Screw- Vs. Cement-Retained Implant Restorations: What Emerging Trends Should Clinicians Consider?

The biological attachment of a tooth is quite dissimilar to that of an implant, and the working depths at which cement margins tend to be placed are also different.⁶ These factors contribute to cementation being a controlled and reliable means of restoring a natural tooth but a potential liability when restoring an implant. Excess cement is known to be a contributory factor to peri-implant disease.⁷ The type of cement used also influences microbial interactions and foreign-body host reactions that do not occur with tooth restorations.^8-10

Several factors are associated with retention of cement around implant restorations, with the most significant being the relationship between the cement margin site and peri-implant gingival tissue; the deeper the position of the margin, the more undetected cement remaining there may be.¹¹ Sites that have a horizontal subgingival undercut or horizontal concavity of the abutment also have a greater tendency to trap cement.¹² Furthermore, changing the abutment contour at the tissue level to correct the angulation of the implant can lead to facial overcontour, which can lead to potential gingival recession since excessive pressure may be exerted on the soft tissues.

Techniques have been developed for performing extraoral indirect cementation of the implant crown onto a duplicate die or replica of the implant abutment that has a smaller dimension than the actual abutment.^13,14 The crown is filled with cement and seated onto the replica. This allows for removal of excess extraoral cement with cotton roll or pellets, with the crown still retaining an adequate amount of cement. The benefit is that most of the cement is removed extraorally. The crown is removed from the die before the cement sets and then seated onto the abutment intraorally. This technique can be used to control the amount of sulcular cement that needs to be removed.

Screw retention also has its own issues.¹⁵ Surgical placement must be exact, otherwise screw access can be a challenge. Since dentists are not trained as engineers, even something as routine as tightening a screw may be poorly understood. Few dentists can relate to preload or Hooke's law¹⁶ and the physics behind screw fastening. Additionally, the tools, such as torque wrenches, often are used incorrectly and not calibrated routinely.¹⁷ These factors all contribute to why screw loosening is the most frequent prosthetic complication in single-implant restorations.¹⁵

To address implant angulation issues, some novel screws and screwdrivers have been designed to allow angulation changes up to nearly 30 degrees.¹⁸ However, these solutions, too, present limitations mainly with regard to torque as the off-axis angle increases,¹⁹ adversely affecting the screw-joint clamping force and the abutment-implant joint.

When possible, clinicians should avoid the use of cementation for dental implant restorations, but if cement must be used it may be better controlled by making supragingival margins and through careful selection of cement material. Although screw-retained implant restorations are preferred, having an understanding of how to optimize screw tightening and properly calibrate equipment is essential. With the use of angle-changing screw-channel abutments care must be exercised in high-function patients and bruxers.

With concerns about peri-implantitis along with a desire for retrievability, clinicians generally prefer screw-retained implant restorations over their cement-retained counterparts. A recent development in implant dentistry has been the introduction of angulated screw-channel (ASC) solutions to correct undesirable implant angulation. This method is an effective alternative to the use of cemented restorations and their associated issues or intermediary angulated abutments with their potential esthetic problems and additional height requirements.

ASC systems allow for the restorative screw channel to be placed at various angulations up to 30 degrees off the center axis of the implant in a 360-degree circle. In the anterior, where the use of screw-retained restorations had been precluded, the access hole can now be placed toward the palatal aspect. ASC can also be used in the posterior where the access needed to screw in a restoration may be restricted, such as in a patient with limited mouth opening or trismus.

Various implant designs provide angle correction up to 30 degrees. Both multiunit and single implant restorations are available. Multiunit restorations can be fabricated using cobalt-chromium (Co-Cr)-, titanium-, or zirconia-based materials. Single or multiunit implant-supported ceramic restorations can be manufactured as a one- or two-piece zirconia prosthesis with a titanium base. Research has demonstrated that two-piece zirconia abutments with a metallic base present higher fracture resistance and superior strength compared to one-piece ceramic abutments.^20,21

ASC ceramic abutment design and degree of angulation correction influences the fracture and fatigue resistance of the ceramic and retaining screw.The design of these zirconia restorations may incorporate a titanium base that is clamped between the implant and abutment or cemented to a titanium sleeve. Titanium sleeves come in various heights, and clinicians/lab technicians use the manufacturer's software to produce the final restoration via CAD/CAM. The design software enlarges the screw-access opening to allow for passage of the retaining screw and driver. Therefore, with larger angulation correction, the crown access opening is larger and there is less circumferential ceramic material in the cingulum region of anterior restorations. The taller titanium bases should be used to optimize retention of the luted zirconia ceramic to the titanium base along with adherence to proper luting protocols. Manufacturers provide defaults in their software to maintain minimal thickness levels of zirconia. A recent study testing fatigue resulted in a high incidence of fractures at low loads originating from the cingulum region with the clamped manufactured restorations.²²

It should be noted that the corresponding screw for the ASC uses a different screw head design and driver (hexalobar design). Use of the hexalobar ASC system reduces the ability of the screw to obtain optimum preload and may result in premature screw loosening as off-axial angulation increases.^19,23,24 Clinicians should be aware of the different torque levels and associated drivers based on manufacturer guidelines. Early clinical results appear promising, however clinicians should use precaution in areas of high load.

The quest to achieve the simplest, most retrievable, and esthetic implant restoration has taken on many different forms over the years. Nowadays, predictable screw retention for a single-tooth implant restoration can be attained under any circumstance when using "prosthetic angle correction" at the bone level. This is accomplished at the surgical stage of implant placement by using an angled neck implant.²⁵ A new, innovative threaded dental implant design with an angled prosthetic platform correction allows implant surgeons to utilize existing bone while maintaining the correct restorative platform at an angle that enables a screw-retained restoration in most cases.²⁶

This implant design was originally based on the anatomical relationship between the tooth root, crown, and bony housing of maxillary incisors. It features five different prosthetic connection types/interfaces, with platform corrections of 12 degrees, 24 degrees, or 36 degrees depending on range and diameter. The multiple platform angles were added to allow for use in any tooth position and situation where prosthetic angle correction is required at bone level.

Thus, the biological and mechanical complications associated with an angulated abutment could be something of the past.²⁷ In addition, the elimination of an angled abutment frees up valuable interarch restorative space. The implant body is tapered for improved primary stability with immediate placement in extraction sockets. The surface is available either roughened along the entire length of the implant, or in a machined-surface coronal, external hex configuration, where the top 3 mm of thread is left as machined with a specific surface roughness for improved resistance against biofilm.

This design offers significant benefits during immediate implant placement. In the case of a maxillary incisor, the surgeon can place the implant into the apical third of the palatal socket wall where most of the bone volume exists. Compulsory angle correction is unnecessary, thus reducing the incidence of apical perforation while allowing a restorative platform that enables screw retention in most clinical situations. The use of cement at the delivery of an immediate restoration is eliminated, and restoration retrievability for adjustments or final restoration is made easy and predictable. Parallelism in multi-implant cases is increased, and the optimized orientation allows for simplified restoration.²⁸

This approach greatly contributes to predictable esthetics, especially in the anterior regions. The end result is an implant that allows practitioners to more readily negotiate problematic anatomical constraints without sacrificing quality in prosthetic restorations.²⁹

Chandur Wadhwani, DDS, MSD
Affiliate Assistant Professor, University of Washington School of Dentistry, Seattle, Washington; Adjunct Assistant Professor, Loma Linda University School of Dentistry, Loma Linda, California; Clinical Assistant Professor, Oregon Health & Science University School of Dentistry, Portland, Oregon; Private Practice, Bellevue, Washington

Anthony P. Randi, DDS
Former Assistant Clinical Professor, College of Dental Medicine, Columbia University,
New York, New York; Private Practice
specializing in Prosthetic and Implant Dentistry, Garden City, New York

Andre Hattingh, BChD, MChD(OMP)(Pret)
Periodontist, Private Practice,
Kent, United Kingdom