KNEE JOINT INJURY MECHANISMS AND INJURY CRITERIA IN … · KNEE JOINT INJURY MECHANISMS AND INJURY...

IRCOBI Conference – Madrid (Spain), September 2006 319

KNEE JOINT INJURY MECHANISMS AND INJURY CRITERIA IN FULL

²SCALE TESTS ACCORDING TO IMPACT POSITION

ARNOUX P.J.1, THOLLON L.

1, BEHR M.

1, BRUNET C.

1 CESARI D.

2

1 Laboratoire de Biomécanique Appliquée, UMRT 24 Faculté de Médecine - INRETS, Université de la

Méditerranée, Marseille, France. 2 INRETS, Scientific direction, Bron, France.

ABSTRACT

In pedestrian safety research, the definition of an injury criterion for the knee joint should provide

issues to future regulation procedures. Also, do the existing injury criteria (based on knee lateral

shearing levels and flexion levels for sub-segment testing) remain efficient for full scale tests? Is the

impact location on the leg influencing injury mechanisms and injury criteria assumptions? Lastly,

what are the potential consequences of these differences to the pelvis component during the first phase

of impact? The LLMS lower limb model coupled to a hybrid 3 was used to investigate these questions.

This work was based on pedestrian full scale experiments performed in the Laboratory and a

sensitivity analysis regarding various impact positions. The results obtained for this car geometry

showed that injury mechanisms (i. e. the proportion of shearing and lateral flexion) for the knee joint

and the pelvis are directly related to the impact position. Lastly they lead to improve existing injury

criteria by adding combination of two injury modes.

Keywords: PEDESTRIANS, INJURY CRITERIA, KNEES, LEGS, FINITE ELEMENT METHOD

IN PEDESTRIAN CAR ACCIDENT a large field of research is devoted to the development of

vehicle safety countermeasures directed towards the response of the human knee joint. A large portion

of this research concerns the experimental investigation of human body behaviour through cadaver

experiments, the design and validation of human lower limb models or mechanical surrogates. All

these works try to investigate injury mechanisms, injury criteria by correlation to trauma situation and

consequently car safety efficiency.

From an experimental point of view, during a pedestrian impact, knee injuries could result from a

combination of lateral shearing and bending of the knee (Kajzer 1990 and 1993, Teresinski 2001, Bose

2004). Pure shearing induces collateral tibial and anterior cruciate ligaments failure while a primarily

bending mainly induces medial collateral ligament failure. More recently, Bose (2004) performed 3-

point bending tests on isolated knee joints in order to obtain a combination of shearing and bending

effects, and confirmed injuries to medial collateral and anterior cruciate ligaments. It can be noted that

knee injuries are not restricted to the injuries described above. Tibia fractures (especially with at the

tibial eminence in contact with the intercondylar notch at impact), posterior cruciate ligament injuries,

fibula and femur fractures can also be observed. From all these studies, it appears that the main

challenge for improving leg protection should focus on knee ligament damage and failure

minimization. This challenge should not be managed separately as what’s happened on the hip joint

components. With various car designs, lower limb behaviour can differs according to the shape of the

car and the impacts position and modify knee joint loadings and the induced kinematics.

In the specific field of numerical simulation, multibody mathematical models were used in order

to provide an overall insight to the kinematics involved in pedestrian impact loading (Yang et al.,

1994, Wismans et al., 1980, Serre et al. 2005). In order to provide an accurate investigation of trauma

chronology during the impact, finite elements model has been design since the past 10 years. The first

models, designed by Bermond (1994) and Yang (1996) were dedicated to pedestrian loading cases

especially sub segment tests performed by Kajzer (1990 & 1993). The more recent lower limb FE


models (Schuster et al. (2000), Arnoux et al. (2002) and Beillas (2001), Chawla et al. (2004)) focused

on an accurate description of anatomical components that are involved in joint mechanics or are

injured during trauma situations. In these models, material properties were obtained from individual

tissue testing and try to integrate damage and failure of deformable bone and soft tissue structures.

These models are then validated against sub-segment and full scale post mortem human subject

(PMHS) in various loading configuration in order to evaluate mechanical behaviour of the whole

structure. Besides validating for overall impact response some of the more advanced models, as they

can record data not available experimentally, are capable of predicting injury mechanisms and thus

help define injury tolerances (Takahashi et al. (2003), Nagasaka et al., (2003), Chawla et al. (2004),

Arnoux et al. 2005). In Nagasaka pedestrian lower limb model, injuries are based on the height and

angle of the impact to include a wide range of vehicle geometry and impact configuration. Based on

similar objectives of understanding lower limb injury mechanisms and estimating injury thresholds we

have evaluated the ultimate bending angle and shear displacement threshold for ligament failure to be

15-200 and 13-15mm respectively on the base of available sub segment tests (Arnoux et al. 2005) and

Bose et al. (2006)). The LLMS model used for the study was developed by Arnoux et al. (2001) and

Beillas et al. (2001). An accurate geometric model of a 50th % adult male was reconstructed using MRI

measurements. The total number of elements was close to 35,000 with characteristic length chosen to

obtain initial time step ranging from 0.8 to 1ȝs. Material properties for each tissue component were

determined from previously published PMHS studies. To validate the material response of the model,

validation was performed at three levels: isolated tissue tests, sub-segment tests and finally entire

lower limb tests (Table 1)

Table 1 : Validation tests performed on LLMS model

Validation Tests Parameters

Soft Tissue Tensile Tests

Anterior Cruciate Ligament Force, Displacement

Posterior Cruciate Ligament Force, Displacement

Medial collateral Ligament Force, Displacement

Lateral collateral Ligament Force, Displacement

Patellar Tendon Stress, Strain

Long Bone Tests

Femur Bending (3-points) Force, Displacement

Tibial Lateral-Medial Bending Force

Tibial Antero-posterior Bending Force

Sub-Segment Loading

Patellar impact on flexed knee Force

Tibia impact on flexed knee Force

Anterior posterior flexed knee Force, Displacement

Quasi-static leg compression Force, Displacement

Quasi-static tibia/fibula comp. Force, Displacement

Ankle Inversion/Eversion/dorsiflexion Moment Rotation

Three point knee bending Moment Rotation

Four point knee bending Moment Rotation

Whole Lower Limb tests

Pedestrian lateral bending Force, Rotation

Pedestrian lateral shearing Force

Frontal Sled test Force

The present study aims at investigate these injury mechanisms regarding real full scale pedestrian

crash situations. In particular, for a defined car model, the question is related to the influence of impact

position on injury mechanisms and injury criteria definition on the knee joint. Additionally the

possible consequences at the hip joint level regarding kinematics and transmitted loads were

investigated. To achieve this, the injury prediction capability of LLMS-hybrid III were investigated

regarding a real pedestrian experimental tests performed in the laboratory. The injury mechanisms

evaluation was based on certain model parameters (e.g. local strain in soft tissues, Von Mises stress in

bones, internal energy) which cannot be measured in experimental tests. In particular, by assuming

that ligaments failure is related to strain level, the knee joint injury threshold is based on the

relationship between admissible strain levels in the ligaments and overall knee joint kinematics such as

lateral flexion and shearing, torsion effects.


INJURY MECHANISMS & CRITERIA EVALUATION IN PEDESTRIAN IMPACTS

THE LLMS MODEL FOR PEDESTRIAN used for this work was coupled to an Hybrid III 50

percentile rigid dummy model in order to take into account the effects of the whole human body

kinematics during the test. The details of LLMS model (from design to validation) were not reported

in this paper as they were already largely published (Arnoux 2001, 2004, 2005, Beillas 2001).

The coupling consisted in adding a part of the pelvis bone to the initial pelvis rigid body component.

The initial geometry of a hybrid pelvis was modified in order to ensure repositioning of the model up

to the standing position (cf. figure 1.). The upper proximal femur to the head of femur was considered

as a rigid body. The hip joint was then defined using a mathematical joint at the centre of the femoral

head. Rotations at the hip joint were defined using torque versus angle user functions in a local skew

system (cf. figure 1).

Fig. 1 – Overview of LLMS Hybrid 3 coupling on the pelvis segment

The model reference pedestrian impact test was relevant with experimental tests performed in the

context of a French APPA (“Amélioration de la protection des piétons lors de collisions avec des

automobiles”) PREDIT (“Programme de Recherche et d’inovation dans les transports terrestres”)

project. It consists in putting the model (50 percentile one as for experiment) in standing position in

front of a “Euroncap supermini (segment B)” class finite element car model. The car model was then

put in braking conditions (pitching angle ~ 2°) with an initial velocity of 10.88ms-1 and, according to

the acceleration recorded during experiments; we postulated a constant deceleration of 5.58 ms-2. As it

was performed during experiment, the pedestrian model and more particularly the impacted leg was

set into a light flexion at the same height of the knee joint in front of the car bumper (figure 2). The

analysis performed in this work focus only on lower limb behaviour for the first phase of the

pedestrian impact until first injuries appears on the leg (figure 2).

Fig. 2 –Illustration of LLMS Hybrid 3 kinematics in the first phase of the impact and Von Mises

curve on bones components

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THE INJURY MECHANISMS EVALUATION was based on same methodology as in previous work

(Arnoux et al., 2004).Although the material definition of the FE knee model did not include failure

behaviour, however specific parameters like ligament strain bone stress were recorded in balance to

joint kinematics to estimate failure and injury mechanisms timings. To measure ligament strain in the

model, several uni-axial low stiffness springs (length approximately equal to twice element

dimension) were assembled in series along the main axis of each ligament and pasted to ligament

structure. During post processing individual spring extension was recorded and computed to provide

strain level by comparison to initial length of each spring. Then the global strain for each ligament

(computed as the mean strain level along the ligament) was used to illustrate strain levels in the

ligament. In the present work, the failure threshold values (Table 2) were compared to the global strain

level. The maximum curve of local strain level was obtained by defining the curve of maximum of

local strain. It was used as an additional indication to the structure strain field (homogenous for lateral

ligaments whereas strong dispersions appeared for cruciates). High local strain level could be

understood as additional information to determine the failure risk and location.

In previous studies (Arnoux et al. 2004, 2005), injury criteria were obtained through numerical

investigation on sub segment tests. Once the model is assumed to be validated, the next step of this

approach was to record not experimentally available data such as ligament strain level. Assuming that

ultimate strain is a failure criteria for ligament, the injury criteria previously investigated was based on

the relationship between knee ligaments strain level and knee joint kinematics. This previous work led

to postulate on an injury criteria in pure shearing (13-15mm) and in pure lateral flexion (15-20°).

Table 2: Tissue failure threshold values used for predicting model failure (Arnoux et al., 2005)

Failure Threshold ValuesLigaments Strain

Collateral MCL, LCL 28%

Cruciate ACL, PCL 22%

Bones Ultimate Stress

Femur 125 MPa

Tibia Metaphysis 130MPa

Epiphysis 110MPa

Patella 125MPa

Fibula Head 125MPa

Diaphysis 100MPa

For this test, the collateral medial ligament and posterior cruciate ligament were both injured in

the first phase of the impact with an approximated time for failure at around 9-10ms (figure 3). A

complementary analysis of strain distribution along the ligaments showed very high strain level at

ligament insertion which let us to postulate on a potential failure at ligament insertions

Fig. 3 –knee ligaments (maximum and total) strain level in the first phase of the impact

From von Mises distribution, damage and failure on bones occurs on the proximal fibula due to the

contact with the bumper (figure 4). No femur or tibia failure was reported, but Von Mises distribution

Total Strain level in S3-PED2B

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was located on distal femur metaphysic and proximal tibia metaphysis with maximum amplitude

around 100-120MPa.

With the analysis of rotation and lateral shearing of the knee, we observed lateral rotation effects

(around 12-15°) but a very small lateral shearing (lower than 5 mm) (figure 5). An additional

significant translation along the limb axes was observed and reached 10mm. Note that in this case,

torsion effects (reaching 5°at the first injury time) were observed. This phenomenon also observed for

sub segment was described as a natural safety countermeasure for ligaments recruitment in the knee

joint.

Fig. 4 –Von Mises stress distribution (MPa) on tibia fibula with failure location on fibular

component

Fig. 5 –Knee joint rotations and shearing level (Y for lateral shearing).

In previous work, the knee joint injury criteria were postulated on the bases of Kajzer and Bose

experimental tests. The results obtained with real car impact show that the ligaments injury seems to

be related to the combination of the two classical lateral shearing and flexion, and a new one

(regarding numerical simulation) which is the stretching on the knee joint along the leg axes. The

shearing and lateral flexion levels obtained are below already defined injury thresholds. These results

lead us to assume that the injury criteria should be improved by defining a relation between shearing,

bending (and also by stretching effects).

INFLUENCE OF IMPACT POSITION

Due to knee joint mechanics, the impact position in pedestrian is assumed to be determinant to the

injury mechanisms observed and consequently the definition or relevance of injury criteria. In order to

investigate quantitatively this situation, several numerical simulations were performed with various

realistic impact position from the medial femur, femur metaphysic, distal femur metaphysic, centred

on knee joint, and lastly on proximal tibia metaphysic. This position were obtained through a vertical

positioning of the car model. For these various situation, the same methodology as described in

Knee shearing in C3-PED2B

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previous section was applied in order to evaluate injury mechanisms.

FOR THE UPPER FEMUR IMPACT, the front car bumper was set to the 2/3 inferior of the femur

(figure 6) which is 165mm above the reference impact position. The conditions of the impact car

velocity and deceleration are the same as those described in previous section. In the beginning of the

shocks (for the first 12ms), ligaments strain level recorded on lateral were largely below injury

threshold with strain level below 18%. For cruciate ligaments, maximum local strain curve show an

inhomogeneous strain field with a tibial ligament insertion highly recruited (it reach the ultimate

threshold) due to shearing effects. At the end of the first phase, after bone failure (see below), the knee

joint structure is highly recruited with and LCA potential failure.

Knee joint kinematics showed differences with the reference test. Whereas stretching effects

(Shear Z) were constant, the shearing effects were dominant in the first 10ms with level below the

13mm criteria value defined in previous works. On the opposite, once bone failure occurred, the

shearing effects decrease and lateral flexion reach 20° at the time of anterior cruciate ligament

potential failure.

Regarding Von Mises Stress curve, if stress concentration seems to be located on the same area as

for previous tests, due to front car shape, the femur is recruited as a three point flexion in the opposite

directions as in the reference test. This loading leads to bone failure at time closed to 11ms.

Fig. 6 – Four steps of lower limb kinematics and Von Mises stress iso curves on bones for the

pedestrian upper femur impact.

In the upper femur impact test, the knee joint kinematics seems to be a combination of dominant

lateral shearing coupled with lateral flexion until the bone failure which is first injury mechanisms

observed. Once bone failure occurred, the knee joint exhibit a dominant lateral flexion leading to

ligaments injury with amplitude of 20° which is relevant to previous criteria.

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Fig. 7 – Upper femur impact : Knee ligaments maximum and total strain level, Joint kinematics

with Shear Y for anterior posterior degree of freedoms, Shear Y for lateral shearing, Shear Z

for stretching effects on the leg

FOR DISTAL FEMUR IMPACT, the front car bumper was set in front of the femur distal metaphysic

which is 95mm above the reference impact position. Impact conditions are same as described in

previous tests. The strain curves analysis on ligaments indicates a potential failure of the medial

collateral ligament and posterior cruciate ligament (for time ranged from 12 to 14ms). Regarding knee

joint kinematics (figure 8 & 9), this potential failure was obtained for of coupled lateral flexion (~12 to

16°) and lateral shearing effect (~11 to 13mm). This result showed that combination of the two

mechanisms leading to ligament failure. Note that front car shape, in particular the lower bumper is

simultaneously in contact with lower tibia during the injury mechanisms which could limit the lateral

flexion level. For femur and tibia bones, the Von Mises curves showed same location with flexion

effects on femur metaphysic which lead to same failure situation as for the upper femur impact at

20ms (later than ligaments injuries).

Fig. 8 – Three steps of lower limb kinematics and Von Mises stress iso curves on bones for the

pedestrian distal femur impact.

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Fig. 9 – Distal femur impact : Knee ligaments maximum and total strain level, Joint kinematics



FOR KNEE JOINT IMPACT, the impact occurs on the middle of the knee joint (30mm above

reference impact tests) with both primary contact with tibia and femur bones and with same conditions

as previous tests (figure 10 & 11). Regarding strain level distribution, the posterior cruciate ligament

and medial collateral ligament seems to be injured in the first 7 to 9 ms and then anterior cruciate

ligaments. The maximum local strain recorded confirmed these assumptions. The knee joint

kinematics is similar to the previous test with a combination of lateral flexion (8-10°), lateral shearing

(10-11mm) and also stretching effects. The front car shape, in particular the lower bumper, is

simultaneously in contact with lower tibia during the injury mechanisms which could limit the lateral

flexion amplitude. This result indicates that injury criteria should be here a combination between

shearing and lateral flexion. Regarding bone structures, Von Mises curves reach values closed to

failure limits but didn’t fail.

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Fig. 10 – Knee joint impact : Knee ligaments maximum and total strain level, Joint kinematics





pedestrian knee joint impact.

FOR METAPHYSIS TIBIA IMPACT, the contact with front car occurred on the proximal tibia

metaphysic (figure 12) 45mm below reference impact position. The knee ligaments failure was

postulated firstly for the two cruciate ligaments at 8ms and then for the medial collateral ligament

closed to 10ms. Regarding knee joint kinematics (figure 12 & 13), lateral flexion effects is closed to 5°

whereas lateral shearing is dominant and reaches the failure criteria of 13mm. Von Mises stress curves

showed that bone structure were highly constraints with failure after ligaments injuries. This failure

was observed on the 1/3 of tibia bone and seems to be induced by tibia lateral flexion.


pedestrian metaphysic tibia impact.

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Fig. 13 – Metaphysis tibia impact : Knee ligaments maximum and total strain level, Joint

kinematics with Shear Y for anterior posterior degree of freedoms, Shear Y for lateral shearing,

Shear Z for stretching effects on the leg

DISCUSSION CONCLUSION

The Lower Limb Model for Safety has been designed to describe multiple loading configurations.

Once the model is assumed to be well validated, it can be used for predicting injury response for a

much wider range of loading parameters. The degree of uncertainty associated with identifying injury

events is reduced by defining injury thresholds based on specific parameters like ligament strain and

bone stress which can also related to joint kinematics. In previous works (Arnoux et al. 2004, Bose et

al. 2006), we had investigated the definition of a knee joint injury criteria for pedestrian impacts on the

base of Kajzer and Bose-Kerrigan previous works (Kajzer et al. 1990, 1993, Bose 2004, Kerrigan

2003). The first criteria, 13mm in lateral shearing and 18° in lateral flexion, was efficient for pure

shearing or lateral flexion but was not really studied for the combination of the two mechanisms. For

full scale pedestrian simulations, we observed that the two mechanisms occurred simultaneously with

various amplitudes according to the impact location (table 3).

Table 3: Injury mechanisms synthesis; The numbering (from 1 to 3) in colon was add in order to

describe the first mechanisms. The injury criteria column let to complete the injury criteria

definition

Impact location Injury mechanisms Joint injuries Injury criteria Bones injuriesTibia Prox Shearing dominant 1. The two cruciates

2. Medial collateral ligament

13 mm 3. Tibia

Knee Joint Shearing : 10-11mm

Flexion : 8-10° + Stretching

1. Posterior cruciate ligament

2. Medial collateral ligament

10° & 10mm

Femur Distal Metaphysis Flexion: 12-16°

Shearing : 11-13mm

1. Medial collateral2. Posterior cruciate ligament

14° & 11mm 3. Femur

Upper Femur

1. Shearing dominant

2. Flexion dominant 3. Medial collateral ligament 20°

1. Femur

From these results, as impact occurred on leg, the shearing is dominant with a 13mm lateral

shearing injury criteria. If impact occurs on knee joint, there is a combination of lateral shearing and

flexion. On the opposite, an impact located on femur induces a dominant shearing with an associated

minor lateral flexion. Consequently, we can improve injury criteria by adding the coupled 14° lateral

flexion and 11mm of shearing in the definition of knee joint injury criteria. Notes these results are

directly related to the front car geometry studied in this work which induce a limitation of lateral

flexion effects for impact on knee joint. In a next step, they should be confirmed for various front car

geometries. Moreover, the knee gets impacted with structures of different stiffness (due to various

positions). This point need to be also investigated in order to evaluate the potential influence of the

local car component stiffness on knee joint injury mechanisms.

Regarding the injury risk for knee joint, more the impact location was high, more the ligaments


injuries occurred later. On the contrary this trend increases the bone failure risk with a three point

failure mode for the femur. Hence, if high impact location seems to minimize (or add delay) on knee

joint loading and its potential injury. The consequences for hip joint and consequently pelvis seem to

follow the same trends i. e. increase of transmitted force and kinematics with height of impact

location. This point should be carefully investigated in further works. Then, regarding safety

regulation directive, the future choices for knee joint impact location should be linked to those for

pelvis impact. If the impact location is centred on knee joint or on tibia, we can assume that needs to

investigate pelvis behaviour regarding first impact phase are minor. On the contrary, for impact

occurring on femur component or upper, it could be essential to investigate pelvis behaviour.

The results obtained in this work show that stretching effects occurred during some tests which

leads to increase strain levels on ligaments (especially with cruciates ones). It is therefore difficult to

conclude on the relevance of these effects. Passive and Active muscle contribution is also expected to

affect the knee joint stability in emergency pedestrian-car impact situations and its implementation to

numerical models such as this should be investigated (only passive behaviour of the muscle groups is

described). The further tests will be performed on model version including active or passive muscle

behaviour (Behr et al. 2006).

The LLMS model presents also limitations in terms of descriptions of mechanical properties of

biological tissues. More particularly, the damage and unsymmetrical behaviour laws were not included

in the model at this stage, which limits the model validity to the first ligament failure. These first

results have to be improved with further developments, including damage and failure properties. The

questions of model sensitivity need to be further investigated (through mesh influence, ligaments and

bone ultimate thresholds and also main mechanical parameters such as Young modulus). Current work

in progress will lead to improve model definition and will let to confirm and to describe more

accurately, mechanisms investigated in this work.

Acknowledgment

We would like to acknowledge the technical staff of the laboratory which were in charge of experiments

essential to the validation and evaluation of the numerical model. We would also acknowledge Mecalog for their

close collaboration as co-designer of the lower limb model. Lastly we would like to acknowledge all partners of

the PREDIT APPA project (Mecalog, Faurecia, INRETS MA, INRETS UMRETTE, UTAC and LAB PSA-

Renault) for their contribution especially providing the numerical model of the car front.

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Lower Limb Model for safety: application to lateral impacts, International Radioss user Conference.

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