Forensic Engineering Expert Witness Blog

Concussion and Whiplash Injury

Posted on Thu, Mar 16, 2017 @ 08:00 AM

Can a concussion be caused by a low-speed rear-end crash? The prevailing science suggests the answer is no, but new research* has identified some cases where the answer could be yes.

Most people link whiplash injuries to low-speed rear-end crashes and concussions to direct head impacts. These links are sufficiently pervasive that the type of impact—rear-end crash versus a direct head impact—may influence a clinical diagnosis for what can be a similar pattern of symptoms. From a biomechanical perspective, rear-end crashes generate head impacts with the head restraint and direct head impacts generate forces in the neck, which mean that mechanisms for both whiplash injury and concussion often co-exist.

To answer the question of whether a concussion can occur in a low-speed rear-end crash, we compared how a computer model of the brain responds during sport-related head impacts to how it responds during head restraint impacts that occur in rear-end crashes.

We used head impact data from football and rear-end crash tests as input to the brain model and then examined the resulting brain strain, i.e., how much the brain stretches and distorts. We then compared these results to data from concussed NFL players whose head impacts were reconstructed from game video, and in particular to two players whose impacts were to the back of the head, the region of the head that strikes the head restraint. We found that football impacts caused higher brain strains than most of the rear-end crash tests we examined, but there were some instances where both types of impacts produced similar amounts of strain in the brain. In one crash involving a 15 km/h speed change and a low head restraint, the dummy’s head wrapped onto the top of the head restraint and caused similar brain strains to head impacts that were more severe than the two responsible for concussions in NFL players.

Image_Concussion_2017_brain strain.jpgFigure - Two images of the peak strains on the brain’s surface as calculated by the SIMon brain model. The left image shows the largest brain strains produced by a rear-end collision, which consisted of a 15 km/h (9 mph) vehicle speed change with a low head restraint. The right image shows the largest brain strains produced during a low rear football impact, which simulated a helmeted head impact with a closing speed of 33 km/h (21 mph). The simulated football impact was more severe than two low rear impacts that have caused concussions in NFL players.

These findings show that brain strains associated with some concussions can occur during rear-end crashes in some special cases. They also show that most exposures to rear-end crashes probably don’t generate the amount of brain strain associated with concussion. For clinicians, our findings mean that they should not rule out a concussion just because their patient experienced a rear-end car crash, particularly if the engineering analysis indicates a higher speed change combined with a poorly positioned head restraint.

* Elkin BS, Elliott JM, Siegmund GP (2016). Whiplash injury or concussion? A possible explanation for concussion symptoms in some individuals following a rear-end collision. Journal of Orthopaedic & Sports Physical Therapy 46(10):874-85.

Tags: conscussion, whiplash, brain strain

Distracted Driving: How it Affects Liability

Posted on Fri, Jan 22, 2016 @ 11:00 AM

Distraction has always affected drivers, but the addition of interactive devices like cell phones and navigation systems has raised both the opportunity for distracted driving and awareness of its perils. Although official US statistics show that only 17% of injury-causing collisions involved distraction,1 a yearlong study monitoring drivers during regular vehicle use showed that inattention (which includes distraction) was present in the three seconds before 78% of all crashes2. Based on these data, distracted driving is a big problem. 

Proving that a driver was distracted before a collision is difficult, but understanding how distraction affects driver performance can help determine whether a driver’s ability to avoid a collision was impaired by distraction.

Effects of Cognitive Distraction

Cognitive distraction, which is anything that takes a driver’s mind off the road and away from the primary task of driving, affects a driver’s visual behaviour and response time. Drivers who are cognitively distracted spend more time focused on the road ahead and less time scanning the periphery3-6. This narrowed focus is commonly called cognitive tunnel vision and can lead to missed or late detection of hazards outside of the forward field of view.

Cognitive distraction also increases response time7-8. A recent driving simulator study9 showed that response times vary with the type and location of a hazard, but in all cases cognitively distracted drivers had longer response times than undistracted drivers. Response times increased by as much as 0.4 seconds, which adds two extra car lengths to the stopping distance at 80 km/h or 50 mph.

Was a Driver Distracted?

Establishing liability based on distraction typically means proving that the driver was distracted (e.g., on their cell phone) and then showing the distraction affected the likelihood or severity of the collision. Witness evidence or phone records can establish the driver was engaged in a distracting task. Proving the distraction affected the collision is more nuanced and requires an investigator to perform a series of analytical steps. For instance, to establish that the distraction affected a driver’s ability to avoid a collision, the following steps might be needed:

  • Use the physical evidence from the collision to estimate the driver’s response time,
  • Compare the driver’s response time to the typical response times for drivers facing a similar situation, and
  • Assess whether a typical response time would have avoided the collision.

In some cases, a slow response time alone may be sufficient to show that the driver was inattentive or distracted before the collision, but this generally requires a response time significantly longer than the range for the general population.

Distractions reduce driver performance and can be partially or completely responsible for a collision and the resulting damages. Ask your collision reconstruction engineer or human factors expert if distraction played a role in your next case, and in the meantime, put away your cell phone and minimize your other distractions while driving.

  1. National Highway Traffic Safety Administration (2013). Distracted Driving 2011, DOT HS 811 737. NHTSA’s National Center for Statistics and Analysis: Washington, DC.
  2. Dingus TA et al (2006). The 100-Car Naturalistic Driving Study, Phase II – Results of the 100-Car Field Experiment, DOT HS 810 593. Virginia Tech Transportation Institute: Blacksburg, VA.
  3. Hammel KR, Fisher DL, and Pradhan AK (2002). Verbal and Spatial Loading Effects on Eye Movements in Driving Simulators: A Comparison to Real World Driving. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, vol. 46, no. 26, pp. 2174–2178.
  4. Harbluk JL, Noy YI, Trbovich PL, and Eizenman M (2007). An on-road assessment of cognitive distraction: Impacts on drivers’ visual behavior and braking performance. Accident Analysis and Prevention, vol. 39, pp. 372–379.
  5. Recarte MA and Nunes LM (2000). Effects of verbal and spatial-imagery tasks on eye fixations while driving. Journal of Experimental Psychology: Applied, vol. 6, no. 1, pp. 31–43.
  6. Reimer B (2009). Impact of Cognitive Task Complexity on Drivers’ Visual Tunneling. Transportation Research Record: Journal of the Transportation Research Board, vol. 2138, pp. 13–19.
  7. Caird, J. K., Willness, C. R., Steel, P., & Scialfa, C. (2008). A meta-analysis of the effects of cell phones on driver performance. Accident Analysis & Prevention, 40(4), 1282–1293.
  8. Horrey, W. J., & Wickens, C. D. (2006). Examining the Impact of Cell Phone Conversations on Driving Using Meta-Analytic Techniques. Human Factors: The Journal of the Human Factors and Ergonomics Society, 48(1), 196–205.
  9. D’Addario P (2014). Perception-response time to emergency roadway hazards and the effect of cognitive distraction, Master of Applied Science. University of Toronto. 

Tags: Human Factors, distracted driving

Safely Towing a Trailer

Posted on Thu, Jun 11, 2015 @ 10:30 AM

Trailers are an important part of many family vacations, but they can also spell tragedy if they are not properly equipped, properly set up and properly towed.

Tow vehicles handle, brake and corner differently when towing a trailer. Stopping distances can be twice as long when towing a trailer, and during cornering the trailer’s wheels track inside of the tow vehicle’s wheels. Thus wide turns are needed to avoid the trailer striking the inside curb. Drivers, particularly those with little or no trailering experience, should drive slowly while they learn how the combined tow vehicle and trailer handle and stop.

As vehicle speed increases, the stability, stopping performance and the ability to swerve or maneuver are reduced. Avoid exceeding the speed limit and slow down before cornering and descending a hill. Trailer stability is reduced in turns because the trailer pushes the rear of the tow vehicle towards the outside of a turn – a phenomenon that can lead to jackknifing.

Trailer sway is a side-to-side oscillation of the trailer that can lead to a control loss and possibly a crash or rollover. Trailer sway can be induced by a sudden steering maneuver, lane change, or even a wind gust. Trailer sway will often diminish after a few oscillations, but high speeds and improper trailer weight distribution can cause the sway to increase with each oscillation until the vehicle loses control.

Trailer_sway_image

The likelihood of trailer sway increases if too little of the trailer’s weight rests on the hitch. The hitch load should generally be between 10% and 15% of the total trailer’s weight (including cargo). To achieve this hitch load, some trailer manufacturers recommend the 60/40 rule: put 60% of the trailer’s total weight forward of the trailer axles and 40% behind the axles.

When a trailer starts to sway, the best response is to hold the steering wheel straight, let off the gas and apply the trailer’s brakes. Applying only the trailer’s brakes counters the sway and will help realign the trailer with the tow vehicle. To prevent further sway, drive slower and redistribute the load if necessary.

Safe trailering begins with the proper equipment, proper setup and proper driving. Consult your owner’s manuals for instructions on how to properly equip, set up and drive your tow vehicle and trailer.

Tags: Failure Analysis, Trailers

Can A Priori Risk Establish Injury Causation?

Posted on Thu, Mar 26, 2015 @ 07:09 AM

The risk of sustaining an injury during a specific kind of crash can be calculated from real world accident data. We simply take the number of injured individuals following a crash and divide this by the number of individuals exposed to this kind of crash. The resulting number, expressed as a percentage, is the a priori risk of that injury for that kind of crash. Here, “a priori risk simply means the risk of being injured before the crash occurs.

Once a crash occurs, however, the a priori risk of injury becomes less meaningful to the exposed individual, who is either injured following the crash or not injured following the crash.  This diagnosis of an injury is made by a medical professional.

In our work, we are routinely asked to establish or refute the causal link between a diagnosed injury and an event alleged to have caused that injury. This question may be asked despite a medical diagnosis for various reasons, including: i) time has elapsed between the incident and when the injury was reported, ii) there is disagreement regarding the diagnosis, or iii) there may be other plausible explanations for the injury.

To assess this causal link, we routinely calculate the force applied to the injured tissue and then compare this force to that shown to cause the injury during controlled experiments. When the applied forces cannot be reliably calculated or the forces associated with the injury are not known, we sometimes turn to a priori risk data. These risk data, however, must be used carefully.

A priori risk data can tell us that a specific injury is possible in a specific kind of crash. This is useful because it confirms that other individuals exposed to a similar crash have been diagnosed with the injury, though these data alone fall short of demonstrating that the injury was probably caused by the crash.

A priori risk values are sometimes used to conclude incorrectly that a diagnosed injury is not causally linked to a specific crash. The argument goes like this: Since only 6% of individuals in a study sustained this specific injury in this specific kind of crash, it is unlikely that the injured party sustained his/her injury in this crash. This logic is flawed because it applies data from many individuals (the study population) to a single individual. Just as the a priori risk generated by the entire study population cannot be applied to a single subject within the study, the a priori risk cannot be applied to a single person outside the study.

In summary, estimates of the a priori risk of injury can help establish that an injury is possible in specific kinds of crashes; however, they cannot be used—in isolation—to conclude that an injury is probably or probably not related to a specific kind of crash.

Tags: Injury Biomechanics

When Should I Replace My Helmet?

Posted on Thu, Mar 12, 2015 @ 11:00 AM

It’s common knowledge that a helmet should be replaced after a significant impact. Exposure to extreme heat or chemicals, like gasoline, ammonia and solvents, can also degrade a helmet. But aside from impact, heat or chemical damage, does a helmet need to be replaced just because it is old? Do the impact attenuating properties of the helmet diminish with time? Put more simply, do helmets have an expiry date?

Owners’ manuals for motorcycle, bicycle and snow-sport helmets often suggest that helmets should be replaced every 2 to 10 years. The scientific basis for these recommendations is unclear, and a cynical consumer might view the short end of this range as self-serving for helmet manufacturers.

Few published studies have evaluated the effect of age on helmet impact performance. Previous tests we did showed that unused 10-year-old motorcycle helmets attenuated impacts as well as new helmets (see this link). But showing that one old helmet model performs like an array of newer helmets does not answer the more general question of whether helmet impact performance degrades with age.

To address this bigger question, we acquired and tested hundreds of undamaged used bicycle helmets of various makes, models and ages. Our preliminary findings, presented at the World Congress of Biomechanics in July 2014, showed that helmet age does not have a significant effect on the impact performance of bicycle helmets. Based on these data, age-related degradation in helmet impact performance is not a compelling reason to replace your old helmet.

As with all scientific studies, our study has limitations. Helmet performance relies on more than just the energy absorbing properties of the helmet’s foam. The straps, buckles, and other helmet features also affect helmet performance, and we did not study how age affects these components. We also tested only helmets without visible damage or significant wear and tear, and therefore our results do not mean that any old helmet will provide the same protection as a new helmet. Nevertheless, our results show that the impact properties of bicycle helmet foam are not significantly affected by age.

Helmet_headinjury_braininjury helmet_closeup

Figure 1 – Exterior and interior of a damaged helmet

 

Impactperformancechart

Figure 2 – Impact performance (head acceleration) does not change significantly with the year of helmet manufacture.

Tags: helmet safety research, Injury Biomechanics, Helmets, head injury

Guardrail End Treatments

Posted on Tue, Dec 09, 2014 @ 02:01 PM

Roadside guardrails make our roads safer, but the ends of these guardrails need protection, or end treatments, to prevent them from becoming roadside hazards of their own. Recent court cases involving some end treatments have focused media and public attention on the potential hazards associated with these special features of guardrails.

The ends of guardrails are designed to absorb the energy of vehicles that crash head-on into them. There are many different designs to achieve this goal, but two “extruder-type” end treatments have received most of the recent attention[i] (Figure 1). These two extruders are mounted onto the end of common W-shaped steel guardrails and are then pushed along the guardrail by the vehicle during a crash. The moving extruder flattens the guardrail and bends it back on itself, a process that absorbs energy and slows down the vehicle. The posts supporting the guardrail also break away, absorbing additional energy.

 Extruder_guard_rail_expert

                                Figure 1: Extruder type guide rail end treatment

Extruder end treatments are designed for primarily frontal vehicle impacts, and therefore are most commonly installed on straight roadways. Extruder end treatments can fail to operate properly under some conditions: if they are improperly installed, if their design is defective, or if vehicles strike them too obliquely. These failures can cause three outcomes:

  • The extruder head bends back with the steel beam and allows the vehicle to get behind the guardrail before slowing the vehicle;
  • The extruder head drops to the ground and the vehicle may then ride up and over the extruder and guardrail; or,
  • The extruder head “locks” on the steel beam and penetrates (spears) the vehicle.

All guardrail end treatments must pass a series of crash tests before being approved for roadside installation. Early tests conformed to a standard issued in 1981[ii], but most currently installed end treatments meet a revised, more comprehensive standard issued in 1992[iii]. Since 2009 end treatments are required to meet a third standard[iv] that is similar to the second standard. The crash tests in these standards attempt to capture typical impact conditions, and include passenger cars and larger pickups striking the end treatments at speeds up to 60 mph (100 km/h) at a maximum angle of 25 degrees. The test vehicles are driven along a straight path and only the front of the vehicle strikes the end treatment. In practical terms, this means that there are real-world crash conditions (e.g., angled or side impacts) that fall outside of the standard test conditions and may result in failures of the end treatments.

Cases involving the potential failure of a guardrail end treatment need to be properly investigated. Key steps in the investigation process include i) examining the vehicle, guardrail and end treatment, ii) determining if the installed device was approved, iii) assessing the appropriateness of the end treatment for the specific location, and iv) documenting how the end treatment was installed and maintained. All of these factors then need to be interpreted in the context of the specific vehicle and collision dynamics. A roadside safety engineer can help evaluate how well a guardrail end treatment functioned and whether a properly functioning device could have affected the collision outcome.

 


[i] Trinity ET-Plus and its predecessor the ET-2000 are the extruders involved in the recent court cases

[ii] National Cooperative Highway Research Program: NCHRP-230 “Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances”, TRB, 1981

[iii] NCHRP-350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features”, TRB, 1993

[iv] “Manual for Assessing Safety Hardware (MASH)”, American Association of State Highway Officials (AASHTO), 2009

Tags: accident reconstruction

Three Factors for Assessing a Driver's Emergency Response

Posted on Wed, Nov 19, 2014 @ 03:47 PM

A driver who fails to avoid a roadway hazard, such as a pedestrian, cyclist or other car, could have his pre-crash responses closely scrutinized if the crash is investigated.  Driver responses are generally scrutinized in three ways: 1) response speed, 2) response choice, and 3) response magnitude.

Response Speed

The time it takes a driver to respond to a hazard is called the perception/response time. This time begins the instant a hazard becomes visible and ends when the driver begins to steer or brake. It captures everything from figuring out what the hazard is doing, deciding how to respond to the hazard, and then contracting the muscles needed to either steer or brake the vehicle.

Differences in individual drivers, hazards and environmental conditions affect the length of perception/response times.  By adjusting for these differences, a driver’s actual perception/response time can be compared to those measured in detailed studies of driver behaviour in similar situations. This comparison can then establish if a specific driver’s response was normal or unusually delayed.

Response Choice

When faced with a complex hazard, a driver may not know if braking, steering or doing both is the best way to avoid a crash.  For instance, a driver might brake and steer to the left to avoid a child who runs onto the road from their right.  This would be the right choice if the child sees the car and stops, but could be the wrong choice if the child continues running (see figure). In the latter case, not steering at all would have avoided the collision.

This sort of after-the-fact analysis of a driver’s choice is affected by hindsight bias1, which means that, with the benefit of hindsight, the analyst can clearly see the best choice because he knows what the child actually did.  At the time the driver had to choose a response, the driver did not know what the child would do.

For this reason, some legal jurisdictions acknowledge that drivers facing the “agony of collision” should not be held to a standard of perfection, but rather to a standard of what a “reasonable” driver would do in similar circumstances.  Thus, information about how typical drivers respond to a situation can help judges and juries see that a specific driver’s response was reasonable, despite the unfortunate outcome.

Driver_response_photo

Figure 1: With hindsight, we can see that steering left away from the incoming child, contributed to the collision, whereas not steering would have avoided an impact.

Response Magnitude

Most drivers do not apply their vehicle’s brakes maximally in an emergency. A study of drivers who encountered a child dummy suddenly crossing their path found that 86% of drivers had a two-stage braking response: they quickly applied their brakes to an initial submaximal level and then paused to assess the situation before braking harder2. Drivers who used two-stage braking were 17% more likely to strike the dummy than drivers who braked hard immediately. Here again an analysis that assumes a driver braked hard immediately might show the collision could be avoided when the driver’s less perfect braking behaviour was the same as a typical driver.

Summary

Response speed, choice and magnitude need to be considered carefully in the context of a driver’s specific collision conditions to properly assess whether their response was typical of most drivers or atypical and possibly responsible for the collision.


[1] Dilich M, Kopernik D, Goebelbecker J (2006)  Hindsight judgment of driver fault in traffic accident analysis – Misusing the science of accident reconstruction.  Transportation Research Record: Journal of the Transportation Research Board, No. 1980, 1-7.

[2] Prynne K & Martin P (1995).  Braking Behaviour in Emergencies, 950969.  Society of Automotive Engineers.

 

Tags: accident reconstruction, Human Factors

Why Don’t Commercial Airplane Passengers Have Parachutes?

Posted on Tue, May 27, 2014 @ 10:05 AM

Among the pilot's last words on Air France 447 that crashed in the Atlantic in 2009 were "This can't be happening”, denying the tragic fact that there is no chance of survival on an aircraft that experiences a catastrophic malfunction at cruise altitude. There is always hope for survival in land and sea accidents, because these forms of transport lend themselves to safety devices like air bags, escape hatches and life boats. Commercial air transport does not lend itself to such measures though, and denial of the tragic outcome raises suspicion that something more could have been done: hence the seemingly radical idea of parachutes.

While parachutes function effectively in military ejection seats and even when attached to light aircraft, they will not work for 90% of commercial airliner accidents because those accidents occur during take-off or landing at altitudes that are too low. In the 10% of accidents that occur at cruise altitude parachutes are highly unlikely to work because the challenges of getting ready, getting out and getting down are too great.

Getting ready: Donning and adjusting a parachute takes a long time - trained personnel require at least four minutes to put on a parachute with the luxury of standing on the ground. Doing it right in the crowded confines of an airplane cabin would take longer. However the Air France accident took about four minutes from the onset of the problem until the crash occurred.

Getting out: The doors on commercial aircraft are mechanically prevented from opening until the aircraft is below approximately 10,000 feet. In a scenario like Air France 447 the descent rate is so high that the act of opening the door at 10,000 feet would consume most of the time remaining to impact, so a parachute would be of no benefit. Were the doors to open at 10,000 feet, evacuation of the air in the pressurized cabin would expel some passengers (unrestrained because they were putting on parachutes) before they had a parachute on.

Getting down: Once outside a cruising aircraft you would experience a sudden violent deceleration into cold air. A collision with the wing or tail of the aircraft would be fatal, but it is possible to miss. In that case the deceleration from being hit by the air outside is initially over 20g’s and is a form of blunt trauma. Accounting for windchill, the temperature outside will be about 40 degrees Celsius lower than on the ground. The blunt trauma and cold temperature are survivable on their own, but add to the challenge of operating a parachute for the first time.

The very short time available to put on a parachute and get out of the aircraft, coupled with the challenges of operating a parachute once outside mean that a parachute is highly unlikely to save you on a doomed commercial airliner at cruise altitude.

Your best outcome aboard a malfunctioning airliner is achieved by preparing beforehand. If you fasten your seatbelt, study the safety card, listen to flight crew instructions, know where the exits are and be prepared to leave your carry-on baggage behind, then you can take comfort knowing you did all you could.

Tags: aviation investigations

Withstanding the Pressures of Failure Analysis using Engineering Fundamentals

Posted on Tue, Apr 08, 2014 @ 09:15 AM


 Of the four consumer products in the figure below, one does not belong. Which one is it?

Products-resized-600

The disposable propane cylinder is made of steel and holds liquid petroleum gas, which is a two-phase mixture of liquid and gaseous propane. At normal indoor temperatures the pressure inside the container, called its internal pressure, is about 130 psi (pounds per square inch).

The soda can is made of aluminum alloy 3004 for the body and 5182 for the top (both are highly formable alloys of mostly aluminum with some magnesium). Like the propane cylinder, the soda can has an internal pressure, in this case from the carbon dioxide dissolved in the contents. The internal pressure of a soda can is approximately 60 psi.

The CO2 cartridge is made of ferritic stainless steel and holds carbon dioxide at an internal pressure of around 1000 psi. These cartridges are used for air pistols, paintball guns, tire inflators and even slip-test tribometers.

The water filter housing is made of a common plastic called polypropylene. Such filter housings are part of water filtration systems designed for domestic use with house plumbing where the water pressure, and hence the internal pressure of this water filter housing, is around 75 psi.

All four products have in common that they must withstand internal pressure in order to function as intended, and all four employ thin-walled cylindrical bodies. But what are the differences?

The most obvious difference among the four is that three are made of metal and one is made of plastic. Pressurized plastic is not a problem though, as long as the designer appreciates that polypropylene has a different set of strength and stiffness properties compared to steel, aluminum and stainless steel.

The most important difference in the four products is that three have domed ends - the propane and CO2 cartridges are convex and the soda can is concave - and one is flat. The significance is that according to solid mechanics theory the tensile stress in a flat end is about ten times more than in a domed end. This was significant for the water filter because although the stress in its cylindrical body was amply resisted by the plastic from which it was made, the much higher stress in its flat bottom caused the polymer to crack so that the whole bottom broke off. Water flowed under pressure out the bottom for hours creating a costly amount of water damage.

Spotting the significance of the water filter’s flat bottom is an example of how failure analysts, applying fundamental engineering principles, find the root cause of failures without necessarily being, in this example, plumbing experts. A mastery of fundamental engineering principles serves any failure analyst well when confronting a vast assortment of product, machinery, device and system failures.

 

Tags: Failure Analysis

Biomechanical Analysis of Concussion

Posted on Tue, Mar 11, 2014 @ 09:15 AM

Like most other injuries, concussions occur in the blink of an eye. Unlike most injuries, however, concussions can occur without leaving a physical mark.

Even when a concussion is diagnosed, the question may remain whether the event alleged to have caused the concussion was indeed responsible. From a biomechanical perspective, it is sometimes possible to confirm or refute causation.

Concussions require that the head experience sufficient trauma to transiently affect brain function. To determine whether a traumatic event had the potential to cause a concussion requires a reconstruction of the event to characterize and quantify the type and magnitude of forces that were applied to the head. The applied forces are then compared to the forces that cause concussion. These forces are often quantified as linear acceleration (reported in units of gravitational force, g) and rotational acceleration (in units of radians per second2).

Physical evidence such as external trauma to the head (e.g., lacerations or extracranial swelling), damage to contacting structures (e.g., a dent in a wall or vehicle hood), and overall scene geometry (e.g., dimensions of a staircase or vehicle interior measurements) can be used to reconstruct a head impact. Witness evidence can also be used to estimate factors such as posture, pre-impact activity and relevant environmental variables. Data from scientific studies of instrumented surrogate headforms (dummies) or cadavers are used to estimate the forces applied during the impact. If published data are not available, then case-specific tests can be performed to estimate the head impact exposure.

Recently, head impact exposure and concussion incidence have been characterized through reconstruction and in situ measurements of head accelerations in professional, collegiate, high school, and youth sports. Although most head impacts do not generate concussions, biomechanical risk curves have been developed from impacts that have caused concussion. These data are analogous to the introduction of “black boxes” in motor vehicles that provided the first real-world data on the relationship between collision exposure and whiplash injury. Similar to whiplash, the percentage of individuals that sustain a concussion is low compared to the number of exposures, so it is important to understand that exposure to forces above concussion thresholds is necessary but not sufficient to cause a concussion. Armed with threshold data, the possibility that a particular traumatic event was capable of causing a concussion can be evaluated.

At MEA Forensic we have the biomechanical expertise required to perform this analysis and help confirm or refute a causal link between a diagnosed concussion and a specific event.

Tags: Injury Biomechanics