A Computerized Tooth-Width Analysis
Tooth-width ratios are used diagnostically toquantify the harmony between the mandibularand maxillary arches. A number of analyseshave been developed to compare the tooth-widthratios of malocclusions to standardized norms.1-4The Ho-Freer Graphical Analysis of Tooth-Width Discrepancy (GATWD) provides a simplegraphic description of tooth-width relationshipsbetween the arches, a comprehensive representationof tooth-width ratios in various arch segments,and a method of localizing tooth-widthdiscrepancies.5,6
Although the GATWD is more comprehensivethan previous analyses, it is also more laborintensive.Now, a personal-computer-based versionof the GATWD enables clinicians to incorporatethe analysis into their routine diagnosisand treatment planning, as well as to identifytooth-size mismatches that may develop towardthe end of treatment.
To generate baseline data, tooth-width measurementswere obtained from a sample of pretreatmentorthodontic study casts. Nine cumulativepercentage ratios relating maxillary teeth tomandibular teeth were derived5 (Table 1), and atemplate was created to show ranges of two standarddeviations from the means (Fig. 1).
A software engineer was hired to design acomputer version of the GATWD that permitsdirect input of 24 tooth-width measurements perpatient from study casts, using digital calipers, ormanual input via the computer keyboard. Theprogram, developed using Microsoft VisualBasic 3.0 for Windows, provides a quick calculationof tooth-width ratios and tooth-widthexcesses. In addition, it incorporates the templateof mean tooth-width ratios for comparison of anymalocclusion with the mean values. Reports suchas tooth-width ratios and tooth-width excessesand a graphic display of the analysis can be printedwith the Windows Print Manager, and thehard copies can then be placed in the patient?streatment folder.
The beaks of a Mitutoyo 6"/150mmDigital Caliper Zero Set (Fig. 2) are grounddown to tapered points to allow precise positioningon the mesial and distal contact points of theteeth. The digital calipers are connected to aMitutoyo Digimatic Mini Processor (Model No. DP-1HS) via a Mitutoyo Digimatic cable. AnRS-232C interface cable links the Mini Processor to an IBM-compatible personal computer.
The program has five main screens:
Patient details screen (Fig. 3): Basic patientinformation is entered here. The malocclusionline has a pull-down menu with choices of ClassI; Class II, division 1; Class II, division 2; ClassII indefinite; and Class III. A field is available toindicate extraction or nonextraction treatment.Pull-down extraction codes include four firstbicuspids; maxillary first and mandibular secondbicuspids; maxillary first bicuspids alone; foursecond bicuspids; and maxillary second andmandibular first bicuspids. Other extraction patternscan be entered by typing in the ISO/FDItooth numbers. The patient information can beused to generate overview reports on the practice?sentire patient population.
Mesiodistal tooth-width measurement screen(Fig. 4): Measurements of the 24 permanentteeth from first molar to first molar are inputdirectly into the appropriate fields by pressingthe data key on the digital calipers. Editing ispossible by deleting the incorrect entry and manuallyentering the tooth width.
Tooth-width ratio screen (Fig. 5): When the "DerivedRatios" button is activated (Fig. 4), the programperforms the calculation of tooth-widthratios. Tooth-width excess can then be viewedtooth by tooth or in graphic form.
Tooth-width excess screen (Fig. 6): The amountof tooth-width excess is displayed for each tooth.Graphic display of tooth-width analysis screen(Fig. 7): The graphic display, based on the templateshown in Figure 1, provides an immediatevisual assessment of any incompatible maxillary-to-mandibular tooth widths. Tooth-widthratios greater than two standard deviations aboveor below the mean are readily recognizable.
In the Class I case with mandibular anteriorcrowding demonstrated here (Fig. 8), theGATWD indicated a significant tooth-widthexcess in the mandibular anterior segment--about the width of a lower incisor. Therefore, themandibular left central incisor was extracted.The post-treatment results show a harmoniousrelationship with the 14 maxillary teeth occludingon the 13 mandibular teeth (Fig. 9).
In a different case with a near-ideal Class Iocclusion (Fig. 10), the GATWD showed alltooth-width ratios falling within two standarddeviations of the mean, indicating harmonybetween the maxillary and mandibular toothwidths (Fig. 11).
Orthodontic diagnosis should alwaysinclude an analysis of the relative sizes of maxillaryand mandibular teeth, especially in the anteriorregion, where relative tooth sizes control theamount of overbite, overjet, crowding, and spacing.Tooth-width excesses of greater than 2mmshould be taken into account during treatmentplanning. Maxillary-to-mandibular discrepanciescan have a pronounced effect on the occlusionthat may not become apparent until the finishingstages of treatment.
Previous tooth-width analyses have requiredmathematical calculations and referenceto standard tables or formulas to determine thelocations and amounts of tooth-width discrepancies.1-4 Now that the more comprehensiveGATWD5,6 has been incorporated into a computersoftware program, these inconveniences canbe avoided.
The use of digital calipers with direct inputinto the computer program can virtually eliminatemeasurement transfer and calculation errors,compared to analyses that require dividers,rulers, and calculators. Although some measurementerror may be associated with the positioningof the calipers on the mesial and distal surfacesof the teeth, this method is certainly morereliable than manual measurements. It provides adiagnostic tool that is convenient, consistent, andeasy to operate.
ACKNOWLEDGMENT: This research was supported by theAustralian Dental Research Foundation.