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					                                                                                                   Handout # 9




           Disadvantages of Screw-Retained
                     Implant Prostheses
1)   Screws have to be torqued to each manufacture's specifications to 50% to 75% of their yields
     strength.
2)   Fulcrums or pivot points are created on one side of the casting (crown) were it meets the head of
     the implant. This causes screw loosening and breakage.
3)   Vertical loading over the implant head can compress the casting and cause screw loosening.
4)   Offset loading can cause the screw to be stretched, broken or loosened from the rocking force.
5)   Micro gaps exist between castings and implants causing misfits, which allow for compression of
     the casting and the framework. It is worsened by offset loading.
6)   Screw loosening and breakage is a major problem in screw-retained restorations.
7)   3mm diameter screws take up at least 50% of the occlusal table of molars and more than 50% of
     the occlusal table of bicuspids.
8)   The location of the screw hole is a great obstacle to generating an optimum occlusion and axially
     load the implants.
9)   Implant screw holes 3mm plus wide are highly unesthetic.
10) Screw retained implant prosthesis may lack the proper anatomy on the cuspids and central incisors
    for the smooth transition into protrusive and lateral protrusive movements; thus, anterior guidance
    may be compromised.
11) Castings are routinely torqued into position placing inappropriate stress on the screw as well as the
    bone implant interface and the prosthetic components.
    ____________________
     Reference
     HEBEL, KS, GAJJAR, RC Cement-Retained Versus Screw-Retained Implant Restorations: Achieving
     Optimal Occlusion and Esthetics in Implant Dentistry.
     Prosthetic Dent J 1997; 77; 28-34
                                                _______________________________________

12) The bending strength of a screw is proportional to the fourth power of the diameter.
    Example:       A 2mm screw is 16 times stronger than a 1mm screw.
13) Repeated removal of a screw-retained prosthesis may result in wearing of a screw and/or implant,
    which may contribute to component fracture.
14) It is virtually impossible to fabricate a perfectly fitting, passive multiple implant prosthesis. Dental
    laboratory technicians cannot fabricate a framework within 25-50 microns of accuracy.
15) Many implant components are not manufactured to 50 micron tolerance.
16) A screw-retained prosthesis offers little room for error.
17) Inevitable casting inaccuracies will cause undesirable stresses to the implant, bone, and prosthesis.
18) The fabrication of screw-retained restorations requires special knowledge, training, and skills by the
    laboratory technician.
19) Screw-retained prostheses are generally more complex in their fabrication.
20) The additional components and laboratory procedures lead to higher costs and more remakes as
    compared with cement-retained prostheses.
21) The screw-retained prosthesis requires more components as compared with the cement-retained
    restoration.
22) At a minimum, implant or abutment impression posts, implant or abutment laboratory analogs,
    abutments, waxing sleeves, and abutment screws are needed to fabricate the restoration.
23) The laboratory fees for screw-retained prostheses are typically 50 to 100 percent higher than for similar
    cement-restorations.
24) Margin changes in screw-retained systems often require new abutments and possibly additional chair
    time to make new master casts.
25) Correction of misalignments beyond those allowed by the screw abutment taper may require the use of
    complex and expensive angled components.
26) The access hole of the screw-retained restoration is filled with acrylic resin or composite that wears faster
    then the surrounding porcelain and metal, which can lead to occlusal disharmony.
27) Porcelain fracture is much more common in screw-retained prostheses as compared with cement-retained
    restorations, as the access hole creates an inherent weakness in the occlusal surface of the restoration.
28) There are few components commercially available to create screw-retained provisional restorations.
29) The clinician may be forced to fabricate an expensive custom provisional, place and prepare additional
    abutments for a cement-retained provisional, or choose not to put provisionals at all.
30) Removal of a screw-retained prosthesis often involves a considerable amount of time and effort by the
    clinician.




    Reference
    DARIO, LJ Implant Angulation and Position and Screw or Cement Retention: Clinician Guidelines
    Implant Dent. 1996; 101-103.




                                                                                                            07 Oct 06

				
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