Minutes of meeting  May 2, 2002
Submitted by George F. Green, D.D.S.
 

Introduction - Dr. Gantes and Dr. Yamada

Introduction of previous speakers at study club: Dr. Robert Simon, prosthodontist / private practice, and Dr. Wayne Campagni, prosthodontist / Loma Linda Dental School.

Announcements:
June 5 - orthodontic meeting . . . please invite your orthodontist. Dr. Joe Caruso (chairman / ortho dept. Loma Linda) will discuss the Newtom / radiographic imaging device.
October 3 - Staff meeting (to replace originally scheduled meeting with Torsten Jemt)
November 7 - Literature review
December 12 - Torsten Jemt / register early / Long Beach Marriot Hotel / "digital impression making "

Tonight's presentation is by Dr. Guillermo Bernal, prosthodontist, Loma Linda Dental School

Implant Restorations: Cemented or Screw-Retained? - Dr. Guillermo Bernal

At Loma Linda Dental School, about 80% of implant cases are external-hexed (Branemark) and 20% internal-hexed (ITI).

A. Historical Background
    1. 1981 (20 yrs ago) - we know implants work, lowest long-term maintenance of any tooth replacement
                                          procedure
    2. 1994 - partially edentulous cases can also be successfully treated with implants
    3. Evolution of screw-retained (prosthetic preference) to cement retained (more esthetic / porcelain occlusal)
    4. Problems of screw loosening solved with WP fixtures, pre-loading of screws
    5. 1996 - single tooth implants successful, decrease in screw loosening by:
          use of gold alloy screw in place of Ti screw
          alteration of occlusal scheme  (decrease area of occlusal table, decrease cusp inclination)
    6. Currently, at Loma Linda, primarily use custom abutments / fixture-level impressions

B. Treatment Modalities
   1.  Screw - Retained
        Advantages
            1. retrievability
                 re-tighten abutments
                 simplify replacement of fractured components
                 perform hygiene
            2. limited inter-occlusal space = INDICATION
                for cemented restorations: need 5mm with external hex, 6mm with internal hex
                with screw-retained, can restore with
            3. Avoid sub-gingival excess cement
        Disadvantages
            1. requires precise implant placement
            2. necessary passive fit difficult to achieve on multiple units, particularly difficult with internal hex cases
                 Dr. Wayne Campagni: "A casting is a miracle"
                 Torsten Jemt: study demonstrated "misfits" of 75-250 microns resulted in no additional bone loss,
                                          but did result in screw-loosening and fractures
            3. screw holes in occlusal surface
                 compromised esthetics
                 disruption of occlusal surface
    2. Cement - Retained
        Advantages
            1. improved esthetics, maintain occlusal scheme
            2. passive fit easier to achieve (Guichet IJOMI 2000; 15:239  better stress distribution with
                                                                    cement-retained)
            3. physical strength of occlusal porcelain maintained
        Disadvantages
            1. removal of sub-gingival excess cement ( could use lateral set-screw in place of cement)
            2. abutment screw loosening
                Jemt: 10-56% screw loosening with Ti screws
                original hex height = 0.75 mm, currently WP hex height = 1.0 mm
                must use torque driver to deliver greater than 20 N-cm (must have torque driver calibrated)
                internal spline, Morse taper = no screw loosening problems
            3. non-retrievability
        Factors Influencing Retention
            1. total convergence angle of abutments
            2. abutment surface area and height
            3. abutment surface texture
            4. type of cement (following in order of least to most retentive:)
                temp-bond with vaseline
                tempbond , Zinc Phosphate (statistically similar)
                Improv (SteriOss)
C. Conclusions
    1. Screw-reatined is treatment of choice when implant position is optimal, and is mandatory
        with limited inter-occlusal space
    2. Passive fit = difficult to achieve with conventional techniques, increase in risk of screw loosening with
        increase in misfit
    3. Adequate occlusion in repeatable position, narrow occlusal table, minimal cusp inclination = may
        decrease forces leading to better prosthetic stability
    4. NP implants with external hex are contraindicated in posterior areas
    5. Implant location and esthetics influence choice of cement retained or screw retained.
    6. New implant designs may decrease risk of abutment screw loosening making cement and screw more
        reliable.
    7. New screws with increased yield strength require torque drivers.
    8. Lateral set screws: improve retrievability, increase cost, difficult to manipulate clinically
    9. Cement -retained = problems with removal of excess cement leading to soft tissue problems
    10. total angle of convergence usually more parallel than ideal creating difficulties with retrievability, even
         with temporary cement.
    11. Use of die spacer increases likelihood of passive fit.
    12. With short abutments: use auxillary retentive features ( grooves, boxes, etc.)
    13. Permanent cement or Improv may be indicated for cement-retained.
    14. TempBond with vaseline may be suitable for splinted teeth where retrievability is necessary.
    15. Blood contamination during cementation can drastically reduce cement strength.

D. Questions and Answers
   In the case presented, why were the upper implants splinted?
    To compensate for minimal retention: short abutment length and large crown:"root" ratio.

   What is the ideal convergence angle for Procera abutments?
    Depends upon length of abutment . . . with short abutment, need less convergence (2 deg - 6 deg). With
    long abutment, can increase convergence to 15 deg.

    Misfit: most critical in horizontal dimension.

Meeting adjourned at 8:40PM