A ‘Graft Less’ Approach to Posterior Implant Sites

INTRODUCTION
In the past, a common guideline for selection of an implant size was to choose the maximum length without impinging on anatomic boundaries. When treating the atrophic ridge, surgeons would often favor the option of vertical bone augmentation to allow the placement of longer implants. Several advancements have led to the greater use of, and better outcomes with, shorter implants. Microtextured implant surfaces enhance earlier bone formation and a higher percentage of bone-implant contact.¹ Design improvements in the connection between the implant and abutment have resulted in greater stability and better fit. Stronger grades of titanium, including titanium alloy, improve the implant strength and decrease the risk of fracture. The use of cone beam computed tomography (CBCT) and virtual implant planning allows clinicians to better evaluate constrained bone volumes for short implant placement (CS 9000 [Carestream Dental]).

There has been a trend over time toward minimally invasive treatment options in implant dentistry. This approach attempts to avoid more complicated procedures involving bone augmentation for implant placement. In the posterior maxilla and mandible, the use of short implants (≤ 8.0 mm) can reduce the need for vertical bone grafting. Patients often prefer this approach over more complex procedures that can cause complications, increased morbidity, greater treatment length, and higher costs.²

Short Implant Outcomes
Recent evidence-based reviews on short implants with textured surfaces reveal similar survival rates compared to standard-length implants.^3,4 A systematic review and meta-analysis performed by Lemos et al4 evaluated 13 short implant studies. The implant survival rate was similar between standard and short implants (97.3% vs 96.1%, respectively). A meta-analysis found no statistically significant difference between standard implants and short implants placed in the posterior regions. Furthermore, there were no significant differences between implant lengths in the maxilla or mandible. The analysis of studies evaluating marginal bone loss showed no significant difference between short and standard implants. These positive findings support the view that an 8.0-mm length may well be included in the standard implant category. However, when evaluating implants with a length < 8.0 mm, there were slightly lower survival rates than with standard implants. Therefore, shorter implants (< 8.0 mm) should be used with some discretion.

One consistent finding is that when short implants do fail, it typically occurs early on during the healing period.⁴ This may be due to reduced bone-implant contact and difficulty in obtaining primary stability in a constrained bone volume with compromised bone quality.⁵ Short implants with a tapered body and/or a more aggressive thread design may provide better anchorage and improved stability (BioHorizons Tapered Short Implants [BioHorizons]). Resonance frequency analysis can also be used to measure the implant stability at placement and during healing (Osstell USA).

Some clinicians question using short implants, as this may create an unfavorable crown-to-implant (C/I) ratio. Biomechanical studies have shown that the implant crown height, including the abutment, is much more influential than the implant length.⁶ Three systematic reviews revealed that marginal bone loss and the survival rate of implants did not seem to be influenced by the C/I ratio of implant prostheses.^7-9 However, greater implant crown height can increase the incidence of technical problems. Several studies have found that an unfavorable C/I ratio could be considered a potential risk factor for screw loosening and abutment fractures in posterior areas.^8,10 When using short implants to replace multiple teeth, the implant crowns may be splinted to help reduce this complication.¹¹

Vertical Augmentation vs Short Implants
Treatment options for managing posterior partial edentulism with jaw atrophy include vertical bone augmentation to allow the insertion of standard-length implants or the placement of short implants. In the posterior maxilla, the sinus floor can often limit the available bone for implant placement. The management of maxillary atrophy and sinus pneumatization for dental implant placement has evolved over the years. When sinus bone grafting was first developed, clinicians favored the use of long dental implants. This was thought to be necessary for optimal biomechanical loading of the implant and prosthetic support. In addition, shorter machined surface implants (≤ 10 mm) showed lower survival rates in the posterior maxilla.¹² Under these constraints, it was often necessary to perform sinus bone grafting through a lateral window approach to allow the placement of longer implants. Improvements in implant materials, designs, and surface properties have led to the use of shorter dental implants. Many studies have shown that the survival of short implants in native bone below the sinus is the same as longer implants placed into grafted sinuses.¹³ Sinus bone grafting has a higher incidence of complications and a greater cost and requires additional surgical and healing time.

The clinical trend is to favor shorter implant lengths in the posterior maxilla. The use of shorter implants may avoid the need for grafting or reduce the volume of bone graft material that is needed for sinus floor augmentation. It may also allow the surgeon to consider an internal sinus floor lift for shorter implant placement rather than using a lateral window technique.¹⁴ For example, a vertical bone height of 6.0 mm below the sinus floor would allow the placement of an 8.0-mm implant via a transcrestal lift. Following preparation of the implant osteotomy short of the sinus, an osteotome may be used to fracture the floor superiorly. Another alternative is to use Densah Burs (Versah) in a counterclockwise rotation to elevate the sinus floor and lift the membrane a few millimeters. If a lateral window approach is used for sinus grafting with a simultaneous implant placement, it may also be advantageous to consider using a shorter-length implant. The bone graft surrounding the portion of the implant within the sinus cavity provides no additional primary stability. Less sinus mucosa manipulation and bone graft material is needed to place a shorter implant. A smaller volume of graft material also requires less healing time for bone formation (MinerOss [BioHorizons]). If a lateral window technique is used for sinus bone grafting and delayed implant insertion, shorter implant lengths may also be considered.

Increasing bone height in the posterior mandible is more complex and less predictable than vertical bone augmentation in the maxilla via sinus bone grafting. In the posterior mandible, vertical bone growth can be achieved by guided bone regeneration, block bone grafting, titanium mesh grafting, interpositional grafting, or distraction osteogenesis. There is no consensus on the best method for vertical bone augmentation.¹⁵ Vertical onlay bone augmentation in this area usually requires a harvest of autogenous bone or the use of a growth factor, such as recombinant bone morphogenetic protein 2.¹⁶ Vertical bone augmentation procedures are more technically difficult and require advanced surgical skills. Even when performed by experienced surgeons, complications are not uncommon, and failure can occur.¹⁷ These procedures also increase costs and risk greater morbidity for the patient. Most clinicians stage the augmentation procedure and place implants secondarily. This increases the overall length of treatment. Several studies on treating the atrophic posterior mandible with short implants vs various bone augmentation procedures for longer implants have all concluded that the short implant option was preferred.¹⁸ Implant and prosthetic survival is similar in comparison studies, but there are more complications in the augmented group.

Another viable option to consider for managing the partially edentulous patient with bilateral posterior atrophy is to extract the remaining teeth for implant placement in the anterior mandible. As few as 4 implants can be inserted between the metal foramina.¹⁹ In this situation, an immediate-load provisional prosthesis can be placed on the implants as well. This alternative may be preferred if the prognosis of the remaining teeth is guarded or the caries risk is high. Overall, this option may be more costly, but the patient benefits from immediate implant teeth and a shorter treatment length compared to bone augmentation.

When placing short dental implants in the posterior mandible, it is critical to maintain a safe distance of a few millimeters from the mandibular canal. This allows for potential inaccuracies in radiographic measurements, drilling depth, and implant insertion. For example, if an 8.0-mm implant was planned in the atrophic posterior mandible, then at least 10.0 mm of available bone would be needed. Therefore, vertical bone augmentation may not be required for placement of extra-short implants (6.0 mm) unless there is less than 8.0 mm of bone height superior to the canal. When multiple posterior teeth are missing, several short implants can be inserted for the support of splinted crowns.

In some instances, there is adequate bone height, but the posterior ridge may be resorbed medially with deficient width. In these cases, it may be possible to perform a more predictable horizontal bone augmentation for the placement of short implants instead of attempting to gain additional bone height for longer implants. This has been termed the “graft less” approach to managing the atrophic ridge. Horizontal augmentation may be accomplished by guided bone regeneration, block bone grafting, titanium mesh grafting, or ridge expansion. The choice of technique may depend on the residual ridge dimension, the amount of bone gain needed, and the operator’s preferred method. A block bone graft may be harvested from the mandibular ramus for horizontal augmentation of the narrow posterior mandible.20 Particulate autogenous bone can be harvested using a bone scraper for guided bone regeneration. Ridge expansion can be performed for simultaneous implant placement or staged for delayed implant insertion after healing. However, a ridge width of at least 3.0 mm is required for the ridge split procedure.

CASE REPORTS
Case 1
A 54-year-old male presented with failed abutment teeth supporting a fixed bridge (Figure 1). There was minimal bone height (6.0 mm) below the sinus floor for molar implant placement. Two dental implants were planned for a 3-unit fixed bridge (teeth Nos. 3 to 5). The molar implant osteotomy was prepared with Densah burs in a counterclockwise rotation (Figure 2). The Densah Burs were used to elevate the sinus floor (Figure 3). A BioHorizons Tapered Short Implant (5.8 × 7.5 mm) was placed in the molar site (Figures 4 and 5). The implant had good primary stability, and healing abutments were inserted. The postoperative radiographs revealed the sinus floor had been elevated above the implant apex (Figure 6).

Case 2
A 58-year-old female presented with partial edentulism and atrophy in the left posterior mandible (Figure 7). A CT scan found there was inadequate ridge width and minimal available bone height for dental implant placement. A tenting screw was inserted, and a composite bone graft of particulate autogenous bone mixed with bovine bone mineral (BioOss) and platelet-rich fibrin (L-PRF, IntraSpin [BioHorizons]) was placed onto the atrophic ridge (Figures 8 and 9). The grafted site was covered with a collagen membrane (Ossix Plus [Datum Dental]) and an L-PRF membrane (Figure 10). After 6 months of healing, 2 dental implants were inserted (BioHorizons Tapered Internal Implants) (Figure 11). The implants were restored with a screw-retained fixed bridge (Figure 12).

CLOSING COMMENTS
The successful use of short implants for prosthetic support offers clinicians and patients a simplified approach to the replacement of posterior teeth. When inadequate available bone is present, bone augmentation procedures may be performed for the purpose of placing short implants. This allows clinicians to choose augmentation methods that are more predictable and less invasive than those used in the past.

Acknowledgment:
Dr. Misch would like to acknowledge Rick Sonntag, RDT at 4Points Dental Designs, St. Petersburg, Fla, for the fabrication of the implant bridges.

References

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15. Chiapasco M, Casentini P, Zaniboni M. Bone augmentation procedures in implant dentistry. Int J Oral Maxillofac Implants. 2009;24(suppl):237-259.
16. Misch CM. Bone augmentation of the atrophic posterior mandible for dental implants using rhBMP-2 and titanium mesh: clinical technique and early results. Int J Periodontics Restorative Dent. 2011;31:581-589.
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18. Lee SA, Lee CT, Fu MM, et al. Systematic review and meta-analysis of randomized controlled trials for the management of limited vertical height in the posterior region: short implants (5 to 8 mm) vs longer implants (> 8 mm) in vertically augmented sites. Int J Oral Maxillofac Implants. 2014;29:1085-1097.
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Dr. Misch received certificates in postgraduate prosthodontics and oral implantology as well as a Master of Dental Science degree from the University of Pittsburgh. He also completed a residency in oral and maxillofacial surgery in Pittsburgh and is board certified by the American Board of Oral and Maxillofacial Surgery and the American Board of Oral Implantology/Implant Dentistry. Dr. Misch practices as a dual specialist in Sarasota, Fla. He is a clinical associate professor at the University of Florida, the University of Alabama, and the University of Michigan in the departments of periodontics and prosthodontics. Dr. Misch serves as editor in chief of the International Journal of Oral Implantology. He lectures extensively on the topics of bone augmentation, implant surgery, and prosthodontics and has published over 40 articles and 15 textbook chapters. He can be reached at mischimplants@yahoo.com.

Disclosure: Dr. Misch is a consultant for BioHorizons and Datum Dental.

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