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Short versus standard implants for single-crown restorations in the posterior region: A systematic review and meta-analysis

Short versus standard implants for single-crown restorations in the posterior region: A systematic review and meta-analysis



Short versus standard implants for single-crown restorations in the posterior region: A systematic review and meta-analysis




Journal of Prosthetic Dentistry, 2020-11-01, Volume 124, Issue 5, Pages 530-538, Copyright © 2019 Editorial Council for the Journal of Prosthetic Dentistry


Abstract

Statement of problem

Whether implant-supported crowns on short or standard implants have similar clinical outcomes in the posterior alveolar bone is unclear.

Purpose

The purpose of this systematic review and meta-analysis was to compare clinical outcomes, including survival rates, marginal bone loss (MBL), and complications associated with short implants and standard implants supporting a single crown in the posterior alveolar bone.

Material and methods

This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) principles and was registered with PROSPERO (CRD42018112978). The authors identified eligible trials published before August 2019 by searching PubMed, EMBASE, and the Cochrane Library. Only randomized controlled trials (RCTs) were included in the study, and quality assessment was performed by using the Cochrane Collaboration Risk of Bias tool. Relevant information was extracted by using a standardized form, and a meta-analysis was performed by using a software program.

Results

A total of 1954 references were identified. Five eligible trials were included in the quantitative synthesis. The survival rate of the short implants (≤6 mm) was similar to that of longer implants (>6 mm) in the short term ( P =.72; RR: 0.99; 95% CI: 0.97-1.02); however, long-term follow-up showed that short implants had a poorer survival rate than standard implants ( P =.01; RR: 0.94; 95% CI: 0.90-0.99). There was no significant difference in the MBL ( P =.94; MD: 0.00; 95% CI: −0.10 to 0.11).

Conclusions

The present study suggested that, although short implants have a higher crown-to-implant (C/I) ratio, they do not affect MBL. However, long-term follow-up comparisons indicated that short implants (≤6 mm) have a poorer survival rate than standard implants (>6 mm) ( P =.01). Nonsplinted crowns supported by short implants should be used with caution in the posterior alveolar bone.

Clinical Implications

Numerous clinical trials have indicated that short implants and standard implants have similar survival rates. The present study focused on short implants (≤6 mm) used for the prosthetic rehabilitation of single-teeth or nonsplinted crowns in the posterior alveolar bone and indicated that compared with longer implants, short implants had a lower survival rate. Nonsplinted crowns supported by short implants should be used with caution in the posterior alveolar bone.

Initially, the dental implant literature suggested longer implants were preferred when allowed by bone height because longer implants provide better primary stability and larger bone-to-implant contact (BIC). However, the alveolar ridge undergoes a resorption process after tooth loss. Therefore, many patients do not have adequate bone volume to receive a standard dental implant. However, with treatment to modify the surface topography and surface energy of implants, short implants with rough surfaces have demonstrated superior clinical outcomes to those with smooth surfaces. Although the definition of short implants varies in the literature, , recent studies have focused mainly on implants with lengths ≤6 mm. Therefore, the present study adopted the definition of a short implant as that with a length ≤6 mm.

Short implants can alleviate the need for additional surgical bone augmentation, reduce postoperative complications and morbidities, and reduce treatment time and cost. Therefore, the use of these implants is a valid option for patients who have insufficient bone volume, and short dental implants have become widely used. However, controversial clinical outcomes have been reported regarding the use of short implants in the posterior alveolar bone. Some authors have noted that short implants achieve a similar survival rate to standard implants, , while others have reported that short implants have lower survival rates than standard implants. , Previous meta-analyses and systematic reviews have been performed to clarify this controversy, but most of these studies , , , , combined both nonsplinted crown and splinted crown restorations, which may influence the results as splinting implants can disperse the stress on single implants and may reduce the incidence of mechanical complications and implant overload. , Therefore, it is necessary to conduct a meta-analysis to determine whether short implants can achieve similar survival rates for single crown restorations.

The purpose of this meta-analysis was to compare clinical outcomes, including survival rate, marginal bone loss (MBL), technical complications, and biological complications, of short implants (≤6 mm) and standard implants (>6 mm), with a single crown placed in the posterior alveolar bone. The first null hypothesis was that there is no difference between short and standard implants regarding the implant survival rate, and the second null hypothesis was that there is no difference in MBL.


Material and methods

This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) principles and was registered with PROSPERO (CRD42018112978). According to the patient, intervention, comparison, and outcome (PICO) scheme, the focused research question was whether short implants and standard implants have similar clinical outcomes supporting single crowns in the posterior alveolar bone. The patients included people who had lost teeth in the posterior alveolar region. The intervention group consisted of patients who received short implants to replace the missing teeth. The comparison group consisted of patients who received standard implants with or without bone augmentation. The primary outcome was the survival rates of the implants. The secondary outcomes were MBL, technical complications, and biological complications.

The eligible studies were required to meet the following criteria: RCTs comparing short implants (≤6 mm) and standard implants (>6 mm) supporting single crowns in the posterior region of the maxilla or mandible of partially edentulous patients in the same study, with a minimum of 10 implants per group and at least 1-year follow-up period after loading. Given multiple publications involving the same group of participants, only the article with the longest follow-up period was included. Non-English language articles were discarded.

The detailed search strategy is shown in Table 1 . Literature screening was conducted in 3 databases (MEDLINE via PubMed, EMBASE, and Cochrane Library) by 2 independent reviewers (X.X., Y.X.) for articles published before August 2, 2019. Disagreements were resolved by discussion or by involvement of a third reviewer (L.X.).

Table 1
Search strategy
Element Contents
Population Tooth extraction[MeSH Terms] OR Alveolar bone loss[MeSH Terms] OR Jaw, edentulous, partially[MeSH Terms] OR Dental implants[MeSH Terms] OR Tooth loss jaw[MeSH Terms] OR Posterior jaw OR Partially edentulous OR Single tooth loss OR Posterior implant OR Single crown OR Nonsplinted crown
AND
Intervention and comparison Bone substitutes[MeSH Terms] OR Alveolar bone grafting[MeSH Terms] OR Vertical ridge augmentation OR Sinus lift OR Longer implant OR Standard implant OR Long implant OR Short implant OR Short dental implant OR Bone augmentation OR Sinus floor elevation
AND
Outcome Dental restoration failure[MeSH Terms] OR Survival rate[MeSH Terms] OR Implant survival OR Implant failure OR Cumulative survival rate OR Marginal bone loss OR Peri-implant bone loss OR Marginal bone level OR Complication
AND
Study design Randomized Controlled Trial[Publication Type] OR Controlled clinical trial[Publication Type] OR Randomized[Title/Abstract] OR Randomly[Title/Abstract] OR Clinical trials as topic[MeSH Terms]

The relevant information about publication year, participants, interventions, and outcomes was extracted by 2 reviewers (X.X., Y.X.) by using a standardized form. The corresponding authors of the article were contacted for relevant missing, unclear, or unpublished data.

Two independent reviewers (B.H., Q.L.) performed the quality assessments of the selected trials by using the Cochrane Collaboration Risk of Bias tool. When there was a major disagreement, a third reviewer (L.X.) participated in the discussion until consensus was reached.

Statistical analyses were performed by using a meta-analysis software program (RevMan 5.3; Cochrane). Dichotomous variables were compared by using the Mantel-Haenszel (MH) method, and continuous variables were compared by using the inverse variance (IV) method. The outcomes were aggregated and analyzed by using a random-effects model or a fixed-effects model. When I 2 >50%, the random-effects model was used to merge data; otherwise, the fixed-effects model was used. The survival rates and complications were assessed by the risk ratio (RR), and MBL was assessed by the mean difference (MD) with corresponding 95% confidence intervals (CIs). The chi-square test and the I 2 statistic were used to assess heterogeneity.


Results

In total, 1954 references (MEDLINE via PubMed: 723; EMBASE: 794; Cochrane Library: 437) were identified. After removing the duplicate references, 1147 references were selected for title and abstract screening, and 19 articles were selected for full-text review. After applying the inclusion or exclusion criteria, 5 RCTs , , were included in this study ( Fig. 1 ).

Search strategy according to PRISMA statement.
Figure 1
Search strategy according to PRISMA statement.

Studies were excluded for the following reasons: 8 studies , used different types of restoration in the prosthetic procedures; 3 studies reported data from the same group of participants; 1 study contained only 2 implants with a length ≤6 mm; 1 study had a follow-up time <12 months; and 1 study was not an RCT.

Table 2 shows the characteristics of the 5 included studies. In total, 372 implants (group short: 184; group standard: 188) were installed in 281 participants with a mean age of 52 years. In all 5 studies, participants who received 6-mm-long implants were divided into the test group. The control groups contained a variety of implant lengths ranging from 10 to 15 mm.

Table 2
Characteristics of studies included (
n =5)
Author Year Country Number of Participants (Participants Dropped out) Number of Implants Arch Test and Control Group (mm Long × mm Wide Implants) Crown-To-Implant Ratio Implant Systems Mean Age Loading Protocol Bone Graft Follow-up Outcomes Measures
Standard Implants Short Implants
Gulje, F. L. 2014 Netherlands Short: 21 (0)
Long: 20 (1)
Short: 21
Long: 20
Maxilla Test: 6×4
Control: 11×4
Short: NR
Long: NR
Astra Tech OsseoSpeed 49 Conventional Standard implants with maxillary sinus floor augmentation 1 y Survival rate: 100%
Fail implants: 0
Biological complications: 0
Technical complications: 0
MBL (mean and SD):
0.1 (0.3)
Survival rate: 100%
Fail implants: 0
Biological complications: 0
Technical complications: 0
MBL (mean and SD):
0.1 (0.2)
Rossi, F. 2015 Italy Short: 30 (0)
Long: 30 (0)
Short: 30
Long: 30
Maxilla and Mandible Test: 6×4.1
Control: 10×4.1
Short: 1.49 (SD, 0.36)
Long: 0.95 (SD, 0.21)
Straumann 48 Early Bone augmentation not performed 5 y Survival rate: 96.7%
Fail implants: 1
Biological complications: NR
Technical complications: 0
MBL (mean and SD):
0.18 (0.57)
Survival rate: 86.7%
Fail implants: 4
Biological complications: NR
Technical complications: 0
MBL (mean and SD):
0.14 (0.49)
Naenni, N. 2018 Switzerland Short: 47 (3)
Long: 47 (1)
Short: 47
Long: 47
Maxilla and Mandible Test: 6×4.1
Control: 10×4.1
Short: 1.75 (IQR, 1.50 to 1.90)
Long: 1.04 (IQR, 0.95 to 1.15)
Straumann 58 Early Standard implants with maxillary sinus floor augmentation 5 y Survival rate: 100%
Fail implants: 0
Biological complications: NR
Technical complications: NR
MBL (mean and SD):
0.12 (0.8)
Survival rate: 91%
Fail implants: 4
Biological complications: NR
Technical complications: NR
MBL (mean and SD):
0.13 (0.77)
Thoma, D. S. 2018 Switzerland Short: 50 (6)
Long: 51 (5)
Short: 63
Long: 70
Maxilla Test: 6×4
Control: (11, 13, or 15)×4
Short: 1.86 (SD, 0.23)
Long: 0.99 (SD, 0.17)
Astra Tech OsseoSpeed 51 Delay Standard implants with maxillary sinus floor augmentation 5 y Survival rate: 100%
Fail implants: 0
Biological complications: 9
Technical complications: 14
MBL (mean and SD):
0.18 (0.96)
Survival rate: 98%
Fail implants: 1
Biological complications: 5
Technical complications: 21
MBL (mean and SD):
0.12 (0.54)
Weerapong, K. 2019 Thailand Short: 23 (0)
Long: 23 (0)
Short: 23
Long: 23
Mandible Test:
Long: 6
Wide: Not report
Control:
Long: 10
Wide: Not report
Short: NR
Long: NR
PW+ 51 Immediate Bone augmentation not performed 1 y Survival rate: 95.65%
Fail implants: 1
Biological complications: NR
Technical complications: 2 (interim restoration)
MBL (mean and SD):
0.26 (0.27)
Survival rate: 91.30%
Fail implants: 2
Biological complications: NR
Technical complications: 3 (interim restoration)
MBL (mean and SD):
0.33 (0.47)
IQR, interquartile range; MBL, Marginal bone loss; NR, not reported; SD, standard deviation.

Table 3 shows the quality assessment of the included studies. The Cochrane criteria indicated a low risk of bias for the random sequence generation and allocation concealment. However, it is difficult to blind the surgeon and participants during bone augmentation or the dental implant placement procedure even though the blinding of participants was attempted. All studies reported complete results without selective outcome reporting ( Figs. 2 , 3 ).

Table 3
Risk of bias assessment of included studies
Bias Gulje, F.L. Rossi, F.
Random sequence generation Low risk: A block randomization sequence was used. Unclear risk: no information provided
Allocation concealment Low risk: The information of treatment allocation was enclosed in a sealed envelope. Low risk: Sealed numbered envelopes were prepared from the monitor.
Blinding of participants and personnel High risk: difficulty of blinding the surgeon and participants during bone augmentation or procedure of placement dental implants High risk: difficulty of blinding the surgeon during the procedure of placement dental implants
Blinding of outcomes assessment Unclear risk: no information provided Unclear risk: no information provided
Incomplete outcome data Low risk: drop-out or lost to follow information provided Low risk: drop-out/lost to follow information provided
Selective reporting Low risk: reported all the intended outcomes described in the methodology of this study Low risk: reported all the intended outcomes described in the methodology of this study
Other bias Low risk: The study appears to be free of other sources of bias. Low risk: The study appears to be free of other sources of bias.
Naenni, N. Thoma, D.S. Weerapong, K.
Low risk: Randomization was performed according to a computer-generated randomization list. Low risk: A block randomization sequence was used. Low risk: Randomization was performed according to a computer-generated randomization list.
Unclear risk: no information provided Low risk: The randomization was performed at the day of surgery after flap elevation by using a sealed envelope. Unclear risk: no information provided
High risk: difficulty of blinding the surgeon and participants during bone augmentation or procedure of placement dental implants High risk: difficulty of blinding the surgeon and participants during bone augmentation or procedure of placement dental implants High risk: difficulty of blinding the surgeon during the procedure of placement dental implants
Low risk: Statistical analyses were performed with the average values of 2 independent examiners’ measurements. Low risk: Clinical evaluation of the outcome was performed by an independent examiner without the knowledge of the group allocation. Low risk: Clinical evaluation of the outcome was performed by 2 independent examiners.
Low risk: drop-out/lost to follow information provided Low risk: drop-out/lost to follow information provided Low risk: drop-out/lost to follow information provided
Low risk: reported all the intended outcomes described in the methodology of this study Low risk: reported all the intended outcomes described in the methodology of this study Low risk: reported all the intended outcomes described in the methodology of this study
Low risk: The study appears to be free of other sources of bias. High risk: Eleven participants dropped out for various reasons that may affect the outcome. Low risk: The study appears to be free of other sources of bias.

Risk of bias summary.
Figure 2
Risk of bias summary.

Risk of bias graph.
Figure 3
Risk of bias graph.

Three studies , , were followed up to 5 years. Gulje et al reported 1-year follow-up data in a manuscript published in 2014; therefore, the reviewer contacted the corresponding author and learned that the 5-year report of the trial was under revision and the number of surviving implants at the 5-year follow-up was obtained. One participant (11-mm group) died before the 12-month evaluation, 1 participant (6-mm group) moved without leaving an address, 1 participant (6-mm group) lost an implant at 4 years, and the remaining 38 participants completed 60 months of follow-up. To minimize bias, the unpublished data were included in the meta-analysis.

The meta-analysis indicated that short implants (≤6 mm) had a survival rate similar to that of standard implants (>6 mm) at the 1-year follow-up ( P =.72; RR: 0.99; 95% CI: 0.97-1.02) ( Fig. 4 ); however, long-term follow-up showed that short implants had a poorer survival rate than standard implants ( P =.01; RR: 0.94; 95% CI: 0.90-0.99) ( Fig. 5 ). The heterogeneity test did not reach statistical significance, suggesting low between-study heterogeneity.

Forest plot of survival rate comparing short implants with standard implants in 1 year. CI, confidence interval.
Figure 4
Forest plot of survival rate comparing short implants with standard implants in 1 year. CI, confidence interval.

Forest plot of survival rate comparing short implants with standard implants in 5 year. CI, confidence interval.
Figure 5
Forest plot of survival rate comparing short implants with standard implants in 5 year. CI, confidence interval.

Four studies , , , reported no significant difference in the MBL between the test group and the control group. Only Rossi et al reported significant differences. However, the marginal bone resorption between the time of prosthesis implantation and after 5 years of function was only 0.14 mm in the short group and 0.18 mm in the standard group, with limited clinical significance. Naenni et al reported the interquartile range of MBL; therefore, the corresponding author was contacted for the mean values and standard deviations of MBL. In general, the MBL stabilized after the first year of function. It was impossible to extract 1-year follow-up MBL data for 1 study. Therefore, only the final follow-up data for MBL were used for the meta-analysis. Figure 6 shows the MBL forest plot. The fixed-effects model did not show a statistically significant difference ( P =.94; MD: 0.00; 95% CI: −0.10 to 0.11).

Forest plot of marginal bone loss comparing short implants with standard implants at final follow-up measurement. CI, confidence interval.
Figure 6
Forest plot of marginal bone loss comparing short implants with standard implants at final follow-up measurement. CI, confidence interval.

Thoma et al reported the exact number of biological complications (group short: 5; group standard: 9) and technical complications (group short: 21; group standard: 14) that occurred on the implant level during the entire study period. Naenni et al reported a few technical complications that occurred but did not report an exact number of events. Weerapong et al reported that 5 interim crowns fractured (group short: 3; group standard: 2), but no restoration failure occurred with the definitive restorations. Because of various complications reported with different definitions, a meta-analysis of complications could not be performed in the present study.


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