Stability of One-Step Spray-on Splint for Lower Extremity Fractures During Splinting, MEDEVAC, and Impact

ABSTRACT

Introduction

Military transport can induce whole-body vibrations, and combat almost always involves high impact between lower extremities and the ground. Therefore, robust splinting technology is necessary for lower extremity fractures in these settings. Our team compared a novel one-step spray-on foam splint (FastCast) to the current military standard structured aluminum malleable (SAM) splint.

Materials and Methods

Ten cadaveric specimens were subjected to complete tibia/fibula osteotomy. Specimens were fitted with custom accelerometer and gyroscope sensors superior and inferior to the fracture line. Each specimen underwent fracture and splinting from a standard of care SAM splint and an experimental FastCast spray foam splint in a randomized order. Each specimen was manually transported to an ambulance and then released from a 1 meter height to simulate impact. The custom sensors recorded accelerations and rotations throughout each event. Repeated-measures Friedman tests were used to assess differences between splint method within each event and between sensors within each splint method.

Results

During splinting, overall summation of change and difference of change between sensors for accelerations and rotations were greater for SAM splints than FastCast across all axes (P ≤ 0.03). During transport, the range of acceleration along the linear superior/inferior axis was greater for SAM splint than FastCast (P = 0.02), as was the range of rotation along the transverse plane (P < 0.01). On impact, the summation of change observed was greater for SAM splint than FastCast with respect to acceleration and rotation on the posterior/anterior and superior/inferior axes (P ≤ 0.03), and the cumulative difference between superior and inferior sensors was greater for SAM than FastCast with respect to anterior-axis rotation (P < 0.05).

Conclusion

FastCast maintains stabilization of fractured lower extremities during transport and impacts to a significantly greater extent than SAM splints. Therefore, FastCast can potentially reduce the risk of fracture complications following physical stressors associated with combat and extraction.

INTRODUCTION

Musculoskeletal injuries are a burden to military readiness as 34.7% of service members report lost time due to non-combat injury, of which, 25-50% are fractures.¹ In Afghanistan, 40% of all musculoskeletal injuries were fractures.¹ Furthermore, military medical evacuation (MEDEVAC) is known to induce whole-body vibrations. This is true for transport over water,² air,^3,4 and land.^4,5 When transporting wounded soldiers with extremity fractures in an austere environment, vibration is known to induce additional stress to the fracture site. In addition to military transport, battlefield events, such as falling to the ground, that deliver impact through a direct blow of force to the fracture site can induce high amounts of stress to lower extremities.⁵ It is especially important in these settings that effective splints be implemented for fractured lower extremities to reduce the risk of further complications from the fracture.

The current standard of care for fractures in the military is the use of a structured aluminum malleable (SAM) splint, which has frequently been used for such applications as immobilization of humeral fractures.⁶ However, there is limited data regarding the efficacy of SAM splints in keeping fractured lower extremities intact during transport and extraction. Complications that can arise from limb instability following a fracture include soft tissue injury, vascular injury, acute compartment syndrome, and deep venous thrombosis.^7–10 Furthermore, although external fixation helps increase the stability of a fractured limb, this procedure is performed at higher echelons of care, not at the point of injury. A critical military healthcare need exists to develop a product that couples the efficacious benefits of external fixation fracture stabilization with ease of deployment that matches a splint.

Thus, our team has aided in the development of a single-stage spray-on foam splint called FastCast. Rapid, rigid stabilization of a fractured limb via FastCast foam can provide multiple direct benefits including minimized risk for secondary injury including fracture hemorrhage; management of pain control; and facilitation of medical evacuation.¹¹ Foam materials have been successfully used for stabilizing extremities following reparative orthopedic surgery, including supracondylar humerus fracture repairs.^12,13 Foam splints are beneficial for reducing tissue damage and swelling following the repair of supracondylar humerus fractures, thereby increasing stabilization of the fracture site and allowing for more favorable healing outcomes.⁸ FastCast is designed to simplify splint application at the point of injury by allowing lay personnel to apply immobilization in situ.¹¹ In situ application minimizes fracture manipulation while providing rigid stability that can last the duration of extended field care. Despite knowledge that foam can improve system rigidity, it remains unclear as to weather this stabilization is functionally realized during transport tasks following a traumatic fracture event. Such technology could offer significant advancement in fracture stabilization and warfighter extraction ease immediately after acute extremity fractures on the battlefield or in training.

There is a paucity of literature on the efficacy of lower extremity splint materials from a biomechanical standpoint. The overall purpose of our study is to evaluate the efficacy of FastCast in maintaining biomechanical integrity under duress during vehicular transport and high-impact events compared to SAM splints. As FastCast foam is self-forming and naturally cures to a subject-specific mold in each application, we hypothesized that the application of FastCast would induce less displacement on a fractured lower extremity compared to SAM splints. We further hypothesized that with the FastCast splint, fractured lower extremities would exhibit better biomechanical stability under duress during transport and high-impact events compared to SAM splints.

METHODS

This was a laboratory study comparing acceleration and rotation occurring at a lower extremity fracture site during the splinting process and transport rigidity for SAM splints and single-stage FastCast spray foam splints. This cadaveric study was exempt from IRB approval as per standard policy at The Ohio State University. A cohort of 10 cadaveric specimens was procured following a surgical skill laboratory. Specimens originated from Body Donation Program at The Ohio State University, Columbus, OH, USA. These specimens were intact from the tibial plateau through the sole of the foot. Before any experimental activities, complete osteotomies were performed by an Army orthopaedic surgeon at the metaphyseal-diaphyseal junction of the distal tibia in the transverse plane. Following osteotomy, inertial measurement unit (IMU) sensors were adhered both superior and inferior to the fracture site. The superior sensor was adhered to skin superficial to the medial aspect of the tibial tubercle and the inferior sensor was adhered to skin superficial to the medial malleolus. Sensors were custom IMU devices from the Mayo Special Purpose Processor Development Group (Mayo Clinic, Rochester, MN, USA) that were able to continuously assess acceleration and rotation across three axes. Sensors were positioned to align axes such that the X-axis was posterior(+)/anterior(−), the Y-axis was superior(+)/inferior(−), and the Z-axis was lateral(+)/medial(−) (Fig. 1A). An adherent dressing (Tegarderm, St. Paul, MN, USA) was used to adhere each sensor to the skin, which was then wrapped in coban bandage (3 meter, St. Paul, MN, USA) to minimize artifactual motion and vibration (Fig. 1B). Once secured, sensors were activated and recorded data continuously at 100 Hz for the duration of a given specimen’s testing. Accelerations were measured in “4g” which is equivalent to 4 * 9.81 m/s². These values were then converted to acceleration meter/second² for reporting.

Each specimen was evaluated through two forms of fractures splinting, which were applied in a randomized order. One splint consisted of a SAM splint (SP500-OB-EN, SAM Medical, Tualatin, OR, USA), which is the military standard for acute fracture stabilization. The second splint consisted of FastCast spray foam (US Army Medical Research and Development Command, Fort Detrick, MD, USA). SAM splinting was performed per the U.S. Army First Aid handbook circa 2018 (Fig. 1C, 1D).¹⁴ FastCast splints were applied with the limb in a prone position with the dorsal side in contact with the ground. FastCast was shaken and dispensed around the medial and lateral aspects of the specimen for the full length of the limb. Supporting crosslinks were sprayed transversely across the anterior aspect of the specimen approximately every 4-6 inches (Fig. 1E). After spraying, foam was allowed to cure for ∼120 s without disruption, which exceeds the designated curing time for the foam to harden.¹⁵ Each of these splints was performed by the same medical professional who was part of a single local Emergency Medical Services (EMS) squad.

Splinted specimens were subjected to two activities representative of tasks that may occur during emergency extraction from an austere environment. The first task was designated ‘Transport’ and consisted of moving each specimen from the laboratory location to an ambulance and back to the laboratory. To accomplish this task, specimens were loaded onto a transport blanket in groups of four (Supplementary Fig. 2). The blanket was then hoisted by the aforementioned EMS team and carried down a 22 m hallway, down three flights of stairs (38 linear meters), and loaded onto a gurney. The gurney was wheeled 50 m over a concrete walkway and loaded into an ambulance. Due to cadaveric use restrictions, we were unable to transport the specimens any distance via ambulance. After specimens were loaded, they were subsequently unloaded and the transport tasks were performed in reverse to return to the laboratory. The second task was designated “Impact” and was performed by dropping the specimens to the floor from a 1 m height. Specimens were held statically at a 1 m height off the floor in a prone position, with the dorsal side of the limb facing the floor. From this position, specimens were released to gravity and allowed to impact the floor uninhibited. The Transport task was representative of multiple modes of transport and transfers that might occur during extraction. The Impact task was representative of a fall or ground impact that might occur during extraction under duress. Before the start and completion of each task, specimens were allowed to rest in a static position to ease identification of start and stop points for each task. These static starting positions were considered neutral alignment for each specimen.

IMU sensor data were extracted from each individual sensor using custom MATLAB code (version 2022b, The MathWorks Inc., Natick, MA, USA). Before and after the completion of the splinting process, specimens were allowed to rest statically on the ground. This static position was considered neutral and was used to identify the start and stop of each splint procedure as acceleration and rotational data were likewise static while the specimens were at rest. All values were zeroed to the neutral starting position and calculations were made with respect to this calibrated position. Three variables were calculated from the continuous acceleration and gyroscope data: summation of change from start to finish of each task, summation of difference in change between the Superior and Inferior sensors from start to finish of each task, and range of maximum to minimum values for each task. Each variable was calculated individually across each of the three axes previously identified. Because not all variables were normally distributed, non-parametric repeated-measures Friedman Tests were used for statistical evaluation. Since each splint method was applied to each limb, a repeated-measures design was appropriate. Statistical significance was determined at α < 0.05.

RESULTS

The summation of change observed during Splinting was greater for SAM splint than FastCast with respect to acceleration and rotation on all three axes (Table I, P < 0.01). For instance, the superior/inferior summation of change in linear acceleration for the SAM splint was greater than that of the FastCast foam by 3.08 m/s². The summation of overall change was greater for SAM splint than FastCast with respect to both accelerations and rotations on the posterior/anterior and superior/inferior axes during the Impact Event (Table I, P ≤ 0.03). For instance, the superior/inferior summation of change linear acceleration for the SAM splint was greater than that of the FastCast foam by 0.14 m/s². No significant differences in summation of change were observed for any axes during the Transport Event (Table I, P ≥ 0.50).

Cumulative difference between the two sensors during Splinting was significantly greater for the SAM splint than for FastCast with respect to both acceleration and rotation along all axes (Table II, P < 0.01). For instance, the cumulative difference between sensors along the superior/inferior axis of linear acceleration for the SAM splint was greater than that of the FastCast foam by 4.74 m/s². No significant summation of change differences were observed between the superior and inferior sensors for FastCast during Transport or Impact (Table II, P ≥ 0.10). However, for the SAM splint, there were summation of change differences between the superior and inferior sensors along the lateral/medial-axis rotation for Transport and Impact (Table II, P ≤ 0.03) as well as along the superior/inferior axis for acceleration during the Transport Event (Table II, P = 0.01). The cumulative difference between the superior and inferior sensors during the Impact Event was greater for the SAM splint than FastCast with respect to anterior-axis rotation (Table II, P < 0.05). For instance, the value for the SAM splint along the superior/inferior linear acceleration axis was 0.34 m/s² greater than that of the FastCast foam. No significant cumulative differences between sensors were observed between Splint Material during the Transport Event (Table II, P ≥ 0.60).

For the Splinting event, the range between the maximum and minimum values along each axis was greater for the SAM splint than for FastCast with respect to both acceleration and rotation variables across all three axes (Table III, P < 0.01). For instance, this value for the SAM splint along the superior/inferior linear acceleration axis was 0.0113 m/s² greater than that of the FastCast foam. During the Transport Event, the maximum-minimum range for acceleration along the linear superior/inferior axis was greater for SAM than FastCast by 0.010 m/s² (Table III, P = 0.02). This was also true of the maximum-minimum range of rotation along the transverse plane during Transport, and the absolute value of the difference was 71.5° (Table III, P < 0.01). During the Impact Event, the maximum-minimum range of acceleration along the linear posterior/anterior axis was greater for SAM splints than FastCast by an absolute difference of 0.03 m/s² (Table III, P < 0.01), as was the maximum-minimum range of rotation for the frontal plane by an absolute difference of 547.9° (P < 0.01).

DISCUSSION

Our data show that FastCast splints significantly reduced acceleration and rotation versus SAM splints during the actual fracture splinting procedure, which supports our hypothesis. These data indicates that the splinting process for FastCast was less aggressive and less traumatic to the fracture site than SAM splints, which means the injured limb was more representative of a rigid body during FastCast splinting. SAM splint application requires the leg to be lifted and the splint to be wrapped around the leg, which can induce additional displacement to the fracture site. The also data supports our second hypothesis that with the FastCast splint, fractured lower extremities will exhibit greater biomechanical stability during transport and high-impact events compared to SAM splints. SAM splints exhibited up to 14.8 times the acceleration and 14.6 times the rotation exhibited by FastCast splints. Thus, during the same tasks, FastCast splints reduced acceleration and rotation to a greater extent than did SAM splints. This is true both cumulatively across the entire limb body as well as between both segments separated by the fracture site. Combat environments can necessitate extraction under duress, where impacts to the injured limb may occur. The reduced acceleration and rotation in FastCast splints indicate that they performed better in maintaining rigid stability of the fractured limb during impact and multiple transfers than SAM splints. Both the DoD and the Congressionally Directed Medical Research Program place emphasis on expedited extraction from austere environments with reduced impact on warfighter unit effectiveness. The current data demonstrate that, relative to standard of care, FastCast splints offer advantage in stabilizing the tibia fractures during splinting and transport, such as “buddy carries” and “buddy drags” leading up to placement on a litter, which contributes to the stated program directives.

The higher acceleration and rotation associated with the use of the SAM splint indicates that extremities splinted with SAM were less stable during the splinting process and were more susceptible to additional injuries that could lead to greater complications during extraction and medical treatment. It is critical to maintain a stable environment during fracture splinting to promote injury healing, especially when neurovascular structures are compromised.¹⁶ Compared to external fixation, splint immobilization for ankle fractures is associated with higher frequency of soft tissue complications simply because external fixation allows for higher stability than splints.¹⁷ Therefore, given that external fixation is less feasible on the battlefield than splint immobilization, it is important to maximize the stability and minimize the displacement that is possible with a splint immobilization. A recent study compared FastCast with SAM splints as applied to cadaveric specimens with cervical spine fractures; following immobilization, orthopaedic surgeons rated the radiographic alignment based on their clinical expertise. FastCast passed the radiographic alignment following immobilization in 100% of cases, compared to 66% for SAM splint cases.¹⁸ This is consistent with the results of our present study and indicates that FastCast provides a more stable environment during fracture splinting.

Unlike a controlled laboratory, combat injuries often occur in austere high-stress situations and point of injury care is often provided by the injured service member (self-aid), or as per the Tactical Combat Casualty Care (TCCC) guidelines anther layperson (buddy-aid). Given the lack of training and high stress, we would expect significantly more acceleration and motion of the fracture site under these conditions. The in situ application of FastCast simplifies the process and provides rigidity to allow for buddy drags and carries, while avoiding pressure points allowing it to be maintained until on a stretcher until higher echelons of care are reached.

The fact that specimens splinted with SAM exhibited significantly higher levels of acceleration and rotation is consistent with the high rates of complications associated with lower extremity fractures, including infection, hemorrhage, and secondary soft tissue injury.¹ The data show that SAM splints are associated with higher magnitude of acceleration and rotation both superior and inferior to the fracture site, thus indicating instability of the SAM splint relative to FastCast, as well as lower efficacy in maintaining the integrity of the tibia under mechanical stimulation representative of medical transport and impact from falling. The whole-body vibration induced by land transport was evident in the superior axis for the SAM splint, but not for the other axes. Given that our Transport occurred over smooth, paved surfaces, which are more likely favorable conditions than most military austere environments, this likely contributed to a lack of significance in differences observed between SAM and FastCast during the Transportation Event. Further study would be warranted to simulate transport over difficult terrain as well as air evacuation conditions.

Military combat often involves repeated impact between the ground and lower extremities, leading to increased tibial stress.¹⁹ While such impacts would expect to be avoided during transport of injured warfighters, drops and unanticipated impacts cannot be excluded when operating in a hostile, austere environment. In the current study, the effect of the Impact event was more pronounced in the SAM splint. Greater differential between sensors indicated that the SAM splint resulted in a less rigid body at the fracture site. If static rigidity of the bone was maintained, differences between the inferior and superior sensors would be minimal or non-existent. Therefore, our data indicates that FastCast is a mechanically-efficacious alternative to SAM splints for mitigating the effects of tibial stress on fractured lower extremities in austere settings.

This study is not without limitations. First, the summation of change variable does not account for whether the superior and inferior sensors moved in the same or opposite directions as we calculated the absolute values of the change in acceleration between time points. Nonetheless, our data remain valid as the absolute magnitudes of acceleration and rotation indicate overall stability, regardless of direction. Second, this was a cadaveric study analyzing the mechanical behavior of limbs in isolation from the rest of the human body. Intuitively, splinting is only applied to battlefield injuries where the lower extremity has not been dismembered from body. Presence of a complete body is likely to alter biomechanical exertion and restriction on the lower extremity during the events examined during this study. However, other cadaveric models have demonstrated that lower extremity mechanics and injury events can be emulated in a relevant context even without full body attachment.^20,21 Additionally, the age and bone density of the cadavers was not measured or controlled; osteoporotic bone is likely to have been less biomechanically stable. The present data remain valid because we aimed to demonstrate that the FastCast can reduce motion under duress in general. Third, the sensors were left turned on after removing the first splint material, which may have introduced potential noise and drift, and thereby affected the acceleration and rotation data for the second splint material. Because IMU sensors are known to drift over time, each specimen was allowed to rest in a static position prior to task execution and after task completion. This allowed the research team to calibrate collected data relative to sensor positioning recorded in the static state prior to each task. In addition, the IMUs were adhered to skin, allowing the potential for skin artifact. However, such error was mitigated by two factors: (1) IMU placement was selected for locations where the soft tissue volume between the sensor and bone was minimal and (2) IMU placement was selected for the lateral aspect of the shank where each sensor was adhered directly deep to the splint material. Thus, the artifactual motion should have been limited by the surrounding Ace bandage for the SAM splint and the surrounding foam for the FastCast splint. In this study, sensors were not mounted independent of the splint material as the intention was to quantify splint rigidity of the lower extremity. Future study could use Stineman pins to directly mount the IMUs to bone independent of the splints themselves. Finally, the effects of the FastCast foam on moisture retention and whether constriction of the leg would cause compartment syndrome, the ease of removal of FastCast splints in the operating room, the effectiveness of FastCast in stabilizing multiple fracture types remains undetermined.

Regarding future studies, our team obtained radiographs of the specimens both before and after the Transport and Impact events. An in-depth qualitative analysis of these radiographs will be helpful for understanding how well FastCast visually maintains relative positioning of fractured bones compared to SAM splints. Further, while this study compiled acceleration and rotation data for each sensor; future work may benefit from the application of statistical parametric mapping to determine how well the continuous behavior of Superior and Inferior sensor data for each specimen and splint type are correlated. In addition, since our study was meant to simulate real-life combat events, piloting trials of FastCast foam splinting in live animal models or on human subjects who suffer fracture in a controlled environment (such as a sporting event) would provide valuable biomechanical stability knowledge in real-time and in context to the full human body.

CONCLUSIONS

The present data that FastCast presented reduced disruption to the fracture site during splinting and were able to effectively maintain stabilization of fractured lower extremities during vibration-inducing Transport and Impact events. This stability exceeded that of SAM splints in several instances, which indicates that FastCast may offer stability advantages over the current military standard of fracture splinting. These results are promising in that FastCast can potentially withstand the physical stressors associated with combat and extraction. Continued development is needed such that FastCast product can be implemented as a safe and practical tool to minimize fracture complications in austere warfighter environments.

ACKNOWLEDGMENTS

We thank the Surgical Skills Lab at The Ohio State University Wexner Medical Center for providing the cadaveric specimens for this study. We would like to thank all those who generously donated their bodies through the Body Donation Program at The Ohio State University, whom without this gift we would not have been able to conduct this project.

CLINICAL TRIAL REGISTRATION

Not applicable.

INSTITUTIONAL REVIEW BOARD (HUMAN SUBJECTS)

This study was exempt from approval by The Ohio State University Wexner Medical Center Institutional Review Board.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC)

Not applicable.

INSTITUTIONAL CLEARANCE

Not applicable.

INDIVIDUAL AUTHOR CONTRIBUTION STATEMENT

C.G.P.H. analyzed the data and drafted the original manuscript. J.H. analyzed the data. N.A.B., K.A., and F.P. collected the data. C.H. and C.F. developed the accelerometer and gyroscope sensors used for data collection. N.A.B., A.T.G., K.D.M. designed this research, reviewed and edited this manuscript. All authors read and approved the final manuscript.

SUPPLEMENTARY MATERIAL

SUPPLEMENTARY MATERIAL is available at Military Medicine online.

FUNDING

None declared.

CONFLICT OF INTEREST STATEMENT

K.D.M. is a consultant for Triton Systems, Inc., and is the inventor of FastCast foam. There are no other disclosures for the research team.

DATA AVAILABILITY

The data that support the findings of this study are available on request from the corresponding author. All data is freely accessible.

REFERENCES

Author notes