Cement-retained restorations, posterior zone
- The practitioner may chose to cement retain the final restoration.
- The choice cement final restorations mirrors conventional prosthodontics.
- There are multiple abutment choices for cement retention.
- The practitioner must decide to use a provisional or permanently cement the restoration.
Implant restorations may be cemented in the posterior zone and are sometimes are the only option due to the resulting implant placement. Understanding the relationship of the implant to adjacent structures (implant / tooth) to preserve the inter-proximal bone, to the buccal plate, and the depth at which the implant is placed related to the osseous base will result in a predictable result. (Tarnow et al. and other articles)
In order to place implants in a ideal position it is recommended that a surgical guide be utilized, after the diagnostic wax phase, and planning with NobelClinician software. This serves as an invaluable aid for implant planning and placement. The value of three dimensional imaging cannot be underestimated in the posterior maxilla and mandible as vital structures such as blood supply, nerve supply and sinus proximity are present. Additionally the interproximal bone, and circumferential height of bone, proximity of adjacent teeth and or implants are all critical.
Once the implant has been placed (depending on the implant) there are multiple abutment options. If an occlusal access is achieved with the placement of the implant, a stock / or custom abutment is considered. The stock abutments can be made of titanium, zirconia, or other metal compositions. These can be placed directly on the implant and prepared with copious amounts of water. This will achieve the ideal marginal form, and preparation characteristics for ideal retention and resistance form. A provisional is then fabricated, placed and the soft tissue contours developed and allowed to mature prior to final impressions. These abutments can also be prepared in the dental laboratory, with the provisional and placed, thus avoiding intra oral preparation.
With the advent of CAD/CAM a custom abutment of ideal geometry and form can be fabricated. The literature is conclusive that these abutments have at least equal accuracy of fit to the implant surface with the advantage of being able to use a wide choice of materials (Zirconia, Titanium, Gold, Metal ceramic alloys etc). Machining is done precisely and the abutment is “clean” of any residual materials that may cause soft tissue irritation. They are also able to withstand the posterior occlusal forces. One must also consider the minimum amount of interocclusal distance that is needed for sufficient thickness of the final restoration. This will vary depending on the material of choice (Zirconia, Lithuim Disilicate, Metal ceramic etc)
The final choice for the practitioner is choice of cement. The first is whether to use a provisional or permanent cement. The ability to gain access to the abutment screw cannot be over emphasized. Even with following all the manufactures recommendations and achieving the desired torque values, screws come loose. This complication is easily dealt with if the restoration can be removed, the screw tightened and the restoration salvaged. If one uses a permanent luting agent the restoration will need to be destroyed to reach the abutment screw in most instances.
The choices of permanent luting agents are diverse, with resin cements becoming more popular due to their superior characteristics while being insoluble. These cements are also available in a variety of shades to optimize esthetics depending on the material choice for the final restoration.
There is evidence that the presence of cement not completely removed acts as medium for bacterial colonization contributing to late implant failure due to progressive bone loss. Since some of the resin cements are not radiopaque and may extrude along the implant surface. It is strongly recommended that a meticulous cementing procedure be followed.