The reliability of study model measurement in the evaluation of crowding

A New Method for Prediction of Dental Arch Perimeter

Purpose

Dental arch length, width, and perimeter are considered to be important for the diagnosis and treatment of orthodontic cases. This study aimed to utilize dental arch width and length to create an equation for predicting dental arch perimeter.

Materials and methods

Sixty-seven pairs of study models for patients with normal occlusion who received dental treatment were included in this study. Dental arch width at the level of the canines, first premolars, and first molars, in addition to dental arch length and perimeter, were measured using a digital vernier with 0.01mm accuracy. Data were subjected to step-wise regression analysis to determine the major predictors of arch perimeters and develop regression equations for both arches. The predicted arch perimeters were compared with the actual measured values using paired sample t-test.

Results

For both arches, the perimeter showed a direct, moderate to strong, highly significant correlation with the length and width measurements. Findings from step-wise regression analysis indicated that there was a strong correlation between arch perimeter and the inter-canine width and arch length, which explained 67.7% of the variation in arch perimeter in the maxillary arch. In the mandibular arch, inter-molar width, inter-canine width, and arch length explained 55.1% of the variation in the dental arch perimeter. The arch perimeter values predicted from the developed equations were not significantly different from the actual values that were measured.

Conclusion

New regression equations based on dental arch width and length at the level of the premolars, molars, and canines were developed to predict dental arch perimeters for both the mandibular and maxillary arches.

Dental and alveolar arch forms in a Caucasian population compared with commercially available archwires

INTRODUCTION

The aim of this study was to identify an arch form, comprising dentition and alveolus, representative of the Caucasian population, and to compare it with the shape of the main archwires on the market.

MATERIALS AND METHODS

The study sample comprised 35 pairs of dental casts, taken from the arches of 35 Southern Europeans with ideal natural occlusion. After the dental casts were scanned, the reference points of the dental arches (FA points) and alveolar bone (WALA ridges) were identified with 3D software. Cartesian coordinates and intercanine and intermolar diameters were calculated for each patient, and curves representing the upper and lower dentition, as well as those showing the shape of the lower supporting bone, were traced and compared with those pertaining to different types of commonly used orthodontic archwires.

RESULTS

Ideal shapes of the mandibular alveolus and of the upper and lower archwires were calculated and compared with those actually on the market. Statistical analysis showed significant differences between the two, particularly at the upper and lower molars and canines. Likewise, there were significant differences between the shapes of the available archwires and the WALA ridge, except for the intercanine width.

CONCLUSIONS

None of the commercial archwires examined faithfully represented the shape of the 'ideal' dentition we calculated, particularly at the molars and canines. The bone structure of the mandibular support cannot be used as a guide to the shape of the arch form during orthodontic treatment.

Computational formulation of orthodontic tooth-extraction decisions. Part I: to extract or not to extract

OBJECTIVE

To develop a mathematical model that simulates whether or not to extract teeth in optimizing orthodontic treatment outcome and to formulate the morphologic traits sensitive to optimizing the tooth-extraction/nonextraction decisions.

MATERIALS AND METHODS

A total of 188 conventional orthodontic records of patients with good treatment outcomes were collected, and dentofacial morphologic traits, along with their degrees of influence in the optimized model, were determined.

RESULTS

The rate of coincidence between the recommendations given by the optimized model and the actual treatments performed was found to be 90.4%. The major morphologic traits and their corresponding influences in improving the simulation accuracy of the model were the incisor overjet (3.0) and the size of the basal arch relative to the sum of the mesiodistal crown diameters of the upper dentition (2.4) and the lower dentition (2.0). The remaining 22 morphologic-trait variables were also found to be indispensable in achieving robust simulation readings.

CONCLUSION

A mathematical model that simulates whether or not to extract teeth in optimizing orthodontic treatment outcomes with a success rate of 90.4% at its prediction performance was developed. This model has 25 morphologic traits with four major categories (sagittal dentoskeletal and soft tissue relationship, vertical dentoskeletal relationship, transverse dental relationship, and intra-arch conditions) that affected the accuracy in determining optimal tooth extractions/nonextractions.

Methodologies for evaluating long-term stability of dental relationships after orthodontic treatment

A comprehensive review of literature which considers methodologies for studying long-term occlusal stability is presented. This article focuses on the evaluation of plaster study models because occlusal changes are best reflected in longitudinal casts. Of particular interest is the assessment of crowding in the dentition and the various physical and mathematical procedures used to evaluate the measurement of space available. Indices used to assess the overall occlusal results of treatment are also presented.

The assessment of crowding without the need to record arch perimeter. Part II: Crowded and irregular arches

A mathematically determined technique which seemed to be valid for the calculation of crowding and spacing in arches with acceptable alignment, was tested on a series of arches with varying degrees of crowding and irregularity. Upper and lower study casts of 30 individuals with sufficient malalignment and crowding to warrant orthodontic treatment with fixed appliances were examined. Models were measured by three operators, using a reflex microscope interfaced to an IBM compatible PC and a customized calculation program. The degree of crowding was compared with a "clinical' assessment of each arch, in which the irregularity was measured directly using a steel ruler. Both the computer calculated and clinical values varied between observers, but the agreement between observers was better using the reflex microscope. Correlation coefficients ranged from 0.92 to 0.96 for the reflex microscope, and between 0.86 and 0.91 for the clinical assessment. Inter-observer agreement for the three operators was high: 0.93 for the reflex microscope and 0.87 where a ruler was used. This suggests that the computer program used to calculate the crowding was valid for the crowded arches under consideration.

The assessment of crowding without the need to record arch perimeter. Part I: Arches with acceptable alignment

A simplified, mathematically determined technique for calculating arch perimeter (the overlap method) is described and its validity in determining an accurate assessment of crowding is tested. Indices, expressed in millimetres of crowding or spacing, were computed for both the whole arch and the labial segment alone. Study casts of 36 individuals with clinically acceptable lower arches were used to validate the method. The degree of crowding or spacing calculated was compared with a "clinical' assessment of each arch, in which the irregularity was measured directly using a steel ruler. Various calculation schemes were tested. Depending on exactly how the overlap was determined, the results varied slightly. Although the arches showed acceptable alignment, it was preferable to include a strategy for normalizing the positions of any rotated teeth before the overlaps were calculated. Repositioning any bucco-lingually displaced teeth into the line of the arch, however, was not useful. For the complete arch good agreement with the clinical assessment was reached on 31 occasions and for the labial segment, all but one appraisals were within 0.5 mm of each other. In the remaining instances (five complete arches and one labial segment), the degree of crowding or spacing was between 0.5 and 1 mm of the clinical assessment. Considering that clinical measurement of minor degrees of crowding and spacing cannot be precise, these results were considered acceptable. The method was easy to use, relying only on the recording of mesio-distal tooth widths and was acceptably reproducible. The technique would therefore appear to provide a valid yet simple research tool with which to record the degree of crowding. Its ability to cope with irregular and crowded arches will be the subject of a subsequent review.

Distal movement of buccal segments with the "en masse" removable appliance: its value in treating patients with mild Class II, Division 1 malocclusion. Part II: the model measuring system and results

The model of 25 children with mild Class II, Division 1 malocclusions who had their upper buccal segments moved distally with an en masse appliance were measured at the beginning of treatment and at the completion of buccal segment retraction. A reflex microscope, interfaced to a personal computer, was adapted for this purpose. A custom-made jig and linear stepping motor permitted the recording of both buccal and incisal measurements of the teeth in occlusion, as well as individual arch parameters. Software was designed to record and calculate the required measurements. The method error associated with this approach ranged from 0.01 to 0.5 mm. Results indicated that mean distal movement of the buccal segments approximated 6 mm, equivalent to a full cusp of buccal segment retraction. A small spontaneous reduction in overjet was seen. The upper arch showed spontaneous alignment and increases in width, length, and perimeter. In the lower jaw, transverse expansion was accompanied by a small increase in arch perimeter. Arch length, however, was slightly reduced. It was concluded that the system was an acceptable method of recording occlusal changes during orthodontic treatment, supplementing routine cephalometry. Furthermore, a full unit of buccal segment retraction could be expected, by using a removable appliance/headgear technique.

Distal movement of buccal segments with the "en masse" removable appliance--its value in treating patients with mild Class II, Division 1 malocclusions: Part I, clinical techniques (how to do it)

This retrospective study examines the therapeutic changes during the buccal segment retraction phase of treatment in 25 children with mild Class II, Division 1 malocclusions and for whom distalization of buccal teeth was the treatment of choice. Where significant mandibular retrognathia was diagnosed clinically, a functional treatment approach was indicated, and these children were not included in this study. Therapy was carried out with an upper removable expansion appliance and extraoral traction. It will be shown in Part II of the investigation that a full unit of buccal segment retraction can be expected, with a combination removable appliance/headgear technique.

Individualized catenary curves: their relationship to arch form and perimeter

Study casts of the lower arches of 35 children in whom arch alignment was considered acceptable, were examined using a reflex microscope. Arch perimeter was calculated mathematically from a method which required measurement of the mesio-distal widths of the teeth only. This was described as the 'overlap' method. Arch perimeter was also calculated using individualized catenary curves for each subject. Two calculations were made, recording arch width either at the distal contact points of the first permanent molars or between their mesio-buccal cusps. Arch perimeter measured from the length of the catenary curves was consistently shorter than that calculated by the overlap method. The variation ranged from 0.02 to 4.58 mm. The catenary constructed using the distal molar contacts gave a better representation of the actual arch form: the mean discrepancy was 2.36 mm, compared with 2.86 mm with the second method where arch width was measured between mesio-buccal cusps. Both techniques for calculation of arch perimeter were highly reproducible. The catenary curve only approximated arch form well when the arch was relatively narrow across the inter canine region. For square arches this method was unsatisfactory. It is suggested that alternative techniques would be more reliable and the overlap method described here is considered satisfactory.

An assessment of the fit of a parabolic curve to pre- and post-treatment dental arches

Study casts of the teeth are routinely used for diagnosis and to assess treatment change: the crowding of the teeth or shortage of space available within the dental arch is usually assessed visually. A full cast analysis program has been developed previously, making use of the three-dimensional Reflex Plotter linked to a computer. This study examines the validity of the fit of computer generated parabolic curves to dental arches, as performed in the measurement of crowding. Using a visual analogue method it was found that the parabola appeared to fit lower post-treatment dental arches best. However, reservations regarding the fit of symmetrical curves are expressed and other solutions suggested.