In situations where the degree of tooth destruction does not indicate a direct restorative technique, such as in the case of medium to large lesions, the indirect restorations are indicated. Naturally, there are significant differences between the direct and the indirect restorations.

See below some of the main characteristics of preparation for indirect restorations 👇🏻

Expulsiveness :

Probably this is the main difference between the preparations for direct and indirect restorations. Since the fabrication, finishing & polishing steps are performed extra orally, the preparation for an indirect restoration must allow for its removal and reinsertion, whenever it is necessary. For this purpose, it is essential that the preparation present expulsiveness, which is basically related to the angulation of the surrounding walls.

⛔️ The degree of expulsiveness of the preparations is very important to the retention of the restorations; the lower the expulsiveness, the greater the retention & vice-versa.

🧐 while the statement mentioned above suggests firstly that the expulsiveness should be kept as minimal - since this improves the retention - one should realize that the absence of the proper expulsiveness impairs adaptation and makes luting procedures more difficult.

⛔️ The recommended degree of expulsiveness varies according to each type of preparation “ mentioned before in previous posts “. 

Retention :

Retention in indirect restorations mainly depends on the luting agent that fills the space between restoration and the tooth remnant.

⛔️ Obviously, this doesn’t mean that the geometry of the preparation is not important, rather, as mentioned earlier, the degree of expulsiveness affects directly the potential of retention. 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 cements, the retention to the abutment is purely frictional and the geometric principles of the macromechanical retention ( i.e, angulation of walls and prep height ) are more relevant.
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 🦷.
Because of this effect, in cases where there is possibility of establishing good adhesion both to the remaining tooth as the prosthetic piece, the adhesive cementation enables some types of preparation that would not be possible with non-adhesive cements.

✔️ one example is the preparation for the laminate veneers, entirely without macromechanical retention.
It is worth remembering that even in situations where restoration is adhesively cemented, it is advantageous to have preparations that meet the macromechanical requirements of retention, provided that they don’t result in excessive sacrifice of healthy tooth structure.

✔️ For instance, preparations for indirect restorations such as endocrown are based essentially on adhesive cementation. However, it is possible to choose to prepare a groove throughout the marginal perimeter, in order to enhance the retention and stability of the restoration because of the frictional retention between the restoration and the preparation. 💪🏻

👉🏻 Finally, it is worthy to say that the current trend is that, the indirect restorations are increasingly cemented adhesively with resin cements. 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 cements are 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.

Resistance :

When performing a preparation for an indirect restoration, one of the most important aspects is to obtain adequate thickness for the restoration material.
Whether due to its mechanical properties or particularities of the techniques involved in the production of the restoration, 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. 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. 👌🏻

⛔️ 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.

Marginal finish :

It is essential that the edges are sharp and well defined in all indirect restorations.
When these conditions are not met, the impression does not clearly define the preparation margins and therefore there is a risk that the restoration remains poorly adapted or presents subcontouring or overcontouring. 

👉🏻 It is also important that the termination provides sufficient thickness and present shaping that favors the stratification of ceramics.

⛔️ Although there are several types of terminations, all modern systems require indirect preparation of rounded shoulder or deep chamfer - both characterized by uniform thickness and rounded internal angles.

👉🏻 Finally, in addition to ensuring space for the restorative material, the cervical end should be assessed as to its relationship with the periodontal tissues.
Ideally, the end should be maintained as far as possible from the gingiva ( supragingival ) 💥 , since this is the most favorable situation for the periodontium. However, for several reasons - aesthetics, retention, the extent of preexisting lesions - the preparation can be extended to the gingival level ( juxtagingival / equigingival ) 💥 or slightly inside the sulcus ( intrasulcular / subgingival ) 💥. These three alternatives satisfy the biological distances and are well tolerated by the periodontium.

Of course, whatever the position of the finish line, the procedures of preparation and cementation should be as atraumatic as possible ( eg, it is useless to have a juxtagingival termination if excess cement is left after cementation, which could compromise the periodontal health ).