Implant-supported restorations can be secured to implants with screws (screw-retained), or they can be cemented to abutments which are attached to implants with screws (cement-retained). This literature review discusses the advantages and disadvantages of each method of retention from different aspects. These aspects include: ease of fabrication and cost, esthetics, access, occlusion, retention, incidence of loss of retention, retrievability, clinical prosthesis fit, restriction of implant position, effect on peri-implant tissue health, provisionalization, immediate loading, impression procedures, porcelain fracture, and clinical performance. Peer-reviewed literature published in the English language between 1955 and 2010 was reviewed using PubMed and hand searches. Since the choice of using either method of retention is still controversial, this review article offers some clinical situations that prefer one method of retention over the other. The review demonstrated that each method of retention has certain advantages and disadvantages; however, there are some clinical situations in which it is better to select one method of retention rather than the other.
With the high rate of implant success for edentulous, partially edentulous, and single tooth restorations, the concept of implant therapy is now a highly predictable treatment modality.
Implant dentistry has seen rapid and remarkable progress in recent years. Several questions have been raised concerning materials as well as designs of both implants and implant abutments to achieve maximum clinical success rates.
One of the debates is the choice between screw- and cement-retained implant prostheses, which has long been discussed, but the best type of implant prosthesis remains controversial among practitioners.
There are few publications that comprehensively compare the 2 types of retention. The aim of this review of the literature was to provide an overview of the advantages and disadvantages of the cement- and screw-retained restorations, and also to suggest some clinical situations that advocate for one method of retention over the other.
The factors that are affected by different methods of retention of the prostheses to the implants are: ease of fabrication and cost, esthetics, access, occlusion, retention, incidence of loss of retention, retrievability, passivity of fit, restriction of implant position, effect on peri-implant tissue health, provisionalization, immediate loading, impression procedures, porcelain fracture, and clinical performance.
Ease of Fabrication and Cost
The fabrication of cement-retained restorations is easier than that of screw-retained restorations because conventional laboratory and clinical prosthodontic techniques are used for making cemented restorations.1,–3
The screw-retained restorations are usually more expensive because of the extra components needed, such as plastic sleeves, laboratory fixation screws, and the fixation screws themselves.4
Nevertheless, the increased cost of the screw-retained restoration that allows for predictable retrievability must be compared to the potential costs of damaging the cemented restoration if biologic or technical complication occurs.5
When the implant is placed in the ideal position, predictable esthetics can be achieved with either screw- or cement-retained restorations.6 One of the debates regarding using screw-retained restorations is the screw access channel that may be placed in an esthetic area.1,2,4,7 When there is difficulty in placing the implant in an ideal position for any anatomic limitation, the preangled or custom abutments can be used so that the screw access channel is relocated away from esthetic area.6
The use of an opaquer in combination with a resilient composite offered a significant esthetic improvement of implant restoration.8
Ideal and stable occlusal contacts can be established with cement-retained restorations because there are no occlusal screw access holes.1,–3,7 These screw access holes will also interfere with protrusive and lateral excursions and, therefore, anterior guidance may be compromised.1
The screw-retained restorations where the screw access hole occupies more than 50% of the intercuspal occlusal table require an occlusal restorative material to cover the screw access channel; these restorative materials are susceptible for wearing under functional forces and so the occlusal contacts will be less preserved than when using cement-retained restorations with intact occlusal surface.1,10,–12
Moreover, the difficulty in achieving stable occlusal contacts when using screw-retained restorations because of the presence of restoration material will affect the direction of occlusal loads which will be distributed as lateral forces to the implant instead of being axially directed.3
The security of retention is considered one of the most important factors affecting implant prostheses longevity.2
Taper greatly affects the amount of retention in cement-retained restorations13 machined abutments have mostly 6° of taper depending on the concept of ideal tapering proposed by Jorgensen for natural teeth.14
Regarding surface area and height, the subgingival placement of the implants provides longer implant abutment walls and usually more surface area than prepared natural teeth.1,15 The minimum abutment height to use cement-retained restorations with predictable retention was documented to be 5 mm.15 Therefore, when the interocclusal space is as little as 4 mm, screw-retained restorations may be used,6 since these restorations can be attached directly to implants without intermediate abutment.18
Increased surface roughness will offer increased mechanical retention for cements,17 and so roughening the implant abutments using diamond burs or grit blasting will provide higher retention.1 However, because of the ideal 6° taper and long surface provided by implant abutments, there will usually be no need for roughening abutment surface to increase retention.1
Cement selection is one of the most important factors controlling the amount of retention attained for cement-retained restorations.17 The cement used with implant restorations can be either permanent or provisional, and it is the clinician's decision to choose a certain type of cement based on the clinical situation.1,19,–21 The concept of using provisional cementation is considered to achieve restoration retrievability without endangering the implant restoration components when loose restoration or abutment screw loosening occurs.17
With regard to screw-retained restorations, retention is obtained by a fastening screw. The loss of retention in screw-retained restorations is demonstrating itself as screw loosening.22 Factors including insufficient clamping force, screw settling, biomechanical overload, off-axis centric forces (forces that are not directed along the long axis of the implant), implant components and prosthesis misfit, differences in screw material and design, and finally hex height and implant diameter will affect the amount of retention of screw-retained restorations.1,22,–33
To achieve sufficient clamping force the screws should be torqued 50% to 75% of their yield strength, so it is imperative that all screws be tightened to manufacturers' specifications using a torque control wrench in the initial phase of screw tightening.1,22
Screw settling or embedment relaxation will occur shortly after screw tightening due to compressing the microscopically rough areas of the screw threads and opposing flanges during screw tightening23; therefore, retorquing the screw 5 minutes after initial torquing and again a few weeks later is recommended.26
During function and biomechanical overload, both compressive and tensile forces will cause screw loosening. The compressive forces will cause disengagement of mating threads when applied in amount equal to or greater than the preload, the tensile forces may cause plastic deformation of the screw, thereby decreasing the clamping forces that hold the components together.23,24
Off-axis centric forces are detrimental to screw-retained restorations. Therefore, excessive implant angles, cantilever prosthesis, and connecting implant to natural teeth using fixed partial dentures should be evaluated and eliminated whenever possible to prevent screw loosening.22
Screw loosening is also affected by implant component and prosthesis misfit. Poor fit between implant and components could increase stress in the screw leading to screw loosening.25,–27 The same is applied to nonpassive prosthesis that will apply additional load to the system leading to bending moments constantly loading the implant components and surrounding bone tissue.28,29
To prevent screw loosening, various modifications to the screw and implant were reported.26,30,–33 It was found that gold screws can be tightened more effectively than titanium ones and therefore will provide better retention.31
The screw design will affect screw retention, so it has been shown that screw heads with internal hexagon remain tighter than those with slots.32 Tapered head screws have been abandoned because the head/shaft load ratio was found to be 4∶1 as opposed to flat head screws using a 1∶1 head/shaft ratio, and this will lead to strained interfaces when using tapered screws that will increase the susceptibility of screw loosening.27
Incidence of Loss of Retention
The screw loosening is a major problem with screw-retained restorations.34,–37 The incidence of screw loosening was 65% for single tooth implant restorations in one study,34 whereas the incidence of unretained cemented implant restorations was reported to be less than 5% in other studies.38,39
However, the improvements in implant systems, including the advent of internal implant-abutment connections, enhancement of torque drivers, and screw materials and design, led to reduction in the incidence of screw loosening.4,35,36,40,41
On the other hand, the screw loosening of screw-retained restorations can be considered as an important advantage since the weakest component within the implant-supported restoration will be the prosthetic screw; this will allow for assessing the implant-supported restoration before more serious complications develop, such as implant fracture at screw level especially in implant systems using internal connections.42,43 Using screw-retained restorations will enable assessing the preload of implant abutment screws over time, since the preload is not constant with ongoing application of forces associated with occlusion.
The main advantage of screw-retained restorations is the predictable retrievability that can be achieved without damaging the restoration or fixture.1,4 Therefore, the prosthodontic components can be adjusted, the screws can be refastened, and the fractured components can be repaired44 with less time and at lower cost than would be the case with cement-retained restorations.2,45,46
Several suggestions and techniques have been introduced to facilitate the removal of cement-retained restorations.
One of the techniques described is the incorporation of screw into the cemented restoration to be used later to lift the restoration off the abutment if activated. Compared with conventional screw retention, this technique improves esthetics and occlusion since the access hole can be placed in the most ideal position without regard to implant position.47 Another method proposed is to prepare a cylindrical guide hole on the lingual surface of the abutment and an access hole in the lingual side of the restoration. Then, by inserting a removing driver into the guide hole through the access hole and turning it to generate a shear force, the cement will disintegrate and, in turn, the restoration can be easily removed.48
Other techniques that have been suggested depend mainly on locating the screw access opening of the abutment screw in cement-retained restoration, in turn, to allow access to the abutment screw with least damage in the future. These techniques are achieved by using abutment screw access guide or placement of a well-defined small ceramic stain on the occlusal surface of restoration where the screw access opening is located.49
Using abutment inserts is a technique developed so that there will be no need for either screw or cement for connecting the restoration to the abutment. In this technique a standard abutment with perforation on the lingual side is screwed to the implant. An insert is cast to fit tightly into the abutment in a lock and key fashion, and the same insert lodges into the screw of the implant to secure it. This insert has a perforation to match the lingual perforation of the abutment. Then, the restoration is made with a lingual hole to match the abutment and insert through a spring-locked pin. An explorer can be used to push the spring to release the crown for removal.52
Provisional cement is frequently used as final cement for cement-retained implant-supported restorations to allow for future retrieval.6
In spite of all the proposed techniques to improve the retrievability of cement-retained restorations, screw retention becomes more necessary in extensive cases where prosthesis needs more maintenance, so cantilevered prostheses and full arch implant reconstruction are best restored with screw retention.10,53
Clinical Prosthesis Fit
The passive fit of implant prostheses has been stressed because of the ankylotic character of implant abutments and because poor fit is correlated with biologic and mechanical complications.28,31,43,54,55
Many authors believe that a cement-retained restoration is more likely to achieve passive fit than a screw-retained one.1,7,10,39,56,57 This increased passivity of cement-retained restorations rests on the assumption that the cement could act as a shock absorber and reduce stress to bone and implant-abutment structure.1,56,–58 Conversely, screw-retained prosthesis without precise fit between crown and abutment may create substantial stress within the prosthesis, the implant, and surrounding bone.7
However, the main factors that affect the prosthesis fit depends on accuracy achieved in the fabrication process, including impression technique, master cast accuracy, component tolerance, casting tolerance, and skill of the technician, while the type of retention does not play a role in transferring or compensating for inaccuracies of prosthesis fabrication.59
Screw-retained restorations have been found to produce tighter margins than their cemented counterparts.60 As a consequence with cement-retained restorations there is always a risk of colonization of the space with microflora which may result in cement dissolution and gingival inflammation.61,62
To enhance the fit of single cast framework spark erosion is another proposed technique.
One of the most recent approaches to improve passivity of fit is using the laser scanned computer numeric controlled–milled titanium (computer aided design/computer aided manufacturer).66
Restriction of Implant Position
Screw-retained implant-supported restorations require precise placement of the implant to achieve predictable esthetics.10,47 However, the use of cement-retained restorations allows for greater freedom in implant placement.6
As the manufacturers have not provided angled abutments of less than 17° for screw retention until now, malaligned implants with divergence axis less than 17° have to be restored with cement-retained restorations.10,67
In general with good treatment planning and precision surgery using surgical guides, the implant can be placed at its ideal position.6
Effect on the Health of Peri-Implant Tissue
Some authors reported gingival inflammation when using cement-retained prosthesis because of difficulty in removing excess cement, especially when the restoration margin is greater than 3 mm subgingivally. This is particularly common in the anterior region when it is recommended to place the implant 3 to 4 mm apical to the cementoenamel junction or the facial gingival margin of adjacent teeth to develop proper emergence profile.68
It has been shown that incomplete removal of cement may result in peri-implant inflammation, soft tissue swelling, soreness, bleeding or exudation on probing, and resorption of peri-implant bone.10,68,–70 The solution for these clinical situations is using either screw-retained restorations or custom abutments for cement restoration with margin following the anterior gingival contours.68
One of the techniques proposed for removal of excess cement is using plastic scalers, but even this may result in scratches on the implant surfaces which may encourage plaque accumulation and compromised soft tissue health.71,–73
Reducing the amount of cement placed in the restoration before cementation can be achieved by placing a luting agent only on the occlusal half of the intaglio of the restoration. In addition, seating the restoration after placing the cement on the fitting surface on the abutment analogue extraorally before the restoration is cemented intraorally can be used to reduce the excess cement.68
Creating a lingual vent hole in the implant-supported crowns is another technique to reduce the amount of excess cement being lodged in the sulcus.74
The gingival response is found to be better when using screw-retained crowns since no cement is used. However, if prosthetic retaining screws and abutment screws become loose, granulation tissue accumulates between the prosthesis and the abutment and also between implant and abutment leading to fistulae formation, plaque deposition, and screw fracture. Therefore, it is recommended to retighten the screws in full arch fixed prosthesis every 5 years.32
Provisional restorations are frequently used for immediate or early implant loading to achieve better esthetics and to mold soft tissue for proper emergence profile for definitive restorations.75
Using screw-retained provisional restoration is preferred over cement-retained restoration because the screw can be used to seat the provisional restoration and to expand peri-implant mucosa.6,76 Also, screw-retained provisional restoration can be screwed into the master impression to translate additional information to the technician about the contours.6
The major disadvantage of cement-retained provisional restoration is the difficulty associated with removing excess cement and managing bleeding at the same time. Moreover, cement residues may cause gingival inflammation.6
Screw-retained restoration is considered the restoration of choice for immediate loading; this is because using this restoration eliminates the need for cement and the associated difficulty in removing excess from the peri-implant area that may interfere with healing and implant integration.71 In addition it has been shown that the interface of machined components is superior to any cement margin that can be developed.60
In addition, screw retention provides the most definitive and rigid splinting when multiple implants are used and therefore enhances implant primary stability.6
Screw-retained provisional restorations can be screwed in the master impression so as to transfer soft tissue contours to master cast. As a result the definitive restorations will be easily seated without soft tissue impingement.6
Porcelain fracture is a common complication observed in implant-supported restorations.43 This is most commonly seen in screw-retained restorations because the screw access hole disrupts the structure continuity of porcelain leaving some unsupported porcelain at the screw access hole.1,3,10,11,77,78
The success rate of cement- and screw-retained implant-supported restorations were evaluated in several studies.11,79,–81 Most of these studies showed that screw-retained restorations have more complications during follow-up periods than their cemented counterparts. However, the percentage of these complications was generally small and most of them were controllable.
Some Situations Prefer One Method of Retention over the Other
It was stated that the selection of an implant system is the first step in determining the feasibility of either a cement or screw retention for the prosthesis.10 The current implant systems that employ a conical interface between the implant and the abutment or other internally designed connection features have reduced the incidence of screw loosening and other problems associated with traditional hex-top systems. Therefore, it is believed that it is easier and simpler to utilize the traditional cementation methods with these current systems for retaining definitive prostheses. However, there are some situations where it is better or more suitable to use one method of retention rather than another. These situations are summarized in the Table.
The authors do not prefer one type of restoration over the other because both types of restorations, screw-retained and cement-retained, have certain advantages and disadvantages. However, based on reviewing the related literature, it has been demonstrated that one type of restoration is more appropriate than the other in some clinical situations.