Available Bone

Available bone describes the amount of bone in the edentulous extraction site considered for implantation. It is measured in width, height, length, angulation, and crown height space (Fig. 14.4). As a general guideline, 1.5 to 2 mm of surgical error is maintained between the implant and any adjacent landmark. This is especially critical when the opposing landmark is the mandibular inferior alveolar nerve. However, the implant may be placed without complication through the cortical plate of the maxillary sinus or inferior border of the mandible. The implant may also be positioned closer to the cribriform plate of a natural tooth.⁶ When placing an implant in an immediate-extraction site, the surgeon needs to consider the socket dimension and the defect between the labial plate of bone and the implant. The faciopalatal dimension of an anterior tooth, for example, is often greater than its mesiodistal dimension. When an anterior tooth requires extraction, during the extraction process the thin facial cortex often becomes compromised or lost. As a result, it is most always several millimeters apical to the palatal cortical plate, and frequently bone grafting and/or membrane placement in conjunction with the implant insertion are needed. Immediate implant placement in the anterior region using a round implant often requires that the osteotomy and implant insertion engage the lingual wall of the alveolus and penetrate halfway to two thirds of the way down the extraction site into the remaining lingual apical bone for rigid fixation (Fig. 14.5). This surgical approach is more challenging than preparing the osteotomy in a homogenous bone density. The best implant size is often 4 to 5 mm in diameter for a central incisor because the associated extraction socket is often greater than 6 mm (especially in the faciopalatal dimension), so a surgical defect as large as 2 mm remains around the implant. More or less broad, oval, or kidney-shaped spaces have been described to run coronoapically along the entire surface of the socket next to the implant.⁷ The natural resorption of the facial plate may not be halted by implant insertion, and bone implant contact may be reduced when the facial plate resorbs.⁸

Available Bone Height

The height of available bone is measured from the crest of the edentulous ridge to the opposing landmark. The anterior regions are limited by the maxillary nares or the inferior border of the mandible. The anterior regions of the jaws have the greatest height because the maxillary sinus and inferior alveolar nerve limit this dimension in the posterior regions. The maxillary canine eminence region often offers the greatest height of available bone in the maxillary anterior.⁹ In the posterior jaw region, there is usually greater bone height in the maxillary first premolar than in the second premolar, which has greater height than the molar sites because of the concave morphology of the maxillary sinus floor. Likewise, the mandibular first premolar region is usually anterior to the mental foramen and provides the most vertical column of bone in the posterior mandible. However, on occasion, this premolar site may present a reduced height compared with the anterior region because of the presence of an anterior loop of the mandibular canal. The nerve courses anteriorly below the foramen and proceeds superiorly, then distally, before its exit through the mental foramen. Posterior nerve anatomy has particular significance with regard to immediate implant placement. Primary stability for immediately placed implants is frequently achieved using bone apical to the extraction site. In the posterior mandible, the course of the inferior alveolar nerve can vary from Type 1 to Type 3 with associated available apical bone ranging from nonexistent to sufficient and surgical risk varying accordingly. In addition, variants of the mental foramen exist that can increase the possibility of injury to the inferior alveolar nerve during immediate implant placement in the region (Fig. 14.6). The available bone height in an edentulous site is the most important dimension for implant consideration because it affects both implant length and crown height. Crown height affects force factors and esthetics. In addition, vertical bone augmentation, if needed, is less predictable then width augmentation.

Bone Angulation

Bone angulation is an additional determinant for available bone. The initial alveolar bone angulation represents the natural tooth root trajectory in relation to the occlusal plane. Ideally, it is perpendicular to the plane of occlusion, which is aligned with the forces of occlusion and is parallel to the long axis of the prosthodontic restoration. The incisal and occlusal surfaces of the teeth follow the curve of Wilson and curve of Spee. As such, the roots of the maxillary teeth are angled toward a common point approximately 4 inches away. The mandibular roots flare, so the anatomic crowns are more lingually inclined in the posterior regions and labially inclined in the anterior area compared with the underlying roots. The mandibular first premolar cusp tip is usually vertical to its root apex. The maxillary anterior teeth are the only segment in either arch that does not receive a long axis load to the tooth roots but instead are usually loaded at a 12-degree angle. As such, their root diameter is greater than the mandibular anterior teeth. In all other regions the teeth are loaded perpendicular to the curves of Wilson or Spee. The anterior sextants may have labial undercuts that often mandate greater angulation of the implants or concurrent grafting of the site after insertion. The narrower width ridge often requires a root form implant design that is likewise narrower. Compared with larger diameters, smaller-diameter designs cause greater crestal stress and may not offer the same range of custom abutments. In addition, the narrower width of bone does not permit as much latitude in placement regarding angulation within the bone. This limits the acceptable angulation of bone in the narrow ridge to 20 degrees from the axis of the adjacent clinical crowns ora line perpendicular to the occlusal plane.⁶ The angulation of available bone in the maxillary first premolar region may place the adjacent cuspid at risk during implant placement (Fig. 14.7).

Type of Prosthesis

The clinician must always be aware of the anticipated final prosthesis and its associated dimensions of crown-height space, whether for a single tooth crown or full arch prosthesis. In cases where tooth extraction will result in an edentulous arch, the frequent need for alveoloplasty may result in almost complete elimination of the residual socket, thus making it comparable to an implant placement protocol for a healed site.

When considering immediate implant placement for partially edentulous patients, the dimension of the tooth space(s) being replaced, in the context of the desired final tooth positioning within and between the arches, may require orthodontic evaluation and treatment. Typical examples would be extruded and tipped teeth.

Bone Density

Bone quality or density refers to the internal structure of bone and reflects a number of its biomechanical properties, such as strength and modulus of elasticity. The density of available bone in a potential implant site is a determining factor in treatment planning, implant design, surgical approach, healing time, and initial progressive bone loading during prosthetic reconstruction. The quality of bone is often dependent upon the arch position. The most dense bone is usually observed in the anterior mandible, with less dense bone in the anterior maxilla and posterior mandible, and the least dense bone typically found in the posterior maxilla. In addition to arch location, several independent groups have reported different failure rates related to the quality of the bone. Johns et al reported 3% failure of implants in moderate bone densities, but a 28% implant failure in the poorest bone type.¹¹ Smedberg et al reported a 36% failure rate in the poorest bone density.¹² The reduced implant survival most often is more related to bone density than arch location. In a 15-year follow-up study, Snauwaert et al reported early annual and late failures were more frequently found in the maxilla.¹³ Hermann et al¹⁴ found implant failures were strongly correlated to patient factors, including bone quality, especially when coupled with poor bone volume. Bone quality is directly related to the ability to achieve an acceptable level of primary fixation for immediate implant placement as well as long-term success for all placement protocols.

Anatomic Location

For immediate implant placement, an awareness of the bone characteristics of the proposed anatomic location will help dictate the appropriate treatment plan modifications for short- and long-term success. Regional variations in both available bone and bone density have already been described. The initial treatment plan prior to surgery suggests the anterior maxilla be treated as D3 bone, the posterior maxilla as D4 bone, the anterior mandible as D2 bone, and the posterior mandible as D3 bone. Bone remodeling, including loss of bone density, is primarily related to the length of time the region has been edentulous and therefore not loaded, the initial density of the bone, and mandibular flexure and torsion. Immediate implant placement can take advantage of the fact that implant placement can be performed before the bone density in the jaws begins its usual decline after tooth loss.

Presence of Bacteria/Existing Pathology

Immediate implant placement is generally recognized as a more complex procedure in contrast to implant placement in a healed ridge of adequate bone quality. The presence of infection adds an additional variable to this complexity. A site can be classified as having either periapical, endodontic, perioendodontic, or periodontal infection. Multiple studies have found the survival rates for implants immediately placed in infected sockets similar to those placed in noninfected sockets or healed ridges.^15–17 These reviews, however, should be interpreted taking into account the classification of infection was often vague and varied among the studies.

Biomechanical Overload Issues

Immediate occlusal loading on temporary crowns positioned immediately on implants placed in fresh extraction sockets have been shown to reduce treatment time. A number of different immediate load protocols have been described in the literature (Schnitman, Tarnow, Misch).^18–20 An acknowledged common concern, however, is the risk of occlusal overload. Often the risks of this procedure are perceived to be highest during the first week after the implant insertion surgery. In reality, the bone interface is stronger on the day of implant placement than it is 3 months later.²¹ As a result of the surgical placement, organized, mineralized lamellar bone in the preparation site becomes unorganized, less mineralized, woven bone of repair next to the implant.²² The implant-bone interface is weakest and most at risk of overload at 3 to 6 weeks after surgical insertion because the surgical trauma causes bone remodeling at the interface that is least mineralized and unorganized during this time frame. A clinical report by Buchs et al²³ found immediately loaded implant failure occurred primarily between 3 and 5 weeks after implant insertion from mobility without infection. In addition to procedures and techniques needed to manage the immediate implant placement, the desire to add immediate load requires the clinician to address and control the issues of occlusal overload and bone remodeling.

Learning Curve

Clinical experience has been shown to affect the outcomes of dental implant treatment. Lambert reported surgical experience may influence the success or failure of dental implants from initial placement to second-stage surgery.²⁴ Preiskel et al concluded that a 2-year differential of surgical experience can have a major impact on the failure probability of unloaded implants.²⁵ Geckili noted a decrease in failure rate from 4.6% to 1.6% owing to the presumptive improvement in the skill of the surgeon over a 5-year period.²⁶ The added variable of perioperative bone grafting, frequently performed during immediate implant placement, was thought to negatively affect implant success rates in a 6-year study by Smith. That study reported an overall 1-year survival of 94% and a 5-year survival of 92.8%. In addition, there were a relatively high proportion of technical failures due to errors in treatment planning or surgical technique, which accounted for the intraoperative failures (33%) and late failures (14%) that were attributed to the experience levels of the surgical trainees.²⁷

Prosthesis type, bone density, and anticipated load factors are treatment plan modifiers affecting implant size, design, number, surface condition, and the need/method of progressive loading.

The implant dimension of width is often limited by adjacent teeth or implants. As a general rule the implant should be at least 1.5 mm from an adjacent tooth and 3 mm from an adjacent implant. This dimension not only allows for surgical error but also compensates for the width of an implant or tooth crestal defect, which is usually less than 1.4 mm. As a result, if bone loss occurs at the crest module of an implant or from periodontal disease with a tooth, the vertical bone defect will not spread horizontally and cause bone loss on the adjacent structure.²⁸ The ideal implant width for single-tooth replacement or multiple adjacent implants is often related to the natural tooth that is being replaced in the site. The tooth has its greatest width at the interproximal contacts, is narrower at the cement-enamel junction (CEJ), and is even narrower at the initial crestal bone contact, which is 2 mm below the CEJ. The ideal implant diameter corresponds to the width of the natural tooth 2 mm below the CEJ, if it also is 1.5 mm from the adjacent tooth. In this way the implant crown emergence through the soft tissue may be similar to that of a natural tooth (Table 14.2).⁶ The ability to select the most ideal implant size based on these parameters is much less difficult in a healed site as compared to an extraction site. An ideal treatment plan would include implant length of 12 mm or greater with a 4-mm diameter for most anterior implant sites and 5 mm or greater in the molar regions.²⁹ When the ideal implant size cannot be inserted because of inadequate bone, an alternative to bone augmentation may be to increase the surface area of the implant by modifying the implant body design.⁶

Implant Design

The most predictable aspect of implant dentistry appears to be the surgical success rate from implant insertion to uncovery; it is usually higher than 98% regardless of implant design or size.^30–36 Implant design considerations, however, should be made with intent of successful long-term outcomes, not just short-term surgical success; designing an implant for surgical ease does not appear to be the most important aspect of the long-term overall implant prosthodontic-related process to reduce the incidence of complications. Given the desire for long-term implant survival and health, careful selection of implant design should include the features of titanium alloy material, roughened surface treatment, tapered crest module, and square thread design.^37,38

Many biocompatible materials are unable to withstand the type and magnitude of loads that may be imposed on dental implants. Titanium and titanium alloys have a long history of successful use in dental and orthopedic applications. The excellent biocompatibility of titanium and its alloy has been well documented. Titanium-aluminum-vanadium alloy (Ti-6Al-4V) has been shown to exhibit the most attractive combination of mechanical and physical properties, corrosion resistance, and general biocompatibility of all metallic biomaterials.^39,40 The primary advantage of titanium alloy as compared with other grades of titanium is its strength. Ultimate strength and fatigue strength are primary considerations given the ramifications of the loading profiles to which dental implant bodies are subjected and that can still place an alloy at fracture risk (Fig. 14.8).

Crest module design has a significant influence in regard to overall implant body design. There are at least six causes of marginal bone loss at the crestal bone region of implants, including the formation of a “biologic width” and occlusal overload after the implant is in function.⁶ Ultimate strength and fatigue strength are primary considerations given the ramifications of the loading profiles to which dental implant bodies are subjected and that still can place an implant of ideal dimensions at fracture risk. The crest module of an implant should be slightly larger than the outer thread diameter of the implant body (Fig. 14.9). In this way the crest module may completely seal the osteotomy, providing a barrier and deterrent for the ingress of bacteria or fibrous tissue during initial healing after insertion in a healed site. The contact created by the larger crest module may also provide for greater initial stability of the implant following placement, especially in softer unprepared bone, because it compresses the crestal bone region. In the case of immediate implant placement, the crestal bone contact, if any, will usually be on the lingual or palatal aspect and can contribute to initial stabilization. The next consideration of the crest module is related to occlusal loading. Most of the occlusal stress occurs at the crestal region of an implant design (Fig. 14.10). A smooth, parallel-sided crest module will increase the risk of bone loss after loading, whereas any crest module design that incorporates an angled geometry or grooves to the crest module, coupled with a surface texture that increases bone contact, will impose a beneficial compressive component to the contiguous bone and decrease the risk of bone loss (Fig. 14.11).

The surface condition of the implant is particularly important for healing of immediately placed implants. The bone-implant contact (BIC) of roughened implants has been shown to be increased during initial healing as compared to smooth metal.⁴¹ Lastly, although selection of thread shape should be optimized for long-term load function, thread shape may have an influence on the initial healing phase of osseointegration. An animal study by Steigenga et al compared three thread shapes with identical implant width, length, thread number, thread depth, and surface condition. The V-shaped and reverse buttress thread shapes had similar BIC percent and similar reverse torque values to remove the implant after initial healing. The square thread design (Fig. 14.12) had a higher BIC percent and a greater reverse torque test value.³⁸

Potential Complications Related to Immediate Placement Protocol

In cases where the facial bone is missing, the bone regenerated over the facial aspect of the implant with guided bone regeneration (GBR) is often immature woven bone, which is more prone to resorption because of occlusal overload. To improve GBR success, techniques for immediate implant placement after extraction typically include countersinking the implant 2 mm or more below the facial plate (which is already more apical than the palatal plate) and placing a biomaterial such as deproteinized bovine bone, calcium phosphate (CaPO₄), resorbable hydroxyapatite (HA), allograft, and/or autologous bone to fill the labial defect, with or without the addition of connective tissue grafts and/or membranes. Many classifications and protocols have been published with regard to immediate implant placement. The implant will obtain rigid fixation with nearly all of these techniques. However, the goal of implant therapy is not limited solely to rigid fixation. The inability to achieve proper esthetic and health parameters constitutes a compromised result and increased risk of esthetic or implant failure.⁴² When the implant is countersunk below the facial bone, the implant platform may be as much as 4 mm apical to the CEJ of the adjacent teeth, which increases the anatomic crown height and the pocket depth, especially after crestal bone loss during the first year. In addition, synthetic grafts, if used, placed around the titanium implant grow less dense quality bone that is also limited in implant contact. The capacity of this less dense bone promoted by barrier membranes around implants to withstand loading seems to be limited, and animal studies indicate as much as 85% may be lost after loading.⁴³ An explanation may be that no blood vessels arise from the implant; to the contrary, it reduces the number of bony walls of the defect and limits blood supply to the facial bone graft. As a result, bone is less likely to form, and when it forms it is less dense and more at risk of resorption once the implant is loaded. Although primary closure of the soft tissue provides a more predictable result when bone grafting is performed, it may be more difficult with an immediate extraction technique. Although not advocated, the labial tissue is often reflected to approximate the tissue over the socket defect. This technique further compromises the blood supply to the labial cortical bone and also decreases the amount of facial keratinized gingiva because the facial tissues are placed over the extraction socket. As a consequence, some type of mucogingival corrective surgery may be indicated after stage I healing to restore the facial attached and keratinized tissue. The labial bone usually remodels to 0.5 mm below the abutment-implant connection (which was, in most cases, already countersunk below the facial bone and several millimeters below the palatal bone). Bone loss may continue in the region to the first thread (as a result of the implant crest module design), then stabilize in a region of greater bone density. As a consequence, reports often illustrate soft tissue pocket depths greater than 7 to 8 mm at the midfacial tooth position. The presence of anaerobic microorganisms in soft tissue pockets of 5 mm or more has been documented. With good hygiene the soft tissue often recedes, with a resulting lengthened clinical crown and “black triangles” in the interproximal areas caused by the absence of properly developed interdental papillae, which compromises long-term esthetics and/or contributes to soft tissue complications. When judicious case selection has not been exercised and thorough debridement has not been performed, an increased risk of postoperative infection exists around the implant with an immediate insertion owing to the presence of bacteria that were part of the cause of tooth loss. The presence of exudate lowers the pH, which causes a solution-mediated resorption of the grafted bone and contaminates the implant body with a bacterial smear layer, which in turn reduces bone contact. An improved bone interface may be obtained if the large-diameter extraction site is grafted before implant placement. If the labial plate is compromised, additional intraorally harvested bone and/or GBR are indicated. The delayed implant insertion method appears to enhance capillary propagation and trabecular formation before implant placement, facilitating the formation of an implant-bone interface.⁴⁴ A staged protocol allows the soft tissue to granulate over the augmented extraction site, creating an increased zone of attached gingiva. The result of the augmentation can be evaluated before implant placement, rather than dealing with compromises after implant integration. In this way the implant may be placed in an ideal position in relation to the crestal bone and the adjacent teeth and within the exact contours of the final restoration.⁶

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