Head Impact Measurement Devices.

CONTEXT: Concussive injuries are at the forefront of sports medicine research. Recently, researchers have used a variety of head- and helmet-based impact-monitoring devices to quantify impacts sustained during contact sport participation. This review provides an up-to-date collection of head accelerometer use at the youth, high school, and collegiate levels. EVIDENCE ACQUISITION: PubMed was searched for articles published between 1980 and 2015 using the terms accelerometer and concussion, impact sensor and concussion, head impact telemetry system, head impact telemetry, and linear acceleration and concussion. An additional Google search was performed to capture devices without publications. STUDY
DESIGN: Clinical review. LEVEL OF EVIDENCE: Level 4.
RESULTS: Twenty-four products track and/or record head impact for clinical or research use. Ten of these head impact devices have publications supporting their utility.
CONCLUSION: Head impact measuring devices can describe athlete exposure in terms of magnitude and/or frequency, highlighting their utility within a multimodal approach for concussion assessment and diagnosis.

There is growing concern regarding concussions in football and contact sports, prompting legislation, clinical protocols, and research to improve prevention and treatment. To prevent injury, the mechanism of injury must be understood. Quantifying the biomechanical properties of a head injury may elucidate targets of prevention. Multiple head impact devices have been developed for in vivo use in athletics to relate mechanical force and acceleration to the clinical manifestation of concussion.[2]

Investigations have used head impact devices to monitor the number and magnitude of head impacts sustained in sports participation that do not result in concussion.[9,13,15,16,28,53,62] Unfortunately, no linear and/or rotational concussive threshold has been established.[31,35,69] The objectives of this clinical review are to provide an up-to-date assessment of head impact devices used for concussion monitoring and to provide guidance on their clinical utility at the college, high school, and youth levels.

Methods

Research studies were identified via the PubMed database (1980-2015) through searches of keyword phrases: accelerometer and concussion, impact sensor and concussion, head impact telemetry system, head impact telemetry, and linear acceleration and concussion. From this search, 121 unique articles were identified. Peer-reviewed publications were then screened for inclusion based on the following criteria. Inclusion—athlete population, in vivo studies, and reported magnitude (eg, linear acceleration); exclusion—review/commentary papers, case study, review article, accelerometer type and hardware not listed, and in vitro studies (n = 14). In total, 48 articles met the criteria. Whenever possible, impact count, top 1% and top 5% linear acceleration, rotational acceleration, and impact severity values were also collected. Three reviewers (RMW, MD, KLO) independently extracted relevant data from the studies. Of the 48 articles, 7 head impact systems were identified. Additional commercial head impact devices advertised for concussion were identified through a Google search using the terms sports accelerometers and head impact accelerometers, which identified an additional 17 head impact–monitoring devices. A total of 24 devices were identified (Table 1 in Appendix, available at http://sph.sagepub.com/content/by/supplemental-data).

Accelerometer Systems

The earliest in vivo research on head impact biomechanics was completed in the 1970s using triaxial accelerometers to measure head acceleration during football games.[50,56] More recently, a single triaxial accelerometer was used inside the helmets of 1 high school hockey player and 2 football players.[53] A single accelerometer fixed to the head or helmet provides limited information because headband systems are known to slip and helmet motion is largely independent of head motion.[42] The Head Impact Telemetry System (HITs) was the first modern era impact accelerometer system to estimate motion after impact.[19,42] The HITs implements 6 single axis accelerometers recording at 1000 Hz to capture data on linear acceleration. It also calculates rotational acceleration, impact duration and location, Gadd Severity Index (GSI), Head Injury Criterion (HIC15), and Head Impact Telemetry severity profile (HITsp). The HITs has been used extensively in head impact biomechanics (Table 2 in Appendix, available at http://sph.sagepub.com/content/by/supplemental-data). The specific outputs vary by system, but in general, all systems report linear and rotational acceleration, impact location, a time-stamp, GSI, HIC15, and HITsp (Table 2 in Appendix).

Results

Ten of 24 (41.6%) market- or research-based head impact–monitoring devices have publications supporting their utility.[1,13-15,17,22,24,26,28,33,34,41,43,62,63,66,68] This suggests that some marketed head impact–monitoring devices have limited to no research supporting their use. The HITs is the most widely used head impact–monitoring device and has captured impacts from all levels of play for football and ice hockey (Table 1).

Table 1.

High school football and college head impact frequency and magnitude

	Total Season Impacts	Impacts per Player per Season	Linear Acceleration, g	Rotational Acceleration, rad/s2	Most Frequent Impact Location
Football
College	3312-90,054[20,21,34,62]	223-1354[16,20,47]	20-35[16,28,47,62]	1187-6990[22,34,59]	Top[47]
High school	413-652[8,60-63]	413-652[8,60-63]	21-27[13,14,43]	Up to 7701[9,63]
Youth	748-11,978 [18,24,51,68]	106.9-252 [18,23,24,51,68]	16-22[18,23,48,55,68]	4-12,322[18,51]	Top, front, back[24,51]
Ice hockey
College	28,178			1187-6990.5[22,34,59]	Top[47]
Male	15,281	347[7]	31.2[65]-43.7[7]	2,881.065-4,7647
Female	12,897	179.2[7]	28.54[65]-44.9[7]	1,766.865-3,7097
High school  and youth	12,25349	22349	18.4[49]-35g[53]	1,464.549	Side49

Concussion Threshold

Head impact measurement devices offer the potential for improving concussion diagnosis and accelerating concussion management if a biomechanical injury threshold can be identified.[35]

At the college level, the average linear acceleration for a concussive event ranges from 55.8g to 168.8g[17,36,62] with no consistency. The average rotational acceleration for collegiate concussive episodes was between 163.4 and 15397.1 rad/s2.[28,36,62] In a cohort of high school and collegiate athletes, 17 concussions occurred with 75% of the concussive impacts exceeding 96g and 7235 rad/s2.[34] At the high school level, linear acceleration values range from 74g to 146g, with mean linear acceleration reported at 105g.[15,18,23] During a concussive episode, the average linear acceleration value was 93.6g, and rotational acceleration ranged from 5582.6 to 9515.6 rad/s2.[10,13] This suggests that concussion threshold varies in both college and high school football players.[31] The fact that many impacts at similar magnitudes do not result in a concussive injury suggests that the individual injury threshold is dynamic.[31] Covariates to consider include sex, age, genotype, and history of concussion.

Clinical Implications

Head impact devices may be used in conjunction with other assessment tools to provide additional information regarding impacts.[33,37] Data from impact-monitoring devices may indirectly reduce concussion risk by influencing rule and coaching changes. Additionally, the ability to identify multiple subconcussive impacts may be important to calculate cumulative effects, as repeat subconcussive impacts may increase concussion risk.[5] Unfortunately, the lack of specificity with head impact devices precludes the practical application of accelerometer systems as a diagnostic tool.

Limitations

The fixation of the accelerometer to the head, helmet fit, and device hardware should be considered when evaluating an impact-monitoring device. Player helmet fit is a significant component of head impact–monitoring data variability. Athletes in nonhelmeted sports have accelerometers affixed to their skin or through a headband, which may cause error or variation in acceleration due to skin or device movement and sweat. Despite limited diagnostic utility, these devices identify which athletes sustain large-magnitude and frequent impacts.

Conclusion

Without an established injury threshold, clear accuracy and validity, impact-monitoring devices cannot replace a clinician’s clinical judgment regarding a concussive event. Head impact–monitoring devices should not be used as a single assessment tool for concussion.