Measurement and Morphological Analysis of Mandibular Growth Using Computer-Aided Radiological Approaches
Date Issued
2014
Date
2014
Author(s)
Lin, Hsien-shu
Abstract
Background:
Mandibular growth patterns play a very important role in the foundation of modern dentistry. Orthodontists and oral surgeons are especially interested in this topic. For many years, the earliest scholars used a measurement method based on two-dimensional radiographic images to record mandibular growth. However, this method could not provide a complete picture of three-dimensional bone mass because it was limited to only a two-dimensional plane view.
With advances in technology, three-dimensional computer tomography has been developed and used in the study of mandibular growth. However, little related research on mandibular growth has been conducted because of its high costs and high radiation doses. In recent years, improved cone beam computed tomography (CBCT) has been developed, eliminating the problems with computer tomography (CT). Therefore, many scholars have begun to use CBCT to record and analyze mandibular growth.
In using radiological imaging technology to investigate mandibular growth, the most important factor is the repeatability of the identification of anatomical marker points. In contrast, two-dimensional radiography entails a problem with head-positioning. In three-dimensional space, any slight deviation in the X, Y, or Z axis is bound to affect interpretation of the images.
Therefore, when using two-dimensional images in the absolute standard position and in the offset position, it is necessary to further investigate the repeatability of identification of anatomical marker points. Once this problem is clarified, it can be done accurately in two-dimensional or three-dimensional images to provide accuracy in morphological measurement.
Since medical ethics do not permit long-term radioactive irradiation of humans to obtain mandibular radiographic images, animal models must be used. Previous scientific research has determined that the miniature pig’s mandible patterns, chewing patterns, and bone metabolic rate are very similar to those of humans. Therefore, in this study, Lee Sung strain miniature pigs were used as the experimental animal for the observation of mandibular growth in this study.
This study used computer-assisted radiological imaging techniques to measure mandibular growth and conduct morphological analysis for the following research purposes:
(1) to assess the repeatability of the morphological parameters of human mandibles between measurements and those measurements among different measurement cycles under various mandibular conditions of the absolute standard, random offset angle and certain angle of rotation using CBCT.
(2) to use CBCT to collect measurements of mandibular growth in miniature pigs over the long term and thereby create a basic database for use in further study,
(3) to study the continuity of the geometric changes of the mini pig’s mandibles, and
(4) to compare the measurement error of morphological parameters between
three-dimensional CBCT and the synthetic two-dimensional CBCT images of mandibles.
Materials and Methods:
The experimental apparatus used in this study was a dental cone beam computed tomography (CBCT) system. In the first part of the study, CBCT data of twelve mandibles were obtained and used to generate synthetic cephalograms via a digitally reconstructed radiography (DRR) technique. Eleven landmarks describing the key morphological features of the mandible on each DRR-synthesized cephalogram were identified manually 6 times by one senior and one junior dentist in the neutral and randomized rotated positions. The procedure was repeated 5 days later. Twelve parameters based on inter-landmark line segments and their angles were calculated. Test-retest reliability was assessed in terms of intra-class correlation coefficient (ICC) and coefficient of variation (CV) using a two-way mixed-effects model. The paired-sample t-test was used to compare differences between examiners and sessions. A one-sample t-test was employed to assess whether differences between the examiners were significantly different from zero.
Eight Lee-Sung strain miniature pigs were chosen in the second part of the study. The mandibles of each of the pigs were scanned with CBCT 12 times, once every 4 weeks beginning at the age of four weeks, to accumulate a total of twelve sets of CT data. Each of the CT data sets was used to reconstruct a 3D model of the mandible. In total, 17 anatomical landmarks on the mandible were marked as morphological measurement parameters by an experienced dentist. Line segments were then defined for selected pairs of markers and were used to measure the growth patterns of the miniature pigs'' mandibles and build up baseline data of mandibular growth.
The third part was adapted from the three-dimensional model of the second part. A
new method was developed to search for corresponding points on two consecutive
models with the highest likelihood of the anatomical and morphological features.
The growth rate patterns of the mandibles for each month were described using color
maps on the models over the monitoring period.
In the fourth part of the study, mandibles of six miniature pigs were scanned with CBCT at one-month intervals for 12 months, and the data were used to reconstruct 3D bone models. The DRR technique was used to generate simulated 2D cephalograms. Five anatomical landmarks were identified on each bone model, and the inter-marker distances, monthly distance changes, and their errors were calculated with the 3D measurements as the gold standard. Differences in the variables measured using the 2D and 3D methods were compared using a paired t-test.
Results:
The results indicated very good intra-rater (senior: ICC>0.93; junior: ICC=0.78 for CdP-GOP, ICC>0.91 for other parameters), inter-rater (ICC=0.62 for CdP-GOP, ICC>0.84 for other parameters), and inter-session reliability (ICC>0.84 for all parameters and examiners; ICC=0.74 for CdP-GOP for junior examiner) in measuring mandibular morphological parameters in the neutral position. These results suggest that very good reliability can be achieved via manual identification of the anatomical landmarks, without the effects of factors such as malpositioning of the head during imaging. The results for reliability of the randomized rotated position were not quite as high, especially in the trials measured by the junior dentist. This difference can be explained by the malpositioning of the head during imaging, which indeed affected identification of anatomical landmarks and thus the accuracy of morphological measurements.
In the study of long-term miniature pigs’ mandibular growth patterns, the baseline data of the mandibular growth has been successfully established. The mandibular volume increased nonlinearly with time, growing rapidly during the first five months and more slowly in subsequent months. The growth of the mandible was found to be anisotropic and non-homogeneous within the bone and non-linear over time, with faster growth in the ramus than in the body. These growth patterns appeared to be related to the development of dentition, providing necessary space for the teeth to grow upward for occlusion and for the posterior teeth to erupt.
This is the first study to track the continuous morphological changes of the mandible in miniature pigs during growth in the first 12 months in three dimensions with continuous color maps over the surface of the mandible. This was achieved by integrating CBCT and the new analytical approach, which quantifies the nonlinear growth patterns and the nonlinear rate of their changes in different growth regions and the whole mandible.
Comparison and analysis of measurement errors between the three-dimensional images and two-dimensional images revealed significant errors in measurements using 2D imaging. The mandibular dimensions, their monthly changes in the early stages of growth, and total annual growth were underestimated. These results should be considered in dental treatment planning at the beginning of treatment in order to control more precisely the treatment process and outcome.
Subjects
牙科錐型電腦斷層
顎顏面發育
下顎骨生長
迷你豬
模擬的放射影像
下顎骨
信賴度
敏感度分析
測顱攝影片
組內相關係數分析
Type
thesis
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