The objective of the current study was to assess GT with non-invasive methods. Lower incisors were in focus since change in their inclination or torque may introduce a risk factor to gingival recessions marking this as an area of major concern, not only in functional, but also in aesthetic respect.
Direct measurement is regarded as a fairly objective method for GT assessment. Nevertheless, since it involves tissue penetration, its clinical applicability is associated with some limitations;  these are often linked with measurement errors, probably originating from instruments’ rounded tips and thickness .
An USD showing a high reproducibility [12, 13, 21, 25], was selected as the first, non-invasive method, for measuring GT. The second selected method was CBCT imaging that has been shown to have a high diagnostic applicability [2, 16].
According to the present results, the difference between USD and CBCT measurements of gingival thickness was not zero. In general, CBCT measurements were consistently higher than the USD measurements. This difference was independent of the magnitude of GT measurement. CBCT measurements were constantly higher than USD, with the difference ranging from 0.13 mm to 0.21 mm (Table 4). It is difficult to attribute the difference reported to one methodology or the other. If a possible explanation was to be given, It could possibly be the ultrasound procedure, due to measuring imprecision such as misangulation of the ultrasound transducer or over-compression of the soft tissue.
There was no evidence of significant differences between the repeated measurements made by the first examiner on the mandibular right central incisor (p-value=0.104). Although the respective results for the mandibular left central incisor indicated that the measurements were not exactly true replicates from a statistical point of view, the magnitude of the point estimate of bias was small and possibly not clinically significant (Bias= 0.06 mm; p-value=0.014).
Moreover, there was evidence of a small systematic difference between the CBCT measurements made by the two examiners on the mandibular left central incisor (bias= 0.06 mm, 95% CI= 0.01, 0.11). However, this difference was minor, and again, clinically unimportant. On the other hand, the respective analysis on the mandibular right central incisor showed no evidence of a significant difference between the two examiners (bias= 0.05 mm, 95% CI= -0.01, 0.11).
Numerous dental procedures require accurate measurement of GT, since respect of gingival phenotype is vital and appears to influence the outcomes of various treatment strategies. Gingival phenotype evaluation through simple visual appraisal is shown to be inaccurate [10, 26], mostly due to its subjective nature; it relies, at least to a great extent, on clinical competence. Thick gingiva, i.e. more than 0.8- 1mm of thickness, is shown to be relatively resistant to gingival recession following surgical or restorative therapies [27-30], whereas thin-scalloped gingiva is considered at risk because it has been associated with a compromised response following the same treatments [5, 27-32]. These findings point clearly to the need of a thorough diagnosis, through a straightforward and reproducible method, of these high-risk patients, before various interventions involving the gingiva. At this point it has to be outlined, as far as accuracy is concerned, that this term refers to closeness of the measurements to the true value of gingival thickness. By definition, true value cannot be measured by methods, as those in the present study. There is only an estimation of the true value. This is the reason why it is important to describe repeatability, reproducibility and the correlation of the methods tested.
Our study is not free of limitations, although efforts were made to minimise them. Firstly, the clinical measurements did not take into account potential differences in dental arch crowding or tooth inclination that may influence the clinical handling of the USD transducer probe, although it was anticipated that this wouldn’t lead to a large method error. Secondly, at present, CBCT conducting for assessment of GT might not be justified due to the associated amount of radiation, as CBCT includes higher doses than two-dimensional imaging. Moreover, CBCT Images have a certain degree of inaccuracy attributed primarily to image generation, processing, voxel size and various types of artefacts that might be present. In general, the smaller the voxel size, the higher the precision/resolution of the information provided. Larger voxels may include different tissues, and thus, the subsequent grayscale value may not indicate clearly one specific tissue, such as bone. This issue is primarily evident at the limits between neighbouring tissue types of different radio-density. However, at the same time, the smaller the voxel size the higher the motion artefacts. Thus, based on the above considerations and also on the need to keep the radiation exposure as low as possible, a specific CBCT image can only reach a certain degree of detail on the information it provides [33-35].
Finally, it has to be pointed out that the lack of a gold standard measuring procedure of soft tissue thickness may downgrade the clinical significance. Finding a gold standard measurement for humans is, nevertheless, almost impossible, because all clinical or imaging methods present an inherent measurement error, which is not always easy to assess during implementation. On the other hand, significance of the current study lies in the fact that it includes both imaging and clinical procedures and provides robust data for the comparison of the tested methods.