CATEGORY 86 AUSTRALASIAN DENTIST is still invariably screwed down to the fixture. Given the description above, cementretained implant prostheses should be defined in the literature as prostheses that are cemented clinically in the patient’s mouth, after which there is no other means of accessing the prosthetic or abutment screw without cutting the crown. Examples of cement-retained restorations as defined above, and inferred by the literature, will be described below with illustrations. Definition and Examples of ScrewRetained Implant Supported Prostheses The literature infers that screw retention is any prosthesis “attachment mechanism that sacrifices occlusion and esthetics for retrievability”. This means that there is a screw channel present through the prosthesis, for the purposes of access to an internal screw. This internal screw may be an abutment screw, in the case of a directto-fixture prosthesis; or a prosthetic screw, in the case of a prosthesis that connected to the abutment via a second screw. The screw channel needs to be filled with a restorative material, and the occlusal contacts on the prosthesis must then be shifted away from this channel, meaning all occlusal forces become non-axial relative to the implant. These non-axial or cantilevering forces are considered to be destructive, and can result in screw loosening or fracture, and increased micro leakage.11 The loss of integrity due to the screw channel also increases risk of prosthetic fracture.7 It is interesting to note that cross-pin restorations do not have a screw channel, and do not sacrifice occlusion or aesthetics, but are still considered to be screw retained restorations. Given the description above, screwretained implant prostheses should be defined as prostheses where there is direct access to the prosthetic or abutment screw when inserted into the patient’s mouth. The screw channel, if present, may then be covered in any other restorative material. Examples of screw-retained restorations as defined above, and inferred by the literature, will be described below with illustrations. Clinical Identification and Retrieval of Cement-Retained vs Screw-Retained Prostheses From a clinical standpoint, cementretained implant prostheses can be distinguished by the absence of either a screw access channel covered in restorative material, or the absence of a cross pin screw. If the internal abutment screw becomes loosened or disengages completely from the fixture, retrieval techniques include attempted removal of crown (crown removers or completely cutting the crown off) or by cutting a hole through the crown, essentially converting it into a direct-tofixture restoration. Screw-retained restorations can be clinically identified by the presence of a screw-access channel or a cross-pin screw. Retrieval or re-tightening a loose screw is much simpler and involves removal of the restorative material covering the internal screw; or in the case of a cross-pin, direct access to the cross-pin screw is immediately obtainable from the palatal/lingual surface. Conclusions Cement-retained implant prostheses should be defined in the literature as prostheses that are cemented clinically in Figure 4 shows a customised base metal abutment (a) which is screwed via an abutment screw (as) to the fixture (f). There is a feldspathic ceramic layer (c) fired directly to the surface of the abutment, obviating the need for cement. There is a screw channel present to gain access to the abutment screw. Since the prosthesis and abutment are already joined before insertion to the fixture, this is also known as a direct-to-fixture restoration. Figure 6 shows a stock one piece titanium abutment (a) which has a conjoined screw. The abutment itself is screwed to the fixture (f). There is a porcelain-fused-to-metal crown (c) cast onto a metal coping (cop), and there is a screw channel present in the crown to gain access to a 2nd screw, the prosthetic screw (ps), that holds the crown and coping to the abutment. Figure 5 shows a stock titanium abutment (a) often referred to as a “Ti-base” which is screwed via an abutment screw (as) to the fixture (f). There is a full zirconia crown (c) pre-cemented with a layer of cement (cem) to the abutment, but there is a hole present to gain access to the screw channel. Despite the use of cement, this is still regarded as a screw-retained prosthesis. This is also a direct-tofixture restoration. Figure 7 shows a customised titanium abutment (a) which is screwed via an abutment screw (as) to the fixture (f). There is a porcelain-fused-to-metal crown (c) that is screwed to the abutment from the palatal surface using a cross-pin prosthetic screw (ps). The crown covers the screw access channel allowing occlusal forces to be directed through the axis of the fixture and improves aesthetics, but this is not regarded as a cement-retained prosthesis. the patient’s mouth, after which there is no other means of accessing the prosthetic or abutment screw without cutting the crown. Screw-retained implant prostheses should be defined as prostheses where there is direct access to the prosthetic or abutment screw when inserted into the patient’s mouth. The screw channel, if present, may then be covered in any other restorative material. u For a complete list of references email gapmagazines@gmail.com The Restorative Implantology course is run by Dr Alex Loh. Register online at http://edentistry.net.au/academy LINICAL
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