Partial Crowns

Unlike a full crown, a partial crown does not encircle the tooth to be restored. It is used to stabilize only partial areas of the tooth crown, while leaving unbroken any substance that is still resilient. After preparation (grinding) of the tooth, the partial crown is fabricated indirectly (outside the mouth) and – depending on the material used – fixed adhesively (with plastic) or conventionally (with cement). Extensive defects of the tooth structure, such as those that occur after the removal of large carious lesions (holes caused by tooth decay) or as a result of trauma (dental accident), can no longer be treated with fillings, which should be surrounded by largely intact and stable cusp tips on the occlusal surface of a tooth. With a partial crown, the occlusal surface is reshaped and one or more tooth cusps are stabilized by overcoupling. Accordingly, the preparation margins (circumference of the milled tooth areas) are extended beyond the occlusal and proximal surfaces (the masticatory and interdental surfaces). Usually, several tooth cusps are overcoupled (included in the preparation margins). However, unlike a full crown, not all cusps are included and the preparation margin is not lowered to gingival level (the height of the gum line) in an overall circular fashion. For many decades, cast gold alloy restorations have been established and proven as the so-called “gold standard” for the restoration of extensive defects. Since pure gold is too soft and cannot withstand chewing pressure, numerous gold alloys are available with additives that affect strength, hardness, elasticity, grain size and other properties:

  • Palladium (Pd)
  • Platinum (Pt)
  • Silver (Ag)
  • Copper (Cu)
  • Zinc (Zn)
  • Indium (In)
  • Ruthenium (Ru)
  • Iridium (Ir)
  • Rhenium (Re)

Due to the desire for better aesthetics, procedures that preserve tooth structure and biocompatible materials, ceramic restorations have entered the field of dentistry. This has been made possible not only by the ceramic materials themselves, but also by improvements in the micromechanical bond between ceramics and tooth structure through adhesive technology. Ceramic partial crowns are now considered to be as scientifically accepted as cast partial crowns. Alloys – usually high-gold precious metal alloys – are still used for partial crowns today, but increasingly ceramics are being used. One advantage of ceramic materials is that they are bioinert (free from interaction with the organism). However, in the case of adhesive cementation, hypersensitivity reactions to the methacrylate-based luting resin can negate this advantage. Glass-infiltrated and zirconia-based advanced ceramics can also be cemented with conventional (conventional) cements such as zinc phosphate or glass ionomer cement. However, these do not achieve the bond that is achieved by micromechanical anchorage using adhesive technology.

Indications (areas of application)

The indication for a partial crown results primarily from loss of tooth structure, which makes it impossible to restore the tooth with a filling, inlay or overlay. For example, undermined cusps require stabilization by their overcoupling and possibly adhesive (bonding) technique, unilateral circular decalcifications (along the gingival margin) are included in the course of the partial crown by lowering the preparation margin. Also in case of loss of bite height, for example due to years of bruxism (teeth grinding), it may be necessary to reshape the occlusion (the final bite and the guiding surfaces for chewing movements) and protect the remaining tooth structure with partial crowns. Furthermore, the indications depend on the material planned for the partial crown:

Indication for a ceramic partial crown

  • Esthetics that can not be achieved with metal partial crowns.
  • Reasons arising from the pre-treatment of a tooth – the thin dentin walls (dentin) of endodontically treated teeth (with root filling) require stabilization by the adhesive technique.
  • Proven incompatibility against cast alloys.
  • Lack of residual tooth substance or the insufficient length of the clinical dental crown make retentive preparation of the tooth impossible for a cast partial crown, which is essentially anchored by friction (fit by friction).

Indications for a cast partial crown

  • Subgingival cavities (holes extending below the gingival margin) that no longer permit adhesive cementation, as is usually required and useful for ceramic restorations
  • Proven intolerance to adhesive luting materials (resin-based).
  • Bruxism (teeth grinding and clenching).

Contraindications

  • Small tooth structure defects
  • High risk of caries – indication for a full crown
  • Circular (circumferential) decalcification around the entire tooth – Here results in the indication for the full crown.
  • For cast partial crown: length of clinical crown or degree of destruction do not allow friction (primary fit by friction).
  • For ceramics: cavities (“holes”) that extend deep subgingivally (deep into the gingival pocket), so that drainage for adhesive cementation technique is not guaranteed. In this case, a partial gingivectomy (partial surgical removal of gums to reduce a gingival pocket) may be helpful to allow the adhesive cementation method of ceramic restoration after all. Alternatively, it is necessary to switch to conventional cementing, for example, zirconia ceramics.
  • Ceramic materials have a higher microhardness than enamel, so this can result in increased abrasion of the antagonists (abrasion of the teeth of the opposing jaw), especially in bruxism (teeth grinding).
  • For ceramics: incompatibilities towards the luting components.

Before the procedure

a thorough diagnosis is required, taking into account the clinical and usually radiological parameters for planning the partial crown.

The procedures

I. Procedure for a partial ceramic crown

The procedures are described separately under the article Ceramic Partial Crown. II. procedure for a cast partial crown.

Unlike the direct filling technique, restoration with cast restorations fabricated indirectly (outside the mouth or in the dental laboratory) is divided into two treatment sessions. II.1 First treatment session – preparation.

  • Excavation (caries removal) and, if necessary, placement of a build-up filling (made of cement) for substance compensation.
  • Preparation (grinding of the tooth) of the tooth tissue as gently as possible, with sufficient water cooling and with the least possible removal of substance. The following criteria must be observed:
    • Preparation angles of the inner walls – must diverge slightly in the pull-off direction so that the future partial crown can be removed from or placed on the tooth without jamming or leaving undercut areas unprovided. On the other hand, the friction must be so strong that the crown can only be removed from the tooth with resistance, even without cement. In this, the preparation technique differs from that for a partial ceramic crown, whose subsequent hold is based on the micromechanical bond tooth – luting composite – ceramic.
    • Occlusal substance removal (in the occlusal surface area) – Following the central fissure (main furrow in the occlusal surface relief, runs in sagittal direction, i.e. from “front to back”), a box (box-shaped preparation form) is created, the walls of which enclose an angle of 6° to max. 10° diverging in the removal direction. In addition to the preparation angle of the outer walls, the box preparation contributes significantly to the friction of the cast object. In the area of the cusps to be overcoupled, the enamel is removed.
    • Approximal preparation (in the interdental area): also only slightly divergent box.
    • Preparation angle of the outer surfaces – slightly conical: total convergence angle 6° to max. 15° – the shorter the available outer surfaces, the smaller the total angle should be.
    • Proximal contact (contact with the neighboring tooth): must be in the area of the partial crown, not in the tooth substance area.
    • Spring margin – Applied in areas limited by enamel with a width of max. 1 mm, it protects the enamel prisms running out at the preparation margin and, after cementation, enables the cast object, which inevitably runs out just as thin here, to be fined against the tooth, thus minimizing the cement gap.
    • Finishing the preparation – All traces of coarse diamond grinding and roughness are removed with fine-grained rotary instruments.
  • Bite taking and opposing jaw impression – serve to spatially match both jaws and design the occlusal relief of the partial crown.
  • Impression of the preparation with detailed and dimensionally stable impression materials, e.g. addition-curing silicone.
  • Fabrication of a temporary (transitional) crown made of acrylic to protect the tooth and insertion with temporary cement.

II.2. working steps in the dental laboratory.

  • Pouring the impressions with special plaster.
  • Making a saw model – The model of the prepared tooth, called the die model, can be removed, making the preparation accessible all around.
  • Wax modeling of the partial crown freehand, thereby alignment along the preparation margin and orientation to the occlusal relief of the opposing tooth.
  • Embedding of the wax model in investment material, from which the wax is burned out by heating. This creates a hollow mold.
  • Casting of the molten gold alloy into the hollow mold with the help of a centrifuge, which assists in shooting the melt into the hollow mold.
  • After cooling, bedding out the cast object.
  • Finishing and polishing

II.3. second treatment session – incorporation

  • Control of the completed partial crown
  • Provided that the preparation margins allow this: Installation of rubber dam (tension rubber) to protect against saliva ingress and against swallowing or aspiration (inhalation) of the partial crown.
  • Cleaning the prepared tooth
  • Try-in of the partial crown, if necessary with the help of thinly flowing silicone or color spray to find places that interfere with the internal fit
  • Checking the proximal contact
  • Control and correction of occlusion (final bite and chewing movements).
  • Cementing the partial crown – e.g. with zinc phosphate, glass ionomer or carboxylate cement.
  • Removal of excess cement after its curing.
  • Finishing – finishing the edges with ultra-fine grit polishing diamonds and rubber polishers.

After the procedure

After a wearing period of about two weeks, it is recommended to re-fine the wafer-thin edges of the feather edge to minimize the cement gap.

Possible complications

Possible complications can arise from the many intermediate steps in the fabrication process, such as:

  • Fracture (breakage) of a partial ceramic crown.
  • Loss of a cast partial crown due to insufficient friction (primary fit due to friction).
  • Tooth sensitivity (hypersensitivity) or pulpitides (pulpitis) due to preparation trauma or errors in adhesive cementation of a ceramic restoration
  • Allergy to a component of a casting alloy
  • Allergy to an adhesive luting material; the decisive role here is the unavoidable low residual content of monomer (individual components from which the larger and thus hardened polymers are formed by chemical combination) in the finished polymerized material; diffusion of monomer into the pulp can lead to pulpitis (pulp inflammation)
  • Marginal caries in the area of the joint between the tooth and partial crown by washing out cement.
  • Marginal caries caused by poor oral hygiene – bacteria preferentially adhere to the luting material in the cement joint