This article was published in The International Journal of Oral & Maxillofacial Implants, Vol 14, No 1, 1999.

I. Winston Chee

Winston Chee, BDS, serves at the University of Southern California School of Dentistry in several capacities: He is an Associate Professor and the Ralph W. and Jean L. Bleak Endowed Professor of Restorative Dentistry, Director of Implant Dentistry, and Codirector of Advanced Prosthodontics. He is a Diplomate of the American Board of Prosthodontics and maintains a private prosthodontic practice in Pasadena, California.

Screw-retained implant restorations have an advantage of predictable retrievability but demand precise placement of the implant for optimal location of the screw access hole. Deviation from this optimal direction can lead to an unesthetic restoration if screw retention is to be used. Also, obtaining passivity of frameworks that are screw-retained is difficult due to dimensional discrepancies inherent in the fabrication process (Hebel and Gajjar 1997, Guichet 1994).

To eliminate the presence of the screw access hole in esthetically demanding areas, other methods have been used to connect implant restorations to implant abutments or implants. These include the use of preangled abutments, which allow screw-retained restorations; cemented implant restorations to angled or custom abutments; and lateral set screws in the restoration, which allow for retention to the abutment. Unfortunately, none of these methods are ideal, and each exhibits advantages and disadvantages. Preangled abutments can redirect screw access openings to the occlusal or cingulum areas of implant restorations. However, to allow the abutment to be retained on the implant and still provide sufficient abutment structure to house a retention screw for an implant restoration, the long axis of the implant and the path of the retention screw for the restoration must diverge sufficiently.

At present, the minimum amount of divergence required to permit the use of a screw-retained preangled abutment is 17 degrees (SDCA 378, Nobel Biocare USA, Westmont, IL). If the divergence of the screw path is less than 17 degrees, the use of preangled abutments for screw-retained restorations is not possible.

Cementation of implant restorations eliminates unesthetic screw access holes. Cemented restorations also have the potential to compensate for any minor dimensional discrepancies in the fit of restorations to abutments, which can contribute to lack of passivity (Hebel and Gajjar 1997, Guichet 1994). Minor dimensional discrepancies may be compensated for by using cement and cement space.

Preformed titanium or ceramic abutments can be secured to the implant and prepared in much the same way that teeth are prepared for fixed restorations. Custom abutments can also be formed by waxing and casting to premachined cylinders that are screw-retained on the implant. Restorations can then be fabricated and cemented to the abutments, similar to conventional fixed restorations.

When cemented abutments are considered, it is imperative that the cement margin is shaped to maintain a relationship with the mucosa margin. With many manufactured abutments the cement margin is circular, which can lead to deeply located cement margins. Cement removal becomes difficult, if not impossible, and it can lead to soft tissue irritation (Agar et al 1997). The ability to cement implant-supported restorations using techniques similar to conventional fixed prostheses simplifies treatment planning and the restoration of implants. However, a disadvantage of cementing implant-supported restorations is the potential difficulty in retrieving the restoration. Should an abutment loosen or any repair of the restoration become necessary, the restoration may be destroyed during the removal procedure if the cement seal cannot be easily broken. Further, when an abutment screw loosens under a cemented multiunit implant restoration, the restoration is usually uncemented from abutments firmly seated to implants and firmly attached to the loosened abutment(s). Any force applied to a restoration on a loosened abutment has the potential to damage the internal threads of the implant.

The use of set screws allows a retrieval screw to be placed in a position where a displacing force can be applied in the direction of the abutment to break the cement seal and allow removal of the restorations (Chee et al 1998). The access hole can be placed in a variety of positions, independent of the direction and position of the implant body. Cement can be mixed without using lubricant and still allow the predictable retrieval of the restoration. This technique has the potential to reduce stress to splinted implants, since the effects of minor misfit of the framework are not transferred directly to the implants, as is the case with prosthesis-retaining screws. In addition, the exposure of screw access holes in esthetic areas of the mouth can be avoided.

The predictable retention and retrieval of screw-retained implant restorations must be weighed against the passivity and lack of screw access of cemented restorations. In my experience, the ability to predictably retrieve restorations and confidently retain them leads me to choose a screw-retained restoration whenever the implant position permits. From a facial and buccal view, esthetics can be achieved with either screw or cement retention. When cemented restorations are required, a mechanism to predictably separate them from the abutment should be incorporated.

II. David A. Felton

David A. Felton, DDS, MS, FACP, is Chair of the Department of Prosthodontics at the Universtiy of North Carolina School of Dentistry. He is currently principal investigator of a 5-year prospective clinical trial investigating the immediate loading of implants in an overdenture population. He is an original member of UNCs Clinical Implant Program and has lectured nationally and internationally on the prosthetic reconstruction of dental implants. He has just completed a term on the Board of Directors of the American College of Prosthodontists and holds memberships in the ACP, AO, AADR/IADR, ICP, ADA, ICD, and AADS.

P-I Brånemark moved successful implant dentistry into the United States in the mid-1980s. American practitioners, lacking understanding of the concepts he was promoting, immediately began to challenge his precepts and push the envelope of what had become a most successful dental implant system. With a lack of scientific scrutiny, and in many cases, a lack of approval from the Institutional Review Board, the BrR.nemark implant system was utilized in ways that had never been evaluated and for which it had not been designed. One such way was to change what had fundamentally been a screw-retained prosthesis into a cement-retained prosthesis.

The single advantage of a screw-retained implant system is that of retrievability. This includes removal of the prosthesis to retighten bridge or abutment screws, replace failed or fractured components, or to perform routine hygiene. The disadvantages of a screw-retained implant system are numerous. First, there is the problem of a lack of esthetics at the screw access channel, particularly if the channel is cast in metal. Second, if the metal is cut back to hide the nonesthetic metal, porcelain fracture around the screw access channel may occur. Third, screw-retained prostheses generally require both the abutment screws and bridge screws to be tightened using a torque driver to effect preload of the screws. This torquing appears to have lowered but not eliminated the incidence of screw loosening. Finally, screw-retained systems generally leave a microgap beneath the gingival crest, resulting in chronic gingival inflammation (Keller 1998, Lindhe 1998).

An evaluation of the literature suggests that screw fracture and screw loosening are common occurrences in the traditional hex-top implant systems, regardless of whether the prosthsis is cemented or screw-retained. Various studies indicate an incidence of screw loosening or fracture of between 10 and 56% (Jemt 1991 and 1993, Parein 1997). The cost of long-term maintenance of these prostheses has not been investigated; nor have comparative studies of the effects of screw versus cement retention of prostheses been performed. Clearly, unless other provisions are made to successfully recover a cemented prosthesis (use of temporary cement, lateral retaining screw, etc), screw loosening in the abutment remains an ongoing concern for traditional hex-top systems. Finally, few hands on implant courses are offered in the dental curriculum to predoctoral dental students that adequately train them to restore and maintain implants in their practices.

Selection of the implant system is the first step in determining the feasibility of either cement or screw retention of the prosthesis. Current second generation implant systems that employ a conical interface between the implant and abutment (ASTRA Techs Conical Seal design, or Straumanns Morse Taper design) or other internally designed connection features (Friatec, etc) may provide solutions to the screw loosening problems experienced with traditional hex-top systems. When compared with traditional hex-top systems, these second generation systems provide the following advantages:

1. The systems rely on increased surface area contact between the implant and the abutment (a 2- to 5-fold increase) via a modified abutment-implant interface.

2. The large abutment/screw design increases the rigidity of the implant-abutment complex, which may improve stress distribution in the cortical bone.

3.The new connectors provide a geometric lock against displacement from nonaxial loads. This geometric lock may lower the incidence of screw loosening, as the screw itself is not solely responsible for providing resistance to abutment displacement, as is the case with many traditional hex-top systems.

The increased surface area and rigidity, along with the geometric lock, should result in fewer problems with screw loosening. In our institution, we have 14 years of experience placing and restoring numerous implant systems. With the ASTRA Tech and Straumann systems, there appears to be a very low incidence (less than 2%) of abutment loosening with cemented prostheses compared with all other systems we have evaluated. Both systems appear to have simplified patient care for the provider by utilizing traditional methods (cementation) for retaining the permanent prostheses. As providers, our goal now is to determine the long-term associated costs for successful maintenance of the implant-retained prostheses that we provide for our patients.

III. Peter F. Johnson

Peter F. Johnson, DMD, is an Associate Clinical Professor at the University of Southern California School of Dentistry. He also maintains a private practice in prosthodontics in La Mesa, California, and is a Diplomate of the American Board of Prosthodontics.

Cemented and screw-retained implant prostheses present distinct advantages and limitations. The major difference between the 2 strategies is that a screw-retained prosthesis can be removed and replaced by the clinician, while a cemented restoration is not intended to be retrieved. Each can be the best option, depending on the objectives of the prosthesis, the attributes of the implant system, and the philosophy of the practitioner.

A major benefit of a screw-retained prosthesis is retrievability. This feature permits necessary servicing of the implant stack as needed and is particularly desirable in multiple-unit, full-arch, or cantilever prostheses, where some maintenance of restorative materials or structures, components, or implants would probably be necessary during the lifetime of the prosthesis.

In addition, screw joint systems provide a great variety of transmucosal and prosthetic components, work well in patients with limited occlusal space, require no removal of subgingival cement, and cause no negative sequela when the cement is not removed. This is particularly important if the transmucosal-to-implant junction is greater than 3 to 4 mm subgingival.

However, the screw joint has limited stability, particularly when used in single-tooth and partially edentulous situations (Henry et al 1996, Jemt et al 1992), and it is particularly susceptible to buccolingual off-axis loading. This liability is usually surmounted by splinting multiple implant units, and is most effective when the splinted implants are not in a straight line. Screw-retained units generally have screw access openings, which can compromise esthetics, weaken the porcelain around the openings and at cusp tips, and establish unstable occlusal contacts. The centric contact at a screw access opening, which is often in the central fossa and may occupy 50 to 66% of the intercuspal occlusal table, is usually developed with the head of a screw or a composite restorative material.

New designs of the abutment-implant connection feature enhanced component coupling and increased stack stability and rigidity. These systems utilize external splines or internal hexes and conical tapers for joining components, and they provide lateral surface-to-surface contact of 1.0 to 3.2 mm. Internally fitted components provide resistance through the transfer of forces from the lateral surfaces of the abutment extension to the wall of the implant, reducing or eliminating the strain placed on the fastening screw, micromovement and loosening of the screw, deformation of the component stack, and separation of components.

The more secure component stack permits the use of conical, preparable abutments that provide conventional crown-and-bridge retention and resistance form for cemented restorations (Solnit and Schneider 1998, Levine et al 1997). These abutments are most often fabricated with a 6- to 7.5-degree taper. I restore partially edentulous segments with single implants, which avoids the problems of framework misfit, splinting and tripoding, torquing and flexure of bone, and the reliance on 0.7-mm hexes for lateral resistance and prosthesis stability. The proximal and occlusal contacts are adjusted and the units are cemented with a permanent cement. Treatment planning is concentrated on the use of the single dental unit; this avoids overloading, avoids the use of precarious teeth as abutments for conventional prostheses, and minimizes the treatment of complications of the implant teeth.

Employing these systems simplifies implant procedures because there is a limited selection of abutments, and standard crown-and-bridge procedures can be used in most situations. Systems that incorporate the transmucosal element as a part of the implant (eg, ITI) and others that use implant-level impressions and laboratory modification of the abutments (eg, Frialit, Calcitek, and Paragon) develop a cemented junction slightly subgingival or supragingival, permitting easy removal of cement. The treatment can be accomplished in fewer and shorter appointments, standard crown-and-bridge laboratory techniques can be employed, and more stable occlusal contacts are created because of lack of the screw access opening. The cost of the implant tooth is reduced as the result of the use of routine and more expedient clinical procedures.

IV. Daniel Y. Sullivan

Daniel Y. Sullivan, DDS, is a Diplomate of the American Board of Prosthodontics. In addition to his full-time private practices in Washington, DC, and McLean, Virginia, Dr Sullivan currently serves as Adjunct Professor at the University of Pennsylvania School of Dentistry and lectures extensively in North America, Europe, and Asia. He is active in several dental organizations, serving as President of the Academy of Osseointegration from 1992 to 1993 and as an Officer and Board Director of the American Academy of Esthetic Dentistry.

This topic is particularly timely, as implant manufacturers produce and market an increasing number of cementable abutment options for restorations.

There is an overriding biomechanical issue that needs to be addressed first in any discussion of this issue, and that is the stability of the abutment-to-implant screw joint. Long-term stability through reduced micromotion is the single most important requirement for cemented restorations. The same abutment-to-implant stability is essential for screw-retained restorations.

Manufacturers have developed adequate materials, implant sizes, and manufacturing techniques and protocols to resolve this issue. Implant diameters have been increased; in combination with increased implant table size, this has resulted in greater overall implant strength and resistance to tilting forces. Improved abutment-to-implant fit, particularly at the hex (external or internal) interface, using interference-type machining likewise has dramatically improved abutment stability. Finally, improvement to the abutment screws through design changes, refinement of materials, surface coating, and exact torque protocols now produce higher clamping forces to the joint and improved initial preloads.

Therefore, the arguments against cementation for fear that the abutment complex may loosen are now merely historic. Indeed most of the early criticism of repeated screw loosening in screw-retained restorations can now be answered by illustrating the same improved abutment interface biomechanics, since failure at the abutment level is the forerunner of restorative set screw loosening. One can now safely utilize either a screw-retained or cemented restoration with more confidence than several years ago.

The question of which modality is better depends on the clinical situation. Sometimes only one possible prosthetic solution exists, and this needs to be understood as well.

Generally, single-tooth implant restorations are best cemented. I believe they should be treated similar to conventional single crowns on natural teeth. Implant alignment in the anterior should be toward the incisal edge and in the posterior toward the central fossa. Both situations can utilize standard or custom abutments with permanent cementation. The only reason to use screw retention would be if the implants long axis is too palatal in the anterior region. Short-span fixed partial dentures can be permanently cemented as well, assuming that implant table size, implant numbers, and abutment screw torque can be optimized.

Contraindications for cement retention for partial denture candidates include extremely limited interocclusal space that limits the vertical wall heights on abutments and malaligned implants, in which axial wall reduction becomes so extreme that in attempting to achieve parallelism, individual abutment retention is lost.

Large, full-arch implant reconstructions are likewise best treated with screw retention. I firmly believe that retrieval of these reconstructions is far less traumatic to the patient—and far more predictable to the restorative dentist—than their cemented counterpart. The argument that cements of differing retentive capacity can be titrated to somehow function as effectively as screws in either removal or retention is fallacious. Equally unfounded is the argument that cement retention provides a more passive, stable environment than screw retention. This might have been true 10 years ago, but the advent of electric discharge machining (EDM) has changed forever the field of implant dentistry. Large, unsoldered, screw-retained castings can be machined prior to ceramic application using EDM technology to fit within 5- to 10-µm gaps at the manufactured abutment restoration interface. Even after ceramic application, with its possible distortion to the framework, further EDM procedures can refine the fit with no effect on the finished ceramics.

Combining EDM technology with intraoral verification index techniques chairside can greatly reduce clinician time, and one can often eliminate the need for metal try-in with these cases. This EDM technology is widely available. Although some practitioners would prefer to use crown-and-bridge techniques for full-arch cases, I prefer not to operate chairside with implant-level impressions, subsequent intraoral custom abutment refinements, and retraction cord procedures with anesthetic. I do prefer to operate at the machined abutment level, with machined impression copings, using no anesthetic or retraction cord and stabilizing intraoral occlusal records with screw-retained platforms. In terms of expense, reducing chairside time is far more productive than saving small amounts on laboratory or manufactured abutment-related expenses.

The final arguments in favor of screw retention are the esthetics of the final restoration and occlusal design. The esthetics of the screw chamber are easily addressed with gold plating of the metal portion of the chamber, which reduces the need for opaque composite materials. Proper shade matching of composite is all that is required. Finally, the idea that somehow the screw chambers, with their resulting composite fillings, disrupt the occlusal design is not clinically relevant. We place occlusal access holes routinely for endodontic purposes. A well-diagnosed implant treatment would have only 4 to 6 chambers in most routine full-arch situations, and this is hardly disruptive to the occlusion.