The main reasons to fabricate custom abutments are miss aligned implants, excessive tissue above the implant abutment interface and as a work around in cases where there are no zirconia abutments available for the particular implant diameter or brand.
Custom abutments are waxed and cast with indexed UCLAs. All plastic or gold plastic combinations are utilized for custom abutments. All plastic UCLAs are considerably cheaper but sacrifice the precision fit of gold plastic combination UCLAs. Choose appropriate platform diameter UCLAs from the implant manufacturer or clone parts
Remove the soft tissue moulage and position the UCLA on to the analog on the working model, ensure proper seating and engaging the anti rotational feature. Use a lab screw if available to secure the abutment in place, final screws should not be used while working on the case. The final screws are in some instances coated or surface treated to prevent screw loosening and the coating or surface treatment might get damaged during the production process.
Reduce the plastic tube pressed on to the machined abutment-implant interface with a rubber wheel or a carbide bur in height to allow 2mm of room for the restorative material. Next reduce the buccal and lingual surfaces as needed to allow adequate room for the restorative material. Often the mesial and distal surfaces also need reduction to accommodate the restorative material and draw purposes.
Wax up the remaining abutment to ideal contour, creating a collar and finish line for the restorative material. The margin should not be placed more than 1.5mm below the crest of the tissue because it is difficult for the restoring dentist to clean excess cement or bonding agents from the sulcus if it is placed deeper than 1.5mm below the tissue. Placing the margin less than 1.5mm below the tissue might expose the collar of the custom abutment in the future because of bone loss and tissue recession around the implant. Dr.'s often have specific preferences how far below the tissue the margin should be placed. Develop the emergence profile according to the restoring dentists preferences. Some Dr.'s prefer zero tissue displacement while others dictate total disregard of the existing tissue contour.
Remove the finished wax up from the working model and meticulously clean the abutment-implant interface with a cotton swab and rubbing alcohol to remove any wax residue.
Sprue the wax up following the long axis of the sprue as close as possible. This will allow the investment to freely flow through the screw access channel of the UCLA when pouring the investment into the casting ring. The investment expansion should be adjusted to achieve zero expansion, no special investment liquid, just distilled water. Cast the wax up in a yellow gold alloy (If the cast custom abutment is being made because no zirconia abutment is available for the particular implant brand or diameter at hand, use white precious metal for porcelain application). Do not use non precious alloys for any implant restoration because this might cause a galvanic reaction due to the dissimilar non precious alloy and the titanium of the implant. Take care not to exceed the solidus of the UCLA during preheating. Pre heat time, heat climb rate, hold temperature and time need to be fine tuned to avoid "flash cast on" on to the implant abutment interface.
Divest the cast custom abutment with great care as not to damage the abutment-implant interface with hand tools or sand blasting. Instead place the abutment in acid and a ultra sonic unit to remove any investment material.
Cut the sprue and screw the abutment on to a second analog to protect the abutment-implant interface.
Refine the cast abutment to final contour and chamfer margin.
For anti rotational purposes cut grooves in the mesial, distal and lingual surfaces of the abutment.
Polish and high shine the collar portion of the abutment.
 Mark the buccal surface of the abutment with a round bur to ease proper abutment transfer and positioning for the restoring dentist at the time of delivery or fabricate seating index (Tutorial).
Alternative custom abutment cast in white precious metal with porcelain application for situations where no zirconia abutment is available.