Year : 2018 | Volume
: 8 | Issue : 1 | Page : 9--19
Screw versus cemented implant restorations: The decision-making process
Herman Ostrow School of Dentistry of USC, Los Angeles, California, USA
Dr. Saj Jivraj
Anacapa Dental Art Institute, 2821, North Ventura Road, Oxnard, CA 93036
Improved skills, techniques and materials, case selections and proved designs have witnessed higher rates of success and survivals of osteointegration with dental implants. But prosthesis failures especially mechanical types have been constantly reported. These prostheses have been secured to the integrated implants with cement or screws. Controversies are rife in literature regarding the choice of retention. This review article provides an overview of the various clinical aspects, abutment designs and materials and procedures used, which contribute to selection of type of retention whether screw or cement and highlighting their clinical significance. The clinician can then use these information for optimizing the retention in an given clinical condition and minimize the risk of complications.
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Jivraj S. Screw versus cemented implant restorations: The decision-making process.J Dent Implant 2018;8:9-19
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Jivraj S. Screw versus cemented implant restorations: The decision-making process. J Dent Implant [serial online] 2018 [cited 2019 Nov 14 ];8:9-19
Available from: http://www.jdionline.org/text.asp?2018/8/1/9/237588
Implant-supported prosthetic reconstructions can either be screw retained or cement retained to the implant fixtures or both. The choice for screw or cement retention appears largely to depend on the clinician's personal preference rather than on scientific basis. Many factors have been presented in the literature recently to advocate for both methods. While there are many advantages to either approach, there are also inherent risks and drawbacks that can negatively affect the long-term success of the entire prosthesis.
Some authors advocate the advantages of cemented restorations being versatility in esthetics, passivity of fit, and easier control of occlusion.,,,,, However, the possibility of leaving excess cement in the peri-implant tissues is a major drawback., Conversely, clinicians report that screw-retained restorations are advantageous for their retrievability, making the evaluation of oral hygiene easier and maintenance procedures simpler., However, reports of screw loosening can be considered a problem for both the patient and the practitioner., The purpose of this article is to review the recent literature on screw- and cement-retained implant restorations in an attempt to clarify when a clinician may choose to use one retention system versus the other. Clinical significance of the review is the main focus.
With patients demanding more esthetic restorations today, clinicians are continually seeking the most biomimetic techniques or materials. It is well documented in the literature that clinicians believe cement-retained restorations are more esthetic.,,,,, This thought arises solely from the lack of a visible screw access hole. However, selecting a cement-retained restoration exclusively based on esthetic outcome is unfounded; the esthetic outcome has little to do with the method of retention to the implant. Rather, esthetics is multifactorial and depends on patient selection, tissue volume, tissue type, and implant position. The trajectory of the implant will only determine the type of retention method, whether it be cement or screw retained. For anterior restorations, the use of pre-angled abutments, angulated screw channels (ASCs), or dynamic abutments can redirect a screw access opening to the cingulum area where it is not visible. For posterior restorations, several esthetic techniques exist to blend the screw access hole with the restoration utilizing a silicone plug and resin opaque  or a pressed ceramic plug.
Esthetic success is not dependent on the use of a screw- or cement-retained restoration. Both can be used to achieve the same esthetic result.
Some clinicians prefer cement-retained restorations over screw retained due to instability of occlusal contacts around the screw access channel., Hebel and Gajjar  stated that the size of occlusal access is determined by the retaining screw diameter which, for larger implants (for example, in the posterior regions of the mouth), obliterate a large portion of the occlusal table. In addition, the screw access hole is often in the central fossa of the crown where centric contact should be. Since the access hole can often involve up to 50%–60% of the occlusal table, Vigolo et al. argued that the opposing occlusion is usually developed with the head of a retaining screw or on composite restorative material instead in the screw hole. Chee and Jivraj  stated the importance of building opposing contacts on the restoration itself versus the screw access filling material. In their opinion, no untoward wear or instability to the occlusal contacts occurs provided that the screw access filling material is not needed to maintain the occlusion.
In regard to an optimal and stable occlusion with the opposing dentition, screw-retained restorations can achieve the same predictable result as cemented restorations provided that the occlusion resides on the crown and not on the screw channel filling material.
Several in vitro studies have shown that the screw access channel in metal-ceramic crowns weakened the surrounding porcelain resulting in a higher incidence of porcelain fracture.,, This is also seen clinically, as reported in a study by Nissan et al. where 221 implants were followed up over 15 years; ceramic fracture occurred at a statistically significantly higher rate in screw-retained (38%) than in cement-retained (4%) metal-ceramic restorations. The researchers proposed that the opening cuts off the structural continuity of the porcelain and represents a locus minoris resistentiae, or an area of less resistance to fracture. In contrast, in cemented restorations, the integrity of the metal-ceramic bond is not interrupted by any screw access channel and occlusal forces can be evenly distributed throughout the bulk of the crown material. Utilization of a screw access channel modifies the position of the center of mass of the ceramic bulk and occlusal forces must be redirected to the peripheries of occlusal table. The result is often local failure of the metal-ceramic bond and detachment of the porcelain. Interestingly, Sailer et al. performed a systematic review of 59 clinical studies and found that chipping of the veneering ceramic tended to occur more frequently with the screw-retained restorations for single crowns. Yet for full-arch restorations, ceramic chipping was observed more frequently with cement-retained prostheses.
This led others to question whether the screw access channel is the primary cause for weakened porcelain and increased fracture incidence. In their 10-year randomized controlled trial, Vigolo et al. reported a lack of prosthetic complications related to porcelain fracture in their screw-retained metal-ceramic restorations. Although only a small sample size of thirty implants was used, they ensured accurate evaluation of the occlusal scheme and provided appropriate variations to the occlusal contacts (both static and dynamic) to reduce technical failure rates. Similar results have been found in a recent in vitro study using newer materials (zirconia and lithium disilicate). The researchers found no significant differences in fracture rates between the crowns with or without an occlusal screw access channel.
Despite reports that the screw access channel reduces the strength of screw-retained metal-ceramic restorations resulting in porcelain fracture, careful evaluation of the occlusal scheme and appropriate adjustments to the static and dynamic occlusion can reduce the incidence of screw-retained porcelain fracture comparable to that reported with cement-retained crowns.
Interocclusal Space and Retention
When using cement-retained implant restorations, many of the same principles used for conventional fixed prosthodontics for natural teeth can be applied. Sufficient implant abutment dimensions (principally the height) are required for adequate retention of the crown. In situ ations where minimal interocclusal space exists or the implant is severely malaligned, it may not be possible to achieve adequate retention to retain the restoration with cement. However, fixture-level screw-retained restorations can be restored predictably with as little as 4 mm of space from the implant fixture to the occluding surface of the opposing dentition. In some situations, this can avoid preprosthetic surgery and invasive restorative treatment in an attempt to gain more restorative space for the implants.
In situations with limited interocclusal space or malaligned implants, cement-retained implant restorations are contraindicated; a screw-retained prosthesis should be chosen instead.
Provisionalization and Gingival Molding
Gingival molding is a critical step during the healing period to achieve satisfactory esthetics with natural-looking tissue profile around implant crowns. A provisional restoration is required for this process as healing abutments do not have the proper size or emergence profile that the restoration needs. Chee and Jivraj  reported that a screw-retained provisional restoration can be used with ease to incrementally expand the peri-implant tissues until fully seated. In addition, following implant surgeries where the provisional restoration is to be placed immediately, a screw-retained option is the preferred method; it is difficult to manage the bleeding and cement a restoration in a clean environment for ideal tissue health. Another advantage of a screw-retained provisional restoration is that it can be used as a pick-up type impression coping. A soft tissue cast is poured around the exposed provisional after an impression coping is attached, yielding a soft tissue cast which is identical to the soft tissue form intraorally. This provides the laboratory with a model of an exact replica of the emergence profile that should be transferred to the definitive restoration.
A screw-retained provisional has several advantages over cemented provisionals in achieving ideal tissue contours, maintaining tissue health, and transferring the soft tissue profile in an impression for laboratory communication.
In a screw-retained prosthesis, torque that is applied to a screw forces the mating screw threads together until the shaft of the screw begins to elongate. This produces a clamping force within the system known as preload. Not all the torque that is applied to the screw is converted into preload. Slight discrepancies between the two mating components will create frictional and misfit resistance within the screw. The screw bends and deforms to compensate for the strain at the interface and results in a lower clamping force. Ultimately, a lower clamping force will result leading to future screw loosening or fatigue fracture.
Passivity refers to a state of existing without resistance, and when applied to implant prosthetics, it translates into a lack of any misfit forces being generated within the prosthetic system. It is technically very difficult to achieve a completely passive framework in a screw-retained prosthesis. Most of the frameworks being used currently may not be totally passive, yet are functioning normally. Passivity is listed as an advantage of a cement-retained prosthesis.,,,,, Individual abutments are screwed onto the implants and the superstructure is cemented overtop. The cement layer (typically about 40 μm) compensates for unintended dimensional discrepancies between the abutment and the restoration effectively acting as a buffer space.
Cement-retained prostheses are listed as inherently more passive due to the cement space buffer. Passivity is technically more challenging to achieve a screw-retained prosthesis as any dimensional discrepancy can translate into a static load and misfit stress on the screw which could result in more screw-related complications (loosening or fatigue fracture).
Technical Complications (Screw Related)
Implant screw joints are susceptible to technical complications due to the oral forces and strength limitations of the components themselves. The literature contains many reports regarding screw loosening and screw fractures.,,,, In a systematic review that evaluated the 5-year survival rates of screw-retained implant-supported single crowns, abutment screw loosening was reported at 12.7%. Other studies have shown similar screw loosening rates of 5.8% and 6.7%. Screw fractures appear to be less common in screw-retained prostheses occurring at 1.5% in one study and 3.9% in another study. Sailer et al. reported that over a 5-year period, single-crown abutment screw loosening occurred more commonly in screw-retained restorations while screw fracture occurred more often in cement-retained restorations. For full-arch restorations, abutment screw loosening was more common in screw-retained prostheses. No screw fractures were observed in the cement-retained full-arch prostheses.
As described in the previous section, screw-retained restorations are susceptible to implant-abutment misfit discrepancies which lowers the amount of clamping force a screw is able to generate. Lower than normal buccolingual off-axis, loading forces can then induce screw loosening. It has been observed that screw fractures occur primarily at the first occlusal screw thread., Freitas et al. proposed that the failure occurs in this region because bending forces are concentrated the most at this level of the screw.
Screw retightening has been reported to be necessary, especially within the 1st year of functional loading. The goal is to reestablish the optimum preload stress on the abutment screw to counter the possibility of screw loosening. This should be performed with caution though as Yilmaz et al. found that second torques applied to the screws can rotationally displace abutments as much as 9 μm. A rotation >5° can cause a reduction of 63% in screw joint stability. If a patient presents with a loosened abutment screw, Assenza et al. stress the importance of replacing the screw entirely instead of simply retightening the existing one. The old screw has most likely undergone deformation changes due to misfit stress and a re-torqued screw may experience fatigue fracture if the same preload stress is applied.
The majority of technical failures in the past were blamed on the inaccuracy of fitting components which allowed for micromovement. With the introduction of more precise abutments and screws which improve the abutment-to-implant fit, fewer technical complications are being seen with both screw- and cement-retained crowns., In fact, a recent systematic review reports that they found no statistically significant difference between cement- and screw-retained restorations for technical outcomes.
Screw loosening and screw fracture are the most common technical complications seen in both screw- and cement-retained restorations. Current evidence shows no statistical difference for the frequency of technical complications among the groups. A loose screw must be replaced before retightening the abutment.
The biology of an implant in oral tissue is very different than a natural tooth. The periodontium of a healthy tooth has supracrestal collagen fibers (Sharpey's fibers) that insert into the cementum to hold the soft connective tissues to the tooth. The barrier of collagen limits bacterial ingress and resists damage from physical trauma. The arrangement of the fibers results in compartmentalization that localizes disease and limits its spread. Implants do not have inserting Sharpey's fibers. Instead, they have circumferential fibers that sling around the implant and attach through hemidesmosomes (a weak attachment mechanism). This creates a single “compartment” so any disease will affect the entire implant. Bacterial infection is a major factor leading to bone loss and implant failure in healthy individuals. Any subgingival irregularity (such as calculus or residual cement) may assist in the microbial colonization of implants and lead to peri-implant disease.
Recent studies have demonstrated that peri-implant tissues around screw-retained restorations present with fewer biological complications.,,In vitro studies demonstrate that cement-retained prostheses luted to titanium abutments with simulated margins have been shown to leave a surprisingly large quantity of cement remnants., Clinically, Wilson  found an 81% correlation between excess cement left in the peri-implant tissue and the occurrence of sulcular bleeding or suppuration. Weber et al. found that after 6 and 12 months of implant loading, cement-retained crowns consistently demonstrated a higher degree of sulcular bleeding and plaque accumulation than screw-retained crowns.
When restorations are luted to the implant abutment, extruded cement has enough hydraulic pressure to tear the delicate tissues surrounding the implant instead of being deflected out. Even on smooth surface implants, Agar et al. demonstrated that it was not possible to completely remove a resin cement. With the surfaces of newer implants purposefully roughened for better healing, cement is expected to have even greater adherence and cleaning becomes significantly more challenging. This is of even greater importance in an immediate loading situation where there is often space between the wall of the extraction site and implant body where cement could flow.
Some researchers have found the opposite, with more soft tissue complications around screw-retained prostheses. However, the problem was associated only with screw-retained single crowns that had loose abutment screws creating a microgap. The inflammation healed soon after the retightening of the screws once the gap was closed. When properly interfaced, the gap distance between the machine-surfaced implant and abutment is superior to any cement margin that can be developed. A screw-retained interface will allow less bacterial penetration and less peri-implant complications will be seen.
A screw-retained restoration demonstrates fewer biologic complications compared to cement-retained prostheses. Cement extrusion and retention in the peri-implant tissues can result in microbial colonization and soft/hard tissue damage.
Overall Complications, Retrievability, and Long-Term Treatment Planning
As discussed previously, both screw- and cement-retained restorations suffer from technical (screw loosening, screw fracture, and porcelain fracture) and biologic (peri-implant infections, swellings, recession, and bone loss) complications. A systematic review of 59 clinical studies reports that, for single crowns, significantly more technical complications occurred with screw-retained prostheses. For full-arch restorations, the opposite trend was seen toward fewer technical complications with screw-retained restorations. Regarding biological complications for single crowns, screw-retained restorations were found to have a higher incidence rate overall, except for recession and bone loss which occurred more with cemented crowns. A similar trend was seen in full-arch restorations where bone loss was more commonly observed in cement-retained prostheses.
Regardless of the technical or biologic complications that may occur, implant-supported prostheses may need to be removed for necessary restoration servicing, oral hygiene maintenance, or surgical treatment; the major benefit of the screw-retained restoration is that it permits for this retrievability with ease. Instead of treating implants such as natural teeth and definitively cementing restorations, clinicians should take the extra care to plan for future retrieval with a screw in the event it is required. When abutment screws of cement-retained restorations loosen, the crowns are not always predictably removed and often require the crown be cut off and removed. Once the screw is retightened, the restoration must then be refabricated.
When treatment planning with implants in partially edentulous patients, the clinician should always assess the prognosis of the remaining teeth, especially for teeth adjacent to proposed implants. In the case of teeth with a poor or questionable prognosis next to implants, the implant restoration should always be designed with the eventual loss of those teeth in mind. An implant restoration can always be modified and used to support new pontics in the future, should the teeth require extraction. However, it takes diligent and strategic planning of metal framework designs (to allow for soldering). Recycling and addition to an existing prosthesis is simplified with screw-retained restorations as they can be removed intact. Cement-retained restorations are often extensively damaged beyond repair during the removal process.
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.
Cement-Retained Prosthesis Potpourri
The cement-retained implant-supported prosthesis was initially introduced for esthetic reasons and to compensate for loosening problems that were encountered with screw-retained restorations. Perhaps the reason why they have become so widely used is that they have more in common with fixed prosthodontics of natural teeth, and as such have a wider appeal to practitioners of any experience level. This section will discuss several important aspects of cement-retained restorations.
When a cemented abutment is considered, it is imperative that the cement margin is shaped to maintain a relationship with the mucosal margin. A custom abutment facilitates the formation of anatomical gingival topography with a natural emergence profile and proper spatial design at the cervical margin. Stock abutments with deep subgingival margins (especially interproximally) should be avoided as they would make it difficult to remove cement residues., In the anterior, however, a deep cementation margin position often cannot be avoided for esthetic purposes. In the posterior region, however, the cement margin can easily be placed paramarginally without esthetic compromise. Despite the use of a custom abutment, the absence of residual excess cement cannot be guaranteed., Wadhwani et al. found that modifying the abutment with vent holes can statistically significantly decrease the amount of extruded cement from 90% (with a standard abutment) to 36% (with an internal vents abutment).
A customized abutment that follows gingival contour is preferred over a stock abutment but does not guarantee the absence of residual cement. Vent holes may decrease the amount of extruded cement.
Type of Cement
There is a lack of consensus regarding the type of luting cement to use for cement-retained implant prostheses. Cements are usually chosen arbitrarily, usually because the clinician is familiar with them for cementation procedures to natural teeth. Research has shown that some cement types were more likely to cause remnants in the peri-implant tissues and increase the risk of biological complications., Ideally, the luting agent should be strong enough to retain the restorations, but weak enough so that the restorations can be removed easily if required.
One study evaluated glass ionomer (Ketac-Cem), resin (Panavia-21), and zinc phosphate (Fleck's) cements. According to the researchers, the excess resin cement was most difficult to remove. This is problematic as resin cements promote substantially higher bacterial biofilm growth compared to other cements. In an in vitro study, Behr et al. found that zinc phosphate cements leave powdery remnants, which remain even after cement removal. Zinc oxide-eugenol (TempBond) was found to be easiest to remove and to have the cleanest surface after removal.
In a retrospective clinical study, a methacrylate cement that was used (premier implant cement) was found to cause complications in many of the patients in the form of suppuration around the implants at follow-up. The crowns and abutments were removed, and in all cases, excess cement was found between the abutment and the peri-implant tissue. After recementing with TempBond, all signs of suppuration and inflammation disappeared in all patients within 3–4 weeks. TempBond is a zinc oxide-eugenol cement that has antibacterial effects. TempBond also has a tendency to dissolve upon contact with fluid, so possibly this characteristic of the cement causes less excess cement in the peri-implant sulcus with fewer complications in the long term. If the clinician believes that the metal-to-metal bond with TempBond will be too strong, it can be weakened by mixing it with petroleum jelly. Methacrylate cements have very low viscosities and can flow quite easily into the peri-implant tissues. They have also been shown to promote biofilm formation. These characteristics make methacrylate cements a poor choice for luting to implants.
Polycarboxylate cements (for example, Duralon) contain fluoride and are a chemical known to etch titanium when used in an acidic environment, resulting in pitting corrosion on both turned and milled surfaces as well as the formation of reactive oxidative species that cause inflammation., Polycarboxylate cements should not be used for luting to implants.
No clear recommendations exist as to which type of cement is best for use for various scenarios. It is suggested that if the clinician is most concerned with retrievability and lowest amount of cement remnants, TempBond is the clear choice., However, the crown material must also be considered, as lithium disilicate requires a permanent resin cement for proper luting.
Cement application techniques appear to be used arbitrarily with little understanding by clinicians regarding how or where to apply the cement. Many studies report phrases such as “the cement was loaded into the crown,” but advice is not given as to how this procedure was performed. In addition, protocols for amount of cement to use are lacking. Wadhwani et al. found that for the same crown form to be cemented, clinicians used cement weights ranging from 3.2 mg to 506.4 mg. A formula  was created to determine actual amount of cement needed (assuming a 40 μm cement space) and was calculated to be 13.6 mg, or 3% of the crown's total volume. Clearly, some crowns were underfilled and many were overfilled resulting in cement extrusion. To prevent extrusion into sulcular peri-implant tissues, Wadhwani  suggests performing a preextrusion step extraorally before cementing. Excess cement is removed extraorally from the crown using a custom copy abutment, then cemented intraorally.
Using a computational fluid dynamics model, Wadhwani et al. were able to demonstrate that the ideal location to place cement was a small bead circumferentially around the crown margin for ideal cement coverage. Other techniques (circumferential placement at the occlusal third, brush-on application, and gross fill) demonstrated poorer results in comparison. They also found that seating the crown slowly was more ideal as rapid seating would cause too rapid a flow due to the shear thinning properties of the cement and leave an incomplete sealing of the margin. Abutment modification with internal vent holes resulted in less air trapping and less cement extrusion.
Following cementation of the restoration, Ramer et al. emphasized the importance of follow-up appointments. A recommendation of 1–2 weeks, 1 month, 3 months, and 6 months postoperative checks should be performed. The early follow-up visit is recommended to collect baseline data which will serve as a reference at future appointments. Early detection of cement remnants and successful treatment can prevent disease progression.
Cement application should be minimal and localized to the circumference of the crown margin only. A preextrusion procedure can be performed to remove excess cement extraorally, and then the crown should be seated slowly onto the abutment. The strict follow-up protocol should be adhered to reduce the risk of complications.
Residual Excess Cement
Residual excess cement is one of the most common problems of cement-retained implants, found on up to 81% of subgingival spaces in one study. During cementation, hydraulic pressure builds up and the cement flows to the direction of least resistance beyond the crown margin to the gingival sulcus. The weak peri-implant tissues are less resistant to this pressure and the extruded cement continues to flow subgingivally. Numerous clinical studies report incidences of excess cement left in patients,,,,], which is suspected as being a primary cause of peri-implant disease.,,,,,,, In fact, the American Academy of Periodontology includes residual implant cement as a risk factor for peri-implant disease (peri-mucositis and peri-implantitis). Korsch et al. found that larger implant diameters were more significantly associated with the presence of excess cement and are therefore at risk for peri-implant disease. However, this is more likely related to the fact that larger diameter implants are used in the posterior regions of the mouth which are more difficult to access for cleaning and cement removal.
Wadhwani  provides an excellent summary of the multifactorial causes of peri-implant disease encompassing (i) a microbiological origin, (ii) the host response, (iii) an allergic response, and/or (iv) alteration in implant surface. Although the role of cement in the development of peri-implantitis is still controversial, excess cement in the subgingival spaces can be described as an artificial calculus, thus altering the implant surface to become more rough and creating a niche for bacteria. If the patient's host response is already active from preexisting periodontal disease, three of the four risk factors are present. Systematic literature reviews have shown that 8.6%–14.4% of restored cement-retained implants are prone to develop peri-implantitis within 5 years. Initial signs of peri-implant disease can occur as early as 4 months (early peri-mucositis) or as late as 9 years (delayed peri-implantitis) after cementing the restoration.,,, This time frame can be accelerated in patients with a history of active periodontitis.
If a patient presents with signs of peri-implant disease, the clinician must first confirm a diagnosis before treating., In most cases, dentists are unable to evaluate and detect excess cement that is subgingival.,,, Even radiographs are poor diagnostic aids for evaluation of residual cement because its detection depends largely on the type and thickness of the cement used.,, The radiographic material varies directly with the extent that it can be visualized on a radiograph. Zinc-containing cements are radio-opaque (zinc phosphate and zinc oxide-eugenol) but are often not thick enough to be diagnostic. Out of 53 restorations that had residual excess cement, Linkevicius et al. could only detect cement remnants radiographically in ten cases. The only reliable method to diagnose residual-excess-cement-induced peri-implant disease is to remove the crown and unscrew the abutment, which is paradoxical in that the crown must be destroyed to accurately inspect the tissues.
Once an initial diagnosis is confirmed, generally, a nonsurgical treatment regimen should precede any surgical interventions. This would include mechanical debridement, irrigation with 0.12% chlorhexidine gluconate solution, and replacement of the restoration with TempBond. Titanium scalers are better overall at cleaning cement from abutment surfaces compared to resin scalers. However, they tend to produce more scratches on the smooth abutment surfaces. Zirconia abutments were found to be less prone to scratching by titanium scalers and easier to clean.,
Healing (reduction of peri-implant disease signs) typically occurs within 3–4 weeks after cement removal.,,
Residual excess cement in the peri-implant tissue is a risk factor developing peri-implant disease, with symptoms developing either early (after a few months) or late (after a few years). Diagnosis is most accurate by removing the crown and abutment; use of radiography is a poor diagnostic aid. Nonsurgical debridement should be attempted first with titanium scalers to remove the cement remnants and assess for healing before considering surgical treatment. Healing can be expected after 3–4 weeks.
Retrieval of Cemented Restorations
Retrieval of cemented implant restorations is often more difficult than from natural teeth. Even despite the use of a provisional cement, it may function more like a definitive luting agent between a metal–metal interface. If the crown must be removed due to a loosened abutment screw, any force applied to the restoration on a loosened abutment has the potential for damaging the internal threads of the implant and it often becomes safer to simply cut the crown off.
Several techniques have been proposed to increase the ease of retrievability of cement-retained implant crowns. A lingual access channel extending through the crown and into the abutment near the cervical crown-abutment interface is made. The crown is then cemented and the channel is filled with resin to serve as a locking mechanism. If the removal is warranted, the resin is simply removed and an ultrasonic device or crown remover is used to lift the crown. A drilling template can be fabricated based on an angulation analysis from a radiograph as a way to more accurately locate the screw access hole. Unique surface stains can be applied on the occlusal surface at the location of the screw access channel to identify its location. Finally, a combination implant crown utilizes principles from both a screw- and cement-retained prosthesis. The definitive crown is cemented to the abutment extraorally where excess is easily removed. Then, the cemented unit can be screwed onto the implant intraorally through a screw access channel which is later closed with composite resin.
Several techniques exist to aid in the removal of a cement-retained implant crowns. Despite their use, removing a cement-retained crown is still arguably more challenging and less predictable than a screw-retained restoration.
Screw-Retained Prosthesis Potpourri: New Developments
Ideally, implants should be placed parallel to one another and be aligned vertically with axial forces. However, anatomical constraints can sometimes prevent the clinician from placing implants at ideal angulations to allow for a screw-retained prosthesis. Historically, restoring with a cemented restoration, performing surgical correction (for example, bone grafting) or relocating the implant position/trajectory used to be the only options., Several prosthetic components have been introduced that can compensate for and redirect the angulation of the prosthesis into a more favorable position to restore. This section will discuss pre-angled abutments, dynamic abutments, and ASCs as alternatives for cement-retained prostheses on unfavorably angulated implants. At the time of writing, the latter two components are quite new and currently have very few peer reviewed articles.
Pre-angled abutments can redirect screw access openings to the occlusal or cingulum areas of implant restorations through the use of two off-axis screws: one screw to secure the abutment into the implant and the second screw at an angle to secure the crown into the bulk of the abutment structure. However, to provide sufficient abutment structure to house a retention screw for the restoration, the long axis of the implant and path of retention screw must diverge significantly. At minimum, Nobel Biocare provides a screw-retained pre-angled abutment with a divergence of 17°. A 30° component is also available for more severely angled implants.
Some research suggests that off-axis loading induces excessive lateral stress onto the supporting implants, supporting bone, and the prosthesis which could trigger complications. While this concept is true, Clelland et al. and Celletti et al. found that the principal strains were considered to be within the physiologic zone for bone and the restorative materials. Goodacre et al. found a higher proportion of abutment screw loosening in angled abutments; however, the screws were reported to be titanium. The incidence of screw loosening dropped considerably once they were replaced with gold abutment screws (which can produce higher preloads when torqued). It is suggested that occlusion is minimized on inclines of the restoration, and if possible, avoid lateral excursions on teeth with pre-angled abutments.
There is a general consensus that pre-angled screw-retained abutments produce no additional clinically evident physiological disadvantages over traditional abutments. From a biomechanical point of view, the pre-angled abutment may be a suitable restorative option for implants with a nonideal angulation.,,,
In 2004, a biomechanically novel abutment design was introduced: The dynamic abutment (Talladium International Implantology). This allows for 360° variability in abutment angulation up to 28° off-axis. Unlike the pre-angled abutment which has thick labial margins and requires more apical placement to hide the components, the benefit here is that the labial dimensions are quite thin so implant placement does not have to occur further apically. The abutment consists of a base with an occlusal semi-sphere, on which sits a burnout chimney that has 28° of rotation freedom. The angle is chosen and set prior to casting the abutment. Once fabricated, the abutment is secured using a screw with a uniquely fluted screw head and a screwdriver containing a sphere head with 1.3 mm hexagonal facets.
Preliminary research from the Herman Ostrow School of Dentistry by Lee et al. showed that deviation of the restoration from the implant axis did not have any significant influence on the screw removal torque values and fracture resistance of the screw after 5 years of simulated loading compared to a standard 3i Biomet gold screw.
Dynamic abutments may be the preferred option over pre-angled abutments as the components are thinner and less intrusive on the peri-implant tissues. A custom angle can be selected anywhere from 0° to 28°.
Angulated Screw Channel
Most recently, Nobel Biocare launched a variation on the dynamic abutment: The ASC abutment. This system offers a much simpler alternative to the cement-retained prosthesis in the restoration of tilted implants. The technician can “bend” the screw access channel into a more favorable position up to 25° with 360° of variability. The abutment is secured using an Omnigrip driver with rounded facets to engage the screw head. The head itself has multi-fluted reliefs cut into the walls to provide space for maximum angulation of the driver. With this design, torque can be applied consistently between 0° and 25° of angulation.
The use of the ASC abutment appears extremely promising, but additional research should be performed first to assess complication rates.
Screw- and cement-retained implant prostheses both have advantages and disadvantages. A review of the literature shows that neither method can be used in every clinical situation, so it remains up to the clinician to make the most evidence-based decision as to which retention method will be most effective. Major risk factors to consider are porcelain-related failures (chipping or fracturing), screw loosening or fracture, and peri-implant disease secondary to residual excess cement in the peri-implant tissues. Retrievability should also be at the forefront of decision-making to be able to easily manage technical or biological complications. ASCs and dynamic abutments are making screw-retained restorations more versatile, especially in the anterior esthetic zone.
For single-crown restorations, screw-retained prostheses experienced more technical complications whereas cement-retained prostheses experienced more biological complications. The clinician must decide what he/she is willing to maintain more during follow-up visits. Full-arch implant restorations should be screw retained. They are more easily retrieved for management of technical complications and also seem preferable from a biological perspective.
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