Introduction

The leveling and aligning phase represents the foundational stage of comprehensive orthodontic treatment, establishing dental form, sagittal and vertical relationships, and arch coordination essential for successful final detailing and long-term stability. This initial phase typically comprises 6-12 months of treatment, during which irregularly positioned teeth are systematically aligned into harmonious dental arches, anterior-posterior dental relationships are refined, vertical relationships are normalized, and transverse deficiencies are corrected when present.

The leveling phase demands meticulous biomechanical understanding of wire-bracket interactions, force application principles, and three-dimensional tooth control. Success during this phase substantially influences subsequent treatment efficiency and ultimate clinical outcomes. This comprehensive review examines leveling phase biomechanics, wire sequencing strategies, force magnitude optimization, and management of common complications encountered during initial alignment.

Objectives and Treatment Goals

Primary objectives of the leveling phase include:

Arch alignment: Correcting rotations, mesiodistal tipping, and axial inclination variations of individual teeth, establishing uniform dental arch form suitable for systematic space closure or correction. Leveling: Eliminating vertical discrepancies within individual arches (anterior deepbite, open bite, segmental extrusion) by establishing common vertical plane of occlusal contact. Anterior-posterior relationship refinement: Reducing overjet through careful sagittal movement of maxillary anterior teeth and mandibular molar distalization, establishing class I molar and canine relationships. Transverse correction: Expanding maxillary arches or correcting lingual inclination of maxillary posterior teeth when transverse deficiency exists; correcting lingual inclination or crowding of mandibular posterior teeth. Interarch coordination: Establishing compatible maxillary and mandibular arch forms enabling subsequent mechanics for class I relationship establishment.

Specific goals vary by individual malocclusion severity, skeletal patterns, and anticipated final mechanics requirements. Early treatment planning determines whether extraction or non-extraction approaches will be employed, directly influencing leveling phase mechanics.

Wire Selection and Sequencing Strategy

Wire selection and sequencing represents the critical biomechanical parameter determining leveling phase efficiency. The progression from flexible, low-force wires to increasingly stiff wires enables systematic tooth movement with controlled force magnitude and direction.

Nitinol wire characteristics: Superelastic nickel-titanium (nitinol) wires demonstrate unique properties ideal for initial leveling. Unlike conventional stainless steel wires that produce high force at large deflection magnitudes, superelastic nitinol wires deliver relatively consistent, light forces (50-100 grams per tooth) over large deflection ranges. Austenite-martensite phase transitions at specific temperature points enable the wire to maintain relatively constant force as teeth move within the bracket slot, reducing force decay and producing more consistent tooth movement.

Contemporary superelastic nitinol wires are available in multiple temperature-transition (Tt) formulations:

  • Conventional Tt (25-27°C): Optimal for room-temperature clinical use, producing consistent forces
  • Heat-activated Tt (35°C): Intended for mouth-temperature activation, though clinical benefits remain unclear
  • Intermediate Tt formulations: Offering variable force properties

Most orthodontists utilize conventional-Tt superelastic nitinol wires for leveling phase mechanics due to their reliable performance characteristics. Wire sequencing progression: Systematic wire progression from smaller to larger cross-sectional dimensions and from more flexible to stiffer materials provides graduated force increase as dental compliance improves.

Typical sequence: 1. 0.014" nitinol: Initiates alignment, particularly valuable in severely crowded cases where initial horizontal control is minimized to reduce pain and patient discomfort 2. 0.016" nitinol: Provides increased vertical control while maintaining flexibility for lateral adjustments 3. 0.018" nitinol: Further increases vertical control while permitting continued three-dimensional adjustments 4. 0.018"x0.025" nitinol: Introduces rectangular wire geometry enabling torque control (axial inclination) while maintaining flexibility 5. 0.020"x0.025" or 0.021"x0.025" nitinol: Provides comprehensive three-dimensional control approaching completion of leveling phase

Wire sequencing duration depends on individual response: typical intervals range from 4-8 weeks between progressions, with some cases requiring extended intervals (8-12 weeks) in high-friction extraction cases or when minimal tooth movement occurs.

Stainless steel wire utilization: Heavy stainless steel wires (0.018"x0.025" or 0.021"x0.025") provide superior control and force consistency compared to nitinol, useful during later leveling stages when high-force mechanics are appropriate and tooth positions are relatively normalized. However, stainless steel wires produce force discontinuities (rapid force decrease with tooth movement), potentially producing longer intervals between appointments or requiring frequent wire adjustments.

Force Magnitude and Biological Response

Optimal force magnitude during leveling phase reflects the balance between efficient tooth movement and biologically sustainable tissue remodeling. Excessive force produces hyalinization (tissue necrosis around tooth root), temporary cessation of tooth movement, potential permanent root resorption, and patient pain. Insufficient force produces minimal movement, treatment delays, and patient frustration.

Research establishing optimal force magnitudes for aligned teeth moving through minimal spaces suggests:

  • Incisors: 50-75 grams per tooth
  • Canines: 75-100 grams per tooth
  • Premolars: 100-150 grams per tooth
  • Molars: 150-200 grams per tooth

However, these guidelines apply to teeth in relatively good alignment. Severely malpositioned teeth, particularly those requiring rotation, respond favorably to higher forces (100-200 grams for incisors, 200-250 grams for canines). The principle of "light but continuous forces" represents an oversimplification; more precisely, the magnitude should produce consistent movement without hyalinization or root resorption.

Superelastic nitinol wires automatically deliver force magnitudes near biological optimums across the range of tooth positions, providing a primary advantage for leveling phase. Stainless steel wires require manual force adjustment as tooth positions change to maintain biological force magnitude.

Bracket Engagement and Slot Mechanics

Bracket slot characteristics fundamentally influence wire-bracket interactions and force magnitude delivered. Modern brackets typically employ 0.022" slot dimensions (historically 0.018" slots were standard). Larger slot dimensions increase wire-to-slot clearance, reducing friction during initial alignment phases while providing inferior control later in treatment.

Wire-to-slot clearance directly influences friction resistance. When wire cross-section is substantially smaller than bracket slot (early leveling with 0.014" or 0.016" wires in 0.022" slots), significant clearance permits tooth movement with minimal friction. As wire size increases (0.018"x0.025" or 0.021"x0.025" in 0.022" slots), slot fill increases substantially, increasing friction and enabling greater three-dimensional control.

Bracket torque—the root inclination built into the bracket—influences initial alignment phase mechanics. Brackets with zero or minimal torque enable faster alignment without resistance from torqued wires fighting misinclined tooth roots. As leveling progresses, brackets with progressively greater torque values enable systematic root inclination correction. The progression from low-torque to standard-torque to high-torque brackets throughout treatment enables graduated torque increase matching treatment needs.

Three-Dimensional Tooth Control

Leveling phase mechanics ideally address three-dimensional tooth position changes simultaneously: mesiodistal position (sagittal), vertical position, and axial inclination (torque or root position).

Sagittal plane mechanics: Sequentially moving teeth mesiodistally establishes proper arch form and reduced overjet. Early phases focus on aligning teeth within their arches without aggressive sagittal movements. Posterior tooth movements (molar distalization in non-extraction cases or minimal mesial movement in extraction cases) establish space for anterior alignment. Anterior overjet reduction occurs gradually throughout leveling phase and subsequent mechanics phases. Vertical plane mechanics: Deepbite correction represents a primary leveling phase objective. Excessive overbite (deepbite) reduction mechanisms include:
  • Intrusion of maxillary incisors (decreasing their vertical position)
  • Extrusion of mandibular posterior teeth (increasing posterior vertical dimension)
  • Extrusion of maxillary posterior teeth (increasing maxillary posterior vertical dimension)
Leveling phase wires naturally extrude posterior teeth during early alignment, increasing posterior vertical dimension. In open bite cases, posterior extrusion is often contraindicated; these cases benefit from special mechanics emphasizing anterior extrusion while maintaining or reducing posterior vertical dimensions. Transverse plane mechanics: Maxillary arch width expansion occurs naturally during leveling through lingual bracket positioning creating transverse forces directing maxillary teeth facially. Additional transverse correction may employ:
  • Quad helix or similar passive expansion appliances
  • Rapid palatal expansion (RPE) for severe transverse deficiency
  • Selective wire bending introducing transverse force components

Common Complications and Management Strategies

Inadequate space for alignment: Severe crowding may preclude complete alignment within available arch perimeter. Management options include:
  • Selective stripping (proximal enamel reduction) creating space for alignment
  • Extraction of one or more teeth (planned extraction protocol modification)
  • Arch expansion through transverse and sagittal adjustments
  • Combination approaches
Anterior open bite development: Some cases demonstrate anterior open bite development during leveling phase as posterior teeth extrude. Management includes posterior intrusive mechanics (using headgear or intraarch springs) or anterior extrusive mechanics emphasizing anterior tooth elongation rather than posterior movement. Root resorption: Aggressive initial forces or extended treatment periods can produce root resorption (shortening of roots). Light force magnitudes, appropriate force directions, and periodic panoramic radiographs monitoring root status minimize this risk. Cases showing early signs of root resorption warrant force reduction or treatment termination. Undesirable tooth movements: Teeth occasionally move in directions counter to therapeutic goals—lingual inclination of maxillary molars, buccal inclination of mandibular molars, rotation of teeth despite bracket positioning efforts. These movements may require temporary bracket repositioning, interarch elastics to redirect forces, or localized wire bending. Bracket loosening or debonding: Repeated bracketed tooth movement, occlusal forces, and adhesive degradation can produce bracket loosening. Systematic rebonding and adhesive reapplication maintain bracket integrity throughout treatment.

Patient Compliance and Treatment Efficiency

Leveling phase duration and efficiency depend substantially on patient compliance with appointment scheduling, dietary modifications (avoiding hard and sticky foods), and oral hygiene maintenance. Irregular appointments prolong treatment; excellent oral hygiene prevents decalcification and carious lesions during treatment. Patient education emphasizing these factors accelerates treatment while maintaining oral health.

Transition to Space Closure Phase

Leveling phase completion criteria include:

  • All teeth well-aligned within individual arches
  • Anterior deepbite correction to normalized overbite (3-4 millimeters)
  • Transverse discrepancies corrected or managed
  • Wire sequence advanced to final leveling wire
  • Excellent oral hygiene with minimal gingival inflammation
  • Patient motivation and compliance demonstrated

Upon completion, treatment transitions to space closure phase (in extraction cases) or anterior mechanics phase (in non-extraction cases), focusing on anterior-posterior relationship correction and final detailing.

Conclusion

The leveling and aligning phase establishes the foundation for successful comprehensive orthodontic treatment through systematic alignment of irregularly positioned teeth, correction of vertical and transverse discrepancies, and refinement of dental relationships. Meticulous wire sequencing progressing from flexible superelastic nitinol wires delivering light, consistent forces through increasingly stiff wires enabling enhanced three-dimensional control optimizes leveling efficiency. Understanding force magnitude optimization, bracket engagement mechanics, and three-dimensional tooth control principles enables clinicians to navigate complications and maintain efficient treatment trajectories. Optimal leveling phase outcomes depend on careful biomechanical planning, periodic reassessment and mechanical adjustment, and patient education regarding compliance factors essential for consistent progress.