A Virtual Investigation of the Change in Arch Length from Leveling a Curve of Spee: A Pilot Study

Abstract Background Leveling a Curve of Spee (COS) is often a primary treatment intention in orthodontic and oral rehabilitation cases. It is a prevalent belief that additional arch length or “space” is always required to level a COS to avoid undesirable incisor proclination. The objective of the study was to evaluate changes in arch length from virtually leveling a COS and compare these virtual changes to estimated changes using several published formulas. Methods Sixteen pretreatment digital mandibular casts were randomly selected and imported to and evaluated using uDesign® software. COS of 1 mm (n = 4), 2mm (n = 6), 3mm (n = 4), and 4mm (n = 2) were identified. The AP distance from a midpoint between the central incisors perpendicular to a tangent contacting the distal surfaces of the second molars (D1) was calculated. Each arch was virtually leveled and the AP distance from the incisor midpoint to the second molar tangent (D2) was recalculated. The difference (+ or -) between D1 and D2 was multiplied by 2 to calculate total arch length changes. Virtual arch length changes were compared to changes calculated using several published formulas. Results Virtually calculated arch length changes for leveling were: 1mm COS (mean 0.43mm, SD +/-0.06, range 0.84mm; 2mm COS (mean 0.63mm, SD +/- 0.91, range 3.0 mm); 3mm COS (mean 1.43mm, SD +/- 0.15, range 2.02mm); and 4mm COS (mean 1.85mm, SD +/- 5.04, range 2.17mm). The mean difference between the virtual assessments and across all formula-based assessments was + 1.53mm with a range of 6.7 mm. Conclusions A virtual simulation of leveling demonstrated variation in arch length changes among COS of similar depth. Considerable differences were found in arch length changes between virtually leveled COS and changes estimated using published formulas. Clinical Relevance Virtual simulation of leveling a COS can give reliable information for both orthodontic and oral rehabilitation treatment planning.