Measurement of tooth size and shape in subjects with hypodontia and a control group using a new image analysis technique
The literature referred to here provided information concerning the epidemiology of hypodontia and its association with other developmental anomalies. Current research with the major developments in molecular biology has focused on the genetic component of the aetiology mainly on animals. Limited information has been gained from human studies concerning the morphology of different teeth in hypodontia. The present research is both methodological and investigative in nature. The aims of this study were to measure crown morphology of the permanent dentitions for hypodontia subjects divided according to severity of hypodontia into mild, moderate and severe groups and a control group, and then to compare the measurements of groups. All subjects were of white Caucasian origin and were unrelated. The study groups were balanced for size (N = 40), gender (N = 20) and age. The effect of both severity of hypodontia and gender were examined. The total study population was N= 161. A new measurement system using image analysis and new measurement index for tooth taper were developed, validated and then utilised in the main study. The image analysis technique and tooth taper measurement were validated against the classical manual measurements and subjective scoring respectively. The errors of measurements were assessed by duplicate measurements to determine the intra- and inter-observer reproducibility. It was found that measurements for all study variables were repeatable without systematic error and with small method error. Comprehensive measurements (N = 15) were obtained from both the buccal and occlusal views for the following dimensions of tooth crowns: The principal mesiodistal (MDb and MDo) and buccolingual (BL), proportional mesiodistal (MD25, MD50 and MD75) and buccolingual (BLm and BLd), occlusogingival (OG), perimeter (Pb and Po), area (Ab and Ao) and the distance between MDb and occlusal level (Db) and between MDo and buccal border of tooth crown (Do). A number of indices (N = 5) to evaluate tooth crown morphology were also calculated: Tooth taper determination index (MD50/MD75), crown indices of buccal morphology (CIBMI and CIBM2 i. e. MDb/OG and Db/OG respectively) and crown indices of occlusal morphology (CIOMI and CIOM2 i. e. MDo/BL and Do/BL respectively). Two-way analysis of variance followed by multi-comparison tests with Bonferroni adjustment for the significance levels were employed in the main study. The results of the main study revealed significant differences between hypodontia groups and control subjects for the following measurement variables: MDb, Db, OG, Pb, Ab, MD25, MD50, MD75, tooth taper, CIBM2, MDo, Do, BL, Ao, Po, BLm and BLd. Hypodontia subjects demonstrated significantly smaller measurement values than controls except Db, tooth taper and CIBM2. The severity of hypodontia affected the degree of difference in tooth morphology; for many variables the more severe the hypodontia the greater the difference from controls. There were only a few significant differences between hypodontia and control groups for the variables CIBM1, CIOM1 and CIOM2. Gender differences (within groups) indicated that males tend to show larger measurements than females although few findings reached statistical significance. The symmetry of right and left measurements (MDb and tooth taper), the number of cusps in the premolar and molar teeth and the intermaxillary ratios were investigated. The findings suggest that asymmetry in bilateral measurements appeared to be increased in hypodontia subjects. Descriptive data also suggest that all the intermaxillary ratios tend to be larger in hypodontia than the control group. There was a tendency for the reduction in the number of cusps for hypodontia subjects than controls. Again, these findings were related to severity of hypodontia. Thus, variation in tooth morphology in patients with hypodontia was clearly established. The differences were related to the severity of the condition. However, there were also variations in the measurements between individuals with hypodontia of the same severity group. A possible explanation is the multifactorial aetiology of the condition. In certain groups, especially with severe hypodontia, the small numbers of some tooth types which developed may also have influenced the results.