A novel post-processing strategy to improve the accuracy of complete-arch intraoral scanning for implants: an in vitro study

OBJECTIVES

To develop a new post-processing strategy that utilizes an auxiliary device to adjust intraoral scans and improve the accuracy of 3D models of complete-arch dental implants.

MATERIALS AND METHODS

An edentulous resin model with 6 dental implants was prepared. An auxiliary device, consisting of an opaque base and artificial landmarks, was fabricated and mounted onto the resin model. Twenty intraoral scans (raw scans) were taken using this setup. A new post-processing strategy was proposed to adjust the raw scans using reverse engineering software (verified group). Additionally, ten conventional gypsum casts were duplicated and digitized using a laboratory scanner. The linear and angular trueness and precision of the models were evaluated and compared. The effect of the proposed strategy on the accuracy of complete-arch intraoral scans was analyzed using one-way ANOVA.

RESULTS

The linear trueness (29.7 µm) and precision (24.8 µm) of the verified group were significantly better than the raw scans (46.6 µm, 44.7 µm) and conventional casts (51.3 µm, 36.5 µm), particularly in cross-arch sites. However, the angular trueness (0.114°) and precision (0.085°) of the conventional casts were significantly better than both the verified models (0.298°, 0.168°) and the raw scans (0.288°, 0.202°).

CONCLUSIONS

The novel post-processing strategy is effective in enhancing the linear accuracy of complete-arch implant IO scans, especially in cross-arch sites. However, further improvement is needed to eliminate the angular deviations.

CLINICAL SIGNIFICANCE

Errors generated from intraoral scanning in complete edentulous arches exceed the clinical threshold. The elimination of stitching errors in the raw scans particularly in the cross-arch sites, through the proposed post-processing strategy would enhance the accuracy of complete-arch implant prostheses.

Does the geometry of scan bodies affect the alignment accuracy of computer-aided design in implant digital workflow: an in vitro study?

OBJECTIVES

To compare 2 implant scan bodies with different geometry on the accuracy of the virtual alignment process in the digital workflow.

MATERIALS AND METHODS

A master model of the edentulous maxilla with 6 implants and multi-unit abutments (MUA) inserted was fabricated. Six dome-shaped and cuboidal scan bodies were mounted on the MUAs respectively and consecutively scanned by a laboratory scanner 10 times. The original scans were imported to a dental-specific CAD software and virtually aligned with the default CAD model in the implant library. Thus, 10 aligned models were created. Both the original scans and the aligned models were evaluated by an inspection software for deviation of the scan body surfaces, the centroids of scan body and MUA, the scan body centre-axis and the inter-MUA distances/angles. The two-sample T-test/Mann Whitney U test were used to analyse the data with the level of significance set at 0.05.

RESULTS

The cuboidal group showed significant greater deviations of the model surface (13.9 µm vs 10.7 µm) and the MUA centroids (31.7µm vs 22.8 µm) but smaller deviation of the inter-MUA angle (0.047° vs 0.070°) than those of the dome-shaped group (P<0.05). No significant differences in the deviation of scan body centroids, centre-axis and the inter-MUA distances between the 2 groups were found.

CONCLUSIONS

Virtual alignment of implant scan body affected the accuracy of the digital workflow for complete-arch implant-supported prostheses (up to ~30 µm/0.09°). Different geometries of the implant scan body could also influence the transfer accuracy in the CAD process.

Accuracy of CBCT in the Identification of Mental, Lingual, and Retromolar Foramina: A Comparison with Visual Inspection of Human Dry Mandibles

The present dry-mandible study evaluated the presence of the mental (MF), lingual (LF), and retromolar (RMF) foramina to assess the accuracy of CBCT in detecting these anatomical structures. In total, 38 human dry mandibles were analyzed (30 men, 8 women; mean age: 61.9 ± 13.7 years). CBCT scans were taken using low-dose parameters, and LF, MF, and RMF were assessed visually and radiographically. Both the presence (yes/no) and the count (n) of each foramen were compared between CBCTs and visual assessment. For the presence assessment, only RMF exhibited a significant difference between CBCT and visual inspection (P = .035). For count, the RMF (P = .049) and paramedian LF (P = .003) exhibited differences between the two methods. The agreement between CBCT and visual inspection was excellent for the MF, moderate-excellent for the LF, and low-moderate for the RMF. Intra- and interassessor agreement varied from excellent (MF), to moderate-excellent (LF), and low-moderate (RMF). The LF and RMF represent challenging structures to identify on CBCT images due to their limited dimension. False negative findings in CBCTs in the assessment of foramina, especially LF, might lead to surgical complications during implant surgery.

Evaluation of laboratory scanner accuracy by a novel calibration block for complete-arch implant rehabilitation

OBJECTIVES

To compare the accuracies of 4 laboratory scanners using a new custom-made block for complete-arch implant rehabilitation.

MATERIALS AND METHODS

A block comprised 4 cylinders, with 2 in the anterior (0° angulation) and 2 in the posterior region (45° distal angulation) (Experimental group) and a standard block with 2 parallel cones in 16° taper (ISO group), were fabricated. Both blocks were scanned consecutively for 15 times by 4 laboratory scanners: IScan, Zfx, 3Shape, and KaVo. Measurements were also made by a coordinate measuring machine (CMM) as the reference. Acquired digital models were inspected with a metrology software. Linear and angular distortions were computed evaluation of trueness, precision and expanded uncertainty of scanners. Effects of blocks and scanners on the scanning trueness and precision were analyzed by Two-way ANOVA (α = 0.05).

RESULTS

The linear trueness and precision of scanners was significantly poorer when the Experimental block instead of ISO block was used. Significant greater distortions were noticed at the 45° sites than the 0° sites and no significant effect of inter-implant distance on the scanning accuracy was found. Zfx, IScan and 3Shape exhibited comparable expanded uncertainties (10.6∼11.8) but KaVo showed the greatest (19.3) in complete-arch implant scans.

CONCLUSION

The ISO block might not be more suitable than the Experimental block for evaluating the accuracy of laboratory scanner for complete-arch implant scanning. All the scanners tested except one demonstrated acceptable accuracy for complete-arch implant digitization. Scanning accuracy was compromised by unfavourable angulation of implants.

CLINICAL SIGNIFICANCE

It is questionable whether the laboratory scanners validated for tooth-borne cases could also perform well for implant cases. It might be necessary to develop a new calibration object specifically for complete-arch implant scenarios to verify the capability of laboratory scanners in the implant workflow.