Evaluation of the cone-beam computed tomography accuracy in measuring soft tissue thickness in different areas of the jaws.

PURPOSE: Due to the extensive use of cone-beam computed tomography (CBCT) in dentistry, especially in measuring thicknesses of hard and soft tissues, and the fact that CBCT has low contrast resolution, the aim of this study was to evaluate the accuracy of CBCT in measuring soft tissue thickness.
MATERIALS AND METHODS: In this cross-sectional study, pink baseplate wax as a soft tissue simulation was overlaid in different areas of the jaws on the dry human skull, and the probe was used to determine the thicknesses of 1, 2, 3, and 5 mm. These thicknesses were then measured accurately with a digital caliper by one person four times at interval of a week. The skull was scanned using two CBCT scanners, and the different thicknesses of wax were measured by two observers four times with a week interval. The CBCT measurements were compared with digital caliper measurements as a reference.
RESULTS: Statistical analysis showed no significant difference between CBCT and digital caliper measurements for thickness < 2 mm (P > 0.05).
CONCLUSIONS: Statistically, the difference between digital caliper and CBCT measurements was < 0.1 mm, but this difference is not clinically important. The accuracy of CBCT in measuring soft tissue thickness was 0.1 mm.

INTRODUCTION

Originally, cone-beam computed tomography (CBCT) was developed at the Mayo Clinic for angiography procedures in 1982.[1] CBCT has been used for the craniofacial imaging since 1998. CBCT because of its high performance, low cost, lower radiation exposure, and better spatial resolution is preferred to conventional CT.

Distortion and overlapping as the disadvantages of extraoral radiography are not seen in CBCT images. CBCT provides three-dimensional images with high detail and resolution. CBCT images can provide valuable information for the physician including bone morphology, pathological findings, and anatomical location. These benefits have led to the use of CBCT in most dental specialized fields.[23]

Today, dentists, especially in the field of implantology, are demanding cross-sectional images along with hard tissue measurements for implant placement in the edentulous areas. Hard tissue measurements are clinically acceptable for implant placement with an accuracy of 1 mm.[4] One of the disadvantages of CBCT images is the low soft tissue contrast resolution due to the scatter radiation as a result of imaging the structures outside the field of view (FOV).[14]

Along with the evaluation and measurement of hard tissue, the soft tissue evaluation and its thickness measurement have also been important. For example, in periodontal plastic surgery which is performed to correct or eliminate the traumatic, anatomic, or developmental deformities of the gingiva or alveolar mucosa, the graft was taken from the palatal mucosa in order to create or widen the attached gingiva around the tooth or implant. Estimating the thickness of the palatal mucosa is predictive of the surgical outcome. If the donor tissue is too thick, reperfusion of the graft may be prevented, and if this tissue is too thin, the graft necrosis may be happened. The ideal graft thickness is between 1 and 1.5 mm.[5678]

Furthermore, in the orthodontic practice for anchorage control, while planning the treatment for patients with dental and skeletal malocclusions, the palate is a popular site for temporary anchorage placement due to thick keratinized mucosa. Measuring the thickness of the palatal mucosa to place the temporary anchorage device is important.[9]

Soft tissue evaluation is one of the significant aspects in forensic science. The main purpose of forensic facial reconstruction is to reconstruct the face of a deceased person based on the skull. For proper reconstruction of the human face, it is necessary to know the mean thickness of the soft tissue, especially in the areas of the face and scalp that are used in the reconstruction.[1011]

In orthognathic surgery for the position correction of the maxilla, mandible, or chin, though dental-skeletal relationships are the primary predictors of surgery requirement, soft tissue profile is one of the most important components in the treatment plan and surgical procedure. The thickness of the soft tissue is various in the maxillofacial area and depends on the site and person's phenotype. In orthognathic surgery, an accurate analysis of soft tissue is a prerequisite to improve the facial esthetics. In the preoperative evaluation, the esthetic balance should be considered by measuring the soft and hard tissues.[1213]

So far, various methods have been used to measure the thickness of palatal soft tissue, such as the use of local anesthesia and measurement with prob that is a painful and aggressive method and also this measurement is performed right before graft removal; therefore, it is not effective in planning of the surgical procedure. Another method is the use of an ultrasonic device which has a certain degree of difficulty and unreliable results.[56814]

Many studies have also been conducted to measure the soft tissue thickness of the oral cavity using CBCT,[68121415161718192021] but few of them have evaluated the accuracy of CBCT in measuring soft tissue thickness. The aim of this study was to evaluate the accuracy of CBCT in measuring soft tissue thickness in the oral cavity.

MATERIALS AND METHODS

This cross-sectional study was approved by the Ethical Committee of Babol University of Medical Sciences (Ethical number: IR-MUBABOL.REC.1397.004). In this cross-sectional study, pink baseplate wax as a soft tissue simulation was placed in different regions of the maxilla and mandible on the dry skull (anterior of maxilla: buccal, lingual, and crestal; posterior of maxilla: buccal, lingual, and crestal and the same regions on the mandible) so that the waxes were uniform and free of bubbles [Figure 1].

Figure 1

(a, b) anterior of maxilla: buccal, lingual, and crestal and posterior of maxilla. (c, d) buccal, lingual, and crestal and posterior regions on the mandible

The baseplate wax was overlaid on the skull, and the probe was used to determine the thicknesses of 1, 2, 3, and 5 mm. Then, these different thicknesses of pink wax were accurately measured by one person at a weekly interval four times with a digital caliper (ERSTE QUALITY/GERMANY/MEASUREMENT ACCURACY: 0.01 mm). The orthodontic wire was inserted into the pink baseplate wax and marked with a marker parallel to the wax surface. This distance was then measured with digital calipers [Figure 2]. A small piece of gutta-percha as a reference point and guide for the path of measurement was placed inside the hole created by probe measurement.

Figure 2

(a) The probe being used to determine the thickness. (b) The orthodontic wire was inserted into the pink baseplate wax and marked with a marker parallel to the wax surface. (c) This distance being measured with digital calipers

The skull was scanned using two CBCT scanners:

NewTom 5G (QR Srl, Verona, Italy) – FOV(field of view): 16 cm × 18 cm, kVp(Peak kilovoltage): 110 kV, tube current: 0.57 mA, exposure time: 3.6 s, and voxel size: 0.01 mm. In NNT software, the cross-sectional images with thickness and interval of 0.5 mm were scrutinized and thickness of base plate wax was measured.

ACTEON (X-MIND® TRIUM, Italy) – FOV: 110 mm × 80 mm, KVP: 80 kV, tube current: 6 mA, exposure time: 6 s, and voxel size: 0.15 mm. In OnDemand3D software, the cross-sectional images with thickness and interval of 0.5 mm were scrutinized and thickness of base plate wax was measured.

These measurements were performed four times by two oral and maxillofacial radiologists at interval of a week using two related software. Observers used software features such as magnification as well as contrast and brightness changes [Figure 3].

Figure 3

Measurement of pink baseplate wax thickness in the buccal region of posterior of mandible, using the OnDemand3D software for the ACTEON X-MIND TRIUM (a) and NNT software for the NewTom 5G scanner (b)

Statistical analysis

Data were analyzed using SPSS version 17 (IBM Company, Armonk, New York, USA). An intraclass correlation coefficient (ICC) and Bland–Altman comparison and Pitman's test were used to assess the agreement between two observers. One-way ANOVA was used to compare the mean measurements of three devices (a digital caliper and two CBCT scanners) and also independent t-test was applied for comparing two mean measurements (digital caliper measurements and CBCT measurements).

RESULTS

High ICC indicated high reliability among the direct physical measurements obtained from the four-time measuring results using a digital caliper (0.961). Further, the intra- and interobserver reliabilities of CBCT measurements were high with 0.995 and 0.998, respectively [Figure 4].

Figure 4

Bland–Altman comparison of observer 1 and observer 2 agreements

Analysis of data obtained from digital caliper and CBCT measurements showed no significant difference between CBCT and digital caliper measurements in thickness <2 mm (P > 0.05 and mean difference = 0.03 mm), and the significant difference was observed for thicknesses >2 mm (P < 0.05 and mean difference < 0.1 mm).

The comparison of the measurement accuracy of the two CBCT scanners is illustrated in Table 1.

Table 1

Comparison of the measurement accuracy of two cone-beam computed tomography scanners in different thicknesses of pink baseplate wax

Thickness (mm)	Measuring device	Mean (mm)	SD	Mean difference (mm)	P*
T <2	Digital caliper	1.34	0.15
	CBCT scanner 1	1.26	0.15	0.07	0.02
	CBCT scanner 2	1.33	0.16	0.004	0.9
3> T ≥2	Digital caliper	2.33	0.16
	CBCT scanner 1	2.23	0.14	0.10	0.001
	CBCT scanner 2	2.26	0.14	0.07	0.1
4> T ≥3	Digital caliper	3.42	0.16
	CBCT scanner 1	3.40	0.17	0.02	0.9
	CBCT scanner 2	3.34	0.17	0.08	0.01
6> T ≥5	Digital caliper	5.20	0.12
	CBCT scanner 1	5.14	0.15	0.06	0.05
	CBCT scanner 2	5.12	0.16	0.07	0.01

*P<0.05 considers statistically significant. SD – Standard deviation; CBCT – Cone-beam computed tomography; P – Probability value

Comparing the obtained measurements of digital caliper and CBCT in the anterior and posterior regions of the jaws in different thicknesses indicated no significant difference between CBCT and digital caliper measurements in the posterior region (P > 0.05) [Table 2].

Table 2

Measurement accuracy of cone-beam computed tomography in the anterior and posterior regions of the jaws in different thicknesses of pink baseplate wax

Thickness (mm)	Region	Measurement devise	Mean (mm)	SD	P*
T <2	Anterior	Digital caliper	1.37	0.17	0.01
		CBCT	1.27	0.16
	Posterior	Digital caliper	1.30	0.13	0.6
		CBCT	1.32	0.16
3> T ≥2	Anterior	Digital caliper	2.36	0.17	0.001
		CBCT	2.24	0.14
	Posterior	Digital caliper	2.30	0.14	0.08
		CBCT	2.25	0.14
4> T ≥3	Anterior	Digital caliper	3.46	0.18	0.002
		CBCT	3.33	0.16
	Posterior	Digital caliper	3.38	0.14	0.6
		CBCT	3.40	0.17
6> T ≥5	Anterior	Digital caliper	5.18	0.11	0.002
		CBCT	5.08	0.14
	Posterior	Digital caliper	5.22	0.14	0.2
		CBCT	5.18	0.16

*P<0.05 considers statistically significant. CBCT – Cone-beam computed tomography; SD – Standard deviation; P – Probability value

Variable results were obtained from a comparison between CBCT and digital caliper measurements in the lingual, buccal, and crestal regions of the jaws [Table 3].

Table 3

Measurement accuracy of cone-beam computed tomography in the lingual, buccal, and crestal regions of the jaws in different thicknesses of pink baseplate wax

Thickness (mm)	Region	Measurement devise	Mean (mm)	SD	P*
T <2	Lingual	Digital caliper	1.31	0.15	0.7
		CBCT	1.33	0.17
	Buccal	Digital caliper	1.43	0.15	0.005
		CBCT	1.30	0.16
	Crestal	Digital caliper	1.27	0.12	0.9
		CBCT	1.27	0.14
3> T ≥2	Lingual	Digital caliper	2.38	0.14	0.005
		CBCT	2.26	0.14
	Buccal	Digital caliper	2.29	0.14	0.1
		CBCT	2.23	0.14
	Crestal	Digital caliper	2.32	0.18	0.03
		CBCT	2.23	0.14
4 >T ≥3	Lingual	Digital caliper	3.46	0.15	0.2
		CBCT	3.41	0.16
	Buccal	Digital caliper	3.45	0.18	0.2
		CBCT	3.34	0.17
	Crestal	Digital caliper	3.36	0.14	0.8
		CBCT	3.35	0.17
6> T ≥5	Lingual	Digital caliper	5.19	0.13	0.1
		CBCT	5.13	0.17
	Buccal	Digital caliper	5.19	0.14	0.1
		CBCT	5.12	0.17
	Crestal	Digital caliper	5.22	0.10	0.04
		CBCT	5.14	0.13

*P<0.05 considers statistically significant. CBCT – Cone-beam computed tomography; SD – Standard deviation; P – Probability value

Comparing the obtained measurements of digital caliper and CBCT in the maxilla and mandible suggested that there was no significant difference between CBCT and digital caliper measurements for thicknesses < 2 mm, thicknesses between 3 and 4 mm, and thicknesses between 4 and 5 mm in both the jaws (P > 0.05), and the significant difference was found in the thickness between 2 and 3 mm in both the jaws (P < 0.05).

DISCUSSION

In the analysis of data obtained from digital caliper and CBCT measurements, no significant difference was observed between CBCT and digital caliper measurements in thickness <2 mm (P > 0.05 and mean difference = 0.03 mm), but the significant difference was seen for thicknesses >2 mm (P < 0.05). Mean difference between the CBCT and digital caliper devices was < 0.1 mm.

Mallikarjun et al. compared the difference of soft tissue measurements in the anterior of the maxilla, using radiovisiography and CBCT, and concluded that there was no significant difference between two methods (P > 0.05).[19] In their study, they did not use the reference to evaluate the measurement accuracy of two methods and only compared the obtained measurements from these two methods.

Chittineni Nirosha and Krishnajaneya et al. who examined the difference of measurements using probe and CBCT in assessing the relationship between the thickness of the alveolar bone and the thickness of its overlying soft tissue in the anterior mandible indicated that no significant difference was seen between the thicknesses of the soft and hard tissues in three areas (P < 0.05), and the findings were various. They concluded that CBCT is an accurate tool for measuring the soft tissue.[22] Their study has been a pilot one and requires a larger sample size to be evaluated.

Patcas et al. evaluated the accuracy of CBCT and CT in measuring hard tissue thickness and the accuracy of CBCT in measuring soft tissue thickness in comparing with the physical measurements using a digital caliper. It was shown that CBCT was accurate in measuring soft tissue (mean difference = 0.15 mm) and more accurate in measuring soft tissue than hard tissue (the limit of agreement was smaller for soft tissue than hard tissue). CBCT is also more accurate than CT scan in measuring hard tissue. In their study, the CBCT image quality was much better than CT images because of less metal artifact that affected the measurements of areas.[1] Similar to our study, CBCT measurement was considered accurate in 0.1 mm.

Fourie et al. assessed the accuracy of CBCT in soft tissue measurements. In this study, eleven sites were selected on the faces of seven cadavers; the thickness of these sites was measured using digital caliper, and then, these sites were measured using CBCT too. It was concluded that CBCT is very reliable for measuring soft tissue thickness in the facial region.

The absolute error and absolute error percentages were used to measure CBCT accuracy. Although they did not mention P value, the tables of their study illustrated that the accuracy of CBCT was 0.1 mm similar to our study.[11]

Lau et al. assessed the accuracy of CBCT in measuring the gingival thickness and its biotype. This study was carried out on the mandible of four pigs with high-resolution scan. No difference was found between direct physical and CBCT measurements (P > 0.05).[18] In our study, the same results were obtained for thickness < 2 mm. Furthermore, we use low-resolution protocol in scanning the skull.

Our study revealed that measurement accuracy was more in the posterior than anterior regions of the jaws (P > 0.05), which could be due to the cone-beam geometry of radiation.

In this study, it was found that CBCT measurements were accurate for all thicknesses (except thickness, 2–3 mm) in both the jaws (P > 0.05), indicating that different jaw arches have no effect on the accuracy of CBCT measurement.

Although various data were obtained by comparison of the measurement accuracy in the lingual, buccal, and crestal regions, these measurements were accurate in the range of 0.15 mm (mean difference ≤0.15 mm).

In the medical field, various modalities have been used to evaluate the soft tissue, and the best modality is the magnetic resonance imaging. Further, CT scan imaging, in addition to hard tissue evaluation, is also used for the soft tissue evaluation, and in this modality, the various tissues of the body with different tissue contrasts can be seen. Today, with the extensive use of CBCT in the maxillofacial area, especially in hard tissue measurement with high accuracy and lower radiation dose compared to CT scans, soft tissue along with hard tissue can be assessed although CBCT has lower contrast resolution than CT scan.

In hard tissue studies, especially in evaluating and measuring the area for implant, CBCT has high accuracy of 1 mm.[4] The ideal thickness of the soft tissue graft during periodontal surgery is between 1 and 1.5 mm.[7] It is noteworthy that in the NNT software, the measurement accuracy is at 0.1 mm. Therefore, considering the statistical analysis, software characteristics of CBCT system, and clinical properties of soft tissue, in the current study, CBCT measurement accuracy is 0.1 mm and measurements < 0.1 mm are not clinically important.

CONCLUSIONS

Statistically, the difference between digital caliper measurements as a golden standard and CBCT measurements was < 0.1 mm, but this difference is not clinically important. The soft tissue measurement is accurate using CBCT with an accuracy of 0.1 mm.

Financial support and sponsorship

This article presented the results of a research project in Babol University of Medical Sciences (by Grant No: 9645524).

Conflicts of interest

There are no conflicts of interest.