Short and Ultra-Short Implants

Douglas Deporter

• Publisher: Quintessence Publishing Co, Inc
• Year: 2019
• ISBN:
  9780867157864
• Topic: Medicine
• Subtopic: Dentistry

Research has shown that short implants are not only a viable option but oftentimes a superior one that carries fewer risks for the patient and dentist, especially in resorbed jaw sites. As clinical trials continue to underscore the safety and efficacy of short implants, more dentists are considering their use with real interest, and this book provides the information clinicians need to incorporate short implants into their own practice. The book reviews the clinical effectiveness of short implants and then describes treatment protocols for the various types of short implants and their placement in different areas of the mouth. Case presentations demonstrate the recommended techniques and showcase the results.

It has been more than 50 years since the first successfully osseointegrated implant procedure was performed by Dr Per-Ingvar Brånemark, and yet controversy remains regarding the optimal shape and size of solid endosseous root-form dental implants. There are few medical fields with this degree of uncertainty despite the wealth of relevant scientific data. The recommended lengths of dental implants are a striking example. A quick search on PubMed in December 2016 identified 5,400 articles mentioning short implants, but the majority of articles focused on more complex solutions, relegating short implants to the rank of emergency fallback solutions only. Consequently, the mindset persists that the longer the implant, the more successful it will be both in the short and long term. Advanced, costly, and technique-sensitive collateral procedures often need to be performed to be able to use standard-length implants, such as autogenous (and other) bone block grafting, vertical alveolar ridge augmentation, mandibular nerve repositioning, and open sinus grafting. Interestingly, however, the first implants developed and tested successfully by Brånemark in the 1960s were all under 8 mm in length, and some were even shorter than 5 mm.

The reluctance of clinicians to use short implants derives largely from their reading of statistical assessments of implant failure while neglecting other considerations like the patient’s sex, the size of the patient’s mouth, the risk of complications with more complex procedures, and the feasibility of such procedures being undertaken by nonspecialists in a private practice setting. If the intention is to provide the simplest, least invasive, least complicated, and least stressful procedure, it is worth asking: why should we avoid using short implants? Arguments commonly put forward include the following:

•Their success rate may be lower compared with standard-length implants.

•Stress distribution with loading of short implants may result in increased biomechanical risk due to the poor crown-to-implant (C/I) ratio.

•There may be a greater risk of failure if crestal bone loss occurs due to mechanical overload or inflammation and infection (eg, mucositis or peri-implantitis).

•It is difficult to move away from more familiar ways of doing things.

This chapter addresses these concerns.

Success Rates

Are the success rates for short implants worse than those of procedures that use standard-length implants? The poor reputation of short implants is—for the most part—based on early implant literature using the original Brånemark-type implant: A commercially pure titanium threaded screw with a machine-turned (ie, minimally rough) surface finish.^1–4 In each of these reports, the authors stated in their abstracts and/or conclusions that failure of short (ie, ≤ 10 mm) implants was higher than that of longer ones. This was enough to establish a dogma that was not easily challenged. However, a closer reading of these articles reveals that although the failure rates with short implants were higher, the difference compared with longer implants was less significant than the authors implied. For example, in the report by Friberg et al² examining 4,641 implants, the failure rate of short maxillary implants covering everything from single crowns to full-arch reconstructions in fully edentulous patients was only 7%. Looking only at the results for partially edentulous patients in this study, the failure rate of short implants plummeted to 1.3%. Moreover, the authors stated that most of the failures were early—once short implants had been fully integrated in bone, they behaved just like longer implants. In 1991, these clinicians were not only using cylindric, machined-surface implants; they were also following a standard and identical drilling protocol for both short and longer implants and regardless of the bone density encountered.

Likewise, a closer look at the report by Lekholm et al⁴ shows the failure rate for short implants as not significantly different from that of procedures using longer implants. In the study by van Steenberghe et al,¹ short implants were separated into those with lengths of 7 mm and 10 mm, and the 7-mm implants had a higher success rate than those with lengths of 10 mm. The adaptation of these findings to comply with the general consensus at the time (ie, that implants shorter than 10 mm fail more often despite contrary objective data) is termed confirmation bias. This type of bias is a very common cognitive behavior: Once a decision has been made or a “fact” learned, the human brain will always look for data that corroborate the preconceived notion in question and dismiss data challenging these notions. It is an unfortunate but frequent occurrence in many scientific fields.⁵

As implant research continued, the influences of length and diameter on implant survival and success have been studied using a more objective approach to scientific publications: reviewing the facts prior to drawing conclusions. For instance, Renouard and Nisand⁶ published a systematic review of articles published between 1990 and 2005. Using Medline resources, papers were initially selected if implants were placed in healed sites in human subjects and if the studies provided (1) relevant data on implant lengths and diameters, (2) implant survival rates that were either clearly stated or calculable, and (3) clearly defined criteria for implant failure. A total of 34 studies fulfilled these inclusion criteria, and of these, 13 were devoted to short implants. The investigators in these 13 studies were able to report outcomes for a total of 3,173 implants in 2,072 patients. Implants from a number of manufacturers were included, and the mean implant length was 7.9 mm. Observation periods ranged from 0 to 168 months with a median period of 47.1 months. A total of 9.5% of the study patients were reported as having dropped out, leading to a mean implant survival rate of 95.9%, which is more or less the same as for longer implants of the time. However, some authors reported noticeably lower success rates than this mean, and readers need to understand why.

In 2005, Hermann et al⁷ analyzed a large number of failed implant procedures and reported that short implants had usually been used in sites with low bone volume and density, whereas longer implants were nearly always placed in denser bone. This observation challenges implant length as the cause of failure. For example, did an implant in the posterior maxilla fail because of its length, or was the result caused by low bone density and the clinician’s failure to appropriately modify osteotomy preparation?

If a short implant is to be eliminated as a treatment choice, the clinician and patient need to be aware of the likely risks, complications, and possibility of implant failure if longer implants are to be used in conjunction with maxillary sinus grafting or a vertical ridge augmentation procedure. Recent research addressed some of these issues using randomized controlled clinical trials in humans comparing short implants alone with longer implants placed in previously grafted bone. Unlike a lot of the early work with short implants, the investigators employed moderately rough (eg, particle-blasted or acid-treated) rather than machine-surfaced implants.⁸ In resorbed posterior mandibles, Esposito et al⁹ provided a 3-year report on the use of 6.3-mm-long implants versus 9.3-mm-long implants placed in sites where earlier vertical ridge augmentation had been done using interpositional block xenografts. Each patient received two or three implants that were allowed submerged healing and ultimately restored with splinted fixed prostheses. Two short implants failed and three standard-length implants failed, all in different patients. However, there were significantly more complications in the augmented patients with standard-length implants (22 complications in 20 augmented patients) than those with short implants (5 complications in 5 patients). In a similar vein, Pieri et al¹⁰ compared 6- to 8-mm implants with 11-mm implants placed in conjunction with a lateral window sinus grafting procedure. After a mean functional time greater than 3 years, the implant survival rates were similar, but again there were significantly more surgical complications with the sinus group (10 complications in 9 patients with standard-length implants compared with 1 complication in the short implant group).

Choosing not to use short implants because of their supposed inferior performance then appears not to be a wise decision. Indeed, it seems clear that short implants should be the treatment of first choice in sites with low bone volume unless there are other considerations (eg, esthetics) that would contraindicate short implant use.

Implant Stress Distribution and C/I Ratio

In the 1990s, articles began to appear on the impact and patterns of mechanical stress distribution with functioning dental implants. Using finite element analyses, as early as 1992 Meijer et al¹¹ predicted that when dental implants are subjected to lateral (ie, off-axis) loading, stress will be chiefly concentrated in crestal bone surrounding the neck regions of implants. It was concluded that changing implant length had no significant impact on stresses being received by peri-implant crestal bone. Similar conclusions were later drawn by Pierrisnard et al.¹² In line with basic mechanical principles, the greater the angle at which a force is received, the greater the stress concentration at the implant neck, with this stress diminishing toward the implant apex.

The ratio of crown length to tooth root length (C/R) has long been considered a key factor in designing traditional, tooth-supported prostheses. Having a C/R ratio greater than 1 was considered a risk factor for failure, and this precept was initially thought to apply to dental implant performance as well. With the anticipated implant C/I ratio significantly greater than 1, many clinicians preferred to use collateral surgical techniques to allow for the placement of longer implants. In the posterior mandible, for example, some clinicians have gone as far as to recommend lateralization of the mandibular nerve to enable longer implants to be placed—even though no prospective trials have been reported comparing this risky procedure with short implants. However, numerous authors have now shown that C/I ratio is not an issue for the most part, meaning that once again short implants may be the treatment of first choice.^13–15 This is not to say that there are no upper limits for C/I ratio because there certainly may be; however, whether an implant is 5 mm long or 15 mm long, it will ultimately experience the same stress patterns and minimize the importance of C/I ratio on biologic complications. Nevertheless, a long implant-abutment complex will increase the length of the lever arm (measured to the implant neck) and therefore increase the risk of prosthesis overload. This means that selection of the number of short implants used and the use of splinting are important considerations.

Risk of Bone Loss and Peri-implantitis

The long-standing Albrektsson criteria¹⁶ for dental implant success continue to be used in clinical practice, including the recommendation that bone loss should not be greater than 1.2 mm during the first year of implant function ...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright Page
• Contents
• Foreword
• Preface
• Acknowledgments
• Contributors
• 1 Why Avoid Using Short Implants?
• 2 The Performance of Short and Ultra-Short Implants
• 3 A Single Practitioner’s 20-Year Experience with Short Implants
• 4 Using Short Implants for Overdenture Support
• 5 Threaded Implants in the Posterior Maxilla
• 6 Threaded Implants in the Atrophic Posterior Mandible
• 7 Press-Fit Sintered Porous-Surfaced Implants
• 8 Plateau Root Form Implants
• 9 Ultra-Wide Threaded Implants for Molar Replacement
• 10 The Way Forward
• Index