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Department of Prosthodontics

PDM_Preclinical_I_L5 -_Engl

Functional – mechanical equilibrium of the periodontium. Muscle

forces. Masticatory pressure. Periodontal pressure. Biomechanics of

Mandibular movements. Basic movements. Theory of Mandibular movements

Slide 1

The masticatory system is a complex musculoskeletal system, which consists of the

jaws.And anatomically complex of muscles, involved in mastication. These muscles bring about

mandibular motion and generate forces, which can influence morphology and Material properties

of the osseous components. The activation and coordination of the jaw muscles determine the

direction of jaw movement, control occlusal force, and load the bones of the skull in various

ways. The jaw muscles are involved in a much broader range of motor tasks.

Slide 2

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Under physiological conditions, the muscle is activated until nervous stimulation ceases.

The level of muscle activation, and with it the amount of force generated, is controlled by two

major mechanisms: rate modulation, altering the frequency of neuronal action potentials at the

neuromuscular junction and recruitment modulation, altering the number of active motor units in

the muscle.

Slide 3

From a classical anatomical perspective, the jaw muscles are divided into elevator and

depressor groups. As you already know, The elevator group consists of the masseter, temporalis, and medial pterygoid muscles, while the depressor group consists of the geniohyoid, mylohyoid, and digastric muscles. The lateral pterygoid muscle completes the system. Because its two heads have different actions, this muscle cannot be regarded exclusively as elevator or Depressor. Jaw-muscle activity has mostly been characterized by the relative duration of muscle activation during a specified period the so-called “duty time”, which is regarded as a measure for the overall neuromuscular activity. Differences in the duty time between muscles or muscle groups provide information on differential neuromuscular activation by the central nervous system.

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Dental absence interferes in the physiological functioning of the masticatory system, promoting occlusal and functional alterations. These data evidence the strong influence of

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dental loss over the maximal bite force and small correlation between bite force and electromyographic activity. Changes in vertical jaw opening may affect the relative contributions of various masticatory muscles to bite force production. EMG activity usualy has recorded simultaneously from the masseter, and anterior, middle, and posterior temporalis muscles, during controlled isometric biting at different force levels and vertical jaw openings.

Slide 6

Every individual muscle group develop muscle activity with a certain force. The size of

the masticatory pressure is proportional to the size of this aggregate and absolute muscle strength

and it is inverse to the size of the occlusal surface. The Absolute muscle force is in direct

proportion to the mass of muscle groups. This slide shows data for the masticatory muscles as

follows: The thickness determines the absolute muscle strength, which includes the synergetic

action of these symmetrical muscles.

Slide 7

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Three main types of muscle action determine the size of the masticatory pressure on the future fixed partial prosthesis. Absolute and specific are the most important.

Slide 8

It is a great honour for our Department the formulation of the main design principle for

prostheses construction by prof. Balabanov : the chewing surfaces of bridge prosthesis must

comply with state and reserve forces of the abutments periodontium. The corresponding perio

reactions are appeard directed against of the occlusal surfaces. In this reason the abutment’s

periodontium is loaded functional better when reducing the size of the retainer’s chewing

surfaces. The Balabanov Law of the functional - mechanical equilibrium of the abutments

periodontium can be expressed by the above equation, shown on the bottom of the slide down. It

states that the abutments periodontium is in their functional mechanical equilibrium when the

ratio between the total occlusal surface with total periodontal surface is constant and does not

exceed the result 2/1. Total occlusal surface is also mixed including different elements –

retainer’s occlusal surfaces and pontics ones.

Biomechanics of the Temporomandibular Joint Temporomandibular joint (TMJ) connects the mandible or the lower jaw to the skull and

regulates the movement of the jaw. The TMJ is one of the most complex, delicate and highly

used joints in a human body. The most important functions of the TMJ are mastication and

speech.

Slide 9

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TMJ is a bi-condylar joint in which the condyles, the movable round upper ends of the

Mandible, function at the same time. Between the condyle and the articular fossa is a disc made

of fibrocartilage that acts as a cushion to absorb stress and allows the condyle to move easily

when the mouth opens and closes. Mandibular movement around the horizontal axis is an

opening and closing motion. It is referred to as a hinge movement, and the horizontal axis

around which it occurs is therefore referred to as the hinge axis (Figure 4-2). The hinge

movement is probably the only example of mandibular activity in which a “pure” rotational

movement occurs.

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Slide 13

Mandibular movement refers to the muscle- and ligament-activated border and/or

intraborder movements of the lower jaw. There are five types of mandibular movements

including rotational, horizontal axis, frontal axis, sagittal axis, and translational. Mandibular

movement is affected by several factors such as the muscles used in suspending the jaw,

mandibular articulation, and the synovial joint system. Study of this movement is important for

the fields of dentistry and orthodontics as it describes the concepts related to dental occlusion

and the masticatory processes of the jaw.

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Slide 28

Functional conditions of the mandible: theory and physiology

The functional conditions of the mandible are differentiated according to the number of

kinematic degrees of freedom assigned to each mandibular movement. As mandibular

movements reflect the functional morphology of temporomandibular joint (TMJ), the occlusal

morphology of each tooth may be functionally related to its antagonist, to the TMJ and to the

other components of the stomatognathic system in reasonably precise ways.

Slide 29

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General types of articulators

Slide 30

Dental articulators are mechanical instruments that recreate the relationship between the

temporomandibular joint (TMJ) and the jaws, by mounting upper and lower print patterns on the

instrument.

The articulator simulates the movements of the patient's jaw; it provides static and

dynamic relationships to observe these malocclusions or dysfunctional movements of extraoral

form. Thus, the dentist can study the treatment guidelines without fatigue or discomfort for the

patient.

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Slide 31

Aesthetic perspective – the casts are mounted with a facebow so that the upper member

of the articulator corresponds approximately to a plane passing through the ears and the infra-

orbital notch. The alignment of the incisal edges to the horizontal can be observed.

Fully adjustable articulators - Fully adjustable articulators are more complex devices

allowing the clinical scenario to be most closely reproduced. Instead of flat tracks and planes that

reproduce the condylar movements on semi-adjustable articulators, fully adjustable articulators

have further components that can be adjusted and use curved condylar inserts that can more

accurately reproduce the three-dimensional nature of the glenoid fossa anatomy.

Slide 32

An articulator is a mechanical device wth upper and lower components to which maxillary and

mandibular casts are attached and which is intended to reproduce the static relationship of a

patient's maxilla to mandible (in intercuspal or retruded contact positions [RCPs]) and may

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provide to a limited extent for lateral and protrusive movements. Semi adjustable articulators are

the most often used in dental practice.

Articulators are used for the following:

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Study individual teeth and full dental arches for diagnosis and treatment planning

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Allow adjustment of fixed and removable prostheses and indirect dental restorations

Articulators have been the cornerstone requirement of prosthodontics and restorative

dentistry. However, their key role is changing with the progressive incorporation of

technological advances. The development of digital records and virtual articulators is

transforming all aspects of dental practice; as the most far-reaching and definitive change

for case management and education, it is an exciting transformation.

There are many designs of articulators, but in general there are four different types:

•Simple hinge

•Average value (plane-line)

•Semiadjustable

•Fully adjustable

Average value articulators have their condylar angle fixed at 30°. There is no provision for an

adjustment for condylar side shift but they may have an adjustable incisal guidance (Fig. 9-1, B).

Semiadjustable articulators allow adjustment of condylar inclination and side shift (Bennett

angle or progressive side shift) and in some designs for Bennett movement or immediate side

shift. Intercondylar width is usually fixed at 110 mm, but some articulators allow different

intercondylar width settings.

Реферат на тема : Движения на долната челюст и видове артикулатори