Taking the guesswork out of mandibular symphyseal distraction osteogenesis.

Mandibular symphyseal distraction osteogenesis has recently been introduced as a means of resolving arch length deficiencies in the anterior segment and as a method of reducing large vestibular spaces related to a narrow mandible. Accurately relating the required distraction for a given anterior tooth mass and desired future anteroposterior location of the central incisors has not been possible until recently. The relationship between these 3 controlling factors has been mathematically described by the hyperbolic cosine function and a computer program designed for easy use by the clinician. Two clinical cases illustrate the application of the program. A Web site where the program can be downloaded at no cost is mentioned.

The relationship of arch length to alterations in dental arch width.

An accurate method is presented for forecasting alterations in arch length related to various width increases in each dental arch. It is based on combined beta and hyperbolic cosine functions which express the expanded dental arches with correlation coefficients of r = 0.98, between measured data and representations of the dental arch. When the midpalatal suture is expanded, canine width and molar width alterations are not equal because the line of action of the expanding force is anterior to the center of resistance of the dentomaxillary complex. Therefore, canine to molar width ratio alterations of 1:1, 1.25:1, and 1.5:1 are examined, and simple linear functions are presented for purposes of predicting changes in arch length.

Stress analysis of human tooth root using various root canal instruments.

This study compared external root stresses from instrumentation by several motor-driven nickel-titanium instruments to stresses developed by hand files. Twelve curved first molar mesial roots were invested in PLM-9 photoelastic material to disclose net stress transmitted from the canal through the dentin. Minimal fringe patterns were observed during instrumentation with all techniques. Light-speed yielded the least change. In no case was a complete 60 psi fringe change noted.

An evaluation of the shape of some popular nickel titanium alloy preformed arch wires.

The mathematical Beta function is shown to be an accurate planar representation of the natural human arch form defined by the spatial coordinates of the labial and buccal dental/bracket interfacing surfaces in the maxillary and mandibular arches. Graphic planar representations of the corresponding bracket base spatial coordinates of 33 popular preformed nickel titanium arch wires and bracket assemblies were superimposed on each of the relevant maxillary and mandibular natural forms. The arch forms of the preformed nickel titanium arch wires and bracket assemblies did not emulate the natural human arch form. The average mandibular natural human arch form first molar/canine width ratio is 2.38/1; the same preformed arch wire/bracket ratio is 1.87/1. These ratios for the maxillary arch are 1.92/1 and 1.54/1, respectively. The average canine width exceeded the natural canine width by 5.95 mm in the mandibular arch and 8.23 mm in the maxillary arch. The corresponding mandibular first molar and maxillary first molar widths exceeded the natural human first molar arch width by 0.84 mm and 2.68 mm, respectively. These findings have implications with respect to posttreatment stability and facial esthetics. "Round tripping" teeth resulting from subsequent change to stainless steel arch wires to restore a more natural human arch form and size may result in deleterious tissue effects.

A new accurate approach to the anterior ratio with clinical applications. Part II: a nomographic solution.

Evaluating the anterior ratio is useful to forecast potential tooth mass discrepancies and related overjet or underjet. Because the arcs of the six anterior teeth have recently been described by the mathematical hyperbolic cosine function, it has permitted the design of a nomograph for ready use in clinical practice. A nomograph is a simple chart on which one can draw a straight line that will intersect three scales representing variables that satisfy an equation. A clinician may now evaluate and forecast the effects on overjet with purposeful alterations in the sums of the six maxillary and mandibular anterior tooth masses for various cross-arch canine widths with an accuracy of +/-0.5 mm.

A new accurate approach to the anterior ratio with clinical applications. Part 1: a computer program.

The arcs of the six anterior maxillary and mandibular teeth have recently been described mathematically by the hyperbolic cosine function with a maxillary correlation coefficient (r ) of 0.885 and a mandibular correlation coefficient (r ) of 0.951. Because the geometric relationships of the anterior dental arcs are known when the occlusion is Class I, a computer program has been developed for use in clinical practice. Rapid forecasting of the interrelationships between the maxillary and mandibular arc depths (related to overjet) with variations in the mesiodistal sums of the six maxillary and mandibular anterior teeth for various intercanine widths is now possible with ease and accuracy (+/- 0.1 mm). Clinical applications are illustrated.

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.

Dynamic relationships of the mandibular anterior segment.

The hyperbolic cosine function is shown to be an accurate representation of the form of the mandibular anterior teeth from the canine/first premolar contact on one side around the perimeter to the opposite side (r = 0.951). On the basis of this mathematical function, the changes in canine width, anterior segment depth, arch perimeter, and their related incisor angular alterations are forecastable. This knowledge will allow the clinician to predict the effects on various aspects of the anterior segment arch form as one or more of these variables are altered without resorting to trial and error or performing a wax-up. For example, the clinician can predict the change in the anterior segment arch depth and incisor angulation that would occur with alterations in canine width.

The curve of Spee revisited.

Through the use of a sophisticated measuring device and support computer technology, accurate arch circumferences were determined for 27 casts that exhibited moderate to severe curves of Spee. Arch circumference differences were subsequently obtained by comparing the measured arch length to a planar projection formed by the center of the incisal tips anteriorly and the distobuccal cusp tips of the second molars distally. A general relationship has been derived for the arch circumference differential, resulting from the elimination of the curve of Spee, versus the severity of the curve. The arch circumference reduction is considerably less than that found by earlier investigators, implying that the incisor protrusion often associated with leveling the curve of Spee is not primarily due to the aforementioned differential, but rather more directly due to the mechanics used in leveling the curve of Spee.

A study of maximum bite force during growth and development.

Bilateral bite force was measured in a sample of 457 subjects (231 males and 226 females) from 6 years through 20 years. The mean maximum bite force was found to increase from 78 Newtons at 6 to 8 years to 176 Newtons at 18 to 20 years. While earlier studies have shown adult males have a greater mean bite force than females, this difference is not evident during growth and development. Gender-related bite force difference likely develops during the postpubertal period in association with greater muscle mass development in males.

A study of bite force, part 1: Relationship to various physical characteristics.

A new device for measuring and recording bilateral bite force in the molar/premolar region has been developed. Because this new device is elastic and conforms to the occlusal surfaces of the teeth, and because the sensing element is relatively comfortable, it is believed that experimental subjects are less reluctant to register true maximal forces than in earlier studies. Potential correlations of maximum bite force to gender, age, weight, body type, stature, previous history of orthodontic treatment, presence of TMJ symptoms (jaw motion limitation, clicking with pain, or joint pain), or missing teeth were studied in a sample of 142 dental students. The mean maximum bite force of the sample was found to be 738 N, with a standard deviation of 209 N. The mean maximum bite force as related to gender was found to be statistically significant, while the correlation coefficients for age, weight, stature, and body type were found to be low. Even so, all data scatterplots exhibited relatively positive relationships. Correlations of maximum bite force to an earlier history of orthodontic treatment or to the absence of teeth were not found. Subjects reporting TMJ symptoms did not exhibit a significantly different maximum bite force than subjects without symptoms.

A study of bite force, part 2: Relationship to various cephalometric measurements.

Maximum bilateral bite force, determined in 129 dental students, was evaluated with regard to six skeletal and eight dental measurements acquired from conventional lateral cephalometric radiographs. Statistically significant correlations for three of the skeletal measurements were found. Maximum bite force increased with regard to decreasing mandibular plane/palatal plane angle and to decreasing mandibular plane angles. Maximum bite force increased with an increasing ratio of posterior facial height to anterior facial height. Significant statistical correlation for only one of the eight dental measurements was found: maximum bite force related directly with increasing maxillary and/or mandibular dentoalveolar heights, and unexpected finding.

Distal radio-ulnar ligament motion during supination and pronation.

The dorsal and palmar distal radio-ulnar ligaments (DRUL) play an important role in the stability of the distal radio-ulnar joint (DRUJ). Various authorities, however, hold opposite opinions regarding DRUL motion during DRUJ pronation and supination, thus implying opposite techniques for reconstruction of the unstable DRUJ. With the hypothesis that relative displacement would increase in the dorsal DRUL during pronation and would increase in the palmar DRUL during supination, measurements were made of the relative DRUL displacement with a Hall-effect displacement transducer during DRUJ pronation and supination in six fresh cadaver wrists. The hypothesis was confirmed that the dorsal radio-ulnar ligament undergoes relative displacement during pronation, while the palmar radio-ulnar ligament undergoes relative displacement during supination.

Evaluation of the vertical forces generated by the cervical biteplate facebow.

The biteplate facebow has been recommended for use in the correction of Class II malocclusions with deep overbites. This facebow is similar in design to the conventional cervical facebow with the addition of an inner bow metal plate. The plate presses against the maxillary incisors and prevents the patient from fully closing, thus acting as a biteplate. A test apparatus was constructed to simulate the force system present during application of the facebow. In this study, high resolution force transducers were used to measure the intrusive forces on the maxillary and mandibular incisors. Static force analysis techniques were then used to calculate the vertical force component of the first molars. Analyses were performed using a wide range of relative bow angles, neck strap tensions of 200 grams and 400 grams, and various mandibular incisor occluding forces. The molar eruptive forces of the biteplate facebow are found to exceed those of the standard cervical facebow by a low of 158% to a high of 537%, depending on the neck strap tension and the inner bow/outer bow angle. While the intrusive forces on the maxillary incisors were excessive, no intrusion is anticipated because the biteplate disarticulates the posterior teeth and the eruption of the unopposed maxillary molars would likely cause the occlusal plane to tip in a counter-clockwise direction. Consequently, the overbite correction would be obtained through maxillary molar eruption accompanied by occlusal plane tipping. Before considering use of the biteplate facebow, a patient's anticipated growth pattern, the magnitude of the intrusive forces and the treatment objectives should be evaluated.