CATEGORY 84 AUSTRALASIAN DENTIST LINICAL Introduction The use of cement-retained or screwretained implant prostheses has long been debated in the literature. Articles have been published from more than 20 years ago1 till very recently2 discussing the advantages and disadvantages of either approach. Despite the numerous articles published, many fail to clearly define the terms screw-retained or cement-retained. The exact definition of either of these terms is missing from the articles3,4,5,6,7,8,9,10, or at best, is vague1. The aim of the first of this three part article is to clarify the meaning of these terms used in the literature, and then use this knowledge in the final part of this article to describe the solution to a clinical case involving a patient’s restorative complication when implant components are used incorrectly. The second part will discuss the concept of abutment indexing, when they are required, and how it relates to the solutions of the clinical case report. Lack of Definition Clarity for CementRetained and Screw-Retained Prostheses Implant abutments act as the connector between the fixture and the prosthesis. It is important to understand that all abutments, regardless of their intended connection to the prosthesis are always screwed down to the fixture through a screw channel, whether it be with a separate screw or, very infrequently, as a one piece abutment where the screw is “built in” to the abutment (Figure 2). Therefore, even prostheses that are cemented and have the intention of covering the screw channel, will still have an abutment with a screw fixation mechanism to the fixture. The presence of this abutment screw does not determine what is referred to as cementretained or screw-retained. To make definitions even more vague, prostheses can be cemented to the abutment not by the clinician in the patient’s mouth, but rather done so in the laboratory before being connected to the fixture. In these cases, there is a “hole” or channel cut into the prosthesis so that the pre-cemented prosthesis and abutment can be screwed down to the fixture in one piece. These are known as direct-to-fixture prostheses, and despite still using cement to fixate the prosthesis to the abutment, they are not considered “cement retained” by the literature. Definition and Examples of CementRetained Implant Supported Prostheses The fewarticles that doprovide somedegree of clarification of definitions essentially infer that cement retention is any implant restoration that is not only fixated to the abutment using dental cements, but also covers the screwchannel in order to achieve better occlusion and better aesthetics. The improved occlusion is due to the fact that load can be applied directly through the axis of the fixture. This is because the prosthesis covers the screw channel, and the screw channel will always be in line with the axis of the fixture. If this screw channel is not covered by the prosthesis, occlusal forces cannot be applied to the prosthesis through the axis, and this creates a cantilevering force which is considered to be destructive to the fixture, abutment and prosthesis complex.11 The improved aesthetics is a result of the absence of a visible screwchannel, which even if covered in opaque composite, rarely matches the colour of the prosthesis exactly.12 In these cement-retained prosthesis situations, it is important to understand that the abutment Clinical Case Report of Misused Implant Componentry: Part 1 – Clarification of Definitions regarding Screw-Retained and Cement-Retained Prostheses By Dr Alex Loh, BDSc(Hons) GradDipClinDent(Impl) MClinDent(Orth) FRACDS FICCDE FIADFE Figure 1 shows a stock (or pre-fabricated) titanium abutment (a) which is screwed via an abutment screw (as) to the fixture (f). There is a full zirconia crown (c) covering the screw channel cemented with a layer of cement (cem) to the abutment. Figure 3 shows a customised titanium abutment (a) which is screwed via an abutment screw (as) to the fixture (f). There is a full zirconia crown (c) covering the screw channel cemented with a layer of cement (cem) to the abutment. Figure 2 shows a stock one piece titanium abutment which has conjoined screw (a). The abutment itself is screwed into the fixture (f) with a full zirconia crown (c) cemented with a layer of cement (cem) to the abutment. There is no screw channel to cover since the abutment is a one piece abutment.
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