Dental Implants: Past, Present, and Future
Nicolas Elian, DDS
Replacement of missing teeth using various materials and methods has been documented for centuries. Replantation, transplantation, implantation, and many prosthetic options have been used with limited success. The biocompatibility of different materials and biologic response were among the many reasons for infections, bone resorption, and failure. In the early 18th century, the introduction of gold intraosseous anchorage fixtures as roots for replacement of missing teeth generated several case reports on the use of artificial materials. However, metal did not become the focus for tooth replacement until the early 19th century. Researchers and clinicians embarked on studying the properties and biocompatibility of gold, silver, aluminum, zinc, copper, iron, nickel, lead, carbon steel, and other metals histologically. A reaction-free, electrically inert metal was considered ideal. Based on these findings, vitallium along with the screw design for implantation and implant stability were introduced as critical factors for success. Multicenter support and advocacy developed for titanium as the metal of choice in orthopedics for its resistance to corrosion, mechanical, and physical properties. The clinical observations of Per-Ingvar Brånemark while studying blood flow in animals using titanium chambers led to the term osseointegration. The typical designs of the early 1960s were subperi- osteal frames, blade vents, or transmandibular devices with poor clinical results. The first Brånemark dental implant was placed in 1965, and the following 5 years showed limited success. However, subsequent changes in design, surgical protocol, healing period, and prosthetic component improved clinical outcomes for patients.
The first international team to learn the principles of osseointegration, led by George A. Zarb, DDS, was inspired to duplicate Brånemark’s work in Toronto. These efforts, combined with the 1982 Toronto Conference on Osseointegration in Clinical Dentistry, proved to be an enormous break-through for osseointegrated dental implants. The longitudinal study by Zarb and coworkers resulted in multiple publications; the classification of bone quality and quantity and the opportunities for many surgeons and prosthodontists to be involved led to the broad acceptance of this new era in implantology. This early excitement focused on osseointegration and the treatment of patients who had ill-fitting complete mandibular dentures, provided adequate bone volume was present between the mental foramina. The implant consisted of a machined titanium screw threaded into the alveolar bone in a two-stage approach. Following second-stage surgery, healing abutments were placed to allow soft-tissue healing and finally the “standard abutment” to start the prosthetic phase of therapy. Major emphasis was placed on remote incision design; surgical protocol and sterility; healing time; and loading to ensure positive outcomes. The transition from the mandibular arch to the maxilla and partially edentulous cases resulted in a multitude of significant modifications, including surface treatment, thread design, internal vs external connection, abutment selection, and incision design. Macro- and microdesign features to respond to loading and quality of bone became very critical, particularly in the posterior maxilla. The recognition of surface roughness by André Shroeder and colleagues and its influence on bone-to-implant contact in soft bone allowed greater degree of success in type IV bone. The impact of macro- and microgeometry, surgical protocol, initial stability, and cross-arch stabilization led clinicians to the arena of immediate occlusal loading, a significant departure from the original principles of osseointegration.
The partially edentulous cases—more specifically in the esthetic zone—redefined and possibly reoriented both design and therapy. An increase in esthetic demands modified incision design to improve soft-tissue quality and quantity. Clinicians no longer accepted implants in a less-than-ideal restorative position. The implant was designed to achieve excellent esthetics and implant restoration indistinguishable from a natural tooth. Because available bone and soft tissue could not dictate surgical placement, prosthetically driven implant dentistry was on the horizon. Clinicians had to surgically regenerate and, in many cases, overbuild the recipient site. Site development protocols using autogenous, alloplastic, xenograft, and barrier membrane materials widened the scope and level of care. Techniques for horizontal and vertical regeneration and sinus augmentation became an integral part of an implant practice. The partnership between surgeons, restorative dentists, laboratory technicians, and the industry will further enhance this modality.
Diagnostic modalities such as wax-ups of pink and white missing teeth and gingiva, opaque radiographic templates, computed tomography scans, 3-dimensional treatment planning software, and surgical guides will aid in determining implant position and the necessity for augmentation. Other factors include the selection of implant type and its ability to maintain crestal bone, the choice of one-piece or two-piece implants, the decision for splinting to natural teeth, and determination of whether two adjacent implants should be placed in the esthetic zone. In addition, choices now include wide-diameter implants for molar teeth or tooth-specific implant diameter; narrow-diameter implants (NDIs) for cases with very thin ridges that cannot be augmented; and tapered implants and contoured abutments to mimic the root and cement-enamel junction of natural teeth. The early versions of NDIs were developed for transitional therapy. Based on histologic evidence that an NDI can have similar percentage of bone-to-implant contact as a standard diameter implant and predictable outcome, these implants are now approved for long-term treatment. The clinical necessity for such a diameter in many clinical situations in both partially and fully edentulous patients and the approval for long-term use will expand both indications and acceptance of this modality. Finally, to move implant therapy closer to conventional dentistry, custom abutments for cementable restorations and prefabricated abutments with snap-on impression copings deliver a familiar concept.
As design at both the macro and micro levels and predictable regeneration continued to improve, achieving successful results became standard. Immediate implant placement into an extraction socket was added to the armamentarium, along with the “dilemma of immediate or delayed implant placement” and a number of related questions. The body and thread design, initial stability, and surface texture of the implant combined impact decision making. Also, the gap distance and grafting of the space are considerations. Can the healing be modulated, and should a provisional be placed? What abutment should be selected, and should it be the final one? One abutment placed and never removed is referred to as “one abutment one time.” How does this modality influence the biologic width reformation combined with platform switching? Can supracrestal biologic width around an implant be demonstrated histologically?
The development of cone-beam computed tomography (CBCT) and the introduction of this technology to private practice will further change the implant office. Dentists will no longer have to refer patients to radiology centers. Many practices today in North America and globally integrate CBCT as part of the new implant practice. CBCT will eventually replace the panoramic and cephalometric radiographs due to its vast amount of information-providing, diagnostic, and planning capabilities. This will make CBCT not only part of the new implant practice but also an essential and integral part of the advanced, digital, and modern dental office regardless of the specialty. The integration of CBCT and virtual planning in conjunction with computer-aided design/computer-aided manufacturing (CAD/CAM) technology has advanced the concepts of minimally invasive surgery. Flapless, punch flap, and transgingival operations are performed using stereolithographic and 3D-generated surgical guides for osteotomy preparation, final insertion of the implant, abutment, provisional restoration, and potentially abutment design and final restoration.
In the early stages of its introduction, guided surgery became popular in anticipation of expanding the scope of minimally invasive surgery and broadening the numbers of dentists performing the surgical phase. Using the same concepts, computer-generated frame and abutment design using various materials such as titanium and zirconia was developed. However, the great value of navigation surgery in medicine benefited from very limited exposure in dentistry. Navigation surgery offers the possibility of intraoperative modifications based on available bone and real-time monitoring of drilling and implant placement without a guide. Alternatively, guided surgery cannot be adjusted and limited by the volume of bone and quality of soft tissue present.
Adequate bone and soft-tissue volume remains the challenge. While autogenous graft continues to be the gold standard, osseoinductive particulates may be equally predictable without donor-site morbidity and limited volume. The barrier membrane and its characteristics will impact the outcome. Biologic modifiers such as plasma-rich platelets influence postoperative soft-tissue response only. Connective tissue substitutes (cellular and acellular) are critical variables in enhancing the volume and quality of the surrounding gingival complex. The use of bone morphogenic protein in oral and maxillofacial surgery for certain indications, recently approved by the Food and Drug Administration, might be promising and could lead to improved results and a decreased regenerative timetable. In contrast, the macrogeometry of the implant can compensate for inadequate bone volume and, in many cases, eliminate the need for augmentation. This geometry focuses primarily on the collar and neck of the implant. A preangled implant with 12°, 24°, or 36° reduces the number of surgical interventions and delivers an ideal restorative platform. As practitioners favor preservation to augmentation, the previously discussed design will play a pivotal role in implant selection and placement, both immediate and delayed, based on ideal tooth position. A superwide diameter implant for immediate molar placement provides more options.
Gradually, preservation will take center stage and the macrogeometry will dominate design to support biomechanical and chemical bonding of the surfaces that will result in a “bioactive implant.” Tissue engineering, biologic modulators, and 3D structures that “smart graft” to correct ridge deformity will have various degrees of mineralization, different cell types, and time-dependent growth factor release that will define the “new standard.” Electrical, electromagnetic, and laser therapy will also be used to enhance and accelerate wound healing and regeneration. Once ideal implant design and site development have been achieved, research will move toward guided navigation and finally robotics. The dentist will be able to focus on biology, diagnostics, and treatment planning. These innovations could lead to a larger implant team. Ultimately, with the advances in genetics and tissue engineering, the macro- and microstructures of an implant may not be of great importance and the material will be irrelevant. New teeth will become the implant of choice.
About the author
Nicolas Elian, DDS
Division Head of Implant Dentistry, New York University College of Dentistry, New York, New York;
Private Practice, Englewood Cliffs, New Jersey