Anatomy physiology of the totally edentulous

Anatomy physiology of the totally edentulous

Introduction

Total edentulism is often progressive, following periodontal disease, or the impossibility of carrying out conservative treatment. It results from total edentulism of changes in the various anatomical structures.

The exploitation and respect of the anatomical elements of the totally edentulous oral cavity are essential for the integration of the total removable prosthesis.

1. Physical appearance of total edentulism

The face of the totally edentulous person is characteristic from the front, from the side, at rest and in movement.

At rest and in profile, the facies of the totally edentulous person is characterized by:

• the reliefs of the nose and chin are particularly accentuated due to the sagging of the jugal musculature and the invagination of the lips which follow the recession of the alveolar profile.
• The decrease of the lower level of the face 
• A mandibular prolapse
• The collapse of soft tissues and the accentuation of wrinkles and skin furrows.
• The skin is pale and thin

2. Topography of the oral cavity

The mouth is made up of an articulated bony framework, muscles and mucous membrane lining the whole. It is divided into two parts by the alveolar ridges: one peripheral or vestibule, the other central or oral cavity proper.

• the vestibule of the mouth: is included between the alveolar arches on the one hand, the lips and cheeks on the other hand. It is lined by the buccal mucosa. On the median line, the frenulum of the lip extends from the attached mucosa to the mucosa of the medial surface of the lip. Opposite each tuberosity, the empty space described by Eisenring is called the ampullary pocket, at the mandibular level, this region of the vestibule is called the Fisch pocket.
• The oral cavity itself: limited in front and on the sides by the residual arches, above by the palate, below by the floor on which the tongue rests.

The palatine vault is formed at the front by the hard palate and at the back by the soft palate.

3. The mucosa:

The oral cavity is covered, depending on the region considered, by 3 distinct types of mucous membranes:

• Masticatory mucosa, fibromucous or attached mucosa at the level of the hard palate and ridges
• The covering mucosa or free mucosa: inner face of the cheeks, lips, floor
• The specialized mucosa, on the dorsal surface of the tongue.

1. Histology

The oral mucosa consists of an epithelium and a richly vascularized underlying connective tissue. The junction between the two tissues is ensured by the basement membrane.

1. Epithelium: multi-layered squamous, keratinized at the level of the palatine vault and ridges, or non-keratinized (inner surface of cheeks, lips, etc.)
2. The epithelial-connective junction: it has a glove-finger architecture, it plays a dual role: ensuring the junction and anchoring between the epithelium and the connective tissue; and creating hermeticity between these two tissues. Its structure can be modified by inflammation.
3. Connective tissue: The biomechanical properties of connective tissue arise from its histological characteristics.

It is subdivided into two zones, the lamina propria and the submucosa.

The lamina propria corresponds to the connective tissue located under the basement membrane. It includes cellular elements (fibroblasts, macrophages, inflammatory cells, etc.) and an extracellular matrix whose composition determines the resistance and elasticity properties of this tissue. Resistance to mechanical stress results from the presence and number of collagen fibers. Grouped into bundles, these fibers create a perfectly organized network playing a “buffer” role.

Elasticity results from the presence of elastic fibers. There are few of them in the mucous membranes of the support surface. 

The submucosa contains fatty deposits and minor salivary glands, it separates the mucosa from the underlying muscle or bone tissues. It is particularly present in the lateral regions of the palatine vault at the level of the so-called Schroeder areas.

Anatomy physiology of the totally edentulous

2. Biomechanics of the oral mucosa

When a force is applied to the oral mucosa, the behavior of the tissues depends on their viscoelastic properties and the hemodynamic behavior of the tissue microcirculation. Viscoelasticity results from the combination of the elastic and viscous characteristics of the mucosa. If a force is applied to an elastic material, the displacement is immediate until the application of the force stops. Conversely, a viscous material is deformed progressively until reaching its equilibrium state. Viscoelasticity is characterized by the combination of the two previous behaviors. Hemodynamic properties concern the flow of blood and formed elements in arterioles and venules as well as the behavior of the fluids of the extracellular matrix.

Viscoelasticity and tissue behavior

The basic studies on mucosal viscoelasticity were carried out by Kydd et al (1976), who reported that the application of any pressure to the mucosa results in immediate elastic deformation, followed by delayed deformation. This deformation is accompanied by a reduction in the thickness of the epithelium, and a flattening of the epithelial-connective tissue junction. The extent of the alterations is directly related to the intensity and duration of the force applied. While the pressureless covering of the palatal mucosa with a prosthetic base does not result in tissue modification, the application of pressures results in changes proportional to their intensity.

Viscoelasticity and aging

Age is also an important parameter regarding the behavior of the mucosa with respect to occlusal loads. Indeed, the results show that if the curves concerning compression are identical, the behavior of the tissues in the recovery phase is a function of age: the older the patient, the less the immediate elastic recovery is important, and the slower the delayed recovery.

Viscoelasticity and microcirculation

Pressures exerted on the mucosa cause displacement and deformation of the tissues and, consequently, they cause disturbances in the mucosal microvascularization. As soon as pressure is applied, blood flow is reduced, the mucosa whitens, a phenomenon that persists as long as the pressure is maintained. This impairment of vascularization is important with regard to the development of an inflammatory state. Indeed, the vascular system is involved in the first place in the release of neurotransmitters, which will be involved in the establishment of inflammation, and beyond in the phenomena of bone resorption. When the pressure is stopped, the blood flow immediately returns to its initial level, so “recovery” is much faster than at the mucosal level.

Oral mucosa and total removable prosthesis

Mechanical stimulation associated with wearing a prosthesis initially leads to a thickening of the epithelium and increased keratinization. However, the stratum corneum then decreases. In addition, a flattening of the basement membrane and a reduction in the number of epithelial-connective digitations occur. This development is interpreted as a tissue adaptation to the loads applied to the support surface.

If the prosthesis is not correctly designed (edges too wide or not wide enough, unbalanced prosthesis, etc.) lesions of the oral mucosa are observed (ulcerations, hyperplasia, floating ridges, etc.)

4. The bone

The bones related to the total prosthesis are: the maxillary bone, and the mandibular bone.

1. anatomy
2. The maxilla

Paired, large, massive, irregular bone, in the shape of a truncated pyramid. The anatomical elements coming into contact with the prosthesis are:

• The palatine vault: has a double concavity, forms a variable ogive according to the degree of resorption. On the median line is the median raphe which can present tori
• The alveolar crest: the volume of which depends on the degree of resorption.  
• The maxillary tuberosities: each occupy a posterior end of the crest, they are sometimes voluminous and have undercuts
• The incisive canal (the retro-incisive papilla): this canal provides passage to the anterior palatine vasculo-nervous bundle.
• The anterior nasal spine: may be located on the crest if the resorption is significant.

2. The mandible

Mobile, odd, median, symmetrical bone. The elements that are in contact with the prosthesis are: 

• The alveolar crest: of variable morphology depending on the degree of resorption.
• Basilar bone: may persist alone after significant resorption
• The external oblique line 
• The internal oblique line (mylohyoid line)
• The geni apophyses
• The retromolar trigone: bony surface delimited by the bifurcation of the insertion crest of the temporal muscle. The mucous ridge that covers it is called the retromolar tubercle.

2. Bone resorption
3. Definition

“Resorption means the total or partial disappearance of an organ whose elements are gradually taken up by the blood or lymphatic circulation” ACKERMANN.

2. Etiology 

The etiology of ROA is complex and involves several factors:

1. The age factor

Bone, like other living tissues, has an internal clock that programs its own aging characterized by a reduction in calcified bone tissue.

The age factor intervenes as an associated factor:

• Alveolar bone is destroyed more quickly in an elderly person than in a young person under the same overload conditions.
• The degree of involution is linked to the age of the edentulism.
• There is a significant reduction in alveolar ridge height with age regardless of periodontal disease and presence of teeth.

2. The biological factor

The authors incriminate, among others, in alveolar resorption:

• The role of hormones
• The role of nutritional factors and enzymes
• Senile osteoporosis (especially in the jaw)
• Calcium Absorption Failure
• Bone mineralization deficiency (vitamin D deficiency)
• Stimulation of neuropeptides in the case of defense against mechanical bone dysfunction.

Anatomy physiology of the totally edentulous

3. The factor related to dental anatomy

The resorption is all the more accentuated as the extracted teeth are numerous and their roots are wide.

To preserve residual bone capital, GAUDY recommends:

• Perform tooth extraction before bone loss extends beyond the apex
• Apply avulsion techniques saving the smallest bone fragment.

4. The stability factor

A marked decrease in PTA stability contributes to crest destruction.

5. The biomechanical factor

Tension is the triggering factor for bone apposition while pressure causes resorption.

The three main causes of ROA are:

• The pressure continues;
• Discontinuous pressure separated by too short rest intervals, which acts as if it were continuous.
• The absence of any pressure (atrophy due to non-function).

6. Prosthetic factors:

• Materials: Ceramic teeth preserve the stability of the prostheses by maintaining the occlusal relationships. But in case of flat ridges, flat resin teeth are recommended.
• The occlusal surface of the prosthetic teeth: it is recommended to reduce the vestibulo-lingual dimension of the posterior teeth in order to avoid high pressure on the support surfaces.
• The assembly of artificial teeth according to the balanced bilateral occlusion scheme, and their equilibration best preserve the visco-elastic qualities and the bone capital of the support surfaces.
• Prolonged wearing of prostheses leads to progressive resorption of the ridges.

3. Mechanisms of alveolar bone resorption

Alveolar bone resorption affects both cancellous and cortical bone and can occur symmetrically or asymmetrically on edentulous arches, it affects the mandible much more than the maxilla and is accomplished along the axis of implantation of the teeth. Progressive and irreversible, it is rapid and invasive after edentulation, then occurs more slowly and with less loss of bone volume.

1. In the maxilla:

Physiological resorption is centripetal in the frontal, horizontal and sagittal planes, it reduces the width and concavity of the palatine vault. This results in a flat palate shape and a triangular appearance, the nasal spine and pterygoid processes become prominent.

Atwood describes four stages of ROA:

• Stage I ridge poorly resorbed

• Stage II: ridge in the process of resorption (moderate bone loss)
• Stage III: flat crest
• Stage IV: negative crest.

Anatomy physiology of the totally edentulous

2. In the mandible:

Physiological resorption is very marked the first year following tooth loss, it is 

• centripetal at the parasymphyseal level
• centrifugal in the premolar-molar regions due to the lingual inclination of the molars.
• Stronger in height.

5. The muscles 

Muscles affecting prosthetic stability are:

• The orbicularis oris , whose fibers cross those of the following small skin muscles:
• The incisive muscles
• The zygomaticus minor and major muscles
• The canine
• The nasolabial lifters
• The risorius
• The square chin
• The triangular lips

These muscles help form the modioli.

Anatomy physiology of the totally edentulous

• The temporalis muscle, which inserts on the coronal, and which can interfere with the edge of the upper prosthesis during lateral movements
• The masseter: whose anterior edge has an impact on the stability of the lower prosthesis
• The buccinator: constitutes the jugal strap. Its insertions constitute the limits of the depth of the vestibule.
• The mylohyoid is inserted on the mylohyoid line limiting the depth of the lingual side of the prosthesis
• Tongue muscles: play a vital role in the stability of prostheses during various functions.

6. The ATM

Consisting of a temporal articular surface, a mandibular articular surface , and a disc that interposes between the two.

Toothlessness causes disturbances that alter joint structures.

7. Saliva

In patients with complete dentures, saliva fulfills many important functions. These functions are of course the preservation and maintenance of the integrity of the tissues of the oral cavity, but also biomechanical and functional roles. The response to these functions depends on the quantity and quality of the salivary fluid secreted.

Saliva or total saliva is a physiological fluid secreted by three pairs of major salivary glands (parotid, sublingual and submandibular) and by minor glands (labial, buccal, lingual and palatine)

The major glands produce nearly 90% of the total flow in the absence of stimulation. But the quality of saliva produced by the minor salivary glands is very different. These are called glands

mucous membranes, secreting saliva rich in mucins (glycoproteins) which play a crucial role for the complete removable prosthesis. These mucins have low solubility, high viscosity and elasticity and, finally, a high adhesion coefficient. Thus, they give the mucous saliva essential properties of lubrication and protection of the mucosa against mechanical aggressions, directly or indirectly linked to the wearing of the prosthesis, but also a role of adhesion and retention. In addition, the presence of mucins facilitates chewing, swallowing and phonation.

The parotids are salivary glands called serous, the sublingual and submandibular glands are mixed glands. The saliva produced is “more watery”.

Anatomy physiology of the totally edentulous

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Anatomy physiology of the totally edentulous