RESEARCH AND FEA STUDIES EXAMINE STRANGULATION HAZARDS TO AID IN THE DESIGN OF SAFER CHILDREN'S PRODUCTS

Children between one and two years of age are most susceptible to hazards that could lead to ligature strangulation, the interruption of the blood supply to the brain. Children of this age are often mobile, but lack the balance of older children, and are curious about their environment.

June 18, 1999, Pittsburgh, Pennsylvania - Cribs, child car seats, strollers, swings, pacifiers, clothing, toys -- we provide these for our children to keep them warm, happy and safe. However, each year manufacturers recall hundreds of children's products because they pose serious suffocation, choking, strangulation or entrapment hazards. Approximately 22 percent of accidental child deaths in the United States from 1990 to 1993 resulted from strangulation due in part to playground equipment, children's furniture, clothing with drawstrings or straps and cords from mini-blinds.

While caregiver vigilance is key to protecting children from harm, product manufacturers also must attempt to ensure safety in their product designs. Manufacturers' efforts to minimize strangulation risks for children's products have been stymied by the absence of quantitative methods or tools for assessing product hazard potential. Data about the limits of the material properties of neck tissue are needed, but have not been available, for developing quantitative assessment methods. Without these methods, manufacturers cannot assess if their products present a strangulation hazard, which could result in the accidental application of pressure to the neck that exceeds the limits of tissue strength.

Based on the occurrence of strangulation, lack of assessment methods and absence of data about the limits of neck tissue, the RAM Consulting division of Intertek Testing Services, Inc., Oak Brook, Illinois, in conjunction with Denver Children's Hospital, has spearheaded an award-winning, extensive research project to study this problem. Using a multi-faceted approach that integrates science, medical research and engineering technology, specifically finite element analysis (FEA) and Mechanical Event Simulation software from Pittsburgh-based ALGOR, Inc., RAM Consulting is working to develop assessment criteria that accurately simulate and assess strangulation hazards.

RAM Consulting engineers used ALGOR's FEA-based simulation software to replicate the pressure needed to compress veins and arteries in the neck of an 18-month old child. The analysis results enabled researchers to explore the virtual behavior of veins, arteries and other tissues of the neck and create an accurate physical prototype made of materials that simulate the properties of human tissue.

This research to understand the limits of the material properties of human tissue under the compressive forces that cause strangulation is much like testing that is performed to determine the strengths of materials for manufacturing. Manufacturers of children's products must know the limits of human tissue in order to design products that minimize the possibility of strangulation.

RAM Consulting Develops Multi-Faceted Strangulation Research Project
Founded in 1988, RAM Consulting uses a multidisciplinary approach to providing scientific solutions for engineering safe, effective consumer products. According to Gene Rider, president and co-founder of RAM Consulting, the use of CAD and FEA software to create virtual product designs is critical in helping clients develop safer products. "Virtual designs can be quickly and creatively modified without significant investments in time or prototyping costs," Rider said. "We find that our customers are more open to investigating concept changes at this stage in product development."

Figure 1. Each year manufacturers recall hundreds of children's products because they pose serious suffocation, choking, strangulation or entrapment hazards. Approximately 22 percent of accidental child deaths in the United States from 1990 to 1993 resulted from strangulation due in part to playground equipment, children's furniture, clothing with drawstrings or straps and cords from mini-blinds.

RAM Consulting began studying the hazards associated with strangulation after recognizing that strangulation is the second leading cause of accidental death for children in the United States, trailing behind suffocation, which comprises approximately 60 percent (see figure 1) of deaths. During the preliminary stages of developing the project, researchers discovered that little research data was available about the potential risks of strangulation to help manufacturers design and consumers purchase safer children's products.

Recognizing the lack of pertinent data and the impact it had on product design, RAM Consulting devised a four-part plan for researching and developing a quantitative method of assessing strangulation hazards:

• Analysis of Injury Data. Researchers collected information about the circumstances of documented strangulation cases.
• Assessment of Human Factors. A Human Factors Specialist compared the compiled injury data with anatomical factors to establish trends in strangulation incidents.
• Collection of Medical Data. Medical researchers conducted tests to study how a child's airway, carotid arteries and jugular veins respond to external pressure.
• Development of Virtual and Physical Prototypes. Engineers used finite element models to examine the behavior of human tissues under compressive force using a computer and made physical prototypes to replicate this behavior using bio-simulating materials.

Injury Data Analysis Links Closely to Human Factors Assessment
In the first phase of the project, RAM Consulting researchers collected available information about strangulation from death certificates, autopsy reports, medical papers and studies. A database was developed to compile factors about individual strangulation cases. Then, the researchers analyzed the compiled data, identifying common traits upon which they could create a general strangulation criterion. Some of the factors considered in the database development included age, gender, action at time of incident, caregiver vigilance, anatomical point of strangulation and characteristics of any involved products.

"After studying the factors of documented strangulation cases, we began to understand that two types of strangulation exist: ligature and suspension. Distinguishing between the two types is very important in developing strangulation criteria that can be applied to a variety of individual product types," explained Rider. "In the second phase of research, our Human Factors Specialist was able to define differences between the two types by examining the characteristics of typical incidents for each type."

About 30 percent of the strangulation deaths studied can be classified as ligature strangulation, which involves the interruption of the blood supply to the brain. Children's garments with drawstrings or straps as well as cords from window blinds can be responsible for this strangulation type. Furthermore, ligature strangulation occurs most often in children between one and two years of age, who are often mobile, but lack the balance of older children, and are curious about their environment. Incidents have been documented involving children up to four years of age.

Suspension strangulation is the second type and comprised approximately 70 percent of the strangulation cases studied. It occurs when a child's airway is occluded by external pressure, which cuts off airflow to the body and brain. Researchers have found that playground equipment and children's furniture are most often responsible for suspension strangulation.

This frequently occurs in children under one year of age because they are often top-heavy, unstable and may be unable to lift themselves up after placing themselves in a dangerous situation. Through the medical research performed at Denver Children's Hospital, researchers discovered that just the pressure of a thumb pressing underneath the chin is enough to cause suspension strangulation, illustrating how easy a child could be strangled by furniture or playground equipment. For instance, if an infant is allowed to stand in a crib with the rails too low, he could become suspended on a rail caught under his chin since he does not have the stability or lower-body strength to pull himself upward.

Part of the Human Factors Specialist's role was to determine the typical cross-sectional area of the neck at which pressure is applied in ligature strangulation. The specialist examined MRI images to identify a cross-section most representative of the general area to be used as a basis for the ALGOR FEA model and ultimately the physical prototype of ligature strangulation. Further examination of injury data led to the formulation of preliminary compressive forces that were applied to the FEA model.

Medical Research Yields Valuable Clinical Findings
RAM Consulting enlisted the help of Denver Children's Hospital for the third phase of the project. The two organizations cooperatively designed and conducted a groundbreaking, award-winning clinical project in the area of suspension strangulation research. It was selected for the prestigious 1999 Seymour Cohen Award, awarded by the American Broncho-Esophagological Association for the best original research conducted by a resident, fellow or attending physician. The goals of the project were to better understand the mechanisms of airway obstruction and the amount of external pressure required for airway occlusion in children.

Figure 2. RAM Consulting and Denver Children's Hospital have cooperatively designed and conducted a groundbreaking, award-winning clinical project in the area of suspension strangulation research. This medical research was conducted on small children during same-day surgery procedures to better understand the mechanisms of airway obstruction and the amount of external pressure required for airway occlusion in children.

The research took place during same-day surgical procedures of 100 children between the ages of 5 months and 6 years at Denver Children's Hospital. Wearing a penny-sized pressure gauge adapter on one finger, a researcher applied pressure at two different positions on each child's neck while an anesthesiologist monitored the level of airway occlusion (see figure 2). The researchers concluded that minimal pressure applied in a superior and posterior direction caused airway occlusion.

"The results of this study provide an essential building block in the development of our suspension strangulation hazard assessment criterion," Rider said. "Without clinical data about the pressure required to close off a child's airway, we could not adequately advise product manufacturers about possible strangulation hazards."

Rider has observed the importance of knowing forces at work in product design. The U.S. wearing apparel industry agreed to remove from the market clothing with drawstrings because they lack an anatomical tool to evaluate the potential strangulation hazards related to these features.

Future clinical research is planned for ligature strangulation using some similar operating room procedures to determine the amount of force needed to cut off the blood supply to the brain. In addition, the researchers hope to verify the behavior of human tissues that was predicted by linear stress analyses performed with ALGOR software.

ALGOR FEA Model Provides Questions and Answers
In the final phase of research, RAM Consulting engineers used ALGOR's modeling and analysis capabilities to develop a finite element model of a cross-section of the neck of an 18-month old child, a common age at which ligature strangulation occurs. The purpose of the analyses was to learn more about the amount of force needed to completely compress the jugular veins and carotid arteries that carry blood to and from the brain. The analyses also enabled engineers to understand the behavior of the neck tissues and how force is transferred through them.

"Our engineers chose to use ALGOR software because it offers the modeling and analysis features they need at an affordable price," Rider said. "ALGOR also works very well within the PC network structure of our research facility. Our engineers can easily access files in ALGOR directly over the network."

Because the neck cross-sectional model was their first attempt at modeling human tissue, RAM Consulting researchers needed to first identify its engineering properties so they could be applied to the FEA model. Through the research of scientific literature, the engineers located a reference titled "Strength of Biological Materials" by Hiroshi Yamada, MD, Professor of Anatomy, The Kyoto Prefectural University of Medicine, Kyoto, Japan and edited by F. Gaynor Evans, Ph.D., Professor of Anatomy, the University of Michigan Medical School, Ann Arbor, Michigan. The modulus of elasticity of the jugular vein, carotid artery, skeletal muscle, cervical vertebra, esophagus and trachea cartilage were obtained from this text and used in the model.

The engineers used Superdraw III, Algor's single user interface for FEA and finite element model-building tool, to draw the 2-D solid model (see figure 3). A plane-stress model was specified because the engineers assumed no deformation in the longitudinal direction of the neck. Supergen, an automatic 2-D mesh engine, was used to quickly produce an FEA mesh and refine the curved areas around the arteries, veins, esophagus and airway. In addition, the engineers used groups to organize the various neck components for easier application of the different material properties. Boundary conditions were specified to fix the neck model at the location of the bones of the neck vertebrae.

After setting up the FEA model, RAM Consulting ran a linear analysis to verify that the model contained no errors and to get conservative estimates of deflection from compressive forces that were specified on the side of the neck. However, in the initial run, the engineers found that the modulus of elasticity for human tissue was too low to be analyzed using a linear material model. In subsequent runs, the engineers specified a higher elasticity modulus for the fat, muscle and skin portions of the model.

"We will be performing nonlinear analyses in the future that will use the precise material properties of human tissue," said Tao Xu, Ph.D., the technology manager of global safety engineering at RAM Consulting. "We also expect these analyses to yield deflection results that show accurate large deformation, which cannot be determined using linear analysis."

Figure 4. The ALGOR linear deflection results show how easily a vein deforms under pressure because of its location outside the artery, low pressure and large diameter. RAM Consulting researchers used the results to help define future physical medical research with the Denver Children's Hospital.

Examining deflection results from the modified linear analysis (see figure 4), the engineers noted how much more severely the areas surrounding the veins and arteries deform during ligature strangulation when compared to airway occlusion data acquired during the medical research of suspension strangulation. The engineers also compared the deformation experienced at the carotid artery versus that seen at the jugular vein, which they have used to design the planned medical research of ligature strangulation.

"The ALGOR linear analysis helped us to identify the most important information we need to gather from the medical community," Xu explained, referring to the physical studies at the Denver Children's Hospital. "Before conducting the analysis, we determined that we should focus on the compression of the veins in the neck versus the arteries because the veins are located outside of the arteries, have lower pressures and larger diameters, making them easier to compress. The analysis results confirmed our hypothesis."

According to Xu, future nonlinear analyses will explore other aspects of ligature strangulation, including how the size and shape of straps and cords affect tissue behavior and the likeliness of strangulation. With size and shape information and additional data about compressive forces from medical research, RAM Consulting will be able to set specifications for size, shape and breaking force that will minimize the risk of ligature strangulation. In addition, the ALGOR analysis results can be used as an educational tool in conjunction with a physical prototype to help RAM Consulting clients understand the significance of designing products that pose minimal ligature strangulation hazards.

"ALGOR is an educational tool," Xu said. "It has helped us to explain how a vein will compress and serves as an illustration tool to a lay person or customer."

Figure 5. After the ALGOR finite element model was completed and analyzed, RAM Consulting researchers created a 3-D CAD model of a planned physical prototype using AutoCAD 14. RAM Consulting researchers will be able to perform strangulation scenarios using actual objects in conjunction with the physical prototype to further define the ligature strangulation criteria.

RAM Consulting researchers are continuing to work on the development of a 3-D physical prototype of the neck made of two-part castable silicones with diluent and two-part castable polyurethanes. These bio-simulating materials have dynamic properties that replicate actual human tissues. Modeled after the ALGOR finite element model and drawn with AutoCAD 14, the prototype (see Figure 5) features a circulatory interface and circulatory system that work together to simulate the flow of blood through arteries and veins in the neck. RAM Consulting researchers will be able to perform strangulation scenarios using actual objects to further define the ligature strangulation criteria.

While many questions remain unanswered from an engineering standpoint, from a scientific standpoint, the use of ALGOR software propagated discussion and helped direct future research. "The ALGOR FEA model raised as many new questions as it answered," Rider said. "We were enlightened by this phenomenon because it truly represented the dynamic nature of the human tissue and facilitated exploration into all matters of tissue behavior early in the research process."

Though much work remains on developing suspension and ligature strangulation criteria, RAM Consulting is exploring the possibility of using ALGOR FEA software in other aspects of risk analysis and management. According to Rider, RAM Consulting is developing the idea of using ALGOR's heat transfer analysis software for burn injury studies. "As we realize the implications of ALGOR on risk analysis and management, we continue to seek out new areas of product safety research that could benefit from FEA."

Contributors
Gene Rider, President, RAM Consulting
Scott Milkovich, Ph.D., Technical Manager of Research, RAM Consulting
Tao Xu, Ph.D., Technical Manager of Global Safety Engineering, RAM Consulting
Jill C. Scandridge, Safety Engineer, RAM Consulting
Daniel Stool, Human Factors Specialist, RAM Consulting
Amy Marrinan, Marketing Communications Specialist, RAM Consulting