What is known

Recent studies support the use of short (lengths ≥6 and ≤8 mm) implants as a predictable method of rehabilitation in posterior regions only if splinted. Moreover, most of the studies reported follow-ups of less than 5 years, with the absence of a specific assessment of different types of implant design.

What this study adds

This retrospective study reports promising 5-year outcomes in terms of implants survival for short and ultra-short implants with a plateau design, a locking-taper implant-abutment connection and restored with single crowns.

1 INTRODUCTION

In the last decades implant placement has become a widespread and predictable methodology of treatment for partially edentulous posterior regions. However, in some instances the long-term absence of teeth has resulted in extensive resorption of the alveolar ridge.1 As a consequence, there may not be a sufficient volume of bone available for standard implant rehabilitation without subjecting the patient to difficult, invasive, and costly surgical grafting procedures.2 In these specific conditions, the placement of short (lengths ≥6 and ≤8 mm) implants3 may represent a valid alternative therapy to augmentation procedures for maxillary and mandibular jaw atrophies. Moreover, the use of short implants reduces the number of surgeries, potential morbidity, and time and costs of the treatment. Several meta-analyses showed no significant differences in survival rates between short and standard diameter implants.4-7 With a better understanding of the mechanisms underlying osseointegration and the development of new technologies, consisting in modifications of implant surfaces8 (through traditional methods9 or bioactive components10) and improvement of macrodesign features,11, 12 short implants have shown high-quality performances.

The success of short implants has led to the development of a new class of implants, ultra-short implants (length < 6 mm).13 Recent meta-analyses and systematic reviews focusing on implants with lengths ≤6 mm14-19 have supported the use of these implants as a predictable method with stable results, but most of the studies reported follow-ups of less than 5 years. Moreover, most of them combined splinted crown and single-crown restorations2, 4-7, 20, 21: the absence of a specific assessment of different types of restoration design may certainly represent a serious limitation in reaching effective conclusions regarding the survival and success of ultra-short implants.

In a previous investigation,22 short implants were used to restore multiple adjacent teeth loss with unsplinted single crowns, which may be suggested as a gold standard in terms of oral hygiene procedures and framework. On the other hand, the analysis of implant survival and success has been strictly connected with peri-implant marginal bone loss, a critical issue in the case of unsplinted prosthetic rehabilitations characterized by disproportionate crown-to-implant ratios (CIRs).23 Authors of the present study support the hypothesis that these issues are of particular importance in case of ultra-short implants, as the stability of marginal bone levels appears to be of crucial importance for implant survival at the medium and long-term follow-ups. Despite discrepancies among different authors regarding the definitions of marginal bone loss and success criteria22, 24-27 in the evaluation of short and ultra-short locking-taper implants, the threshold for bone loss compatible with implant success was set at 1 mm after 5 years. This rigorous value was proposed considering that a threshold of 2 mm after a medium-term follow-up, even if allowable for 8-mm length implants, cannot be considered acceptable for 6.0- and 5.0-mm length implants (the shortest groups), for the involvement, in terms of marginal bone loss, of almost half of implant length.

Given the limits of previously published investigations (especially short-term 3-year follow-up), and since controversial outcomes are still present in the literature for ultra-short implants supporting single crown restorations,5, 28-30 the aim of this retrospective study was to evaluate whether the clinical and radiographic results of ultra-short (5-mm length) locking-taper implants supporting single crowns are comparable to short implants (6- and 8-mm length) after 5 years of follow-up.

2 MATERIALS AND METHODS

A retrospective study with a 5-year follow-up was conducted in 2020 on patients who had been referred between February 2007 and June 2015, for edentulism (tooth loss caused by trauma, caries or periodontal disease) in the posterior areas of maxilla and mandible at the Dental and Maxillo-Facial Surgery Clinic at the University of Verona. The study was approved by the University Institutional Review Board (Prot. 34 939, CROWNMAXMAND, 30/05/18). The nature and aim of the study, together with the anonymity in the scientific use of data, were clearly presented in a written informed consent form, and signed by every patient. All procedures accorded with Helsinki Declaration and good clinical practice guidelines for research on human beings.

Patients enrolled for the study matched the following inclusion criteria22, 31-33: aged between 18 and 90 years; having had single-tooth replacement of at least one 8.0, 6.0, or 5.0 mm locking-taper implant supporting a single crown; had no previous consent for bone augmentation procedures; had a history of treated chronic periodontal diseases, or were never affected by any form of periodontal disease; and who were compliant with a regular maintenance program (professional oral hygiene sessions every 4 months). Exclusion criteria of the study were previously described22, 31-33 (see Appendix).

The locking-taper (Morse taper or Morse cone) dental implant system (Bicon Dental Implants; Bicon LLC) used in this study is characterized by a convergent crest module, platform switching, plateau root-form design, and an Integra CP surface (hydroxyapatite treated and acid-etched).22, 31-33

All surgical treatments were conducted by a single clinician, as previously described22, 31-33 (see Appendix). After 4–6 months the implants were surgically uncovered, healing abutments were placed, and the mucosal flaps readapted around them. After 3 weeks of soft-tissue healing, impressions were taken using a polyether material (3M ESPE Impregum Impression Material). Definitive single-crown porcelain or composite restorations were placed within 2 weeks. The choice for restorative materials (porcelain or composite) was based on patients' preference, which was guided by personal economic resources in most of the cases (see Appendix). The technique used for the composite restorations was the Integrated Abutment Crown (IAC), in which crowns are conventionally fabricated but also extraorally cemented to the abutment, excess cement is removed and then the one-piece abutment and crown are inserted (see Appendix).34

Recall appointments were established to manage prosthetic complications as needed, and a maintenance program was designed to provide patients a professional oral hygiene session every 4 months. Clinical and radiographic examinations22, 31-33 were performed during the follow-up 5 years from loading time, one time per year at regular intervals.

The postsurgery evaluations and the follow-up evaluations were performed by two other operators both of whom were different from the clinician who performed the surgical phase.

Implant lengths considered in the study were 8.0, 6.0, and 5.0 mm; implant diameters were 3, 3.5, 4.0, 4.5, 5.0, 6.0, and 6.5 mm. Covariates included were: sex, age, smoking history, history of periodontal disease35 (see Appendix), American Society of Anesthesiologists (ASA) status, number of oral hygiene sessions per year, use of interproximal oral hygiene devices, arch involved, tooth site, prosthetic material, CIR.22, 31-33

Study outcomes were implant survival, marginal bone loss, and implant success after 5 years of follow-up, which were assessed according to covariates. In regard with implant survival, failure was considered as the need for implant removal either before loading (due to absence of osseointegration), or after loading (due to excessive bone loss). Implant survival was considered as the implant's state of being in function at the 5-year follow-up evaluation, for example, symptom-free, without mobility, radiolucency, or bone loss so severe as to warrant implant removal.33, 36-38

A descriptive analysis was conducted between loading time and the 5-year follow-up time, according to covariates. This included assessment of crestal bone level (CBL; average bone level around implants at mesial and distal sides, expressed in mm), first bone-to-implant contact (F-BIC; in mm)39-41 with their variations ΔCBL (average bone loss) and ΔF-BIC (average apical shift of the “first bone-to-implant contact point” position) (see Appendix).

Peri-implant soft tissues were assessed using a periodontal probe (Florida Probe; Florida Probes Company) and applying a force of mild intensity (0.25 N). For each implant site, four parameters were assessed. The Modified Bleeding Index (mBI), and the Modified Plaque Index (mPLI), as reported in the literature by Mombelli,27 were used to record the appropriate values for the mesial, central, and distal on the buccal and lingual/palatal sides of each implant. Similarly, the peri-implant probing depths (PPD) were performed on the same six sites. The amount of keratinized tissue (KT) was assessed by measuring the distance between the zenith of the buccal gingival margin and the mucogingival line.

Biological complications after loading were also assessed at the 5-year recall appointment. According to the latest updates,42 peri-implant mucositis was defined as at least one soft-tissue peri-implant surface with positive BOP or pus on probing, PPD ≥4 mm, and no radiographically detectable bone loss. It should be noted that visual signs of inflammation can vary and that peri-implant mucositis can exist around implants with variable levels of bone support.35 We diagnosed peri-implantitis when an implant had simultaneously one surface with positive BOP or pus on probing, increasing PPD compared to previous examinations or PPD ≥5 mm in the absence of the previous examination data, and presence of radiographically detectable bone loss greater than 1 mm when compared with the loading measurements. As opposed to earlier 3-year studies on locking-taper implants, the threshold for bone loss at a longer follow-up of 5 years was set at 1 mm. This was done in recognition of the fact that in the present study implant length was highly reduced compared to other longer implant types, for which a threshold of 2 mm can be considered acceptable.35, 42 In case of 6.0- and 5.0-mm length implants, a marginal bone loss of 2 mm, representing slightly less than half of the entire implant length, appears to be underestimated after 5 years of follow-up.

Implant success was defined according to the following criteria43, 44 and to the defined bone loss threshold: absence of persistent pain, dysesthesia, or paraesthesia in the implant area; absence of peri-implant infection with or without suppuration; absence of perceptible mobility of the implant; and finally, absence of persistent peri-implant bone resorption greater than 1 mm during the time interval from loading to 5-year follow-up. Once the failed implants were excluded, implant success can be considered implants without signs of peri-implantitis.

For data collection, a database including all patients evaluated in the study was created with Microsoft Excel. All data analysis was carried out using Stata v.13.0 for Macintosh (StataCorp). The normality assumptions for continuous data were assessed by using the Shapiro–Wilk test; mean and standard deviation were reported for normally distributed data, median and interquartile range (IQR) otherwise. For categorical data, absolute frequencies, percentages and 95% confidence intervals were reported. The association between categorical variables was tested with χ2 test; if any of the expected values was less than 5, a Fisher's exact test was performed. The comparison between the means of continuous variables in two different times was performed by using paired Student's “t”-test or Wilcoxon matched-pairs signed-rank test. The comparison between the means of two different groups was performed using unpaired Student's “t”, or Wilcoxon rank-sum test. The comparison of the means among more than two groups was done using one-way analysis of variance, or Kruskal–Wallis equality-of-populations rank test. A multivariate analysis (logistic regression) was carried out to find factors associated with implant success. Significance level was set at 0.05.22, 31-33

By way of illustration, Figures 1A–D and 2A–D report some radiographic cases.

Single implants placed in 3.4, 3.5, and 3.6 sites (5 × 6 mm, 4 × 5 mm, 5 × 5 mm): (A) Preoperative radiograph before implants placement; (B) radiograph obtained at time of loading; (C) radiograph obtained at 3-year follow-up; (D) radiograph obtained at 5-year follow-up

Single implants placed in 2.4, 2.5, and 2.6 sites (4 × 8 mm, 4.5 × 8 mm, and 5 × 6 mm): (A) Preoperative radiograph before implants placement; (B) radiograph obtained at time of loading; (C) radiograph obtained at 3-year follow-up; (D) radiograph obtained at 5-year follow-up

3 RESULTS 3.1 Demographics

A total of 142 patients (65 men and 77 women) received at least one 8.0-, 6.0-, or 5.0-mm length single-crown dental implant. Of the patients, 78.87% (112/142) were nonsmokers, 59.15% (84/142) had an ASA status I, and 52.82% (76/142) had a history of periodontal disease. All patients were compliant with the maintenance program, receiving on average, three oral professional hygiene sessions in a year; 74.65% (106/142) of them used interproximal oral hygiene devices daily. Mean age at placement was 53.48 ± 10.39 (range 29–78) years.

Of the 333 implants placed, 127 (38.14%) were 8-mm length, 115 (34.53%) were 6-mm length, and 91 (27.33%) were 5-mm length. The majority of 8-mm length implants were placed in premolar regions, while 5- and 6-mm length implants were placed in molar regions. One implant in the posterior maxilla failed before loading, thus 332 implants in 141 patients (64 men and 77 women) were finally rehabilitated with single crowns. The mean CIR was 1.94 ± 0.85 (range 0.91–3.81) and 45.48% of the implants presented a CIR ≥2. CIR in 8-, 6-, and 5-mm length, respectively, was 1.45 ± 0.32 (range 0.91–3.06), 2.01 ± 0.48 (range 1.09–3.03), and 2.57 ± 0.59 (range 1.80–3.81), with statistically significant differences among groups (p < 0.001).

The implants distribution, analyzed according to length-groups, is presented in Table 1.

TABLE 1. Overall implants and length-groups distribution according to study variables. Age at follow-up and oral professional hygiene/year, respectively, are presented as mean ± standard deviation and median (IQR) Variable Overall 5 mm 6 mm 8 mm Test statistic df p Value Sex Male 150 (45.05) 38 (41.76) 59 (51.30) 53 (41.73) χ2 = 2.78 2 NS (p = 0.24) Female 183 (54.95) 53 (58.24) 56 (48.70) 74 (58.27) Age at follow-up 59.57 ± 10.52 58.55 ± 9.82 60.34 ± 9.86 59.62 ± 11.56 χ2 = 5.49 2 NS (p = 0.06) Smoking history No 268 (80.48) 67 (73.63) 97 (84.35) 104 (81.89) χ2 = 3.97 2 NS (p = 0.13) Yes 65 (19.52) 24 (26.37) 18 (15.65) 23 (18.11) ASA status I 159 (47.75) 47 (51.65) 53 (46.09) 59 (46.46) χ2 = 0.76 2 NS (p = 0.68) II 174 (52.25) 44 (48.35) 62 (53.91) 68 (53.54) Oral professional hygiene/year 3 (2) 3 (2) 3 (2) 3 (2) F = 0.74 1/332 NS (p = 0.47) Use of interproximal oral hygiene devices No 75 (22.52) 20 (21.98) 29 (25.22) 26 (20.47) χ2 = 0.80 2 NS (p = 0.67) Yes 258 (77.48) 71 (78.02) 86 (74.78) 101 (79.53) History of periodontal disease No 120 (36.04) 25 (27.47) 44 (38.26) 51 (40.16) χ2 = 4.07 2 NS (p = 0.13) Yes 213 (63.96) 66 (72.53) 71 (61.74) 76 (59.84) Implant tooth site Premolar 146 (43.84) 35 (38.46) 30 (26.09) 81 (63.78) χ2 = 36.29 2 <0.001 Molar 187 (56.16) 56 (61.54) 85 (73.91) 46 (36.22) Arch Posterior mandible 197 (59.16) 52 (57.14) 74 (64.35) 71 (55.91) χ2 = 1.99 2 NS (p = 0.37) Posterior maxilla 136 (40.84) 39 (42.86) 41 (35.65) 56 (44.09) Implant diameter 3 mm 1 (0.30) 0 (0.00) 0 (0.00) 1 (0.79) 3.5 mm 11 (3.30) 0 (0.00) 0 (0.00) 11 (8.66) χ2 = 173.38 6 <0.001 4 mm 81 (24.32) 43 (47.25) 1 (0.87) 37 (29.13) 4.5 mm 112 (33.63) 0 (0.00) 55 (47.83) 57 (44.88) 5 mm 109 (32.73) 30 (32.97) 58 (50.43) 21 (16.54) 6 mm 18 (5.41) 17 (18.68) 1 (0.87) 0 (0.00) 6.5 mm 1 (0.30) 1 (1.10) 0 (0.00) 0 (0.00) Prosthetic material Resin 47 (14.16) 14 (15.38) 18 (15.79) 15 (11.81) χ2 = 0.93 2 NS (p = 0.62) Porcelain 285 (85.84) 77 (84.62) 96 (84.21) 112 (88.19) Crown-to-implant ratio <2 181 (54.52) 3 (3.30) 56 (49.12) 122 (96.06) χ2 = 195.16 2 <0.001 2–2.99 133 (40.06) 73 (80.22) 56 (49.12) 4 (3.15) >2.99 18 (5.42) 15 (16.48) 2 (1.75) 1 (0.79) Note: For all other variables, values are presented as n (%). Abbreviations: ASA, American Society of Anesthesiologists; df, degrees of freedom; IQR, interquartile range; NS, not statistically significant. 3.2 Implant survival

One early failure in one patient was assessed, and 12 implants were lost and removed after functional loading in 12 different patients. The overall implant-based survival at 60-month follow-up was 96.10% (320/333). Failures features are recorded in Table 2. The overall patient-based survival was 91.55% (130/142). No association was found between survival and failure groups, and any of the covariates considered (Table 3).

TABLE 2. Failures features Site 16 24 24 26 34 35 35 44 45 46 46 47 17 Diameter 5 4 4 5 4.5 4 4.5 5 6 4 4.5 5 5 Length 6 5 8 6 8 5 6 8 5 5 8 6 6 Sex M F F F M F M M M F M F M Smoking history Yes No No Yes Yes No No No No No No No No ASA status II I I II II II I II I II I II I