Screw- Versus Cement-Retained Implant Restorations: Current Concepts

[drsushant]

RETRIEVABILITY

A major advantage proposed with the use of screw- retained restorations is that allow the retrieval of the restoration without damage to the restoration or fixture. Therefore, the screws and restorations can be restorqued, fractured components can be repaired, implant and soft tissue can be evaluated , calculus can be removed, and cleaning of the components is possible. The retrievability may be most valuable in extensive cases, where the prosthesis needs to be retrieved for maintenance purpose. However, the importance of retrieving an implant crown was questioned because the rate of complications remains low in comparison with natural teeth. As cementing a final crown on a natural tooth is a very common procedure, there are questions raised as to why one should treat an implant crown in a more rigorous way. The importance of retrievability remains under the discretion and beliefs of each clinician.

The technique used to remove a screw- retained prosthesis involves removal of the occlusal restoration, the intermediate cotton pellet, and the coping screw. When the restoration is removed, it can be screwed back in subsequently. A new cotton pellet is placed and the acces hole is then restored with materials such as composites.

As for the cement-retained prosthesis, retrievability is not necessarily impossible because implants abutments do not typically have undercuts and there is no chemical adhension between the cement and the abutment. A temporary cement composed of zinc oxide-eugenol or mixed with petroleum jelly is frequently used as a final cement for implant crowns to allow for future retrieval . In fact, an in vitro study performed by Kim et al demonstrated that decreased stress to the implant was obtained with provisional cement-retained prosthesis in comparison with permanent cement- and screw- retained prostheses. Nevertheless, the ability to remove the crown with a temporary cement is still very unpredictable, with the crown either strongly cemented or prematurely loosened. Another technique used to retrieve a cement –retained crown was described using set screws. The idea is to place a retrieval screw in an area where the cement seal can be broken without damaging the restoration. Other reported techniques involve the use of guides to reach the screw underneath or the use of screws to move the abutments and then retrieve the restoration. If the abutment does indeed become loose,if the restoration requires a repair and cannot be uncemented, or if the crown cannot be salvaged then fabrication of a new crown is often suggested .As a consequence, additional cost is charged to the patient. In addition, fabrication of a temporary restoration may be necessary in the esthetic zone.

RETENTION

Multiple properties of the abutment affect the retention of a cement-retained implant,prosthesis, such as its degree of tapering, surface area and height, and surface roughness. Most manufacturers fabricate the implant abutment with a taper of approximately degrees based on the concept that this is the ideal taper reported in natural teeth. In terms of the restoration height, the margins of the implant-supported crown and abutment are usually located to mm subgingivally, which increases the surface area in comparison with the natural dentition. At least mm of abutment height is needed to ensure the retention of a cement-retained restoration. As a consequence, when the interarch space is limited(i.e.<4, mm ),a screw-retained restoration may be indicated.

Another indication for a screw-retained prosthesis is in a case of malpositioned implants because a cement-retained prosthesis would require excessive axial wall reduction.

Retention is also influenced by the surface finish of the abutment. If additional retention is required, the abutment can be roughened with diamond burs or grit blasting, in a way similar to natural teeth. Moreover, the type of cement is also a factor that influences the retention of the cement-retained restoration, as discussed previously. The concept of progressive cementation may also be a consideration in which stronger cements are progressively used until the desired retention to keep in mind the at retention values for the same cement differ on natural teeth compared with implants.