University of Detroit/ Mercy Dental School                                   Course 824  Prep FPD fmc pfm

Prosthodontic Department                                                                                Michael L. Linebaugh DDS MS

Fixed Prosthodontic 11                     

Preparation of Abutments (FMC & PFM crown)

for a Three Unit Posterior Fixed Prosthesis

There are ten steps in the preparation of the

full metal crown and porcelain metal crown for a fixed prosthesis.

1. Evaluating occlusal plane 

2. Buccal/lingual and mesial/distal angulations of abutments

3. Breaking contact

4. Occlusal or incisal reduction

5. Buccal and lingual reduction, shoulder design, anterior lingual reduction

6. Proximal reduction

7. Secondary bevel for functional cusp

8. Evaluating line of draw

9. Line angel rounding

10. Axial and occlusal smoothing

Step #1 Evaluating occlusal plane

Evaluate the opposing dentition and the abutment for proper occlusal plane. Have pre-preparation diagnostic casts available to aid in evaluation.

Step #2 Buccal/lingual and mesial/distal angulations of abutments

Evaluate the angulations of the abutment teeth in a BL and MD direction. This will determine the best line of draw for removing the least amount of tooth structure.

Note: It is important to select the smaller of the abutments to begin your preparation. The smaller tooth has less surface area to work with. Thus the larger abutment can be adjusted more easily for draw etc.

Step #3 Breaking contact  

Breaking contact is completed first to create enough space interproximal to allow for the finish line preparation without jeopardizing the adjacent teeth. This is accomplished with a thin cone shaped straight diamond burr. The diamond is used with a sawing motion.

Step #4 Occlusal or incisal reduction

Occlusal or incisal reduction is the second step.  For a full metal crown the occlusal surface is reduced in a uniform manner that will allow for a 1.5mm clearance between the opposing tooth. For porcelain coverage 2mm of occlusal reduction is required. The occlusal reduction proceeds by first making tracer cuts or depth orientation grooves. These grooves will act as a reference to determine adequate reduction. The tooth surface left between the grooves is then removed.  The diamond burrs that will best accomplish this task are the parallel sided (cylinder) chamfer burr.

Step #4.5 Shoulder design The buccal or facial surface is reduced with a shoulder diamond creating enough room for metal, opaque and porcelain on the replacement crown.  A 1.2 mm finish line is created in the form of a shoulder. This shoulder will extend just lingual to the interproximal contact area.  The shoulder finish line will be .5mm above the free gingival margin on the typodont.  In the clinical situation the shoulder finish line may be subgingival. There is a secondary plane reduction on the buccal or labial which follows the occlusal /incisal contour. This reduction will allow for adequate thickness of porcelain in the incisal area.

Step #5 Buccal and lingual reduction  The buccal and lingual walls are reduced and a chamfer finish line is created at the same time.  The finish line is created by the tip of the diamond which is a bullet shape with 1/2 of the diamond creating a chamfer finish line.  The finish line should be smooth, regular and definite. There needs to be 1mm+ reduction on both the buccal and lingual.

Step# 5.5 Lingual reduction: anterior teeth  The lingual of anterior teeth are reduced  so the original contour of the tooth will remain.  This same is best maintained by the #379 football shaped diamond.  There needs to be 1.5mm reduction on and lingual.

Step #6 Proximal reduction 

Once there has been sufficient room for maneuvering the chamfer burr then the walls are reduced in a manner that allows for the proper taper. At the same time the finish line is also formed and concocted to the finish line previously completed during buccal/lingual axial wall reduction. The diamond burr used for the proximal reduction is a #877 parallel sided diamond.  This diamond has a smaller diameter which allows it fit the interproximal  without contacting the adjacent tooth.

Step #7 Secondary bevel for the functional cusp

The secondary  bevel is placed on the functional cusp to give more thickness to the metal casting.  On the mandibular molar it is the buccal incline and on the maxillary molar the lingual incline.  This is an area of greater force on the occlusal surface and subjected to increase wear.  The bevel can be created with either the parallel sided diamond or the football shaped diamond.

Step #8 Evaluating line of draw

This step is the final evaluation of the line of draw.  In order for the three unit fixed prosthesis to seat properly to the finish lines on the preparation the abutments must be parallel to one another. You must evaluate:

            A. BL inclinations of the axial walls of the preparations

            B. MD inclinations of the axial walls of the preparations

C. Overall path of withdrawal to the long axis of the abutment and adjacent teeth

Step #9 Line angel rounding

The line angles are  slightly rounded to create a easier metal casting form. The sharp angles created by the preparation are difficult to cast.  Rounding the line angles can be created by using the

Step #10 Axial, proximal, occlusal  and finishing line  smoothing
The smoothing of the preparation removes rough areas that will interfere with the casting and helps to make a smooth and continuous finish line.