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Introduction
The use of implant-supported crowns has become a well-established and preferred approach to compensate missing single teeth. There are two different types of prosthetic restorations fixed on dental implants: Screw-retained and cemented restorations. The choice of method is usually based on the clinician's preference. Retrievability And lack of cement are the main advantage of screw-retained crowns that would make it more favorable to many clinicians. It allows better control on the hygiene of the implants and surrounding mucosa, also, crowns can be easily repaired in case of crown fracture. On the other hand, cases when the access hole is on the incisal edges or cusps teeth or easier access to the posterior area of the mouth is needed, cemented crown restorations could be more practical.

Indication and principle considerations

Screw-retained restorations are designed to be screwed either directly onto the implant or onto a screw-retained abutment positioned on the implant (Multi-unit Abutment). Screw-retained restorations represent a secure and easy way to maintain a prosthetic restoration.

Retrievability
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.

For cement-retained crowns, retrievability is not a major drawback. Cemented crowns may be recovered if the correct cement is used. Nevertheless, while the screw-retained crown is certainly retrievable, removing a cement-retained crown can be a questionable undertaking if strong cements are used.

Esthetics
Esthetics is another factor to consider when deciding between screw-retained and cement-retained crowns. In anterior screw-retained crowns, the implant is placed lingually to allow screw emergence through the cingulum area. The restoration is cantilevered facially from the implant body, which results in offset loading of the implant. Lingual implant placement also results in a porcelain ridge lap, which compromises hygiene. An implant for an anterior cemented restoration is placed under the incisal edge. An angulated abutment is then used, which eliminates the ridge lap and replicates a more natural emergence profile.

In posterior screw-retained restorations, it is favorable that the access hole will exit through the central fossa of the prosthetic tooth. Due to the anatomical restrictions in many cases the screw channels exits through the buccal side, which is not only a cosmetic compromise but an structural one, therefore angular multi-units are used, to move the exit of the screw channel to the occlusal part, the more common angulated multiunits are 17°and 30°, some companies make 45° and 60° also. As the straight multiunits they come also in different heights. The cementable crown obviously has no entrance cavity. Allowing the forces of occlusion to be distributed along the axial inclination, congruent with the long axis of the tooth, is easier.

Hygiene
excess cement left behind cemented restoration is a major problem and can result in soft tissue damage, bone loss, and/or chronic inflammation. The literature shows that the soft tissue surrounding screw-retained crowns are healthier than the peri-implant mucosa surrounding cemented restorations. Recent studies shown that even removing cement thoroughly, the risk of leaving cement subgingivally is reduces but cannot be eliminated as there is no direct view for the doctor and the X-ray can't record small leftovers of cement.

As on cemented crowns, in screw retained restorations the connection abutment-crown will preferably be on the gingiva level. For that there are different multiunit heights for different gingiva height. Most multiunits are coming up to 5mm height, 6-7mm are less common but can be found in scenarious.

Retention
Abutment height, degree of taper and surface area are all factors that affect the retention of cemented crowns. Abutment height is an important factor for proper retention. Longer abutment walls have more surface area, consequently are more retentive. At least 5 mm of abutment height is needed for proper retention of cemented crowns. Therefore; screw-retained crowns are necessary in situations when limited inter-arch space dictates an abutment that would be shorter than 5 mm.