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

The Relationship between Biomechanics and Clinical Symptoms

Sport-related concussion typically results from forces directly imparted to the head or indirectly through the neck, resulting in a combination of rapid acceleration and deceleration. Such forces create linear and/or rotational acceleration/deceleration on the brain. The animal literature on this topic suggests that rotational acceleration is more significant than linear acceleration and can lead to more serious effects on the brain,[25] but we do not know that this is necessarily the case with sport-related concussion. Concussive injury presents with varying types of symptoms and different levels of symptom severity. This presentation of symptoms also may vary widely depending on the biomechanical forces involved. It has long been understood that the severity of the pathological injury depends on the magnitude, location, and distribution of the forces across the brain tissue, and several research groups have attempted to examine and quantify the accumulated accelerations during sport impacts and how those accelerations are transmitted to the brain of an athlete.[24,27,28,30,33,36,37] However, very few researchers have conducted data collection in real time on the field in the environment in which athletes typically encounter concussive impacts and attempted to study the relationship of clinical outcome to the biomechanical measures.

Biomechanical analysis can provide valuable clinical insights into the causes and factors contributing to head loadings and stresses to the brain. Such techniques have involved both empirical and analytical approaches. Empirical methods measure kinematics and kinetics exerted on the body as opposed to analytical methods, which predict bodily responses by replicating the impact dynamics.[30] Kinematic analysis techniques provide information on body motions and tend to consist of cinematography and motion-tracking systems providing two- or three-dimensional information on body segments. The kinematic measurement techniques include linear accelerometry for direct measure of head impact response.[24] Kinematic measurement generally involves sensors mounted onto the head for direct measurement in one axis or combined with multiple units to quantify accelerations in two or three dimensions. Angular accelerometer techniques also are included in kinematic analysis methods. The angular accelerations about one or more axes are derived from multiple linear acceleration measurements and require stringent implementation to ensure accuracy.[37]

Observation and assessment of the biomechanics of injuries in American football also have provided some insight into concussive injury.[27,28,32,36] Pellman and colleagues[28] reconstructed head impacts observed in concussion footage. Measures of head acceleration for concussed athletes versus uninjured struck athletes and uninjured striking athletes show a significantly greater rapid change in head velocity for the concussed athlete. Translational acceleration from impacts on the face mask or the side of the helmet or falls on to the back of the helmet was most frequently associated with concussions.[28] Helmet-to-helmet contact occurs regularly in American football. Viano and colleagues[32] sought to assess the collision mechanics resulting in injury to the struck player and the biomechanics of the striking players in laboratory tests. A number of translational and rotational head accelerations were examined to determine how the striking player executed the concussive blow. Results revealed the key to the execution of a concussive blow is the head-down position. This occurs when the striking athlete lowers his or her head, bringing his or her head, neck, and torso into alignment, allowing an exertion of maximum force on the struck athlete, whose head and neck resist the impact. The concussion was observed to occur as a result of the greater inertia of the striking athlete behind the impact. The head-down position was found to increase the mass of the striking athlete up to 67% by coupling the torso into the impact and transferring more momentum to the struck athlete.[32] Viano and colleagues[32] sought to compare the head impact from a boxing blow with impacts observed in American football. The findings show that the punches inflicted by the boxers had high-impact velocity but lower head injury criteria - a measure of the likelihood of head injury arising from an impact - and translational acceleration than in American football impacts but cause proportionally more rotational acceleration because of a lower effective punch mass.[32]

Our ongoing study at the University of North Carolina on injury biomechanics in American football players uses a real-time helmet accelerometer data collection methodology in Division I college football players. Our findings suggest a higher propensity of top-of-the-head impacts and a higher relative risk of concussion for those impacts. In this regard, 6 of 13 concussions occurred from impacts to the top of the head, this is in contrast to four, two, and one concussion occurring to the front, right, and back, respectively (Fig. 2).[11] Our findings suggested that football players are concussed by impacts to the head that occur at a wide range of magnitudes (60.51g-168.71g linear acceleration), and that clinical measures of acute symptom severity, balance, and neuropsychological function all appear to be largely independent of impact magnitude and location. There was no relationship between impact magnitude or location, and clinical outcomes of symptoms, balance, or neuropsychological performance (Table). In short, the concussions sustained as a result of lower end magnitudes tended to present with just as many clinical deficits as those with higher end magnitudes. Thus, despite the literature suggesting that high magnitudes of head impact, particularly with high-angular acceleration, result in more serious clinical outcomes in cases of moderate or severe TBI,[15,25] the magnitude and location likely do not predict clinical recovery in cases of mTBI.

Figure 2.

Pictorial representation of 13 concussive impacts by location and the specified magnitudes of linear acceleration in terms of gravity force (g). No concussions resulted from impacts to the left side in our sample. (Reprinted from Guskiewicz KM, Mihalik JP, Shankar V, et al. Measurement of head impacts in collegiate football players: relationship between head impact biomechanics and acute clinical outcome after concussion. Neurosurgery. 2007; 61(6):1244–52. Copyright © 2007 Lippincott Williams & Wilkins. Used with permission.)

The uniqueness of this study was that it combined impact biomechanics captured in real time with clinical measures of symptom severity, neurocognitive function, and balance captured during the acute period after concussive injury. The findings supported the notion that the threshold for mTBI (concussion) is elusive and added further to the debate as to whether it is lower or higher than previously thought. Given the findings of our companion articles,[17,19] one could make the argument that, on average, the threshold must be higher than 80g or 90g. Impacts greater than 90g, in the absence of self-reported concussion symptoms, did not result in a diagnosed concussive episode[17] and fewer than a half percent (<0.35%) of all impacts greater than 80g resulted in a diagnosed concussion.[19] If the threshold were in the 80g to 90g range for mTBI, one would expect more prevalence of symptoms in impacts of these high-end impacts. Our data also suggest that top-of-helmet impacts may result in larger postural stability deficits after mTBI.[11] We speculated that top-of-helmet impacts might result in a coup-contrecoup mechanism occurring in a superior-to-inferior direction, causing the cerebellum to impact the base of the skull and recoil superiorly into the cerebellar tentorium. Interestingly, our data also indicate that top-of-helmet impacts typically result in relatively lower rotational acceleration values compared with injuries after impacts to the other areas of the head. However, we observed that these impacts to the top are at least six times more likely to result in impact magnitudes greater than 80g of linear acceleration than side or front impacts.[19] These findings bring into question the notion that rotational acceleration is the leading precursor to injury and are suggestive that the type of acceleration, in combination with impact location, may be a better determinant for both onset and severity of injury.

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