EVOLUTION AND MECHANICS OF HEAD PROTECTION
George Salvaterra
The Department of Athletics, Sport Medicine Center, The Pennsylvania State University, Lasch Building, University Park, PA 16802, gfs@psu.edu
Abstract: The major focus of this chapter is two-fold: 1) to deliver a clear message that athletic injuries, including traumatic brain injury, are not simply accidents, but instead, they have definite patterns and distinct non-random and more or less predictable characteristics; and 2) to elaborate on current understanding of head protection by means of a scientific database approach to the mechanics of helmetry. It is the responsibility of athletes' health care providers and sport clinicians to continually make adjustments to rules and protective devices in order to reach an optimal level of safe participation without changing the integrity of the game. These steps may ensure that protective devices are indeed performing as expected and not causing harm to athletes and to other participants.
Keywords: Helmetry; Concussion; Multi-sport Helmets; Pneumatic helmets; Head gear; Database approach; Types of head impact.
1. INTRODUCTION
According to the Centers for Disease Control and Prevention: 'Injury is probably the most unrecognized major health problem facing the nation today, and the study of injury presents unparalleled opportunities for reducing morbidity and mortality and for realizing significant changes savings in both the financial and human terms-all in return for a relatively modest investment" (Viano, 1990). There is a conventional wisdom among coaches and medical practitioners that injury is an unfortunate risk that is an unavoidable part of athletics. Therefore it is quite predictable that most athletes, especially in contact sports such as football, hockey, rugby etc.
experience some type of injury during their athletic carriers. As a result, and not surprisingly we observe a growing number of sport-related injuries despite advances in coaching techniques, protective devices and safety rules.
However, my personal experience is in agreement with the well-forgotten
notion suggesting that "it is important to realize that injuries are not
accidents. Instead they have definite patterns and distinct non-random
characteristics" (Doege, 1978). There are definitely athletes at risk for
injury, there are predictable risky athletic situations, and there are harmful
environmental conditions predisposing to injury. The characteristics of the team, the quality of the way the helmets are being fit/maintained and other numerous factors are very important in terms of prediction and prevention of injuries in athletics. These should be seriously considered when analyzing the causes of athletic injuries and developing appropriate strategies aimed at prevention of injuries in sports. Head injuries, commonly occurring in athletics, are probably the most unrecognized health problem facing the nation today and there are unparalleled opportunities for reducing morbidity and mortality. We definitely have in our hands numerous elements allowing for the reduction of morbidity and mortality in the sport environment.
Among these elements are: a) helping with the maintenance of the properly fit helmets; b) developing different coaching strategies with the emphasis on proper, skillful and safe techniques; c) improving general injury reporting/grading systems; and d) developing a scientifically based return to play criterion. With advances in brain imaging technology, including MRI/MRS, CT, and EEG, research methodology including evaluating patients in pre and post concussion time frames, we may soon identify individuals who are at risk for injury and should not participate in any type of contact activities. If we could reduce the morbidity and mortality of these types of situations, then we could provide a tremendous service to our athletes.
In this chapter, the scientific database approach to the mechanics of helmetry and different types of helmets will be discussed with respects to prevention of brain injuries in athletics. Specifically, database approach consists of collecting good epidemiological data and then matching that up with good mechanical data. The information for database, including biomechanical and empirical data, has been obtained from animal and cadaver studies, automotive industry, military, Snell Foundation, and various other institutes, committees, societies and organizations. Considering helmetry, in general, it is important to evaluate different types of helmets in terms of thier quality and suitability for the demands of each individual sport, and to address controversial questions such as: Are multi-sport types of helmet effective? Is this particular helmet appropriate for all extreme aspects of a particular event? Is a multi-sport type of helmet appropriate and safe for use in pole vaulting? To answer these questions, one should understand the mechanics of helmets; how the shell and padding convert energy and attenuate force; or how to manage the crash by slowing the head.
These are very important issues because every sport has different crash
management characteristics requiring different impact protection. The fit of
a helmet is also an important factor especially with pneumatic helmets. The
existing notion that pneumatic features of helmets do nothing for shock
absorption is highly erroneous. There is empirical evidence that pressure
change inside the helmet effects the fit and therefore the ability to attenuate
force. Specifically, deflated helmets versus fully inflated helmets have
different shock absorption characteristics. Therefore, it is extremely important that the proper size helmet fits correctly with the adequate amount of inflation to maintain an appropriate stopping distance. Thus, avoiding the risk for brain injury upon impact or head-to head collision is at least partly defined by the proper characteristics of helmets. Unfortunately, the constant monitoring of the PSI (pressure inside the helmet) is problematic. Pressure inside the helmet may change as a result of change in atmospheric pressure.
Therefore, it is critically important, especially after a road/airplane trip, to check pneumatic helmets to make sure that the valves fit properly and have not leaked and that the air bladder is properly filled.
2. EVOLUTION OF PROTECTIVE EQUIPMENT 2.I. ^^Non-scientific, field testing approach''
Until the second half of the 20^^ century the evolution of helmetry was
"unscientific" and driven by a trial and error approach to equipment development. Little or no scientific evidence and objective laboratory data were employed while developing protective devices including helmets.
Moreover, no field testing was conducted to control for safety and reliability of protective devices. Finally, no design modifications were proposed based on scientific evidence of used equipment. Overall, the evolution of protective equipment has been driven by morbidity, mortality and litigation.
Examining the sequence of events that have happened from 1896 to 2000, it has been found out that the drivers of new modifications in helmetry were epidemic of fatalities that occurred. In 1939, the NCAA required the use of helmets in collegiate football. Prior to that, athletes played football with or without leather helmets at their own choice. Not surprisingly, there was a high mortality rate in collegiate athletics. In 1943, the NFL required the use of helmets as well.
Originally, helmets were introduced in 1896, and they were basically
made by harness makers where the leather was stripped together and covered
the head almost like a basket. These original helmets were later converted
into the leather caps and eventually formed into the leather suspension
helmets that covered the ears. Additional features of these early helmets
included the face mask attachments and nose attachments. Overall, these
helmets were a combination of various types of padding and leather
suspension under a fiber crown. Unfortunately, when these leather helmets
were introduced, the amount of head injuries, including concussions
increased dramatically due to increased number of head-to-head collisions
and face tackling. This tendency drove the industry to find different
methods to protect the head and face from concussive accidents. The partial
solution came from advances in aviation technology prior to World War II.
In 1937, the development of a plastic shell influenced production in the helmet industry. The original plastic shells were introduced that consisted of two separate half-molds glued together with a one inch lap joint band.
However, they often split during collision. The plastic shells were supported by a web suspension system that kept the helmet off the skull. The plastic helmet covered the entire skull and diverted blows from any direction and distributed force over the entire head. Moreover, individuals wearing this helmet experienced tremendous reverberation upon impact and collision.
Not surprisingly, the number of head injuries increased as a result of this
"head protective innovation". Later on, the helmet proceeded from the web suspension style to a padded style of suspension and then to more intricate style of padding using different types of foam and fluid bladders, pneumatic air cushions and pneumatic airliners. In 1940, the webbed style suspension was replaced by a padded style suspension. This different type of technology with use of the foam padding helped to better absorb a shock, but basically, the protection of the helmet was due to the plastic flexing shell. In the 1950's, a more fully padded style of helmet was introduced. This was a new epoch in evolution of padding systems similar to our modern style of helmets. This style of fully padded helmet was stiff and it took a long time to break in the foam padded liner. Athletes often complained about headaches as they used the stiff foam padding. Fortunately, this stiff padding was a tremendous shock converter. The full padded design was the best shock absorbing helmets at the time on the market. The next cornerstone in the evolution of helmetry was a hydraulic helmet. Inside the shell there were a series of hydraulic pads filled with alcohol. The hydraulic fluid shot through the plastic coating that covered the enclosed padding. The helmet design was trying to suspend the shell on the head and protect the head from blows in various directions as the fluid was pushed through the padded pockets. However, this "innovation" ended up being problematic because of the pocketed leaking alcohol through the padding.
In 1957 the Snell Foundation was created. The Snell Foundation was
initially focused on the development of bicycle, motorcycle and other
vehicular types of accident helmets. The Snell Foundation, however,
gathered and produced a tremendous amount of valuable information about
the level of human tolerance to injury. Via their funding initiatives a lot of
new technology was launched in the development of helmetry. Since its
formation in 1969 the National Operating Committee on Safety Equipment
(NOCSAE) has worked to develop safety standards for athletic equipment
and headgear. The football helmet standard was revised in 1977 to include
procedures for recertifying the previously (NOCSAE) certified helmets. In
1978 the National Collegiate Athletic Association and the National
Federation of State High School Association made it mandatory that all
players must wear helmets that meet NOCSAE test standards. In the early
70's the American Society for Testing Materials (ASTM) F-8 Committee on
Sports Equipment and Facilities developed a test method for assessing shock attenuation in football helmets. The method was published in 1976 (Standard F 429 -75). That was probably one of the largest advances in the standardization of safety equipment in athletics. It basically set the standard for all types of helmetry, everything from skateboarding, to kayaking. This is an ongoing charge of the sub-committees to make every sport safe by further improving the quality of protective equipment.
The automotive industry also played a critical role in determining human beings' tolerance to injury. Gurdjian, Lissner, Patrick and later Hodgson and the Biomechanics Research Department at Wayne State University developed the Wayne State Tolerance Curve (WSTC) through the Society of Automotive Engineers and the National Highway and Safety Institute. The National Highway Safety Institute help to develop an acceleration time tolerance curve by examining the fracture patterns of animal and human cadaver skulls by various linear and "rotatory impactors". The WSTC is considered to be a boundary curve; forces of magnitude and duration above the curve are considered dangerous, while those below the curve are tolerable. A series of injury indices were developed using modification of acceleration -time pulses. The Gadd Severity Index (GSI) and Head Injury Criterion (HIC) are still the most widely used indices to predict injury potential. Researchers, manufactures and "reconditioners" use these indices in evaluating head protection and the development of better helmets. In
1978, NOCSAE became a certifying agent for football helmets using the GSI. The NOSCAE warning label was used in 1980 and became standard for sports head gear protection. The ASTM uses linear acceleration attenuation measured in 'g's. ASTM's F-8 Committee is actively engaged in the development of standards for all types of sport helmets today. The National Collegiate Athletic Association (NCAA) through it's rules committees for individual sports has mandated the use of NOSCAE approved head gear for the following sports: football, men's lacrosse, men's and women's ice hockey, goalies in field hockey and women's lacrosse, batters ,base-runners, catchers & hitters in baseball, and softball.
The next prominent step forward in the technology of helmetry was
observed in the late 70'th-early 80's when aeronautical engineers introduced
different types of foam and plastic. The foam and plastic technologies
allowed stiffer polycarbonate alloy shells and thinner resilient vinyl air cell
padding. As a result of these innovations, a variety of new helmets were
introduced. For example, a wide variety of pneumatic helmets (i.e.,
BIOLITE Inflatable pad systems & PRO-AIR) were produced by Riddell
and Bike. These are a type of fully padded helmet lined with pneumatic *'air
cells" with soft and condensed foam inside and pure air helmets with
pneumatic single and double bladders which produce different type of
suspension and shock attenuation. These helmets are supported by various
types of nitril foam. It should be noted that the double bladder mechanism
does not have the same shock attenuation capabilities as the single bladder or foam cells. The inflatable double bladder seems to give better protection against tangential blows because of the greater shell stand-off due to the double bladder mechanism. Overall, these pneumatic helmets are more comfortable than the previous styles of helmet and they offer more protection because of the quality of fit and foam padding.
Over the last eight years, the increased awareness to the serious consequences of concussions has driven manufactures to substantial increase helmet research and design. It is a common practice now to see a liter helmet that has large thin plastic shell and numerous air vents and extruded ridges for strength and thicker padding. This should be worrisome to the consumer, because the polycarbonate alloy losses strength with the thinning shell and increased venting. The construction techniques of fluting the plastic, however, does help to promote strength along the ridge. Most of the weight loss in the plastic is made up in the reinforcement of thicker molded padding. The compromise in construction may cause crumpling of the shell with focal loading. The aero design may also cause crumple zones where the shell has been elongated and face mask, shields and retention straps are attached. The compromises in construction, however may give a shell the necessary strength at an inferior load site if the plastic is reinforced by molding foam directly to the shell. Various helmet manufacturers have proved that by fluting out the back of the helmet and grossly improving back pack may help to better absorb the forces. Of course, fluting out the back of the helmet greatly improves the quality of the plastic. It might be critically important when developing a light pole vaulting helmet, where focal loading of a thin plastic will need to be reinforced. This may provide a tremendous amount of shock attenuation for a single focal load.
2,2. ^^Scientific Approach" to Helmetry
In the 1960's a more scientific approach started to develop in helmetry.
Within the scope of this approach the initial impression of the problem and elaboration of the solution have to be a driving force in the development of protective devices. Does the sport of football really demonstrating a high risk of head injury? Is pole vaulting at higher risk for traumatic brain injury?
Is concussion an unavoidable phenomenon in contact sports? These and
other relevant questions should be addressed first prior to addressing specific
questions regarding the safety of sporting equipment. It is important that
sport safety equipment designers carefully analyze epidemiologic data to
determine the incidence and severity of sport related brain injuries and
analyze the athletic events and circumstances that might be associated with
injury to ascertain the risk of the individual and therefore to society.
First, the video analysis of athletic events may be considered as an important and affordable tool to satisfy this approach. Through film inquiry one may examine the mechanism of the trauma and determine a definitive pattern and non random characteristics that would separate it from an accident. Analysis requires an examination of the body mass, velocity, amount and direction of forces during the impact. The severity of impact and body position, environmental conditions etc. may all determine causality of injury and identify athletes at risk for injuries. Various video formats are currently in use to examine brain trauma relative to real time impact force obtained from accelerometers worn within helmet lining. This may lead to modeling of brain trauma and a better understanding of the mechanism of brain injury. During sport practices and competitions, the events that cause brain injury are usually witnessed and/or caught on video, thereby enabling an accurate identification of the mechanism of injury. This affords the opportunity to determine a cause-effect relationship between injury mechanism and injury symptom resolution. The ability to link the exact mechanisms of injury, such as type of concussive blow, to specific symptoms may provide a greater understanding of how damage to specific brain areas occur, and how this type of damage affects brain functioning.
The relationship of the type of impact at the moment of accident on concussion symptoms and time-course of its resolution could be developed and validated. Specifically, various patterns of impact are identified and categorized based on the videotape analyses of the accidents leading to mild brain traumatic injury. A list of identified movement categories leading to mild brain traumatic injury is shown below. In the final analyses, the frequency counts of these categories at the time of the accident can be correlated with the functional sequelae of brain injury and symptoms resolution.
1. Landing on the head following the collision;
2. Head-to-head collision;
3. Head-to-torso collision;
4. Blow to the back, causing head acceleration into extension;
5. Blow to the torso, causing rotational head acceleration;
6. Blow to the face;
7. Collision with stationary heavy objects;
8. Collision with moving objects;
9. No physical impact;
10 Blows from the side;
11 Other unknown and overlooked categories.
Second, epidemiological studies may provide an invaluable input to
determine which injuries are preventable with appropriate use of protective
equipment and what kind of sport/athlete is at a higher risk for traumatic
brain injury. With this approach, we could identify the number of head injuries within the sport and between sports, location of injury, types and severity of injury, gender and age effects etc. For example, when we first considered the development of pole vaulting helmets, we were led to believe that most of head injuries in this sport were just accidents. But, when we carefully looked at the epidemiological data of pole vaulting injuries, we found that there were 47 catastrophic injuries in this sport since 1982.
Testimony of the accident and some video of the mechanisms made it was clear that there was a need to improve pole vaulting safety by educating coaches on technique, proper pole selection, and improving the pole vaulting landing systems. These changes can significantly reduce the risk of brain injury while not changing the integrity of the sport. It is important to note the ongoing debate regarding the role of helmetry in pole vaulting. Many pole vaulting advocates are concerned that the integrity of the sport may be affected as a consequence of the use of helmets. The production of a light, comfortable and aerodynamically sound helmet for pole vaulting may resolve existing controversies. In particular, one must be sure that these
"protective devices" are not going to cause any harm to the athlete and any other participant on the field. Once it is determined that sufficient epidemiological data indicates the use of protective devices, we can implement these devices with the possibility of changing some rules.
Ongoing injury data collection will determine a change in injury rate and other effects. Then performance standards must be prepared to ensure that these devices meet the safety requirements and reduce the probability of injury. Standards then have to be put into action and developed by various certification councils to ensure that protective devices that are sold meet these standards. NOSCAE, ASTM, HEC, and other societies are there to protect and implement standards and to make sure that protective devices are used properly. Continuous data collection is then required so that any rules and standards can be modified to maintain safety.
3. ISSUES OF MECHANICS IN HELMETRY
The major concern for the designers of headgear is to reduce the relative velocity of the head and the object it will impact to zero without causing tissue damage. This must take place over a sufficient distance. Therefore, the same relative velocity could cause significant damage or no damage dependant on the stopping distance or time it takes to bring the head to zero velocity.
To design adequate head protection, an understanding of the types of
injuries, types of impacts, and the magnitude of forces that may be
detrimental and damaging brain tissues must be developed. It is critically important that in order to develop a proper protective device, one must know the type of impact the person encountered at the time of injury. It should be noted that brain injury can be inflicted via three distinct mechanisms: direct impact, indirect impact (impulse) or qusai-static impact (compressive).
Direct impact entails linear accelerative force which makes contact with the head (i.e., head lands on hard surface, head hit with ball). Indirect impact (impulse), on the other hand refers to a tangential accelerative force which sets the head in motion without directly striking it (i.e., whiplash). Qusai- static impact (compressive) refers to a crushing type of force when the head is caught between two objects. Regardless of how the concussive blow is produced, it exerts its effects via a process of inertial forces or loading.
Inertial forces possess two chief components. These are translational or linear acceleration/deceleration and rotational or angular acceleration/deceleration. Translation of the brain may be defined as movement in a straight line which passes through the head's center of gravity. In contrast, rotation of the brain occurs when the head is accelerated tangentially and moves through an arc around its center of gravity.
o 2 5 0 -
200"
1 5 0 -
1 0 0 "
50"
\
- \
\ Acceleration-Time Tolerance
x^^^
— 1 1 1 1
5 10 15
T i m e ( m s )
2 0
Fig, 1. Acceleration-time tolerance level in humans (based on cadaver tests). High levels of acceleration can be tolerated for longer times. Forty-two g's can be tolerated by humans for several seconds without serious injury, according to tests. Re-drawn with permission from:
Gurdjian, ES, Protection of the Head and Neck in Sports, JAMA, 182(5) pp. 509-12)
"Copyright @ 1962, American Medical Association. All right reserved.
The types of impact include direct impact, inducing linear acceleration,
indirect impact inducing tangential acceleration, and static/quasistatic impact
inducing compression forces. These are the types of impacts a helmet is
suppose to stop at the time of collision. Different types of blows result in
various types of brain damage mediated by types of forces that may damage
brain tissues. Compressive forces may press tissue together while tensile
and shear forces may tear tissues apart. There are three types of injury in terms of structural damage that these forces cause; soft tissue injury (basic laceration), bone injury (skull and/or facial fracture) and brain tissue injury.
If adequate protection is to be provided to the head, it is necessary to establish impact tolerance levels and provide optimum design of the protective devices by decreasing force onset rate and the peak force within acceptable limits. It should be remembered, hov^ever, that it is absolutely impossible to stop all types of traumatic forces in the athletic environment. But, it is possible to reduce the amount of these traumatic forces below the threshold of injury. Schematics of tolerance level as a function of linear head acceleration and dui'ation of impact is commonly represented by the tolerance curve as shown above in Fig.l.
Linear acceleration forces above the curve are considered to be extremely dangerous. Forces below the curve are considered to be tolerable.
Clearly, it is important to consider that a helmet decrease linear acceleration forces below the tolerable levels so athletes may be able to tolerate various impact forces over a given time. A few mathematical derivations from this curve were proposed leading to development of two basic standards: Gadd Severity Index (GSI) and Head Injury Criterion (HIC). The Severity Index is used by NOCSAE and most football helmet manufacturers. The GSI of 1,200 is the tolerable limit for headgear that uses the NOSCAE testing standard. Gadd Severity Index was at one time the standard for head injury criteria. There is also another standard of head injury criteria, used mostly in bicycle helmet manufacturing. Acceleration energy measured in "g"s as a unit of stress is used by ASTM, Snell Foundation, CPSCNASA, and ANSI to assess the tolerance level. The stress limit measured in ''g"s is 250-300 depending on the test standard.
In the world of sport various demands are placed upon the head and it may need protection from various velocities of impact. High velocity may cause focal tissue damage whereas low velocities may cause defuse tissue deformation. Protective headgear must be constructed to manage high velocity impacts and distribute the energy over a large area within the shell;
while managing low velocities by deformation within the lining.
Fig. 2 depicts well-known relationship between protective padding
requirements and impact velocities. When impact velocity is doubled,
padding needed for protection must be quadrupled. Thus, in order to protect
an athlete from a 12 foot drop (impact velocity 27.8 ft./sec), like is pretty
standard for pole vaulting, one need to quadruple the padding of a helmet
designed to protect an athlete from a 3 foot drop (impact velocity 13.9 ft/sec)
a standard bicycle helmet. The impact energy quadruples from 1062.66ft/lbs
to 4250.62 ft/lbs. Therefore, it would be improbable to assume that a bicycle
helmet could possibly meet the needs of pole vaulting. The stopping time is
also crucial. It is necessary to decrease the impact velocity within a certain time frame so that the crash energy is converted in the form of heat or foam deformation. The stopping time may be calculated based on impact velocity-time relationship. Again, as can be seen from Fig.2, with increased impact velocity it is necessary to magnify the size of the padding in an ideal crash management situation.
SLOPE EQUALS:
ACCELERATION OF HEAD 2V1=V2
Time
Fig, 2. When the velocity is doubled, padding needed for protection (same acceleration) must be quadrupled. Area "cde" equals distance head moves after contact in coming to rest. Area
"abc" is 4 times area 'cde". Re-drawn with permission from: Gurdjian, ES, Protection of the Head and Neck in Sports, JAMA, 182(5) pp. 509-12) "Copyright @ 1962, American Medical Association. All right reserved.
It is also important to consider the characteristics of crash energy
management with different material lining the helmet. It should be noted
that hard foam (B2) has quite opposite characteristics than soft foam (Bl)
and the ideal (B) which produces the minimum acceleration. Sofi foam has
a tendency to deform more quickly. Hard foam has a tendency to have
greater acceleration at first along with decreased deformation. Moreover,
h^vd foam has a tendency to be more uncomfortable while soft foam has the
tendency to be more comfortable, but does not posses the crash management
capacity prior to bottoming out over time. In most helmets there is a mix of
both types of padding to manage the crash. Soft foam may reduce the force
initially more comfortably. The harder padding takes over the energy
management over a longer period of time. Basically, this is ideal crash
energy management where deformation occurs in the padded lining. The
impact energy transforms into the helmet causing its deformation. Heat is
then released as deformation occurs, and the impact energy is reduced to a
safe level as the head comes to rest.
Key point: The slowing of the head is dependent on the foam characteristics and foam thickness.
It is very important to realize that efficient crash management is dependent on the proper selection of foam or cushioning material. Every sporting activity has different demands (single very high impacts or numerous low impacts), therefore one must take into consideration what type of foam or cushioning material is lining the protective headgear and its thickness. Overall, the slowing of the head is dependent on the foam characteristics, and foam thickness.
There are two types of foam and one style of cushioning material on the market. The two types of foam can be categorized as very dense stiff crushable foams or softer rubbery foams that are compressible. The stiffer crushable foams can vary in their density. The density enables the lining to tolerate various impacts before it meets its limit and bottoms out. The higher the impact the greater the needs for more dense foam that will crush upon impact and gradually slow the head. EPS (Expanded PolyStyrene), EPP (Expanded PolyPropylene), and EPU (Expanded PolyUrethane) are a few of the most widely used crushable foams. EPU and EPS are both dense foams.
They have very uniform cell structures that can be reinforced with additive resins, plastic, nylon or carbon fiber to increase cell density that stops the foam from splitting. These foams are typically not reusable once they have met their crushable limits. EPP is a crushable foam that has some elasticity and will recover after impact. It is used in some multiple impact headgear that requires a level of safe restitution. There are new EPP styles of foam that contain EPS resins which appear to demonstrate the ability to with stand multiple high impacts. The softer foams have a rubbery texture and are easily compressible. They are found in all multi-impact helmets like football and hockey. These foam liners frequently need re-conditioning to make sure that they meet the necessary standard of protection. Zorbium, PVC (Polyvinylchloride/Nitrile), Polyurethane, Polyethylene and Polyester are a few of the most widely used rubbery foam that are fabricated in open and closed cell structure for comfort and force attenuation. They are also easily molded in various shapes and coated with vinyl covering to protect them from sweat. Typically the softer foams of the same thickness do not have the same shock attenuation as the crushable foams. The newest cushioning material is a TPU (Thermoforming polyurethane) that is constructed in twin- hemisphere sheets called SKYDEX. It is more durable than foams and gels with higher shock attenuation. It is also impervious to the elements.
Type of liners:
• Suspension (web)
Suspension (padded) Fully Padded
EPP(expanded polypropylene) EPS (expanded polystyrene) Skydex (TPU)
Polyurethane Polyethylene
Polyvinylchloride/Nitrile Pneumatic Airliner
Pneumatic Air Cushions - Pads Hydraulic - Liquid Filled
The quality of the shell is very important in terms of the shock attenuation which is a function of the hardness. The softer the helmet shell the more focal an impact would be with a higher peak force. The more rigid a shell the more diffuse the impact would be with lower peak force. There are many types of shells available on the market today. Basically, these are polycarbonate/polyester alloy and polycarbonate/polyolefin alloy types of shells making up the biggest part of the football helmets. The helmets are typically molded spherically to deflect blows. Newer model have thin plastic for decreased weight with venture venting for cooling and fluted construction for strength. There are also Acrylonitrial Butadienes Styrene (ABS) and Polyethylene terephthalate (PET) types of shells. The ABS and PET shells can come in various levels of hardness and can be molded in various shapes. ABS can also withstand the heat of having an EPS liner molded within the shell. This provides the shell with more construction strength. ABS and PET typically make up the shells of bicycle, skateboard and water sport helmets. In addition, there are Carbon/Fiberglass and Carbon/Kevlar shells available on the market. These are brittle shells that can distribute the forces over a greater area before the shell may crack.
These brittle shells are usually used in so-called "once and done" situations (for example pole vaulting, cycling, skiing, and motor sport). These shells are useful in sports where a high peak force must be attenuated upon impact.
The softer shells like PET will have more centralized pressure site due to continuation of force at impact. With these moderate shells, like the ABS, we have a complete spectrum of impact forces that must be attenuated.
Considering a hockey helmet, one must be aware of the high velocity of a puck at the impact and possibility of a diffuse blow when hitting the ice or glass multiple times. The shells should meet the sport specific needs to be safe and meet the protective standard.
Types of shells:
• Acrylonitrial Butadienes Styrene(ABS)
Polyethylene terephthalate (PET) Carbon/Fiberglass
Carbon/Kevlar
Polycarbonate/polyester alloy Polycarbonate/polyolefin alloy
4. SELECTING THE PROPER HELMET
Many factors must be considered when designing a headgear for sports.
The optimum design of the helmet must meet impact tolerance levels which are a function of the characteristics impactors of the sport. The impact tolerances are a function of the impactor mass, velocity, direction, location, repetition, area and surface of impact. All of these factors are specific to the sport. The surface of the impactor includes the roughness and density. The harder the impactor the more focal the impact site. The softer the impactor the more diffuse the distribution of impact force. The roughness will influence the coefficient of friction and may require the headgear to have a specific retention system built for the headgear to keep it in place and not cause further damage to another body part. At the time of impact the direction of the impactor may cause tangential forces resulting in rotation of the head increasing the potential risk for injury. The location is vitally important because various locations of the head are more sensitive to impact forces requiring reinforcement of the headgear construction. The mass and velocity are two of the major characteristics of the impactor that directly effect headgear design. The mass is related to the area, shape, and velocity.
A light object at a given velocity may cause little focal impact, while a heavy object with the same velocity may cause severe diffuse damage. A high velocity impactor of light mass could cause serious focal damage while a low velocity impactor of greater mass may cause little if any damage.
Lastly the headgear must always meet the sport standard developed by a standard writing committee (ASTM, ANSI, NOSCAE). It must then be certified to assure the consumers that the headgear has meet the specific performance standard of the sport (HEC, NOSCAE ,CPSC).
Characteristics of impactors in sports:
Area
Surface
Mass
Velocity
Direction
Location
Repetition
General Helmets Specifications:
Weight ~ 2 lbs.
1" to 1/2" Stopping Distance = deformation of shell and liner Load must be distributed by the shell to prevent penetration Liner must attenuate the force of acceleration to tolerable level The energy absorbing system must be resilient
Low coefficient of friction on the surface to minimize angular motion Compressive loading must be minimized
Edges of the shell must not contact the head or neck causing injury Helmet must be able to fit head sizes from 6 Vi - i 3/8. Comfort and maximum protection
Retention system must hold helmet in place and not cause injury
The protective properties must be resistant to decomposition from head oils, the elements and organic elements.
5- THE IDEAL HEADGEAR
The ideal headgear is one that has an energy management system that meets the challenges of extreme environmental conditions and various impactors and keeps the forces within tolerance limits. In order for a headgear to meet all the specifications of standardized testing there are compromises that must be made but some general specifications must always be met. The headgear must always have a shell hard enough to stop a penetrating impact and distribute the load over the entire surface. The surface must be smooth and spherical and minimize friction and tangential acceleration. It must be resilient enough to tolerate crushing blows and cover specific areas that constitute all of the documented impact sites while not coming in contact with the neck causing injury. The energy management system of the shell and liner must be approximately 1-1/2"
thick allowing deformation but not bottoming out and maintaining a safe stopping distance. The headgear must be able to withstand various environmental conditions without decomposition and loss of shock attenuation. It must also come in various sizes to get exact sizing. Lastly a retention system must keep the helmet in place so that it does not cause injury.
CONCLUSION
What is really important to realize is that head injuries are not accidents.
Instead they have definite patterns and distinct non-random characteristics.
There are several critical elements that may be under our control reducing
the risk of brain injuries in athletics. Among these elements are: a) helping with the maintenance of a helmet that properly fits the athlete; b) developing different coaching strategies with the emphasis on proper, skillful and safe techniques; c) improving general injury reporting/grading systems; and d) developing a scientifically based return to play criterion. It is extremely important that the proper size helmet fits correctly with the adequate amount of inflation to maintain a safe stopping distance. Avoiding the risk for brain injury upon the impact or head-to head collision is at least partly defined by the proper characteristics of sporting helmets.
REFERENCES
Viano, D. C. (1990). Public Health Report, 105(4):329-333.
Doege, T.C. (1978). Sounding board: Any injury is no accident. New England Journal of Medicine, 298(9), 509-510.
Gurdjian, E. S. (1962). Protection of the Head and Neck in Sports. JAMA, 182(5), 509-12.
