1-Historical Overview, Definitions, and Principles of Artificial Crowns and Tooth Preparation

Preclinical Fixed Prosthodontics 1: Lecture 1

Lecturer: Prof. Dr. Nikola Gigovski
Literature: Shillingburg T. H. et al. ‘Fundamentals of fixed prosthodontics’. Quintessence Publishing Co, Inc; Chicago, 1997 (third edition).

Overview of Fixed Prosthodontics

Fixed prosthodontics focuses on the rehabilitation of damaged or lost teeth, similar to mobile prosthetics, but with distinct mechanical and biological differences.

Advantages of Fixed Prosthetic Constructions

Attachment: The construction is strongly attached to the remaining tooth or teeth; it is non-removable by the patient.
Force Distribution: It transfers masticatory forces physiologically through the root and parodontium of the natural tooth (or teeth) onto the surrounding bone structure.
Integration: These constructions are significantly smaller than mobile ones, typically matching the approximate size of the natural teeth they replace, allowing for quick integration into the masticatory system.

Historical Overview of Fixed Prosthodontics

Ancient Foundations

Etruscan Prosthesis: Dating back several centuries BCE, this is considered the oldest prosthetic replacement. It consisted of natural teeth bound together and secured to remaining natural teeth using gold wires.
Roman Restorations: Archaeological excavations in Falera, Tarquinia, and Vetulonia revealed human remains with restorations made of bovine teeth joined via gold splints.

The Middle Ages and Renaissance

Dental prosthetics largely stagnated during the Middle Ages.
Notable academic contributions included: * Guy de Chauliac (1298–1368): Wrote ‘Chirurgia Magna’, referencing tooth and jaw anatomy. * Eustachius (1563): Wrote ‘Libellus de Dentibus’, documenting dental diseases and anatomy.

Development of Modern Dentistry

Pierre Fauchard (1678–1761): Known as the father of modern dentistry, he published the first scientifically based work, ‘Le Chirurgien Dentiste,’ which included illustrations of prosthetic constructions for missing anterior teeth.
Philipp Pfaff: A German contemporary who made significant contributions to the field during the 18th century.
19th Century Advancement: The discipline transitioned to a rigorous scientific approach aligned with developments in chemistry, physics, and medicine.

Evolution of Crown Designs and Materials

Late 18th Century Prototypes:
* Richmond Crown: An artificial tooth featuring a post for retention.
* Jacket Crown: A full porcelain crown fired on platinum foil.
20th Century Materials:
* Acrylic (methyl methacrylate) was discovered in Germany before WWII.
* Harold Vernon: The first to apply methyl methacrylate in prosthodontics.
Technological Drivers: Development was accelerated by heat-resistant investment materials, improved casting techniques, and high-quality dental alloys.

Objectives and Types of Restorations

Primary Goals

Single Tooth Reconstruction: Restoring anatomical shape and correcting developmental or structural abnormalities using artificial crowns.
Missing Teeth Replacement: Restoring the dental arch in one or both jaws using dental bridges.

Restoration Categories

Artificial crowns
Dental bridges
Cast posts and cores
Coat Ceramics
Inlays and Onlays
Laminates (Veneers)

Artificial Tooth Crowns: Definitions and Characteristics

An artificial tooth crown is defined by three main characteristics:

It is a fixed restoration used to reconstruct a damaged or lost natural crown.
It is permanently attached to the remaining tooth or root.
Mastication forces are transmitted to the bone via the periodontium (periodontal/dental transmission) and, to a lesser degree, through contact points to adjacent teeth.

Indications for Fabrication

Extensive carious lesions
Morphological defects
Traumatic fractures
Discolorations
Alignment anomalies
Abrasion
Erosion
Bite correction
Bridge abutments
Increasing vertical dimension (OVD)

Contraindications

Absolute: Healthy teeth (not requiring restoration), mobile teeth, teeth with periapical lesions, advanced periodontal disease, or untreated teeth.
Relative: Teeth currently undergoing endodontic or periodontal treatment.

Classification of Artificial Crowns

By Duration

Definitive (Permanent)
Temporary

By Surface Coverage

Partial: Half crowns, three-quarter ( $3/4$ ) crowns, four-fifth ( $4/5$ ) crowns.
Full: Full-coverage crowns.
Post-and-core: Crowns used when only the root remains.

By Material

Entirely metal
Non-metal (Composite or Ceramic)
Combined (Veneered)

By Fabrication Method

Monoblock cast
Electrochemical (Galvanic)
Sintered
CAD/CAM-fabricated

Essential Requirements for Artificial Crowns

Morphology: Must reproduce anatomical features (cusps, fissures, contact points, and the Curve of Spee).
Occlusion: Harmonized occlusion without premature supra-contacts or infra-contacts.
Marginal Integrity: The crown margin must end at the finish line and fit intimately to the cervical part of the tooth to avoid gingival irritation.
Retention: Primary retention is achieved via the shape of the prepared tooth and intimate fit. Cementation serves as secondary retention.
Structural Integrity: Must be thick and strong enough to resist fracture or perforation.
Posts: For post-and-core crowns, the post must have sufficient length and thickness for long-term retention.

Fabrication Stages

Clinical Stages

Tooth preparation.
Impression taking.
Try-in and adjustment.
Cementation.

Laboratory Stages

Pouring the impression to obtain working models with removable dies.
Bite registration template fabrication (optional).
Mounting models in an articulator.
Wax-up (starting with a coping; may use wax, acrylic, or prefabricated components).
Investing and casting.
Divesting (opening the mold), cleaning, processing, and fitting the cast onto the stone model.
Veneering.
Polishing to a high gloss.

Principles of Tooth Preparation

Definition: Preparation is the clinical stage involving the removal of hard tooth substance by grinding to provide a specific shape and space for the crown. The prepared tooth is the "abutment."

Instrumentation: High-speed turbine handpieces exceeding $300,000\,\text{rpm}$ with diamond burs.

Geometric Principles of the Abutment

The abutment should retain the basic shape of the natural tooth in reduced dimensions.
It must be widest at the cervical part (at the finish line/demarcation line).
It must taper gradually toward the occlusal or incisal surface, creating a slightly conical shape.

The Conical Taper

Ideal Taper: The lateral walls should taper toward the long axis between $2^\circ$ and $6^\circ$ .
Recommended Range: $3^\circ\text{--}4^\circ$ .

Comparison of Preparation Shapes

Slightly Conical (Optimal): Allows easy seating, provides sufficient friction for retention, and ensures stability.
Cylindrical (Undesirable): Difficult to place/remove during try-in, unclear preparation margins, and risks over-contoured artificial crowns due to insufficient substance removal.
Strongly Conical (Undesirable): Risk of iatrogenic pulp exposure, increased pain during preparation, poor retention, and high risk of de-cementation.

Material Removal Measurements (Averages)

Incisal/Occlusal: $1.5\text{--}2.0\,\text{mm}$ .
Axial/Lateral Surfaces: $1.0\text{--}1.5\,\text{mm}$ .
Cervical Region: $1.0\,\text{mm}$ .

The Finish Line (Preparation Margin)

This is the demarcation line where the prepared portion of the tooth transitions into the unprepared part. The crown edge must terminate precisely here.

Location

Sub-gingival: $0.5\text{--}1\,\text{mm}$ below the gingiva.
Para-gingival: At the level of the gingiva (entrance to the sulcus).
Supra-gingival: Above the gingiva.

Design Shapes

Tangential (Knife edge)
Chamfer: Characterized by a rounded internal angle.
Shoulder
Shoulder with beveled outer edge

Note: Each finish line type corresponds to a specific crown margin design for optimal fit and möaesthetics.