Prosthodontic and Replacement Procedures

Prosthodontic and Replacement Procedures

When a tooth is heavily compromised by extensive caries, catastrophic fracture, or severe structural wear, or when teeth are entirely missing, the scope of restorative dentistry advances into the specialized realm of prosthodontics. Prosthodontics focuses on the design, manufacture, and fitting of artificial replacements for teeth and other parts of the mouth. The field is broadly divided into two primary disciplines: fixed prosthodontics (restorations permanently cemented or screwed in place) and removable prosthodontics (appliances the patient can insert and remove themselves).

Fixed Prosthodontics

Fixed appliances are designed to be permanent, relying on remaining healthy tooth structure, root canal treated roots (often requiring a post and core), or surgically placed dental implants for their support and retention. Because they are fixed, they most closely replicate the natural dentition in terms of comfort, masticatory function, and proprioception.

Crowns: A crown (often colloquially referred to as a 'cap') is an indirect artificial restoration that completely encases a prepared natural tooth, restoring its original anatomical morphology, functional integrity, and aesthetics.

• Porcelain Bonded to Metal (PBM) / Porcelain Fused to Metal (PFM): These traditional crowns feature a strong, cast metal alloy substructure that provides immense compressive strength, overlaid with tooth-colored feldspathic porcelain for aesthetics. While durable, the opaque metal core can sometimes result in a less natural appearance, particularly if the gingival margin recedes, exposing a dark metal line.
• All-Ceramic / Zirconia Crowns: These represent the gold standard for highly aesthetic regions. Utilizing modern high-strength monolithic ceramics (like lithium disilicate or zirconia), these crowns are entirely metal-free. They allow light to transmit naturally through the material, mimicking the optical properties of natural enamel while providing exceptional durability.
• Full Cast Gold / Metal Alloy Crowns: These restorations offer unparalleled longevity and require the least amount of healthy tooth structure removal during preparation. However, their highly visible metallic appearance restricts their use almost exclusively to non-visible posterior teeth (molars).

Bridges: A bridge is a fixed dental prosthesis used to replace one or more missing teeth by permanently anchoring an artificial, suspended tooth (the pontic) to the adjacent, natural teeth or implants (the abutments).

• Conventional Bridges: These demand aggressive, irreversible preparation of the adjacent abutment teeth to accommodate full-coverage retainers (crowns). While highly retentive, the biological cost in terms of sacrificed healthy tooth structure is significant.
• Resin-Retained (Maryland) Bridges: A far more conservative alternative requiring minimal to no preparation of the abutment teeth. The pontic is suspended and anchored via thin cast metal or ceramic wings that are chemically bonded directly to the intact lingual or palatal surfaces of the adjacent teeth.
• Pontic Design: The design of the pontic is critical for maintaining hygiene. A ridge lap pontic is aesthetically pleasing but difficult to clean, while an ovate pontic sits within a surgically sculpted depression in the gum to give the illusion that the tooth is growing naturally out of the tissue.

Veneers: Veneers are ultra-thin, custom-made ceramic or composite resin shells meticulously bonded to the labial (front) surfaces of anterior teeth. They are a highly popular aesthetic solution designed to mask severe intrinsic discoloration, close diastemas (gaps), correct minor malalignment, or restore chipped incisal edges. Preparation is extremely conservative, often only involving the removal of 0.5mm of labial enamel. During cementation, the internal fitting surface of a porcelain veneer must be treated with hydrofluoric acid and a silane coupling agent to ensure a powerful micromechanical and chemical bond to the underlying resin cement.

The Fixed Prosthodontic Workflow

Providing a definitive indirect crown or bridge typically requires a highly coordinated, two-visit clinical sequence separated by a laboratory manufacturing phase:

Visit 1: Clinical Preparation and Impression

1. Local Anaesthesia is administered for profound patient comfort.
2. Tooth Preparation: Utilizing specialized high-speed diamond burs under copious water coolant, the clinician systematically reduces the tooth circumferentially and occlusally to create sufficient three-dimensional space for the chosen restorative material. A distinct, smooth margin (typically a chamfer or shoulder configuration) is established at or just below the gingival crest.
3. Gingival Retraction: To ensure the impression captures the entire preparation margin perfectly, retraction cord is gently packed into the gingival sulcus using a blunt instrument. This cord physically pushes the gingival tissues away from the tooth and is often impregnated with a haemostatic agent (like aluminium chloride or ferric sulfate) to halt localized bleeding and control crevicular fluid seepage.
4. Final Impression: A highly accurate elastomeric impression material (such as addition-cured silicone or polyether) is employed. The clinician typically syringes a highly flowable light-body 'wash' directly around the prepared tooth and into the retracted sulcus, while the dental nurse simultaneously loads a rigid heavy-body putty into the impression tray. Digital intraoral scanning is increasingly replacing this physical step in modern practices.
5. Shade Selection: Using a standardized shade guide under optimal lighting (preferably natural daylight), the exact color and translucency profile are mapped out for the technician.
6. Provisionalisation (Temporisation): The prepared tooth must never be left exposed. A temporary crown is fabricated directly chairside—often using a bis-acryl composite resin within a pre-operative alginate matrix—and cemented using a weak, non-eugenol temporary cement (such as Temp-Bond NE). This protects the sensitive dentine, maintains the periodontal health, and prevents the tooth from drifting out of alignment.

Visit 2: Try-In and Definitive Fit

1. The provisional restoration is carefully removed, and the preparation is thoroughly cleaned of any residual temporary cement.
2. The definitive laboratory-made restoration is tried in. The clinician meticulously evaluates the marginal integrity with a probe, the tightness of interproximal contact points using dental floss, the aesthetics with the patient's input, and the occlusal relationship using articulating paper.
3. Once approved, the restoration is permanently cemented using a definitive luting agent, such as a traditional glass ionomer cement, a resin-modified glass ionomer, or a dual-cure adhesive resin cement, depending on the material of the prosthesis.

Removable Prosthodontics

When multiple teeth are missing bilaterally, or when the terminal abutment teeth are absent, removable dentures often become the treatment of choice, restoring function and facial support.

Acrylic (PMMA) Dentures: Constructed entirely from rigid polymethyl methacrylate resin, these tissue-bearing dentures are highly cost-effective and allow for relatively straightforward modifications if further natural teeth need to be extracted and added to the plate in the future. However, because they are purely mucosa-supported (resting entirely on the compressible gums), they can accelerate the resorption of the underlying alveolar bone and often feel bulky to the patient.

Cobalt-Chromium (Co-Cr) Cast Dentures: These represent a vastly superior biomechanical option. They feature a rigid, custom-cast metal alloy framework onto which aesthetic acrylic saddles and teeth are processed. Co-Cr dentures are significantly thinner, lighter, and more comfortable. Crucially, they are heavily tooth-supported; they utilize meticulously designed metal occlusal rests that sit on prepared seats on the natural teeth, transferring the heavy forces of mastication directly down the long axis of the strong natural abutments rather than crushing the fragile soft tissues.

The Comprehensive Denture Workflow

Fabricating a functional and aesthetic denture involves a multi-stage process requiring precise clinical records and iterative laboratory stages:

1. Primary Impressions: Taken in alginate using standardized stock trays. These capture the broad, generalized anatomy of the edentulous ridges, muscle attachments, and palate to allow the laboratory to pour primary study models.
2. Secondary (Master) Impressions: The laboratory constructs customized 'special trays' specifically fitted to the patient's primary models. These are used clinically with a highly accurate, dimensionally stable material (like silicone elastomer or zinc-oxide eugenol paste) to capture the detailed functional anatomy of the vestibules under muscle action.
3. Bite Registration (Jaw Relationship): Wax occlusal rims (bite blocks) constructed on rigid bases are placed in the mouth. The clinician modifies the wax to record the patient’s precise jaw relationship in centric occlusion and to establish the correct vertical dimension of occlusion (VDO)—the anatomical distance between the nose and chin when the jaws are closed at rest. The smile line and midline are also scribed into the wax.
4. Try-In Stage: The laboratory mounts the models on an articulator and sets up the chosen artificial teeth in wax. This allows the clinician, nurse, and patient to critically evaluate the trial denture in the mouth for aesthetics, phonetics (speech), and occlusal harmony before finalizing the design.
5. Fit and Delivery: The laboratory permanently processes the denture (converting the wax pattern into hard acrylic via a flask and pack technique). At the fit appointment, the finished denture is inserted, and any high spots on the tissue-fitting surface are relieved using specialized pressure-indicating paste. The occlusion is fine-tuned, and the patient is provided with comprehensive instructions on denture hygiene, insertion, removal, and overnight storage.