Sajal Chirvi

An Examination of Isolated and Interaction-Based Biomechanical Metrics for Potential Lower Neck Injury Criteria

Lower neck injuries inferior to C4 level, such as fractures and dislocations, occur in motor vehicle crashes, sports, and military events. The recently developed interaction criterion, termed Nij, has been used in automotive safety standards and is applicable to the upper neck. Such criterion does not exist for the lower neck. This study was designed to conduct an analysis of data of lower neck injury metrics toward the development of a mechanistically appropriate injury criterion. Axial loads were applied to the crown of the head of post mortem human subject (PMHS) head-neck complexes at different loading rates. The generalized force histories at the inferior end of the head-neck complex were recorded using a load cell and were transformed to the cervical-thoracic joint. Peak force and peak moment (flexion or extension) were quantified for each test from corresponding time histories. Initially, a survival analysis approach was used to derive injury probability curves based on peak force and peak moment alone. Both force and moment were considered as primary variables and age a covariate in the survival analysis. Age was found to be a significant (p 0.05). A lower neck Nij formulation was done to derive a combined interactive metric. To derive cadaver-based metrics, critical intercepts were obtained from the 90% injury probability point on peak force and peak moment curves. The PMHS-based critical intercepts derived from this study for compressive force, flexion, and extension moment were 4471 N, 218 Nm, and 120 Nm respectively. The lower cervical spine injury criterion, Lower Nij (LNij), was evaluated in two different formulations: peak LNij and mechanistic peak LNij. Peak LNij was obtained from the LNij time history regardless of when it occurred. Mechanistic peak LNij was obtained from the LNij time history only during the time when the resulting injury mechanism occurred. Injury mechanism categorization included compression-flexion, compression-extension, and those best represented by a more pure compression-related classification. Mechanistic peak LNij was identified based on the peak timing of the injury mechanism. Peak LNij and mechanistic peak LNij were found to be significant (p<0.05) predictors of injury with age as a covariate. The 50% injury probability was 1.38 and 1.13 for peak LNij and mechanistic peak LNij, respectively. These results provide preliminary data based on PMHS tests for establishing lower neck injury criteria that may be used in automotive applications, sports and military research to advance safety systems.

Role of age and injury mechanism in cervical spine injury tolerance under head contact loading.

ABSTRACT Objective: The objective of this study was to determine the influence of age and injury mechanism on cervical spine tolerance to injury from head contact loading using survival analysis. Methods: This study analyzed data from previously conducted experiments using post mortem human subjects (PMHS). Group A tests used the upright intact head–cervical column experimental model. The inferior end of the specimen was fixed, the head was balanced by a mechanical system, and natural lordosis was removed. Specimens were placed on a testing device via a load cell. The piston applied loading at the vertex region. Spinal injuries were identified using medical images. Group B tests used the inverted head–cervical column experimental model. In one study, head–T1 specimens were fixed distally, and C7–T1 joints were oriented anteriorly, preserving lordosis. Torso mass of 16 kg was added to the specimen. In another inverted head–cervical column study, occiput–T2 columns were obtained, an artificial head was attached, T1–T2 was fixed, C4–C5 disc was maintained horizontal in the lordosis posture, and C7–T1 was unconstrained. The specimens were attached to the drop test carriage carrying a torso mass of 15 kg. A load cell at the inferior end measured neck loads in both studies. Axial neck force and age were used as the primary response variable and covariate to derive injury probability curves using survival analysis. Results: Group A tests showed that age is a significant (P < .05) and negative covariate; that is, increasing age resulted in decreasing force for the same risk. Injuries were mainly vertebral body fractures and concentrated at one level, mid-to-lower cervical spine, and were attributed to compression-related mechanisms. However, age was not a significant covariate for the combined data from group B tests. Both group B tests produced many soft tissue injuries, at all levels, from C1 to T1. The injury mechanism was attributed to mainly extension. Multiple and noncontiguous injuries occurred. Injury probability curves, ±95% confidence intervals, and normalized confidence interval sizes representing the quality of the mean curve are given for different data sets. Conclusions: For compression-related injuries, specimen age should be used as a covariate or individual specimen data may be prescaled to derive risk curves. For distraction- or extension-related injuries, however, specimen age need not be used as a covariate in the statistical analysis. The findings from these tests and survival analysis indicate that the age factor modulates human cervical spine tolerance to impact injury.

Biomechanical tolerance of whole lumbar spines in straightened posture subjected to axial acceleration: BIOMECHANICAL TOLERANCE OF WHOLE LUMBAR SPINES

Quantification of biomechanical tolerance is necessary for injury prediction and protection of vehicular occupants. This study experimentally quantified lumbar spine axial tolerance during accelerative environments simulating a variety of military and civilian scenarios. Intact human lumbar spines (T12‐L5) were dynamically loaded using a custom‐built drop tower. Twenty‐three specimens were tested at sub‐failure and failure levels consisting of peak axial forces between 2.6 and 7.9 kN and corresponding peak accelerations between 7 and 57 g. Military aircraft ejection and helicopter crashes fall within these high axial acceleration ranges. Testing was stopped following injury detection. Both peak force and acceleration were significant (p < 0.0001) injury predictors. Injury probability curves using parametric survival analysis were created for peak acceleration and peak force. Fifty‐percent probability of injury (95%CI) for force and acceleration were 4.5 (3.9–5.2 kN), and 16 (13–19 g). A majority of injuries affected the L1 spinal level. Peak axial forces and accelerations were greater for specimens that sustained multiple injuries or injuries at L2–L5 spinal levels. In general, force‐based tolerance was consistent with previous shorter‐segment lumbar spine testing (3–5 vertebrae), although studies incorporating isolated vertebral bodies reported higher tolerance attributable to a different injury mechanism involving structural failure of the cortical shell. This study identified novel outcomes with regard to injury patterns, wherein more violent exposures produced more injuries in the caudal lumbar spine. This caudal migration was likely attributable to increased injury tolerance at lower lumbar spinal levels and a faster inertial mass recruitment process for high rate load application. Published 2017. This article is a U.S. Government work and is in the public domain in the USA. J Orthop Res 36:1747–1756, 2018.