Preparation Features of Dental Crowns

 

Features of a PFM crown preparation

The crown preparation should start from incisal aspect, then labial, then moving towards proximal contact area and finally to the lingual aspect in cases of anterior tooth preparation. 

For posterior tooth preparation, one should start from occlusal surface, then buccal surface from  mesial to distal direction and then move to break the contact areas. After breaking contact points, start doing reduction of lingual surface from mesial towards distal direction. 

The above view of the sequence of tooth reduction is of the Author, though, in the book "Fundamentals of Fixed Prosthodontics" by Herbert T. Shillingburg jr., it is given differently. The author found it better suited for himself after modifying the Shillingburg's recommended sequence to get better access.

Always remember the type and final dimensions of your preparations. The recommended are given in the following tables. Always aim to reduce the tooth surface 0.1-0.2 mm less than the dimension given below so that you can do final finishing and not doing over reduction.

Before making final impression, always check for any irregularity, sharpness, undulations on prepared surfaces and finish lines or any lacunae in your tooth preparation.

 As per Shillingberg, the following features should be incorporated in a crown preparation.  

Recommended preparation features of posterior crowns
Crown type	Posterior crown preparation features
	Occlusal reduction	Finish line depth and configuration
VMC	1 mm. non-functional cusp 1.5 mm. functional cusp	0-1.0 mm Chamfer, knife-edge, shoulder or shoulder with bevel
HSPC	2 mm non-functional cusp 2.5 mm functional cusp	0.8-1.0 mm Shoulder or heavy chamfer
PFM	Same as VMC if metal surface 2 mm for non-functional cusp 2.5 mm functional cusp	1.2 mm labial shoulder 0.5 mm lingual chamfer

# VMC - Veneered Metal Crown

# HSPC - High Strength Porcelain Crown 

# PFM - Porcelain Fused to Metal 

Recommended preparation features of anterior crowns
Crown type	Anterior crown preparation features
	Occlusal reduction	Finish line depth and configuration
PJC	2 mm. incisally 1 mm- labial & lingual aspect	0.8-1.0 mm Shoulder
RBPC	2 mm incisally 0.5-1.0 mm-lingual aspect	>0.4 mm chamfer
PFM	2 mm incisally 0.5-1.0 mm lingual aspect (porcelain guidance requires greater clearance)	1.2 mm labial shoulder or heavy chamfer 0.5 mm lingual chamfer

# PJC - Porcelain Jacket Crown

# RBPC - Resin Based Polymer Crown

# PFM - Porcelain Metal Crown 

# Where the tooth is tilted or vertical dimension is to be increased, the amount of reduction required will vary.

# Too deep a reduction for diminutive teeth or for long clinical crowns where a metal collar is preferable

Pontic Design in Prosthodontics

The pontic, \pŏn΄tĭk\, is an artificial tooth on a fixed partial denture that replaces a missing natural tooth, restores its function, and usually restores the space previously occupied by the clinical crown. An ideal correctly designed posterior pontic should have the following features. 

1. All surfaces should have convexity with proper finish. 
2. The contact with buccal contiguous slop should be minimal (pinpoint)) and with pressure free (modified ridge lap). 
3. Buccal and lingual shunting mechanisms should conform with those of the adjacent teeth. 
4. The occlusal table should be in functional harmony with the occlusion of all of the teeth. 
5. The overall length of the buccal surface should be equal to that of the adjacent abutment teeth or pontic. 

Figure 1. Schematic presentation of various pontic designs; (a) Sanitary pontic; has no contact with the edentulous ridge, (b) ridge lap pontic; forms a large concave contact replacing the contours of a missing tooth, (c) modified ridge lap; shows illusion of a tooth but it has all or nearly all convex surfaces for easy cleaning and minimize plaque accumulation, (d and e) bullet/conical; rounded and cleanable smaller tip in relation to overall size, (f) ovate; round end design currently in use where aesthetics is a primary concern[1].

The Pontic design can be classified into two types:

A. Pontic having mucosal contact 

1. Sanitary/Hygienic/Fish-belly
2. Modified sanitary type 

B. Pontic with no mucosal contact      

1. Saddle/Ridge lap 
2. Modified Ridge Lap 
3. Conical
4. Ovate 

Sanitary or Hygienic Pontic or Fish-Belly  

Sanitary pontic makes no contact with the edentulous ridge. It is made in an all-convex configuration, facio-lingually and mesio-distally. The space between the pontic and the mucosa should be 2 or 3 mm [2]. It is the most commonly used mandibular molar replacement pontic design. It has the advantage of providing good access for maintaining hygiene with the disadvantage of poor esthetic. 

Sanitary pontic design

Modified Sanitary 

The tissue facing surface of the modified sanitary design pontic has a hyperbolic parabola. The pontic is designed as a concave archway mesio-distally while the under surface is convex facio-lingually. It is indicated in molar replacement, provides access to the under-surface for good hygiene, along with poor esthetic.  

Modified sanitary pontic design-note the concavity at the cervical region.

Saddle/Ridge Lap 

It looks like crown of the tooth because it replaces all the contour of the missing tooth. It maintains a large concave contact with the underlying ridge and obliterates facial, lingual and proximal embrasures. Esthetic wise it provides the best result but due to impossible access to under-surface for cleaning, its use is limited for the replacement of maxillary incisors. 

Saddle/ridge lap pontic design

Modified Ridge Lap 

The modified ridge lap combines esthetic with easy cleaning and makes contact with ridge tissues in the shape of ‘T ‘ whose vertical arm ends at the crest of the ridge. This design is most commonly used in the area of the mouth that is visible during function, e. g. anterior teeth, premolars and sometimes maxillary molars.  

Modified ridge lap pontic design: note the half concave underneath surface in contact with mucosa.

Conical 

Conical pontic design is mostly limited to the replacement of thin knife edged ridges in the non-display zone of the mouth. It has a convex surface with only touching the center of the residual alveolar ridge. It helps maintain good hygiene with poor esthetic. Conical design is indicated for the replacement of molars. 

Image of conical pontic design- note the egg shaped design in green circle.

Ovate 

Ovate pontic design is the most aesthetically suitable design, that looks like emerging from the gingiva. When ridge resorption is corrected by ridge augmentation, ovate design appears to be emerging through gingiva just like natural tooth. It is indicated for the replacement of maxillary incisors, canines, and premolars. It has the advantage of best esthetic, negligible amount of food entrapment and easy cleaning. The disadvantage is the requirement of surgical preparation of the receiving site before prosthesis fabrication. 

Ovate pontic design-note the convex under-surface of pontic embedded in mucosa (green circle).

 

References:

1. Eur J Dent. 2018 Jul-Sep; 12(3): 375–379. doi: 10.4103/ejd.ejd_232_18
2. Rosenstiel & Tyllman respectively
3. Pontics