Biomechanics of lower jaw movements: Spee's curve, Bennett's angle, muscles, occlusal compass, transversal plane

When considering in detail the issue of the biomechanics of the movements of the lower jaw, the thought of the perfection and harmony of everything created by the hands of nature involuntarily comes to mind.

In the human body, however, as in other biological nature, everything is aimed at the implementation of the fundamental principle - expediency.

From the molecular structure of any substance to a complex biological structure, everything is tied to the implementation of a single idea and the answer to the question - for what and in the name of what?

Without such a strict organization, the biological functioning of any organism is impossible.

Purpose, structure and functioning of the dentoalveolar system

Understanding of a complex process called the biomechanics of the chewing apparatus of the dentoalveolar system, contributes to the timely detection of pathology in the development of muscles, articular structures, teeth closing and fortunes

periodontium (tooth - Greek. odontos, lat. dente - hence the formation: odontology is the science that describes odontos, periodontitis is a disease of periodontal tissues). It is from a healthy periodontium - a complex of tissues surrounding the tooth, which is a single component temporomandibular joints, its normal work depends.

It follows from this that the biomechanical functions of the periodontium are determined by the anatomical and physiological features of its structure and are closely tied to the work of other elements.

The laws of the biomechanics of the dentition are successfully applied in orthopedics at the stages of designing and creating various prostheses, as well as some auxiliary devices.

Apparatus for reproducing the movements of the lower jaw include:

1. Occluder. Patient care device that allows the design and correct fitting of the prosthetic structure.
2. Face bow. This device allows you to make an impression as accurately as possible for further bite corrections.
3. Articulator. They are of various types: universal, medium (simplified). This device is used for the manufacture and fitting of removable and fixed dentures and bridges, crowns and drip.

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It should be noted that the articulator is an extremely important device that helps to correctly and exclusively fit different prostheses. After all, it is articulation, which in dentistry is perceived as a multi-vector movement of the lower jaw (lat. mandibula) relative upper, which occurs during compression and tension chewing muscles, decisively determines the intelligible and articulate pronunciation.

If some kind of pathology associated with LF is formed, then speech, chewing food, laughter, swallowing are immediately disturbed.

In the table of movements of the lower jaw, in a compressed form, the basic positions and determining factors of the dominant theories of articulations are set out, the authors of which are Ganau, Gizi, Monson. Despite some discrepancies in the interpretation of the processes, their authority is indisputable, and the role in the development of orthopedics is beyond doubt.

Articulation theories of the construction of dentition	Basic provisions	Determining factors
Gizi's theory	The slope of the articular pathway determines the vector of displacement of the mandibula, which is influenced by the size and shape of the articular tubercle	Accurate definition of the articular pathway. Incisal path recording. Determination of the sagittal compensation curve. Determination of the transversal compensation curve of the line.
Monson's theory	Complex vector displacements of the LF are determined not by the articular pathways, but by the surfaces of the dental cusps, which give direction to the advances
Ganau theory	The theory is similar to that of Gizi, which analyzes the entire system of articulation. In particular, she highlights the differences between the position of the prostheses in the articulator and in the mouth due to the decrease in the elasticity of the muscle tissue.	Inclination of the articular path Compensation curve depth Inclination of the reference plane Inclination of the upper incisors The height of the hillocks
Balancing theory	Takes into account: the angle of inclination of the sagittal compound path; the angle of inclination of the sagittal incisal path; the angle of inclination of the transversal articular path; the angle of inclination of the transversal incisal path; the angle of inclination of the cusps of artificial teeth; the angle of inclination of the occlusal curves; directions of the occlusal plane.
Spherical theory	Provides: articulatory balance in the phase of chewing movements; vector freedom of displacements; fixing the position of the central occlusion while obtaining a functional impression; the formation of a tuberculous chewing plane.

In addition, correct and healthy breathing, aesthetic emotions (expression) are impossible if the muscles pushing the lower jaw forward are subject to obstruction (spasm, remission).

Full chewing of food occurs only if the teeth of the upper and lower jaw enter the correct contact - occlusion. Therefore, it is precisely the closure of the dentition that is the defining characteristic of the chewing movements.

All connecting elements of the LF move as a result of the synchronous interdependent action of the temporomandibular joint (TMJ), muscular masticatory tissues and teeth. Their actions are organized, coordinated and controlled by the central nervous system.

Displacements of a spontaneous and reflex nature are entirely subordinated to the neuromuscular apparatus and are able to reproduce sequentially.

Initial voluntary movements include the process of biting off food and directing it into the mouth. And already following them chewing and swallowing are reflex-unconscious actions.

Due to the tasks that are defined for the jaw, its complex structure is determined.

First of all, it is the only movable bone of the facial skull, which vaguely resembles a horseshoe.

This structure is due not only to the defining purpose as a responsible component of the chewing process, but also to its development, which comes from the first branchial arch.

Mandibula structure:

1. Body.
2. The edge of the body where the cells for the teeth (alveoli) are located is the alveolar ridge.
3. Chin hole. It serves as a communicator for nerves and blood vessels.
4. Injection.
5. Head.
6. Mandibular canal and foramen.
7. Branches.
8. Articular and coronary processes.

Bone formations would remain permanently in a static position if it were not for the muscle tissue connecting them.

The muscles that move the lower jaw are called chewing muscles.

Moreover, each muscle structure, or rather their groups, produce certain movements:

1. The medial pterygoid, masticatory and temporal ones raise the jaw.
2. The digastric, maxillary-hyoid, chin-sublingual are involved in the process of lowering.
3. Lateral movement is possible thanks to the lateral pterygoid muscles.

Directions of movement of the lower jaw

During the active phase, the biomechanics of the masticatory apparatus ensures the work of the LF in three vector directions or planes of motion, simultaneously producing rotational and sliding displacements of it heads:

• vertical;
• sagittal;
• transversal.

Vertical movement

It is possible with the active work of bilateral muscle tissues extending from the LF to the hyoid bone. This movement is characteristic when opening and closing the mouth.

The weight of the bone itself in this case acts as an auxiliary factor.

Three phases characterize this process, i.e. opening the mouth directly:

• insignificant;
• significant;
• maximum.

The maximum vertical displacement can be up to 5 centimeters.

The reverse movement is carried out thanks to the same muscle group, but already with their contraction.

Raising and lowering occurs in the lower part of the joint between the head of the bony structure and the cartilaginous disc.

To identify anomalies in the structure of the jaws of the dentition in the vertical vector of displacement, as well as to calculate linear and angular sizes of the skull and temporomandibular joint, in 1884 at the congress of anthropologists in Frankfurt was adopted and consolidated the term "Frankfurt horizontal".

Sagittal movement

The sagittal axis of displacement is expressed by the forward-backward movement vector. It is realized as a result of the work of the lateral pterygoid muscle tissues in the upper part of the joint, between the articular surface of the temporal bone and the cartilaginous disc.

At first glance, forward bone traffic is a simple biomechanical process. In fact, it consists of rather complex components, which are divided into two phases:

1. The first. The cartilaginous disc, together with the head, moves along the articular surface of the tubercles.
2. The second. At this stage, its hinge movement around its own axis is simultaneously connected to the sliding displacement of the head. The vector of this axis itself passes directly through the head of the main bone structure.

This traffic is synchronized on both the left and right. The LF structure allows the head to be pushed down and forward along the articular tubercle to a distance of up to one centimeter.

The distance that the articular head travels when moving forward is called the sagittal articular path.

It is worth recalling that this movement or path is not purely linear, but passes at a certain angle, which is formed when the intersection of vectors lying in the occlusal plane and the sagittal line - in the plane of the sagittal articular path.

A logical question arises - what, in this case, is the angle of the sagittal articular path?

Alfred Gizi, an authoritative university professor from Zurich, already in the last century - in 1908, measured and substantiated the relationship between the angles of inclination of the incisal and articular paths.

According to him, which no one disputes, the angle of the sagittal path is 33 °.

The traffic that the lower incisors make when moving the bone structure is called the sagittal incisal path by the same scientist.

When the line of this path is crossed with the occlusal plane, an angle of the sagittal incisal path is formed. And it fits within the range from 40 to 50 degrees.

By the way, A. Gizi made a significant contribution to the development of gnathology, a science that studies the coordinated work of the dentoalveolar apparatus. These and other discoveries allowed the eminent scientist, already in 1912, to create an unregulated articulator, which became the prototype of today's orthopedic devices.

Transversal movement

Lateral displacements occur in the horizontal or transversal plane and are carried out by contraction (compression) of the lateral pterygoid muscles.

Here you need to correctly understand the vector directions. Simply put, the horizontal displacement is carried out to the left and right relative to the horizon, but in the frontal plane, if you look in the face (front) of a person.

If the joint moves to the right side, then the left lateral muscle is working and vice versa.

In this case, the jaw head from the offset side rotates around the vertical axis. It slides simultaneously with the disc along the articular surface of the tubercle - down and slightly inward. Simply put, the head makes a lateral articular path, which is also at an angle to the sagittal plane.

The angle of the transversal articular path in dentistry is called the Bennett angle and is equal to 17 °.

The position of the teeth will change if the bass moves to the left or right. These offsets have an angular projection called the transversal incisor path or gothic angle. With lateral displacements, it determines the span of the incisors, which fit within the range from 100 to 110 °.

Knowledge and understanding of the functioning of the apparatus for advancing the lower jaw forward and backward, as well as other vector components, allows you to correctly take into account the general factors that are extremely necessary in the creation of high-quality orthopedic designs.

It is these factors that decisively affect articulation:

1. Sagittal occlusal curve.
2. The height of the cusps of the chewing teeth.
3. The angle of inclination of the sagittal articular path.
4. The angle of inclination of the sagittal incisal path.
5. Transversal occlusal curve.

Also without knowledge and consideration of the Bonneville-Ganau laws of articulation, which determine the linear arrangement and close synchronous interconnection of all components of the LF, it will not be possible to correctly manufacture and install artificial teeth in dentures on toothless jaws.

Temporomandibular joint problems

TMJ dysfunction is a malfunctioning joint structure and muscle tissue that connects the upper jaw and lower jaw.

There is no doubt that this process, or rather the absence of it, is associated with various pathologies. It can be innate and acquired.

Temporomandibular joint dysfunction can manifest itself under the following causal circumstances:

1. There is a defect in the dentition.
2. Increased abrasion.
3. Pathology of a traumatic nature.
4. Improper contact (bite).
5. Errors made in the manufacture of orthopedic structures.
6. Congenital jaw anomaly and malformed teeth.

Symptoms of TMJ dysfunction:

1. When opening and closing the mouth, chewing food, clicking or clicking sounds appear.
2. The patient suffers from a migraine-like headache and pain in the ears and behind the eyes.
3. Pain when yawning and opening the mouth wide.
4. Weakening of the jaw muscle tissue.
5. When closing and opening the upper and lower jaw, pain and general physical discomfort are manifested.

Clinical studies determine which muscles, ligaments, bones and cartilage that move the lower jaw are working with abnormalities.

In addition, the results or final conclusions are needed to take measures of a local or large-scale nature, restore the functions of movement of the lower jaw and the temporomandibular joint in general.

Research methods are subdivided:

1. Clinical: survey, analysis of bite, articular noise and low frequency movement, palpation of the joint, masticatory muscle structure and pain points on the face.
2. X-ray. Computer tomograms, orthogram of the jaw, radiographs according to the Schüller et al.
3. Graphic:
   • Electromyography is a study of the bioelectrical capabilities of masticatory muscle fibers.
   • Masticatiography - recording of the chewing movements of the lower jaw.

Additional studies are also carried out: biochemical blood samples for rheumatism, psychosomatic and dental neurological examinations, etc.

To determine anomalies in the anatomical setting of the teeth, the following research method is used:

1. Vnerotova: recording the nature of jaw advancement, incisal slip angle and lateral displacements.
2. Intraoral: it is based on the use of the Christensen phenomenon, which fixes the lumen in the area of ​​the molars.

When eliminating the identified deviations and for the individual manufacture of prostheses, the technician needs to determine the occlusion and the central ratio of the jaws.

For this, a special technique for determining the central occlusion and the ratio of the jaws is recommended. It defines the sequence of actions, tools, criteria and evaluation of results.

Recall that occlusion Is a static and dynamic contact of the upper and lower jaw during various functional actions.

Perhaps, here it is worth declaring three "golden" rules of occlusion related to the topic of the TMJ dysfunction subsection:

1. Correct bilateral contact of the posterior group.
2. Canine separation and management of this group.
3. Unimpeded provision of the above functions.

When considering issues related to occlusion, other concepts and terms that relate to LF traffic are invariably used:

1. Spee Curve Is a sagittal occlusal curve that virtually touches the apex of the mandibula.
2. Wilson curve Is a transverse occlusal curve. It, as it were, repeats the figurative geography of the same hillocks, but in the vector of lateral displacement.
3. Occlusal plane belongs to the most important landmark in dentistry. It is an imaginary surface that runs along the tops of the frontal and chewing dente.
4. Occlusal compass in dentistry it is used to simulate the movement of teeth during the manufacture of certain orthopedic structures.

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Stable functioning of both jaws is possible due to the uniform fissure-tubercle contact of the lateral dental units. They just provide the correct axial load and relieve excessive periodontal stress.

Speaking about TMJ dysfunction, it is difficult, or rather impossible, to grade problems in terms of the depth and nature of the pathology.

But probably the greatest suffering for a person is caused by occlusion anomalies. Indeed, because of them, a person loses his attractiveness, suffers emotionally and mentally.

Types of malocclusion:

1. Distal. This is an anomaly of occlusion in the sagittal direction. When there is a disproportion in the development of the jaws - an underdeveloped lower and outstripping development of the upper.
2. Mesial. This anomaly repeats the previous one, but exactly the opposite.
3. Open. Vertical malocclusion. In this case, a gap is formed due to the non-closure of the dentition.
4. Deep. This is the most common problem when the upper dentition overlaps the lower one (protrudes forward) by a distance that exceeds the length of the dental unit.
5. Cross. This transversal anomaly occurs due to the weak development of one of the sides of the LF. As a result, the chewing tubercles of the lower canine seem to bulge forward relative to the upper teeth.
6. Dystopia. The arrangement of the unit of the dentition is not in its place in the row, i.e. shifted to the side.
7. Diastema. Formation of a gap (up to 6 mm) between the central incisor teeth of the upper or lower row, which is less common.

For a long time, the dispute about three-point occlusion does not subside. It looks like this - one contact point is on the front teeth, and the other two are on the cusps of the third molars.

This condition was investigated by Bonneville and named after him - Bonneville's three-point contact.

The number of supporters and opponents of this statement was divided equally, but has not changed to this day. Some consider this a deviation, others the norm.

In addition, he also introduced the concept of Bonneville's triangle, in which he calculated the distance between the articular heads and the incisal point, which is equal to 10 cm. This discovery formed the basis for the construction of most anatomical articulators.

Continuing the thought about TMJ dysfunction, it should be noted: fractures - This is the most severe and dangerous LF pathology.

This is a consequence of various mechanical injuries related to industrial, household, criminal and other circumstances.

The International Classifier of Diseases of the 10th revision (ICD-10) assigns its own exclusive code to each type of fracture. Whether it be articular fractures, alveolar, condylar, coronoid process, branch, angle, or the LF itself.

This makes it possible for a doctor in any country to understand the nature of the damage without detail and explanation.

Treatment and elimination of congenital and acquired pathology is a long and painstaking process that requires specialists in deep knowledge and understanding of the processes of NP biomechanics.