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PREDICTING THE LOAD TRANSFERRED TO THE CLAVICLE FROM STRAIN DATA DURING A SHOULDER LATERAL IMPACT –
PRELIMINARY RESULTS
Sonia DUPREY, Damien SUBIT, Hervé GUILLEMOT, Richard KENT University of Virginia, Centre for Applied Biomechanics, Charlottesville VA, USA
ABSTRACT
QUANTIFYING THE LOAD transferred to the clavicle when the shoulder is laterally impacted will assist automotive safety system designers interested in load paths during side impacts. This paper presents two non-invasive methodologies to estimate the force transferred to the clavicle by using multiple strain gage measurements taken in situ during a controlled shoulder impact.
The first methodology involved beam theory application to estimate compressive force from the strains measured in situ. The second methodology involved an additional set of bench-top experiments to reproduce the in situ strain distribution in vitro and thereby relate the measured strain to the applied force. Keywords : Side Impact, Shoulder.
INJURIES TO THE TRUNK account for the bulk of serious injuries in vehicle side impacts (Haland et al., 1993). The shoulder is an important load path in lateral vehicle impacts since it can off-load the more vulnerable thoraco-abdominal structures and organs (Melvin et al., 1998). Determining the in situ load in the clavicle during full scale side impact is a challenge and would help defining countermeasures (side impact air bags, padding, etc.) that increase this off-loading while maintaining a non-injurious load on the shoulder.
Previous studies have investigated either the clavicle response (Kemper et al., 2006, Harnroongroj et al., 2000; Duprey et al., 2008) or the shoulder response to lateral impacts on post mortem human subjects (PMHS) (Bolte et al., 2000, Compigne et al., 2004; Koh et al., 2005). However, we are aware of no study that has specifically focused on the response of the clavicle in situ.
The use of a load cell for in situ measurements on the clavicle of PMHS would significantly alter the shoulder structure and modify the load path, whereas several strain gages can be placed on the clavicle with minimal disruption of the load-bearing structures.
For long bones, linear beam theory has been successfully applied to characterize the strain field in a cross-section and to estimate the force applied to the bone from strain measurements (Rybicki et al., 1977; Funk et al., 2006). While the long bones are almost straight (tibia, femur), the clavicle geometry presents two curves forming an S-shape (Fig. 1).
The purpose of this study is to develop a minimally invasive methodology to estimate the load transmitted into the clavicle during shoulder lateral impacts. The current study investigates two methods.
METHODOLOGY TWO METHODOLOGIES WERE evaluated. The first method was based on the assumption that
the straight-beam theory applies to the clavicle. This method is therefore called “Beam theory method”. Compression tests were performed on nine isolated clavicles at quasi-static rates. Three strain gages were pasted around the most anterior cross-section of the bone. The straight-beam theory was applied to apportion the internal compression and bending and to determine the force transferred through the clavicle. The calculated force was then compared to the force experimentally applied.
The second method consisted of two steps. First, in situ strains in five clavicles were measured at six locations in full-scale impacts to the shoulder. Next, these clavicles were harvested and tested
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under loading conditions that replicated those measured during the full-scale impacts. The load transferred to the clavicle was then estimated by matching the strains measured in the isolated clavicle with the strains measured in the full-scale impacts. This method is called “Reproduction of in situ strains”.
These two methodologies are described in detail in the following subsections. FIRST METHOD – BEAM THEORY METHOD : Nine clavicles were harvested from six subjects
(four males, two females, average age: 62 years old) in accordance with ethical guidelines approved by a “University of Virginia – Center for Applied Biomechanics” oversight committee. The depths of the medial and lateral curves of the bones were measured (Fig. 1) on the 3D clavicle geometries reconstructed from Computed Tomography (CT) images.
Axis going through the centers of the clavicle extremities
l
L L = Depth of the medial curve l = Depth of the lateral curve
Fig. 1 - Geometric parameters of the test clavicles measured in the clavicle transversal plane
Bone extremities were potted in cylindrical cups with FastCast® resin (GoldenWest Inc., USA).
Unidirectional strain gages (Vishay measurements, CEA-06-062-UW-350) were pasted on the bones around the cross-section at the level of the maximum of the posterior concavity in order to measure longitudinal strains. This location was chosen because (1) it can be reliably identified on every clavicle, (2) it would not require cutting any muscles when performing in situ tests, and (3) the cross section is perpendicular to the axis going through the center of the clavicle extremities.
The specimens were mounted on a linear stage machine which imposed displacements in the clavicle transversal plane at a quasi static rate (0.63mm/s) to the lateral extremity of the bone (Fig. 2). The axial force generated by the imposed displacement was measured by a load cell. The clavicles were loaded up to 400 N, which is below the failure force (Duprey et al, 2008).
6 axis load cell
X
YZ
Strain gages
Free rotation around Y
Displacement with different angles
Fig. 2 - Test set-up – First methodology
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Geometric properties of the cross-section of interest (area A and centroid coordinates) were obtained from the CT images. Using the same method as Funk et al. (2006) and Kam (2007), the neutral axis was obtained by fitting a line through the points of zero strain.
The strain recorded by each gage was decomposed into the component due to axial loading (εaxial) and the component due to bending (εbend). εaxial is by definition equal at all points in the cross-section. Furthermore, in pure bending the neutral axis passes through the centroid of the cross-section. Thus, the level of strain that, when subtracted from each measured strain, caused the recalculated neutral axis to pass through the centroid was defined as the strain component due to pure axial loading εaxial. The axial force, Faxial, was calculated from the axial strain obtained at a force level of 400 N, for each tested clavicle:
AEF axialaxial ⋅⋅= ε The relationships between the force applied in the Z direction and the measured surface strains
were observed to be very close to linear in all tests and for all strain gages. Thus, it was assumed that the analysis results were independent of force magnitude. The analysis was therefore done at a single force level of 400 N in all tests, which is approximately 90% of the maximum of the applied load.
The Young modulus was taken from Iwamoto et al. (2000) and an uncertainty interval of 25 % was considered: E=11 000 ± 2750 MPa. Since the cross section equipped with gages is perpendicular to the loading vector, it is assumed that the axial force should match 400N. The error between the expected 400N and the calculated axial force was investigated in order to assess the method.
SECOND METHOD – REPRODUCTION OF IN SITU STRAINS : First, the shoulders of three
male subjects (Table 1) were impacted at the humeral head level (a detailed description of the tests is available in “Response of the torso in side and oblique impacts under constant velocity impacts” by Subit et al., 2009). The subjects were kept seated without a backrest by means of cables and were released just at the impact moment. These tests were non-injurious and six strain gages, positioned on the clavicle of the impacted shoulder (see Fig. 3), documented the strain distribution in the clavicle.
Table 1. Description of the tests and test subjects
Subjects Age Weight Size Shoulder impacted
Shoulder Impact velocity
Shoulder Impact
direction
Isolated clavicle test velocity
Left 1 m/s 0º 85 mm/s 427 79 y 79 kg 181 cm Left 3 m/s 0º 170 mm/s Left 3 m/s +15º 150 mm/s 430 74 y 47 kg 173 cm Right 3 m/s 0º 300 mm/s Left 3 m/s -15º 200 mm/s 420 59 y 93 kg 180 cm Right 3m/s +15º 200 mm/s
Positioning of the gages on the clavicle in situ After the preparation Fig. 3 - Preparation of a PMHS shoulder for the impact tests
Then, the clavicles were harvested from the test subjects without removing the strain gages used during the full scale lateral tests. The remaining soft tissues were removed and the clavicle extremities
IRCOBI Conference - York (UK) - September 2009 87
were potted in a square-shaped aluminum mold with a resin (FastCast®, GoldenWest Inc., USA). Just before the tests on the isolated clavicles, the previous strain gages were finally removed and new uniaxial strain gages were fixed at the exact same locations (See Fig. B1 in Appendix B).
The clavicle was then positioned in the testing fixture (see Fig. 4). During this positioning procedure, the strain gage readings were monitored to make sure that the strain levels did not change beyond the sensor noise levels, i.e. that no initial strain was introduced onto the clavicles during this adjustment procedure. A ramp displacement was applied at a rate that generated strain rates matching the ones reached during the in situ tests. The target peak force was set under the injurious threshold (2000N) in order to prevent any fractures.
The strain results were compared with the data from the lateral impact that was performed previously. This comparison allowed for an estimation of the time at which the strain in the in vitro test matched the maximum strain during the in situ test. The corresponding force, i.e. the force that generated this strain level, was then taken from the force-time history curve. This force was considered to be the force that was transferred to the clavicle during the full scale shoulder lateral impact, since it created a similar strain distribution (See Appendix A figures). This was performed for each of the six gages, when possible. Gage concurrence, i.e. the fact that several gages did not provide comparable transferred force estimates, was also investigated in order to better understand the methodological challenges of this procedure.
Fig. 4 - Test set-up – Second methodology
RESULTS
FIRST METHOD – BEAM THEORY METHOD : In order to study the range of validity of the
strain based approach presented here, geometrical parameters were measured on the clavicles, and links between these parameters and the accuracy of the calculated load were investigated.
The depths of the medial curve (L) and lateral curve (l) measured on the specimens (Fig. 5) are consistent with the results of Andermahr et al. (2006). The clavicles were divided into two groups for further analysis: 1) the clavicles #411 (male), #217 (male), #323 (male) which have a medial curve (L) deeper than the average reported by Andermahr et al. (2006), and 2) the clavicles #229 (female), #218 (male), #234 (female) which present a smaller medial curve.
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Geometric parameters
364
217
411
229218
323
4
6
8
10
12
14
16
12 14 16 18 20 22Depth of the medial curve - L (mm)
Dep
th o
f the
late
ral c
urve
- l
(mm
)
Female clavicleMale clavicleMean (Andermahr et al)
l
L
Fig. 5 - Geometric parameters of the test clavicles
The calculated compression strains and axial forces Faxial are listed in Table 2. Fig. 6 shows their
distribution and illustrates also the expected 400N.
Table 2. Axial strain and force results for the first methodology (the force interval results from the 25% interval of uncertainty on the Young’s Modulus)
217R 218R 218L 229R 229L 323R 323L 364R 411L εaxial (μstr) 243 331 396 715 464 146 223 455 100 Faxial (N) 182±46 354±88 424±106 518±130 336±84 203±51 310±77 324±81 120±30
Fig. 6 - Calculated force for each test clavicle. The error bars result from the 25% interval of
uncertainty on the Young’s Modulus
The method did not provide acceptable results for the clavicles in the group 1: the calculated force was smaller than the 400N reference value. Mann-Whitney U tests performed on the force values derived from E=11000MPa showed that clavicles from the group 1 have a significantly higher error (p<0.05) for the calculated axial force. The averages and standard deviations of this error are:
− 49±19.7%, when considering the clavicles in the group 1, − 16.3±8.8%, when considering the clavicles in the group 2.
The global average and standard deviation of the error between the expected and the calculated axial force is 30.5±22.5%. The large variability of the results and the large error, which seem to depend on the clavicle geometry made this method not reliable enough.
SECOND METHOD – REPRODUCTION OF IN SITU STRAINS : Table 3 lists the average times
and estimated forces transferred to the clavicle for each test. Some gages did not allowed to determine the time of the clavicle test when the strain is at the same level as the maximum strain of the shoulder impact (See Appendix A figures that illustrate the strain-time history curves from the in vitro and in situ tests). This happened:
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- when one test (in vitro or in situ) provided compression while the other test provided tension; - when the in situ tests provided a complex strain signal, i.e. compression (or tension) and then
tension (or compression); - when the strain-time history did not provide a clear maximum value. In Table 3, the number of gages used to estimate the transferred force is specified, as well as the
variability of the results. Gage concurrence generates a large variability of the results (up to 69%) and only few gages from the six strain gages allowed a force estimation. The causes of these phenomena are discussed in the next section.
Table 3. Force results for the second methodology (‘Avg ± StD’ means Average ± Standard Deviation)
Test Gage numbers
Time (Avg ± StD)
Estimated transferred force (Avg ± StD)
Result variability (StD/Avg * 100)
427L, 1 m/s, 0º 4 11.1±3.5 ms 351±101 N 29 % 427L, 3 m/s, 0º 4 14.1±4.3 ms 788±258 N 32 %
430L, 3 m/s, +15º 2 11.6±6.3 ms 959±661 N 69 % 430R, 3 m/s, 0º 1 7.2 ms 552 N -
420L, 3 m/s, -15º 1 8.2 ms 614 N - 420R, 3 m/s, +15º 3 4.4±0.7 ms 310±92 N 30 %
Avg ± StD 40 % DISCUSSION
This study is a first step towards a final methodology to predict from in situ strains the force transferred to the clavicle during a shoulder impact.
The S-shape geometry of the bone does not allow the application of a methodology based on the beam theory as shown by the assessment of the first method.
The second methodology allowed an estimation of the force transmitted to the clavicle by replicating the in situ loads from full-scale side impacts. However few gages were exploitable and the gage concurrence provided a high result variability. These two issues illustrate the disagreement between the in vitro boundary conditions and the in situ boundary conditions. Since the subjects were released at the moment of the impact, the thorax could move freely after the impact. Furthermore, the impacted shoulder could “escape” the impactor by either moving backward, having the scapula translating on the thorax, or moving forward and thus loading the clavicle. This may result in a complex clavicle loading direction and complicated joint behaviors; whereas in vitro, the loading was uniaxial and the sternoclavicular joint was modeled as a pin joint.
The motion of the impacted shoulder bones during the full-scale tests were reconstructed, making the loading 3D displacement available. However reproducing a complex dynamic 3D displacement for the in vitro tests may not be feasible. A possibility could be to perform the full-scale tests without releasing the subjects or by releasing them only in one direction. This could provide boundary conditions easier to replicate. Another solution could be the use of clavicle subject-specific finite element models: finite element softwares would allow to simulate a complex loading pattern and would allow to study the strain distribution in the clavicle.
Furthermore, the fact that few gages were usable may be due to the locations of the gages. Indeed, the preparation of the isolated clavicles included their embedment into molds with a compound resin. Some of the six gages positioned on the bone were very close to the molds. Thus, their behaviors were not expected to be correct as the boundary conditions were very different from the in situ boundary conditions. This partly explains the fact that some of the six gages could not be used for the force estimation whereas a proper signal was recorded.
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CONCLUSION This study points out the issues encountered to estimate the force transferred to the clavicle from in
situ strains during a shoulder impact. Further work is still needed to better understand the in situ boundary conditions and how they should be replicated to reproduce the complex strain patterns generated in situ.
ACKNOWLEDGEMENTS This study was sponsored by Toyota Motor Company, though the conclusions are those of the
authors alone. We would like to thank the technical team at University of Virginia Center for Applied Biomechanics for their support in these tests. REFERENCES Andermahr J., Jubel A., Elsner A., Johann J., Prokop A., Rehm K.E., and Koebke J., 2007, Anatomy
of the Clavicle and the Intramedullary Nailing of Midclavicular Fractures, Clin Anatomy, 20, pp.48-56.
Bolte IV J.H., Hines M.H., McFadden J.D., and Saul R.A., 2000, Shoulder Response Characteristics and Injury due to Lateral Glenohumeral Joint Impacts, STAPP Car Crash Journal, 44, pp.261-280.
Compigne S., Caire Y., Quesnel T., and Verriest J-P., 2004, Non-injurious and Injurious Impact Response of the Human Shoulder - Three Dimensional Analysis of Kinematics and Determination of Injury Threshold, STAPP Car Crash Journal, 48, pp.89-125.
Duprey S., Bruyere K., and Verriest J-P., 2008, Influence of Geometrical Personalization on the Simulation of Clavicle Fractures, J Biomech, 41(1), pp.200-207.
Funk R., and Crandall J., 2006, Calculation of Tibial Loading using Strain Gages, Proc. Rocky Mountain Bioengineering Symposium & International ISA Biomedical Sciences Instrumentation, pp.160-165.
Haland Y., Lovsund P., and Nygren A., 1993, Life-threatening and Disabling Injuries in Car-to-Car Side Impacts - Implications for Development of Protection Systems, Accid. Anal. Prev., 25, pp.199-205.
Harnroongroj T., Tantikul C., and Keatkor S., 2000, The Clavicular Fracture: a Biomechanical Study of the Mechanism of Clavicular Fracture and Modes of the Fracture, J Med Assoc Thai, 83, pp.663-667.
Iwamoto M., Miki K., Mohammad M., Nayef A., Yang K.H., Begeman P.C., and King A.I., 2000, Development of a Finite Element Model of the Human Shoulder, STAPP Car Crash Journal, 40, pp. 281-297.
Kam C., 2007, Three-dimensional strain assessment of pedestrian lower extremities during a vehicle collision, MS Thesis, University of Virginia, Charlottesville.
Kemper A., Stitzel J., Gabler C., and Duma S., 2006, Biomechanical Response of the Human Clavicle subjected to Dynamic Bending, Proc. Rocky Mountain Bioengineering Symposium & international ISA Biomedical Sciences Instrumentation Symposium, pp.231-236.
Koh S.W., Cavanaugh J.M., Mason M.J, Peterson S.A., Marth D.R., Rouhana S.W., and Bolte IV J.H., 2005, Shoulder Injury and Response due to Lateral Glenohumeral Joint Impact: an Analysis of Combined Data, STAPP Car Crash Journal, 49, pp.291-322.
Melvin J., Baron K., Little W., Gideon T.W., and Pierce J., 1998, Biomechanical analysis of Indy race car crashes, STAPP Car Crash Conference, 42, pp.337-359.
Rybicki E.F., Mills E.J., Turner A.S., Simonen F.A., 1977, In Vivo and Analytical Studies of Forces and Moments in Equine Long Bones, J Biomech, 10, pp.701-705.
Subit D., Duprey S., Guillemot H., Lessley D., Lau S., Kent R.W., Response of the torso in side and oblique impacts under constant velocity impacts, Article submitted to Traffic Injury Prevention.
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APPENDIX A. SECOND METHOD - STRAIN-TIME HISTORIES
Gage 6
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Fig. A1: Strain-time history for the clavicle test 427 L, 1m/s, 0° (Black curves = isolated
clavicle tests; gray curves = full scale shoulder tests. Dashed lines illustrate the process of determining the time in the clavicle test corresponding to the strain peak in the full scale tests)
Gage 1
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Fig. A2: Strain-time history for the clavicle test 427 L, 3m/s, 0° (Same legend as Fig. A1)
92 IRCOBI Conference - York (UK) - September 2009
Gage 1
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Fig. A3: Strain-time history for the clavicle test 430 L, 3m/s, +15° (Same legend as Fig. A1)
Gage 1
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Fig. A4: Strain-time history for the clavicle test 430 R, 3m/s, 0° (Same legend as Fig. A1)
Gage 1
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0 10 20 30 40Time (ms)
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Fig. A5: Strain-time history for the clavicle test 420 L, 3m/s, -15° (Same legend as Fig. A1)
IRCOBI Conference - York (UK) - September 2009 93
Gage 1
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Fig. A6: Strain-time history for the clavicle test 420 R, 3m/s, +15° (Same legend as Fig. A1)
94 IRCOBI Conference - York (UK) - September 2009
APPENDIX B. SECOND METHOD – GAGE LOCATIONS
427 L
430 L
430 R
420 R
420 L
Fig. B1: Second methodology – Location of the strain gages: superior view of the clavicles
(N.B. Clavicle 420R: the antero-superior view is illustrated, the fracture shown here did not happen during the tests studied here)
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