Key points

Brief history of implants

People’s desire to replace missing teeth predates all recorded treatises on dentistry. Human osseous remains carbon dated as far back as 600 ad show the Mayan practice of using seashells carved into tooth-shaped pieces placed into empty sockets.

The modern era of root form endosseous implants begins with Dr P.I. Brånemark’s discovery of osseointegration in 1952 and subsequent placement of the first Brånemark implants in human patients in 1965. Dr Brånemark’s presentation in 1982 at the Toronto Osseointegration Conference in Clinical Dentistry included incomparable scientific documentation going back to 1952 and data on human research from 1965. Such data in implantology had never before been collected.

Brief history of implants

People’s desire to replace missing teeth predates all recorded treatises on dentistry. Human osseous remains carbon dated as far back as 600 ad show the Mayan practice of using seashells carved into tooth-shaped pieces placed into empty sockets.

The modern era of root form endosseous implants begins with Dr P.I. Brånemark’s discovery of osseointegration in 1952 and subsequent placement of the first Brånemark implants in human patients in 1965. Dr Brånemark’s presentation in 1982 at the Toronto Osseointegration Conference in Clinical Dentistry included incomparable scientific documentation going back to 1952 and data on human research from 1965. Such data in implantology had never before been collected.

Early history of implant lengths

Early implants ranged in length from 7 mm to 20 mm. The most widely available implant diameter at the time was 3.75 mm with a machined or turned surface. At first, the implant length was considered paramount and the diameter not as important, even though a linear relationship between length and success had not been proved.

What are short implants?

There is no general consensus in dentistry as to what constitutes a short versus a long implant. Various investigators have considered various lengths of less than or equal to 7 mm up to 10 mm as short. For the purposes of this article, lengths less than 10 mm are considered short. Implants 10 mm or greater in length are considered long or standard length.

Why long implants were preferred

As stated earlier, long implants were considered most desirable. Reasons for this opinion was probably 2-fold.

First there was early evidence that short Brånemark implants (6–10 mm) with traditional turned/machine surfaces had inferior success rates compared with longer fixtures.

Friberg and colleagues reported on 4641 consecutively placed Brånemark machined implants that were followed from implant surgery to prosthesis insertion. They concluded that, “A preponderance of failures could also be seen among the shortest fixtures (7 mm)” compared with the longer 10-mm to 20-mm fixtures.

Wyatt and colleagues reported in 1998 on 230 machined Brånemark implants followed for up to 12 years (mean 5.4 years). Of the 7-mm implants placed, 25% failed, whereas the 10-mm fixtures had an 8% failure rate and the 13-mm and 15-mm implants had failures rates of only 5% and 2% respectively.

Bahat followed a total of 660 implants placed in the posterior maxilla from 5 to 12 years. Of the 3.75-mm diameter short implant fixtures, including 7 and 8.5 mm, 17% failed.

In 2003, Attard and Zarb showed a 15% failure rate for 7-mm implants, whereas 10-mm and 13-mm implants had failure rates of 6% to 7%.

Weng and colleagues reported in 2003 on a multicenter prospective clinical study evaluating the success of 1179 3i machined surface implants for up to 6 years. Of the 1179 implants, 48.5% were considered short (≤10 mm). These short implants (7–10 mm) accounted for 60% of all failed implants, with a cumulative success rate of only 88.7%. The 10-mm long implants accounted for 10% of the failures, whereas the 8.5-mm and 7-mm implants accounted for 19% and 26% of failures respectively. The cumulative success rate for the long implants (>10 mm) was 93.1%. The overall cumulative success rate was only 89%.

Herrmann and colleagues described in 2005 a multicenter analysis of 487 Brånemark System; Nobel Biocare implants followed for 5 years in the hope of predicting implant failures based on patient and implant characteristics. They found a 10.1% failure rate for 10-mm implants and a 21.8% failure rate for the 7-mm implants.

Second, dental training in conventional fixed prosthodontics, specifically Ante’s law, possibly skewed clinicians’ thought processes. 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. From that law, the radiographic calculation of the crown/root ratio (CRR) was used to decide a tooth’s suitability as an abutment. A variety of ratios are reported in the literature. A CRR of 1:2 was considered ideal, but is seldom found in clinical practice. Shillingburg and colleagues suggested that a CRR of 1:1.5 was optimal and a ratio of at least 1:1 necessary for a satisfactory result. Even though Ante’s law has subsequently been disproved, the concept of longer roots being better abutments than short roots still prevails. With these ideas already established, short implants just looked wrong and, whenever possible, long implant fixtures were placed.

So why, after so many of the early studies showed generally decreased success with short implants, did researchers and clinicians continue to pursue the idea of short implant fixtures? The answer is because there are numerous clinical situations in which the placement of long implants is problematic because of anatomic boundaries. Some of the more commonly encountered problems are the position of the inferior alveolar nerve and canal, low-lying maxillary sinuses with insufficient residual alveolar bone height, and alveolar ridge deficiencies. In an effort to overcome these anatomic problems, advanced surgical techniques were developed. To increase the alveolar bone height, guided bone regeneration, block grafting, maxillary sinus floor grafting, and distraction osteogenesis procedures were performed. To bypass vital structures such as the inferior alveolar nerve, nerve transpositioning techniques were used. All of these advanced surgical procedures can be challenging, technique sensitive, time consuming, and costly, and can increase surgical morbidity and prolong overall treatment time.

With early disappointing results such as these, why would anyone want to continue to use short implants? The answer lies in improvements in surface technology, implant to abutment connections, a better understanding of implant to bone biomechanics using finite elemental analysis (FEA), and weighing the risks versus benefits of advanced bone augmentation procedures as opposed to using short implants.

Cost/benefit and risk/benefit of advanced bone grafting surgery versus short implants

A systematic review by Milinkovic and Cordaro of different alveolar bone augmentation procedures for partially edentulous and fully edentulous jaws documented the mean implant survival rate (MISR) and the mean complication rate (MCR) for vertical augmentation procedures, including guided bone regeneration (GBR), bone blocks (BBs), and distraction osteogenesis (DO). In partially edentulous patients with GBR, the MISR ranged from 98.9% to 100%, with an MCR 13.1% to 6.95%. BBs had an MISR of 96.3% and MCR of 8.1%. The greatest vertical gain was noted with DO, but it also had the highest MCR (22.4%) and MISR (98.2%). In fully edentulous patients, the BB MISR was only 87.75%. The complication rate was highly variable, contingent on whether the different donor sites or recipient sites were being analyzed. The overall MCR was calculated as 21.9%. For Le Fort? I grafts, the MISR was 87.9%, with complications ranging from 24% to 30%. With sinus graft there are multiple different complications possible, including intraoperative and postoperative complications. Vasquez and colleagues documented the complication rate in 200 consecutive sinus lift procedures. They reported that the most common intraoperative complication, at 25.7%, was schneiderian membrane perforation. Previous reports note a range of 7% to 56% in the rate of perforation. After surgery, 19.7% had some type of complication. The most frequent were wound infection (7.1%), sinusitis (3.9%), and graft loss (1.6%).

For atrophic mandibles, it is the posterior segment, distal to the mental foramen, that poses the greatest challenge. When the remaining posterior vertical alveolar bone is deemed inadequate for 10-mm or longer implants, the only option is either vertical grafting as outlined previously or inferior alveolar nerve transpositioning surgery. Hassani and colleagues documented initial postoperative sensory impairment after inferior alveolar nerve transposition of almost 100%. Normal sensory function returned in 84% of cases, with 16% of patients left with a permanent and irreversible condition. Neurosensory disturbances are so prevalent with this procedure that many surgeons consider sensory disorders such as these to represent a normal and predictable postsurgical state and that they should not be considered untoward sequelae of treatment.

The concept of placing short implants was introduced as a way to avoid the need for advanced surgical procedures and their associated risks.

Nisand and Renouard in 2014 reviewed multiple studies comparing short implants with standard-length implants with various vertical augmentation procedures and found similar survival rates. However, the use of short implants resulted in faster and lower-cost treatment with reduced morbidity ( Table 1 ).

Rough/textured surface versus machine/turned surface

Over the years research progressed on the surface technology of implant fixtures, leading to textured or rough-surface implants.

If clinicians were to still accept 1.5 mm of crestal bone loss in the first year and no more than 0.2 mm of bone loss per year in succeeding years, then short implants would effectively become even shorter, potentially increasing the negative effects. These criteria for success were written in 1989, at a time when most implants had only a machined/turned surface. With the advent of rough/textured surfaces, these old criteria are no longer valid.

Renouard and Nisand reviewed 53 human studies of the impact of implant length and diameter on survival rates. They found that 12 of the studies indicated an increased failure rate with short implants, which was associated with operator experience, routine surgical preparation (irrespective of bone density), machined surface implants, and placement in areas of poor bone density. Twenty-two later publications showed comparable survival rates between short and long implants when rough-surface implants and adapted surgical protocols based on bone density were used.

Pommer and colleagues indicated that rough-surfaced implants showed significantly lower failure rates than machined implants. Balshe and colleagues found in their retrospective study of 2182 smooth (machined) surface implants versus 2425 rough-surfaced implants that there was no statistical difference in the 5-year survival rates (94% vs 94.5% respectively). However, when implants of less than or equal to 10 mm were evaluated separately, the estimate of survival for rough-surface implants was 93.7%, whereas for smooth-surface implants it was only 88.5%. A prospective study by Nedir and colleagues on a 7-year life table analysis on International Team for Implantology of Institute Straumann (ITI) rough-surface (titanium plasma-sprayed [TPS] and sandblasted, large grit, acid-etched [SLA]) implants loaded for at least 1 year showed that the cumulative success rate for short implants (8–9 mm) was as high as for the longer implant groups (10–11 mm and 12–13 mm).