Documentation:FIB book/Mouthguards as Injury Prevention Devices

Introduction

Mouthguards are a piece of protective equipment often used in sports. They were first introduced in boxing, then football, and now are recommended in 29 sports or exercise activities including acrobatics, basketball, skateboarding, skiing, and wrestling by the American Dental Association and the International Academy Sports Dentistry . Mouthguards are designed to separate the upper and lower dentition and a portion of teeth from the surrounding soft tissues and absorb high impact energy. The mouthguard primarily protects from orofacial injuries but there is additionally debate on whether they also protect against concussions, a common traumatic brain injury.

According to Woodmansey’s review, an athlete is 60 times more likely to sustain a dental injury when not wearing a mouthguard. According to Glassman reviewed by Tuna and Ozel, an athlete has a 10% chance of receiving orofacial injuries every season of play and the chance increases to 33-56% throughout the athlete’s entire playing career. As well, sport related concussions represent 20% of all diagnosed traumatic brain injury. Patients with concussions experience short term neurological function deficiencies and can occationally experiance loss of conciousness, often taking up to a month to recover with an increased risk of future concussion. These injuries can affect patients’ well-being physically, psychologically, and socially; therefore, injury prevention of both concussions and facial injuries from sports is an important area of research. The following literature review summarizes the current research on mouthguards and their effectiveness as well as future improvements and uses to understand and prevent concussions and orofacial injuries.

Facial Injuries

Anatomy of Mandible

In this section, we will focus more specifically on the mandible. Although valid dental injuries can also be prevented with a mouthguard, this literature review will focus on the effect of mouthguards on the mandible.

Temporomandibular joint (TMJ) injuries as well as mandibular fractures are main areas of focus of research connecting mouthguards to injury prevention of the mandible. The TMJ, as shown in the figure, is the joint that connects the mandible to the skull. Injuries to this joint can cause long term chronic pain that can result in limited jaw movement and difficulty chewing. Most studies found that the use of a mouthguard decreases the likelihood and severity of mandibular injuries through many different methods of analysis. However, other studies report improper use of mouthguards may increase likelihood and severity of injuries.

It can be observed through epidemiological reports that mouthguards generally decrease risk of mandibular injury. This can be supported by an empirical study done by Takeda et al. An artificial skull was attached to a pendulum, and data from strain gauges and accelerometers on the skull was collected on the distortion of the mandibular angle with and without a mouthguard. The distortion of the mandibular angle was reduced by 39.9% when a mouthguard was in use.

In addition to the main investigation into general effectiveness of mouthguards at preventing mandibular injuries, two supporting points of interest are presented: thickness and occlusion of the mouthguard.

Thickness of Mouthguard

A relationship between shock absorption and thickness of mouthguard was observed through two different studies with two separate methods. The first study used an artificial skull – the same one used in the previously mentioned study done by Takeda et al. The second study used finite element analysis to measure the maximum principal stresses in the facial bones. As the effectiveness of the mouthguard’s shock absorbency increases, the likelihood of an injury decreases, since the forces that were once directed at the mandible are now being absorbed by the mouthguard. Since both studies have the same conclusion, it further strengthens the argument that the thickness of the mouthguard is proportional to the thickness.

Anatomy of temporomandibular joint with disc alignment

Occlusal Protection

Another variable that changes from mouthguard to mouthguard is its precise positioning within the mouth. Another study by Takeda et al shows that the improper alignment or insufficient occlusal protection of mouthguard to mandible can result in fracture. The study suggests if the mouthguard does not fit properly, the wearer of the mouthguard may clench more than necessary to ensure it does not fall out.

This can exacerbate previous injuries. An experiment involving volunteers was conducted to evaluate the disc displacement of clenching the jaw while wearing a mouthguard. As seen in the figure, if there is disc displacement, there would be misalignment. Magnetic resonance imaging (MRI) was used to evaluate volunteers who had anterior disc displacement and those without. The displacement worsened in those with pre-existing disc displacements when clenching their jaws with a mouthguard on. In addition, the thicker the mouthguard, the more displacement there was.

Despite these studies showing that improper use of mouthguards may increase risk of mandibular fracture and severity of pre-existing conditions, no regulatory standards exist for use of mouthguards in sports. These potential standards could greatly improve mandibular injury prevention. Something as simple as requiring mouthguards to be custom fit by a dentist may reduce any injuries that may occur with incorrect use or application.

Injury Prevention Against Concussions

A concussion is clinically described as a type of traumatic brain injury (TBI) which results in loss of neurological function from an impact causing an impulsive force to the head. A concussion diagnosis is based on a set of symptoms which are temporary but long-term effects have also been recognized. Considering that each year there are millions of cases of concussions from sports alone and that many concussions are unreported due to having no visible signs, developing protective equipment to protect athletes against concussions is an important issue at hand. Researchers have hypothesized that mouthguards may reduce the risk of concussion in athletes; however, the literature on the subject is unclear and contradictory.

Research Showing a Correlation Between Mouthguards and Reduced Concusssions

Studies connecting mouthguards as a possible piece of injury prevention equipment have been studied as early as 1985. There have been biomechanical laboratory studies such as Takeda et al. as mentioned previously using an artificial skull and a pendulum to impact the skull with and without a mouthguard. This study found that the mouthguard reduced the acceleration acting upon the head. Assuming that there is a correlation between linear acceleration of the head and concussions, there is the possibility of the mouthguard reducing risk of concussion. Additionally, a 2020 study by Chisholm et al. found through 315 youth hockey players that there was a significant 69% decrease in concussions when wearing a mouthguard.

Research Showing No Correlation Between Mouthguards and Reduced Concussions

However, the correlation has been debated profusely over the last 20 years in papers such as McCrory, who argues the sample sizes for most studies are too small to show a strong effectiveness at injury prevention and that cadaver studies such as Hickey et al. which impacted skulls to measure cranial pressure which was then equated to an increase of concussion risk. These two arguments are connected to the problem that there is no injury criterion which gives a clear indicator of whether concussions exist. Therefore, much of the biomechanical studies are speculative based on whether an acceleration or a force measured in the head is thought to cause the concussion. Further, there is not a lot of data on what forces and accelerations are felt by athletes during impacts which cause concussions. In other words, it is unclear whether a reduction in acceleration or force on a human surrogate in a lab would prevent a concussion. Additionally, in terms of epidemiology, Knapik et al. found little to no evidence of a reduction of concussion from those wearing mouthguards.

Clearly there is contention and no clear agreement of whether mouthguards prevent concussions. In the future, more research into injury criterion for concussions is necessary. Once a consistent injury criterion is found, it may be possible to adjust the design or shape of the mouthguard to better protect against concussions. Until then, there continues to be no clear correlation between concussions and wearing mouthguards.

Emerging Technologies

Multi-material structures and Material design

Impact absorption is one of the areas of research still in development for mouthguards, however this is highly tied to improvements gained from developing new materials. Two main factors can play a role in improving impact absorption properties, maximizing contact with the mandibular teeth and improving the shock absorption properties of the overall construction and materials used. Miyahara et al.’s study consisted of impact testing composite layups consisting of candidate mouthguard materials to try and find a more effective design for improving this impact absorption property. The tests consisted of sandwiching ethylene vinyl acetate (EVA) around a foam substrate. The form was not in the complex shape of a mouthguard but was created to get comparisons of the material properties of this novel mouthguard material. Improvements were observed in the shock absorption of the tests done on the sample; however, some plastic deformation was observed on the foam after impacts were complete. These promising results suggest that the use of composite materials in mouthguards may eventually provide more effective solutions.

Although impact absorption is critical to the types of high energy events that would cause injury, there is another area of development for improving the safety of mouthguards: sanitation. Dirty mouthguards can contain dangerous concentrations of bacteria, yeast, and molds that could cause infections and disease to people who improperly use them. Yoshida et al., tried to improve the innate ability for mouthguards to be cleaned by imbedding silver particles into an EVA surface and observed the antibacterial properties on samples of bacteria like E. coli. The study yielded positive results where the material exhibited similar shock absorption properties to market materials while gaining these antibacterial properties, but the direct implementation of this approach is prevented by the possibility of silver ingestion. Over a long period of time this silver ingestion may provide harmful effects on those using mouthguards. The idea of creating mouthguards which can prevent injuries while minimizing the dangers of a dirty or improperly maintained mouthguard may be achieved in other ways.

Mouthguards for Experimentation

Rather than actual improvements on the performance of mouthguards, sensors can be outfitted into custom made mouthguards to help characterize the impacts experienced by athletes in the real world. This enables natural experiments to be made as was done in a study trying to characterize which individuals were subjected to dangerous impacts in football collisions. By implementing sensors into a natural experiment, researchers are able to study full scale impacts that would never be conducted in a lab experiment due to the likely deleterious effect on the subject. As noted by Cecchi et al., if similar research were done on a large enough scale, the data could be used to compare impacts of different types of safety equipment such as helmets.

Discussion/Future Research

It is important to understand the ways that injury occurs in the variety of impact sports that mouthguards are deployed to prevent. These are broken down into the major topics of dental, orofacial, and intracranial injuries that participants in sports may experience. Mandibular injury prevention and concussions were covered in this report to outline where the current research is with respect to the effectiveness of mouthguards in preventing injury. Improvements in impact absorption and maximizing tooth contact have been identified as further places of development for reducing the danger of mandibular injuries. It is near unilaterally agreed in both metastudies and on a case-by-case analysis that mandibular and orofacial injuries are less likely to occur in relevant sports when an individual is properly using a mouthguard.

Concussions are an area of debate on if mouthguards play a real role in prevention when looking primarily at metadata on if current designs had the desired effect of preventing these injuries. A metadata analysis across a large cross section of research on this topic found no significant difference between mouthguards increasing or decreasing the probability of concussion. An important distinction is that this and many other studies are characterizing the effectiveness of current designs. Given the uncertainty in how concussions actually occur, further research is needed to properly characterize these mechanisms. After which, an evaluation on if mouthguards are capable of effectively mitigating these injury conditions given typical impacts should be conducted.

As seen through the papers examined above, there was significant diversity in methodology for testing of materials and quantification of mouthguard properties. This is warranted for experiments trying to isolate specific properties like material, and test the impact that these have. However, when this is applied to how safe the mouthguards are, comparisons across studies are difficult. This is something that is a key problem in testing the effectiveness of mouthguards against current and alternative designs. Knapik et al. identifies that many studies on orofacial and concussion injuries that are currently available have low methodological quality. Problems were identified ranging from papers not identifying why the experiment was being conducted to factors like poor external generalizability and low statistical significance of the results.