The aims of this study were to: (1) determine whether the incidence and severity of concussion have changed over the surveillance period in club and international rugby union; (2) quantify the likelihood of a player sustaining concussion relative to the number of rugby union matches; (3) establish whether concussion is associated with an increased risk of subsequent injury in professional rugby union; and (4) examine if concussion confers greater risk of specific types of subsequent injury.

Concussion was associated with 60% risk of subsequent injury (of any type) in rugby union 7 and 50% in association football (soccer), 8 in addition to a greater risk of sustaining lower limb injuries in college-level contact sports. 9 10 Subsequent injuries also occur more quickly following a concussion than a non-concussive injury. 7 However, it is not known what tissue types and body areas are at increased risk in rugby union.

At the professional level, injury risk also appears to vary between club and international rugby union, with international match incidence being twice that of club rugby. 6 Yet, no study has directly compared injury rates that include data from players who are involved with both the international and club levels, while also ensuring similar injury surveillance methodology. Additionally, recent RFU injury data indicate that concussion incidence has been increasing at club level over recent years, but injury severity appears to have remained relatively unchanged. 2 However, this longitudinal data source has not yet been assessed at the international level.

A high proportion of professional rugby union players sustain multiple injuries over consecutive seasons. 1 Of the common injuries, concussion has the highest match injury incidence, with the English Rugby Football Union (RFU) reporting a rate of 15.8/1000 player-match-hours in 2015/2016. 2 This contrasts with earlier injury surveillance in 2006 reporting concussion incidence to be 1.4/1000 player-match-hours within South African professional rugby union, 3 and a mean incidence of 4.7/1000 player-match-hours cited in a 2014 worldwide rugby union injury meta-analysis. 4 However, the low injury incidence in earlier years may represent under-reporting, meaning recent injury data from 2012 onwards could provide better estimates of true concussion incidence due to the roll-out of the Head Injury Assessment and concussion education programmes. 5

Following this step, players were sorted into those who sustained at least one concussion and those who had not. The injury data for players who had suffered no concussions formed the non-concussive injury set used for reference comparisons. Players who had suffered a concussion had their injury data split into two sets: (1) injuries sustained prior to the first concussion and (2) injuries sustained following the first concussion. For the non-concussive injury set a ’first' injury was randomly chosen from each player’s injury data to allow a similar split to be determined. The longest possible interval between injuries was 4 years because no time frame limit was imposed. Models were evaluated using the log-likelihood test. A p value greater than 0.05 indicated that the null hypothesis should be assumed and all HRs were consistent with unity. The R programming language (V.3.3.1) and the survival library (V.2.40–1) were used to perform the survival analysis. 17 18

The Andersen-Gill extension to the Cox proportional hazards model was used to quantify the risk of subsequent injury following return to play. 15 16 HRs were calculated to compare players who had suffered concussions with those who had not suffered a concussion. Several preprocessing steps were taken to prepare the data. First, only active players on team rosters were selected to ensure each player had a complete set of injuries; if a player was not on a team roster for a Welsh club the injury was excluded. Club and international injuries were combined to avoid undercounting injuries to players who represented the national team.

where n is the number of player matches and is the incidence of an injury per player match, that is, 80 player-minutes of exposure (see online supplementary 1 for the derivation of this formula). The smallest n such that is the number of matches where there is a higher probability than not of an injury being sustained by a single player. This metric represents the average risk over the whole team, and does not take into account differences between players. Club and international injuries were combined to avoid undercounting concussion injuries. As such the metric will be referred to as the risk to the average player. The Python programming language (V.2.7.12) with numpy (V.1.11.2), scipy (V.0.18.1), statsmodels (V.0.6.1) and matplotlib (V.1.5.3) libraries was used for incidence, severity and risk calculations. 14

The standard metric used to report injury incidence is the number of injuries per 1000 player-match-hours. This metric allows comparisons to be made across different sporting populations, but it is difficult to infer specific player risk. Therefore, the incidence was used to calculate the risk of injury to a single player. The metric used to quantify the risk was the number of matches required for an injury to be more likely than not. The formula used to compute this quantity was:

Injury severity refers to the number of days lost of training and match play due to an injury. The distribution of injury days lost was found to be skewed and did not fit well to any probability distribution so the median was used to represent days lost. The positive (right) skewness originates from most injuries resolving in a relatively short time, with a small number of injuries requiring longer recoveries. Uncertainties for injury severity were calculated using bootstrapping. 13 Comparison of severity distributions was done using a Kolmogorov-Smirnov two-sample test. A weighted linear least squares fit was used to determine whether incidence and severity changed over time. Weights for the fit were taken to be the reciprocal of the size of the 95% CI, meaning points with narrower CIs were weighted more highly.

Injury frequency for concussion sustained during matches was characterised by injury incidence, which is the mean number of concussions sustained in 1000 player-match-hours. Incidences were calculated for club and international teams separately, and collectively by combining the club and international injuries and exposure. To compare two injury incidences a rate ratio (RR) was calculated. The 95% confidence interval (CI) was calculated using normal distribution, since the log of the ratio of two incidence rates is normal. Two incidences were significantly different if the 95% CI for the RR did not intersect with unity.

The injury definition and data collection procedures complied with the international consensus of injury surveillance in rugby union. 11 Injuries were coded using the Orchard Sports Injury Classification System V.10. 12 Additionally, each injury included information about the activity at time of injury (eg, match/training, contact/non-contact), mode of onset and number of days lost per injury. Exposure was calculated by assuming each match lasted 80 min and each team consisted of 15 players. Occurrences where players were withdrawn due to injury or foul play (yellow/red cards) leaving fewer than 15 players per team in play were not accounted for.

The Welsh Rugby Union injury surveillance programme has established an injury database collected from the Wales senior men’s international team and the four professional Welsh rugby clubs (Cardiff Blues, Dragons, Ospreys and Scarlets) for the 4 years spanning the 2012/2013–2015/2016 seasons. Ethics approval was obtained from the Cardiff Metropolitan University School of Sport Ethics Committee and informed consent was obtained from players involved in the first team squads each season. The teams’ physiotherapist recorded all time loss injuries during each season and submitted injury records at the end of each month, or end of each tournament in the case of the international team, to Cardiff Metropolitan University (ISM). Any discrepancies in injury records were immediately checked and reconfirmed following the submission of the data to the independent researcher. For the purpose of this study, only injuries sustained during matches were used for calculations of incidence, severity and subsequent injury risk, with all other time loss presented to provide a contextual understanding.

Risk of injuries to specific body regions was greater after concussion than prior to concussion ( table 2 ). Injuries to the head and neck (HR 1.34; 95% CI 1.06 to 1.70), upper limb (shoulder and arm, HR 1.59; 95% CI 1.19 to 2.12), pelvic region (buttock and groin, HR 2.07; 95% CI 1.18 to 3.65) and the lower limb (leg, ankle and foot, HR 1.60; 95% CI 1.21 to 2.10) were more likely in players following a concussion than to players who did not suffer a concussion. Injuries to joints and ligaments (HR 1.49; 95% CI 1.22 to 1.81) and to muscles and tendons (HR 1.38; 95% CI 1.12 to 1.70) were also more likely following a concussion ( table 3 ).

There was a greater subsequent injury risk following a concussion compared to following a non-concussive injury (HR 1.38; 95% CI 1.21 to 1.56), with players who sustained a concussion having a 38% greater injury risk than players who did not sustain a concussion. Injury risk was assessed before and after players had sustained a concussion, with players having a 23% greater injury risk after concussion than before concussion (HR 1.23; 95% CI 1.05 to 1.44). No difference in injury risk was found between players prior to sustaining a concussion and players prior to sustaining a non-concussive injury (HR 1.13; 95% CI 0.97 to 1.31). The median time to next injury following a concussion was 36 days (95% CI 32 to 40), which was shorter than the time to next injury following an injury other than a concussion (49 days; 95% CI 44 to 53).

The match injury incidence (injuries per 1000 player-match-hours) of concussion in the first and last seasons of the injury surveillance period, and the proportionality constant from a weighted least squares fit of the incidences from all seasons

Concussion was the most common injury (10% of club and international injuries combined) and the incidence was shown to increase over the four seasons of club, and club and international rugby combined ( table 1 ; figure 1 ). The severity of concussion did not change over time in club, international, and club and international rugby combined. The median severity of concussion over the time period was the same for club, international, and club and international rugby combined (9 days; 95% CI 8 to 9). The average player had a higher risk of sustaining a concussion than not after 25 matches (95% CI 19 to 33) in club rugby, after 24 matches (95% CI 12 to 55) in international rugby and after 25 matches (95% CI 19 to 32) in club and international rugby combined, based on the incidence from the 2015–2016 season. Considering the squad size and player-specific match exposure per season across the 4 years, the proportion of the squad playing more than 25 matches per season was 10%.

The match injury incidence over the surveillance period was 94.5 injuries/1000 player-match-hours (95% CI 89.1 to 100.2). There was a lower match injury incidence in club rugby (87.0 injuries/1000 player-match-hours; 95% CI 81.5 to 92.7) compared with international rugby (177.0 injuries/1000 player-match-hours; 95% CI 151.9 to 205.1; RR 2.04; 95% CI 1.73 to 2.39). The overall median severity for all injuries was 10 days lost (95% CI 9 to 10). There was no difference between the median severity for club rugby (10 days lost; 95% CI 10 to 11) and international rugby (9 days lost; 95% CI 8 to 10).

The data set contained 2441 injuries to 367 players, from a total player pool of 429 players. Therefore, 86% of players sustained an injury during the surveillance period. Match injuries accounted for 1602 injuries, while there were 514 injuries during training. The remaining 325 injuries were sustained during other activities. The total match exposure was 11 960 player-match-hours, of which 1000 player-match-hours were from international matches.

Discussion

In Welsh Rugby Union, concussion injury incidence increased over the four seasons at a club level, but not international level; severity remained unchanged at both levels. Players were more likely than not to sustain a concussion after 25 matches and there was a 38% greater injury risk after concussion compared to following a non-concussive injury. Specifically, the subsequent injury risk was greater for the head and neck, upper limb, pelvic region and lower limb.

Change in incidence and severity of concussion Our finding of an increasing secular trend in concussion incidence in elite Welsh Rugby Union, when combined with findings from the RFU Injury Surveillance Project,2 suggests that concussion incidence is rising throughout the professional game. At the international level, the concussion incidence was higher than in previous reports.6 19–21 However, the low level of exposure for the international team led to larger uncertainties. Combining longitudinal injury surveillance data from multiple international teams is recommended to see whether the recent increase in concussion persists at the international level. World Rugby, rugby union’s international governing body, has implemented several concussion policies at the professional level. Others have speculated that the Head Injury Assessment protocol led to a rise in the reported incidence of concussion following its introduction as a pilot in 2012 and officially in 2014.2 As a result, in the 2014/2015 season, concussions may have been reported that previously may not have been included. However, the continued rise in incidence in the 2015/2016 season implies there are additional factors other than stakeholder awareness that contribute to the increased concussion incidence. Identifying concussion risk factors is an important next step. Researchers may need to take into account surface-specific match play demands (eg, tackles made and tackle technique) as grass has a greater concussion incidence than artificial surface.22 Furthermore, 50% of concussions occur in the tackle event7 and recent reports show that an upright posture by the tackler increases the risk of the tackler needing a Head Injury Assessment compared with a bent at the waist posture.23 Note that the latter technique is outlined as best practice in World Rugby’s coaching education.24 Therefore, assessing whether tackling behaviour has changed over recent years may provide further insight into changes in concussion incidence.

Concussion risk The average rugby union player had a higher risk of sustaining a concussion than not after 25 matches; this rate was three times higher than the next most frequent injury in our data (thigh haematoma). Additionally, based on current season lengths (mean 34 matches) the data show that 1 in 10 players will surpass the 25-match threshold of concussion risk each season. However, with concussion incidence rising year on year and 17% of professional rugby union players reporting a concussion in the 2015–2016 season,2 this appears to be a conservative estimate possibly due to being an ‘average’ player metric. Nevertheless, the data suggest that longer seasons would potentially expose more players to the risk of concussion due to a greater proportion of the squad playing more than 25 matches each season, meaning decisions regarding season length should consider the injury risk implications.

Subsequent injury risk The subsequent injury risk to players following a concussion was 38% higher than for players who did not sustain a concussion. Although the increased risk was significant, it was lower than the 60% reported for similar level players in the RFU Premiership7 and lower than the 47% reported in professional association football.8 When considering studies in rugby union, the reported risks are consistent with each other, within calculated errors, with the combined estimate of subsequent injury risk in rugby union from this study and Cross et al’s7 study being 1.49 (95% CI 1.37 to 1.61). When the association football (soccer) data are included,8 the combined estimate of subsequent injury risk is 48% (HR 1.48; 95% CI 1.30 to 1.67). It appears that the increased injury risk (all injuries) following concussion is not specific to a single jurisdiction or sport. The mechanisms underpinning this increased injury risk are not known. However, with the consistency between the two rugby union studies it could be speculated that the current length of the graded return to play protocol is insufficient because players can go from complete rest to full match play within 6 days.25 Previous research has shown that increases in weekly workload that are greater than 10% were associated with an increased injury risk.26 Consequently, if a longer graded return to play protocol was implemented, workloads could be progressed gradually to safer levels, which may mitigate some of the subsequent injury risk by ensuring ‘spikes’27 in acute workloads are avoided. There were several body regions, in addition to joint, ligament, muscle and tendon injuries, that had a greater injury risk after concussion. A greater lower extremity injury risk was reported in collegiate athletes from multiple sports.9 10 Possible underlying mechanisms include altered balance strategies28 and impaired dynamic stability,29 30 in particular dynamic balance28 and walking gait,29 30 which are negatively affected following concussion. This may also explain part of the increased injury risk to the joint, ligament, muscle and tendon structures, as loading across these structures may differ following concussion. We contend that small changes to balance and gait after concussion may increase subsequent injury risk.7 This is the first study to identify the head and neck, and upper extremity body regions to also have an increased risk following concussion. It is conceivable that the nature of contact in rugby union exposes players to a heightened risk to these body areas, with the tackle event a common injury mechanism.2 6 Additionally, if players are tackling higher up the body (eg, shoulder and above), a Head Injury Assessment is four times as likely than if tackling lower down the body.23 Therefore, the high initial risk of concussion and subsequent risk to upper body areas may be a by-product of tackle technique that does not follow recommended guidelines.24 However, contact is unavoidable in rugby union and it is unlikely that all concussions are due to improper tackle technique. As a result, further mechanistic work is required to understand whether the increased risk to these body areas is preventable within rugby union by considering whether factors such as oculomotor and vestibular dysfunction,31 and impaired dynamic stability and proprioception29 30 affect tackle technique. The combination of epidemiological and experimental data supports the recent concussion consensus statement32 that graduated return to play protocols should look to include multifaceted strategies such as neuromuscular control, and oculomotor and vestibular protocols to both aid identification and quantification of postconcussion symptoms, and mitigate subsequent injury risk.