Friday, December 7, 2012

Disc Injuries in Frontal, Side, and Rear-end Crashes


Disc Injuries in Frontal, Side, and Rear-end Crashes
               
         In a trail, two of the biomechanical and surgical opinions that defense experts often testify about are disc bulges and herniations after MVCs, proposing that all are preexisting phenomena and that single-event MVCs cannot cause herniations of the disc until after the spine fractures to some massive degree. One study often quoted by these experts as the basis for this opinion concluded, “With regard to the relationship between disc rupture and impact loading on the spine, it can be safely said that disc ruptures do not occur as the result of a single loading event, unless there are associated massive bony injuries to the spine.” Although controlled laboratory studies have shown that vertical compression of a spine are associated with bony fractures and disc injuries, this is not representative of real-world frontal, side, or rear impacts. It is deceptive to imply that vertical loading studies from pilots rejecting from airplanes, or two cadaver vertebrae (muscles removed) that are mounted on a testing device and then compressed until fracture/disc bulging occurs can be compared to horizontal loading in MVCs. In most collisions where rollovers do not occur, most of the forces are in the horizontal axis and the vertical components are significantly less. The sources that King cites used axial loading for their opinion or used cadaver spines that were not allowed all six-degrees of motion (flexion, extension, rotation in both directions, and lateral flexion in both directions).
               Post-traumatic disc injuries, including annular rears, disc fissures, disc bulging, and herniations are commonly seen in clinical practice. Many of these cases have a recent or prior history of being involved in a MVC. Invertebral disc injuries are routinely seen in occupants involved in rear-end and side collisions, and are seen less often and at higher delta-Vs in frontal impacts in my experience. Circumstances of the collision, occupant age, proneness to injury, out-of-position (OOP) issues, and other factors can change the delta-V threshold for any collision type. These disc injuries are seen in low-speed impacts as well as higher velocity crashes. Although some defense biomechanists and DMEs contend that it is impossible to herniated a human disc without previously fracturing vertebrae, this is not supported in cadaver tests that replicate real-world collisions using entire cadavers, which allow for typical human kinematic motion experienced in a MVC. Some defense experts who opine that the fracture of the vertebrae is a requirement for a traumatic disc herniation to occur may rely on airplane ejection studies that subjected the cadaver to rapid, high force vertical loading or used cadaver spines mounted on plates and forced into various motions. The majority of these tests did not allow the spine to move in all sex-degrees of freedom in same manner as seen in a MVC. For example, the spine may be compressed and analyzed for damage and then flexed in a separate test. Unfortunately, there has been little research of disc injuries following frontal, side, or rear-end crashes.
               I have seen several cases in my office where an MRI was taken of a person’s spine before and after an MVC, confirming a disc bulge. These pre-crash MRIs were usually taken to rule out the cause of a prior episode of neck or back pain with out without radiculopathy, and were found normal. In some instances there was a confirmed contained minimal disc bulge with post-collision MRI showing significantly larger disc protrusion with spinal cord impingement. The majority of disc injuries occur without concomitant fractures of the spine. However, there are many cases where the MRI will detect some degenerative changes in the disc or surrounding joint.
               It is common for a defense biomechanist or defense doctor to opine that the x-rays do not show any abnormalities, thus providing that there is no objective evidence of any injuries and all post-traumatic complaints were not due to the crash event. The assumption used by some of those defense experts is that if the x-ray looks normal, then the injury doesn’t exist. However, this premise has little scientific merit after a close examination of the available cadeveric research and the clinical literature.
               In fact, contrary to some defense theories, several real-world studies using cadaver spines that were allowed to move in all six degrees of motion showed that disc tears/hemorrhage and bulging often occur without any fractures of the spine. Taylor and Taylor performed autopsies of 109 humans who died following blunt trauma, where MVCs accounted for 72 of the fatalities, 34 from blows/falls, and 3 from sporting activities. The study noted that, “Most injuries involved the joints rather than the vertebrae; e.g., injuries to the discs were four times more frequent than fractures to the vertebrae.” The study also found that severe disc avulsions, disruptions, and traumatic herniations were located primarily at the C5-C6-C7 regions. There were almost equal numbers of disc injuries and facet injuries in the autopsy analysis. When determining the type of force vector from the police reports, 54% were from extension or extension-compression injuries and 27% from flexion or lateral-flexion injuries. The C2 and C6 vertebrae were the most common sites for fractures. In the upper cervical spine, bruising of the posterior synovial folds was the most common injury noted with hematoma around the C2 nerve. The study also compared radiologists’ diagnosis of 58 sets of x-rays with the findings at autopsy, concluding that the radiologist failed to detect 199 of the 309 lesions noted in autopsy.

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