COMPARISON OF LEVELING A DEEP CURVE OF SPEE WITH EXTRACTIONS VERSUS NON-EXTRACTION: A BIOMECHANICAL PERSPECTIVE

Introduction: This study evaluated and compared orthodontic forces from different archwire designs and materials used to correct a deep curve of Spee. Methods: A three-dimensional orthodontic force tester (OFT) was used to measure forces and moments exerted by archwires on brackets from central incisors to first molars. Two setups were tested: one with a full dentition from central incisor to first molar, and another simulating bicuspid extractions with the first premolar removed. Dentoform models representing a deep curve of Spee were created using cone-beam computed tomography (CBCT) data. The setup employed 0.022 × 0.028-inch zero-prescription brackets secured with elastomeric ligatures. Three types of 0.016 × 0.022-inch archwires were tested (n = 10 per group): reverse curve of Spee nickel-titanium (rNiTi), stainless steel with step-down bends (SSSD), and titanium-molybdenum alloy with step-down bends (TMASD). The rNiTi wires had a preformed reverse curve, while SSSD and TMASD included bilateral 1.0-mm step-down bends between the canine and lateral incisor and between the lateral and central incisor. Analysis of variance (ANOVA) evaluated the effect of wire design and material on the magnitude and direction of forces and moments on incisors and molars, with p < 0.05 considered statistically significant. Results: Among the archwires tested, the lowest intrusive force occurred with non-extraction rNiTi (2.06 N), while the highest was seen in the extraction SSSD group (2.23 N). First molars experienced intrusive forces in the extraction setup (0.003–0.006 N) and extrusive forces in the non-extraction setup (–0.02 to –0.07 N). The rNiTi archwire produced the greatest buccal-crown moment on the lateral incisors (6.11–6.22 Nmm), followed by SSSD, with TMASD generating the least. Molars exhibited buccal-crown moments ranging from 0.88 to 1.31 Nmm across all tests. In the extraction scenario, rNiTi created a 3.2 Nmm lingual-crown moment on the second premolar, while TMASD produced a –0.71 Nmm buccal-crown moment in the non-extraction setup. Conclusions: Force distribution varied across all archwires. Material, wire size, and severity of overeruption significantly influenced tooth loading, particularly in buccal-lingual moments. The most notable difference occurred on the second premolar between extraction and non-extraction setups. Although varying archwire materials are expected to behave differently, in vivo studies are needed to compare the efficiency and efficacy of Curve of Spee leveling.