PART SELECTION (cementable) ZIRCONIA ABUTMENT

There are fife key determinants in choosing a stock zirconia abutment for a cementable all ceramic (Lava, Procera Empress a.o.) restoration. If a stock abutment can not be used because of excessive tissue thickness above the implant or angulation issues  for example, a custom abutment can be fabricated for most implant brands.

1. Tissue height

Collar height

Most stock zirconia abutments are machined with a collar or finish line already and various collar heights are available from many manufacturers. Placement of the margin on the abutment for the final restoration should be around 1.5mm below the crest of the tissue on the facial side of the abutment. Placing the margin deeper will make the removal of excess cement or bonding agents difficult. Any incomplete cleaning might lead to tissue inflammation and possible implant failure. Placement of the margin less than 1.5mm might lead to abutment exposure due to tissue recession over the lifetime of the implant.

2. Emergence profile

Emergence profiles

Some implant manufacturers offer healing abutments in different flare diameters. Biomet 3i for example offers healing abutments for their implants that flare from the original platform diameter to up to 7.5mm diameter. This information sometimes gets lost when the restorative dentist utilizes a impression coping of a different diameter flare than the healing abutment.

3. Angulation

Pre angled abutment

Miss aligned implants can be compensated with pre angled stock abutments but few manufacturers carry pre angled zirconia abutments. Often it becomes  a necessity to make a custom zirconia abutment in case of less than ideal implant placement. Due to strength limitations of zirconia especially in small diameter implants the degree of correction should be limited to 15 degrees.

4. Anti rotational feature position

Fig. 1

The relationship of the indexing feature (hex, tri lobe, octagon a.o.) of the implant to the angulation it was placed in is the key factor in determining which preangled abutment to choose because preangled abutments are angled towards or away, in relation to a pre determined aspect of the antirotational feature (hex, tri lobe, octagon a.o.) of the abutment.
For example; if the antirotational feature of the abutment is a hex, the angulation of the abutment is machined either towards or away from the flat side of the hex or the point of the hex.

Fig. 2

The interplay of the antirotational feature of the implant (Fig. 1 and Fig. 2) and the angle it was placed in, in relation to the preangled abutment angulation to its own antirotational feature makes the selection of the proper abutment a challenging task. Some manufacturers offer try in abutment kits to simplify this task. The more positions the antirotational feature allows, the easier the selection process will be. A tri lobe (NobelReplace™, Camlog™ a. o.) feature will allow only three positions whereas a dodecagon (Biomet 3i™ Certain®) allows 12 positions for angled abutments. If the position of the antirotational feature of the implant is not in sync with its angled position a custom abutment needs to be fabricated through CAD CAM technology.

5. Bonded indexing feature

Indexing feature

Some manufacturers offer their zirconia abutments machined through and through in zirconia (BioHorizons), including the indexing feature. This would be most preferable because the abutment can be modified by baking porcelain to it. Others prebond a metal indexing feature to the abutment (Biomet 3i) while some utilize a removable press fit metal index feature (Nobel Biocare, Replace). A metal indexing feature can be machined to tighter tolerances than a zirconia indexing feature but the draw back is a dark shadow at the zirconia metal junction and limited prepability. This can be detrimental in cases of very thin tissue resulting in a discoloration of the tissue from the underlying metal.