ammphs – mandible protection · ammphs – mandible protection requirements and specifications e....

AMMPHS – Mandible ProtectionRequirements and Specifications

E. Fournier, N. Shewchenko

Prepared By:

Biokinetics and Associates Ltd. 2470 Don Reid Drive Ottawa, Ontario, K1H 1E1

Biokinetics Report No.: R08-06 Project Leader: Ed Fournier, (613) 736-0384 DRDC Contract Number: W7701-061933/001/QLC Contract Scientific Authority: Amal Bouamoul (418-844-4000 ext. 4588)

Defence R&D Canada – ValcartierContract Report

DRDC Valcartier CR 2009-007October 2008

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and thecontents do not necessarily have the approval or endorsement of Defence R&D Canada.

CR 2008-XXX iii

AMMPHS – Mandible Protection Requirements and Specifications

E. Fournier, N. Shewchenko

Biokinetics and Associates Ltd.

2470 Don Reid Drive

Ottawa, Ontario, K1H 1E1

Biokinetics Report No.: R08-06

Project Leader: Ed Fournier, (613) 736-0384

Contract No.: W7701-061933/001/QLC

Task Authorization No.: W7701-8-2483

Scientific Authority: Amal Bouamoul, (418) 844-4000

The scientific or technical validity of this Contract Report is entirely the responsibility of Biokinetics and Associates Ltd., and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Biokinetics and Associates Ltd. est entièrement responsable de la validité scientifique ou technique de ce rapport de contrat, et le contenu de ce rapport n’est pas nécessairement approuvé ni entériné par R et D pour la défense Canada.

Defence R&D Canada - Valcartier DRDC Valcartier CR 2009-007

October 2008

iv CR 2009-007

This report constitutes the final deliverable for Public Works and Government Services, Contract No. W7701-061933/001/QLC, Task Authorization No. W7701-8-2483. The opinions expressed herein are those of Biokinetics and Associates Ltd. and do not necessarily reflect those of Defence Research and Development Canada Valcartier.

Authors

Ed Fournier, Senior Engineer Biokinetics and Associates Ltd.

Nicholas Shewchenko, President Biokinetics and Associates Ltd.

Publication approved by

Amal Bouamoul, Research Scientist Defence Research and Development Canada - Valcartier

© Her Majesty the Queen as represented by the Minister of National Defence, 2008

© Sa majesté la reine, représentée par le ministre de la Défense nationale, 2008

AMMPHS – Mandible Protection: Biokinetics Report no. R08-06

CR 2009-007 v

Abstract

Military helmets must constantly evolve to improve head protection against ever changing threats that can be encountered on the battlefield. A helmet’s protection is no longer limited to projectile penetration but also includes its capacity to mitigate behind armour blunt trauma, blunt impact trauma, as well as blast effects. Additionally, facial injuries are becoming a bigger concern and consequently the Advanced Multi-threat Modular Protective Headwear System (AMMPHS) development program incorporates a mandible guard in its design.

To assess the effectiveness of a mandible guard an appropriate test methodology and injury based performance criterion must be developed. A specification for the performance of a mandible guard is proposed. Performance levels are based on biomechanical tolerance levels of the mandible region that were identified in the scientific literature. Surrogate headforms and test protocols for assessing performance are evaluated along with appropriate injury tolerance thresholds.

Résumé

Les casques militaires doivent constamment évoluer afin d’améliorer le niveau de protection contre les différents types de menace rencontrés sur le champ de bataille. Le casque ne doit plus protéger uniquement contre la pénétration de projectiles. Il doit aussi prévenir les traumatismes reliés aux effets arrières, les traumatismes contondants ainsi que ceux causés par les effets de souffle. De plus, les lésions traumatiques de la face sont également devenues une préoccupation importante. Conséquemment, un protecteur de mâchoire inférieure a été inclus dans le programme de développement du système de protection modulaire multi menaces (AMMPHS).

Pour évaluer l’efficacité d’un protecteur de mâchoire inférieure, une méthode d’essai et un critère de performance basé sur le niveau de blessure doivent être développés. Un devis de performance pour le protecteur de mâchoire inférieure est proposé. Les niveaux de performance sont basés sur les critères de tolérance biomécaniques pour la mâchoire inférieure tel qu’identifiés dans la littérature scientifique. Différentes fausses têtes et protocoles d’essai sont évalués en fonction des seuils de blessure appropriés.


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CR 2009-007 vii

Executive Summary

Military helmets must constantly evolve to improve head protection against ever changing threats that can be encountered on the battlefield. Furthermore, the injury prevention afforded is no longer limited to projectile penetration but also includes the capacity to mitigate behind armour blunt trauma (BABT), blunt impact trauma (BIT) as well as blast effects. Additionally, facial injuries are becoming a bigger concern with sources of trauma that include direct or indirect fragmentation strike from IED’s and RPG’s, direct ballistic strike from small arms fire, and being struck by larger objects or from the soldier striking fixed objects including vehicle compartments and hatches. Consequently, the Advanced Multi-threat Modular Protective Headwear System (AMMPHS) development program incorporates mandible protection as one aspect of protection the helmet is to provide the soldier.

A draft AMMPHS mandible performance specification, prepare by DRDC, incorporates requirements for operational stability, retention system securement, protection from fragmentation, low velocity impact protection, blast protection, weight, coverage and speech comprehension. The current work, however, addresses only the fragmentation and low velocity impact requirements. Moreover, it is assumed that the specified mandible ballistic requirements can be met and therefore the issue of concern is the resulting loading to the face from the backface deformation caused by the defeated ballistic threat.

The draft specification calls for no contact to occur between the mandible guard and the face when struck, which is not possible within the practical limits of helmet design. An alternate approach is proposed that allows contact to occur however, the forces transmitted to the mandible must be kept below injurious levels. As a means of establishing acceptable loading thresholds, a review of published research into the fracture tolerance of the mandible was conducted.

It was apparent from the literature that a fracture tolerance is strongly dependent on load distribution. A contact area of 13 cm2 was identified as a threshold between distributed and concentrated loading of the mandible. The loads resulting from the back face deformation caused by a non-penetrating ballistic strike could likely be considered a concentrated load whereas the loading cause by a blunt impact could be considered in the distributed loading regime.

Testing of available head/jaw surrogates indicated that the surrogate headforms are too stiff resulting in measured jaw loads that are much higher than that reported in literature for similar impacts to the jaw. Despite the high measured loads it is recommended that the Hybrid III with a rigid load sensing mandible be used as an interim headform surrogate for assessing the performance of possible AMMPHS mandible guard solutions. However, further investigation of a headform with an articulating load sensing mandible is also recommended. This headform is currently under development and has been shown to exhibit the correct force displacement characteristics for impacts to the mandible.


viii CR 2009-007

Sommaire

Les casques militaires doivent constamment évoluer afin d’améliorer la protection à la tête contre les menaces rencontrés sur le champ de bataille. La prévention des blessures ne doit plus se limiter à contrer la pénétration de projectiles mais doit aussi inclure la réduction de traumatismes reliés aux effets arrières (BABT), de traumatismes contondants (BIT) et de ceux causés par les effets de souffle. De plus, les lésions traumatiques de la face sont également devenues une préoccupation importante avec des sources de blessure incluant des frappes directs ou indirects de fragments provenant de bombes artisanales (IED) ou de grenades propulsées par fusée (RPG), des frappes balistiques directs provenant de projectiles d’armes légères, des impacts par de plus gros objets ou lorsque le soldat frappe des objets fixes tel que des composantes de véhicule ou écoutilles d'accès. Conséquemment, un protecteur de mâchoire inférieure a été inclus dans le programme de développement du système de protection modulaire multi menaces (AMMPHS) comme un aspect supplémentaire de protection offert aux soldats.

Une ébauche de devis de performance pour le protecteur de mâchoire inférieure préparée par RDDC inclus des exigences pour la stabilité opérationnelle, la fixation du système de rétention, la protection contre les fragments, les impacts à basse vitesse, l’effet de souffle, le poids, la superficie de protection et la compréhension de la parole. Le travail actuel ne traite cependant que de la protection contre les fragments et les impacts à basse vitesse. De plus, il est supposé que les exigences de pénétration reliées aux projectiles balistiques peuvent être rencontrées. Ainsi, le type de blessure considéré dans le cas d’impacts balistiques est le résultat de la déformation arrière du protecteur appliquant une force sur le visage.

L’ébauche de devis de performance ne permet pas de contact entre le protecteur de mâchoire inférieure et le visage lors d’impact. Cette contrainte est physiquement impossible compte tenu des limites pratiques des casques de protection. L’alternative proposée permettrait qu’un contact se produise en autant que la force transmise à mâchoire inférieure soit moindre que le seuil de blessure. Une revue des travaux de recherche publiés sur la tolérance de la mâchoire inférieure à se fracturer suite à un impact a été effectué afin de déterminer un seuil de force acceptable.

La littérature scientifique suggère que la tolérance à la fracture est fortement liée à la distribution de la force d’impact. Une surface de contact de 13 cm2 sur la mâchoire inférieure a été identifié comme limite entre une force distribuée et une force concentrée. Les forces engendrées par la déformation arrière causée par un impact balistique non pénétrant peuvent être considéré comme étant concentrées alors que le chargement dû à impact contondant est considéré comme distribué.

L’évaluation en laboratoire de têtes/mâchoires de substitution indique que ces fausses têtes sont trop rigides car elles produisent des mesures de force beaucoup plus élevées que ce qui est reporté dans la littérature pour des impacts comparables. En dépit de ces mesures élevées, il est recommandé d’utiliser initialement la tête Hybrid III équipée d’une mâchoire inférieure rigide et instrumentée pour mesurer la force transmise afin


CR 2009-007 ix

d’évaluer d’éventuels concepts de protecteur de mâchoire inférieure. Des études supplémentaires concernant la fausse tête équipée d’une mâchoire inférieure articulée et instrumentée pour mesurer la force transmise sont aussi recommandées. Cette fausse tête est présentement en développement mais il a déjà été démontré qu’elle possède les caractéristiques de force / déplacement recherchées pour les impacts à la mâchoire inférieure.


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CR 2009-007 xi

Table of Content

1. Introduction ............................................................................................................1

2. Mandible Guard Protective Requirements .............................................................2

3. Fracture Threshold for Facial Structures................................................................4

4. Proposed Mandible Tolerance Values....................................................................9

4.1 Non-Penetrating Ballistic Impact ...............................................................9

4.2 Blunt Impacts .............................................................................................9

5. Proposed Mandible Guard Specification..............................................................11

6. Head Surrogates ...................................................................................................13

6.1 Hybrid III with Instrumented Chin...........................................................13

6.2 Facial and Ocular Countermeasure Safety (FOCUS) Headform .............14

6.3 Articulating Mandible Headform.............................................................15

6.4 Head Surrogate Evaluations .....................................................................16

7. Discussion and Summary .....................................................................................23

8. References ............................................................................................................25


xii CR 2009-007

List of Tables

Table 1: Mandible guard fragmentation protection requirements (from draft specification [Ref. 1])...................................................................................2

Table 2: Mandible guard low velocity impact requirement (from draft [Ref. 1])..........3

Table 3: Summary of fracture tolerance levels for the mandible, zygoma and maxilla.8

Table 4: Alternative - Mandible guard fragmentation protection requirements (focal loading).......................................................................................................11

Table 5: Alternative - Mandible guard low velocity impact requirement (distributed loading).......................................................................................................12

Table 6: Impact configuration for focal and distributed loading. ...............................17

Table 7: Test matrix. ....................................................................................................18

Table 8: Impact test results for Hybrid III with instrumented chin..............................19

Table 9: Impact test results for the FOCUS headform.................................................20

Table 10: Proposed tolerance levels for the Hybrid III headform with instrumented chin. ............................................................................................................21

List of Figures

Figure 1: Facial bones that may be protected by a mandible guard [Ref. 2]. ................5

Figure 2: Hybrid III with rigid load sensing mandible.................................................14

Figure 3: FOCUS headform (photos from R.A. Denton Inc.)......................................15

Figure 4: Biokinetics Articulating Mandible Headform. .............................................16

Figure 5: Typical impact test setup showing the FOCUS headform. ........................18


CR 2009-007 1

1. Introduction

Military helmets must constantly evolve to improve head protection against ever changing threat levels from fragmentation and ballistic projectiles that can be encountered on the battlefield. Furthermore, the injury prevention afforded by a ballistic helmet is no longer limited to projectile penetration but also includes the capacity to mitigate behind armour blunt trauma (BABT), blunt impact trauma (BIT) as well as blast effects.

The Advanced Multi-threat Modular Protective Headwear System (AMMPHS) development program incorporates mandible protection as one aspect of protection a helmet is to provide to the soldier. In the recent conflict in Afghanistan, facial trauma has been noted as a possible source of serious injury to the soldiers. Sources of trauma may include direct or indirect fragmentation strike from IED’s and RPG’s, direct ballistic strike from small arms fire, and being struck by larger objects or the soldier striking fixed objects including vehicle compartments and hatches.

To assess the protective benefits a mandible guard may have on facial injuries, an appropriate injury criterion must be identified and a method by which a mandible guard can be evaluated must be developed. This report will summarize the proposed requirements for a mandible guard [Ref. 1] and makes recommendations for improvements were applicable. Performance levels are established based on biomechanical tolerance levels of the mandible region that are available in the scientific literature. Surrogate headforms and test protocols for assessing the performance of a mandible guard are also recommended along with appropriate injury tolerance thresholds.


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2. Mandible Guard Protective Requirements

The draft AMMPHS mandible specification [Ref. 1] incorporates requirements for:

• Operational stability, • Retention system securement, • Protection from fragmentation, • Low velocity impact protection, • Blast protection, • Weight, • Coverage, • Speech.

The current work addresses only the fragmentation and low velocity impact requirements for the AMMPHS mandible. It is assumed that the specified mandible ballistic requirements can be met and thus the issue of concern is the resulting backface deformation loading to the face from the defeated ballistic threat.

A summary of the draft AMMPHS technical requirements for fragmentation protection and low velocity impact protection are presented in Table 1 and Table 2.

Table 1: Mandible guard fragmentation protection requirements (from draft specification [Ref. 1]).

Non-Penetrating Ballistic Strike

Functional Requirement AMMPHS Technical Requirement

The backface transient deformation of the AMMPHS mandible guard shall not contact the mandible.

Mandatory:

• No contact with the mandible for a 16 grain FSP at V50-50 m/s

Desirable:

• No contact with the mandible for a 16 grain FSP at V50-50 m/s

Following fragment impact, the backface deformation of the AMMPHS shall not result in mandible fracture.

Mandatory:

• No jaw fracture for a 96 grain steel sphere at V50-50 m/s

Desirable:

• No jaw fracture for a 96 grain steel sphere at V50-50 m/s

Mise en forme : Puces etnuméros






CR 2009-007 3

Table 2: Mandible guard low velocity impact requirement (from draft [Ref. 1]).

Low Velocity Impact


The AMMPHS mandible guard shall protect against low velocity impact to the mandible when tested at ambient conditions as well as at -40 °C and +50 °C

Mandatory:

• 10 impacts at 40 J, with no contact

• 1 impact at 50 J, with no jaw fracture

Desirable:

• 10 impacts at 40 J, with no contact

• 1 impact at 90 J, with no jaw fracture

Range of temperature between -25 °C and +40 °C. Tests performed on the front-centre of the mandible and 60° off-axis from the front-centre of the mandible in the horizontal plane. All tests are conducted with a flat plate anvil.

The attachment system used to secure the mandible guard to the helmet must remain undamaged and fully functional following both the low and high level impacts.

Avoiding contact between the mandible guard and the face under all plausible ballistic and impact conditions is difficult to achieve. To do so would require a helmet suspension/retention system that provides a rigid attachment of the helmet to the head to avoid excessive shell rotation. Since a perfectly rigid attachment of the shell cannot be achieved, the mandible guard would require a significant standoff from the face resulting in a larger, heavier and more cumbersome helmet.

An alternate approach to defining a specification for the AMMPHS mandible guard would be to accept that contact may occur and to focus the requirement instead on limiting the magnitude of the forces transmitted to the face to sub-injurious level. As a means of establishing acceptable loading thresholds, published research into the fracture tolerance of the mandible has been conducted and is presented in Section 3 followed by a proposed AMMPHS specification for the performance of the AMMPHS mandible guard. The specification considers available head/mandible surrogates that can be used to evaluate mandible guard effectiveness at dissipating the loading from a blunt impact or non-penetrating ballistic strike.




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3. Fracture Threshold for Facial Structures

As the name would suggest, a mandible guard offers protection to the mandible but other facial regions such as the zygoma and maxilla may also benefit from such a guard as is exemplified by a full-face motorcycle helmet. However, from a practical perspective, in a military application the extended coverage necessary to offer the additional protection would impinge on the field of view. Therefore it is best that protection for these other facial regions is provided with the use of a visor.

To define an appropriate tolerance specification for the mandible guard an investigation into facial fracture tolerance levels has been carried out. Although research regarding fracture tolerance of facial structures has been conducted since the early days of automotive research, few papers have been published that document the fracture tolerance of the mandible, maxilla and zygoma (see Figure 1). The research that has been identified is summarized in following sections. Of particular importance for the current effort is the tolerance data for the mandible nevertheless, tolerance data for the zygoma and maxilla has been included for possible future reference.

The early research studies focused on the fracture of facial bones resulting from automotive accidents. Impacts to the facial region from steering wheel rims, hubs and instrumentation panel typically guided the test conditions for the experimental studies. Their appropriateness for assessing BABT and BIT in defence applications remains to be seen.


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Hodgson [Ref. 3] investigated the impact tolerance of facial bones to blunt impact to determine the parameters affecting fracture and to aid in the safe design of vehicles. Nineteen post mortem human subjects (PMHS), ranging in age from 53 to 87 and mass from 52 kg to 108 kg, were struck on the symphysis menti with a 6.54 cm diameter cylindrical rod with a 2.54 cm polyurethane foam covering. The line of action of the impact was through the temporal Mandibular joint with the applied force derived from an accelerometer mounted on the impactor. The average peak load for mandible fractures to occur was 2362 N. Impacts to the zygoma resulted in an average peak fracture load of 2220 N for a 33.55 cm2 impactor compared to 1363 N for a 6.45 cm2 impactor. A further investigation into the effect of loading area on the force to fracture revealed that the required force to fracture the zygoma increased by as much as 150% to 250% as the area of loading increase from 6.45m2 to 33.55 cm2.

The results of impact tests on 10 PMHS ranging in age from 55 to 81, conducted by Nahum et al [Ref. 4], indicated that fractures of the zygoma were observed with forces that ranged from 911 N to 3470 N when struck with an impactor with a contact area of approximately 6.45 cm2 and covered with 5.08 mm layer of crushable nickel foam. A load cell was incorporated into the impactor to measure the applied forces. Impacts to the lower maxilla produced fracture loads that ranged from 778 N to 934 N. Impacts to the mandible were also performed at two locations: symphysis and mid-

Figure 1: Facial bones that may be protected by a mandible guard [Ref. 2].

Zygoma

Maxilla

Symphysis

Mental Protuberance

Mandible

Mandible Body

Temporo Mandibular Joint


6 CR 2008-XXX

body. In these two locations the fracture threshold ranged from 1557 N to 1779 N and from 1290 N to 1446 N respectively. The results suggested that the loads to fracture were insensitive to loading rate and that the fracture level for female subjects was lower than that for the males.

Schneider and Nahum [Ref. 5] conducted additional impact testing, on 17 cadavers ranging in age and mass from 45 to 80 and 41 kg to 121 kg respectively, to refine the force tolerance levels for the mandible, maxilla and the zygoma. A similar impactor to that used in the previous testing was also employed. The average peak fracture load of the fore-aft mandible, lateral mandible, the zygoma and the maxilla were 2845 N, 1570 N, 1710 N and 1149 N respectively.

Hopper et al [Ref. 6] conducted dynamic drop tests on the mental protuberance of five, unembalmed cadavers heads disarticulated from the neck that were affixed to the carriage of a drop tower to investigate basilar skull fracture resulting from impacts to the mandible during motor vehicle accidents. The heads were dropped onto an impact surface with a compliance that was varied to assess the influence of loading rate on the force response. The force of impact was measured with a load cell beneath the impact surface. An additional quasi static test was also conducted in a hydraulic test frame to look at significantly different loading rates. The average fracture tolerance of the mandible under dynamic and quasi-static loading was determined to be 5270 N. Similarly to Nahum [Ref. 4], Hopper concluded that the peak fracture load was insensitive to loading rate. However, the energy absorbed during fracture varied from 11.4 J to 119 J for rigid and compliant impact surfaces respectively. The average mandible fracture tolerance of 5270 N found by Hopper was much larger than the average anterior posterior fracture load of 3103 N found by Schneider et al [Ref. 5]. The difference was believed to be partly due to the size of the impact face; 127 cm2 compared to 6.45 cm2 used by Schneider.

Unnewehr at al [Ref. 7] conducted successive increasing severity pendulum tests on 7 human mandibles from PMHS with ages ranging from 35 to 71 years. The contact area of the pendulum was not defined. The objective of the work was to investigate mandible fracture patterns and their potential forensic usefulness in determining the intensity of violent assaults. The jaw bones were removed from the head and secured in a test fixture. The impacts were applied in two different directions: fronto-median with impacts to the mental protuberance and laterally at 90o to the mandible body. The forces of impact were measured with strain gauge strips. For the fronto-median direction the mandible fractured at an average load of 2876 N whereas in the lateral direction the average force to fracture the mandible was 676 N. In considering these fracture loads it must be remembered that the mandible experienced several impacts prior to the resulting fracture. Although low-level control impacts were performed before each test to verify the integrity of the mandibles, the dependability of the measured fracture levels is questionable.

Viano et al [Ref. 8] conducted blunt ballistic impact tests to the forehead, zygoma and the mandible of six cadavers to establish corridors for high-speed low mass facial impacts representative of kinetic energy less lethal weapon projectiles. The projectiles were 37 mm in diameter with a mass ranging from 25 g to 35 g and a flat rigid impact face. The speed at impact was 42±10 m/s. The impact forces were determined from


CR 2009-007 7

an accelerometer mounted in the projectile. The impacts were centered on the mental protuberance of the mandible and at the zygomatic process of the maxillary bone. Anterior maxilla fractures were observed at a force level of 2251 kN from impacts to the zygoma. No fractures were observed to the forehead or mandible at force levels of 3.5±0.9 kN or 3.0±1.0 kN respectively. In comparing his results to other published low speed fracture data Viano concludes that the fracture of the mandible and zygoma is not sensitive to loading rate. Based on his analysis, fracture tolerance levels of 1.6 kN and 1.9 kN were suggested for the zygoma and mandible respectively.

In the same paper by Viano, a contact area of 13 cm2 was cited as a threshold between concentrated loading and distributed loading albeit it was in relation to compressed fractures of the cranium. It was also suggested that as contact area decreases below 5 cm2 a depressed skull fracture may transition into a punch-through failure the size of the projectile. Therefore in establishing loading tolerances for the mandible the area of loading must be considered in establishing an injury tolerance level, which is corroborated by Hopper’s findings. The loading to the mandible from the back face deformation caused by a non penetrating ballistic strike could likely be considered a concentrated load whereas the loading cause by a blunt impact could be considered in the distributed loading regime.

A summary of the mandible, zygoma, maxilla fracture loads identified by the various researchers is presented in Table 3 below.


8 CR 2008-XXX

Table 3: Summary of fracture tolerance levels for the mandible, zygoma and maxilla.

Mandible: Symphysis/Mental Protuberance

Fracture Loads (N) Impactor Description Range Average

Reference

- flat, circular, 33.55 cm2 - 25.4 mm polyurethane padding

1601 to 2669 2362 Hodgson Ref. 3

- flat, circular, 6.45 cm2 - 5.08 mm crushable nickel foam face

1557 to 1779 na Nahum Ref. 4


1890 to 4120 2845 Schneider Ref. 5

- flat, circular, 127 cm2 - various compliant surfaces on anvil

4460 to 6740 5270 Hopper Ref. 6

- flat, circular, unspecified impactor - contact are not defined

2465 to 3122 2876 Unnewehr Ref. 7

- flat, circular,10.8 cm2 - rigid

na 4301 Viano Ref. 8

Mandible: Mid-Body (Lateral) Fracture Loads (N) Impactor Description Range Average

Reference



- flat, rectangular, 25.4 mm x 101.6mm - 5.08 mm crushable nickel foam face


- flat, circular, unspecified impactor - contact are not defined

633 to763 676 Unnewehr Ref. 7

Zygoma Fracture Loads (N) Impactor Description Range Average Reference


1601 to 3363 2220


1121 to 1664 1364 Hodgson Ref. 3


911 to3470 1775 Nahum Ref. 4



- flat, circular, 10.8 cm2 - rigid

1612 to 2890 2251 Viano Ref. 8

Maxilla Fracture Loads (N) Impactor Description Range Average Reference






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4. Proposed Mandible Tolerance Values

From the fracture data presented in Table 3, alternate tolerance levels for fragmentation and low velocity impacts are extracted. The rationale for the establishment of these levels are discussed in the following sections.

4.1 Non-Penetrating Ballistic Impact

In reference to the fracture levels presented in the Table 3, it is proposed that the mandatory tolerance level for impacts to the mandible in the fore-aft direction be 1724 N which is the average of the lower fracture levels reported by Nahum and Schneider [Ref. 4 and Ref. 5] for impacts on the Mental Protuberance or Symphysis (chin) with a 6.45 cm2 impactor. A more stringent (or desirable) requirement would be to set the tolerance level at the lowest fracture level recorded by the researchers, namely 1557 N. The relatively small contact area used in these test configurations can be considered somewhat representative of the focal loading from the backface deformation of a non-penetrating ballistic strike. Consequently, this tolerance level would be introduced in the AMMPHS mandible specification under fragmentation protection from non penetrating ballistic impact whether it is from a 16 grain FSP or a 96 grain sphere.

Similarly, for lateral impacts to the mandible body, it is proposed that the mandatory tolerance level be established at 1290 N for focal loading during non penetrating ballistic strikes. This value is the lowest fracture level documented by Nahum [Ref. 4]. Applying a 10%1 reduction in loads for the fore aft direction a more stringent (or desirable) threshold value, for the non penetrating ballistic lateral impacts to the mandible, of 1160 N is recommended. The data from Unnewehr was not considered because the jaw bones were tested independently from the head, the impact sites may have been struck multiple times before fracture was recorded and the dimension of the impact face was not documented.

4.2 Blunt Impacts

For blunt impacts to the mandible in the fore aft direction a mandatory fracture tolerance of 3030 N is recommended. This is the average of the lowest fracture loads documented by Hodgson and Hopper [Ref. 3 and Ref. 6] for distributed loading (ie contact area greater or equal to 13 cm2). A more stringent (or desirable) requirement of 1600 N is recommended based on the lowest fracture level recorded by Hodgson.

Considering Hodgson’s finding that fracture levels generally increase with increased impact area, the lateral fracture loads documented by Schneider, in a distributed loading configuration, appear low when compared to Nahum’s data from tests

1 As data from only one research paper is relied upon for the lateral direction, a 10% reduction is proposed based on a similar reduction seen in the fore-aft mandible loading.


10 CR 2008-XXX

conducted with a concentrated loading area. In examining Schneider’s data, the lowest fracture level of 820 N appears to be an anomaly with the next lowest fracture level being 1210 N. The average fracture load of 1570 N presented by Schneider may be a more appropriate mandatory tolerance level to select for AMMPHS while using the lower range of 1210 N as a tolerance specification for a more stringent (or desirable) requirement. The data from Unnewehr for lateral impacts to the mandible was not considered because the jaw bones were tested independently from the head, the impact sites may have been struck multiple times before fracture was recorded and the dimensions of the impact face was not documented.


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5. Proposed Mandible Guard Specification

A revised specification for the AMMPHS mandible component under blunt and non-penetrating ballistic impacts is presented in Table 4 and Table 5. The proposed requirements accept the possibility of facial contact with the mandible but instead impose a limit on the loads that can be transmitted to levels that should not result in fracture. The suggested threshold levels were derived from the published research data on the fracture tolerance of the mandible presented in Table 3 and discussed in Section 4. Although two criteria levels were discussed in Section 4 and presented in the draft AMMPHS specification, only a single mandatory tolerance level is proposed for the mandible performance requirement for both fragmentation and blunt impact protection.

Table 4: Alternative - Mandible guard fragmentation protection requirements (focal loading).


The backface deformation of the mandible during a non-penetrating ballistic strike must not impart injurious loads to the jaw.

Mandatory:

• The force transmitted to the mandible following a non-penetrating ballistic strike from a 16 grain FSP and from a 96 grain sphere at V50-50 m/s must be:

- frontal impact < 1724 N - lateral impact < 1290 N


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Table 5: Alternative - Mandible guard low velocity impact requirement (distributed loading).

Low Velocity Impact


The AMMPHS mandible guard shall protect against low velocity impact to the mandible when tested at ambient conditions as well as at -30 °C2 and +50 °C

Mandatory:

• 10 impacts at 40 J, with jaw forces below: - fore aft < 3030 N - lateral < 1570 N

• 1 impact at 50 J, with jaw forces below: - fore aft < 3030 N - lateral < 1570 N

Tests performed on the front-centre of the mandible and 60° off-axis from the front-centre of the mandible in the horizontal plane. All tests were conducted with a flat plate anvil.

The attachment system used to secure the mandible guard to the helmet must remain undamaged and fully functional following both the low and high level impacts.

The suggested tolerance levels presented in Table 5 limit the allowable force transmitted to the jaw to levels below that which can lead to jaw fracture. Applied loads that are below these levels however, do not preclude other lower severity injuries from occurring. Oral-dental injuries, temporomandibular chronic injuries or mild traumatic brain injury (MTBI) may still occur and are a possible consequence of allowing jaw contact to occur. The alternative approach of attempting to prevent jaw contact completely would likely result in a helmet/mandible system that is uncomfortable and or excessively large as discussed previously.

2 The AMMPHS draft specification stipulates a cold conditioning temperature of -40 °C. A helmet standard search was performed for helmets used in cold environments. The coldest conditioning temperature that was found was -28 °C. Also, helmet liner materials at such a low temperature as -40 °C have not been very well characterized and is hypothesized that the helmet materials would breakdown quickly. It is therefore proposed that a cold conditioning temperature of -30 °C be adopted until such time that the performance of energy absorbing materials can be evaluated at -40 °C or colder.




CR 2009-007 13

6. Head Surrogates

In order to verify compliance of an AMMPHS mandible guard design/configuration a head surrogate must be identified that has the capability to measure the transmitted forces from a blow to the mandible guard to the mandible and to replicate the biofidelity of the human head. Three such headforms have been identified:

1. Hybrid III 50th percentile male head with an instrumented rigid chin,

2. Facial and Ocular Countermeasure Safety (FOCUS) headform,

3. Biokinetics Articulating Mandible headform.

Each headform is briefly described in the following sections.

6.1 Hybrid III with Instrumented Chin

A standard Hybrid III headform was modified by cutting off the chin and re-attaching it through a three axis load cell to study the load paths to the cranium through helmet retention systems and face guards of American football helmets. Compliance of the structure is obtained strictly from the standard Hybrid III head skin. The head is shown in Figure 2.


14 CR 2008-XXX

6.2 Facial and Ocular Countermeasure Safety (FOCUS) Headform

FOCUS is a rigid mechanical headform that has been designed for the US Army primarily to test and evaluate head protective devices under impact conditions. In addition to being able to measure the forces on the eye, sensors installed in the head allow for the measurement of forces acting on the frontal, maxilla, zygoma, mandible and nasal bones. Skin thickness is varied to provide appropriate biofidelity in the different facial regions. The FOCUS headform is shown in Figure 3.

Figure 2: Hybrid III with rigid load sensing mandible.


CR 2009-007 15

6.3 Articulating Mandible Headform

An articulating mandible headform was developed for the National Football League and is based on the Hybrid III crash test dummy head with an articulating and force-sensing mandible and mechanical teeth. Its biofidelity has been verified in a series of chin impact experiments with post mortem human subjects (PMHS) [Ref. 9]. This headform is capable of measuring brain cavity acceleration in 6 degrees of freedom making it suitable for assessing the risk of mild traumatic brain injury. Although the headform can measure the forces exerted through the jaw and upper dentition, this manikin is not specifically intended to measure the dental protection offered by mouth guards from direct blows to the mandible but rather for the dynamic assessment of mouth guards in the football environment as they relate to brain injury. The headform is shown in Figure 4.

Figure 3: FOCUS headform (photos from R.A. Denton Inc.)


16 CR 2008-XXX

6.4 Head Surrogate Evaluations

At the time of this work, the Articulating Mandible Headform was still under development and was not available for evaluation. The remaining two headforms were subjected to impact tests to assess their biofidelity with respect to published mandible fracture tolerance levels. Of the research that was reviewed, only the work by Schneider [Ref. 5] is sufficiently documented such that the test setup could be reproduced. These comprised focal loading to the mandible in the anterior posterior direction and distributed loading to the mandible body in the lateral direction.

The Hybrid III with the rigid chin and the FOCUS headform were subjected to focal loading on the mental protuberance (front of the chin) and distributed loading on the mandible body (lower part of the jaw). The parameters of each test configuration are presented in Table 6.

Figure 4: Biokinetics Articulating Mandible Headform.


CR 2009-007 17

Table 6: Impact configuration for focal and distributed loading.

Focal Loading Distributed Loading

Impact Location mental protuberance mandible body

Impact Direction inline - mandible condyle normal to mandible body

Impactor Contact Area 6.45 cm2 2.54 cm x 10.2 cm

Contact Geometry circular rectangular

Impactor Material Steel Steel

Contact Surface of Impactor

2.00 mm nickel foam 4.00 mm nickel foam

Drop Mass 3.12 kg 3.81 kg

Impact Velocity 5.46 m/s 5.73 m/s

Head Support soft polyurethane foam wedges

The headforms were supported by several foam wedges on a heavy pedestal and positioned/supported as per Schneider’s test setup. In Schneider’s testing crushable nickel foam was attached to the strike face of the impactor. The nickel foam was 2.54 mm and 5.08 mm thick for the focal and distributed loading tests respectively. However, after an exhaustive search, only nickel foam with a thickness of 2.00 mm was found. One layer was used for the focal loading test and two layers were used for the distributed loading test. An accelerometer was mounted to the impactor such that the impact force could be calculated and ultimately correlated back to the Schneider results. The forces measured by the headforms and the impactor accelerations were recorded following standard SAE J211 practices. All the testing was conducted under ambient environmental conditions (21 ±2 °C).

Impact tests with both the Hybrid III with instrumented chin and the FOCUS headform were conducted at increasing velocities up to the velocities employed by Schneider. Each test was repeated three times. The test matrix is highlighted in Table 7 with a typical setup shown in Figure 5. The peak forces measured during the impact testing are summarized in Table 8 and Table 9.


18 CR 2008-XXX

Table 7: Test matrix.

Impact Velocity Hybrid III with Instrumented Chin

FOCUS Headform

1.50 x 3 x 3

3.00 x 3 x 3

4.50 x 3 x 3 Foc

al

5.46 x 3 x 3

1.50 x 3 x 3

3.00 x 3 x 3

4.50 x 3 x 3

Dis

trib

ute

d

5.73 x 3 x 3

Figure 5: Typical impact test setup showing the FOCUS headform.


CR 2009-007 19

Table 8: Impact test results for Hybrid III with instrumented chin.

Nominal Impact Parameters Impact Description Drop Height

(m) Velocity

(m/s)

Peak Chin Force

(N)

Peak Impactor Force3

(N) 757 970 766 954 0.115 1.50 746 942

2067 2448 2083 2554. 0.459 3.00 2029 2467 4261 5033 4689 4984 1.032 4.50 4181 4819 7787 9932

- -

Focal Loading Impacts to the Mental Protuberance

1.519 5.46 - -

914 1097 905 1093 0.115 1.50 908 1093

1962 2480 1982 2491 0.459 3.00 2016 2531 4063 5282 3970 4993 1.032 4.50 4057 5123 6031 8001 6126 8240

Distributed Loading on the Mandible Body

1.519 5.73 6046 8109

3 The peak impactor force is calculated based on the measured impactor acceleration and the impactor mass of 3.12 kg for focal loading and 3.81 kg for distributed loading.


20 CR 2008-XXX

Table 9: Impact test results for the FOCUS headform

Nominal Impact Parameters Impact Description Drop Height

(m) Velocity

(m/s)

Peak Chin Force

(N)

Peak Impactor Force4

(N) 1102 1121 1146 1149 0.115 1.50 1175 1181 2608 2460 2593 2527 0.459 3.00 2643 2524 5740 5370

1.032 4.50 See note See note

Focal Loading Impacts to the Mental Protuberance

1.519 5.46 See note See note

0.115 1.50

0.459 3.00

1.032 4.50

Distributed Loading on the Mandible Body

1.519 5.73

See note See note

Note: Testing was terminated or not conducted because the peak loads being measured at low impact velocities were approaching the capacity of the load cells used in the FOCUS headform.

Correct headform biofidelity is required to ensure proper biomechanics of the impact event and hence better correlation of the headform response in relation to the injury parameter being measured. The compliance of both the Hybrid III with rigid load sensing chin and the FOCUS head is achieved strictly through the vinyl skin covering the headforms, resulting in structures that are stiffer than the PMHS used in Schneider’s testing. This is clearly seen in the results presented in Table 8 and Table 9 where the loads measured by Schneider were exceeded at much lower impact severities of 3.0 m/s to 4.5 m/s impacts as opposed to Schneider’s 5.46 m/s to 5.73 m/s impacts for the focal and distributed loading test configurations.

4 The peak impactor force is calculated based on the measured impactor acceleration and the impactor mass of 3.12 kg for focal loading and 3.81 kg for distributed loading.


CR 2009-007 21

The fracture threshold for the FOCUS mandible proposed by United States Army Aeromedical Research Laboratory (USAARL), the developers of the headform, is 1780 N which is similar to the 1724 N proposed in Table 4 for focal loading to the chin. In discussions with USAARL, it was determined that their proposed thresholds are based on the same data that is available from published literature and used in establishing the threshold for AMMPHS. However, the headform has never been validated or tested according to the procedures upon which the headform tolerance levels were established. USAARL agreed that the loads that would be measured under similar impact conditions to that used in the PMHS testing would be larger due to the rigidity of the headform in comparison to a cadaver’s articulating jaw.

Given the limitation of the FOCUS’ instrumentation it is recommended that the Hybrid III headform with the rigid chin be used for the evaluation of possible AMMPHS mandible guard solutions. However, it must be noted that the headform may affect the penetration resistance of the mandible components being testing as their deformation may be limited due to the headform rigidity.

Ideally, for force based criteria, the force-time history needs to be similar between PMHS and surrogates to get representative data and realistic momentum/impulse transfer to the head. A shift in absolute values is normally tolerated but trend reversals and nonlinearity is not acceptable. The shift or increased loads measured in the testing presented above for the Hybrid III with the instrumented chin would replace the initially proposed tolerance levels until such time as a more biofidelic headform is identified. The proposed tolerance levels for the Hybrid III headform are summarize in Table 10 and compared to the tolerance levels of PMHS subjects.

Table 10: Proposed tolerance levels for the Hybrid III headform with instrumented chin.

PMHS Hybrid III load sensing jaw Loading

Fore-aft Lateral Fore-aft Lateral

Focal < 1427 N < 1290 N < 9900 N < 8000 N

Distributed < 3030 N < 1570 < 9900 N < 8000 N

Note:

Although tests were not conducted for the shaded cells, it is expected that the response would be similar whether a small area impactor or large area impactor were used because of the rigidity of the jaw element of the Hybrid III.


22 CR 2008-XXX

The PMHS replication testing described above is a preliminary effort to quantify the headform biofidelity but this is all that can be done at present under the scope of the current project. However, it is proposed that the articulating mandible headform be considered for assessing mandible protection because it has been shown to exhibit the correct force displacement characteristics for impacts to the mandible [Ref. 9].


CR 2009-007 23

7. Discussion and Summary

A draft AMMPHS mandible specification, prepare by DRDC, incorporates requirements for protection from fragmentation and low velocity impact protection. For these requirements it is assumed that the specified mandible ballistic requirements can be achieved and that the issue of concern is therefore the resulting loading to the face from the backface deformation.

The initial draft specification called for no contact to occur between the mandible guard and the face when struck which is not possible within the practical limits of helmet design. An alternate approach is proposed that allows contact to occur but limits the forces that may be transmitted to the face/mandible to below injurious levels.

As a means of establishing acceptable loading thresholds, published research into the fracture tolerance of the mandible was conducted. The studies have shown dependency between the fracture threshold and contact area of the impactor. An impact resulting in a loading area of 13 cm2 was identified as a threshold between distributed and concentrated loading regimes. The loads resulting from the back face deformation caused by a non-penetrating ballistic strike could likely be considered a concentrated load whereas the loading cause by a blunt impact could be considered in the distributed loading regime. Fracture threshold for the mandible were defined for each of theses regimes and whether the impact is to the mandible body or the mental protuberance. It can also be noted that a wide variation in tolerance loads was observed and is likely due to differences in the specimens and/or test protocols. As a conservative estimation of fracture tolerance, the lower loads required to cause fracture were employed for the mandatory requirements of the AMMPHS performance specification.

Testing of a Hybrid III headform with an instrumented chin and of the FOCUS headform indicated that the mandible of both headforms are too stiff resulting in measured jaw loads that are much higher than that reported in literature for similar impacts to the jaw. Despite the high measured loads it is recommended that the Hybrid III with an instrumented chin be used as an interim headform surrogate for assessing the performance of possible AMMPHS mandible guard solutions. However, further investigation of a headform with an articulating load sensing mandible is also recommended. This headform is currently under development and has been shown to exhibit the correct force displacement characteristics for impacts to the mandible and should better correlate with PMHS data.

The proposed specification for the AMMPHS mandible guard is based solely on the loading levels required to cause mandible fracture in a constrained head. Although the loads required to cause fracture would not change, such a constraint of the head is not a loading scenario likely to be encountered in the field. Consequently, it is recommended that testing with surrogate headforms be conducted with the headform mounted to a flexible neck element such as the Hybrid III neck. Furthermore, other injuries such as dental or soft tissue injuries are not addressed by the proposed


24 CR 2008-XXX

mandible specifications and yet these injuries may also affect, although to a lesser extend, military personnel's ability to continue with their daily responsibilities. It is possible that such injuries may be averted by the appropriate selection of padding or comfort materials used on the inner surface of the mandible guard. These types of injuries should be investigated further once a mandible guard design has been developed that will protect against mandible fracture.


CR 2009-007 25

8. References

Ref. 1 “Advanced Modular Multi-threat Headwear System (AMMPHS) Requirements”, Draft Document - Version 3.0, February 10, 2008.

Ref. 2 Skull and facial bones image obtained online from Wikipedia, http://en.wikipedia.org/wiki/Image:Gray188.png

Ref. 3 Hodgson, V. R., “Tolerance of the Facial Bones to Impact”, American Journal of Anatomy, 1967 pages 113 to 122.

Ref. 4 Nahum, A. M., Gatts, J. D., Gadd, C. W., Danforth, J., “Impact Tolerance of the Skull and Face”, Proceedings of the Twelfth Stapp Car Crash Conference, Society of Automotive Engineers Paper # 680785, 1968.

Ref. 5 Schneider, D. C., Nahum, A. M., “ Impact Studies of Facial Bones and Skull”, Proceedings of the 16th Stapp car crash conference, Society of Automotive Engineers, Paper # 720965, 1972.

Ref. 6 Hopper, R. H., McElhaney, J. H., Myers, B. S., “Mandibular and Basilar Skull Fracture Tolerance”, Society of Automotive Engineers, Paper Numer: 942213, 1994.

Ref. 7 Unnewehr, M., Homann, C., Schmidt, P. F., Sotony, P., Fischer, G., Brinkman, B., Bajanowski, T., DuChesne, A., “Fracture Properties of the Human Mandible”, International Journal of Legal Medicine (2003), pages 326-330.

Ref. 8 Viano, D. C., Bir, C., Walilko, T., Sherman, D., “Ballistic Impact to the Forehead, Zygoma, and Mandible: Comparison of Human and Frangible Dummy Face Biomechanics”, The Journal of Trauma Injury, Infection, and Critical Care, 2004, Volume 56 page 1305.

Ref. 9 Craig, M., Bir, C., Viano, D., “Jaw Loading Response of Current ATDs”, under review for publication with the Society of Automotive Engineers.

dcd03e

UNCLASSIFIED SECURITY CLASSIFICATION OF FORM

(Highest classification of Title, Abstract, Keywords)

DOCUMENT CONTROL DATA

1. ORIGINATOR (name and address)

DRDC Valcartier

2459 Pie-XI Blvd. North

Quebec, Qc

G3J 1X8

2. SECURITY CLASSIFICATION

(Including special warning terms if applicable)

UNCLASSIFIED

3. TITLE (Its classification should be indicated by the appropriate abbreviation (S, C, R or U)

AMMPHS - Mandible Protection, Requirements ans Specifications

4. AUTHORS (Last name, first name, middle initial. If military, show rank, e.g. Doe, Maj. John E.)

E. Fournier

5. DATE OF PUBLICATION (month and year)

October 2008

6a. NO. OF PAGES

37

6b. NO. OF REFERENCES

9

7. DESCRIPTIVE NOTES (the category of the document, e.g. technical report, technical note or memorandum. Give the inclusive dates when a specific reporting period is covered.)

Contractor's report

8. SPONSORING ACTIVITY (name and address)

AMMPHS TDP

9a. PROJECT OR GRANT NO. (Please specify whether project or grant)

9b. CONTRACT NO.

W7701-061933/001/QLC

10a. ORIGINATOR’S DOCUMENT NUMBER

CR 2009-007

10b. OTHER DOCUMENT NOS

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11. DOCUMENT AVAILABILITY (any limitations on further dissemination of the document, other than those imposed by security classification)

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12. DOCUMENT ANNOUNCEMENT (any limitation to the bibliographic announcement of this document. This will normally correspond to the Document Availability (11). However, where further distribution (beyond the audience specified in 11) is possible, a wider announcement audience may be selected.)

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dcd03e



13. ABSTRACT (a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual).

Military helmets must constantly evolve to improve head protection against ever changing threats that can be encountered on the battlefield. A helmet’s protection is no longer limited to projectile penetration but also includes its capacity to mitigate behind armour blunt trauma, blunt impact trauma, as well as blast effects. Additionally, facial injuries are becoming a bigger concern and consequently the Advanced Multi-threat Modular Protective Headwear System (AMMPHS) development program incorporates a mandible guard in its design. To assess the effectiveness of a mandible guard an appropriate test methodology and injury based performance criterion must be developed. A specification for the performance of a mandible guard is proposed. Performance levels are based on biomechanical tolerance levels of the mandible region that were identified in the scientific literature. Surrogate headforms and test protocols for assessing performance are evaluated along with appropriate injury tolerance thresholds.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

Military helmets, AMMPHS TDP, Mandible

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