Screw- Versus Cement-Retained Implant Restorations: Current Concepts

[drsushant]

PASSIVITY

Passivity is a desired feature of restorations because reduced stresses are applied on the bone and the implant. A lack of passivity has been correlated with biologic and prosthetic complications. If a nonpassive prosthesis is forced into place, stresses are applied to the suprastructure, bone,and implant, which may lead to bone loss, implant fracture, or even mobility. The development of a microflora at the interface of the implant and abutment is also possible, which may result in chronic gingival inflammation. Moreover, a casting with inadequate fit can cause deviations in the vertical loading of the implant and subsequently screw loosening or fracture.

With the screw-retained prosthesis, a precise fit between the crown and abutment with no space between them is ideal. A soldering technique is used to correct the passively, In this techniques, the casr is first separated; a new transfer impression is taken and then sent to the laboratory. This importance of an accurate technique is reinforced when using a screw-retained prosthesis to minimize deviations in angulations

In the cement-retained prosthesis each abutment is screwed to the implants, and the cement space of approximately 40um permits passivity. When the cement –retained prosthesis is not passive, the problem can be corrected by reshaping the abutment or casting. This additional space, if minor, can be compensated by the cement, which acts as a shock absorber and reduces stresses to the bone. This results in reduced laboratory cost and patient chairside time in contrast to the repair technique of the screw- retained prosthesis

When comparing the passicity obtained in screw- mversus cement- retained prosthesis, most studies reported no difference with either technique. However, one study found that cement-retained restorations had more equitable stress distribution and are therefore biomechanically preferable to screw-retained restorations.

Nonetheless. It is intresting to note that most prostheses do not show a completely passive fit, and however are still functional, suggesting a certain biologic tolerance for known to cause distortion (e.g., impression material and porcelain shrinkage. Dental stone, and investment material expansion), it is a challenge to achieve a perfectly passive structure.

OCCLUSION

Because there is no access hole on the occlusal surface of a cement-retained implant-supported prosthesis, an axial load may be directed to the implant, which is preferable to lateral forces. Hence, occlusal function is promoted with cement-retained implant restorations.

The screw-retained prosthesis requires an occlussal restoration, such as amalgam or composite, to cover the screw access channel. However, the durabiltity of these restorations is inferior to an intact full coverage crown. Moreover, as the size of the implant increases so is thescrew access hole, thus leading to a large occlusal restoration, which may compromise the long-term durability. In fact, the screw access hole occupie about 50% of the occlusal table in molars and 75% in premolars. These screw access holes not only interfere with centric occlusal contacts but alsoaffect protrusive and lateral excursive movements.