Evolution of Biomaterials in Implant Dentistry Leads to Enhanced Tissue Quality

Significant advancements in implant macro- and microdesign in the 1990s, mainly spurred by improved implant surface treatment technologies, greatly increased implant surface area and thereby optimized osseointegration in regions of inferior bone density so that implant success rates in the maxilla could finally match those achieved in the mandible. Mixing dental implants with teeth in the treatment of partially edentulous patients, or single-tooth replacement, only became a viable treatment option when predictability of osseointegration was achieved independent from bone density.

Advancements in implant design simplified treatment protocols for implant placement and restoration, enabling implant dentistry to be practiced by many clinicians instead of only few. However, bone volume still dictated treatment, and implants were placed with little attention paid to ideal implant position and angulations. Restoring dentists and lab technicians struggled to find ways to restore some of these cases, forcing the development of customized restorative abutments and components. Hence, many implants were placed and achieved successful osseointegration, but were never restored. It was not until the concept of restoratively driven implant dentistry was proposed that surgeons started using surgical guides as a reference for implant dentistry. Since implant placement solely based on bone availability was no longer acceptable, surgeons were now faced with a new challenge: overcoming the lack of alveolar bone volume in the most ideal restorative position where implants should be placed.^3,4

Early Concepts

The first concepts of bone augmentation in implant dentistry were imported from orthopedics and oral and maxillofacial surgery, where bone augmentation was used for treatment of trauma and tumor resection patients. Autogenous bone transplantation from extraoral sources (eg, iliac crest, ribs, calvaria) was the most reproducible method of alveolar ridge augmentation. However, predictability rarely reached 50%, the procedure caused significant morbidity, and the cost to the patient was high since it involved hospitalization and the need for an orthopedic surgeon to harvest graft material. With the proposed use of intraoral sources of autogenous bone (eg, symphysis, ramus, tuberosity, torus), treatment was simplified and patient morbidity decreased.^5-7

The use of commercially available bone replacement materials for alveolar bone augmentation started with the management of maxillary sinus pneumatization and sinus augmentation procedures.^8-13 With the evolution of sinus augmentation techniques and materials used, the estimated survival rate of implants placed in sinus-augmented areas is approximately 90% after 3 years in function.^14-17 Bone replacement materials for alveolar ridge augmentation and implant site development were proposed based on the concept of guided tissue regeneration (GTR) for treatment of periodontal defects, where exclusion of unwanted cells (eg, epithelium, connective tissue) from the wound with barrier membranes allowed for the migration of desirable cell groups (eg, periodontal ligament, bone) to improve outcomes of regenerative periodontal therapy. Guided bone regeneration (GBR) is simple compared to GTR since only one tissue type is developed. Bone replacement grafts derived from other humans (allograft), animals (xenograft), or synthetics are used as scaffolds (osteoconduction) during cell recruitment from adjacent bony walls, migration, proliferation, and subsequent maturation. Biologic materials (eg, proteins, growth factors) in combination with bone scaffolds provide additional stimulatory effects during wound healing (osteoinduction). Successful graft material consolidation relies on the progressive apposition of newly formed bone, followed by functional remodeling and progressive replacement of the graft material by vital tissue.¹⁸ This process requires the presence of a stable bone scaffold, adequate formation of new blood supply, and migration of cells with osteogenic potential.¹⁹ Successful maturation and consolidation of grafted sites relies primarily on the proper formation of a functional graft-vital bone complex.^13,20

Graft Materials and Membranes

Graft material selection for GBR has given preference to dense mineralized materials (ie, bovine bone) to maximize space maintenance during the wound healing process and higher bone density in augmented areas. Bovine bone is a naturally derived product that maintains a crystalline structure, porosity, and carbonate content similar to human bone mineral, which favors osteoconduction.²¹ This selection of dense materials has been influenced by the historic lower osseointegration success rates in lower-density bone. Dense graft materials undergo an extremely slow process of graft material turnover that may take years or decades, and some may never turn over completely.²² Superior bone density achieved with this type of dense graft material is largely due to residual graft material particles rather than superior bone quality. Residual graft material particles provide additional resistance during implant osteotomy preparation and mechanical stabilization during implant placement, giving the clinician the appearance of ideal bone quality. Bone quality is defined by superior percentage of vital bone within augmented areas, and not by bone density experienced due to resistance during implant osteotomy preparation and mechanical stabilization during implant placement. Only vital tissue is functional and may undergo bone remodeling. Hence, sites augmented with dense, mineralized graft materials provide inferior bone quality when compared to sites augmented with graft materials that have the capacity to turn over.²³ Both wider (>4 mm) and longer (>11 mm) implants have been proposed and recommended for these sites to compensate for the inferior bone quality in grafted sites.

Barrier membranes have played a secondary role with dense graft materials, acting as a dressing material to contain the graft rather than shield it against unwanted cells. Barrier function became relevant with the evolution of and transition to lower-density graft materials, such as cancellous-based mineralized allograft and demineralized allograft. Due to the much faster turnover of these materials, both absorbable (collagen) and non-absorbable (e-PTFE, d-PTFE) membranes are crucial to clinical success by preventing encapsulation of the graft material by soft tissue from the flap and assisting in space maintenance. Technical adjustments were necessary to maximize surgical outcomes. Barrier membrane stabilization proved to be a key element for clinical success by providing wound stability during wound healing. Treatment predictability, superior bone quality, long-term stability, low morbidity, and reproducibility established GBR based on barrier membrane and lower-density bone replacement materials as the new "gold standard" method for correction of mild to moderate alveolar ridge deficiencies for implant treatment.

Cross-Linking of Collagen

Prolonged barrier function is needed when graft materials of fast turnover rates are used. In theory, this should be better achieved with non-absorbable barriers since these devices should exclude soft-tissue cells from the augmented area for as long as they stay in place. However, such membranes offer inferior biocompatibility when compared to absorbable membranes (collagen) and have been associated with higher postoperative complication rates related to wound dehiscence, premature barrier membrane exposure, and infection secondary to early membrane exposure.²⁴ To delay absorption of collagen membranes and prolong barrier function, cross-linking of collagen is used. The most used method for cross-linking of collagen is through glutaraldehyde. A limitation of this method is the potential cytotoxicity on the newly formed tissue of these chemicals during membrane absorption. An alternative method of cross-linking based on ribose significantly improves collagen membrane biodurability without affecting biocompatibility.²⁵ Tissue-based absorbable membranes (eg, pericardium, dermis) also provide prolonged barrier function without releasing unwanted chemicals in the surgical wound. Without these advancements in absorbable membranes, the introduction of the latest generation of bone graft materials would not have been possible.

Fully Absorbable Bone Scaffolds

Fully absorbable collagen-based bone scaffolds offer unique advantages to traditional methods of bone replacement. Since the construct consists primarily of cross-linked collagen, absorption times can be controlled during manufacturing through modification of the intensity of cross-linking. These materials provide adequate mechanical properties for space maintenance while still undergoing an active absorption/incorporation process that takes 4 to 6 months in the oral cavity, leading to newly formed bone without residual graft material properties. Promising results have been reported in extraction site management, mild to moderate ridge defects post extraction, management of the gap between immediately placed implants and socket walls, maxillary sinus augmentation, and correction of buccal concavity defects for optimal implant esthetic outcomes.

Other clinical applications such as management of severe horizontal ridge deficiencies and vertical bone augmentation may require further advancements of surgical techniques and the combination of biologic materials to optimize bone regeneration, and these fully absorbable bone replacement materials in combination with long-lasting absorbable collagen membranes may eventually replace conventional methods of ridge augmentation. Superior bone quality achieved with fully absorbable scaffolds in GBR should create more stable peri-implant bone that is more resistant against peri-implant disease and bone loss and allow for the current trend of both narrower (≤4 mm) and shorter (<10 mm) implants in healed sites to also be used in augmented sites, minimizing risk to patients and facilitating future retreatment, if necessary.

Conclusion

Restoratively driven implant dentistry would not have impacted the dental treatment of multitudes of patients over the past three decades without the development of surgical techniques and materials for alveolar ridge augmentation. Recent advancements in this field, including the development of fully absorbable scaffolds and long-lasting absorbable barriers, should further enhance treatment outcomes by providing superior tissue quality, thus enabling clinicians to use shorter and narrower implants in bone-augmented areas.

About the Author

Rodrigo Neiva, DDS, MS
Chairman and Clinical Professor, Department of Periodontics, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania