|Year : 2014 | Volume
| Issue : 2 | Page : 66-70
Short implants: A new dimension in rehabilitation of atrophic maxilla and mandible
Sanath Shetty, Naushad Puthukkat, S Vidya Bhat, K Kamalakanth Shenoy
Department of Prosthodontics, Yenepoya Dental College, Mangalore, Karnataka, India
|Date of Web Publication||15-Oct-2014|
Department of Prosthodontics, Yenepoya Dental College, Mangalore, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Insufficient alveolar bone height is a common clinical situation encountered more in the posterior jaws. Advanced surgical procedures such as bone grafting, sinus lifting, and nerve repositioning are required to overcome this condition and make implant treatment possible for such patients. Prolonged healing period, increased morbidity, and longer duration of the implant treatment accompanies these procedures. Short implants are considered as a viable alternative in patients with reduced alveolar bone height to avoid more invasive surgical procedures. They simplify the implant treatment, reduce patient morbidity, shorten the duration of treatment, and make it less expensive. In the past, when machined implants were used, rehabilitation with short implants showed increased failure rate in comparison to longer implants. With the improvements in the surface topography of implants, which increase the bone implant contact, and use of adapted surgical protocols similar survival rates as that of regular implants have been reported even with short implants. Various methods to increase the functional surface area and decrease the stress on the prosthesis have greatly contributed to the increased success rate of short implants.
Clinical Relevance to Interdisciplinary Dentistry
- Successful outcome of implant treatment depends on the coordinated efforts of various specialties
- Proper technique of implant placement by the surgeon and prior planning of the prosthesis by the prosthodontist is essential
- Maintenance and periodic evaluation of periodontal health are necessary.
Keywords: Bone grafting, bone implant contact, functional surface area, short implants, surface topography
|How to cite this article:|
Shetty S, Puthukkat N, Bhat S V, Shenoy K K. Short implants: A new dimension in rehabilitation of atrophic maxilla and mandible. J Interdiscip Dentistry 2014;4:66-70
|How to cite this URL:|
Shetty S, Puthukkat N, Bhat S V, Shenoy K K. Short implants: A new dimension in rehabilitation of atrophic maxilla and mandible. J Interdiscip Dentistry [serial online] 2014 [cited 2020 Sep 26];4:66-70. Available from: http://www.jidonline.com/text.asp?2014/4/2/66/142935
| Introduction|| |
Implant treatment becomes more challenging to the clinicians and more enduring to the patients when the alveolar bone height is insufficient. The condition is more common in the posterior jaws due to the resorption of alveolar bone and pneumatization of the maxillary sinus. Advanced surgical procedures are required to overcome this clinical situation and make implant treatment possible for such patients. These ancillary procedures like ridge augmentation or sinus grafting with simultaneous implant placement has shown more intra and postoperative complications.  Prolonged healing period, increased morbidity, and longer duration of the implant treatment accompanies these procedures. The expertise of a specialist surgeon is required to perform these surgeries.
As far as possible, rehabilitation with implants should be simple, cost effective, highly predictable, and of shorter duration. Short implants are considered as a viable alternative in patients with reduced alveolar bone height to avoid more invasive surgical procedures. ,, They simplify the implant treatment, reduce patient morbidity, shorten the duration of treatment, and make it less expensive. In the past, when machined implants were used, rehabilitation with short implants showed increased failure rate in comparison to longer implants. With the improvements in the surface topography of implants and use of adapted surgical protocols similar survival rates as that of regular implants have been reported even with short implants. [5 ]
| Scientific rationale for short implants|| |
When stress is applied to the natural tooth, it is distributed in to the underlying bone along the entire root length due to the presence of periodontal ligaments as the tooth tends to pivot around the center of the root. However in case of implants, where periodontal ligament is absent, the greatest magnitude of stress concentration is seen at the crest which tapers apically up to 5 mm from the crest. Stress concentration in the apical region is much less.  Most endosteal dental implants are fabricated from alloyed or pure titanium with a modulus of elasticity (stiffness) approximately 5 times greater than dense cortical bone. A basic mechanical principle states that when two materials of different moduli of elasticity are placed together with no intervening material and one is loaded, the stress concentration can be observed where the two materials first come into contact.  These stress contours form a v-shaped or u-shaped pattern, with greater magnitude near the point of first contact, which corresponds to the crest of the bone.  The phenomenon of higher crestal stresses next to the implant is confirmed in photoelastic and two-dimensional or three-dimensional finite element analysis (FEA) studies when an implant is placed within a bone simulant and loaded [Figure 1]. ,
|Figure 1: Finite element analysis model showing the V-shaped stress pattern in the crestal 5 mm of implant body|
Click here to view
Increase in implant length will increase the total surface area of the implant and improve the primary stability by increasing the bone implant contact (BIC). But the area that transfers the compressive and tensile loads to bone that is, functional surface area (FSA) is confined to the crestal 5-7 mm. Increasing the length of the implant will not change this where as a short implant with a wider diameter provides both, improved primary stability and increased FSA. 
How short is short?
There is no consensus in the literature on the definition of a short implant. 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 defined short implants as an implant with a designed intra bony length of 8 mm or less.  6 th European consensus conference of European Association of Dental Implantologists in 2011 approved the classification of implants given by Olate et al., which states, implants are usually referred to as short if their length measures <8 mm, 9-13 mm in length as a medium and long implants are usually understood to be over 13 mm in length. 
Merits of short implants
The main advantage of using short implants is that it simplifies the implant surgery by avoiding the more invasive procedures like bone grafting, sinus lifting, nerve repositioning, etc., and thus decreases morbidity and reduces the healing period. Advanced imaging modalities may not be required which will reduce the radiation exposure. Patient acceptance will be more as it avoids the need for complicated surgeries, reduces the duration of treatment period and cost.
Reduction of the implant surface area is one of the presumed reasons for the lower survival rate of short implants which will lead to less bone to implant contact after osseointegration. The functional forces after loading will be transferred to the crestal bone through this reduced surface of force distribution, which will lead to crestal bone loss. Compromised crown to implant the ratio is thought to be another problem, which will affect the success of the treatment. The poor quality of bone in the posterior region, especially in the maxilla, where short implants are mostly used is another contributing factor.
Earlier clinical trials have shown conflicting reports regarding the treatment outcome and long-term survival of short implants. However recent clinical trials have shown that success rate of short implants is comparable with that of regular implants. ,, This difference was due to the various methods they adopted to overcome the abovementioned shortcomings of the short implants. This includes modification of implant characteristics and biomechanical considerations for stress reduction.
Increasing the diameter of the implant is an effective method to increase the implant surface area. Wider diameter short implants will have increased FSA and improved primary stability. It allows engagement of a maximal amount of bone and better distribution of stress in the surrounding bone. An increase in the diameter reduces stress at the implant neck and is associated with good distribution of force compared with increases in implant length.  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 increase the surface area by 30-200% depending on the implant design.  A three-dimensional FEA demonstrated that increasing the implant diameter resulted in a 3.5-fold reduction in crestal strain, whereas increasing the implant length resulted in 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. This is achieved by increasing the diameter, change in the design, increasing the number, and splinting of the roots, but not by increasing the length of the roots. Similar approach is logical for short implants. If wider implant cannot be placed, each molar can be supported with 2 short implants, thereby increasing the FSA.
Most of the earlier studies using short implants showed less favorable results as compared to longer implants because of the use 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 proven in various studies. ,, This can be achieved by either subtractive processes like blasting, etching and oxidation, or additive processes like titanium plasma spraying, hydroxyapatite and other calcium phosphate coating and ion deposition. Rough implants offer extensive area for osseointegration. It increases the BIC and FSA in addition to improve the wettability of the implant surface.
Photofunctionalization of implants
Treatment of implants with ultraviolet (UV) light has been found to increase the BIC from 55% to near maximum level of 98.2%. This resulted in 3-fold increase in the strength of osseointegration. ,, This increase is attributed to the generation of superhydrophilicity, a significant decrease in surface hydrocarbons, and improvement in the electrostatic status of titanium surfaces after UV treatment. The biological effects along with UV-enhanced surface properties are collectively defined as photofunctionalization of titanium implants. 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 strength doubled and the disadvantage of short implants was eliminated.  A recent human study has demonstrated the effectiveness of photofunctionalization in complex cases using short implants with lesser diameter.  It allows for the placement of short implants in the alveolar ridges which are not wide enough to allow the placement of larger diameter implants.
Macro geometric design
Increasing the diameter is a logical option for increasing the surface area of short implants. But there is an anatomical limit to how much the diameter can be increased. Modifications in the macro geometry of the implant are advantageous in providing more area for BIC and FSA. Various thread shapes such as square, v-shaped, and reverse buttress are available for implants of which square threads provide more 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 also enhance the FSA of short implants.[Figure 2]
Bone density is directly proportional to its strength. Less dense bone may demonstrate a reduction of its strength by 50-80% compared to higher density bone. Poor bone quality is strongly linked to higher failure rates in implants.  Increased failure rates of short implants in the early trials were attributed to the use of machined implants in poor quality bone, especially in the posterior maxilla. This negative effect is somewhat dampened by rough surfaced implants now. Use of self-tapped implants has also brought down the failure rates.  Use of bone expanders/condensers during osteotomy procedure also improves the bone density and there by increases the success of a short implant.  A two-stage implant placement approach was suggested by Gentile et al. while using short implants as it was associated with higher success rates.  But there are other studies which shows no statistical significant differences in the success rate between single stage and two stage protocols.  However, some authors have recommended a two-stage approach with submerged implants for completely edentulous arches. 
Crown implant ratio
A consensus conference defined a desirable crown height space for a fixed prosthesis to be between 8 and 12 mm (bone level to opposing dentition).  This height leaves 3 mm for soft tissue (includes biologic width and soft tissue coverage of implant collar), 2 mm for an occlusal porcelain, and an abutment 5mm high. Increased height of the prosthesis increases the risk of component and material fracture due to elevated forces on the restoration. Therefore, increased crown height has to be considered as a factor that can affect clinical outcomes both technically and biologically.
Increased crown implant ratio (CIR) is a major concern with short implants. A 1:1.5 crown root ratio is suggested as most favorable and 1:1 as a minimum for a tooth abutment.  But the same need not be applicable for short implants and the ideal CIR has not been established. Various studies have demonstrated high success rates with a CIR of up to 2 and increased CIR did not result in additional peri-implant bone loss. ,, This was possible by giving due considerations for various stress reduction methods like avoiding lateral loads, cantilevers, etc.
Biomechanical methods for stress reduction
Biomechanical methods to decrease the stresses to short implants are a critical factor in deciding the success of the treatment. These include decreasing force to the implant prosthesis and increasing implant surface area of prosthesis support.
By avoiding lateral contacts in mandibular excursions and eliminating cantilevers, detrimental forces to which implant prosthesis is subjected can be reduced. The occlusal height of the crown should not affect the force moment along the vertical axis, because if it is centered, its effective moment arm is nonexistent.  Apart from increasing the diameter and surface area, increasing the number of implants and splinting them together can increase the area of forces applied to the prosthesis.
| Conclusion|| |
Insufficient alveolar bone height for implant placement is a commonly seen problem in the posterior jaws. Traditional way of overcoming this difficulty is by undergoing adjunctive surgical procedures. Though they are proven to be successful, these procedures result in delayed healing, increased morbidity, and prolonged treatment period. 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 the stress reduction to the implant prosthesis have made short implants a viable and more predictable alternative to advanced surgical interventions.
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[Figure 1], [Figure 2]