Back face deformation (BFD) isn’t just a cosmetic dent or overhyped marketing number; it’s a direct measure of how much impact force a helmet transfers to the wearer’s head. While some manufacturers downplay BFD to promote lighter and rifle‑rated helmets, extensive research shows that high BFD correlates with behind‑helmet blunt trauma (BHBT), including skull fractures, concussions, traumatic brain injury (TBI), and other long‑term brain injuries.

Buyers should insist on full test data, including BFD measurements, standoff distance, and padding configuration, rather than rely solely on penetration ratings and marketing claims.

Challenging Idea that BFD Does Not Matter

In a May 2025 Police Magazine article, Rewriting the Ballistic Helmet Playbook, a product specialist at a helmet manufacturer argued that back face deformation (BFD), the inward dent made in a ballistic helmet when it stops a bullet, should not concern law enforcement buyers. The author wrote that penetration is the only factor that determines survivability and claimed that agencies have lost “virtually none” of their officers to BFD.

This narrative is convenient for manufacturers whose ultra-high molecular weight polyethylene (UHMWPE) and rifle-resistant helmets meet specific user needs, but at the cost of an optimal BFD. It ignores a substantial body of research linking BFD to behind-helmet blunt trauma (BHBT), skull fractures, and traumatic brain injury (TBI).

Why BFD & BHBT Matter

What is back face deformation, and why measure it?

Back face deformation (BFD) refers to the inward displacement of the inside surface of a helmet when it stops a bullet. Unlike penetration, BFD does not create a hole, but the energy transfer creates a dent that pushes against the wearer’s skull. A high BFD means more force is transmitted to the head, according to research found on core.ac.uk.

Behind helmet blunt trauma (BHBT) is the suite of injuries – skull fracture, brain contusions, hematoma, concussions and diffuse axonal injury (DAI) – caused by this energy transfer even when the helmet does not perforate. Research shows that BHBT injuries range from skin lacerations to life altering brain damage and occur because high energy bullets cause the helmet shell to deform and transmit force to the head, according to MKU.

Ballistic standards such as the U.S. National Institute of Justice (NIJ) set maximum allowable BFD depths to limit blunt trauma. Yet modern helmets are getting lighter to reduce soldier burden; as penetration resistance increases, BFD can increase, according to Core.

Evidence From Scientific Studies

Postmortem Human Specimen (PMHS) Tests

A landmark study published in the Journal of Forensic Sciences (Rafaels et al., 2015) shot 9 mm bullets at seven UHMWPE helmets worn by human cadaver heads. Velocities ranged from 404 m/s to 459 m/s.

The researchers observed an “increasing trend of injury severity” as velocity increased, with injuries progressing from simple linear skull fractures to combinations of linear and depressed fractures. Their results demonstrated a high risk of skull fracture due to BHBT, even though the helmets were not perforated. The authors recommended that preventing BHBT should be a design factor in future protective gear. Source: DOI # 10.1111/1556-4029.12570

Design Modelling & Biomechanical Studies

A 2018 review of composite helmet design noted that as penetration resistance increases, large deformations increase the probability of BHBT. Excessive BFD can range from mild head injuries to irreversible trauma and skull fracture (core.ac.uk). The review acknowledges that current standards assess BFD by using clay mannequin heads, but this does not evaluate internal head damage. Several findings stand out:

•Experiments with cadaver heads and plates showed that 11–12 mm of distance between the helmet and skull is sufficient to prevent skull fracture and dramatically reduce dangerous intracranial pressure.

•In the Rafaels et al. PMHS tests, moderate velocity hits (440–445 m/s) produced linear skull fractures and depressed fractures at the impact site, while higher velocities produced more severe fractures.

•Tests on surrogate heads showed that removing helmet foam pads led to moderate cranial fractures, whereas full cushioning eliminated injuries—demonstrating that padding and BFD mitigation matter.

•Finite element modelling revealed that stiffer helmet shells reduce stresses on the skull but increase strains in the brain. Soft padding decreased both skull fracture risk and brain strain. Hence, designers must balance shell stiffness, padding, stand off distance and weight. Source: core.ac.uk

Epidemiological & Military Research

The U.S. Army Research Laboratory’s report on “Recent Research in Behind Armor Blunt Trauma and Traumatic Brain Injury” (govinfo.gov) finds that TBI among soldiers is a major concern and emphasizes the need to understand BHBT mechanisms. The report summarizes modelling work showing that blunt impacts can transmit stress waves into the brain that are not captured by simple penetration tests.

The National Academies’ review of DoD helmet test protocols explains that non-penetrating impacts can cause brain injuries through contact of the deforming helmet shell with the skull (ncbi.nlm.nih.gov). It notes that blunt trauma can produce concussion, hemorrhage, hematoma, skull fracture, anoxic injury, and diffuse axonal injury. Many injuries arise from differential motions and strains within the brain; the brain’s surface moving against the skull produces tissue contusions, vascular tears, and hemorrhages. These mechanisms cause long-term problems, including cognitive decline and endocrine dysfunction. The chapter concludes that reducing blunt brain trauma is the best approach because current clinical care cannot reverse long-term effects. Source: ncbi.nlm.nih.gov.

Industry-Sponsored Research & Data

One article by MKU (mku.com) on BHBT warns that a non-penetrating bullet can deform the helmet, transmitting forces or shock waves to the brain. The article notes that BHBT can result in skull fracture, hematoma, concussion, and diffuse axonal injury, and may range from skin lacerations to brain damage and skull fracture. It explains that differential motion of the brain against the skull causes tissue contusions, vascular tears and hemorrhages, while acceleration–deceleration injuries lead to axonal injury; these injuries have been associated with TBI.

An infographic from Hard Head Veterans (hardheadveterans.com), which offers a UHMWPE-forward helmet, summarizes the 2015 PMHS study and notes that 57% of non-penetrating ballistic impacts resulted in skull fractures, and four of seven specimens showed definitive evidence of skull fracture on dissection. The graphic illustrates that impact velocity correlates with fracture likelihood, and that high velocity impacts always caused fractures. It also identifies diagnostic challenges—radiology sometimes failed to detect fractures that were found during dissection. The article highlights a design conundrum: as helmets get lighter to reduce soldier burden, they may need to deform more to stop a projectile, increasing BHBT risk. Source: hardheadveterans.com & Rafaels et al., 2015.

Why the “BFD Doesn’t Matter” Narrative Is Misleading & Dangerous

1. Lack of context: In the Police Magazine article, the author emphasizes that major police departments report no officer deaths due to BFD. Deaths are a crude endpoint; research shows that skull fractures and TBIs often occur without fatalities and can have life-changing consequences. The 2015 PMHS study and subsequent modelling work show that BHBT can cause skull fractures, concussions, and diffuse axonal injury without penetration. Source: researchgate.net & ncbi.nlm.nih.gov.

2. Cherry picking metrics: The article posits that penetration and V50 (the velocity at which 50% of bullets penetrate the shell) should be the only metric for an effective helmet. While penetration is of course critical, it is not sufficient as a standalone metric. Effective ballistic helmets are not a catcher’s mitt for bullets—they must effectively transfer the energy away from the wearer’s head. Large deformations can still injure the user, even without penetration. Focusing exclusively on V50 ignores the tradeoff between penetration resistance and BFD, and dodges the critical question about how energy is properly transferred (core.ac.uk).

3. False comparison: The article suggests that minor dents are simply cosmetic, and that a 0.7-inch dent is a win if it stops the bullet. In the case of a rifle-resistant helmet, there may be truth to this statement. But on the whole, research contradicts it. Even moderate BFD can cause fractures and brain injuries, often resulting in a severely reduced quality of life (core.ac.uk). The NIJ provides explicit guidance on BFD allowances for this reason.

4. Ignoring brain injury research: Medical literature describes numerous brain injuries from non-penetrating impacts – concussions, hemorrhages, diffuse axonal injuries – and notes that many are due to differential motion of the brain. These injuries may not immediately incapacitate, but they cause long-term cognitive and psychological issues (ncbi.nlm.nih.gov). Minimizing BFD helps reduce the forces that initiate these injuries.

5. Commercial motive: UHMWPE helmets weigh less, and a standalone rifle-resistant helmet is a tremendous benefit, but both necessarily come with a higher BFD because the material crumples more to absorb energy. Likening this compromise in BFD to the crumple zone of a motor vehicle is disingenuous. The crumple zone in a car is carefully engineered to absorb energy before it reaches the frame and cabin of a vehicle, while giving time for additional life-saving mechanisms to activate (i.e. airbags). The BFD in a helmet, however, is nothing more than the quantifiable measure of the sole protective layer’s ability to prevent inertia from reaching the cranium and beyond.

6. Downplaying BFD conveniently deflects scrutiny of a helmet’s BFD performance. Buyers should demand complete test data, including BFD, stand-off distance, padding configuration, and head form type, rather than rely on marketing narratives.

Practical Recommendations for Buyers & End Users

1. Request full ballistic test data.

Look for BFD measurements in conjunction with penetration/V50. Check lab reports to determine whether tests used clay head forms, hybrid head forms, or PMHS surrogates, and inquire about stand-off distance and padding.

2. Understand the standoff–padding trade-off.

Research shows that an 11 to 12-mm standoff allows margin for BFD while significantly reducing skull fracture, and that proper padding reduces non-penetrating injuries (core.ac.ukcore.ac.uk). Helmets that minimize BFD by using a heavier material, better pads, and better suspension may be safer, even if they weigh slightly more.

3. Evaluate materials critically.

UHMWPE boasts a lighter product, while a rifle-resistant helmet clearly has its merits, but in each case, the loss of BFD protection is not without consequence and is not always the better option. Don’t assume “newer” is automatically safer. Look for helmets that balance penetration resistance, low BFD, and reasonable weight. A multitude of discomforts can be compensated for with a thoughtfully designed padding system, such as that of BAC’s Bastion® helmet (ballisticarmorco.com).

4. Account for human variability.

Real heads differ from clay forms; BFD that seems acceptable on a test block may perform wildly different depending on the wearer. Consider adjustable padding systems and in all circumstances, ensure a snug-but-not-tight fit.

5. Protect against blast and blunt impacts.

Helmets should not only protect from bullets but also manage blast waves and blunt impacts. Look for designs that integrate blast attenuating features and test results beyond standard ballistic benchmarks.

Conclusion

Back face deformation is not a trivial cosmetic dent; it is a measure of how much energy a ballistic impact transfers to a wearer’s head. Contrary to claims that “BFD doesn’t matter,” extensive biomechanical research shows that excessive BFD causes behind helmet blunt trauma, leading to skull fractures, concussions, diffuse axonal injury and other TBIs (core.ac.uk) (ncbi.nlm.nih.gov).

Studies using cadaver heads and advanced models demonstrate that even moderate velocities produce fractures, and that proper standoff and padding are essential to minimize BFD. Buyers should demand transparency about BFD and not be swayed by marketing narratives that focus solely on penetration.

Ultimately, a truly protective helmet balances penetration resistance with low BFD, comfortable fit, and blast and blunt impact mitigation.

Helmets with specific use-cases, such as a rifle helmet or a helmet designed for submersion, should be optimized for their use case. Both have their benefits and tradeoffs, but neither is a well-rounded helmet suited for general-purpose, everyday patrol.

Alex Poythress is the co-founder and CEO of Ballistic Armor Co., the Oregon-based maker of the U.S.-built Bastion ballistic helmet. With eight years' experience in ballistic helmet research and design, manufacturing partnerships, and performance testing in the personal protection industry, he’s overseen everything from composite layup design to impact-attenuation testing and international ballistic compliance. His background combines hands-on engineering collaboration with field testing and feedback from law enforcement and defense professionals. This article was authored Poythress and edited following POLICE editorial standards and style. Opinions expressed may not reflect those of POLICE.