Tips to Overcome Common Mistakes in Conversion Protocol for Fixed Hybrids

Treating patients with terminal dentition by utilizing fixed hybrids has become a popular surgical/restorative treatment alternative.

The technique often consists of doing extractions, bone reduction, implant placement and allowing the treating team to perform an immediate loading protocol that includes converting an immediate denture to a fixed provisional, implant-supported prosthesis.

Once the implants are placed, SRA or transmucosal abutments are normally utilized (Figure 1) in order to correct:

1. The implant angulation.
2. To bring the prosthetic connection to an equigingival or slightly supragingival level, which facilitates insertion and visual aid for fit assessment.

Once these SRA abutments have been secured, non-engaging temporary cylinders are then secured on top of the SRAs (Figure 2) and the conversion protocol consists of picking these temporary cylinders with the immediate denture, so that they will ultimately be embedded within the prothesis.

Once the temporary cylinders have been picked up (Figures 3A-3C), the contours of the denture are modified to ensure a flat or slightly convex intaglio surface to promote hygiene during the osseointegration phase, as well as modifying the distal extension of the prosthesis, where care must be taken to minimize cantilever extension during this transitional period.

When doing the conversion prosthesis protocol, the treating team may often experience a few clinical mistakes or shortcomings. Some of these include:

1. Prosthesis lock-in.
2. Faulty pickup of temporary cylinders.
3. Closing the vertical dimension of occlusion.
4. Canted occlusal plane.

Prosthesis lock-in

As in any other procedure where we are making impressions, relining provisional restorations or picking up components, it is imperative to block out potential undercuts while doing the temporary abutment pickup into the conversion prosthesis. Carelessness here can result in locking in the provisional restoration, which is quite problematic to retrieve.

The easiest way to block out undercuts during the conversion protocol consists of utilizing a rubber dam or Teflon tape to make sure the pickup material is contained above the undercut and retrieval of the prosthesis is simplified (Figure 4).

Faulty pickup of temporary cylinders

It is common to perform the pickup of the temporary cylinders only to discover that one or more is not perfectly stable or rigidly picked up within the denture.

This is common due to problems like:

• Lack of isolation/humidity control.
• Inappropriate delivery or injection of the pick-up material around the cylinder.
• Insufficient concentric opening or hollowing of the provisional denture around the areas of the implants, which does not allow the material to flow 360 degrees around the cylinder.

Care must be taken to ensure that:

• Concentrically all cylinders have at least 1mm of clearance from the denture in order to insure a 360-degree pickup (Figures 5A-5C).
• Ideally, the cylinders should be previously sandblasted and – depending on the pick-up material to be utilized (acrylic or composite) – a thin coat of the same material should be applied to the cylinder and cured prior to injecting the pickup material.

Closing the vertical dimension of occlusion

Since the immediate denture is normally soft tissue-supported, it would be relatively easy to exert excessive pressure compressing the soft tissue and ultimately picking up the cylinders at a reduced vertical dimension.

It is therefore suggested that the adequate vertical dimension of occlusion is recorded. This may be accomplished by utilizing a marker and painting a dot on the patient"s nose and chin and recording the ideal distance between them preoperatively. Then as we do the reline or pickup, we do so at the same vertical dimension.

For maxillary dentures, the palate normally serves as the vertical stop of the provisional prior to relining. For the mandible, it is easier to displace the tissues vertically while performing the pickup protocol, so it is common to end up with a reduced vertical dimension.

Care should be taken not to exert excessive pressure and to verify that the vertical dimension appears to be adequate during the conversion protocol.

Canted occlusal plane

It is not unusual to see that once clinicians have finalized their conversion prosthesis protocol, there is a cant in the incisal edge/occlusal plane of the posterior teeth.

Once again, this should be something we closely monitor during the relining or pickup procedure – ensuring that the incisal edge is parallel to the horizon, or perfectly perpendicular to the midline.

It should be noted that newer available CAD/CAM digital systems like Straumann's Smile in a Box allow the conversion protocol pickup while the provisional is rigidly held in place through anchor pins (Figures 6A-6C). This facilitates the process considerably since it also minimizes the risks of positioning mistakes during the pickup, as well as facilitating the occlusal adjustment that is paramount to ensure a successful outcome.

Getting the three-dimensional position of the conversion prosthesis as close to ideal as possible is paramount because it truly simplifies the remaining steps toward the fabrication of the definitive prosthesis. Once the implants are fully osseointegrated, it becomes the best reference for the dental technician in designing the definitive fixed hybrid restoration.

Ricardo Mitrani, D.D.S., M.S.D., is a member of Spear Resident Faculty.