Introduction

The original Branemark high water design fixed prosthesis was a watershed moment in dental history. It addressed the vexing problem of mandibular complete denture therapy by creating a predictable fixed prosthetic alternative for those patients lacking lower teeth. This treatment alternative had only one method of retaining the prosthesis to the transmucosal implant; abutment‐screw retention. As implantology evolved, sectional fixed prosthetic and single tooth prosthetic designs were developed after the success of the Branemark full arch screw‐retained prosthesis.

An early solution for partial edentulism in either arch was to utilize either a solid or angle‐correcting stock abutment, or a custom cast then later a custom‐milled abutment that was designed to retain the replacement crown, held in position by the selected luting agent. Many factors common to tooth preparation were employed: features to optimize retention and resistance form, and margin placement all evolved over time, driven by idealizing patient treatment outcomes.

Design features vary between the single tooth and fixed partial denture abutments. A single implant has been used in conjunction with a natural tooth to retain a three‐unit fixed partial denture. In this instance, the only means of retaining the prosthesis is to utilize a luting agent. A review of the scientific literature reveals high success rates for this approach, although it is not considered a mainstream dental therapeutic [1–3].

A commercially available solid abutment can be attached to an implant at the manufacturer’s recommended screw torque value. A prosthetic finishing line can then be developed and demarcated either on an analog cast or intraorally, and a conventional or digital impression can be generated. A final implant crown can be cast, milled, or printed, and attached to the abutment with a luting agent. Sometimes, a manufacturer‐supplied prefabricated solid abutment cannot adequately support the required tooth form due to implant body angulation issues. This requires a workaround of some sort. Either a machined angle correcting abutment can be attached to the implant, or a more costly custom‐fabricated abutment can be manufactured via a casting or milling process [4].

The major disadvantage of cement retention is the process of safely applying the correct volume of luting agent. The selected agent could be a provisional or final cement. Ideally, supra‐, or epi‐gingival margin design is incorporated into the abutment fabrication, as well as optimal digital or analog cement spacing is also incorporated into the process. Various strategies have been undertaken to minimize excess cement in the gingival sulcus. An inadvertent excess of cement in the sulcus can be associated with the development of peri‐implant mucositis or peri‐implantitis [5] (Figure 17.1). The unexpected outcome of cement in the implant sulcus can lead to chronic peri‐implant inflammation proceeding on to crestal bone loss and ultimately can result in implant failure (Figure 17.2). Strategies to minimize excess cement include internal venting of the supporting abutment. Wadhwani et al. [6] advocate the placement of two holes in abutments, one high and one low in the abutment profile. This allows the excess luting agent to vent internally into the screw access chamber which minimizes the amount of cement that can be squeezed into the gingival sulcus by crown seating. Haas and Haas [7] advocate an approach described as the “Cement shield membrane technique” in which a specified length polytetrafluoroethylene (PTFE) is stretched over the implant abutment and adjacent teeth, and then the luting agent is applied to the intaglio surface of the implant restoration. The crown is removed, and the PTFE is removed, leaving a nearly optimal amount of cement to retain the crown atop the abutment.

Retrievability

One of the main advantages of screw retention is retrievability in the event of screw loosening, screw fracture, or other prosthetic and biological complications. Without a screw‐access hole, cement‐retained restorations can be difficult if not impossible to remove and often require irreversible damage to the restoration and potential damage to the implant. There are solutions to combat the issue of retrievability with these types of restorations, both of which can be incorporated into the design of the prosthesis itself or employed if separating the restoration from the abutment(s) intraorally is not possible.

One method of improving retrievability for cement‐retained restorations is the inclusion of additional screws into the framework design, such as the lingual set screw. The crown is cemented using the luting agent of choice and the screw is inserted. If the crown must be removed, removing the screw will cause shear forces that break the cement adhesion. Additional retention from the screw also makes it possible to use an interim cement more predictably, which can make removal easier. Disadvantages to this approach are that the additional screws/access channels can be bulky, especially in the lingual area, which can be uncomfortable to the tongue and cause plaque accumulation. The design of such a restoration also takes additional laboratory time and skill, which can be costly.

Another method of retrievability is the lingual retrieval slot mechanism, proposed by Schweitzer et al. [8] for metal ceramic implant prostheses. A 3 mm deep horizontal slot is incorporated in the mid‐marginal position of the lingual cast metallic surface, and this area is not covered by ceramic. If the crown must be removed, an instrument can be inserted into this slot and turned to break the cement barrier. Though slightly less laboratory technique sensitive in its fabrication than the previously described design, this will also take additional time and skill to fabricate and can also be a plaque trap. This area can be filled on a provisional basis with composite to negate the plaque trap, but this material would require removal by rotary instrumentation.

A screw access channel can be cut through the crown to access the abutment screw if the crown is cement‐retained. This will cause weakening of the ceramic and potential for future fracture, as it will then need to be filled with a different material such as composite resin after completion of the screw‐tightening procedure. Without knowing precisely where the abutment screw access is, this hole can become quite large and compromise the integrity of the restoration. Several techniques have been proposed to register the location of the screw‐access hole to minimize the damage. Farzin et al. [9] proposed designing a ledge in the framework in the location of the screw access hole to support the remaining porcelain after perforation of the restoration.

Doerr [10] proposed the fabrication of a retaining‐screw location guide utilizing the original implant– and abutment–level casts. A cast of the cemented restoration is made and used to fabricate a vacuum clear guide, like a surgical guide, with the screw‐access hole prepared.

Another option is to place the abutment on the master cast and mount this on a surveyor. After recording the screw‐access position using a surveyor pin, the restoration is placed on the abutment and placed back on the surveyor. The surveyor pin is replaced with a fine‐tip brush loaded with brown or white stain, which can be applied to the occlusal surface of the restoration in the screw‐access hole area. This requires that a small concavity first be made in the restoration so that the stain can be received and not worn away. However, this is limited due to esthetic concerns as well as the potential for removal of stains if the restoration needs intraoral adjustment after insertion or the patient exhibits parafunctional habits.

Several authors proposed the use of digital photographs prior to the insertion of the restoration to document the position of the screw‐access hole. The camera is mounted on a tripod to maintain the same position for both photos. One photo is taken of the definitive abutment(s) on the cast and one with the definitive prosthesis placed on the abutment(s) on the cast. The photos are superimposed to locate the screw – access hole using a photo‐editing software of presentation software program and can be stored digitally in the patient’s file.

Cement Selection

Material properties such as viscosity, flow rate, and working time will influence the potential for remaining excess cement. Factors such as the amount, site, and radiographic density will influence the detection of residual cement [11]