Did the claimant sustain a head injury in the incident?Would the diagnosed head injury have been prevented if the claimantwas using a protective device such as a helmet, seat belt, orairbag? Biomechanical engineers frequently answer injury causationand prevention questions like these in personal injury claims.

During the course of an investigation, biomechanical engineersmay compare the forces or accelerations experienced by the head inan incident to those required to cause the diagnosed injury. Theincident forces/accelerations are calculated through reconstructionof the event, and the forces/accelerations required for the injuryare typically determined from published experimental data. If theforces/accelerations in the incident are of the magnitude requiredfor the injury, then the injury is likely consistent with theincident; however, if they are not, then the injury likely is notconsistent with the incident.

This type of professional analysis can be useful when injurieslike concussion or mild traumatic brain injury (MTBI) are beingclaimed. The terms concussion and MTBI are typically used todescribe the same injury and are often used interchangeably.Concussion is difficult for medical doctors to diagnose becauseobjective evidence of the injury can be lacking. Recent mediaattention on concussion in football and other sports highlights thesignificant short- and long-term effects these injuries can have aswell as the controversies surrounding proper diagnosis andtreatment.

When an individual claims an injury such asMTBI, proper use of a biomechanical engineer can be valuable inassessing the validity of the claim. A common case involves aconcussion claim following a relatively low speed rear-endcollision. In this type of collision, the driver initially movesrearward relative to the forward-moving vehicle. The driver's backcompresses the seat cushion and his head rotates rearward until itcontacts the head restraint. Following this rearward motion, thedriver rebounds forward into the seat belt, but typically avoidshead contact with any other structures.

In this case, the peak head acceleration occurs during the headcontact with the head restraint. Many experimental tests simulatingthis type of collision have been conducted using human volunteers,cadavers, and crash test dummies. From these tests, the driver'speak head acceleration exposure is estimated. This value is thencompared to published levels that have been associated withconcussion. For low speed rear-end collisions, the head restraintpadding and compliant seats of most vehicles typically result inlow head accelerations with a very low concussion risk.

Necessary Precautions
Safety equipment such as helmets, seat belts and airbags canmitigate or prevent head injuries when used properly. Biomechanicalengineers often answer questions about the use and effectiveness ofthese safety devices.

Helmets for motorcycling, bicycling, and other activities aredesigned to mitigate and prevent brain, skull and superficial headinjuries. Brain injuries and skull fractures are prevented as thehelmet attenuates the head acceleration and distributes the impactforce to a larger region of the head. This is achieved primarilythrough compression and cracking of the helmet's energy absorbingliner (seen in Figure 1). Superficial headinjuries such as lacerations and abrasions are prevented in theregions of the head that are covered by the helmet.

While most certified helmets are made up ofthe same general components, not all helmets provide the same levelof protection. Full-faced helmets cover a larger area of the headthan shorty or beanie helmets, and therefore may protect a largerarea of the head from lacerations and abrasions. They will alsoprotect against skull/brain injuries for impacts over a larger areaof the head, though often not the entire area covered by thehelmet.  Furthermore, just because a helmet has a stickerindicating it is certified to a specific standard (for example, DOTand/or Snell for motorcycle helmets and CPSC for bicycle helmets)does not mean that it actually meets the requirements of thatstandard. Fake labels are readily available and are even sold onsome online auction sites. In addition, random testing of DOTmotorcycle helmets conducted from 2000-2008 shows that 44 percentof the DOT labeled helmets tested actually failed some aspect ofthe standard.

Helmet impact testing illustrates the significant difference inperformance between different helmets, particularly those that arenot certified. A common question we answer is whether or not a“better” helmet would have mitigated or prevented a diagnosed headinjury. The presence of an adequate energy absorbing liner(typically at least 1-inch thick) is generally associated with a“good” helmet. Testing of non-certified beanie helmets (seeFigure 2) clearly shows their inferior performancerelative to DOT certified shorty and full-face helmets.

More On Risk Reduction
When used properly, seat belts and airbags can also mitigate orprevent head injuries. Seat belts are designed to limit and controloccupant motion within the vehicle during an impact orrollover.  Seat belts are most effective in frontalimpacts, where they can limit the forward motion of occupants andprevent or minimize body contacts with vehicle interior structures.Eliminating head contact in an incident substantially reduces orprevents head injury risk.

A variety of airbags exist in today'sautomobiles and include frontal airbags, side airbags, curtainairbags, and knee bolster airbags. Each of these airbags isdesigned to prevent specific injuries for a specific direction ortype of impact (for example, frontal, side, rollover). Seat beltsand airbags perform best when used together and with “normally”seated occupants. The effectiveness of these safety devices can bechallenged by occupants that are “out of position,” such assleeping. In “out of position” cases, when the airbag deploys itcan inadvertently strike the occupant as it is deploying. Sinceairbags deploy at a very high speed, this type of interaction canresult in large head accelerations and severe injuries.

Typically, the largest head injury-risk reduction with vehiclesafety equipment use occurs in cases where an unbelted occupantstrikes his head on a window frame (or some other stiff interiorvehicle structure). Head contact with stiff structures can resultin large peak head accelerations over a very short period of time.Under the same conditions with a seat belt, the head motion iscontrolled and head contact (if it occurs at all) is against arelatively soft fully deployed airbag (Figure 3)or head restraint. Head contacts with these softer structurestypically result in lower peak head accelerations that occur over alonger period of time and are generally less injurious.

Personal injury claims involving severe head injuries can besubstantial. In these claims, biomechanical engineers arefrequently used to investigate issues of injury causation andprevention. By analyzing the mechanics of the head impact in theincident biomechanical engineers can show or refute injurycausation. They can also evaluate the effectiveness of protectiveequipment or devices that could or should have been used.