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

2014; Elsevier BV; Volume: 20; Issue: 1 Linguagem: Inglês

10.1053/j.sodo.2013.12.006

1558-4631

Alexandros K. Tsourakis, Lysle E. Johnston,

Dental Radiography and Imaging

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-to-end, 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. 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-to-end, 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. In 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 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.1Moyers R.E. Handbook of Orthodontics for the Student and General Practitioner. 3rd ed.?>. Yearbook, Chicago1973Google Scholar (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.2Solow B. The dentoalveolar compensatory mechanism.Br J Orthod. 1980; 7: 141-161Google Scholar, 3Björk A. Skieller V. Growth and development of the maxillary complex.Inf Orthod Kieferorthop. 1984; 16: 9-52PubMed Google Scholar 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 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.Table 1Molar Relationship: Distribution by Stage of Occlusal Development According to Previous StudiesReference(s)NDeciduous DentitionMixed DentitionPermanent DentitionCross-sectional dataBaume4Baume L.J. Physiological tooth migration and its significance for the development of occlusion. I. The biogenetic course of the deciduous dentition.J Dent Res. 1950; 29: 123-132Crossref PubMed Scopus (120) Google Scholar, 5Baume L.J. Physiological tooth migration and its significance for the development of occlusion. II. The biogenesis of accessional dentition.J Dent Res. 1950; 29: 331-337Crossref PubMed Scopus (37) Google Scholar, 6Baume L.J. Physiological tooth migration and its significance for the development of occlusion. III. The biogenesis of successional dentition.J Dent Res. 1950; 29: 338-348Crossref PubMed Scopus (27) Google Scholar3086% Flush terminal plane––14% Mesial stepClinch7Clinch L. An analysis of serial casts between three and eight years of age.Dent Pract Dent Rec. 1951; 71: 61-72Google Scholar6143% Flush terminal plane––26% Mesial step31% Distal stepBonnar8Bonnar E.M.E. Aspects of the transition from deciduous to permanent dentition.Dent Pract Dent Rec. 1956; 7: 42-54Google Scholar863% Mesial step––Longitudinal dataCarlson and Meredith9Carlson D.B. Meredith H.V. Biologic variation in selected relationships of opposing posterior teeth.Angle Orthod. 1960; 30: 162-173Google Scholar10955% Mesial step59% Class I–32% Flush terminal plane24% Cusp-to-Cusp13% Distal step15% Class II1% Class IIIArya et al.10Arya B.S. Savara B.S. Thomas D.R. Prediction of first molar occlusion.Am J Orthod. 1973; 63: 610-621Abstract Full Text PDF PubMed Scopus (51) Google Scholar11838% Flush terminal plane49% Cusp-to-Cusp59% Class I48% Mesial step27% Class I38% Class II14% Distal step23% Class II2% Class III1% Class IIIBishara et al.11Bishara S.E. Hoppens B.J. Jakobsen J.R. Kohout F.J. Changes in the molar relationship between the deciduous and permanent dentitions: a longitudinal study.Am J Orthod Dentofacial Orthop. 1988; 93: 19-28Abstract Full Text PDF PubMed Scopus (97) Google Scholar12162% Mesial step–62% Class I29% Flush terminal plane34% Class II9% Distal step4% Class III Open table in a new tab According to Baume,4Baume L.J. Physiological tooth migration and its significance for the development of occlusion. I. The biogenetic course of the deciduous dentition.J Dent Res. 1950; 29: 123-132Crossref PubMed Scopus (120) Google Scholar, 5Baume L.J. Physiological tooth migration and its significance for the development of occlusion. II. The biogenesis of accessional dentition.J Dent Res. 1950; 29: 331-337Crossref PubMed Scopus (37) Google Scholar, 6Baume L.J. Physiological tooth migration and its significance for the development of occlusion. III. The biogenesis of successional dentition.J Dent Res. 1950; 29: 338-348Crossref PubMed Scopus (27) Google Scholar 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,7Clinch L. An analysis of serial casts between three and eight years of age.Dent Pract Dent Rec. 1951; 71: 61-72Google Scholar 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, 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 flush-terminal-plane subjects. According to Maher12Maher JF. Mandibular Arch Development in the Late Mixed Dentition. Master’s Thesis. Ann Arbor, MI: The University of Michigan School of Dentistry; 1955.Google Scholar and Moyers,1Moyers R.E. Handbook of Orthodontics for the Student and General Practitioner. 3rd ed.?>. Yearbook, Chicago1973Google Scholar 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 > maxilla), this differential movement, the so-called “late mesial shift,” is thought by many (e.g., Baume,4Baume L.J. Physiological tooth migration and its significance for the development of occlusion. I. The biogenetic course of the deciduous dentition.J Dent Res. 1950; 29: 123-132Crossref PubMed Scopus (120) Google Scholar, 5Baume L.J. Physiological tooth migration and its significance for the development of occlusion. II. The biogenesis of accessional dentition.J Dent Res. 1950; 29: 331-337Crossref PubMed Scopus (37) Google Scholar, 6Baume L.J. Physiological tooth migration and its significance for the development of occlusion. III. The biogenesis of successional dentition.J Dent Res. 1950; 29: 338-348Crossref PubMed Scopus (27) Google Scholar Clinch,7Clinch L. An analysis of serial casts between three and eight years of age.Dent Pract Dent Rec. 1951; 71: 61-72Google Scholar and Moorrees et al.13Moorrees C.F.A. Grøn A.M. Lebret L.M.L. Yen P.K.J. Fohlich F.J. Growth studies of the dentition: a review.Am J Orthod. 1969; 55: 600-616Abstract Full Text PDF PubMed Scopus (135) Google Scholar) 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,14Murray 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.Google Scholar Paulsen,15Paulsen H.U. Changes in sagittal molar occlusion during growth.Danish Dent J. 1971; 75: 1258-1267Google Scholar White,16White RC. The Role of Mandibular Growth in Occlusal development. Master's Thesis. St. Louis: Saint Louis University; 1983.Google Scholar and Kim et al.,17Kim Y.E. Nanda R.S. Sinha P.K. Transition of molar relationships in different skeletal patterns.Am J Orthod Dentofacial Orthop. 2002; 121: 280-290Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar 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 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 men'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.Table 2Details of Occlusal Development as a Function of MethodologyReference(s)NFindingsCastsBaume4Baume L.J. Physiological tooth migration and its significance for the development of occlusion. I. The biogenetic course of the deciduous dentition.J Dent Res. 1950; 29: 123-132Crossref PubMed Scopus (120) Google Scholar, 5Baume L.J. Physiological tooth migration and its significance for the development of occlusion. II. The biogenesis of accessional dentition.J Dent Res. 1950; 29: 331-337Crossref PubMed Scopus (37) Google Scholar, 6Baume L.J. Physiological tooth migration and its significance for the development of occlusion. III. The biogenesis of successional dentition.J Dent Res. 1950; 29: 338-348Crossref PubMed Scopus (27) Google Scholar60In 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 shiftClinch7Clinch L. An analysis of serial casts between three and eight years of age.Dent Pract Dent Rec. 1951; 71: 61-72Google Scholar61Mandibular growth plays a role in Class I adjustment from deciduous to mixed dentition. No closing of primate spaces was observedMaher12Maher JF. Mandibular Arch Development in the Late Mixed Dentition. Master’s Thesis. Ann Arbor, MI: The University of Michigan School of Dentistry; 1955.Google Scholar43Leeway space is not a critical factor for occlusal adjustment in the transition between the mixed and permanent dentitionBonnar8Bonnar E.M.E. Aspects of the transition from deciduous to permanent dentition.Dent Pract Dent Rec. 1956; 7: 42-54Google Scholar58Forward movement of the mandibular dentition can occur during the deciduous dentition and at the transition between the deciduous and mixed dentitionLamont18Lamont IG. Arch Length-Tooth Size and Distocclusion: Their Relationship to A–B Point Differences. Master's Thesis. Ann Arbor, MI: University of Michigan School of Dentistry; 1964.Google Scholar96Leeway space might be a main contributor for molar adjustment toward Class I when the Wits A–B differential does not changeMicklow19Micklow JB A Cephalometric and Casts Analysis of the Changes Which Occur at the Terminal Plane From the Primary to the Mixed Dentition. Master's Thesis. Ann Arbor, MI: University of Michigan School of Dentistry; 1964.Google Scholar10The predominant factor in molar adjustment is excess mandibular growthMoorrees et al.13Moorrees C.F.A. Grøn A.M. Lebret L.M.L. Yen P.K.J. Fohlich F.J. Growth studies of the dentition: a review.Am J Orthod. 1969; 55: 600-616Abstract Full Text PDF PubMed Scopus (135) Google Scholar94Class I molar relationship is achieved from an initial cusp-to-cusp relation by greater mesial shifting of the mandibular molarsCephalometric radiographsMurray14Murray 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.Google Scholar20The principal factor in the occlusal adjustment of the buccal segments during the mixed dentition is the relative growth/displacement of maxillary and mandibular basal boneWhite16White RC. The Role of Mandibular Growth in Occlusal development. Master's Thesis. St. Louis: Saint Louis University; 1983.Google Scholar34The normal pattern of facial growth (mandible more than maxilla) is the decisive factor in molar adjustment from the mixed to the permanent dentitionKim et al.17Kim Y.E. Nanda R.S. Sinha P.K. Transition of molar relationships in different skeletal patterns.Am J Orthod Dentofacial Orthop. 2002; 121: 280-290Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar40Skeletal growth difference between the jaws influenced the change in molar relationshipCasts and cephalometric radiographsBrin et al.20Brin I. Kelley M.B. Ackerman J.L. Green P.A. Molar occlusion and mandibular rotation: a longitudinal study.Am J Orthod. 1982; 81: 397-403Abstract Full Text PDF PubMed Scopus (5) Google Scholar60Differential growth of the maxilla and the mandible plays a significant role in occlusal relation if dentoalveolar compensation cannot compensate for the differentialPaulsen15Paulsen H.U. Changes in sagittal molar occlusion during growth.Danish Dent J. 1971; 75: 1258-1267Google Scholar52More mandibular growth could contribute to a Class I relation. Change in molar occlusion during the process of growth and development is a multifactorial phenomenonBishara et al.11Bishara S.E. Hoppens B.J. Jakobsen J.R. Kohout F.J. Changes in the molar relationship between the deciduous and permanent dentitions: a longitudinal study.Am J Orthod Dentofacial Orthop. 1988; 93: 19-28Abstract Full Text PDF PubMed Scopus (97) Google Scholar121No correlation was found between molar relationship and the difference in the leeway space between the maxillary and mandibular arches Open table in a new tab 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 superimposition3Björk A. Skieller V. Growth and development of the maxillary complex.Inf Orthod Kieferorthop. 1984; 16: 9-52PubMed Google Scholar, 21Johnston Jr, L.E. Balancing the books on orthodontic treatment: an integrated analysis of change.Br J Orthod. 1996; 23: 93-102Crossref PubMed Scopus (44) Google Scholar, 22Duterloo H.S. Planché P.-G. Handbook of cephalmetric superimposition. Quintessence, Chicago2011Google Scholar—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 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.17Kim Y.E. Nanda R.S. Sinha P.K. Transition of molar relationships in different skeletal patterns.Am J Orthod Dentofacial Orthop. 2002; 121: 280-290Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar 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, White16White RC. The Role of Mandibular Growth in Occlusal development. Master's Thesis. St. Louis: Saint Louis University; 1983.Google Scholar 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 McNamara's23McNamara Jr, J.A. Components of Class II malocclusion in children 8–10 years of age.Angle Orthod. 1981; 51: 177-202PubMed Google Scholar 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. Donaghey24Donaghey JB. A Cephalometric Evaluation of Tooth Movement and Growth of the Jaws in Untreated Individuals, Ages 11–15, Master's Thesis. St. Louis: Saint Louis University; 1986.Google Scholar 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 (Lande25Lande M.J. Growth behavior of the human bony facial profile as revealed by serial cephalometric roentgenology.Angle Orthod. 1952; 22: 78-90Google Scholar and Harvold26Harvold E.P. Some biologic aspects of orthodontic treatment in the transitional dentition.Am J Orthod. 1963; 49: 1-15Abstract Full Text PDF Scopus (41) Google Scholar). If an occlusion goes to Class II, the probably favorable pattern of subsequent growth (mandible > 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. 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 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 so-called “functional occlusal plane,” a best-fit line drawn by inspection through the buccal-segment occlusion (Jenkins27Jenkins D.H. Analysis of orthodontic deformity employing lateral cephalometric radiography.Am J Orthod. 1955; 41: 442-452Abstract Full Text PDF Scopus (17) Google Scholar). A subject was assigned, somewhat arbitrarily, to the mesial-step terminal-plane group if the lower primary second molar was ≥0.5 mm mesial to the upper primary second molar (N = 6) and to the distal-step terminal-plane group if it was ≥0.5 mm distal to the upper (N = 9). The remaining subjects between these two boundaries were assigned to the flush-terminal-plane group (N = 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 post-treatment cephalograms, subjects from the flush-terminal-plane group were assigned to the Class I sub-group if their molar relation at age 15 years was > +1 mm. Molar relations between −1 and +1 were assigned to the end-to-end subgroup and < −1 mm to the Class II subgroup. 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 Björk'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 so-called “Pitchfork Analysis” (Fig. 1; Johnston21Johnston Jr, L.E. Balancing the books on orthodontic treatment: an integrated analysis of change.Br J Orthod. 1996; 23: 93-102Crossref PubMed Scopus (44) Google Scholar; also see Duterloo and Planché22Duterloo H.S. Planché P.-G. Handbook of cephalmetric superimposition. Quintessence, Chicago2011Google Scholar). 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