Evaluating Limited Vertical Space When Replacing Posterior Teeth

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As restorative dentists, we often are faced with cases in which the patient is missing one or more posterior teeth. If implant-supported crowns or bridges are being considered as one of the prosthetic options, the restorative space must be evaluated before the implant is placed (Table 1). If the bite has collapsed or the opposing teeth have supra-erupted, the vertical space may be limited. The table below lists the average crown lengths as a reference.

Treatment Planning & Diagnostic Wax-Up

• Initial assessment of the edentulous space should be done during the clinical examination. This would include evaluating the vertical space when the patient is closed into centric occlusion.
• Articulated study models provide a standard diagnostic tool. The space can be easily measured and compared to the patient's remaining teeth (Fig. 1, Fig. 2a, Fig. 2b).
• A Diagnostic Wax-Up is an invaluable treatment planning tool, as well as a case-presentation and patient-education aid (Fig. 3). If you are utilizing digital treatment planning, the scan appliance/Radiographic Guide is fabricated to illustrate the position and dimensions of the tooth (teeth) to be replaced (Fig. 4).
• Because the restoration begins at the top of the implant, the planned position of the implant becomes critical. Appropriate radiography should be taken to evaluate the quantity of bone in the proposed implant site. These images can also be used as an aid in evaluating the restorative space (Fig. 5). CT scans and treatment planning software are excellent diagnostic tools to plan the case from the surgical and prosthetic perspectives (Fig. 6).

Treatment Options
The patient should be presented with the prosthetic options as well as the consequences of not restoring the space. There are many ways the options can be presented. One is the "good, better, best" approach. As detailed in the article by Dr. David Schwab on page 18 of this issue, if the patient is missing one or more posterior teeth, a partial denture may be a

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good option, a fixed partial denture a better option and an implant-supported crown or bridge the best option (because prepping of adjacent teeth is unnecessary, hygiene is simpler and bone height is more easily retained, etc.).

If there is limited vertical space, the patient should be advised of the situation. Ideal treatment to create the appropriate space for the restoration should be presented. If the patient is unable or unwilling to undergo treatment, he or she should be aware of the compromise. It could eliminate the patient as a candidate for implant restoration in severe cases.

Prosthetic Options
From a laboratory standpoint, there are minimal height requirements for cemented as well as screw-retained restorations*:

• For a cemented crown or bridge over an implant, a minimum of 7 mm is needed from the top of the implant to the opposing dentition (Fig. 7a, 7b). This provides space for the abutment and the crown. The abutment can be shortened to the head of the abutment screw. Approximately 2 mm of occlusal space is needed for the crown.
• Because it is one piece, a screw-retained crown requires less vertical space than its cement-on counterpart. This restoration requires a minimum of 5 mm from the top of the implant to the opposing dentition (Fig. 8a, Fig. 8b). The head of the screw should be at least 1 mm out of occlusion so the access opening can be sealed.

Note: If you are going to take an impression of an abutment in the mouth, the occlusal space must be at least 2 mm. An abutment reduction guide can be fabricated and used as an aid.

* These are minimum heights to allow space for the components and restorative materials. They will result in short clinical crowns. Reduced retention for cemented crowns will also result. To achieve the desired esthetics, additional vertical space may be required