THE RELEVANCE OF THE MANDIBULAR INCISOR IN ORTHODONTICS

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INTRODUCTION

Although great emphasis is placed on measurements taken from the lower incisor crown and the lower incisor inclination in orthodontics, it is difficult to obtain reliable reference points regarding the lower incisor on lateral cephalometric radiographs. The purpose of the study was to investigate an alternative and possibly more accurate method to determine the central lower incisor position and inclination on lateral skull radiographs. The placement of radiographic markers on the clinical crown of a lower central incisor should contribute to more exact measurements taken from the specific tooth, as it should be able to clearly distinguish the dimensions of the tooth crown in the sagittal plane.

PLANNING FOR PATIENTS

The mandibular incisor position is of critical importance in orthodontics. The main purpose of orthodontics can usually be defined as the creation of the best balance between occlusal relationships and dental and facial aesthetics. Stability of the result as well as the long-termmaintenance of the dentition are also critical (4). The lower incisors have a central role to all these goals (Figure 1.1).

Magnification

X-ray beams are not parallel with all the points of the object to be examined with resultant magnification. The difference between image size and object size is explained in Figure 2.3 which in principle illustrates that the further the target (x-ray tube) is from the object (patient), the less will be the magnification.

Distortion

The image of the face and skull that the lateral cephalometric radiograph provides is a two-dimensional image of a 3-dimensional object. Most landmarks used for cephalometric analysis are located in the mid-sagittal plane (Sella, Nasion, ANS, PNS, A-point, B-point, Pogonion and upper/lower incisors – see Figure 2.6) at the standardized 150cm target-object distance. The ruler that enables calculation of magnification is also in the mid-sagittal plane.

CHAPTER I THE RELEVANCE OF THE MANDIBULAR INCISOR IN ORTHODONTICS
1.1 INTRODUCTION
1.2 THE RELEVANCE OF MANDIBULAR INCISORS IN TREATMENT PLANNING FOR PATIENTS
1.3 THE RELEVANCE OF MANDIBULAR INCISOR POSITION IN THE SCIENTIFIC VALIDITY OF ORTHODONTIC RESEARCH
CHAPTER II RADIOGRAPHIC PRINCIPLES AND CEPHALOMETRY
2.1 INTRODUCTION
2.2 ERRORS OF PROJECTION: MAGNIFICATION AND DISTORTION
2.2.1 Magnification
2.2.2 Distortion
2.3 ERRORS IN LANDMARK IDENTIFICATION
2.3.1 Validity
2.3.2 Reproducibility
2.4 CONCLUSION
CHAPTER III ANATOMY OF THE LOWER INCISORS
3.1 INTRODUCTION
3.2 GENERAL DIMENSIONS
3.3 ROOT CURVATURE
3.4 LABIAL SURFACE INCLINATION
3.5 SUMMARY
CHAPTER IV DIFFERENT METHODS CURRENTLY USED TO DETERMINE LOWER INCISOR INCLINATION
4.1 INTRODUCTION
4.2 CLINICAL ASSESSMENT OF LOWER INCISOR INCLINATION
4.3 STUDY MODELS TO DETERMINE INCISOR INCLINATION
4.4 DETERMINING LOWER INCISOR INCLINATION ON LATERAL CEPHALOMETRIC RADIOGRAPHS
4.4.1 Tracing from incisor edge to incisor apex
4.4.2 Tracing from incisor edge parallel to labial surface
4.4.3 Tracing to the middle of the symphysis
4.4.4 Tracing from incisor edge through the midpoint of the crown
4.5 COMPUTED TOMOGRAPHY
4.6 CONCLUSION
CHAPTER V RESEARCH METHODOLOGY
5.1 INTRODUCTION
5.2 MAIN AIM AND SUB-AIMS
5.3 VERIFICATION OF TEETH USED IN THE STUDY
5.3.1 Origin of teeth used in the research project
5.3.2 Comparison of teeth used in the study to normal dimensions
5.4 TEMPLATE VERIFICATION
5.5 RADIOGRAPHIC MARKERS
5.6 IN VIVO PILOT STUDY
5.7 MAIN AIM REALISATION
5.7.1 Set-up of teeth
5.7.2 Radiographs taken
5.7.3 Tracing of radiographs
5.7.4 Transfer of data for electronic processing
CHAPTER VI RESULTS AND DISCUSSION
6.1 INTRODUCTION
6.2 VERIFICATION OF TEETH USED IN THE STUDY
6.3 TEMPLATE VERIFICATION
6.4 RADIO-OPAQUE DENTAL MARKERS TO DEFINE THE INCISOR CROWN ANATOMY
6.5 DETERMINING LOWER INCISOR INCLINATION UTILISING RADIOGRAPHIC MARKERS
6.5.1 Presentation of the data
6.5.1.1 Graphical presentation of data
6.5.2 Comparison of observers’ tracings to the true inclination
6.5.2.1 Standard deviation and mean
6.5.2.2 Teeth estimated more than 4º off the true inclination
6.5.2.3 Verifying trends at different true inclinations
6.5.3 Method comparison
6.5.3.1 Statistical analysis
6.5.3.2 Results
6.5.3.3 Discussion
6.5.3.4 Conclusion
6.6 SUMMARY
CHAPTER VII CLINICAL EXAMPLES OF DENTAL RADIOGRAPHIC MARKERS
1 GENERAL DISCUSSION
2 CONCLUSION
CHAPTER VIII CONCLUSION AND RECOMMENDATIONS
8.1 INTRODUCTION
8.2 VALUE OF THIS INVESTIGATION
8.3 CRITICAL EVALUATION OF THE STUDY
8.4 COMMERCIALLY AVAILABLE RADIOGRAPHIC MARKERS
8.5 FURTHER INVESTIGATION RECOMMENDED
REFERENCES

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UTILISING RADIOGRAPHIC INCISOR CROWN MARKERS TO DETERMINE INCISOR INCLINATION ON LATERAL HEADFILMS – AN EXPERIMENTAL STUDY ON EXTRACTED TEETH