Class II malocclusion: The aftermath of a “perfect storm”

Class II malocclusion: The aftermath of a “perfect storm” Alexandros K. Tsourakis, DDS, MS, and Lysle E. Johnston Jr, DDS, MS, PhD, FDS RCS(E) To characterize the relative contribution of skeletal growth and tooth movement to occlusal development. Longitudinal cephalograms were obtained for 39 untreated subjects between 5 and 16 years of age. The sample was divided into three terminal-plane groups: mesial step, flush terminal plane, and distal step. Based on their final occlusion, the flush group (24 of the 39 patients) was sub-divided into three sub-groups: Class I, end-toend, and Class II. Regional superimposition was used to measure yearly increments of skeletal and dental change. The mesial- and distal-step groups tended to maintain their initial Class I or Class II molar relationships. In the three flush-terminal-plane sub-groups, occlusal progression could be explained by neither an early nor a late mesial shift, both of which featured more upper molar movement than lower. Instead, the groups differed in terms of the timing of the mandibular excess and mesial movement of the upper molars. Mandibular excess and mesial movement of the maxillary molars seems to be the most significant determinants of occlusal development in flush-terminal-plane subjects. The present data argue that the strategy of holding lower leeway space and “distalizing” the upper molars is a rational early-treatment strategy. (Semin Orthod 2014; 20:59–73.) & 2014 Elsevier Inc. All rights reserved.

I

n orthodontics, controversies tend to be nearly immortal, none more so than the contumely surrounding the treatment of Class II malocclusion. Is it to be early or late? Fixed or functional? Extraction or nonextraction? Unfortunately, at least for the Class II patient, if not for the clinician, rational decision-making is distorted by popular, clinically seductive myths that are largely innocent of reason or proof of efficacy. Magic is for entertainment in night clubs; belief in magic is for children, not those who would treat them. The etiology of malocclusion is said to be “multifactorial.” That truism having been said, we would suggest that the persistence of cusps divides a continuous, ratio-scale mixture of jaw

Saint Louis University, St. Louis, MO; The University of Michigan, Ann Arbor, MI. Address correspondence to Lysle E. Johnston Jr, DDS, MS, PhD, FDS RCS(E), Box 595, Eastport, MI 49627. E-mail: lejjr@umich. edu & 2014 Elsevier Inc. All rights reserved. 1073-8746/12/1801-$30.00/0 http://dx.doi.org/10.1053/j.sodo.2013.12.006

growth and tooth movement into an artificial trichotomy—Normal/Class I, Class II, and Class III. An important determinant of the final outcome of this interaction is the so-called “terminal-plane relationship,” the antero-posterior relationship between the distal surfaces of the maxillary and mandibular second deciduous molars. In the case of “distal” or “mesial” steps, the first permanent molars are guided immediately into Class II or Normal/Class I occlusions, respectively.1 (In the present communication, the term “Class I” will be used to denote the molar relationship common to Normal occlusions and Class I malocclusions.) Further, the phenomenon of “dentoalveolar compensation” (a “scientific” term for the fact that intercuspated teeth apparently move in response to differential jaw growth) is said to ensure that, once formed, a molar relationship tends to be stable.2,3 Accordingly, in orthodontics, the study of occlusal development tends to emphasize the so-called molar “flush” terminal plane, a common relationship (Table 1) that guides the first molars into a presumably unstable end-to-end occlusion. The ultimate resolution of this

Seminars in Orthodontics, Vol 20, No 1 (March), 2014: pp 59–73

59

60

Tsourakis and Johnston Jr

Table 1. Molar Relationship: Distribution by Stage of Occlusal Development According to Previous Studies Reference(s)

N

Deciduous Dentition

Mixed Dentition

Permanent Dentition

–

–

–

–

–

–

59% Class I 24% Cusp-to-Cusp 15% Class II 1% Class III 49% Cusp-to-Cusp 27% Class I 23% Class II 1% Class III –

–

Cross-sectional data Baume4–6

30

Clinch7

61

Bonnar8

8

86% 14% 43% 26% 31% 63%

Flush terminal plane Mesial step Flush terminal plane Mesial step Distal step Mesial step Longitudinal data

Carlson and Meredith9

109

55% Mesial step 32% Flush terminal plane 13% Distal step

Arya et al.10

118

38% Flush terminal plane 48% Mesial step 14% Distal step

Bishara et al.11

121

62% Mesial step 29% Flush terminal plane 9% Distal step

relationship—the formation of a Class I, II, or III occlusion—is a key clinical consideration and, as such, has been the subject of extensive research. In general, three phenomena are invoked: an “early mesial shift,” a “late mesial shift,” and excess mandibular growth. According to Baume,4–6 if there is spacing in the deciduous dentition, when the permanent first molars erupt, there is a mesial “shift” of the lower deciduous molars into the primate spaces of the deciduous dentition. Specifically, Baume found that the antero-posterior distance between the distal surface of the maxillary and mandibular deciduous canines does not change, whereas the mandibular deciduous molars move mesially relative to the maxillary with the closure of the primate spaces. He concluded, therefore, that the lower deciduous molars drift forward with the eruption of the permanent first molars so that there can be a transition from a flush terminal plane during the deciduous dentition to a mesial step during the mixed dentition. Many, however, disagree. Clinch,7 for example, argued, again from the study of serial dental casts, that the replacement of the mandibular deciduous incisors by their permanent successors could result in a distal drifting of the mandibular deciduous canines into the lower primate space, and excess mandibular growth could explain the mesial shift of the lower deciduous and first permanent molars. Clearly,

59% Class I 38% Class II 2% Class III 62% Class I 34% Class II 4% Class III

the two interpretations cannot be resolved from a study of dental casts. Today, it is more common to emphasize the role of “leeway space,” the difference in the mesio-distal size of the primary buccal segments and their permanent successors (perhaps a bit less than “E-space”), in the transition to a Class I molar relationship in flushterminal-plane subjects. According to Maher12 and Moyers,1 after incisor alignment, the leeway space on each side is 1.20 mm in the maxilla and 2.16 mm in the mandible. This excess space is thought to allow the mandibular permanent first molars to drift forward more than the maxillary first molars when the deciduous second molars exfoliate. Augmented by the normal pattern of growth (mandible 4 maxilla), this differential movement, the socalled “late mesial shift,” is thought by many (e.g., Baume,4–6 Clinch,7 and Moorrees et al.13) to be the mechanism by which an end-to-end molar relationship in the mixed dentition can change to a Class I in the permanent dentition. Cephalometric studies by Murray,14 Paulsen,15 White,16 and Kim et al.,17 however, seem to contradict the general concept of a late mesial shift. According to these workers, there is a mesial movement of the permanent first molars into the leeway space, but there seems to be no relationship to the difference between the leeway spaces in the maxilla and the mandible and final molar relationship. Further, cephalometric

61

Class II malocclusion

radiographs permitted them to differentiate between skeletal growth and tooth movement. Their studies seem to show that the actual mesial movement of the maxillary first molar during the period between the mixed and the permanent dentitions often exceeds that of the mandibular first permanent molar relative to mandibular basal bone. Inferences from the various types of research are much like the blind men0 s description of an elephant. In general, studies of dental casts permit an assessment of tooth movement; however, they say little about the role of skeletal growth and tooth movement relative to basal bone. By way of contrast, cephalometric studies can provide additional information on the effect of differential growth and the details of tooth movement. The findings from a variety of studies/methodologies are summarized in Table 2. In order to detect an influence of jaw growth and tooth movement on occlusal development, a number of studies (Table 2) have used dental casts, cephalometric radiographs, or a combination of the two. Dental casts are perhaps the best way to depict changes in occlusal relationship

over time; however, when it comes to explaining the etiology of occlusal change, casts cannot differentiate between skeletal and dental components of the change. Skeletal change can be mis-perceived as tooth movement and vice versa. Cephalometric studies, on the other hand, offers, at least in potential, the opportunity to examine and quantify skeletal and dental components with the aid of structural regional superimposition3,21,22—superimposition based on structures that Björk has shown to be stable. This approach would seem to provide the necessary information to differentiate between tooth movement and skeletal growth as determinants of occlusal changes. Based on this sort of cephalometric data, skeletal growth seems to be the most important factor in the occlusal adjustment at the transitional phases of occlusal development. Just as with blind men describing an elephant, each study no doubt depicts a portion of the truth. Integration and interpretation, however, is difficult. A major problem is the increasingly common observation that the details and significance of the late mesial shift are subject to

Table 2. Details of Occlusal Development as a Function of Methodology N

Findings

Casts Baume4–6

60

Clinch7

61

In spaced deciduous dentitions, Class I molar adjustment takes place with an early mesial shift. In closed deciduous dentitions, adjustment occurs by way of a late(er) mesial shift Mandibular growth plays a role in Class I adjustment from deciduous to mixed dentition. No closing of primate spaces was observed Leeway space is not a critical factor for occlusal adjustment in the transition between the mixed and permanent dentition Forward movement of the mandibular dentition can occur during the deciduous dentition and at the transition between the deciduous and mixed dentition Leeway space might be a main contributor for molar adjustment toward Class I when the Wits A–B differential does not change The predominant factor in molar adjustment is excess mandibular growth Class I molar relationship is achieved from an initial cusp-to-cusp relation by greater mesial shifting of the mandibular molars

Reference(s)

12

43

Bonnar8

58

Maher

18

Lamont

96

Micklow19 Moorrees et al.13

10 94

Cephalometric radiographs Murray14

20

White16

34

Kim et al.17

40

Casts and cephalometric radiographs 60 Brin et al.20 15

Paulsen

52

Bishara et al.11

121

The principal factor in the occlusal adjustment of the buccal segments during the mixed dentition is the relative growth/displacement of maxillary and mandibular basal bone The normal pattern of facial growth (mandible more than maxilla) is the decisive factor in molar adjustment from the mixed to the permanent dentition Skeletal growth difference between the jaws influenced the change in molar relationship Differential growth of the maxilla and the mandible plays a significant role in occlusal relation if dentoalveolar compensation cannot compensate for the differential More mandibular growth could contribute to a Class I relation. Change in molar occlusion during the process of growth and development is a multifactorial phenomenon No correlation was found between molar relationship and the difference in the leeway space between the maxillary and mandibular arches

62

Tsourakis and Johnston Jr

question. Firstly, a leeway space differential of a millimeter or so cannot account for a half-cusp change in the molar occlusion. More significantly, it is probable that the mesial drift of the upper molars commonly exceeds that of the lower. For example, Kim et al.17 conducted a serial cephalometric study of occlusal development in which they divided their sample into three groups based on the long-term pattern of growth. In the group that featured an excess of mandibular growth relative to maxilla (Group A), the mesial drift of the upper molars was much greater than that of the lowers. Given that the growth in this group is consistent with the normal pattern, one is forced to the preliminary conclusion that the leeway space may not be a key to molar occlusal development. To characterize the key events in the transition from a flush terminal plane, White16 examined a sample of 34 Bolton Study subjects whose selection criterion was an end-to-end molar occlusion in the early mixed dentition (upper and lower first molar mesial contact points within 1 mm, parallel to the occlusal plane). He followed them up into the permanent dentition and used regional superimposition to study jaw growth and tooth movement parallel to an averaged functional occlusal plane. He found that the subjects who developed a Class I occlusion had a significantly greater mandibular excess than those who did not. Further, in all groups, the upper molars came forward on basal bone about twice as far as did the lowers. A mandibular growth deficit might seem to support McNamara0 s23 contention that Class II is at bottom a mandibular problem whose rational correction would be to grow more mandible; however, the problem is considerably more complex. Donaghey24 used regional cephalometric superimposition to the compare growth seen in Class II and Class I subjects. In young Class II subjects, his findings extend those of White: Class II subjects from ages 9 to 11 years continue to grow less well; however, when he followed up the same subjects for an additional 2 years, the growth deficit was no longer present. A tentative interpretation of these findings might be that Class II patients were unlucky in that they did not get the right growth at the right time. Although Class I and Class II subjects show about the same overall pattern of growth, the gain is not smooth

and continuous (Lande25 and Harvold26). If an occlusion goes to Class II, the probably favorable pattern of subsequent growth (mandible 4 maxilla) would have no impact, given that dentoalveolar compensation tends to maintain an intercuspated occlusion in the face of a wide range of maxilla–mandibular growth differentials. The idea that many Class II patients are merely unlucky victims of bad timing and the stability of an intercuspated, dentition, although interesting, seems to require a more detailed exploration. To date, much of the disagreement in the literature is methods-related. Accordingly, it would seem that a study of occlusal development must be capable of measuring and integrating both jaw growth (i.e., bodily displacement) and tooth movement (relative to basal bone). Serial cephalograms, therefore, would seem to be an appropriate vehicle for such an investigation. The key to the present communication, however, is the assumption that increments of change— both skeletal and dental—must be measured in an integrated fashion. To this end, we argue that the occlusal plane is, both literally and figuratively, the “bottom line” at which all components of occlusal change are summed. As such, an analysis based on this frame of reference should permit a relatively un-confounded assessment of the relative contributions from growth and tooth movement to molar occlusal development.

Materials and methods The sample consisted of 39 healthy (as certified by their family physician), untreated subjects (17 males and 22 females) from the Bolton-Brush Growth Study Center, Case Western Reserve University, Cleveland, OH. Our goal was to document their terminal-plane relationship in the deciduous dentition and then to follow-up the course of their first-molar adjustment on into the permanent dentition. Essentially, the only inclusion criteria were that a series be available at the time of our visits to the Center and that it has annual cephalometric radiographs from ages 5 or 6 years to 15 or 16 years. Because of this variation in the ages at start and finish, statistics for ages 5 and 16 years are based on fewer than 39 subjects. Although it was largely a sample of convenience, it should be noted that some attempt was made to over-sample flush-terminal-plane

Class II malocclusion

subjects. As a result, the mixture of occlusions must be expected to differ somewhat from the historical summary of Table 1. Subjects were excluded based on premature loss or absence of teeth, evidence of primary second molar ankylosis, extensive restorations (subjects with Class II restorations would be included, but not subjects with full-crown restorations in deciduous or permanent teeth), evidence of orthodontic treatment/appliances, and inadequate film quality (in the opinion of the junior author, who executed the tracings). Whenever a marked, apparently temporary anterior displacement of the mandible was seen in one film of a series, it was assumed to have been the result of a mandibular functional shift; the affected film was excluded from the series, and the resulting 2-year increments of change were divided between the intervals adjacent to the missing film. The final sample was divided into three groups according to the initial terminal-plane relationship at age 5 or 6 years: mesial step, flush terminal plane, and distal step. The initial terminal-plane relationship was defined by the distance between perpendiculars erected through the distal surfaces of the second primary molars from the socalled “functional occlusal plane,” a best-fit line drawn by inspection through the buccal-segment occlusion (Jenkins27). A subject was assigned, somewhat arbitrarily, to the mesial-step terminalplane group if the lower primary second molar was Z0.5 mm mesial to the upper primary second molar (N ¼ 6) and to the distal-step terminalplane group if it was Z0.5 mm distal to the upper (N ¼ 9). The remaining subjects between these two boundaries were assigned to the flushterminal-plane group (N ¼ 24). The flush-terminal-plane group was divided further into three sub-groups according to their occlusion at the end of the series (age 15 or 16 years): Class I, end-to-end, and Class II. The molar relationship was measured cephalometrically as the distance parallel to the functional occlusal plane of perpendiculars erected through the mesial of the upper and lower first permanent molars. Based on a pilot study of posttreatment cephalograms, subjects from the flushterminal-plane group were assigned to the Class I sub-group if their molar relation at age 15 years was 4 þ1 mm. Molar relations between 1 and þ1 were assigned to the end-to-end subgroup and o 1 mm to the Class II subgroup.

63

All radiographs in a given series were traced at a single sitting. Bilateral structures were averaged. To maximize consistency in the interpretation of the structural details on which regional superimposition would be based, the tracings were executed in adjacent pairs, starting in the center of the series and working forward and backward pair wise to the beginning and end. Although there is considerable change over a decade, adjacent films are usually similar enough to permit coordinated tracing and regional superimposition based on Björk0 s putatively stable structural details in cranial base, midface, and mandible. To measure maxillary and mandibular bodily translation relative to cranial base and maxillary and mandibular tooth movement relative to basal bone, the tracing pairs (ages 5 and 6 years, 6 and 7 years, 7 and 8 years, etc.) were superimposed regionally according to the socalled “Pitchfork Analysis” (Fig. 1; Johnston21; also see Duterloo and Planché22). Each superimposition was preserved by way of cranial base, maxillary, and mandibular fiducial lines transferred throughout the series. The upper and lower molar movement relative to basal bone up to 2 years after the eruption of the first permanent molars served as an estimate of the “early” mesial shift, and the movement after the loss of the second deciduous molars (on average, 2 years,

Figure 1. Cephalometric regional superimposition (“Pitchfork Analysis 21”)—maxillary displacement relative to cranial base and mandibular excess. This superimposition also is used to measure upper tooth movement. Lower tooth movement is measured from a separate mandibular superimposition: mean functional occlusal plane orientation and D-point registration (by perpendiculars dropped from the occlusal plane).

64

Tsourakis and Johnston Jr

7 months), an estimate of the “late” mesial shift. In addition to the increments of change measured from one year to the next, maxillary and mandibular arch depth (central incisor tip to first-molar mesial contact) and molar relationship (upper molar mesial contact to lower molar mesial contact) were measured on each tracing. All measurements of yearly change were executed parallel to a common mean functional occlusal plane constructed from two films in the center of the series and passed forward and backward by way of maxillary regional superimposition. For the “pitchfork” data, the increments of change (measured to the nearest 0.1 mm with digital calipers) were given signs according to the nature of their contribution to the achievement of a Class I occlusion: positive if the increment helped and negative if it hurt. Because the measurements were executed along a common occlusal plane, their algebraic sum was equal to the change in molar relationship and the various components of the sum, the source/cause of the change. Among- and between-groups differences were analyzed for the three terminal-plane groups and separately in the three flush-terminal-plane subgroups. Analysis was performed with PASW Statistics 18.00 (IBM SPSS, Armonk, NY). Normality was tested with Q–Q plots. Given normal distributions, analysis of variance (ANOVA) and Tukey0 s “honestly significant difference” were used to test for among- and between-groups differences in the various cumulative increments. In the case of non-normal distributions, the Kruskal–Wallace and Mann–Whitney tests were used instead. Additionally, multiple linear correlation was used to examine the relationship between the various increments of change and the total change in molar relationship between 5 and 16 years. An additional line of analysis was developed during data collection and thus must be segregated from procedures planned in advance. When mandibular excess was graphed against time, it was low from 5 to 11 years in the subjects who went to Class II; however, from 11 to 16 years, it was high. Overall differences among- and between- the sub-groups were tested separately in these two intervals. Further, among-groups differences in the rate of change were tested by Rao’s28 method of comparing regression slopes for mandibular excess plotted against time,

Table 3. Error Estimates for Increments of Change Increment

Error SD

R

Maxillary displacement (mm) Mandibular displacement (mm) Mandibular excess (mm) Upper 6 movement(U6) (mm) Lower 6 movement(L6) (mm) Maxillary basal rotation (1) Mandibular basal rotation (1)

1.12 1.21 0.56 0.25 0.34 0.42 0.34

0.80 0.80 0.85 0.90 0.90 0.95 0.95

again by way of parametric and nonparametric analyses. Approximately 2 months after the first tracing, two series were chosen at random and completely re-done—tracing, superimposition, and measurement of change. Error standard deviations29 and indices of reliability30 (the correlation between replicate measurements) were calculated for the 20 double-determinations of each of the seven increments of change. The results are summarized in Table 3.

Results The distribution of molar relationships by initial terminal-plane classification during the stages of occlusal development studied here—early mixed, late mixed, and early permanent—are summarized in Table 4. The progression of the three terminal-plane groups over time is depicted in the averages of Fig. 2. Arranged by final molar relationship, the cumulative skeletal and dental changes seen in entire sample and in the flushterminal-plane sub-group are presented in Table 4. Molar Relationship by Initial Terminal-Plane Classification and Stage of Occlusal Development Stage of Occlusal Development

Initial mesial step (N ¼ 6) Early mixed dentition Before E lost Final occlusion

Molar Relationship (N ¼ 39) Class I

End-to-End

Class II

4 5 5

2 1 1

– – –

15 11 6

4 4 5

1 1 2

8 8 7

Initial flush terminal plane (N ¼ 24) Early mixed dentition 5 Before E lost 9 Final occlusion 13 Initial distal step (N ¼ 9) Early mixed dentition Before E lost Final occlusion

– – –

65

Class II malocclusion

Figure 2. Occlusal progression over time: yearly averages for all groups and sub-groups from ages 5 to 16 years. Note the stability of the mesial- and distal-step groups.

Tables 5 and 6. Based on fiducial lines linked to Björk0 s stable structures, relative to cranial base, there was a mean forward maxillary rotation of 2.51 and a mandibular rotation of 10.11. There were no statistically significant differences among terminal-plane groups with respect to the cumulative change in the dental and skeletal variables measured here. Note that the mesialstep subjects tended to develop stable Class I occlusions and distal-step subjects, stable Class II malocclusions. The cumulative skeletal and dental changes seen in these two groups are charted in Figs. 3 and 4. Flush terminal planes, in contrast, were “balanced on a knife-edge” and thus were analyzed in greater detail. The flush-terminal-plane subjects developed one of three “final” (at age 15 or 16 years) molar relationships; cumulative change in these subjects is summarized in Figs. 5–7. Again, there were no statistically significant among-groups differences in overall skeletal and dental change. For mandibular excess, however, there was a significant interaction between growth and time. Specifically, although at the end of the study, there was no cumulative among-groups difference in mandibular excess, it may be seen

in Figs. 7 and 8 that there was a marked delay in the Class II group. The among-groups comparison of regression slopes for mandibular excess within the flushterminal-plane category showed that, from 5 to 11 years, those with a Class II outcome had a lower rate than the Class I sub-group (ANOVA and Tukey post hoc), whereas, from 11 to 16 years, the Class II slopes were significantly higher (Kruskal–Wallis ANOVA by ranks and Mann– Whitney U.). The effect of this difference between early and late growth rates can be seen in the cumulative curves for mandibular excess depicted in Fig. 8. It also seems to be reflected in the pattern of upper molar movement. Yearly one-way analyses of variance showed that, for ages 7–8 years and 11–15 years, the mesial movement of the upper molars was consistently greater in the “flush” subjects whose occlusions went to Class II. As depicted in Fig. 9, this effect coincides roughly with timing of the mandibular “catch-up” growth. Further, multiple regression showed that variation in the fate of the flushterminal-plane subjects could be “explained” (R ¼ 0.859) by mandibular excess and upper molar mesial drift. Lower molar movement was

Table 5. All Subjects: Cumulative Skeletal and Dental Change From 5 to 16 Years (mm) Measure Final Occlusion

N

Maxilla

Mandible

Mandibular Excess

U6

L6

U Depth

L Depth

Class I End-to-end Class II

18 8 13

5.41 7.39 7.92

13.25 13.17 14.41

7.84 5.78 6.49

7.36 5.80 6.00

0.26 1.14 0.10

0.56 1.58 0.91

1.91 1.16 0.72

66

Tsourakis and Johnston Jr

Table 6. Initial Flush-Terminal-Plane Subjects: Cumulative Skeletal and Dental Change From 5 to 16 Years (mm) Measure Final Occlusion

N

Maxilla

Mandible

Mand. Excess

U6

L6

U Depth

L Depth

Class I End-to-end Class II

10 9 5

7.68 7.02 7.08

14.70 12.38 11.75

7.02 5.36 4.67

5.64 5.46 6.98

0.64 1.44 1.59

0.38 2.82 1.27

1.62 1.48 0.96

not a significant factor. Estimates of “early” and “late” upper and lower molar “shifts” are presented in Table 6. Note that in all instances, the average mesial drift of the upper molars was greater than that of the lowers.

Discussion Firstly, it is important to emphasize that, although more than 400 films were traced and superimposed, the present study details the mechanics of occlusal development in 39 subjects divided into five sub-groups ranging in size from 5 to 13. Of these, perhaps the most interesting— the flush-terminal-plane subjects who progressed to Class II—is the smallest. Accordingly, it goes without saying that the findings may or may not mirror the expected pattern of occlusal development in other subjects. At the very least, the present describes events in subjects who were healthy, untreated, and who, on average, showed a “normal” pattern of “counter-clockwise” growth rotation and mandibular excess. An analysis of the way these occlusions developed thus should constitute a useful heuristic in a consideration of

the prevention and treatment of Class II malocclusions. It is a discussion that reduces to an examination of the relationship between upper molar movement and mandibular growth. In the present study, the mesial- and distal-step terminal planes produced stable, intercuspated Class I and II occlusions in which cumulative upper molar mesial movement relative to basal bone seemed an exact mirror image of the mandibular excess relative to maxillary basal bone. To a first approximation, this mesial movement of the upper molars can be interpreted as a dentoalveolar compensation for the excess growth of the mandible.2,3 It must be noted, however, that a single maxillary superimposition was used to measure both upper molar movement and mandibular excess. As a result, inevitable superimposition errors would be shared and thus would elevate, but probably not be completely responsible for, the significant correlation between the two processes. As shown in Table 3, the error standard deviations for mandibular excess and upper molar movement were 0.56 and 0.25 mm, respectively. These errors are small, but so too are the increments of change with

Figure 3. Cumulative mandibular excess and U6 and L6 mesial movement in mesial-step subjects, ages 5–16 years. Note the lack of a L6 mesial shift (actually some distal uprighting) and the apparent balance between mandibular excess and U6 mesial drift.

Class II malocclusion

67

Figure 4. Cumulative mandibular excess and U6 and L6 mesial movement in distal-step subjects from ages 5 to 16 years. On average, there may be a slight, temporary mesial shift of about 1 mm at age 10 years.

which they are associated. In any event, the present data are consistent with that of other reports (e.g., Björk and Skieller3 and Lundström and McWilliam31) on naturally occurring compensations in Class I subjects. Together, they support the concept of some sort of physical relationship between differential growth and molar movement and seem to provide an adequate explanation for the stability of the

mesial- and distal-step occlusions. In passing, it is perhaps useful to recall Moyers0 1 view of the effect of retained cusps on the occlusion (p. 173): Among people whose diet includes coarse, rough food…the occlusal surfaces of the primary teeth wear to a great extent. This removal of cuspal interferences permits the mandible, which is growing more at this time

Figure 5. Cumulative mandibular excess and U6 and L6 mesial movement in flush-terminal-plane subjects who progressed to a Class I molar relationship. On average, there was no obvious late mesial shift until about age 12 years, and then only about 1 mm.

68

Tsourakis and Johnston Jr

Figure 6. Cumulative mandibular excess and U6 and L6 mesial movement in flush-terminal-plane subjects who went on to an end-to-end molar relationship. In this group, there was on average a 1–2 mm late mesial shift spread throughout the span of the study, but most pronounced for ages 11–13 years.

than the maxilla, to assume a forward position more easily. Under these circumstances, the result for Greek mountain children at age 5 or 6 often is more an edge-to-edge incisal relationship and a distinct mesial step terminally. Moyers0 observations can be seen as a version of the “trapped mandible” hypothesis: the lower jaw often yearns to be free (e.g., Class II, Div. 2

malocclusions) and will jump forward if given a chance. Given a lack of evidence that this phenomenon actually occurs, the mesial-step occlusion of the Greek mountain children is perhaps more easily seen as the result of a lack of maxillary dentoalveolar compensation—the normal pattern of growth would change the terminal-plane relationship by advancing the mandible while the absence of intercuspation would allow the upper buccal segments to be left

Figure 7. Cumulative mandibular excess and U6 and L6 mesial movement in flush-terminal-plane subjects who settled into a molar Class II relationship. Note the mandibular growth deficit relative to maxilla up to age 12 years, while the upper molars continue to drift forward into a Class II relationship.

Class II malocclusion

69

Figure 8. Cumulative increments of mandibular excess from age 5 to 16 years in the three sub-groups of the flushterminal-plane group. Note the early (8–12 years) deficit in the mandibular excess for the Class II subgroup. This deficit is followed by what appears to be “catch-up” growth from ages 12 to 16 years.

behind. Flush-terminal-plane subjects—an oversampled 62% of the present sample (about twice the weighted average of the prevalence in the longitudinal data presented in Table 1)—lack a molar intercuspation and thus may, in effect, be similar to the flat-plane occlusions of Moyers0 Greek wartime sample and thus may shed light on Moyers0 conjecture. Starting from a presumably unstable end-toend relationship, a differential mesial shift of the lower molars—“early” or “late”—is a commonly invoked mechanism of occlusal adjustment. Unfortunately, the lack of study models made it impossible to distinguish the spaced occlusions in

which an early shift might occur from the unspaced occlusions in which it would not. Be that as it may, the present averages argue against a lower shift being a major factor in occlusal development: the average upper mesial shift was greater than the lower, independent of initial terminal-plane relationship or timing (Table 7). Further, it should be noted that, on average, only about half of the overall reduction in lower arch depth (1.4 mm) can be attributed to a late mesial shift of the lower first molars. Indeed, in our sample, the lower molars often became more upright, a maneuver that moved their crowns distally and was perhaps a dentoalveolar

Figure 9. Cumulative maxillary permanent first-molar mesial movement in the three sub-groups of the flushterminal-plane group. Note the continuous mesial drift of the upper molars, with the greatest mesial drift seen in the Class II group from ages 13 to 16 years, roughly coinciding with the mandibular “catch-up” growth depicted in Fig. 7.

70

Tsourakis and Johnston Jr

Table 7. First-Molar “Mesial Shift” By Initial Terminal-Plane Relationship Mesial Shift Early

Late

Initial Terminal Plane

N

U6

L6

U6

L6

Mesial step Flush Distal step

6 24 9

1.76 0.91 0.96

0.28 0.27 0.59

2.69 2.18 2.96

1.14 0.68 0.14

compensation for a favorable pattern of mandibular growth. Consistent with this interpretation, it should be noted that this distal shift was seen most often in those subjects who transitioned successfully to a Class I occlusion (Figs. 3 and 5). Conversely, the most pronounced approximation of a late mesial shift was seen in the subjects whose occlusion either stayed end-on or went on to a Class II malocclusion (Figs. 6 and 7). Their occlusal progression features a number of interesting, potentially significant events. Firstly, for the flush-to-Class II subjects, it is clear that there was an early lag in the extent to which the mandible advanced relative to the midface. This deficit, however, was not mirrored in the upper arch: the upper molars came forward about the same amount as was seen in the other flush sub-groups. This independence implies that mesial upper molar movement is not always/entirely a compensation for excess mandibular growth. Even without a mandibular excess, the molars came forward. There must, therefore, be additional mechanisms in play. For example, Southard et al.32,33 have measured a mesial component of occlusal force that could explain the forward drift of the upper molars (not to mention lower incisor irregularity). Perhaps excess mandibular growth can add to—but not subtract from—the effects of this mesial component. Given a mandibular excess, maxillary dentoalveolar compensation will cancel the effect; absence of an excess, the upper molars will come by an amount determined by other factors. The finding that a mandibular growth deficit contributes to the formation of a Class II malocclusion might seem to support the concept that Class II is a “disease” of small mandibles whose only rational treatment is “growth modification.” The invidious corollary is that the maxilla is “the wrong jaw.” The present results actually contradict both concepts.

It has long been known (e.g., Lande25) that, on average, Class II and Class I patients show about the same pattern of growth. Were it not so, successful individual growth prediction—the ability to predict different outcomes for two children of the same age and sex—could be based solely on molar classification. Instead, growth “prediction,” such as it is, has come to be based solely on averages and is independent of molar classification. In the present study, the five flush-terminal-plane subjects who went to Class II underwent very little excess mandibular growth (Figs. 7 and 8; flat regression slopes, age 5–11 years) during the transition to the permanent dentition. This early growth deficit was perhaps the proximate cause of the development of a Class II molar relationship. Later, however, the growth rate of mandible relative to maxilla increased to the point that, by the end of the study, the mandibular excess was not significantly different from that seen in the other flush subgroups (Fig. 8). Unfortunately, given that an intercuspated Class II already had been established, this “catch-up” growth could have no impact on the occlusion. Instead, it seems to have been “absorbed” by maxillary dentoalveolar compensation (Fig. 9, ages 11–16 years). As noted earlier, this analysis was suggested by the data and thus must be considered with care. An early, temporary growth deficit, however, is supported by earlier studies. Other investigations, however, have mirrored these findings. White16 noted a reduced mandibular excess in flush-terminal-plane subjects who transitioned to Class II. Similarly, Donaghey24 noted a similar deficit in Class II subjects age 9–11 years, but none in the same subjects from 11 to 13 years. For these subjects, the development of occlusion appears to have been based on bad luck, rather than some overall deficit: those who had a favorable mandibular excess at the right time went to Class I (or stayed end-to-end) and those

Class II malocclusion

Figure 10. Terminal-plane management. In this diagrammatic summary of the present findings, note that mandibular excess and upper molar movement tend to balance (⫾ 6–8 mm) and that lower molar “mesial drift” is, on average, negligible in comparison (1 to þ2 mm). A. Given any terminal plane, a major concern might be the preservation of lower leeway space. Flush and distal-step terminal planes, however, require additional steps to achieve a Class I molar relationship. B. Flush terminal plane—the effect of a maxillary holding device (Nance button, pendulum, NiTi coils, Distal Jet, Headgear, etc.) to produce a 1/2 cusp change by preventing maxillary dentoalveolar compensation. C. Distal step—U6 “distalization” (1/2 cusp), plus a period in which the upper molars are prevented from compensating for an additional 1/2 cusp of normal mandibular excess. The total of B and C would produce a Class I molar relationship.

who were unlucky in terms of timing went to Class II. This “perfect storm” scenario implies that there are times and events that can be exploited in the management of occlusal development and the treatment of Class II malocclusion. The present study suggests that a late mesial shift may not be a common, important phenomenon in occlusal development. In contrast, leeway space–about 5–6 mm in the lower arch—is real and, if preserved, as suggested independently by Dugoni34,35 and Gianelly,36–39 can assist in achieving incisor alignment. Indeed, given the approximation that, after the anchorage loss attendant to space closure, bicuspid extraction can free up 8 mm, the use of a lower lingual archwire is about 60% efficient relative to extraction. The correction/ prevention of Class II molar relationship is somewhat more complicated. For flush-terminal-plane subjects, molar occlusal adjustment is the algebraic sum of three components: upper molar movement, lower molar movement, and excess mandibular growth. Which of these constitute realistic therapeutic opportunities? The literature and the magnitude of the effects seen here seem to favor

71

the Dugoni–Gianelly approaches to early treatment: preserve lower leeway space and hold/ “distalize” the upper molars as needed. It is perhaps appropriate to examine this approach in light of the present findings. Growth modification (mandibular growth modification) is, for a variety of reasons, a popular strategy. A basic problem with this approach is that it probably cannot make the mandible longer. Indeed, even if it could, one would have to deal with dentoalveolar compensations that probably would prevent the extra growth from changing the occlusion. Lager40 apparently saw all of this when he argued that functional appliances do what they do by disarticulating the teeth so that excess mandibular growth (of the kind generated by the normal pattern of growth) can change the occlusion. What about the lower molars? Many think that a major purpose of premolar extraction is to achieve a Class I occlusion. The differential anchorage loss from this treatment, however, is only about a millimeter; far short of the width of a cusp.41 Other approaches are considerably more purposeful in their attack on the lower arch. Compared with classic “Tweed” orthodontics, many of our newer treatment op-

Figure 11. Early treatment with functional appliances. A. Mandible advanced by the functional appliance (which advancement is assumed to have no impact on the normal, favorable pattern of growth). B. For a time, the mandibular excess would produce no maxillary dentoalveolar compensation, thereby allowing displacement A. to be converted into a change in the occlusion by the normal pattern of mandibular excess. C. Variable lower anchorage loss caused by the functional appliance. The sum, B þ C, more or less equals the distal-step changes illustrated in Fig. 10. Both approaches make use of the usual pattern of growth (mandible 4 maxilla) by controlling maxillary dentoalveolar compensation. The choice between the two is a practice management decision, although the functional appliance probably would produce a slightly more protrusive dentition.

72

Tsourakis and Johnston Jr

tions—e.g., non-compliance and nonextraction straight wire—tend to “mesialize” the lower dentition.41 It is an unavoidable side effect that is justified by convenience, popularity, imaginative concepts of facial esthetics, “slenderizing,” and lifetime retention. There must, however, be a limit on the extent to which anchorage loss can be relied on as a routine means of preventing or correcting Class II malocclusion. For many patients and orthodontists alike, 3.5–7 mm of mesial movement is just too much. That leaves the maxilla…the “wrong jaw.” Given the magnitude of the effects documented here, we would argue that the upper dentition presents several obvious, uncomplicated options for guiding occlusal development and correcting malocclusion. Mandibular growth is usually favorable; however, our data suggest that its timing is both important and unpredictable. Accordingly, for the flush-terminalplane subject, a useful strategy would seem to be some sort of “holding” appliance to prevent maxillary dentoalveolar compensations until, in the fullness of time, the mandible outgrows the maxilla enough to adjust the occlusion to Class I (Fig. 10A). In the case of a distal step, it probably would be necessary to go beyond “holding” (Fig. 10B) to actual “distalization” (Fig. 10C). Although it is common to dismiss extra-oral traction with the sweeping argument that “patients would not wear a facebow,” it is an obvious option. For those who think Isaac Newton got it all wrong, there are many intra-oral, “non-compliance” treatments that can at least deal with the molar occlusion. The end result of this approach can be seen to be the essentially same as would be achieved with a functional appliance (Fig. 11)—both manipulate the position/drift of the upper molars. The only obvious difference is the probability that functional appliances would achieve part of their effect by way of lower anchorage loss.42–45 The “take-home” message of this article is simple: from the standpoint of prevention and correction of Class II malocclusion, the maxilla is the right jaw.

References 1. Moyers RE. Handbook of Orthodontics for the Student and General Practitioner. 3rd ed., Chicago: Yearbook; 1973. 2. Solow B. The dentoalveolar compensatory mechanism. Br J Orthod. 1980;7:141–161. 3. Björk A, Skieller V. Growth and development of the maxillary complex. Inf Orthod Kieferorthop. 1984;16:9–52.

4. Baume LJ. Physiological tooth migration and its significance for the development of occlusion. I. The biogenetic course of the deciduous dentition. J Dent Res. 1950;29: 123–132. 5. Baume LJ. Physiological tooth migration and its significance for the development of occlusion. II. The biogenesis of accessional dentition. J Dent Res. 1950;29: 331–337. 6. Baume LJ. Physiological tooth migration and its significance for the development of occlusion. III. The biogenesis of successional dentition. J Dent Res. 1950;29: 338–348. 7. Clinch L. An analysis of serial casts between three and eight years of age. Dent Pract Dent Rec. 1951;71:61–72. 8. Bonnar EME. Aspects of the transition from deciduous to permanent dentition. Dent Pract Dent Rec. 1956;7:42–54. 9. Carlson DB, Meredith HV. Biologic variation in selected relationships of opposing posterior teeth. Angle Orthod. 1960;30:162–173. 10. Arya BS, Savara BS, Thomas DR. Prediction of first molar occlusion. Am J Orthod. 1973;63:610–621. 11. Bishara SE, Hoppens BJ, Jakobsen JR, Kohout FJ. Changes in the molar relationship between the deciduous and permanent dentitions: a longitudinal study. Am J Orthod Dentofacial Orthop. 1988;93:19–28. 12. Maher JF. Mandibular Arch Development in the Late Mixed Dentition. Master’s Thesis. Ann Arbor, MI: The University of Michigan School of Dentistry; 1955. 13. Moorrees CFA, Grøn AM, Lebret LML, Yen PKJ, Fohlich FJ. Growth studies of the dentition: a review. Am J Orthod. 1969;55:600–616. 14. Murray JJ Jr. A Cephalometric Analysis of Occlusal Adjustment of the Molars in the Mixed Dentition. Master’s Thesis. Ann Arbor, MI: The University of Michigan School of Dentistry; 1959. 15. Paulsen HU. Changes in sagittal molar occlusion during growth. Danish Dent J. 1971;75:1258–1267. 16. White RC. The Role of Mandibular Growth in Occlusal development. Master0 s Thesis. St. Louis: Saint Louis University; 1983. 17. Kim YE, Nanda RS, Sinha PK. Transition of molar relationships in different skeletal patterns. Am J Orthod Dentofacial Orthop. 2002;121:280–290. 18. Lamont IG. Arch Length-Tooth Size and Distocclusion: Their Relationship to A–B Point Differences. Master0 s Thesis. Ann Arbor, MI: University of Michigan School of Dentistry; 1964. 19. Micklow JB A Cephalometric and Casts Analysis of the Changes Which Occur at the Terminal Plane From the Primary to the Mixed Dentition. Master0 s Thesis. Ann Arbor, MI: University of Michigan School of Dentistry; 1964. 20. Brin I, Kelley MB, Ackerman JL, Green PA. Molar occlusion and mandibular rotation: a longitudinal study. Am J Orthod. 1982;81:397–403. 21. Johnston LE Jr. Balancing the books on orthodontic treatment: an integrated analysis of change. Br J Orthod. 1996;23:93–102. 22. Duterloo HS, Planché P-G. Handbook of cephalmetric superimposition. Chicago: Quintessence; 2011. 23. McNamara JA Jr. Components of Class II malocclusion in children 8–10 years of age. Angle Orthod. 1981;51:177–202.

Class II malocclusion

24. Donaghey JB. A Cephalometric Evaluation of Tooth Movement and Growth of the Jaws in Untreated Individuals, Ages 11–15, Master0 s Thesis. St. Louis: Saint Louis University; 1986. 25. Lande MJ. Growth behavior of the human bony facial profile as revealed by serial cephalometric roentgenology. Angle Orthod. 1952;22:78–90. 26. Harvold EP. Some biologic aspects of orthodontic treatment in the transitional dentition. Am J Orthod. 1963;49: 1–15. 27. Jenkins DH. Analysis of orthodontic deformity employing lateral cephalometric radiography. Am J Orthod. 1955;41: 442–452. 28. Rao CR. Some statistical methods for comparison of growth curves. Biometrics. 1958;14:1–17. 29. Dahlberg G. Statistical Methods for Medical and Biological Students. New York: Interscience Publications; 1940. 30. Houston WJB. The analysis of errors in orthodontic measurements. Am J Orthod. 1983;83:382–390. 31. Lundström A, McWilliam JS. Dento-alveolar compensation for antero-posterior variations between the upper and lower apical bases. Eur J Orthod. 1984;6:116–122. 32. Southard TE, Behrents RG, Tolley EA. The anterior component of occlusal force, Part 1. Measurement and distribution. Am J Orthod Dentofacial Orthop. 1989;96: 493–500. 33. Southard TE, Behrents RG, Tolley EA. The anterior component of occlusal force, Part 2. Relationship with dental alignment. Am J Orthod Dentofacial Orthop. 1990;97: 41–49. 34. Dugoni SA, Lee JS, Varela J, Dugoni AA. Early mixed dentition treatment: postretention evaluation of stability and relapse. Angle Orthod. 1995;65:311–320.

73

35. Dugoni SA. Comprehensive mixed dentition treatment. Am J Orthod Dentofacial Orthop. 1998;113:75–84. 36. Gianelly AA. Leeway space and the resolution of crowding in the mixed dentition. Semin Orthod. 1995;1:188–194. 37. Gianelly AA. A strategy for nonextraction Class II treatment. Semin Orthod. 1998;4:26–32. 38. Gianelly AA. Distal movement of the maxillary molars. Am J Orthod Dentofacial Orthop. 1998;114:66–72. 39. Gianelly AA. Treatment of crowding in the mixed dentition. Am J Orthod Dentofacial Orthop. 2002;121:569–571. 40. Lager H. The individual growth pattern and stage of maturation as a basis for treatment of distal occlusion with overjet. Eur Orthod Soc Trans 1967:137–145. 41. Johnston LE Jr. Commentary. Am J Orthod Dentofacial Orthop. 2006;129:e1–e4. 42. Johnston LE Jr, A comparative analysis of Class II treatments. In: Vig PS, Ribbens KA, eds. Science and Clinical Judgment in Orthodontics, 19. Ann Arbor, MI: Center for Human Growth and Development; 1986: Ć103–148. 43. Johnston LE Jr. Functional appliances: a mortgage on mandibular position. Aust Orthod J. 1996;14:154–156. 44. Johnston LE Jr, Growing jaws for fun and profit: a modest proposal. In: McNamara JA Jr, ed. What Works, What Doesn’t, and Why, 35. Ann Arbor, MI: Center for Human Growth and Development, The University of Michigan; 1998:63–86. 45. Johnston LE Jr, If wishes were horses. In: McNamara JA Jr, ed. Early Orthodontic Treatment: Is the Benefit Worth the Burden, 44. Ann Arbor, MI: Department of Orthodontics and Pediatric Dentistry and Center for Human Growth and Development, The University of Michigan; 2007: Ć39–51.