INTRODUCTION
As clinicians, we must often make decisions on the restoration of our dental implant design. In years past, screw-retained prostheses were the norm, often with custom-prepared screws or screw-retained infrastructures. Issues with past implant designs caused some problems with this type of crown. Screws could loosen, causing stress to the practitioner. Design corrections were made with advent of the UCLA abutment,1,2 which would engage the implant body and allow for either a screw-retained or cement-on crown. Since most implants today are internally designed with some type of internal stabilizing connection (such as a hex, tri-lobes, or morse tapers), loosening of abutments and screws has been dramatically reduced as long as the screws are properly torqued to the manufacturer’s requirements.
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| Dr. Kosinski, educator with the Engel Institute Implant Training Program. |
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| Dr. Kosinski coaching dentists on the surgical and prosthetic design. |
As screw-retained crowns were replaced by screw-retained abutments and cement-on crowns, other issues began to arise. Most problems with cement-on crowns were the result of abutments loosening under a cemented-on implant-retained crown, resulting in the need to remove the crown to retorque the abutment or drill an access opening through the crown to tighten the abutment screw.
Another issue becoming popular in the literature is the periodontal bone loss around implants restored with cemented-retained crowns. Depending on the type of cement used, such as silicone cements, glass ionomers, and resin cements, if the practitioner does not completely remove the excess subgingival cement, the body’s natural response is to lose bone around the necks of the implants.3 Obviously, this problem can be minimized with supragingival margins or margins placed at, or very near, the gingival crest level. The problem is serious enough that screw-retained prostheses have again become popular. So, it may be appropriate to review when screw-retained crowns and cement-on implant-retained crowns are to be used.
Choosing Between Screw-Retained and Cement-Retained Crowns
As previously mentioned, nonresorbable cements that are not thoroughly removed subgingivally can result in peri-implantitis, which is inflammation of the soft tissue around the implant that can result in bone loss.4 Obviously, eliminating the cement with a screw-retained crown will eliminate this potential problem. As abutments are torqued into place in a cement-on crown situation, any issues with mobility of the abutment throughout time can be stressful, depending on the type of cement used to retain the crown. Using a soft or semi-flexible temporary cement (such as Temp-Bond/Temp-Bond Clear [Kerr], Retrieve [Parkell], Improv [Alvelogro], TempSpan [Pentron], or Telio CS Link [Ivoclar Vivadent]) allows for retrievability of the crown from the abutment; however, this can also be an issue if the crown comes off during use by the patient. Any type of porcelain fracture or movement of teeth resulting in open contacts can be difficult to repair if a permanent cement is used. Screw-retained crowns can be removed and corrected if problems arise.
Also, it is imperative that the clinician evaluate the circumstances prior to any surgical intervention and implant placement. What does that mean? The interocclusal distance must be evaluated. An abutment for a cemented-on crown must have a minimal height of 5.0 mm, from the prepared margin to the top of the abutment, to have adequate retention. More height is even better, but with any less than 5.0 mm, the cemented crown may not be retentive long-term and may possibly result in several recementations, so a screw-retained crown is indicated.
Placement of the implant must be made ideal when considering a screw-retained crown. This means palatal in the anterior maxilla, in the cingulum area, and ideally in the center of a posterior tooth, down the long axis of the crown. Angulation issues with our implants can often be corrected more easily with custom prepared abutments and cement-on crowns.
The clinician must be aware of the advantages and disadvantages of having a screw hole in the occlusal surface of posterior surface, or lingual surface in anterior teeth, of a new crown. Also, we must observe the emergence of the screw in a screw-retained crown. Of course, a screw cannot come out of the facial aspect of the crown in the aesthetic zone. The patient must be made aware that the screw access may show a bit. Our best composites do a nice job in filling the access hole, but for some patients, this may not be aesthetic enough.
Finally, screw-retained implant-retained crowns can end up being slightly more expensive than preparing stock abutments and fabricating a basically normal crown over the abutment. Custom abutments, when circumstances warrant, can be more expensive to the practitioner.
Indications for cement-on implant-retained crowns over prepared abutments and those indicated for screw-retained prostheses will be demonstrated with the following 5 clinical cases.
CASE REPORTS
Cases 1 and 2 describe the use of screw-retained implant crowns. With the use of either CT scanning and diagnosis or evaluation of conventional hard models made from conventional impression techniques, the dental laboratory team can create the implant position, interocclusal distance from the opposing arch, and interproximal distances from the adjacent natural teeth.
Case 1
In this case, our dental laboratory team created a custom contoured healing abutment and transitional crown using digital design (Figure 1). The Glidewell tapered dental implant with internal hex design (Glidewell Laboratories) is nicely positioned in the center of the edentulous ridge in the mandibular second bicuspid area using standard surgical protocol. The implant was torqued to 35 Ncm and the custom-contoured abutment was immediately positioned into the implant (Figure 2). The concept is to create ideal tissue contours for the delivery of the final implant-retained crown. The transitional crown was cemented onto the prepared abutment with a temporary cement (Temp-Bond). The margins of the crown were placed at the soft-tissue level, allowing for easy cleanup of the excess cement, thus eliminating any potential periodontal issues from the very beginning. After approximately 3 months of integration of the implant, the cemented transitional crown was removed, showing the incredibly healthy tissue response to the digitally designed transitional abutment. Healthy tissue contours were created by using a custom-fabricated abutment that allowed for ideal emergence profile in the final implant-retained crown (Figure 3). As can be seen by the transitional abutment, the interocclusal distance was limited. Following the guideline of having at least 5.0 mm of abutment height for predictable retention, it was determined that a more effective long-term restoration would be a screw-retained monolithic zirconia crown (BruxZir [Glidewell Laboratories]) (Figure 4). The implant crown was passively seated into the implant (Figure 5), and them proximal and occlusal contacts were checked. A torque wrench was used to completely seat the retaining screw to 25 Ncm. Ideal emergence profile was established using the screw-retained implant crown (Figure 6), and a digital periapical radiograph was taken to ensure the complete seat of the prosthesis (Figure 7).
| Case 1. Mandibular Second Bicuspid | |||||||||||||
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Case 2
Maxillary second molar implant replacement areas should be evaluated carefully for interocclusal distance. A short abutment and cement-on crown in this area can prove to be disastrous due to inadequate retention with a high potential for crowns to become uncemented. Case 2 demonstrates the use of a screw-retained implant crown to replace the missing maxillary left second molar. The dental laboratory team created a soft-tissue model with a screw-retained crown due to the limited interocclusal distance (Figure 8). A prepared abutment would have been much too short to have successfully retained a cement-on crown. Figure 9 shows the stock healing abutment in place approximately 3 months after implant placement. After the healing abutment was removed, good tissue health around the tapered dental implant (Glidewell Laboratories) could be observed. The implant crown was then positioned and torqued intraorally (Figure 10). The aesthetic and functional maxillary second molar exhibited nice emergence profile. The final periapical radiograph (Figure 11) indicated a complete seating of the screw-retained implant crown.
| Case 2. Maxillary Left Second Molar Screw-Retained Implant Crown | ||||||||
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Case 3
Even with the increased popularity of screw-retained implant crowns, in my practice, I tend to use more cement-on crowns over prepared stock or custom abutments. The important issues to remember are that abutment margins should be just slightly subgingival, if using a titanium material, but can be at the tissue level when using tooth-colored monolithic zirconia material. Being aware of margin design with your abutment can make the final result aesthetic and periodontally sound. Using stock abutments may hinder your ability to manage margin position, so considerations for custom abutments should be made and considered when treatment planning and determining proper fees. Figure 12 illustrates a healing abutment placed in the mandibular right first molar area. The implant and tissue were allowed to heal for about 3 months. After removal of the healing abutment, there was good tissue health around the Glidewell implant. The implant itself was approximately 2.0 mm subgingival (facial aspect). Because there is plenty of interocclusal space, a prepared titanium abutment was seated into position and torqued to 25 Ncm. The interocclusal distance allowed for a tall and retentive prepared custom abutment for a cemented crown (Figure 13). The abutment, ideally contoured and in the center of the ridge, allowed for occlusal forces to be maintained down the long axis of the implant (Figure 14). The implant-retained crown was cemented to place and the occlusion checked. I cement implant-retained crowns with a temporary cement (Temp-Bond). These cements are strong enough to hold the crown in place as long as the abutment is properly formed and yet, if any problems were to arise down the road, such as abutment loosening, they could be more easily handled. Figure 15 shows the final radiograph of the cemented crown. No excess cement was left behind because the subgingival margin, at about 1.0 mm, was not placed deeply.
| Case 3. Cement-On Implant-Retained Mandibular First Molar Crown | ||||||||
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| Dr. Kosinski reviewing risks, benefits, and alternatives with the doctors and patient. |
Case 4
Dental implants in certain circumstances can also be used to maintain bridges. This case illustrates the use of an maxillary posterior, implant-retained, cemented fixed partial denture (FPD). Once the healing abutments were removed, healthy tissue was observed around the Glidewell dental implants (Figure 16). A soft-tissue model showed that the margins of the abutments were only slightly subgingival, allowing for emergence profile, proper aesthetics, and easy maintenance of any cement (Figure 17). With the Glidewell implant system, seating stents are provided (Figure 18). The seating stent allows for proper placement of the prepared custom abutments. A torque wrench was used to properly seat the prepared abutments to 25 Ncm. The abutments were positioned for a cemented 3-unit, implant-retained FPD (Figures 19 and 20). The laboratory team was able to ideally contour the abutments for proper draw and emergence profile. A radiograph was taken to ensure complete and proper seating of the prosthesis (Figure 21).
| Case 4. Cement-On, Implant-Retained Maxillary Posterior Bridge | ||||||||||
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Case 5
In this, the final case presented, the preoperative radiograph of a non-restorable mandibular right second molar is shown (Figure 22). After approximately 4 months of healing, a Glidewell implant was surgically placed and torqued to 35 Ncm. Upon removal of the healing abutment after an additional 3 months, the tissue contours were good and the tissue was healthy (Figure 23). The interocclusal distance allowed for a long and retentive prepared custom abutment to be torqued into position to 25 Ncm (Figure 24), and the monolithic zirconia crown (BruxZir) was cemented into place (Figure 25). The final periapical radiograph was taken to verify proper seating and demonstrated no excess cement remained (Figure 26).
| Case 5. Cement-On Mandibular Second Molar | ||||||||||
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CLOSING COMMENTS
Preparation for the surgery of dental implants must be predicated by an understanding of the final prosthetic reconstruction. Visualizing the case finished before, along with any needed surgical intervention, is important if we keep in mind that dental implants are prosthetically driven. Mounted diagnostic casts are important in helping the clinician and laboratory team determine interocclusal space. Discussions about aesthetic requirements/expectations with our patients should be done prior to the placement of any implants. The benefits and risks of cement-on implant-retained crowns versus screw-retained crowns must be clearly understood.
With the advent of new polycrystalline dental materials (such as monolithic zirconia and monolithic lithium disilicate (IPS e.max [Ivoclar Vivadent]), clinicians can deliver aesthetic and strong restorations in the posterior regions, nearly eliminating the risks of fracture as experienced historically using traditional porcelains. These new high-strength, all-ceramic materials (used for abutments, crowns, and FPDs) can also improve the aesthetics around the access opening of the screw-retained crown. Ideal placement of implants so that occlusal forces are maintained down the long axis of the implant can be determined by proper planning. The newest techniques for designing the proper form and function for an implant-retained crown make decisions that much easier. CT diagnostics can allow for determining the ideal position, type, and size of implant to be utilized prior to any surgery. The final crown can even be designed before ever bringing a surgical bur to the patient’s mouth.5
We all want what is best of our patients. With proper clinical training and an experienced and skilled laboratory team, the modern materials and techniques that are now available allow clinicians to provide patients with the best in implant and restorative services.
References
- Wu T, Liao W, Dai N, et al. Design of a custom angled abutment for dental implants using computer-aided design and nonlinear finite element analysis. J Biomech. 2010;43:1941-1946.
- Lewis SG, Llamas D, Avera S. The UCLA abutment: a four-year review. J Prosthet Dent. 1992;67:509-515.
- Sheets JL, Wilcox C, Wilwerding T. Cement selection for cement-retained crown technique with dental implants. J Prosthodont. 2008;17:92-96.
- Wilson TG Jr. The positive relationship between excess cement and peri-implant disease: a prospective clinical endoscopic study. J Periodontol. 2009;80:1388-1392.
- Ganz SD. Restoratively driven implant dentistry utilizing advanced software and CBCT: realistic abutments and virtual teeth. Dent Today. 2008;27:122-127.
Dr. Kosinski received his DDS from the University of Detroit Mercy Dental School and his mastership in biochemistry from Wayne State University School of Medicine. He is a Diplomate of the American Board of Oral Implantology/Implant Dentistry, the International Congress of Oral Implantologists, and the American Society of Osseointegration. He is a Fellow of the American Academy of Implant Dentistry and received his Mastership in the AGD. He has received many honors, including Fellowship in the American and International Colleges of Dentists and the Academy of Dentistry International. He is a member of OKU and the Pierre Fauchard Academy. He is currently an adjunct clinical professor at the University of Detroit Mercy School of Dentistry, serves on the editorial review board of REALITY and the Michigan Dental Association Journal, and became the editor of the Michigan AGD, of which he is a past president. He has published more than 150 articles on the surgical and prosthetic phases of implant dentistry and was a contributor to the textbooks Principles and Practices of Implant Dentistry and 2010’s Dental Implantation and Technology. He was the University of Detroit Mercy School of Dentistry Alumni Association’s “Alumnus of the Year,” and in 2009 and 2014 received the AGD’s Lifelong Learning and Service Recognition Award. He was featured on Nobel Biocare’s Nobelvision and lectures extensively. He can be reached at (248) 646-8651 or drkosin@aol.com.
Disclosure: Dr. Kosinski reports no disclosures.
