Short implants have been proposed as an alternative to eliminate or reduce the need for vertical bone augmentation procedures, which are often associated with additional costs, longer treatment time, increased postoperative morbidity, and greater risk for complications. However, the long-term efficacy of short dental implants has been a topic of controversy in the dental implant literature. Whereas some studies reported lower survival rates for short compared to longer implants, other reports, including a number of systematic reviews, more recently concluded that survival rates of short implants are similar to longer implants placed in pre-existing or grafted bone. The majority of studies does not include direct comparisons of the performance of short and longer implants. The interpretation of the literature is also complicated by the fact that authors have defined “short dental implants” differently. Some have considered <10 mm as short, whereas in other studies, short implants were 8 mm or less, 7 mm or less, or 6 mm or less.
The purpose of this study was to systematically review randomized controlled clinical trials (RCTs) reporting on long-term survival as well as complication rates of short implants (≤6 mm) versus longer implants (>6 mm) in posterior jaw areas of partially edentulous patients.
The main goal and primary outcome of this systematic review and meta-analysis was to compare long-term survival rates between short implants (≤6 mm) and longer implants (>6 mm) in posterior jaw areas.
Secondary outcomes were as follows:
- Radiographic bone levels
- Prosthesis survival
- Implant complications
The present systematic review is based on 10 randomized clinical trials including 775 patients (392 with short and 383 with longer implants) representing a total of 1,290 implants (637 short and 653 longer implants). The follow-up period ranged from 1 to 5 years.
Sufficient data were available to perform a meta-analysis of the primary outcome (implant survival). Only descriptive analyses were possible for the secondary outcomes radiographic bone levels, prosthesis survival, and biologic complication rates for implants.
When interpreting the results, it is important to realize that only three of the 10 studies evaluated the performance of short and longer implants in a randomized manner in sites allowing the placement of both types of implants. The other seven studies compared the use of short implants to longer implants in conjunction with augmentation procedures. In other words, these seven studies compare different treatment approaches and not necessarily implant lengths per se. This difference needs to be kept in mind when comparing the results of these studies.
- Short implants (≤6 mm) exhibit similar survival rates compared to longer implants (>6 mm) after periods of 1–5 years in function. The mean survival rate was 96% (range: 86.7%–100%) for short implants, and 98% (range 95%–100%) for longer implants. The meta-analysis showed a risk ratio of 1.29 (95% CI: 0.67, 2.50, P = 0.45) for failure when short implants were used. This statement is based on a meta-analysis of 10 RCTs including 775 patients (392 patients with short, 383 with longer implants) and 1,290 implants (637 short, 653 longer implants).
- Time in function may reduce the survival rate of short implants more than that of longer implants. This statement is based on one RCT with a follow-up of 5 years including 45 patients and 60 implants (30 short, 30 longer). This is additionally confirmed by a recently published RCT with a 5-year patient follow-up that could not be included as it was published after the cut-off date for inclusion in the systematic review.
- Short and longer implants present similar amounts of radiographic interproximal bone level changes. Following a period of 1–5 years, the radiographic interproximal bone level changes for the short implants ranged from +0.06 to −1.22 mm, whereas the corresponding values for the longer implants ranged from +0.02 to −1.54 mm. This statement is based on 10 RCTs including 775 patients (392 patients with short, 383 with longer implants) and 1,290 implants (637 short, 653 longer implants).
- The rate of surgical and postsurgical complications is higher in the longer implant group (mean: 32.8%; range: 0–90%) compared to the short implants (mean: 6.8%; range: 0–26%) (1). In the longer implant group, the majority of complications were associated with bone grafting procedures (2).
- Prosthesis survival for short and longer implants following a period of 1–5 years is similarly high. The mean prosthesis survival rate was 98.6% (range: 90%–100%) for the short implants, and 99.5% (range: 95%–100%) for the longer implants. This statement is based on nine RCTs including 625 patients (317 patients with short and 308 with longer implants).
(1) This statement is based on eight RCTs including 590 patients (298 patients with short, 292 with longer implants) having 1,022 implants (500 short, 522 longer implants).
(2) This statement is based on six RCTs including 305 patients (134 patients with short and 171 with longer implants) and confirms previous consensus reports.
- What are the current indications for short implants?
Short implants are a valid option in situations of reduced bone height when it is important to avoid possible morbidity associated with augmentation procedures or to reduce treatment time. They may also be preferred when the possibility of damage to adjacent structures can be significantly reduced. Adjacent structures include maxillary sinuses, blood vessels and nerves, tooth structures and existing implants.
- Should longer implants be the first choice?
The selection of the length of an implant depends on site-specific local anatomical and patient conditions. When sufficient bone height exists, implants longer than 6 mm are preferred when they can be placed without increasing surgical risk.
- Can short implants be immediately loaded?
The loading times for short implants reported in the literature ranged from 6 weeks to 6 months. At the present time, no evidence-based recommendation can be made for immediate loading.
- Does implant diameter affect the survival of short implants?
Based on the findings from the studies included in this review, short implants with a diameter of 4 mm or greater should be used.
- Should adjacent short implants be splinted?
Based on the findings from the studies included in this review, the clinical recommendation is made to splint restorations involving adjacent short implants.
- What are the occlusal considerations for restorations on short implants?
Although the reviewed literature does not give specific recommendations regarding occlusion, a greater risk of occlusal overload of short implants has to be considered. Caution is especially advised when indicating short implants in patients presenting with single missing molars and/or parafunctional habits. Changes in occlusion should be assessed and adjusted as necessary during regular maintenance visits.