Determination of reference values for normal cranial morphology by using mid-sagittal vector analysis in Japanese children

Summary

Mid-Sagittal Vector Analysis (MSVA) is a method of measuring the distance from a defined central point on the skull surface in the entire mid-sagittal plane and provides a clear description of the lateral view of the skull. We used a series of images of normal skulls of Japanese children to determine normal MSVA values.

For this cross-sectional study, we first constructed a database of head CT and MRI images of children aged 0–6 years (41.5 ± 24.9 month (mean ± SD)) who showed no abnormality of cranial development and growth at the time of imaging. Measurement errors due to lateral shifting of the sagittal plane during MSVA were examined, CT and MRI images taken in the same patients at the same time were compared, and measurement differences were examined. Finally, MSVA was carried out, and the mean of the measured values was calculated according to age group.

Two hundred ninety-five images were included in the database. When the lateral shifting of the sagittal plane was within 4 mm from the true mid-sagittal plane, the mean errors were less than 1 mm at all measurement points. Between the CT and MRI images from the same patients, most differences in MSVA values were within ±1 mm. These differences were thus acceptable for use in clinical settings. After the above verifications, 220 images were extracted for determination of normal MSVA values. We established a normal dataset of MSVA for Japanese children that can be used effectively for preoperative diagnosis, surgery planning, and postoperative assessment of cranial deformities.

Introduction

Normalization of cranial morphology is an important goal in the surgical treatment of craniosynostosis.1, 2 To achieve this goal, establishing an evaluation method and a normal dataset of cranial morphology is necessary for comparisons during preoperative assessments, surgery planning, and evaluation of therapeutic efficacy. In addition, the evaluation method should allow for detailed description of morphology, and measurements should be easy to perform.

The cephalic index (CI) is an example of a widely used index, describing the percentage of the ratio of the maximum width of the skull to the occipitofrontal length; normal ranges of CI values have been established for different ethnicities.³ The CI has been used in a number of publications for the evaluation of therapeutic efficacy in the treatment of cranial deformities.2, 4, 5 It can however not be used for complex morphological abnormalities, and only allows for assessment of one horizontal plane; therefore, the CI alone is not enough to evaluate cranial morphology.6, 7

In contrast, several morphological evaluation methods using measurements conducted on three-dimensional images have recently been reported, and have made it possible to examine cranial morphology in a highly detailed manner.7, 8, 9, 10, 11, 12

Three-dimensional image analysis provides a detailed and accurate description of cranial morphology, but because it requires the use of expensive dedicated software and a specific workstation, it may take a long time for this method to be widely used.

A method evaluating cranial morphology in detail without an expensive workstation is thus needed. In this article, we focused on the Mid-Sagittal Vector Analysis (MSVA) that Marcus et al. have previously reported.⁶ MSVA is a two-dimensional morphological analysis for the mid-sagittal plane, and was developed to address inadequacies of CI while maintaining clinical applicability. This is performed by measuring each distance from the top of the dorsum sella summit to the skull surface, on radial lines in 10-degree increments, originated from the top of the dorsum sella summit on the mid-sagittal plane. MSVA can clearly represent the lateral morphology of the skull, and can be performed relatively easily without an expensive device.

Few reports have examined normal reference data on cranial morphology in Japanese children,3, 13 and particularly the normal morphology of the lateral view remains unknown. The main purpose of our study was therefore to determine normal MSVA values in Japanese children.

Additionally, we included cases with not only CT but also MRI scans available for analysis, in order to collect as many images as possible. We therefore provide auxiliary verification of the hypothesis that MSVA can achieve clinically acceptable results on the mid-sagittal plane, even when shifted, and in the sagittal plane on MRI images.