The Impact of Different Application Techniques on Fiberglass Casts: A Mechanical Experimental Study

Abstract

Background

Cast immobilization is a part of treatment in most of the orthopaedic injuries. The fiberglass material has several advantages over plaster of Paris which makes it more preferable. Some techniques are frequently used in daily fiberglass cast application to ease molding, shorten curing time and increase stiffness. The aim of this study is to assess the effects of two techniques on fiberglass cast strength and curing time.

Methods

A cruris model was prepared to mimic a patella tendon bearing (PTB) cast. Three groups were created for the study (n = 30 each). The casts in the first group were treated with foam soap during molding. The casts in the second group were wrapped with a wet bandage after application. The third group was the control group. The samples were mechanically tested in the 5th, 15th and 30th min. Maximum load, elastic strength, and Young’s modulus were assessed via 3-point bending. All data were compared using linear regression analysis and p < 0.05 was determined as statistical significance.

Results

The results showed statistically significant improvement in cast mechanics in the wet bandage group at full curing time by means of maximum load, elastic strength and Young’s modulus (p < 0.0001). Although the foam soap group had higher values in the 5th and 15th min, there was no statistically significant difference from the control group at full curing time (p > 0.05).

Conclusions

This study revealed that wet bandage wrapping over circular fiberglass casts improved the cast strength. The use of foam soap during fiberglass cast molding did not alter cast mechanics at full curing time.

Introduction

Bone fractures are common injuries which may be treated with conservative or operative managements, depending on patient (age, function, expectation, etc.) and/or fracture-related (location, classification, soft tissue involvement, etc.) parameters. Fracture reduction and cast immobilization is the standard treatment in most of the closed simple fractures [1]. Although plaster of Paris is still widely used in fracture treatment, the fiberglass casts have several properties that make them more preferable including lesser weight, stronger construct, shorter curing time, decreased heat exposure and radiolucency in radiological images [2, 3]. Fiberglass casts are usually favored by patients for avoiding discomfort and limitations in daily activities by being lighter, less dull, and water-resistant [4, 5]. Additionally, due to their stronger construct, fiberglass casts may let patients mobilize sooner by allowing early weight-bearing [6, 7].

Methods such as applying foam soap or wrapping the cast with a wet bandage while curing are frequently used in daily practice as they are proposed to ease molding, shorten curing time, increase stiffness, and improve cast smoothness. There are studies reporting that cooling or heating the cast material effects the curing time while hardening is an exothermic reaction [8, 9]. To best of our knowledge, there is only one study in the literature that evaluates the effects of additives on stiffness and setting time of fiberglass splints. The authors of the mentioned study evaluated the mechanical properties of the fiberglass splints in the 5th, 10th and 15th min of the initial application of additives and found that foam sanitizer significantly weakened the strength while liquid soap and ultrasound gel did not alter cast mechanics [10].

The aim of this study was to assess the effects of two popular techniques (foam soap molding and wet bandage wrapping) on fiberglass cast material strength and curing time. We hypothesized that both methods could increase stiffness and decrease the curing time of the fiberglass cast.

Materials and Methods

Prior to this mechanical experimental study, we conducted a survey on 67 orthopedic surgeons from 3 different institutions to determine their habits while performing fiberglass casts. The two most common techniques (molding with foam soap and firm wrapping with wet bandage) were selected to be studied.

Sample Preparation

One cruris model with a broken synthetic tibial bone (Sawbones^®, 1179-7, USA) in the center of a silicone envelope was prepared to mimic a properly applied a patella tendon bearing (PTB) cast for the treatment of a simple closed adult tibial shaft fracture (Fig. 1). A circular cast was applied to the cruris of a 30-years-old male volunteer to obtain the negative mold for the modeling process. After the preparation of this negative model, the synthetic tibial bone was centered inside the mold with temporary K-wires and the positive mold was then prepared with black colored molding silicone (Smooth-On 3030 RTV 2 Silicone, Smooth-On Inc., USA). At the end of curing time, the temporary K-wires and the negative mold have been removed from the positive mold. This positive mold was used as a silicone cruris model. All fiberglass tapes (Freely^® Cast, 5-inch Medical Synthetic Casting Tape, Beijing, People’s Republic of China) were wrapped around the same silicone cruris model for the standardization of study. The fiberglass tape rolls were immersed in room temperature (20 °C) water for 10 s. The rolls were then taken out of water and squeezed gently to allow the excessive water to drain. Immediately after that, the rolls were wrapped around the silicone model to form a three layers circular cast for every individual sample.

Fig. 1.

Cruris model as positive mold processing steps. Synthetic tibia with a transverse fracture at the midshaft region was centered with temporary K-wires in a fiberglass negative mold. a Superior view, b anterior view, c finished positive cruris mold placed in the testing stand after removal of K-wires and negative mold

Three groups were created for the study: The first group was the foam soap group (group 1, n = 30) and each circular cast was treated with 3 cc foam soap (Limpex^® Foam Soap, Turkey) during molding (molding time was 1 min for each cast). The second group was the wet bandage group (group 2, n = 30) and each circular cast in this group was wrapped with a wet gauze bandage lasting for 1 min. The third group was the control group (group 3, n = 30) and each circular cast was molded for 1 min without using additives or bandages in this group.

Mechanical Testing

Each study group was further divided into three subgroups for mechanical evaluation. The circular casts were tested in the 5th, 15th and 30th min after the application was completed (n = 10 for each subgroup in the 5th, 15th, and 30th min, respectively). Maximum load, elastic strength, and Young’s modulus at 40 mm deflection were assessed via 3-point bending with a mechanical testing system (Lloyd-Ametek EZ-50 Material Testing Machine, UK) featuring a 5000-N force transducer (Fig. 2). The cruris model was placed in a custom-made stand that maintained the fracture position constant for all samples. The span between the lower supports of the stand was 210 mm, the diameter of the casts was 78 mm at the bending point, and the experiments were run in 100 mm/min rate. The data were collected with Nexygen software ver 4.5.1 issue 3.

Fig. 2.

Mechanical testing machine and the cruris model with circular fiberglass cast placed in three-point bending test stand

Statistical Analysis

All data were analyzed using Graphpad Prism 6 software by Graphpad Inc. The groups were compared using linear regression and the time was determined as the axis X. p < 0.05 was determined as statistical significance for all comparisons. Graphs for each comparison were created and each data point on graphs represented an independent sample. Data points, best-fit lines, and confidence intervals were given same color codes for each group as follows: green straight line = control, blue dashed line = foam soap, red pointed line = wet bandage on all graphs.

Results

The maximum load, elastic strength and Young’s modulus of bending data were gathered for each sample to assess the study. All mechanical properties were significantly (p < 0.0001) increased by time in all groups.

The Maximum Load

The maximum load of soap and bandage groups was significantly greater than control group in the 5th and 15th min (p < 0.0001). Foam soap group had slightly greater results than bandage group in the 5th min, but there was no statistically significant difference between these two groups in the 15th min (p > 0.005). Bandage group had significantly greater values compared to other two groups in the 30th min (p < 0.0001) while soap and control group’s values were closer (Table 1).

Table 1.

The maximum load values of the three different groups (mean ± SD)

	Control (N)	Foam soap (N)	Wet bandage (N)
5th min	1533.28 ± 121.86	1988.61 ± 190.48	1891.46 ± 189.22
15th min	2104.88 ± 96.40	2890.13 ± 82.99	2802.37 ± 174.04
30th min	2766.14 ± 242.84	2542.86 ± 434.36	3038.47 ± 227.06

The Load Rates

Bandage and soap groups had higher load rates between 5 and 15 min (91.09 N/min and 90.15 N/min) than control group (57.16 N/min). There was a decrease of mean maximum load (− 9.35 N/min) at soap group between 15 and 30 min. The load rate of bandage group also decreased between 15 and 30 min (15.74 N/min), but bandage and control groups had similar rates in general (45.88 N/min for bandage and 49.31 N/min for control) (Fig. 3).

Fig. 3.

Maximum load measurements in 5th, 15th and 30th min

The Elastic Strength

The elastic strength of soap and bandage group was significantly greater than the control group in the 5th and 15th min (p < 0.0001). Foam soap group had significantly greater results than bandage group in the 5th min (p < 0.0001), but both groups had closer results in the 15th min (p > 0.05) (Fig. 4). Bandage group samples had significantly greater values compared to other groups in the 30th min (p < 0.0001). Soap and control groups’ values were not significantly different (p > 0.05) (Table 2).

Fig. 4.

Elastic strength measurements in 5th, 15th and 30th min

Table 2.

The elastic strength values of the three different groups (mean ± SD)

	Control (MPa)	Foam soap (MPa)	Wet bandage (MPa)
5th min	10.49 ± 1.34	13.13 ± 2.00	11.24 ± 2.54
15th min	15.42 ± 1.20	20.23 ± 1.48	18.46 ± 1.65
30th min	20.19 ± 2.74	19.19 ± 1.92	21.56 ± 2.90

The Strength Rates

The same phenomenon at maximum load rates appeared at strengthening rates as bandage and soap groups had greater starting values and rates (0.72 and 0.71 MPa/min), but the strength decreased at the soap group between 15 and 30 min (− 0.06 MPa/min). Bandage group had higher strengthening rate (0.42 MPa/min) than soap and control groups (0.24 and 0.38 MPa/min) in general.

The Young’s Modulus

The Young’s modulus of bending of soap group (mean 50.29 MPa) was significantly greater than other groups (45.62 MPa for control and 44.31 MPa for bandage groups) in the 5th min (p < 0.0001) (Fig. 5). All groups had closer results in the 15th min with slightly greater values at the control group. Bandage and foam soap groups had significantly lesser (p > 0.05) values (73.35 and 67.65 MPa) in the 30th min compared to control group (84.38 MPa) (Table 3).

Fig. 5.

Young’s modulus measurements in 5th, 15th and 30th min

Table 3.

The Young’s modulus values of the three different groups (mean ± SD)

	Control (MPa)	Foam soap (MPa)	Wet bandage (MPa)
5th min	45.62 ± 5.32	50.29 ± 5.36	44.31 ± 6.63
15th min	68.69 ± 11.61	71.38 ± 2.67	67.41 ± 4.88
30th min	84.38 ± 12.18	67.65 ± 6.11	73.35 ± 7.52

Discussion

The use of additives in fiberglass cast application is quite common among healthcare professionals [10, 11]. Some adjuvants which were often used during casting have been recently studied [10], and no significant improvement on curing time and total strength was found. It’s a well-known fact that the dip water temperature effects curing time and heat exposure since cast curing is an exothermic reaction [8]. The use of isopropyl alcohol was also reported in effort to decrease curing temperature [2]. Despite these reports, the exact reason to use adjuvants on fiberglass casts is not clearly known. Therefore, we evaluated two different techniques that were commonly preferred during fiberglass casting to demonstrate potential effects on curing time or mechanical strength. Our study was designed to mimic a clinical scenario of a PTB cast applied for the treatment of a nondisplaced closed adult tibial shaft fracture.

The results of this current study showed statistically significant improvement of cast strength in both soap and bandage groups in short term. Especially the results in soap group in the 5th min were found to be significantly higher than the other two groups. The strength of the casts both in the soap and the bandage groups at mid-curing (15th min) were still significantly higher than the control group, but closer to each other than the starting point. The results were surprisingly different at the technically given “full curing” time (30th min) by the manufacturer. The measurements of the bandage group were significantly higher than the other two groups. The foam soap and the control group had no difference in the estimated full curing time. The application of wet bandage did not cause any significant change in the hardening rate, but foam soap application had a significant decreasing effect especially in the 15th min. Also, our study showed no difference at strength and maximum load between the foam soap and the control groups at full curing time. The findings of this current study demonstrated that application of foam soap did not have positive effects on the cast structure.

The use of different techniques in fiberglass cast application may seem reasonable in an effort to improve cast strength and decrease curing time. On the other hand, our study confirmed that the use of foam soap may cause negative effects on fiberglass cast while the hardening is a chemical process. Different variables may affect the chemical reaction of bounding and therefore may potentially impair cast mechanics [1, 7]. Further studies about the potential effects of different types of adjuvants over fiberglass cast materials are needed. The tapes in this study were infused with water-activated polyurethane resin. The wet bandage wrapping was assumed to increase the cast’s time of water exposure and therefore may continue to promote the linkage of polyurethane polymers.

According to this current study, wet gauze bandage wrapping over the circular fiberglass cast could safely improve the cast strength. On the other hand, foam soap application did not show any advantages on fiberglass casts. Additionally, the use of foam soap may impair cast mechanics and may compromise patient safety. Early cast wear or decomposition caused by the use of adjuvants might lead to prolonged healing and unnecessary economic losses.

There are some limitations of our study. First of all, the experimental design of this study was a drawback. Additionally, we did not evaluate mechanics of a model mimicking a weight-bearing extremity. Cyclic repetitive loading tests for longer periods may reveal valuable data. Also, we evaluated the effects of two different techniques in fiberglass casting according to our survey results. The use of different additives during molding may alter the results. Finally, we did our measurements in the 5th, 15th and 30th min. 30 min is the expected curing time as described by the manufacturer’s manual. Further mechanical changes at 24–48 h after the cast application remain unknown.

Compliance with Ethical Standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical standard statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed consent

For this type of study informed consent is not required.