Harry L. Legan

The form of the human dental arch.

The human dental arch form is shown to be accurately represented mathematically by the beta function. The average correlation coefficient between measured arch-shape data and the mathematical arch shape, expressed by the beta function, is 0.98 with a standard deviation of 0.02. Forty sets of casts--15 Class I, 16 Class II, and 9 Class III--were examined. A precision machine tool device was used to record the X-, Y-, and Z-coordinates of selected dental landmarks on all casts to 0.001 mm accuracy. The coordinates were processed through a computer curve-fitting program. The Class III mandibular arches had smaller arch depth and greater arch width (beginning in the premolar area) than the Class I arches. The Class II mandibular arches exhibited generalized reduced arch width and depth compared with the Class I arches. Maxillary arch depths were similar in all three groups. However, the Class III maxillary arch widths were greater from the lateral incisor-canine area distally compared with the Class I maxillary arch, and the Class II maxillary arch form was narrower than the Class I arch form from the lateral incisor-canine area distally. The beta function more accurately described the dental arch form than representations previously reported.

On the management of extraction sites

Extraction sites may be needed to achieve specific orthodontic goals of positioning the dentition within the craniofacial complex. The fundamental reality that determines the final position of the dentition, however, is the control exercised by the clinician in closure of the extraction sites. A specific treatment objective may require the posterior teeth to remain in a constant position anteroposteriorly as well as vertically, while the anterior teeth occupy the entire extraction site. Another treatment objective may require the reverse, or any number of purposeful alternatives of extraction site closure. An appliance system developed over time, which provides this control, is described. The system takes advantage of aspects of continuous arch therapy that provides constant, positive orientation of the anterior and posterior groups of teeth to each other in three-dimensional space across an extraction site, combined with aspects of the segmented arch technique that permit definable and predictable force systems to be applied to these teeth. Consequently, the clinician has the ability to forecast treatment outcomes with confidence.

Changes in occlusion related to the cant of the occlusal plane

It is known that dental occlusion is influenced by changes in the cant of the occlusal plane. This study has defined the geometric and mathematical relationships between dental occlusion and rotations of the occlusal plane in the sagittal view. As a general clinical guide, each degree of rotation of the occlusal plane will result in a half millimeter change in the dental occlusal relationship. This is of importance, because changes in the cant of the occlusal plane are sometimes unintentional, as well as intentional, during orthodontic therapy. An earlier study has also documented that the occlusal plane rotates naturally upward and forward approximately 6 degrees during growth and development. This phenomenon tends to develop a Class II dental relation and therefore has important implications for the developing dentition.

Correction of severe vertical maxillary excess with anterior open bite and transverse maxillary deficiency.

Patients requiring correction of large anterior open bites have historically been among the most challenging treatments for orthodontists. Adding to that fundamental challenge for the adult patient in this case was vertical maxillary excess, a severe transverse maxillary deficiency as well as an arch length inadequacy, even though the patient had prior orthodontic treatment. The prior orthodontist had included arch expansion and extracted four first bicuspids, which limited current treatment options. Various treatment modalities that have traditionally been used to correct transverse maxillary deficiency and the accompanying arch length inadequacy include extractions, labial and buccal dental tipping, segmental maxillary osteotomies, and rapid maxillary expansion with or without surgical assistance. Transverse maxillary distraction osteogenesis is a modification (ie, using a latency period and specific rate and rhythm of distraction) of the surgically assisted rapid maxillary expansion technique developed 25 years ago. This case demonstrates the relationship of transverse maxillary deficiency as well as vertical maxillary excess to apertognathia. Considerations regarding the use of segmental maxillary osteotomy vs transverse distraction osteogenesis are discussed. This case report illustrates the benefit of a team approach using transverse maxillary distraction osteogenesis, effective orthodontic mechanics, and orthognathic surgery to correct a severe dentofacial deformity.

Surgical correction of patientswith asymmetries

Facial asymmetry is quite common and, when sufficiently severe, can warrant surgical orthodontic intervention. The causes of facial asymmetry are numerous and can be generally classified as congenital, developmental, or subsequent to pathology or injury. A systematic and comprehensive examination, diagnosis, and treatment plan are requirements for successful correction of facial asymmetry. Several of the more common facial asymmetries presenting for treatment to the orthodontist and surgeon are discussed in this article. Some fundamental surgical and orthodontic principles for treating these deformities are described. The purpose of this article is to provide a brief overview of a surgical and orthodontic approach to the evaluation and treatment of some of the more common facial asymmetries.

Masseter muscle adaptation following surgical correction of vertical maxillary excess

This study was designed to examine the adaptive response of the human masseter muscle following surgical correction of abnormal facial form. Biopsies of the deep surface of the anterior superficial masseter muscle were obtained from five patients demonstrating vertical maxillary excess (VME), one at the time of corrective surgery, and a second at a long-term postoperative time interval (mean, 8 months). Control biopsies were also obtained from five individuals (three cadavers and two patients) with normal dentofacial morphology. A standard regimen of histologic and histochemical staining was used to classify individual muscle fibers as either type 1, type 2, or intermediate. In both VME patients and normal subjects type 1 fibers predominated, with the control group displaying a higher percentage (50% vs. 43%). On the average, type 1 fibers also had the largest areas. Following surgery there was an increase in type 2 fibers (30% vs. 52%). Two patients receiving maxillary surgery only exhibited either no change in mean fiber area or only a slight increase. The three remaining patients who underwent concomitant maxillary and mandibular surgery all showed a significant decrease in mean fiber area. Two persons in the latter group also showed features consistent with a denervation-reinnervation process. The results of this study indicate that the human masseter responds to surgical manipulation of the jaws in a histochemically demonstrable manner, with the nature and magnitude of the response associated with the particular surgical procedure(s) performed.

A reexamination of various extraoral appliances in light of recent research findings.

The location of the center of resistance of the dentomaxillary complex has recently been identified more accurately than before. Based on this new finding, various modifications of the common facebow are presented for use in protraction therapy. Clinical applications for specific treatment objectives are also reviewed. Orthopedic and biomechanical implications of various standard retraction type extraoral appliances are also analyzed.

Treatment of a patient with multiple impacted teeth.

An impacted or missing permanent tooth can add significant complications to an otherwise straightforward case. When multiple impacted teeth are present, the case complexity increases further. Developing a treatment sequence, determining appropriate anchorage, and planning and executing sound biomechanics can be a challenge. The following case report illustrates a patient reportedly diagnosed with mild scleroderma as an adolescent. He presented for orthodontic treatment as an adult with multiple retained primary teeth and multiple impacted teeth. Diagnosis, treatment planning, and various methods of managing guided eruption of impacted teeth will be discussed. Following orthodontic treatment that required extraction of multiple primary and permanent teeth as well as exposure and ligation of multiple permanent teeth by an oral surgeon, the patient finished with a significantly improved functional and esthetic result.

Variable modulus orthodontics advanced through an auxiliary archwire attachment.

Reducing the load deflection rates of orthodontic springs is important, for it provides relative constancy of the moment-to-force ratio applied to the teeth with concomitant, forecastable dental movement. Increasing patient comfort and reducing the number of office visits while lowering potential tissue damage are additional features of lower load deflection rate springs. A simple auxiliary attachment, which can be crimped into position on an archwire or onto segments of an archwire, is described. This attachment permits the clinician to incorporate a relatively high rate stiff wire to enhance the anchorage of the reactive teeth in one area of the dental arch, while allowing the use of lesser stiffness (lower load deflection rate spring) to engage teeth targeted for movement. The auxiliary allows the clinician various stiffness through the use of wire of one modulus (stainless steel, for example) in one area of the arch, and wire of a differing modulus (NiTi, for example) in another area of the same arch. The advantages and disadvantages of choosing wires of differing moduli are reviewed. Alternative methods of transforming the spring rate through changes in wire cross-section or length are also reviewed. Practical clinical applications of the auxiliary attachment are shown.