Complete secondary edentia. Prosthetics for patients with missing teeth External oblique line
TOPOGRAPHANATOMICAL.
FEATURES OF TOOTHLESS JAWS.
The causes of complete tooth loss are most often caries and its complications, periodontitis, trauma and other diseases; Primary (congenital) adentia is very rare. Complete absence of teeth at the age of 40-49 years is observed in 1% of cases, at the age of 50-59 years - in 5.5% and in people over 60 years old - in 25% of cases.
With complete loss of teeth due to the lack of pressure on the underlying tissues, functional disorders are aggravated and ♦ atrophy of the facial skeleton and the soft tissues covering it rapidly increases. Therefore, prosthetics of toothless jaws is a method of restorative treatment, leading to a delay in further atrophy.
With the complete loss of teeth, the body and branches of the jaws become thinner, and the angle of the lower jaw becomes more obtuse, the tip of the nose drops, the nasolabial folds are sharply expressed, the corners of the mouth and even the outer edge of the eyelid droop. The lower third of the face decreases in size. Muscle sagging appears and the face takes on an senile expression. Due to the patterns of bone tissue atrophy, to a greater extent from the vestibular surface on the upper and from the lingual surface on the lower jaw, the so-called senile progeny is formed (Fig. 188).
With complete loss of teeth, the function of the masticatory muscles changes. As a result of a decrease in load, the muscles decrease in volume, become flabby, and atrophy. There is a significant decrease in their bioelectrical activity, with the bioelectrical rest phase dominating in time over the period of activity.
Changes also occur in the TMJ. The glenoid fossa becomes flatter, the head moves posteriorly and upward.
The complexity of orthopedic treatment lies in the fact that under these conditions atrophic processes inevitably occur, as a result of which the landmarks that determine the height and shape of the lower part of the face are lost.
Prosthetics in case of complete absence of teeth, especially on
Rice. 188. View of a person with complete absence of teeth, and - before prosthetics; b - after prosthetics.
lower jaw is one of the most difficult problems in orthopedic dentistry.
When making prosthetics for patients with toothless jaws, three main issues are resolved:
How to strengthen dentures on toothless jaws?
How to determine the required, strictly individual size and shape of prostheses so that they best restore the appearance of the face?
How to design the dentition in dentures so that they function synchronously with other organs of the masticatory apparatus involved in food processing, speech formation and breathing?
To solve these problems, it is necessary to have a good knowledge of the topographic structure of toothless jaws and mucous membrane.
In the upper jaw, during examination, first of all, attention is paid to the severity of the frenulum of the upper lip, which can be located from the top of the alveolar process in the form of a thin and narrow formation or in the form of a powerful cord up to 7 mm wide.
On the lateral surface of the upper jaw there are cheek folds - one or several.
Behind the tubercle of the upper jaw there is a pterygomaxillary fold, which is well expressed when the mouth is opened strongly.
If the listed anatomical formations are not taken into account when taking impressions, then when using removable dentures, bedsores will occur in these areas or the denture will be discarded.
The border between the hard and soft palate is called line A. It can be in the form of a zone from 1 to 6 mm wide. The configuration of line A also varies depending on the configuration of the bony base of the hard palate. The line can be located up to 2 cm in front of the maxillary tubercles, at the level of the tubercles, or up to 2 cm extending towards the pharynx, as shown in Fig. 189. In a prosthetic dentistry clinic, blind holes serve as a guide to the length of the posterior edge of the upper denture. The posterior edge of the upper denture should overlap them by 1-2 mm. At the apex of the alveolar process, along the midline, there is often a well-defined incisive papilla, and in the anterior third of the hard palate there are transverse folds. These anatomical formations must be well represented on the cast, otherwise they will be pinched under the rigid base of the prosthesis and cause pain.
The suture of the hard palate in the case of significant atrophy of the upper jaw is sharply pronounced, and during the manufacture of dentures it is usually isolated.
The mucous membrane covering the upper jaw is immobile, with different pliability observed in different areas. There are devices from various authors (A. P. Voronov, M. A. Solomonov, L. L. Soloveichik, E. O. Kopyt), with the help of which the degree of pliability of the mucous membrane is determined (Fig. 190). The mucous membrane has the least compliance in the area of the palatal suture - 0.1 mm and the greatest in the posterior third of the palate - up to 4 mm. If this is not taken into account when making plate prostheses, the prostheses can balance, break, or, by exerting increased pressure, lead to pressure sores or increased atrophy of the bone base in these areas. In practice, it is not necessary to use these devices; you can use a finger test or the handle of tweezers to determine whether the mucous membrane is sufficiently pliable.
On the lower jaw, the prosthetic bed is much smaller than on the upper jaw. With the loss of teeth, the tongue changes its shape and takes the place of missing teeth. With significant atrophy of the lower jaw, the sublingual glands may be located at the top of the alveolar part.
When making a prosthesis for the lower edentulous jaw, it is also necessary to pay attention to the severity of the frenulum of the lower lip, tongue, lateral vestibular folds and ensure that these formations are well and clearly displayed on the cast.
Rice. 190. Voronov’s apparatus for determining the compliance of the mucous membrane.
there is a so-called retromolar tubercle. It can be dense and fibrous or soft and pliable and must always be covered with a prosthesis, but the edge of the prosthesis should never be placed on this anatomical formation.
The retroalveolar region is located on the inner side of the angle of the lower jaw. From behind it is limited by the anterior palatine arch, from below - by the bottom of the oral cavity, from the inside - by the root of the tongue; its outer border is the inner corner of the lower jaw.
This area must also be used in the manufacture of plate prostheses. To determine the possibility of creating a “wing” of the prosthesis in this area, there is a finger test. An index finger is inserted into the retroalveolar region and the patient is asked to extend his tongue and touch the cheek on the opposite side with it. If, with such a movement of the tongue, the finger remains in place and is not pushed out, then the edge of the prosthesis must be brought to the distal border of this zone. If the finger is pushed out, then creating a “wing” will not lead to success: such a prosthesis will be pushed out by the root of the tongue.
Lower jaw (mandibula) unpaired, horseshoe-shaped, the only movable one. It consists of two symmetrical halves, completely fused by the end of the 1st year of life. Each half has a body and a branch. At the junction of both halves in old age, a dense bony protrusion forms.
In the body (corpus mandibulae) there is a base (basis) and an alveolar part (pars alveolaris). The body of the jaw is curved, its outer surface is convex and its inner surface is concave. At the base of the body the surfaces merge into one another; in the alveolar part they are separated by alveoli. The right and left halves of the body converge at an angle that is individually different, forming a basal arch. The shape of the basal arch is one of the main features characterizing the shape of the lower jaw. To characterize the basal arch, the latitudinal-longitudinal index is used (the ratio of the distance between the angles of the lower jaw to the distance from the middle of the chin to the middle of the line connecting the angles of the lower jaw). There are jaws with a short and wide basal arch (index 153-175), with a long and narrow one (index 116-132) and with an intermediate shape. The height of the jaw body is greatest in the area of the incisors, the smallest is at the level of the 8th tooth. The thickness of the jaw body is greatest in the region of the molars, and the smallest in the region of the premolars. The cross-sectional shape of the jaw body is not the same in different areas, which is determined by the number and position of the roots of the teeth. In the area of the front teeth it approaches triangular with the base facing downwards. In areas of the body corresponding to large molars, it is close in shape to a triangle with the base facing upward (Fig. 1-12).
A - top view: 1 - head of the lower jaw; 2 - pterygoid fossa; 3 - coronoid process; 4 - mandibular pocket; 5 - molars; 6 - body of the lower jaw; 7 - premolars; 8 - fang; 9 - incisors; 10 - mental tubercle; 11 - chin protuberance; 12 - interalveolar septa; 13 - dental alveoli; 14 - chin hole; 15 - interroot partitions; 16 - angle of the lower jaw; 17 - outer wall of the alveoli; 18 - oblique line; 19 - inner wall of the alveoli; 20 - retromolar fossa; 21 - buccal ridge; 22 - cutting of the lower jaw; 23 - tongue of the lower jaw; 24 - neck of the lower jaw. ; B - rear view: 1 - incisors; 2 - fang; 3 - premolars; 4 - molars; 5 - coronoid process; 6 - condylar process; 7 - tongue of the lower jaw; 8 - mylohyoid groove; 9 - maxillohyoid line; 10 - submandibular fossa; 11 - pterygoid tuberosity; 12 - digastric fossa; 13 - mental spine; 14 - sublingual fossa; 15 - angle of the lower jaw; 16 - canal of the lower jaw; 17 - neck of the lower jaw.
. IN - inside view: 1 - buccal ridge; 2 - temporal crest; 3 - mandibular notch; 4 - head of the lower jaw; 5 - neck of the lower jaw; 6 - tongue of the lower jaw; 7 - opening of the lower jaw; 8 - mylohyoid groove; 9 - mandibular ridge; 10 - pterygoid tuberosity; 11 - maxillohyoid line; 12 - angle of the lower jaw; 13 - submandibular fossa; 14 - sublingual fossa; 15 - digastric fossa; 16 - compact substance of the lower jaw; 17 - spongy substance of the lower jaw; 18 - incisors; 19 - fang; 20 - premolars; 21 - molars
In the middle of the outer surface of the jaw body there is a chin protuberance (protuberantia mentalis), which is a characteristic feature of modern humans and determines the formation of the chin. The angle of the chin relative to the horizontal plane in modern humans ranges from 46 to 85°. On both sides of the mental protuberance, closer to the base of the jaw, there are mental tubercles (tubercula mentalia). Outside of them is the mental foramen (foramen mentale), which is the outlet of the mandibular canal. The vessels and nerves of the same name exit through the mental foramen. Most often, this hole is located at the level of the 5th tooth, but it can move anteriorly to the 4th tooth, and posteriorly to the space between the 5th and 6th teeth. The dimensions of the mental foramen range from 1.5 to 5 mm, its shape is oval or round, sometimes it is double. The mental foramen is removed from the base of the jaw by 10-19 mm; on toothless jaws of adults with an atrophied alveolar part, it is closer to the upper edge of the jaw.
In the lateral areas of the body of the lower jaw there is an obliquely located roller - an oblique line (linea obliqua), the anterior end of which corresponds to the level of the 5-6th tooth, and the rear end, without sharp boundaries, passes to the anterior edge of the lower jaw branch.
On the inner surface of the jaw body, near the midline, there is a bone spine, sometimes double, - the mental spine (spina mentalis). This place is the beginning of the geniohyoid and genioglossus muscles. Below and lateral to the mental spine is the digastric fossa (fossa digastrica), in which the digastric muscle begins. Above the digastric fossa there is a flat depression - the sublingual fossa (fovea sublingualis) - a trace from the adjacent sublingual salivary gland. Further posteriorly, the mylohyoid line (linea mylohyoidea) is visible, on which the superior pharyngeal constrictor and the mylohyoid muscle begin. The maxillary-hyoid line runs between the digastric and sublingual fossae at the level of the 5-6th tooth and ends on the inner surface of the jaw branch. Under the maxillary-hyoid line at the level of the 5-7th tooth there is a submandibular fossa (fovea submandibularis) - a trace from the submandibular salivary gland located in this place.
Alveolar part of the jaw body contains 8 dental alveoli on each side. The alveoli are separated from each other by interalveolar septa (septa interalveolaria). The walls of the alveoli facing the lips and cheeks are called vestibular, and the walls facing the tongue are called lingual. On the surface of the body, the alveoli correspond to alveolar elevations (juga alveolaria), which are especially well expressed at the level of the canine and 1st premolar. Between the alveoli of the incisors and the mental protuberance there is a subincisal depression (impressio subincisiva). The shape, depth and width of the alveoli, the thickness of their walls are different for teeth of different groups. The alveoli of the incisors (especially the central ones) are compressed from the sides, their bottom is shifted to the vestibular compact plate, therefore the thickness of the lingual wall of the alveoli is greater than the vestibular one. The alveoli of the canine and especially the premolars are rounded, the lingual wall is thicker than the vestibular one. The deepest alveoli of the canine and 2nd premolar. The thickness of their walls is greater than the alveoli of the incisors. The alveoli of molars are distinguished by the presence of interradicular septa. In the alveoli of the first two molars there is one septum separating the anterior and posterior chambers for the corresponding roots. The alveolus of the 3rd molar is varied in shape and number of septa, which is associated with the variability of the shape of this tooth. Most often the alveolus is conical, without septa, but may have one and sometimes two septa. The walls of the alveoli of the molars are thickened due to the oblique and mylohyoid lines. This strengthens the lower molars and protects them from loosening in the buccolingual direction during transversal lateral chewing movements.
The area located behind the 3rd molar is triangular in shape and is called the posterior molar fossa (fovea retromolaris). Laterally from this fossa, on the outer plate of the alveolar part, there is a mandibular pocket (recessus mandibulae), which stretches from the 2-3rd molar to the coronoid process (Fig. 1-13).
Rice. 1-13. The structure of the lower jaw, the outer surface (diagram according to V.P. Vorobyov ), part of the dense bone substance of the outer plate is removed: 1 - condylar process; 2 - coronoid process; 3 - opening of the lower jaw; 4 - tongue of the lower jaw; 5 - buccal ridge; 6 - retromolar fossa; 7 - incisors; 8 - alveolar elevations; 9 - chin eminence; 10 - fang; 11 - premolars; 12 - tooth roots; 13 - canal of the lower jaw; 14 - angle of the lower jaw; 15 - chewing tuberosity; 16 - notch of the lower jaw; 17 - tongue of the lower jaw (external view); 18 - molars
The structure of the alveoli of the lower jaw similar to the structure of the alveoli of the upper jaw. The wall of the upper third consists of two layers: solid and compact plates (inner and outer). In the area of the bottom and lower third of the alveoli, under the hard plate there is a spongy substance.
In the spongy substance of the body of the lower jaw There is a canal of the lower jaw (canalis mandibulae), through which vessels and nerves pass. The canal begins with the opening of the lower jaw (foramen mandibulae), on the inner surface of the branch and ends with the mental opening on the outer surface of the body. The canal has an arcuate direction with a convexity facing down and forward, lies closest to the bottom of the alveoli of the 2-3rd molar and passes between the chambers for their roots. Small tubules extend from the canal, through which vessels and nerves pass to the roots of the teeth; they open at the bottom of the alveoli. Medially from the mental foramen, the mandibular canal continues in the form of a small tubule to the midline and along this length gives off lateral branches to the bottom of the alveoli of the anterior teeth.
Branch of the lower jaw (ramus mandibulae) has outer and inner surfaces, anterior and posterior edges, which pass respectively into the coronoid process (processus coronoideus) and into the condylar process (processus condylaris). These processes are separated by the notch of the lower jaw (incisura mandibulae). The coronoid process serves to attach the temporal muscle, the condylar process serves to form. The shape of the mandibular ramus varies individually (Fig. 1-14).
Rice. 1-14. , bottom view: A - wide and short; B - narrow and long
Condylar process has a head (caput mandibulae) with an articular surface for connection with the mandibular fossa of the temporal bone and a neck (collum mandibulae). On the anteromedial surface of the neck of the condylar process there is a pterygoid fossa (fovea pterygoidea) - the attachment site of the external pterygoid muscle.
Head of the articular process flattened and occupies a position in which the axes drawn through the largest size of both heads intersect at the foramen magnum at an angle of 120-178°, open anteriorly. The shape and position of the head are individually different and depend on the operating conditions of the TMJ and the condition of its components. Deviations leading to changes in the volume and direction of movement in the joint change the shape and position of the articular heads.
Anterior edge of the mandibular ramus Laterally it passes on the outer surface of the jaw body into an oblique line, and medially it reaches the posterior alveoli, thus limiting the retromolar fossa. The medial part of the ridge, formed at the site of the transition of the anterior edge into the walls of the posterior alveoli, stands out under the name of the buccal crest (crista buccinatoria), from which the buccal muscle begins.
Posterior edge of the branch passes into the base of the jaw, forming an angle (angulus mandibulae), the value of which ranges from 110 to 145° (usually 122-133°) and changes throughout life. In newborns it is close to 150°, decreases in adults with preserved teeth and maximum chewing load, and increases again in old people with complete loss of teeth (Fig. 1-15).
Outer surface of the branch contains a masticatory tuberosity (tuberositas masseterica), which occupies most of the ramus and angle of the jaw and is the site of attachment of the masticatory muscle. On the inner surface of the branch in the area of the angle and adjacent sections there is a pterygoid tuberosity (tuberositas pterygoidea) - the place of attachment of the medial pterygoid muscle. On the same surface, in the middle, there is an opening for the lower jaw (foramen mandibulae), which is covered in front and above by an inconsistently pronounced bony protrusion - the tongue (lingula mandibulae). Above and anterior to the uvula is the mandibular ridge (torus mandibularis) - the place of attachment of two ligaments: the maxillary-pterygoid and the maxillary-sphenoid.
Branches of the lower jaw usually turned outward, so that the distance between the condylar processes of the right and left branches is greater than the distance between the outer points of the angles of the jaw. Can be distinguished as extreme forms of the jaw with maximally and minimally deployed branches. The degree of divergence of the branches depends on the shape of the upper half of the face. With a wide upper half of the face, the branches of the lower jaw are less developed than with a narrow upper half of the face. The smallest width of the branch, which usually falls in the middle of its height, ranges from 23 to 40 mm (usually 29-34 mm). The width and depth of the jaw notch are also individually different: the width of the notch is from 26 to 43 mm (usually 32-37 mm), the depth is from 7 to 21 mm (usually 12-16 mm). People with a wide upper half of the face usually have jaws with the greatest width of the notch and vice versa.
Biomechanics of the lower jaw
The forces that compress the teeth create more stress in the posterior sections of the branches. Self-preservation of living bone under these conditions consists of changing the position of the branches, i.e. The angle of the jaw should change; it occurs from childhood through maturity to old age. The optimal conditions for stress resistance are to change the jaw angle to 60-70°. These values are obtained by changing the “external” angle: between the basal plane and the rear edge of the branch (see Fig. 1-15).
Overall strength of the lower jaw under compression under static conditions it is about 400 kgf, less than the strength of the upper jaw by 20%. This suggests that arbitrary loads when clenching teeth cannot damage the upper jaw, which is rigidly connected to the cerebral part of the skull. Thus, the lower jaw acts as a natural sensor, a “probe”, allowing the possibility of gnawing, destroying with teeth, even breaking, but only the lower jaw itself, without damaging the upper jaw. These indicators must be taken into account when making prosthetics.
One of the characteristics of the compact bone substance is its microhardness, which is determined using special methods using various instruments and amounts to 250-356 HB (Brinell). A higher rate is observed in the area of the sixth tooth, which indicates its special role in the dentition.
Rice. 1-15. Changes in the value of the “external” angle of the human lower jaw in connection with his age and the presence of teeth
The microhardness of the compact substance of the lower jaw ranges from 250 to 356 HB in the area of the 6th tooth.
In conclusion, let us point out the general structure of the organ. Thus, the branches of the jaw are not parallel to each other. Their planes are wider at the top than at the bottom. The toe-in is about 18°. In addition, their front edges are located closer to each other than the rear ones, almost a centimeter. The basic triangle connecting the apexes of the angles and the symphysis of the jaw is almost equilateral. The right and left sides are not mirror-like, but only similar. Ranges of sizes and structural options depend on gender, age, race and individual characteristics.
Materials used: Anatomy, physiology and biomechanics of the dental system: Ed. L.L. Kolesnikova, S.D. Arutyunova, I.Yu. Lebedenko, V.P. Degtyareva. - M.: GEOTAR-Media, 2009
The lower jaw is under the continuous action of masticatory and facial muscles; these functional features leave a sharp imprint on both the relief and its internal structure. The outer and inner sides are replete with irregularities, roughness, pits and depressions, the shapes of which depend on the method of muscle attachment. Attachment of a muscle with a tendon leads to the formation of bumps and roughness of the bone tissue. The direct attachment of muscles to the bone, in which the muscle bundles (their shells) are woven into the periosteum, leads, on the contrary, to the formation of pits or a smooth surface on the bone (B. A. Dolgo-Saburov). There is another explanation for the morphological features of the bone at the site of muscle attachment. When the muscle acts perpendicularly on the bone, a depression is formed, and when the muscle acts at an angle relative to the bone, a tuberosity occurs. On the inner surface of the body of the lower jaw, near the midline, there is a single or bifurcated mental spine, spina mentalis (the origin of the geniohyoid and genioglossus muscles). At its lower edge there is a depression - the digastric fossa, fossa digastrica, a trace of the attachment of the digastric muscle. Lateral to the digastric fossa there is a bone ridge running upward and posteriorly. It is formed as a result of the action of the mylohyoid muscle attached to this roller. This line is called the internal oblique, or mylohyoid line, linea mylohyoidea (the mylohyoid muscle and the maxillopharyngeal part of the superior pharyngeal constrictor begin here). Above the anterior part of the maxillary-hyoid line there is a depression formed due to the adherence of the sublingual salivary gland. Below 
In the posterior jaw of this ridge there is another depression, to which the submandibular salivary gland is adjacent. In the middle of the inner surface of the branch there is an opening of the lower jaw, foramen mandibulae, limited from the inside and in front by a small bony protrusion - the tongue of the lower jaw, lingula mandibulae. This hole leads into the canal of the lower jaw, canalis mandibulae, in which vessels and nerves pass. The canal lies in the thickness of the spongy bone. Below the mandibular foramen is the maxillary-hyoid groove (sulcus mylohyoideus) - a trace of the contact of the maxillary-hyoid branch of the mandibular artery and the maxillary-hyoid nerve.
Outer surface of the lower jaw.
The outer surface of the lower jaw is distinguished by the following anatomical features: the chin protuberance (protuberantia mentalis) is located in the symphysis area - at the fusion of the two halves of the lower jaw. Fusion occurs in the first year of extrauterine life of the child. Subsequently, this part of the chin fuses with the chin bones. These bones also take part in the formation of the chin protrusion.
The mental protrusion is limited on the side by the mental foramen (foramen mentale), which serves as the exit point for the mental nerves and vessels and is located between the first and second premolars. An external oblique line stretches upward and posteriorly from the opening, located on the border between the body of the lower jaw and the alveolar process. On the outer surface of the angle of the lower jaw there is a roughness formed as a result of the traction of the masticatory muscle attached in this place, the so-called masticatory tuberosity (tuberositas masseterica). The external oblique line, like the internal one, serves to strengthen the lower molars and protect them from loosening in the bucco-lingual direction during transversal chewing movements (A. Ya. Katz). Between the articular head and the coronoid process there is a mandibular notch (incisura mandibulae).
It is interesting to briefly dwell on the phylogeny of the mental protuberance (protuberantia mentalis). Different authors explain the formation of the chin in different ways.
Some attribute the appearance of the chin to the action of the pterygoid muscles. The external and internal pterygoid muscles, acting on both sides in opposite directions, create an area of dangerous section in the area of the mental protuberance and stimulate the bone tissue in the mental region to grow and thicken, which protects the lower jaw from fracture. This theory is one-sided.
Others explain the formation of the chin with the emergence of articulate speech and rich facial expressions, which distinguishes modern man from his ancestors. Various emotional experiences, reflected on the face and requiring continuous and specialized mobility of the facial muscles, cause increased functional irritation of the bone tissue and, as a result, the formation of a chin protrusion. This idea is confirmed by the fact that all modern people have a pronounced chin, while primitive people, who stood at a low level of the phylogenetic ladder, had no chin.
Still others explain the formation of the chin by reduction of the alveolar process due to the reverse development of the lower dentition; the basal arch of the lower jaw therefore protrudes.
Branch of the jaw, ramus mandibulae, extends upward on each side from the posterior part of the body of the lower jaw. At the top, the branch of the lower jaw ends in two processes: the anterior one, the coronoid, processus coronoideus (formed under the influence of the traction of the strong temporal muscle), and the posterior condylar, processus condylaris, is involved in the articulation of the lower jaw with the temporal bone. Between both processes a notch incisura mandibulae is formed. Towards the coronoid process, the crest of the buccal muscle, crista buccinatoria, rises on the inner surface of the branch from the surface of the alveoli of the last large molars.
Condylar process has a head, caput mandibulae, and a neck, collum mandibulae; in front of the neck there is a fossa, fovea pterygoidea (place of attachment of m. pterygoideus lateralis).
To summarize the description of the lower jaw, it should be noted that its shape and structure characterize modern humans. Along with this, a person began to develop articulate speech, associated with the strengthened and delicate work of the tongue muscles attached to the lower jaw. Therefore, the mental region of the lower jaw, associated with these muscles, functioned intensively and resisted the action of regression factors, and mental spines and a protrusion appeared on it. The formation of the latter was also facilitated by the expansion of the jaw arch, associated with an increase in the transverse dimensions of the skull under the influence of the growing brain. Thus, the shape and structure of the human lower jaw were influenced by the development of labor, articulate speech and the brain that characterize a person.
33812 0
(mandibula), unpaired, horseshoe-shaped (Fig. 1). This is the only movable bone of the skull. It consists of two symmetrical halves, completely fused by the end of the 1st year of life. Each half has a body and a branch. In old age, a bony protrusion forms at the junction of both halves.
IN body (corpus mandibulae) differentiate base of the lower jaw (basis mandibulae) And alveolar part (pars alveolaris). The body of the jaw is curved, its outer surface is convex and its inner surface is concave. At the base of the body, the surfaces transform into one another. The right and left halves of the body converge at individually different angles, forming the basal arch.
The height of the jaw body is greatest in the area of the incisors, the smallest is at the level of the 8th tooth. The thickness of the jaw body is greatest in the region of the molars, and the smallest in the region of the premolars. The cross-sectional shape of the jaw body is not the same in different areas, which is determined by the number and position of the roots of the teeth. In the area of the front teeth it approaches triangular with the base facing downwards. In areas of the body corresponding to large molars, it is close to a triangle with the base facing upward.
Rice. 1.
a — topography of the lower jaw;
b — side view: 1 — coronoid process; 2 - notch of the lower jaw; 3 - pterygoid fossa; 4 - head of the lower jaw; 5 - condylar process; 6 - neck of the lower jaw; 7 - chewing tuberosity; 8 - angle of the lower jaw; 9 - base of the lower jaw; 10 - mental tubercle; 11 - chin protuberance; 12 - chin hole; 13 - alveolar part; 14 - oblique line; 15 - branch of the lower jaw;
c — view from the inner surface: 1 — condylar process; 2 - coronoid process; 3 - tongue of the lower jaw; 4 - opening of the lower jaw; 5 - maxillary-hyoid line; 6 - mental spine; 7 - sublingual fossa; 8 - mylohyoid groove; 9 - mandibular ridge; 10 - pterygoid tuberosity; 11—submandibular fossa; 12—digastric fossa; 13 - angle of the lower jaw; 14 - neck of the lower jaw;
d — top view: 1 — alveolar arch; 2 - retromolar fossa; 3 - temporal crest; 4 - coronoid process; 5 - tongue of the lower jaw; 6 - pterygoid fossa; 7 - head of the lower jaw; 8 - oblique line; 9 - mandibular pocket; 10—base of the lower jaw; 11 - mental tubercle; 12—chin protuberance; 13 - dental alveoli; 14 - interalveolar septa; 15 - chin hole; 16 — interroot septa; 17 - neck of the lower jaw; 18 - condylar process;
d — position of the opening of the lower jaw; e - the magnitude of the angle of the lower jaw
In the middle outer surface the body of the jaw is located chin protuberance (protuberantia mentalis), which is a characteristic feature of modern man and causes the formation of a chin. The angle of the chin to the horizontal plane in modern humans ranges from 46 to 85°. In the apes, Pithecanthropus, Heidelberg man and Neanderthal, the mental protrusion is absent, the angle of the chin in the first three is obtuse, and in the Neanderthal it is straight. From 1 to 4 participate in the formation of the human mental protuberance chin bones (ossicula mentales), which arise at the time of birth and later fuse with the jaw. On both sides of the mental protuberance, closer to the base of the jaw, there are mental tubercles (tubercula mentalia).
Outward from each tubercle is located mental foramen (foramen mentale)- outlet of the mandibular canal. The vessels and nerve of the same name exit through the mental foramina. Most often, this hole is located at the level of the 5th tooth, but can move anteriorly to the 4th tooth, and posteriorly to the space between the 5th and 6th teeth. The dimensions of the mental foramen range from 1.5 to 5 mm; it is oval or round, sometimes double. The mental foramen is removed from the base of the jaw by 10-19 mm. On the jaws of newborns this hole is located closer to the base, and on the toothless jaws of adults with an atrophied alveolar part - closer to the upper edge of the jaw.
An obliquely located ridge runs along the lateral half of the outer surface of the body of the lower jaw - oblique line (linea obliqua), the anterior end of which corresponds to the level of the 5-6th tooth, and the posterior end, without sharp boundaries, passes to the anterior edge of the lower jaw branch.
On inner surface of the body of the jaw, near the midline, there is a bone spine, sometimes double, - mental spine (spina mentalis). This place is the beginning of the geniohyoid and genioglossus muscles. Below and lateral to the mental spine is determined digastric fossa (fossa digastrica), in which the digastric muscle begins. Above the digastric fossa there is a flat depression - sublingual fossa (fovea sublingualis)- trace from the adjacent sublingual salivary gland. Further posteriorly visible mylohyoid line (linea mylohyoidea), on which the muscle of the same name and the superior pharyngeal constrictor begin. The mylohyoid line begins below the hyoid fossa and ends on the inner surface of the jaw branch. In some cases it is barely noticeable, in others it is represented by a strongly pronounced bone ridge. Under the maxillary-hyoid line at the level of the 5-7th tooth there is submandibular fossa (fovea submandibularis)- a trace from the submandibular salivary gland located in this place. Below and parallel to the maxillary-hyoid line there is a groove of the same name, to which the vessels and nerve are adjacent. The groove begins on the inner surface of the jaw branch near the opening of the lower jaw and ends under the posterior part of the mylohyoid line. Sometimes for some distance it turns into a canal.
The lower jaw is a movable bone of the facial skeleton, consisting of a body, a branch, and an angle.
The body consists of basal and alveolar parts.
The branch has two processes - the condylar, ending in the head of the lower jaw, and the coronoid.
The ratio of the height of the branch to the length of the body of the jaw in an adult is 6.5-7:10. The angle of the lower jaw is normally 120 degrees ± 5 (Tridents).
The shape of the dentition is parabolic.
The lower jaw is a horseshoe-shaped unpaired bone consisting of a body, two branches that end in two processes, a coronoid and an articular, and between the processes a semilunar notch.
The lower edge of the body and the posterior edge of the branch form an angle of 110-130°
Inner surface:
1. In the area of the central incisors there are mental spines;
2. Next to them is the digastric fossa, the place of attachment of the muscle of the same name;
3. Laterally (from the fossa) the bone ridge is the internal oblique line (mylohyoid);
4. In the area of the angle on the inner side there is the pterygoid tuberosity, the place of attachment of the muscle of the same name;
5. On the inner surface of the lower jaw branch there is a hole, the exit point of the neurovascular bundle.
Outer surface:
1. Mental protuberance, mental foramina in the area of the second premolars;
2. The external oblique line goes superiorly and posteriorly, merging with the internal oblique line to form a space behind the retromolar;
3. In the area of the corner there is a masticatory tuberosity.
