Dental Implants Survival After Nasal Floor Elevation: A Systematic Review

Dental implant placement has become a routine technique, increasingly in demand by patients.¹  Frequently, clinicians must face the challenge of the treatment in cases of severe vertical and/or horizontal bone atrophy.^1–3  Recent advances in implant design and size (short, extra-short, and narrow implants) have provided new solutions or have enhanced older ones, for the treatment of different types of atrophy.^3–5 

In cases where implants cannot be placed directly, because the height or width of the residual bone does not allow it, there are accessory techniques for recovering the lost bone volume.^6,7  In the posterior maxilla, different sinus lift techniques have been broadly used with high success rates.^4–7  The sinus lift can be performed with a lateral or transcrestal approach, placing the implants in the same surgical time or in a second stage and using diverse grafting materials.^6,7  Although differences between the different approaches have been reported, almost all show acceptable success rates when performed following updated indications and procedures.^6–8 

Based on the principles of sinus lift, at the end of the 1980s and during the 1990s the first nasal floor elevation (NFE) techniques were developed to treat the vertical atrophy in the premaxilla. Amongst others Adell et al,⁹  Jensen et al,¹⁰  or Garg et al¹¹  described different procedures for the NFE and grafting. In some of the first patients treated with these approaches, the residual height bone was up to 11 mm and the aim of performing a NFE procedure was to place longer implants (10 to 16 mm) trying to ensure a bicortical anchorage.^12,13  Later, some reports of patients treated with shorter implants and less bone height available were published.^14,15  As the sinus floor elevation, NFE may be performed simultaneously (1-stage approach) or prior to implant placement (2-stage approach), through a lateral or transcrestal access (Figure 1) and using different types of grafting materials.^2,9–14 

NFE has some advantages for implant survival compared to sinus floor elevation, such as a thicker mucosa that means a lower risk of membrane perforation.^16,17  Moreover, premaxillary bone usually presents a higher density and higher crestal cortical bone thickness than in the posterior maxillary area, facilitating a better stabilization of the implants.¹⁸ 

Although NFE procedures have been performed for more than 3 decades, data from implant survival and success in the literature is still scarce. In addition, the effect of different approaches employed on implant outcomes has not been clearly stated. Therefore, the aim of this study was to assess the survival of dental implants placed in the premaxilla along with a NFE and to collect information that may be of interest to the clinician during the planning of the treatment.

A systematic review was carried out following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement recommendations¹⁹  to answer the following the PICO questions: “In patients undergoing dental implant placement in the maxillary anterior region (P), Do implants placed after nasal floor elevation (I) have a different survival (O) from those implants placed without grafting procedures (C)?” and as secondary, the following questions were formulated and answered in a narrative manner:

1. Do any of the techniques described for NFE provide better outcomes?

2. Are there differences in implant survival related to the type of graft employed?

The aim of the review was to answer with the best available evidence all these questions to help clinicians with common uncertainties when planning the treatment.

A register in the International Prospective Register of Systematic Reviews—PROSPERO (NIHR)—was obtained before starting (CRD42021229479). The PRISMA guidelines for systematic reviews were used to conduct the review process.¹⁹ 

Four electronic databases including MEDLINE, Cochrane Library, Directory of Open Access Journals, and Scopus were searched. To build the search strategy (PICO), the following considerations were applied: Patient, patients undergoing dental implant placement in the maxillary anterior region; Intervention, NFE and implant placement; Comparison, implants conventionally placed in the same area; Outcomes, implant survival. The main question built was then: “In patients undergoing dental implant placement in the maxillary anterior region, do implants placed after nasal floor elevation have a different survival from those implants placed without grafting procedures?”

The search was carried out using the following terms: “dental implants” (Mesh), “nasal floor elevation,” and “nasal floor augmentation” (Free terms). The search strategy was defined as follows: ((“dental implants"[MeSH Terms] OR (“dental"[All Fields] AND “implants"[All Fields]) OR “dental implants"[All Fields]) AND “nasal floor elevation"[All Fields]) OR “nasal floor augmentation"[All Fields].

This electronic search was complemented by:

• Review of the full-text selected articles reference lists.

• Manual searches in the same databases including other free terms such as “nasal floor,” “nasal lift,” and “nasal floor lifting.”

• Grey literature (WorldCat, State Library of New South Wales, CORE, Jisc, Bielefeld Academic Search Engine, Open Grey, WorldWideScience.org, and Doctoral Thesis Database of The Spanish Ministry: TESEO)

• Internet free search.

No restrictions of language or time were applied. Two authors independently assessed the publications by title and abstract. The inclusion or exclusion criteria for the studies were as follows:

1. Inclusion criteria:

• All types of clinical studies in humans were included: randomized clinical trials (RCTs), prospective and retrospective cohort or case-control studies and case series/reports.

2. Exclusion criteria:

• Studies without information about the implants placed after the NFE or a follow-up period shorter than 6 months after implant placement

• Studies including patients treated with nonconventional implants (basal implants, bi-cortical implants, or others)

• Studies including patients with a history of previous severe trauma (gunshot injury or others).

The study selection was performed by the same 2 independent reviewers and an additional reviewer acted in case of disagreement. After article selection based on the abstract and the article selection criteria, both reviewers read the complete articles and determined whether they met the inclusion criteria for this review. Agreement in the selection process was calculated using Cohen's kappa coefficient, with a k value of 0,82.

Data from all articles was collected in duplicate by both researchers independently and then pooled in the same worksheet. The following information was extracted from each selected study: year of publication, type of study, number of patients and implants, sex and age of patients, information related to smoking habit, implant average length and position, type of grafting material employed, mean bone height available before surgery, mean bone height gain, type of prosthesis, follow-up period, survival rate, and success rate.

Data from the identified and relevant publications were extracted and, if indicated, presented in evidence tables. Overall findings were summarized in a narrative manner.

The main outcome analyzed was implant survival, defined as the presence of the implant in function in the mouth after the end of the follow-up period established in each study. Secondary outcomes analyzed were the rate of implant success and prosthetic complications after the full follow-up time.

The methodological quality of the included studies was assessed using The National Institutes of Health “NIH quality assessment tools” for case series, and “The Newcastle-Ottawa scale for assessing the quality of nonrandomized studies” for case-control and cohort studies.^20,21  Although “NIH quality assessment tools” were initially conceived to help reviewers, these tools have been broadly used in many recent systematic reviews to assess the study quality.^20,21  For case reports, the Joanna Briggs Institute “JBI critical appraisal tools for case reports” was employed.^20,21  The risk of bias was measured independently by 2 authors, and in cases of disagreement, a third author participated to solve it. To rate the articles, a follow-up period ≥24 months was considered as “adequate” to rate this item.

All variables were collected in a database and analyzed with IBM SPSS statistics v. 20-0 (IBM Corp., Armonk, NY, USA). For the univariate description, we employed basic descriptive statistics.

The initial search provided 30 articles. Additional searches allowed ability to identify 11 more articles. Fifteen duplicated articles were eliminated and after reading the abstract, 11 were excluded because they were beyond the scope of this review. From the 15 articles assessed for eligibility, only 12 were finally included in the qualitative synthesis after excluding the articles that did not fulfill the inclusion criteria.^{12–16,22–28} Figure 2 summarizes the study selection process in a flow diagram.

These 12 articles corresponded to 3 cohort studies, 4 case series (2 multicentric), and 5 case reports, that involved a total of 151 patients and 460 implants. No RCTs or previous systematic reviews were found.

The study published in 2014 by Lorean et al²²  was a long-term retrospective case series including cases previously reported in the cohort published by Mazor et al¹³  and new cases. From the study published by Keller et al¹² Groups of study 1 and 3 were excluded and only data from the Group 2 (patients with bone grafts placed in the nasal floor via anterior intraoral nasotomy exposure) were included. Group 1 of this study included patients with bone grafts in the sinus floor but not in the nasal floor, and Group 3 included patients treated via an intraoral Lefort I osteotomy downfracture exposure. A more complex surgical procedure that could influence the results.²⁹  Moreover, although in some patients of this Group 3 a NFE was also performed, it is unclear how to identify in how many of them and how many implants were placed in these sites.

The final selected studies included data from 151 patients (79 female/72 male) with a mean age of 59 years, in which a total of 460 implants were placed. From the 101 patients in which this information was available, 30 (30%), were smokers. Sixty-one of the implants were placed in the lateral incisor position, 22 in the central position, 49 in the canine position, and in 328 implants (71%) the exact position was not stated.

The surgical approach was lateral (frontal) in 148 patients (98%) and transalveolar (transcrestal) in 2 patients (2%). Implants were placed in the same surgical time (1-time approach) in 119 patients (79%), in a second time in 17 patients (11%), and in 15 patients the time sequence was nor reported (10%). The most employed grafting material was xenograft (120 patients, 79.5%), followed by oral and/or extraoral autologous bone (25 patients, 16.5%). A mixture of xenograft and autologous bone was employed in 2 patients (1.3%), bone substitutes in 2 patients (1.3%), allograft in one patient (0.7%), and in another patient (0.7%) elevation was performed without any grafting material. No statistical differences were observed between the type of surgical approach or the grafting material used and implant survival.

The weighted average length of the placed implants was 12.5 mm. (range: 9–18). Information of mean implant length was not available from 33 implants (7%). Previous weighted bone height available was 8.68 mm. (range: 4.4–11.2), in the 122 patients (81%) in which this information was available. The weighted mean bone gain after implant placement and healing was 3.6 mm. (range: 1.1–7), within the 119 patients (79%) with available information.

In 12 patients (7.9%) missing teeth were restored using fixed prostheses, in 11 (7.3%) overdentures, 4 (2.6%) fixed-removable, and in another one (0.7%) a hybrid resin full-arch prosthesis. The type of restoration was not clearly stated in 123 patients (81.5%).

After a weighted follow-up of 32.2 months, the weighted survival rate was 97.64% (range: 89.2%–100%). No further complications were reported, except for one implant osseointegrated but nonfunctional (1/56 implants) in the study from Keller et al¹² In the 6-patient case series from El-Ghareeb et al¹⁵  a 75% rate of success was reported. Table 1 summarizes the most important data collected from the selected articles.

Individual study quality assessment was performed using the Newcastle-Ottawa tool for cohort studies, with 2 of the identified cohort studies rated as poor quality and one good quality. The NIH Study Quality Assessment Tool for case series was applied in the 4 identified case series, with 3 of them rated as fair, and a rating of poor for the remaining one. From the 5 case reports assessed with the “JBI critical appraisal tools for case reports,” 4 were rated as fair and 2 as poor quality (Table 2). Applying SORT (Strength Of Recommendation Taxonomy),¹⁹  no article was rated as level 1; 3 articles were level 2; and 9 had level 3 evidence. Due to the type of study design in selected studies and the great heterogeneity found in methodological aspects, a quantitative analysis followed by meta-analysis was not possible.

The level of evidence was grade 2b according to the Centre for Evidence-Based Medicine (University of Oxford) levels of evidence. In absence of randomized studies, the level of evidence was low, applying the GRADE (Grading of Recommendations, Assessment, Development and Evaluations) system. Based on the outcomes inconsistency and study quality level, the strength of evidence attending the SORT taxonomy was B.

Implants placed after NFE presented a good survival rate (97.64%) after a follow-up period of 32.2 months. No significant differences were observed in implant survival when compared to implants placed in the same area without a previous NFE.^30–34  Using contemporary dental implant systems, the overall implant survival rate after a long follow-up period has been estimated in the recent literature to be higher than 96.5%.^35–37  Implant site is one of many factors that can influence the success or failure of dental implants³⁸  and the failure rate of implants that were placed in the maxillary anterior region has been stated to be higher than in other regions.³⁹  Despite this, survival rate in maxillary anterior region in recent studies ranged between 96.2%³⁴  and 100%.³²  The use of narrow implants³  or short implants^40–42  did not seem to have a significant influence on these survival rates.

The good survival rates after NFE and the absence of reported complications in the included articles could be related to anatomical considerations, such as the resistance of the nasal membrane, the density of the bone at this area, or the stabilization of the implants thanks to the bicortical anchorage.^16–18  Lorean et al²²  also hypothesized that these surgeries were commonly performed by highly skilled and experienced specialists, positively influencing in the result.

The lateral access and the one-time surgical approach for the NFE have been much more widely reported than other approaches. The scarcity of data with the transcrestal approach and the 2-stage procedures did not allow to ascertain significant differences in implant survival rates. This is not to say that there was no influence of the surgical approach on the outcomes. To clarify this, new prospective studies or RCTs would be desirable.

The most employed grafting material was xenograft (79.5%), of bovine origin in almost all cases, followed by autologous bone. No differences have been observed related to implant survival in dependence of the grafting material employed. The use of autologous bone from an extraoral donor site was reported only in 2 studies, both performed more than 20 years ago.^12,23  The use of this donor site in the first procedures described implied some disadvantages, such as higher morbidity.⁴²  Probably for this reason, in more recent treatments the use of intraoral donor sites to harvest autologous bone and/or xenografts were preferred.

The exact position of the implant was reported in less than 30% of implants, being the lateral incisor the most common site. As there could be differences in bone density, implant angulation, diameter, or length, and other factors that could influence the outcomes,^39,44  it seems important to include this information in future studies.

The influence of smoking could not be definitively assessed in the present study. Although in the past there were some controversies on this topic, in a recent systematic review and meta-analysis, Naseri et al⁴⁵  observed a significantly enhanced risk of dental implant failure in smokers, with an increase in relation with the number of cigarettes smoked per day.

In relation to the mean length of the implants placed after NFE and the mean height of previously available bone, what is noteworthy is a progressive decrease of both variables in more recent studies^14,26,28 The development of increasingly shorter implants has made it possible to place short and extra-short implants in the atrophic premaxilla without the need for auxiliary surgical techniques in many of the cases in which it was previously necessary to perform a NFE. Moreover, the need for bone gain height required to allocate shorter implants could be reduced too. Long-term studies with short and extra-short implants after NFE were not found but are desirable to assess the outcomes of more updated standard practices.

No prosthetic complications were reported in the included studies, but the information related to the type of prosthesis or incidences is scarce and did not allow to obtain statistically significant conclusions. Neither surgical nor late complications were reported among the included studies, so NFE could be considered as a safe procedure. Nevertheless, complications such as nasal airflow reduction, exposition of the implant in the nasal cavity, or oronasal communication have been reported in the literature.⁴⁶ 

One of the limitations of the present study is the absence of RCTs, prospective studies, or other studies with a higher level of evidence. A meta-analysis was not possible to obtain and, as stated before, the treatment indications of older studies could be partially outdated. On the other hand, no previous systematic review has been conducted on the topic to the best knowledge of the authors, and the results obtained could encourage to perform new well-designed studies to clarify the important lack of information in some key points for clinical practice.

Within the limitations of this study, NFE could be considered as a predictable and safe procedure with a high rate of success. Despite the long history of this procedure the information available is scarce and partially outdated. New studies are needed to ascertain the best surgical approach or grafting material with a higher level of evidence.

Ethics approval and consent to participate is not applicable. Also, consent for publication is not applicable. Regarding competing interests, EA is the Scientific Director of BTI Biotechnology Institute, a dental implant company that explores the fields of oral implantology and PRGF-Endoret technology, and the president of Eduardo Anitua Foundation. MHA is a researcher at BTI Biotechnology Institute. MHA, AT, LP, and AE are researchers at the Eduardo Anitua Foundation. As for funding, this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. All authors have made substantial contributions to the conception or design of the work or the acquisition, analysis, or interpretation of data for the work; AND drafting the work or revising it critically for important intellectual content; AND have given final approval of the version to be published; AND agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.