Accessing screw access channels on a full arch fixed prosthesis utilizing digital dentistry

by Nicholas Miller, DMD, MS, FACP, Board Certified Prosthodontist | Georgia Prosthodontics

Cynthia Aita-Holmes, DMD, FACP

The implant-supported full-arch fixed restoration is frequently chosen by the prosthodontist as the treatment of choice in the rehabilitation of the edentulous patient.

These types of prostheses are not without failures and their retrieval, when cement-retained, can present a complex clinical situation. Conservative retrieval will preserve integrity of the prosthesis and allow it to serve as a provisional throughout the duration of treatment.

A patient presented with a maxillary full-arch implant-supported fixed prosthesis on nine implants (Fig. 1). The prosthesis was made of a zirconia substructure with individually cemented lithium disilicate crowns covering all screw access channels (Fig. 2).

Fig. 1. Retracted frontal view of prosthesis

Fig. 2. Occlusal view of prosthesis

The patient disliked the shade and contours of the gingival porcelain and requested to have the prosthesis remade. She also stated she was experiencing an occasional bad taste and odor, presumably coming from under the maxillary prosthesis.

Clinical examination revealed good oral hygiene and apparent healthy peri-implant tissues, although the majority of the implant sites could not be visualized or probed with the prosthesis in place.

The prosthesis presented with inadequate anterior tooth proportions (too narrow), chipping of the pink porcelain at the interdental papilla between #9 and #10, and an uncemented crown at #2. A radiographic examination revealed that all 9 implants were poorly angulated and lacked parallelism making access to the abutment screws a very unpredictable and challenging procedure (Fig. 3).

The treatment plan for this patient involved fabricating a new maxillary full arch implant supported fixed prosthesis. In 2018, Asiri et al. described a technique to fabricate a guide to conservatively retrieve a single implant cement retained crown using a CBCT and implant planning software.

A very similar technique was used in this clinical case. A cone-beam computed tomography (CBCT) (J. Morita Accuitomo, Irvine, California) of the patient was obtained and exported as a digital imaging and communications in medicine (DICOM) file.

Additionally, an optical scan (inEos X5 scanner, Dentsply Sirona, York, PA) of the prosthesis was obtained and saved as a standard tessellation language (STL) file. The DICOM and STL files were uploaded and correlated into implant planning software (Blue Sky Bio, Libertyville, IL). Because the lithium disilicate crowns did not create any scatter on the CBCT, this correlation procedure was straightforward.

When scatter is anticipated, such as seen with metallic or zirconia restorations, a scan appliance with fiduciary markers should be used.

Virtual implants were added in the implant software and precisely aligned with the existing implants and their long axis was extrapolated beyond the prosthesis using virtual abutments. Three separate screw access guides were designed, 3D printed and used to conservatively access all abutment screws using diamond burs (seeFig. 4 for digital workflow).

After removal of the prosthesis, purulence was noted around the implant at #9-10 site. Presenting with a poor prognosis and angulation, this implant was removed. The prosthesis was reinserted and used as a provisional while the patient awaited fabrication of a new prosthesis (see Figs. 5-10 for clinical documentation).

The technique described above allowed predictable and conservative retrieval of a complex implant supported fixed prosthesis and eliminated the need to fabricate a provisional thereby reducing treatment cost and time.

Fig. 5. Occlusal view of guide in place on anterior portion of prosthesis

Fig. 6. Buccal view of guide in place in posterior right portion of prosthesis

Figure 7: Utilization of guide to access abutment screws

Fig. 8. Access to all abutment screws completed

Fig. 9. Maxillary occlusal view with prosthesis removed

Fig. 10. Purulence at implant site #9–10

The technique described above allowed predictable and conservative retrieval of a complex implant supported fixed prosthesis and eliminated the need to fabricate a provisional thereby reducing treatment cost and time.

References: 
Asiri W, Domagala D, Cho S, Thompson GA. A method of locating the abutment screw access channel with cone-beam computed tomography and a 3D-printed drilling guide. J Prosthet Dent 2018; 119: 210-13.

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