This article was published in The International Journal of Oral & Maxillofacial Implants, Vol 16, No 1 (January/February 2001).

Bernard T. Carr, DDS, maintains a private practice in Alexandria, Virginia, emphasizing gnathological, aesthetic, and implant prosthetics. He is a fellow of the American College of Dentists, an Active Member of the Academy of Osseointegration, and a board member of the Northern Virginia Implant Society.

When I draw up a treatment plan using endosseous implants, the following are the most important clinical and laboratory factors that influence my choice of an implant system. The initial need for different implant systems to be developed in implant dentistry arose from the loose screw syndrome with the original external hex design. The loosening of small gold cylinder screws, as well as the larger titanium screws used to secure gold cylinders and machined abutments to the implants, was a constant problem when the restoration lacked a passive-fitting prosthetic framework. This lack of a passive fit would cause microvibrations, which would loosen the screws. These vibrations were the result of built-in stress from the forced fitted framework returning to its original dimension. Obviously, there were far fewer problems with screws loosening in a passive-fitting prosthetic framework.

All of the original-design frameworks were screw-down implant prostheses. The need and desire to restore intact and stronger occlusal morphology with porcelain-fused-to-gold crowns led to the development of the custom-milled cast UCLA abutment for a cemented prosthesis. The advent of the 32 N/cm square drive gold screw, used to secure the custom abutment directly to the implant, as well as improved manufacturing tolerances, has practically eliminated the loose screw problem.

Therefore, designs of the other implant systems, although they may have higher and stronger engineering values, limit prosthetic versatility, both from a laboratory and clinical point of view, in restoring implants that are surgically or positionally compromised. The external hex system provides more versatility for the technician to solve problems with emergence profile and esthetics, since the technician is able to bring the porcelain of a porcelain-fused-to-gold crown closer to the implant interface.

If the use of screws was intended to prevent implant fracture in cases of excessive occlusal load, then the internal hex implant may be more prone to fracture because of its stronger design at the abutment/implant interface.

The MicroMiniplant, used to replace congenitally missing maxillary lateral incisors, has an internal hex interface. It is easier to engage the impression and/or final custom abutment in the internal hex design. However, if the implant platform level is not significantly subgingival, esthetic problems can arise at the abutment/implant interface. These are easier to correct with a custom UCLA abutment and the external hex design system.

Strong consideration should be given to a tapered root-form implant where the apical root forms of the natural roots adjacent to the implant site are too close to permit placement of a cylinder implant.

Personally, there seem to be no problems with the esthetics and longevity of the external hex design implant system, especially if the necessary density and volume of bone is available at the implant site. Clinically, I have had few problems with the external hex implant design and do not see the need for using another implant system.

David A. Dersh, DMD, maintains a full-time private practice in Westfield, New Jersey, emphasizing implant reconstruction and cosmetic and general dentistry, with special interest in salvage of failing implant situations. He is a Fellow in the International College of Dentists and the Academy of General Dentistry. Dr Dersh is on the medical-dental staff of Overlook Hospital and Rahway Hospital in New Jersey. He is a past president of the New Jersey Central Dental Society and has served at the state level as well. He has been active in the Academy of Osseointegration and has lectured on dental implants locally, nationally, and internationally.

The purpose of implant dentistry is to resolve the prosthetic concerns of the patient, both functional and esthetic. The years of experience that now exist in implant dentistry, both of clinicians and manufacturers, has yielded the result that osseointegration can be achieved in a highly predictable manner. These days, I rarely wonder whether implants will integrate. The rapid evolution of the various implant systems in the marketplace is driving these improvements, resulting in ease of use, simplification of techniques, and predictable results when used in a compromised clinical situation. Continuing trial and error will keep this process moving forward, as surely there is still room for improvement.

The basic factors that we all seek in any implant system are delineated in the results of the Toronto Symposium of April 24, 1998, which are:

1. The resultant implant does not prevent placement of the planned restorative prosthesis, either esthetically or functionally.
2. Implants are clinically immobile at uncovering and at any future time.
3. No pain or discomfort is attributable to the uncovered fixture or resultant prosthesis.
4. Bone height remains stable over the long term.1

When in harmony, these factors result in an environment that will lead to success.

However, one must not lose sight of the other clinical factors that also come into play in the selection of an implant system, especially conditions specific to a patient. These include bone quantity, quality, and contours in 3 dimensions. If bone and soft tissue are missing, is the region amenable to bone and/or soft tissue grafting for cosmetics or for implant placement? How does the bone relate to adjacent teeth remaining in that arch? How does the bone remaining relate to the opposing arch and the existing or needed occlusal scheme of the patient? The ravages of bone loss are a major factor that must be considered during treatment plan formulation, since the results of bone loss can alter the implant system needs and restorative needs that are to be supported by those implants. The patient’s general overall health comes into play as well: Is the patient a smoker, diabetic, osteoporotic, immunocompromised, or arthritic (which may limit maintenance abilities)?

Psychologic expectations must also be taken into consideration. What is the patient’s motivation, not only at present but also in the future, after rehabilitation is complete? Is the patient motivated to maintain the prosthesis and implants? Many of these patients have lost teeth and bone because of lack of motivation and must therefore be carefully evaluated for suitability during treatment planning. This final question is the most important to me as the restorative and maintenance dentist, because once a patient’s dentition is restored, I have the responsibility to maintain the prosthesis and occlusion of this patient forward in time.

It is at this point that selection of the system and restoration becomes critical. Will the final prosthesis be fixed, removable, porcelain, acrylic, cementable, screw-down, hybrid, etc? How easily can the patient maintain it now, and in the future, from their personal physical point of view? How stable will the occlusion be, and most important from my viewpoint, how easily can repairs be made to the prosthesis and implant/abutments? What can be done in the event of implant loss or prosthesis breakage or damage? Can the final prosthesis and/or the implants be re-used for a new prosthesis if necessary? During a repair or salvage procedure, do their hexes or locking mechanisms remain undisturbed, or are they easily damaged? Can the prosthesis be removed without damaging or bruising the implant or abutment components? Can the abutments be removed without damage to the implants or to the osseointegration? Can either implant or abutment be modified if necessary, without compromising osseointegration? As can be seen, I view as a primary concern for selection that of retrievability or salvageability, should it become necessary. In addition, minimalization of instruments needed for placement and restoration and disposable carriers are my preference. Mixing different systems within one patient generally creates havoc and should be avoided. My experience has shown that different systems need not be used in different regions of the mouth. Different implant styles within a system can be used in different regions, however.

With respect to laboratory factors, the most obvious criterion I seek is passive fit in multiple-unit restorations. This has been an issue since the beginning of osseointegrated implant dentistry, and I still have not seen a fully acceptable alternative. I am not going to debate screw-down versus cementable prostheses here, but implant systems should support both approaches, as well as hopefully other options in the future. The use of only a single approach is not appropriate, since screws, posts, cements, porcelain, and all other dental materials have been shown to fail.

Other laboratory concerns for the most part are to keep the technicians job to a minimum. Properly placed implants will accomplish this, and the need for custom abutments, underframes, or attachments should be minimized. If needed, however, the implant system should offer versatility in customization. I prefer machined interfaced components where feasible, and, obviously, biocompatible metals. The system should allow for prosthetic provisionalization for gradual loading when deemed necessary.

I would like to see further improvement in prosthetic components, since during the early years of osseointegration most advancements appeared to be in surgical technique and implant development. In addition, I seek a manufacturer that is supportive to the surgeon, prosthetic dentist, laboratory, and patient in terms of management and assistance where needed in continued innovation. Finally, the most important factor in selecting an implant system and prosthetic protocol is whether or not the patient’s well-being and quality of life will be improved both physically and psychologically by this treatment modality.

References

Wayne R. Harrison, DDS, graduated with honors from Loma Linda University School of Dentistry. For 15 years he maintained a private practice in Chula Vista, California, emphasizing cosmetic, reconstructive, and implant dentistry. Dr Harrison has lectured around the world on a variety of dental implant–related topics and has been a featured clinician at the Hinman Meeting, the Yankee Dental Congress, and the Dallas Mid-Winter Meeting. He is a member of several professional organizations, including fellowship status in the Academy of Osseointegration, and chairs the Ad Hoc Committee on Practice Management for the AO.

With the numerous dental implant systems and related restorative components available today, the final decision as to which system to use can be more than a little confusing to the clinician. In an evidence-based treatment era, any decision about what implant system to use should be based on science.

As a restorative dentist, I am concerned about the long-term success of the implant treatment for the patient. The investments of time and finances the patient has made demand predictable results surgically, functionally, and esthetically. My criteria for selection include but are not limited to the following:

• Long-term documented clinical survival rates
• Ease of use
• Restorative versatility and innovation
• Ongoing research and development

Long-term documented clinical survival rates

There is no question that the clinical success of an implant depends heavily on treatment planning, implant placement, design and fabrication of the final prosthesis, and the patient’s personal hygiene. It is imperative that the implant system selected has well-documented, long-term clinical data so that we can confidently assure the patient of predictable results.

In keeping with the aforementioned parameters, there are many choices of dental implant systems: straight-walled or tapered; threaded or cylindric; machined, coated, or enhanced titanium or titanium alloy; and external or internal connection. There is abundant clinical research demonstrating that surface coatings and surface enhancements improve the biologic effect in poor-quality bone. Additional issues to consider when selecting an implant system include the location of the implant, esthetic requirements (anterior or posterior), quality/quantity of bone and soft tissue, and the planned surgical protocol (immediate placement, 1-stage surgery, 2-stage surgery, or immediate loading).

For the completely edentulous mandible and partially edentulous situations, I generally select straight-walled, externally hexed, threaded implants. In clinical situations involving immediate implant placement (single-rooted teeth), my choice is a tapered, threaded implant (external hex or internal connection). Whether an external hex or an internal connection is used depends on the amount of bone and soft tissue available, the surgical protocol, and the abutment requirements. Laboratory concerns focus on the implant-abutment connection. There are 2 primary choices: either a segmented abutment (abutment, abutment screw, gold cylinder, and prosthetic screw) or a non-segmented abutment (direct to the implant). The final choice is based on the available interarch space, the desired emergence profile, and the esthetic demands of the patient. If segmented abutments are chosen, additional interarch space is obtained with an internal connection. If non-segmented abutments are used, additional interarch space is obtained with an external hex (assuming placement of the implant with the top of the hex coincident with the crest of the ridge).

Ease of use

Restorative components must be user-friendly. The philosophy of one screw, one driver is very important. The fewer parts the better, ie, less is more. The system chosen should have an accurate and simple implant-level impression or indexing system to facilitate abutment selection in the laboratory and outside of the clinical environment, where time is money. A closed-tray impression system is preferred over an open-tray impression system. Implant-level impressions provide our laboratory partners an opportunity to work with mounted soft tissue master casts to evaluate which abutment best achieves restorative goals. Abutments used in multi-unit, screw-retained restorations should be simple to deliver chairside, such as a system with no internal hex in the abutment.

Restorative versatility and innovation

The selected system should provide both screw-retained and cement-retained restorative options. The pendulum has moved from predominantly screw-retained to cement-retained restorations. We made this change in our practice because we could place implants in better anatomic positions for ideal emergence profile (especially in the anterior), avoid screw access holes, and create a more ideal occlusal relationship in the laboratory. Large screw access holes are predominantly found in the cusp-fossa area and require chairside restorative materials to close and create the ideal occlusal scheme. With cement-retained restorations, this can be accomplished by the laboratory technician. As a result of this paradigm shift, the system of choice must have clinically proven abutment screw technology with easily measured and applied torque.

Furthermore, it is important that the laboratory have castable abutments available that provide a premachined component to avoid the misfits and resultant microgaps created with plastic castable patterns.

Also, a ceramic abutment capability in the esthetic zone is a must in the chosen system. Patients who present with thin gingival tissue and/or with implants placed too coronally can ill afford the unsightly appearance of a titanium abutment.

Ongoing research and development

We live in a very now society, where patient demands and expecations are at an all-time high. To satisfy our patients, we need ongoing research and development of innovative products. We have to be ready, able, and willing to accept these changes and, further, be able to easily incorporate them into our practices. Implant surface enhancements are available today that provide more predictable results in poor-quality bone. Impression techniques that will decrease chair time and increase precision of fit (photogrammatic) will be available in the immediate future. Restorative components that can be customized for each patient (with CAD/CAM), thereby enhancing the esthetic results, are available today.

All this said, we can’t throw caution to the wind. Regardless of the advancements and innovations, we should remember that whatever dental implant system is selected, it must be based on science.

Richard P. Kinsel, DDS, is an Assistant Clinical Professor in the Department of Restorative Dentistry at the University of California, San Francisco, where he also serves as the Director of the Implant Dentistry Program, Buchanan Dental Center. In addition, he maintains a private practice in Foster City, California.

Undeniably, osseointegrated dental implants have profoundly altered prosthodontic treatment options for partially and fully edentulous patients. However, there are a myriad of dental implant manufacturers, each claiming significant advantage over the others. Therefore, prudent decision-making necessarily requires both a critical review of the relevant literature and clinical experience. Important considerations include long-term maintenance of the bone and soft tissue levels surrounding the implant, life table analysis, surface microtopography, reactions of the soft and hard structures to the implant, and the significance of the implant-abutment microgap and its relationship to biologic width. Prosthodontic concerns include the mechanical strength of the implant and reliability of prosthetic components, the ease of modification of both the implant and abutment for different restorative situations, cost-effectiveness of prosthetic fabrication, and reliability of the manufacturer.

For both oral endosseous and orthopedic implants, there appear to be significant biomechanical advantages with a roughened titanium surface versus a machined, smooth surface. Several animal studies and limited human studies have repeatedly shown significantly greater bone contact to a rough surface, without an intervening proteoglycan layer, resulting in greater resistance to mechanical removal forces. Although there is insufficient clinical research to quantify the minimal level of bone-to-implant contact necessary for long-term survival, intuitively, a greater percentage of bone contact would be an admirable goal. Several in vitro studies have shown that the specific microtopography of rough titanium surfaces can favorably influence such diverse elements of the osseointegration process as cell differentiation, rate of cell activity and growth, and stimulation of biochemical effectors in the process of cell differentiation into osteoblasts.

The supracrestal soft tissues around endosseous dental implants generally exhibit structures and features similar to gingiva around teeth. However, there are significant differences in the dimensions of both the soft tissue and crestal bone surrounding nonsubmerged 1-piece and submerged 2-piece implants. The 2-piece dental implant design that places the implant-abutment interface at the level of the crestal bone typically displays rapid but limited bone loss following surgical exposure to the oral environment and a long junctional epithelium that can proliferate to the alveolar crest.

Several studies have indicated that bacteria populate the internal surfaces of all dental implants, their restorative component parts, and the implant-abutment interface. This inevitable microleakage results in a zone of inflammatory cell infiltrate that is consistently present within the deeper peri-implant areas surrounding the 2-piece design. Although the 1-piece implant also has bacterial colonization of the microgap between the implant and the abutment, the surgical protocol and implant design place the microgap 2 to 3 mm coronal to the crestal bone. Therefore, any adverse affect upon the bone and deeper soft tissue attachments caused by microbial contamination would not be applicable.

The unavoidable microgap at the implant-abutment interface has led to the inevitable question: Does the concept of biologic width apply to dental implants? Substantial evidence in the literature strongly suggests that this is true. Perhaps the most definitive studies to date on this question were published by Cochran et al1 and Hermann et al,2 which consistently showed crestal bone loss of 2 mm apical to the microgap, regardless of the apicocoronal placement of the implant.

Therefore, the clinician should consider an implant design that has shown long-term survival without continued bone and soft tissue loss, a rough titanium surface in bone contact and a smooth surface adjacent to soft tissue, biologically stable tissue reactions, and an implant-abutment interface that is at least 2 mm coronal to the alveolar crest. Other practical concerns include life table analysis for the specific implant, surgical and restorative ease of use, adaptability to a variety of prosthetic demands, low incidence of post-delivery complications such as fractures or abutment and screw loosening, and the reliability of the implant manufacturer.

References