Unfavorable implant placement may cause difficulties in making implant impressions or scans. If the impression posts or scan bodies cannot be properly seated on the implants because of contact between them, adjustment with a rotary instrument or the use of gold cylinders is necessary. This article introduces a technique for obviating the clinical difficulties without the need to adjust the impression posts or scan bodies or the use of gold cylinders.
Unfavorable implant placement may occur because of inadequate treatment planning, anatomic limitations, surgical inexperience, preference for primary stability, lack of the use of a surgical guide, or lack of understanding of prosthetics. When implants are placed in close proximity, inadequately positioned, or at compromised angulations, contact between implant impression posts (IIPs) or scan bodies (SBs) may occur. As the contact precludes the complete seating of the impression components to the implants, the position of the implants will be incorrectly transferred to the laboratory.
Several methods have been presented for avoiding or removing the contact interferences. McCartney used gold cylinders as alternative IIPs. Michalakis et al modified a metal IIP with a rotary instrument. Chaimattayompol et al recommended the use of screw-retained titanium or frictional fit plastic IIPs to reduce chair time. Selecman and Wicks used solid plastic, press-fit, closed-tray IIPs to facilitate the modification procedure.
With the aid of intraoral scanners (IOSs), SBs, and computer-aided design and computer-aided manufacturing (CAD-CAM) technology, a fully digital workflow has been developed. However, unfavorable implants placement may also complicate intraoral digital scanning. To overcome the difficulties, Alqarni et al removed the interferences between SBs by using a rotary instrument. During the SB modification, extreme care was required not to damage the critical areas essential to aligning the scanned SBs with the corresponding reference SBs. Moreover, the modification procedure may reduce the structural durability of SBs and be time consuming when the scan region of SBs is made of metal.
IOS software programs and dental CAD software programs have been developed to widen the scope of clinical applications and improve user convenience. This article presents an alternative technique for fabricating a digital cast of dental implants placed in limited space and with adverse angulations.
A dentiform was used to simulate 2 unfavorably positioned implants in the maxillary right molar sites ( Fig. 1 ). A lock surface function provided by an IOS software program (TRIOS 2015-1 188.8.131.52; 3Shape A/S) can protect previously digitized areas from subsequent scan procedures. As some IOS software programs may not have the function, methods for generating a digital cast of the implants with and without the lock surface function are presented.
With lock surface function
Make an order with the IOS software program. Select icons of the maxillary right molars on the screen. Touch the “Implant” icon.
Remove the healing abutment from the mesial implant and manually tighten an SB. If a healing abutment on the distal implant precludes the proper seating of the SB, remove the healing abutment. Scan the maxilla and the SB by using an IOS (TRIOS Color Cart; 3Shape A/S) ( Fig. 2 A). Choose the “Tools” and “Lock surface” icons. Rub the whole surface of the SB on the screen ( Fig. 2 B). Retrieve the SB and place it on the distal implant. Rescan from the adjacent tooth to the SB ( Fig. 3 ). Digitize the opposing arch and maxillomandibular relationship. Transmit the scan data to a laboratory.
Open the scan data by using a CAD software program (Dental System 2015-1 × 64; 3Shape A/S). Align the reference SB stored in the library to its scanned image by using a 3-point matching technique ( Fig. 4 ). Design custom abutments and an implant-supported crown ( Fig. 5 ).
Without lock surface function
Create an order with the IOS software program. Touch the icon of the maxillary right second molar on the screen. Choose “Implant.” Select the icon of the maxillary right first molar on the screen. Check “Inlay/Onlay” and “Pre-preparation” (“Pre-preparation” is the scan process of interim restorations in the software program). In other words, regard the mesial implant as the first molar and deem the placed SB as an interim crown.
After removing a healing abutment on the mesial implant, manually tighten an SB on the implant. If the contact between the SB and a healing abutment on the distal implant is identified, remove the healing abutment. Make a prepreparation scan by using the IOS ( Fig. 6 A). According to the predefined workflow of the prepreparation function, the mesial implant site is automatically cut out to make space for digitizing the mesial implant ( Fig. 6 B), as the software program considers the mesial implant to be the first molar. Retrieve the SB on the mesial implant and place it on the distal implant. Rescan the molar sites ( Fig. 6 C). Digitize the antagonist and maxillomandibular relationship. Send the scan data to a laboratory.
Open the “Raw Preparation scan” file by using a dental CAD software program (exocad DentalCAD: Engine build 5164; exocad GmbH) ( Fig. 7 A). Click the right mouse button on the background and select “Add/Remove mesh.” Load the “PrePreparationScan” file as a gingival scan ( Fig. 7 B). Save the aligned data into a single standard tessellation language (STL) file.
Open the STL file by using the CAD software ( Fig. 8 ). Continue the design process in the previously described manner. ,
A completely digital workflow to digitize the closely placed and mesiodistally tilted implants was described. With the help of the digital technologies, the time-consuming modification procedure of the IIPs or SBs was avoided. As the lock surface function preserved the previously acquired image of the mesial SB during the distal SB scan process, the position information of the inadequately positioned implants was efficiently obtained.
When the lock surface function was not applied, modification of a scan order was required. According to the predefined process of most IOS software programs, an interim crown is digitized before an abutment tooth or an implant is scanned. As the SB on the mesial implant was regarded as the interim crown, it was captured during the prepreparation scan process. After digitizing the SB on the mesial implant, the SB was removed and then placed on the distal implant. As a result, the incomplete seating of the SBs was prevented.
If the intraoral digital scans, , , , superimposition process of SBs, and manufacturing of SBs are accurate and precise, the described technique can be applied in more complex clinical situations. However, the technique has limitations. When inadequately positioned, implants cannot function, provide esthetics, or help maintain oral health; the prosthetic procedure may not proceed; and implant removal should be considered. In addition, the lock surface function may disappear or the terminology may change with IOS software updates.