Biomechanics of Sport Concussion: Quest for the Elusive Injury Threshold

Kevin M. Guskiewicz; Jason P. Mihalik

Disclosures

Exerc Sport Sci Rev. 2011;39(1):4-11. 

In This Article

Abstract and Introduction

Abstract

Previous concussion biomechanics research has relied heavily on the animal model or laboratory reconstruction of concussive injuries captured on video footage. Real-time data collection involves a novel approach to better understanding the medical issues related to sport concussion. Recent studies suggest that a concussive injury threshold is elusive and may, in fact, be irrelevant when predicting the clinical outcome.

Introduction

As many as 3.8 million sport-related traumatic brain injuries may occur annually in the United States,[13] and the incidence between sports widely varies. Fortunately, most of these injuries are classified as mild traumatic brain injuries (mTBI) or concussions, yet the incidence in some sports (e.g., women's ice hockey) may exceed that of all other injuries sustained in a season.[1] The U.S. Centers for Disease Control and Prevention also report a high incidence of recurrent concussions in several sports such as football and warns that the likelihood of serious sequelae increases with repeated injury.[2] Our earlier research has indicated that a history of previous concussion increases the risk for future concussion, as well as the acute outcomes after subsequent concussion.[10,13] Other factors such as learning disabilities[3] and age[6] also have been shown to influence the risk for and outcomes after concussion. Research on the influence of various biomechanical factors for predicting outcomes after sport-related concussion is inconclusive, but new technologies may lead us to more answers.

The biomechanics of head impacts have been investigated in a variety of laboratory settings over the past six decades. Ommaya and Gennarelli[25] were among the first to describe in detail linear and rotational accelerative mechanisms of injury using animal models that helped to better explain the role of linear versus rotational acceleration for brain injury. More recently, the National Football League (NFL) commissioned an investigation of sport-related concussions. One of these studies performed laboratory reconstructions of video-recorded concussions using helmeted Hybrid III dummies,[28] and it was suggested that an injury threshold of 70g-75g existed for sustaining concussion based on the translational (linear) acceleration of a football player's head.

Linear and rotational head accelerations are hypothesized to be the primary risk factors for concussion during an impact. Both direct and inertial (i.e., whiplash) loading of the head may result in linear and rotational head acceleration. Head acceleration induces strain patterns in brain tissue, which may cause injury. Current science has not identified an exact threshold for concussive injury, and direct measurement of brain dynamics during impact is extremely difficult in humans. Head acceleration, on the other hand, can be more readily measured; its relationship to severe brain injury has been postulated and tested for more than 50 yr. Both linear and rotational accelerations of the head play important roles in producing diffuse injuries to the brain. However, the relative contributions of these accelerations to specific injury mechanisms have not been conclusively established. The numerous mechanisms theorized to result in brain injury have been evaluated in cadaveric and animal, surrogate,[26] and computer[34,35] models. Prospective clinical studies combining head impact biomechanics and clinical outcomes have been strongly urged but have been relatively void in the literature.

Concussions are often referred to structurally as "diffuse axonal injuries" and result in some degree of functional impairment but differ from more moderate to severe TBI in that the impairment is transient in nature. Diffuse axonal injury, in addition to linear coup-contrecoup mechanisms of injury, can result in disruption to centers of the brain responsible for breathing, heart rate, and consciousness, but more typically result in memory loss, cognitive deficits, balance disturbances, and a host of other somatic symptoms. Our prior work has identified recovery curves for symptoms, cognitive function, and balance, with deficits typically lasting 7–10 d after concussion in high school and college athletes.[12,18] Reflection upon these findings has led to the question, What is the relationship between clinical outcome measures from our earlier work and biomechanical factors? The literature has not adequately addressed this question. We hypothesize that within the spectrum of concussion or mTBI, the biomechanical threshold for sustaining the injury is not only elusive, but impact severity (measured in acceleration/deceleration) may be clinically irrelevant. This review aims to collate findings from our recently conducted studies investigating biomechanical relationships with various factors such as playing position, types of play, concussive versus subconcussive impacts, location of impacts, and clinical measures of concussion.

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