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The purpose of this research was to compare the reliability of two different methods for cranial midline localization through cephalometric analysis of mandibular condyle asymmetries. A retrospective cohort study was performed analyzing consecutively the SMV radiograms of 47 patients undergoing oral surgery before orthodontic treatment at the Dental School, University of Trieste (Italy) from 2003 to 2008. Two different cephalometric analyses were used to identify the basicranium midline (Tracing 1: initial landmarks = craniostat ear rods; Tracing 2: initial landmarks = spinosum foramina), and the left/right symmetry ratio (SR) for four parameters (condylar length, condylar angle, intra-condylar hemidistance, extra-condylar hemidistance) was calculated. The main result showed that no significant statistical difference between the SRs of the intra-condylar and extra-condylar hemidistance obtained with the same tracing was found (

Without considering major facial deformities typically associated with various syndromes, a small degree of craniofacial asymmetry is always present in all subjects with normal biometric parameters, although this asymmetry is rarely appreciable and is often unnoticed [

Craniofacial asymmetry is often a cause of major diagnostic difficulties in orthodontics. Diagnosis of asymmetry and of its localization could be essential for planning treatment and evaluating the results of orthognathodontics or maxillofacial surgery [

Asymmetry of the craniofacial complex can be evaluated only with appropriate radiological projections and cephalometric measurements on posteroanterior and submentovertex (SMV) radiograms or by using three-dimensional (3D) computed tomography (CT) imaging [

Furthermore, prior to combined orthodontic/orthognatic surgery, radiological images are recommended to evaluate if there were preexisting temporomandibular pathologies in the patients to identify or prevent temporomandibular disorders that could heavily influence the postsurgical function. Altered anatomical condylar position and bone degeneration (osteoarthrosis) are often associated with the angle jaw discrepancies/malocclusion [

Although magnetic resonance represents the “gold standard” in TMD diagnosis [

Ritucci and Burstone and Nahoum et al. [

The system of cephalometric coordinates to evaluate the symmetry of bone structures of splanchno- and neurocranium takes as its main reference the sagittal symmetry axis, whose identification has been examined by numerous authors over the last thirty years [

Cheney proposed a midsagittal plane passing through the nasion and the anterior nasal spine considering that this plane crosses the prosthion and the menton in subjects with a symmetrical face [

More recently, in a 3D CT study, Katsumata et al. [

Although the key to evaluating asymmetries is defining the criteria to determine the cranial midline, the existence and utilization of different procedures to identify the ideal midline underlines that no clear, universally accepted, method currently exists for the evaluation of craniofacial asymmetries. Furthermore, until now no study exists evaluating the comparison between the utilization of ear rods and spinosum foramina as starting points for drawing basicranium midline on SMV radiograms.

The aim of the study was to compare the validity and reliability of two different methods for localization of the cranial midline through a cephalometric analysis of asymmetries of the mandibular condyles in a representative adult population.

The hypothesis that the spinosum foramina represent the most reliable starting points for tracing the coordinate system to identify transverse craniofacial asymmetries on SMV radiograms, was tested.

This retrospective cohort study was performed by analyzing the SMV radiograms of 47 patients (26 females and 21 males; age range: 21–56 years; mean age

Cranial radiographs in SMV projection were obtained by the same operator (M.M.) with an Axial Tome EX II unit (Axial Tome Corporation, San Carlos, CA, USA). The choice of a single operator responded to the need to reduce inter-operator bias during the positioning of the patient and the insertion of the ear rods. Radiograms were performed with the following technique: ear rods were positioned and each patient was asked to rotate the head posteriorly until the Frankfurt plane became parallel to the film cassette [

Scheme of the submentovertex radiograms and corresponding digital acquisition. (A) right transverse condylar width, (B) right condylar horizontal angle, (C) left transverse condylar width, (D) left condylar horizontal angle, (E) neck of the right condyle, (F) neck of the left condyle, (G) intracondylar distance, (H) extracondylar distance, (K) midline (orthogonal to the transporionic axis).

The anatomic landmarks used in this study were extrapolated from the SMV analysis developed by Lew and Tay [

(a-b) Anatomic landmarks and reference planes used in submentovertex cephalometric analysis (Tracing 1). TPA, transporionic axis: line passing through the left and right tip of ear rods corresponding to the line passing through the midpoint of external auditory meatus (LM, left Mei; RM, right Mei); MP, midsagittal axis: perpendicular bisecting TPA; RCoL, right condylion lateralis: most lateral aspect of right condyle; RCoM, condylion medialis: most medial aspect of right condyle; LCoL, left condylion lateralis: most lateral aspect of left condyle; LCoM, left condylion medialis: most medial aspect of left condyle; RCoL-MP, right condylion lateralis-midline: distance from right L-point to MP; RCoM-MP: right condylion medialis-midline: distance from right M-point to MP; LCoL-MP, left condylion lateralis-midline: distance from left L-point to MP; LCoM-MP, left condylion medialis-midline: distance from left M-point to MP.

(a-b) Anatomic landmarks and reference planes used in submentovertex cephalometric analysis (Tracing 2). SPR, right foramen spinosum: geometric centre of right foramen spinosum; SPL, left foramen spinosum: geometric centre of left foramen spinosum; TSA, transspinosum axis: line passing through the geometric centre of right and left spinosum points; MSP, midsagittal axis: perpendicular bisecting TSA; RCoL, right condylion lateralis: most lateral aspect of right condyle; RCoM, condylion medialis: most medial aspect of right condyle; LCoL, left condylion lateralis: most lateral aspect of left condyle; LCoM, left condylion medialis: most medial aspect of left condyle; RCoL-MSP, right condylion lateralis-midline: distance from right L-point to MSP; RCoM-MSP: right condylion medialis-midline: distance from right M-point to MSP; LCoL-MSP, left condylion lateralis-midline: distance from left L-point to MSP; LCoM-MSP, left condylion medialis-midline: distance from left M-point to MSP.

A single operator (F.C.) performed a digital tracing of each radiogram for five times with the method described below. The resulting mean values of lengths and angles were calculated and considered for statistical analysis to reduce the measurement error.

Two different cephalometric analyses were chosen to trace the basicranium midline. The first (Tracing 1) considers the craniostat ear rods as initial landmarks. The straight line connecting the tip of the left and right ear rods passes through the left and right midpoint (or left and right mei—LM and RM) of the external acustic meatus (transporionic axis, TPA) [

The outlines of the mandibular condyles were traced on each radiogram, and the medial and lateral poles were identified (RCoL, right condylion lateralis: most lateral aspect of right condyle; RCoM, condylion medialis: most medial aspect of right condyle; LCoL, left condylion lateralis: most lateral aspect of left condyle; LCoM, left condylion medialis: most medial aspect of left condyle).

Quantification of the condylar asymmetry was performed using eight parameters:

left and right condylar width,

left and right condylar angle (the horizontal condylar angle is the angle formed by the straight line passing through the condylar poles and the straight line perpendicular to the midline) [

intracondylar hemidistance defined as the distance from LCoM and RCoM to MP (Tracing 1) or MSP (Tracing 2),

extracondylar hemidistance defined as distance from LCoL and RCoL to MP (Tracing 1) or MSP (Tracing 2).

The anatomic landmarks and reference planes used are shown in Figures

On each radiogram, the left/right symmetry ratio (SR) was calculated for Tracing 1 and Tracing 2 with this simple formula:

The left-side measurement was used as a reference. A

All statistical analyses were performed with the SPSS software package (Statistical Package for Social Sciences, Windows 98, version 10.0, SPSS, Chicago, Ill) using the Student’s

Table

Symmetry ratio of the variables analyzed.

Tracing 1 | Mean | St Dev | Left/right symmetry ratio (SR) |
---|---|---|---|

Left condylar width (mm) | 22.1 | 3.7 | |

Right condylar width (mm) | 22.5 | 3.9 | |

Horizontal left | 21.5 | 7.9 | |

Horizontal right | 22.9 | 7.2 | |

LCoM-MP (mm) | 50.1 | 4.9 | |

RCoM-MP (mm) | 47.3 | 4.4 | |

LCoL-MP (mm) | 71.2 | 6.8 | |

RCoL-MP (mm) | 68.4 | 5.9 | |

Tracing 2 | Mean | St Dev | Left/right symmetry ratio (SR) |

Left condylar width (mm) | 22.1 | 3.7 | |

Right condylar width (mm) | 22.5 | 3.9 | |

Horizontal left | 20.9 | 7.5 | |

Horizontal right | 22.8 | 6.6 | |

LCoM-MSP (mm) | 48.5 | 4.7 | |

RCoM-MSP (mm) | 48.8 | 4.4 | |

LCoL-MSP (mm) | 69.1 | 5.6 | |

RCoL-MSP (mm) | 70.3 | 4.5 |

The mean values of the condylar widths and angulations fell within the physiological range [

SRs were

Statistical analysis did not reveal any significant difference in the comparison of the SRs of the intracondylar and extracondylar hemidistance using the same tracing (Tracing 1:

Statistical analysis comparing the differences in the symmetry ratio using Tracing 1 and Tracing 2.

Left/right | Tracing 1 | Tracing 2 |
---|---|---|

Intracondylar | LCoM-MP/RCoM-MP: ^{a, c} | LcoM-MSP/RCoM-MSP: ^{b, c} |

Extracondylar | LCoL-MP/RCoL-MP: ^{a, d} | LCoL-MSP/RCoL-MSP: ^{b, d} |

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Table

Scheme of the distances between the ear rods and the spinosum foramina for the five control cases.

control cases (years) | Ear rods distance (mm) | Spinosum foramina distance (mm) |
---|---|---|

BC 18 | ||

BC 21 | ||

GF 4 | ||

GF 5 | ||

SC 25 | ||

SC 28 | ||

PC 8 | ||

PC 11 | ||

KM 5 | ||

KM 8 |

As reported by Haraguchi et al. [

In this context, the SMV radiographic technique represents a useful method to examine the cranial base and to evaluate the rate of asymmetry of the anatomic structures in the axial plane [

The analysis of asymmetries requires that all anatomic parameters have to be compared to a symmetry axis (or midline) that is established using stable anatomic references. Williamson et al. [

Our study population was composed of 47 adult subjects needing orthodontic treatment. The sample comprised 26 females and 21 males considering that no gender-associated difference in craniofacial asymmetry has been reported in the adult population [

Statistical analysis revealed the substantial equivalence and reliability of the two tracing methods for performing a cephalometric analysis in a representative population. This reliability results from the fact that the left/right discrepancy for Tracing 1 and 2 is not statistically significant (Table

Our data indicate that some parameters may be more pronounced on the left or the right side, but that the side of prevalence of the same parameter can change if a different midline is established (Table

Comparing the SR of the intra and extracondylar hemidistances, the statistical analysis showed a lower SR for Tracing 2 (

The SR calculated using the MSP agrees with data obtained in a study on dry skulls performed by Marmary et al. [

The SMV images of the five control cases allowed us to verify the impact of successful therapy on the modification of the radiographic landmarks used to trace the symmetry axis. The distances between spinosum foramina and ear rods were identified on the radiograms to evaluate their changes over time. The repeatability of the cephalometric measurements before and after therapy was ensured by the same angle of incidence of the X-ray beam on the film thanks to the craniostat that maintains the position of the head unaltered. This projection allows also appreciation of the minimal variations in condylar dimensions and the relationship between mandibular condyles and basicranium [

In the three growing subjects, the millimetric increase of the absolute values between ear rods, corresponding to the distance between the external acoustic meatuses, is clearly evident. This change is the direct consequence of physiologic development: the advancing of the temporal bones and the new orientation of the glenoid cavities directly influence the position of the temporomandibular joint and acoustic meatus.

In all five cases, the distance between spinosum foramina remained unvaried (Table

As found by Moss and Salentijn [

Williamson et al. [

Consequently, if the analysis of the condylar asymmetries is performed in growing subjects, utilization of anatomic references such as the neurovascular foramina seems to guarantee a lower error than nonfixed references. This hypothesis needs to be confirmed by a larger cases series to test its statistical significance and understand whether this margin of error is clinically relevant for the precise quantification of craniofacial asymmetries.

One of the limitations of this retrospective analysis is that it lacks the comparison with 3D images although van Vlijmen et al. [

Submentovertex radiograms can provide assistance in diagnosing condylar asymmetries and planning the most appropriate treatment; furthermore, the reliability of this examination allows assessment of the anatomic variations induced by the orthognathodontic or surgical therapy [

The extent of the asymmetry can be quantified by using as a reference the midline, which should be as much as possible superimposable to the ideal midsagittal axis and not change during cranial development.

The results of this study validate our hypothesis indicating that the midline traced using the spinosum foramina as references more closely approximates the ideal midsagittal axis and represents the most reliable line to trace the coordinate system for identifying craniofacial asymmetry during cranial development on submentovertex radiograms.