high enough to cause burns (p < 0.05) (see Appendix). Similarly, if a
twelve-ply plaster splint was applied and the end was folded over, creating a
twenty-four-ply area, the risk of thermal injury in that area was significant
(p < 0.05) (Fig. 2). While
the average peak temperature was slightly higher for the small limbs
(50.9°C ± 1°C) and lower for the standard (50.5°C ±
1°C) and large (49.9°C ± 1.7°C) limbs, these differences
were not significant. Application of a cast to an L-shaped limb consistently
produced higher temperatures in the concavity (average, 60.1°C) and lower
temperatures on the convexity (52.1°C). The peak temperature could be
decreased significantly (p < 0.05) by reinforcing the convexity with a slab
of plaster and reducing the number of circumferential wraps by 50% (six-ply)
on the concavity.
Overwrapping a curing plaster cast with fiberglass caused temperatures high
enough to cause thermal injury (p < 0.05, compared with the temperatures of
plaster and fiberglass alone) (Fig.
3). Similarly, placement of a limb with a twelve-ply cast on a
pillow yielded temperatures on the interior surface that were high enough to
cause thermal damage. In these runs, the peak temperatures on the top,
uncovered part of the limb were significantly lower than those on the bottom
(p < 0.05) (Fig. 4). This
increase in temperature was averted by simply placing ice packs between the
pillow and the cast (Fig. 5).
Application of isopropyl alcohol to the exterior of the casts that were prone
to lead to thermal injury did not significantly decrease the interior peak
temperature or decrease the risk of injury
(Fig. 6).
Humans feel the painful stimuli of conducted thermal energy when
temperatures are applied in the range of 47°C to
49°C8,13.
On the basis of the results of experimental and theoretical modeling, it is
thought that living tissues can accommodate some changes in temperature
without
damage7-10,12.
Once the thermal energy exceeds a critical level, the tissue can no longer
accommodate increases without
damage6-10,12.
Henriques and Moritz defined this relationship more than fifty years ago, and
their work is still being used to model thermal
injury8,10.
Near the time of its inception, the time-temperature relationship was believed
to reflect the thermal alteration in proteins, alteration in enzymatic and
nonenzymatic metabolic processes, and nonprotein-induced alterations in the
physical properties of cells. Henriques suggested that his findings were
similar to observations based on protein
denaturation10.
Since then, Xu and Qian proposed that Henriques' data could be modeled by
attributing the damage to the deactivation of cellular
enzymes11. More
recently, Despa et al. used computer modeling to determine which cellular
macromolecules are most thermally
sensitive6. They
found the lipid membrane to be the most thermally sensitive and believed DNA
to be particularly thermally stable. The ability to model and predict thermal
injury has been used to help prevent such
injury14,15.
While thermal injuries from the application of plaster casts are more
likely to be encountered than reported in the literature, there have been
reports of clinical and experimental
findings1,4,16.
Previous investigators have evaluated risk factors associated with thermal
injury, such as the brand of cast material, cast thickness, dip-water
temperature, and placement of the limb on a pillow during the curing process.
These studies focused only on the use of circumferential casts on similarly
sized straight limbs, and the experimental limb used in some of these studies
was essentially a water-filled glass cylinder. While this model can adequately
maintain temperature and provide enough stability to allow application of a
cast, it was not possible to test variables such as size and shape as we did
in the present study. Recent technology also allowed easy real-time
measurements to be made every ten seconds, thereby generating temperature
curves that are better than those obtained in previous studies. In addition,
we compared several different methods for predicting burns (see Appendix) and
evaluated much of our data with two of them (those of
Henriques10 and
Lavalette et al.2).
Both methods led to identical results and conclusions for almost every
variable tested.
In this study, the temperatures, temperature differentials (skin surface
compared with the exterior of the cast), and curve patterns for the human limb
were similar to those for the experimental limb. Small differences in limb
size, cotton thickness, and ambient temperatures were noted, but the
similarity between the experimental and human limbs demonstrates the
usefulness of this model. Goto and Ogata measured the temperatures generated
while applying splints to models and human
limbs4 and found the
mean peak temperatures in their model (54°C) to be significantly higher
than those of the human limb (46°C). They, however, used only plaster
splints and did not clearly describe their model, which was probably different
from the one presented in our study. They also did not describe the method of
statistical evaluation of their data; thus, it is difficult to make
comparisons with the present study. It should be noted, however, that, despite
the lower temperatures in the human subjects, several were found to have
first-degree burns following the experiments.
Obviously, one could not ethically test the extreme variables presented in
our report on a human subject as they could lead to injury. We attempted to
use our higher dip-water temperature (50°C) to apply a mid-range-thickness
(twelve-ply) cast to the arm of an investigator, but the test had to be
aborted before completion (i.e., when the internal temperature reached
43°C to 47°C) because it became too painful. This experience
correlated with the findings in our model, which predicted that those
variables would generate temperatures on the verge of causing a first-degree
burn. In addition, the values derived from the experimental model and the
human limb were similar when we compared the internal and exterior
temperatures of the thinner casts applied with lower dip-water temperatures.
On the basis of the validation testing, we believe that our experimental model
simulates changes in skin surface temperature when a cast is applied.
Williamson and Scholtz described individual differences between some of the
subjects in their study, but noticed that general trends
occurred12. There
may be some individual differences between subjects as a result of differences
in age, body fat percentage, amount of muscle, osseous prominences, and
vascularity. However, we believe that elucidating these differences in
response to heat, high enough to cause damage, is not possible in human
subjects for ethical reasons.
Limitations of this study should be pointed out. We may have underestimated
the true thermal damage caused by each of the variables. The plots developed
by Henriques10 and
Williamson and
Scholtz12 and
adopted by Lavalette et
al.2 are based on a
constant temperature maintained for a period of time. In our study as well as
that by Lavalette et al., the temperature of the cast material followed a
bell-shaped curve. One can only plot the data obtained during cast application
on the curves presented by Henriques and Lavalette et al. if time zero is
assumed to be the time of the peak temperature. This essentially ignores the
thermal energy and its potential to cause injury as the cast is "heating
up." The curve during this process is rather steep and the overall
thermal effects were thought to be minor, but they could have added to the
overall effects that were measured. In addition, the previous burn data are
based on small areas of tissue exposed to varying temperatures. None of the
authors of these studies measured the effects of exposing an entire limb
segment to an increased temperature. Also, each of our conclusions regarding
the risk of thermal damage was based solely on in vitro data plotted on graphs
developed primarily from in vivo studies. The validity of this type of
transfer is unknown, and the effects on actual patients may be different;
however, as stated previously, we believe the trends and conclusions would be
similar. Despite the above concerns, we concur with the conclusions of Pope et
al.5 that synthetic
tape is relatively thermally safe throughout the range of dip-water
temperatures tested. However, we updated these findings by showing that they
hold true for the new waterproof cast padding.
We have shown that the external temperature of a cast correlates well with
the internal temperature at the skin surface. Lavalette et
al.2 noticed that
the external temperature of the casts followed curves that were similar to,
but lower than, the curves followed by the internal temperatures. However, in
that study, no attempts were made to correlate the two temperatures and to use
the external temperature to predict the internal temperature. In our study, we
found a slight lag time between the peak internal and external temperatures
and slightly lower temperatures were measured externally. Although these
differences appear to increase with the plaster thickness, even with the
thickest plaster tested, which was most likely thicker than generally would be
used clinically, these differences were less than ninety seconds and 7°C,
respectively. Monitoring the external temperature may provide the clinician a
way to estimate the temperature inside of a recently applied cast or
splint.
We hypothesized that a smaller limb would be more at risk for a burn than a
larger limb; however, we could not demonstrate this. While the surface area of
a limb or cylinder increases in a linear fashion with increasing limb
diameter, the amount of plaster also increases. This is perhaps why we did not
see a significant difference in thermal risk among limb sizes. On the other
hand, we showed that, as a result of the increased amount of cast material
that accumulates in the concavity of an L-shaped limb, the risk of thermal
injury is increased in this area. Using a plaster slab for reinforcement over
the convexity and limiting the number of circumferential wraps minimize the
amount of plaster in the concavity and therefore decrease the peak
temperatures in this area.
We believe that the splinting data in this study are important as there is
a risk of thermal injury when a fracture is splinted postoperatively. All too
often, a splint is folded over when it has been cut to an inappropriate
length, but this essentially doubles its thickness in a small area and thus
increases the local exothermic reaction. Our recommendation is to trim excess
plaster and resist the temptation to fold over the ends.
We have offered potential interventions for reducing thermal risk, such as
placing ice packs between the limb and the pillow. Safer ways to avoid the
risk of thermal injury in this setting would be to hold the limb or allow it
to hang free during the curing process. The use of isopropyl alcohol to
decrease the temperature of a curing cast has been reported
anecdotally17. Our
attempts to use it in this way revealed that, while the external temperature
of the cast may go down, the internal temperature is minimally affected. In
fact, the clinician may be fooled into thinking that the interior of the cast
is cool because he or she can feel the external surface of the cast cooling
(Fig. 7).
In conclusion, the data on plaster material that were derived with the
current model support previous authors' findings, in particular that
application of thicker plaster casts with use of higher dip-water temperatures
and placement of the limb on a pillow may lead to temperatures high enough to
cause thermal injury. In addition, we were able to determine the temperature
at various locations on the limb under the cast. This information is important
so that clinicians realize that, while the temperature at one location of the
cast may feel safe, the temperature elsewhere (i.e., in contact with the
pillow) may not be. We also showed that overwrapping a curing plaster cast
with fiberglass significantly increases its internal temperature. Allowing
time for the plaster to cure completely before it is overwrapped with
fiberglass should eliminate this problem.
Three additional figures, including a plot of skin surface and external
temperatures of a twelve-ply cast applied to the arm of an investigator, a
comparison of skin (human) and surface (model) temperatures after application
of a twelve-ply cast (37°C), and a graph of representative plots of
temperatures causing thermal injury (reference lines) are available with the
electronic versions of this article, on our web site at
(go to
the article citation and click on "Supplementary Material") and on
our quarterly CD-ROM (call our subscription department, at 781-449-9780, to
order the CD-ROM). ?
Note: The authors thank the University of Wisconsin School of
Biomedical Engineering students Stacey Hoebel, Kristin LaFortune, Katie Mantz,
and Liz Thottakara for their help in the development and design of the
experimental limbs used in this study.