The placement of short implants, which measure less than 10 mm in length, requires the practitioner to have a thorough comprehension of implant dentistry to achieve acceptable results. Innovation of the rough-surface implant and the progression of the implant-abutment interface from an external hex to an internal connection have considerably influenced the longevity of short implants. Dentists are better equipped to serve their patients because the utilization of short implants may preclude the need for advanced surgical bone-grafting procedures.
Key points
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The placement of short implants, which measure less than 10 mm in length, requires the practitioner to have a thorough comprehension of implant dentistry to achieve acceptable results.
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Innovation of the rough-surface implant and the progression of the implant-abutment interface from an external hex to an internal connection have considerably influenced the longevity of short implants.
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Dentists are better equipped to serve their patients because the utilization of short implants may preclude the need for advanced surgical bone-grafting procedures.
History of implants
Archeologists have discovered that even early civilizations desired to replace missing teeth. Excavations of ruins roughly dating back to approximately 600 AD demonstrate that the Mayan people used a variety of materials to insert into empty tooth sockets, notably carved seashells.
The contemporary age of root form endosseous implants commenced in 1952 when a Swedish physician, Dr. Per-Ingvar Branemark, discovered osseointegration. Subsequently, in 1965, Dr. Branemark placed the first titanium dental implants in a human patient. His extraordinary clinical research and investigations were unparalleled. In 1982, when Dr. Branemark presented his work on osseointegration at a dental conference in Toronto, the field of dental implantology was revolutionized.
Implant length
Initially, the length of implants ranged from 7 mm to 20 mm. The most common implant diameter accessible to dentists early on was 3.75 mm. The consensus among practitioners at the time was that the length of the implant was considerably more important than implant diameter. However, the research had not yet demonstrated a linear relationship with regard to the aforementioned concept.
Numerous researchers have deemed various implant lengths as short. Some assert that implants up to 7 mm or less are short whereas others have proclaimed up to 10 mm or less. In the field of dentistry there is no universal agreement on what comprises short and long implants. Throughout this article, long or standard-length implants are 10 mm or longer, whereas short implants are considered to be less than 10 mm.
Partiality for long implants
There were 2 prevailing reasons why long implants were considered superior. First, initial clinical research showed that short Branemark implants with traditional machined surfaces, which were 6 mm to 10 mm in length, had lower success rates than longer implants. Friberg and colleagues investigated 4641 Branemark machined fixtures from the time of implant insertion until prosthetic restoration. Their findings led them to conclude that long implants, ranging from 10 mm to 20 mm in length, were significantly more successful compared with short implants. Wyatt and Zarb reported on a 12-year investigation (mean of 5.4 years) of 230 Branemark machined fixtures. In their study, 25% of short implants, which were 7 mm in length, failed; whereas only an 8% failure rate was associated with 10-mm implants. Longer implants, measuring 13 mm and 15 mm, only failed 5% and 2% of the time, respectively. Bahat reported a 17% failure rate with short implants, measuring 7.0 mm and 8.5 mm, after following 660 fixtures for 5 to 12 years. Attard and Zarb reported that short implants (7 mm) had a 15% failure rate, whereas long implants measuring 10 mm and 13 mm only failed 6% to 7% of the time.
Weng and colleagues conducted a multicenter prospective clinic study for 6 years investigating 1179 3i implants. In their study, short implants were considered to be 10 mm or less in length and comprised 48.5% of all implants evaluated. In 2003, their report showed that short fixtures, measuring 7 mm to 10 mm, had an overall success rate of only 88.7% and were attributed to 60% of all failed fixtures. The 7.0-mm and 8.5-mm implants had a failure rate of 26% and 19%, respectively, whereas the 10-mm implants had a 10% failure rate. Long implants, measuring longer than 10 mm, had an overall success rate of 93.1%. Herrmann and colleagues conducted a multicenter investigation for 5 years following 487 Nobel Biocare implants. In 2005, their report showed that 7-mm implants had a failure rate of 21.8%, whereas 10-mm implants had a 10.1% failure rate.
The second reason why long implants were considered more desirable was based on dental education regarding fixed prosthodontics, which conceivably may have altered the judgment of practitioners. Ante’s law states that the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced. Based on this postulate, the crown-to-root ratio was used to determine if a tooth was an appropriate abutment. The literature reports a variety of ratios. Although infrequently found in clinical practice, a crown-to-root ratio of 1:2 was considered ideal. Shillingburg and colleagues stated that a crown-to-root ratio of at least 1:1 was required for an adequate result, and a crown-to-root ratio of 1:1.5 was most favorable. The thought of longer roots serving as more advantageous abutments still endures although Ante’s law has subsequently been refuted.
To understand why clinicians and researchers were persistently exploring the concept of short implants, after numerous investigations demonstrated the lower success rate of short fixtures, one must be cognizant of the fact that there are abundant clinical conditions that preclude the application of long implants. Some of the anatomic difficulties encountered are alveolar ridge deficits, maxillary sinus pneumatization with inadequate alveolar ridge height, and the location of the inferior alveolar nerve. Innovative surgical procedures were established to solve these anatomic problems. Block grafting, guided bone regeneration, and maxillary sinus floor grafting were performed to augment alveolar bone height. These advanced surgical techniques can be time-consuming, challenging, costly, can extend total treatment time, and increase surgical morbidity.
Surgical bone-grafting procedures
Milinkovic and Cordaro conducted a systematic review of various surgical techniques that vertically augmented partially edentulous and completely edentulous alveolar ridges. Their report contrasted the mean implant survival rate and the mean complication rate between block grafting, guided bone regeneration, and distraction osteogenesis. Guided bone regeneration in partially edentulous patients had a mean implant survival rate of 98.9% to 100% and a mean complication rate of 6.95% to 13.1%. Block grafts in partially edentulous patients had a mean implant survival rate of 96.3% and a mean complication rate of 8.1%. Distraction osteogenesis yielded the greatest gain in vertical height, but concurrently had the highest mean complication rate of 22.4% while having a mean implant survival rate of 98.2%. Block grafts in completely edentulous patients had a mean implant survival rate of only 87.75%. Its mean complication rate varied widely depending on which donor sites and recipient sites were examined.
Vasquez and colleagues reported both the intraoperative and postoperative complication rate of 200 maxillary sinus floor graft surgeries. Their work showed that sinus membrane perforation was the most common intraoperative complication encountered 25.7% of the time. Prior studies found a sinus perforation rate in the range of 7% to 56%. Postoperative complications occurred 19.7% of the time. They included surgical wound infection at 7.1%, sinusitis at 3.9%, and loss of bone graft at 1.6%.
The posterior segment of atrophic mandibles poses a great challenge to practitioners. When the residual vertical height of posterior mandibular alveolar ridges is not amenable for the placement of implants 10 mm or greater in length, then the only recourse is bone grafting as already stated above or surgical transposition of the inferior alveolar nerve. Hassani and colleagues reported a sensory deficit approaching 100% in the initial postoperative period following transposition of the inferior alveolar nerve. They observed that 84% of patients regained their baseline nerve function, whereas 16% of cases had permanent nerve impairment.
To circumvent the risks associated with advanced surgical procedures, short implants were introduced. , In 2014, Nisand and Renouard conducted a review of various reports comparing the survival rates of long implants with concurrent bone-grafting procedures with the survival rates of short implants. Similar survival rates were found for both groups being studied; however, the placement of short implants yielded a shorter treatment period, was more cost-effective, and resulted in considerably less morbidity ( Table 1 ).
| Author/Year | Patients/Implants | Follow-up, mo | Test Implant | Control Implant/Sx | Cumulative Success Rate |
|---|---|---|---|---|---|
| Penarrocha-Oltra et al, 2014 | 37/80 | 12 | 5.5 mm (intrabony length) | >10 mm + block vertical graft (>8.5 mm intrabony length) | T: 1 short failed C: 2 long failed (preload); 7 grafts deficient needed to use short implants; 21 needed additional grafting |
| Gulje et al, 2013 | 95/208 | 12 | 6 mm | 11 mm | T: 3 short failed; 2 preloading; 1 postloading C: 1 long failed |
| Pieri et al, 2012 | 68/144 | 36 | 6 mm | >11 mm + sinus graft | T: 98.6% success C: 96% success |
| Esposito et al, 2011 | 60/121 | 36 | 6.3 mm | >9.3 mm + vertical bone graft | T: 2 short failed C: 3 long failed; 2 grafts failed |
| Felice et al, 2011 | 28/178 | 5 | 5.0–8.5 mm | >11.5 mm + vertical bone graft | T: 2 short failed C: 1 long failed; 2 grafts failed |
| Felice et al, 2010 | 60/121 | 12 | 7 mm | >10 mm + vertical bone graft | T: 1 short failed C: 3 long failed; 2 grafts failed |
Implant surface
In the late 1980s, it was accepted that there would be 1.5 mm of crestal bone loss in the first year of implant placement and subsequent bone loss not exceeding 0.2 mm during each successive year. Based on this axiom, short implants are inherently at a disadvantage when considering long-term clinical outcomes. However, when the criteria for success were first published most implant fixtures had a machined/turned surface. As time passed, research on the technology of implant surface advanced and the corollary was rough-surface or textured implants. Through the innovation of rough/textured surfaces, the former criteria were no longer justified.
Renouard and Nisand examined the effect of implant length and diameter on survival rates. During their review of 53 studies, they found that 12 cases demonstrated a higher failure rate with short implants which were associated with machined surface fixtures, preparation of the osteotomy site regardless of bone quality, clinician experience, and placement in regions with inadequate bone density. Twenty-two reports that followed displayed similar survival rates between short and long implants when rough-surface fixtures were placed and a modified osteotomy technique corresponding to bone quality was used.
When published in 2011, a meta-analysis conducted by Pommer and colleagues showed that the failure rates of rough-surface fixtures were significantly lower compared with machined surface fixtures. Balshe and colleagues conducted a retrospective analysis of 2425 rough-surface fixtures and 2182 machined surface fixtures and found that there was no statistical difference in the 5-year survival rates. The rough-surface implants had a survival rate of 94.5%, whereas the machined surface implants had a survival rate of 94%. In addition, the survival rates for rough-surface and machined surface implants were 93.7% and 88.5%, respectively, when implants measuring 10 mm or less were studied separately. Nedir and colleagues reported on a 7-year life table analysis of success rates between short and long rough-surface implants. In their prospective study, they found that the cumulative success rate of short rough-surface fixtures measuring 8 to 9 mm was equivalent to long rough-surface fixtures measuring 10 to 13 mm when loaded for at least 1 year.
Implant-abutment interface
The progression of the implant to abutment interface from an external hex to an internal connection, such as a Morse taper or conical design, has considerably influenced the longevity of short implants. De Castro and colleagues reported that abutments with the Morse taper connection sustain less marginal bone resorption than ones with an external hex connection. Furthermore, their study found that the internal conical connection might allow for bone growth over the shoulder of the fixture in close proximity to the abutment.
Weng and colleagues reported on crestal bone loss among fixtures with an internal connection and those with an external hex. In their study, they placed implants with an internal conical connection at the level of the bony crest and also 1.5 mm subcrestal, and then compared the amount of crestal bone loss incurred by those implants with that of the external hex implants that were correspondingly placed at the bony crest level and 1.5 mm subcrestal. They found that the implant-abutment interface with an internal connection had significantly less crestal bone loss than the external hex implants. In addition, external hex fixtures that were placed subcrestal sustained the largest amount of crestal bone loss. These findings demonstrate that crestal bone loss is not a substantial component in the survival of short implants.
Implant-bone interface
The implant-to-bone interface can be analyzed via a computer program, called Finite Element Analysis, that is capable of anticipating how the amount of stress that is applied to an implant will affect it and also how the applied stress will affect the association between the implant and adjacent bone. The program generates a lattice of points in the form of the fixture that encompasses data about the fixture which can be analyzed at all points. Finite Element Analysis enables clinicians to try to predict what will occur to the implant-bone interface during various situations as they modify aspects such as bone density, implant diameter, and length.
Using 3-dimensional computer models created by Finite Element Analysis, Himmlova and colleagues investigated the stress that occurs at the implant-bone interface. The diameters of their simulated fixtures ranged from 2.9 mm to 6.5 mm and the lengths ranged from 8 mm to 18 mm. When comparing an 8-mm implant with a 17-mm implant, there was not much variance in area affected by maximum stress, and the maximum amount of stress was concentrated at the superior 5-mm to 6-mm portion of the implant. Only a 7.3% difference in stress was noted. In addition, there was a negative correlation between implant diameter and stress. The narrow 2.9-mm implant had maximum stress values that were 60% higher than those of the wider 6.5-mm implant. Therefore, implant diameter was a more significant factor than length regarding stress distribution.
Using the computer program with 5 commercially accessible fixture designs, Baggi and colleagues evaluated the impact of fixture diameter and length on stress distribution. The simulated implants had lengths of 7.5 mm to 12 mm and diameters of 3.3 mm to 4.5 mm. The results of their study led them to conclude that implant diameter had greater efficacy than implant length in withstanding biomechanical stress.
Implant-crown ratio
There have been numerous case series and studies that have investigated crown-to-implant length ratios specifically concerning short implants. Blanes conducted a systematic review of the effect of crown-implant ratios on implant survival rates and peri-implant crestal bone resorption. The analysis, which included rough-surface and machined implants, showed that implants with crown-implant ratios greater than or equal to 2 had a 94.1% survival rate after a mean follow-up of 6 years. Furthermore, the study found that crestal bone resorption was not affected by crown-implant ratios.
Tawil and Younan conducted a prospective clinical study following 269 machined surface fixtures for a period of 12 to 92 months. In the study, standard-length fixtures measured 10 mm, whereas short fixtures had lengths of 6 mm to 8.5 mm. The diameters of the fixtures ranged from 3.3 mm to 5 mm. The survival rates for the short and standard-length fixtures were 93.1% and 97.4%, respectively. These findings were not statistically significant. In addition, they found that the 3.75-mm-diameter fixtures had a survival rate of 96.3%, the 4-mm-diameter fixtures had a survival rate of 96.0%, and the 5-mm-diameter fixtures had a survival rate of 94.5%. Again, the findings were not statistically significant when analyzing survival rates based on the various diameters. The 3.3-mm-diameter fixtures were not evaluated. Furthermore, the average peri-implant crestal bone resorption was 0.71 mm.
Nedir and colleagues conducted a 7-year life table study of 528 rough-surface fixtures with regard to their crown-implant ratios. In their analysis, the crown-implant ratios ranged from 1.45 to 1.97 for the short implants which had lengths of 6 mm to 9 mm. The crown-implant ratios ranged from 1.05 to 1.3 for the long implants which had lengths of 10 mm to 13 mm. Their findings demonstrated a cumulative success rate of 99.4% and showed that short implants did not have a higher failure rate compared to long implants although the short implants had greater crown-implant ratios.
A systematic review of 33 studies conducted by Atieh and colleagues focused on short implant placement in the posterior regions of the maxilla and mandible. A total of 3573 short implants measuring 8.5 mm or less in length were studied. Fifty-one percent of the implants were in the mandible, 38% were in the maxilla, and it was unknown where the other 11% of implants were placed. The mean follow-up was 3.9 years with a range of 1 to 7 years. In total, there were 67 implants that failed, and 71% of the failures occurred before implant loading. The finding of implant failure before loading is congruous with other analyses, , , and indicates that forces concomitant with loading of implants and a decrease in the implant-bone interface are not the predominant factors in the definitive failure or success of short implants.
Risk factors
Contemporary literature supports the placement of short fixtures; however, there are risk factors such as low-quality bone, smoking, and the experience of the clinician. A systematic review of 29 studies conducted by Telleman and colleagues focused on short implant placement in the posterior regions of the maxilla and mandible. A total of 2611 short implants measuring 5.0 mm to 9.5 mm in length were studied. They evaluated several factors including the type of implant surface, implant length, if the patient was a smoker or not, and whether the implants were placed in the maxilla or mandible. Their study found that rough-surface implants had a 29% higher success rate than machined implants. The overall survival rate enhanced as the length of the implants increased. The 5-mm implants had a survival rate of 93.1%, the 6-mm implants had a survival rate of 97.4%, the 7-mm implants had a survival rate of 97.6%, the 8-mm implants had a survival rate of 98.4%, the 8.5-mm implants had a survival rate of 98.8%, the 9-mm implants had a survival rate of 98.0%, and the 9.5-mm implants had a survival rate of 98.6%. The heavy smoker group, composed of patients who smoked at least 15 cigarettes per day, had a 57% higher failure rate compared with nonsmokers. The study also found that implants placed in maxilla, which generally has low-quality or low-density bone, had a twofold increase in their failure rate in contrast to implants placed in the mandible.
In 2014, Nisand and Renouard conducted a review of various reports comparing the survival rates of long implants with short implants. Short implants were considered to have a length of 8 mm or less. In their analysis of long implants, they evaluated 5 reviews that consisted of 58,953 implants, and found an overall survival rate ranging from 93.1% to 99.1%. In their analysis of short implants, they evaluated 29 case series that consisted of 9780 implants, and found an overall success rate of 96.67%. The findings are congruent with other studies that validate the effectiveness of short implants when placed in suitable clinical conditions.
Several studies that have evaluated short implants demonstrate promising success rates ( Table 2 ). A total of 493 short implants were analyzed in these 4 studies with an average follow-up ranging from 2 years to 10 years. The studies included rough-surface and machined implants, implants that were placed in high-density and low-density bone in both the maxilla and mandible, implants with lengths that ranged from 5.5 mm to 10 mm, and implants that were placed in patients who were smokers and nonsmokers. The short implants had an overall success rate of 95.5%.
