Cone beam computed tomography analysis of the relationship between the morphology of the spheno-occipital synchondrosis and the prevalence of skeletal malocclusions: A cross-sectional observational study

INTRODUCTION

The dynamic mechanism of the skull bones can be better understood by examining the skeletal and dental patterns in relation to the joints and neuromuscular complex.^[1] Craniofacial sutural movement is influenced by the biodynamic forces acting upon the craniofacial complex, directing the displacement of bones connected through their articular action. Consequently, these skeletal sutures function as hinges for the connecting bones.^[2]

The neurocranial base consists of a series of connections established between the frontal, ethmoid, sphenoid, and occipital bones through sutures in each bone. These sutures deals with the variation of the mechanical strength of the skull bones, preserving the entire skull base harmony.^[2] Among the skeletal sutures, the parts with a very important joint action are the lower parts of the pyramidal base of the temporal bone and the spheno-occipital synchondrosis (SOS).^[2] The force generated by the occlusal function from the action of the masticatory muscles is transmitted to the temporal bone through the temporomandibular joint. An unnatural interaction during the development period may impair the harmony of movement between these bones.^[3]

Coro et al. and Tanaka and Sato^[4,5] affirm that there is a relationship between the inclination and the height of the maxillary posterior occlusal plane and the mandibular position, consistent with the etiology of the different dentoskeletal patterns. An alteration in vertical dimension in addition to an inadequate inclination of the posterior occlusal plane may be related to the development of skeletal malocclusions.

The prediction of growth is a primary goal in craniofacial biology and a major concern in orthodontics, as it is the key point in the diagnosis, prevention, interception, and treatment of malocclusions. Given the limited scientific evidence regarding the influence of craniofacial base structures on growth, the objective of this study was to evaluate the relationship between the morphology of the SOS and the prevalence of anteroposterior skeletal discrepancies in growing patients.

MATERIAL AND METHODS

Variables analyzed, data sources, and measurements

Digital imaging and communications in medicine data were obtained using a Kodak 9500 Cone Beam 3D system (90 kW, full field of view: 200× 184 mm, 0.3 mm and 2–15 mA voxel resolution; Kodak) imported and rendered with Dolphin Imaging software (Patterson Dental Supply, Inc., Mendota Heights, Minnesota). CBCT examinations were performed with the patients standing, with their heads positioned at the Frankfort horizontal plane.

All exams were anonymized and assigned identification numbers in a random sequence.

After positioning the SOS in a sagittal view, posterior sphenoid and posterior occipital points were demarcated three-dimensionally through the software, and a line was drawn joining the two points, demarcating the uppermost part of the spheno-occipital synchondrosis (USOS). The anterior sphenoid and anterior occipital points were demarcated three-dimensionally, and a linear line was drawn joining the two points, also demarcating the lowermost part of the spheno-occipital synchondrosis (LSOS) [Figure 1].

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Figure 1:

Tomographic sagittal section of the spheno-occipital synchondrosis. Demarcation of points of interest for calculation of the ΔSO. PS: Posterior sphenoid, PO: Posterior occipital, AS: Anterior sphenoid, AO: Anterior occipital, USO: Upper spheno- occipital, LSO: Lower spheno-occipital, ΔSO: Spheno-occipital delta.

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The spheno-occipital delta (ΔSO) was defined as the difference between the USO value and the LSO value. Table 1 details all the points and measurements used.

Table 1: Definition of points and measurements.

Abbreviation	Description
Posterior Sphenoid	Most posterior point of the upper limit of the spheno- occipital synchondrosis in contact with the sphenoid bone
Posterior occipital	Most posterior point of the upper limit of the spheno- occipital synchondrosis in contact with the occipital bone
Anterior sphenoid	Most anterior point of the lower limit of the spheno- occipital synchondrosis in contact with the sphenoid bone
Anterior occipital	Most anterior point of the lower limit of the spheno- occipital synchondrosis in contact with the occipital bone
Upper spheno- occipital (USO)	Distance in millimeters from the upper limit of the spheno-occipital synchondrosis
Lower spheno- occipital (LSO)	Distance in millimeters from the lower limit of the spheno-occipital synchondrosis
Spheno-occipital delta (ΔSO)	Difference resulting from subtracting the distance values from the upper limit by the lower limit of the spheno-occipital synchondrosis (ΔSO=USO-LSO)

These reference points were selected in the three-dimensional volume, reconstructed, and refined in the axial, coronal, and sagittal slices using the software’s slice locator. A blinded operator, previously calibrated, selected the anatomical points and performed the cephalometric analysis.

RESULTS

Descriptive data

The ICC was 0.93 between repeated transversal linear and angular measurements (P < 0.01), which is classified as excellent reproducibility.

The measured values were Class I APDI 81.27 ± 2.01 (°) and ΔSO 0.25 ± 1.08 (mm), Class II APDI 73.89 ± 2.64 (°) and

ΔSO −0.75 ± 0.56 (mm), and Class III APDI 87.22 ±1.37 (°) and ΔSO 0.94 ± 0.80 (mm). The descriptive analysis of the sample is presented in Table 2.

Table 2: Descriptive analysis of the sample and division of participants into groups by Kim’s APDI measure.

Classification	n	Mean	95% confidence interval
			Lower	Upper	Standard deviation
APDI (°)
Class I	106	81.27	80.88	81.66	2.01
Class II	95	73.89	73.36	74.43	2.64
Class III	54	87.22	86.84	87.59	1.37
ΔSO (mm)
Class I	106	0.25	0.04	0.45	1.08
Class II	95	−0.75	−0.87	−0.64	0.56
Class III	54	0.94	0.73	1.16	0.80

The CI of the mean assumes that sample means follow a t-distribution with n - 1 degrees of freedom. APDI: Anteroposterior dysplasia indicator, ΔSO: Spheno-occipital delta.

Box-plot graphs were created for a visual analysis of both variables [Figures 2 and 3].

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Figure 2:

Box-plot analysis of the distribution of the dependent variable (APDI) between groups. APDI: Anteroposterior dysplasia indicator.

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Figure 3:

Box-plot analysis of the distribution of the predictor variable (ΔSO) between the groups. ΔSO: Spheno-occipital delta.

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DISCUSSION

The study suggests that during the developmental phase, between 6 and 13 years of age, the morphology of spheno- occipital synchondrosis is related to the facial characteristics of Class I, II, and III malocclusions, but we cannot say that it is a causal factor and a question remains: What happens first?

Does the morphology of the spheno-occipital suture adapt to dentofacial discrepancy or does this morphology directly influence the development of skull structures during growth , resulting in dentofacial changes?

According to Blum,^[1] SOS has balanced characteristics in relation to the anatomical space in which it is found when other cranial structures have a dynamic functional development that may adapt to the occlusion of the patient. When growth begins to show some variations, it is possible to identify some signs in its morphology.

An increased ΔSO indicates a flexion of the SOS, which, according to several authors,^[4,5,10,11] can cause greater vertical growth of the posterior region of the naso-maxillary complex, resulting in morphological characteristics of patients with Class III malocclusions. In contrast, a reduced ΔSO represents an extension of the SOS, which can cause less vertical growth of the posterior region of the naso-maxillary complex, resulting in morphological characteristics of patients with Class II malocclusions [Figure 4].

Costa et al.^[12] proposed that when the sphenoid bone flexes or extends, this force is transmitted by the vomer to the maxillary process and palatine, increasing or decreasing, respectively, the eruption force of the posterior upper teeth, thus establishing the height of the vertical dimension of the posterior occlusal plane and making the mandible establish its position through the adaptation process.

A previous study by Basili et al.^[13] observed that children with flatter cranial bases had a slightly shorter posterior base region, and the mandibular condyles were located farther back and upward, indicating a strong tendency toward Class II. The condyles of the groups with closed cranial base angles were more forward and downward, but none of the children had Class III occlusions, and most of them were normal or Class I, which suggests that changes in the vertical dimension and in the occlusal plane greatly influence the growth of the mandible, which consequently may help to establish Class I occlusion during growth and development. The relationship between skull base flexion and skeletal pattern of the jaws appears to be established before the age of 5 years.^[14,15]

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Figure 4:

Representative 3D reconstructions and schematic illustrations of the spheno-occipital synchondrosis in skeletal Class I (balanced), Class II (extended), and Class III (flexed) patterns. The double-headed red arrows indicate the linear measurements (in mm) of the anterior and posterior widths of the synchondrosis.

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In contrast, Thiesen et al.^[16] stated in their study that there was no difference between the mean values of the skull base deflection angle (SNBa) in the different facial patterns (I, II, III), although they found a statistically significant difference for the mean values of the posterior skull base (S-Ba) for the Pattern III group. In this group, the size of the posterior skull base was reduced when compared to the patterns I and II groups.

This study used a robust sample comprised of CBCT scans, which allowed for an isolated and accurate analysis of the spheno-occipital suture. The R² value (0.32) makes it clear that this is not the only factor related to the development of dentofacial discrepancies but emphasizes that it is a point to be considered, especially due to the adaptation capacity of the joints during the period of growth. The β value obtained allowed us to quantify how much the predictor variable (x = ΔSO) influences the dependent variable (y = APDI). Using the linear regression equation (y = α+βx), you can estimate how balanced this relationship is or not. Hypothetically, a patient with a APDI^[8] (81.4 ± 3.79º) may have a balanced ΔSO indicating Class I dentofacial development or an unbalanced ΔSO, suggestive of Class II development for negative values and Class III for positive values.

It is essential for understanding malocclusion, as well as developing a treatment plan, to identify the etiology. If the etiology is not understood, treatment may be unpredictable and unstable. Understanding that the dynamic relationship between the bones at the base of the skull may influence mandibular position is useful in the diagnosis and treatment of malocclusions.

CONCLUSION

Flexion or extension of the spheno-occipital synchondrosis (ΔSOS) is related to the prevalence of dentofacial discrepancies during the growth and development phases (P < 0.001) but is not the only variable involved (R² = 0.32). Class I, II, and III patients presented ΔSO 0.25 ± 1.08 (mm), -0.75 ± 0.56 (mm), and 0.94 ± 0.80 (mm), respectively. ΔSO values close to zero suggest balanced dentofacial development, while negative values suggest an imbalance for Class II alterations and positive values for Class III.