Does implantoplasty affect the failure strength of narrow and regular diameter implants? A laboratory study

Overt peri-implantitis lesions regularly require a surgical intervention to achieve disease resolution [1, 2]. Depending on defect morphology and treatment approach, implantoplasty (IP), i.e., the mechanical removal of the implant threads and smoothening of the implant surface [3, 4], can be part of the surgical treatment protocol for implants with a rough surface. IP aims to achieve implant surface decontamination and also to reduce the risk of reinfection, and is recommended at those aspects of the implant, where bone healing and/or re-osseointegration is not expected. Although the clinical significance of IP (e.g., reduced bleeding indices and/or probing pocket depths, improved bone levels, etc.) has been confirmed only in a single randomized controlled clinical trial [3, 4], positive results have been reported in several case series (e.g., [5‐12]), and IP appears as a widely used procedure.

Nevertheless, IP unavoidably causes a reduction of the implant mass, and thus it may weaken implant strength and increase implant fracture rate. A recent systematic review [13] summarized the available information on mechanical and/or biological complications due to IP. In 2 out of 3 laboratory studies identified [14‐16], IP reduced implant strength; i.e., standard/regular diameter implants suffered up to 40% strength reduction [14, 16]. However, several other factors (e.g., implant type/design, implant material, etc.) may additionally affect implant strength after IP, but were not addressed in those studies.

Therefore, the present laboratory study aimed to assess whether the impact of IP on implant strength depends on implant type/design, diameter, and/or material.

Peri-implantitis treatment requires in most cases a surgical approach to get access to the implant surface for decontamination. One approach for implants with a rough surface includes removal of the implant threads and smoothening of the implant surface (i.e., implantoplasty, IP) at the aspects of the implant, where bone healing and/or re-osseointegration is not expected. Since IP unavoidably causes a reduction of the implant mass, it may also weaken the implant and lead to implant fracture. The present laboratory study confirmed that IP causes statistically significant reduction of the maximum implant failure strength, irrespective implant type/design, diameter, and material. Up to now 4 laboratory studies [14‐16, 18] and one finite element analysis [19] are available on this topic, describing that the impact of IP on implant failure strength appeared to depend on the implant diameter and connection type. Specifically, while wide diameter implants (i.e., 4.7 mm diameter) were not significantly affected by IP [14], contradicting results were reported for regular diameter implants (i.e., 3.75 to 4.3 mm diameter), with 2 studies [15, 18] showing no significant impact of IP on the fracture strength of regular diameter implants, and 2 studies demonstrating statistically significant reduction (up to 40%) in implant failure strength [14, 16]. Further, reduction in fracture strength varied among connection types, with a Morse taper connection being least affected [16]. Herein, IP resulted in significant reduction in implant failure strength in both standard and narrow diameter implants. However, IP seemed to affect more the narrow diameter implants than standard diameter implants, and implant type/design was also shown as a relevant parameter. In contrast to previous data [16], the range of the maximum implant failure strength among the implants within each specific group appeared unaffected by IP; i.e., herein, the range did not increase relevantly in implants subjected to IP. The fact that TL implants are weaker than BL implants is at least partly explained by the fact that TL implants were exposed (i.e., out of the plexiglass holder) at a larger extent compared to BL implants; i.e., 3 mm of the rough surface plus 1.8 mm of the TL neck, thus resulting in a bigger lever. Although previous studies [20, 21] had already indicated that a higher “marginal bone loss” further reduces implant strength, this approach was chosen to simulate a similar amount of horizontal marginal bone loss in both implant types.

In the present study, the impact of IP on narrow diameter implants (i.e., ≤ 3.5 mm) was assessed for the first time. It appears obvious that a smaller diameter implant, which has a thinner metal wall compared to regular/standard diameter implants, would also be more affected from IP. Indeed, fractures during dynamic loading occurred only among narrow diameter TL implants (i.e., 5 out 6 fractures), mainly those subjected to IP. In this context, fracture rate during dynamic loading was clearly lower in narrow TiZr implants compared with narrow Ti implants; i.e., only a single TiZr implant subjected to IP fractured vs. 5 Ti implants. Indeed, the results of the separate multiple linear regression analyses showed that TiZr implants had a statistically significant increased maximum implant failure strength compared with Ti implants, among TL implants and among regular diameter implants. The lack of statistical significance among the narrow diameter implants is most likely due to the high “drop-out rate” among the narrow Ti TL implants subjected to IP. However, the higher fracture rate among narrow Ti TL implants subjected to IP compared with that of narrow TiZr TL implants subjected to IP (i.e., 4 vs. 1 fractures, respectively), in combination with the fact that the highest maximum load value of the narrow Ti TL implants subjected to IP was lower than the lowest maximum load value of the narrow TiZr TL implants subjected to IP, gives a strong indication for an effect of the material also among the narrow implants. Previous laboratory studies have indeed indicated a higher strength of TiZr compared with Ti implants (for overview see: [22]); however, the clinical relevance of these reports is yet unknown. A recent systematic review [23], assessing the clinical performance of narrow diameter Ti and TiZr implants, showed that similar success rates in terms of survival and marginal bone loss, independent of the region in the mouth, are obtained from both types of implants at least on the short-term.

The results herein showed that despite the fact that IP resulted in a statistically significant reduction of the maximum implant failure strength, the forces required to fracture or deformate all regular diameter implants and narrow BL implants remained high (i.e., > 650 and 440 N, respectively). Forces occurring in the natural dentition during regular mastication range between 100 and 300 N [24]. Single implants as well as implant-supported fixed bridges appear to be loaded with similar or slightly lower forces [25‐27], while loading forces decrease in implant-supported cross-arch restorations, and even more in implant-supported overdentures [28‐30]. Indeed, no study/case report describing implant fracture after IP was identified in a recent systematic review on mechanical and/or biological complications due to IP [13]. Nevertheless, direct comparison of forces derived from laboratory studies to those from clinical studies should be made with care, due to limitations such as differences in the loading mechanism (i.e., only vertical forces in the laboratory vs. a combination of vertical and horizontal loading forces in the mouth) or in the superstructure geometry (i.e., standardized hemispherical shaped, purpose-made abutments in the laboratory vs. anatomically shaped crowns in the mouth) [31].

The present study shows some important differences/advantages in terms of study design compared with previous laboratory studies on IP. In contrast to previous studies, all implants herein were subjected to dynamic loading prior to loading until failure, to simulate regular mastication and add a certain “aging effect” on the implants [17]. The implants were loaded 2,000,000 cycles, which correspond to the masticatory activity of a couple of years, and the forces applied were within the range of regular chewing forces (i.e., up to 300 N) [24]. Further, IP was performed with a computer-controlled torn, instead of “free hand,” which was used in most of the previous studies [14, 15, 18], to ensure removal of a standardized amount of implant material. Although this approach does not represent the true clinical situation, it ensured that exactly the same amount of metal was removed from every single implant of the various groups. Next, in the present study, a horizontal bone loss of 3 mm was simulated in contrast with the previous laboratory studies simulating 5 to 6 mm bone loss. Thus, the results of the present study may be applicable only in cases of incipient to moderate peri-implantitis (i.e., about 3 mm of bone loss) than to advanced (i.e., ≥ 5 mm of bone loss) peri-implantitis cases. Finally, herein, implant failure was defined as (1) implant fracture or (2) severe deformation of the implant (i.e., bending of the implant and/or prosthetic component > 30°), whichever appeared first. It appeared reasonable that an implant, which is already bended beyond 30°, should be considered failure although fracture may occur only at a later timepoint.

In conclusion, within this laboratory setting, IP significantly reduced maximum implant failure strength, irrespective implant type/design, diameter, or material. However, the maximum implant failure strength of regular diameter and narrow BL implants remained high despite IP (i.e., > 650 and 440 N, respectively), while > 50% of the narrow Ti TL implants subjected to IP were fractured already during dynamic loading, simulating regular mastication. Thus, IP seems to have no clinically relevant impact on the majority of cases, except from those of single narrow Ti TL implants, which may have an increased risk for mechanical complications. The latter should be considered for peri-implantitis treatment planning (e.g., communication of potential complications to the patient), but also in the planning of implant installation (e.g., choosing TiZr instead of Ti for narrow implants).