Dental Implant Placement

Background

Dental implant placement is no longer performed only by oral surgeons and periodontists; general dentists are also increasingly providing difficult surgical implant services. Dental implants may be used to replace single teeth, replace multiple teeth, or provide abutments for complete dentures or partials. This topic focuses on the placement of single-tooth dental implants.

In the 1960s, Branemark introduced the concept of osseointegration as it applies to dental implants. ^[1] Since that time, a multitude of different dental implant types have been introduced into the market and are being used in daily dental practice. Osseointegration, as applied to dental implants, refers to the postinsertion result in which medullary bone has grown up to or into the material of the implant without interposition of a connective tissue layer between the bone and the alloplastic implant material.

Osseointegration can be contrasted with fibroosseous integration, in which there is a soft-tissue interface that is viewed as the equivalent of the periodontal ligament that surrounds the tooth. Both mechanisms can facilitate retention of an implant, but osseointegration is considered more suitable for long-term implant success.

Relevant Anatomy

Awareness of the anatomic structures associated with the mandible and maxilla is essential. Avoiding life-threatening hemorrhage and nerve and sinus injury is imperative.

The mandible (see the image below) is a U-shaped bone. It is the only mobile bone of the facial skeleton, and, since it houses the lower teeth, its motion is essential for mastication. It is formed by intramembranous ossification. The mandible is composed of 2 hemimandibles joined at the midline by a vertical symphysis. The hemimandibles fuse to form a single bone by age 2 years. Each hemimandible is composed of a horizontal body with a posterior vertical extension termed the ramus.

The maxilla (see the image below) has several roles. It houses the teeth, forms the roof of the oral cavity, forms the floor of and contributes to the lateral wall and roof of the nasal cavity, houses the maxillary sinus, and contributes to the inferior rim and floor of the orbit. Two maxillary bones are joined in the midline to form the middle third of the face.

For more information about the relevant anatomy, see Facial Bone Anatomy. Also see Mouth Anatomy, Facial Nerve Anatomy, and Tooth Anatomy.

Indications

Implants can be used to replace single teeth, multiple teeth, or used to provide abutments for complete dentures or partials.

Contraindications

In addition to evaluation of the patient’s underlying health, it is important to assess for preexisting periodontal disease as peri-implantitis, a periodontal condition similar to periodontitis that is associated with gingival tissue inflammation, pocket formation, and ultimately loss of bone that can lead to implant failure. Implants that are placed adjacent to teeth with periapical lesions may also be at greater risk of failure. ^[2]

Another patient factor that has been associated with peri-implant microbiota and potential disease and implant failure is smoking. ^[3]  A meta-analysis demonstrated a significant increase in the relative risk of implant failure in patients who smoked more than 20 cigarettes per day compared with nonsmokers. ^[4]  A meta-analysis by Mustapha et al concluded that the failure rate of dental implants in smokers was 140.2% higher than the failure rate in nonsmokers. ^[5]

Other factors associated with implant failure include bruxism, ^[6]  radiation therapy, and osteoporosis. ^[7]

The presence of dental caries on adjacent teeth is not a contraindication for implant placement, but lesions should be eliminated and teeth restored prior to proceeding with surgical procedures.

Outcomes

From a biomechanical standpoint, several factors appear to be important in determining the success of implant placement as they relate to technique: stiffness of the tissue-implant interface, the quality of the supporting tissues, and implant diameter, particularly as it applies to short implants. Bone density around submerged implants that failed was found to be significantly less than in those implants that survived. Additionally, it was found that there was a tendency towards higher failure rates of machined (smooth) surface implants versus those with rough surfaces and in those placed where there was poor bone quality. ^[8]

There has been a significant lack of well-controlled prospective longitudinal studies comparing the success rates of different implant systems. Many published studies are retrospective in nature and thus must be interpreted with caution. ^[9] Population size is also problematic in many studies.

With respect to survival rate, the probability of failure for industry-associated clinical trials was significantly lower than that that reported in nonindustry research, as reported in a systematic review of 38 published clinical trials. ^[10] Based on the analysis of these studies, funding sources may have a significant effect on the reporting of implant failure. Thus, all survival data must be interpreted with caution.

In a retrospective study evaluating 206 immediate implants using a flapless technique and immediate loading, the cumulative survival rate at a mean of 23 months was 98.77%. ^[11]

Complications

Tissue Loss

Tissue loss is of particular concern in the anterior maxillary and mandibular region. The causes of tissue loss include smoking, flap design, and poor oral hygiene. Postplacement plaque control is also important. It is important to have a good peri-implant soft tissue seal and the preservation of a keratinized gingival zone to promote plaque control. Patients receiving implants should be instructed on the importance of plaque control and monitoring for implant survival.

Crestal Bone Preservation

The loss of crestal bone around an implant results in an environment conducive to anaerobic colonization, which can lead to peri-implantitis and implant failure. Early bone loss has been associated with several factors, including the effect of tissue manipulation during surgery on blood flow, osteotomy preparation, bacterial colonization, lack of biologic seal around the implant, plaque accumulation at the implant-transmucosal abutment interface, ^[12] implant design features, flap elevation, ^[13] and stress. ^[14]

It has been reported that crestal bone loss is a fairly normal consequence of implant placement when it involves two stages. It does not typically occur when the implant remains completely submerged but can occur during the exposure surgery and placement of the abutment. There is evidence that platform shifting at the time of abutment placement can act to prevent crestal bone resorption. ^[15]

When implants placed with conventional drilling techniques were compared with those placed via osteotome technique for crestal bone loss, it was found that at month 3 the osteotome group experienced significantly more crestal bone loss. However, by months 6 and 12, there were comparable bone levels. ^[16]

Treatment Failure

Implants can fail for a number of reasons, including the following:

• Lack of osseointegration

• Fracture of bone during the osteotomy

• Graft failure

• Prosthetic design

• Postimplant soft tissue defects

• Improper selection of implant type

• Patient force factors

• Miscalculation of bone density or tissue thickness

• Patient health and lifestyle factors

• Comorbid periodontal disease

• Poor drilling technique

However, research suggests that cumulative survival rates for dental implants are excellent, even if training is of a limited nature. Success rates for dental implants were reported to be greater than 90% for both maxillary and mandibular implants, with mandibular implants having a greater survival rate than maxillary implants. ^[17]

In patients with placed implants, there is evidence that the survival of the initial implant correlates with the survival of subsequent implants and vice versa.

A study investigated if the patient can have a role in reporting early peri-implant complications. This study demonstrated that by using validated questions, an educated patient can perceive peri-implant health/disease. The authors also add that this can play a role in the early diagnosis of peri-implant complications. ^{[18, 19]}

Pre-Procedure Planning

Diagnostic imaging is perhaps the most important technique for preimplant treatment planning. Imaging is also used during the surgical process to assist in defining the location of the implant after bone preparation. The objective of imaging is to identify bone quantity, quality, and density, as well as to identify potential pathology that could interfere with implant placement. It is also used to identify the optimum angulation, orientation, and position of the implant.

Imaging studies may include the following:

• Periapical radiology, which can help in defining disease but has little value otherwise

• Occlusal radiography, which distorts bone morphology

• Cephalometric radiographs, which are useful in assessing the completely edentulous patient

• Panoramic radiography, which distorts the vertical and horizontal planes, does not provide good information regarding bone density and quality, and does not allow cross-sectional reformatting

• Computed tomography, which not only allows for visualization of jaw structure in three planes but incorporates software that can provide the presurgical electronic placement of an implant and the formation of diagnostic templates that guide the surgical process—techniques that are useful for complex multi-implant treatments

Determining Bone Density

Bone density is one of the most important confounders of implant success or failure. Bone density varies considerably based on location. For example, the density of bone in the anterior mandible is greater than that of the anterior maxilla.

A number of bone density classification schemes have been published over the years. The Misch scale defines bone density based on macroscopic cortical and trabecular bone characteristics. The Houndsfield scale uses computerized tomography scans and Simplant software to define an objective bone density scale. ^[20] In addition to the these taxonomies, the Lekholm and Zarb classification systems are also used.

A number of different tests are used to access bone density values for immediate postimplant placement, including using a metallic instrument to tap the implant with assessment of sound, the insertion reverse torque test, and the periotest, which involves an instrument that uses an electrically driven and electronically monitored tapping head with a pressure sensitive tip. ^[21]

Pre-Procedure Bone Augmentation

In cases where there has been extensive alveolar bone loss following extraction, it may be necessary to provide bone augmentation prior to implant placement. Bone grafts are classified on the basis of how they produce new bone: osteoconduction, osteoinduction, and/or osteogenesis.

Osteoconductive materials are classified as alloplasts or zenografts. These are synthetic biocompatible materials that allow bone growth via apposition from the existing bone. Bioactive ceramics such as synthetic hydroxyapatites allow for the formation of bone around the graph. Xenographs, which are deproteinated inorganic nonhuman bone, can also be used to increase bone thickness in advance of implant placement. It should be appreciated that there may be resorption of these materials over a prolonged period of time.

Osteoinductive materials are divided into allografts and autografts. The former is obtained from processed cadaver bone and comes in three forms: frozen, freeze-dried, and demineralized freeze-dried material. The literature suggests that there is wide variability in results in the use of this material.

Autogenous bone taken from the iliac crest when grafted will produce an osteogenenic response. Successful bone grafting depends on good soft-tissue closure, choice of appropriate grafting material for the defect, nutrient blood vessels, growth factors, systemic conditions, and surgical technique. Healing time of 4-6 months in advance of implant placement is recommended, but this may be highly variable depending on the type of graft material used. ^[22]

For maxillary implants, a one-step procedure that provides for a lifting of the sinus floor can be pursued at the time of implant placement. Problems that have been associated with sinus grafting include implant migration. ^[23]

Choosing a Dental Implant

Choosing an implant design and system may be the most difficult problem the dentist will face. The number of companies is producing implant systems is quite large and growing yearly. The number of implant types is even greater with new designs, surface coatings, and implant composition ever enlarging at a rapid pace.

The number of actual implant types available in the marketplace is staggering. Over 50 different body designs are sold in the United States alone in three basic categories: the cylinder, the screw, or the combination root form. Body designs can also be perforated, solid, hollow, or vented. The external surface can be smooth and noncoated, coated, or textured.

As a general rule, the choice of a dental implant should be based on these factors: where it is to be placed, relative bone density, how it is going to be used (eg, freestanding abutment, overdenture abutment), aesthetics, and stress determinants.

In choosing an implant system, the problem of selection becomes more complicated and other factors become equally important. These factors include ease of use (eg, using color-coded drills or stopper boring drills and fixture platforms, having the implant with abutment together in one package), the level of basic research (including human and animal studies) that provide support for the implant design, company support and suspected longevity of the company providing the implant system, and the accompanying restorative options provided with the implant.

Other practical factors that need to be considered include the number of small/large and short/tall fixtures provided with the system, the threading systems (tall-big threads and deep grooves), the presence of microthreads in the crestal zone of the available fixtures, surface characteristics of the implant, adequate prosthetic components, and allowance for platform switching (ie, being able to use a smaller platform diameter with the implant than that provided by the fixture itself). The reader should keep in mind, however, that manufacturer warranties related to implant failure may only apply if the company’s parts are used exclusively with implant restoration.

As noted previously, bone density is a critical determinant of implant success or failure. Soft bone requires an implant with greater surface area to affect bone-implant contact than denser bone. To accommodate this, most manufacturers have developed implant designs that are engineered for the variations in bone density that might be encountered.

Research suggests that osseointegration is facilitated by the shape and surface characteristics of the implant. The shape impacts micromotion of the implant, which can have a deleterious effect on bone growth, primarily because micromotion allows fibrous growth at the implant/bone interface. Research suggests that large-diameter and standard-length implants appear to be better at reducing micromotion effects than short or narrow implants.

In addition to the implant material itself, ^{[24, 25, 26]} implant superstructure design and material can also interact with the implant structure to effect load transmission and potential force to impact long term stability. ^[27] Of all of the different implant materials, titanium or titanium alloy has been most studied. The latter include a mixture of aluminum and vanadium alloy and is available in four grades.

Zirconium implants are relatively new. Because they are white, they offer improved aesthetics, particularly because their use eliminates the blue-grey halo under the cervical gingival tissue that can be associated with titanium implants. Zirconium implants may have similar osseointegration characteristics as the titanium implants, but there may also be an increased risk of fracture. Because they are new, the long-term efficacy of zirconium implants has not been fully established. ^[28]

The results of a systematic review by Fernandes et al suggest that zirconia implants may have a lower survival rate over time than other types of dental implants. The survival rates ranged from 92.6% to 100% for titanium implants, 95.8% to 100% for titanium-zirconium implants, and 87.5% to 91.25% for zirconia implants. ^[29]

Approach Considerations

In general, surgical approaches are determined by region, bone density, and bone volume. Current techniques include flapped or flapless (immediate implant) procedures. Single or multiple implants can be placed in edentulous bone, grafted bone, or fresh extraction sockets. However, a Cochrane review on the timing of implant placement after tooth extraction noted that there is presently insufficient evidence to define the disadvantages or disadvantages of immediate, immediate-delayed, and delayed implants. None of these techniques appear superior to the others in terms of implant success or failure.

First and foremost, the dentist providing implant services needs to be aware of the anatomic structures associated with the mandible and maxilla. Avoiding life-threatening hemorrhage and nerve and sinus injury is imperative.

In general, certain criteria are important to adhere to in preparing the bone for implant placement. Thermal and mechanical trauma of the bone must be avoided. Thus, during drilling of the surgical site, the area needs to be profused with cool irrigation fluid. Bone trauma from heating is reduced by using slow rotational speed of the drill and increasing drill size during the procedure. Dense bone requires a greater number of drills to prepare the implant osteotomy. Bone death can also be reduced by the application of intermittent pressure during drilling.

With regards to irrigation, there is controversy regarding internally cooled versus externally cooled drills. Externally cooled drills are recommended with the Nobel Biocare implant system and the BioHorizons implant system. Drills should be changed once they are damaged or dulled. Some authors have recommended a bone tap be used before actual implant placement to prevent damage to the implant and for removal of drill remnants in the site.

Once the osteotomy is prepared, the implant can be inserted with a hand ratchet or handpiece with slow rotation. It should not be placed to the full extent of the surgical preparation, as doing so can result in microfractures of the bone. After the implant is in its final position, it should be rotated a half-turn, particularly in dense bone to reduce bone stress.

The clinician placing the implant also needs to be concerned with crestal ridge height. The smooth portion of the terminal aspect of the implant may be placed above the ridge or it can be countersunk to permit the placement of the implant superstructure.

Host Site

Implants can be used to replace single teeth, multiple teeth, or used to provide abutments for complete dentures or partials. For adjacent implants, the standard is to allow at least 2 mm between implants. Historically, implants were placed and allowed to stabilize for up to 9 months prior to loading. More recent techniques for immediate placement and loading have been developed and have been found to be equally as efficacious as those placed using the more standard technique. ^[30]

In areas where there has been bone atrophy, the alveolar ridge is typically modified via bone augmentation prior to implant placement. In the maxilla where there is minimal bone between the alveolar ridge crest and the maxillary sinus, implant techniques can involve sinus elevation and the application of grafting material at the same time the implant is placed.

Tooth extraction leads to loss of alveolar bone and residual ridge changes that have been identified and characterized via several classification systems. The available bone must be evaluated carefully in implant planning and correlated with implant height, length, and cross-section. This is particularly important with respect to crestal bone.

These factors must also be correlated with bone density as well. For example, highly dense bone can accommodate a shorter implant, whereas less dense bone requires a longer implant. There is also variability with respect to mandibular and maxillary implant planning, particularly as it relates to the maxillary sinus and bone angulation of the maxillary anterior ridge. This topic is beyond the scope of this topic but is reviewed thoroughly in the major texts on implants.

Presurgical Oral Disinfection

Historically, implants were placed in a hospital environment under strict sterile guidelines. At present, less stringent protocols are used in the presterilization of the oral cavity prior to implant surgery. Currently, most texts recommend intraoral and extraoral scrubbing with iodine povidone-iodine or 0.12% chlorhexidine gluconate. Some authors advocate a one-stage process to disinfect the oral environment, which includes presurgical periodontal scaling and tooth prophylaxis, as well as use of antibacterial rinses such as chlorhexidine. ^[31]

Although there is a great deal of literature addressing general sterilization techniques and sterilization of the implant material itself, there are no published standards for oral disinfection before or after implant insertion. There is literature to suggest that pretreatment antibiotic coverage may improve implant complications. ^{[32, 33]} Specifically, the evidence from two randomized controlled trials that met inclusion criteria in a recent meta-analysis suggests that providing 2 g of amoxicillin taken orally 1 hour preoperatively may significantly reduce failures of dental implants that are placed in nonextraordinary conditions. However, it remains unclear whether postoperative coverage provides an additional effect.

Mandibular Surgery

Basically there are two mandibular procedures. The two-step procedure begins with surgical flapping of the attached gingival tissue. Subsequently, sequenced drills are used, with larger diameters each time they are used. The final drill is designated for the implant type based on bone type.

The implant is inserted with a low-torque handpiece or wrench. The implant screw is placed, with resuturing of the flap for complete tissue coverage. The covered implant left in place for 3-9 months.

The one-step procedure involves the same steps but occurs without the flapping of the gingival or covering of the implant. The one-step procedure can involve the use of computer-designed selective laser sintering surgical guides. The idea behind the surgical guide is that it may allow the most ideal angulation of the implant via precise drilling angles determined by computed tomography virtual assessment.

However, use of these guides may result in apical deviation and complications such as lingual soft-tissue tearing during drilling, insertion of a wider implant than planned, implant instability, prolonged pain, midline deviation of the prosthesis, and prosthesis fracture. ^[34]

Maxillary Surgery

As a general rule, maxillary bone is porous and of low density. This can make the surgical procedure more difficult because bone softness can lead to lateral perforations and a stripping of the facial plate during the osteotomy. Placement of the implant itself can also be problematic. Further, the presence of the nasal floor, the angulation of the anterior maxilla, the absence of crestal bone, and the maxillary sinus can present additional challenges to surgical preparation. Soft-tissue aesthetic considerations are also important, particularly in the anterior region. The hydroxyapatite-coated and treaded implant has been recommended for placement in the maxillary ridge.

Placement of an implant in the region of the maxillary sinus offers special challenges. There is often inadequate bone height and volume as the floor of the sinus is typically associated with the roots of the maxillary teeth; with their loss, it expands to sometimes join the residual alveolar ridge. To gain proper supporting bone mass, there is a need for bone augmentation.

To accomplish the bone augmentation, there has to be a lifting of the floor of the maxillary sinus without perforation of the sinus membrane. In this case, graft material is also placed into the osteotomy to promote bone growth at the apex of the implant. Another technique uses the osteotome for fracturing the sinus floor with manipulation of the segment but does not include additional graft material.

The surgical technique for the placement of a posterior implant in the maxillary sinus area involves surgical flapping of the attached gingival tissue. Initial drilling is performed with sequential drills to within 2 mm of the sinus floor, using an osteotome to sequentially widen the osteotomy. The largest osteotome is pushed 2 mm further to elevate the sinus membrane.

Graft material is applied if there is elevation of 3 mm or greater of the sinus floor. The graft helps to tent up the membrane and stabilize the implant apex. An alternative procedure involves lifting the sinus by a balloon sinus lift, followed by graft and implant placement.