## 4 Discussion
### Splinting ISBs to Improve the Accuracy of IOSs  
In 2020, [PERSON] et al. compared the accuracy of an original and two newly designed ISBs (CAD/CAM) used for digital scans with one another as well as with conventional implant impressions (group 4) for a completely edentulous mandibular model [35]. The original ISB (Straumann) (group 1) was compared to two CAD/CAM ISBs, the first without extensional structure (group 2) and the second with a one-piece unit extensional structure (group 3). All these scans were compared to a conventional splinted open-traym impression using abutment-level impression copings connected to the abutments on the reference model and splinted using self-cured pattern resin (GC Corporation). The digital scans were made using the IOS Trios3 (3 Shape) and the conventional impressions were made using siloxane impression material (Silagum putty/light; DMG). This is the first study to introduce and evaluate a scan body with a rigid extensional structure to make the ISBs \"splinted\" together. A rigid extensional structure was designed to provide more characteristic reference points at the inter-implant regions than most of the commercially available cylindrical scan bodies, which could facilitate the identification and stitching procedures. The CAD/CAM ISBs made for this study were comprised of titanium alloy. The surface was sandblasted prior to the scanning procedure to reduce surface reflection. When compared to PEEK ISBs, titanium alloy ISBs have advantages related to decreased distortion associated with repeated use and sterilization cycles. The trueness and precision values were shown using the median and interquartile range (IQR) of the RMS values, which indicated significant differences for both trueness (\(p\) = 0.001) and precision. The median (IQR) of trueness was 35.85 (29.80-49.10) \(\upmu\)m for group 1, 38.50 (35.35-52.58) \(\upmu\)m for group 2, 28.45 (24.88-36.43) \(\upmu\)m for group 3, and 25.55 (22.98-28.90) \(\upmu\)m for group 4. The median (IQR) of precision was 48.40 (40.80-57.90) \(\upmu\)m for group 1, 48.90 (38.70-85.40) \(\upmu\)m for group 2, 27.30 (22.50-35.50) \(\upmu\)m for group 3, and 19.00 (15.70-22.75) \(\upmu\)m for group 4. Conventionally splinted open-traym impressions yielded the highest accuracy, followed by the digital impressions made using the CAD/CAM scan bodies with extensional structure. Digital impressions using the original ISBs and the CAD/CAM ISBs without extensional structure showed relatively low accuracy. The innovative design of the extensional structure used to splint the ISBs could serve as a new approach with which to increase the accuracy of a digital scan in full-arch rehabilitation [35]. In their 2022 study, [PERSON] et al. investigated the accuracy of a novel implant complete-arch intraoral digital impression using ISB splitting (ISS) [36]. An edentulous mandibular model with four implant analogues (Nobel-Parallel RP 4.3, NobelBiocare) was fabricated. In an attempt to reproduce real clinical situations in vitro, the four implants were positioned with different depths, inclinations, and inter-implant distances. The IOS Trios3 (3 Shape) was used to scan the model, and 30 scans were performed before the ISS and 30 after the ISBs were splinted. The ISS was fabricated using a 3D-printed (Accura resin XM24.7 and Ipro 8000, 3 DSystems, Rock Hill, SC, USA) modular chain with an opaque detailed surface which was easy to assemble according to the implant positions and anatomy of the edentulous arch ridge. The modular chain was secured to the ISBs with a light-curable flow composite (Clearfil Majesty ES Flow, NoritakeKuraray). The investigated implant and complete-arch intraoral digital impression with ISB splitting (ISS+) improved the image stitching. In clinical scenarios, such as the one reproduced in this study, involving an edentulous patient rehabilitated with several implants, different depths and inclinations and long inter-implant distances could lead to a deterioration in accuracy and a slowing down of the scan [23].