Screw- Versus Cement-Retained Implant Restorations: Current Concepts

[drsushant]

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.