Exploring Low to Moderate Velocity Motor Vehicle Rear Impacts as a Viable Injury Mechanism in the Lumbar Spine

Abstract

Epidemiological research suggests that up to 50% of individuals involved in low speed rear impact collisions develop acute onset low back pain. Given that little information is known about the low back injury mechanisms as a result of these collisions the overarching goal of this thesis was to explore low to moderate velocity rear-end collisions as a potential low back injury mechanism. Using a combination of data mining, in vivo and in vitro mechanical testing of porcine functional spinal units, the global purposes of this thesis were to (i) explore the types of low to moderate velocity collisions that frequently result in claims of low back pain and injury (ii) explore the influence of low velocity rear impact collisions on peak in vivo joint loading, occupant pain reporting and passive tissue response of the lumbar spine, and (iii) characterize the effects of these mechanical exposures and explore facet joint capsule injury as a potential source of injury and pain generating pathways following low to moderate severity impacts. In-line with these global purposes, four independent studies were conducted, each with their own focused objectives.

Study I - Exploring Low Velocity Collision Characteristics Associated with Claimed Low Back Pain Background: Up to fifty percent of individuals involved in low to moderate velocity collisions report low back pain. However, our understanding of the specific collision or occupant characteristics that result in such claims of low back pain remains limited. Objectives: The primary objective of this study was to define the circumstances of low velocity motor vehicle collisions that result in litigation in Ontario with claims of low back injury. Methods: Data for this investigation were obtained from a forensic engineering firm based in Toronto, Ontario, Canada. The database was searched and only cases with an evaluation of the injuries sustained in passenger vehicle to vehicle collisions, with a collision severity of 24 km/hour or less were included in this analysis. Each identified case was reviewed for collision characteristics, pre-existing medical conditions and injuries claimed. Descriptive statistics (mean, SD and ranges) across low back injury claims were computed for documented variables. Results: Out the 83 cases reviewed, 77% involved a claim of low back injury. Specific to those who claimed low back injury, examination of the medical history revealed that pre-existing low back pain (LBP) or evidence of lumbar disc degeneration were particularly common with 63% of claimants either having had a history of LBP or evidence of lumbar disc degeneration, or both. Of all low back injury claims, 97% were accompanied by a whiplash and/or whiplash associated disorder claim. For low back injury claims, a rear-end impact was the most common configuration (70% of all low back injury claims involved a rear-end collision). The majority of all low back injury claimants experienced a change in velocity of 13 km/hour or less (69%), with 42% of all low back injury claims falling between collision severities of 10 – 12 km/hour. Conclusions: Results indicate that rear-end collision severities of 10 – 12 km/hour appear to be particularly common with respect to low back injury reporting; more severe collisions were not associated with greater low back injury reporting. This result contrasts with previously published neck injury risk data, which demonstrated the risk of neck injury symptom reporting increases with collision severity. Evidence of lumbar disc degeneration was particularly common across claimants with low back injury claims.

Study II - Characterizing Trunk Muscle Activations During Simulated Low Speed Rear Impact Collisions Background: The internal forces generated by the musculature of the lumbar region create most of the mechanical load placed on the spine. Thus, despite the anticipated low external forces generated between the occupant and the automobile seatback during a low speed rear impact collision, increased muscle tension may influence the resultant peak joint loads experienced in the lumbar spine. Consequently, the risk of low back injury may be altered by muscle activation. Objective: The purpose of this study was to evaluate the activation profiles of muscles surrounding the lumbar spine during unanticipated and braced simulated rear-end collisions. Methods: Twenty-two low speed sled tests were performed on eleven human volunteers (△v = 4 km/h). Each volunteer was exposed to one unanticipated impact and one braced impact. Accelerometers were mounted on the test sled and participants’ low back. Six bilateral channels of surface electromyography (EMG) were collected from the trunk during impact trials. Peak lumbar accelerations, peak muscle activation delay, muscle onset time and peak EMG magnitudes, normalized to maximum voluntary contractions (MVC), were examined across test conditions. Results: While not statistically significant, bracing for impact tended to reduce peak lumbar acceleration in the initial rearward impact phase of the occupant’s motion by approximately 15%. The only trunk muscles with peak activations exceeding 10% MVC during the unanticipated impact were the thoracic erector spinae. Time of peak muscle activation was slightly longer for the unanticipated condition (unanticipated = 296 ms; braced = 241 ms). Conclusions: Results from this investigation demonstrate that during an unanticipated low speed rear-end collision, the peak activation of muscles in the lumbar spine are low in magnitude. As such, muscle activation likely has minimal contribution to the internal joint loads that are experienced in the lumbar intervertebral joints during low speed rear impact collisions.

Study III - Characterizing In Vivo Mechanical Exposures of the Lumbar Spine During Simulated Low Velocity Rear Impact Collisions Background: Historically, there has been a lack of focus on the lumbar spine during rear impacts because of the perception that the automotive seat back should protect the lumbar spine from injury. As a result, there have been no studies involving human volunteers to address the risk of low back injury in low velocity rear impact collisions. Objectives: The primary objectives of this study were to explore lumbar kinematics and joint reaction forces in human volunteers during simulated rear impact collisions and to examine the influence of lumbar support on the peak motion and forces experienced in the lumbar spine. A secondary objective was to evaluate lumped passive stiffness changes and low back pain reporting after a simulated rear impact collision Methods: Twenty-four participants (12 male, 12 female) were recruited. A custom-built crash sled was used to simulate unanticipated rear impact collisions, with a change in velocity of approximately 8 km/h. Randomized collisions were completed with and without lumbar support. Measures of passive stiffness and flexion-relaxation-ratio (FRR) were obtained prior to impact (Pre), immediately post impact (Post) and 24 hours post impact (Post-24). LBP reporting was monitored over the next 24 hours leading up to the final Post-24 measures. For collision simulations inverse dynamics analyses were conducted, and outputs were used to generate estimates of peak L4/L5 joint compression and shear. From the passive trials, lumbar flexion/extension moment-angle curves were generated to quantify time-varying changes in the passive stiffness of the lumbar spine, Post and Post-24 relative to Pre. FRRs were computed as the ratio of thoracic erector spinae and lumbar erector spinae muscle activation in an upright posture to muscle activation in a flexed position Results: Average [± standard deviation] peak L4/L5 compression and shear reaction forces were not significantly different without lumbar support (Compression = 498.22 N [±178.0]; Shear = 302.2 N [± 98.5]) compared to with lumbar support (Compression = 484.5 N [±151.1]; Shear = 291.3 N [±176.8]). Lumbar flexion angle at the point of peak shear was 36 degrees [±12] without and 33 degrees [±11] with lumbar support, respectively, with 0 degrees being the lumbar posture in upright standing. No participants developed clinically significant levels of LBP after impact. Time was a significant factor for the length of the low stiffness flexion and extension zone (p = 0.049; p = 0.035), the length of the low stiffness zone was longer in the Post and Post-24 trial for low stiffness flexion and longer in the Post-24 for low stiffness extension. Conclusions: Findings demonstrate that during a laboratory-simulation of an unanticipated 8 km/hour rear-impact collision, young healthy adults do not develop LBP. Lumbar support did not significantly influence the estimated L4/L5 joint reaction forces. Changes in the low stiffness portion of the passive flexion/extension curves were observed following impact and persisted for 24 hours. Changes in passive stiffness may lead to changes in the loads and load distributions within the passive structures such as the ligaments and intervertebral discs following impacts.

Study IV - Exploring the Interaction Effects of Impact Severity and Posture on Vertebral Joint Mechanics Background: To date, no in vitro studies have been conducted to explore lumbar soft tissue injury potential and altered mechanical properties from exposure to impact forces. Typically, after a motor vehicle collision, the cause of a reported acute onset of low back pain is difficult to identify with potential soft tissue strain injury sites including the facet joint and highly innervated facet joint capsule ligament (FCL). Objectives: The purpose of this investigation was to quantify intervertebral translation and facet joint capsule strain under varying postures and impact severities. A secondary objective was to evaluate flexion-extension and shear neutral zone changes pre and post impact. Methods: A total of 72 porcine cervical FSUs were included in the study. Three levels of impact severity (4g, 8g, 11g), and three postures (Neutral Flexion and Extension) were examined using a full-factorial design. Impacts were applied using a custom-built impact track which simulated impact parameters similar to those experienced in low to moderate speed motor vehicle collisions. Passive flexion-extension and shear neutral zone testing were completed immediately prior to and immediately post impact. Intervertebral translation and the strain tensor of the facet capsule ligament were measured during impacts. Results: A significant main effect (p > 0.001) of collision severity was observed for peak intervertebral translation and peak FCL shear strain (p = 0.003). A significant two-way interaction was observed between pre-post and impact severity for flexion-extension neutral zone length (p = 0.031) and stiffness (p>0.001) and anterior-posterior shear neutral zone length (p = 0.047) and stiffness (p>0.001). This was a result of increased neutral zone range and decreased neutral zone stiffness pre-post for the 11g severity impact (regardless of posture). Conclusions: This investigation provides evidence that overall the peak vertebral translations observed across 4g to 11g impacts are below previously published ultimate shear failure displacements. FSU’s exposed to the highest severity impact (11g) had significant NZ changes, with increases in joint laxity in flexion-extension and shear testing and decreased stiffness, suggesting that soft tissue injury may have occurred. Despite observed main effects of impact severity, no influence of posture was observed. This lack of influence of posture and small FCL strain magnitudes suggest that the FCL does not appear to undergo injurious or permanent mechanical changes in response to low to moderate MVC impact scenarios.

Study V - Characterizing the Mechanical Properties of the Facet Joint Capsule Ligament Background: The facet joint capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Previous work has demonstrated that under physiological motion the FCL is subjected to significant deformation with FCL strains increasing in magnitude with increasing flexion and extension moments. Thus, it is important to characterize the mechanical response of the FCL for investigations into injury mechanisms. Sub failure loads can produce micro-damage resulting in increased laxity, decreased stiffness and altered viscoelastic responses. Thus, the objective of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been exposed to an impact. Objectives: The purpose of this investigation was to quantify the mechanical properties and viscoelastic response of control and impacted FCL. Methods: 200 tissue samples were excised from the right and left FCL of 80 porcine cervical functional spinal units (FSU’s). Tissue samples were excised from FSU’s obtained from Study 4. Twenty FCL tissue samples served as the control group. The remaining 180 FCL tissue samples were randomly obtained from FSU’s that had been exposed to one of nine impact conditions (impacted tissue). Each specimen was loaded uniaxially, collinear with the primary fiber orientation. The loading protocol was identical for all specimens: preconditioning with 5 cycles of loading/unloading to 5% strain, followed by a 30 second rest period, 5 cycles of 10% strain and 1 cycle of 10% strain with a hold duration at 10% strain for 240 seconds. The same protocol followed for 30% (cyclic-30% & 30%-hold) and 50% strain (cyclic-50% & 50%-hold). All loading and unloading were performed at a rate of 2%/sec. All impacted FCL properties were compared back to controls. Measures of stiffness, hysteresis and force-relaxation were computed for the 30% and 50% strain conditions. Results: No significant differences in stiffness were observed for impacted specimens in comparison to control (30% Control = 2.64 N/mm; 4 g = 2.20 N/mm, 8 g = 2.07 N/mm, 16 g = 2.16 N/mm)(50% Control = 5.06 N/mm; 4g = 4.60 N/mm, 8 g = 4.07 N/mm, 16 g =4.64 N/mm). Impacted specimens from the 8g Flexed and 11 g Flexed and Neutral conditions exhibited greater hysteresis during the cyclic-30% and cyclic-50%, in comparison to controls. In addition, specimens from the 8g and 11g Flexed conditions resulted in greater force relaxation for the 50%-hold conditions. Conclusions: Results from this study demonstrate viscoelastic changes in FCL samples exposed to moderate and highspeed impacts in the flexed posture. However, it is interesting that these viscoelastic changes were not accompanied by changes in stiffness. Findings from this investigation provide novel insight and provide mechanical and viscoelastic properties of the FCL both in control and impacted scenarios.

Global Summary: Findings from this thesis demonstrate that (i) rear-end collision severities of 10 – 12 km/hour appear to be particularly common with respect to low back injury reporting (ii) during a laboratory-simulation of an unanticipated 8 km/hour rear-impact collision, young healthy adults do not develop LBP, however, changes in the low stiffness portion of the passive flexion/extension curves were observed following impact and persisted for 24 hours and (iii) the observed peak displacements in porcine functional spinal units exposed to varying impact severities are below ultimate shear failure displacements and does not support a lumbar spine injury mechanism resulting in acute traumatic bone fractures and/or acute traumatic IVD herniations in previously “healthy” tissues. Overall, the small FCL strain magnitudes during impacts and unchanged FCL mechanical properties post-impact suggest that the FCL does not undergo injurious or permanent mechanical changes in response to low to moderate MVC impact scenarios. Collectively, the findings from this thesis indicate that there are no direct mechanical changes that would indicate the high incidence of low back pain reporting following low to moderate severity rear-end motor vehicle impacts. However, changes in passive tissue properties were observed, and if persistent over time, may predispose individuals to secondary pain pathways. It is also important to note that this thesis tested healthy conditions and the results do not directly apply to pre-existing LBP cases being exposed to the same impacts.