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

Concussion Injury Threshold

Whereas our research has provided important insights about recovery after a sport-related concussion,[10,12,13,18] we have become increasingly interested in the relationship between the clinical recovery curves and impact biomechanics. Why are some athletes able to withstand very high magnitude impacts without much deficit, whereas others struggle with significantly lower-end impacts? A number of contemporary studies have investigated impact biomechanics and have sought to shed light on proposed injury thresholds for mTBI. In an earlier work, Hodgson et al.[14] imparted short-duration impacts (1–2 ms) to six monkeys and reported that the linear accelerations of the impacts causing concussion ranged from 2000g to 5000g. Unterharnscheidt and Higgens[31] report from their study that rotational accelerations in excess of 200 rad·s−2 produced cerebrovascular injury to most subjects in their sample.

More recently, Gurdjian[8] proposed an mTBI injury threshold in terms of linear acceleration. In their report, they proposed that an mTBI would likely result from a blow to the head exceeding 80g to 90g that was sustained for greater than 4 ms. During the past 7 yr, the NFL has recently published a sequence of studies in Neurosurgery, describing many facets of concussion and mTBI in their league. The initial study from these efforts pertained to the laboratory reconstruction of concussive injuries captured on video;[28] they represented the most sophisticated method of analyzing concussive injuries in professional football players at the time. The studies are not without limitations. For one, the laboratory retrospective reenactments were based on game video footage, and important mathematical derivations were extrapolated from relatively low-speed video capture frequencies. Second, only 31 of 182 cases reviewed were reconstructed in the laboratory. Conclusions were made based on this very small and selective sample of cases. Based on these data, Pellman et al.[28] suggest that mTBI in helmeted impacts are likely to occur between 70g and 75g. This contrasts with data we have previously published, where only 7 (<0.38%) of 1858 head impacts exceeding 80g resulted in a diagnosed case of mTBI.[19] One reason for the discrepancy may be explained by the fact that Pellman et al.[28] studied professional football players, whereas we investigated this phenomenon in college football players. Given similarities in player size, these differences are unlikely to be explained by the different samples.

Zhang et al.[38] shortly thereafter proposed injury threshold values using the Wayne State University brain injury model. This model replicates a 50th percentile adult male head and includes anatomical structures including the dura, falx cerebri, tentorium and falx cerebelli, cerebrospinal fluid, cerebrum, cerebellum, and brainstem. Before analyses, the model was prevalidated against cadaveric intracranial and ventricular pressure data previously published. Twenty-four head-to-head impacts sustained in professional football were duplicated using their finite element head model to predict injury thresholds based on brain tissue responses. They reported that resultant linear accelerations of the head center of gravity of 66g, 82g, and 106g were associated with a 25%, 50%, and 80% probability of mTBI, respectively. These values are similar to those proposed by Ono et al.,[26] who suggested that impacts of 90g sustained for 9 ms or longer would result in mTBI. The Wayne State University Concussion Tolerance Curve, published in 1964 by Gurdjian et al.,[9] is a function of impact duration and impact magnitude. It deemed that an 80g impact noninjurious and an impact greater than 90g could produce an mTBI. Zhang et al.[38] also propose rotational accelerations more in line with those we have collected in our own ongoing work. They associate rotational accelerations of 4600 rad·s−2, 5900 rad·s−2, and 7900 rad·s−2, with a 25%, 50%, and 80% probability of sustaining an mTBI. Our University of North Carolina data suggest that there is far from a 50% probability of sustaining an mTBI with an impact exceeding 82g or 5900 rad·s−2.[11,19]

Our work continued in this area, attempting to better understand the effect of sustaining head impacts in excess of previously published injury thresholds. We studied how football players performed on concussion clinical measures after a game or practice session in which they sustained an impact exceeding 90g.[17] Athletes were tested only in the absence of a concussion diagnosis within 16–24 h after the session. The most important finding was that nonconcussed football players did not exhibit a decline in balance and cognition after an exposure in which they sustained at least one high impact greater than 90g, which is a proposed theoretical injury threshold. Our findings suggest that clinicians should not expect a single impact greater than 90g to necessarily result in immediate symptoms of a concussion or subsequent balance or cognitive deficits that would suggest the impact affected their overall function 24 h later. Our findings would seem to contradict the notion that a rigid threshold for concussion can be set, given that all 22 players in our high-impact condition sustained impacts well above the proposed threshold of 70g-75g.

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