Servosystem /certified fixed orthodontic courses by Indian dental academy
SERVOSYSTEM THEORY OF
INDIAN DENTAL ACADEMY
Leader in continuing dental education
COMPONENTS OF A SERVOSYSTEM
PRIMARY AND SECONDARY CARTILAGES
CONTROL OF MAXILLARY GROWTH
CONTROL OF MANDIBULAR GROWTH
THREE LEVEL ARBORIZATION
MODE OF ACTION OF FUNCTIONAL APPLIANCES
Last 20 years have seen an increasing awareness of the
potential of functional appliances as a valuable tool in the
armamentarium of orthodontists.
In late 1960’s Petrovic & co-workers produced first rigorous
demonstration that condylar cartilage’s growth rate & amount
can be modified by using appropriate functional & orthopedic
Later he employed the model of cybernetics & control theory
to describe craniofacial growth patterns & method of operation
of functional & orthopedic appliances.
The term “CYBERNETICS” (Greek kybernetes means
steersman) was coined by mathematician Norbert
Wiener in 1948 to encompass the entire field of
control and communication theory, whether in the
machine or in the animal.
Cybernetics is concerned with scientific investigation
of systematic processes of a highly varied nature,
including phenomenon such as regulation,
information processing, storage, adoption, self
organisation and strategic behavior.
It grew out of Shannon's information theory –
designed to optimise transmission of information
through communication channels and the Feed
back concept used in engineering control systems.
The concept of cybernetics and control theory was
put forth by Petrovic (1977,1982) to describe
craniofacial growth mechanisms and the method of
operation of functional and orthopedic appliances.
The theory refines orthodontic concepts by
demonstrating a qualitative and quantitative
relationship between observationally and
experimentally collected findings.
Helps in a broader understanding of orthodontic
problems as the language of cybernetics is compatible
with the rapidly expanding use of computers among
Phylogenetic explanation for
the peculiarities of condylar cartilage
According to Symons, in mammalian embryo, the
condylar cartilage develops independently of the
The response of the condylar cartilage growth to
local factors may explain the extraordinary success of
the phylogenetically new mammalian joint between
the skull and the lower jaw.
Condylar cartilage growth is integrated into an
organised functional whole that has the form of a
servosystem and is able to modulate the lengthening
of the condyle so that the lower jaw adapts to the
upper jaw during growth.
In the absence of such an adjustment the forces of
occlusion would expose the pdl structures to repeated
trauma and loss of teeth.
This adjustment hence allows proper mastication and
facilitates high basic metabolism.
WHAT IS CYBERNETICS?
Cybernetics is based on the communication of
Any cybernetically organized system operates
through signals that transmit information (which may
be physical, chemical or electromagnetic in nature).
Any cybernetic system, when provided an input (or
stimulus), processes such an input and produces an
output. The output is related to the input by a transfer
function that characterizes the physiologic system.
No feedback loop
THE SERVO SYSTEM
Main input constant
or follow up system
Main input not constant
It is –ve feedback
In an open loop, the output does not affect the input. There are
no feed back loops or comparators.
In a closed loop system, a specific relation is maintained
between the input and output and are characterized by a
feedback loop and a comparator.
Feed back loop
The input is fed into a comparator which analyses the
input and judges the degree to which the transfer
function needs to be carried out to obtain a certain
The output is fed back to the comparator (through a
feed back loop) and is analyzed for its adequacy. If
found inadequate, the transfer function is carried out
The feed back loop can have a positive or enhancing
affect or a negative or attenuating affect.
A regulator type of closed loop is one in which the
input is constant. Any disturbance in the input will
cause the comparator to initiate a regulatory feedback
system, which will restore the input to its normal
Eg. The temperature regulation system of the bodyAny change in body temperature acts as the input into
the comaparator (the hypothalamus), which causes an
action (pilorection and shivering) which ultimately
brings the body temp back to normal.
Servosystem in this the main input is constantly
changing with time and the output is constantly
adjusted in accordance to the input.
Craniofacial growth is an extremely complex process
involving a multitude of factors.
The connections between constituents are complex,
although the constituents themselves are not.
The identification and analysis of the feed back loops
(regulation processes) is among the main tasks in the
field of craniofacial growth.
Cybernetic language has been the best to accurately
describe the intricacy and complexity of craniofacial
morphogenesis and the means to influence it
The following set of approaches may be useful in
relating scientific findings and the method of
operation of orthopedic and orthodontic appliances.
COMPONENTS OF A
COMMAND :a signal established independent of the
servosystem and is not affected by the output of the
system. It tells the system what has to be done.
REFERENCE INPUT : is a signal established as a
standard of comparison.
REFERENCE INPUT ELEMENTS :establish the relationship
between command and the reference input.
COMPARATOR (PERIPHERAL) :It is a component that
analyses the reference input and judges the
performance of the system through performance
CENTRAL COMPARATOR : the performance judging
elements then transmits a deviation signal to the
central comparator which sends a signal to various
components – the actuator, coupling system and the
This ultimately brings about an output/controlled
THE PRIMARY AND SECONDARY
Stutzman (1976) emphasized the following PRIMARY CARTILAGE - dividing cells, differentiated
chondroblasts, are surrounded by a cartilaginous
matrix synthesized by them, that isolates them from
local factors able to restrain or stimulate cartilaginous
growth. Chondroblasts undergo maturation and are
later transformed into hypertrophied chondroblasts.
Deeper in the cartilaginous matrix, calcium is
deposited and endochondral ossification begins.
Epiphysial cartilages of long bones
Cartilages of synchondroses of cranial bones.
Nasal septal cartilage.
Lateral cartilaginous masses of ethmoid
Cartilage between greater wings and body of
SECONDARY CARTILAGES the dividing cells,
prechondroblasts, do not synthesize a cartilaginous
matrix, hence are not isolated from local factor
influences. Once they mature into chondroblasts,
they become surrounded by cartilaginous matrix
and do not divide.
Coronoid and condylar cartilage
Mid palatal suture cartilage
Post fracture callus
According to studies carried out by Chartlier,
Petrovic and Stutzmann on organ culturesDividing chondroblasts (in primary cartilages) are
more susceptible to general extrinsic factors,
especially growth hormone, stomatomedin, and sex
hormones. The cartilaginous matrix surrounding the
mature chondroblasts, isolates them from the effects
of local factors.
Local biomechanical factors can only modify the
direction of growth and not the amount of growth
at these sites.
In the secondary cartilages, where prechondroblasts
are the dividing cells, general and local extrinsic
factors can affect the growth.
The amount of growth of these cartilages can be
affected by altering the local extrinsic factors.
Adaptive to both extrinsic & local biomechanical &
Condylar cartilage growth is integrated into an
organized functional whole that has form of
Servosystem & able to modulate lengthening of
condyle so that lower jaw adapts to upper jaw during
Specific features of condylar cartilage
Fibrous capsule -fibroblasts and type I collagen.
Zone of growth (mitotic compartment) –skeletoblasts and
prechondroblast type II, not surrounded by the cartilaginous
matrix with type I collagen.
Zone of maturation - functional and hypertrophied
Zone of erosion
Zone of endochondral ossification.
Correlation between growth direction of
condyle & sagittal distribution of dividing
cells in condylar cartilage
Anatomic, microscopic and histologic studies have
shown that the growth direction of the condyle
coincides in general, with the axis of individual
trabeculae, located just inferior to the central part of
Hence the condylar growth direction can be
determined by measuring the main axis of
endochondral bone trabeculae in the condyle and the
angle it forms with the mandibular plane.
A histologic & radioautographic study was made of
distribution of dividing cells in a sagittal section of
condylar cartilage of juvenile rats.
Condylar cartilage divided into 4 equal sections from
anterior to posterior & cells counted.
Each experimental group was subjected to specific
Results showed that both treatment with the postural
hyperpropulsor & with the growth hormone produced
significant increase in growth rate of condylar
cartilage compared to control group (Charlier et al,
1968, 1969; Petrovic et al , 1975)
Condylar growth is not exclusively a result of the
lengthening of pre-existing endochondral bone
trabeculae under condylar cartilage but also a result
of growth of bone trabeculae (mesenchymal cells)
that are formed in parallel & posteriorly oriented in
Stutzmann angle- the angle formed between main
axis of endochondral bone trabeculae in condyle with
mandibular plane as viewed on lateral cephalogram.
In anterior growth rotation there is closing of angle as
seen in treatment with growth hormone.
In posterior growth rotation there is opening of angle
as seen in treatment with postural hyperpropulsor
Resection of LPM & retrodiscal pad
Experimental studies on juvenile rats were carried out
in which LPM were resected.
The interruption of circulatory dependence on the
blood supply originating directly from LPM &
indirectly through retrodiscal pad may contribute to
inhibited differentiation of skeletoblasts.
It was observed that growth of condylar cartilage &
lengthening of mandible continued but significantly
Intrinsic regulation of condylar cartilage growth
A “negative feed back signal” originates from the
proximal part of the chondroblastic zone and exerts a
restraining effect on the prechondroblastic
This concept can help explain the effects of some
orthopedic and orthodontic appliances and of a
hormone such as thyroxine.
The earlier commencement of chondroblastic
hypertrophy and the subsequent decrease in the
prechondroblastic division-restraining signal are
important intermediary steps in growth stimulating
effects of class II elastics, mandibular hyperpropulsar
The acceleration of the chondroblastic maturation rate
is similarly an intermediary step for the growth rate –
stimulating effect of thyroxine. (Stutzmann, Petrovic,
CONTROL OF MAXILLARY
Increase in length of maxilla
Is caused by growth at the premaxillomaxillary and
maxillopalatine sutures and by subperiosteal deposition of
bone in the anterior region.
Increase in width of maxilla
Is due to growth at the mid palatal suture and bone deposition
along lateral areas of alveolar ridge.
Mid palatal suture - secondary cartilage.
Mechanisms controlling growth of the
STH-somatomedin, testosterone and estrogen play
primary roles in extrinsic control of post natal growth
of the upper jaw.
They have direct and indirect effects.
Represents almost the entire influence of the
hormones on growth of spheno-occipital
synchondrosis and nasal septal cartilage.
Small part of the effect of hormones on growth of
cranial sutures is direct. Effects the responsiveness of
preosteoblasts to regional and local factors,
stimulating the skeletal cell multiplication.
In secondary cartilage - effect seen in multiplication
and responsiveness of prechondroblasts
CONTROL OF MANDIBULAR
The variation in direction and magnitude of condylar growth is
partly a quantitative response to changes in maxillary length.
Variation in maxillary growth can be induced through
resection of nasal septal cartilage or administration of growth
hormone or testosterone or by orthopedic appliances.
As long as growth alteration does not exceed a certain limit, no
significant changes in saggital relationship of dental arches
The physiologic adaptation of mandibular length to maxillary
length occurs through a variation in both growth rate and
direction of growth of condylar cartilage.
Growth hormone- somatomedin affects the lengthening of
mandible (through condylar growth) to a greater extent than its
affects on the lengthening of maxilla.
If this hormonal effect remains within physiological limits,
the occlusion is not significantly altered, as concomitant
reduction an angle between ramus and corpus of mandible,
decreases the length of the mandible.
The release of somatomedin represents the command
(command to grow).
Reference input elements are the nasal septal cartilage,
septopremaxillary frenum, labionariary muscles and
premaxillary and maxillary bones. The position of maxillary
dental arch is constantly changing reference input of the
Lower arch is controlled variable.
The “operation of confrontation” between the upper and lower
dental arches is the “ peripheral comparator” of the
Owing to the forward and outward growth of maxilla, there is
obvious change in relation of the teeth. What was originally a
cusp to fossae relationship becomes a cusp to cusp
Hence the peripheral comparator (occlusion), senses this, due
to change in performance or efficiency of mastication. Due to
improper mastication there is increases force on periodontium,
teeth, muscles and TMJ, which serve as performance
analysing elements. The performance analyzing elements send
signals to the central comparator (controller) represented by
The CNS is equipped with a SENSORY ENGRAM.
The sensory engram is a collection of feedback loops, which
record the activity of masticatory muscles corresponding to a
particular habitual mandibular position.
It operates on the principle of OPTIMALITY OF
Any particular muscle action or mandibular position that gives
the minimal deviation signal is recorded in the sensory
engram. i. e. when any new mandibular position is dictated to
the patient, unless the newer position causes a smaller
deviation signal than the older position, the CNS will tend to
make the mandible relapse to its older position, where in
function was more ideal.
The CNS compares the present muscular position with the
ideal position stored in sensory engram and sends a deviation
signal to an actuator-motor cortex to correct this discrepency.
The actuator then sends an actuating signal to the coupling
system of the lateral pterygoid muscle and retrodiscal pad.
The LPM positions the mandible forward and the activity of
retrodiscal pad induces mandibular growth at the condyle.
The resultant output or controlled variable is the forward
growth of mandible which results in an ideal cusp to fossa
Once growth at the condyle occurs, the posterior border of
the mandible becomes more concave in shape, causing a
negative piezoelectric effect to develop at the posterior border
of mandible and bone apposition occurs.
At the same time anterior border becomes more
convex, positive piezoelectric current
Thus length of mandible increases.
It is a morphogeneticic classification of human facial
By Lavergne and Petrovic (1983).
The first level, based on the quantitative
determination of the difference between maxillary
and mandibular sagittal growth, has three main
The second level based on variations in the direction
of mandibular and maxillary growth, relates to
growth inclinations and growth rotations of both
maxilla and mandible.
The third level, based on the occlusal relationship that
functions as the peripheral comparator of the
Servosystem, has subdivisions representing either an
aggravation or a melioration of malocclusions
resulting from the first two arborizational levels.
Occlusal relationships play a significant role in the
process of controlling facial growth.
The peripheral comparator has several stable
positions, each corresponding to some type of class I,
II or III intercuspations.
Any given occlusal relationship is stable with respect
to limited fluctuations and disturbances.
Each cusp to cusp unstable position corresponds to a
functional discontinuity-a topologic bifurcation type
instability, described by Thom(1972)and
The concept of discontinuity connotes that at critical
points, the servosystem behavior goes through some
basic switch, implying the existence of continuous
quantitative variations that appear qualitative.
Occlusal development involves two phases.
First phase consists of all morphogenetic process
leading to a stable occlusion, during this phase all the
parts of the servosystem are already existent and
functional, but stable occlusal relationship capable of
serving as a peripheral comparator has not yet been
A reference point for the development of sensory
engram is not possible
Hence mandibular morphogenesis cannot be
regulated through information originating from
The beginning of the second phase coincides with the
establishment of a stable occlusion to serve as a
peripheral comparator – required for the development
of a sensory engram.
The subsequent morphogenesis of the face is
regulated to minimize possible deviations from
achieved stable occlusal adjustment, regardless of
whether this corresponds to a class I, II or III
Depending on the relationship of maxilla to mandible,
the dentition as a whole or in part (peripheral
comparator may be located near molars or incisors,
sometimes near canines.) may be operating as a
peripheral comparator of the servo system.
In posterior rotating mandible - molars
In anterior rotating mandible - incisors and canines.
The action of the peripheral comparator is an
important part of both orthodontic and orthopedic
Whenever a curative measure alters the position of
group of teeth operating as a part of the peripheral
comparator in a growing child (incisor – canine group
in anterior rotating mandible, molar group in
posteriorly rotating mandible, or whole dentition in
some cases), the clinician is dealing not only with an
orthodontic treatment (moving teeth) but also with an
orthopedic one (modifying the rate, amount and
direction of growth in facial skeleton.)
MODE OF ACTION OF
Appropriate functional appliances that place the
mandible in forward postural position increases
condylar cartilage growth rate & amount.
Periodic increase in thickness of postural
hyperpropulsor, produces increase in LPM activity &
of retrodiscal pad, consequently increasing rate &
amount of condylar cartilage growth.
If appliance removed after growth completed – little
or no relapse.
If removed before growth completed- no relapse if
If good intercuspation has not been achieved before
the growth is completed - then the comparator of
Servosystem imposes an increased or decreased
growth rate until state of good intercuspation
Class II elastics
Class II elastics not only move teeth but act also act
as a functional appliance capable of stimulating the
growth rate & amount of condylar cartilage.
The stimulating effect of the Class II elastics on the
lengthening of the condyle appears to be mediated
primarily through the retrodiscal pad.
HERREN (L.S.U) ACTIVATOR
(Louisiana state university)
It opens the construction bite beyond the postural rest
According to Herren (1953) & Auf der maur (1978)
the wearing of appliance does not bring about any
increased activity of LPM as no free movement of
TWO STEP ACTION
When appliance is wornForward positioning of mandible is the cause of
reduced increase in length of LPM.
New sensory engram
When appliance is not wornMandible functioning in more forward position
More stimulation of retrodiscal pad activity
Repetitive activity of pad leads to earlier onset of
condylar chondroblasts hypertrophy.
Decrease in no of functional chondroblasts.
Decrease in prechondroblasts multiplication
Increase in condylar cartilage growth.
FRANKEL LATERAL VESTIBULAR SHIELD
The appliance acts by stimulating midpalatal suture
growth & to lesser extent by increasing bone
apposition on external subperiosteal layer of maxilla.
Buccal shield --- eruptive pathway of teeth at the
critical time in their development.
The relief of pressure from the cheeks in the
dentoalveolar area seems to allow a more downward
and outward eruptive path at a time of maximal
variability, permitting horizontal and vertical
adjustment of osseous tissues involved.
Summary of method of operation of functional
Class II elastics, postural hyperpropulsar, Frankel
regulator, Balters bionator, Clark twin block all exert
effects mainly through movement of mandible. Their
stimulating effects are produced mainly during
wearing of appliance.
Herren & L.S.U activators & extraoral forward
traction on mandible seem to exert their effects
mostly through sagittal repositioning of mandible.
Regardless of differences in mode of action, the
following causal chain is involvedFunctional appliance
Increase contractile activity of LPM
Intensification of repetitive activity of retrodiscal pad
Increase in growth stimulating factors
Enhancement of local mediators
Reduction of local mediators (factors causing negative feedback
Additional growth of condylar cartilage
Additional subperiosteal ossification of posterior border of
Supplementary lengthening of mandible
According to the principle of optimality of function, a
condition which results in maximum efficiency is one
that is instilled in the brain. Hence the tendency for
relapse will be less if we achieve an optimal
Functional appliance therapy should be extended until
growth is completed, or should achieve a good
intercuspal relation, if growth is not completed.
If Treatment ends with teeth in poor occlusion, during
growth phase, relapse is more likely to occur.
The sensory engram is poorly developed in children.
Hence they respond better to functional appliance
Hormonal activity is highest during pubertal growth
spurt. As hormones are very important for growth,
one must take full advantage of the increased
hormonal activity if any growth modulation is
Proper functioning of LPM-RDP, is essential for
growth. (Petrovic and Stutzmann)
Dentofacial Orthopedics with Functional
Appliances. Graber TM, Rakosi T, Petrovic AG.
Second edition. Chapters 1-3.pg 1-73.
Petrovic A, Stutzmann JJ. Does the frankel
appliance produce forward movements of the
mandibular premolars? EJO 1982;4;173.
Lavergne J, Petrovis A. Discontinuities in occlusal
relationship and the regulation of facial growth.A
cybernetic view. EJO 1983;5,269.
Stutzmann JJ, Petrovic A. Intrinsic regulation of the
condylar cartilage growth rate. EJO 1979,1:41.
Stutzmann JJ, Petrovic A, Shaye R. Relationship
between mandibular growth rotation and alveolar
bone turnover rate. J Dent Res 1980: 59,448.
Orthodontics – Current principles and techniques.
third edition. Graber, Vanarsdall.
Contemporary orthodontics. third edition, Proffit.
For more details please visit