Crown for Dental Implant
The screw-retained crown has both benefits and liabilities. The main advantage is retrievability. The crown is not only recoverable, but no damage occurs upon removal of the crown. In the event of loosening or fracture, the crown can easily be removed. Cleaning, screw replacement and assessment of surrounding tissue is also possible. Many dental professionals consider a yearly cleaning and replacement of screws a prudent approach. The longer the span, the more important salvaging becomes. Most dental professionals believe a long restorative span, cantilever or full arch dictates screw retained crowns. To recover a crown or change a screw for maintenance, the restoration is removed, the cotton pellet is removed and the screw is accessed. Once repairs and/or alterations are concluded, the screw is torqued, a new cotton pellet is placed and composite or acrylic is used to seal the opening.
The screw-retained crown has both benefits and liabilities. The main advantage is retrievability.
For cement-retained crowns, retrievability is not a major drawback. Cemented crowns may be recovered if the correct cement is used. Adding a water soluble gel to the cement may ease rescue of a crown. Nevertheless, while the screw-retained crown is certainly retrievable, removing a cement-retained crown can be a questionable undertaking if strong cements are used.
PROS AND CONS OF CEMENT
Cement-retained implant-borne restorations offer several advantages, including the elimination of un- esthetic screw access holes and greater resistance to porcelain fracture. Standard crown & bridge procedures can also be used in most situations. However, excess cement left behind inadvertently is a major problem and can result in soft tissue damage, bone loss and/or chronic inflammation.1 In a 2006 study by Weber et al., soft tissue surrounding screw-retained implant crowns was found to be healthier than soft tissue surrounding cemented restorations.2 To help address this issue, custom abutments can now be designed with supragingival margins that allow for easy and complete cement removal.
Custom abutments can now be designed with supragingival margins that allow for easy and complete cement removal.
RETENTION AND RESISTANCE
Factors affecting the retention and resistance of cement-retained crowns on natural tooth abutments can also be applied to cemented crowns on implant abutments. Abutment height, degree of taper and surface area and roughness all affect the retention and resistance of a cemented crown.The ideal taper of opposing surfaces in natural tooth preparations is reported to be six degrees.3 Parallelism of implants has a direct effect on the taper of their abutments. Implants that are not parallel may require further preparation and tapering of their abutments to enable an ideal path of insertion of the prosthesis. Overtapered abutments may lack adequate retention for the cemented restoration and may necessitate a screw-retained prosthesis. To counter this and to increase retention for a cemented crown, an irregular abutment surface and/or stronger cement may be indicated.
Abutment height is an important factor for proper retention and resistance of cemented crowns. Longer abutment walls will have more surface area and will therefore be more retentive. The length of the abutment wall also plays an important role in the resistance to tipping forces. To prevent tipping forces from dislodging the cemented restoration, the length of the abutment wall must be great enough to interfere with the arc of the casting, pivoting about a point on the margin on the opposite side of the restoration.4 Custom abutments offer an excellent option to correct the line of draw and provide appropriate resistance and retention.
At least 5 mm of abutment height is needed for proper retention and resistance of cement-retained crowns.5 Therefore, screw-retained crowns are required in situations when limited interarch space dictates an abutment that would be shorter than 5 mm.
The primary advantage of a screw-retained superstructure is the lower profile retention of the abutment system.6 These low-profile abutments offer a significant advantage for bar-retained overdenture applications. The lower height of the screw-retained bar offers greater room for denture teeth and greater thickness of acrylic, which is needed for strength of the restoration.