Longitudinal Tibia Stress Fracture Risk During High-Volume Training: A Multiscale Modeling Pipeline Incorporating Bone Remodeling.

Tibia stress fractures are prevalent during high-intensity training, yet a mechanistic model linking longitudinal training intensity, bone health, and long-term injury risk has yet to be demonstrated. The objective of this study was to develop and validate a multiscale model of gross and tissue level loading on the tibia including bone remodeling on a timescale of week. Peak tensile tibial strain (3517 µstrain) during 4 m/s running was below injury thresholds, and the peak anteromedial tibial strain (1248 µstrain) was 0.17 standard deviations away from the mean of reported literature values. An initial study isolated the effects of cortical density and stiffness on tibial strain during a simulated eight week training period. Tibial strains and cortical microcracking correlated with initial cortical modulus, with all simulations presenting peak anteromedial tensile strains (1047-1600 µstrain) near day 11. Average cortical densities decreased by 7-8% of their nominal value by day 11, but the overall density change was <2% by the end of the simulated training period, in line with reported results. This study demonstrates the benefits of multiscale models for investigating stress fracture risk and indicates that peak tibial strain, and thus injury risk, may increase early in a high intensity training program. Future studies could optimize training volume and recovery time to reduce injury risk during the most vulnerable training periods.

Whole Body PMHS Response in Injurious Experimental Accelerative Loading Events.

Previous studies involving whole-body post-mortem human surrogates (PHMS) have generated biomechanical response specifications for physically simulated accelerative loading intended to reproduce seat and floor velocity histories occurring in under-body blast (UBB) events (e.g.,. References 10, 11, 21 These previous studies employed loading conditions that only rarely produced injuries to the foot/ankle and pelvis, which are body regions of interest for injury assessment in staged UBB testing using anthropomorphic test devices. To investigate more injurious whole-body conditions, three series of tests were conducted with PMHS that were equipped with military personal protective equipment and seated in an upright posture. These tests used higher velocity and shorter duration floor and seat inputs than were previously used with the goal of producing pelvis and foot/ankle fractures. A total of nine PMHS that were approximately midsize in stature and mass were equally allocated across three loading conditions, including a 15.5 m/s, 2.5 ms time-to-peak (TTP) floor velocity pulse with a 10 m/s, 7.5 ms TTP seat pulse; a 13 m/s, 2.5 ms TTP floor pulse with a 9.0 m/s, 5 ms TTP seat pulse; and a 10 m/s, 2.5 ms TTP floor pulse with a 6.5 m/s, 7.5 ms TTP seat pulse. In the first two conditions, the seat was padded with a ~ 120-mm-thick foam cushion to elongate the pulse experienced by the PMHS. Of the nine PMHS tests, five resulted in pelvic ring fractures, five resulted in a total of eight foot/ankle fractures (i.e., two unilateral and three bilateral fractures), and one produced a femur fracture. Test results were used to develop corridors describing the variability in kinematics and in forces applied to the feet, forces applied to the pelvis and buttocks in rigid seat tests, and in forces applied to the seat foam in padded seat tests. These corridors and the body-region specific injury/no-injury response data can be used to assess the performance and predictive capability of anthropomorphic test devices and computational models used as human surrogates in simulated UBB testing.

Severe Calcaneus Injury Probability Curves Due to Under-Body Blast.

The lower extremity is the most frequently injured body region to mounted soldiers during underbody blast (UBB) events. UBB events often produce large deformations of the floor and subsequent acceleration of the lower limb that are not sufficiently mitigated by the combat boot, leaving the calcaneus bone vulnerable to injury. Biomechanical experiments simulating UBB loading scenarios were conducted in a laboratory environment using isolated postmortem human subject (PMHS) leg components. Each leg component was tested twice: one sub-injurious test followed by a injury-targeted test. This enabled the use of interval censoring for each specimen in the survival statistical analysis to generate the human injury probability curves (HIPCs). Foot contact forces were measured in both the hindfoot and forefoot. Strains and acoustic emission signals at the calcaneus and distal tibia were utilized to determine injury timing. The footplate velocities of the injury tests ranged 8-13 m/s with time-to-peak velocity of 1.8-2.5 ms while the velocities of non-injury tests ranged from 4 to 6 m/s with the same time-to-peak. The majority of the injuries were severe calcaneus fractures (Sanders III-IV). Secondary injuries included fractures to the distal tibia, talus, cuboid and cuneiform. These injury outcomes were found to be consistent with those reported in UBB injury literature. The HIPCs for the severe calcaneus fracture were developed using the vertical heel contact force as the injury correlation measure through survival analysis statistical method in the form of lognormal function. This work represents the first set of HIPCs dedicated to the severe calcaneus fracture using the biomechanical force measurement closest to the injury location. This injury probability curve will enable biomechanical response validation of computational models, development of ATD injury assessment reference curve, and injury prediction capability for computational models or ATDs in the UBB environment.

Evaluation of the Whole Body Spine Response to Sub-Injurious Vertical Loading.

It is critical to understand the relationship between under-body blast (UBB) loading and occupant response to provide optimal protection to the warfighter from serious injuries, many of which affect the spine. Previous studies have examined component and whole body response to accelerative based UBB loading. While these studies both informed injury prediction efforts and examined the shortcomings of traditional anthropomorphic test devices in the evaluation of human injury, few studies provide response data against which future models could be compared and evaluated. The current study examines four different loading conditions on a seated occupant that demonstrate the effects of changes in the floor, seat, personal protective equipment (PPE), and reclined posture on whole body post-mortem human surrogate (PMHS) spinal response in a sub-injurious loading range. Twelve PMHS were tested across floor velocities and time-to-peak (TTP) that ranged from 4.0 to 8.0 m/s and 2 to 5 ms, respectively. To focus on sub-injurious response, seat velocities were kept at 4.0 m/s and TTP ranged from 5 to 35 ms. Results demonstrated that spine response is sensitive to changes in TTP and the presence of PPE. However, spine response is largely insensitive to changes in floor loading. Data from these experiments have also served to develop response corridors that can be used to assess the performance and predictive capability of new test models used as human surrogates in high-rate vertical loading experiments.

Kinematic and Biomechanical Response of Post-Mortem Human Subjects Under Various Pre-Impact Postures to High-Rate Vertical Loading Conditions.

Limited data exist on the injury tolerance and biomechanical response of humans to high-rate, under-body blast (UBB) loading conditions that are commonly seen in current military operations, and there are no data examining the influence of occupant posture on response. Additionally, no anthropomorphic test device (ATD) currently exists that can properly assess the response of humans to high-rate UBB loading. Therefore, the purpose of this research was to examine the response of post-mortem human surrogates (PMHS) in various seated postures to high-rate, vertical loading representative of those conditions seen in theater. In total, six PMHS tests were conducted using loading pulses applied directly to the pelvis and feet of the PMHS: three in an acute posture (foot, knee, and pelvis angles of 75°, 75°, and 36°, respectively), and three in an obtuse posture (15° reclined torso, and foot, knee, and pelvis angles of 105°, 105°, and 49.5°, respectively). Tests were conducted with a seat velocity pulse that peaked at ~4 m/s with a 30-40 ms time to peak velocity (TTP) and a floor velocity that peaked at 6.9-8.0 m/s (2-2.75 ms TTP). Posture condition had no influence on skeletal injuries sustained, but did result in altered leg kinematics, with leg entrapment under the seat occurring in the acute posture, and significant forward leg rotations occurring in the obtuse posture. These data will be used to validate a prototype ATD meant for use in high-rate UBB loading scenarios.

Response of individual thoracolumbar spine ligaments under high-rate deformation.

Under-Body Blast (UBB) has emerged as the predominant threat to ground vehicles and Warfighter survivability. The force transference from the vehicle structure to the human body has resulted in serious injuries, with the thoracolumbar spine frequently damaged. Computational models of the human body are being generated to model human response and develop injury mitigation strategies. To effectively model the spine mechanics, the thoracolumbar ligaments, which serve varying roles in contributing to spine stability, must be characterized at relevant strains and strain rates. Adaptation of cervical spine testing methods has allowed for testing of isolated spinal ligaments including the Anterior Longitudinal Ligament (ALL), Posterior Longitudinal Ligament (PLL), and Ligamentum Flavum (LF). A high-rate servo-hydraulic test machine was used to execute a tensile test protocol for 24 complexes with loading rates ranging from 240 - 2800 mm/s and displacements of 25%, 50%, 75%, 100%, and 300% of the measured ligament length. Non-contact strain field measurements were recorded to produce a three dimensional strain field of the ligament surface. In order to provide the ligament data in a form which can be incorporated in the human computational models, analytical methods for modeling the ligament response are being investigated. Ultimately, this model will be optimized to be utilized in computational models of the lumbar spine.

Method for obtaining simple shear material properties of the intervertebral disc under high strain rates.

Predicting spinal injury under high rates of vertical loading is of interest, but the success of computational models in modeling this type of loading scenario is highly dependent on the material models employed. Understanding the response of these biological materials at high strain rates is critical to accurately model mechanical response of tissue and predict injury. While data exists at lower strain rates, there is a lack of the high strain rate material data that are needed to develop constitutive models. The Split Hopkinson Pressure Bar (SHPB) has been used for many years to obtain properties of various materials at high strain rates. However, this apparatus has mainly been used for characterizing metals and ceramics and is difficult to apply to softer materials such as biological tissue. Recently, studies have shown that modifications to the traditional SHPB setup allow for the successful characterization of mechanical properties of biological materials at strain rates and peak strain values that exceed alternate soft tissue testing techniques. In this paper, the previously-reported modified SHPB technique is applied to characterize human intervertebral disc material under simple shear. The strain rates achieved range from 5 to 250 strain s-1. The results demonstrate the sensitivity to the disc composition and structure, with the nucleus pulposus and annulus fibrosus exhibiting different behavior under shear loading. Shear tangent moduli are approximated at varying strain levels from 5 to 20% strain. This data and technique facilitates determination of mechanical properties of intervertebral disc materials under shear loading, for eventual use in constitutive models.