Dental News - Screw-retained, implant-supported fix partial denture (FPD)

Michael Nawrocki, DMD, MD, MS, and Dov M. Almog, DMD

Mon. 20 December 2010

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A screw-retained implant-supported fixed partial denture (FPD) has certain physical advantages. However, according to several studies they require precise positioning of the implant for optimal location of the screw access hole.1 Also, obtaining passivity of frameworks that are screw-retained is difficult due to dimensional discrepancies inherent in the fabrication process.2, 3, 4

Anchorage of prosthetic fixed partial dentures to implants can be achieved in two ways: some clinicians cement the prosthetic construction to implant abutment, while others suggest that screw retention is preferable.

Screw-retained implant restorations have an advantage of predictable retention and retrievability, and the lack of potentially retained excessive sub-gingival cement.

On the other hand, a few disadvantages exist: obtaining passivity of screw-retained framework that is difficult due to dimensional discrepancies inherent in the fabrication process. 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.

Cementation of implant restorations eliminates unaesthetic 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 a lack of passivity.

It has the potential to reduce stress to splinted implants because 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. On the other hand, any excess retained cement extruding from the prosthesis/abutment interface, especially when located sub-gingivally, can cause inflammation, infection and periodontal complications.

As more and more dental practitioners are focusing on implant-supported fixed partial dentures, restoring dentists need to understand the restorative options they may have. Many dental practitioners and dental labs will persistently use a screw-retained implant-supported fixed partial denture, and thereby promote choices that offer the utmost in serviceability, cosmetic result and maintenance of optimized bite possible.⁵

At the same time, in recent years, the utilization of adjunctive state-of-the-art cone-beam CT and technologies and 3-D derived virtual planning software solutions altered the manner in which we pulled together diagnostic data, planned and executed both simple and complex implant cases.

As a result, more and more implant trajectories are consistent with the planned prosthetic trajectories. Yet, some cases are still driven by the residual bone trajectories and are left to the restoring dentists’ decision as far as the final restorative option.

In other words, when the implant trajectories are inconsistent with the planned prosthetic trajectories, the screw-retained implant-supported fixed partial denture systems offer an opportunity to minimize any controversy between the surgeons, restorative dentists and the labs, creating greater understanding, appreciation and professional camaraderie.

Case report

Patient presented for implant-supported FPD after having teeth #8, 9 and 10 extracted with socket preservation.

A CBCT study was performed with the iCAT CBCT machine (Imaging Sciences International, Hatfield, Pa.) and revealed reasonable alveolar dimensions, both vertical and horizontal.

However, by utilizing ImplantMaster software (iDent Imaging, Inc., Foster City, Calif.), it was discovered that the residual bone trajectory and the planned prosthetic trajectory were in conflict, that is, projecting a compromised restorative trajectory lingually in implant site #9 and buccally in implant site #11 (Fig. 1).

Nevertheless, following a treatment planning conference, rather than considering bone grafting, a decision was made to proceed with these angulations and a 3-D reconstruction of the patient’s anatomy was attained and a virtual surgical guidance template was designed and computer-manufactured with precise drilling holes’ distribution and trajectory for implants #9 & 11.

The palatal trajectory of the implant in tooth position #9, the patient’s deep bite which resulted in severely limited space for prosthetic components, dictated a screw-retained prosthetic FPD construction solution for the case.

The extremely buccal angulation of the implant replacing tooth #11 resulted in a buccaly located screw access opening, which compromised esthetics and potentially weakened the porcelain around the screw opening in the proposed screw-retained three-unit FPD.

The esthetic dilemma could be solved by either gold plating of the metal portion of the screw chamber, which can reduce the need for opaque composite material, or by metal cut back to hide the non-esthetic metal. We chose to overcome this esthetic and structural obstacle by using a separate telescopic crown design to cover the metal substructure of the screw-retained in #11 location.

Conclusion

As more and more dental practitioners are focusing on implant-supported fixed restorations, restoring dentists need to understand the restorative options they may have to deal with.

Dental practitioners and dental labs need to be prepared to use a screw-retained implant-supported fixed partial denture, and thereby promote choices that offer the utmost in serviceability, cosmetic result and maintenance of optimized bite.

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Figs. 1a, 1b: CBCT study was performed with the iCAT CBCT machine (Imaging Sciences International, Hatfield, Pa.). By utilizing ImplantMaster software (iDent Imaging Inc., Foster City, Calif.), it was noted in the 3DVR (a) and virtual surgical template (b) that the residual bone trajectory and the planned prosthetic trajectory were in conflict, projecting compromised restorative trajectory lingually in implant site #9 and buccally in implant site #11.

Figs. 2a–2c: The screw-retained restoration was made by CQC a DTI Dental lab in Rochester, N.Y. Different views of final screw-retained restoration emphasize the extreme lingual trajectory of implant #9 (2a) and extreme buccal trajectory of implant #11 (2b). Note telescopic design crown on #11 (2b and 2c).

Figs. 3a, 3b: Intraoral views of the screw-retained restoration. Note the implants’ prosthetic platforms (3a) emphasizing the actual trajectories of implants #9 and #11 in the patient’s maxillary ridge. Note telescopic design crown on #11 (3b).

References

1 Winston Chee, David A. Felton, Peter F. Johnson, Daniel V. Sullivan. Cemented versus screw-retained implant prostheses: Which is better? Int J Oral Maxillofac Implants 1999; 14(1):137–41.

2 Hebel KS, Gajjar RC. Cement-retained versus screw-retained implant restorations: achieving optimal occlusion and esthetics in implant dentistry. J Prosthet Dent. 1997; 77(1):28–35.

3 Guichet DL. Load transfer in screw- and cement-retained implant fixed partial denture design [abstract]. J Prosthet Dent 1994; 72:631. Guichet DL, Caputo AA, Choi H, Sorensen JA.

4 Passivity of fit and marginal opening in screw or cement-retained implant fixed partial designs. Int J Oral Maxillofac Implants. 2000; 15:239–46.

5 Implant Bridge Mounting Choices: Cemented vs Screw Mount. www.dental--implants.com/fixed_bridge_implants.html (last viewed 10-8-10).

Michael Nawrocki, DMD, MD, MS, prosthodontist, VA New Jersey Health Care System.

Dov M. Almog, DMD, prosthodontist, chief of the dental service, VA New Jersey Health Care System.

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