Soft-shell headgear in rugby union: a systematic review of published studies

To review the rate of soft-shell headgear use in rugby union, consumer knowledge of the protection potential of soft-shell headgear, incidence of concussion reported in rugby headgear studies, and the capacity of soft-shell headgear to reduce acceleration impact forces. A systematic search was conducted in July and August 2021 using the databases SPORT Discus, PubMed, MEDLINE, CINAHL (EBSCO), Scopus, and Science Direct. The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The protocol for this systematic review was registered on PROSPERO (registration number: CRD42021239595). Rates of headgear use, reports of estimated protection of headgear against head injury, incidence of concussion and magnitude of impact collisions with vs. without headgear, impact attenuation of headgear in lab studies. Eighteen studies were identified as eligible: qualitative (N = 4), field (N = 7), and lab (N = 7). Qualitative studies showed low rates of headgear use and varying understanding of the protection afforded by headgear. Field studies showed negligible association of headgear use with reduced impact magnitude in headgear vs. non-headgear cohorts. Lab studies showed increased energy attenuation for thicker headgear material, poorer performance of headgear after repetitive impacts and increased drop heights, and promising recent results with headgear composed of viscoelastic polymers. Rates of adoption of soft-shell headgear remain low in rugby and any association between its use and reduction in acceleration impact forces remains unclear. Lab results indicating improved impact attenuation need to be validated in the field. Further headgear-related research is needed with youth and female rugby players.


Introduction
The need for a systematic review into the effectiveness of rugby headgear takes place within a wider health context. Traumatic brain injury (TBI) is a major global health issue, affecting up to 55 million people in a given year [1]. The majority (70-90%) of TBI are classified as mild (mTBI), with sports-related concussion (SRC), including rugby, a significant contributor to these numbers [2]. In the United States it is estimated 1.1-1.9 million SRC occur every year in children aged 0-18 [3]. Sports-related concussion numbers are increasing in parallel with public awareness of concussion [4].
There is increasing concern regarding concussion in contact sports, including rugby union, and if practical measures, such as headgear use, can mitigate its risk [5,6]. In most concussion cases, symptoms resolve spontaneously and within a month [7], but some players develop a post-concussive syndrome with a variety of somatic, cognitive, and emotional symptoms [8]. Various factors for reducing concussion incidence have been proposed including rule changes, playing techniques, and the use of protective equipment, including helmets and headgear [9].
Rugby union is a popular contact sport played by 9.6 million people in 123 countries [10]. Collisions are inherent in the game. Peak linear acceleration (PLA) forces have been recorded up to 165 g [11] and have occurred at velocities of up to 13.8 m/sec [12]. Rugby players are, therefore, at a higher risk of injury compared to other contact sports [13]. While there is no currently agreed threshold on what impact results in a concussion [14], data from professional rugby in England reports increasing incidence (up to 17.9 concussions per 1,000 player hours in the 2017-2018 season [15]) while a 2021 review listed rugby as the highest risk contact sport with incidence of 28.25 per 10,000 athlete exposures [16]. Rugby tends to be over-represented in concussion incidence numbers versus other sports [17] and concussion-related injuries account for 25% of all days lost from rugby participation [5].
Within this wider context, the current systematic review seeks to review the use and effectiveness of softshell headgear in rugby union, i.e. what is the established research base regarding: (1) Use rates of soft-shell headgear in rugby union (2) Consumer understanding of protection provided by soft-shell headgear in rugby union (3) The effectiveness of soft-shell headgear in rugby union to mitigate concussion and head injury (4) The capacity of soft-shell headgear to reduce linear and rotational impact forces Regarding the first three aims, current rates of headgear use appear to be low. Barnes et al. quoted 36% of players wear headgear regularly [18], and this usage rate has remained essentially the same over the last twenty years ranging from 12 to 33% in earlier studies [5,19]. There is wide variety in invested parties' understanding of the protection headgear affords against head injury or concussion (37-62%) [18][19][20][21]. Measuring the effectiveness of this protection is more challenging to establish; Benson et al. [22] reported that although protective equipment, such as helmets, has been shown to be effective in concussion reduction in pursuits such as cycling and American football, less evidence has been demonstrated with rugby headgear [22].
Regarding the final aim, the need for the current paper addressing effectiveness of headgear in rugby union is timely as World Rugby (WR) recently approved a medical device trial process allowing thicker, denser headgear to be used in games designed to improve player welfare [23]. Previously WR had only permitted one class of headgear, and this was primarily used to prevent lacerations, scalp injury, and cauliflower ears. This change in WR policy has led to more recent studies incorporating headgear, such as NPro, which is composed of impact-reducing viscoelastic polymers [11,24]. NPro claims its headgear is "scientifically proven to provide impact protection." [25].
Previous systematic reviews have looked at headgear effectiveness across a diverse range of sports, leisure pursuits, and study designs: soft-shell headgear and concussion in youth collision sports [26], effectiveness of interventions in reducing brain injuries in rugby [27], concussion prevention of all protective equipment [22], all concussion prevention strategies [6], strategies to reduce concussion risk in all sport [28], concussion in rugby [17], incidence of concussion in youth sport [29], helmet design in sport [30] and considerations for requirements of soft-shelled headgear [31]. At the time of writing, no such review has been published looking specifically at rugby union headgear regarding the above aims.

Methods
The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [32].

Quality and level of evidence assessment
The review utilized the following reporting standards to evaluate article quality and reduce risk of bias: Observational studies were analysed according to the Strengthening and Reporting of Observational Studies Epidemiology (STROBE) [33].
Qualitative studies were analysed according to the Standardized Reporting of Qualitative Research (SRQR) [34].
Laboratory studies were analysed according to an author-generated checklist based on recommendations adapted from Benson et al. [22]. This checklist was generated in consultation with co-author (ND) and was generated using recommendations from the Benson et al. article, namely that studies of this type are of sufficient power to determine true concussion risk, specify exact properties of tested equipment, assess headgear that is or would be used in gameplay, and study injury mechanisms that are consistent with examples from the field. The article further recommended a clear operational definition of concussion be used rather than the more generic term of head injury. Such parameters were included in the checklist visible in Table 6.

Data sources
A systematic search was conducted in July and August 2021 using the databases SPORTDiscus, PubMed, MEDLINE, CINAHL (EBSCO), Scopus, and ScienceDirect (see Fig. 1). Reference lists of selected studies and relevant reviews were also hand-searched to identify pertinent articles not identified by initial search strategies.
The protocol for this systematic review was registered on PROSPERO and can be accessed at https:// www. crd. york. ac. uk/ prosp ero/ displ ay_ record. php? Recor dID= 239595

Study selection
Studies were included when the population included rugby union players (junior, senior, male and female, amateur and professional) and coaches. Studies were included within the following parameters: incidence studies, observation studies, interventions, and perceptions of use. Studies were included with the following research designs: randomised controlled trials (RCTs), observational, intervention, surveys, questionnaires, focus groups. Duplicates removed (n = 78)

Records screened (n = 274)
Records excluded (n = 232) Full-text arƟcles assessed for eligibility (n = 41) Full-text arƟcles excluded, with reasons (n = 23) Review: n = 7 Commentary: n = 1 Not specific to rugby: n = 6 Full text does not assess headgear: n = 9 Studies included in systemaƟc review (n = 18) QualitaƟve: n = 5 Field: n = 6 Lab: n = 7 Studies were excluded when they were reviews, commentaries, not related to the use of soft-shell headgear, or when related to sports other than rugby union.
Two review authors (SH and KA) independently screened manuscripts using title and abstract, selecting studies based on the inclusion of keywords (head injury, brain injury, concussion, rugby, rugby union, headgear, scrumcap, head guard, soft-shell headgear). The reviewers then screened the selected manuscripts in full text. The two review authors (SH and KA) identified the same studies to be included, some papers were included or excluded after review author (SH) performed a closer reading of each included study. Included studies have been reviewed using a narrative approach due to the heterogeneity present in their design. Due to this heterogeneity, a meta-analysis was not conducted.

Results
Of the 274 records identified after duplicates were removed, 41 were screened in full text for eligibility (the full text of one eligible study could not be accessed). Of these 41, 23 were excluded. Reasons for exclusion are listed in Fig. 1. After screening, 18 studies were included.

Study characteristics
All included studies were published between 2000 and 2021. Qualitative studies (N = 4) included three survey and one questionnaire design. Mixed methods designs were included in this section as they report qualitative data. A total of 870 participants (ranging from 63 to 545) took part in all studies, of which 106 were female (12.2%). Age ranged from 14 to 33, with one study looking exclusively at youth (under 15) and one study not stating its age range. Playing level ranged from junior (youth/under 15) to professional. All studies surveyed players, one study additionally surveyed coaches. No studies reported on socio-economic or racial factors. All studies reported rates of headgear use and perspectives on the level of protection provided by headgear. Four studies reported reasons for/against wearing headgear. Two studies reported players' self-report on concussion incidence. Results are summarised in Table 1.
Field studies (N = 7) included three prospective cohort studies (PC), two randomised controlled trials (RCT), one retrospective cohort study (RC), and one survey design. A total of 9,905 participants took part in all studies, of which just 87 were female (0.88%). The effect studied regarding headgear was incidence of concussion of players wearing headgear vs. players not wearing headgear. One study additionally assessed modified vs. standard headgear, while another additionally assessed days absent from concussion in headgear vs. non-headgear users. Two studies reported time loss from concussion. All studies looked at concussion incidence during match-play, one study also looked at training. Although the Marshall et al. [5] study conducted telephone interviews, it was included in this category as its primary outcomes were injury rates. The Kahanov [20] study was included as it reported quantitative data (closedended questions) in its survey. Playing level ranged from under-15 to professional. Two studies exclusively looked at youth (under-20) populations. Results are summarised in Table 2 Lab studies (N = 7) included a variety of methods for dropping headforms onto a surface. Drop heights ranged from 20 to 91.2 cm. Numbers of headgear tested ranged from two to ten. Criteria for drop testing followed standards set by World Rugby (WR) [35] or the National Operating Committee on Standards for Athletic Equipment (NOCSAE) [36]. Six studies looked at repetitive impacts. All studies used a variety of impacts sites on the headforms. Five studies measured peak linear acceleration (g), two measured impact energy (J) or the Head Impact Criterion (HIC), and one measured impact velocity (m/s) or the Gadd Severity Index [37]. One study additionally measured peak rotational acceleration (rad/s/s). Results are summarised in Table 3.

Quality and levels of evidence results
Two reviewers (SH and ND) independently assessed the quality of studies.

Qualitative studies
Qualitative studies were analysed according to the Standards for Reporting Qualitative Research (SRQR) checklist [34]. Results can be seen in Table 4. Results show commonalities of research design principles present in most studies. Titles, abstracts, results, and discussion sections generally adhered to the checklist, although three studies [21,38] provided only partial information for item 4 (purpose or research question). Methods sections were less adherent, with no studies providing adequate information for item 5 (qualitative approach and research paradigm) and one study [38] providing information for item 6 (researcher characteristics and reflexivity). Information on data collection methods, instruments, and processing was minimal in two studies [19,21]. Similarly, the 'other' section of the checklist revealed discrepancies: one study did not provide information on conflicts of interest [19], and only one study provided information on funding [38].

Field studies
Observational field studies were analysed according to the Strengthening and Reporting of Observational  Studies Epidemiology (STROBE) [33]. Results can be seen in Table 5. Direct comparison of all studies was problematic due to variability in research design and concussion definition. Common limitations of field studies included unclear objectives, lack of clearly defined variables (especially potential confounders), unclear explanation of quantitative variables (such as why groupings were chosen), and inadequate descriptions of study limitations. Regarding headgear use, there is a general lack of specific information on headgear wearing rates such as not reporting on the number of participants who actually wore headgear in each group. The Hollis et al. and Kahanov et al. studies relied on a questionnaire or survey asking about headgear use at a single point in the season [39]. This is problematic, as the reported rates of concussion are reliant on accurate description of headgear use. The Marshall et al. study was found to be susceptible to selfreport bias as the authors were reliant on obtaining injury data from players on a weekly basis, and such reports were gathered by telephone [40]. By contrast, other studies involved direct observation by trained personnel. Video verification was used in the Stokes et al. study (all games) [41] and the McIntosh and McCrory study (six randomly chosen games) [42].
HG headgear, PLA peak linear acceleration, PRA peak rotational acceleration, HIC head impact criterion  Regarding validity, the Kemp study used a wide definition of concussion which may have overestimated incidence rates [43]. The Marshall et al. study included wide demographic variation (different age groups, levels of play, different genders) which may decrease the specificity of its findings towards a particular target population [40]. The McIntosh and McCrory study had a lack of control for confounding variables [42]. All other studies adjusted for confounding variables, thereby improving internal validity. External validity appeared sound; no evidence exists to suggest the staff, places, and facilities described in studies were not representative of the environments to which participants would normally be exposed.
Studies did not consider that higher risk-taking behaviour may negate benefits of headgear prevention, but there is also potential bias in incidence data if players wearing headgear played more conservatively. Training exposure was only captured in one study [40]. The Hollis et al., Stokes et al., and Marshall et al. studies included adjusted rate ratios accounting for previous injury [39][40][41], therefore the remaining three studies may have over-estimated incidence by not considering this variable [42][43][44].
Studies consistently refer to the challenging nature of objectively defining concussion. Participants may have sustained sub-clinical concussions or other head trauma, making them more susceptible to concussion despite the use of protective equipment. There is variation on the standards used to measure injury and concussion incidence: four studies used player-hours [39,41,43,44], the remaining two used concussion time-loss [40,42]. The Hollis et al. study lists its outcome as mTBI rather than concussion [39]. Baseline concussion history was only reported by Hollis et al. [39]. The Kahanov et al. study did not report an outcome measure of effect, had self-reported concussions, and did not use a denominator to calculate incidence rates [20].

Lab studies
Experimental lab studies were analysed. Direct comparison of all studies was problematic due to variability in measurements used to assess impact acceleration and attenuation. Given the lack of a standard quality assessment tool for lab studies, a checklist was generated using a model adapted from Benson et al. [22]. Results can be seen in Table 6. Labbased study designs lack external validity, namely, game and practice conditions differ from laboratory settings and the impact forces required to produce concussion are not agreed upon [14]. All studies reported on the validity and variability of the testing apparatus used. Levels of evidence in lab studies are dependent on multiple factors. Hollis et al. [39] McIntosh et al. [44] Kemp et al. [43] Marshall et al. [40] McIntosh et al. [42] Kahanov [20] Title  [45]. Commercially available headgear is more likely to be worn by players but tended to perform worse on impact tests. The three most recent studies used a no headgear condition to act as a control [11,24,46]. Therefore, these studies provided more valid comparisons of headgear performance vs. a baseline. The two McIntosh studies provided information on foam testing [45,47]. All studies reported on the headgear used (density range 45-87 kg/m 3 ; thickness range 7-20 mm). Two studies did not provide detail on the thickness and density of headgear used but do provide information on brands used [24,46]. The Knouse et al. and Draper et al. studies provided the greatest detail in makeup of the type of headgear (honeycomb vs. flat panel), thickness, and density [11,12]. The Hrysomallis and McIntosh studies (N = 3) did not report on the cellular makeup of the foam present in the headgear [45,47,48].
All studies except Hyrsomallis [48] used repetitive impact testing and its rationale for use, i.e. mirroring repetitive impacts experienced in gameplay. Number of repetitions varied from two to ten times. The Ganly and McMahon study carried out a separate repeated impact test on the NPro headgear, involving 1,920 impacts, to simulate up to three years of use [24]. All studies reported on the sites of impacts tested. Three studies reported on the lack of padding at the back of headgear [11,12,48], suggesting impacts received to the unprotected occipital region might result in higher impacts than front or side impacts. The Ganly and McMahon study included pendulum testing on two moving headforms thereby replicating on-field head-to-head contacts, but did not test impact to the back of the head [24]. The Draper et al. study justified inclusion of side impacts as the most common impact site in gameplay [11].
Most studies did not report testing headgear in different environmental conditions (wet, humid, temperature). Four studies [12,45,47,48] reported testing took part in ambient temperature (20-22 °C) while the McIntosh et al. study reported testing headgear in ambient, hot (50 °C), and cold (− 10 °C) temperatures [47]. All studies reported using spherical headforms in testing, McIntosh and McCrory provided more detail in describing a deformable skin more like a human head [45]. Four studies [11,24,45,47] reported the use of a Hybrid III headform [49]. Two studies reported on details of the glycerine "brain" encased inside a sealed cranium and included details on the "neck" of the headform [12,48]. All studies reported on the details of the impact surface used. The McIntosh Table 6 Laboratory studies analysed using a model adapted from Benson et al. [22] PLA peak linear acceleration, PRA peak rotational acceleration

Checklist item
Draper et al. [11] Frizzell et al. [46] Ganly and McMahon [24] Hrysomallis [48] Knouse et al. [12] McIntosh et al. [47] McIntosh  [47]. All studies reported on the makeup of the impact pad, with three studies reporting on the thickness of the impact pads used (1-1.6 cm, 1.3 cm, 2.5 cm, respectively) [11,12,47]. The Frizzell et al. study used an artificial pitch impact pad to mimic gameplay [46]. Regarding the measurement of rotational as well as linear acceleration, one study (Ganly & McMahon) reported data on peak rotational acceleration (PRA) but provided less detail on its results [24]. The Draper et al. study asserted there is no currently accepted method to measure PRA [11]. Regarding outcome values, two studies reported peak acceleration (g) values only [46,47]. Three studies reported both peak acceleration and HIC [11,45,48]. The Knouse et al. study also used the Gadd Severity Index [12]. The Ganly and McMahon study also measured rotational acceleration (rads/s 2 ) and velocity (km/h) [24], but two studies reported PRA is not currently able to be studied adequately under lab conditions at this time [11,46].
All studies reported on the details of the drop testing rigs used to measure linear acceleration. The Ganly and McMahon study also used a pendulum rig to simulate head-to-head player contact [24]. All studies report on height and impact energy. The McIntosh et al. and Frizzell et al. studies did not report mass [46,47]. The Knouse et al. and Frizzell et al. studies did not report velocity [12,46]. The Knouse et al. and Draper et al. studies provided detail on the use of twin wires to guide the headform onto the anvil and reported on calibration of the testing rig prior to study commencing [11,12].
Regarding the quoting of authority standards used in approving equipment and procedures, four studies reported on using NOCSAE standards [11,24,46,48]. The Frizzell et al. study did not report on World Rugby standards for headgear [46]. The three most recent studies [11,24,46] quoted EN960 standards for headforms used [50]. The McIntosh et al. study provided minimal information on approval specifications for the testing rig [47], while the Hrysomallis study calculated mean impacts for concussion from previous field reports [48]. Frizzell et al. quoted popularity of artificial pitches in choosing its impact pad [46].
Finally, reporting on statistical analyses varied. The two McIntosh studies did not report statistical analysis used [45,47]. The Hrysomallis study did not explain which statistical analysis was used in its methods section, but quoted correlation co-efficients in its results section [48]. The four remaining studies utilized multiple data analysis methods, which may improve internal validity [11,12,24,46].

Discussion
The aim of this systematic review was to review the use and effectiveness of soft-shell headgear in rugby union. Despite the variation across different study types and designs, several commonalities and trends can be observed. The use of headgear in rugby fits within a larger framework of both (a) protective equipment use and injury prevention within the sport and (b) concussion mitigation strategies, including rule changes such as lower tackling height, and proper tackling techniques which may better protect the head in contact. Outcome variables (numbers of papers related to each objective of this review vs. study type) are summarised in Table 7, and the discussion section is organised according to each objective.

Use rates of soft-shell headgear in rugby union
Although helmets have a long history of use in sport [51] and headgear use has been sanctioned by World Rugby since 1993 [52] headgear usage rates in rugby remain low. To date Japan is the only country to mandate headgear use in rugby [52]. Marshall et al. reported overall headgear use of 14.2%, with 32.6% of schoolboys and just 1% of women players wearing it [5]. This is generally reflective of headgear use rates quoted in qualitative studies used in this review. Little evidence is present to suggest a change in headgear use trend, although junior players tend to wear headgear at higher rates [5,38].
Many factors were cited for the use or non-use of headgear. Reasons cited in the wider literature include players being told to wear headgear by their parents [53] or due to parental concern over concussion as awareness of SRC increases [54,55]. Other usage reasons were often repeated in studies included in this review. These may be broadly arranged into the following categories: cost, comfort, utility (scalp/laceration protection), utility (concussion prevention), and aesthetics. Even if studies can show improvement in one targeted area, namely utility: concussion prevention, the other categories may remain as barriers to increasing usage rates. The qualitative studies in the review all report headgear usage rates, but some may not have high levels of external validity. One reason for this may be lack of representative samples used in these studies. Although female players represent 23% of players worldwide [28], just 10.6% of respondents in quoted studies were female [18][19][20][21]38]. Although usage rates and attitudes towards headgear of female players appears to mirror that of males [53], better adherence to items 5 (qualitative approach and research paradigm) and 6 (researcher characteristics and reflexivity) on the SRQR checklist could result in higher reporting quality for gender, socio-economic, and socio-cultural factors. No included qualitative studies adequately addressed these issues in their methodology.
External validity may also be impacted by issues of compliance in headgear use and reliance on self-reporting. McIntosh et al. expressed concern over consistency of headgear use throughout the season [44]. Other studies mentioned players may only wear headgear due to another extraneous factor, such as when recovering from head or scalp injury [18,38]. In a clinical commentary on the McIntosh et al. study, Swartz and Marshall describe this lack of compliance as a "profound" limitation of the study [56]. This issue may be partially solved by having participants as own controls. If players choose to wear headgear then compliance may improve as by doing so they are demonstrating an understanding of the reasons for headgear use. Subsequently, the reliability of results in studies with similar designs may increase.

Consumer understanding of protection provided by soft-shell headgear in rugby union
Multiple relevant points regarding understanding of protection were raised by qualitative and field studies. Several studies reported the supposed link between use of protective equipment and the concept of risk compensation first described by Hagel a21nd Meeuwisse [57]. Of note Menger et al. reported players who believed headgear prevented concussion were four times more likely to play aggressively []. This concern has been mentioned in media reports [58], but has not been backed up by other studies. Patton and McIntosh found that players who stated they would "tackle harder" when wearing headgear were less likely to continue to hold that opinion after a season of playing [31]. Rivara et al., in a consensus statement, concluded there is insufficient data to suggest such risk compensation exists [59].
Generally, studies show players were not confident in the ability of headgear to prevent concussion. Pettersen and Kahanov et al. reported more than half of the players in their studies believe headgear "often" or "sometimes" prevents concussion [19,20]. This is not generally reflected in the wider literature however, and a more recent study reported just 10% of participants wore headgear to prevent concussion [18]. Moreover Reid et al. reported 98% of their study's participants recognise headgear does not prevent SRC [60]. Although conflation of the terms head injury and concussion is problematic, this may represent a trend in increased awareness of headgear effectiveness and concussion knowledge in general. The validity of this trend may be backed up by Zuckerman et al. who found no significant difference in performance on neuro-cognitive tests between cohorts of helmeted and un-helmeted athletes after sustaining an SRC [61].
These findings are particularly relevant to the youth population. Junior players tend to wear headgear at higher rates and may have more confidence in its protective qualities [5,[18][19][20][21]. Children play contact sport at a higher rate than adults and are generally over-represented in concussion numbers [2,4,62]. Kuhn et al. reported concern over children regarding the relative size of their heads compared to their body, their less developed neck musculature resulting in higher risk of injury at the cranio-cervical juncture, and that children may experience greater angular acceleration given tangential impact [30]. The authors suggest none of these factors would be lessened by the use of headgear.

The effectiveness of soft-shell headgear in rugby union to mitigate concussion and head injury
The outcomes of all studies suggest the effectiveness of headgear in preventing or mitigating the effects of concussion remains unclear. Qualitative studies generally did not report incidence, but Kahanov et al. listed 32 concussions in participants wearing headgear vs. 104 without (but no effect of measurement was given) [20]. In field studies incidence varied, but only Kemp et al. reported a significant protective effect of headgear with an incidence in the headgear group of 2.0/1,000 players hours vs. 4.6 for the non-headgear group [43]. Hollis et al. (2009) reported reduced risk for players always wearing headgear (IRR 0.57) [39]. Lab studies, particularly more recent studies involving more recently developed technology, reported generally more promising findings, but these have yet to be generalised to the field [11,24,46]. Although not looked at specifically in this paper, animal studies have found headgear offered significant protection from moderate mTBI symptoms in animal models [63].
Regardless of the quality of reporting in the reviewed studies multiple factors regarding concussion pathophysiology cannot be adequately addressed. Lab and field studies focus on direct impact to the head, but TBI and concussion can also be caused by non-direct, inertial impacts (commonly referred to as "whiplash") for which helmets or headgear likely offer little protection [14]. The Concussion in Sport Group (CISG) defines this as a blow "elsewhere on the body with an impulsive force transferred to the head" [64]. The acceleration-deceleration of these events has been suggested as injurious to the brain in contact sports [51,[65][66][67]. Moreover, Sone et al. suggested it is more difficult to design helmets that reduce low-impact accelerations and peak pressures that are distributed throughout the brain [51].
Rugby has relatively few catastrophic injuries [68,69] suggesting repetitive, low impact collisions may be more strongly associated with concussion. King et al. estimated the average amateur rugby player can expect 77 impacts per game [70]. Many of these impacts are low impact, whereas large linear forces, such as one causing a skull fracture, is not necessarily associated with TBI [71]. Moreover, multiple studies show the tackle as the primary concussion mechanism in gameplay [68,72] and that lowering tackle height does not decrease concussion incidence [73,74]. This suggests the inertial and rotational forces of tackle impacts, even without direct contact to the head, can still result in concussion. Similarly, the headforms used in lab studies often lack a "neck" and therefore cannot replicate rotational impacts typically associated with concussion [75].
An obstacle in attempting to compare these studies is a lack of a consistent operational definition of concussion. Although all field studies reported some definition of concussion, or reported concussion was verified by a physician or trainer, reviews often quote this as being a potential source of bias [6,22,26,27,64]. In addition to having trained personnel making observations in the field, it has been suggested that video analysis with software coding [72,[76][77][78][79] and the use of instrumentation such as mouthguards [70,80] or skin patches [81,82] would help in verification of collision events [31,69].

The capacity of soft-shell headgear to reduce linear and rotational impact forces
Despite recent lab studies showing promising impact attenuation of NPro and Gamebreaker units [11,24], the most recent field study [41] did not find evidence of headgear reducing concussion risk, suggesting the trend over time has not changed. Although headgear is positively correlated with prevention of scalp and laceration injuries [83] concussion risk appears to be mostly unchanged. This is inclusive of the youth population, where headgear usage rates are higher. Another confounding variable is the lack of knowledge and agreement on the concussion threshold. King et al. reported concussive events as low as 54.9 g [70], but Guskiewicz and Mihalik concluded the threshold is elusive and may be irrelevant [14]. Therefore, clarification of the size of an impact that will likely result in concussion remains unclear, but scales such as the Head Impact Criterion (HIC) remain a useful way to measure impact severity [11].
Biomechanical lab studies conducting drop tests using headforms have recently produced more promising results in impact attenuation of headgear [11,24,46]. Lab studies tend to use more objectively measurable outcomes, such as HIC and NOCSAE [37,84]. Recently the need for experimental validation in all theoretical aspects of biomechanical lab studies has been emphasized [85,86]. Challenges in the generalisation of these studies remain though. Draper et al. stated that peak rotational acceleration (PRA) cannot be adequately studied as no accepted method of quantifying it currently exists [11]. Hernandez et al. suggested headform testing is unable to produce acceleration directions common in field impacts due to the physical limitations of lab equipment [75]. Rowson and Duma stated combined information on PRA and PLA is needed to assess the probability of concussion [87].
The makeup and design of rugby headgear is also relevant to this analysis. In lab studies as early as 2000 authors were making recommendations for minimum thickness of 12.5-16 mm to provide adequate thickness for impact attenuation [45,47,48]. Concern was also raised by Knouse et al. regarding the relatively open back design of rugby headgear, potentially increasing the likelihood of occipital region concussions [13]. Earlier studies reported flat panel headgear had superior attenuation performance [12,47] compared to honeycomb headgear designs, but McIntosh et al. only reported this difference at higher drop heights [47]. In reviews of these earlier studies, Benson et al. described the results as inconclusive [22] while Patton and McIntosh (2016) suggested including oblique tests would better assess concussion risk reduction [31].
More recent studies, conducted since World Rugby's approval of headgear as a medical device, have reported on the density of the foam used in headgear. The better performing headgear in two studies were either composed of open-cell polyurethane foam or ethylene vinyl acetate (EVA) foam with a layer of impact-absorbing foam composed of viscoelastic polymers [11,24]. These studies suggest better impact attenuation than previous studies. Frizzell et al. reported superior impact reduction on the side of the head [46], which is significant as the temporo-parietal region has been reported as the most common site for head impact during play [79,88]. Although increased density may help with impact attenuation, increased headgear bulk may influence wearing preference, as well as increasing mass on top of the player's neck [48]. The most recent reviews did not look at lab studies [26,64].

Directions for future research
Despite the risk of concussion in rugby much of the literature mentions the generally positive impact of youth sports outweighing its risks [59]. More research is needed in multiple areas, namely (a) the involvement of female players and teams; (b) the use of instrumented mouthguards and better systems of surveillance; (c) more field studies to validate lab findings; (d) a greater focus on youth populations; (e) objective measurement of impacts and collisions in field studies, rather than using concussion incidence as the primary outcome measure; (f) to establish a threshold for concussions and an understanding of against what types of impacts protection is required; (g) improved methods to study headgear performance at impact protection; and (h) reducing concussion risk itself within the framework of rugby rules and regulations.
An additional consideration for headgear-based field studies regards the ethical considerations around performing randomized trials involving headgear vs. no-headgear cohorts [51] and the fact blinding cannot be used in direct observational studies. Additionally, the problem of using concussion incidence as an outcome measure remains due to a lack of clear operational definition, ongoing issues with the accuracy of reporting, and disagreement over the threshold required for concussive injury [22,31,64]. If studies purport to measure concussion, or an impact that may result in concussion, then a consistent operational definition of concussion should be clearly stated [17,26,59]. If a survey or questionnaire is given to players asking about head injury this may skew results. Head injury is a wider term which includes skull fracture, lacerations, facial fractures, dental injuries, and eye injuries and so should be avoided in concussion studies [22]. Current best practice suggests using more objective outcome measurement of collision impacts and matching these to physician-verified concussive events. This is valid as the link between head impacts and concussion has been well documented [5,17,67]. Ideally more than one measurement (such as video analysis and instrumentation) should be used to rule out false positives and quantify collision frequency, velocity, and acceleration. Results of field studies could then be replicated under lab conditions to confirm findings. If such headgear is shown to be effective in impact attenuation in the lab and in the field this could result in more players electing to wear it.
The wider literature also strongly suggests combining any use of headgear and protective equipment with a wider program of concussion prevention. This may include collaboration with coaches (Salmon et al. reported coaches as the primary point of contact for concussion reporting [62]), and education programs such as RugbySmart [89,90]. Evidence on physical conditioning and rule changes in preventing concussion is more equivocal. Garnett et al., in a scoping review, showed limited results (especially in the adolescent population) [91] while Hislop et al. demonstrated reduced concussion incidence in an intervention group who had undertaken an exercise programme [92]. Two studies reported increased concussion rates when rugby laws were changed to lower tackle height [73,74].
Other factors should be considered when assessing the role of protective equipment in rugby. Discussion of concussion mitigation speaks to a wider issue of injury safety. Rugby in New Zealand remains very popular, but youth participation rates have fallen by 24.1% in the last twenty years [93]. Parental concerns regarding safety in rugby and other football codes, has been raised in the media where it is often linked to concussion and decreasing playing numbers [94,95]. Parental concerns regarding sport injury safety and risk have been mentioned in the literature with particular emphasis given to rugby and other football codes [96,97]. Future studies should consider these variables.

Limitations
This systematic review was subject to multiple limitations. The heterogeneity of included studies resulted in difficulty comparing studies across different designs and populations. This necessitated a narrative approach to analysis which may hinder objectivity and negated the ability to perform a meta-analysis. Comparison between study designs (qualitative vs. field vs. lab) is problematic, but even comparison within these areas is difficult due to their heterogeneity. This is particularly true in field studies, which may use varying outcome measures. Moreover, the results of these diverse studies are challenging to generalize to a wider research area. The efficacy of helmets and headgear are specific to a particular sport (in this case rugby), necessitating the use of smaller, more localized cohorts, thereby decreasing the statistical power of findings. Any discussion of headgear must also involve a long list of confounding variables in its use and effectiveness, such as reasons for and against wearing headgear (qualitative studies), differences in age and skill level (field studies), and testing in different environmental conditions (lab studies). If studies purport to measure concussion, or an impact that may result in concussion, then a consistent operational definition of concussion should be clearly stated. Head injury is a more generic term and should not be conflated with concussion.

Conclusion
This review focused on the perceived and measurable ability of headgear to reduce concussion in rugby union. As concussion is difficult to define, future research could focus on measuring collisions. Such research could benefit from multiple verification measures such as instrumented mouthguards. Qualitative studies show improved understanding of headgear use, while also highlighting the need for further education. While recent lab studies show promising results in new headgear design, these results need to be replicated in the field. Any recommendations for headgear use must be used in conjunction with clear protocols regarding rules of the game, head injury assessment, education, and tackling techniques. Other risk factors, such as concussion history, should also be considered. Psychological, gender, and sociocultural factors may also play a role in the implementation of any concussion prevention strategy, of which headgear use may be considered one component. Funding Open Access funding enabled and organized by CAUL and its Member Institutions.

Data availability statement
The datasets generated during and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare no competing interests.

Conflict of interest
The authors declare no financial or non-financial interests directly or indirectly related to this work.
Ethical approval and Informed consent Ethical approval and informed consent were not sought for this systematic review as it does not involve direct research with human participants. The protocol for this systematic review was registered on PROSPERO and can be accessed at https:// www. crd. york. ac. uk/ prosp ero/ displ ay_ record. php? Recor dID= 239595.
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