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 Table of Contents  
REVIEW ARTICLE
Year : 2022  |  Volume : 16  |  Issue : 1  |  Page : 61-65

Short implants a plausible alternative to conventional dental implants - A review


Command Military Dental Centre, Lucknow, Uttar Pradesh, India

Date of Submission09-Mar-2021
Date of Acceptance27-Nov-2021
Date of Web Publication05-Apr-2022

Correspondence Address:
Sheetal Anand Asija
Command Military Dental Centre (CC), Lucknow - 226 002, Uttar Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jodd.jodd_12_21

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  Abstract 


The success rate of implants positioned in reduced bone quality can be highly compromised especially as observed in the posterior dental arches. A critical aspect in implant placement and design like bone length can be augmented using various bone reconstructive procedures like sinus lift, bone grafting, alveolar distraction, and many more but attached with these techniques comes a high rate of morbidity, lengthy treatment times, and healing periods to which the patient may not appear very pleased about. Hence, short implants can be used as a viable alternative utilizing all of the available bone and taking into account the biomechanics and biological conditions of each case. As compared to vertical bone augmentation of poorly resorbed ridges, short implants perform better, especially in areas of proximity to anatomical structures such as mandibular canal and maxillary sinus. The merits of short implants mainly include ease of insertion of implant fixture, not too elaborate an osteotomy, and less likelihood for undue heat generation in the alveolus. This article is an attempt to provide a review on short implants with the basic aim of overcoming the myths and barriers attached with this concept and will help to open global doors into this less ventured space for achieving a successful implant outcome.

Keywords: Reduced bone quality, resorbed mandibles, short implants, simplified osteotomy


How to cite this article:
Asija SA, Roy S. Short implants a plausible alternative to conventional dental implants - A review. J Dent Def Sect. 2022;16:61-5

How to cite this URL:
Asija SA, Roy S. Short implants a plausible alternative to conventional dental implants - A review. J Dent Def Sect. [serial online] 2022 [cited 2022 Jun 30];16:61-5. Available from: http://www.journaldds.org/text.asp?2022/16/1/61/342636




  Introduction Top


Oral implantology is one of the mainstays of contemporary dentistry. It is a conservative therapy that circumvents the preparation of healthy teeth and allows bone salvation with implant loading in areas where teeth have been lost. In the past 30 years, the use of osseointegrated dental implants has resulted in increased survival rates. Some prospective studies have shown an accumulative survival rate of 94.4% with fixed prosthesis, versus 95.7% with implant-retained overdentures.[1] Oral implant therapy has become a common and putative treatment. Patients are better informed and more demanding, and dental professionals have more tools and well-documented techniques available to them to create clientele satisfaction. In favorable implant cases with sufficient bone and soft tissue available, the treatment option could be simple surgery with a standard, noninvasive technique. On the other hand, cases with bone or soft-tissue atrophies present a challenge and require alternative techniques in terms of surgical approach. Bone reconstruction, short implants, or complete removable dentures are among the common options. The clinician's goal is to offer a simple and predictable treatment with a successful and satisfactory result.

Prosthodontic rehabilitation has been revolutionized after the introduction of dental implants with the underlying biological principle of osseointegration for replacement of missing teeth rather than resorting to conventional fixed prosthodontics.[2] Posterior dental arches, be it maxillary or mandibular, pose challenges for implant placement inspite of a high success rate.[3] A greater number of implants failed in posterior region as compared to anterior areas.[4] The reason is that the posterior dental regions have a paucity of available bone and also these areas are subjected to higher occlusal stresses[5] as compared to anterior regions.

Tooth loss is often succeeded by alveolar bone resorption in unequivocal proportions, especially in posterior ridges, which greatly impede implant placement due to proximity to maxillary and mandibular anatomic landmarks. To overcome such difficulties a lot, many kinds of surgical intervention procedures have been devised ranging from nerve repositioning to bone graftings.[6] However, by employing such invasive techniques, the chances of developing postoperative infections, prolonged healing periods, and morbidity are greatly accentuated. Therefore, in such cases, utilizing short implants [Figure 1] instead of conventional implants without extra surgical mediation seems to be a more acceptable option.[7]
Figure 1: IOPAR view of short implant in mandibular second molar region

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  Understanding the Length Paradox Associated with Short Implants Top


Natural tooth when under stress shows a response in which the periodontal ligament fibers take the load along the whole length of the root while transferring the force to the surrounding bone as the tooth hinges along the root axis. However, in implants as there is the absence of periodontal ligament, the largest degree of stress is observed at the crest as compared to the apical region.[8] The use of photoelastic and two or three dimensional finite element analysis demonstrates a higher degree of crestal strain right next to the implant placed in a bone simulator and when subsequently loaded.[9],[10] Therefore, in contrary to a natural tooth with surrounding periodontal ligament, which bears all the functional and parafunctional stresses and transmits these ahead to the surrounding bone, an implant support system causes all the stresses to get concentrated at the crestal bone region that is the point of first contact and thus the role of length of implant in stress distribution is questionable.

It is generally agreed that implants <10 mm in length are considered short implants. In short implants, all the implant threads have to be osseointegrated and loaded as differing from long implants [Figure 2]. Therefore, implant surface treatment should optimize short implant success rates. Rough surfaces contribute to a higher bone to implant contact, which results in a better distribution of forces through better implant geometry. The literature does not unanimously define short implants in a very clear way. Various authors have used different lengths of implants as short implants. Earlier studies considered 10 mm as standard length for implants and anything less than that as short implants. Renouard and Nisand have stated that short implants have an intrabony length of 8 mm or less.[11] European Association of Dental Implantologists in 2011 at the 6th European consensus conference approved the classification of implants given by Olate et al., which states that implants are usually mentioned to as short if their length is <8 mm, medium if between 9 and 13 mm, and long implants if more than 13 mm in length.[12]
Figure 2: Comparison of length between conventional and short implants

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  Advantages of Short Implants Top


The focal benefit of using short implants is that they lessen the complexity attached with the implant surgery by avoiding the more invasive techniques such as bone grafting, sinus lifting, and nerve repositioning and thus decrease morbidity and reduce the healing period. Advanced imaging modalities may not be required, which will subsequently reduce the radiation exposure. The patient acceptance will be more as it avoids the need for complicated surgeries and reduce the duration of treatment period and cost. Extensive research work has shown that short implants have comparable success rates as compared to implants with conventional length. In posterior maxilla, an overall summative rate of success of 7 mm to 9 mm implants was found to be 95.1%.[13] Another study found a success rate of 100% for small length implants in posterior maxilla.[14] In one such study, 96 short implants in the range of 6 mm to 8.5 mm in length were placed in posterior maxilla, and after 2 years of loading, a 94.6% success rate was obtained.[15] In yet another pioneering study, 437 implants <10 mm in length were placed maximally in posterior mandible and maxilla and were loaded for a minimum period of 18 months and they showed a 99% survival rate right from first stage to the time of insertion of prosthesis.[16] The implants and restorations were followed for at least 18 months and up to 3 years. There was no incidence of implant loss or failure of restoration observed. An improvement in surgical technique and alteration in the design of implant has led to increased success rate of short implants, as suggested by other researchers [Figure 3].
Figure 3: Short implants in various shapes and surface configuration

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  Then why is Short Implant Performance Questionable? Top


Shortened implant surface area is one of the supposed reasons for the lower sustenance rate of short implants, which will lead to less bone to implant contact after osseointegration. The functional forces after loading will be transmitted to the crestal bone through this reduced area of force distribution, which will lead to crestal bone reduction. Compromised crown to implant ratio is believed to be another issue, which will affect the success of the treatment. The poor quality of bone in the posterior region, particularly in the maxilla, where short implants are mostly used due to proximity to vital anatomic structures such as maxillary sinus, is another factor to be taken into consideration.

Although earlier clinical trials have shown incongruent reports regarding the treatment outcome and long-term survival of short implants, the present-day clinical trials have shown that the success rate of short implants is commensurate with that of regular implants.[17],[18],[19] This difference was due to the various methods adopted to overcome the above-listed limitations of the short implants. This includes modification of implant physiognomies and biomechanical aspects for stress reduction, which shall be discussed in depth in the subsequent paragraphs to ensure greater usage of this less invasive and practiced concept.


  Ways to Improvise Short Implant Performance Top


Widening the implant diameter

Enlarging the implant diameter is an effective way to increase the implant surface area. Greater diameter short implants have increased functional surface area and improved primary stability, as they allow a maximum amount of bone to be engaged, resulting in better stress distribution. An increase in the diameter reduces stress at the implant collar and is associated with better distribution of force compared with an increase in implant length.[20] Implant strength and fracture resistance can be improved by increasing the diameter of the implant. Wider implants also facilitate the creation of a better emergence profile, especially in the posterior segment. An increase in diameter by 1 mm will amplify the surface area by 30%–200% depending on the implant design.[21] A three-dimensional finite element analysis demonstrated that increasing the implant diameter resulted in a 3.5-fold reduction in crestal strain, whereas increasing the implant length resulted in an only 1.65-fold reduction in crestal strain. In natural dentition, molars are subjected to high occlusal forces, and the root surface area of molars is 200% more than that of other teeth. Hence, an optimal force dissemination can be achieved by enhancing the diameter, changing the design, increasing the number, and splinting of the roots but not by increasing the length of the roots. A similar approach is logical for short implants. If a wider implant cannot be placed, each molar can be supported with two short implants, thereby increasing the functional surface area.

Alteration of implant surface morphology

Most of the earlier studies using short implants showed less encouraging results as on comparison to lengthier implants due to the usage of machined surface implants. Studies conducted using rough surface implants showed similar survival rates for both the types. The fact that alteration of the implant surface can influence the success of osseointegration has been proved in various studies.[22],[23],[24] This can be achieved by either subtractive processes such as blasting, etching, and oxidation or additive processes such as titanium plasma spraying, hydroxyapatite and calcium phosphate coating, and ion deposition. Rough implants offer an extensive area for osseointegration by increasing the bone to implant contact and functional surface area, in addition to improving the wettability of the implant surface.

Surface treatment of implants with ultraviolet (UV) light has been found to increase the bone to implant contact (BIC) from 55% to near maximum level of 98.2%. This caused a three-fold accentuation in the strength of osseointegration.[25] This increase is ascribed to the generation of superhydrophilicity; the surface hydrocarbons are reduced greatly and enhancement of the electrostatic status of titanium surfaces after UV treatment. Photofunctionalization of surface of titanium implants has been defined as the collective biological effects produced along with UV-perpetuated surface properties. An animal study showed implants with 40% shorter length resulted in a 50% or more decrease in the strength of osseointegration, but after photofunctionalization, the osseointegration value doubled and the shortcoming of short implants was eradicated.[26] A recent human study has revealed the efficacy of photofunctionalization in intricate cases using short implants with smaller diameter, thereby permitting the placement of short implants in the alveolar ridges which are not sufficiently wide to allow the placement of larger diameter implants.

Impant macrodesign

Greater surface area produced by increasing the diameter of the short implant is a valid option, but there is an anatomical limit to how much the diameter can be increased. Alterations in the external configuration of the implant are more beneficial in providing more area for BIC and greater functional surface area. Different kinds of thread shapes such as square, v-shaped, and reverse buttress are obtainable for implants and out of which square threads provide the greatest surface area for a given length of the implant. Increasing the number of threads per unit area (decreased thread pitch) and increasing the thread depth, self-tapping threads, tapered profiles, and flared necks also augment the functional surface area of short implants and thus enhance the primary stability.

Optimizing crown to implant ratio

An elevated crown-to-implant ratio (CIR) is the foremost concern with short implants. A crown root ratio of 1:1.5 is considered as ideal and 1:1 as a minimum for a tooth abutment. Several studies have validated high success rates with a CIR of up to 2, and increased CIR did not result in added peri-implant bone loss, and this was made plausible by incorporating various biomechanical stress reduction approaches such as avoiding lateral loads and avoiding cantilevers.[27] An achievable crown height space for a fixed dental prosthesis should be between 8 and 12 mm, according to a consensus conference.[28] Out of which soft tissue occupies 3 mm, occlusal ceramic 2 mm, and the abutment ≥5 mm in height. Chances for component and material failure are high if height of prosthesis is increased beyond a certain limit. Consequently, an elevated height of the crown can influence the clinical outcomes both technically and biologically.

Minimizing forces

The most important concern while using short implants is optimal stress distribution due to decreased area in accordance with a reduced length. Therefore, incorporation of certain stress reducing options will help to counter balance the biomechanical forces like elimination of cantilevers in restorations, avoidance of lateral contacts in mandibular excursions, increasing the number of implants, and splinting together of implants. The force moment along the vertical axis should not be affected by the coronoapical height of the crown due to nonexistent moment arm if centered.[29]


  Bone Density Property Top


Misch states that the internal architecture of bone is described in terms of quality or density, which reveals the strength of bone. Less dense bone may exhibit a reduction of its strength by 50%–80% in comparison to a bone greater in density. Low bone quality is strongly associated with higher failure rates in implants.[27] Increased failure rates of short implants in the early trials and tribulations were attributed to the use of machined implants in poor quality bone, chiefly in the posterior maxilla. This negative effect is somewhat stifled by rough-surfaced implants now. The use of self-tapping implants has also had greatly reduced the failure rates.[21] The success of short implants can also be improved by using expanders or condensers during osteotomy. Gentile et al. proposed a two-stage implant placement while using short implants as it was associated with higher success rates.[30] The minimum available bone length for long-term implant survival is related to the bone density. Cases where high density bone is existing are viable for short implant placement. Reduced quality bone, normally located in the posterior maxilla, has mechanically low resistance in comparison with dense bone. It is also unable to support similar forces as cortical bone, which is normally located in the interforaminal region of the mandible. Hence, the type of bone is a key factor in terms of force distribution in cases of short implant placement.


  Conclusion Top


The available alveolar bone is more in anterior than posterior regions of the oral cavity, and therefore chances of failure of implants are higher in the latter. Short implants have emerged as a boon in patients with atrophic posterior ridges, and moreover, patients are not very conducive to the idea of any kind of invasive procedure. The performance of short implants though questioned by many research groups has been supported by clinical and statistical data that short implants can give good results if improvised by incorporation of certain design criteria and techniques so as to ensure satisfying results. Short dental implants have been successfully used in such situations with comparable survival rates with that of longer implants. Various methods to increase the surface area and BIC along with stress reduction to the implant prosthesis have made short implants a viable and more predictable alternative to conventional dental implants placed with or without additional advanced surgical interventions.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Misch CE, Bidez MW. Occlusal considerations for implant-supported prosthesis: Implant protected occlusion. In: Misch CE, editor. Dental Implant Prosthetics. St. Louis: Elsevier/Mosby; 2005. p. 472-510.  Back to cited text no. 16
    
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25.
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26.
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29.
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30.
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Introduction
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Then why is Shor...
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