Matrix Systems for Restorative Dentistry

Fundamentals of Restorative Matrix Systems

• A matrix system acts as a mechanical mold providing a "temporary wall" during the restoration of tooth preparations where one or more proximal walls have been removed (Class II, Class III, and Class IV).
• Key biological and mechanical benefits of matrix systems include:
  • Stabilizing and confining the restorative material during condensed placement.
  • Supporting and retaining the restorative material throughout the bench set-time or light-curing process.
  • Restoring physiological interproximal anatomical contours.
  • Establishing appropriate, tight interproximal contact with the adjacent tooth to prevent food impaction and periodontal degradation.
  • Creating a smooth external surface that minimizes finishing time and plaque accumulation.
• Anatomy of cavity preparation walls involved in matrix application:
  • External Walls: Cavity boundaries extending to the external tooth surface, including distal, facial (buccal), lingual, and gingival walls.
  • Internal Walls: Cavity surfaces not extending directly to external enamel:
  • Pulpal Wall: Positioned over the pulp, oriented perpendicular to the long axis of the tooth.
  • Axial Wall: Located along the pulpal aspect, oriented parallel to the long axis of the tooth.
  • Cemento-Enamel Junction (CEJ): Serves as an anatomical landmark when preparations extend subgingivally toward the root.

G.V. Black's Classification of Caries and Cavity Preparations

• Class I Lesions:
  • Defined as all pit-and-fissure caries and restorations.
  • Involve three anatomical location groups:
  • Occlusal surfaces of premolars and molars.
  • Occlusal two-thirds of the buccal and lingual surfaces of molars.
  • Lingual surfaces of anterior teeth (specifically maxillary incisors near the cingulum).

• Class II Lesions:
  • Involve the proximal surfaces (mesial and distal) of posterior teeth (premolars and molars).
  • Clinical cavity access is established through the occlusal tooth surface, terminating in a proximal box.

• Class III Lesions:
  • Involve the proximal surfaces (mesial and distal) of anterior teeth (incisors and canines).
  • May extend lingually or facially for operative access, but do not involve the incisal line angle.

• Class IV Lesions:
  • Involve the proximal surfaces of anterior teeth that do involve the incisal line angle or incisal edge.
  • Clinically identified as disto-incisal ($\text{DI}$) or mesio-incisal ($\text{MI}$) defects resulting from extensive caries or traumatic fractures.

• Class V Lesions:
  • Involve the cervical third of the facial or lingual surfaces of all teeth.
  • Include proximal cervical surfaces of posterior teeth only when the marginal ridge is intact and not involved in the cavity preparation.
  • Strictly exclude pit and fissure lesions.

• Class VI Lesions:
  • Involve defects on the incisal edges of anterior teeth or the cusp tips of posterior teeth.
  • Primarily result from developmental defects, attrition, abrasion, or localized wear facets.

Universal (Tofflemire) Retainer System

• The universal retainer, also designated as the Tofflemire retainer, is a mechanical clamp designed to secure and adjust a flexible matrix band around a posterior tooth.
• Assembled chairside by the dental assistant ($\text{DA}$) and added to the restorative instrument tray setup.
• Typically positioned from the buccal (facial) aspect of the arch; contra-angle designs are available for lingual placement.
• Component architecture of the universal retainer:
  • Outer Knob: Knurled control located at the distal end used to tighten or loosen the internal spindle pin within the diagonal slot, clamping the matrix band ends securely in place.
  • Inner Knob: Knurled control used to adjust the diameter of the matrix band loop by advancing or retracting the sliding carriage (vise).
  • Spindle Pin: An internal threaded screw that advances into the diagonal slot to lock the folded ends of the matrix band.
  • Diagonal Slot: An angled aperture inside the sliding vise where the ends of the matrix band slide; must point toward the gingiva when positioned in the mouth.
  • Guide Slots: Located at the working tip of the retainer head, featuring channel openings to project the matrix band loop straight, to the left (for maxillary right or mandibular left quadrants), or to the right (for maxillary left or mandibular right quadrants).

Posterior Metal Matrix Bands: Design and Clinical Criteria

• Fabricated from thin, flexible, medical-grade stainless steel in varied thicknesses and shapes.
• Standard band configurations:
  • Universal Band: Noncontoured or flat strip selected for Class II preparations with normal proximal box depth and width where anatomical cusps remain intact.
  • Extension Band: Features cervical extensions (gingival aprons) selected for deeper Class II proximal boxes extending subgingivally toward the $CEJ$.
  • Contoured Premolar and Molar Bands: Pre-shaped bands configured to fit the anatomical crown heights and cervical constrictions of bicuspids and molars.

• Circumference Orientation Rule:
  • Bringing the ends of a matrix band together produces an open, truncated cone.
  • Smaller Circumference: Corresponds to the gingival edge and must always be directed apically toward the gingiva.
  • Larger Circumference: Corresponds to the occlusal edge and must always be directed coronally toward the occlusal surface.
• Band Contouring Procedure:
  • Eliminates flat contours to guarantee a physiologically tight interproximal contact point.
  • The band is laid flat on a resilient paper mixing pad.
  • An egg-shaped burnisher, ball burnisher, or the rounded stem of a mouth mirror handle is rubbed across the inner surface of the proximal contact area with firm pressure until the band edges curl slightly upward.
• Placement Criteria:
  • The diagonal slot of the retainer must open toward the gingiva.
  • The retainer body is oriented along the buccal surface, with the handle projecting from the corner of the patient's mouth.
  • The gingival edge of the seated band must extend approximately $1\,\text{mm}$ below the prepared gingival margin (cervical floor).
  • The occlusal edge must extend no farther than $1.5\,\text{mm}$ to $2\,\text{mm}$ above the adjacent marginal ridge height.

Wedges and Interproximal Adaptation

• A matrix band alone does not produce the anatomical seal or contour required at the cervical cavosurface margin.
• Primary Functions of Wedges:
  • Press the matrix band firmly against the gingival tooth structure, closing gaps at the gingival-axial seat.
  • Provide slight temporary physical separation of adjacent teeth to compensate for the thickness of the matrix metal ($\approx 0.038\text{--}0.050\,\text{mm}$), ensuring a solid contact once the band is removed.
  • Guard against common restorative defects:
  • Overhang: Excess restorative material forced past the gingival margin into the interproximal periodontium.
  • Cupping: Indentation or concave distortion on the interproximal surface from an unbraced, loose band.
• Wedge Types and Material Properties:
  • Manufactured in triangular and round profiles in wood (e.g., sycamore wood) or plastic.
  • Sycamore Wood Wedges: Absorb oral fluids, expanding slightly to adapt closely against irregular gingival margins.
  • Size selection requires a wedge slightly wider than the interproximal cervical gap to apply pressure to both apical and gingival walls.
• Clinical Placement Technique:
  • Wedges are inserted firmly using cotton pliers or Howe ($\#110$) pliers.
  • In posterior Class II restorations, insertion occurs from the lingual embrasure because the lingual embrasure space is wider than the buccal embrasure space.

Anterior Matrix Systems (Mylar / Celluloid Strips)

• Indicated for anterior Class III and Class IV preparations restored with composite resin or glass ionomer cements.
• Employs a clear, flexible plastic strip, historically termed a celluloid or Mylar strip.
• Functions of clear plastic matrix strips:
  • Placed interproximally prior to applying etchant and adhesive primer to isolate and protect adjacent teeth from acid and resin.
  • Pulled securely around the tooth after composite placement to mold anatomical facial and lingual convexities.
  • Allows transillumination so curing light can penetrate directly through the clear matrix for rapid polymerization.
• Handling and Contouring Technique:
  • Strips are contoured prior to placement by pulling them lengthwise over the rounded shank of cotton pliers or a mouth mirror handle.
  • During resin adaptation and curing, the strip is maintained under tension using finger pressure, an interproximal wedge, or a matrix clip.

Sectional Matrix Systems for Posterior Composites

• Circumferential Tofflemire bands can flatten interproximal contours and produce open contacts when restoring posterior composite resins due to the inability to laterally condense composite like dental amalgam.
• Sectional matrix systems solve this issue by combining anatomically contoured sectional bands with spring-loaded separator rings.
• Prominent commercial systems include Palodent, Triodent (V3 System), Garrison Dental Solutions, and FenderMate.

• Features of Modern Sectional Systems (e.g., Palodent, Triodent):
  • Strong Nickel-Titanium ($\text{Ni-Ti}$) rings deliver spring memory and positive separation force between adjacent teeth.
  • Glass-fiber reinforced plastic tines feature a $V$-shaped notch to accommodate the wedge.
  • Pre-contoured sectional bands incorporate an anatomical marginal ridge crest and a gingival apron to prevent cervical flash at the gingival-axial line angle.
  • Band grip tabs enable placement and retrieval with pin-tweezers.
  • Specialized wedges (e.g., Adaptive Wedges) feature hollowed undersides allowing placement of a second wedge from the opposite side.
• Placement Sequence:
  1. Place the anatomical sectional matrix band into the proximal box.
  2. Insert a contoured wedge at the gingival margin to close the cervical floor.
  3. Engage the $\text{Ni-Ti}$ tension ring with specialized ring forceps, expand the ring, and seat its tines straddling the wedge between the matrix band and the adjacent tooth.

Retainerless Circumferential Systems (AutoMatrix)

• Designed for extensive Class II preparations, complex multi-surface cuspal build-ups, or teeth where a bulky Tofflemire retainer cannot be stabilized.
• Completely eliminates the external retainer frame, maximizing visual access and patient comfort.
• Available in four standardized band sizes: Narrow Regular, Medium Thin, Medium Regular, and Wide Regular.
• Operative mechanism:
  • Each preformed metal band contains an integrated autolock coil loop.
  • The band is seated over the tooth and adapted to the cervical margin.
  • The Automate III tightening device (wrench) is engaged into the coil and rotated clockwise to tighten the band circumferentially.
  • The wrench is disengaged, leaving only the coiled matrix on the tooth during restorative placement.
  • Following restoration placement, the wrench is turned counterclockwise to loosen the loop, and dedicated AutoMatrix removing snippers/pliers are used to sever the coil for band removal.

Pediatric Matrix Systems for Primary Molars

• Standard universal adult retainers and matrix bands frequently slip off primary molars due to marked cervical constrictions, short clinical crown heights, and prominent buccal cervical bulges.
• Dedicated pediatric solutions include:
  • Miniaturized pediatric Tofflemire retainers paired with ultra-thin bands.
  • Custom T-bands.
  • Custom spot-welded matrix bands.
• T-Band Matrix Technique:
  • Composed of a thin, malleable brass or copper strip shaped like the letter "T".
  • Preparation Steps:
  • The two wings of the "T" are bent upward to create a $U$-shaped channel.
  • The long, straight tail of the band is threaded through the $U$-shaped channel.
  • The wings are folded down flat against the tail to form a loop.
  • The loop is positioned over the primary molar with the tail facing buccally.
  • The tail is pulled to tighten the band around the cervical margin, folded back over the clasps, and stabilized with a wooden wedge.

• Spot-Welded Matrix Band Technique:
  • Employs flat stainless steel matrix strip material.
  • Fabrication Steps:
  • Cut a length of matrix strip measuring approximately $\frac{3}{4}\text{ to }1\,\text{inch}$ ($\approx 19\text{--}25\,\text{mm}$).
  • Adapt the material manually around the prepared primary tooth.
  • Use Howe ($\#110$) pliers to pinch the loose ends together on the buccal surface, adapting the band closely to the tooth circumference.
  • Remove the pinched band coronally in an occlusal direction.
  • Place the pinched seam on the contact plates of an electric spot welder.
  • Apply spot welds at three distinct points along the seam to fuse the metal into a custom loop.
  • Cut away excess metal beyond the weld line, smooth any sharp edges, seat the band onto the tooth, and place a wedge.

Clinical Armamentarium and Handling Instruments

• Cotton Pliers:
  • Available in locking and non-locking designs.
  • Constructed with angled arms, parallel internal tip grooves, precision serrations, guided alignment pins, and knurled handle grips to ensure stable handling of small matrix bands and wedges.

• Howe Pliers ($\#110$ Pliers):
  • Multipurpose dental utility pliers featuring straight or curved beaks with rounded, serrated, pad-like working tips.
  • Used for placing and removing wedges, contouring band margins, and pinching custom spot-welded bands.

• Pin-Tweezers:
  • Fine-point handling pliers with an integrated locking pin designed to securely grasp sectional matrix bands through their retention holes without bending or distorting the matrix.
• Ring Placement Forceps:
  • Heavy-duty sprung forceps designed to engage the bilateral notches of $\text{Ni-Ti}$ sectional matrix separator rings, opening them safely for placement over the wedge.
• Hemostats:
  • Straight or curved locking forceps used to grasp and pull tight matrix bands or stubborn wedges from the interproximal embrasure during post-restoration disassembly.