The oronasopharyngeal space and renewed formalization of the functional matrix hypothesis

Conceptualization of the functional matrix hypothesis (FMH) by Melvin L. Moss [1,2] remains a pillar of orthodontic theory to explain compensatory growth of the craniofacial skeletal complex. Functional matrices are described as periosteal, comprising functional muscle/tendon attachments to skeletal units, and capsular, enclosing a mass or surrounding a functional space. Because it is easy to intuit, the neurocranial capsular matrix was heralded to convey FMH concepts.

While operational demands of the oronasopharyngeal functional space relate to metabolism, it was described as an intrinsic space whose tissue boundaries grew mitotically from embryonic life [3]. Yet, if anomalous functional matrices cause abnormal growth [4], why do perfectly normal teeth comprising the functional matrix of alveolar bone [5] ever become crowded? Moss would surely have attributed responsibility to extrinsic environmental factors, suggesting that orofacial capsular matrices have contents exerting pressure and stimulating compensatory growth of their skeletal units.

That is, if you agree with the FMH’s premise. In a more extensive and formal treatise of the FMH, this response was elicited:

The FMM [functional matrix model] is untestable. There was never anything substantial about the model that could be tested empirically. Instead, you either were an advocate or a naysayer, with almost no evidence on either side. Having read through this manuscript, I see that nothing much has changed since 2006 when Dr. Moss passed away. (Comment from a reviewer of my formerly submitted manuscript to AJODO.)

This sentiment notwithstanding following are features of orofacial capsular matrices for improving our understanding.

The oropharyngeal cavity

To discover what contents fill the oropharyngeal cavity, we ask, what are human teeth for? The hierarchy of attributes conveying mechanical efficacy from macro- to nanoscopic scales in enamel is revealing. At the macroscopic scale are loads and stresses absorbed by teeth due to incursive, intercuspal, and excursive contacts pursuant to masticatory function [6]. A level down the hierarchy is human molar enamel thickness, among the thickest of all primates, at 2 mm thick and more [7] and an adaptation to hard and tough diets [8]. Further down, enamel retains decussating enamel prisms near the enamel-dentine junction (EDJ), forming Hunter-Schreger bands (HSBs) in mammals having hard and tough diets [9]. Unions between decussating groups of prisms form enamel prism discontinuities that resist crack propagation. Occlusal loads drive the enamel shell into softer underlying dentine, causing peak tensile stress at the EDJ, to which brittle solids, like enamel, are vulnerable. HSBs are thus situated in exactly that region of the enamel to resist bottom surface crack propagation [10]. Human molar enamel [11] and the EDJ [12] also contain gradient compositional hardness and elastic modulus discontinuities. At the micrometer scale, complex arrangements of enamel crystallites create discontinuities at junctions between prisms that are anti-crack propagating [13], and crystallites themselves are packed to provide sliding planes at nanometer scales. Cracks emerging at the micron and nanometer level will arrest at these length scales.

Human teeth are designed for processing hard and tough foods

What is the relationship between what teeth are for and their bone? It is an axiom of hard tissue biology attributed to Wolf’s Law that bone adapts its architecture to the habitual intensity and frequency of mechanical loads [14]. Biological loads and mildly overloaded biological regimes stimulate growth, expanding the oral cavity and oropharynx, permitting uncrowded occlusion. Underloading has no effect on modeling drift [14]. Bones that do not experience strain thresholds to which they are adapted during growth will not stimulate sufficient modeling. Chronic underloading leads to maxillary and mandibular insufficiencies and tooth crowding.

Among modern-day hunters and gatherers, average fracture toughness of underground plants consumed is about 2000 Jm² [15]. Foods of industrialized countries have an average fracture toughness of roughly 200 Jm² [16]. We know orofacial muscle tonus is positively associated with mechanical food properties [17], that softer diets are associated with reduced oral processing [18], and that mandibles are more gracile in low-masticatory-loaded individuals [19].

The vast majority of maxillary and mandibular insufficiencies of interest to orthodontists are explained by deficiencies in mechanical properties of diet alone.

The functional matrix of the oral cavity is hard and tough food.

The nasopharyngeal cavity

To understand what contents fill the nasopharyngeal cavity, we ask what is respiration’s role in facial growth? We address this by outlining its physiologically important attributes in relation to the necessary exchange of carbon dioxide and oxygen, and olfaction: humidity added to inspired air facilitates lung function; nitric oxide release in the upper respiratory tract destroys pathogens and assists in regulating carbon dioxide and oxygen concentrations; filtering of particulates from air protects lungs’ delicate structures; thermal conditioning of air by warming facilitates gas exchange; vagal tone is stimulated during deep diaphragmatic breathing, facilitating health; and immune defenses are mounted against airborne pathogens.

What distinguishes these benefits is that none of them occurs during habitual mouth breathing, but rather describe the purpose of obligate nasal breathing. While nasal breathing is more work [20], it has hidden efficiencies [21]. Anomalies of facial development and dental health attributed to mouth breathing, typically in relation to nasal obstruction, include aberrant inferoposterior mandibular rotation, overjet, long face, narrowing of the dental arches, overcrowding, open bite, crossbite, poor lip seal and posture, and relatively small external nares, which are universally regarded as harmful to health.

The selection pressure to preserve water by itself explains obligate human nasal breathing. Water loss when switching from nasal to mouth breathing increases 42% [22].

Obligatory nasal breathing is for promoting normal facial growth and function

What mechanism relates nasal breathing to facial growth? Resistance to airflow by tissues overlying bones of the nasal passage is significant [20]. Turbulent airflow occurs above 40–80 Pa [23]; thus, nasal breathing may be 100 Pa and higher during intense physical exercise, such as vigorous play. Bernoulli forces of this magnitude represent 10 gm of compression pressure per sq cm and are many orders of magnitude larger than proliferative pressure exerted by mitotically growing tissues [24].

High intranasal pressures may expand the passage’s delicate bones, distending sutures, and stimulating compensatory modeling. Not to discount masticatory contributions from hard and tough diets generating high strains [25], we know the number of chews is decreased when mouth breathing [26]. However, orthodontic maxillary expansion may not significantly improve nasal airflow [27]; thus, Bernoulli forces are worth consideration.

The functional matrix of the nasal cavity is air.

Intrinsic genetic and extrinsic epigenetic factors are interacting causes of craniofacial growth. Masseter muscle genetic variants, fiber phenotype, and physiology, as well as salivary genotypes, vary in relation to skeletal growth, facial morphology, and malocclusion groups. But their effects on facial morphology are mediated via periosteal matrices, not capsular matrices.

The orthodontic community has failed to address the public health advantages of treating to the purpose of orofacial capsules. Orthodontics acknowledges the many risks of treatment [28], but such risks endure by treating symptoms in deference to causes. Sir Arthur Keith wrote, “Civilization … is anti-evolutionary in its effects; it works against the laws and conditions which regulated the earlier stages of man’s ascent” [29]. If dentistry permitted evolutionary medicine into its curriculum, then some Board examination questions required for licensure could be replaced with ones relevant to improvement in public health.