11. Protective equipment (low level of certainty that a risk factor for concussion)

Thirteen studies (6 mouth guard, five padded headgear, 3 face visors) investigated the ability of protective gear to reduce concussion risk 

• Mouth guards:
  • The effect of mouth guards is unclear.  "Surprisingly, a trend for increased concussion risk was shown in American football players wearing mouth guards, possibly explained by the phenomenon of "risk compensation," which occurs when protective equipment use induces an increase in risky behavior that may negate the possible effect of such equipment.
  • 5 of the 6 studies found no significant effect of mouth guard use on concussion risk.
  • 1 level II study showed a decrease in concussion rate after introducing customized mouth guards in 28 American football players, although the study's methodology has been heavily criticized, including the fact that the players were older when they used the customized mouth guards and there was an overrepresentation in concussion rates when using the standard mouth guards as both match and practice injuries were included compared with only match injuries for the customized mouth guards.
  • The relation between mouth guards and concussion prevention is constrained by the use of static cadaver skulls to mimic dynamic biomechanical and biological processes, the limited evidence for an association between force transduction and concussion induction, and few sport concussions resulting from mandibular impact.
• Padded headgear was show to decrease the risk of abrasions and lacerations, but its effectiveness in preventing concussions needs to be determined.
  • A trend for padded soccer headgear in a level II prospective cohort of adolescent soccer players showed a 2.5-fold greater concussion risk, but the study's authors said the finding needed to be interpreted "cautiously" because no 95% confidence index was given and concussions were based on recall of symptoms.  There is also biomechanical evidence that commercially available soccer headgear products do not have the structural ability to prevent concussions.
  • Studies of rugby headgear yielded mixed results:
    • 2 studies indicated that headgear had no significant effect on concussion risk;
    • 1 large level II prospective cohort study of adult amateur rugby playersd found that the use of padded headgear decreased concussion risk.
    • 1 large level I study that included 81 concussions found that rugby headgear did not significantly decrease concussion risk in professional rugby union teams.
• Two studies observed the effects of facial protection on concussions in ice hockey:
  • No difference was found in concussion rates in players wearing full, partial or no facial protection.
  • One study found that players who complained that the face mask obscured their vision were 5 times more likely to be concussed.

The researchers concluded that the "overall effect of protective equipment is inconclusive," thus warranting assigning a low level of certainty for protective equipment as a concussion risk.

12. Body checking (low level of certainty that a risk factor for concussion)

Body checking is thought to increase concussion risk, especially in younger players. 

• 4 of the 6 studies showed a greater concussion risk due to body checking in ice hockey players ages 11 to 16
• A prospective study of 986 ice hockey players reported an increased concussion risk for those permitted to body check (13-14 and 15-16 years) compared to those who were not (9-10 years), but the age difference between the comparative groups makes drawing conclusions about the effect of checking on concussion risk difficult.
• Although prohibiting body checking at younger age groups is thought to increase injury risk due to poor technique, one study found no difference in concussion risk between players with body-checking experience (checking allowed at 9-10 years old) and novices (no checking alllowed)
• Only 2 of the 5 studies on body checking were of a high level of evidence of I, with the remaining studies being level II and III.

A limitation is that most lacked age-matched comparative groups, thus a low level of certainty was given for body checking as a concussion risk estimate.

13. Environment (low level of certainty that a risk factor for concussion)

3 studies analyzed whether environmental factors affect concussion risk:

• Neither of two level II studies comparing concussion risk on natural and artificial grass in elite soccer players found a significant difference  in concussion risk between the two surfaces, although one noted a tendency for match concussion to be reduced on the artificial turf. Both lacked the large numbers of concussion often required to identify significant associations.
• A large scale level II retrospective study found that youth football-related concussions were more likely to occur at school compared with at a recreational facility or home.

"Owing to the limited number and size of studies observing the effect of playing environment, a low level of certainty was assigned," the study said.

14. Other factors (low level of certainty that a risk factor for concussion)

The following risk factors that were assessed by a single study were assigned a low level of certainty:

• rugby union players with less training and below average body mass had higher concussion rates
• junior ice hockey players in the lowest body weight quartile were at increased concussion risk;
• players who used a wheelchair sustained less concussions
• trends for fewer concussions were reported in youth ice hockey winning teams and football players with low aerobic fitness
• a trend has been identified between the temporal side of the head and concussions in soccer and football, which may be due to the fact that impacts to the side of the head are often outside of the player's field of vision, limiting the abilty to engage the neck muscles necessary to decrease head acceleration after impact, thus increasing concussion risk; the side of the head may also be biomechanically more vulnerable to an impact force.

 Level of Certainty (high-low) Definitions Used for Risk Assessment

"The investigation of risk factors is an important step twoards understanding [what causes] concussion. Although several risk factors were identified in the appraised studies, poor study methodology caused constrained estimation of concussion risk for almost all of the investigated risk factors, except previous concussion and match play," Abraham writes.    

1. Abrahams S, McFie S, Patricios J, Posthumus M, September AV. Risk factors for sport concussion: an evidence-based systematic review.  Br J Sports Med. 2014;48:91-97.  doi:10.1136/bjsports-2013-0927734.