Physiology of Stomatognathic
System
Presented by-
Sara Veqar (66)
BDS IVth year
BBD University

Content
Definition
Components
Functional osteology:
● Bone turnover- modelling and remodelling
● The trajectorial theory of Culmann, Meyer, Wolff, Thoma and Roux;
● Stress trajectories/ Beninghoffs lines
Myology
● Jaw reflex types
● Buccinator mechanism
● Tongue
● Equilibrium theory
Temporo Mandibular Joint
● Anatomy of temporo mandibular joint
● Response of tmj to abnormalities

Stoma: mouth
Gnathia: jaws
Stomatognathics
Salzmann defines stomatognathics as the approach to the practice of orthodontics, which takes
into consideration, the interdependence of form and function of the teeth, jaw relationship,
temporomandibular articulation, craniofacial conformation and dental occlusion.
Stomatognathics deals with the functional anatomy. Stability of the orthodontically moved teeth
depends on the integration of the stomatognathic components.
Components
The components of the stomatognathic system are teeth and their supporting structure, jaw
bones and their functional osteology, muscles of the face and head, temporomandibular joints,
tongue, nerves, vascular supply and related structure.

Functional Osteology
Although bone is one of the hardest materials in the body, it is one of the most plastic and most responsive
to functional forces.
Bone turnover
The physiologic concept of bone turnover is largely attributed to important biologic activities, like
remodeling and modeling.
Bone Remodeling
It is the term representing the physiological turnover of the mineralized tissue without changing its overall
appearance. Both catabolic (resorptive) and anabolic (osteogenic) events are coupled as a sequence.
Remodeling is active throughout life and serves to modify the shape of skeleton, architecture, and bone
volume, and repair micro-damage
Bone Modeling
It is a mechanically mediated adaptive process for changing a bone’s size, shape, or position. Bone modeling
is an uncoupled process (i.e. where anabolic and catabolic sites are controlled independently).
Osteoblasts and osteoclasts act over a large surface area, removing or forming large volumes of bone mass,
which is an activity primarily found during growth and is responsible for the final shape of the bones.

The density of bone is modulated by a group of cells, including osteoclasts, which are multi-nucleated cells that resorb bone and osteoblasts
which refill the resorption cavities created by osteoclasts.
Osteoclasts anchor themselves to the surface of bone.
This creates a microenvironment underneath the cell which is referred to as the sealed zone.
Within this zone, the osteoclasts create an acidic environment that dissolves the bone's mineral content.
Once the mineral content of the bone has been dissolved, enzymes released from osteoclasts remove the remaining collagenase bone
matrix to complete the process of resorption.
Following resorption, osteoblasts move into the resorption space, and start to produce and deposit organic matrix called osteoid.
Osteoid, a substance made predominantly of collagen forms a scaffold in which minerals including calcium and phosphate begin to
crystallize.
Some active osteoblasts become trapped within the matrix they secrete and thereby become osteocytes.
Other osteoblasts will undergo apoptosis or will revert back to lining cells, which cover the surface of bone.
This cycle of bone resorption and formation is referred to as remodeling.
There is also a process where bone formation by osteoblasts occur without prior bone resorption by osteoclasts.
This results in an increase in bone mass, and is referred to as bone modeling.
Bone modeling promotes the growth of bones and is important for maintaining bone strength.
The important element of growth of the skeleton is bone modeling and functions as incremental process for bone mass adaptation and
architecturing to its functional needs over lifetime.

The trajectorial theory of Culmann, Meyer, Wolff, Thoma and
Roux;
The trajectorial theory of force states that the lines of orientation of the bony trabaculae correspond to the pathways of maximal pressure and tension
and that bony trabaculae are thicker in region where the stress is greater
Trajectorial Theory of Bone Formation
In 1867, an anatomist named Meyer, with the help of the mathematician Culmann, propounded the trajectorial theory of bone formation.
Cullman was mathematician. In this seminal work, he described how the transmission of stresses in structures could be determined with the
use of graphical analysis
Illustrated in Culmann’s text are two solid structures, a crane and a beam, showing calculated stress trajectories, which ultimately influenced
von Meyer’s ideas about the mechanical relevance of trabecular architecture His approach was later used in the design and engineering of the
Eiffel Tower
Von Meyer drew a crane similar to the shape of the upper end of the femur and asked Culmann to draw in the tension and pressure lines
(trajectories) to be calculated by him for this purpose, having already drawn trabeculae that were significant—in his opinion—on another
piece of paper.
The illustration by Culmann corresponded with that of Meyer.
Meyer pointed out that the alignment of the bony trabeculae in the spongiosa followed definite engineering principles. If lines were drawn
following discernible columns of oriented bony elements, these lines showed a remarkably similar structure to the trajectories seen in a
crane.
Many of these trajectories crossed at right angles—an excellent arrangement to resist the manifold stresses on the condyle of the femur.

Law of Orthogonality and Law of Transformation of Bone
In the 1870s, Julius Wolff carried this theory one step further.
He claimed that the trabecular alignment was due primarily to functional forces.
A change in the intensity and direction of these forces would produce a demonstrable change in
the internal architecture and external form of the bone.
He thought that his observation could be expressed by definite mechanical mathematic laws.
This concept was referred to as the law of orthogonality.
Wolff observed that bones are living, highly vascular-ized structures that can change shape
during life (re-model), and hypothesized that such changes would in some way systematically
improve their capacity to resist such external loading.
Simply put, Wolff’s law (referred to as the ‘law of bone transformation’ in 1883) holds that bone
is deposited where it is needed and resorbed where it is not needed.
Wolff’s Law states that “Every change in the form and the function of a bone or of their function
alone is followed by certain definite changes in their internal architecture, and equally definite
secondary alterations in their external confirmation, in accordance with mathematical laws”

Stress Trajectories/Benninghoff Lines
Benninghoff made an exhaustive study of the architecture of the cranial and
facial skeleton, and of the so-called stress trajectories, similar to those seen in
the head of the femur.
He showed that these trajectories, or lines of stress, involve both the compact
and spongy bones.
Benninghoff showed that the stress trajectories obeyed no individual bone
limits, but rather the demands of the functional forces.
Following his reasoning, the head is composed of only two bones—the
craniofacial skeletal unit and the mandible, the only movable bone.

Stress Trajectories of the Craniofacial unit
Division of stress trajectories in the zygomatic area

Stress trajectories, emanate from above the teeth in the maxillary arch and
pass superiorly to the zygomatic or jugal buttress.
There are three main vertical pillars of trajectories, all arising from the alveolar
process and ending in the base of the skull: the canine pillar, the zygomatic
pillar and the pterygoid pillar.
Frontonasal vertical pillar/buttress: Purpose of this pillar or buttress is to
transmit pressures from the incisors, canines and first premolar.
It runs vertically along the piriform aperture and crest of the nasal bones and
ends in the frontal bone.
Zygomatic vertical pillar/buttress: It transmits stress from the posterior teeth.
It also receives force of the masseter muscles.
Pterygoid vertical pillar/buttress: It runs vertically and transmits stress from
the conchae of the nasal cavity and posterior teeth.

These trajectories curve around the sinuses and nasal and orbital cavities
The supraorbital and infraorbital bony eminences and the zygomatic buttresses
are horizontal reinforcing members for the vertical stress trajectory columns.
In the zygomatic area, they split into three parts and finally end in base of the
skull.
Also included with these buttressing structures are the hard palate, the walls of
the orbits, and the lesser wings of the sphenoid bone. Actual stress trajectories
crossing the palatal structure themselves also exist.
Sicher emphasizes the importance of the supraorbital rim as a receptor of the
forces transmitted to it by the canine and zygomatic pillars.
He believes that the development of the supraorbital ridge in lower primates and
man is an adaptive response to the strong prognathism and heavy masticatory
pressures.

Stress Trajectories of Mandible
The mandible, because it is a unit by itself and
a movable bone, has a trabecular alignment
different from that of the maxilla
Trabecular columns radiate from beneath the
teeth in the alveolar process and join
together in a common stress pillar, or
trajectory system, that terminates in the
mandibular condyle. The mandibular canal
and nerve are protected at the same time by
this concentration of trabeculae,
demonstrating the ‘unloaded nerve’ concept.
The thick cortical layer of compact bone along
the lower border of the mandible offers the
greatest resis-tance to the bending forces
Stress trajectories in the mandible

Myology
JAW REFLEX TYPES
Myotatic Reflex
It is the tonic contraction of the muscles in response to a stretching force, due to stimulation of
muscle proprioceptors. It is also called Liddell-Sherrington reflex, muscular reflex, and stretch reflex.
The elevator muscles are maintained in a mild state of contraction called muscle tonus in response
to the forces of gravity acting on the lower jaw. If there were complete relaxation of all the muscles
that support the jaw the forces of gravity would act to lower the jaw and separate the articular
surfaces of the TMJ.
Clasp Knife Reflex
This phenomenon is produced by stretching an extensor muscle against a background of increased
extensor muscle tone. The result is a relaxation of the muscle being stretched, i.e. the muscle now
lengthens easily after initial resistance. Clasp knife reflex is also called autogenic inhibition or inverse
myotatic reflex.
Example of clasp knife reflex is lengthening of the mandible with use of activator. Mandible assumes
static position in contact with the appliance and is prevented from reaching the occlusion. The
elevators and retractors remain contracted, fatigue of the muscle occurs. Muscle relaxes and the
mandible drops down. When the muscles have recovered the cycle starts again

Jaw-Closing Reflex
Jaw-closing reflex is the most basic reflex in the facial and oropharyngeal area. Jaw-closing
reflex is sometimes referred to as jaw jerk reflex.
Demonstration: A sharp downward tap on the chin when the mandible is held loosely in the
rest position results in contraction of the masseter muscle to bring the teeth into occlusion
Jaw-Opening Reflex
This reflex is the first reflex movement to make its appearance in the orofacial region of
human beings at about 8.5 weeks of intrauterine life. This is sometimes known as the
linguomandibular reflex, since it also occurs with brief application of a noxious stimulus to the
tongue.
Occurs as a result of mechanical or electrical stimulation of the lips, oral mucosa or teeth. A
slight opening movement occurs due to inhibition of activity in the mandibular elevators
without simultaneous contraction of the depressors.

Buccinator mechanism
Jt is a continuous muscle band that encircles the dentition and is anchored at
the pharyngeal tubercle.
The buccinator, along with orbicularis oris and pharyngeal constrictor, forms a
functional unit (buccinator mechanism) which is essential for orofacial
functions (swallowing, sucking, whistling, chewing, vowel pronunciation)
Although bone is the hardest tissue in the body, it is one of the most
responsive to change when there is an alteration in the environmental
balance. The major factor in this environmental balance is the musculature.
Aberrations of muscle function can and do produce marked malocclusions.
The restrictive, guiding role of the buccinator mechanism must be recognized
and emphasized.

Starting with the decussating fibers of the orbicularis oris muscle, joining right
and left fibers in the lips,
⬇️
the buccinator mechanism runs laterally and posteriorly around the corner of
the mouth, joining other fibers of the buccinator muscle
⬇️
that insert into the pterygomandibular raphe just behind the dentition.
⬇️
At this point, it intermingles with fibers of the superior constrictor muscle and
continues posteriorly and medially to anchor at the origin of the superior
constrictor muscles, the pharyngeal tubercle of the occipital bone.

Opposing the buccinator mechanism is a very powerful muscle—the
tongue.
The teeth and supporting structures are constantly under the influence of
the contiguous musculature.
The integrity of the dental arches and the relations of the teeth to each
other within each arch and with opposing members are modified by the
active functional forces of the muscles.

Tongue
It is relatively one of the best developed
structures in the human body at birth.
It is also relatively larger than contiguous
structures and thus assumes a posture
interposed between the gum pads, rather than
completely contained within them.
Winders has shown that during mastication and
deglutition, the tongue may exert two to three
times as much force on the dentition as the lips
and cheeks at any one time
but the net effect is one of balance as tonal
contraction, peripheral fiber recruitment of the
buccal and labial muscles and atmospheric
pressure team up to off-set the momentarily
greater functional force of the tongue.

Equilibrium Theory
The equilibrium theory of tooth position proposes that a stable dentition exists in a
state of balance—where the net resting pressure of the tongue, lips, cheeks, and
periodon-tium is zero. If this balance is disrupted, then the teeth will move until a
new state of equilibrium is reached.
Recent studies have shown that there is no balance of force between the tongue
and lips. Tongue exerts more force than lips during swallowing and rest
position.The dental apparatus is well developed to resist short-acting forces that
are generated during speaking, swallowing, and chewing.
It is not the magnitude of the force that is important but the duration of the force.
Hence lips and tongue forces during mastication alone do not contribute to
equilibrium.
The most significant factor in dental equilibrium seems to be the resting pressures
of the tongue and lips along with the forces generated by the periodontal ligament.

Temporomandibular Joint

RESPONSE OF TMJ TO ABNORMALITIES
A small change in any of the variables affecting the temporomandibular joint
(TMJ) may cause pathology.
Normally, temporalis muscle and masseter muscle exhibit relatively equal
magnitude of contraction in the closing maneuver from PVD to OVD.
Excessive interocclusal space and over-closure, or ‘deep bite’, may change this
smooth action and lead to a change in muscle habit patterns during the
closing cycle.
This lack of harmony of the structures are clinically observed in the form of
clicking and crepitus

References
Textbook of Orthodontics by Sridhar Premkumar
Orthodontics the Art and Science- S.I. Bhalajhi (5th edition)
John G. Skedros, MD and Richard A. Brand, MD: Biographical Sketch: Georg
Hermann von Meyer (1815–1892)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3183195/#!po=57.5000
https://youtu.be/0dV1Bwe2v6c

Thank you