1. Introduction
The placement of standard-length implants in conjunction with vertical bone augmentation and major reconstructive procedures usually implies longer treatment times and increased risk of post-operative complications [
1]. As implant dimensions have considerably decreased in length and diameter during the last decades [
2,
3], these drawbacks can be easily avoided by choosing minimally invasive alternative treatments [
4], which provide various advantages both for clinicians and patients. On that note, the use of short (length ≥ 6 mm and ≤8 mm) and ultra-short (length ≤ 5 mm) [
5] implants in the rehabilitation of extreme maxillary and mandibular atrophies is reported to be as effective as the use of longer implants [
6,
7,
8] in terms of implant survival and bone level stability at medium-term follow-up. On the other hand, recent RCTs (randomized controlled trials) [
9] assessed lower cumulative survival rates for short and ultra-short implants, also considering long-term follow-up (at least 5 years) [
10]. Furthermore, it is reported [
11] that short narrow-diameter implants supporting single-crown can be associated with greater marginal bone loss compared to standard implants.
While outcomes such as implants survival and marginal bone loss were widely evaluated in many studies [
12,
13,
14,
15], the influence of biological complications on implants failure was less investigated [
16,
17]. As implant success directly regards the onset of mucositis and peri-implantitis [
18,
19,
20,
21], their prevention and management [
22] are essential in long-term maintenance of healthy hard and soft peri-implant tissues. Peri-implant mucositis [
23] is characterized by bleeding on gentle probing; erythema, swelling, and/or suppuration may also be present. An increase in probing depth is often observed in the presence of peri-implant mucositis, due to swelling or decrease in probing resistance. It is not possible to define a range of probing depths compatible with peri-implant health, as it can also exist around implants with reduced bone support [
23]. There is strong evidence from animal and human experimental studies [
23] that plaque is the etiological factor for peri-implant mucositis. Thus, peri-mucositis associated with poor plaque control [
24] can be reversed with efficient measures aimed at eliminating the deposits and preventing the development of a subsequent peri-implantitis.
Peri-implantitis [
23] is a plaque-associated pathological condition occurring in tissues around dental implants, characterized by inflammation in the peri-implant mucosa and subsequent progressive loss of supporting bone. Peri-implantitis sites exhibit clinical signs of inflammation, bleeding on probing, and/or suppuration, increased probing depths and/or recession of the mucosal margin, in addition to radiographic bone loss (greater than 2 mm [
25,
26]).
In addition to implant-related and prosthesis-related variables considered for the assessment of implants survival and success, there is an emerging matter about the importance of patient-related factors, such as systemic diseases, smoking [
26,
27,
28,
29] and history of periodontal disease. The latter may be considered a preponderant risk factor for the occurrence of peri-implantitis [
30,
31]. However, the evidence [
26,
32,
33] concerning clinical and radiographic outcomes of short and ultra-short implants placed in patients with treated periodontitis is still scarce, in addition to a lack of homogeneous follow-up terms in the current studies.
The aim of this 3-year retrospective study was to evaluate implant survival, marginal bone loss and implant success in 326 short and ultra-short implants restored with single crowns. The implants were placed in the maxillary and mandibular, edentulous posterior regions of patients with history of periodontal disease (PP), and without history of periodontal disease (NPP).
4. Discussion
Peri-implantitis is defined as inflammation of the peri-implant mucosa, plaque association and non-reversible, radiographically detectable bone loss that exceeds normal physiological remodelling [
42]. This condition, in the absence of treatment, seems to progress in a non-linear and accelerating pattern [
19,
43,
44]. A strong similarity between the bacterial composition of sites with periodontitis and sites with peri-implantitis has been observed [
45,
46,
47]. This could be considered a crucial point in endorsing the implant placement in patients without a history of periodontal disease, in order to avoid the possibility of serious peri-implant complications. Furthermore, residual pockets at the end of active periodontal therapy represent a significant risk for the development of peri-implant bone loss in patients susceptible to periodontitis [
48], even if the patient is compliant to an established maintenance protocol.
Current reported prevalence of peri-implant diseases is not unequivocally determined in literature [
23,
26,
49,
50], because of multiple discrepancies regarding different definition, implant-related characteristics, prosthetic protocols and bone loss threshold indicative of destructive process. A systematic review based on an average follow-up of 3 years [
20] reported an implant-based prevalence of peri-implant mucositis and peri-implantitis of 29.48% and 9.25% respectively.
Nevertheless, recent studies showed that implants placed in NPP demonstrate fewer failures, and consequent higher percentages of implant survival, compared to those placed in PP. Karoussis et al. stated [
51] that implants in patients with history of periodontitis usually encounter less survival (90.5%) compared to implants in patients with no past history of periodontitis (96.5%) after a long-term follow-up. Hardt et al. [
52] considered 346 implants placed in the posterior maxillary areas with a follow-up of 5 years: the survival was 96.7% and 92% for NPP and PP respectively. Roccuzzo et al. [
53] found a 10-year survival rate of 96.6%, 92.8% and 90% for 61, 95 and 90 implants placed respectively in periodontally healthy patients, patients with a history of moderate periodontitis and patients with a history of severe periodontitis.
Concerning the increased risk for developing peri-implantitis due to the susceptibility to periodontitis, stated by many authors [
26,
50,
54,
55,
56,
57], Changi et el. [
50], in a 3.5-year study on 6129 implants, demonstrated that radiographic evidence of periodontitis is one of the principal risk-factor statistically associated (odds ratio (OR) = 3.6) with peri-implantitis. Renvert et al. [
57] found a OR even equal to 4.5 assessing the likelihood of association between peri-implantitis and history of periodontitis. Moreover, insurgence of peri-implantitis seem to be higher in PP: Karoussis et al. considered [
51] 112 ITI dental implants, comparing 21 implants placed in PP and 91 implants in NPP, both following regular supportive therapy for 10 years, and found that incidence of peri-implantitis in NPP (5.8%) was lower compared to PP (28.6%). In a 3- to 5-year cross-sectional study, Arunyanak et al. found [
58] that prevalence of peri-implantitis was significantly higher in PP (25% in 72 patients) compared to NPP (10.9% in 128 patients).
On the other hand, investigations involving short implants (length ≥6 mm and ≤8 mm) and considering a history of periodontal disease as a variable with potential correlation with failure and biological complications, are still scarce in literature. Hasanoglu et al., [
33] in a multicenter long-term retrospective study on 460 short implants (4 to 9 mm in length) placed both in posterior and anterior regions of maxilla and mandible of 299 patients, found an overall implant survival of 95.86% and a prevalence of peri-implantitis of 10% after a follow-up of up to 9 years, with 73.91% of failures caused by peri-implantitis; in this study, 70.85% of implants were placed in patients without a history of periodontal disease. Zhang et al. [
59], in a study on 214 implants, whose length was less than 8 mm in 25 implants, assessed implant-related variables (e.g., length, diameter and position) and periodontal-related variables (e.g., soft tissue indexes and marginal bone-level alterations), identifying residual pockets and posterior region as predictors for peri-implantitis. Akram et al. [
32], in a 3-year follow-up study, compared the clinical and radiographic conditions between teeth of healthy patients (11) and short implants placed in patients treated for aggressive periodontitis (48); soft tissues parameters of PI, BOP, PD and CAL were recorded, finding a significantly greater attachment loss in implants compared to teeth. Correia et al. [
60], in a retrospective study on 689 implants in 202 patients, found an overall implant survival of 95.8% for NPP (214 implants) and 93.1% for PP (475 implants), after 3 years of follow up, with no statistically significant differences between groups; moreover, short implants showed a survival of 97.3% and 93% for NPP (74 implants) and PP (157 implants) respectively, with no statistically significant differences between groups.
Similarly, in this 3-year retrospective study, a history of periodontitis seemed not to be correlated to implant failure, as no statistically significant differences in implant survival were found between PP and NPP.
Excessive bone loss after loading can influence both implant survival and success: our results showed that bone level stability was preserved after 3 years, without significant differences between implant placed in PP and NPP. It is also worth noting that the implant system examined in the study presents a screw-less locking-taper implant-abutment connection, which increases mechanical stability with no micromovements or micro-gaps at the implant-abutment interface and provides minimal bone resorption [
61]. Moreover, the convergent crest module in short and ultra-short implants seems to have an important influence on marginal bone loss. Referring to biomechanical models which compare different crest modules, the quantity of bone present around the neck of the implant is fundamental for the distribution of the occlusal forces [
62,
63]. The transmission of vertical, horizontal and rotational forces on F-BIC is thus more favorable and homogeneous in implants with convergent crest module compared to implants with divergent crest module with the same diameter. Furthermore, the sloping shoulder guarantees a platform switching at implant level with bone growth over the neck, assuring successful long-term functioning together with the specific plateau root-form design [
62].
The literature supports a general agreement that implants can be successfully placed in periodontal patients if proper supportive protocols of maintenance are applied before and after loading [
64,
65], in order to prevent peri-implant mucositis and peri-implantitis. Some authors [
66] claimed that current definitions of peri-implant health and diseases are still greatly debated and controversial, as healthy implant mucosa may bleed upon probing, thus leading to high number of false-positives. Nevertheless, an increase of probing pocket depth values over time is not necessarily associated with loss of supporting bone around dental implants. It is also suggested that bleeding on probing should be used as a diagnostic tool and as an indicator for treatment in association with probing pocket depth of at least ≥4 mm, the presence of abundant plaque deposits, and radiographic detection of bone loss [
66]. Furthermore, the evidence is equivocal regarding the effects of keratinized mucosa (which was statistically different between length-groups in our study) on the long-term health of the peri-implant tissue, such as patient comfort and ease of plaque removal [
23].
In the present study, where patients adhered to a strictly observed protocol of TPS, low inflammatory indexes were generally assessed, with a positively significant correlation with the number of oral hygiene interventions administered per year. Finally, only 15.41% of the implants presented signs of mucositis, with no statistically significant differences between PP and NPP. Similar results were found in a study by Zorzano et al. [
67], where 786 implants were placed in 239 periodontally compromised patients, who regularly received supportive periodontal therapy; after a mean follow-up of 63 months, 12.8% of the implants were affected by peri-mucositis and 9.8% by peri-implantitis.
However, the present study, being retrospective, presents some critical issues. The medium sample size, the relatively short evaluation (3 years of follow-up) and a non-homogeneous distribution among implant length-groups, arch-groups and PP/NPP-groups are the product of its retrospective nature. The single-center setting, involving a university dental clinic, could also have introduced an important bias, suggesting that our results cannot be generalized.
Another issue that could represent a critical limit for the study is that most of the implants were placed in patients characterized by a history of periodontal disease: nonetheless, after 3 years of loading, the main strengths of our study rests on a positive assessment of the proportion of surviving implants and bone level stability when placing short and ultra-short single-crown locking-taper implants both in PP and NPP. Furthermore, all patients enrolled in the study showed a positive compliance to the maintenance program.
Prospective long-term (5-year follow-up or longer) studies are necessary for a better evaluation of larger homogenous samples, and a more balanced distribution between patients with and without a history of periodontal disease is desirable.