Crown dimensions and the alignment or crowding of mandibular incisors

The recent study, “Incisor crown shape and crowding” (Shah AA, Elcock C, Brook AH. Am J Orthod Dentofacial Orthop 2003;123:562-7), revisits a subject dear to me. Over 30 years ago, my late brother Harvey and I introduced to orthodontists a tooth shape index (MD/FL) to evaluate 2-dimensional anatomical differences among homologous teeth, particularly the often-crowded mandibular incisors.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar Our conclusion was that “A consideration of tooth shape and the MD/FL index appears essential for the successful orthodontic management of mandibular incisor irregularities.”As rightly noted by Shah et al, our article was followed by many studies from others, examining the relationship between mandibular incisor crowding and incisor crown size or shape. Their study, and most of those before them, found “no predictors of lower incisor crowding…from mandibular incisor crown shape.” Their article provides an opportunity for me to discuss and critique these findings.Most orthodontists acknowledge the following clinical facts: some degree of mandibular incisor crowding is virtually endemic in orthodontic populations, pretreatment and long-term posttreatment; the etiology of this condition is multifactorial. So is it any wonder that studies of subjects with incisor crowding have routinely yielded confounding, equivocal, or statistically unremarkable results? We tried to avoid these pitfalls in designing our research on mandibular incisor tooth shape in the 1970s.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar, 2Peck S. Peck H. Crown dimensions and mandibular incisor alignment.Angle Orthod. 1972; 42: 148-153PubMed Google Scholar We examined nonorthodontic subjects selected for their naturally well-aligned incisors, not crowding. In studying the absence of crowding, we attempted to neutralize the confounding factors associated with the mandibular incisor crowding phenomenon, thus permitting a “noise-free” focus on incisor crown dimensions. This is the same scientific rationale that drives molecular biologists today to construct “knock-out” experiments to be able to focus on the functions of a single gene (or factor). Why, then, would Shah et al and most of the investigators before them select study samples with multifactored dental crowding that give little promise of yielding clear-cut results on associations between crowding and specific factors like the size and shape of the mandibular incisors? I do not know the reason, but I have a guess: samples with crowding are used in these studies simply because they are much easier for orthodontists and students to collect than untreated ideals.Another methodological problem in Shah et al (and in other mandibular incisor studies) involves inadequacies of the irregularity index (IrI), which they used to measure mandibular incisor crowding. The IrI is a collective measure of lower anterior tooth displacement. Although this method might be a serviceable epidemiological tool, it fails to quantify accurately most mandibular incisor crowding configurations. Lower incisors with arch space deficiency tend to compensate as much by rotation and overlap crowding —which the IrI mismeasures—as by displacement crowding; the type of crowding expressed is usually a function of the incisor's shape.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar Furthermore, the IrI cannot measure pure rotational irregularities, such as midline incisor winging often seen among Asian patients. Of course, these complications could be avoided by studying a sample without incisor irregularity.A third methodological fault in Shah et al is inaccurate and arbitrary measurements of tooth dimensions. The authors performed their odontometry indirectly, applying metric software to digital images of dental casts, a process involving 2 transformations of the teeth to be measured: (1) the fabrication and modification of the plaster cast, and (2) its subsequent computer imaging from which tooth measurements were extracted. Their reported incisor MD widths are considerably smaller than European and North American tooth-size norms.2Peck S. Peck H. Crown dimensions and mandibular incisor alignment.Angle Orthod. 1972; 42: 148-153PubMed Google Scholar This is a red flag, because their samples were from a pretreatment group of young adults with “varying degrees of crowding,” patients whose tooth widths should have been larger than average, not smaller. Moreover, I cannot understand the reason behind their construction and measurement of a nonanatomic “midpoint level” of a mandibular incisor in relation to its crowding/alignment status, except that it could be an easy target for their imaging methods. I suspect that a motivating force behind the article might have been the requisite electro-optical image analysis system in their laboratory. This would not be the first time a well-meaning biometric study is derailed by a new research method or analysis system that investigators want to showcase or validate.I believe these intrinsic flaws are largely responsible for the weak results in the Shah et al article (and in some of the similar earlier studies they cite). Given the multiple etiologic factors intertwined in the incisor crowding/recrowding conundrum, an exact relationship between mandibular incisor crowding and variations in incisor morphology might never be elucidated. This acknowledgment does not diminish the useful association between mandibular incisor tooth shape and the incisor alignment/irregularity equation that we uncovered from studying a sample with naturally well-aligned mandibular incisors and then applied with inferential reasoning to mandibular incisor crowding. The MD/FL index identifies atypically shaped incisors that experienced clinicians recognize often as candidates for reproximation (mesiodistal enamel reduction). A tooth shape analysis incorporating an accurately (intraorally) measured MD/FL index is highly effective in these circumstances to establish limits for mandibular incisor reproximation on a precise anatomical basis,3Peck H. Peck S. Reproximation (“enamel stripping”) as an essential orthodontic treatment ingredient.in: Cook J.T. Transactions of the Third International Orthodontic Congress, London. C. V. Mosby, St. Louis1975: 513-523Google Scholar rather than under the earlier haphazard rubric of “judicious stripping.” The recent study, “Incisor crown shape and crowding” (Shah AA, Elcock C, Brook AH. Am J Orthod Dentofacial Orthop 2003;123:562-7), revisits a subject dear to me. Over 30 years ago, my late brother Harvey and I introduced to orthodontists a tooth shape index (MD/FL) to evaluate 2-dimensional anatomical differences among homologous teeth, particularly the often-crowded mandibular incisors.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar Our conclusion was that “A consideration of tooth shape and the MD/FL index appears essential for the successful orthodontic management of mandibular incisor irregularities.” As rightly noted by Shah et al, our article was followed by many studies from others, examining the relationship between mandibular incisor crowding and incisor crown size or shape. Their study, and most of those before them, found “no predictors of lower incisor crowding…from mandibular incisor crown shape.” Their article provides an opportunity for me to discuss and critique these findings. Most orthodontists acknowledge the following clinical facts: some degree of mandibular incisor crowding is virtually endemic in orthodontic populations, pretreatment and long-term posttreatment; the etiology of this condition is multifactorial. So is it any wonder that studies of subjects with incisor crowding have routinely yielded confounding, equivocal, or statistically unremarkable results? We tried to avoid these pitfalls in designing our research on mandibular incisor tooth shape in the 1970s.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar, 2Peck S. Peck H. Crown dimensions and mandibular incisor alignment.Angle Orthod. 1972; 42: 148-153PubMed Google Scholar We examined nonorthodontic subjects selected for their naturally well-aligned incisors, not crowding. In studying the absence of crowding, we attempted to neutralize the confounding factors associated with the mandibular incisor crowding phenomenon, thus permitting a “noise-free” focus on incisor crown dimensions. This is the same scientific rationale that drives molecular biologists today to construct “knock-out” experiments to be able to focus on the functions of a single gene (or factor). Why, then, would Shah et al and most of the investigators before them select study samples with multifactored dental crowding that give little promise of yielding clear-cut results on associations between crowding and specific factors like the size and shape of the mandibular incisors? I do not know the reason, but I have a guess: samples with crowding are used in these studies simply because they are much easier for orthodontists and students to collect than untreated ideals. Another methodological problem in Shah et al (and in other mandibular incisor studies) involves inadequacies of the irregularity index (IrI), which they used to measure mandibular incisor crowding. The IrI is a collective measure of lower anterior tooth displacement. Although this method might be a serviceable epidemiological tool, it fails to quantify accurately most mandibular incisor crowding configurations. Lower incisors with arch space deficiency tend to compensate as much by rotation and overlap crowding —which the IrI mismeasures—as by displacement crowding; the type of crowding expressed is usually a function of the incisor's shape.1Peck H. Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors.Am J Orthod. 1972; 61: 384-401Abstract Full Text PDF PubMed Scopus (93) Google Scholar Furthermore, the IrI cannot measure pure rotational irregularities, such as midline incisor winging often seen among Asian patients. Of course, these complications could be avoided by studying a sample without incisor irregularity. A third methodological fault in Shah et al is inaccurate and arbitrary measurements of tooth dimensions. The authors performed their odontometry indirectly, applying metric software to digital images of dental casts, a process involving 2 transformations of the teeth to be measured: (1) the fabrication and modification of the plaster cast, and (2) its subsequent computer imaging from which tooth measurements were extracted. Their reported incisor MD widths are considerably smaller than European and North American tooth-size norms.2Peck S. Peck H. Crown dimensions and mandibular incisor alignment.Angle Orthod. 1972; 42: 148-153PubMed Google Scholar This is a red flag, because their samples were from a pretreatment group of young adults with “varying degrees of crowding,” patients whose tooth widths should have been larger than average, not smaller. Moreover, I cannot understand the reason behind their construction and measurement of a nonanatomic “midpoint level” of a mandibular incisor in relation to its crowding/alignment status, except that it could be an easy target for their imaging methods. I suspect that a motivating force behind the article might have been the requisite electro-optical image analysis system in their laboratory. This would not be the first time a well-meaning biometric study is derailed by a new research method or analysis system that investigators want to showcase or validate. I believe these intrinsic flaws are largely responsible for the weak results in the Shah et al article (and in some of the similar earlier studies they cite). Given the multiple etiologic factors intertwined in the incisor crowding/recrowding conundrum, an exact relationship between mandibular incisor crowding and variations in incisor morphology might never be elucidated. This acknowledgment does not diminish the useful association between mandibular incisor tooth shape and the incisor alignment/irregularity equation that we uncovered from studying a sample with naturally well-aligned mandibular incisors and then applied with inferential reasoning to mandibular incisor crowding. The MD/FL index identifies atypically shaped incisors that experienced clinicians recognize often as candidates for reproximation (mesiodistal enamel reduction). A tooth shape analysis incorporating an accurately (intraorally) measured MD/FL index is highly effective in these circumstances to establish limits for mandibular incisor reproximation on a precise anatomical basis,3Peck H. Peck S. Reproximation (“enamel stripping”) as an essential orthodontic treatment ingredient.in: Cook J.T. Transactions of the Third International Orthodontic Congress, London. C. V. Mosby, St. Louis1975: 513-523Google Scholar rather than under the earlier haphazard rubric of “judicious stripping.” Crown dimensions and the alignment or crowding of mandibular incisors: Authors' responseAmerican Journal of Orthodontics and Dentofacial OrthopedicsVol. 124Issue 4Preview Full-Text PDF