An Analysis of The Potential Causes and “Cures” of Malocclusion

An Analysis of The Potential Causes and “Cures” of Malocclusion

 

Hello there. This is an evidence summary I’ve created on the potential causes and cures (or treatments) of malocclusion. It has been written with the guidance and suppourt of countless individuals who I am incredibly grateful to. I wanted to begin by thanking those people, as without them, this would not have been possible.

 

If you’re reading this, chances are it is because I asked you to. I just wanted to say, thank you too! I hope you find this informative, or at the very least an engaging read. I must warn you, however, that this is still in its early stages, and so the work is largely conversational while I try and piece the evidence together. I also want to state that I am one admittedly biased layman, not a dentist or orthodontist. My background is in nutrition and dietetics, which I have a Masters degree in. Because of this, I doubt I will have the level of understanding and comprehension to truly make a professional thesis on the subject – but health is my passion, and I wanted to give this an honest shot, in the hope that it might contain something of value to someone. Anyway, Let us begin by identifying the issue at hand.

“From a broad perspective, only about one-third of the U.S. population has normal occlusion, while two thirds have some degree of malocclusion. In the malocclusion group, only a small minority (not more than 5%) have problems attributable to a specific known cause; the remainder are the result of a complex and poorly understood combination of inherited and environmental influences.” (Proffit, Fields & Sarver. 2006.)

 

In other words, it’s multifactorial, and these factors are not well understood. This work, “Contemporary Orthodontics”, a leading authoritative text in orthodontic education, was published in 2006. And yet, nearly 15 years on, we don’t seem to have learned much more. Malocclusion, which refers to crookedness of the teeth and jaws, still appears to be sparsely understood, and there unfortunately doesn’t seem to be a significant push by orthodontic bodies into conducting research to identify its potential “root” cause(s). In the world of health, where new data and mechanisms are constantly appearing in order to deepen understanding and more effectively treat everything from heart disease, cancer, and mental illness, the aetiology of malocclusion remains an inexplicably neglected anomaly.

 

While some could certainly argue that the incentive of providing a costly treatment to a significant chunk of the population may be a factor in this lack of interest (In many areas of health, prevention is often much cheaper than treatment), I believe that the vast majority of orthodontic professionals care deeply about their patients. I don’t think that they are actively trying to hide information, or exploit them, which can be a common sentiment whenever the aetiology of diseases is discussed. I think we may just be so acclimatised to the fact that most of us are affected by these issues, so much so that we fail to notice or question them. Ultimately, however, 60% of us experience a poorly understood condition which can be uncomfortable, painful, expensive, and potentially detrimental to physical and mental wellbeing, to the point of it being life threatening. And if only 5% of total malocclusion cases can be linked to conditions such as dental diseases, syndromes and physical traumas, impacting teeth position or hindering jaw growth, what could the remaining factors responsible for the other 60% be? I think this is a fascinating question, one that could potentially improve the lives of billions of people. It deserves our immediate attention.

Image result for mandibular ankylosis
Image result for mandibular ankylosis

While there are countless schools of thought, there is generally one accepted hypothesis, which states that malocclusion is a result of genetic changes throughout recent human evolution, leading to the development of smaller jaws. Because the size of the teeth did not change along with this, there is a tooth size to jaw size discrepancy. The end result is crooked teeth. Another idea is that some element of our diet, such as its nutritional content, or toughness may be a factor. Lets explore these ideas.

 

According to the Australian Society of Orthodontists, “when extra teeth or abnormally large teeth create a malocclusion…the culprit is usually genetic in nature. Other inherited traits involve jaws that are too small to accommodate a full set of teeth and misaligned jaws that did not form properly. In most cases, underbites, overbites, or crooked teeth are genetic and can’t be avoided. Orthodontic treatment with braces will be necessary to correct the condition once your child is old enough to wear them.” (ASO, 2019). Malocclusion can also contribute to a range of health problems, including wear and tear, gum disease and injury, as well as tooth decay. (Betterhealthvic, 2019).

 

Beyond standard crooked teeth, wisdom tooth impaction is another highly frequent issue, affecting 72% of the population in the mandible (lower jaw) alone. Dodson et al. (2010) highlighted that impacted wisdom teeth “can cause inflammatory dental disease manifested by pain and swelling of infected teeth and may destroy adjacent teeth and bone.” Only 11.6-37% of retained wisdom teeth are free of disease and other symptoms (Dodson et al., 2010 & 2012). Therefore, most people in the modern world need to have them removed. Given the high rate of wisdom teeth impaction, as well as the frequency of potentially severe side effects, this begs the question as to why this is such an issue in the first place. Wisdom teeth impaction, by its definition, is a result of insufficient space in the jaws to house all 32 teeth, but why is this the case? If this is a deadly genetic issue that has existed for millennia, why does it exist in such high frequencies today, seemingly unaffected by natural selection? It’s hard for me to believe that our ancestors frequently suffered these detrimental health effects, which would have likely led to their deaths in an already incredibly harsh environment, let alone passed the genes responsible down at such high frequencies. it certainly calls into question the “wisdom” of such an evolutionary path. 

 

I think we should also take into consideration the importance of properly functioning jaws to our survival, at least, prior to modern times. This might sound surprising, as today, even severe malocclusions may not have a huge impact on our ability to eat a diet that is very soft and calorically dense (think mashed potatoes, pasta, liquid calories such as milk, etc.).

 

However, history provides a different, but fascinating dietary story. Our ancestors, for most of our evolution, subsisted on uncooked or partially cooked animal and plant foods, which were mostly calorically dilute. Notice the image on the left (from Proffit, Fields & Sarver. 2006), of an Indigenous Australian man eating a piece of kangaroo with bare hands. We can see from the striations present that he is using significant amounts of not only jaw but upper body strength for this meal. To survive on this kind of diet (knife, fork and blender not included), properly functioning jaws and teeth would be essential, as its toughness and low calorie nature would require a prolonged amount of intense chewing daily. It isn’t hard to imagine how, say, a substantial underbite, or teeth that don’t meet together correctly, would, to put it bluntly, be a death sentence in these scenarios.

 

Finally, given that malocclusion is also typically seen as less attractive, we would expect sex selection to play a role here as well (Pithon et al. 2016). And yet, despite all of these factors, which, presumably, would encourage malocclusion to occur at low frequencies, we are currently faced with something of a pandemic that affects most of the populace. For these reasons, and more, I’m just not convinced that genetics is the primary factor that it’s made out to be.

 

For my next point, observe the faces below.

There are 5,146 species of mammal (I realised later that sharks are not mammals, but still, those are impressive chompers). However, as we can see, they don’t seem to be struggling with severe malocclusions as humans do. When was the last time you saw a tiger with an underbite, for example, or a chimpanzee that required up to 8 teeth extractions, due to small jaws? Is it just our genes which are defective? I’m skeptical about this.

 

Perhaps by examining the occlusion of other primates, who are closest interspecies relatives, we may be able to get a slightly better understanding of the role, or lack thereof, of our genes.

Research by the dental historian Colyer, presented in the work of Mills (1963) indicates that overall, the rate of malocclusion across mammals is substantially lower than modern norms in humans. However, while the existence of malocclusion in other primates is still substantially lower than our own, at 27.3%, it still is something of an anomaly here in the animal kingdom. Let’s take a closer look.

When this data is broken down further, we find that O.W (old world) monkeys have a higher malocclusion rate than most other primates. However, Mills notes that “many of the irregularities were confined to very slight rotations of the premolars, so slight as to be insignificant. The sixth column shows the percentage of irregularity in these two groups when such cases were excluded.”

 

Mills goes on to say that “They are usually mild, and affect the occlusion but little. They fall into Angle’s class I (where the posterior teeth and specifically first molars are in normal antero-posterior relation), and similar conditions are seen in man, where the condition is often more severe.”

 

Regarding Class II malocclusion, in which the upper jaw overlaps the lower jaw, there was not a single identifiable case of this occurring. This is fascinating, as the prevalence of this form of malocclusion in humans can occur at a rate of up to 40% (Bilgic, Gelgor & Celebi 2015).

 

The final column gives the percentage of cases in which the lower incisors occlude in front of the uppers. This is known as a Class III malocclusion, or an underbite. While the rate of 24.7% in Colobinae may seem high, research indicates that it may in fact be an evolutionary response to their particular diet high in leaves, for efficient mastication. “The results suggest that a combination of mechanical pressures and idet may explain the underbite characteristic and that it is an adaptive trait to these dietary pressures” (Knowles & Sirianni, 2014). Unfortunately, the same cannot be said for Class III malocclusions in humans, which is considered to be maladaptive, and can significantly interfere with eating food.

Here is a side by side comparison of a “typical deep underbite” in a P. melalophos male of the Colobinae subfamily (Zingeser 1970) compared to a substantial underbite in a human. Clearly there is a difference in severity and the effect on mastication and overall function of these two images. Something is up here.

Colyer’s research also found that captive state specimens often had far greater levels of malocclusion than their wild state counterparts. This gives an indication that something environmental may be at play. I think this evidence paints a fascinating picture.

 

So, why is it that our closest relatives in the animal kingdom have a significantly lower rate of malocclusion compared to us? What makes our own rate of malocclusion so severe, compared to every other life form on the planet? Let’s delve into this, and begin with the occlusion of early humans.

 

According to anthropology professor, Peter Lucas, author of ‘Dental Functional Morphology’, “Virtually any mammalian jaw in the wild that you look at will be a perfect occlusion, a very nice Hollywood-style dentition.” Lucas argues that the mechanical process of chewing, combined with the physical properties of foods in the diet, will drive tooth, jaw, and body size, particularly in human evolution.

 

Evolving to Eat Mush  https://www.sciencedaily.com/releases/2005/02/050223144712.htm

 

If this is a genetic issue, what kind of selection process may be shrinking the jaws, causing insufficient space for teeth, and therefore malocclusion? One idea is that as our brain grew, our jaws shrunk, but I don’t see why these have to be competitive processes in any way. Plenty of animals have far bigger brains than ours, and don’t have these issues.  The idea presented in this same article is that, perhaps due to the softness of cooked foods, our jaws have gradually evolved to become smaller, as we don’t require jaws as large and strong to mechanically process our cooked, soft diets. We will explore that soon.

 

According to the general scientific consensus / wikipedia, we’ve been anatomically human since over 200,00 years ago. Perhaps we can observe the occlusion of our early ancestors to learn more. What do the teeth of these early humans tell us? If there is some modern environmental factor at play, studying the skulls of pre-agricultural humans would be a great place to start.  They would have similar brain sizes to us, after all. If their occlusions are significantly different from ours, it would provide credence to the idea that there is something about our modern environment that is responsible.

 

Let’s keep in mind that even small observable changes can require hundreds of thousands of years of evolution to occur. Given this, we would therefore expect malocclusion to have existed at a similar percentage for Homo sapiens, hundreds of thousands of years ago. Except, the evidence paints a different picture. 

 

100,000 year old specimens from the Krapina cave in Yugoslavia. Skeletal remains from approximately 80 individuals present “near-perfect alignment or minimal crowding was the usual finding in this group.” (Proffit, Fields & Sarver. 2006.)

 

Notice the broad, U-shaped dental arches and perfect occlusion in figure A. Figure B had the biggest teeth in the group, yet only experienced very minor crowding. Fully functional wisdoms in every specimen. Keep in mind these individuals have room for their wisdom teeth as well, something that is very rare in modern peoples.

Related image
Related image
Image result for kennewick man
Image result for kennewick man

Here we see a 10,00 year old native American male skull.  Notice the robust, horizontal growth of the skull and broad features, as well as the healthy occlusion of all teeth, including the wisdom teeth.

 

https://www.cdapress.com/archive/article-7bcae138-1a25-11e6-8843-d3a01c727354.html

https://www.ancient-origins.net/news-history-archaeology/spirit-cave-mummy-0010987

 

According to professor of Anthropology, Ron Pinhasi, after studying almost 300 skulls from Anatolia to Europe (28,000 to 6,000 years ago), “Our findings show that the hunter gatherer populations have an almost “perfect harmony” between their lower jaws and teeth,” he explains. “But this harmony begins to fade when you examine the lower jaws and teeth of the earliest farmers.” They report the jaw “shrinking.” http://www.ucd.ie/news/2015/02FEB15/050215-Malocclusion-and-dental-crowding-arose-12000-years-ago-with-earliest-farmers-study-shows.html

 

The hunter gatherer mandible on the left is robust, horizontally grown, wide, and can fit all teeth, including the wisdom teeth, and space behind the wisdom teeth. In contrast, while the agrarian mandible has wisdom teeth in position, there is significant incisor crowding. Also, notice the V-shaped dental arch, as opposed to the U-shaped ones characteristic of hunter gatherers. 

 

Skulls from the Jomon period of Western Japan (ca. 6,000-7000) years ago indicate this same pattern of broad arches, healthy occlusion and room for all 32 teeth. It might also be worth pointing out the extensive tooth wear which may be indicative of intense and prolonged masticatory activity.

 

https://onlinelibrary.wiley.com/doi/full/10.1002/ajpa.10329

On the right, we have Dr Janet Monge, anthropologist and keeper of the extensive Physical Anthropology section At Penn Museum, which contains thousands of human crania. As we can see, the specimen she is holding has the same features we described before, and room for the wisdom teeth, with extra space behind them. Monge states, “Nobody in the past had dental problems…it’s like the upper jaw, the maxilla, and lower jaw, the mandible, are actually kind of perfectly in unity with each other and the interesting thing is that was everybody in human history. Monge states that there was an emergence of crooked smiles around 150-200 years ago. “It happened fast. Something significant happened, and it’s almost global.”

 

What could that “something” be? If this were a genetic issue, it does not seem possible that a gene could have cropped up 200 years ago and spread across the globe. In the article, Monge refers to two hypotheses. As we’ve heard before, she believes the soft diet of modernity may play a role in jaw development, as well as tongue position. She argues that when the tongue is on the roof of the mouth, this creates a wider palate and affects facial growth, providing room for the teeth.

 

https://whyy.org/segments/could-old-skulls-help-us-understand-why-we-have-crooked-teeth/

Dr Monge kindly sent me this Morton skull from Moravia, which shows the same physical characteristics we’ve described. 

Prominent researcher Robert Corruccini and Elsa Pacciani assessed the occlusion and facial development of more recent Etruscan skulls from the 8-9th Century B.C. “Specimens dating back to the VIII Century B.C. indicate Etruscans may have been the first people to employ orthodontic bands to improve tooth alignment. A survey of dental occlusion in Etruscan cranial remains, however, shows very good typical occlusion and almost no crowding. Thus, these people do not represent the earliest development of epidemiologically high prevalence of malocclusion, a feature instead reserved for the later industrial world.” Below, this skull from the National Museum of Archaeology displays what the researchers refer to as displaying “ideal classic dental occlusion.” (Corruccini & Pacciani, 1989).

 

“Figure 3. A most complete and typical Etruscan specimen…slight degree of vertical incisor overbite, and near-perfect dental alignment throughout lower and upper arches. Although incisor relations were generally difficult to determine for most Etruscan specimens due to postmortem tooth loss and poor preparation and storage, the other occlusal traits are, like this, typically of very low variance from the figured ideal.”

 

“Etruscans resemble hunter-gatherer (and other non-acculturated) people much more than the frequently maloccluded modern western people, confirming the very recent epidemiological proliferation of malocclusion.

 

“Some of the low scores for Etruscans must result from inability to score the relations among missing teeth (although the alveoli could nevertheless be observed), and more reduction in the apparent level of occlusal variability would occur if maloccluded adults were more likely to become edentulous and hence be unscorable. Nevertheless, these nonrandom factors must be fairly minor influences on the results; it is reasonable to infer that the Etruscan samples fall into the lower range of occlusal variations shown by human foragers and primitive agriculturalists, as opposed to modern and industrialized samples.”

 

“Aside from suggested genetic mechanisms, various dietary factors are major potential determinants. Among these, the interproximal attrition brought about by dietary grift may increase archspace in aboriginals. We favor an old anthropological idea that dietary consistency and toughness promote alveolar remodelling and proper permanent tooth eruption, bringing about ideal adult occlusion; when nonresistant processed foods become ubiquitous after industrialisation, malocclusion shows a rapid rise. Etruscan diet, even for the nobility, was not intensively refined. Meat was derived mostly from domestic animals…Cereals, wheat and grains for a relatively coarse bread were the staple foods.”

 

All 50 of these Etruscan skulls were compared with worldwide samplings of aboriginal / forager, acculturating / agricultural, and industrialised / modern populations in regards to crossbite and buccal segment relation traits, with the results viewable below, indicating a link with malocclusion and modernization.

 

As we can see, Etruscans had the lowest incidences of crossbite, followed by Aboriginal peoples, acculturated peoples who were more industrialised than not, and fully industrialised peoples.

So far we have seen that as people move from hunter gatherer to agricultural diets, malocclusion increases, but could it be the case that as people become more and more modernised, so too do the rates of malocclusion increase?

 

Scandinavian researchers tested this idea, and observed 146 medieval 16th century skulls, comparing them with 99 modern children  (Evensen & Øgaard, 2007).

“Only 36% of the medieval group showed objective assessed needs for orthodontic treatment, compared with 65% in the present-day sample…7% of the medieval skulls had severe malocclusion, compared to 21% of the modern sample…This study indicates a significant increase in both the prevalence and the severity of malocclusions during the last 400 to 700 years in Oslo, Norway.”

36% in the middle ages, while still lower than the modern norm by a substantial margin, is certainly higher than the research available from hunter gatherer skulls. Given that these skulls were found merely hundreds of years ago, it is unlikely that this near doubling of malocclusion has a genetic basis.

It seems we are seeing an interesting pattern. Pre-agricultural skulls seem to indicate very low rates of malocclusion, which tended to be mild and may have been related to disease or physical trauma. As we transition to an agricultural / medieval diet, malocclusion rates and severity increase, and in recent modernity, they increase significantly. Impacted wisdom teeth extractions, while common today, don’t seem to have been issues for paleolithic peoples, who had space for all 32 teeth, and fully functional wisdom teeth.

Dr Corruccini studied trends in malocclusion rates using the treatment priority index, often comparing people a generation apart. In this diagram, youths who are presumably more modernised than their parents were more likely to have more serious malocclusions that required treatment. The same trend was found in urbanised individuals compared to those living in rural areas.  This appears to be, as Dr Corruccini puts it, “a disease of civilisation.”

 

All this evidence indicates that this is beyond a genetic issue. It is difficult to believe that our jaws could have shrunk in such a short period of time, or even why our jaws would evolve in such a detrimental way, that could potentially be lethal if we did not have access to very modern medicine. Based on this data, I think it is safe to conclude that there must be some environmental factor or factors at play, which are at the very least contributing to this modern surge. But what could have happened in between the hunter gatherer period, agricultural period and modern period to influence this?

An epidemiological transition in dental occlusion in world populations.

http://www.sciencedirect.com/science/article/pii/S0002941684900356

One idea, although not nearly as well received, is that this change in jaw size is not genetic, but a result of some form of nutrient deficiency. It was popularised by Weston A. Price, a dentist in the early 1900s and author of “Nutrition and Physical Degeneration”. He noticed that in his younger patients, the prevalence and severity of malocclusion was much greater than their parents, which struck him as alarming. Let’s keep in mind that during the 1900s, flour, sugar and canned, refined foods started to become dietary staples in young people, not so much their parents. Could this dietary change be responsible in some way for the rise in malocclusion? What about the isolated pockets of “less modernised” peoples around the world, who ate more traditional diets? To answer these questions, Dr Price travelled the world, seeking out closely connected modern and “tribal” groups of people, to determine if there were indeed changes in occlusion following modernisation. These are some of his photographs. 



In his book, “Nutrition and Physical Degeneration”, published in 1939, Dr Price writes: “While the primitive groups constantly presented well-formed faces and dental arches reproducing the tribal pattern, the new generation, after the adoption of white man’s foods, showed marked changes in facial and dental arch form.” (Price, 2004)

 

In his analysis of facial and dental changes, he noticed a typical pattern of “narrowing of the features and the lengthening of the face with crowding of the teeth in the arch.”

 

In reference to the Melanesian peoples, he writes that while on a diet of native foods of animal life from the sea as well as plants from the land, they developed “well formed faces and dental arches.” However, in “the succeeding generations after the parents had adopted the modern foods, there occurred distinct change in facial form and shape of the dental arches”.

 

But there was something else that caught my eye. In his book, Dr Price makes reference to something fascinating. “While in the isolated groups not a single case of a typical mouth breather was found, many were seen among the children of the lower-plains group.” Could this mean something?

I also wanted to include this photo of the skull of an Indigenous Australian man Dr Price included in his book. Just like we’ve seen before, it has the characteristic wide dental arches, robust facial skeleton and room for all teeth that characterises pre-agricultural humans.

 

Dr Price argued that there may be a nutrient responsible for these physical changes, which he referred to as “activator x”.

 

However, as interesting as these photos are, I would not consider them to be evidence.

 

There is no research I am aware of which indicates that any nutrient could promote a wider facial structure or room for all teeth. In modern society, severe nutrient deficiency and malnutrition aren’t exactly common, and although we are familiar with the effects of different nutrient deficiencies on physical health, there is no correlation I am aware of with nutrient deficiency and malocclusion. It seems likely to me that if there were, we would have discovered it by now. But then why do we see low rates of malocclusion in modern hunter gatherer peoples?

 

Okay, so we have established that the argument for a genetic basis for malocclusion is somewhat sketchy. There certainly does seem to be an increase in malocclusion as societies become more modernised, but there isn’t evidence of a link with any nutrient and malocclusion. But, what if there was something else about the diet that was responsible, such as its toughness? Regarding the Etruscan skulls, Corruccini made the argument that the tough nature of the diet may have promoted the facial bones to develop differently in a way to accommodate all of the teeth. What does more recent research suggest?

 

It turns out, there may be something to this idea. Researcher Cramon-Tabubadel (2011) assessed whether human mandibular (lower jaw) variation reflected differences in agricultural and hunter-gatherer subsistence strategies. She found that hunter-gatherers have “consistently longer and narrower mandibles than agriculturalists.” Rather than a genetic pattern, “these results suppourt notions that a decrease in masticatory stress among agriculturalists causes the mandible to grow and develop differently. This developmental argument also explains why there is often a mismatch between the size of the lower face and the dentition, which, in turn, leads to increased prevalence of dental crowding and malocclusions in modern postindustrial populations.”

 

Other research comparing modern Finnish skulls with specimens from the 15th and 16th centuries found a marked difference in dental attrition or tooth wear, with modern individuals having teeth that were virtually unworn (Varrela 1990). It was found that “the gonial angle…were significantly smaller in the skull sample than in the present-day sample…The results indicate that intensive mastication affects mandibular growth by advancing its anterior rotation.”

 

Experimental research conducted in hyraxes have mimicked this to a degree. Hyraxes raised on a softer diet had significantly less growth (approximately 10%) than those raised on natural diets, resembling many of the differences evident between humans raised on highly processed versus less processed diets. The results support the hypothesis that food processing techniques have led to decreased facial growth in the mandibular and maxillary arches in recent human populations” (Liberman et al. 2004).

 

In one study, squirrel monkeys were raised either on naturally tough or on artificially soft foods, with “significant differences in occlusal features. Animals raised on soft foods show more rotated and displaced teeth, crowded premolars, and absolutely and relatively narrower dental arches. Dietary consistency may be a determinant of occlusal health” (Corruccini et al. 1982). This has been suppourted by research in another primate, baboons. Those raised on a soft diet during growth wer more likely to have disruption of normal occlusion (Corruccini & Beecher 1984).

 

Research hypothesizes that, in humans, “increased loading of the jaws due to masticatory muscle hyperfunction may lead to increase sutural growth and bone apposition, resulting in turn in an increased transversal growth of the maxilla and broader bone bases for the dental arches…An increase in the function of the masticatory muscles is associated with anterior growth rotation of the mandible and with well-developed angular, coronoid and condylar process” (Kiliaridis 1995a).

 

A Swedish study aimed to investigate the craniofacial structure differences in normal adults with those who had significant occlusal wear, indicating greater usage of the masticatory muscles (Kiliaridis 1995b).

 

They found that a less steep mandibular plane  were found for the occlusal wear sample. A smaller gonial angle was also a characteristic for men and women of the occlusal wear sample. Essentially, they had more anteriorly / forward grown faces.

“The results support the hypothesis that functional hyperactivity of the masticatory system imposed increased stress on the bony structures of the craniofacial complex with possible influences on its structure”. Other research demonstrates that in adults, thickness of the masseter muscle (which is used for chewing), is associated with a less steep mandibular angle, and a longer mandibular ramus (Kubota et al. 1998). Individuals with thicker masseter muscles also have vertically shorter facial patterns due to the less steep mandibular angles (Şatıroğlu et al. 2005) and  increased transverse facial growth, or width, indicated by wider maxillary dental arches (Tircoveluri et al. 2013)

 

This research is also suppourted by data on people with myotonic dystrophy, a form of genetic disorder which results in muscular dystrophy, including that of the masticatory muscles. Individuals with this condition have facial features that seem to contrast starkly from healthy individuals, and even moreso from individuals with advanced occlusal wear. Their faces are more posteriorly grown, with steeper gonial angles and longer, more recessed faces.  (Kiliaridis 1995a).

“A high prevalence of malocclusions….were found among these patients (with myotonic dystrophy). Their craniofacial morphology showed a vertical aberration, characterized by…a steep mandible. These findings seem to be most pronounced in patients with an early onset of the disease and suppourt the hypothesis that reduced muscle function may cause changes in the craniofacial morphology.”

Below is an example of the facial growth pattern associated with reduced masseter usage from this study. The facial changes were more severe in those who developed myotonic dystrophy earlier in life, further suppourting the role of masseter usage in facial development.

There is one study I am aware of (Ingervall & Bitsanis 1987) which directly quantified human facial growth response to tough chewing by examining the effects of chewing a hard resinous gum for 2 hours a day for one year in children aged between 7-12. Children who chewed the gum were more likely to have facial growth characterized by anterior (forward) mandibular rotation, “considerably greater than would be expected during normal growth.”

 

Ultimately, it appears that masseter strength has a convincing relationship with facial growth, one that could potentially explain the rise of malocclusion and vertical facial growth patterns which are so prevalent in the modern world today.

 

As discussed, for most of our evolution, human beings subsisted on extremely tough diets. We would have developed very thick masseter muscles as a result of the intense, prolonged chewing necessary to survive in harsh, wild environments. As we transitioned to agrarian and medieval periods, our diets would have become more grain based, processed, and we would also have access to milk (liquid calories) due to animal rearing, which was previously not possible to our nomadic ancestors.

 

This softer diet may explain why the transition from hunter gatherer lifestyles to agrarian and medieval ones was associated with a significant increase in malocclusion, though not quite as severe as in the modern world today.

 

Even in comparison to the diets of medieval and agrarian peoples, our current highly processed diets require almost no masticatory effort whatsoever. This may have further negatively affected our facial growth, causing our skulls to grow in a narrower, more vertical fashion.

 

This may explain why so many of us have jaws which lack the horizontal and transverse space needed for all 32 teeth, leading to crookedness of teeth and impacted wisdom teeth, and possibly other issues such as overbites and overjets.

 

This might also explain why modern day hunter gatherer societies seem less affected by problems in industrialised peoples. Perhaps their diets are still tough enough to encourage a kind of facial growth that is similar to our ancestral norms?

 

Frankly, I’m convinced this is a large factor, but it may not be the only factor. During my research, I also came across studies looking at the effects of oral vs nasal breathing on craniofacial growth, in both animals and humans, and was stunned by what I found. I’ve summarised many of the articles below.

 

Harvold et al. (1981) developed mouth breathing in primates via obstruction of the nasal passages with nose plugs. “All experimental animals gradually acquired a facial appearance and dental occlusion different from those of the control animals. All experimental animals acquired a facial appearance and dental occlusion different from those of the controls.” While dentition developed normally in controls, the experimental group experienced a wide array of craniofacial abnormalities. “The common finding was a narrowing of the mandibular dental arch and a decrease in maxillary arch length, causing an incisor crossbite. Animal 7042 developed the most severed dental malocclusion of this type, a full class III…Some animals developed other types of malocclusion….Animal 8108 developed a severe open-bite….while animal 16440 developed a maxillary overjet and overbite”



“The primates in these experiments developed an oral airway in response to nasal obstruction. The response was not uniform among the animals. However, some traits were common: increased face height, steeper mandibular plane, and  larger gonial angles.”

 

Another study conducted by Britta S. Tomer found the same results in 16 monkeys. The eight of the 16 with induced oral respiration had a lowering of the chin, a steeper mandibular plane, and an increase in the gonial angle compared with the control animals. The same malocclusion results were found as well.

 

Primate experiments on mandibular growth direction. http://www.ncbi.nlm.nih.gov/pubmed/6961782

 

But they’re monkeys, what about humans?

 

The effect of mouth breathing on dentofacial morphology of growing child.

http://medind.nic.in/jao/t12/i1/jaot12i1p27.htm

 

“Changed mode of respiration was associated with increased facial height, mandibular plane angle and gonial angle…longer faces.”

 

Mouth breathing in allergic children: Its relationship to dentofacial development.

https://www.ncbi.nlm.nih.gov/pubmed/6573147

 

“The upper anterior facial height and the total anterior facial height were significantly larger in the mouth breathers. Angular relationships of the sella-nasion, palatal, and occlusal planes to the mandibular plane were greater in the mouth breathers, and their gonial angles were larger. The mouth breathers’ maxillae and mandibles were more retrognathic. Palatal height was higher, and overjet was greater in the mouth breathers. Maxillary intermolar width was narrower in the mouth breathers and was associated with a higher prevalence of posterior cross-bite. Over all, mouth breathers had longer faces with narrower maxillae and retrognathic jaws. This supports previous claims that nasal airway obstruction is associated with aberrant facial growth.”

 

Breathing mode influence in craniofacial development. https://www.ncbi.nlm.nih.gov/pubmed/16446911

 

“It was observed that the measurements for the inclination of the mandibular plane (SN.GoGn) in mouth breathing children were statistically higher than those in nasal breathing children. The posterior facial height was statistically smaller than the anterior one in mouth breathing children (PFH-AFH ratio). Thus, the upper anterior facial height was statistically smaller than the lower facial height (UFH-LFH ratio)…We concluded that mouth breathing children tend to have higher mandibular inclination and more vertical growth. These findings support the influence of the breathing mode in craniofacial development”

 

Skeletal and occlusal characteristics in mouth-breathing pre-school children. https://www.ncbi.nlm.nih.gov/pubmed/15366619

 

The skeletal pattern measurements…indicated a tendency to mouth-breathing children presenting a dolichofacial pattern. According to occlusal characteristics, only the intermolar distance showed a significant correlation with a narrow maxillary arch in mouth-breathing subjects. Based on the results of this study, mouth-breathing can influence craniofacial and occlusal development early in childhood.

 

The effect of mouth breathing versus nasal breathing on dentofacial and craniofacial development in orthodontic patients. https://www.ncbi.nlm.nih.gov/pubmed/20824738

 

“Mouth breathers demonstrated considerable backward and downward rotation of the mandible, increased overjet, increase in the mandible plane angle, a higher palatal plane, and narrowing of both upper and lower arches at the level of canines and first molars compared to the nasal breathers group. The prevalence of a posterior cross bite was significantly more frequent in the mouth breathers group (49%) than nose breathers (26%), (P = .006). Abnormal lip-to-tongue anterior oral seal was significantly more frequent in the mouth breathers group (56%) than in the nose breathers group (30%) (P = .05)…Naso-respiratory obstruction with mouth breathing during critical growth periods in children has a higher tendency for clockwise rotation of the growing mandible, with a disproportionate increase in anterior lower vertical face height and decreased posterior facial height.”

 

Effect of mouth breathing on dental occlusion.

https://www.angle.org/doi/pdf/10.1043/0003-3219(1973)043%3C0201:EOMBOD%3E2.0.CO%3B2

 

“From the present study it can be concluded that the effect of mouth breathing was confined to the changes in maxillary arch dimensions. There was contraction of maxillary arch and increase in maxillary arch length. An increased overjet and deep overbite were present in these cases. The palate appeared high, not because its height was actually increased, but due to contraction of the maxillary arch. A higher percentage of Class II, division 1 malocclusion was seen in mouth breathers.”

 

Facial characteristics of children who breathe through the mouth. https://www.ncbi.nlm.nih.gov/pubmed/6718117

 

“Thirty children with allergy, aged 6 to 12 years, who had moderate-to-severe nasal mucosal edema on physical examination and who appeared to breathe predominantly through the mouth and 15 children without allergy who had normal findings from nasal examination and who appeared to breathe predominantly through the nose were evaluated…In comparison with children who breathed through the nose, children who breathed through the mouth had longer faces with narrower maxillae and retruded jaws. This supports the hypothesis that children with nasal obstruction and who appear to breathe through the mouth have distinctive facial characteristics.”

 

Prevalence and factors related to mouth breathing in school children at the Santo Amaro project-Recife 2005 http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2179-64912011000400005&lng=en&nrm=iso&tlng=en

 

“mouth breathing prevalence was of 53.3%. There was no significant difference between gender, age and type of breathing. Facial alterations were: incomplete lip closure (58.8% X 5,7%), fallen eyes (40.0% X 1.4%), High palate (38.8% X 2.9%), Anterior open bite (60.0% Versus 30.0%), Hypotonic lips (3.8% X 0.0%), Circles under the eyes (97.5% Versus 77.1%)…There were significant differences between physical traits and breathing pattern.”

 

Influence of Mouth Breathing on the Dentofacial Growth of Children: A Cephalometric Study http://europepmc.org/articles/pmc4295456

 

“All subjects with mouth-breathing habit exhibited a significant increase in lower incisor proclination, lip incompetency and convex facial profile. The presence of adenoids accentuated the facial convexity and mentolabial sulcus depth.”

 

Effect of Naso-respiratory Obstruction with Mouth Breathing on Dentofacial and Craniofacial Development https://www.nepjol.info/index.php/OJN/article/view/21343

 

“The mouth breathers had backward and downward rotation of mandible with increased overjet, increased mandibular plane angle, higher palatal plane, and constriction of upper and lower arches at the level of cuspids and first molars when compared with nasal breathers group. The prevalence of posterior cross bite was observed greater in mouth breathers group (40%) than the nose breathers (20%) (p =0.006). Abnormal lip-to-tongue anterior oral seal was seen more in the mouth breathers group (55%) than in nose breathers group (25%) (p = 0.05).

 

 Conclusion: Naso-respiratory obstruction with mouth breathing during growth periods in children has a greater tendency for clockwise rotation of growing mandible, with an irregular increase in anterior lower vertical face height and decreased posterior facial height.”

 

Dental consequences of mouth breathing in the pediatric age group

http://www.ijohsjournal.org/article.asp?issn=2231-6027;year=2013;volume=3;issue=2;spage=79;epage=83;aulast=Malhotra

 

“Mouth breathers demonstrated considerable increase in palatal height and increased overjet, and statistically significant narrowing of the upper arch at the level of the molar. Conclusion: Changed mode of respiration during critical growth periods in children has a higher tendency for increased palatal height and overjet, reduced overbite and maxillary intermolar width.”

 

So, it seems that not only masseter usage, but also mouth breathing, seems to be strongly linked to certain undesirable facial growth and malocclusion patterns, but why is this? Well, some argue that it relates to force. One hypothesis is that it is necessary for the tongue to lie on the roof of the mouth during a child’s growth period, providing a gentle upwards force that encourages horizontal growth of the face. If the child is mouth breathing, however, the tongue would then likely rest on the floor of the mouth. Because the tongue cannot provide the needed upwards force, the face instead develops vertically due to the downwards pulling force of gravity, which would otherwise be counter-balanced by the tongue.

We might think bone is static and unchanging, but below is an example of how growth of bone can be manipulated by binding the skull while it is growing. Of course, this isn’t quite the same as what we’re talking about, but I do think it goes to show how malleable bones are while they are developing.

But why are humans breathing from their mouths? Is it a more common problem in the modern world, and are there other causes for lowered tongue posture? We’re nearing the end of this essay, but lets go through these, too.

 

Abreu et al. (2008) found that in a sample of mouth breathing children, the three most common causes were allergic rhinitis, enlarged adenoids, and enlarged tonsils.

 

Allergic rhinitis does seem to be strongly linked with environmental factors. Research by Chrinstensen et al. (2016) found an inverse link with the risk of allergic rhinitis and the level of urbanisation during upbringing. They point to the possible beneficial effects of microbial diversity as a factor. They state that the prevalence of allergic diseases has increased rapidly since the mid-20th century and has become a major public health problem, particularly in modern industrialised populations, with allergic rhinitis being the most common of all allergic diseases.

 

Diet may be another factor here. Some research indicates that consumption of healthy foods such as fruits, vegetables, nuts and fish, also known as a traditional medietarrrnanea diet, is associated with lower prevalence of allergic rhinitis and asthma (Chatzi et al. 2007). Given that allergies are considered to be a kind of inflammatory response, I must wonder whether our current inflammatory diets which are often loaded with sugar and highly processed foods could be contributing?

Adenoids are a gland found in the back of the throat which are involved in dealing with infections. However, they can hypertrophy, making it difficult or impossible for children to breathe through their noses. Research indicates some common causes of adenoid hypertrophy are chronic infection and allergy (such as allergic rhinitis), and pollution (Rout et al. 2012)

 

I did want to mention that the link between adenoid hypertrophy and aberrant facial development is a strong one. Research indicates that children with thicker adenoids had significant increase in anterior face height, and a retro-positioned, posterior rotated mandible (Koca et al. 2016). This growth pattern is commonly referred to as “adenoid facies”.


Usage of a pacifier may also be implicated, as it encourages open mouth posture, as well as a lowered tongue position. Even though it would not discourage nasal breathing, if the tongue is not on the roof of the mouth, the same negative growth patterns and malocclusion may still occur. Indeed, a significant association is found between malocclusion and pacifier usage, most notably, open bites (Katz et al. 2004).

 

Breastfeeding may be protective, as short duration of breast-feeding is associated with malocclusion, such as posterior cross bite and no maxillary space (Chen et al. 2015). Breastfed infants have wider dental arches (Galan-Gonzalez et al. 2014) and were less likely to develop malocclusion compared to those that were bottle-fed.

 

Infants that are bottle fed are more likely to go on to develop a dolichocephalic (steeper) mandibular plane, overjet, and retruded mandibles, compared to breast fed infants (Sanchez-Mollinz 2010). Personally, I think that it may be the case that breastfeeding encourages infants to raise their tongues to the roof of the mouth in order to release milk from the breast. Bottle-feeding, on the other hand, doesn’t seem to rely on much muscular usage by comparison, and allows the baby to passively suck as they receive milk, with a lowered tongue as well. This might affect the position of the tongue long term.

 

Below, I have included an array of photos which compare the two main growth patterns discussed. On the left hand side are faces which aim to demonstrate the kinds of characteristics associated with poor muscle usage as well as mouth breathing. Namely, vertically grown, narrow, retruding, oval faces and narrow dental arches with crooked teeth. On the right are images of people whose skulls appear more in line with those who breathe nasally (with correct tongue posture) and stronger masseter muscles. This consists of wider, more horizontally grown, square faces with shallower mandibular angles, wide dental arches and straighter teeth.

 

Try to take into consideration the significant skeletal differences between the two sets.






The woman on the left is a surgery patient who uploaded her x-ray prior to surgery. When I first saw this, I was stunned. This skull looks nothing like the paleolithic examples we have seen, which universally were very robust and angular, with the mandible angle nearing 90 degrees, similar to Margot Robbie, on the right hand side. The x-ray, however, shows a skull that almost looks like it has partially melted downwards.







One thing a friend told me when viewing these images, is that the people on the right look more like human beings are “naturally” supposed to look, whereas the people on the left look like there is something “unnatural” about them, as if human beings are not “supposed” to look like that. Could it be that on a biological, instinctual level, we recognise what humans are supposed to look like, and also recognise what humans are “not” supposed to look like? Maybe our brain matches horizontally grown faces onto some kind of instinctual blueprint, and sees vertically grown faces as having a deformity, as our ancestors would not have looked like them? I’m not sure, but I think ultimately these images do a good job of showing the two different growth patterns that human faces can have, generally speaking.

 

So, to conclude, I believe that we could explain the rise of malocclusion and aberrant facial growth patterns in the modern age with two central factors, usage of the masseter muscles and correct, upwards tongue position with nasal respiration. Our ancestors would have subsisted on tough diets, encouraging horizontal facial growth, and would have likely had correct tongue posture due to breastfeeding, and a lack of baby bottle and pacifier usage, and longer term breastfeeding may also reduce the risk of thumb sucking. Their environments would have had low levels of pollution, microbially rich and their diet anti-inflammatory, reducing the risk of allergic rhinitis and other issues which could lead to oral breathing.

 

However, in our modern world, soft and highly processed diets require little effort to consume, leading to weak masseter muscles, which may affect growth. Polluted and inflammatory diets and environments with weak microbial diversity may make us more prone to issues such as allergic rhinitis, enlarged tonsils and adenoids, causing us to mouth breathe, and modern inventions such as baby bottles and pacifiers as opposed to breastfeeding may also affect tongue posture. I’m not sure regarding the nuances of how weak masseter muscles and low tongue posture affects the risk of malocclusion and vertical facial growth differently. There does seem to be overlap in the effects, however.

 

If this hypothesis is true, then by simply addressing these factors, we may be able to turn the tide of malocclusion and vertical growth, creating a generation of people that more closely match our ancestral norms, the way human beings looked and functioned for millennia, with horizontally grown faces, straighter teeth, and room for all 32 teeth. Not only would this have substantial health benefits, but I also think the psychological benefits of looking more like we “should” genetically may also be important for us.

 

Possible Recommendations based on this research?

 

I do want to make it clear that I am not recommending anyone do anything as a result of this article necessarily, but I did want to juggle some possibilities that maybe we should be approaching to ensure the correct development of our children, from day 1, or even before conception.

 

  1. Maternal diet is linked with symptoms of allergic rhinitis in their offspring. It may be beneficial for parents to try and eat as healthy a diet as possible, to reduce the risk of these problems occurring.

  2. When the baby is born, it may be beneficial to assess for lip ties or tongue ties, and deal with them appropriately. My understanding is that for newborn babies, tongue ties can easily be clipped.

  3. It might be a good idea to ensure the baby is brought up in an environment that is not polluted, but this is obviously very difficult to control.

  4. It may be beneficial to promote breastfeeding over bottle feeding, to try and ensure correct tongue posture, and avoid pacifiers, which also lower tongue posture, and possibly sippy cups as well. Perhaps cups should be the only thing in which babies should drink from?

  5. Possibly baby led weaning?

  6. Maybe parents could assess whether or not their young children are breathing through their mouths either during the day or at night, or have other symptoms of chronically blocked noses or breathing difficulties? In this case, it might be a good idea to speak to an ENT who might be able to diagnose issues such as enlarged adenoids or tonsils, and respond?

  7. Some research indicates that exposure to pets is associated with a reduced risk of allergic rhinitis, so maybe parents should get a pet? As long as they aren’t allergic of course, and also it might be important to observe the child to see if they are responding negatively with wheezing or other allergic symptoms. If a young child is showing allergic symptoms, seeing a professional and a dietitian might be a good idea, as it could be food related?

  8. It might be a good idea to encourage chewing of gum such as resinous gum at an appropriate age, such as around 6 years old?

  9. Ultimately, doing whatever possible to try and ensure the tongue is at the roof of the mouth, the child is nasally breathing, and they are using their jaw muscles appropriately. These are possible recommendations to be made based off of this research, I think.

 

Some other stuff I’m not sure how to include properly 

 

Something I also find interesting is, if we all have this genetic blueprint for horizontally grown faces, which are affected by our environment, we would expect to see significant changes in the first few years of life in children who are most affected, and I think we do see this. Most children appear to be born with good looking, square shaped faces. However, years of mouth breathing, soft diets and other issues in these critical growth stages seem to lead to dramatic changes in facial form. One thing I’ve noticed about people with correct facial growth patterns, is that they look most like themselves when they were very young children, while many people with aberrant facial growth look very different to themselves as young children. Below are some examples of what I’m talking about.


Here are an assortment of animal skulls. Notice their mandibular angle is close to 90 degrees, which is similar to the infant skull shown in the right, as well as the hunter gatherer skulls we’ve observed so far.



Observe the difference in the mandibular angle between the hunter gatherer on the left and the vertical grower on the right. While the one on the left has a robust skull with an angle of around 90 degrees, the person on the left has a growth pattern which I have never seen in a hunter gatherer, but is somewhat common in humans today. The angle is much steeper and the face is much more recessed and shallower. I think it is interesting to show that our hunter gatherer ancestors had the same growth pattern which other animals have, and which can also be seen in infants. In this way, modern adults are the outlier, furthering suppourt for the idea that this growth pattern is largely an environmental issue.

References in progress

 

Abreu, R.R., Rocha, R.L., Lamounier, J.A. and Guerra, Â.F.M., 2008. Etiology, clinical manifestations and concurrent findings in mouth-breathing children. Jornal de pediatria, 84(6), pp.529-535.

 

ASO, 2019. Why are my child’s teeth crooked? https://www.aso.org.au/node/20204

 

Betterhealthvic, 2019. Orthodontic Treatment https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/orthodontic-treatment?viewAsPdf=true

 

Bilgic, F., Gelgor, I. E., & Celebi, A. A. (2015). Malocclusion prevalence and orthodontic treatment need in central Anatolian adolescents compared to European and other nations’ adolescents. Dental press journal of orthodontics, 20(6), 75–81. https://doi.org/10.1590/2177-6709.20.6.075-081.oar

 

Chatzi, L., Apostolaki, G., Bibakis, I., Skypala, I., Bibaki-Liakou, V., Tzanakis, N., Kogevinas, M. and Cullinan, P., 2007. Protective effect of fruits, vegetables and the Mediterranean diet on asthma and allergies among children in Crete. Thorax, 62(8), pp.677-683.

 

Chen, X., Xia, B. and Ge, L., 2015. Effects of breast-feeding duration, bottle-feeding duration and non-nutritive sucking habits on the occlusal characteristics of primary dentition. BMC pediatrics, 15(1), p.46.

 

Corruccini, R.S. and Beecher, R.M., 1982. Occlusal variation related to soft diet in a nonhuman primate. Science, 218(4567), pp.74-76.

 

Corruccini, R.S. and Beecher, R.M., 1984. Occlusofacial morphological integration lowered in baboons raised on soft diet. Journal of Craniofacial Genetics and Developmental Biology, 4(2), pp.135-142.

 

Corrucini & Pacciani, 1989. Orthodontistry and dental occlusion in Etruscans. https://sci-hub.tw/10.1043/0003-3219(1989)059%3C0061:OADOIE%3E2.0.CO;2

 

Christensen, S.H., Timm, S., Janson, C., Benediktsdóttir, B., Forsberg, B., Holm, M., Jogi, R., Johannessen, A., Omenaas, E., Sigsgaard, T. and Svanes, C., 2016. A clear urban–rural gradient of allergic rhinitis in a population-based study in Northern Europe. European clinical respiratory journal, 3(1), p.33463.

 

Dodson et al., 2010. Impacted wisdom teeth https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2907590/

 

Dodson 2012. How many patients have third molars and how many have one or more asymptomatic, disease-free third molars? https://www.ncbi.nlm.nih.gov/pubmed/22916698

 

Galán-Gónzalez, A.F., Aznar-Martin, T., Cabrera-Dominguez, M.E. and Dominguez-Reyes, A., 2014. Do breastfeeding and bottle feeding influence occlusal parameters?. Breastfeeding Medicine, 9(1), pp.24-28.

 

Ingervall, B. and Bitsanis, E., 1987. A pilot study of the effect of masticatory muscle training on facial growth in long-face children. The European Journal of Orthodontics, 9(1), pp.15-23.

 

Katz, C.R.T., Rosenblatt, A. and Gondim, P.P.C., 2004. Nonnutritive sucking habits in Brazilian children: effects on deciduous dentition and relationship with facial morphology. American Journal of Orthodontics and Dentofacial Orthopedics, 126(1), pp.53-57.

 

Knowles & Sirianni. The Underbite Characteristic in Old-World Monkeys: An analysis of possible causes of the trait in six different leaf eating species (Presbytes melalophus, P. obscurus, Colobus badius, C. guereza, Pygathrix nemaeus, and Nasalis larvatus) https://www.researchgate.net/publication/263846744_The_Underbite_Characteristic_in_Old-World_Monkeys_An_analysis_of_possible_causes_of_the_trait_in_six_different_leaf_eating_species_Presbytes_melalophus_P_obscurus_Colobus_badius_C_guereza_Pygathrix_ne

 

Kiliaridis, S., 1995a. Masticatory muscle influence on craniofacial growth. Acta Odontologica Scandinavica, 53(3), pp.196-202.

 

Kiliaridis, S., Johansson, A., Haraldson, T., Omar, R. and Carlsson, G.E., 1995b. Craniofacial morphology, occlusal traits, and bite force in persons with advanced occlusal tooth wear. American journal of orthodontics and dentofacial orthopedics, 107(3), pp.286-292.

 

Koca, C.F., Erdem, T. and Bayındır, T., 2016. The effect of adenoid hypertrophy on maxillofacial development: an objective photographic analysis. Journal of Otolaryngology-Head & Neck Surgery, 45(1), pp.1-8.

 

Kubota, M.U.N.U.T.S.U.G.U., Nakano, H., Sanjo, I., Satoh, K., Sanjo, T., Kamegai, T. and Ishikawa, F., 1998. Maxillofacial morphology and masseter muscle thickness in adults. The European Journal of Orthodontics, 20(5), pp.535-542.

 

Lieberman, D.E., Krovitz, G.E., Yates, F.W., Devlin, M. and Claire, M.S., 2004. Effects of food processing on masticatory strain and craniofacial growth in a retrognathic face. Journal of human evolution, 46(6), pp.655-677.

 

MILLS, J. R. E. (1963). OCCLUSION AND MALOCCLUSION OF THE TEETH OF PRIMATES. Dental Anthropology, 29–51. doi:10.1016/b978-0-08-009823-4.50007-0

 

https://sci-hub.tw/10.1016/B978-0-08-009823-4.50007-0

 

Proffit, W.R., Fields Jr, H.W. and Sarver, D.M., 2006. Contemporary orthodontics. Elsevier Health Sciences.

 

Price, W.A., 2004. Nutrition and Physical Degeneration. 1939. Reprinted. La Mesa, CA: The Price-Pottenger Nutrition Foundation.

 

Pithon et al. 2016 . Impact of malocclusion on affective/romantic relationships among young adults. https://www.ncbi.nlm.nih.gov/pubmed/26367312

 

Fikret Şatıroğlu, Tülin Arun, Fulya Işık, Comparative data on facial morphology and muscle thickness using ultrasonography, European Journal of Orthodontics, Volume 27, Issue 6, December 2005, Pages 562–567, https://doi.org/10.1093/ejo/cji052

 

Rout, M.R., Mohanty, D., Vijaylaxmi, Y., Bobba, K. and Metta, C., 2013. Adenoid hypertrophy in adults: a case series. Indian Journal of Otolaryngology and Head & Neck Surgery, 65(3), pp.269-274.

Sánchez-Molins, M., Grau, C.J., Lischeid, G.C. and Ustrell, T.J., 2010. Comparative study of the craniofacial growth depending on the type of lactation received. European journal of paediatric dentistry, 11(2), p.87.

 

Tircoveluri S, Singh JR, Rayapudi N, Karra A, Begum M, Challa P. Correlation of masseter muscle thickness and intermolar width – an ultrasonography study. J Int Oral Health. 2013;5(2):28-34.

 

Varrela, J., 1990. Effects of attritive diet on craniofacial morphology: a cephalometric analysis of a Finnish skull sample. The European Journal of Orthodontics, 12(2), pp.219-223. https://watermark.silverchair.com/12-2-219.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAArIwggKuBgkqhkiG9w0BBwagggKfMIICmwIBADCCApQGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMmQO2PglFP6kg1NSkAgEQgIICZft7uryxLmDrUA9TdXSN2s1Syco5nhCwRf_0fLkHeroFUgWqI_ni0VZd0rboTeMVPZmcGYahVXAipzkQ_vDhmP656LLh5g95UggBNBMF1oqPa4xA8d2dnu_zAuddeuGq3yv4CdnS51VkptsGeGoRD7NmnpSsFfbv5gfQCG3dmVbFGe0j3bWBRyXt0WU_oVHw_0qPt02c7o208sLYrPTrHioVXTh40HZwESBg4rGX3VPORmlucodztwtEKEQf0a4yXQ-u9mapYx5D_EYSNTMKXJfv2RFeIBE6j0jnN-wMSzLRs4B4Ci1F7l3JyNakULCPI6cDsq2EIqALzNDl7osSfjCYRa1h5j9M69OxqlFS8t7YF7y4QZL2qskEle1vIf1g8wO4CK4R6nuA–vqDOHc73NgIDFEomATthG-CGnjmCds9MLsxHUBATykMB5YzCGtTLat6H1MQHPK8klJqk6hqgxV9QSYtXxG47GbZTbYjXstMKd8DFbywZ2dprcdnXcetgTxZYVkU5DjZZyM0JMFKKFeOPjYf5ilv8R7rLCVPLfL-hSZj9g2SZ83bkJKI0thcJfv4CraO7IdnsJplbZ7AihnXnZap2HRa4xlXwbB1mRbpJUNKBqNMMnmZep-iW9T9L4F1x5StHwoozDNFi8PX-wdA62DJf3FIvQbsHJZ3CeiHMjoHICjfhCtvkpe333dmklZGQUHiC2nimmjRxiD6cClVTTVmq53zubSKIVXFD7okc80RJLpzGCfH_dn63gnWBhxywLZcGNEdaBv_bG_kv02yz04OsFVZBE-PI9iJhrrCLtkR2c

 

von Cramon-Taubadel, N., 2011. Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies. Proceedings of the National Academy of Sciences, 108(49), pp.19546-19551.https://www.pnas.org/content/pnas/108/49/19546.full.pdf

 

Zingeser, M.R., 1970. The morphological basis of the underbite trait in langurs (P. melalophus and T. cristatus) with an analysis of adaptive and evolutionary implications. American journal of physical anthropology, 32(2), pp.179-186. https://onlinelibrary.wiley.com/doi/pdf/10.1002/ajpa.1330320204

 

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