Inlay and Onlay Restorations

Inlay and onlay restorations, types of intracoronal restorations, are made using metal, composite, or porcelain materials through a casting process. Inlays are placed within the tooth structure, while onlays cover the entire occlusal surface of the tooth. Inlay restorations have several significant advantages over conventional amalgam or composite restorations: Inlays...
Close up of female mouth with retractor. Doctor flossing the teeth
Inlay and Onlay Restorations
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Inlay and onlay restorations, types of intracoronal restorations, are made using metal, composite, or porcelain materials through a casting process. Inlays are placed within the tooth structure, while onlays cover the entire occlusal surface of the tooth.

Inlay restorations have several significant advantages over conventional amalgam or composite restorations:

  1. Inlays and onlays can establish excellent contact relationships with adjacent teeth.
  2. They can easily prevent the common problem of overhang in Class 2 restorations.
  3. Intracoronal restorations utilize a type of retention that generates outward forces on the tooth. A successful restoration requires a design that can withstand these forces.
  4. If an inlay is applied to a tooth with extensive structure, the tooth itself can withstand these forces. However, in cases like MOD (mesio-occluso-distal) restorations, where buccal and lingual cusps are isolated, destructive forces might affect the tooth structure. In such cases, additional features might be required for the restoration to withstand these forces, necessitating an onlay restoration.
  5. An inlay should be applied only if a specific tooth structure is undamaged and is intended to repair the lost tooth tissue.
  6. The design of an inlay should be able to distribute forces evenly and broadly over larger surfaces.

Adhering to these principles, inlay and onlay restorations can be effective treatment options employed by dentists.

 

Preparation of Class II Proximal-Occlusal Inlays and MOD Onlays

The principles of inlay preparation are similar to those of conservative dentistry, yet inlays have specific characteristics. The following features apply to inlays and MOD onlays:

For Inlays:

  • Walls should be parallel.
  • A single path of insertion is required.
  • Sharp internal angles should be present.
  • A smooth floor is necessary.
  • Marginal bevels should be added.

Inlay preparation begins with processing the occlusal surface. The isthmus cavity on the occlusal aspect is extended to the outermost boundary, encompassing fissures. Cavity walls are created with an outward angle of approximately 6 degrees. In the proximal area, the bur is inserted apically inward to prepare the cavity floor, and this area is shaped into a box form. The proximal box cavity is flared outward and its walls are smoothed. The inlay preparation is completed with a bevel that starts from the edges of the occlusal cavity, extending to about 1/3 of the occlusal portion, and is formed with an outward angle of 15-20 degrees.

MOD Onlays:

  • A MOD onlay is a modified inlay design that covers a portion of the occlusal surface to reduce stress concentration.
  • MOD onlays may be indicated in the following situations:
    • Teeth with completely damaged buccal or lingual cusps due to fractures.
    • When more than half of a tooth’s bucco-lingual dimension is compromised and an MOD cavity is present.
    • For protective purposes in endodontically treated teeth, if the buccal and lingual cusps are undamaged.
  • In onlay preparation, an occlusal cut is made with a round-ended bur. For the upper jaw, a cutting depth of 1.5 mm is created on the functional palatal cusp and 1 mm on the buccal cusp.
  • A wide bevel is made on the palatal slope of the palatal cusp.
  • An occlusal step is prepared at the start of the occluso-palatal slope. This step should be 1 mm wide and positioned 1 mm below the lowest occlusal contact.
  • This step is finished with a bevel.
  • Then, the occlusal cavity is prepared.
  • Proximal box preparations should be made to be compatible with a single path of insertion and parallel.
  • The edges of the proximal cavity are flared outward and merged with a bevel about 0.7 mm wide at the gingival floor edge.
  • In cases of extensive material loss, pinholes may be opened to increase retention.
  • Bevels of 0.5-0.7 mm are made at the lingual and occlusal finishing lines, while the buccal bevel, if aesthetically significant, is prepared perpendicular to the path of insertion.

Adhering to these principles, inlays and MOD onlays can be successfully prepared and used in restorative dentistry practices.

 

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