Dental Model Analysis

A dental model analysis is an essential component in the evaluation of patients requiring orthognathic surgery. It is done at least twice: before orthodontics and prior to surgery. The first analysis—the initial dental model analysis—guides orthodontic treatment; the second—the progress dental model analysis—establishes readiness for surgery.

In orthognathic surgery, surgeons and orthodontists collaborate to normalize the jawbones and the occlusion. In the first stage of treatment, an orthodontist aligns the upper and lower teeth to their corresponding jaw, creating normal dental arches. A surgeon then aligns the arches with one other during surgery. The orthodontist’s task is complex. He/she must coordinate the dental arches so that they can be placed in normal intercuspation at surgery. Coordination of dental arches entails giving the dental arches (upper and lower) corresponding forms. An initial dental model analysis shows the clinician how the pretreatment form deviates from the target, which is essential to planning correction.

An initial dental model analysis includes the following:

• Analyses of shape
  • Arch shape correspondence
  • Dental alignment
  • Dental leveling
  • Curve of Spee
  • Buccolingual inclinations
  • Analyses of size
    • Spacing
    • Arch width
    • Bolton assessment

A dental model analysis has multiple components, which the authors have classified into two groups: Appraisals of shape and appraisals of size. The first appraisal of shape is arch shape correspondence. For teeth to fit into a normal occlusion, the shapes of the upper and lower dental arches must be similar. This is called arch shape correspondence. To assess it, one looks at the occlusal surfaces of both models simultaneously, mentally comparing the shapes of both arches. Dissimilar shapes are a problem; for instance, a “U” shaped lower arch will not fit a “V” shaped upper arch, and a square lower arch will not fit a “U” shaped upper arch.

The second appraisal of shape, evaluates dental alignment. With perfect alignment, the edges of the incisors and the buccal-cusp-ridges of canines, premolars, and molars make a catenary arch. Misalignment occurs when the teeth are not aligned in an arch, because of malrotation, displacement, or tipping.

The third shape appraisal evaluates dental leveling. Leveling refers to the vertical position of teeth in relation to the occlusal plane. The occlusal surfaces of all teeth should be on the plane.

The fourth shape appraisal assesses the curve of Spee.²⁸ The curve of Spee is the up-down curvature of the occlusal plane. It starts in the canine and extends back, to the last molar. An ideal curve of Spee is flat or has minimal upward concavity.²⁹

The fifth and final shape assessment appraises the buccolingual inclination of posterior teeth. In the mandible, the lingual cusps should be 1 mm lower than the buccal. In the maxilla, the buccal cusps should be 1 mm higher than the palatal. Buccolingual inclination is assessed with a straightedge. In the mandible, the straightedge is placed on corresponding buccal cusps and the gap between the tool and the lingual cusps is measured. In the maxilla, the straightedge is placed on the palatal cusps and the gap between the instrument and the buccal cusps is measured.

The next group of measurements appraises size. Among them is spacing, which is a comparison between the space available for the alignment of teeth and the space required. In the first step, one calculates the available space:  the arch perimeter from one first molar to the other. In the second step, an examiner measures the space needed, which is the sum of the widths of individual teeth—premolars, canines, and incisors.¹³

Another essential component of the size appraisal is arch width, which is measured at the first molars. In an ideal Class I occlusion, the mesiopalatal cusps of the upper first molars occlude with the distal fossae of the lower first molars. Thus, the distance between the mesiopalatal cusps of the maxillary first molars should be the same as the distance between the distal fossae of the mandibular first molars. A discrepancy between these measurements may reveal an underlying apical-base deformity.

The final measure is the Bolton assessment. This analysis originated from the observation that, in order to obtain the proper interdigitation and arch coordination in a Class I relationship, the width of the lower teeth must be proportional to the width of the upper teeth. Bolton discovered that a Class I canine occlusion is only possible when the upper and lower anterior teeth have a specific proportion. The sum of the widths of the lower anterior teeth must be 77% of the sum of the widths of the upper anterior teeth.¹³ Failure to account for a Bolton discrepancy commonly results in a lack of arch coordination.