15.1 Malocclusion Classification, Diagnostic Records & Fixed Appliances

15.1 Malocclusion Classification, Diagnostic Records & Fixed Appliances

Orthodontics is the specialized dental discipline devoted to the surveillance, guidance, and correction of growing and mature dentofacial structures. The intra-oral dental assistant plays an integral role throughout orthodontic therapy, from obtaining diagnostic records and placing interdental separators to preparing enamel for direct bracket bonding, placing and ligating archwires, and educating patients on appliance maintenance.

A rigorous understanding of craniofacial growth, the biological etiology of malocclusions, Angle's classification, and the mechanical armamentarium of fixed appliances is foundational for clinical practice and national certification.

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Etiology of Malocclusion

Malocclusion refers to any deviation from physiologically ideal occlusion. The factors contributing to malocclusion are classified into three primary categories:

1. Genetic and Hereditary Factors

Genetics determine the basic dimensions, morphology, and growth vectors of the skeletal craniofacial complex and the dentition:

• Discrepancy Between Jaw Size and Tooth Size: Inheriting jaw dimensions from one parent and tooth dimensions from the other frequently results in severe disharmonies. If a child inherits large teeth within small dental arches, severe dental crowding, rotation, and ectopic eruption occur. Conversely, small teeth in large jaws result in widespread generalized spacing and diastemas.
• Skeletal Discrepancies: Inherited micrognathia (an abnormally small mandible or maxilla) or macrognathia (an abnormally large jaw) establishes severe skeletal Class II or Class III malocclusions that may require combined orthodontic and orthognathic surgical intervention.
• Congenital Anomalies: Congenitally missing teeth (hypodontia or oligodontia, commonly involving maxillary lateral incisors 12 and 22, or mandibular second premolars 35 and 45) disrupt arch continuity and allow adjacent teeth to drift.

2. Environmental Factors and Deleterious Habits

Non-nutritive oral habits exert continuous mechanical forces against alveolar bone and teeth, remodeling the pliable alveolar process:

• Prolonged Thumb- or Digit-Sucking: If persistent beyond age 4 to 5 (when permanent incisors begin descending), the digit exerts an upward and labial force on maxillary incisors and a lingual force on mandibular incisors. This results in maxillary incisor proclination, excessive overjet, an anterior open bite, and a narrowed, V-shaped maxillary arch with posterior crossbite caused by negative intraoral pressure during sucking.
• Tongue-Thrust Swallowing: In an abnormal infantile swallow pattern, the tongue thrusts forward between the anterior teeth during deglutition rather than elevating against the hard palate. This continuous forward muscular pressure prevents incisal eruption, maintaining an anterior open bite and bimaxillary protrusion.
• Chronic Mouth-Breathing: Upper airway obstruction caused by hypertrophied adenoids, tonsils, or chronic allergic rhinitis forces the child to maintain an open-mouth posture with a low, depressed tongue posture. Without the internal balancing outward support of the tongue against the palate, the buccinator muscles compress the maxillary arch inward, producing maxillary constriction, high palatal vault, bilateral posterior crossbite, and an elongated facial profile known as adenoid facies.

3. Local Factors

• Premature Loss of Deciduous Teeth: When a primary molar is lost prematurely to caries or trauma, the permanent molar situated posterior to the space drifts and tips mesially into the gap. This blocks the eruption path of the underlying permanent premolar, causing impaction or ectopic eruption.
• Prolonged Retention of Deciduous Teeth: If a deciduous root fails to resorb normally, the permanent successor is deflected out of its normal eruption path, erupting lingually (common in mandibular incisors) or buccally.
• Supernumerary Teeth: Extra teeth, most notably the mesiodens (a conical supernumerary tooth located at the maxillary midline between teeth 11 and 21), physically obstruct the eruption of central incisors or create a wide midline diastema.
• Impacted Teeth: Teeth unable to erupt due to physical obstruction or abnormal angulation (most commonly permanent maxillary canines 13 and 23, or mandibular third molars 38 and 48).

Angle's Classification of Malocclusion

Developed by Dr. Edward H. Angle, this system uses the permanent maxillary first molar as the fixed anatomical reference point ("the key to occlusion"), postulating that its position within the maxilla is stable relative to the cranial base.

The Reference Relationship

The diagnostic standard assesses the relationship between:

• The mesiobuccal (MB) cusp of the permanent maxillary first molar (tooth 16 or 26)
• The mesiobuccal (MB) groove of the permanent mandibular first molar (tooth 36 or 46)

Class I Malocclusion (Neutroclusion)

• Molar Relationship: The mesiobuccal cusp of the maxillary first molar occludes precisely within the mesiobuccal groove of the mandibular first molar. The skeletal base relationship between the maxilla and mandible is normal (orthognathic profile).
• Canine Relationship: The maxillary permanent canine occludes in the embrasure between the mandibular canine and the mandibular first premolar.
• Characteristics: Although the anteroposterior molar relationship is ideal, individual teeth or groups of teeth display localized malalignments, including crowding, rotations, spacing, localized anterior or posterior crossbites, or abnormal axial inclinations.

Class II Malocclusion (Distoclusion)

• Molar Relationship: The mandibular first molar is positioned distal (posterior) to its normal Class I relationship with the maxillary first molar. Anatomically, the mesiobuccal cusp of the maxillary first molar occludes anterior (mesial) to the mesiobuccal groove of the mandibular first molar. The patient typically exhibits a convex facial profile with a retrognathic mandible.
• Class II, Division 1:
  • Maxillary anterior incisors are severely proclined (flared labially).
  • Excessive horizontal overjet is present.
  • The maxillary arch is frequently narrow and V-shaped, accompanied by an incompetent upper lip and deep labiomental groove.
  • Patients are at elevated risk for traumatic fracture of the protruding maxillary incisors.
• Class II, Division 2:
  • Maxillary central incisors are retroclined (tipped lingually), while the maxillary lateral incisors are flared labially or overlap the centrals.
  • Overjet is typically normal or slightly decreased, but a severe, deep vertical overbite is present (the mandibular incisors may impinge directly onto the palatal gingiva).
• Subdivision: When a Class II molar relationship occurs unilaterally (on one side only) while the opposite side is Class I, it is documented as a Class II Subdivision (e.g., Class II Division 1, Subdivision Right).

Class III Malocclusion (Mesioclusion)

• Molar Relationship: The mandibular first molar is positioned mesial (anterior) to its normal Class I relationship with the maxillary first molar. The mesiobuccal cusp of the maxillary first molar occludes posterior (distal) to the mesiobuccal groove of the mandibular first molar (often between the mandibular first and second molars).
• Canine Relationship: The maxillary canine occludes posterior to the interdental space between the mandibular canine and first premolar.
• Characteristics: The facial profile is concave with a prominent, prognathic chin ("underbite"). Maxillary incisors typically occlude lingual to the mandibular incisors, creating an anterior crossbite.

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Summary Comparison: Angle's Classification System

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Spatial Relationships and Occlusal Discrepancies

Clinical evaluation requires measuring dental relationships across three spatial dimensions:

1. Overjet (Horizontal Dimension): The horizontal overlap between the labial surface of the mandibular central incisors and the incisal edge of the maxillary central incisors. Measured in millimeters using a periodontal probe held parallel to the occlusal plane. Normal overjet is 1 to 3 mm. Excessive overjet occurs in Class II Division 1, whereas negative overjet (underjet) characterizes anterior crossbites in Class III.
2. Overbite (Vertical Dimension): The vertical overlap of the incisal edges of the maxillary central incisors over the labial crowns of the mandibular central incisors. Normal overbite is 1 to 2 mm (or approximately 20% to 30% coverage of the mandibular crown). An excessive vertical overlap is termed a deep overbite (or closed bite); if the incisal edges fail to overlap vertically when posterior teeth are occluded, an anterior open bite exists.
3. Crossbite (Transverse Dimension):
   • Posterior Crossbite: Maxillary posterior teeth occlude lingual to the mandibular posterior teeth (normal relationship: maxillary buccal cusps overlap mandibular buccal cusps). Can be unilateral or bilateral and frequently results from maxillary skeletal constriction.
   • Anterior Crossbite: One or more maxillary anterior teeth occlude lingually relative to the mandibular anterior teeth.

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Fixed Orthodontic Appliances and Chairside Armamentarium

Fixed orthodontic appliances ("braces") are bonded or cemented directly onto teeth and cannot be removed by the patient. They deliver continuous, precisely controlled forces to produce tooth movement through alveolar bone remodeling.

1. Orthodontic Separators (Spacers)

Teeth have tight interproximal contacts that prevent the seating of molar bands. Separators are placed to gently wedge teeth apart, creating temporary interproximal space:

• Elastomeric Ring Separators: Small, resilient polyurethane rings. Placed using specialized separating pliers (which stretch the ring) or by looping two pieces of dental floss through the ring and pulling it through the contact point with a seesaw motion until the lower half sits gingival to the contact while the upper half rests occlusal to the contact.
• Brass Wire and Spring Separators: Steel springs or dead-soft brass wires threaded beneath the contact point, twisted tightly, and pigtails cut to 3 mm and tucked into the embrasure.
• Clinical Timing: Separators remain in place for 1 to 2 weeks prior to the banding appointment. Patients must be instructed not to floss between separated teeth or chew sticky foods (e.g., gum, caramel) that could dislodge the spacer.

2. Molar Bands

Preformed, stainless steel rings precisely shaped to fit the anatomical contours of permanent molars. Bands provide the anchorage required to withstand heavy masticatory forces and anchor heavy headgear, lip bumpers, or palatal expanders:

• Band Anatomy: Each band has an anatomical occlusal and gingival edge. Pre-welded to the buccal surface is a buccal tube (which accepts the main archwire and facebow headgear inner bow) and an auxiliary hook. The lingual surface may feature a lingual sheath for transpalatal arches or quad-helix appliances.
• Selection and Fitting: The band is selected by size and tried onto the tooth. The assistant or clinician pushes the band down over the tooth using finger pressure, then positions an orthodontic band seater (bite stick) against the band's seating lug. The patient is asked to bite down gently, utilizing their own occlusal force to seat the band past the interproximal heights of contour. A hand-held band pusher is then used to burnish and adapt the malleable margins into the developmental grooves.
• Cementation: Glass ionomer cement or zinc phosphate cement is mixed. The interior of the band is filled with cement, with the buccal tube covered with utility wax to prevent cement blockage. The band is seated, margins adapted, and excess cement removed cleanly with a scaler once the material reaches its initial set.

3. Direct Bonded Brackets

Brackets transmit mechanical forces from the archwire directly to the teeth:

• Components:
  • Mesh Base: Micro-etched mesh pattern providing micromechanical retention for composite resin adhesive.
  • Bracket Slot: Precision horizontal channel (standard dimensions: 0.018-inch or 0.022-inch width) that houses the archwire.
  • Tie Wings: Projections (usually twin or edgewise design) around which ligatures are tied to secure the wire.
• Bracket Types: Stainless steel (standard, strong, low friction), ceramic/polycrystalline alumina (esthetic, tooth-colored, brittle, carries risk of enamel abrasion against opposing teeth), and self-ligating brackets (feature a built-in sliding labial door or mechanical clip that locks the archwire into the slot, eliminating elastomeric or wire ligatures and reducing friction).
• Direct Bonding Sequence:
  1. Isolate teeth with cheek retractors, salivary ejectors, and dry angles; clean enamel surfaces with oil-free, fluoride-free pumice slurry.
  2. Etch enamel for 30 seconds with 37% phosphoric acid gel; rinse thoroughly with water and desiccate until enamel displays a uniform, chalky-white matte appearance.
  3. Apply liquid resin primer and light-cure or air-thin according to manufacturer guidelines.
  4. Apply light-cure composite resin paste to the mesh base of the bracket.
  5. Transfer bracket using bracket placement forceps (bracket tweezers); clinician positions the bracket precisely along the long axis and clinical crown center using an orthodontic bracket height gauge.
  6. Remove excess adhesive flash around bracket margins with an explorer before photopolymerization.
  7. Light-cure with blue LED light for the specified duration (typically 10–20 seconds per bracket).

4. Orthodontic Archwires

The archwire serves as the pattern and engine of orthodontic force. Held in the bracket slots, it deforms when engaged into crooked teeth, and as it attempts to recover its original preformed arch shape, it exerts gentle, continuous biological forces that stimulate osteoclastic bone resorption on the pressure side and osteoblastic bone apposition on the tension side of the periodontal ligament.

• Archwire Alloys:
  • Nickel-Titanium (NiTi): Exceptional elasticity, flexibility, and shape memory. Delivers light, continuous, physiologic forces over wide deflections without permanent deformation. Used during the initial stages of treatment for leveling and alignment.
  • Stainless Steel: Stiff, rigid, with a high modulus of elasticity and low surface friction. Used during intermediate and finishing stages for space closure, arch form consolidation, and controlling root torque.
  • Beta-Titanium (TMA / Titanium-Molybdenum Alloy): Combines intermediate stiffness, high springback, and formability; can be bent chairside for detailing and finishing bends.
• Wire Cross-Sectional Geometries:
  • Round Archwires (e.g., 0.014-inch, 0.016-inch): Contact only the base/floor of the bracket slot. Permit tipping movements; ideal for initial alignment and unraveling crowded teeth.
  • Rectangular / Square Archwires (e.g., 0.016 × 0.022-inch, 0.019 × 0.025-inch): Fill the rectangular slot completely. Engage all four slot walls, generating three-dimensional control over torque (buccolingual root inclination) and angulation (mesiodistal root tip).

5. Orthodontic Ligatures

Ligatures secure the archwire firmly into each bracket slot:

• Elastomeric Ligatures (O-Rings): Polyurethane elastic donuts available in various colors. Placed over bracket tie wings using a Mathieu needle holder or a hand-held ligature director. Over time, they absorb oral fluids and lose elasticity, requiring replacement at each adjustment appointment.
• Wire Ligatures: Thin (0.009- or 0.010-inch) soft stainless steel wires looped around tie wings and twisted tightly using Mathieu pliers. The twisted twisted "pigtail" is cut with a pin-and-ligature cutter to a length of 3 mm, then carefully tucked gingivally beneath the archwire toward the tooth using a ligature director to prevent laceration of the patient's buccal mucosa and lips.
• Power Chains (Elastomeric Chains): Continuous interconnected elastomeric rings used to close spaces (diastemas) or consolidate teeth into a single segment.

6. Specialized Orthodontic Hand Instruments

• Howe Pliers & Weingart Pliers: Utility pliers with serrated, rounded tips used for placing, guiding, and removing archwires from buccal tubes.
• Bird-Beak Pliers (#139): One round conical beak and one square pyramidal beak; used for precise wire bending and forming loops.
• Distal-End Cutters: Placed intraorally at the distal exit of the molar buccal tube. Cuts the excess archwire flush against the tube while an internal safety mechanism mechanically grips and retains the severed wire fragment, preventing it from flying into the oral cavity or being aspirated.
• Posterior Band-Removing Pliers: Features a nylon/teflon padded cylindrical rest that seats onto the occlusal cusp of the molar, and a slender metal chisel blade that slips under the gingival margin of the band. Squeezing handles creates vertical leverage, popping the band occlusally without damaging enamel.