Gene Rider
President

Tao Xu
Technical Manager of
Global Safety Engineering

Daniel Stool
Human Factors Specialist

RAM Consulting
Oak Brook, Ill.
www.itsram.com

About 22% of accidental child deaths in the U.S. from 1990 to 1993 resulted from strangulation partially due to playground equipment, children’s furniture, clothing with drawstrings or straps, and cords from mini-blinds. Manufacturers of such equipment must work to ensure safe products for all foreseeable users.

Designers have been stymied, to some degree, by the lack of quantitative methods or tools for assessing a product’s hazard potential. Because there are few good assessment methods and little data on neck tissue, we worked with Denver Children’s Hospital to spearhead extensive research into the problem. Project goals are to better understand how airways are obstructed and the amount of external pressure that can block a child’s airway (trachea). Early efforts in the project aim to develop criteria that accurately simulates and assesses strangulation hazards using an approach that integrates science, medical research, and FEA technology from Pittsburgh-based Algor Inc.

The project is to study the causes of strangulation, collect physical data from a hospital, and build and validate linear and nonlinear FEA models that use the physical data. From that information we’ll build physical models of human anatomy so product designers can use the information in their work.

The plan
Strangulation is the second leading cause of accidental death among children between the ages of six months and two years in the U.S. (Suffocation tops the list at about 60% of deaths.) During preliminary project development stages, we found little research on potential strangulation risks that could help manufacturers or consumers. Because the lack of pertinent data hampers product design, researching and developing a quantitative method of assessing strangulation hazards had to include:

• Analzying injury data that would come from collecting information on the circumstances of documented strangulation cases.
• Assessing human factors by comparing compiled injury data with anatomical factors to establish trends in strangulation incidents.
• Collecting medical data from research to study how a child’s airway, carotid arteries, and jugular veins respond to external pressure.
• Developing virtual and physical prototypes using finite-element models to examine the behavior of human tissues under compressive force and make physical prototypes to replicate this behavior using biosimulating materials.

The first phase of the project involved collecting strangulation information from death certificates, autopsy reports, hospital charts, and medical studies. In addition, we searched our existing database for factors on individual strangulation cases. Analyzing the compiled data allowed identifying common traits upon which to create general strangulation criteria. The database information included age, gender, action at time of incident, caregiver vigilance, anatomical point of strangulation, and characteristics of involved products.

Our human factors specialist found that strangulation data fits in two categories, suspension and ligature. Children die by suspension strangulation (about 70% of the time) when their airway is closed by external pressure, cutting off airflow to the body and brain. 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. Playground equipment and children’s furniture are big culprits, usually of children under one year of age because they are often top-heavy, unstable, and may be unable to lift themselves from a dangerous situation. Research performed at Denver Children’s Hospital showed that just a thumb pressing underneath the chin (a 4 or 5-lb force) is enough to cause suspension strangulation.

Ligature strangulation, responsible for about 30% of the deaths, interrupts blood flow from the brain. For example, children’s garments with drawstrings or straps as well as cords from window blinds can be responsible for this type. Furthermore, this type occurs most often in children between one and two years of age, who are often mobile, curious about their environment, but lack the balance of older children.

The project also requires describing a typical cross section of a child’s neck where pressure is most likely to cause ligature strangulation. Denver Children’s Hospital assisted designing and conducting a clinical project to research suspension strangulations. Our human factors specialist examined MRI images with physicians from the hospital to identify a cross section most representative of the general area to be used as a basis for the 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.

The Denver research took place during surgical procedures on over 100 children. Wearing a penny-sized pressure gage on one finger, a physician applied pressure at two different positions on each child’s neck while an anesthesiologist monitored the level of airway occlusion. It didn’t take much pressure in a superior and posterior direction to cause airway occlusion. Without solid information provided by the test, we could not adequately advise product manufacturers about possible strangulation hazards.

The finite-element analyses so far have revealed more about the amount of force needed to completely compress the jugular veins and carotid arteries in the neck of an 18-month-old child, a common age for strangulation. These analyses also help understand the behavior of the neck tissues and how force is transferred through them.

Makers of children’s products must know the limits of human tissue to generate designs that minimize the possibility of strangulation. Virtual designs can be quickly and creatively modified without significant investments in time or prototyping costs. Clients are more open to investigating concept changes at early stages in product development.

A linear stress analysis was performed to verify the finite-element model. It produced conservative estimates of deflection from compressive forces that were applied to the side of the neck. The initial run showed the modulus of elasticity for human tissue was too low to be analyzed using a linear material model. In subsequent runs, our engineers specified a higher elasticity modulus for the fat, muscle, and skin portions of the model. Deflection results from the modified linear analysis showed that ligature strangulation more severely deforms areas around veins and arteries than does suspension strangulation.

Before conducting the analysis, we decided to focus on the compression of the veins in the neck rather than arteries because veins are located outside the arteries, have lower pressures and larger diameters which make them easier to compress. Analysis results confirmed the hypothesis.

The next step
Greater precision will require nonlinear analyses to simulate more complex material properties of human tissue. These analyses should yield deflection results that show accurate large deformation, which cannot be determined using linear analysis.

Future nonlinear analyses will explore other aspects of ligature strangulation, such as how the size and shape of straps and cords affect tissue behavior and the likelihood of strangulation. With size and shape information, and additional data about compressive forces from medical research, we will be able to set specifications for size, shape, and breaking force that will minimize the risk of ligature strangulation.

In addition, analysis results can be used as an educational tool in conjunction with a physical prototype to help clients understand the significance of designing products that pose minimal ligature strangulation hazards.

One 3D physical prototype of the neck has been made of two-part cast silicones and polyurethanes. These biosimulating materials have dynamic properties that replicate actual human tissues. A later prototype, based on the finite-element model, will feature a circulatory interface and system that work together to simulate blood flow through neck arteries and veins. It will also let us perform strangulation scenarios using physical objects to further define ligature strangulation criteria.

In addition, future clinical research is planned for ligature strangulation using similar operating room procedures to find the force needed to cut off the blood supply from the brain.

Many questions remain unanswered from an engineering standpoint. But from a scientific standpoint, FEA software has propagated discussion and helped direct future research. The FEA model raised as many new questions as it answered. We are enlightened by this phenomenon because it represents the dynamic nature of human tissue and encourages exploration into all matters of tissue behavior early in the research process.

Finite-element analysis also makes it possible to explore other aspects of risk analysis and management. For example, one possibility is to apply the software’s heat-transfer capability to burn-injury studies. As we realize the implications of FEA on understanding product related risks, we continue to seek out new areas of product safety research that could benefit from FEA.

Additional contributors to this article include: Jill C. Scandridge, Safety Engineer, Scott Milkovich, Technical Manager of Research, and Amy Marrinan, Marketing Communications Specialist, all of RAM Consulting

Where to find soft tissue models
Because the cross-sectional model of the neck was a first attempt at modeling human tissue, it was necessary to identify its engineering properties so they could be applied to the FEA model. Through researching 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, Mich. 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 stress-strain curves for soft tissue are extremely nonlinear. An accompanying graph shows their general shape. Each of the tissues mentioned has a distinct curve which make a nonlinear analysis all the more essential.

Later analysis must also be 3D because pressing on a neck causes some tissue to move up and some down. The model will be further complicated by the relative motion of adjacent tissues so each tissue group will be surrounded by constraint elements.

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