The reality of backface deformation and blunt trauma

It’s 1987 and on a dark street, a police detective is walking along, lighting a cigarette. Along comes a dark colored sedan, a shotgun pointed out the window and BAM! the detective is hit in the chest, lifted off his feet, and blown through a plate glass window to land some ten feet away. His partner shows up, helps him to his feet, checks that he doesn’t have any holes in him and they go on their way. The character was Martin Riggs played by Mel Gibson in “Lethal Weapon.” That movie, along with tons of other movies and television shows, has depicted not only unrealistic capabilities of firearms but also that of the body armor law enforcement wears.
We have all seen movies wherein the hero character, or some significant bad guy, gets shot with any one of a selection of weapons from handguns to long guns, and the individual in question gets blown through a window or into a car or whatever. It looks highly energetic, devastatingly powerful and yet…the hero inevitably gets up, shakes it off and goes about arresting/fighting the bad guy. His ability to get up and function is attributed to the body armor he’s wearing, quite often (mistakenly) referred to as a “bullet-proof vest.”
We’re going to discuss two things, both of them related to ballistic armor, and both of which would directly impact that hero’s ability to get back up, or (more likely) not.
There are a couple of terms we need to understand and a few reasonable assumptions that need to be in place for this conversation to take place. First bullet-proof vests aren’t. They are bullet resistant. The large majority of body armor worn by law enforcement today is comprised of soft armor; layers of a fabric that has a higher tensile strength than steel. It’s woven to stop high velocity projectiles by partially wrapping around the incoming bullets as they spin in and by having multiple layers to absorb the energy of the impact.
The energy delivered to the body armor by the projectile is kinetic energy and causes blunt force trauma to the tissue behind the projectile’s impact on the vest. That blunt force trauma—the energy that is delivered through the vest material and into the part of the body behind it—can cause other injuries which we often focus on such as bruises, broken bones, lacerations, abrasions and more. Still, blunt force trauma is preferred to penetrating trauma which would occur if the projectile wasn’t stopped; if it entered the body and did its damage to the tissue and organs inside us.
The projectile impacts the front of the vest. The back of the vest sits (albeit perhaps separated by a few layers of clothing) against our body. As the vest stops the projectile, which takes time and distance, the vest is deformed backward toward the body. How much it is deformed depends on the protective level of the vest and the energy of the projectile. Without getting into all of the physics involved: the heavier the projectile is and the faster it’s going, the more energy it delivers. That distance of deformation is commonly referred to as “back face deformation,” and by itself it can still be lethal. That’s what we’re here to discuss.
The National Institute of Justice rates body armor at various levels of protection:

• Level IIa: to stop .22s, .380ACP, .38SPL, 9mm, .45ACP,
• Level II: to stop all above plus .22lr
• Level IIIa: to stop all above plus .44 Magnum (and presumably .357 Magnum as well)
• Level III: to stop all above plus .30 Carbine, 5.56mm, 7.62mm NATO & .30-06
• Level IV: to stop all above plus .30 Armor Piercing

In addition to those levels of protection to be had from (mostly) soft armor, you can also get hard armor plates that offer a higher level of threat protection. Usually that higher level of threat protection is dependent on combining the hard armor plates with a soft armor vest worn under them. Most Level IV armor is in the form of hard plates and it’s near unimaginable to consider the impact of a 7.62 NATO or .30-06 round hitting the hard plate you’re wearing. That still has to be one heck of an impact and cause a LOT of blunt force trauma. If it’s being stopped by a hard plate, that trauma will be spread out over most of the face of the plate—usually an 8”x10” or 8”x12” size plate. If that round is being stopped by heavier soft armor, the back face deformation can be estimated and measured in inches: four or more. Take a look down at your torso and ask yourself which part of your body you’d like to have shoved in to a depth of four or more inches at high speed. That is the reality of back face deformation.
The additional reality is that we don’t just wear armor on our bodies. We and the military wear ballistic helmets as well. Traditional ballistic designs do a great job of stopping fragmentation, but not such a great job of high angle penetrative shots from high velocity projectiles. If the helmet doesn’t stop the incoming projectile, it slows it down some and that may serve to minimize the injury received. However, if the helmet does stop the projectile, there is still back face deformation—measured in inches. Which part of your cranium (skull) do you want crushed-in an inch—or two—or more?
Recent (in the past decade) design and engineering upgrades to all types of body armor, including helmet design, have made improvements in minimizing the blunt force trauma transmitted through to the individual and in reducing the depth of the back face deformation. Such research and design improvements are of high value to those who have to wear the protective items. For now, we have to settle for getting “the best we can afford,” or the best the agency can afford. When given the choice between a $400 ballistic vest or an $800 ballistic vest, both with an NIJ Level III rating, many choose the $400 option. It is viable and rated according to certain test standards. If an agency is buying such armor, budgetary concerns inevitably take precedent.
An agency’s research or recommendation team might specify a Level III vest, but NIJ recommends a minimum of Level II. Experts in the industry state that you should never wear armor that won’t at least stop the caliber and ammunition you’re carrying in your duty weapon. When that recommendation makes it to the chief’s desk though, and he has to decide whether to buy X number of $800 vests or twice that number of $400 vests, he may well choose to buy the greater number. If you’re the one wearing it, which do you want to bet your life on? Because that’s what’s being done.
Most of the measuring of the value of a vest or helmet is done based on its protection level in accordance with the NIJ standards. Realistically, there needs to be a specific measurement and rating for how well the vest spreads out and absorbs blunt for trauma, thereby reducing the back face deformation. The body armor industry should have such information available based on its own testing, instead there is—due to consumer demand—a huge focus on armor comfort. Unfortunately, greater comfort usually means a thinner and lighter vest, which most often translates into a lower level of protection. Which is more important to you? A higher threat level protection? Or a greater level of comfort for the totality of your shift?
Our end goal shouldn’t be to develop more comfortable armor with an acceptable minimum protection level, i.e. Level II or III. Our end goal should be to develop reasonably comfortable armor with a sufficient protection threat level that also incorporates a maximum estimated back face deformation measurement of two inches or less. Our ribs can flex a bit. Our soft tissue absorbs impact fairly well. However, bones can break and broken bones tend to tear up the tissue around the break, which can sometimes be important tissue like your heart, lungs, liver, etc. Yes, that’s still better than having a bullet rip through your body, but back face deformation of too great a depth can do nearly equal damage.
Go check what type of armor you wear, if you don’t already know. What level of protection does it offer? How old is it? Do you have any idea how well it will absorb blunt force trauma or minimize back face deformation? If not, you need to find out. The information will be of value to you in the long run.