|Year : 2015 | Volume
| Issue : 3 | Page : 132-137
Short implants - When, where and how?
Anjan Kumar Shah
Department of Implantology, Rajarajeswari Dental College and Hospital, Bangalore, Karnataka, India
|Date of Web Publication||31-Dec-2015|
Anjan Kumar Shah
Department of Oral and Maxillofacial Surgery, Rajarajeswari Dental College and Hospital, Mysore Road, Bangalore - 560 060, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
The success of dental implants has led to research in techniques in minimising surgical complexity and increasing survival. Short dental implants are increasingly being seen as an alternative to more extensive bone augmentation procedures. This paper reviews aspects of short implant literature to give some guidelines in making their use more successful.
Keywords: Short, dental, implant
|How to cite this article:|
Shah AK. Short implants - When, where and how?. J Int Clin Dent Res Organ 2015;7, Suppl S1:132-7
| Introduction|| |
Dental implantology is one of the most dynamic topics in the clinical dentistry today. The impetus is on simplifying the treatment to make it more acceptable to both the patient and clinician. Short dental implants are one of the techniques that have developed with this aim, when anatomical barriers and lack of bone availability prevent conventional dental implant treatment. There is very little good quality evidence regarding the use of short dental implants and the available evidence is sometimes confusing. The aim of this paper is to review the evidence and provide some guidelines to the practitioner on the applications and contraindication to the use of short dental implants. In a consensus paper on bone augmentation procedures in implant dentistry, Chiapasco et al. recommended the consideration of short implants as an alternative to more complex and often expensive augmentation procedures.
| Definition and History|| |
Initially, dental implant literature suggested the need for implants of length 12-20 mm to provide sufficient primary stability. Thomas Driskell invented the Bicon system of dental implants in 1968, with the introduction of the 8-mm implant. Until 1979, the shortest conventional endosseous implants available were 10 mm in length. In this year, Branemark introduced the 7-mm implant. The literature can therefore, be divided into implants that are termed conventional “short implants” ranging from 7 mm to 10 mm length and “ultrashort implants” of length <7 mm. Many companies have implants as short as 5 mm in their armamentarium.
For the purpose of this review, we will look at the evidence of using implants <7 mm in length. The Bicon system itself introduced a 5-mm height implant and received the Food and Drug Administration (FDA) approval in 2008.
More recently, Straumann introduced a 4-mm Roxolid implant, with 5-year data showing a survival of 94% for these implants.
| Is There a Need for Short Implants?|| |
The alveolar ridge undergoes reduction in height and width after tooth loss. In the posterior maxilla and mandible, this resorption results in proximity of the maxillary sinus and inferior alveolar nerve to the alveolar crest. Advanced bone grafting procedures such as sinus augmentation or guided bone regeneration (GBR) with simultaneous implant placement have shown more intra- and postoperative complications. Therefore, placing short implants might provide higher patient satisfaction in terms of surgical procedure and treatment outcomes. Oikarinen et al. conducted a radiographic study in a population of 431 65-year-olds; it was found that the placement of an implant of at least 6-mm length was possible in the lateral part of the maxilla only in 38% of the cases and in only 50% of the mandible cases in the lateral zone. This excludes a huge population from the benefits of dental implants.
The procedures for increasing the available bone height in these two regions are also not without its disadvantages. Maxillary sinus lifts, zygomatic implants in the maxilla, and nerve lateralization and alveolar distraction increase the complication rate of dental implants. They also increase the cost and downtime of the treatment. Graziani  in a systematic review of bone augmentation procedures and their success in the maxilla noted that survival of implants in the nonaugmented maxilla was 73-100% while survival in the grafted maxilla was 36-100%. His paper suggests that although bone grafting with dental implants can be highly successful, there is greater variability of outcome in the grafted sites. Chiapasco  in a systematic review concluded that short implants should be seen as an alternative to more complex augmentation procedures [Table 1].
| Are Short Dental Implants Successful?|| |
The evidence of long-term success is conflicting. A large study in the University of California, Los Angeles (UCLA) suggested failure rates of as high as 30% in 2 years and 40% in 5 years for 7-mm plasma sprayed titanium implants. It must be noted that this paper was from 1994 and the surfaces on implants have changed significantly since then. More recent papers ,, suggest that the success rates of 5-mm dental implants range from 83.7% to 100% survival over 1-8 years from various papers.
A recent systematic review  reported that short implants had a cumulative survival rate of up to 99.1% after a follow-up period of 3.2 ± 1.7 years. In a recent systematic review of short implant survival, Naert  concluded that machined implant of less than 10 mm had a survival rate of 81.5% against a survival of 97.2% for implants that were 10 mm and longer. Whether the same extends to rough surface and surface treated implants is unclear. A study by Deporter  in 2000 reported 100% survival at 11 months of porous surface implants. This variation may have something to do with changing implant surface and design in recent years. Initially, short implants were considered biomechanically unsuitable and their use was seen as simply experimental.
Finite element analysis performed by Pierrisnard et al. showed that maximum stress in the implant area was largely independent of the implant length. Experimental studies on animal models  confirmed that the increase of implant length from 7 mm to 10 mm did not significantly improve implant anchorage in the bone. Finite element analysis  suggests that the maximum stresses on a dental implant are at the crest and the stress levels apical to the top 5 mm was minimal. It also suggests that the stress forces on a dental implant of length 7 mm and 10 mm were the same. Data from finite element analysis certainly suggest that implants as short as 5 mm can be as successful as conventional length implants [Figure 1] and [Figure 2]. Short implants not only appear to distribute forces similarly but the use of short implants in distal areas where dental implants would be contraindicated due to lack of bone height may reduce cantilevers and reduce the overall compressive and tensile stresses on the prosthesis by 34.7% and 19%, respectively.
|Figure 1: failing maxillary short implant used as distal abutment for long span bridge|
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|Figure 2: successfully restored short dental implants in the mandible (courtesy Bicon)|
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| Factors Affecting Success of Short Dental Implants|| |
Studies demonstrate greater success of short implants in the mandible than in the maxilla. This is likely to be due to the poorer density and therefore, lower bone contact in the maxillary posterior bone. It is suggested that whenever possible, bicortical fixation of the sinus floor should be done to increase the survival of short implants in the maxilla.
A prospective multicenter study  was conducted in 1985 with a total of 159 edentulous patients with 558 implants. They used the Applegate-Kennedy classification to classify jaws into class 1, 2, and 4. A standard surgical procedure was followed. A clear correlation was noted with implant length failure, and most failures were represented by short implants of standard diameter, whereas the implants of wider diameter seemed to achieve better results than the standard ones. Implant loss was higher in the maxilla, particularly in type 4 bone quality, and most losses occurred in the early loading periods. This study relates implant length and maxillary bone quality with survival.
A prospective multicenter study  was carried out in which five (7-mm) implants with 3.75 mm diameter were used and were immediately placed. A total of three implants were placed in the maxilla, and all the implants were lost. Two were placed in the mandible and one was lost. The authors concluded an 80% failure rate for the 7-mm implant in this study.
Increasing implant diameter resulted in better engagement of the buccal and lingual cortical plates and more bone-to-implant contact, thus improving stress distribution within the surrounding bone. In a three-dimensional (3D) finite element analysis, it was demonstrated that increasing the implant diameter resulted in a 3.5-fold reduction in crestal strain. On the contrary, increasing the implant length resulted in a 1.65-fold reduction in crestal strain.
Clinical investigations in a study regarding Endopore implants that were placed in patients with an age range of 22-72 years showed that the 7-mm length implants were 32 in number with no failures. The authors attributed the success to the large surface area provided by the implant design, which was three to four times that of machined threaded implants and porous surface implants, and it allows for 3D mechanical interlock with the bone for ingrowth of the bone, and this interface effectively resists interfacial shear forces and also tensile forces created by the transverse component of occlusal loading. The functional time was 32.6 months.
Many papers demonstrate the higher survival rates of wide diameter short implants such as the 5 × 5 mm Bicon implant. The wider diameter implant increases bone contact and distributes forces better. It is suggested that when wide diameter implant placement is not possible, two implants should be considered in place of one implant. A retrospective study conducted on 49 patients with edentulous mandible and bone resorption was shape E and quality 1 according to Lekholm and Zarb. None of the 5 × 6 mm implants were lost in the present study, which may be coincidental or because of their limited number. They recommended this treatment procedure without hesitation for a severely resorbed mandible.
Surface texture of implants
Again, the literature around short implants is confounded by the studies involving machined surfaces and various surface modifications. Roughened surface implants appear to increase implant survival, especially in compromised bone height situations and the maxilla. A systemic review by Telleman et al. suggested that machined surface implants increased the failure rate of short implants by a maximum of 29%.
This is due to mechanical interlocking that provides greater stability as well as the rough surfaces that provide a more amenable surface area for attachment of osteoblasts. Kasten  and associates reported that gingival cells also attached to roughened titanium surfaces three times more frequently than to smoother surfaces.
Numerous other histomorphometric and biomechanical tests, animal studies, human clinical trials, and in vitro experiments have demonstrated that implants with roughened surfaces achieve greater bone-to-implant apposition and interfacial strength than implants with conventional machined surfaces.,
Neldam in a systematic review of short implants reported a failure of 4.3% for short Straumann titanium plasma-sprayed (TPS) implants, 1% for Straumann sandblasted and acid-etched (SLA) implants, and 0% for Astra microthreaded implants.
Implant thread design
Abuhussein  in a review of thread design noted three important variations:
- A decrease in thread pitch positively influences implant stability.
- Increase helix angle, although increasing speed of insertion may reduce implant stability.
- Deeper threads have an important contribution to implant stabilization, particularly in the soft bone.
In addition, the paper noted that the inclusion of microthreads at the crest may increase bone-to-implant contact and reduce crestal bone loss but the evidence for this was not conclusive.
Platform switching and short implants
A review of the literature by Atieh  reports a statistically significant difference in the marginal bone levels between platform matched and platform switched implants.
While this may not be significant for standard length implants, the loss of bone may be significant for short implants. For example, a 1-mm loss of the crestal bone on an 8-mm implant results in a 12.5% loss of bone support.
Linking of implants
Splinting of implants appears to be beneficial and particularly the linking a short implant to the adjacent longer implant does seem to increase its longevity.
Different success rates were found for the splinted (97.7%) and nonsplinted (93.2%) groups. The success of splinted implants was associated with no other variable, whereas nonsplinted implants exhibited a higher risk of failure when placed in men and when implants shorter than 10 mm were used. The failures in men may be due to greater bite forces although this was not clear from the data.
Avoiding immediate loading
Immediate loading of short implants has a deleterious effect to its long-term survival. A one-stage versus two-stage surgical approach did not have a significant effect on the success of short implants. According to a retrospective study, immediate loading of threaded implants of 10 mm or shorter with a rough surface should be performed with caution. But investigations of immediate loading of implants of length (≤7 mm) as per this review are not found. It seemed prudent to follow a two-stage implant placement approach when using short implants, as this approach has been linked with higher success rates. However, according to a review, there is no significant difference in the failure rates between one-stage and two-stage implants. The use of interim implants and the restriction of denture wearing to avoid pressure on the soft tissues during the first 3 months following surgery were additional precautions to optimize conditions for implant survival. A stress-free environment may be important because movement of as little as 10-20 μm during the early stages of wound healing can be enough to direct differentiation of mesenchymal cells into fibroblasts instead of osteoblasts.,
The benefits of a nicotine-free period around the time of surgery have been supported by the results of several animal studies. A study in rats has shown that smoking appears to have more adverse effects on cancellous bone than it does on cortical bone. In a rabbit model of bone graft revascularization, it was shown that inhalation of nicotine decreased vascular ingrowths into autogenouscancellous bone grafts. In a prospective study of implant surgery, Bain  described a smoking cessation protocol in which patients stopped smoking 1 week before and 8 weeks after the implant surgery. Significantly lower implant failure rates were observed in smokers who followed this protocol (11.8%) than in those who did not (38.5%). Failure rates in compromised maxillary bone are slightly higher in smokers as compared to nonsmokers (2.6% versus 1.9%, respectively).
| Conclusion|| |
Based on the review of the literature and personal experience, short implants if applied correctly can be an alternative to other more complex dental implant procedures.
- Short implants should be considered in the treatment plan when considering dental implants.
- Prognosis of short implants in the maxilla is guarded.
- Splinting of short implants may be beneficial for their long-term success and is hence, recommended.
- A staged protocol is suggested when planning the use of short implants.
- Short implants are an advanced surgical technique and should be utilized once the clinician is familiar with a conventional implant protocol as the margin of error in surgical technique is very small.
- Smoking may have deleterious effect on the success of short implants and therefore, following a protocol of cessation by smokers before and after implant surgery is recommended.
- There is insufficient literature at present to comment on the long-term success of short implants versus conventional length implants, therefore implants longer than 10mm are still recommended where height of bone is available.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]