Scaled Staging

To eliminate variation in the test data due to differences in specimen anatomy, the calculated acceleration data were scaled to the 50^th percentile army male using the equal stress equal velocity technique reported by (Eppinger et al., 1984). This method normalizes the response data of the test while maintaining the original loading velocity. The acceleration calculated data for each region were scaled based on the corresponding reference mass obtained from the ANSUR II

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pilot study (Paquette et al., 2009). The normalization factor was determined using the following equation:

Where λm – scale factor, M ref- mass of 50th percentile army male, M i – specimen segment mass

The test subject segment masses were measured using the “CT Image Based Mass Calculation Technique”, which is discussed in the section (4.8.4.1). Assuming that the elastic modulus and mass density are the same between test subjects, the acceleration and time normalization equations are as follows:

Acceleration: A _a = λm^-1 A _b

Time: T a = λm T b

Where subscript ‘a’ indicates normalized data and subscript ‘b’ indicates calculated sensor data.

In addition to acceleration data, the knee and shoulder motion processed data were also normalized. For both motion data, instead of mass ratio, length based normalization technique was used for determining parameter. The parameter for the knee and shoulder were calculated using the femur length and specimen stature height respectively. The reference length were obtained from the ANSUR II pilot study (Paquette et al., 2009).The length scale factor was determined using the following equation:

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Where λ l – scale factor, L ref- length of mass of 50th percentile army male, M i – specimen length

The specimen stature height and the femur length were obtained from the anthropometry measurement. The length and time normalization equations are as follows:

Length: L a = λl L b

Time: T _a = λl T _b

Where subscript ‘a’ indicates the normalized data and subscript ‘b’ indicates the processed kinematic data obtained by tracking the specified target marker on the specimen. The scale factor for both tests are shown in Appendix C

1.18.4.1 CT Image Based Mass Calculation Technique

Acceleration response data are one of the key measurements in the evaluation of biomechanical response due to UBB loading. In general, acceleration is inversely proportional to mass.

Therefore, more mass will attenuate the acceleration impulse transmitted to the body during the impact. Thus, to understand the load distribution through each body region, each segment of mass must be calculated separately. The segmentation plane discussed by McConville at al. was referenced to identify the planes for segmenting each anatomical component (McConville et al., 1980). The following is a brief overview of the anatomical plane implemented for segmentation.

 Head – a plane through the Atlanto-occipital joint that extends along the inferior border of the mandible.

 Knee plane – a transverse plane inferior to the patella that passes through the femoral-tibia joint with the leg extended.

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 Hip plane- the segmentation plane at the hip which starts above the femur head and extends obliquely along the acetabulum rim.

Pre-instrumentation CT scans were used for measuring the segment mass. If the subjects were not positioned ideally on the CT table during the scanning, the orientation of the segmentation planes was affected. Therefore, after segmenting the component, the unwanted segment regions or debris were removed. The protocol to measure segment mass from CT was referenced from (Heymsfield et al., 1979). The tissue threshold levels and the segmented mass calculation procedure are described as follows:

Threshold: MIMICs Version 15 (Materialise, MI) was used for measuring the segment mass.

The WIAMan Scaling Working Group recommended that the Hounsfield unit values for the soft tissues fall in the range of 524 to 1579 HU, while the bone Hounsfield units should measure above 1579 HU. However, in MIMICs the Dicom translation uses (-1023 HU) as the minimum threshold value. Thus, to get a similar threshold value in MIMICs, 1023 HU were subtracted from the recommended threshold values. The program recommended and the MIMICs calculated threshold value for the bone and the soft tissue are reported in Table 4-9.

Table 0-9: Threshold value used for CT mass measurement.

Threshold HU Soft tissue range Bone range WIAMan program recommended 524 ≤ T ≤ 1579 1579 ≤ T

MIMICs calculated -499 ≤ T ≤ 556 556 ≤ T

Mass measurement protocol:

A) CT Dicom images were read into MIMIC software B) Bone and soft tissue threshold values were set

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C) Based on the threshold value, separate 2D masks were created for bone and soft tissue for the desired anatomical regions.

D) Unwanted tissue regions were removed using the edit mask option E) A 3D model was created using the property option

F) The default volume for the 3D model was set to cm³

G) The segment mass was predicted as a product of calculated volume and the pre-defined tissue density (Bone = 1 g/cm³ and Soft tissue = 1.92 g/cm³).

With the normalizing of accelereation and the knee and shoulder motion data, the data processing for the vertical testing came to an end. Appendix B provides a summary of the data processing performed in each stages.

For each stage, a separate readme (.txt file) was created, which included the channel processed in that stage, engineering units, sensor ID and the corresponding filter used. These readme files give a brief overview of each stage. Followed by data processing, the data from all the stages were plotted using DIADEM software to analysis and check the quality of the processed data.

The response data with the indication of fracture were excluded from further analysis and were labeled as bad channels in the corresponding (.TVS format) channel file and in the readme file of all the stages.

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