Maxillary Growth: A Judicious View
Marinho Del Santo, Jr., DDS, PhD

Private Orthodontist, São Paulo, Brazil
Copyright © 2000 by Marinho Del Santo, Jr.

All rights reserved. Written permission from the author is mandatory prior to the reproduction of part of this publication. 

Course Objectives

This course will be provided in three hours, with one 15 minutes break. It will emphasize the clinical application of changes in maxillary form and function. The specific aims are: 1) to introduce methods and tools which will help to refine the student's ability to make clinical assessments, 2) to relate the student's understanding of growth and development to clinical evaluations and treatment strategies and 3) to discuss the pertinent literature, its contribution and limitations. 

Rationale 

Before attending this course, the students must have learned basic concepts about craniofacial growth, primarily at the cellular and tissue levels and specifically about the mechanisms involved in maxillary growth. At the end of the course, the students will be able to make more objective clinical decisions based on growth and development of the maxilla.

Introduction

Why Consider Growth?

The understanding of craniofacial growth is mandatory for the orthodontist or the graduate student to fully diagnose the problem, to better understand the etiology of the problem and to evaluate treatment and posttreatment changes. When comprehensive understanding of these subjects is gained, favorable growth patterns will be identified and advantages will be taken from it. Unfavorable growth patterns must also be assessed because its treatment is more difficult or sometimes impossible to be treated just orthodontically. Lately, orthodontic treatment has become many times appliance-driven. The right way to do it is to understand Biology first, then to identify the appliances that can correct the problem. The most important goal is the valorization of the profession. Nowadays, thousand of people claim themselves orthodontists. With the current technological level of the orthodontic industry, "appliances manipulation" is migrating to the general practitioner hands. The real orthodontist uses appliances to express his/her knowledge.

Historical Theories pertaining to Craniofacial Growth Control

"Craniofacial Biology can be defined as the study of the development, growth and adaptation, both phylogenetically and ontogenetically, of the craniofacial skeleton and related structures" (Carlson, 1985). Scientific interest in the structure and function of the craniofacial complex has considerable historical depth. Because of the broadly extension of this field, it is obviously impossible to cover all areas in details in this manuscript.

Early Research in Craniofacial Growth (1920-1940)

At that time, craniofacial growth studies were based primarily on the static study of the structures of the craniofacial skeleton, with little or no consideration of function. With the advent of the X-rays (Broadbent, 1931), studies started to be based upon comparative anatomy, craniometrics and radiographic cephalometrics. In these studies, anatomical intuition and extrapolation from other areas of the body led to the belief that growth in the craniofacial area was genetically predetermined (Charles, 1925; Sicher, 1947). A consequence of this approach was the premise that if craniofacial growth was genetically determined, its pattern should be practically immutable. Therefore, recognition of facial type by comparison with growth standards would imply on the classification of specific patterns of growth. Moreover, norms would provide a way to predict facial growth, by comparison, for individual patients and influence the choice of treatment. However, because orthodontists did not believe that they could change facial growth, the best they could do was to achieve good dental alignment. 

Craniofacial Growth Research from 1940 to 1960

During this period, there was an increased emphasis on experimental animal research in an effort to understand the actual mechanisms of facial growth (Baume, 1961). Conceptually, orthodontists became aware that there was much more variation within the facial region than would be predict if growth was predetermined genetically, and that this variation could be the result of environmental influences during ontogeny. In this matter, the concepts of function and adaptation to altered function were highlighted (Carlson, 1985). Although there was little evidence at this time that the cephalic cartilages, including those at the cranial base, nasal septum and mandibular condylar cartilages, were not strictly under genomic control, the alternative view that "function" played a role during facial growth started to gather prestige. 

Scott (1953), considered the cranial cartilages as centers of growth, especially the nasal septum. For him, the nasal septum is a strategic structure that could address the midfacial growth forward relative to the cranial base. Sutural growth would be secondary and would answer passively to the growth direction imposed by cartilages.

Craniofacial Growth Research from 1960 to present

In the early 1960's, an alternative approach to the genomic view was formalized. The functional matrix hypothesis was proposed by Moss in 1962. According to Moss, growth and form of all skeletal tissues, including bones and larger skeletal units comprising the craniofacial region, are not genetically determined. Rather, they are the result of functional demands and biophysical phenomena that are part of the epigenetic environment. However, the functional hypothesis did not exclude the possible influence of genetic factors on growth and form of the craniofacial skeleton (Carlson, 1985).

Enlow (1963, 1986), stated that bones grow only by apposition at their periosteal and endosteal surfaces. This position does not necessarily mean that bones grow equally by apposition at all surfaces. Bones can maintain their shape and size by differential bone apposition and resorption, process called remodeling. For Enlow, local extrinsic factors can modulate (stimulate or restrain) the compensatory growth of secondary cartilages. 

van Limborg (1970), partially agreed with Scott. He supported the relationship between cartilages and sutures but added the concept of intrinsic and extrinsic influences changing periosteal growth. In his opinion, growth at the cartilages is almost exclusively controlled by genetics; however, at the periosteum is almost totally controlled by local extrinsic factors. 

Petrovic (1974), proposed that quantitative processes controlling postnatal craniofacial growth are part of a servosystem. The influence of hormones on the growth of primary cartilages has the cybernetic form of a "command". Moreover, growth of secondary cartilages is directly influenced by hormones and also submitted to indirect modulators. At the condylar, coronoid and angular cartilages, indirect modulators can be local factors, as neuromuscular mechanisms relative to postural occlusal adjustment. 

Maxillary Growth

The midface, or maxillary complex, is comprised of the paired maxillae, nasal bones, zygomatic bones, palatine bones and vomer. The midface is connected to the neurocranium by a circummaxillary suture system and, toward the midline, by the nasal septum, a cartilaginous nasal capsule, and vomer (Carlson, 1985). There is also a sagittal suture system comprised of the midpalatal, intermaxillary and internasal sutures. With the exception of the vomer and the inferior turbinates, all the bones comprising the midface are formed intramembranously from a connective tissue mass. In contrary, the nasal capsule and septum grow by means of interstitial cartilaginous growth (Carlson, 1985).

The direction of the interstitial growth within the nasal septum and synchondroses of the cranial base results in the typical downward and forward growth of the entire midface relative to the anterior cranial base. Savara and Singh (1968), found that the midface increases most in height (antero-posteriorly), next in depth (vertically) and least in width (transversely) after age three years of age (post-birth). The anterior and vertical growth of the midface is more expressive during the first seven years of life.

The age of seven years is something of a benchmark for the maxillary growth (Scott, 1953, 1956). Growth of the central nervous system is essentially complete by this age. The spheno-ethmoidal synchondrose fuses at about this time, establishing a relatively stable anterior cranial base. The nasal capsule and nasal septum change significantly. The cartilaginous nasal capsule ossifies and the nasal septum (although cartilaginous throughout life in humans) decreases significantly in growth activity. Bone growth at the circummaxillary sutures, which compensates for the downward and forward displacement of the midface, maintains the articulation between the maxillary complex and neurocranium and decreases at age seven as well. 

Over the next decade, the midface primarily grows downward and forward by the process of remodeling of the maxillary complex (Enlow and Bang, 1965; Enlow, 1982). However, the anterior growth of the frontal bone and base of the nose is also a consequence of growth at the anterior cranial fossa. Deposition of bone at the posterior margin of the maxillary tuberosity results in an increase in length of the entire maxilla and the maxillary dental arch. Furthermore, eruption of the dentition and associated development of the maxillary alveolar process significantly contribute to the vertical increase.

Transversally, the maxilla grows primarily by bone remodeling rather than sutural growth (Scott, 1956; Enlow and Bang, 1965; Latham, 1970). Histological studies have shown that the midpalatal suture remains active as a growth site until adolescence (Persson, 1973; Melsen, 1975), although interdigitation on the midpalatal suture can be noticed also at the pre-birth period (Del Santo et al., 1998). Björk and Skieller (1976), following human subjects from age 10 years to adulthood with the aid of metallic implants in the maxillae, confirmed the growth pattern presented by Scott, Enlow and Latham.

In summary, the fact that the nasal septal cartilage and nasal capsular cartilage are comprised of primary hyaline cartilage, that is derived from the cartilaginous cranial base, allows the assumption that the nasal septal cartilage is capable of tissue-separating forces and significantly contributes to the downward and forward displacement of the maxillary complex away from the cranial base (Carlson, 1985). However, it is clear that the whole understanding of the role of each specific factor in the maxillary growth process has not been achieved yet. The nasal septal growth decreases at age of seven years and the midface still growing down and forward over one more decade. Why? Probably, because the nasal septum contributes to the maxillary displacement but does not drive the midface downward and forward independently of other components as the orofacial and pharyngeal functional complexes (Carlson, 1985). As the midface is displaced anteriorly and inferiorly, the tension created at the circummaxillary sutures stimulates an osteogenic activity to fill in the potential gap created by such displacement. After seven years of age, growth of the nasal septum, like that of the circummaxillary sutures, becomes secondary to the requirements imposed by the oral and pharyngeal areas (Carlson, 1985).

Maxillary Growth Measured on the Living Human Face

Björk (1955), in his first implant study, studied the maxillary growth of a boy followed from 5 to 13 years of age and concluded that the maxilla suffered extensive surface modeling. Björk and Skieller (1972, 1983) confirmed that the rotations of the maxilla and the mandible are accompanied by a continuous surface modeling of the bones, which tends to mask the actual translatory and rotational displacement.

Solow and Iseri (1996), described the maxillary growth (displacement and surface modeling) and the eruption pattern of the maxillary incisors and first molars in a sample of growing girls. The material was obtained from the Björk's archives (1968) and comprised of annual lateral films of 14 girls aged 8-25 years with maxillary implants inserted below the anterior nasal spine and on the lower anterior surfaces of the zygomatic arches.

As reference points, they used:

S= Sella 
N= Nasion
op= orbital posterior
oa= orbital anterior
pm= posterior nasal spine
sp= anterior nasal spine
ss= A point.

As fiducial points, they used:

1. Cranial base: St and Nt. These are the Sella and Nasion on the first film, transferred by superimposition on stable structures of the anterior cranial base. St to Nt represents the "new" cranial base reference line.

2. Maxillary base: pmta and spta. These are the pm and sp on the first film, transferred by superimposition on stable structures of the maxillary base. The "new" maxillary base is represented by the line pmta to spta. The points pmtp and sptp (not shown) are determined by superimposition on implant line but registered on ip.

3. Implants: ip and ia. Posterior and anterior implants. The implant line is represented by the line ip to ia.

4. Point ss: A point. 

5. Point pr: prosthion. 

6. Upper incisor tip: is; upper molar tip: ms. Upper occlusal line: is to ms.

The terminology described by Solow and Houston (1988) for rotation was applied. True rotation is the change of the implant line relative to the cranial base. Remodeling rotation is the change of the maxillary line relative to the implant line. Apparent rotation is the change of the maxillary line relative to the cranial base line. 

Therefore, we have:

True rotation 

Remodeling rotation

Apparent rotation

In order to show the differences between the observed skeletal changes and the actual skeletal changes, this article uses a meticulous superimposition technique. Such differences are very significant and fiducial points provide a better reliability.

Furthermore, the difference between the landmarks superimposition and the fiducial points superimposition increased in degrees throughout life:

The average forward angular displacement of the maxilla in relation to the anterior cranial base was 6 degrees. The angle St-Nt-ia is similar to the angle S-N-ia but eliminates the forward apposition effect at Nasion. Changes in St-Nt-ia truly show the angular displacement of maxilla.

The average path of displacement was inclined about 45 degrees in relation to the SN line from 8-14.5 years, and then changed to almost horizontal. The direction of maxillary growth is similar to the 51 degrees in relation to the Nasion-Sella line reported for boys by Björk in 1966.

The anterior and downward displacement can be spatially represented (in millimeters):

Moreover, the mean distance between anterior and posterior implant points showed a continuous shortening due to maxillary rotation in the transversal plane (mm/year): 

And it can graphically be represented as: 

Using the following landmarks, they measured the anterior vertical maxillary displacement (mm/year), based on the anterior and posterior implants, which are represented in the following graphic: 

And the anterior horizontal maxillary displacement (mm/year):

The average relocation of the A point (in this article called ss) and the anterior nasal spine (ANS; in this article called sp) was about 4 to 4,5 mm downward and 1 mm forward from 8 to 25 years. 

A point:

The anterior nasal spine point can be also spatially represented:

The posterior nasal spine (PNS; in this article called pm) was relocated 6 mm backwards and 1,5 mm downward by surface modeling. Growth in length of the maxillary base occurs almost exclusively on the posterior aspect of the bone (Björk,1968) and corresponds to the 5 mm forward displacement of the maxillary body in relation to the anterior cranial base described by Iseri and Solow (1995). 

Maxillary Rotation

The displacement of the anterior and posterior implants in relation to the cranial base represents the forward true rotation of the maxilla (degree/year):

The displacement of the palatal plane (based on the anterior and posterior nasal spine) in relation to the implant line (based on the anterior and posterior implants) represents the remodeling rotation of the maxilla (degree/year), which happens mainly at the nasal floor:

The apparent rotation of the maxilla (degree/year) represents the true rotation that suffered remodeling. It is based upon the cranial base and measured at the palatal plane:

The following graphic combines all the components of the maxillary rotation:

Maxillary complex 

The posterior implant was lowered more than the anterior one during growth. The implant line (IPLs) changed its inclination in relation to the anterior cranial base by 1.5 degrees from 8.5 to 12.5 years (forward true rotation). This is in agreement with the 2.5 degrees forward rotation of the maxilla previously observed by Björk and Skieller (1972), in a circumpuberal sample of 21 boys and girls.

Palatal plane

The relocation of the ANS (in this article called spta) and PNS (in this article called pmta) is due to the remodeling rotation of the palatal plane, 2.5 degrees backwards. 

As the maxillary complex rotated 1,5 degrees forward in relation to the anterior cranial base (true rotation) and the palatal plane rotated 2,5 degrees backward (remodeling rotation), the result is 1 mm degree increase in the inclination of the palatal plane to the anterior cranial base (backward, represented by apparent rotation). 

Better explaining, the differential remodeling rotation of the nasal surface of the bony palate serves to compensate for the forward rotation of the maxillary complex, maintaining this surface at a relatively constant inclination to the cranial base (just slight increase in angulation).

The last part of the maxillary growth to be considered is dental eruption. The eruption of the maxillary molars is more expressive than the maxillary incisors.

Dental eruption observed by the implants and the cranial base superimposition:

Occlusal plane 

Due to the difference between the continued eruption of incisors and molars (more eruption of the molars than the incisors), there was a mean reduction of 4.5 degrees of the inclination of the upper occlusal plane in relation to the implant line, from 10 to 16 years of age. After this age, no systematic change was observed. In relation to the anterior cranial base, the inclination of the occlusal plane was reduced by about 5 degrees. In relation to the palatal plane, the occlusal plane inclination reduced by about 6.5 degrees.

Decrease in the inclination of the occlusal plane (degrees/year):

The changes in inclination of the occlusal plane in relation to the palatal plane were larger than in relation to the implant line or cranial base. In reality, when we consider the inclination of the occlusal plane in relation to the palatal plane, we are just considering tooth eruption. The inclination of the occlusal plane in relation to the implant line or cranial base also includes the maxillary displacement.

In conclusion, dental eruption is of considerable clinical importance, and should be taken into account in the assessment of long periods of growth.The velocity of eruption of the maxillary incisors decreased gradually from 10 to 18 years of age. The peak of the eruption of the maxillary molars is at the age of 12 years and shows a postpuberal decrease in velocity until the age of 17. A similar relation between rate of eruption of the incisors and rate of growth in height was shown by Siersbæk-Nielsen (1971).

Orbital floor

Previous studies have shown that in boys, the downward growth displacement of the maxillary body is accompanied by apposition on the floor of the orbits (Björk, 1955, 1966). The growth tracks of the orbital points indicate that the apposition on the orbital floor compensates for the vertical, the horizontal and the rotational components of the sutural displacement of the maxillary body.

In the Solow and Iseri's study (1996), there was about 2.5 mm upwards and 2 mm backwards appositional relocation for both the anterior (OA) and posterior (OP) orbital reference points. The growth track of the posterior orbital point was found to be steeper and longer than that of the anterior orbital point, indicating that the apposition on the orbital floor also compensates for the forward rotation of the maxillary body in relation to the anterior cranial base, probably keeping the average inclination of the orbital floor in relation to the anterior cranial base relatively constant. 

Anterior orbital implant and respective displacement:

Posterior orbital implant and respective displacement:

Treatment

Extensive literature could be mentioned in regard to the treatment modalities of the maxillary complex deficiencies. In order to restrict this paper to the scope of our initial goals, we will describe the effects of the headgear for the treatment of Class II cases and of the facial mask for the treament of Class III cases.

Headgear

The literature is very vast and we leave for a revision article or a PhD thesis the commitment of a deep discussion based on significant articles about the headgear appliance. Here, we elected one article to be discussed in details:

Baumrind et al. (1983), studied 188 (in total) patients treated with high pull headgear, cervical pull headgear and modified activator. They evaluated the position of the ANS point, the upper molar cusp point and the upper molar apex point. Superimposition was performed on the best fit of the maxilla to identify the orthodontic displacement (dental) and on the best fit of the anterior cranial base to study the total displacement. The orthopedic effect was defined as the difference between the total and the orthodontic displacements. 

The authors found statistically significant pretreatment differences for a number of measurements but these differences were of no consequence to the interpretation of the effects. The total change observed among the three treatment groups reflected the combined effect both of treatment and growth.

Control Group 

The molar cusp and the ANS point each displaced downward and forward approximately 3 mm, at an angle of approximately 50º to the occlusal plane, purely as a result of the displacement of the maxilla in relation to the anterior cranial base. In addition, the molar cusp displaced downward and forward within the maxilla about 2 mm at an angle of 65º to the occlusal plane (mesial migration of the upper first molar). No cant on the palatal plane was observed.

Cervical Group

The observed changes were quite different than the control group. Anterior displacement of ANS was smaller than that in the control group, and the direction was more vertical in relation to the occlusal plane. At ANS, the mean orthopedic effect of treatment was a mean downward and backward displacement of about 2 mm. There was a slightly greater downward movement at ANS than at the molar apex, increasing the cant of the palatal plane about 1.75 degrees and rotating the molar cusp distally to the same extent. The orthodontic contribution added an additional 0.9 mm of distal displacement plus about 0.4 mm of downward displacement. The total effect was about 3 mm, just sufficient to correct the Class II relationship, with a relative extrusion of just less than 1 mm as compared to the control group. This value for extrusion was much smaller in absolute terms than many practitioners would have anticipated for this treatment modality.
High Pull Group

In the high pull group, the magnitude and type of displacements were strikingly different from those in the other samples. The observed orthopedic displacement included a small but real absolute reversal of the mesial movement of ANS which was seen in the control group and the other two treatment groups. Moreover, a reduction in the downward displacement of the posterior bone palate was observed as well. The combined effects produced an increase of about 2 degrees in the cant of the palatal plane and in distal crown tip of the molar. The magnitude of these rotational changes was very similar to those observed in the cervical group. 

The larger horizontal and vertical components in the high pull group were orthodontic in character. The molar apex roughly moved 2 mm distal to its starting position with an increase in distal crown tip of 4.5 degrees. The location of the molar cusp was about 3.5 mm distal to its original position but its vertical orientation was essentially unchanged with respect to the pretreatment occlusal plane. 

Modified Activator

The treatment effects observed in the intraoral group were smaller in magnitude than those observed for either of the extraoral appliances.

The change in cant of the palatal plane and the equivalent orthopedic position of the first molar were less than for either other treated groups, about 0.6 degree. The orthodontic rotation of the molar was also less than for either of the other treatment groups.

Downward orthopedic displacement of the maxilla was similar to that observed in the control group, both in the region of the molar and in the region of ANS. The orthodontic displacement in both the forward and downward direction was compared to the untreated control group and the combined orthodontic and orthopedic effect was smaller than that in any other sample.

In the vertical direction, there was a mean intrusion of about 0.7 mm, as compared to the untreated controls, all of it being of orthodontic origin. This finding is a quantitative representation of the success of this appliance in retarding the eruption of the upper molar.

Conclusions

1) The use of forces to retract the maxilla did produce substantial effects in the maxilla of both the orthopedic and orthodontic types.

2) The character and magnitudes of the effects observed were different when different appliance systems were used.

3) The high pull appliances, which was also the device with which the largest nominal force values were used, produced the largest changes of both the orthopedic and the orthodontic types in the region of the upper first molar and did so over the shortest mean treatment time. Distally directed tooth displacement in the molar region with this relatively high force system was more orthodontic than orthopedic in character.

4) The cervical appliance, a relatively low force system, showed a smaller orthodontic effect than orthopedic effect.

5) The intraoral appliance produced smaller magnitudes of tooth displacement than did either of the other two. Its total impact was about equally divided between orthodontic and orthopedic effects.

6) As far as changes in the orientation of the palate are concerned, all appliance systems tended to increase the cant of the palatal plane which, in the untreated control group, remained remarkably constant on average. This increase in the cant of the palatal plane was similar in magnitude in the two extraoral samples, measuring 1.73 degrees in the cervical group and 1.89 degrees in the high pull group. However, the mechanisms of change in the two groups appeared to be different. By this, they mean that in the cervical group the change appeared to involve relatively greater downward displacement in the anterior region of the palate while in the high pull group it appeared to be associated with relative upward movement in the posterior part of the palate. The increase in the cant of the palatal plane in the intraoral group was only about one third than in the extraoral groups.

Analysis of Statistical Power Between Significant Differences with p<0.05

Sample Size: Control=50, High Pull=53, Cervical Pull=74

This study presented appreciable statistical power (that indirectly means effect size and varies from 0.001 to 0.999:

High Pull Group

Cervical Pull Group

 

Based on these data, we conclude that the effect size for the orthopedic horizontal displacement, the high pull appliance was not different than the cervical pull. However, for orthopedic vertical displacements, the high pull appliance was significantly more efficient than the cervical pull, holding the downward maxillary growth.

Orthodontically, the high pull headgear effects (horizontal and vertical) were significantly bigger than the cervical pull headgear effects since the high pull appliance prevented upper molar eruption. However, the cervical pull did not extrude upper molars either, as has been commonly proposed.

Maxillary Class III

Before to offer a therapeutic approach it is necessary to assess the cause of the antero-posterior discrepancy. Guyer et al. (1986) studied 144 Class III children between 5 and 15 years of age. Following, we describe their findings:

Although there was not total agreement between the two measures used to indicate antero-posterior maxillary position (SNA and A to Nasion perpendicular), the Class III maxillae were generally retrusive. The effective length of the Class III maxillae (Co-A) was significantly shorter. Based on mean SNA value, the Class III sample exhibited retrusive maxillae, when compared to the Class I sample in all except the 11-13 years group. However, using the linear measure from A point to the Nasion perpendicular to relate the maxilla to the cranial base, the maxillae of the Class III sample were significantly more retrusive only in the 5-7 years group. The mean ANB angles were all negative in the Class III sample, whereas they averaged about +3° in the Class I sample. Similarly, the difference between the average effective mandibular length (Co-Gn) and average effective maxillary length (Co-A) was at least 6 mm greater in the Class III sample at all ages.

The upper anterior facial height measures (N-A) for the two samples were very similar in all age groups. The lower anterior face measures (ANS-Me and A-Gn) were different in several of the age groups. The ANS-Me value, although approximately 2 mm greater in the Class III sample at all ages, was greater by a statistically significant amount only in the 13-15 years group. On the other hand, the mean A-Gn value was significantly larger in the Class III sample in all age groups. These measures seen to indicate that the Class III sample differs most from the Class I sample in the lower part of the face.

The Class III sample generally showed maxillary incisors significantly more prominent than the Class I control group in all age groups. The vertical relation between the upper incisor and the maxilla was not significantly different between the two samples in any age group.

Face Mask

Animal models have demonstrated that the maxilla can be repositioned anteriorly by dissociation from circummaxillary sutures using protraction forces (Adams et al., 1972; Dellinger, 1973; Kambara, 1977; Nanda, 1978; Jackson et al., 1979).

However, there is not significant scientific evidence in the literature to support skeletal effects of the face mask in human beings. Ishii et al. (1987) studied the effects of face mask therapy on 63 skeletal Class III patients and compared the results to the craniofacial templates proposed by Popovich and Thompson (1977). The mean age of the patients at the beginning of the treatment was 10 years and 9 months. Forward movement of the maxilla and backward rotation of the mandible were characteristic treatment effects. In comparing the protraction anchored on the maxillary first molars and the maxillary first premolars, they concluded that the maxilla was displaced more anteriorly and rotated more upward and forward in the first molar protraction group. Therefore, the intraoral site of protraction should be selected by considering the pretreatment vertical dimensions and the amount of forward displacement of the maxilla required in the treatment of the individual patient. 

The difference observed in the antero-posterior position of the ANS point was statistically significant at the 0.01 level of confidence but it is important to stress that such data does not have statistical power and the skeletal changes were, at least in our view, doubtful.

* Significant at the 1% level of confidence, H=Horizontal; V=Vertical

Maxillary Transverse Discrepancy

Korn and Baumrind (1990) used metallic implants to study the transverse development of the human jaws longitudinally in frontal and lateral cephalograms of 31 normal subjects (11 males and 20 females), between 8.5 and 15.5 years of age. The data were derived from the data collection of the Section of Orthodontics of the University of California School of Dentistry. In the maxilla, the results were remarkably similar to those of Björk and Skieller (1974, 1977). Transverse widening was greater in the most posterior part of the palate. Moreover, there is no evidence to support the idea of a progressive slowing down in the rate of spontaneous maxillary widening during the time frame studied.

Transverse Maxillary and Mandibular Discrepancy

Cortella et al. (1997) studied the maxilla and mandibular transverse dimensions in a sample of 35 normal subjects, 18 females and 17 males from the Bolton-Brush Growth Center. The subjects were selected based upon the availability of quality records taken annually. The data were mixed-longitudinal between 5 and 18 years of age. The jugale (at the jugal process of the maxilla) and antegonion (at the antegonial notch of the mandible) were selected as landmarks to be studied in postero-anterior cephalograms. Positive gender differences were found. The mandibular width in boys continues to grow beyond the spurt period, in a pattern similar to developments in facial length and height, which did not happen for girls. Further, for boys between ages 6 to 18 years, there was a compensatory mechanism that allowed preservation of normal occlusion (no crossbite) between the posterior teeth since the increase in maxillary intermolar width follows the increase in the mandibular width. This growth represented about 52% of the interzygomatic distance but the increase in mandibular intermolar width represented just 17 % of the intergonial distance increase for the mandible.

Treatment of Transverse Maxillary Deficiency

Transverse maxillary deficiencies are corrected by maxillary expansion, either rapid or slow, surgically assisted or not. Karl-Nieke (1996) conducted a long-term follow-up study of 226 orthodontically treated patients to evaluate post-retention (at least 10 years) changes in arch width dimension. At the follow-up exam, all patients were adults. They found that arch width relapse (decrease) occurred most frequently in the maxillary intermolar width. Anterior constriction of 2 mm or more was found in 13.8% of the sample. Posterior arch width relapse (decrease of 2.5 mm or more) was found in 25.8% of the upper arches. Intermolar expansion of 4 mm or more and intercanine expansion of 2.5 mm or more was found to be correlated with arch width relapse.

Bishara and Staley (1987) and Bell and Epker (1976) stated that in young patients, who still exhibit a patent midpalatal suture, maxillary expansion can be gained through the use of orthodontic appliances alone, namely through rapid or slow palatal expansion. It is commonly believed that by the late teens, after the maxillary articulation become more complex and fuse, orthodontic maxillary expansion becomes a less feasible option.

Betts et al. (1995) discuss orthopedic maxillary expansion in detail and suggested that it has been successful primarily in children prior to sutural closure as well. Further, they suggested that Rapid Palatal Expansion should be chosen over slow expansion to maximize skeletal changes over dental changes. In contrary, advocates of slow expansion, which takes from 2 to 6 months, believe that if the expansion is slow, the risk of relapse is minimize (Firacelli et al., 1978; Bell, 1982).

Discussion

The maxillary growth is clearly downward and forward (see figures pages 12, 14 and 15). Transversely, it is more expressive at the posterior part of the maxilla than at the anterior part (figures at page 13). One of the most impressive areas of growth is the posterior part of the maxilla, which demonstrates a lot of backward growth (page 17). In terms of rotation, the maxilla, as a facial bone, truly grows downward more in the posterior part than the anterior part (page 18), what is compensated by its remodeling (page 19). The backward apparent rotation (page 20) is also compensated by the differential dental eruption of the molars and the incisors (molars erupt more than incisors, page 22), reducing the inclination of the occlusal plane (page 23). Dental eruption is so significant that can reverse the backward rotation of the maxilla to a significant decrease of the inclination of the occlusal plane or, in different words, to a forward rotation of the occlusal plane.

From the elegant work of Björk (1972, 1974, 1976, 1977) and Solow and Iseri (1996), we learned that the maxillary molars move downward and backward significantly in normal patients. When a skeletal Class II malocclusion is due to a maxillary protrusion, it is reasonable to assume that the entire maxilla moves forward, since we do not see a reduction in the maxillary arch depth, which would support a lack of backward growth at the posterior part of the maxilla but a normal positioning of the maxillary incisors.

Once we know how the maxillary complex grows, we can study the effects of the proposed therapeutic approaches. Before to do that, we would like to discuss a basic concept, many times misunderstood: the difference between skeletal and dental changes. It is very common dento-alveolar changes be described along with basal bone changes, without any distinction between both processes. First, because there is no precise anatomical delimitation between the alveolar bone and the basal bone. However, when changes are not "dental", does not necessarily mean that they are purely skeletal. In other words, we can have changes in the dento-alveolar complex and not in the skeleton, strictly speaking. In our view, the authors should clearly describe that alterations are not just dental, however, not necessarily the facial skeleton has been changed. The feasibility of changes to occur at the dento-alveolar bone level is, without any question, bigger than to occur at any maxillary basal bone part. Our recommendation for the readers is to explore the articles enough to understand if there was a truly skeletal alteration, or the changes have mainly involved the dento-alveolar bone.

Headgear appliance effects were nicely described by Baumrind et al. (1983). According to these authors, we can orthodontically modify the position of the upper molars and orthopedically the position of the maxilla (our note: more precisely, its dento-alveolar portion). High pull appliances, which used the largest force values, produced the largest orthodontic and orthopedic changes, over the shortest period. However, cervical pull appliances also produced significant orthopedic and orthodontic effects, especially horizontally. Vertically, if the appliance did not work as the high pull did, at least, did not extrude molars either, a common concept. It sounds reasonable to us to study the facial type as well as the amount of change targeted before to make any decision. The fact that the cervical pull appliance did not work as the high pull appliance did, not necessarily mean that it can not be used in the most of the cases, nicely correcting Class II malocclusions. High pull appliances may be an overdose in many cases.

The change in the inclination of the occlusal plane was similar in both groups, although based on different methods: the high pull group intrude molars, the cervical pull extrude incisors. The assessment of the patient's "gum line" is an important factor to be considered before to make any decision.

For the Class III cases, the face mask is efficient for the correction of the malocclusion, however, it has not been well demonstrated that the face mask can alter the skeletal position of the maxilla in human beings (i.e. maxillary basal bone; see pages 32 and 33). It has not been negate either. The explanation is based on the fact that the face mask is mainly anchored on teeth and when the objective of the treatment is achieved (Class I, molars and canines), the treatment is suspended. In contrary, when normal animals are treated to a malocclusion, there are not dental goals and the appliance is applied to achieve its best performance. 

The treatment of transverse maxillary discrepancies by expansion is clearly efficient, however, relapse is an important issue to be considered, especially in the posterior part of the maxillary arch (intermolar width). It is also clear from the literature that expansion in young patients is expected to be more stable than in adult patients.

Further studies are necessary, especially to clarify the skeletal effects of the face mask in human beings. No question that sample size is an important issue to be considered, since Class III malocclusions are, by far, the least prevalent type of malocclusion.

Conclusion

In conclusion, maxillary growth can be modified in the three dimensions of space. When the growth potential of each patient is well understood, the appliances' effects masterized by the clinician and the limitations respected, results are very predictable.

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