Mandible Headform Biofidelity Evaluation for AMMPHS E. Fournier Michael Wonnacott Prepared By: Biokinetics and Associates Ltd. 2470 Don Reid Drive Ottawa, Ontario, K1H 1E1 Biokinetics Report No.: R09-04b Project Leader: Ed Fournier, (613) 736-0384 Contract Number: W7701-84308 Contract Scientific Authority: Kevin Williams (418-844-4000 ext. 4238) Defence R&D Canada – Valcartier Contract Report DRDC Valcartier CR 2009-147 September 2009 The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.
Transcript
Mandible Headform Biofidelity Evaluation for
AMMPHSE. FournierMichael Wonnacott
Prepared By:
Biokinetics and Associates Ltd. 2470 Don Reid Drive Ottawa, Ontario, K1H 1E1
Biokinetics Report No.: R09-04bProject Leader: Ed Fournier, (613) 736-0384 Contract Number: W7701-84308 Contract Scientific Authority: Kevin Williams (418-844-4000 ext. 4238)
Defence R&D Canada – ValcartierContract Report
DRDC Valcartier CR 2009-147September 2009
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.
Mandible Headform Biofidelity Evaluation for AMMPHS
Ed Fournier Michael Wonnacott Biokinetics and Associates Ltd. Prepared By: Biokinetics and Associates Ltd. 2470 Don Reid Drive Ottawa, Ontario K1H 1E1 Biokinetics Report No.: R09-04b Contract Project Manager: Ed Fournier, (613) 736-0384 CSA: Kevin Williams, Defence R&D Canada – Valcartier, (418) 844-4000 Ext. 4238 The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.
The Advanced Modular Multi-threat Protective Headwear System (AMMPHS) development program was initiated to address the ballistic, blast and blunt impact protection needs of the soldier in today's battlefield. One aspect of protection incorporated by AMMPHS is mandible protection. To evaluate the protection offered by a mandible guard, an injury criterion with appropriate injury tolerance thresholds has been proposed based on mandible fracture levels determined from published post mortem human subject (PMHS) testing results. To evaluate mandible guard designs a surrogate headform is required. A headform with an articulating mandible was evaluated for use as a head surrogate for assessing the performance of AMMPHS mandible protection systems. The impactor loads in the fore-aft and lateral impacts to the mandible were compared to published results of similar PMHS tests and were found to be higher but the total force measured by the headform was within the range of loads reported for the PMHS. In comparisons to other headforms, the instrumented jaw of the articulating mandible headform is the most suited to the evaluation of AMMPHS mandible guards and appropriate threshold response levels are proposed.
Résumé ….....
Le programme de développement du système de protection modulaire multi menaces (AMMPHS) a été initié afin d’adresser les besoin de protection du soldat d’aujourd’hui contre les menaces balistiques, les effets de souffle et les impacts contondants. Parmi les différents aspects de protection considérés, AMMPHS incorpore un système de protection de la mâchoire inférieure. Afin d’évaluer cet aspect de la protection, un critère de blessure incorporant des niveaux de tolérance appropriés a été proposé. Ce critère est basé sur des résultats d’expériences de fractures mâchoire inférieure effectuées sur des spécimens post mortem. L’utilisation d’une fausse tête instrumentée est également requise pour évaluer les concepts de protection de la mâchoire inférieure. Un modèle de fausse tête incluant une mâchoire inférieure articulée a été évalué pour cette application. Les forces d’impact avant-arrière et latérales mesurées avec la fausse tête ont été comparé aux données publiés pour des conditions similaires. Les résultats obtenus sont plus élevés mais la force totale mesurée par la fausse tête se situe à l’intérieur de la plage de variation observée avec les spécimens post-mortem. En comparaison avec d’autres fausses têtes, celle comportant une mâchoire inférieure articulée et instrumentée est la plus appropriée pour l’évaluation de protecteur de mâchoire inférieure AMMPHS. Et conséquemment des seuils de tolérances appropriés ont été proposés.
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DRDC Valcartier CR 2009-147 iii
Executive summary
Mandible Headform Biofidelity Evaluation for AMMPHS:
Ed Fournier; Michael Wonnacott; DRDC Valcartier CR 2009-147; Defence R&D Canada – Valcartier; September 2009.
The Advanced Modular Multi-threat Protective Headwear System (AMMPHS) development program was initiated to address the evolving ballistic, blast and blunt impact head protection needs in today’s battlefield. Facial trauma resulting from direct or indirect fragmentation strike, small arms fire or blunt impact has been identified as threats that can result in serious injury to the soldiers. Consequently, one aspect of protection incorporated by AMMPHS is mandible protection.
Previous research established an injury criterion for the mandible, based on published fracture levels. A test protocol and injury tolerance thresholds were also proposed that employed a surrogate headform for measuring the loading transmitted to the jaw in an impact. The Hybrid III and the FOCUS headforms, both commercially available, were evaluated for this purpose but were found to be much too rigid. As an alternative, a headform with an articulating mandible originally developed for the National Football League to assess the performance of mouth guards was evaluated for use in assessing the performance of AMMPHS mandible protection systems. This headform which incorporates an articulating jaw can measure the forces exerted through the jaw and upper dentition. This headform's response to impacts in the fore-aft and lateral directions were compared to published results of similar post mortem human subject testing.
The articulating mandible headform was subjected to impacts to the chin and mandible body by a rigid impactor with velocities ranging 1.50 m/s to 5.73 m/s. The peak input loads were measured with an accelerometer mounted to the impactor whereas the reaction loads were calculated from load cells installed in the headform. Although impactor mass and velocity were the same as that used in the PMHS testing, the measured impactor loads were found to be higher for the tests on the mandible headform. These higher loads are due to the rigid steel construction of the mandible that does not fracture and absorb energy as would bone even though the force displacement of the jaw has been found to be biofidelic at lower energy inputs. Although high, these results are more favourable than the results obtained with the Hybrid III and FOCUS headforms and given the suitability of the headform for assessing mandible loads it is proposed that the articulating mandible headform be used for the evaluation of AMMPHS mandible guard solutions. The tolerance thresholds to be used in evaluating AMMPHS mandible options were established according to the measured headform loads that were obtained during the comparative PMHS impact tests.
iv DRDC Valcartier CR 2009-147
Sommaire .....
Mandible Headform Biofidelity Evaluation for AMMPHS:
Ed Fournier; Michael Wonnacott; DRDC Valcartier CR 2009-147; R & D pour la défense Canada – Valcartier; Septembre 2009.
Le programme de développement du système de protection modulaire multi menaces (AMMPHS) a été initié afin d’adresser les besoin de protection du soldat d’aujourd’hui contre les menaces balistiques, les effets de souffle et les impacts contondants. Les traumatismes faciaux causés soit par des impacts directs ou indirects de fragments ou de projectiles d’armes à feu, soit par des impacts contondants ont été identifiés comme des menaces importantes pouvant provoquer des blessures sérieuses aux soldats. Conséquemment, un système de protection de la mâchoire inférieure a été inclus parmi les différents aspects de protection considérés pour AMMPHS.
Une étude antérieure a permis d’établir un critère de blessure basé sur des niveaux publiés quant à la tolérance de la mâchoire inférieure à la fracture. Un protocole d’essai et des niveaux de tolérance à la blessure reliés à l’utilisation d’une fausse tête permettant de mesurer les forces d’impact transmises à la mâchoire ont aussi été proposés. Les fausses têtes Hybrid III et FOCUS, disponibles commercialement, ont été évaluées pour cette application mais elles se sont avérées beaucoup trop rigide. Comme alternative, une fausse tête avec une mâchoire inférieure articulée permettant de mesurer les forces appliquées à la mâchoire et la dentition supérieure a également été évaluée. Cette fausse tête a été développé originalement pour la ligue nationale de football (NFL) afin d’évaluer la performance des protège-dents. Les forces d’impact avant-arrière et latérales mesurées avec cette fausse tête ont été comparé aux données publiés pour des conditions similaires obtenus avec des spécimens post mortem.
La fausse tête avec mâchoire inférieure articulée a été soumise a des impacts au menton et à la mâchoire inférieure pour des vitesse variant entre 1.50 et 5.73 m/s. Les forces appliquées maximales ont été mesurées en utilisant un accéléromètre installé sur l’impacteur alors que les forces de réaction ont été calculées à partir des cellules de charges de la fausse tête. Bien que la masse et la vitesse de l’impacteur étaient les mêmes que celles utilisé lors des testes avec les spécimens pot mortem, les forces appliquées mesurées étaient plus élevées pour les tests avec la fausse tête avec mâchoire inférieure articulée. Le niveau de force plus élevé est dû à la construction rigide de la mâchoire inférieure qui ne se fracture pas et qui absorbe l’énergie comme la matière osseuse le ferait bien que les caractéristiques de force-déplacement de la mâchoire sont biofidèles pour des énergies d’impact plus faibles. Même élevés, ces résultats sont plus favorables que ceux obtenus avec les fausses têtes Hybrid III et FOCUS. Étant donné l’inaptitude de ces fausses têtes à mesurer les forces au niveau de la mâchoire, la fausse tête ayant une mâchoire inférieure articulée est donc la plus appropriée pour l’évaluation de protecteur de mâchoire inférieure AMMPHS Les seuils de tolérances à être utilisés pour ces évaluations ont été établis à partir des mesures obtenues lors des tests comparatifs avec les spécimens post mortem.
Résumé …....................................................................................................................................i Executive summary ................................................................................................................... iii Sommaire ..... .............................................................................................................................iv
Table of contents.........................................................................................................................v List of figures.............................................................................................................................vi List of tables ..............................................................................................................................vi
1 Introduction ..........................................................................................................................1 2 Test Setup.............................................................................................................................3
3 Results..................................................................................................................................6 4 Discussion and Summary....................................................................................................11 5 Conclusion: ........................................................................................................................15
References ................................................................................................................................17 List of symbols/abbreviations/acronyms/initialisms ...................................................................19
Figure 2: Test setup for the lateral impacts to the mandible body. ................................................4
Figure 3: Test setup for the impacts to the chin. ...........................................................................4
Figure 4: Applied force as a function of the impact velocity - impacts to the chin. .......................6
Figure 5: Headform response as a function of the applied force - impacts to the chin. ..................7
Figure 6: Individual loads and computed total force - impacts to the chin....................................7
Figure 7: Applied force as a function of the impact velocity - lateral impacts. ..............................8
Figure 8: Headform response as a function of the applied force - lateral impacts. .........................9
Figure 9: Individual loads and computed total force - impacts to the mandible body. ...................9
List of tables
Table 1: Impact test configurations. .............................................................................................3
Table 2: The proposed impact test matrix. ...................................................................................5
Table 3: Previously defined mandible load requirements for AMMPHS based on the Rigid Hybrid III. ...............................................................................................................12
Table 4: Summary of PMHS and Mandible loads. .....................................................................14
The Advanced Modular Multi-threat Protective Headwear System (AMMPHS) development program was initiated to address the evolving ballistic, blast and blunt impact head protection needs in today’s battlefield. 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. Consequently, one aspect of protection incorporated by AMMPHS is mandible protection.
To assess the protective benefits of an AMMPHS mandible guard on mitigating facial injuries, an injury criterion was proposed based on fracture levels that were established from biomechanical tolerance levels reported in the literature [1]. Test protocols for assessing the performance of a mandible guard are also recommended along with the appropriate injury tolerance thresholds. Two headforms, a rigid Hybrid III headform with a load sensing mandible and the Facial and Ocular Countermeasure Safety (FOCUS) headform, were both evaluated according to the proposed test protocol but were found to be too rigid with measured responses that are well in excess of that recorded in the published cadaver results.
As a possible alternative to the Hybrid III and the FOCUS headforms, a mandible headform originally developed for the National Football League to assess the ability of mouth guards to influence head accelerations, and hence brain injury was evaluated. The force transmission through the upper dentition and the left and right temporo-mandibular joints (TMJ) is measurable with this manikin.
The mandible headform is based on the Hybrid III 50% male manikin head however; the entire lower portion of the skull was removed and replaced with the mandible framework. The mandible has a large triangular slot at the upper end. The TMJ is represented by a metal pin that has one end encased in an elastomeric donut. The pin fits loosely in the triangular slot, so that the mandible can move in and out, slide left and right and pivot. In the neutral position, like in a human, the slot ends up sitting against the pin and there is no ride down room. Upon insertion of a mouth guard, the mandible is relocated such that the pin is in the middle of the slot. Therefore the mandible can move backwards a few millimeters within the free play until hitting the pin. The pin and rubber donut on each side are mounted to 3-axis load cells. The load cell does not measure force until the pin is engaged by the mandible. In this way, the condylar space that opens up in the human jaw when a mouth guard is inserted can be simulated.
The headform is shown in Figure 1. The testing and comparison of the mandible headform's impact response in the fore-aft and lateral directions to the published results of similar cadaver testing is presented in this report.
The assessment of the Articulating Mandible Headform comprised of focal loading on the mental protuberance (front of the chin) and distributed loading on the mandible body (lower part of the jaw) according to the test procedure used by Schneider[2] and from which the AMMPHS mandible performance criteria was based [1]. The parameters of each test configuration are summarized in Table 1 below.
Table 1: Impact test configurations.
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
95% porosity
500 g/m2
40 ppcm pore size
4.00 m nickel foam
95% porosity
500 g/m2
40 ppcm pore size
Drop Mass 3.12 kg 3.81 kg
Impact Velocity 5.46 m/s 5.73 m/s
Head Support soft polyurethane foam wedges
For impacts to the chin, a headform without a neck was supported by foam wedges on a heavy pedestal and positioned/supported as per Schneider’s test setup. For impacts to the mandible body the headform was installed on a Hybrid III neck mounted to a rigid lower neck bracket which was bolted to the test pedestal. In both configurations, the headform positioning was achieved using three targeting lasers fixed to the laboratory reference. The lasers ensured repeatable positioning of three targets affixed to the headform. The two different test configurations are shown in Figure 2 and Figure 3.
4 DRDC Valcartier CR 2009-147
Figure 2: Test setup for the lateral impacts to the mandible body.
Figure 3: Test setup for the impacts to the chin.
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, only nickel foam with a thickness of 2.00 mm is currently available and is reflected in Table 1. One layer was used for the focal loading tests and two layers were used for the distributed loading tests to approximate the original nickel foam thickness1.
The impact force was calculated based on the response of an Endevco 7702A-50 piezzo electric accelerometer mounted to the top of the impactor. The response of the headform mandible was measured directly with three PCB 260A11 triaxial force sensors installed in the headform. The headform's load cells measured the reaction loads in the forward (X), lateral (Y) and vertical directions (Z) at each of the temporo-madibular joints (TMJ) and the upper dentition from which a resultant load was computed. The resultant load is calculated by firsts summing the load in the
1 The nickel foam was purchased from Marketech International, 107B Louisa Street,Port Townsend, WA, Tel: 360-379-6707, Toll Free: 877-452-4910, Fax: 360-379-6907, http://www.mkt-intl.com/.
DRDC Valcartier CR 2009-147 5
in each of the three orthogonal directions from the tree different load cells and then calculating the resultant total load. The measured forces were compared to Schneider’s results.
The forces measured by the headform's load cells and the impactor accelerations were recorded at a sampling frequency of 10 kHz following standard SAE J211 practices. The acceleration data were filtered with CFC 1000 filters whereas the headform's load cell data was filtered using CFC 600 filters. All the testing was conducted under ambient environmental conditions (21 ±2 °C).
The impact velocity was increased from a low impact severity up to the impact velocity used by Schneider. The test matrix is shown in Table 2.
Table 2: The proposed impact test matrix.
Impact Velocity Repeats
1.50 x 3
3.00 x 3
4.50 x 3
Imp
act
s to
th
e
Ch
in
5.46 x 3
1.50 x 3
3.00 x 3
4.50 x 3
Imp
act
s to
th
e
Man
dib
le B
od
y
5.73 x 3
6 DRDC Valcartier CR 2009-147
3 Results
The force applied to the mandible as measured by the impactor for strike velocities ranging from 1.50 m/s to 5.46 m/s are shown in Figure 4 for the focal impacts to the chin. The peak loads calculated from the response of the mandible headform load cells corresponding to the measured impact loads are shown in the Figure 5. A typical example of the measured loads from the three 3-axis load cells and the computed resultant from which the peak loads are obtained are presented in Figure 6 for impacts to the chin.
Applied Force versus Impact Speed
Focal Impacts to the Chin
y = 533.08x1.2877
R2 = 0.9988
0
1000
2000
3000
4000
5000
6000
0 1 2 3 4 5 6
Impact Velocity (m/s)
Pea
k I
mp
acto
r F
orc
e (
N)
Figure 4: Applied force as a function of the impact velocity - impacts to the chin.
DRDC Valcartier CR 2009-147 7
Headform Response versus Applied Force
Focal Impacts to the Chin
y = 0.7838x
R2 = 0.9867
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 1000 2000 3000 4000 5000 6000
Peak Impactor Force (N)
Peak
Res
ult
an
t H
ea
dfo
rm F
orc
e (
N)
Figure 5: Headform response as a function of the applied force - impacts to the chin.
Left TMJ Forces
-1500
-1000
-500
0
500
1000
1500
2000
0 0.005 0.01 0.015 0.02
X
Y
Z
Right TMJ Forces
-1500
-1000
-500
0
500
1000
1500
2000
0 0.005 0.01 0.015 0.02
X
Y
Z
Upper Dentition Forces
-1500
-1000
-500
0
500
1000
1500
2000
0 0.005 0.01 0.015 0.02
X
Y
Z
Total Force
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 0.005 0.01 0.015 0.02
Figure 6: Individual loads and computed total force - impacts to the chin.
8 DRDC Valcartier CR 2009-147
Similarly to the impacts to the chin, the force applied to the mandible as measured by the impactor for strike velocities ranging from 1.50 m/s to 5.73 m/s are shown in Figure 7 for the lateral impacts to the mandible body. The peak loads calculated from the response of the mandible headform load cells corresponding to the measured impact loads are shown in the Figure 8. A typical example of the measured loads from the three 3-axis load cells and the computed resultant from which the peak loads are obtained are presented in Figure 9 for impacts to the to the mandible body.
Applied Force versus Impact Velocity
Lateral-Distributed Impacts
y = 247.43e0.5325x
R2 = 0.9956
0
1000
2000
3000
4000
5000
6000
0 1 2 3 4 5 6 7
Impact Velocity (m/s)
Peak
Im
pac
tor
Fo
rce
(N
)
Figure 7: Applied force as a function of the impact velocity - lateral impacts.
DRDC Valcartier CR 2009-147 9
Headform Response versus the Applied Force
Lateral-Distributed Impacts
y = 0.5077x
R2 = 0.9567
0
500
1000
1500
2000
2500
3000
0 1000 2000 3000 4000 5000 6000
Peak Impactor Force (N)
Pea
k R
es
ult
an
t H
ea
dfo
rm F
orc
e (
N)
Figure 8: Headform response as a function of the applied force - lateral impacts.
Left TMJ Forces
-800
-600
-400
-200
0
200
400
600
800
0 0.005 0.01 0.015 0.02 0.025 0.03
X
Y
Z
Right TMJ Forces
-2500
-2000
-1500
-1000
-500
0
500
1000
0 0.005 0.01 0.015 0.02 0.025 0.03
X
Y
Z
Upper Dentition Forces
-500
-400
-300
-200
-100
0
100
200
300
400
0 0.005 0.01 0.015 0.02 0.025 0.03
X
Y
Z
Total Force
0
500
1000
1500
2000
2500
3000
0 0.005 0.01 0.015 0.02 0.025 0.03
Figure 9: Individual loads and computed total force - impacts to the mandible body.
10 DRDC Valcartier CR 2009-147
The severity of the impact during tests with velocities above 5 m/s was sufficient to cause the mandible to permanently splay open slightly. To prevent this from reoccurring, the specification for the jaw element has been changed from a mild steel component to a heat treated component with a higher yield stress. The headform provided to DRDC included the hardened, heat treated mandible component.
The maximum force measured by any single channel in the articulating headform was below 70% of the rated capacity of the load cells indicating that the headform's instrumentation is operating well within its capacity and therefore should be sufficiently rugged to withstand repeated testing. For obvious reasons this assumes that no penetration occurs when load measurement are made following a ballistic strike to the mandible guard. Also, it is possible that blunt impacts testing under AMMPHS may be of greater severity but the presence of head and mandible protection will absorb and diminish the impact to the headform.
DRDC Valcartier CR 2009-147 11
4 Discussion and Summary
As a means of establishing acceptable loading thresholds, published research into the fracture tolerance of the mandible was conducted [1]. A wide variation in load tolerance was observed in the published literature which is likely due to differences in the specimens and/or test protocols. The studies showed a dependency between the fracture threshold and contact area of the impactor where an impact resulting in a loading area of 13 cm2 was identified as a threshold between distributed and concentrated loading regimes. In relation to military helmets, 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 thresholds for the mandible were defined for each of theses regimes and whether the impact is to the mandible body or the mental protuberance. The originally proposed tolerance levels for the AMMPS specification were based on the PMHS data and are shown in Table 3. Also presented in the same table is the range of loads that were recorded in the published post mortem human subject (PMHS) testing and the corresponding force response from the articulating mandible headform when tested under similar conditions. Results from previous tests conducted on a rigid Hybrid III headform are also included. The results from testing of the FOCUS headform, that were obtained during the same test program as the rigid Hybrid III, are not included in the table due to premature termination of the testing because the measured loads were approaching the capacity of the FOCUS headform's load cells.
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Table 3: Previously defined mandible load requirements for AMMPHS based on the Rigid
Hybrid III.
Rigid Hybrid III Headform Articulated Mandible Headform
Loading
(Peak)
PMHS
Range of
Results (a)
AMMPHS
Requirement
(Original Proposal) [1]
Based on
rigid Hybrid III
Average Impactor Load
(Range)
(N)
Average
Headform
Resultant Load
(N)
Average Impactor Load
(Range)
(N)
Average
Headform
Resultant Load
(N)
Foca
l
1557 N to
4120 N
< 1724 N (b)
9932 (f)
7787 (f)
4776
(4678 - 4962 )
3916
(3839 - 4057)
Fo
re-a
ft
Dis
trib
ute
d 1601 N
to 6740 N
< 3030 N (c)
(g) (g) (g) (g)
Foca
l
1290 N to
1446 N
< 1290 N (d)
(g) (g) (g) (g)
Late
ral
Dis
trib
ute
d 820 N
to 2600 N
< 1570 N (e) 6066
(6031 - 6126)
8817
(8001 - 8240)
5026
(4636 - 5393)
2436
(2209 - 2748)
Notes: a- Range of results from all reported research. b- Average of lowest values reported by Nahum [3] and Schneider [4]. c- Average of lowest values reported by Hodgson [5] and Hopper [6]. Insufficient data is reported in the literature to be able to repeat these tests. d- Lowest value presented by Nahum [3]. e- Average fracture load presented by Schneider [4]. f- Only one test was conducted because loads were approaching the capacity of the instrumentation. g- Although these tests were not conducted, due to insufficient published data, 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 headform.
DRDC Valcartier CR 2009-147 13
The proposed requirements for the AMMPHS specification indicated in column 3 of Table 3, which came from a previous study [1] and are being modified below, were based on the lower range of the PMHS mandible fracture loads reported in the published research results. Modifications to these originally proposed requirements based on the currently reported testing of the mandible headform are discussed below.
For the mandible headform testing, the input conditions of impactor mass and velocity were the same as that used in the Schneider PMHS tests that were sufficiently detailed to reproduce. In comparison to the PMHS tests the measured impactor loads upon striking the mandible headform were found to be higher. The average impactor load in the fore-aft focal loading configuration of the mandible headform was approximately 16 % higher than the range reported for the PMHS testing, whereas, in the lateral direction the average impactor loads measured during the distributed loading configuration were approximately 93 % higher. These loads are likely higher because the mandible is fabricated from mild steel that does not deform/fracture and absorb energy as would bone even though the force displacement of the jaw has been found to be biofidelic at lower energy inputs [7]. Although fabricated from a rigid element, the mandible has the capacity to displace due to the compliance of the TMJ as described earlier. This compliance which provides additional ride down distance for impacts to the jaw is largely responsible for the headform's superior performance to jaw impacts over that of the rigid Hybrid III or FOCUS headform designs where, during similar impact testing, the rigid Hybrid III headform loads were approximately 141% and 212% higher for the focal and distributed loading respectively. Furthermore, testing on the rigid FOCUS headform could not be completed because the measured loads in the headform were approaching the rated capacity of the transducers installed in the headform.
Measuring impact load with an accelerometer on the impactor is a convenient and easily implemented method to use for controlled laboratory testing to assess the performance of the headforms. However, for practical purposes this method is not possible for the likely testing scenarios envisaged for the AMMPHS helmet, such as non penetrating ballistic strike to the helmets mandible guard. As a result, the instrumented jaw of the articulating mandible headform is ideally suited for the evaluation of AMMPHS mandible guard solutions from any possible loading mechanism ranging from ballistic strike to blunt impact.
Accepting the fact that the mandible headform is more rigid than a human's at the energy levels required to cause fracture, it is proposed that the articulating mandible headform be used for the assessment of AMMPHS mandible guard solutions. Given that the input energies of the impacts to the mandible headform were the same as those in the PMHS tests the average response of the mandible headform load cells can be correlated directly to the average injury threshold identified in the PMHS testing. These average loads are summarized in Table 4 below.
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Table 4: Summary of PMHS and Mandible loads.
Loading Direction Average PMHS
Results
Average Mandible
Headform Loads
(see note)
Fore-aft Focal 2845 N 3916 N
Lateral Distributed 1570 N 2436 N
Note The mandible headform loads are derived from the measured TMJ loads and the upper dentition loads.
It is proposed that the AMMPHS mandible guard performance specifications based on the measured response of the articulating mandible headform, replace the originally proposed levels indicated in Table 3 which were based on the PMHS data. The newly proposed AMMPHS mandible guard performance specification is summarized in Table 5 below.
Although tests were not conducted for the shaded cells, due to insufficient published data, it is expected that the response of the headform would be similar whether a small area impactor or large area impactor were used because of the rigidity of the jaw element of the articulating mandible headform.
DRDC Valcartier CR 2009-147 15
5 Conclusion:
Incorporating a mandible guard in the design of AMMPHS necessitates a test surrogate for assessing the performance of a mandible guard solution. Three headforms were evaluated for this purpose, two of which were rigid and the third incorporated an articulating mandible element. In comparing the response of the headforms to fracture data from PMHS trials in which the load required to cause fracture was measured, in both distributed and focal loading conditions, the articulating mandible headform performed the best. Additional published comparison of the articulating mandible headform's force deflection characteristics to those of available PMHS has shown the headform to be biofidelic at low energy impacts that do not result in fracture. However, at impact levels required to cause fracture in the PMHS, the measured force on the impactor was found to be higher. The higher forces are likely due to the rigid steel construction of the mandible component which does not fracture and absorb energy as would bone.
Improvement of the mandible headform's response would require a compliant mandible element with stiffness characteristic similar to those of a human mandible. This would require additional research to be conducted into the force displacement characteristics of the human mandible up to the point of fracture. Until such research data becomes available it is proposed that the computed response from the headform load cells can be correlated directly to the average injury threshold identified in the review of the PMHS data.
Upon impact, the mandible reaction loads are measured with triaxial load cells located in the left and right temporo-madibular joints (TMJ) and the upper dentition. These load cells measure forces in the forward, lateral and vertical directions respectively, from which a total resultant load can be computed by summing the loads in each of the three orthogonal directions and then calculating the resultant.
Fracture tolerance levels based on the mandible headform's response when impact tested using the same methodology as that used in Schneider's PMHS testing, were established at 3916 N and 2436 N for the fore-aft and lateral directions respectively. Until additional data becomes available from which fracture probability curves can be development, it is proposed that measured loads in excess of these values correspond to mandible fracture. It should be noted that less severe injuries, such as broken teeth, may occur at levels below these suggested fracture tolerance levels.
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References .....
[1] Fournier, E., Shewchenko, N., "AMMPHS - Mandible Protection, Requirements and Specifications", Biokinetics and Associates Limited Contract Report to Defence Research and Development Canada Valcartier, Contract No. W7701-061933/001/QLC, Contractor Report No. R08-06, October 2008.
[2] 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.
[3] 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.
[4] 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.
[5] Hodgson, V. R., “Tolerance of the Facial Bones to Impact”, American Journal of Anatomy, 1967, pages 113 to 122.
[6] Hopper, R. H., McElhaney, J. H., Myers, B. S., “Mandibular and Basilar Skull Fracture Tolerance”, Society of Automotive Engineers, Paper Numer: 942213, 1994.
[7] Craig, M., Bir, C., Viano, D., "Jaw Loading Response of Current ATDs", Society of Automotive Engineers, paper 2009-01-0388.
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List of symbols/abbreviations/acronyms/initialisms
AMMPHS Advanced Modular Multi-threat Protective Headwear System
CFC Channel Filter Class
DRDC Defence Research & Development Canada
FOCUS Facial and Ocular Countermeasure Safety headform
PMHS Post Mortem Human Subjects
TMJ Temporomadibular Joints
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DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)
1. ORIGINATOR (The name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Centre sponsoring a contractor's report, or tasking agency, are entered in section 8.)
Biokinetics and Associates Ltd. 2470 Don Reid Drive Ottawa, Ontario K1H 1E1
2. SECURITY CLASSIFICATION (Overall security classification of the document including special warning terms if applicable.)
UNCLASSIFIED
3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S, C or U) in parentheses after the title.)
Mandible Headform Biofidelity Evaluation for AMMPHS:
4. AUTHORS (last name, followed by initials – ranks, titles, etc. not to be used)
Fournier, E.; Wonnacott, M.
5. DATE OF PUBLICATION (Month and year of publication of document.)
September 2009
6a. NO. OF PAGES (Total containing information, including Annexes, Appendices, etc.)
32
6b. NO. OF REFS (Total cited in document.)
7
7. DESCRIPTIVE NOTES (The category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered.)
Contract Report
8. SPONSORING ACTIVITY (The name of the department project office or laboratory sponsoring the research and development – include address.)
9a. PROJECT OR GRANT NO. (If appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant.)
12RR08
9b. CONTRACT NO. (If appropriate, the applicable number under which the document was written.)
W7701-84308
10a. ORIGINATOR'S DOCUMENT NUMBER (The official document number by which the document is identified by the originating activity. This number must be unique to this document.)
R09-04b
10b. OTHER DOCUMENT NO(s). (Any other numbers which may be assigned this document either by the originator or by the sponsor.)
DRDC Valcartier CR 2009-147
11. DOCUMENT AVAILABILITY (Any limitations on further dissemination of the document, other than those imposed by security classification.)
Unlimited
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.))
Unlimited
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.)
The Advanced Modular Multi-threat Protective Headwear System (AMMPHS) development program was initiated to address the ballistic, blast and blunt impact protection needs of the soldier in today's battlefield. One aspect of protection incorporated by AMMPHS is mandible protection. To evaluate the protection offered by a mandible guard, an injury criterion with appropriate injury tolerance thresholds has been proposed based on mandible fracture levels determined from published post mortem human subject (PMHS) testing results. To evaluate mandible guard designs a surrogate headform is required. A headform with an articulating mandible was evaluated for use as a head surrogate for assessing the performance of AMMPHS mandible protection systems. The impactor loads in the fore-aft and lateral impacts to the mandible were compared to published results of similar PMHS tests and were found to be higher but the total force measured by the headform was within the range of loads reported for the PMHS. In comparisons to other headforms, the instrumented jaw of the articulating mandible headform is the most suited to the evaluation of AMMPHS mandible guards and appropriate threshold response levels are proposed.
Le programme de développement du système de protection modulaire multi menaces (AMMPHS) a été initié afin d’adresser les besoin de protection du soldat d’aujourd’hui contre les menaces balistiques, les effets de souffle et les impacts contondants. Parmi les différents aspects de protection considérés, AMMPHS incorpore un système de protection de la mâchoire inférieure. Afin d’évaluer cet aspect de la protection, un critère de blessure incorporant des niveaux de tolérance appropriés a été proposé. Ce critère est basé sur des résultats d’expériences de fractures mâchoire inférieure effectuées sur des spécimens post mortem. L’utilisation d’une fausse tête instrumentée est également requise pour évaluer les concepts de protection de la mâchoire inférieure. Un modèle de fausse tête incluant une mâchoire inférieure articulée a été évalué pour cette application. Les forces d’impact avant-arrière et latérales mesurées avec la fausse tête ont été comparé aux données publiés pour des conditions similaires. Les résultats obtenus sont plus élevés mais la force totale mesurée par la fausse tête se situe à l’intérieur de la plage de variation observée avec les spécimens post-mortem. En comparaison avec d’autres fausses têtes, celle comportant une mâchoire inférieure articulée et instrumentée est la plus appropriée pour l’évaluation de protecteur de mâchoire inférieure AMMPHS. Et conséquemment des seuils de tolérances appropriés ont été proposés.
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.)
