Reading the Disc

Spinal disc injuries in rear-end, low-impact collisions: myth or reality?

July 13, 2020 Photo

Biomechanics, a sub-specialty under the general umbrella of biomedical engineering, is generally known as the application of engineering mechanics to biological systems, such as the human body.

Typically, the most likely questions posed to biomechanical engineers after an accidental event include: “What was the mechanism of injury that resulted into the reported injury?” and “Are these injuries consistent with the motions and forces generated from that event?” 

To that end, biomechanical engineers are frequently faced with determining whether a low-speed rear-end accident resulted in the potential for a spinal disc injury. Biomechanics utilizes the same principles as other materials in nature to determine the injury potential by addressing two main issues: analyzing the response of the human body to external loading (e.g. the motions and specific forces, comprising the location, magnitude and direction of the forces, experienced by the body in the accident); and the effect the developed loads have in relation to potential tissue damage (e.g. how the generated forces from the event compare to the proper mechanism and force levels necessary to damage the specific human tissue questioned). 

In many low-speed rear-end accidents, the occupants of the struck vehicle claim to have sustained an injury to one or more of the intervertebral discs in their cervical and/or lumbar spines. Traditionally, treating physicians have been considered the primary authority when determining the connection between the disc injury being treated and the accident.

Outside of the physician’s standard examination and treatment of the patient, these causation opinions are typically based solely on the temporal relationship between the patient’s reported symptoms and the accident. A more thorough investigation that seeks to explore the forces, motions, and accelerations experienced by the claimant in the accident is required to determine the consistency of the disc injury with the claimant’s accident. This process of determining causation based on the consistency of forces needed to result in a given injury with the forces experienced in the event is known as an injury causation analysis (ICA). Biomechanical engineers are trained in engineering principles such as physics and dynamics as well as in anatomy and physiology and thus are fully equipped to appropriately form these causation opinions.

It is a common misconception that an event that places forces on the human body can result in injury to any tissue in the body.  Many years of biomechanical testing has shown that a specific human tissue fails when subjected to a force with a specific location, magnitude, and direction.

In the case of intervertebral discs, a traumatic herniation occurs when the spine is hyperflexed, or bent excessively in the forward direction, and is simultaneously subjected to a large compressive force. This is commonly represented by the individual who injures his back while lifting a heavy object from the floor.

In the absence of this specific injury mechanism, a disc herniation will not occur in a given event. Therefore, the nature of the forces and motions experienced by occupants in low-speed rear-end accidents must be known to appropriately determine the possible causation of a disc injury. 

Biomechanical engineers have sought to quantitatively define these forces and motions by conducting physical testing with cadavers, anthropometric test dummies (ATDs), and human volunteers. This research has consistently shown that occupants maintain a level of flexion that is within their normal voluntary range of motion throughout the course of a low-speed rear-end impact. The research has also shown that the primary force present in the cervical and lumbar spines during a rear-end accident acts in tension, meaning that the components of the spine are being stretched apart rather than compressed together.

Taken together, these points show that the appropriate injury mechanism for causing a disc herniation is not present for an occupant in a low-speed rear-end accident. To reinforce this point, prior studies, such as “Lumbar Spine Injuries in Rear Impacts of Different Severities,” in 2013 and “Vehicle Rear Impacts and Spinal Disc Herniations in Occupants: Is There a Basis for Causation?” in 2017, have reported frequencies of cervical and lumbar disc herniations of 0.19 percent and 0.06 percent in rear-ended occupants in accidents resulting in a delta-V, or change in speed, of less than 12 mph for the rear-ended vehicle. However, disc herniations or bulges can exist in the absence of this singular event due to the effects of repetitive loading.

It is well-established that the human spine is subjected to significant compressive forces in our daily lives as it bears the weight of the upper body (or the head, in the case of the cervical spine). Laboratory studies have explored the effects of these repetitive forces by subjecting isolated discs as well as functional spinal units to known forces for long periods of time.

One study, “Mechanism of Disc Rupture: A Preliminary Report,” reported incidences of disc herniation when lumbar tissues were subjected to 300 pounds of force applied at a rate of 90 times per minute, while another study, “The Effect of Fatigue on the Lumbar Intervertebral Disc,” reported similar results with 700 pounds of force applied at a rate of 40 times per minute. Since every resident of this planet is bound by the same gravitational forces, no one is immune to the effects of these repetitive loads.

It follows that many discs would exhibit degenerative changes despite a lack of isolated trauma, particularly as people age. However, this damage does not always result in symptoms. Radiologists studying MRIs of asymptomatic individuals have reported disc abnormalities in 33-64 percent of these individuals, including 57-80 percent of asymptomatic individuals over the age of 60.Degeneration is even seen in young people—one study, “Prevalence of Degenerative Imaging Findings in Lumbar Magnetic Resonance Imaging Among Young Adults,” found at least one degenerated disc in 47 percent of young adults between the ages of 20-22.Taken together, these facts show that the presence of injuries in the spine are not necessarily the result of singular trauma.

In some cases, the presence of disc damage in a claimant’s spine is conceded as a condition that pre-existed the accident. In many of these instances, it is claimed that the occupant’s condition was exacerbated or aggravated by the accident. To appropriately investigate this claim, the proper mechanism of injury that would result in this structural exacerbation must be explored.

Conventional wisdom suggests that a degenerated disc is more susceptible to injury than a healthy disc, however this does not hold true for all disc pathologies. For instance, a desiccated disc—or one that has experienced a decrease in water content—has been shown to exhibit more resistance to herniation in the presence of compressive forces than a healthy disc. Additionally, it has been shown that compressive force in the presence of damage to the annulus is not sufficient to result in a herniation of the disc.

Therefore, a causation opinion must consider the specific nature of disc pathology exhibited by the claimant in order to correctly identify the injury mechanism that would result in an exacerbation of that pathology. If the appropriate mechanism is present, then the magnitude of the forces experienced by the occupant must be considered.

As noted above, the primary force present in the spine during a low-speed rear-end accident acts in tension, however the cervical and lumbar spines are also subjected to a compressive force of lower magnitude. Prior studies have quantified these compressive forces—a 5’8” 160-pound male would experience about 250 pounds of compressive force in his lumbar spine in a rear-end accident with a delta-V of 12 mph.

It is very important to note that in the absence of physical testing, the mechanical tolerances of a specific individual’s discs cannot be determined, thus it is inappropriate to build a causation opinion solely on a singular amount of compressive force experienced by a claimant in an accident. However, the compressive forces experienced in the accident can be compared to compressive forces that are regularly placed on an individual’s spine in order to quantify the relative magnitude of the forces resulting from the accident.

Researchers have quantified the compressive forces present in the spine during normal activities of daily living (ADLs). These forces are quite substantial even in seemingly mundane events: In fact, the same 5’8” 160-pound male would be expected to experience over 270 pounds of compressive force in his lumbar spine simply by walking, and this force increases to over 400 pounds while jogging. A comparison of these forces to those experienced in the accident highlights the small likelihood of the structural damage seen in the spine being related to the accident. 

It is a commonly held belief that no one can determine the chances of a disc injury in a given accident or the specific mechanism by which this injury would occur. However, an appropriate injury-causation analysis can answer these questions: Through careful examination of the specific mechanism required to result in the claimed disc injury, and a quantification of the forces experienced by the claimant in the accident, the consistency of the injury with the accident can be determined.

Engineers in biomechanical laboratories around the world have extensively researched the mechanisms of disc injuries and have also thoroughly tested and defined the human response to low-speed rear-end accidents. Application of this scientific research to a specific accident situation can provide an accurate answer of causation in a low-speed rear-end accident.

About The Authors
Evan P. McConnell

Evan P. McConnell is a senior consultant in biomechanical engineering with Rimkus Consulting Group. 

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