Dr Rahul Ravi, Dr Adnan Kheyroolla, Dr Omkar Shetty, Dr Rubina Tabassum, Dr Gaurang Mistr, Dr Kunal Mehta
Dean and Professor,
Department of Prosthodontics and Implantology,
D.Y Patil University, School of Dentistry, Navi Mumbai, India
Implant therapy offers increased longevity, improved function, bone preservation and quality of life. This article discusses the literature related to the dynamics in effect when an implant is placed in the bone. Osseous and soft tissue changes take place around the implant subsequent to their placement. These changes determine the esthetic outcome of the implant.
Keywords : Implants, osseointegeration, esthetics, implant-abutment connection, platform switching
Citations : Ravi R, Kheyroolla A, Shetty O, Tabassum R, Mistry G, Mehta K. The implant - abutment connection and its relation to crestal bone - a review. J Prosthodont Dent Mater 2020;1(1& 2): 51-59.
Dental implants is a widely accepted device used as a predictable and reliable tool for dental reconstruction, however, it is still necessary to ensure that the height of the peri-implant crestal bone is maintained. Directly after insertion of a dental implant, a cascade of biological events occurs during the bone healing process. The change in bone shape and continuity is a result of this bone healing process, contrary to a possible pathological bone loss. Osseointegration is considered to be the phenomenon of direct apposition of bone on an implant surface, which subsequently undergoes structural adaptation in response to a mechanical load. Over time the shape of crestal bone around the implant changes both horizontally and vertically.
One of the criteria for the success of dental implant treatment is the amount of crestal bone change. Albrektsson et al.8 proposed that a dental implant can be considered successful if peri-implant crestal bone loss is less than 1.5 mm during the first year after implant placement and less than 0.2 mm annually after that.
Numerous theories are put forward to explain the crestal bone loss around implants including, surgical trauma due to excessive heat and pressure generation while preparing the osteotomy for the implant and periosteal flap elevation, or secondary to the attachment of prosthetic components due to occlusal overload and the presence of a micro gap between the implant and the abutment & its positioning in relation to the crestal bone. Although the success of a dental implant is probably multifactorial and dependent on some or all of the above factors, this review specifically addresses the relationship of the implant-abutment connection and its relation to crestal bone changes.
Implant-Abutment Connections and Micro gaps:
In the two-stage implant placement technique, the implant is placed at the bone crest level. After 3-6 months, a prosthetic abutment is installed on the implant to connect the implant to future prosthetic restorations (crowns, bridges or dentures). It is postulated that a potential microscopic space exists at the abutment/implant interface, along the abutment screw threads and at the base of the screw chamber known as micro gaps.
Three main factors are identified as possible causes for the formation of micro gaps: occlusal load during physiological function, manufacturing tolerance and micromotion between the implant–abutment connections. Microorganisms occupy this gap and set up a bacterial reservoir, resulting in an area of inflamed soft tissue facing the fixture abutment junction. This microbial leakage at the implant-abutment interface is a chief challenge for the constructing the two-stage implant systems & is a major contributing factor for peri-implant inflammatory reactions.
Fig 1: Microgaps at the Implant-abutment connection
Many studies have shown that this microscopic space between implant and abutment (micro gap) facilitates the infiltration of fluids and macromolecules from crevicular fluid and saliva, enabling bacterial invasion and proliferation, even in patients with good oral hygiene.
On the other hand, bacterial infiltration may also arise during the first stage or the second stage of implant surgery. Moreover, findings from several studies have documented that bacterial infiltration may occur both from an external source to the inner area of an implant, and in reverse. This migration of bacteria is probably facilitated through the unavoidable presence of micro gaps between the fixture and the abutment components of the assembled system.
The bacterial contamination may be correlated with gap sizes or misfits. Gap dimensions ranging from 20μm to 168μm have been reported. The level of contamination depends not only on the precision of fit but also on the degree of the applied micromovement and torque. The incidence of loads and unscrewing of the prosthetic abutment can increase infiltration, whereas optimal adaptation, minimal micromovement and exceptional prosthetic and occlusal planning are factors which can minimize microleakage, but do not seem to prevent it completely.
The presence of a micro gap in close relation to bone plays a role in developing of peri-implant inflammation and bone loss. Put merely, bacterial invasion of the micro gaps can interfere with the osseointegration of an implant during the healing phase of the surgical intervention and cause peri-implantitis. Pathogenic bacterial microflora may also influence the outcome of guided bone regeneration in the treatment of peri-implantitis.
Broggini et al. demonstrated an increase in inflammatory cells in the peri-implant soft tissues at the level or slightly coronal to the implant–abutment junction due to the bacterial presence, which when combined with osteoclasts formation results in alveolar bone loss. This is in contrast to an implant system with a lack of implant-abutment interface that shows little evidence of the presence of inflammatory cells. This infiltration of inflammatory substances is irrespective of the amount of plaque accumulation.
Comparison of Microleakage in Different Implant-Abutment Connections:
Verdugo CL et al. used external connection implant and conical internal connection (Morse taper) implants. The results of the study showed that less microleakage was shown by Morse taper connection implants than external connection implants. A gap of 10 μm was presented by external connection implants more than Morse taper implants with a gap of 2-3 μm.
Canullo L et al. conducted a five year follow-up study on humans for different implant connections under functional loading. The results showed that microbial contamination was seen in all the connections. Internal Hex and conical connection implants showed less leakage of bacteria at the peri-implant sulcus and inside the connection than external hexagon implants.
Do Nascimento C et al., in their in vitro study, used 43 microbial species, which were very common in the human oral cavity. They evaluated prostheses supported by External Hexagon or Morse Cone implants under dynamic loading conditions. Results revealed that higher microbial count was found in External Hexagon implants than Morse Cone implants. Many microbial species including, peri-implant diseases causing organisms were detected in internal part of External Hex implants. Internal surfaces of Morse Cone implants showed no colonization of microorganisms, as micro gaps present in conical connections were much smaller at the implant-abutment interface.
Baggi L et al. in their study, found that tube-in-tube interface implants were more resistant to colonization than flat to flat interface. Contradictory results were obtained by Al-Jadaa A et al. where they found, implants with a flat-to-flat interface (internal hexagonal) mating surfaces showed the best performance with regard to leakage under both static and dynamic conditions. This study also proved that if implants under static conditions were tight and would provide better sealing ability under dynamic conditions.
Koutouzis T et al. evaluated microleakage of internal Morse-taper connection and found that there was minimal penetration of bacteria down to the implant-abutment interface. Dynamic loading increases the penetration of bacteria as there was micro movement at the implant-abutment interface, which causes a pumping effect and leads to detrimental effects on marginal bone stability.13Contradictory results were obtained by Harder S et al. where conical implant-abutment connections do not prevent microleakage on a molecular level in even unloaded conditions.
From the studies mentioned above, it is possible to draw the inference that internal implant-abutment connections and conical (Morse Taper) implant-abutment connections show lesser microleakage than external implant-abutment connections. The external connections, of which the external hexagonal connection is the most common, provides a short and narrow connection with the abutment and provides for only limited screw engagement and a short fulcrum arm, which together allow for frequent screw loosening. The instability of the external connection leads to open the micro gaps.
Besides, it is often challenging to seat components on the hex easily and with confidence, especially in the posterior parts of the mouth, even for an experienced clinician. Minute rotational changes at a single abutment location can result in the misfit of the superstructure. Increasing the flat-to-flat width and the height of the connection has reduced these complications but not eliminated them. Internal connections, due to their design, have a more stable implant-abutment connection resistant to joint- opening forces.
Lateral forces are distributed deep within the implant, and the long internal wall engagement with the implant shields the abutment screw and buffers vibrations. Internal connections are therefore, less prone to micromovements and opening of the implant-abutment connection. Conical connections, specifically, have the advantage of the smallest inherent micro gaps due to frictional fit of the abutment into the implant, virtually removing any gap between the two. However, no implant-abutment interface can completely prevent microleakage in either loaded or unloaded states.
Position of the Implant-Abutment Connection in Relation to Crestal Bone:
Numerous studies have shown that bone resorption around the implant neck does not start until the implant is uncovered and exposed to the oral cavity. It is hypothesized that bacterial contamination of this micro gap during the second stage surgery results in peri-implant inflammation, leading to bone remodeling. Bone remodeling will progress until the biologic width has been created and stabilized to seal off the connective tissue from the colonized micro gap. Not only does this width progress apically, along the vertical axis, but according to studies conducted by Tarnow et al there is also a horizontal component amounting to 1–1.5 mm.
Fig. 2: Establishment of Biologic Width after second stage implant surgery
So, the position of the implant-abutment connection in relation to the crestal bone determines the amount of bone loss around an implant. A study by Broggini et al revealed that as the apical position of the implant-abutment interface was progressively increased, the total number of peri-implant inflammatory cells was increased in parallel, i.e., the deeper the interface, the greater the magnitude of peri-implant inflammation. Further, the maximum density of neutrophils adjacent to supracrestal implants was significantly less than for crestal and subcrestal implants. Moreover, the peri-implant location with maximum neutrophil density was also dependent upon the depth of the implant-abutment interface. Thus, for supracrestal implants, this location was near the implant-abutment interface (ie, above the original bone crest), whereas for subcrestal implants, this location was immediately coronal to the implant-abutment interface (below the original bone crest).
In parallel with differences in peri-implant inflammatory cell accumulation, the apico-coronal dimension of connective tissue was also progressively expanded as the depth of the implant-abutment interface was increased. This primarily reflected increases in the connective tissue compartment apical to the original alveolar crest (ie, alveolar bone loss). Specifically, there was significantly greater bone loss associated with subcrestal implants as compared to supracrestal position.
Therefore, this study demonstrated that moving the interface supracrestally, effectively changing the location of the inflammatory stimulus, also reduces peri-implant bone loss. Thus, minimal inflammation (and bone change) occurred when the interface was above the original bone crest, whereas the greatest inflammation (and bone loss) occurred when the interface was below the alveolar crest. These clinical observations are highly relevant, since the maintenance of crestal bone height appears to be an important predictor of soft tissue margins in both natural dentition and implants. These findings have several important clinical implications relative to limiting inflammation and bone loss around implants.
First, implant design could be either one-part or transmucosal to eliminate the interface. Second, the interface could be positioned supracrestally. Third, the interface might be made in such a way that excludes microbes, i.e. a more stable interface with smaller micro gaps. In these scenarios, inflammation would not be expected to develop near the alveolar crest, consequently reducing the potential for bony changes. Support for this speculation comes from another animal study in which implants were placed with the interface approximately 3mm above the original alveolar crest. Bone loss around these implants was minimal. Further, in patients with transmucosal implants placed so that the implant interface was approximately 3 mm above the original alveolar crest, minimal bone loss was observed over an eight-year period.
Platform Switching Concept:
Platform switching or platform shifting is a method used to preserve the alveolar bone around dental implants. The platform switching effect was first observed in the mid-1980s. At the time, larger-diameter implants were often restored with narrower abutments (Ankylos Dentsply, Friadent, Germany; Astra-Zeneca, Sweden; Bicon, Boston), as congruent abutments were often still unavailable. As it later turned out, this was a remarkable coincidence. The abutments used with conventional implant types are generally flush with the implant shoulder in the contact zone. This results in the formation of micro gaps between the implant and the abutment.
The bacterial contamination of these micro gaps adversely affects the stability of the peri-implant tissues and leads to a reduction of horizontal and vertical marginal bone levels. If the microcrack is located close to the bone, the creation of the biologic width will occur at the expense of the bone. The platform-switching concept requires that this microcrack be placed away from the implant shoulder and closer toward the axis in order to increase the distance of this microcrack from the bone. This generally implies the use of a reduced-diameter abutment. The inward, horizontal repositioning of the abutment inflammatory cell infiltrate (right) will move the abutment inflammatory cell infiltrate away from the crestal bone and into a more confined area.
There appear to be two results of the horizontal inward repositioning of the implant-abutment interface. First, with the increased surface area created by the exposed implant seating surface, there is a reduction in the amount of crestal bone resorption necessary to expose a minimum amount of implant surface to which the soft tissue can attach. Second, and perhaps more important, by repositioning the implant-abutment interface inward and away from the outer edge of the implant and adjacent bone, the overall effect of the inflammatory cell infiltrate on the surrounding tissue as described by Ericsson et al. and Abrahamsson et al. may be reduced, thus decreasing its resorptive effect on crestal bone. It is further suggested that platform switching repositions the inflammatory infiltrate further away from crestal bone and locates it within an approximate ≤90-degree confined area of exposure instead of a ≤180-degree area of direct exposure to the surrounding hard and soft tissues. As a consequence, the reduced exposure and confinement of the platform-switched abutment inflammatory cell infiltrate may result in a reduced inflammatory effect within the surrounding soft tissue and crestal bone.
It is important to note that to benefit from the platform-switching bone preservation technique, reduced-diameter components, beginning with the healing abutment, must be used from the moment that the implant is exposed to the oral environment, because the process of biologic width formation begins immediately following exposure to the oral environment. Thus, whether an implant is placed using a one- or two-stage surgical procedure, the first component placed on the implant must be of a smaller diameter if a horizontally repositioned biologic width is to be accomplished. This is important because after crestal bone has remodeled to a post-restorative resting position around the top of an implant, it will not return to its presurgical level if platform-switching principles are implemented at a later time.
The long-term predictability of dental implants is now a well-documented fact. Virtually all the major manufacturers can document success rates greater than 90%, and the more refined systems have achieved well above that number for more than ten years. A grey area has been the long-term stability of the abutment and prosthesis. Tremendous progress has been made in this area mainly, due to improved clinical machining tolerances. The transition to internal connections has been gradual but profound. The internal connections available today are far more stable, physically stronger, easier to restore, more amenable to excellent esthetics, and definitely more user-friendly.
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