Current Topics in Dentistry
Author: Charles English, DDS/Oded Bahat, BDS, MSD/Burton Langer, DMD, MSD/Cherilyn G. Sheets, DDS
What are the clinical limitations of wide-diameter (4 mm or greater) root-form endosseous implants?
This article was published in International Journal of Oral & Maxillofacial Implants, Vol 15, No 2 (March/April 2000) .
I. Charles English, DDS
Charles English, DDS is a Diplomate of the American Board of Prosthodontists, an active member of the Academy of Osseointegration and the International College of Oral Implantologists, and was the 1993 recipient of the Ralph V. McKinney Jr Award for outstanding authorship and education in implant dentistry. Dr English has experience with more than 12 different implant systems. He is now Vice President for Education and Technical Services for Green Dental Laboratories in Heber Springs, Arkansas. He also serves as Continuing Education Director for the Alliance Dental Institute, a new clinical teaching facility for all aspects of restorative dentistry.
Restorative problems can occur with the misuse of wide platforms in certain clinical situations. I consider 3.0- to 3.5-mm platforms as narrow, 4.0- to 4.5-mm as medium, and 5.0- to 6.0-mm as wide. The restorative issues involve the abutment platform connection and are irrelevant to the outside diameter of the implant threads. Many posterior fixed partial prostheses would be better fabricated with premolar anatomy and occlusion with 2 medium-platform designs, rather than a 5.0- or 6.0-mm implant that is placed in the center of the partially edentulous space. Fourteen millimeters of interproximal clearance are required for the placement of two 4.l-mm platforms for premolar designs. Otherwise, consideration has to be given to proximal stripping with abrasives to gain 1 to 2 mm of needed space, orthodontic distalization of the tipped and migrated molar, or aggressive preparation of the distal abutment with a corrected proximal contour on the mesial surface. Distal-extension surgical placement of 2 implants can be challenging, as the handpiece can engage the tooth, preventing exact positioning of the implant for occlusion, proximal contouring, and esthetic results. Close enough is not justified from a restorative standpoint; nor is it fair to the patient or the referring restorative dentist.
Wide platforms can give a false sense of security to the surgeon, as they may not be optimal for single-tooth stability in molar positions. Although the wide platform contributes to biomechanical stability of the restoration, an optimal abutment antirotation mechanism is needed. Some external hex designs are marginally adequate and not mechanically suited for long-term stability and strength (ie, resistance to rotational forces and parafunctional loads). Newer designs for antirotation with the wide platforms provide superior stability.
The 4.3- to 5.0-mm-wide platforms are quite adequate for most molar sites. Six-millimeter platforms are only used in probably 10% of the patients needing this treatment and usually require recent extraction sites or osseous grafting. There is abuse of the 6.0-mm-wide implant in immediate placement treatment for extracted maxillary central incisors. The problem with this treatment is the overcontoured emergence profile and bulk that is seen in relationship to adjacent teeth. One should consider extraction of the maxillary central incisor, necessary augmentation, healing, and placement of a 4.3- to 5.0-mm platform for enhanced esthetic results.
In summary, 4.3- to 5.0-mm-wide platforms will satisfy most clinical situations involving maxillary central incisors, premolars, and molars. Strong consideration should be given to multiple implants in the partially dentate posterior region, as opposed to a single-wide implant with excessively extended proximal contacts, inadequate proximal tissue, and the resulting food entrapment that can occur. Cantilever rotational forces place the abutment connection at risk for loosening and potential fracture. In the posterior, think premolar, and place one 4.1- to 5.0-mm platform for each tooth position! In the anterior, caution is recommended with the esthetic zone and with maxillary central incisor replacement; proper platform and abutment connection is critical for esthetics and biomechanical stability.
II. Oded Bahat, BDS, MSD
Oded Bahat, BDS, MSD, is a Diplomate of the American Board of Periodontology and maintains a private practice in periodontics in Beverly Hills, California. He is the former Director and Chairman of Postdoctoral Periodontics at the University of Southern California School of Dentistry. In 1994 he was elected a Fellow of the American College of Dentistry.
The initial indications for osseointegrated implants have been expanded to areas of greater need, compromised bone, and greater occlusal forces. Wide-diameter implants provide greater support and better biomechanical characteristics for the final restorations, and so may be desirable for such sites. Clinically, the following parameters will influence the ultimate likelihood of success:
A minimum of 1.5 mm of bone, cortical and cancellous, is needed circumferentially throughout the length of the implant. Lack of cancellous bone may reduce the healing potential after the initial osteotomy and result in advanced bone loss and possible implant loss.
The greater diameter requires the clinician to change the spatial view of the osteotomy site as it relates to the ridge and adjacent structures.
Ridge concavities become more critical, as they reduce the available width and may result in early dehiscence and fenestrations.
The integration of implants is often determined by the initial dimensions and precision of the osteotomies. The wider the implant, the greater the number of steps required to establish the sites. These repeated surgical steps could result in overt instrumentation, directional changes, and loss of primary stability.
The successful use of wide-diameter implants will require changes in treatment planning and surgical protocols. Optimal results can be achieved with proper surgical execution.
III. Burton Langer, DMD, MSD
Burton Langer, DMD, MSD, is a Diplomate of the American Board of Periodontology and is former Associate Clinical Professor of Periodontics and Director of Postgraduate Clinical Periodontics at Columbia School of Dental and Oral Surgery. Dr Langer was one of the first periodontists trained in osseointegration by Professor Per-Ingvar Brånemark in 1983. He has lectured extensively throughout the world and was the 1997 recipient of the American Academy of Periodontology Master Clinician Award. Dr Langer maintains a private practice in New York City.
Wide-diameter implants originated early in the development of osseointegration. Their original purpose was primarily as a rescue implant for conditions in which the standard 3.75-mm-diameter implant could not be stabilized in soft bone. As osseointegration developed in scope, the areas in which implants were placed often were extremely osteoporotic and anatomically wide.
The 4.00-mm-diameter implant was not wide enough to engage the denser lateral cortical plates of bone and immobilize the implant. This was especially evident in the posterior eggshell mandible, when the only denser bone was found in the lateral plates of bone surrounding the center medullary bone or surrounding the inferior alveolar nerve. In the maxilla, a similar condition was prevalent, with the exception that the denser bone was just inferior to the maxillary sinus.
When the 5.00-mm-wide implant was introduced, it was intended to meet the clinical conditions that were untreatable with standard components. Three years of multicenter testing were performed to evaluate the outcome of this new dimension. The results showed that, since its purpose was as a rescue implant, the success rate was less favorable than with standard implants. This was understandable, in that the conditions for its use were not only in compromised areas of bone quality, but also where there was minimum bone height. Clinicians felt that the increased width would also add greater surface area and offer a better result in sites with minimum bone height.
As time progressed, other situations emerged that beckoned the use of wider implants. They were the single-molar replacement, the immediate extraction socket, patients who exhibited severe parafunctional habits that threatened the integrity of the implant (ie, fracture problems), and immediate replacement of damaged or nonintegrated implants.
As an initiator of the wide-diameter implant, and one who has evaluated the short- and long-term results of this technology, I have come to some clinical impressions. Primarily, it seems that while there are a number of indications when no other treatment modality will offer a desirable result, I would still prefer to use an implant that is less than 5.00 mm in diameter. The main reason is that the greater the width, the more heat generated in the drilling process and the more bone damage experienced. This surgical trauma is often self-limiting and stable after loading, but on occasion exposure of the threads could be an initiator of peri-implantitis.
It seems to me that bone circulation is modulated by physiologic processes when it functions around a metal structure of less surface area. The metal surface cannot offer any help to the vascularity of the bone, although in most instances it coexists in a harmonious manner. Therefore, whenever possible, I prefer placing either 3.75-mm-wide or preferably 4.00-mm-wide implants. The latter is my overall choice, since the problem of implant fracture is almost non-existent, and is my standard implant for the partially edentulous posterior arch.
I do prefer using the 5.00-mm-diameter implant for the single molar replacement, because the abutment screw for the wide-platform implant is wider and can be tightened to 45 Ncm. It also occupies space for the prosthetic crown replacement. However, I have restored many molars with 4.00-mm-diameter implants and have not seen a major difference, either in prosthetic appearance or patient comfort. If the space is too wide, I would prefer placing 2 standard implants. I am not concerned about creating bifurcation involvement, as the situation is not the same as with teeth; in fact, in many regards, it mimics the anatomy of the original tooth.
In the case of immediate extraction and implant placement, again I would rather place a standard implant, since I would prefer to have a clot that possesses osteogenic properties surrounding the implant, rather than extra titanium in the position of the blood clot. It will be interesting to see the long-term results of wider-necked implants in these sites over the years. Will the thin buccal bone resorb and cause recession problems?
Certainly, when replacing nonintegrated or damaged implants, the 5.00-mm or wider implants are the only viable immediate solution and are a bonus for the patient. In the situation of weak bone, there is now a standard-diameter tapered implant that appears to be a reasonable alternative. As long as there is some degree of denser bone at the crest of the ridge, these tapered implants can achieve a significant degree of stability. My results so far have been exceptionally good after loading. This looks very promising.
IV. Cherilyn G. Sheets, DDS
Cherilyn G. Sheets, DDS, maintains a full-time private practice in Newport Beach, California, emphasizing esthetics, implants, and reconstructive dentistry. She is past president of the American Academy of Esthetic Dentistry and the American Association of Women Dentists. Dr Sheets is a Professor of Clinical Dentistry in the Department of Restorative Dentistry at the University of Southern California and an Associate Clinical Professor in the Department of Orthopedics at the University of California, Irvine. She is also the founder and executive director of the Newport Coast Oral-Facial Institute, an international teaching and research institute.
For the restorative dentist, the wide-diameter implant has been a good addition to the available choices of implant designs. The improved bioengineering principles of increased stability, greater surface area, and improved force distributions are welcomed benefits in the posterior regions for long- term maintenance of the implant prosthesis. Certainly the larger-diameter implants come closer to replicating the emergence profile of the larger teeth in a dentition. However, as beneficial as the wide-diameter implants have been, we have noticed several limitations with their use.
The circular shape of the wide-diameter implant at the occlusal surface is nonanatomic and therefore limiting. It would be an advantage in many situations to have the occlusal contours of the implant be not circular, but oval, to allow placement of the implant in the narrower resorbed ridge areas. The wider portion could be positioned in the mesiodistal orientation, providing the appropriate emergence profile and esthetics, while the narrower buccolingual dimension would be oriented to potentially fit into the narrower and perhaps more atrophic ridge. The result would be a more natural buccal and lingual emergence profile that is supported by a larger volume of buccolingual bone than can be achieved with the circular large-diameter implants. Several manufacturers are now addressing this problem.
The uniform occlusal-apical dimension of the wide-bodied implants can be limiting in some situations. A tapering root form would allow additional flexibility in wide-bodied implant placement, where there often is sufficient mesiodistal dimension at the occlusal portion of the bone but a narrower bone site at the apex related to angulation of the surrounding natural tooth roots. A tapering root form combined with an occlusal ovoid shape would allow additional flexibility in placement in the more challenging surgical situations. Wide-diameter implants have been tempting to use in special situations in the anterior locations where a wider emergence profile is desired.
Again, the limitation of the circular shape is often significant. Since natural root forms for the anterior teeth are not uniformly circular at the gingival-bone interface, the anterior esthetic compromises and occlusal implications can be significant. Tissue contours, emergence profiles, and lingual and labial cervical anatomy can all become compromised as a result of the uniform diameter of the wide-diameter implant.