Principles of crown preparation

Mechanical principles:


It is the ability of the restoration to resist a displacement in the opposite direction to its insertion axis when subjected to tensile forces. It prevents the removal of the restoration along the path of insertion or long axis of the tooth preparation. This basically depends on the frictional resistance that is the existing contact between the internal surfaces of the restoration and the external surface of the prepared tooth. No cement that is compatible with living tooth structure and the biological environment of the oral cavity possesses adequate adhesive properties to hold a restoration in place solely through adhesion. The geometric configuration of the tooth preparation must place the cement in compression to provide the necessary retention.

An extra-coronal restoration uses opposing external surfaces for retention.


The angulation or occlusal convergence of the preparation was one of the first aspects of crown preparation to receive specific attention. The convergence angles between the opposed surfaces of a full-crown preparation influence their retention and resistance to rotation, with the angle being theoretically optimal between 2 and 4 degrees. Due to the difficulty in fitting and preparing a crown with these angles, angles between 2 and 22 degrees are considered acceptable. 
Because a ceramic restoration is placed on the preparation after the restoration has been fabricated in its final form, the axial walls of the preparation must taper slightly to permit seating of the restoration. This convergence can be used to describe the respective relationships between the two opposing walls of a preparation. The relationship of one wall of preparation to the long axis of that preparation is the inclination of that wall. A tapered diamond or bur will confer an inclination of 2 to 3 degrees to any surface it cuts if the shank of the instrument is held parallel to the intended path of insertion of the preparation. Two opposing surfaces, each with a 3-degree inclination, would give the preparation a 6-degree taper.

Theoretically, retention is closely related to the parallelism obtained by the walls and the prepared area. It seems reasonable to make the preparation as parallel as possible to enable better frictional retention. Thus, the more parallel the axial walls and the greater the surface contact, the greater should be the retention. The most retentive preparation should be one with parallel walls. 

However, parallel walls are impossible to create in the mouth without producing preparation undercuts. Preparation walls are tapered to allow their visualization, prevent undercuts, compensate for inaccuracies in the fabrication process, and permit more nearly complete seating of restorations during cementation. A more parallel preparation complicates the cementation procedure because the flow of the cement is hindered. This causes a misfit of the restoration and increased thickness of the cement film, especially on the occlusal and cervical portion of the preparation.

The degree of convergence will be determined by the individual characteristics of each preparation. Teeth with short clinical crowns require less convergence to provide greater retention, and teeth with long clinical crowns require greater convergence. 

The existing relationship between the height and angulation of the preparation. Shorter teeth should have less taper, longer teeth should have a larger taper.

Tooth preparation taper should be kept minimal because of its adverse effect on retention, a minimum taper is necessary just to ensure the absence of undercuts. The tendency to overtaper preparations is one that must be vigilantly guarded against in order to produce preparations with the least possible taper and the greatest possible retention. Consciously attempting to create a taper can easily result in an over-tapered and non-retentive preparation.

Length & Surface area

Longer preparations will have more surface area and therefore will be more retentive. The shorter the wall, the more important its inclination. The walls of shorter preparations should have as little taper as possible to increase the resistance. However, even this will not help if the walls are too short.
As the frictional retention depends on the contact between the restoration and the preparation surfaces; the greater the clinical crown of a prepared tooth, the larger the contact surface and the greater the final retention. 
Thus, when the teeth are long, the inclination of the walls can be increased to achieve greater occlusal convergence, sufficient to maintain adequate retention. On the other hand, the walls of shorter teeth should be prepared close to parallel to maintain effective retention. It has been proposed that the anterior teeth and premolars should have a minimum occluso-cervical dimension of 3 mm, and the molars a minimum dimension of 4 mm. Teeth that do not have these minimum dimensions should be modified to increase their retention through additional retention.

A full crown preparation is more retentive on a molar than on a premolar because the molar preparation has a greater surface area.

In the same tooth, a partial preparation is less retentive than a total preparation. With teeth with the same height, the same preparation in a premolar is less retentive than on a molar.

Luting agent

Retention also depends on the luting agent that fills the space between restoration and the tooth remnant. Notwithstanding, this potential can be more or less critical, depending on the type of the luting agent. When the cementation is performed with conventional non-adhesive cement, the retention to the abutment is purely frictional and the geometric principles of the macro-mechanical retention (i.e, angulation of walls and preparation height) are more relevant. For instance, zinc phosphate-based cement aids the mechanical interlocking as it takes the advantage of the micro-roughness of the preparation and the restoration. 

On the other hand, when the cementation is performed using resin luting agents, with adhesive properties, the micromechanical retention plays a role in addition to frictional retention. The micromechanical retention is related to the interaction of the adhesive components of the luting agent with the modified dental substrate.

The type of cement used has a direct effect on the cementation of the restorations. It has been found that the bond strength of adhesively cemented preparations is superior to that of preparations with zinc phosphate and glass ionomer cement. 

It is worth remembering that even in situations where restoration is adhesively cemented, it is advantageous to have good preparations that have a wall inclination with adequate frictional retention, complete seating with the lowest possible cement thickness, and resistance to displacement from different masticatory forces, provided that they don’t result in an excessive sacrifice of healthy tooth structure. Finally, it is worthy to say that the current trend is that the indirect restorations are increasingly cemented adhesively with resin cement. They are less soluble and more aesthetic than non-adhesive cements, in addition to presenting excellent mechanical properties. This obviously does not mean there are not situations where conventional, non-adhesive cement is suitably indicated. There are cases where, for technical limitations or problems related to the adhesion of some restorative materials, non-adhesive cementation is a great alternative.

Path of insertion

The path of insertion is an imaginary line along which the restoration will be placed onto or removed from the preparation. It is determined by the dentist before the preparation is begun, and all features of the preparation are cut to coincide with that line. The correct technique must be used to survey a preparation visually because this is the primary means of ensuring that the preparation is neither undercut nor over-tapered. If the center of the occlusal surface of a preparation is viewed with one eye from a distance of approximately 30 cm (12 inches), it is possible to sight down the axial walls of a preparation with a minimum taper. 


To examine a preparation for undercuts, one eye should be closed.


However, it is also possible to sight down the axial walls of a preparation with a reverse (ie, undercut) taper of 8 degrees when both eyes are open. This occurs because of the distance between the eyes, which is responsible for binocular vision. Therefore, it is important that preparations be viewed with one eye closed. 


If both eyes are open when the preparation is viewed, undercuts may remain undetected.



For a preparation to be surveyed in the mouth, where direct vision is rarely possible, a mouth mirror is used. It is held at an angle approximately ½ inch above the preparation, and the image is viewed with one eye. If fixed partial denture abutment preparations are being evaluated for a common path of insertion, a firm finger rest is established, and the mirror is maneuvered until one preparation is centered. Then, pivoting on the finger rest, the mirror is moved, without changing its angulation, until it is centered

over the second preparation.


Preparations in the mouth are viewed through a mouth mirror using one eye.


The path of insertion must be considered in two dimensions: facio-lingually and mesiodistally. The faciolingual orientation of the path can affect the esthetics of the crown. For all-ceramic crowns, the path is roughly parallel with the long axis of the teeth. 

(a) The path of insertion of a preparation for a ceramic crown should parallel the long axis of the tooth. (b) If the path is directed facially, the prominent facio-incisal angle may create esthetic problems of overcontouring or opaque show-through. (c) However, if the path is directed lingually, the facial surface will intersect the lingual surface, creating a shorter preparation. It also may encroach on the pulp.



A facially inclined path of insertion on a preparation for a ceramic crown will leave the facio-occlusal angle too prominent, resulting in overcontouring of the restoration, opaque show-through, or both.


The mesiodistal inclination of the path must parallel the contact areas of adjacent teeth. If the path is inclined mesially or distally, the restoration will be held up at the proximal contact areas (ie, locked out). This is a particular problem when restoring a tilted tooth. In this situation, making the path of insertion parallel with the long axis of the tooth will cause the contacts of the adjacent teeth to encroach on the path of insertion.

The path of insertion of preparation must parallel the adjacent proximal contacts (a) or it will be prevented from seating (b).



It should be noted that even though retention and stability are defined separately, they depend on each other and are always intertwined. The difference between them is the direction of forces exerted on the preparation. Stability is the property of the preparation to withstand the displacement of a restoration due to oblique forces. These can lead to rotation of the restoration such as during mastication or in the presence of parafunctional habits. 

Preparations with a high degree of convergence or short preparations can be subjected to dislodging forces in various directions. The shorter the preparation, the more important the occlusal convergence angle. All measures taken to limit the freedom of movement of restorations subjected to torsional and rotational forces in a horizontal plane will increase their stability. Two alternatives can be employed to obtain stability in preparation: decreased convergence of surfaces and adding grooves to occlusal surfaces.

Shillingburg et al (2007) suggested that the length of the tooth should always be greater than its base to ensure stability. Based on the principles of the height/width ratio of the preparation, Pegoraro et al (2004) suggested that if the width is greater than the height, the radius of rotation increases, impairing adequate stability. For the same height, crowns with larger diameters have comparatively less stability.

Existing relationship between the height and diameter of the preparation. Tooth with smaller diameter (A). T tooth with large diameter (B).

Some authors found that a minimum preparation height of 3 mm was necessary to provide sufficient resistance to lateral displacement of the restoration when the occlusal convergence angle does not exceed 10 degrees. In preparations with a height of 5 mm, changing the convergence angle from 2 to 10 degrees decreases the surface area by 13.9%. An increase of just 1 mm in preparation height results in a considerable gain in surface area. Since the surface area increases, increasing the height of the preparation and reducing the angulation may result in improvement in the function of the luting agent, the resistance to lateral displacement, and the retention of the fixed prosthesis. All means to increase the contact surface result in increased retention and are called auxiliary retentive features. They are categorized as grooves, boxes, and pinholes.

Simply stated, preparations on large teeth are more retentive than preparations on small teeth. This is a factor that must be considered

when a preparation is done on a small tooth, especially when it is an abutment for a fixed partial denture or a splint. Surface area can be increased somewhat by adding boxes and grooves. However, the benefits derived from such features may relate more to their limiting the freedom of movement than to the increase in surface area.




A restoration must contain a bulk of material that is adequate to withstand the occlusal forces. This bulk must be confined to the space created by the tooth preparation. Only in this way can the occlusion on the restoration be harmonious and the axial contours normal, preventing periodontal problems around the restoration.

The structural stability is the minimum material thickness of the restoration to resist the action of masticatory forces without deformation. Indirect restorative materials require a minimum thickness to provide adequate properties. Such thickness varies according to the material and the region of the tooth which is being prepared. For example, regions subject to tension during function require more space. The recommended thickness of the material may vary from one region to another. For all-ceramic restorations, there must be enough space to accommodate 1.5 to 2 mm of the ceramic material. 

Occlusal reduction


One of the most important features for providing adequate ceramic bulk and strength to the restoration is occlusal clearance. The tooth should be prepared in a manner that allows for a restoration of sufficient thickness that is able to withstand masticatory forces. Inadequate clearance makes a restoration weaker. In addition, the inadequate reduction under the anatomical grooves of the occlusal surface will not provide adequate space to allow good functional morphology. The required preparation for good structural stability will depend on the ceramic material of choice. There should be 2 mm of clearance on preparations for all-ceramic crowns. An inadequate tooth preparation can lead to weakness of the restoration due to a lack of material. The attempt to correct the inadequate reduction leads to an overcontoured restoration causing biological damage. An overcontoured restoration will result, and a deflective occlusal contact is likely to occur unless the opposing tooth is reduced.


Inadequate occlusal reduction does not provide the needed space for a ceramic restoration of adequate thickness.

The teeth should be uniformly reduced to ensure a restoration with suitable shape and esthetics. This also helps the laboratory technician to reproduce the natural teeth with the desired color and translucency. Uniform reduction also promotes the production of a normally-contoured restoration. It is important to note that the occlusal reduction should follow the existing anatomy, including pits and fissures, and reproduce the main ridges, allowing an appropriate occlusal morphology of the restoration. A flat occlusal surface may over shorten a preparation that is already of minimal length to provide adequate retention.

Occlusal reduction should reproduce basic inclined planes rather than being cut as one flat plane.


An integral part of the occlusal reduction is the functional cusp bevel. A wide bevel on the palatal inclines of the maxillary palatal cusps and the buccal inclines of mandibular facial cusps provide space for an adequate bulk of the ceramic in an area of heavy occlusal contact.

The functional cusp bevel is an integral part of the occlusal reduction.

If a wide bevel is not placed on the functional cusp, several problems may occur. If the crown is waxed and cast to normal contour, the casting will be extremely thin in the area overlying the junction between the occlusal and axial reduction. To prevent a thin casting when there is no functional cusp bevel, an attempt may be made to wax the crown to optimal thickness in this area. An overcontoured restoration will result, and a deflective occlusal contact is likely to occur unless the opposing tooth is reduced.

Lack of a functional cusp bevel can cause a thin area or perforation in the casting.

Lack of a functional cusp bevel may result in overcontouring and poor occlusion.

Overinclination of the facial surface will destroy excessive tooth structure and lessen retention.

Axial reduction


The axial walls of restoration must restore the anatomical contours of the tooth, and the preparation should facilitate this process. These walls also play an important role in transmitting the masticatory forces to the cervical regions of the preparations. The axial reduction also plays an important role in securing space for an adequate thickness of restorative material. If restorations are made with normal contours over preparations with inadequate axial reduction, they will have thin walls that will be subject to distortion. Laboratory technicians often attempt to compensate for this by overcontouring the axial surfaces. While this intended solution to the problem strengthens the restoration, it can have a disastrous effect on the periodontium.

Inadequate axial reduction can cause thin walls and a weak restoration (a) or a bulbous, overcontoured restoration (b).

Another important characteristic of the preparation regarding the resistance of the tooth remnant is the rounding of the internal angles in order to dissipate more effectively the stresses that affect the tooth-restoration assembly. The angles of the preparations should be rounded to increase the strength of the ceramic restorations and to facilitate laboratory manufacturing steps. Acute angles on the prepared surfaces act as stress concentration regions. Rounding these angles increases the strength of all-ceramic crowns.

Preparation with rounded angles

So, how to remove enough structure, without removing too much structure⁉️.. The importance of such a question becomes clear when one considers that, in general, the resistance of the restorative material benefits from the increased thickness, but the strength of the tooth remnant is also favored by the conservation of the tooth structure, except in those situations where there is not an indication to perform more invasive preparations. Thus, to ensure that the preparation meets both the requirements for the thickness of the material and simultaneously saves as much healthy dental tissue as possible, it is essential to use a technique that allows control over the depth of wear.


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