DENVER (April 13, 2011) − There has been increased awareness in the last year related to head injuries incurred in the NFL as well as in collegiate and high school football. The annual incidence of football-related concussion in the United States is estimated at 300,000 and nearly 47,000 football-related head injuries were serious enough to be treated at U.S. hospital emergency rooms in 2009. While there have been many studies related to concussion in football, and more recently, the development of chronic traumatic encephalopathy related to repetitive brain trauma in sports, this is the first in-depth analysis of the biomechanics of subdural hemorrhage formation specific to American football.
Researchers at Vanderbilt University Medical Center in Nashville, Tenn., analyzed the case of a 17-year-old football player treated at their institution for a subdural hemorrhage he suffered secondary to a helmeted collision during football practice. In performing this analysis, they reviewed previous data that had been published regarding the biomechanical threshold of bridging vessel rupture, in addition to other studies describing patterns of energy transfer during collisions at various levels of football. The results of this study, Biomechanics of Subdural Hemorrhage in American Football, will be presented by Jonathan A. Forbes, MD, 3:01-3:15 pm, Wednesday, April 13, during the 79th Annual Scientific Meeting of the American Association of Neurological Surgeons in Denver. Co-authors are Jeffrey H. Markle, Douglas M. Browne, Tyler S. Severance, and Allen K. Sills, MD. Dr. Forbes will be presented with the Synthes Craniofacial Award for this research.
Review of data collected from the time period of 1945-1994 has shown that subdural hemorrhage (SDH) is responsible for the majority of fatalities in American football. In fact, 352 of 684 fatalities during this time period were secondary to SDH. In a recent analysis of American high school and collegiate football players, it was shown that about 6-7 players a year suffer catastrophic head injury secondary to helmeted collision, with 94 percent of these cases involving SDH.
Case Study
A 17 year-old male came to the emergency department of Vanderbilt University Medical Center about 60 hours after a helmet-to-helmet collision at football practice. Non-contrast CT scan of the head demonstrated a subacute left frontoparietal subdural hematoma, slightly hyperdense to cortex, measuring approximately 11 mm at its widest thickness with 6 mm of left to right midline shift visible on the scan Evaluation of the bony windows demonstrated a focal calvarial irregularity overlying the side of SDH. The patient’s headaches continued to worsen and the decision was made to proceed with craniotomy for evacuation of the clot on the second day of hospitalization. The patient did well following the procedure, was discharged postoperatively after two days in the hospital, and was neurologically intact and without complaints at 6-week follow-up.
To better understand the biomechanical factors associated with SDH following this collision and others like it, an extensive literature review was undertaken of the following factors implicated in SDH formation:
•The material properties involved rupture of bridging veins (BVs), the veins that drain blood from the surface of the brain to the venous sinuses.
•The thresholds of rotational/translational acceleration associated with BV rupture and SDH formation.
•Rotational/translational accelerations observed during collisions at all levels of American football.
•Variables that increase rotational acceleration in a collision: increases in impact force, decreases in rotational stiffness (resistance of an object to deformation and varies with material composition and structure in the neck), and increases in length of the lever arm in relationship to moment (torque that is equivalent to the applied force multiplied by the length of the lever arm).
“The results of this in-depth study yielded interesting findings that have potential for improving the safety of American football players at all levels of play,” said Dr. Forbes. Among the findings:
•Helmeted collisions result in abrupt changes in the velocity of the head. The helmet itself distributes the applied force over a surface area estimated to be about 10 times greater than the surface area of contact in unhelmeted collisions. SDH following helmeted collisions are usually caused by rupture of a BV. Rupture occurs when there is relative motion between the brain and the skull and BVs are stretched.
•Relatively speaking, a much greater degree of translational acceleration (TA) is required to produce BV rupture and SDH than rotational acceleration (RA). The risk of SDH formation has been found to be proportional to both the amount of RA incurred and the duration of the collision.
•SDH formation in helmeted collisions is hypothesized to relate to dangerous levels of RA. In experiments meant to simulate elite level impact conditions in the NFL, rotational accelerations greater than 15,000 rad/s2 have been observed. While the parameter of RA has not been specifically assessed in high school and college impacts, comparable linear accelerations reached in high-energy impacts at these levels indicate that overlap with the critical RA of BV rupture may also present.
•Increased moment arm and decreased rotational stiffness associated with extension about the articulations of the subaxial cervical spine contributes disproportionately more to moment and rotational acceleration than extension at the craniocervical junction.
“Our findings indicate that modification of the current helmet quality assurance standard should include limits on RA, which would have potential for reducing the incidence of catastrophic head injury in American football,” concluded Dr. Forbes.
Founded in 1931 as the Harvey Cushing Society, the American Association of Neurological Surgeons (AANS) is a scientific and educational association with more than 8,000 members worldwide. The AANS is dedicated to advancing the specialty of neurological surgery in order to provide the highest quality of neurosurgical care to the public. All active members of the AANS are certified by the American Board of Neurological Surgery, the Royal College of Physicians and Surgeons (Neurosurgery) of Canada or the Mexican Council of Neurological Surgery, AC. Neurological surgery is the medical specialty concerned with the prevention, diagnosis, treatment and rehabilitation of disorders that affect the entire nervous system, including the spinal column, spinal cord, brain and peripheral nerves.
Disclosure: the author reports no conflicts of interest.
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