Factors that need consideration before deciding upon a fixed vs. removable implant rehabilitation
1.1 Positioning the Maxillary and the Mandibular Incisal Edge
To determine if a fixed or removable restoration would be appropriate, a wax try-in is done without a flange. For a fixed restoration, the clinical crown should ideally end up at the soft-tissue level of the alveolar ridge. In this situation, minimal resorption would have occurred, interarch space will be favourable and an optimal tooth-lip relationship is present. When a large vertical distance exists between the cervical aspect of the tooth and the alveolar ridge but the tooth-lip relationship is favourable, pink ceramic or acrylic may be utilized to disguise the tooth length and a fixed restoration is still possible. When there is both a vertical and horizontal discrepancy between the ideal position of the tooth and the alveolar ridge, and the tooth-lip relationship is not optimal, this may be an indication for use of a removable prosthesis. The flange will provide adequate lip support, and the teeth can be positioned appropriately to satisfy the parameters of aesthetics.
The mandibular incisal edge is positioned for function. The clinician must provide shallow guidance, sufficient to provide posterior disclusion in both protrusive and lateral excursions. Anterior guidance must be smooth and distributed amongst as many anterior teeth as possible.
A thorough evaluation must be made of the existing mandibular incisal-edge position . When patients are missing posterior teeth and have been diagnosed as having lack of posterior support the mandibular incisal edge is often in the incorrect position. The clinician must decide whether to reshape, reposition or restore if the maxillary arch is being considered for implant-supported restorations. Conventional prosthodontic guidelines will place the mandibular incisal edge just at the level of the lower lip with 0.5–1.0 mm of the incisal edge visible. Guidelines in relation to the lower mandibular occlusal plane can also be sought from anatomical landmarks such as the retromolar pad.
If the clinician is planning a fixed implant-supported restoration for the mandible, adequate restorative space must be provided. The overeruption of teeth brings with it an excess of bone, which must be reduced prior to the implants being placed.
1.2 Restorative Space
Insufficient restorative space is the most common error when planning full-arch restorations. Inadequate space results in either premature failure of the restoration or changing the treatment plan from one restoration to another to accommodate the space requirements.
To accommodate adequate designs, different types of restorations require different dimensional tolerances . Accurately mounted casts are critical in assessing prosthetic space limitations. Spatial constraints must be considered as a matter of practicality. The limiting factor in edentulous patients is the available inter-arch space.14. Adequate restorative space is critical, and guidelines exist depending upon the type of prosthesis in the treatment being planned. There must be adequate space for bulk of restorative material that also permits a prosthesis design to establish aesthetics and hygiene. If space is limited, re-establishing a patient’s vertical dimension or altering the opposing occlusion should be considered .
1.3 Lip Support
One of the best diagnostic tools is the patient’s existing maxillary denture. The clinician can evaluate the patient’s denture to determine what are the likes and dislikes regarding aesthetics, speech and function. Each point should be noted for improvements in the new restoration. There is always a tendency for patients to prefer fixed over removable prostheses. It is the restorative dentists’ responsibility to determine if this is feasible. Facial support is an important decision in this regard.
Facial support, if inadequate, is obtained mainly by the buccal flange of a removable restoration. Lip support is derived from the alveolar ridge shape and cervical crown contours of the anterior teeth. Resorption of the edentulous maxilla proceeds cranially and medially and this often results in a retruded position of the anterior maxilla.
If the anticipated position of the teeth and implant results in a large horizontal discrepancy, a number of options must be considered before finalizing implant placement.
Bone reduction and a deeper implant placement to allow the contours of the restoration to satisfy the parameters of lip support and hygiene: Without bone reduction, undesirable contours in the restoration are developed, which make it very difficult for the patient to maintain hygiene (Fig. 1.9).