growth relativity hypothesis

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Growth relativity theory

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The appearance of mandibular condyle varies greatly among different age groups and individuals. Morphologic changes of condyle occur due to developmental variations, remodelling, various diseases, trauma, endocrine disturbances and radiation therapy. Genetic, acquired, functional factors, age groups, individuals have a role in morphologic changes in shapes and sizes of condyle. This study comprised radiographic evaluation of 464 condylar heads in 232 digitalised OPGs taken for routine investigation in a scan centre. Trends occurring in the shapes were evaluated, and their distribution instudy population were identified.Of the four types identified (bird's beak, crooked finger, diamond and the oval), the oval shape is more frequently found and the crooked finger is less.

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Condyle-fossa modifications and muscle interactions during Herbst treatment, Part 2. Results and conclusions

Affiliation.

1 Department of Orthodontics, College of Dentistry, New York University, New York, NY, USA. [email protected]
PMID: 12867894
DOI: 10.1016/s0889-5406(03)00150-1
Am J Orthod Dentofacial Orthop. 2003 Sep;124(3):243

Herbst appliances were activated progressively in growing nonhuman primates, and the results were compared with primate and human controls. The methods and materials of this research are explained in Part 1 of this study. The results are discussed here in Part 2. All experimental subjects developed large super Class I malocclusions, the result of many factors including posterior movement of the maxilla and the maxillary teeth, an increased horizontal component of condylar growth, and anterior displacement of the mandible and the mandibular teeth. The growth modification measured in the glenoid fossa was in an inferior and anterior direction. Restriction of the downward and backward growth of the fossa observed in the control subjects might additionally contribute to the overall super Class I malocclusion. Clinically, these combined effects could be significant at the fossa. The restriction of local temporal bone (fossa) growth cannot be observed clinically; thus, these results might also clarify some Class II correction effects that cannot be explained with functional appliances. Differences in the area and maximum thickness of new bone formation in the glenoid fossa and in condylar growth were statistically significant. The bony changes in the condyle and the glenoid fossa were correlated with decreased postural electromyographic activity during the experimental period. Results from permanently implanted electromyographic sensors demonstrated that lateral pterygoid muscle hyperactivity was not associated with condyle-glenoid fossa growth modification with functional appliances, and that other factors, such as reciprocal stretch forces and subsequent transduction along the fibrocartilage between the displaced condyle and fossa, might play a more significant role in new bone formation. These results support the growth relativity concept.

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Published: 20 November 2018

The dilemma of functional therapy: the new EFA to do or not to do?

Omnia A. Elhiny ORCID: orcid.org/0000-0002-8435-1021 1 &
Mohamed I. El-Anwar 2

Bulletin of the National Research Centre volume 42 , Article number: 23 ( 2018 ) Cite this article

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Metrics details

This finite element analysis was conducted to study the effects produced by a new fixed functional appliance (EFA; Elhiny functional appliance) and hence predict its clinical effectiveness.

Materials/methods

Under ANSYS environment, a simplified 2D finite element model was prepared for this study. The models simulated a clinical situation where the mandible was positioned forward via a new fixed functional appliance design. The models’ components were created on a commercial CAD/CAM package then imported to finite element software. Pushing load of 2 N was applied along the appliance longitudinal direction.

The mandible showed downward and forward deformation in the X and Y directions with the highest deformation at the symphysis and lower border with a total deformation of 80 μm. There was little deformation in the maxilla. The highest strain results were at the condyle, both compressive and tensile in the X and Y directions with a total of 1520 micro strain behind the condyle. The strain in the mandibular tooth bearing area was around zero and in the maxillary tooth bearing area ranged from − 9 to 16.6 micro strain.

Conclusions

Within the limitations of this finite element analysis, it could be predicted that the new appliance (EFA) produces pure functional skeletal results with absolutely no dentoalveolar effects, which provides the opportunity for observing significant skeletal changes.

Introduction

The skeletal effect of functional therapy on the mandible is a highly debatable subject that is yet unresolved. Many trials and systematic reviews have been made in a quest to find whether functional appliances have a skeletal effect on the mandible or is it merely the dentoalveolar compensation that corrects the discrepancy.

Some researchers assert the presence of favorable mandibular growth represented by condylar and glenoid fossa remodeling (Ruf and Pancherz 1999 ; Rabie and Hägg 2003 ; Antonarakis and Kiliaridis 2007 ; Paulsen et al. 1995 ; McNamara Jr and Howe 1990 ; Franchi et al. 1999 ; Woodside et al. 1987 ). Such remodeling resulted from the acceleration of chondrocytic differentiation and the increase in the amount of cartilage matrix formation, hence enhancing growth (Rabie and Hägg 2003 ).

On the other hand, some researchers argue about the significance of the magnitude of such skeletal effects and point to the evidence showing the distinction of the dentoalveolar changes produced (Cozza et al. 2006 ; Franchi et al. 2011 ; Zymperdikas et al. 2016 ; Marsico et al. 2011 ; Cope et al. 1994 ; Darda et al. 2010 ; Küçükkeleş et al. 2007 ). However, these dentoalveolar changes may in turn have acted as a restraint to the full expression of the skeletal enhancement.

It then appears that to be able to ascertain or negate the presence of significant skeletal changes with functional therapy, it was necessary to rule out the dentoalveolar factor through modifying the appliance design.

Consequently, this finite element analysis was conducted to study the effects produced by a newly devised fixed functional appliance (EFA; Elhiny functional appliance) and hence predict its clinical effectiveness.

Materials and methods

The current finite element analysis simulated a clinical situation where the mandible was positioned forward via a new fixed functional appliance design, EFA. The appliance could be easily constructed in the laboratory.

A two-dimensional model was prepared on ANSYS GUI, to simulate the lower part of the skull. The model dimensions were taken from literature (Panigrahi and Vineeth 2009 ). Two types of elements were used to build the model: Shell 3D 4node 181 with 6 degrees of freedom to mesh the bone and Link 180 as spar element to represent the appliance effect (Kohnke 2013 ). The applied force was 2 N propulsive force, as the forces generated by fixed functional appliances range from 150 to 200 g, i.e., 1.47 to 1.98 N (Karacay et al. 2006 ; Nalbantgil et al. 2005 ). The meshing process resulted in 59,386 nodes and 30,459 elements. As presented in Fig. 1 , after the model meshing, the upper line, connecting the model to the skull, was set fixed in place as boundary condition.

Simplified model after meshing

All materials were assumed to be isotropic, homogenous, and linearly elastic, and their properties were listed in Table 1 .

Linear static analysis was performed on a personal computer (Intel Core to Due processor, 2.8 GHz, 4.0 GB RAM), using commercial multipurpose finite element software package (ANSYS version 13.0). The deformation and strain results were analyzed and represented graphically.

The mandible showed forward and downward deformation, in both X and Y directions (Fig. 2 ).

Total deformation in the mandible and its components in horizontal and vertical directions

The mandibular symphysis showed the highest deformation in the Y direction, and the symphysis and lower border in the X direction. As illustrated in Fig. 2 , the maximum horizontal deformation in the mandible was 9.3 μm, while the vertical deformation was of order 7.2 μm. The total deformation at the symphysis was 80 μm.

The maxilla showed very little deformation upwards in the Y direction (about 0.13 μm), in the area representing point A and the anterior nasal spine (ANS). While in the X direction, there was an even less backward than the upward deformation, about 0.016 μm.

The condylar strain results in Fig. 3 showed that the strain in the X direction was a compressive strain of about 1122 micro strain anterior to the condyle, and 122 tensile micro strain behind the condyle. Above the condylar head, the strain ranged from − 71 to + 191 micro strain. While in the Y direction, the strain ranged from 150 to 170 micro strain behind the condyle, from 260 to 395 micro strain in front of the condyle, and an average of 940 micro strain above the condyle. The total condylar strain was about 1520 micro strain behind the condyle.

Condylar strain in X and Y directions and total strain

The mandibular strain increased gradually in the X direction from − 70 to + 13 micro strain and in the Y direction from − 9 to + 23 micro strain. A stress/strain concentration appeared around the point of force application, where in the X direction the strain was about 677 micro strain and in the Y direction it was approximately 86 micro strain. Hence, the total mandibular strain ranged from 0 to 166 micro strain with an average of 150 micro strain at the point of force application. On the other hand, at the tooth bearing area, the strain was approximately zero (Fig. 4 ).

Mandibular strain in X and Y directions and total strain

As presented in Fig. 5 , the strain in the glenoid fossa in the Y direction was around 5.3 to 6.9 micro strain, while the total glenoid fossa strain ranged from 5.6 to 11.2 micro strain whereas the strain in the X direction, in the maxillary tooth bearing area, ranged from − 9 to 16.6 micro strain and the total maxillary strain ranged from 0 to 50 micro strain.

Maxillary strain in X and Y directions and total strain

There has been a wide debate regarding the output of using functional appliances, removable or fixed, for mandibular advancement. In noncompliant and post pubertal patients, the fixed functional appliance was the only successful non-surgical treatment (Panigrahi and Vineeth 2009 ). However, the main issue was the prevalence of dentoalveolar effects over skeletal effects (Cozza et al. 2006 ; Franchi et al. 2011 ; Zymperdikas et al. 2016 ; Marsico et al. 2011 ; Cope et al. 1994 ; Darda et al. 2010 ; Küçükkeleş et al. 2007 ; Panigrahi and Vineeth 2009 ; Nalbantgil et al. 2005 ).

As a result, it was hypothesized, in the current study, that by modifying the fixed functional appliance design into the new Elhiny functional appliance (EFA) design, the dentoalveolar effect would be either reduced or ruled out and the presence or absence of a significant skeletal effect could be discriminated.

The deformation in finite element analysis indicates that a change in size, and accordingly movement, has occurred. In clinical practice, the desirable effects for the correction of class II skeletal malocclusion are enhancing the mandibular growth while restraining the maxillary growth (Antonarakis and Kiliaridis 2007 ; Vargervik and Harvold 1985 ; Harvold and Vargervik 1971 ; Pancherz 1982 ; Macey-Dare and Nixon 1999 ; Collett 2000 ). Similar results were reported in this study; the greatest movement occurred in the forward and downward direction at the symphysis and lower border of the mandible. This was associated with a little backward and upward deformation in the maxilla indicating that some restraining effect was demonstrated as well (Nalbantgil et al. 2005 ). The low deformation values observed at the condyle suggest the absence of pain during treatment.

Different studies in the literature discussed the effects of stress and stress distribution on the condyle and the glenoid fossa and how the tensile and compressive stresses created by mandibular advancement resulted in remodeling (Panigrahi and Vineeth 2009 ; Rabie et al. 2001 ; Rabie et al. 2003a ; Sato et al. 2005 ; Hu et al. 2001 ; Zhou et al. 1999 ; Ress 1954 ). However, there was no known reference value for the optimal physiological range of stresses (Panigrahi and Vineeth 2009 ). It was apparent then, as there were reported values for strain in the literature, that it was the optimum parameter to be investigated even though there were no comparable studies considering strain. It was previously reported that the strain values that resulted in physiological bone modeling and remodeling ranged from 100 to 3000 micro strain (EL- Zawahry et al. 2016 ), and strains from 3600 to 4000 micro strain were considered within the physiological range in living animals (Sugiura et al. 2000 ).

Owtad et al. reported that the biophysical changes that occur as a result of mandibular advancement prompt cellular and molecular changes which result in bone formation and condylar growth enhancement (Owtad et al. 2011 ). These cellular changes could be as a result of the genetic expression of Sox 9 and type II collagen leading to merely an acceleration of the genetically predetermined growth. However, Rabie et al. demonstrated that the expression of such factors did not result in a change in the normal growth pattern; thus, functional therapy could induce true condylar growth augmentation (Rabie et al. 2003b ). On analyzing the strain results in this study, it was deduced that there was physiologic adaptive remodeling in the mandibular condyle in all directions, and mandibular forward and downward movement was demonstrated.

Different growth theories that explained the mechanism of growth modification described that functional adaptation occurred harmoniously in the condyle and the glenoid fossa, yet differently, and contributed to the growth modification process (Voudouris et al. 2003 ). The growth relativity hypothesis explained that the viscoelastic forces applied during functional therapy resulted in growth remodeling in the TMJ complex, which depended on the balance among many factors (Voudouris et al. 2003 ; Voudouris and Kuftinec 2000 ).

On the other hand, the ratchet hypothesis proposed that the condyle was the utmost determinant of the mandibular downward and forward movement (Whetten and Johnston Jr 1985 ). Others also reported that the role of the condyle in the process was exceptionally higher than the glenoid fossa (Owtad et al. 2011 ; Barnouti et al. 2011 ), which conformed to the results of this study in which condylar remodeling was considerably higher, and in contrast to some studies which reported remarkable glenoid fossa adaptation (McNamara Jr et al. 2003 ).

Contrary to all previous clinical reports (Cozza et al. 2006 ; Franchi et al. 2011 ; Zymperdikas et al. 2016 ; Marsico et al. 2011 ; Cope et al. 1994 ; Darda et al. 2010 ; Küçükkeleş et al. 2007 ; Panigrahi and Vineeth 2009 ; Nalbantgil et al. 2005 ), there was no movement in the mandibular dentoalveolar area at all as the observed strain at the tooth bearing area was an average of zero. Similarly, no maxillary dentoalveolar movement was demonstrated.

Accordingly, it could be predicted that the new design would enhance mandibular forward and downward growth without resulting in any dentoalveolar compensations. The absence of such compensations provides the opportunity for observing the presence or absence of significant skeletal changes with functional therapy.

Within the limitations of this finite element analysis, it was concluded that the new Elhiny fixed functional appliance design (EFA) resulted in:

Mandibular forward and downward movement, apparent at the mandibular symphysis and the lower border of the mandible

Physiological remodeling at the condyle, indicative of condylar growth

No dentoalveolar movement

Hence, it could be predicted that the new appliance (EFA) produces pure functional skeletal results with absolutely no dentoalveolar effects. The absence of dentoalveolar effects might allow the full expression of growth.

Furthermore, it can be useful in cases with deficient mandibular growth, increased overjet, proclined lower incisors, and/or retroclined upper incisors.

Recommendations

Clinical studies should be conducted on the newly designed appliance.

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Orthodontics and Pediatric Dentistry Department, National Research Centre, 33 El Bohouth St., Dokki, P.O. 12622, Cairo, Egypt

Omnia A. Elhiny

Mechanical Engineering Department, National Research Centre, Cairo, Egypt

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OE came up with the idea of the appliance and research, interpreted the engineering data orthodontically, and was a major contributor in writing the manuscript. ME designed the study methodology, analyzed and interpreted the data from the engineering point of view, and contributed in writing the manuscript. All authors read and approved the final manuscript.

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Correspondence to Omnia A. Elhiny .

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Elhiny, O.A., El-Anwar, M.I. The dilemma of functional therapy: the new EFA to do or not to do?. Bull Natl Res Cent 42 , 23 (2018). https://doi.org/10.1186/s42269-018-0024-3

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Received : 20 June 2018

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DOI : https://doi.org/10.1186/s42269-018-0024-3

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Angle Orthod
v.81(2); 2011 Mar

A histochemical study on condylar cartilage and glenoid fossa during mandibular advancement

To evaluate cellular hypertrophic activities in the mandibular condylar cartilage (MCC) and the glenoid fossa (GF) during mandibular advancement in the temporomandibular joint (TMJ) of Sprague-Dawley rats, as evidenced by fibroblast growth factor 8 (FGF8).

Methods and Materials:

Fifty-five female 24-day-old Sprague-Dawley rats were randomly divided into four experimental and control groups, with a mandibular advancement appliance on the experimental rats' lower incisors. The rats were euthanized on days 3, 14, 21, and 30 of the study, and their TMJ was prepared for a immunohistochemical staining procedure to detect FGF8.

FGF8 expression was significantly higher among the experimental rats ( P = .002). Patterns of ascension and descension of FGF8 expression were similar in experimental and control samples. The results show an overall enhanced osteogenic transition occurring in both the MCC and the GF in experimental rats in comparison with controls. The level of cellular changes in the MCC is remarkably higher than in the GF.

Conclusion:

In the MCC and the GF, cellular morphologic and hypertrophic differentiations increase significantly during mandibular advancement. It is also concluded that endochondral ossification in the MCC and intramembranous ossification in the GF occur during adaptive remodeling.

INTRODUCTION

Several studies have discussed mandibular advancement as a functional therapy for skeletal Class II malocclusion 1 and have shown that a fundamental factor in regulating cellular activities during tissue morphogenesis is mechanical stress. Forward positioning of the mandible is followed by adaptive remodeling in the mandibular condylar cartilage (MCC) and the glenoid fossa (GF). 2 – 6 Growth modification of the lower jaw during mandibular forward positioning is a successful example of bone remodeling in response to a change in the biophysical environment. 1 Many studies with rats and monkeys have shown that new bone formation in the condyle and the GF occurs in response to mandibular advancement. 6 – 8

This remodeling occurs by expression of endogenous regulatory factors of cells in the mandibular condyle through an endochondral ossification process 1 – 3 and intramembranous ossification in the GF. 4 , 6 , 7

The population size of the mesenchymal cells present in the subperiosteal connective tissues of the MCC directly affects the number of bone-making cells available to engage in the formation of new bone during craniofacial development. Undifferentiated mesenchymal cells in the extracellular matrices (ECMs) give rise to other cell types as the need arises and are present in the ECM of developing bones in the skull, including the temporal bone. 9

The fibroblast growth factors (FGFs) regulate mesenchyme and chondrocyte proliferation in MCC adaptive remodeling. 10 FGF signaling results in a decrease in chondrocyte proliferation and an acceleration of hypertrophic differentiation and morphologic changes in chondrocytes. 5 FGF8 is expressed in highly proliferating, columnar chondrocytes and in early hypertrophic and hypertrophic chondrocytes. 11

The aim of this study is to evaluate hypertrophic activities in the MCC and the GF during mandibular advancement in Sprague-Dawley rats, as evidenced by FGF8. In this study, FGF8 is used as an indicator of cellular chondrogenic and morphologic differentiation and hypertrophic activity, to demonstrate the histochemical nature of the adaptive response of bone to mandibular protrusion.

MATERIALS AND METHODS

Fifty-five female Sprague-Dawley rats at the age of 24 days were randomly divided into the experimental group (n = 35) and the control group (n = 20); the study was approved by the Westmead Animal Ethical Committee (Protocol No: 4113.06-08). All rats were kept in the same well-controlled temperature and humidity environment. They were fed a soft palate diet and had uninhibited access to water 24 hours a day throughout the entire experimental period ( Table 1 ).

Experimental Design

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Object name is i0003-3219-81-2-270-t01.jpg

The animals were sedated and a crown former was positioned on their lower anterior incisors in such a way that it caused mandibular forward-downward positioning during the rats' rest and functional bite ( Figure 1B ). Animals in the control groups were not fitted with an appliance and were untreated ( Figure 1A ). Body weight was monitored throughout the experiment.

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Bite jumping appliance. Normal incisal relationship in control rats. The bite jumping appliance is adjusted on the lower incisors of experimental rats to move their mandible into a forward position during rest and function.

Euthanasia and Tissue Preparation

The animals in each subgroup were euthanized, respectively, by carbon dioxide gas (Aligal 2, Air Liquide Australia Ltd, Fairfield NSW, Australia) on the 27th, 38th, 45th, and 54th days of the rats' age.

Immediately after death, the heads were removed and were fixed in 4% paraformaldehyde for 24 hours. The heads were then decalcified in 20% ethylenediaminetetraacetic acid (EDTA), pH 7 to 7.4, at 4°C to 8°C, for 4 to 6 weeks.

The temporomandibular joint (TMJ) was dissected, and surrounding soft tissues were removed until the TMJ was exposed. Excess tissues were removed, and specimens with the buccal surface of the ramus parallel to the surface of the block were embedded in paraffin. Serial sections 5 µ thick were cut through the TMJ at the parasagittal plane using a rotary microtome (Leitz 1516, Leica Microsystems, Wetzlar, Germany); sectioning was continued until the approximate middle of the condyle was reached. At this level, a few sections were floated onto glass slides coated with poly-L-lysine.

Individual variations occurred in TMJ orientation in the skull. Thus, to make a reliable comparison, the plane of each section throughout each of these anatomic variables was adjusted as identically as possible between samples. Sections cut from each sample were assigned to immunohistochemical staining for fibroblast growth factor 8 (FGF8).

Immunohistochemical Examinations

The specific primary antibody used was FGF8 goat polyclonal antibody collagen (N-19, Cat #SC 6958, Lot #E300, 200 µg/mL; Santa Cruz Biotechnology Inc, Santa Cruz, Calif). The secondary antibody was rabbit antigoat immunoglobulin (Ig)G (HRP, Code No. P0449; Dako A/S, Glostrup, Denmark).

Immunohistochemistry was carried out using a method in which the sections were dewaxed and rehydrated and were treated with glycine, 3% hydrogen peroxide, horse serum, the primary antibody, and the secondary antibody.

Then the slides were dipped in 3,3′-diaminobenzidine (DAB) in chromogen solution (Dako Liquid DAB + Substrate Chromogen System, Code K3467, Dako A/S) and finally were counterstained with Mayer's hematoxylin for background staining. Negative controls were included, in which the primary antibody was replaced by FGF8-blocking peptide (N-19 P, Cat #SC 6958 P, Lot #F268, 100 µg/0.5 mL, Santa Cruz Biotechnology) to ascertain the specificity of the immunostaining.

After the clearing protocol, slides were covered by mounting medium (Fisher Scientific Permount SO-P-15, 500 mL, 1.1 pt; Fisher Scientific, Fair Lawn, NJ) and a coverslip for long-term storage of slides and additional microscopic studies.

Quantitative Imaging

Digital images were taken from stained tissues with a Leica digital imaging microscope and its software (Leica Application Suite Software; Leica Microsystems, Bannockburn, Ill) at 10×, 20×, and 60× magnification.

Expressions of FGF8 were quantified by manually counting the cells of positive reacted immunostaining signals on the computer screen from 20× magnified images. Cells were counted from the middle quarter of the MCC and the distal third of the GF, where the most prominent cellular responses to mandibular repositioning occur. 2 , 12 Cells that were stained with certain intensity were counted, and those that were weakly stained were excluded.

Statistical Analysis

After the first counting, data were collected again 4 weeks later by the same observer, and the method of error (ME) was tested. No statistically significant difference was noted among the registrations. Data were analyzed using a statistical package (Statistical Package for the Social Sciences [SPSS] for Windows, version 16.0, SPSS Inc, Chicago, Ill).

The cytoplasms of early hypertrophic and hypertrophic cells, beneath the layer of cell proliferation and above the erosive zone, were positively stained for FGF8 ( Figure 2D,G ). FGF8 is located mainly in the cytoplasm of osteochondroprogenitor cells, chondroblasts, and chondrocytes before their degeneration, which is shown by extra magnification of a typical immunopositive cell for FGF8 from the hypertrophic layer of the mandibular condylar cartilage of a 38-day-old experimental sample ( Figure 2 ).

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FGF8 in the MCC and the GF. Photograph of a female Sprague-Dawley rat's TMJ (A) shows the anatomic relationship of the condyle (A-MCC) and the articular fossa (A-GF), and the arrow (A) shows the direction of forward-downward displacement of the condyle during mandibular advancement. Photomicrographs show immunostaining for FGF8 expressed in the glenoid fossa of another experimental sample (27-day-old rat, wearing bite jumping appliance for 3 days) (B, C, and D) and the mandibular condylar cartilage of an experimental sample (38-day-old rat, wearing bite jumping appliance for 14 days) (E, F, and G).

The level of FGF8 expression in the condyle and glenoid fossa of experimental rats generally is significantly higher than in relevant control samples. Furthermore, it is clear that the amount of cellular activity in the mandibular condylar cartilage is greatly higher than in the glenoid fossa in both controls and experimental rats at different stages of growth and development. The effect of the appliance on FGF8 expression is generally significant in the MCC and the GF (FGF8c, P = .002; FGF8gf, P = .002). The level and pattern of FGF8 expression are shown in Table 2 and Figure 3 .

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FGF8c and FGFgf: diagram for experimental rats vs controls on different experiment days. In this diagram, the means of the numbers of FGF8-immunopositive cells in the mandibular condylar cartilage (FGF8c) and in the glenoid fossa (FGF8gf) are compared in experimental and control rats on different experiment days.

FGF8c and FGF8gf: Experimental Rats vs Controls on Different Experiment Days a

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The histologic structures of the rats' TMJ are similar to those of humans, with morphologic differences. 13 Because of this similarity and the possibility of a histochemical study on rats based on previous studies, 14 , 15 55 Sprague-Dawley rats were used in this experiment, as in other histologic and biochemical investigations. 3 , 4 , 8 , 12 – 14 , 16 , 17

Even though some studies on monkeys indicate that such adaptive responses are nonexistent and negligible, 18 several other findings, such as those of the current study, indicate positive significant TMJ adaptation in response to mandibular advancement. 2 , 3 , 16 , 19 – 22 However, the pattern of this adaptive response in the MCC is different from that in the GF.

The adaptive response of the MCC-GF complex could be described by the growth relativity hypothesis and the functional matrix theory. The mandible is displaced to a forward position, viscoelastic forces are applied on the MCC-GF complex at the same time in reverse directions, and the forces are transduced by being radiated beneath the articular layers of both the MCC and the GF. 23 – 25

The role of the MCC in the process of growth and development of the TMJ and its adaptive response to mandibular advancement are remarkably greater, particularly during the period of the present study.

This lower amount of activity in the GF might show that adaptation of the whole TMJ occurs mainly by ossification and relocation of the MCC and relocation of the GF, as a harmonized biologic response to mandibular protrusion. Otherwise, it is possible that the GF is not significantly relocated from its initial position, which could be a reason for future relapses of successful functional mandibular treatment. If the GF does not remarkably remodel or relocate, then the soft tissue attachments pull the condyle back to its initial relationship with the GF. For clearer and more detailed information in this regard, these possibilities should be precisely studied and evaluated by a combination of histochemical, cephalometric, and electromyographic methods over a longer period of time.

Signaling molecules of the FGF family regulate endochondral ossification at several levels 26 ; therefore results of the current study are consistent with the fact that the mandibular condyle is a growth site and is ossified through endochondral ossification. 22 However, endochondral ossification in the GF is observed only during initial stages of rats' growth (27 days of rats' age), as indicated by detection of FGF8 at that age and not after that age. This could indicate that osteogenesis generally slows down in rats' GF at this age, or that intramembranous ossification is the dominant ossification process in the GF afterward, and no cartilage tissue is found in the GF during later growth and development. Intramembranous ossification in the GF is similarly reported in other studies. 27

Immunopositive cells for FGF8 molecules are detected in the cytoplasm of chondrogenic cells in the deep columnar proliferative layer and in early hypertrophic and hypertrophic zones. 11 , 26 The location of FGF8 expression is consistent with other reports, which suggests that FGF8 could be known as an indicator for osteogenesis through an endochondral ossification process by patterning and regulating chondrocytes' proliferation and their hypertrophic morphologic differentiation. 11 , 28 , 29

This indicates that FGF8 is more involved in hypertrophic activities than in chondrocyte proliferation. FGF8 plays a role in cellular chondrogenic differentiation and in creating morphologic changes from mesenchymal cells to chondroblasts and from chondroblasts to bone-making cells. This has also been reported by Minina et al. 28 On the molecular level, FGF signaling reduces chondrocyte proliferation and induces hypertrophic differentiation of chondrocytes. 28

The higher amount of FGF8 expression in experimental samples, in comparison with control samples, generally shows enhanced osteogenic transition occurring in both the MCC and the GF, with the exception of FGF8 excretion in the MCC on experiment day 30, during which the level of expression in control animals was slightly, although not significantly, higher than in the experimental rats. Additional long-term studies are required to find out the reason for the lower molecular and cellular activities in the GF than in the MCC.

Evaluating the expression of FGF8 in experimental rats versus controls suggests that mandibular advancement does not change the pattern of molecular activity, but just increases the level of activity. The pattern of FGF8 expression follows the pattern of normal growth and development of the TMJ, in reference to controls.

The FGF8 pick of expression is at approximately the same level in experimental and control rats. Considering this evidence and the fact that FGF8 in the condyle is slightly higher in controls on day 30, it is possible that stepwise advancement may generate more changes in the MCC-GF complex than one-step advancement. This is similarly suggested by other researchers. 16 , 17

The future direction would be to design and perform studies covering longer periods of the rats' lives and comparing the MCC and the GF with other growth centers and growth sites on each sample, such as epiphyseal plates, synchondroses (eg, spheno-occipital synchondrosis), maxillary sutures (eg, intermaxillary suture), cranial sutures, and mandibular symphyses at earlier stages of growth. This could be done with cephalometric evaluations, in addition to histochemical evaluations, performed during the period of the experiment to measure the level and direction of growth and development.

Structural and molecular adaptations occurred in the MCC and the GF of the experimental animals. Hypertrophic differentiations were significantly increased in both parts, which could enhance bone formation during the adaptive response. Therefore, mandibular growth modification takes place as an end result of extracellular morphologic differentiations and hypertrophic changes in the both the MCC and the GF.

Growth relativity hypothesis

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Figure 10 Orthodontic treatment flowchart.

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Repositioning Of The Mandible & What Happens

Improved clinical use of Twin-block and Herbst as a result of radiating viscoelastic tissue forces on the condyle and fossa in treatment and long-term retention: growth relativity.

Understanding mechanisms of action for orthopedic appliances is critical for orthodontists who hope to treat and retain the achieved corrections in patients with initial Class II mandibular retrognathism. That knowledge can help orthodontists produce clinically significant bone formation and avoid compression at the condyle-glenoid fossa region. It also assists us to understand the differences between short-term and long-term treatment results. It was previously thought that increased activity in the postural masticatory muscles was the key to promoting condyle-glenoid fossa growth. By analyzing results from several studies, we postulate that growth modification is associated with decreased activity, which leads to our nonmuscular hypothesis. This premise has its foundation on 3 key specific findings: significant glenoid fossa bone formation occurs during treatment that includes mandibular displacement; glenoid fossa modification is a result of the stretch forces of the retrodiskal tissues, capsule, and altered flow of viscous synovium; observations that glenoid fossa bone formation takes place a distance from the soft tissue attachment. The latter observation is explained by transduction or referral of forces. Evidence is presented, therefore, that the 3 trigger switches for glenoid fossa growth can similarly initiate short-term condylar growth modifications because the 2 structures are contiguous. These are displacement, several direct viscoelastic connections, and transduction of forces. Histologic evidence further shows that stretched retrodiskal tissues also insert directly into the condylar head's fibrocartilaginous layer. The impact of the viscoelastic tissues may be highly significant and should be considered along with the standard skeletal, dental, neuromuscular, and age factors that influence condyle-glenoid fossa growth with orthopedic advancement. These biodynamic factors are also capable of reversing effects of treatment on mandibular growth direction, size, and morphology. Relapse occurs as a result of release of the condyle and ensuing compression against the newly proliferated retrodiskal tissues together with the reactivation of muscle activity. To describe condyle-glenoid fossa growth modification, an analogy is made to a light bulb on a dimmer switch. The condyle illuminates in treatment, dims down in the retention period, to near base levels over the long-term.

“ significant glenoid fossa bone formation occurs during treatment that includes mandibular displacement; glenoid fossa modification is a result of the stretch forces of the retrodiscal tissues, (stretching of the) capsule, and altered flow of viscous synovium”

- Voudouris and Kuftinec

The posterior, anterior, and lateral attachments of the retro- discal articular disc complex are shown. The condyle is usually guided upward and backward to CR by these and other attachments (from an open jaw position).

When the complex is pulled forward opposite to the direction of the arrows, glenoid fossa modification occurs. Studies have shown that not only the pull of these tissues lead to these changes, the altered dynamics of the synovial fluid facilitates growth beneath the hydrophilic condylar fibrocartilage. In the upper chamber, the opposing anterior flow of synovial fluids similarly influences glenoid fossa growth.

THREE GROWTH STIMULI WORK TOGETHER   Displacement + Viscoelastic tissue pull + Referred Force = Remodeling / Growth of bone in the Glenoid Fossa (along with some growth at the posterior superior aspect of the condyle)

The concept that viscoelastic tissue forces affect growth of the condyle and bone of the Glenoid Fossa suggests that modification first occurs as a result of the action of anterior orthopedic displacement.

Second, both the fossa and the condyle are affected by the posterior viscoelastic tissues anchored between the glenoid fossa and the condyle, inserting directly into the fossa and the condylar fibrocartilage.

Finally, it is hypothesized that displacement and viscoelasticity further stimulate (or turn on the switch for) normal condylar growth by the transduction of forces over the fibrocartilage cap of the condylar head.

Dr. Bakr Rabie Faculty of Dentistry Department of Orthodontics The University of Hong Kong

Typical Effects of Functional Appliances for Class II

Posture the mandible forward

Place some distal driving force on the maxilla

Affect the vertical control of the posterior teeth (usually -Christianson effect)

Detorque the upper incisors

Torque the lower incisors forward

Have some effect on speech and esthetics (this should be a positive effect in the long run)

Do you take jaw joint x-rays before advancing the lower jaw?

It’s an important idea because it allows you to know where the condyle was before being advanced and is referenced to determine whether you are done with the functional aspect of treatment

The main result of a Class II correcting functional appliance is the remodeling of the Temporal fossa

HOW CONDYLAR MODIFICATIONS OCCUR

The growth relativity hypothesis has three main foundations:

A: The glenoid fossa promotes condylar growth with the use of orthopedic mandibular advancement therapy. Initially, that displacement affects the fibrocartilaginous lining in the glenoid fossa to induce bone formation locally.

B: This is followed by the stretch of non-muscular viscoelastic tissues

C: Third, and the most interesting aspect is the new bone formation some distance from the actual retro- discal tissue attachments in the fossa. The glenoid fossa and the displaced condyle are both influenced by the articular disc, fibrous capsule, and synovium, which are contiguous, anatomically and functionally, with the viscoelastic tissues.

Condylar growth is affected by viscoelastic tissue forces via attachment of the fibrocartilage that blankets the head of the condyle.

Although there is some growth of the condyle, especially at the posterior aspect, the actual amount of growth of the condyle that results from the change in mandibular position is not great.

Clear Aligner Orthodontic Treatment and Root Resorption

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COMMENTS

Improved clinical use of Twin-block and Herbst as a result of radiating
The growth relativity hypothesis is currently tested experimentally through the use of full occlusal coverage Herbst-Block and Twin-Block appliances for condylar displacement, stimulation of viscoelastic tissue, and transduction forces in primates. This method minimizes condylar compression typically found in a more traditional design of the ...
Improved clinical use of Twin-block and Herbst as a result of ...
It was previously thought that increased activity in the postural masticatory muscles was the key to promoting condyle-glenoid fossa growth. By analyzing results from several studies, we postulate that growth modification is associated with decreased activity, which leads to our nonmuscular hypothesis.
PDF Improved clinical use of Twin-block and Herbst as a result of radiating
The growth relativity hypothesis has three main foundations: A: The glenoid fossa promotes condylar growth with the use of orthopedic mandibular advancement therapy. Initially, that displacement affects the fibrocartilaginous lining in the glenoid fossa to induce bone formation
Improved clinical use of Twin-block and Herbst as a result of radiating
The growth relativity hypothesis is currently tested experimentally through the use of full occlusal coverage Herbst-Block and Twin-Block appliances for condylar displacement, stimulation of. Acknowledgements. We wish to dedicate this article to the memories of Drs Egil Harvold, Aaron Posen, and Frank Popovich. We acknowledge Dr Bruce Ross ...
Improved clinical use of Twin-block and Herbst as a ...
The growth relativity hypothesis explained that the viscoelastic forces applied during functional therapy resulted in growth remodeling in the TMJ complex, which depended on the balance among many ...
Effective temporomandibular joint growth changes after stepwise and
Voudouris and Kuftinec [4], in their "growth relativity hypothesis," also concluded that modification of the condyle and the fossa can be clinically significant when the two structures are separated. With maximum advancement, growth was in the posterior direction, similar to Bolton standards with normal occlusion, whereas with stepwise ...
Condyle-fossa modifications and muscle interactions during Herbst
A review of the literature 1 indicates that the glenoid fossa has the potential to remodel during functional appliance therapy. For decades, the prevailing notion was that condylar growth modification was caused by lateral pterygoid muscle hyperactivity. 2 The growth relativity theory, 3 on the other hand, describes a specific set of reciprocal soft tissue forces that operate during Herbst ...
Condyle-fossa modiﬁcations and muscle interactions during Herbst
growth modiﬁcation.4 The problem appears to be re-lated to earlier EMG methods, which were transcuta-neous and removable. The theory of growth relativity essentially states that bone growth modiﬁcation occurs relative to 2 elements: the retrodiskal tissues are stretched recipro-cally, similar to a large elastic band, between the fossa
(PDF) The Biology of TMJ Growth Modification
The growth relativity hypothesis presents the mechanism of . condyle-fossa growth modification with mandibular advance-ment. It describes viscoelastic forces applied to the TMJ through .
(PDF) Growth relativity theory
The difference between the growth relativity hypothesis and the functional matrix theory is that the former is specific to the C-GF and identifies the soft tissues, fluids, and loci of force transduction that cause growth modifications when using orthopedic appliances. This is a macroscopic overview of a complex mosaic of numerous microscopic ...
Condyle-fossa modifications and muscle interactions during Herbst
The growth modification measured in the glenoid fossa was in an inferior and anterior direction. Restriction of the downward and backward growth of the fossa observed in the control subjects might additionally contribute to the overall super Class I malocclusion. Clinically, these combined effects could be significant at the fossa.
The dilemma of functional therapy: the new EFA to do or not to do
The growth relativity hypothesis explained that the viscoelastic forces applied during functional therapy resulted in growth remodeling in the TMJ complex, which depended on the balance among many factors (Voudouris et al. 2003; Voudouris and Kuftinec 2000).
Condyle-fossa modifications and muscle interactions during herbst
The theory of growth relativity essentially states that bone growth modification occurs relative to 2 elements: the retrodiskal tissues are stretched reciprocally, similar to a large elastic band, between the fossa and the displaced condyle during the expansion of the growing facial complex; and the transduction of these nonmuscular forces has ...
Functional Matrix Hypothesis: A Review of Literature
Growth relativity hypothesis. Paradigm. A conceptual scheme that encompasses. individual theories and is ac cepted by a scientific. community as a model and foundation for further. research.
PDF RESEARCH ARTICLE
Growth relativity theory suggests that bone architecture is induced by the neuromuscular and the contagious non-muscular viscoelastic tissues anchored to the 14359 Sreelatha et al. Response to twin block treatment: A literature review. glenoid fossa and altered dynamics of the fluids enveloping bone (John De Vincenzo, 1991). ...
A histochemical study on condylar cartilage and glenoid fossa during
The adaptive response of the MCC-GF complex could be described by the growth relativity hypothesis and the functional matrix theory. The mandible is displaced to a forward position, viscoelastic forces are applied on the MCC-GF complex at the same time in reverse directions, and the forces are transduced by being radiated beneath the articular ...
Growth relativity hypothesis
Download scientific diagram | Growth relativity hypothesis from publication: Temporomandibular joint and orthodontics -A contemporary review | The Temporomandibular joint is an atypical synovial ...
Repositioning Of The Mandible & What Happens
The growth relativity hypothesis has three main foundations: A: The glenoid fossa promotes condylar growth with the use of orthopedic mandibular advancement therapy. Initially, that displacement affects the fibrocartilaginous lining in the glenoid fossa to induce bone formation locally.