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Class I malocclusions—The development and etiology of mandibular malalignments

  • Peter H. Buschang
    Correspondence
    Address correspondence to Peter H. Buschang, PhD, Department of Orthodontics, Texas A&M University Baylor College of Dentistry, 3302 Gaston Ave, Dallas, TX 75246
    Affiliations
    Department of Orthodontics, Texas A&M University Baylor College of Dentistry, Dallas, TX
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Published:December 26, 2013DOI:https://doi.org/10.1053/j.sodo.2013.12.002
      Class I malocclusion is the most prevalent form of malocclusion, even more prevalent than normal occlusion, and there is no clear consensus concerning its etiology. To understand its etiology, how and when this form of malocclusion develops must be fully appreciated. Understanding the development and etiology of Class I malocclusion is fundamentally important for determining the treatment and post-treatment methods that will be most effective. Rather than thinking of it as a static state, Class I malocclusion should be viewed as a process, as a maladaptation of the dentoalveolar complex. The purpose of this article is to comprehensively review the various factors that help explain this process.
      Individuals with Class I malocclusion have normal molar relationships, but their teeth are not correctly positioned in the line of occlusion due to malposed teeth, rotations, spacing, overbites, open bites, posterior cross bites, or even anterior cross bites. While Class I malocclusion includes a variety of occlusal discrepancies, the primary clinical concern is the malalignment of the anterior teeth, particularly the mandibular teeth.
      Two basic measures have been used to quantify the malalignment of teeth: tooth size–arch length discrepancies (TSALD) and the irregularity index. TSALD are based on the difference, measured in millimeters between the perimeter of the arch where the teeth should be located and the sizes of the teeth. Both anterior (canine to canine) and total (molar to molar) TSALD have been used. The incisor irregularity index (IIRREG) refers to the linear displacement, measured in millimeters, of the five anatomic contact points of the lower incisors.
      • Little R.M.
      The irregularity index: a quantitative score of mandibular anterior alignment.
      While both provide measures of anterior malalignment, IIRREG only explains 25–36% of the variation in TSALD, and vice versa,
      • Harris E.F.
      • Vaden J.L.
      • Williams R.A.
      Lower incisor space analysis: a contrast of methods.
      • Myser S.A.
      • Campbell P.M.
      • Boley J.
      • Buschang P.H.
      Long-term stability: post-retention changes of the mandibular anterior teeth.
      indicating that the two indices are measuring different attributes. Because the mandibular incisors have similar mesiodistal and facio-lingual dimensions,
      • Peck S.
      • Peck H.
      Crown dimensions and mandibular incisor alignment.
      incisor rotation can have a substantial impact on IIRREG and little or no effect on TSALD. Similarly, displacements of incisors that retain the space needed for the correction of the malocclusion will affect IIRREG but not TSALD. Due to these differences, IIRREG and changes in IIRREG over time are usually greater than TSALD and changes in TSALD.
      Class I malocclusion is so prevalent that it is now the norm rather than the exception. Malalignment of the anterior teeth is greater in the mandible than in the maxilla (Fig. 1). In the maxilla, approximately 44% of the US population has ideal IIRREG (0–1 mm), 45% have mild-to-moderate IIRREG (2–6 mm), and 11% have severe or extreme IIRREG (>6 mm).
      • Proffit W.R.
      • Fields Jr, H.W.
      • Moray L.J.
      Prevalence of malocclusion and orthodontic treatment need in the United States: estimates from the NHANES III survey.
      In contrast, only 37% of the population has ideal mandibular IIRREG, 49% has mild-to-moderation IIRREG, and 14% has severe or extreme IIRREG. It has been estimated that approximately 40% of the untreated persons in the US between 15 and 50 years of age have clinically significant IIRREG.
      • Buschang P.H.
      • Shulman J.D.
      Incisor crowding in untreated persons 15-50 years of age: United States, 1988-1994.
      Figure thumbnail gr1
      Figure 1Prevalence of maxillary and mandibular crowding among the US population. (Adapted from Proffit et al.
      • Proffit W.R.
      • Fields Jr, H.W.
      • Moray L.J.
      Prevalence of malocclusion and orthodontic treatment need in the United States: estimates from the NHANES III survey.
      )
      While the molar relations of subjects with Class I malocclusion appear to be stable, the relationships of the more anterior teeth are anything but stable. Based on large samples of untreated individuals with various types of molar relationship evaluated at 19.7 years and again at 54.6 years of age, it has been shown that Class I molar relations are stable (i.e., none of the 69 cases evaluated changed their relations), while Class II and Class III molar relations worsen slightly over time.
      • Harris E.F.
      • Behrents R.G.
      The intrinsic stability of Class I molar relationship: a longitudinal study of untreated cases.
      In contrast, the relationships of the mandibular incisors of individuals change substantially over time, especially during the teenage and young adult years, regardless of their molar relationships (Table). Based on 65 untreated children who had normal occlusion, it has been shown that IIRREG decreases slightly between the mixed and early permanent dentition and then increases significantly between 12 and 20 years of age.
      • Sinclair P.M.
      • Little R.M.
      Maturation of untreated normal occlusions.
      Bishara et al.,
      • Bishara S.E.
      • Jakobsen J.R.
      • Treder J.E.
      • Stasi M.J.
      Changes in the maxillary and mandibular tooth size–arch length relationship from early adolescence to early adulthood. A longitudinal study.
      • Bishara S.E.
      • Treder J.E.
      • Damon P.
      • Olsen M.
      Changes in the dental arches and dentition between 25 and 45 years of age.
      who provide some of the best long-term longitudinal data, showed a 2.4-mm increase in anterior TSALD between 14 and 25 years of age and another 0.7-mm increase between 25 and 46 years of age (Fig. 2). Similarly, Bondevik
      • Bondevik O.
      Changes in occlusion between 23 and 34 years.
      demonstrated that anterior TSALD increases 2.0 mm in untreated subjects 23–34 years of age; Richardson
      • Richardson M.E.
      A review of changes in lower arch alignment from seven to fifty years.
      reported increases of 2.3 mm in total TSALD between 13 and 18 years of age. Based on the available longitudinal data of untreated subjects, the greatest increases in malalignment occur during the early permanent dentition; rates of change slow down rapidly during the teenage years and early 20s (Fig. 3).
      TableChanges in Incisor Irregular and Tooth Size–Arch Length Discrepancies (TSALD) of Untreated Subjects in the Permanent Dentition
      MeasureAge Range (Years)Mandible
      Sinclair and Little
      • Sinclair P.M.
      • Little R.M.
      Maturation of untreated normal occlusions.
      Incisor irregularity12–200.7
      Bishara et al.
      • Bishara S.E.
      • Jakobsen J.R.
      • Treder J.E.
      • Stasi M.J.
      Changes in the maxillary and mandibular tooth size–arch length relationship from early adolescence to early adulthood. A longitudinal study.
      Ant TSALD14–242.4
      Bishara et al.
      • Bishara S.E.
      • Treder J.E.
      • Damon P.
      • Olsen M.
      Changes in the dental arches and dentition between 25 and 45 years of age.
      Ant TSALD25–460.7
      Bondevik
      • Bondevik O.
      Changes in occlusion between 23 and 34 years.
      Ant TSALD23–342.0
      Richardson
      • Richardson M.E.
      A review of changes in lower arch alignment from seven to fifty years.
      6-6 TSALD13–182.3
      Richardson
      • Richardson M.E.
      A review of changes in lower arch alignment from seven to fifty years.
      6-6 TSALD21–280.2
      Driscoll-Gilliland et al.
      • Driscoll-Gilliland J.
      • Buschang P.H.
      • Behrents R.G.
      An evaluation of growth and stability in untreated and treated subjects.
      Ant TSALD14–231.1
      Driscoll-Gilliland et al.
      • Driscoll-Gilliland J.
      • Buschang P.H.
      • Behrents R.G.
      An evaluation of growth and stability in untreated and treated subjects.
      Incisor irregularity14–231.3
      Figure thumbnail gr2
      Figure 2Changes in anterior mandibular TSALD and arch length between 14 and 46 years of age. (Adapted from Bishara et al.
      • Bishara S.E.
      • Jakobsen J.R.
      • Treder J.E.
      • Stasi M.J.
      Changes in the maxillary and mandibular tooth size–arch length relationship from early adolescence to early adulthood. A longitudinal study.
      • Bishara S.E.
      • Treder J.E.
      • Damon P.
      • Olsen M.
      Changes in the dental arches and dentition between 25 and 45 years of age.
      )
      Figure thumbnail gr3
      Figure 3Annualized changes in malalignment of untreated patients between 12 and 42 years of age.
      Importantly, increases in TSALD and incisor irregularity of untreated subjects have been consistently associated with decreases in intercanine width and decreases in arch depth. The 0.7-mm increase in IIRREG reported by Sinclair and Little
      • Sinclair P.M.
      • Little R.M.
      Maturation of untreated normal occlusions.
      in the permanent dentition was associated with a 2-mm decrease in arch length and a 1.5-mm decrease of intercanine width. Bishara et al.
      • Bishara S.E.
      • Jakobsen J.R.
      • Treder J.E.
      • Stasi M.J.
      Changes in the maxillary and mandibular tooth size–arch length relationship from early adolescence to early adulthood. A longitudinal study.
      • Bishara S.E.
      • Treder J.E.
      • Damon P.
      • Olsen M.
      Changes in the dental arches and dentition between 25 and 45 years of age.
      showed that greater increases in malalignment are associated with greater decreases in arch length. This association might be expected because with malalignment, particularly TSALD, the teeth move mesially into a narrower, shorter, part of the arch.
      • Bondevik O.
      Changes in occlusion between 23 and 34 years.
      • Richardson M.E.
      A review of changes in lower arch alignment from seven to fifty years.
      Although malalignment is a summary measure, the actual amount of discrepancy between tooth contacts depends on the teeth and the arch in question. Based on a survey with 9044 adults who participated in the National Health and Nutrition Examination Survey (NHANES) III, maxillary contacts between the canines and lateral incisors showed only slightly greater amounts of discrepancies than the contacts between the lateral and central incisors (Fig. 4A), both of which were substantially greater than the contacts between the central incisors. In the mandible, the contacts between the canines and lateral incisors showed the greatest discrepancies; the discrepancies between the laterals and centrals were only slightly greater than those between the centrals. Regardless of whether crowding is severe or slight, the same patterns (i.e., the greatest discrepancy in the mandible always occurs between the lateral incisors and canines) are maintained. Moreover, patients whose occlusion becomes irregular or crowded after orthodontic treatment exhibit the same pattern.
      • Vaden J.L.
      • Harris E.F.
      • Gardner R.L.
      Relapse revisited.
      Longitudinal changes in incisor irregularity follow the same patterns both for treated and untreated subjects (Fig. 4B).
      Figure thumbnail gr4
      Figure 4Maxillary (Mx) and mandibular (Md) discrepancies (mm) between interproximal contacts (e.g., right canine and right lateral incisor or rC-rI2 and left lateral incisor and left central incisor or lI1–lI2) of (A) a large cross-sectional sample of adults from the NHANES III and (B) similar untreated and treated longitudinal patients.

      Class I malocclusion and the dental compensatory mechanism

      Teeth are maintained in positions of equilibrium between the lingual, labial, and vestibular muscles.
      • Weinstein S.
      • Haack D.C.
      • Morris L.Y.
      • Snyder B.B.
      • Attaway H.E.
      On an equilibrium theory of tooth position.
      In addition to the intrinsic forces of the soft tissues, extrinsic forces (e.g., habits and orthodontic appliances) from dental occlusion, and forces from the periodontal membrane must also be considered in order to understand the changes in tooth position that occur.
      • Proffit W.R.
      Equilibrium theory revisited: factors influencing position of the teeth.
      Teeth can have more than one position of equilibrium; the different positions represent compensations to the forces placed upon the teeth. Weinstein et al. showed that 2-mm onlays cemented on premolars can disrupt a tooth’s equilibrium, causing movements similar to those produced by orthodontic appliances.
      • Weinstein S.
      • Haack D.C.
      • Morris L.Y.
      • Snyder B.B.
      • Attaway H.E.
      On an equilibrium theory of tooth position.
      Their work demonstrated that differential forces, even when they are small, can cause tooth movement if they are applied over sufficiently long periods of time.
      Dentoalveolar compensations refer to the changes in tooth positions that occur between periods of equilibrium. They are often positive, adaptive changes that maintain normal interarch relations under varying skeletal relationships.
      • Solow B.
      The dentoalveolar compensatory mechanism: background and clinical implications.
      However, they can also be negative, maladaptive changes that account for the high prevalence of malocclusion among modern day populations. The three factors thought to be responsible for dentoalveolar adaptations are (1) a normal eruptive system, (2) soft tissue forces exerted on the teeth, and (3) influence of the neighboring teeth.
      • Solow B.
      The dentoalveolar compensatory mechanism: background and clinical implications.
      When one or more of these factors break down, malocclusion results. For example, subjects with systematic problems that influence the eruptive system might be expected to have a less efficient dentoalveolar compensatory mechanism.
      After the teeth emerge and erupt into functional occlusion, they continue to erupt and migrate. The clinical importance of the changes that occur, particularly eruption, is often unappreciated; these tooth movements play a large role in the development of malalignment. Based on superimpositions performed using naturally stable references structures, the maxillary first molars undergo approximately 1.2–1.5 mm of vertical eruption and 0.4–0.6 mm/year mesial migration between 10 and 15 years of age (Fig. 5).

      McWhorter K. A longitudinal study of horizontal and vertical tooth movements during adolescence (ages 10 to 15). Master’s Thesis, Baylor University, 1992.

      The amounts of eruption are considerably greater than the amounts of vertical growth displacement that occur. The mandibular first molars show less eruption over the same time period, averaging 0.6–0.8 mm/year, and slightly more mesial migration (0.6–0.7 mm/year).
      Figure thumbnail gr5
      Figure 5Adolescents (ages 10–15 years) changes in first molar eruption and migration. Yearly increases in malalignment of untreated subjects who were followed up longitudinally.
      Among growing individuals, the amount of eruption that occurs after the teeth achieve occlusal contact depends primarily on the amounts of space created by the vertical displacement of the jaws. Vertical growth explains approximately half of the variation in lower molar eruption.
      • Liu S.S.
      • Buschang P.H.
      How does tooth eruption relate to vertical mandibular growth displacement?.
      The greater the inferior displacement of the mandible, the greater the superior eruption of the molars. This explains why mandibular inferior displacement and vertical molar eruption exhibit adolescent spurts at approximately the same time, with displacement and eruption peaking at 11.8 and 12.1 years of age, respectively. The fact that peak velocity is slightly later and that there is substantially greater variability in eruption than displacement among subjects, supports the notion that mandibular eruption is compensating for, or adapting to, the growth displacements that occur.
      Teeth fill spaces created, whether by growth, occlusal wear, interproximal wear, or tooth loss. For example, supraeruption is a normal compensatory response to offset occlusal attrition and maintain efficient mastication.
      • Sicher H.
      The biology of attrition.
      • Weinmann J.P.
      • Sicher H.
      Bone and Bones.
      Both adult and growing rhesus monkeys with bite splints cemented to their posterior maxillary teeth for 48 weeks showed significantly greater incisor supraeruption than controls.
      • Schneiderman E.D.
      A longitudinal cephalometric study of incisor supra-eruption in young and adult rhesus monkeys (Macaca mulatta).
      American Indians who subsisted by hunting and gathering have larger than expected distances between the alveolar crestal bone and the cemento-enamel junction (CEJ) due to supraeruption compensating for occlusal wear (Fig. 6). Similarly, the substantial amounts of interproximal wear caused by their diets resulted in broader contacts and mesial migration of the dentition. Mesial migration also occurs when there are extensive carious lesions or when teeth are lost prematurely.
      • Northway W.M.
      • Wainright R.L.
      • Demirjian A.
      Effects of premature loss of deciduous molars.
      The movements are compensations for the spaces that are created.
      Figure thumbnail gr6
      Figure 6The dentition of native Americans showing extensive interproximal and occlusal wear and supraeruption of the teeth.
      In fact, the normal changes in dental arches that occur during growth are best understood as dentoalveolar compensations. Maxillary intercanine width increases approximately 3 mm during the transition to the early mixed dentition,
      • Moorrees C.F.
      • Gron A.M.
      • Lebret L.M.
      • Yen P.K.
      • Fröhlich F.J.
      Growth studies of the dentition: a review.
      largely due to the primary incisors being approximately 4-mm smaller than their permanent counterparts. The additional 2-mm increase in maxillary intercanine width that occurs with the eruption of the permanent canines is related to their larger sizes and eruptive paths. When the larger maxillary permanent incisors emerge, arch depth increases because the incisors compensate for the lack of space by proclining. Subsequently, maxillary arch depth decreases approximately 2 mm, compensating for the space created by the loss of the deciduous molars. Mandibular arch depth also compensates for the space created by the loss of the deciduous molars by decreasing 2–3 mm. Thus, the normal changes that occur in maxillary and mandibular arch perimeter represent compensations (Fig. 7) largely due to insufficient space during the early mixed dentition and excess space during the early permanent dentition.
      Figure thumbnail gr7
      Figure 7Normal changes in maxillary and mandibular arch perimeters between 6 and 16 years of age. (Adapted with permission from Moyers et al.
      • Moyers R.E.
      • van der Linden F.P.G.M
      • Riolo M.L.
      • McNamara J.A.
      )
      When they are malaligned and space is created, the teeth again compensate by partially and spontaneously resolving crowding. For example, when extractions—and no other form of treatment—were performed during the mixed and permanent dentition stages, there were marked differences in how the anterior teeth compensated, depending on the amount of initial crowding.
      • Papandreas S.G.
      • Buschang P.H.
      • Alexander R.G.
      • Kennedy D.B.
      • Koyamu I.
      Physiological drift of the mandibular dentition following first premolar extractions.
      The canines drifted laterally and distally into the extraction sites, and IIRREG was reduced from 5.5 to 3.3 mm in the mixed dentition group. The permanent dentition group showed even greater compensations, with incisor irregularity decreasing from 8.3 to 4.2 mm. Similarly, the increases in arch dimension that occur when patients are treated with lip bumpers also partially resolve crowding. On an average, approximately 3.5-mm spontaneous resolution of crowding occurs (Fig. 8).
      • Solomon M.J.
      • English J.D.
      • Magness W.B.
      • McKee C.J.
      Long-term stability of lip bumper therapy followed by fixed appliances.
      • Vargo J.
      • Buschang P.H.
      • Boley J.C.
      • English J.D.
      • Behrents R.G.
      • Owen 3rd, A.H.
      Treatment effects and short-term relapse of maxillomandibular expansion during the early to mid mixed dentition.
      • O’Donnell S.
      • Nanda R.S.
      • Ghosh J.
      Perioral forces and dental changes resulting from mandibular lip bumper treatment.
      • Davidovich M.
      • McInnis D.
      • Lindaur S.J.
      The effects of lip bumper therapy in the mixed dentition.
      • Werner S.P.
      • Shivapuja P.K.
      • Harris E.F.
      Skeletodental changes in the adolescent accruing from use of the lip bumper.
      • Nevant C.T.
      • Buschang P.H.
      • Alexander R.G.
      • Steffen J.M.
      Lip bumper therapy for gaining arch length.
      Figure thumbnail gr8
      Figure 8Decreases (mm) in incisor irregularity associated with lip bumper therapy only.

      What anterior mandibular malalignment is not related to

      To prevent misunderstanding and unnecessary treatments, it is important to determine what malalignment is not related to. First, crowding does not seem to be related to the presence of third molars. In perhaps the best study performed to date, Harradine et al.
      • Harradine N.W.T.
      • Pearson M.H.
      • Toth B.
      The effect of extraction of third molars on late lower incisor crowding: a randomized controlled trial.
      reported no differences in crowding between individuals whose third molars were extracted early and those whose molars were not removed. This and several other studies
      • Lindqvist B.
      • Thilander B.
      Extraction of third molars in cases of anticipated crowding in the lower jaw.
      • Ades A.G.
      • Joondeph D.R.
      • Little R.M.
      • Chapko M.K.
      A long-term study of the relationship of third molars to changes in the mandibular dental arch.
      demonstrated that the removal of the third molars as a preventative measure to reduce late incisor crowding cannot be justified. The adults evaluated by the NHANES who had both their mandibular third molars in the mouth had approximately 1 mm less incisor irregularity than those who had only one or none present.
      • Buschang P.H.
      • Shulman J.D.
      Incisor crowding in untreated persons 15-50 years of age: United States, 1988-1994.
      More importantly, post-retention crowding is not related to either pretreatment crowding or the decreases in crowding that occur during treatment. Most long-term follow-up studies that have looked for an association have not been able to find one. Reviewing the multiple studies performed by the University of Washington, Little
      • Little R.M.
      Stability and relapse of dental arch alignment: University of Washington studies.
      noted that “The degree of post-retention anterior crowding is both unpredictable and variable and no pretreatment variables either from clinical findings, casts, or cephalometric radiographs before or after treatment seem to be useful predictors.” Similarly, none of the many follow-up studies performed at the Baylor College of Dentistry have been able to establish relationships between the pretreatment malalignment or treatment changes of alignment and post-retention changes in alignment. For example, pretreatment incisor irregularity of nonextraction patients provides no clue for estimating their post-treatment changes (Fig. 9).
      • Elms T.
      • Buschang P.H.
      • Alexander R.G.
      Long-term stability of Class II, Division 1 Nonextraction cervical facebow therapy: I. Model analysis.
      Figure thumbnail gr9
      Figure 9Lack of relationship between patients' pretreatment incisor irregularity and post-treatment changes in irregularity.
      Peck and Peck
      • Peck S.
      • Peck H.
      Crown dimensions and mandibular incisor alignment.
      have suggested that the shape of the teeth is a risk factor for the development of crowding. It was thought that individuals whose mesiodistal tooth diameters are larger than expected for their buccolingual diameters are likely to exhibit more crowding. Others, however, have not been able to substantiate their claims.
      • Smith J.R.
      • Davidson W.M.
      • Gipe D.P.
      Incisor shape and incisor crowding: a re-evaluation of the Peck and Peck ratio.
      • Shah A.A.
      • Elcock C.
      • Brook A.H.
      Incisor crown shape and crowding.
      • Freitas M.R.
      • Castro R.C.
      • Janson G.
      • Freitas K.M.
      • Henriques J.F.
      Correlation between mandibular incisor crown morphologic index and postretention stability.
      Recent multivariate assessments show that the mesiodistal/buccolingual tooth size ratio was never a primary predictor or TSALD and does not contribute to any great extent after either mesiodisal or buccolingual tooth size has been accounted for.
      • Agenter M.K.
      • Harris E.F.
      • Blair R.N.
      Influence of tooth crown size on malocclusion.
      Finally, and most importantly, there is little or no support of the notion that orthodontic treatments, when performed within well-accepted guidelines (i.e., no excessive flaring of incisors, no over-expansion of canines, and an adequate retention period to allow reorganization of the periodontal fibers), are responsible for post-retention crowding. The amount of post-treatment crowding that occurs for both extraction (Fig. 10A)
      • Dugoni S.A.
      • Lee J.S.
      • Varela J.
      • Dugoni A.A.
      Early mixed dentition treatment: postretention evaluation of stability and relapse.
      • Glenn G.
      • Sinclair P.M.
      • Alexander R.G.
      Nonextraction orthodontic therapy: posttreatment dental and skeletal stability.
      • Sadowsky C.
      • Schneider B.J.
      • BeGole E.A.
      • Tahir E.
      Long-term stability after orthodontic treatment: nonextraction with prolonged retention.
      • Luppanapornlarp S.
      • Johnston Jr, L.E.
      The effects of premolar-extraction: a long-term comparison of outcomes in “clear-cut” extraction and nonextraction Class II patients.
      • Moussa R.
      • O’Reilly M.T.
      • Close J.M.
      Long-term stability of rapid palatal expander treatment and edgewise mechanotherapy.
      • Kahl-Nieke B.
      • Fischbach H.
      • Schwarze C.W.
      Post-retention crowding and incisor irregularity: a long-term follow-up evaluation of stability and relapse.
      • Ferris T.
      • Alexander R.G.
      • Boley J.
      • Buschang P.H.
      Long-Term stability of combined RPE/lip bumper expansion therapy followed by full fixed appliances.
      • Paquette D.E.
      • Beattie J.R.
      • Johnston Jr, L.E.
      A long-term comparison of nonextraction and premolar extraction edgewise therapy in “borderline” Class II patients.
      • Uhde M.D.
      • Sadowsky C.
      • BeGole E.A.
      Long-term stability of dental relationships after orthodontic treatment.
      • Boley J.
      • Mark J.
      • Sachdeva R.C.L.
      • Buschang P.H.
      Long-term stability of Class I premolar extraction patients.
      • Little R.M.
      • Wallen T.R.
      • Riedel R.A.
      Stability and relapse of mandibular anterior alignment-first premolar extraction cases treated by traditional edgewise orthodontics.
      • Haruki T.
      • Little R.M.
      Early versus late treatment of crowded first premolar extraction cases: postretention evaluation of stability and relapse.
      • McReynolds D.C.
      • Little R.M.
      Mandibular second premolar extraction post-retention evaluation of stability and relapse.
      and nonextraction (Fig. 10B) cases (1.9–2.2 mm) is similar to or less than the 2.2–2.3 mm crowding previously reported for untreated individuals.
      • Bishara S.E.
      • Jakobsen J.R.
      • Treder J.E.
      • Stasi M.J.
      Changes in the maxillary and mandibular tooth size–arch length relationship from early adolescence to early adulthood. A longitudinal study.
      • Bishara S.E.
      • Treder J.E.
      • Damon P.
      • Olsen M.
      Changes in the dental arches and dentition between 25 and 45 years of age.
      • Bondevik O.
      Changes in occlusion between 23 and 34 years.
      • Richardson M.E.
      A review of changes in lower arch alignment from seven to fifty years.
      This being the case, the malalignment that occurs, both after treatment and without treatment, must be due to similar causative factors.
      Figure thumbnail gr10
      Figure 10Post-retention incisor irregularity increases of patients treated with (A) nonextraction and (B) with extractions.

      What anterior mandibular malalignment is related to

      At the most basic level, malalignment of the anterior mandibular teeth is due to slight tooth movements that cause the contacts to slip and be displaced. Once the contacts are broken and the teeth are freed, they move out of alignment because a space is temporarily created. However, the space created is not maintained because the teeth move mesially until a new equilibrium is established. It is the rotations and displacements that occur while the teeth are compensating the increase IIREG and TSALD (Fig. 11).
      Figure thumbnail gr11
      Figure 11Reference chart showing various factors thought to move teeth, disrupt contacts, and lead to malalignment of the anterior dentition.
      The transseptal fibers play an important role in the dynamics of the process. The fibers extend from one tooth to adjacent teeth, and even to other teeth. They extend from the cementum of one tooth, over the inter-dental bone, to the cementum of the adjacent tooth. Transseptal fibers, which link all of the teeth together, maintain inter-dental contacts.
      • Stubley R.
      The influence of transseptal fibers on incisor position and diastema formation.
      The function of the transseptal fibers is to keep the teeth together; they serve as biological splints–nature’s retainers. In order to maintain tooth contacts, the transseptal fibers must also have a role in moving teeth. In order to survive, prehistoric populations had to maintain interproximal contacts while the mesial and distal surfaces of their teeth were being worn away by the rough diets they ate. In their classic study, Moss and Picton
      • Moss J.P.
      • Picton D.C.
      Short-term changes in the mesiodistal position of teeth following removal of approximal contacts in the monkey Macaca fascicularis.
      showed that the transseptal fibers are responsible for moving the teeth mesially when a space is created. They showed that if the occlusal surfaces of the teeth are ground out of occlusion (i.e., to eliminate the anterior component of force) and the interproximal surfaces are ground to create inter-dental space, the teeth move mesially to reestablish contact. On the foregoing basis, it has to be assumed that mesial drift of the teeth is due to the contractile mechanism of the transseptal ligament.
      • Nanci A.
      As previously suggested, treatment can result in crowding if the orthodontist breaks the rules. If teeth are put in a position of disequilibrium and are not retained, they will move and produce crowded arches. For example, Little et al.
      • Little R.M.
      • Riedel R.A.
      • Stein A.
      Mandibular arch length increase during the mixed dentition: postretention evaluation of stability and relapse.
      showed that increasing mandibular arch length during the mixed dentition in order to gain the space necessary to relieve crowding results in unacceptable levels of relapse. Almost 90% or their cases demonstrated clinically unsatisfactory alignment (incisor irregularity >3.5 mm) approximately 8-years post-treatment. Post-treatment incisor irregularity increases have also been related to treatment increases in intercanine width. Patients who have the greatest increases of intercanine width during treatment also show the greatest increases of crowding post-treatment.
      • Årtun J.
      • Garol J.D.
      • Little R.M.
      Long-term stability of mandibular incisors following successful treatment of Class II, Division 1, malocclusion.
      • Goldberg A.I.
      • Behrents R.G.
      • Oliver D.R.
      • Buschang P.H.
      Facial divergence and mandibular crowding in treated subjects.
      One of the major risk factors for malalignment of the anterior teeth is their point-to-point contacts. Broader surface contacts are more stable than point-to-point contacts. Experimentally, it has been shown that simulated arches with teeth that have concave/convex contact surfaces are considerably more stable than arches with point-to-point contacts.
      • Ihlow D.
      • Kubein-Meesenburg D.
      • Fanghänel J.
      • Lohrmann B.
      • Elsner V.
      • Nägerl H.
      Biomechanics of the dental arch and incisal crowding.
      The relationships of the teeth in the anterior, more curved, part of the simulated arches were the least stable. Interestingly, the teeth of prehistoric populations, who exhibited less malalignment than modern day populations, tended to have concave/convex inter-dental relationships of the posterior teeth and flat surface contacts of the anterior teeth.
      • Hinton R.J.
      Differences in interproximal and occlusal tooth wear among prehistoric Tennessee Indians: implications for masticatory function.
      Although the shape differences are not as extensive as in prehistoric populations, modern day populations exhibit thinner enamel on the mesial than distal surfaces of the molars and premolar,
      • Stroud J.L.
      • English J.
      • Buschang P.H.
      Enamel thickness of the posterior dentition: its implications for nonextraction treatment.
      with the mesial surface tending to be more concave. This, along with their broader contacts, could partially explain why the alignment of the posterior teeth tends to be better than the alignment of the anterior teeth. Broader contacts of the anterior teeth results in less crowding and irregularity. This is why post-treatment irregularity increases were significantly less (approximately 1.4 mm) in patients who have had interproximal reductions (i.e., stripping) at the end of treatment than in patients who had point-to-point contacts.

      Alexander JMA. Comparative Study of Orthodontic Stability in Class I Extraction Cases Master’s Thesis, Baylor University, 1996.

      The stability of point-to-point contacts appears to depend, at least in part, on the shape of the arch. Subjects with narrow mandibular arch forms show greater post-treatment crowding than subjects with broad arches.
      • Myser S.A.
      • Campbell P.M.
      • Boley J.
      • Buschang P.H.
      Long-term stability: post-retention changes of the mandibular anterior teeth.
      This relationship explains why teeth that have smaller inter-dental angles (angle between pairs of contralateral teeth) show greater post-treatment crowding than teeth with larger inter-dental angles. There is also an inverse relationship between the contact angles (another measure of arch shape) between adjacent teeth and the distances between their contact points (Fig. 12). The canine and lateral incisors have, as previously indicated, the greatest discrepancies between contact points post-treatment and the smallest contact angles. In other words, the site of greatest arch curvature has the greatest likelihood of slipping contacts and developing discrepancies. Differences in mandibular arch shape could explain why there is significantly more (0.8 mm) post-retention irregularity among extraction than nonextraction patients.
      • Myser S.A.
      • Campbell P.M.
      • Boley J.
      • Buschang P.H.
      Long-term stability: post-retention changes of the mandibular anterior teeth.
      The difference was probably not due to the extractions that they had, but rather due to the fact that patients who need extractions have narrower arch forms.
      Figure thumbnail gr12
      Figure 12Relationship between the contact angles of the six anterior teeth (blue line) and the amount of discrepancy between contacts (green line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
      Three general factors contribute to the malalignment of the anterior teeth. First, anterior-directed forces have the potential to move the teeth, break anterior contacts, and initiate the development of malocclusion. If posterior arch space is lost during the mixed dentition stage of development, the erupting premolars can cause the incisors to move, again setting up the potential to break contacts. Finally, eruption of the anterior teeth associated with inferior growth displacement of the mandible often participates in the development of crowding.

      Anterior components of force

      Anterior-directed forces on the teeth can potentially cause contacts to slip and contribute to the malalignment of the teeth. Due to the axial inclination of the posterior teeth, it is reasonable to think that they tip forward during occlusal loading, producing an anterior component of biting force. This was experimentally validated by Southard et al.,
      • Southard T.E.
      • Behrents R.G.
      • Tolley E.A.
      The anterior component of occlusal force. Part 1. Measurement and distribution.
      who placed 0.0015-in stainless steel strips between teeth and measured the differences in force necessary to pull out the strip when adult subjects were and were not biting down maximally on the right second molars. The differences in interproximal contact strengths showed that there was an anterior component of force, which was greatest between the first molars and second premolars and decreased progressively across to the other side of the arch. A year later, the same group showed that the anterior component of biting force was greater in subjects with amounts of incisor irregularity.
      • Southard T.E.
      • Behrents R.G.
      • Tolley E.A.
      The anterior component of occlusal force. Part 2. Relationship with dental malalignment.
      Patients who were followed up 3.5 years post-retention also show positive relationships between the anterior component of force and IIRREG.
      • Acar A.
      • Alcan T.
      • Erverdi N.
      Evaluation of the relationship between the anterior component of occlusal force and postretention crowding.
      In addition to biting forces, anything else that takes up arch space has the potential to move the teeth and cause the contacts to slip. For example, the teeth can be forced anteriorly by restorations, which are supposed to be “tight.” Orthodontic patients who received post-treatment interproximal restorations exhibited significantly greater increases in incisor irregularity (0.9 mm) and crowding (0.4 mm) over the 15.6-year observation period than those who did not have restorations.
      • Myser S.A.
      • Campbell P.M.
      • Boley J.
      • Buschang P.H.
      Long-term stability: post-retention changes of the mandibular anterior teeth.

      Early space loss

      Although not as important a factor as some think, tooth size is definitely related to crowding. The mesiodistal sizes of the incisors have been correlated—albeit at relatively low levels—to crowding.
      • Fastlicht J.
      Crowding of mandibular incisors.
      Larger incisors are found among individuals with malocclusion than among those with normal occlusion.
      • Norderval K.
      • Wisth P.J.
      • Böe O.E.
      Mandibular anterior crowding in relation to tooth size and craniofacial morphology.
      • Doris J.M.
      • Bernard B.W.
      • Kuftinec M.M.
      • Stom D.
      A biometric study of tooth size and dental crowding.
      More recently, Agenter et al.
      • Agenter M.K.
      • Harris E.F.
      • Blair R.N.
      Influence of tooth crown size on malocclusion.
      showed that the crown dimensions (mesiodistal and buccolingual) of subjects with malocclusion are consistently larger than the same dimensions in subjects with normal occlusion, but the associations that they identified were low, usually explaining less than 10% of the variation.
      With the exception of the third molars, the number of teeth present in the mouth is also related to increased risk of crowding (i.e., the fewer the teeth, the less the crowding). Based on a large sample of untreated adults 15–50 years of age who participated in the NHANES III survey, it has been shown that those who have both of their second molars present have mandibular IIRREG that is 1-mm greater than subjects with one or none of their molars present.
      • Buschang P.H.
      • Shulman J.D.
      Incisor crowding in untreated persons 15-50 years of age: United States, 1988-1994.
      Richardson and Mills
      • Richardson M.
      • Mills K.
      Late lower arch crowding: the effect of second molar extraction.
      previously showed that individuals whose second molars had been extracted exhibited a slight (0.75 mm) decrease in anterior crowding and 0.4–0.6-mm distal movements of the first molars; individuals who did not have the extractions showed significant (2.0 mm) increases in crowding and mesial movements of the first molars. Adults who have both the mandibular first molars, second premolars, and first premolars are also at a higher risk of having irregularities; having both teeth vs one or none of each pair present increases incisor irregularity by approximately 1 mm.
      • Buschang P.H.
      • Shulman J.D.
      Incisor crowding in untreated persons 15-50 years of age: United States, 1988-1994.
      There have also been a number of studies that have linked arch size to crowding. For example, crowding has been reported to be inversely related to arch width and arch depth,
      • Sampson W.J.
      • Richards L.C.
      Prediction of mandibular incisor and canine crowding changes in the mixed dentition.
      • Howe R.P.
      • McNamara Jr, J.A.
      • O’Connor K.A.
      An examination of dental crowding and its relationship to tooth size and arch dimension.
      suggesting that individuals with smaller arches have greater amounts of crowding. While such associations are reasonable, they should be interpreted with caution because the teeth of individuals who exhibit crowding move mesially into a narrower part of the arch, which necessarily decreased arch widths and depths. In other words, the relationship between jaw size and crowding should not be based on measurements of dental arch dimensions. This is not meant to imply that there is no relationship between jaw size and crowding, because there probably is. After all, the small jaw sizes of individuals with maxillary synostosis (e.g., Apert and Crouzon syndromes) cause substantial crowding and even ectopic eruption of teeth.
      Premature space loss and associated tooth movements provide another explanation for crowding. Leeway space, first described by Nance
      • Nance H.
      The limitations of orthodontic treatment: I. Mixed dentition diagnosis and treatment.
      as providing 1.7 mm of space per side in the mandible, actually provides closer to 2.5 mm per side in the mandible and 1.5 mm per side in the maxilla.
      • Moyers R.E.
      • van der Linden F.P.G.M
      • Riolo M.L.
      • McNamara J.A.
      Early loss of leeway space before the permanent premolars erupt substantially increases the risk of crowding. Severe caries or premature exfoliation of the deciduous molars results in 4–5 mm of space loss per side (Fig. 13A), most of which is taken up by mesial migration of the posterior teeth (Fig. 13B).
      • Northway W.M.
      • Wainright R.L.
      • Demirjian A.
      Effects of premature loss of deciduous molars.
      Figure thumbnail gr13
      Figure 13Changes in (A) D + E space and (B) lower molar mesial migration associated with severe caries and premature loss of deciduous molars. (Adapted with permission from Northway et al.
      • Northway W.M.
      • Wainright R.L.
      • Demirjian A.
      Effects of premature loss of deciduous molars.
      )
      There also appears to be an association between the sequence of eruption and crowding. The way in which the leeway space is utilized depends in part on the sequence of eruption of the posterior teeth.
      • Moorrees C.F.
      • Gron A.M.
      • Lebret L.M.
      • Yen P.K.
      • Fröhlich F.J.
      Growth studies of the dentition: a review.
      In the mandible, the canines usually erupt before the first premolars, and this is the favorable sequence. The most unfavorable eruption sequences in the mandible are those in which the canines erupt before the premolars and when the second molars erupt before the canines and premolars.
      • Lo R.
      • Moyers R.E.
      Studies in the etiology and prevention of malocclusion: I. The sequence of eruption of the permanent dentition.
      Recently, Lange

      Lange GM. Correlations of sequence of eruption and crowding. Saint Louis University Master’s Thesis, St. Louis, MO, 2011.

      found substantially less crowding (≈2.5 mm) among untreated subjects who had left and right 345 (Palmer notation) eruption sequences, than among those who had left and right 435 eruption sequences.

      Vertical eruption

      While growth generally helps the orthodontist, it is a potential risk factor for the development of crowding. The predominant type of growth that occurs is vertical growth of the mandible, which is directly associated with eruption. In fact, the inferior displacement of the mandible that occurs between 10.5 and 14.5 years of age explains approximately 53% (R = 0.73) of the variation in lower molar eruption.
      • Liu S.S.
      • Buschang P.H.
      How does tooth eruption relate to vertical mandibular growth displacement?.
      Recall that the vertical eruption is a compensation for the vertical growth that occurs. This was first clearly established in a long-term follow-up study that compared 40 stable (incisor irregularity Δ < 1 mm) to 33 unstable (incisor irregularity Δ > 2 mm) Class I extraction patients.
      • Little R.M.
      • Riedel R.A.
      • Stein A.
      Mandibular arch length increase during the mixed dentition: postretention evaluation of stability and relapse.
      While the anterior displacement of the mandible and mesial movements of the teeth did not differ between the two groups, the inferior displacement of the mandible and superior eruption of the mandibular incisors were both significantly greater in the unstable group. These relationships were later confirmed in subjects who were followed up 13.7 years post-treatment as well as in untreated subjects who were followed up between 14.3 and 23.2 years of age.
      • Driscoll-Gilliland J.
      • Buschang P.H.
      • Behrents R.G.
      An evaluation of growth and stability in untreated and treated subjects.
      Facial height increases and lower incisor eruption were both significantly related to the development of malaligned teeth, particularly in the untreated group, probably because they exhibited greater vertical growth. More recently, it was shown that adolescents who were followed up 15.6 years post-treatment exhibited significantly greater increases in incisor irregularity and the PAR index than adults who were similarly treated and followed up 16.1 years post-treatment. Most recently, Goldberg et al.
      • Goldberg A.I.
      • Behrents R.G.
      • Oliver D.R.
      • Buschang P.H.
      Facial divergence and mandibular crowding in treated subjects.
      again showed that greater vertical growth, greater incisor eruption, and especially facial divergences were all related to greater post-treatment mandibular crowding. Greater amounts of growth require greater eruption to compensate for the vertical space created; eruption decreases the likelihood that the contacts between the anterior teeth will be maintained and increases the risk of crowding.
      The relationship between growth, eruption, and crowding is important because many patients, particularly males, should be expected to exhibit clinically meaningful amounts of vertical growth well into their 20s. For example, 15.5-year-old males have attained only 90% of the ramus height that they will have attained at 22.4 years of age.
      • Buschang P.H.
      • Baume R.M.
      • Nass G.G.
      A craniofacial growth maturity gradient for males and females between 4 and 16 years of age.
      Importantly, individuals in their 20s still have clinically meaningful amounts of vertical growth potential, both in anterior and posterior facial heights (Fig. 14).
      • Forsberg C.M.
      Facial morphology and ageing: a longitudinal cephalometric investigation of young adults.
      • Sarnäs K.V.
      • Solow B.
      Early adult changes in the skeletal and soft-tissue profile.
      • Behrents R.G.
      • Bishara S.E.
      • Treder J.E.
      • Jakobsen J.R.
      Facial and dental changes in adulthood.
      • West K.S.
      • McNamara Jr., J.A.
      Changes in the craniofacial complex from adolescence to midadulthood: a cephalometric study.
      Figure thumbnail gr14
      Figure 14Adult changes in upper anterior face height (UAFH), lower anterior facial height (LAFH), and posterior face height (PFH).

      Conclusions

      There are numerous factors—each explaining relatively small portions of the overall variation seen between individuals—working in combination to produce malalignments of the teeth. Maintaining the contacts between the anterior teeth is the most important way to reduce future crowding and irregularities. Broader contacts decrease the risk of slippages that occur when the teeth move, and the teeth will move for a variety of reasons. If point-to-point contacts are maintained, then orthodontists must either keep patients in permanent retention or do everything possible to mitigate the effects of anteriorly directed forces. For example, referring dentists should be told that large tight restorations are problematic, especially for hyperdivergent patients with narrow arches. Perhaps most importantly, orthodontists should tell their patients to wear their retainers until the end of active vertical growth, at least until the early- and mid-20s for females and males, respectively.

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