Cybernetic Theory by Petrovic. The PPT is made from Graber, Petrovic and Rakosi Chapter -2 . Department of Orthodontics and Dentofacial Orthopedics. The servo-system theory of growth was given by Alexandre Petrovic, Charlier, and Jeanne Stutzmann of Louis Pasteur school of Medicine in the late 1960s. It is based on factorial qualitative analysis and describes the craniofacial growth mechanisms and the modulus operandi of functional and orthopedic appliances. The servo-system theory relies on the principle of cybernetics to describe the growth of the craniofacial complex.

1. Presented by: Dr. Aditi Jain
1st year PG
Popper (1963) Science is not a static acceptance of truth but rather the permanent search of
truth.
Department of Orthodontics and Dentofacial Orthopedics
Santosh Dental College, Deemed to be University

2. INTRODUCTION
The servo-system theory of growth was
given by Alexandre Petrovic, Charlier, and
Jeanne Stutzmann of Louis Pasteur school of
Medicine in the late 1960s.
It is based on factorial qualitative analysis
and describes the craniofacial growth
mechanisms and the modulus operandi of
functional and orthopedic appliances.
The servo-system theory relies on the
principle of cybernetics to describe the
growth of the craniofacial complex.
Alexander Petrovic 1923-2003
(France)
“I worship contradictory

3. CYBERNETICS
The term "CYBERNETICS (comes from
greek word kybernetes which means
steersman) was coined by mathematician
Norbert Wiener in 1948 to encompass the
entire field of control and communication
theory, whether in a machine or in an
animal.
Cybernetics is concerned with scientific
investigation of systematic processes of a
highly varied nature, including
phenomenon such as regulation,
information processing, storage,
adaptation, self organization and
strategic behavior.

4. CYBERNETICALLY ORGANIZED BIOLOGICAL
FEATURES
INPUT
Orthodontic and
orthopedic
appliances.
BLACK BOX OUTPUT
Correction of
malocclusion and
intermaxillary relation.
Genetically
determined and
cybernetically
organized biologic
feature of phenomena
characterizing,
including, controlling
spontaneous and
appliance-modulated
growth relative
primarily to the
following:-
• Maxillary
lengthening and
widening.
The physiological system under investigation is represented by BLACK BOX.

5. Cybernetically Organised System
operate
d
Physiological signals
Transmits information
Physic
al
Electromagn
etic
Chemica
l
Information Processing
Production, Perception, Transmission, And Storage Of
Information.
Not static
Sophisticated
A
D
V
A
N
T
A
G
E
S
Positive &
negative
feedback loops
Self regulation
INPUT OUTPU
T

6. Physiological Cybernetic
System
OPEN
LOOP
CONTROL
SYSTEM
CLOSED LOOP
Regulato
r
Servosyste
m
Feedbac
k
Comparato
r
Positiv
e
Negativ
e
Centra
l
Periphera
l

7. OPEN LOOP CONTROL SYSTEM
Environmental
Stimulus Recepto
r
Reflex
Transmission Perception Passive
Response
INPUT OUTPUT

8. CLOSED LOOP CONTROL SYSTEM
Transfer
Function
INPUT OUTPU
T
COMPARATOR
FEEDBACK

9. REGULATORY TYPE OF CLOSED LOOP

10. SERVOSYSTEM TYPE OF CLOSED LOOP
Control of
movement
s
Neural
Integration
Organic
Control
Receptor
Environmental
Stimulus
Feedback
Input is not
constant

11. SERVOSYSTE
M THEORY
1977
Shannon’s information theory
(1948) - designed to optimize the
transmission of information
through communication channels
The feed back
concept used in
engineering
control systems.
Petrovic and Charlier (1967-1969) – Jaw
growth rate and direction can be modified
through functional appliances.
Chronobiological aspects of cartilage and
bone growth
Jeanne Stutzmann
1976 The role of
LPM & postural
hyperpropulsor
temporomandibul
ar frenum
mediator role.

12. THE FACE AS A SERVOSYSTEM
Input- Any Changes In
Maxilla
(outward and forward
growth)
Output-
Adjustment
of the position
of mandible

13. COMPONENTS OF A SERVOSYSTEM
COMMAN
D
Reference
Input
COMPARATO
R
PERFORMANC
E
Coupling
System
Controlled
System
(Controlled Variable)
OUTPUT
CONTROLE
R
Performance Analyzing
Elements
ACTUATO
R
Constantly
Changing Inpu
Reference Input Elements
Central Comparator/Sensory
Deviation Signal
Peripheral
Comparator
GAIN

14. COMMAND
The command is a signal established
independent of the feedback system
in scrutiny and is not affected by the
output of the system.
It affects the behavior of the control
system without being affected by the
consequences of this behavior.
For instance, the secretion-rates of
growth hormone-somatomedin,
testosterone, and estrogen do not
seem to be modulated by variations of
craniofacial growth.

15. REFERENCE INPUT ELEMENT
They establish the relationship between Command
(Growth Hormone) and the Reference Input (Sagittal
Position of Maxilla).
Septal Cartilage, Septomaxillary Frenum, Labionarinary
Muscle, Premaxillary and Maxillary bone.

16. REFERENCE INPUT
• It is constantly changing
variable.
• Signal established as a
standard of comparison.
• Independent of the feedback
mechanisms.
• Sagittal Position of maxilla.

17. COMPARATO
R
Serves to establish the
relationship between the
reference input and controlled
variable.
“The operation of
confrontation” – Occlusal
contact between upper and
lower jaw.
Any deviation from optimal
occlusal contact detected by
the comparator leads to

18. PERFORMANC
E
The function
occurring by
peripheral
comparator that is
occlusion of
mandible and
maxilla is
mastication which is
the performance.
It is analyzed by

19. PERFORMANCE ANALYZING
ELEMENTS
They are periodontal ligament,
teeth, muscles,
temporomandibular joint.
 Proprioceptors within the
periodontal regions and
temporomandibular joint
perceive even a very small
occlusal discrepancy and
tonically activate the muscles

20. DEVIATION SIGNAL:
 Any input other than the
reference chosen by the
researcher is considered a
disturbance.
 The disturbance is
responsible for deviation of
the output signal and is thus
called deviation signal. It
may act on any element of
the regulating system.
 Abnormal Tooth position,
occlusal interferences,
arthritis, muscle
inflammation, periodontitis,

21. CONTROLL
ER
The BRAIN or the Central
Nervous System is the Central
Compactor also called as
Sensory engram or Controller.

22. ACTUATO
R
The actuator is the Motor Cortex
from where the signal is
transferred.
The actuating signal
corresponds to the output signal
of the controller (i.e., to the
input signal of the controlled
system).
 The activity of the Lateral

23. COUPLING
SYSTEM
The coupling system is
part of the control system
between the actuating
signal and the directly
controlled variable.
An example is the growth
of condylar cartilage via
metabolic blood
interchange in retrodiscal
pad.

24. CONTROLLED SYSTEM
The final output of the
system i.e. the OUTPUT
SIGNAL.
The Sagittal position of
Mandible.
Since it is not constant it is
also controlled variable
which keeps on changing
with the change in input.

25. GAIN
The gain of a system is the output
divided by the input.
 Gain > 1= Amplification.
Gain < 1 = Attenuation.
The pterygo-condylar coupling is an example of gain.
According to petrovic's investigation, the basal value of the
gain is determined genetically but may be amplified by growth
hormone-somatomedin and testosterone or attenuated by
estrogen. This is of great clinical significance.

26. Attractor
 Structurally stable steady state in dynamic system.
 Full interdigitation of the teeth.
 CLASS-I, CLASS-II, CLASS-III with cusp to fossa
relationship.
 The servosystem strives to achieve this stable form.
Repeller
 Unstable equilibrium state.
 Cusp to cusp type of occlusal relationship.
 The comparator detects this kind of disturbance
(Deviation signal) and tries to achieve the cusp to fossa
relationship through servosystem.
Feedback Signal
 The feedback signal is the function of the controlled
variable that is compared to the reference input. In a
regulator or servo-system, it is negative.

28. TYPES OF CARTILAGE
PRIMARY CARTILAGE:
Nasal septum, cartilage
between the body of
sphenoid, epiphysis,
SECONDARY CARTILAGE:
Condylar process, Coronoid
process, Mid- palatal
Suture, Fracture Callus

30. GROWTH AT CELLULAR LEVEL
CHONDROBLAST LOST (hypertrophy or surgically
removed)
INCREASED MULTIPLICATION OF
(Originates from Chondroblastic layer)

31. Functional appliances ( especially class 2 elastics)
Increased activity of RDP
Increased nutrients and growth factors supplied and inhibitors
removed
Increased mitosis and earlier hypertrophy of chondroblast
Reduced negative feedback signal reaching Prechondroblasts
Increased Growth at Condyle

32. Cytoplasmic Junctions between Skeletoblast
reduce
Transmission of inhibitory Factors reduce
Increased mitotic rate and rate of differentiation
into Prechondroblast

33. LOCAL FACTORS AFFECTING THE
CONDYLAR CARTILAGE
Consistent transmembrane ion flux variation.
Intra cellular Na ion concentration increased.
Intra cellular K ion concentration is decreased.
Discharge of H ions from blast cells is increased.
Increased pH.
Intracytoplasmic Ca++ concentration is low.

34. Thrust
Effect Ligame
nt
Muscle
traction
Spheno-occipital synchondrosis
Nasal septal cartilage
Lateral masses of ethmoid
Body and greater wing of
STH-
Stomatomedi
n
CONTROL OF THE MAXILLARY GROWTH
Forward
Growth
Of Septal
Cartilage
Outwar
d
Growth

35. FORWARD GROWTH OF SEPTAL
CARTILAGE
Thrust
Effect
Anterior Extremity of the Nasal Septal Cartilage
Spreads Laterally in Anterio-Inferior Direction
Penetrates into the Premaxillary Bone
Thrust
Effect
Premaxillomaxillary
Suture
Maxillopalatine
Suture

36. SEPTOPREMAXILLARY LIGAMENT
TRACTION
Growth of the
Nasal Septal
Cartilage
Traction effect on the
premaxillary bone through
the septomaxillary ligament.
TRACTION OF THE LABIONARINARI
MUSCLE
Traction of the premaxillary bone through
this muscle.
Biomechanics promotion of the forward
growth of the upper jaw.
Absence of labial muscle attachment on the
nasal septum – cleft lip- bone deformation.

37. OUTWARD
GROWTH
Outward growth of lateral Cartilaginous masses of the
ethmoid and the cartilage between the body and greater
wings of sphenoid.
Produces a lateralization of right and left
alveolar ridges
Stimulates the growth of mid-palatal suture
Outward growth of Maxilla

38. ROLE OF LPM AND RETRODISCAL PAD
Blood circulation:
Increase in open Loop
Factors.
Reduction in negative
feedback factors.
Biomechanical effect

41. For every unit of STH or testosterone that is released,
amount of growth is maximum. Mandible > Maxilla
At normal hormonal level N- good maxilla and
mandible relation is maintained.
At T2- hormonal level- (man> max)- LPM is reduced.
Beyond T2 hormonal level – LPM activity can no longer
be reduced – Prognathism
The opposite happens when hormonal level is below
T1 hormonal level – LPM activity can no longer be
increased- retrognathism.
Within levels T1- T2 , good maxilla and mandible
relation is maintained outside these levels:

43. GROWTH IN LENGTH OF MAXILLA

44. GROWTH IN WIDTH OF MAXILLA

45. Growth
Hormones
[STH-
Stomatomedin]
Sagittal
Position of
maxilla
OCCLUSION
MASTICATION
Lateral
pterygoid
muscle and
retrodiscal
pad
Growth at
condyle,
sagittal
position of
mandible
BRAIN Periodontium, teeth,
muscles and TMJ
Motor Cortex
Constantly
Changing Input
Septal Cartilage, Septomaxillary
Frenum, Labionarinary Muscle,
Premaxillary and Maxillary bone
Central Comparator/Sensory
Engram
Deviation Signal
Peripheral
Comparator
GAIN

47. MODE OF ACTION OF FUNCTIONAL
APPLIANCES
FUNCTIONAL APPLIANCES
INCREASED CONTRACTILE ACTIVITY OF LPM
INCREASE IN GROWTH STIMULATING FACTORS
ENHANCEMENT OF LOCAL MEDIATORS & REDUCTION IN
LOCAL REGULATORS
ADDITIONAL GROWTH OF THE CONDYLAR CARTILAGE
ADDITIONAL SUPER PERIOSTEAL OSSIFICATION
SUPPLEMENTARY LENGTHENING OF MANDIBLE

48. FUNCTIONAL APPLIANCES
1ST - GROUP
POSTULAR HYPERPROPULSOR
TWIN BLOCK
BIONATOR
ANDERSON HAUPL ACTIVATOR
CLASS II ELASTICS
FRANKEL’S APPLIANCE
2ND -GROUP
HERREN ACTIVATOR
LSU ACTIVATOR
HARVOLD WOODSIDE
ACTIVATOR
EXTRA ORAL TRACTION OF THE
MANDIBLE

49. Positions
Mandible
Forward
Increased
Activity of LPM &
RDP
Less fatigable
fibers in LPM
Oudet et el
(1988)
Carlson et el
(1990)
Positions
Mandible
Forward
1ST - GROUP
2ND - GROUP
Open in beyond
rest position
No Increased
Activity of
LPM.Yet increase
in Growth
Herren (1953)
Auf der Maur
(1978)

50. WHILE APPLIANCE IS
WORN
Forward position
 Reduction of length of LPM
New sensory engram
WHILE APPLIANCE IS
NOT WORN
New sensory engram
Functioning in anterior position
 Increased activity of RDP
Activity of 1st
Group
Activity of 2nd
Group

51. POSTURAL HYPERPROPULSOR

52. CLASS – II ELASTICS
Acts primarily through RDP rather LPM.
Alters the intrinsic regulation of the prechondroblastic multiplication.
Enhance the rate of hypertrophy of functional chondroblasts so that the
decreased amount of functional chondroblasts enhance prechondroblast
replication.
Similar to the effect of thyroxine.

53. TWIN BLOCK
Alters the occlusal inclined
planes.
70˚ inclined planes alter the
sensory engram and provide
a horizontal component of
force.

54. L.S.U. OR HERREN ACTIVATOR
Acts when appliance is not worn.
Action not mediated to through the LPM but through the
retrodiscal pad.
Shortening of the LPM when the appliance is worn compared to
other muscles.
A new sensory engram is produced.
The mandible closes in a more anterior position.

55. EFFECT OF CHIN CUP THERAPY
Retropulsion of the mandible results in reduction in number of
dividing cells.
Dividing cells if any are found anteriorly.
Resulting in anterior Growth rotation and decreased mandibular
Length.

56. CLINICAL IMPLICATIONS
• Principle of optimality of function:- less relapse
tendency if post orthodontic treatment muscular
activity produces a lower deviation signal.
• Removal of functional appliances when growth is
complete.
• If removed when growth not complete then we need
proper intercuspation.

57. • First group : full time , second group: part time .
• Proper functioning of LPM and RDP is important for growth –
parent counselling should be done properly.
• Sensory engram is poorly developed in younger children.
• Utilization of high hormonal activity at puberty.

58. CONSIDERATIONS OF SERVOSYSTEM
THEORY
Growth
Rotations
of the
Mandible
Alveolar
Bone
Turnover
Rate

59. BIFURCATION
Topologic Representation of increasing Hormone levels (testosterone or STH- stomatomedin) resulting
in supplementary growth of the mandible combined with lengthening of the maxilla as modulated by
the comparator of the servosystem (cuspal relief). Experimental results are expressed in terms of

60. PETROVIC’S GROWTH CATEGORIES
Growth Inequality of Bases
1 : Mandible equal to maxilla
2: Mandible smaller than maxilla
3 : Mandible greater than maxilla
Rotation of the Mandible
R : Neutral
P: Posterior rotation of mandible
A: Anterior Rotation of Mandible
Posteroanterior State
N: Normal
D: Distal
M: Mesial
Vertical State
N: Normal
OB: Open Bite
DB: Deep Bite

62. LEVEL -1
Cell and molecular
Biology Level
70%
3%
25%
Mandible
less than
Maxilla
Mandible
greater
than
Maxilla
Mandible equal to
Maxilla
Quantitative
determination
of difference
between
maxilla and
mandible.

63. LEVEL -2
Partly on Respiration
Phonation & Deglutition
Anterior Posterior
Neutral Relates to the
Growth
rotation and
growth
inclination of
both maxilla
and mandible

64. LEVEL -3
Unstable occlusion and
Functional Discontinuity
Distal Mesial
Normal
Based on the
occlusal
relationship

65. GROWTH ROTATIONS OF MANDIBLE
Morphogenetic
Rotations
Positional
Rotations
The Change in the
Shape of
Mandible
The Change in the
Position of
Mandible
in relation to the
Adjacent
Structures
Lavergene & Gasson
(1977)

66. ALVEOLAR BONE TURN OVER RATE
Tissue level Growth
Potential :
Level of
subperiosteal
ossification rate and
the level of alveolar
bone turnover rate
Tissue level Growth
Responsiveness:
The degree of
augmentation of the
alveolar bone turnover
resulting from
orthodontic treatment.

67. AUXOLOGIC GROUPS
Six auxologic categories of the
mandible.
Depends on:
1. tissue level growth potential
2. Tissue level Growth responsiveness to
orthodontic, orthopedic and functional
appliances.
Helps define the biologic inter-
individual variation of growing
mandible.
Clinicians must give importance to
tissue level growth responsiveness than

68. FAILURE OF SERVOSYSTEM TO
CONTROL GROWTH
Before development of
Occlusion
Sensory engram not developed
Servosystem does not operate
Genetic influence of mandibular
Growth
Anodontia
After Development of
Occlusion :-
Sensory Engram forms
Peripheral Comparator Control
Minimize possible deviation to
achieve stable occlusion.
Peripheral Comparator Faulty- Caries, Mutilated
Dentition
Discrepancy between rotation pattern (Anterior or
Posterior) and location of Comparator.

69. DRAWBACKS
The author places a lot of importance on the role of hormones in
controlling growth.
The peripheral comparator, the occlusion, itself is unstable.

According to this theory, an end on relation is a Repeller.
This theory places a lot of importance on condyle as growth center. If
condylar cartilage is lost, growth should seize. But some studies done
on Scandinavia show that this does not happen.

70. EVIDENCES AGAINST THE THEORY
Goret – Nicaise, Awn (1983) - Resection of LPM – Fails
to diminish condylar Growth.
Whetten and Jhonston (1985) – bilateral condylectomy
and unilateral LPM in rats – same growth on both sides.
Das, Myer & Sicher ( 1980) – Occlusion remained
unaffected in condylectomy studies.

71. CONCLUSION
While we may no longer seek a single synthesizing theory
for all of craniofacial growth, we now have, because of
petrovic’s work, a convenient model & a language by which
we can describe and relate growth activities to one another,
thus obviating any need for another paradigm.
The cybernetic theory of facial growth and the crucial -
topologic concept of structural and functional stability and
instability are highly useful in a systematic and increasingly
computerized approach to clinical situations.

72. REFERENCES
• GRABER, PETROVIC, RAKOSI. DENTOFACIAL ORTHOPAEDICS AND FUNCTIONAL
APPLIANCES, 2ND EDITION, 1917.
• ZHOU, Z.; LUO, S. (1998-05-01). "[DIFFERENTIAL EXPRESSION OF IGF-I AND ITS MRNA IN
MANDIBULAR CONDYLAR CARTILAGE OF RAT--DIRECT EVIDENCE FOR SERVOSYSTEM
THEORY OF FACIAL GROWTH]". HUA XI KOU QIANG YI XUE ZA ZHI = HUAXI KOUQIANG
YIXUE ZAZHI = WEST CHINA JOURNAL OF STOMATOLOGY. 16 (2): 164–165. ISSN 1000-
1182. PMID 12214426.
• HTTPS://WWW.SCRIBD.COM/DOCUMENT/383886681/PETROVIC.
• ROLE OF THE LATERAL PTERYGOID MUSCLE AND MENISCOTEMPOROMANDIBULAR
FRENUM IN SPONTANEOUS GROWTH OF THE MANDIBLE AND IN GROWTH STIMULATED BY
THE POSTURAL HYPERPROPULSOR J.J. STUTZMANN , A.G. PETROVIC
DOI:HTTPS://DOI.ORG/10.1016/0889-5406(90)70110-X.
• HTTPS://PHAIDRABG.BG.AC.RS/OPEN/O:21881

73. THANK YOU