CHAPTER 26 A NEW CONCEPT OF TAPERED DENTAL IMPLANTS
PHYSIOLOGY, ENGINEERING, AND DESIGN
Implants are becoming the standard of care in dental treatment and there are many implant designs available. To provide the superior treatment outcomes expected in modern dental implantology, the ideal implant should have a unique structure and design, achieve high primary stability, and allow for easy insertion. At the same time, direction of implant placement must be easy to control and create minimal trauma to the bone structure. The implant must be designed with an understanding of the physiological and biological concepts of the healing process, which can be accomplished by reducing the stress from the cortical crestal bone and condensation of the trabecular bone. The implant must be versatile enough to accommodate various surgical indications such as one- and two-stage implant procedures, immediate implantation, and immediate loading.
Modern implants must allow the practitioner to easily achieve pleasing aesthetic results using physiology and prosthetics. Physiologically this is accomplished by reducing bone resorption, increasing volume of the soft tissue, and maintaining the ridge structural architecture. Prosthetically this is accomplished by allowing for all variants of prosthetic solutions such as cemented and screw-retained restorations in various angulated implants, CAD/CAM abutments, and bridges.
Over the last 40 years there has been steady progress toward designing the best dental implant to improve function and aesthetics. An understanding of the biology of the behavior of bone and soft tissue around natural teeth and implants is crucial to the design of a superior implant. In the past there were many types of implants with several approaches for surgical and prosthetic procedures. Most implant manufacturers have tried to imitate the shape of the natural tooth by forming conical implants that would be similar to the tooth root with a wide coronal cervical area and an emerging profile. This means that the implant and the abutment have the same diameter at the connection level.
This chapter takes an in-depth look at the NobelActive implant (Nobel Biocare AB, Göteborg, Sweden), explaining the reasoning that led to the development of the NobelActive tapered implant with a nonlinear tapered core and variable tapered thread design. Important criteria investigated in the development phase include physiological advantages, load transmission, gradual bone condensation, initial stability, and mechanical strength. The biological theory of critical bone volume, physiology, and surgical considerations is discussed. In addition, the mechanical design behind the NobelActive implant is explained. It has taken more than 20 years of experience and experimentation to accomplish this design, in which all of the implant elements function in harmony with dental biology.
The NobelActive implant design is based on the physiology of bone surrounding natural teeth and implants. A clinically successful dental implant must create and maintain integration with hard and soft tissue.1 However, some authors have alluded to the presence of remodeling after implant placement, which manifests in diminished bone dimensions, vertically and horizontally.2 Implant placement also has been unable to preserve the hard tissue dimension after tooth extraction, and the buccal and lingual walls are often resorbed.3,4 The theories surrounding critical bone volume (CBV) and well-known factors about bone physiology are the main concepts that aid in understanding the process involved in maintaining bone dimension.
The CBV theory supports the idea that bone tissue can survive and maintain its original dimensions, providing sufficient volume and/or blood supply to maintain the surrounding bone. Data support the concept of a positive relationship between the densities of blood vessels and the formation of bone.5,6 Consequently, even if the blood supply has been altered in the supporting structures of the natural teeth or dental implants the bone can survive when minimal volume remains.
However, with reduced bone volume there is a tendency toward decreased blood supply, and as a consequence, resorption takes place. Moreover, if there is any stress on the bone (in addition to the minimal bone volume) resorption will occur even more quickly. To compensate for the decrease in the blood supply an increase in the bone volume must occur to maintain the bone supporting structure.
Bone is a living, growing tissue. It is a porous, mineralized structure made up of cells, vessels, calcium salts, and crystals of calcium compounds (hydroxyapatite), the proportion of which varies according to bone types and regions. The human skeleton is made up of two types of bone: cortical bone and trabecular bone.7
Cortical bone, also called compact bone, represents nearly 80% of the skeletal mass. It is called compact bone because it forms a protective outer shell around every bone in the body. Cortical bone has a high resistance to bending and torsion and it provides strength where bending would be undesirable. It has a specialized vascular system, called the Haversian system, with an inside net of vertical and horizontal canals and blood vessels to overcome limited diffusion capability. Cortical bone has a slow turnover rate, progressing approximately 0.6 micron a day. It can be estimated that an injury to the cortical bone can take up to 5 months to repair.7
Trabecular bone comprises only 20% of the skeletal mass but 80% of the bone surface. Trabecular bone is less dense and more elastic than cortical bone, and forms the interior scaffolding that helps the bone to maintain its shape despite compressive forces. Trabecular bone is rigid but appears spongy. It is composed of bundles of short, parallel strands of bone fused together. The center of the bone contains red and yellow marrow bone cells, and other tissues. Trabecular bone has a higher turnover rate than cortical bone, and as a result has a quicker healing capacity, progressing about 20-50 microns a day.6
Cortical bone provides strength and trabecular bone provides nutrition. The combination of both types of bone builds a strong body with the ability for comfortable movement due to the fairly weightless nature of the trabecular bone.
The outer surfaces of cortical bone are covered with the periosteum membrane, which is composed of an irregular type of dense connective tissue. Periosteum is divided into an outer fibrous layer and an inner cambium layer. The fibrous layer contains fibroblasts; the cambium layer contains progenitor cells that develop into osteoblasts. These osteoblasts are responsible for increasing the width of long bones and the overall size of the other bone types. Unlike osseous tissue, periosteum has nociceptor nerve endings that make it very sensitive to manipulation. It also provides nourishment by providing the bone’s blood supply.8 Trauma to the periosteum and cortical bone must be minimized because the most superficial bone layer gets all of its blood supply from the periosteal tissue and 30% of the blood supply to the bone comes from and returns to the periosteum.
Periosteum is attached to bone by strong collagenous fibers called Sharpey’s fibers, which are the ends of principal fibers that insert into the cementum. Sharpey’s fibers intensify the continuity between the periodontal ligament fiber and the alveolar bone (tooth socket), and act as a buffer medium against stress. Sharpey’s fibers in the primary acellular cementum are mineralized fully; those in cellular cementum and bone are mineralized only partially at their periphery,9 which extends to the outer circumferential and interstitial lamellae. They also provide attachments for muscles and tendons.8
It is important to understand the vascular supply to the soft and hard tissue of the oral cavity. The blood supply to the periodontium has three main sources: the blood vessels of the alveolar bone, the blood vessels of the periodontal ligament, and the supraperiosteal blood vessels.10 The teeth are surrounded by the periodontal ligament, gingival tissue (circumferential fibers), and a lamina dura. The periodontal ligament comprises a group of specialized connective tissue fibers that essentially attach each tooth to the alveolar bone within which it sits. These fibers help the tooth withstand the naturally substantial compressive forces that occur during chewing and remain embedded in the bone11 (Figure 26-1).
Figure 26-1. A, The sulcular tissue around a tooth is similar to an implant and so is the junctional epithelial zone. The connective tissue zone, which attaches to cementum on a natural tooth, is completely different around the implant. GM, Gingival margin; PM, peri-implant soft tissue margin; BC, bone crest; aJE, apical termination of the junctional epithelium; CEJ, cemento-enamel junction; AFJ, abutment-fixture junction. B, The peri-implant tissues exhibit histological sulcular and junctional epithelial zones similar to a natural tooth. The primary difference is the lack of connective tissue attachment and the presence of primarily 2 fiber groups, rather than 11 with the natural tooth. C, Relationship of the inferior alveolar canal to the mandibular first, second, and third molar teeth. a, Trabecular bone; b, lamina dura.
The buccal bone adjacent to natural teeth is usually only 0.5-1 mm thick. This bone gets its blood supply from the periosteal layer and the periodontal ligament. The gingival side of the outer periosteum of bone anastomoses with the periodontal ligament intra-alveolar blood vessels.12 Why is the buccal wall adjacent to natural teeth so thin? It has been observed that buccal bone remains thin even after orthodontic movement, and that it follows the movement of the teeth.4 One evolutionary explanation for buccal bone thinness might be protection against dental infections. An infection usually leads to buccal bone perforation with secretion throughout the buccal side (as opposed to the lingual side), which prevents potentially life-threatening infection of other vital structures such as the floor of the mouth or the palate.
Flap elevation can lead to increased bone loss.13 In particular, trauma from drilling procedures affects the vascularity of the bone, which, in turn, leads to resorption.14 The cortical bone is especially vulnerable to resorption because the blood supply comes from the periosteal tissue and the Haversian system, which are damaged during the surgical procedure. Even more drill-related trauma may occur as the drill becomes wider and the flap becomes larger and is exposed for a longer period of time.
According to Wolff’s law, bone in a healthy person adapts to the load placed on it. If loading on a particular bone increases, the bone remodels itself over time to become strong enough to resist that sort of loading. As a result, the external cortical portion of the bone becomes thicker and trabecular bone becomes denser and oriented toward the direction of the forces exerted on it. The converse is also true; if the loading on a bone decreases the bone will become weaker due to turnover. Weaker bone is less metabolically costly to maintain and without increased loading there is no stimulus for continued remodeling required to maintain bone mass.15,16
Occlusal forces are first transmitted to the coronal part of the teeth and then are partially transmitted horizontally to the adjacent teeth through the contact points of the crowns. The rest of the forces are transmitted to the bone through the periodontal ligament. The periodontal ligament structure allows for a special tridimensional architecture of fibers that absorb some of these forces and transmit some to a dense bone layer around the roots (lamina dura), which are connected to the cortical bone. In turn, the lamina dura transmits some forces to the trabecular bone and cortical bone of the jaws. All of these mechanisms of force absorption and transmission also enable orthodontic movements, in which forces reorganize and remodel the periodontal ligament to facilitate tooth movement.17
The internal structure of the surrounding bone changes after implant placement. The bone around the implant is replaced through bone healing and remodeling.18 The dense lamina dura that was present in natural teeth disappears. As a consequence, occlusal forces are transmitted mainly to the cortical bone. This is true for most implants on the market due to their structural design, which lacks features to absorb and transmit occlusal forces and maintain the blood supply from the periodontal ligament. The reduced blood supply and concentration of forces on the crestal cortical bone can lead to resorption of this vulnerable tissue.
Buccal bone is resorbed after tooth extraction. The bone-to-implant contact established during the early phase of socket healing following implant installation is in part lost when the buccal bone wall undergoes continued resorption. After tooth extraction the buccal bone resorbs over several months.1,3,19 It is therefore recommended that the postextraction socket should be filled with a bone augmenting material.20,21,22
The goal of ridge preservation procedures is preservation of the hard and soft tissue volume and architecture and maximization of the supply of osteoprogenitor cells and their ability to invade the area occupied by the osteoconductive scaffold.21 The key factor in ridge preservation is prevention of postextraction bone resorption. There often is extensive resorption of the buccal wall and minimal resorption of the palatal/lingual wall because the palatal wall has a better blood supply from the thick palatal gingiva and presents a volume above the CBV. In most cases, the buccal wall volume decreases to below the CBV. According to the CBV theory, there is a high probability of maintaining bone dimensions without bone augmentation procedures if there is sufficient buccal wall volume of bone and good blood supply.
Between two adjacent teeth there is a thin layer of bone supporting the interdental papilla. The interdental bone and papilla get most of their blood supply from the periodontal ligament of the adjacent teeth. The papilla is configured in a tridimensional orientation of fibers to maintain its architecture. When teeth are extracted the blood supply to interdental bone and the papilla is altered, resulting in bone resorption and papillary dimension decreases.23,24 If only one of the adjacent teeth is extracted, adequate blood supply will still come from the periodontal ligament of the remaining tooth. This leads to more predictable and aesthetic results in single-tooth implants between two natural teeth. It is more difficult to create a good papilla between two adjacent implants.25
When every tooth is replaced with a standard implant with a conical design and a wide coronal neck, the crestal bone volume between two adjacent implants is very small. This inter-implant crestal bone is usually below the CBV and as a consequence it will resorb. This becomes even more evident as the implants are placed closer to each other. If the distance between two adjacent implants is less than 3 mm,26 which is below the CBV, the resorption of the interdental bone increases.
According to the CBV theory, the interdental bone can be preserved even with a 2-mm inter-implant distance between 4-mm-diameter implants in the presence of a wide alveolar ridge (>10 mm). A 2-mm inter-implant distance is possible due to the large three-dimensional volume of bone available around the implants, which improves stress distribution and provides a good blood supply.
The use of a pontic has been advocated to overcome the problem of inadequate bone volume between implants.27,28 It is possible to restore a three-unit bridge on two implants using an ovoid pontic. The papilla can be maintained underneath the pontic because the bone has an adequate blood supply provided by the adjacent periodontal ligament and periosteal tissue in three directions: buccal, lingual, and crestal. The use of an ovoid pontic instead of an implant allows more space for preserving the dental architecture and the underlying bone and soft tissues. A pseudo-papilla can be created by modeling the gingival soft tissue.
A certain amount of bone resorption occurs around implants when they are in contact with the oral cavity. The distance between an implant and the adjacent tooth, the distance between two implants, is as important to maintaining bone as the bone volume around the buccal side of the implant head and in the papillary area. This is true especially for a successful long-term result.29 It is a well-known fact that 1.5-2 mm of bone must surround the implant buccally and lingually.29,30
A thin 0.5- to 1-mm layer of bone, such as the buccal plate, can survive adjacent to a natural tooth because of the blood supply from the periodontal ligament and the periosteal tissue. However, the buccal bone is adjacent to an implant that has no periodontal ligament and no blood vessels; therefore, it is unlikely for the bone to survive around the implant. According to CBV theory, the width of buccal and lingual bone plates must be at least 1.5-2 mm to survive when the blood supply is compromised, such as when a tooth is adjacent to an implant.
In summary, the bone has more blood supply when a tooth is present or absent than when a tooth has been replaced with a dental implant. As an example, the bone around a wider diameter implant is at higher risk for crestal bone resorption because a greater amount of bone is needed to surround the implant.31 It is recommended that the width of the buccal and lingual bone plates around 5- to 6-mm diameter implants must be at least 3-4 mm to achieve the CBV.
One concept in immediate implantation which is of concern placement of the implant in the same position as the extracted natural tooth, adjacent to the thin buccal bone that will resorb after several months. As stated, the blood supply is compromised during the extraction, hence the damage to the buccal plate. Then, a titanium implant is inserted where the vascularity around it is compromised. This implant should be placed more palatally, maintaining a space of approximately 2 mm between the implant and the buccal plate, thus improving the likelihood for bone survival around the implant. Preferably, this space should be filled with augmenting material. Once more, according to CBV theory, more palatal implant will achieve higher buccal volume that will allow buccal bone to survive in this compromised vascularity.
Both bone grafting and distraction osteogenesis are predictable methods for restoring missing tissue. The main cause of bone resorption after distraction osteogenesis is the trauma of laying a flap involving the periosteum, drilling through the bone, and/or the insertion of the fixating screws. When performing a distraction osteogenesis procedure, the bone segments must be wide enough to comply with the CBV theory and maintain bone volume.32,33
Bone resorption near an implant will usually cease when the bone volume increases. Vascularity is proportional to bone volume. If the bone has minimal volume compared to the blood supply, the resorption will slow down or cease. Even a very thin bone can be maintained for long periods of time if the blood supply is adequate.
Augmentation is necessary to prevent resorption. It can occur at or after tooth extraction. There are many techniques for alveolar ridge augmentation, such as guided bone regeneration (GBR); onlay/veneer grafting (OVG); combinations of onlay, veneer, interpositional inlay grafting (COG); distraction osteogenesis (DO); ridge splitting (RS); free and vascularized autografts for discontinuity defects (DD); mandibular interpositional grafting (MI); and socket preservation (SP).34 The main idea behind augmentation is to maintain adequate volume to comply with the CVB theory and avoid bone resorption.
Immediately after insertion of an implant in a two-stage procedure, there are no significant changes in the bone structure around the implant or at the crestal level. Healing after the surgical implant placement procedure results in defect fill and clinically healthier situations.35 When the implant is submerged, it is protected from occlusal loads and bacterial infection. Also, it obtains its vascularity from the crestal periosteum in the adjacent bone.
After exposure to the oral environment, marginal bone loss of 1-2 mm occurs during the first year.36 Possible reasons for this bone loss include micromovement, microgaps, and bacteria infiltration at the implant and abutment interface. Another possible explanation is the damage to the thin crestal bone during placement of implants with wide coronal diameters. After 3-6 months the bone heals by resorption and apposition, but it is not functional bone. Changes start to occur once the implant is exposed to the oral cavity, where greater occlusal forces36,37 are introduced to the crestal bone than to the trabecular bone.
The trabecular bone does become functional and a structure comparable to the lamina dura on teeth develops around the implant. At the same time, the bone resorbs until it achieves its critical size and the blood supply is altered. The resorption will end when a balance is achieved between the functional trabecular bone lamina dura, the resorption and stress distribution of the crestal bone, and an adequate bone supply. At that point the whole implant complex is in harmony with the bone and its surrounding structures and minimal or no changes will occur, since the bone has corrected itself and adapted to the new situation after the damage occurred during the osteotomy.
The understanding of bone behavior around implants has aided the development of an implant that minimizes trauma to the cortical bone and condenses the trabecular bone. The reduction of trauma to the bone will influence the preservation of bone and soft tissue, including the papilla. It is important to consider the support system around an implant. The implant supports the abutment and the abutments support the restoration. However, this whole implant system is held in place by the most important element, which is the bone. The bone is the foundation for and the sustenance for the soft tissue. Box 26-1 lists the requirements for the NobelActive implant.