Postrestorative reductions in crestal bone
height around endosseous dental implants have long been acknowledged to be a
normal consequence of implant therapy involving two-stage hexed implants.1-4
Such remodeling does not typically occur as long as the implant remains
completely submerged, but rather develops when an abutment is connected during
second-stage surgery, when a two-stage implant is placed and connected to an
abutment in a one-stage procedure, or when an implant is prematurely exposed to
the oral environment and bacteria.5
Research by Hermann, et al demonstrated
that crestal bone loss typically occurs approximately 2 mm apical to the
implant-abutment junction (IAJ).6 This position appears to be
constant, regardless of where the IAJ is situated relative to the original
level of the bony crest.6 The researchers also demonstrated that the
addition of a textured, bone-holding surface within 0.5 mm of the IAJ fails to
prevent bone resorption within 2 mm apical to the IAJ.6
Investigations by various researchers have
shed light on why the presence of the IAJ appears to trigger resorption in the
adjacent bone. Ericsson, et al found histologic evidence of inflammatory cell
infiltrate associated with a 1-mm– to 1.5-mm–tall zone adjacent to the IAJ.7
Berglundh and Lindhe concluded that approximately 3 mm of peri-implant mucosa
is required to create a mucosal barrier around a dental implant.8 This
suggests that crestal bone remodeling may occur to create space when inadequate
soft-tissue height is present so that a biological seal can be established,
isolating the crestal bone and protecting it from the oral environment.
These investigations have focused on
implant systems in which the diameter of the implant-seating surface matches
that of the abutment. This ubiquitous design positions the abutment
inflammatory cell infiltrate in direct approximation to the bone at the time of
The term “platform switching” refers to the
use of a smaller-diameter abutment on a larger-diameter implant collar; this connection
shifts the perimeter of the IAJ inward toward the central axis (ie, the middle)
of the implant.5 Lazzara and Porter theorize that the inward
movement of the IAJ in this manner also shifts the inflammatory cell infiltrate
inward and away from the adjacent crestal bone, limiting the bone change that
occurs around the coronal aspect.5 Crestal bone preservation has
been observed on other commercially available implant designs, purportedly
attributed to microthreads at the coronal aspect of the implant, connection
designs, occlusal schemes, or combinations thereof.9
In 1991, 5-mm– and 6-mm–diameter implants were
introduced with seating surfaces (ie, restorative platforms) of the same
dimensions. These large-diameter implants, with a larger surface area, were
intended to increase the amount of bone-to-implant contact when placing shorter
implants in areas of limited bone height, such as under the maxillary sinus or
above the inferior alveolar canal. The ability to increase the bone-to-implant
contact by the use of wide-diameter implants also enhanced the likelihood of
achieving primary stability in areas of poor-quality bone. At the time of the
wide-diameter implants’ introduction, no matching, wide-diameter prosthetic
components were available. Hence, clinicians restored them with standard 4-mm
After a five-year period, the typical
pattern of crestal bone resorption was not observed radiographically in which
platform switching was utilized. The authors theorize that this occurred
because shifting the IAJ inward also repositioned the inflammatory cell
infiltrate and confined it within a 90-degree area that was not directly
adjacent to the crestal bone.
The ability to reduce or eliminate crestal
bone loss can result in significant aesthetic and clinical benefits. In order
to facilitate the practice of platform switching, specific implant systems have
This expanded collar of such implants can
provide better engagement of the bone crest, better sealing of extraction
sockets, and better primary stability. Restoring, for example, a 4.8-mm implant
collar with the corresponding 4.1-mm prosthetic component shifts the IAJ inward,
moving the inflammatory infiltrate away from the surrounding bone. To achieve
this effect and maintain adequate soft-tissue depth, the implant should be
placed crestally if sufficient soft-tissue height and interocclusal space is
present, or subcrestally if insufficient soft tissue height and interocclusal
space is present.
(Continued from page 1 )
A 28-year-old male patient presented with
nonrestorable maxillary central incisors that had previously been treated
endodontically before being fractured by trauma (Figures 1 and 2). The teeth
were carefully extracted and, with the aid of a surgical guide, two 5.0-mm x 13-mm
implants were placed in a single-stage protocol (Figures 3-4-5-6-7). The
specific implant diameters and lengths were selected by the clinician based on
the size and shape of the individual sockets. The implants were placed in a
flapless manner in order to protect the buccal cortical plate from injury to
the vascular supply, which is often associated with a full-thickness flap.
Moreover, great care was taken to avoid touching the buccal plate of the
sockets during implant site preparation.
Healing abutments with 5-mm emergence
profiles and 4.1-mm restorative platforms were immediately placed (Figures 8 and 9). The patient was then discharged with antibiotic and anti-inflammatory
After three days, two 4.1-mm customizable
abutments, prepared by the dental technician on the master cast were inserted
into the internal interface of the implants and torqued to 20 Ncm (Figures 10-11-12-13). These titanium abutments had a gold-nitride coating that would
prevent graying of the marginal gingival tissue. Two acrylic provisional crowns
were then luted to the abutments with temporary cement and adjusted
out-of-occlusal contact, following the protocol of immediate nonocclusal
loading (Figure 14).10 An intraoral radiograph was taken (Figure 15),
and the patient was instructed to avoid loading the crowns for any purpose for
at least eight weeks. Gentle brushing with a toothpaste containing chlorhexidine,
however, was recommended.
two-month healing period (Figure 16), clinical osseointegration was confirmed,
and two metal-ceramic crowns were placed. The prognosis for maintenance of the
interdental papillae was excellent. The definitive crowns were constructed on
duplicate abutments made from the surgical index at the time of implant
No additional implant-level impression
procedure was required due to the technical prosthetic protocol, which allowed
the construction of the definitive crowns on a duplicated model and their
subsequent delivery chairside (Figures 17 and 18).
Clinical observation of the bone-preserving
effects of “platform switching” has been ongoing for more than a decade. This
procedure has been used by a number of clinicians successfully around the
The procedure requires that the “switch” be
in place from the day the implant is uncovered or exposed to the oral cavity in
either a one- or two-stage approach. It cannot be utilized after the
establishment of the biologic width around a conventional implant-abutment
interface configuration to regain crestal bone height. Potential applications
include situations where a larger implant is desirable but the prosthetic space
is limited, in the aesthetic zone where preservation of the crestal bone can
lead to improved aesthetics and where shorter implants must be utilized.
It is important to note that sufficient
tissue depth (approximately 3 mm or more) must be available to accommodate an
adequate biologic width. In the absence of sufficient soft tissue, bone
resorption will result, regardless of the implant geometry.11-14
This sometimes requires that the implant platform be placed below the bone
crest to obtain adequate tissue depth. Additionally, sufficient ridge width (ie,
minimum of 6.8 mm) must be present to accommodate the flared 4.8-mm implant
collar. Case selection and management, however, may influence the clinical
outcome and radiographic evidence of crestal bone preservation.
While bone preservation has been observed
for some time as a result of the use of a standard-diameter abutment on a
wider-diameter implant, the potential for confusion has existed for clinicians
who have attempted to employ this strategy while using standard components.
Dental laboratories and restorative dentists are accustomed to working with
matching-diameter implants and abutments.
Preliminary evidence suggests that the anticipated
bone loss that occurs around two-stage hexed implants may be reduced or
eliminated when implants are restored with smaller-diameter abutments, a
practice termed platform-switching.5 A new implant design has been
developed that facilitates this practice, and initial clinical observations
indicate the preservation of crestal bone results. Definitive clinical trials
are currently underway.
Professor, Department of Periodontics, University of Pennsylvania, School of
Dental Medicine, Philadelphia, PA; private practice, Philadelphia, PA
†Private practice, Verona, Italy
Professor and Head of the Section of Implant Dentistry and Oral Rehabilitation,
Department of Odontology, Galeazzi Institute, Milan, Italy; private practive,
American Academy of Periodontology; Associate Clinical Professor, Department of
Implant Dentistry, New York University College of Dentistry, New York, NY; private
practice, Voorhees, NJ
practice, West Palm Beach, FL
- Albrektsson T,
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- Morris HF, Winkler S, Ochi S, Kanaan A.
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- Cocchetto R, Vincenzi G. Delayed
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FF, Pustiglioni FE, Imbronito AV, et al. Influence of the microgap in the peri-implant hard and soft
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