Overview of Helmet Design and Safety Components

The purpose of a helmet is to reduce the severity or probability of injury that occurs when the head is subjected to a blow. The objective of good helmet design is to insure that, regardless of the characteristics of the striking object, the loading area is high enough that the force does not exceed the injury criteria. If the helmet is too strong, the force can exceed the injury criteria. Conversely, if the helmet is too weak, the deformation will become excessive resulting in forces higher than the injury criteria.

A helmet typically consists of three primary elements, 1) the outer shell; 2) an energy absorbing liner; and 3) a retention system (such as a chin strap). The object of the shell is to provide a hard, strong, outer surface that distributes the impact load over a large area. It also provides a penetration shield against high-speed objects and is intended to protect both the wearer and the underlying liner of the helmet from abrasion with the impacting surface. The shell must be rigid, tough and hard. Shells are typically manufactured from one of two materials, either fiber reinforced plastics (e.g., fiberglass/resin composites) or thermoplastics (e.g., polycarbonates). The thermoplastic shells are cheaper to produce but they tend to be less rigid unless molded with a very high wall thickness. In addition, they are susceptible to stress concentrations around the rivet holes and can be relatively weak in these areas. Thermoplastic materials can also be relatively brittle. Fiberglass or resin composite materials tend to be more expensive but offer a more rigid shell. Most motorcycle helmet shells are made from fiber reinforced plastics, while most football and cycling helmet shells are made of thermoplastic material.

The interior of the shell is the liner of the helmet. Energy is absorbed through the partial destruction of this component. In order to perform its function effectively, it must deform at force levels below that which would cause head injury. Its strength should be largely insensitive to impact velocity, and to maximize net energy absorption it should have slow rebound or recovery characteristics. When the material is nearly fully crushed, it will become very stiff and the forces developed will become very high. When the material is no longer capable of absorbing additional impact energy, the unabsorbed energy is transferred to the head by accelerating it or deforming it and potentially injuring it.

Following this logic, the head can be protected if the following conditions are met:

1. The reduction in the kinetic energy of the head during impact (i.e., that absorbed by the padding) is less than that which would completely crush the padding material.

2. Both the area and the depth of the padding crushed are small enough that the force developed is less than that necessary to produce sufficient relative movement within the head to constitute an injury.

The probability of meeting these two criteria increases with increased padding thickness and increased padding area which maximizes the energy absorbed, as well as uniform and decreased crushing strength of the padding, which minimizes the force developed.

Motorcycle helmet liners are typically made from polyurethane foams or expanded polystyrene bead (EPB) foams. Both of these materials are relatively inexpensive and suitable for the job. Other recreational helmets such as football, hockey or baseball helmets use other types of energy absorbing liners such as an air or liquid bladder, rubber or synthetic foam.

The third component of a helmet that is critical in keeping a wearer safe is the chin strap and fastener. Clearly a helmet can provide protection only if it remains properly situated on the wearer's head. The chin strap must be designed to keep the helmet in place. Motorcycle, bicycle and hockey helmets generally use a ““D ring”” fastener, a knurled bar arrangement or a plastic fastener. Other recreational helmets such as football or equestrian helmets use other types of fasteners such as snap attachments or open metal catches.

Despite the obvious importance of the chin strap and fastener to the helmet's performance, none of the helmet standards test the retention system dynamically. The only tests performed to test the chin strap and fastener are by loading the system to analyze its performance in a static state. Moreover, several standards (such as the standard for football helmets or equestrian helmets) have no test whatsoever for the retention systematic or dynamic.

Helmet Standards and Testing

Helmet standards typically test impact performance through the following approach:

The helmet is placed on a head form and subjected to a drop test. Typically the helmet is dropped on a metal surface from between three and nine feet. These drop tests are meant to emulate the typical impact the helmet would be sustain during its intended application. The linear acceleration of the head form is monitored during the impact and measured against an injury criterion.

In the U.S., helmet development was pursued by the military beginning as early as the 1940s. In 1959, the Snell Memorial Foundation published the first American Performance Standard for protective helmets. The Snell Foundation sets the highest standard for helmet performance design and continues to increase the stringency of its standard. There are many other world wide standards pertaining to helmet performance, including American National Standards Institute ANSI Z90.1, International Standards Organization ISO R1511, and the United Nations Regulation No. 22.

In 1972, the U.S. Department of Transportation initiated Federal Motor Vehicle Safety Standard 218Motorcycle Helmets, which was identical to the ANSI standard. This DOT standard took effect in 1974 and has remained relatively unchanged since it was finalized. Every helmet sold in the U.S. used by motorcyclists and other motor vehicles must meet the minimum performance requirements of FMVSS 218, which is intended to reduce deaths and injuries from head impacts. Under this requirement, helmets are subjected to impacts with both a flat and spherical impactor that tests for impact attenuation and penetration resistance. Peak accelerations of the head form cannot exceed 400g, 200g for more than 2 ms or 150g for more than 4 ms. The helmets are also subjected to a retention test. The Snell Foundation tests to a higher impact standard than FMVSS 218, with the requirement that energy and peak accelerations not exceed the 300g level.

According to consumer advocates, the DOT helmet standards are inadequate and have allowed the industry to turn to their FMVSS 218 compliance tests as a means to demonstrate their due diligence. However, the U.S. DOT standard is a minimum requirement that is no longer state-of-the-art like the more stringent Snell standard.

Interestingly, one survey of helmet compliance tests conducted by the DOT shows that many helmets do not even meet the minimum requirements of the DOT standard. During the investigation and discovery process in helmet litigation, consumers have also discovered incidents of false helmet certification. For an attorney representing plaintiff in a helmet defect case, it is important to request all documents showing compliance for this reason.

Defect Theories

There are two basic types of helmet defects which cause injury. First, there are those cases in which the chin strap or fastener fails, resulting in the helmet becoming displaced or being ““ejected”” from the wearer's head. These cases have often been referred to as retention system failure cases. Second are head impact attenuation cases that involve injuries caused by the helmet's shell or liner failing to effectively distribute or dissipate the force of impact. Each of these will be briefly discussed.

Retention System Failure Cases

How many times on a televised football game does one witness a player's helmet coming off of his head after an especially hard hit? This is a prime example of a retention failure case. The rate of helmet ““ejection”” as this is called is actually considerably higher in motorcycling than in football or any other sport. These cases, in which the helmet comes off, typically result in skull fracture. Despite the significance of the problem and the resulting injuries which are easily understood, attachment devices are not truly analyzed until an accident takes place.

There are several causes of helmet ““ejection”” or displacement which relate to retention system failures, including poor fit, failure of the user to adequately fasten the strap or inadequate design. An example of this is a chin strap with a weak snap that fails upon impact. Another example of a defect claim that has been brought against several motorcycle helmet manufacturers involves the design and manufacture of their chin straps. Suits have alleged that these straps have broken during impact causing the helmet to be ejected. In one poor design, the double D-rings which were part of the strap's fastener did not have rounded edges. As a result, when tension was put on the strap during the crash, the D-rings functioned as a pair of scissors, cutting through the strap and allowing the helmet to be ejected. In other cases manufacturers have used a chin strap and plastic fastener with Velcro on the straps for loose end attachment. Some wearers unknowingly bypassed the plastic connection and simply attached the strap using only the Velcro. Without the fastener, the Velcro was not strong enough to withstand loading following impact, resulting in the helmet being ““ejected”” following impact.

Head Impact Attenuation

If the helmet shell or liner fails to effectively distribute and dissipate the force of an impact, the wearer will experience head impact attenuation and resulting brain injury. There are three basic failure modes which have been typically alleged in these cases. First are those cases involving shell failure in which the shell actually cracks or fractures upon impact. These cases allege that the shell was defectively designed or manufactured with either an insufficiently thick material, or with material that was too weak to withstand the forces of an otherwise survivable crash. An example of this are cases which have been brought against motorcycle helmet manufacturers who manufactured their shells using cheap polycarbonate material. Discovery in these cases revealed that the manufacturers used ““re-grind”” material in their polycarbonate shells, which resulted in structurally inferior shells. In a crash involving even moderate forces these shells crack or shatter. Once a shell has been compromised in this way the wearer is highly likely to experience a severe localized brain injury or skull fracture.

The second head type of head impact attenuation cases involve cases in which the shell does not crack or fracture but nevertheless fails to effectively distribute the impact load. In these cases plaintiffs have alleged that the shell was too thin or too flexible, causing a local buckling of the shell. Immediately after the impact the shell rebounds after failure to its original shape thus making the defect not readily observable upon casual inspection. Similarly, if the liner is made of a deformable material such as the synthetic or rubber foam used in most football and baseball helmets, in these cases there will be no visible indication of the localized impact on the liner.

The third type of head impact attenuation cases involve injuries caused by poor design or manufacture of the liner material. Helmets made with an extremely dense and rigid liner material such as certain types of EPB foams may be too dense to partially collapse or deform during a crash. When this occurs the liner fails to absorb and dissipate the energy from the impact. This failure causes complete transmission of the force of the impact to the head and brain. Often this type of defect will cause a diffuse axonal brain injury.

Expert Witnesses

As with other types of product liability litigation, the plaintiff's attorney in a helmet defect case will need several expert witnesses. These witnesses should include an accident reconstruction expert, a biomechanical expert and a materials expert. Preferably, the plaintiff should also have a witness who has sufficient expertise to testify regarding the design and testing of recreational helmets.

There are several moving parts of the case that these experts will have to help the attorney assemble: these include the specific facts of the accident; analysis of the manufacturer's test results; compliance results from the NHTSA or other regulatory entity; witness statements; photographs; medical records and other similar incidences. All of this information must be diligently obtained by the attorney during the pre-suit investigation, networking with other plaintiff attorneys, and in discovery.

The experts must help the attorney answer the technical questions that will need to be proven during trial. These questions include:

1. What was the particular design or manufacturing defect?

2. How did the particular defect cause the plaintiff's injuries?

3. Would the plaintiff's injuries have been prevented if the subject helmet had been well designed and manufactured? The answer to this question can only be answered after the experts have used the facts and evidence to calculate the forces involved in the crash and upon the plaintiff's helmet.

Due to the complexities and disagreement among experts regarding the cause of brain injuries, the biomechanical expert is probably the most critical expert in the case. Most experts will concede that localized brain injuries and skull fractures are related to direct head impact and can be minimized by a good quality helmet. However, in the more difficult cases involving a diffuse brain injury, the plaintiff's attorney should expect an aggressive defense on the issue of causation. In most helmet cases that go to trial, the case will often turn on whether the plaintiff presents convincing proof on the issue of causation. The biomechanical expert will have to carry burden on the issue of causation and prove that the client's brain or spinal cord injury was caused by the particular defect in your case.

Conclusion

NHTSA studies show that helmets are 75 percent effective in preventing head injuries. Helmet manufacturers, distributors and retailers should be held liable for damages if a helmet fails to protect its wearer. The key is to look for the classic survivable case in which the helmet fails to attenuate reasonable impacts. Look for incidents in which the helmet is broken, the helmet rolls off or the chin strap fails. Have your experts evaluate the construction of the helmet. Some designs have improper thickness for the material chosen or have an improper shell and liner combination. Recreational helmets should be designed to protect the wearers in the event of a crash but too many times, due to poor manufacture or design, fail and cause unnecessary catastrophic injuries.