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August 9, 2008

By Sandy Chang, DMD, and Natalya Bejar, DDS

Advances in implant dentistry have led to widespread interest in the subject by dentists and patients. An implant is basically a titanium alloy cylinder screwed into bone, but the planning of that placement is an involved process. Many factors must be analyzed before placement and restoration can occur. A thorough medical history must be assessed to rule out contraindications, which may include:

• Uncontrolled diabetes
• History of osteomyelitis at site
• Hepatitis
• HIV
• Pregnancy
• Severe alcoholism
• Terminal illness
• History of IV bisphosphonate use
• Malignancies with ongoing chemotherapy
• Head and neck radiation treatment
• Chemical dependency
• Recent heart transplant or artificial heart valve
• Smoking
• Limited opening
• Psychiatric disorders

A comprehensive preoperative evaluation is important. It is also important to have a thorough implant consent form and verbally discuss adverse outcomes and limitations of implant treatment. The preoperative evaluation should include:

• Evaluation of the factors that limit conventional prosthodontic treatment
• Medical history review
• Dental history/examination
• Esthetic assessment
• Behavioral assessment
• Basic radiography (panoramic and/or periapical radiographs)
• Mounted casts and diagnostic tooth arrangement
• Tentative treatment plan
• Radiographic template (when needed)
• Advanced radiography such as a cone-beam CT
• Definitive treatment plan
• Patient presentation, acceptance, informed consent

How do I plan the placement and restoration of a single implant?

There are many factors to take into consideration when choosing a good candidate for replacement of a single tooth with a dental implant restoration. For your first implant case, you should select a patient who needs a premolar or molar restoration. Soft tissue complications in the anterior region may result in an esthetic failure. After you have selected a potential patient, the first question to ask is:

Does the patient have enough bone to place an implant?

It is important to begin your selection process with periapical and panoramic radiographs of the edentulous space in question. Measure the distance vertically from the crest of the bone to the inferior alveolar nerve in the mandible and the maxillary sinus in the maxilla. It is important to have at least 11 mm of bone height to accommodate at least a 10 mm long implant. Longer implants have a better prognosis than shorter implants. Next, measure the buccolingual width of the bone intraorally. It is important to have at least 7 mm of bone to accommodate a 4 mm diameter implant. This measurement allows for at least 1.5 mm of bone on either side of the implant. Remember that soft tissue will account for a few millimeters width of your measurement. Consider having a cone beam CT scan of the edentulous area made if the bone quality or quantity is in question. Bone sounding is an alternate method for determining bone quantity. Although bone sounding is considered accurate, it is more time-consuming yet less costly for the patient. Evaluate radiographs and CTs for anatomic landmarks (inferior alveolar nerve or maxillary sinus) that need to be avoided during the implant surgery. Be aware of adjacent tooth root proximity to the edentulous space. The next question to ask yourself is:

Does the patient have adequate restorative space both mesiodistally and incisal-gingivally?

It is imperative that adequate restorative space is established before implant placement. See Figure 1 and Figure 2.

Figure 1

Figure 2

At least 7 mm of mesiodistal restorative space is needed between the two adjacent remaining teeth for replacement of a premolar and 10 mm for a molar. The restoration type depends on the amount of vertical restorative space present, which is measured from the crest of the bone (or implant platform) to the opposing tooth's occlusal surface. If less than 7 mm of restorative space is present, a screw-retained restoration may be indicated. Restorative space greater than 7 mm gives the practitioner the opportunity to choose between a screw-retained or a cement-retained restoration. See Figure 3 for an example of a cement-retained implant crown and Figure 4 for an example of a screw-retained crown. Remember to ensure that the implant is placed equidistant between the adjacent teeth and it is parallel to the long axis of the adjacent teeth. Be aware of supraerupted teeth that may impose on restorative space, and mesial or distal drifting of adjacent teeth.

Figure 3

Figure 4

It is also important to decide which type of impression will be made — a closed-tray or open-tray technique. If the patient has limited opening, it is necessary to use a closed-tray impression coping. The closed-tray technique utilizes an impression coping which stays in the mouth after the impression and tray are removed from the mouth. In the closed-tray impression technique, the impression coping is unscrewed from the implant and placed into the impression for master cast fabrication. The open-tray technique utilizes an impression coping which is screwed into the implant and retained within the impression material. The impression coping is screwed into place, a hole is made in the impression tray over the impression coping, the impression is made, and the impression material is wiped away over the impression pin to create access to the pin for removal. The pin is then unscrewed and the impression can then be removed from the mouth.

Mandibular overdenture

A patient who currently wears a conventional mandibular complete
denture is the ideal patient to benefit from implant therapy. Conventional mandibular complete dentures often lack retention and stability, which results in patient dissatisfaction with the function and comfort of the dental prosthesis. Placing two implants in the anterior mandible with subsequent attachments between the mental foramina will stabilize the prosthesis significantly. Figure 5 shows the Zest Locator attachment housings in the intaglio surface of an implant overdenture. Dental implants placed in the anterior mandible have the highest success rates of osseointegration. To ensure a successful prosthesis, evaluate the available bone and restorative space for implant placement.

Figure 5

Should you fabricate the mandibular denture first or place the implants first?

You need to make the mandibular denture first, and pick up the attachments in the denture after the implants are placed. This is because where you place the implants depends on the contours of the denture. Fabricating the denture serves as a diagnostic tool to plan for subsequent procedures necessary for implant placement. After fabricating the patient's new mandibular denture, consider the cross-section contours of the mandibular denture. The strength of the prosthesis depends on having ample thickness of acrylic resin to house the attachments necessary for the implant. For your first implant patient, you want to select the patient with at least 9 mm of vertical cross-sectional thickness of acrylic resin of the mandibular denture. This allows for 4 mm for the attachment housing and 5 mm of acrylic for strength of the prosthesis. A patient with a mandibular denture that is thin in the anterior region will need alveoloplasty to reduce the height of the bone in order to gain restorative space prior to implant placement. Failure to reduce the height of bone prior to placement of the implants will result in risk of fracture of the prosthesis. After the mandibular denture is fabricated, the next step is to determine if there is available bone for the placement of implants.

Where should the implants be placed?

The ideal location for placement of implants for a mandibular overdenture is in the anterior mandible between the mental foramina. The anterior location avoids the potential complication of injury to the inferior alveolar nerve. See Figure 6 for an example of implants placed interforaminally with Zest Locator abutments for a mandibular implant overdenture.

Figure 6

Is there enough width and height of bone available for implant placement?

The panoramic radiograph is a sufficient diagnostic tool for evaluating vertical bone height after considering the potential 25% magnification of panoramic radiographs. Obtaining a CT scan is recommended for evaluating bone width available for implant placement. Duplicate the newly fabricated mandibular denture with radiopaque teeth to serve as the radiographic template. Have the patient wear the radiographic template during the CT scan to evaluate thoroughly the relationship of the anterior denture teeth to the available bone. The minimum desirable width and length of bone for a standard 4 mm wide x 11 mm implant is 6 mm bone width and 13 mm bone height.

How many implants should be placed?

As a general rule, the greater the number of implants, the more retentive the prosthesis. However, there is a trade-off to this advantage. When an overdenture is retained by one or two implants, the prosthesis is soft-tissue supported and implant-retained. The prosthesis can flex under masticatory loads. With each additional implant incorporated into the prosthesis, the prosthesis is gradually converted into an implant-supported and implant-retained prosthesis. This transition now requires the fabrication of a rigid framework to reduce risk of fracture of the prosthesis from masticatory forces. Thus, as your first implant case, you want to only place two implants between the mental foramina.

After evaluating the CT scan, convert the radiographic guide to a surgical guide. As a general rule, the two implants being placed should be relatively parallel and within the confines of the mandibular denture.

Dental implants have been a revolutionary advancement in dentistry. It is often the best treatment option for replacing missing teeth. The first step toward incorporating implant dentistry into your practice is selecting ideal patients for your first implant case. These basic principles are essential and should act as a starting point for implant dentistry.

Sandy Chang, DMD, is a prosthodontist in private practice in New York City. Contact her at www.drsandychang.com or email@drsandychang.com.

Natalya Bejar, DDS, is currently in private practice in San Antonio, Texas. She completed her certificate in prosthodontics at the University of Texas Health Science Center San Antonio in 2008. Her training included an emphasis on implant dentistry.

References

Diagnosis and Treatment Planning, Implant Dentistry. DVD-ROM. Loma Linda: Loma Linda University School of Dentistry, 2007.

Misch CE et al. Consensus conference panel report: crown-height space guidelines for implant dentistry — part 1, Implant Dentistry Dec 2005; 14(4):312-318.

Misch CE et al. Consensus conference panel report: crown-height space guidelines for implant dentistry — part 2, Implant Dentistry 2006; 15(2):113-121.

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