The screw-retained crown has both benefits and liabilities. The main advantage is retrievability. The crown is not only recoverable, but no damage occurs upon removal of the crown. In the event of loosening or fracture, the crown can easily be removed. Cleaning, screw replacement and assessment of surrounding tissue is also possible. Many dental professionals consider a yearly cleaning and replacement of screws a prudent approach. The longer the span, the more important salvaging becomes. Most dental professionals believe a long restorative span, cantilever or full arch dictates screw-retained crowns. To recover a crown or change a screw for maintenance, the restoration is removed, the cotton pellet is removed and the screw is accessed. Once repairs and/or alterations are concluded, the screw is torqued, a new cotton pellet is placed, and composite or acrylic is used to seal the opening.
The screw-retained crown has both benefits and liabilities. The main advantage is retrievability.
For cement-retained crowns, retrievability is not a major drawback. Cemented crowns may be recovered if the correct cement is used. Adding a water soluble gel to the cement may ease rescue of a crown. Nevertheless, while the screw-retained crown is certainly retrievable, removing a cement-retained crown can be a questionable undertaking if strong cements are used.
Pros and Cons of Cement
Cement-retained implant-borne restorations offer several advantages, including the elimination of unesthetic screw access holes and greater resistance to porcelain fracture. Standard crown & bridge procedures can also be used in most situations. However, excess cement left behind inadvertently is a major problem and can result in soft tissue damage, bone loss and/or chronic inflammation.1 In a 2006 study by Weber, et al., soft tissue surrounding screw-retained implant crowns was found to be healthier than soft tissue surrounding cemented restorations.2 To help address this issue, custom abutments can now be designed with supragingival margins that allow for easy and complete cement removal.
Custom abutments can now be designed with supragingival margins that allow for easy and complete cement removal.
Retention and Resistance
Factors affecting the retention and resistance of cement-retained crowns on natural tooth abutments can also be applied to cemented crowns on implant abutments. Abutment height, degree of taper, and surface area and roughness all affect the retention and resistance of a cemented crown.
The ideal taper of opposing surfaces in natural tooth preparations is reported to be six degrees.3 Parallelism of implants has a direct effect on the taper of their abutments. Implants that are not parallel may require further preparation and tapering of their abutments to enable an ideal path of insertion of the prosthesis. Overtapered abutments may lack adequate retention for the cemented restoration and may necessitate a screw-retained prosthesis. To counter this and to increase retention for a cemented crown, an irregular abutment surface and/or stronger cement may be indicated.
Abutment height is an important factor for proper retention and resistance of cemented crowns. Longer abutment walls will have more surface area and will therefore be more retentive. The length of the abutment wall also plays an important role in the resistance to tipping forces. To prevent tipping forces from dislodging the cemented restoration, the length of the abutment wall must be great enough to interfere with the arc of the casting, pivoting about a point on the margin on the opposite side of the restoration.4 Custom abutments offer an excellent option to correct the line of draw and provide appropriate resistance and retention.
At least 5 mm of abutment height is needed for proper retention and resistance of cement-retained crowns.5 Therefore, screw-retained crowns are required in situations when limited interarch space dictates an abutment that would be shorter than 5 mm.
The primary advantage of a screw-retained superstructure is the lower profile retention of the abutment system.6 These low-profile abutments offer a significant advantage for bar-retained overdenture applications. The lower height of the screw-retained bar offers greater room for denture teeth and greater thickness of acrylic, which is needed for strength of the restoration.
Reduced Stress to Bone and Implant
In implantology, reduced stress to the bone and implant is a desired feature. This is obtained through a passive fit of the prosthesis on the implant abutments. A passive fit is more difficult, if not impossible, to attain for a screw-retained implant restoration with more than one implant. Distortion of impression material, dental stone, wax patterns and metal castings are all contributing factors to this problem. A passive fit is easier to accomplish in cemented restorations due to die spacers. The die spacer creates an approximately 40 μm cement space, which compensates for laboratory distortions and permits a more passive casting. If the laboratory is utilizing CAD/CAM technology, the cement space can be adjusted based on the substructure.
Esthetics is another factor to consider when deciding between screw-retained and cement-retained crowns. In anterior screw-retained crowns, the implant is placed lingually to allow screw emergence through the cingulum area. The restoration is cantilevered facially from the implant body, which results in offset loading of the implant. Lingual implant placement also results in a porcelain ridge lap, which compromises hygiene. An implant for an anterior cemented restoration is placed under the incisal edge. An angulated abutment is then used, which eliminates the ridge lap and replicates a more natural emergence profile.
In posterior screw-retained restorations, the access hole will exit through the central fossa of the prosthetic tooth. This is not only a cosmetic compromise but an occlusal one. The cementable crown obviously has no entrance cavity. Allowing the forces of occlusion to be distributed along the axial inclination, congruent with the long axis of the tooth, is easier.
Esthetics is another factor to consider when deciding between screw-retained and cement-retained crowns.
Screw apertures may interfere with mutually protected occlusion — that is, centric occlusion, acquired occlusion, and lateral and protrusive movements. The screw opening may account for more than 50 percent of the crown surface. New modalities to combat the dilemma of the screw-entry opening being attractive include the use of lateral set screws and pre-angled abutments. Gold plating of the inner aspects of the crown greatly helps with esthetics. All-ceramic screw-retained crowns such as monolithic BruxZir® Solid Zirconia and monolithic IPS e.max® lithium disilicate (Ivoclar Vivadent; Amherst, N.Y.) eliminate the challenge of masking underlying discoloration from showing through the occlusal access opening once it is sealed.
Screw-retained restorations are associated with more complications than cement-retained restorations. Fractures of the occlusal materials of implant restorations occur more commonly than in natural teeth due to a lack of periodontal ligament stress relief and higher impact forces. Porcelain fracture in screw-retained restorations is more prevalent due to unsupported material around the screw access hole. Implant restorations receive cyclical loading due to the nature of chewing and, consequently, screw-retained restorations experience screw loosening and fatigue fractures of their prosthetic screws. The frequency of screw loosening is reported to be between 10 percent and 65 percent.7 This mainly occurs in posterior areas, with single restorations and in cantilever situations. Using a ratchet wrench to the recommended torque has greatly diminished this prosthetic complication.
Cost may be a factor to consider as well. The cemented implant crown costs considerably less because of lower laboratory fees and fewer components. Fewer and shorter appointments are needed to restore a cement-retained crown, which is more cost effective for the prosthetic dentist. Temporary crowns with cementable implants are far easier to fabricate than screw-retained provisionals.
Accessibility is another concern. Restoring a screw-retained restoration in a patient with a limited opening and/or in the posterior of the mouth can be challenging. The implant abutment connection must line up with the interproximal contacts to allow seating of the one-piece restoration. A cement-retained crown may be easier to deliver in these situations.
The success and long-term survival of endosteal dental implants is largely dependent upon the forces applied to them and the resultant stress transmitted to the implant-bone interface. Avoiding even the slightest movement of the abutment relative to the implant is the overriding concern for cement-retained crowns and screw-retained crowns. Most implant architects have improved abutment screws, torque procedures, implant size and implant tables, and have enhanced the match of the implant to the abutment. This has allowed for less movement and greater stability in both screw-retained and cemented crowns. Machined abutments with matching copings have reduced much of the guesswork in implant prosthetics. With these improvements, prosthetic and esthetic considerations are the main determinants as to which crown is utilized.
Implant treatment plans should consider approaches that eliminate biomechanical overload to the bone supporting the implant-borne prosthesis. Once a proper treatment plan is formulated, creating a harmonious occlusion in the prosthesis is an important factor in further controlling excessive forces.8
Uses for screw-retained implant crowns.
Uses for cement-retained implant crowns.
Applying Occlusal Principles
In general, occlusal principles applied to the restoration of natural dentition should also be applied to implant prosthetics for partially edentulous cases. Modifying an occlusion to establish centric relation coincident with centric occlusion, restoring canine guidance or group function, or establishing mutually protected articulation with anterior guidance are all important guidelines regardless of the type of implant restoration. Screw-retained dentures should follow standard denture principles with bilateral balanced occlusion.
“Implant-protective occlusion” … principles are applied regardless of whether the restoration is cement- or screw-retained.
“Implant-protective occlusion” refers to a set of occlusal principles that are specific to implant-supported prostheses.9 These principles are applied regardless of whether the restoration is cement- or screw-retained. They also follow a philosophy of “design to the weakest arch,” in which rigid implant restorations opposing removable prostheses have occlusal schemes that favor the tissues supporting the removable prosthesis.
Important considerations in the implant-protective occlusal scheme are as follows:
- Elimination of premature contacts
- Timing of occlusal contacts
- Surface area over which the occlusal forces are applied
- Implant angle to occlusal load
- Cuspal inclination
- Implant crown contour
- Crown height
- Occlusal contact position
- Occlusal material
One must be especially aware of evaluating all restorative patients for parafunctional habits. Although ideal occlusal schemes may exist in the restoration, excessive forces created by parafunction can over-load supporting bone around implants and result in failure. If they exist, alterations in the treatment plan will be needed to compensate for these excessive forces, and an appliance to control the noxious habit is recommended.
Case 1 — Replacement of a mandibular right second premolar
A 4.0 mm x 13 mm NobelSpeedy™ (Nobel Biocare; Yorba Linda, Calif.) implant was placed in a long-standing edentulous area. The implant was placed directly over the ridge of bone.
Before the restorative procedures were started, a six-month therapeutic interval elapsed. A lab-fabricated, custom conical abutment with a 20-degree taper was used to allow for a cemented implant crown (Fig. 1). Interocclusal space measured 6 mm. The custom abutment was torqued into place and a custom temporary was created. A porcelain-fused-to-gold crown was fabricated with proper contours to allow for healthy soft tissue and adequate oral hygiene (Figs. 2–5).
A cemented implant crown restoration was used in this case to maximize esthetics. It also allowed for porcelain occlusal contacts within the central fossa, which will not wear over time, and eliminated the risk of porcelain fracture due to unsupported porcelain around a screw access hole. Using the principles of implant-protected occlusion, the crown was adjusted to have light centric contact along the long axis of the implant, with the patient in the clenched position. This took into account compression of the natural dentition within the alveolar sockets. There were no contacts on the buccal cusp tips, which would create off-axis loading of the implant and place it under unfavorable shearing forces. The crown was cemented with temporary cement, allowing for retrievability.
Case 2 — Mandibular screw-retained full-arch FP3 prosthesis
Five 4.0 mm x 13 mm BIOMET 3i NanoTite™ Tapered Certain® implants (BIOMET 3i OSSEOTITE; Warsaw, Ind.) were placed directly over the ridge of bone anterior to the right and left mental foramina. A single 5.0 mm x 10 mm BIOMET 3i NanoTite Tapered Certain implant was placed in each edentulous area of teeth #19 and #30.
A healing period of four months was allowed before restorative procedures were started. BIOMET 3i Certain conical abutments were fastened to each implant, at sizes corresponding to the implant diameter and tissue height. Plastic nonhexed castable cylinders were incorporated into the wax pattern for the multi-unit porcelain fused to metal framework. The framework was cast and its passive fit was confirmed in the mouth using the single screw test. Tooth-colored and pink porcelain were then applied to the framework to reproduce the teeth and gingival tissues. BIOMET 3i Gold-Tite® retaining screws were placed through the framework and threaded into the abutments at 10 Ncm. Cotton pellets were placed over the screw heads, and the access holes were sealed with composite resin (Figs. 1–3).
A fixed implant prosthesis was chosen to restore this case, primarily because the patient did not want a removable prosthesis. A screw-retained design was chosen. The implants do not emerge through the clinical crowns due to resorption of the alveolar ridge post extractions. The metal framework required a low profile in this area so as not to interfere with tongue movement. The shorter profile of the conical abutments allowed for the reduced height of the metal framework in this area, but also necessitated screw fixation of the framework for adequate retention.
There are advantages and disadvantages to using a screw-retained versus a cement-retained crown.1,2,10 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.2,11 These advances have decreased the incidence of screw loosening.12 Many dental professionals would conclude that cement-retained crowns are finer for esthetics and occlusion; similarly, many would conclude that screw-retained crowns are a necessity for multiple units requiring retrievability.13,14 Individual philosophy plays a huge role, however, and deciding which crown to use is best done on a case-by-case basis.15
The aforementioned case studies are representative of the decision-making process when choosing a cement- or screw-retained implant prosthesis. The first case could have been restored either way successfully, but the ultimate decision for a cement-retained crown was made due to the patient’s desire to have the most esthetic crown possible. It was imperative that the second case be screw-retained for adequate retention and ease of retrievability.