Screw- Versus Cement-Retained Implant Restorations: Current Concepts

In Implant therapy,an interdisciplinary approach is an essential aspect and has led to the importance for each specialty to expand its scope of knowledge to improve interaction and offer excellent care to patients. As an implantologist, understanding the concepts that govern the selection between screw- and cement-retained restorations is very important. This helps clinicians to achieve better implant placement and facilitates the restoration of the implant prosthesis while promoting final esthetic outcomes. Articles have been published regarding the differences between screw- and cement-retained restorations. It is important to revisit the literature regarding techniques currently available that may influence the decision to select the type of restoration .The advantages and disadvantages between screw- and cement-retained restorations, as well as the retained for the selection will be discussed. This article will reflect an attempt to characterize the properties of each restoration in an objective way.

The debate between screw- versus cement-retained implant prostheses has long been discussed but the best type of implant prosthesis remain controversial among practitioners. An understanding of their properties will help the clinician in selecting the ideal prosthesis for each clinical case while promoting final esthetic outcomes. With the evolving technology and knowledge, an update of the current trends is necessary. This article provides an overview of the different characteristics of screw- and cement-retained implant restorations, and how they may influence the esthetics, retrivability,retention,passivity,occlusion,accessibility,cost, and provisional restorations. Problems and complications frequently encountered are discussed and treatment solutions are proposed. (Implant Dent 2010; 19:815)

ESTHETICS

When the implant is placed in the ideal position, predictable esthetics can be realized with either a screw- or a cement-retained prosthesis. In the situation, where the implant is placed in a position that will cause the screw access to emerge in the esthetic zone, a cement-retained restoration is the preferred method. The abutment of cement-retained restoration can be prepared in the same way as in natural teeth and the crown can then be cemented

Screw- retained restoration have been criticized because of the screw access channel that may be placed in an esthetic area when the implant is in an unfavorable position. To overcome these problems, preangled/angled/customed abutments have been used so that the screw access opening is relocated to the cingulum or occlusal surface. However, the screw must diverge at least 17 degrees from the long axis of the implant to allow sufficient space for the retention screw. Even in the posterior area, the occlusal restoration may still compromise the esthetics because of the underlying dark metal oxide. It seems that the use of an opaque in combination with resilient composite offers a significant esthetic improvement of the implant restoration.

In general, with adequate treatment planning and the use of surgical guides, the implants should be placed in the ideal position, allowing the restorative dentist with the choice of either screw- or cement-retained restorations.

RETRIEVABILITY

A major advantage proposed with the use of screw- retained restorations is that allow the retrieval of the restoration without damage to the restoration or fixture. Therefore, the screws and restorations can be restorqued, fractured components can be repaired, implant and soft tissue can be evaluated , calculus can be removed, and cleaning of the components is possible. The retrievability may be most valuable in extensive cases, where the prosthesis needs to be retrieved for maintenance purpose. However, the importance of retrieving an implant crown was questioned because the rate of complications remains low in comparison with natural teeth. As cementing a final crown on a natural tooth is a very common procedure, there are questions raised as to why one should treat an implant crown in a more rigorous way. The importance of retrievability remains under the discretion and beliefs of each clinician.

The technique used to remove a screw- retained prosthesis involves removal of the occlusal restoration, the intermediate cotton pellet, and the coping screw. When the restoration is removed, it can be screwed back in subsequently. A new cotton pellet is placed and the acces hole is then restored with materials such as composites.

As for the cement-retained prosthesis, retrievability is not necessarily impossible because implants abutments do not typically have undercuts and there is no chemical adhension between the cement and the abutment. A temporary cement composed of zinc oxide-eugenol or mixed with petroleum jelly is frequently used as a final cement for implant crowns to allow for future retrieval . In fact, an in vitro study performed by Kim et al demonstrated that decreased stress to the implant was obtained with provisional cement-retained prosthesis in comparison with permanent cement- and screw- retained prostheses. Nevertheless, the ability to remove the crown with a temporary cement is still very unpredictable, with the crown either strongly cemented or prematurely loosened. Another technique used to retrieve a cement –retained crown was described using set screws. The idea is to place a retrieval screw in an area where the cement seal can be broken without damaging the restoration. Other reported techniques involve the use of guides to reach the screw underneath or the use of screws to move the abutments and then retrieve the restoration. If the abutment does indeed become loose,if the restoration requires a repair and cannot be uncemented, or if the crown cannot be salvaged then fabrication of a new crown is often suggested .As a consequence, additional cost is charged to the patient. In addition, fabrication of a temporary restoration may be necessary in the esthetic zone.

RETENTION

Multiple properties of the abutment affect the retention of a cement-retained implant,prosthesis, such as its degree of tapering, surface area and height, and surface roughness. Most manufacturers fabricate the implant abutment with a taper of approximately degrees based on the concept that this is the ideal taper reported in natural teeth. In terms of the restoration height, the margins of the implant-supported crown and abutment are usually located to mm subgingivally, which increases the surface area in comparison with the natural dentition. At least mm of abutment height is needed to ensure the retention of a cement-retained restoration. As a consequence, when the interarch space is limited(i.e.<4, mm ),a screw-retained restoration may be indicated. Another indication for a screw-retained prosthesis is in a case of malpositioned implants because a cement-retained prosthesis would require excessive axial wall reduction. Retention is also influenced by the surface finish of the abutment. If additional retention is required, the abutment can be roughened with diamond burs or grit blasting, in a way similar to natural teeth. Moreover, the type of cement is also a factor that influences the retention of the cement-retained restoration, as discussed previously. The concept of progressive cementation may also be a consideration in which stronger cements are progressively used until the desired retention to keep in mind the at retention values for the same cement differ on natural teeth compared with implants. PASSIVITY Passivity is a desired feature of restorations because reduced stresses are applied on the bone and the implant. A lack of passivity has been correlated with biologic and prosthetic complications. If a nonpassive prosthesis is forced into place, stresses are applied to the suprastructure, bone,and implant, which may lead to bone loss, implant fracture, or even mobility. The development of a microflora at the interface of the implant and abutment is also possible, which may result in chronic gingival inflammation. Moreover, a casting with inadequate fit can cause deviations in the vertical loading of the implant and subsequently screw loosening or fracture. With the screw-retained prosthesis, a precise fit between the crown and abutment with no space between them is ideal. A soldering technique is used to correct the passively, In this techniques, the casr is first separated; a new transfer impression is taken and then sent to the laboratory. This importance of an accurate technique is reinforced when using a screw-retained prosthesis to minimize deviations in angulations In the cement-retained prosthesis each abutment is screwed to the implants, and the cement space of approximately 40um permits passivity. When the cement –retained prosthesis is not passive, the problem can be corrected by reshaping the abutment or casting. This additional space, if minor, can be compensated by the cement, which acts as a shock absorber and reduces stresses to the bone. This results in reduced laboratory cost and patient chairside time in contrast to the repair technique of the screw- retained prosthesis When comparing the passicity obtained in screw- mversus cement- retained prosthesis, most studies reported no difference with either technique. However, one study found that cement-retained restorations had more equitable stress distribution and are therefore biomechanically preferable to screw-retained restorations. Nonetheless. It is intresting to note that most prostheses do not show a completely passive fit, and however are still functional, suggesting a certain biologic tolerance for known to cause distortion (e.g., impression material and porcelain shrinkage. Dental stone, and investment material expansion), it is a challenge to achieve a perfectly passive structure. OCCLUSION Because there is no access hole on the occlusal surface of a cement-retained implant-supported prosthesis, an axial load may be directed to the implant, which is preferable to lateral forces. Hence, occlusal function is promoted with cement-retained implant restorations. The screw-retained prosthesis requires an occlussal restoration, such as amalgam or composite, to cover the screw access channel. However, the durabiltity of these restorations is inferior to an intact full coverage crown. Moreover, as the size of the implant increases so is thescrew access hole, thus leading to a large occlusal restoration, which may compromise the long-term durability. In fact, the screw access hole occupie about 50% of the occlusal table in molars and 75% in premolars. These screw access holes not only interfere with centric occlusal contacts but alsoaffect protrusive and lateral excursive movements. SUCCESS RATES The differences in implant survival rates between screw- or cement-retained prostheses were evaluated in a systematic review. No differences were found between the two types of prosthesis in terms of implant urvical or success rates . In regards to prosthesis success rates (>72 months), cement-retained prothess demonstrated a 93.2% success. Compared with 83.4% with screw-retained prothese, although the results did not reach a level of statistical significance (p>0.05).

COMPLICATIONS

Porcelain Fracture

Because implants lack a periodontal ligament, higher forces are exerted on implant crowns. As a consequence, porcelain may be left unsupported because of the screw access channel, screw-retained prostheses are even more susceptible to porcelain fracture In an attempt to minimize this problem the use of a metal occlusal table is gaining popularity in areas with heavy occlusal loads, limited vertical clearance, and without esthetic concerns.

Peri-Implant Tissue Inflammation

Residual cement is a common complication of cement-retained prostheses. The shape of the abutment and crown margin must follow the soft tissue margin to prevent cement margins that are too deep. Soft tissue inflammation and bone loss can occur when residual cement is left. Therefore, it is crucial to remove all residual cement at the time of crown insertion. Multiple techniques have been proposed to each cement removal, such as placing petroleum jelly on the outer surface of the crown, the use of curettes to carefully scale around the crown, and the meticulous use of dental floss.

Clearly, this is not a problem with screw-retained prosthesis because no cement is used. In fact, peri-implant soft tissue response seemed to be more favorable with screw-retained restorations. However, some authors reported gingival inflammation because of the microgap between the restoration and the abutment interface. An intresting study performed by piattelli et al showed bacterial infiltration in the screw-retained implant assembly because of the microgap compared with no bacterial or fuild penetration in the cement-retained implant as assembly. The group conclude the superiority of cement-retained restorations in terms of fluid and bacterial permeability.

Screw Fracture/Loosening

The main disadvantage observed with screw-retained restoration is the screw loosening that occurs during function. The incidence of screw loosening or fractures varies between 10%and 65% depending on the studies and the types of restorations. In general, It occurs most frequently with single tooth implant restorations, restoration in the molar area, and long cantilevers. As this was the main concern with the traditional hexagonal implant dence is greatly reducedwith the advent of newer implant systems(e.g., internal connections with geometric lock,larger,abutments,and screws designs).

To prevent screw loosening,various techniques were reported,such as the antirotational feature,direct mechanical interlock, changes in screw design,and torque controlling mechanisms with torque wrenches. A restoration that is both cemented and screwed retained was also described and known as the ‘Combination Implant Crown.’ The authors claim that the system offers the avantages of both worlds, such as esthetics,retrievability, and antirotational features with the octagon engagement.Detrimental forces including excursive, off-axis centric, interproximal, and cantilever contacts should be thoroughly evaluated and eliminated whenever possible. A more favorable distribution of the forces is obtained when the implant is placed parallel to the occlusal forces. In addition, a nonpassive framework increases the likelihood of screw loosening.

Another observed complication is the fatique failure of the screw. Because the screw holding thescrew-retained prosthesis has a narrow diameter, strength of the prosthesis is compromised. Conversel, the components of the cement-retained restorations are mainly large and, therefore, fatique failure does not occur as often.

COMPLICATIONS

It is more challenging to place a screw-retained restoration in the posterior area when compared with the cement-retained restoration because of the handling of screws and screwdrivers. This problem may be readily observed in cases where the mouth opening is restricted may mimic more treatments of natural teeth and therefore may be preferred by many dentists.

COST

The laboratory cost to fabricate a screw-retained restoration is usually 1.5 to 2 times higher because of the extre time and materials needed (impression transfers, analogs, and screws). There is usually no extra cost to the cement-retianed restoration because no additional training is required to the laboratory technicaian.

PROVISIONAL

Procisional restorations are important in the esthetic area to replace the missing tooth and to shape the soft tissue for a better crown profile.In the posterior area, benefits from a procisional restoration include not only shaping of the soft tissue but also a better assessment of the outcome and anticipation of possible problems with the final crown. For instance, if an adequate thickness of material cannot be optained with the provisional, reshaping the opposing tooth may be necessary before the impression to gain additional interocclucal space.

Cement-retained provisionals are relatively easy to fabricate compared with screw-retained provisionals, because they simulate techniques with natural teeth, which can futher motivate clinicians to make the temporary. Nontheless, the major disadvantage includes the possibility of access cement that can cause tissue inflammation. This is a major problem particularly in cases of immediate loading, where excess cement in the surgical site may compromise healing and implant osseointegration..

Screw-retained procisionals offer the advantage over cement-retained provisionals in the they can be screwed into the master impression to translate additional information to the technician regarding the contour. As the machined surface of screw-rtained restoration are better than cement margins, the selection in cases of immediate loading may favor screw-retained provisionals.

SUMMARY

Tables1-3 list the comparison between screw- and cemented retained implant restoration as well as the advantages and disadvantages of screw- and cemented-retained implant restorations

CONCLUSION

Different philosophies exist regarding the ideal type of restoration. The truth is that most of the decisions are based on the clincian’s personal preference and the actual clinical situation. The literature shows advantage and disadvantages for both screw- and cement-retained implant-supported prostheses(Tables 2 and3). An understanding of how each type of prosthesis influences the esthetics, occlusion, and longevity of the restoration is important in selecting the best case for either a screw- or a cement-retained implant-supported prosthesis. It is generally agreed that the currenttrend tends to favor cement-retained implant restorations for ther superior esthetics, occlusion, ease of fabrication, and reduced chairside time

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