A 3D cephalometric protocol for the accurate quantification of the craniofacial symmetry and facial growth

Abstract

Background: Cephalometric analysis is used to evaluate facial growth, to study the anatomical relationships within the face. Cephalometric assessment is based on 2D radiographic images, either the sagittal or coronal planes and is an inherently inaccurate methodology. The wide availability of 3D imaging techniques, such as computed tomography and magnetic resonance imaging make routine 3D analysis of facial morphology feasible. 3D cephalometry may not only provide a more accurate quantification of the craniofacial morphology and longitudinal growth, but also the differentiation of subtle changes in occlusion. However, a reliable protocol for the computation of craniofacial symmetry and quantification of craniofacial morphology is still a topic of extensive research. Here, a protocol for 3D cephalometric analysis for both the identification of the natural head position (NHP) and the accurate quantification of facial growth and facial asymmetry is proposed and evaluated. A phantom study was conducted to assess the performance of the protocol and to quantify the ability to repeatedly and reliably align skulls with the NHP and quantify the degree of accuracy with which facial growth and facial asymmetry can be measured.

Results: The results obtained show that the protocol allows consistent alignment with the NHP, with an overall average error (and standard deviation) of just 0.17 (9.10e-6) mm, with variations of 0.21 (2.77e-17) mm in the frontonasal suture and 0.30 (5.55e-17) mm in the most prominent point in the chin. The average errors associated with simulated facial growth ranged from 1.83 to 3.75% for 2 years' growth and from - 9.57 to 14.69% for 4 years, while the error in the quantification of facial asymmetry ranged from - 11.38 to 9.31%.

Conclusions: The protocol for 3D skull alignment produces accurate and landmark free estimation of the true symmetry of the head. It allows a reliable alignment of the skull in the NHP independently of user-defined landmarks, as well as an accurate quantification of facial growth and asymmetry.

Keywords: Cephalometry; Craniofacial morphology; Facial growth; Facial symmetry; Phantom study.

Fig. 1

Diagram showing the main steps considered during testing of the new cephalometric protocol, in which (a) a raw model is aligned according to the natural head position (NHP) by a clinical expert to define (b) the gold standard model; next (c) a randomly generated rigid transformation or (d) mesh warping together with a random rigid transformation is applied to the gold standard model to produce several (e) target and (f) warped models, respectively; finally in (g) and (h) the craniofacial symmetry procedures are used to recover the ideal anatomical alignment (i)

Fig. 2

(a) average craniofacial growth between 12 to 15 years (adapted from [13]) with the main cephalometric landmarks used in craniofacial morphology assessment, and (b) ideal gold standard and target model alignment with theoretical 2-year (blue) and 4-year (purple) facial growth geometries

Fig. 3

Modelling of facial asymmetry with: a protrusion of the midface, b unilateral protrusion of the face and (c) lateral displacement of the mandible (gold standard model in grey and asymmetric models in blue)

Fig. 4

Recovering of the symmetry plane after: a coronal and sagittal alignment between the gold standard (grey) and target (blue) models in the presence of the full skull data, and (b) coronal and sagittal alignment of both gold standard (grey) and target (blue) in the presence of partial skull data

Fig. 5

Final craniofacial time-series alignment: a after random transformation and realignment with full skull data, and (b) final alignment of the craniofacial time-series with partial skull data

Fig. 6

Final alignment between the gold standard and target model with: a protrusion of the midface, b unilateral protrusion of the face, and (c) lateral displacement of the mandible (gold standard model in grey and facial asymmetric model in purple)