This paper will give an overview of the clinical and laboratory procedures of two types of implant restorative options, cement-retained and screw-retained. Furthermore, the advantages and disadvantages of screw-retained restorations are discussed in comparison with cemented-retained restorations, along with other manufacturing challenges in the laboratory.
The practice of implant-supported restorations has become a recognised and preferred approach to compensate for missing teeth. There are two different types of prosthetic restorations fixed on dental implants: Screw-retained and cement-retained restorations. The decision of technique is typically based on the clinician’s preference and according to Siamak (2019),some dentists may prefer screw-retained crowns and others cement-retained, it is clinically advantageous that both options are available to address the varying anatomical circumstances presented by each case. Retrievability is the main advantage of screw-retained restorations, making it a favourite amongst clinicians. A study done by Klineberg et al (2011), was that 39 articles on single-tooth implants were critiqued, and an interpretation was realised of potential complications that may occur over the lifetime of a prosthesis and the authors of this study favoured screw-retained because of its ease of retrievability, another study doing a systemic review comparing both cement and screw-retained by Sherif et al (2014) revealed that major failures at 0.71 screw-retained and 0.87 cement-retained failures per 100 years, this revealed that there is no significant difference between the two restorative options and shows that neither is a form of inferior care. Retrievability allows improved control on the health and hygiene of the implant components and assessment of the surrounding soft tissues, also, restorations can be straightforwardly repaired in case of porcelain fracture and cleaning of any calculus. In other cases when the screw access hole is visible on incisal edges, cusps of teeth or easier access to the posterior region of the oral cavity is needed, cement-retained restorations could be more practical. The decision between the clinician and laboratory of either a screw-retained or/versus a cement-retained crown is a challenging and comprehensive decision involving many points of consideration from a laboratory manufacturing and clinical perspective.
A screw-retained prosthesis has one of the foremost advantages, which is retrievability. Not only can the prosthesis be recovered from the oral cavity with ease, but no destruction follows from removal of the prosthesis. Retrievability also helps with regular cleaning of the components, removal of any calculus, assessment of the soft tissues around the implant(s) and if any alterations or additions need to be made by the laboratory. A cement-retained prosthesis can be problematic when trying to remove and may be recovered or more likely destroyed in the process. Jivraj (2018) stated that when the abutment screws of cement-retained restorations become loose, the crowns are not always predictably removed and often require the crown be cut off and removed. Clinicians should plan prostheses for the future. Should a complication arise requiring the removal of the prosthesis to address, a screw-retained prosthesis will simplify the management process according to Jivraj (2018). Screw-retained should be the preferred option for long-span prosthetic restorations due to easier maintenance as they have a higher risk of complications and a lot of the components can be re-used or altered in the laboratory.
Cement-retained restorations may cause peri-implantitis if excess cement is not removed thoroughly after fitting by the clinician, more common in restorations in the posterior region due to limited accessibility for removing that excess cement. Soft tissue around screw-retained restorations is healthier than cement-retained. Siamak (2019) stated that the recent clinical trend toward screw-retained restorations is likely being driven by practitioner concerns over residual cement and its potential to affect the health of peri-implant bone and soft tissue. A retrospective study with a 4year follow-up on 192 patients with cement-retained and screw-retained restorations by Jun-Yu et al (2018) revealed high implant survival rates in both groups, both showed comparable peri-implant conditions, with the present study showing no significant marginal bone loss. With the studies reviewed, it would be apparent that there is no difference between screw-retained and cement-retained regarding the health of soft tissue and per-implantitis.
Abutment height is an important factor for retention on cement-retained restorations along with surface area and degree of taper. (School of Dental Implants, 2017) At least 5mm of abutment height is needed to ensure the retention of a cement-retained restoration. Therefore, when the inter-arch space is limited (i.e. <4, mm), a screw-retained restoration may be indicated. A study was done by Fariba et al (2012) regarding retention of single cement-retained crowns on narrow and wide abutment platforms was that the minimum abutment height for NP and WP single cement-retained restorations were 3mm and 4mm, respectively. To help aid with retention on single and multiple cement-retained implant crowns, an anti-rotational notch can be incorporated in the design of the abutment. It assists in the placement of the cement-retained abutment as most anti-rotational notches are placed lingual and palatal. This also helps the technician locate the crown on top of the abutment more precisely during fabrication, which in turn would stop any overbuild of porcelain on contact points due to not being seated correctly on the working model. With advancements in dental CAD software, parameters are pre-set to ensure that abutment height does not go below the recommended manufacturer standards. The abutment height can sometimes be overlooked when conventionally creating a customised abutment or altering a stock abutment which could potentially cause retention issues based on the literature read.
Aesthetics in interproximal contour areas for screw-retained restorations can be limited due to the insertion angle of the implant abutment, particularly single implant crowns and can cause what is known as “black triangles” and can often be unavoidable during manufacturing, this can be aesthetically unpleasing to the patient and can potentially cause food to be trapped in these areas and failure of the implant/restoration. A recent study by Matsumuto et al (2018) suggested that following rehabilitation procedures after surgery that a provisional fixed prosthesis be created by the laboratory and during this phase, the clinician can add acrylic resin periodically to promote gingival conditioning and help close black triangles ready for the definitive restoration.
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During the fitting of a screw-retained prosthesis, the access hole(s) is masked out with a composite material which can sometimes not match the shade of the restorations. During manufacture in the laboratory, it is possible to create and plug from lithium disilicate to fit in and cover the screw access hole. Wadhwani et al (2011) stated that overcoming the restorative challenges (aesthetic, occlusal) of the screw access channel in a screw‐retained implant restoration is difficult. By fabricating a pressed ceramic over the metal crown and an aesthetic plug these challenges can be dealt with in a predictable manner.
Material choice is another factor to consider during fabrication, the emergence of the abutment can affect the aesthetics and often the opaque layer on metal abutments can show through, particularly at the margin if the metal is thin and close to the screw channel. The same can be said when trying to achieve suitable separation on a porcelain bonded to metal bridge often the opaque layer can show through also. This can be corrected by choosing a Zirconia based material which overall gives natural aesthetics. A study done by Abreu (2017) suggests that both solid and layered zirconia crowns have a strength and consistency that is comparable with traditional porcelain fused to metal restorations, yet they provide far superior aesthetics.
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The emergence profile of any prosthesis, either cement or screw-retained can often be a challenge during manufacturing when trying to blend in with surrounding natural structures, more so when restoring anterior single restorations, where the implant itself can be placed too subgingival due to horizontal bone deficiency. Pink porcelain can be added to either screw-retained implants or a customised abutment for a cement-retained prosthesis on top to create an emergence natural to those structures surrounding the proposed restoration to produce a more natural and pleasing prosthesis. A case study done by Kalman & MacIntosh (2013) stated that the use of pink porcelain through laboratory requisition is a simple option that optimizes aesthetics and masks compromised surgical outcomes. Pink porcelain materials not only blend with soft tissue but also maintain aesthetics over time.
Manufacturing a screw-retained prosthesis in the laboratory has its challenges, most often trying to achieve an aesthetically pleasing prosthesis around the screw hole access(s), especially if the implant is malpositioned. Depending on what implant system has been used, there are options available for angle correction on screw-retained abutments if screw access is compromised, allowing that degree of flexibility to achieve optimal aesthetics. A study done by Pigliacelli (2016) discovered that angle correction varies by implant manufacturer from 17–35 degrees. Angled abutments have a particularly designed screwdriver that has a thin shaft to allow the head of the driver to engage with the head of the screw, the screwdriver head is similar in design to a rose head burr, this allows the indentations to stay engaged with the hex in the head of the screw, even if the shaft of the driver is tilted back.
Cement-retained restorations are not dependant on the angulation of the implant but still have their challenges. If stock abutments are unsuitable due to the path of insertion and angulation, customised abutments can be manufactured instead, which can be precisely designed by using CAD/CAM to conform to the gingival contours of the implant location, which provides an optimal emergence profile, margins and soft-tissue support for the definitive restoration. Iverson Dental (2015) concluded after reviewing several studies that CAD/CAM custom abutments have demonstrated survival rates comparable to conventionally fabricated abutments and have several advantages over other fabrication methods, allowing the technique to be a viable option for manufacturing custom implant abutments.
Further manufacturing challenges
The use of CAD/CAM and additive manufacturing as part of the fabrication process of screw-retained and cement-retained restorations has become more widely available and has been adopted more in dental laboratories.
Traditional or conventional methods of manufacture were often time-consuming and had its list of problems. Fabrication of a customised abutment was often an alternative to a stock abutment which would have a separate crown or bridge made on top, these would be conventionally waxed up on top of a ‘cast on’ pre-made interface part by the chosen implant system being used, invested and then cast in a metal alloy. Miscasting and overheating of the alloy on to the interface part can potentially happen at any time due to equipment or human error which can affect the strength and suitability of the restoration being made on top. CAD/CAM and additive manufacturing has overcome these errors by milling customised abutments from a solid piece of CoCr, titanium or gold coloured alloy with none of these errors present.
The same can be discussed with screw-retained restorations where a pre-made ‘cast on’ interface part was waxed on top, invested and cast in a metal alloy also. Often with screw-retained parts, there is a ledge within the screw channel to seat the screw when torquing. During investing, bubbles within the investment can become trapped around this area and will be present as surface ‘bobbles’, or worse, the whole screw channel blocked with metal alloy due to air being trapped within this area, which can be problematic when trying to rectify or having to start again. CAD/CAM and additive manufacturing have overcome these errors by precision milling of screw-retained substructures.
This literature has given an insight into the clinical aspect of screw-retained and cement-retained implant crowns/restorations and shows their advantages and disadvantages during fabrication in the laboratory with successful outcomes challenged when both implant types are used. The most challenging aspect is getting natural aesthetics on an anterior implant(s) whether it be screw-retained or cement-retained. With software programs being available to assist in implant placement such as coDiagnosticX or NobelClinician they can ensure a precise and predictable definitive restoration is created. A study was done by Mora et al (2014) on software tools and surgical guides concluded that virtual implant planning and guided surgery should be the standard of care in complex dental situations, accuracy errors can be kept to minimal levels when sources of error are kept in mind during the planning of a case. The planning stages are crucial rather than the final stage. With the availability of implant placement software and the dental team working together, plans for the final restorations can be evaluated with a predictable outcome. According to Campbell et al (2019) to overcome the pros and cons of each system, newer implant systems have been developed to include techniques that increase the link between the implant and abutment, use larger abutment screws, and provide a geometric lock.
With each implant patient case there is always a solution to achieve that final dental prosthesis if used the appropriate way. With better prosthetic materials and digital solutions from patient to laboratory and or additive manufacturing this can be accomplished with ease. With sufficient treatment planning and the use of implant placement software and surgical guides, the implants should be placed in the ideal position, allowing the restorative dentist to make various clinical considerations and have the choice of either screw-retained or cement-retained restorations.
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