, Young-Kyun Kim2 As implant treatment become gold standard for edentulous patients, many research has been conducted to overcome insufficient bone height. A short implant, which is defined as the length of less than 8 mm1-3), were recently suggested to avoid complicated bone graft surgery. Short implants have been suggested several advantages with simple surgical protocols, including prevention of overheating, short operative time, easily manageable instruments4).
Poor crown/implant ratio (C/IR) is regarded the highest risk factors of short implants, because of the unsuitable mechanical characteristics compared to the conventional implants5-8). Some research were showed unfavorable clinical outcomes with the machined surface short implants9,10). As the implant surface has been improved, however, short-term survival rate for short implants was increased11). In 2021, Ku et al.12) reported a 14-year retrospective study that short implants (<8 mm) in posterior edentulous areas showed comparable long-term outcomes of marginal bone loss and success and survival rates with conventional implants regardless of other clinical variables such as surgical procedure, crown/implant ratio, prosthetic type, and arch location. However, a long-term poor prognoses of short implants was still controversial in posterior molars especially with single prosthetic11,13-15).
The purpose of this study was to analyze the long-term results of single short implants in posterior mandible, and to analyze the success, survival, and 10-year cumulative survival rates according to the implant length between 7 and 8 mm.
This study was conducted according to the principles of the Declaration of Helsinki for research on humans. The patients for this study were enrolled who received 7 and 8 mm implants in mandibular molar areas between November 2005 and February 2014. Inclusion criteria were as followed 1) Treated by one expert oral and maxillofacial surgeon and by another expert prosthodontist. 2) Visited the clinic for clinical and radiologic follow-up over a year after prosthetic loading. 3) Over 20 years old. Exclusion criteria were as followed 1) Inappropriate or lack of medical records. 2) Uncontrolled systematic diseases 3) Maxillofacial syndromes 4) Additional bone grafts for the implant.
For this retrospective study, medical records were reviewed investigated including age, sex, primary and secondary stability, complications related to surgery and prosthetics, and information of the implant. The primary was examined at the implant placement, and secondary stability was examined at prosthetic impression by Osstell Mentor device (Osstell, Gothenburg, Sweden). The healing period was defined as the time from fixture implantation to prosthetic impression taking. Postoperative complications were defined as peri-implantitis marginal bone loss > 3 mm by using a paralleling cone technique by a digital intraoral radiography device (Heliodent Sirona, Sirona Dental Systems Inc., NY, USA; 60 kVp, 7 mA, 0.16s of exposure time, and bleeding on probing, Fig. 1) and osseointegration failure, and prosthetic complications were defined as screw loosening, screw fracture, and crown fracture. The success of implants was defined following Albrektsson’s criteria; 1) No implant movement; 2) No radiolucency around the implant; 3) No persistent pain, discomfort, or infection; and 4) Bone loss less than 0.2 mm annually after the first year and 1.5 mm during functional loading16). Implant survival was defined as retention of implants to final follow-up regardless of complications17).
Figure 1. Evaluation of radiographic information for single implants12). The lengths of fixture (a) was measured from the fixture apex to the top of the fixture. The crown length (b) was measured from the top to the most occlusal point. Marginal bone loss was measured as the average distance from the marginal bone of the distal (c) and mesial (c’) sides to the top of the fixture. The mesiodistal cantilever was measured as the difference between the distal margin based on a perpendicular line from the implant fixture center (d) and the most occlusal point of the mesial contact (d’). Every measurement value was adjusted according to the distortion ratio of the actual length of the fixture and (a).
The cumulative survival rate at 10-years was calculated by Kaplan-Meier survival curve. Hazard ratios of the clinical and radiographic variables were analyzed by constructing the proportional hazard Cox regression model. Estimates for the hazard ratios were calculated by a univariate analysis (one by one explanatory variable) to estimate the association between success and survival times, and explanatory variables. The differences between subgroups were analyzed using independent t-tests at the significant probability of 95% (SPSS Ver. 25.0, SPSS Korea Institute, Inc. Seoul, Korea).
Of total 81 single implants of 81 patients (58.4±11.5 years, 45 males and 36 females) in the mandibular molar region were analyzed. There were 32 implants 7 mm in length (14 non-submerged and 18 submerged) and 49 implants 8 mm in length (26 non-submerged and 23 submerged). The distribution of implantation procedures was not significantly different between implant lengths (Table 1).
Table 1 . Variables for single implants in mandibular molars according to length
| 7 mm (N=32) | 8 mm (N=49) | P* | |
|---|---|---|---|
| Male:Female | 17:15 | 28:21:00 | 0.726 |
| Age | 54.1 10.2 | 61.2 11.5 | 0.005 |
| Non-submerged:Submerged | 14:18 | 26:23:00 | 0.419 |
| Average of variables (SD) | |||
| Width of implant | 5.3 (0.7) | 5.5 (0.6) | 0.359 |
| Primary stability (ISQ) | 66.7 (21.0) | 73.5 (12.0) | 0.124 |
| Secondary stability (ISQ) | 76.4 (12.5) | 81.1 (6.6) | 0.065 |
| Healing period (months) | 4.7 (3.0) | 3.2 (1.2) | 0.013 |
| Crown/Implant ratio (%) | 1.5 (0.3) | 1.6 (0.3) | 0.17 |
| Follow-up (years) | 6.1 (3.6) | 7.3 (2.2) | 0.075 |
| Marginal bone loss (mm) | 0.5 (1.0) | 0.5 (0.5) | 0.823 |
| Clinical results of implant | |||
| Success rate (%) | 90.6 (29.6) | 95.9 (20.0) | 0.339 |
| Survival rate (%) | 93.8 (24.6) | 98.0 (14.3) | 0.333 |
*Independent t test.
All implants were internal type connection with rough surfaces; resorbable blasted media (RBM) surface titanium implants including 20 Osstem implants (Busan, Republic of Korea; 13 GS IIs and 4 GS IIIs of bone level fixture, 3 SS IIs of tissue level fixture) and sandblasted, large grit, and acid-etched (SLA) surface titanium implants of 61 Dentium implants (Suwon, Republic of Korea; 56 Superline and 5 Implantium of bone level fixture). The average primary and secondary ISQ values were 70.7±15.6 and 76.4±11.1, respectively. The average healing period was 4.7±3.0 months. The prosthetics were fabricated with metal, ceramic, or zirconia occlusion with the mesiodistal cantilever differences within 3 mm. The average C/IR was 1.6±0.3. All implant occlusions were adjusted to the centered contacts and minimized cantilevers18). Over an average period of 7.0±2.8 years, marginal bone loss was 0.5±0.8 mm. The implant failure was occurred for three implants complications at 0.3, 1.2, and 10.2 years after the loading, respectively. The other complications were occurred on one implant with screw loosening after 2 years and another implant with peri-implantitis after 2.2 years.
As shown in Table 1, the 7 and 8 mm implants were on average 5.3±0.7 and 5.5±0.6 mm in width, respectively. Primary stability was 66.7±21.0 and 73.5±13.0 in the 7 and 8 mm implants, respectively. The healing period was longer in the 7 mm implants (4.7±3.0 months) than the 8 mm implants (3.2±1.2 months) (P=0.013). However, the secondary stability was not significantly different between the non-submerged and submerged implants (76.4±12.5 and 81.1±6.6, respectively, P=0.065). The C/IR was 1.5±0.3 and 1.6±0.3 in the 7 and 8 mm implants, respectively. The average observation period was 6.1±3.6 and 7.3±2.2 years and marginal bone loss was 0.5±1.2 and 0.5±0.6 mm the 7 and 8 mm implants, respectively.
Three implants were explanted, and two were excluded to the success criteria. In the implants of 7 mm, the ⌀ 5.0 mm implant (Osstem SSII, Osstem, Korea) was placed with 1-stage protocol of 71 ISQ and occlusion with natural maxillary teeth, but showed marginal bone loss of 2.9 mm at 2.9 years after the loading. Another ⌀ 5.0 mm implant (Superline, Dentium, Korea) was placed with 2-stage protocol of 85 ISQ for primary stability and occlusion with full denture, but explanted due to the marginal bone loss of 5.0 at 1.2 years after the loading. The ⌀ 4.5 mm implant (Osstem GSII, Osstem, Korea) was placed with 2-stage protocol of 76 ISQ for primary stability and occlusion with nature maxillary teeth, but failed at 10.2 years after the loading due to the progressed peri-implantitis. In the implants of 8 mm, the ⌀ 6.0 mm implant (Superline) was placed with 2-stage protocol of 46 ISQ and occlusion with natural maxillary teeth, but explanted at 0.3 years after the loading due to the peri-implantitis. The other ⌀ 5.0 mm implant (Superline) was placed with 1-stage protocol of 75 ISQ and occlusion with natural maxillary teeth, but showed screw fracture at 7.3 years after the loading without the osseointegration failure.
Success rate was 90.6±29.6 and 95.9±20.0% in the 7 and 8 mm implants. Survival rate was 93.8±24.6 and 98.0±14.3% in the 7 and 8 mm implants, respectively. The 7 and 8 mm implants had 96.4±3.5 and 97.6±20.2% 10-year cumulative survival rates (Fig. 2).
Figure 2. Kaplan-Maier estimates of implant failure of 7 and 8 mm single implant on edentulous free-end mandibular molar.
We investigated 7 and 8 mm single implants in the mandibular posterior region (Table 1). We observed marginal bone loss of more than 1.5 mm or 0.2 mm annually in only 6.2% of the sample (5 implants), the mean marginal bone loss was only 0.5 mm, and 96.3% of survival rate was showed during an average of 7 years after loading. These are excellent results that are comparable to those of conventional implants.
Although 7 mm implants had a longer healing period (4.7 and 3.2 months, P=0.013), both 7 and 8 mm implants were successful in terms of minimizing marginal bone loss (0.5 mm), success rate (90.6 and 95.9%), and survival rate (93.8 and 98.0%). Our results agreed with those of a previous study that reported successful outcomes of 93.5 and 95.6% in terms of success and survival rates, respectively, for posterior single short implants less than 8 mm during a period of 51.4 months19). Therefore, a single short implant of 7 mm in length is a good treatment option for mandibular posterior edentulous areas. Ahn et al. reported that wide diameter short implants (5∼7 mm in width, less than 7 mm in length) maximize the bone-implant contact surface and yield successful clinical results20). However, wide implants cause high strain distribution during implantation21), and could represent a high risk of implant failure22). In our study, excellent outcomes were observed for 7 mm implants with a mean width of 5.3 mm. This result supports that the wide short implants are unnecessary.
All implants enrolled in this study were internal, which offer platform switching and stress distribution effects23). Therefore, internal short implants may be used to obtain successful outcomes regarding marginal bone loss. A meta-analysis of conventional micro-roughened titanium implants previously reported that the summary estimate for 10-year survival at the implant level was 96.4% (95% CI 95.2%∼97.5%), and the sensitivity of the meta-analysis estimate of survival was 93.2% (95% CI 90.1% to 95.8%)24). Therefore, our results for short implants (96.7% 10-year cumulative survival rates) were comparable with those for conventional implants.
Considering the fixture lengths of short implants, the loss of marginal bone could lead to deterioration of the bone-implant contact ratio. Therefore, we set a total of 1.5 mm of marginal bone loss as the success criteria in this study, although Albrektsson’s criteria included 1.5 mm of bone loss during the first post-operative years only16). The clinical outcomes of internal short implants were excellent compared with conventional implants. Within the limitations of this study, short implants were cost-effective, non-invasive, and highly predictable long-term treatment options in sites with reduced bone height and unfavorable anatomy. This study has several limitations, including its retrospective design so that we were unable to control for the diameters and type of fixtures (SLA and RBM; bone level and tissue level), and prosthetic materials of implants, and to compare our results with those of conventional implants with bone graft. For short implants to become an alternative standard to conventional implants, prospective studies should be conducted including considerations of soft tissue factors such as vestibular depth, keratinized gingiva, gingival depth, abutment margin level, and subjective patient factors such as oral hygiene and satisfaction.
With regarding the marginal bone loss, prosthetic complications, success rate, and survival rate, single short implants with the latest surface treatments and an internal abutment are excellent prognostic treatment options for mandibular molar regardless of 7 and 8 mm in the length.
The authors report no conflicts of interest related to this study.
Author ORCID Information
Jeong-Kui Ku Young-Kyun Kim |
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