Edentulous sites are often characterized by inadequate bone volume for dental implant therapy. Bone augmentation procedures for site development involve longer healing period and are often invasive, costly, and associated with postoperative morbidity. This article discusses alternatives to invasive bone grafting procedures that are often used to develop implant sites. Owing to the broad nature of this topic, it is presented in two articles. In part I, the use of short and narrow-diameter implants are discussed. Part II reviews the use of tilted as well as fewer implants to support a prosthesis.
Key points
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Alternatives to invasive site development for dental implant therapy include the use of short dental implants and narrow diameter implants.
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These alternatives have the advantages of reduced morbidity, fewer complications, shorter treatment time, lower costs, and better patient acceptance.
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Short dental implants may provide comparable outcomes to standard length implants in vertically augmented sites.
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Narrow diameter implants can be a valid alternative to standard diameter implants in horizontally grafted bone.
Introduction
Dental implant therapy is a widely accepted, long-term treatment option to restore edentulous sites. However, the most favorable outcomes have been reported in sites with adequate native bone or with minor bone regenerative procedures. Placement of longer and wider implants was initially preferred to improve bone-to-implant contact, primary stability, crown-to-implant ratio, esthetics, hygiene, and restoration support. However, edentulous sites are often characterized by inadequate bone quality and quantity.
Implant sites with deficient bone often have to be augmented laterally, vertically, or a combination of both to achieve ideal 3-dimensional implant positioning that is prosthetically driven. However, bone grafting increases the cost, time, and morbidity of treatment. Tooth loss and edentulism also disproportionately afflict low-income populations, for which the cost of both bone augmentation and restoration of edentulous sites with dental implants may be prohibitive. This finding has led to the search for less invasive treatment options with greater patient acceptance.
The broader use of 3-dimensional imaging techniques and computer-aided guided surgery, as well as the introduction of stronger implant alloys with improved surface structures, has led to the development of alternative treatment protocols. These alternatives include the use of shorter, narrower, and tilted implants to avoid bone grafting, as well as fewer implants to restore edentulous arches.
In Part I, we investigate the following questions: (1) Are short dental implants (SDIs) a reasonable alternative to vertically augmented bone with standard length implants? (2) Are narrow diameter implants (NDIs) a valid alternative to laterally augmented bone with standard diameter implants? The use of tilted and fewer implants as an alternative to conventional treatment protocols will be discussed in Part II.
Are short dental implants a reasonable alternative to vertically augmented bone with standard length implants?
When an implant site is completely devoid of bone, missing bone volume must be surgically regained before implant placement. However, there are times when a site has insufficient alveolar ridge height for standard length implants, but is otherwise adequate for SDIs. The use of SDIs is controversial. Higher failure rates have been associated with the use of SDIs in some studies, whereas other reports indicate similar outcomes regardless of implant length. Factors that explain these conflicting outcomes include a lack of standardized definitions for SDIs, and implant primary stability and surgical protocols. The skill and experience level of the clinician, implant surface characteristics, and bone quality are additional contributing factors that influence the predictability of SDIs.
Short Dental Implant Definition
The maximum length of an SDI in the published literature remains controversial, with proposals ranging from 5 to 11 mm. The length of the implant has been defined as the distance from the platform to the apex. However, the intrabony length (ie, the distance from the apex to the most coronal bone-to-implant contact) has a greater relevance. Intrabony length represents the height of the implant that is anchored in bone and determines how external forces are transmitted to surrounding bone.
The first European Association of Osseointegration consensus conference in 2006 defined SDI as an implant fixture with an intrabony length of 8 mm or less. This SDI definition is used in this article because it is gaining acceptance and been adopted in a number of recent studies and reviews.
Surgical Protocol and Primary Stability
Some studies that reported lower survival rate of SDI used a standard surgical protocol regardless of bone density. Such protocols, often using tapping and countersinking, may have resulted in implants with reduced primary stability. Other publications reported favorable clinical outcomes for SDIs when modified surgical protocols that ensured greater primary stability in sites with lower bone quality were used.
Clinician Experience
The clinician’s skill and experience have been postulated to be factors affecting SDI outcomes. A systematic review compared survival and complications for implant-supported prostheses of current studies (studies completed after 2000) with studies published in 2000 or prior years. Higher implant survival rates and lower reported technical, biological, and esthetic complications were found in the more current studies. The authors concluded that the learning curve has a positive impact on implant clinical outcomes.
As an example, a retrospective study reported an 88% implant cumulative survival rate (CSR) over 5 years for 17 patients treated with SDI-retained overdentures from 1989 to 1993. The identical clinical protocol was followed by the same authors via a randomized, controlled trial in 2003. They reported a 100% CSR at 12 months. Although there was a difference in the duration of the reporting period between these 2 studies, the outcomes are comparable, because 7 of the 8 failures in the earlier study occurred in the initial healing period, before prosthetic loading.
Short Dental Implant Surface Structure
Rough textured implant surfaces have been recommended by some authors because they provide greater bone-to-implant contact and higher removal torque. A recent systematic review reported a higher CSR for SDIs with rough surfaces. In contrast, studies documenting increased numbers of SDI failures used machined surface implants. Several studies indicated that the implant length did not influence survival when SDIs with textured surfaces were used. This finding suggests that SDIs with a more favorable surface topography may have greater success than implants with machined surfaces.
Bone Quality
Poor bone quality as well as low bone volume are risk factors for implant failure, particularly with SDIs. Of the 2 factors, bone quality has a greater impact on SDI survival than bone quantity. Short implants are mostly used in the posterior regions, and the posterior maxilla in particular has the poorest quality of bone. A study compared the implant survival of SDIs and standard implants in poor quality bone. SDIs had failure rates of 78%, whereas standard length implants had a failure rate of 0% in the most unfavorable quality and shape of bone. Although the difference was not significant owing to low patient enrollment, SDIs were used in disproportionately high numbers in this study in sites with low volumes of poor quality bone.
Short Dental Implant Outcomes for Fixed Dental Prostheses
A multicenter, randomized, controlled trial compared SDIs with standard length implants placed in vertically augmented bone of the posterior maxilla and restored with single crowns. At 3 years, there were no reported differences between the 2 groups with respect to implant survival or periimplant bone loss.
Another research group compared SDIs with standard length implants placed in vertically augmented bone in posterior sites of both arches. Significantly more complications occurred with grafts in mandibular sites, and greater periimplant marginal bone loss (MBL) was associated with longer implants in grafted sites in both arches compared with SDIs. Although these reported outcomes suggest a therapeutic advantage for SDIs, the weaknesses of these studies include short follow-up periods (≤5 years), small sample sizes, and procedures that were performed by a single, highly skilled and experienced clinical research group. These limitations should be considered before forming a generalized conclusion regarding the use of SDIs ( Fig. 1 ).
Short Dental Implant as Abutments for Mandibular Implant Overdentures
The 2-implant retained overdenture is the standard of care for the edentulous mandible. In contrast, maxillary implant-assisted overdentures are associated with lower rates of implant survival and a greater frequency of prosthetic complications. In a prospective cohort study, 19 functionally dependent elderly adults with conventional dentures received 2 SDIs in the canine regions of their edentulous mandibles. After 6 to 8 weeks of healing, the SDIs received Locator attachments (Zest Dental Solutions, Carlsbad, CA) which were retrofitted to the mandibular complete dentures. Implant survival was 94.7% at 5 years, with acceptable periimplant bone loss and other soft tissue biological parameters. The limitations of this study included a small study cohort, high levels of dropouts, and the lack of a comparison group. However, it demonstrates that cognitive decline and the inability to provide self-care may not be contraindications or risk factors for SDIs. A 10-year study followed 49 patients with severely atrophic mandibles that were restored with fixed or removable full-arch prostheses retained by 260 SDIs. Seventeen implants failed over the study period with a CSR of 92.3%.
Short Dental Implants Outcomes for Full-Arch Fixed Dental Prostheses
Studies comparing SDIs with longer implants in vertically augmented bone for the support of full arch fixed dental prostheses are lacking. One retrospective study reported outcomes for 43 patients with edentulous maxillae rehabilitated with fixed prostheses supported by 172 short and long implants following the all-on-4 treatment concept. After an average observation period of 3 years, 3 short and 3 long implants were lost with a CSR of 95.7% and 95.4% for short and all implants, respectively. MBL for short implants at 3 years was 1.25 mm, with no significant differences in bone loss between the groups. The limitations of this study included the use of outcomes from an individual clinical center as well as a short follow-up periods. In addition, SDIs were splinted to standard and longer length implants in the same arch in many patients. These factors should be considered before generalizing conclusions to full arch fixed dental prostheses supported entirely by SDIs.
Are narrow diameter implants a valid alternative to laterally augmented bone with standard diameter implants?
Owing to volumetric bone loss after tooth extraction, lateral ridge augmentation is often performed before or simultaneously with implant placement. Although bone grafting is a common and a predictable treatment, it is associated with higher levels of complications and increased expense. Therefore, NDIs have been used as a less invasive alternative to laterally augmented sites with standard implant diameters.
Implant diameter is the maximum cross-sectional dimension within the intrabony length of an implant. It is measured from the peak of the widest thread to the same point on the opposite side of the implant. Each implant system has its own inventory of diameters and lengths, with most manufacturers offering narrow, standard, and wide diameter implants.
Implant diameter selection should be steered by site-specific surgical and prosthetic requirements. Surgical considerations include the location of the surgical site, the volume of residual bone, the level of primary stability, and the proximity to adjacent teeth and surrounding vital structures. The implant should engage as much crestal and buccolingual cortical bone as possible, with at least 1 mm of residual bone adjacent to the implant surface. Prosthetic factors include the planned occlusion, gingival esthetics, emergence profile, and the prosthesis design such as the size of the tooth or teeth to be replaced.
Narrow Diameter Implant Definition
Implants with diameters of 3.75 mm and larger are widely regarded as standard or regular diameter implants. However, the definition of an NDI remains controversial. The terms reduced diameter, narrow diameter, small diameter, microimplants, and miniimplants are used interchangeably with overlap in their definitions across studies. A lower narrow diameter threshold of less than 3.0 mm has been proposed in systematic reviews by some authors, whereas others agree with 3.5 mm or less as the upper diameter limit. For this reason, implants with diameters that fall within the range of 3.0 to 3.5 mm are gaining acceptance for meeting the NDI criteria.
Narrow Diameter Implant Limitations
Several factors should be considered before selecting an NDI. NDIs have biomechanical limitations relative to implants with wider diameters. Although NDIs are significantly narrower than implants with standard diameters, they are subject to the same stresses under loading. For this reason, the material and design of NDIs must fulfill similar demands for strength and stability. The thin fixture wall around the abutment or screw, resulting from the decreased implant diameter, can increase the risk for fixture or screw fracture. In addition, NDI abutments have decreased surface areas relative to standard designs. This characteristic may result in greater numbers of prosthetic complications such as retention loss and screw loosening. Finally, NDIs purportedly are at greater risk for fatigue failure than standard and wide diameter implants, particularly in posterior areas subject to high occlusal loads.
NDIs were originally designed for the replacement of teeth with small cervical diameters, such as maxillary lateral and mandibular central and lateral incisors, as well as for implant sites with limited interdental space ( Fig. 2 ). It is recommended that the diameter of the implant should be slightly less than that of the crown for single tooth restorations. A single mandibular incisor has a cervical diameter of 3.5 mm, which is ideally suited for an NDI. However, maxillary and mandibular premolars and molars have diameters that exceed twice those of NDIs. The nonjudicious use of NDIs for these situations can result in unsightly, overcontoured restorations with large gingival embrasures that lead to plaque accumulation, food impaction, periimplant soft tissue inflammation, and gingival recession, all of which can contribute to decreased implant survival.
Narrow Diameter Implant and Anterior Full-Arch Fixed Dental Prostheses
Anterior NDI-supported fixed dental prostheses must be able to withstand the functional forces of mastication and occlusion, as well as meeting esthetic requirements. This means that the implant restoration is expected to provide appropriate protrusive and lateral guidance for the patient without fracture or displacement.
In a prospective, single-arm study on NDIs that replaced missing maxillary lateral and mandibular incisors with single implant crowns, a 95.9% implant survival rate form the time of placement to the time of loading and 100% implant survival rate from the time of loading up to a 5-year follow-up was reported. The mean change in MBL from the day of the surgery to 5 years was −0.15 mm. Although 8.43% of the implants lost more than 1 mm of crestal bone, there was no measurable bone loss for 50.6% of the implants after 5 years. A number of technical complications were reported in this study. Seven implants in 7 patients experienced abutment fracture, and retention loss of 6 crowns occurred for 6 patients. In some cases, patients lost the crowns more than once.
A second prospective, single-arm study reported similar outcomes for NDIs that replaced congenitally missing maxillary lateral and mandibular incisors. An overall 96.8% CSR at 36 months was reported, and none of the implants were lost after loading. Gingival recession was negligible, and the mean change in MBL from implant insertion to 36 months was a loss of 0.21 mm. Technical complications were also similar, with 5 fractured abutments, 1 loose abutment screw, and 8 crowns that needed to be recemented.
Despite these reported complications, the investigators in both previous studies demonstrated acceptable biological outcomes and implant survival over a period of 3 to 5 years when NDIs were used to restore maxillary lateral and mandibular incisor sites. Although prosthetic complications were reported in the 2 studies cited, a direct comparison between NDIs and anterior single crowns supported by standard diameter implants in similar sites was not possible owing to the lack of comparison groups.
A longitudinal study used 20 NDIs at the maxillary lateral incisor sites of 10 patients to support 4-unit partial fixed dental prostheses replacing numbers 7 to 10. After 5 years, no implants were lost and there were no reported technical or biological complications. Marginal bone levels were stable over 5 years for all implants, gingival recession was negligible, and overall patient satisfaction was 96.9%. Although the results of this study illustrate the usefulness of NDIs in multiunit fixed applications, the expertise and clinical skills of this research group should be given consideration before adopting this treatment concept.
Narrow Diameter Implants and Posterior Full-Arch Fixed Dental Prostheses
Posterior natural teeth have cervical diameters much greater than the diameters of NDIs. The occlusal load from the cantilever created by the increased occlusal surface diameter relative to the NDI diameter increases the risk for overload on abutments, abutment screws, and fixtures.
A retrospective study on 247 NDIs in posterior sites (follow-up of ≤11 years) reported a CSR of 95.1%. This study did not report prosthetic complications. A second retrospective study on 98 NDIs placed in premolar and molar sites and restored with single crowns and short span multiunit fixed dental prostheses reported a 96.9% implant CSR at 8 years. There were no implant fractures or abutment screw loosening or fracture. The most common mechanical complication was ceramic veneer chipping (18.4%) and retention loss (5 single crowns). Although 84.6% of the patients were satisfied with restorative function, the main reason for dissatisfaction was gingival recession and food impaction around the restorations. A third study reported the 5- to 10-year outcomes on NDIs placed in anterior as well as posterior sites. An overall CSR of 92.3% was reported at 124 months. In this study, posterior sites demonstrated increased risk for failure and retention loss was the most common prosthetic complication.
Narrow Diameter Implants as Overdenture Abutments
The mandibular edentulous arch is often severely resorbed and characterized by narrow crestal bone. A 2-implant overdenture using standard diameter implants may require either surgically reducing the height of the ridge to a level of adequate buccolingual width, or lateral bone augmentation followed by implant surgery. In addition, patients requesting this treatment are often elderly, under the care of a physician, and have limited self-care capability. Two-implant overdentures retained by NDIs can be the most conservative treatment alternative for these patients.
A longitudinal study reported implant and prosthesis outcomes at 3 and 5 years. The majority of implant sites showed minimal change in crestal bone and an implant CSR of 97.8%. Complications requiring intervention included prosthesis fracture, periimplant inflammation and infection, and replacement of the retentive matrix. A second NDI study that included 24 two- and four-implant overdentures reported an implant CSR of 92.3% at 124 months. Another study investigating immediate versus delayed loading of mandibular NDI overdentures reported 98% implant CSR and very few prosthesis complications at 12 months.
The lack of studies using a comparison group make it difficult to directly compare 2-implant mandibular overdentures retained by NDIs to overdentures retained by standard diameter implants. However, reported clinical outcomes for NDIs are acceptable, and the minimally invasive treatment approach makes this option particularly suitable for patients who are at higher surgical risk with limited economic means.
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