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Contents • • • • • • • • •
Introduction Historical perspective Cephalostat and Obtaining the cephalogram Cephalometric landmarks Uses of Cephalometrics Limitations of Cephalometrics Limitations of some of the common analysis. Conclusion References www.indiandentalacademy.com
Introduction Definition: Cephalometrics is a radiographic technique for abstracting the human head into a geometric scheme. Salzmann “Cephalometrics includes measurements, description and appraisal of the morphological configuration and growth changes in skull by ascertaining the lines, angles and planes between anthropometric landmarks established by physical anthropologists and points selected by orthodontists� www.indiandentalacademy.com
Terminology Anthropometry – Measurement of dimensions of the human body and it’s parts. Craniometry – Branch of anthropometry dealing with measurements of dimensions and angles of bony skull. Cephalometry – Scientific measurement of dimensions of the ‘living’ head.
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Validity\ Accuracy- Is the extend to which the value obtained represents the object of interest. Some cephalometric landmarks and planes do not agree with the anatomic structures because they are chosen on the basis of convenience of identification rather than anatomic validity. Reproducibility\ Precision – Closeness of the successive measurements of the same object. If a measurement is persistently over or under estimated, a systemic error is introduced.
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Historical perspective • Discovery of X-Rays in 1895 by Sir William Conrad Roentgen paved a foundation for the development of Roentgen Cephalometrics • 1915 – Van Loon of Holland developed a method for 3D registration of face and dentition – Cubus Craniophorus • 1922 – Pacini – published “Roentgen Ray Anthropometry www.indiandentalacademy.com of skull”
• 1931 – Herbert Hofrath of Germany published a paper describing a technique to produce lateral Cephalometric head plates. Simultaneously Broadbent Holly of Cleveland,Ohio,USA published “ A new X-Ray technique and its application to Orthodontia” • Broadbent Bolton Cephalometer--the device was initially developed to stabilize and properly orient the patients head at 90° to the central beam of X-Ray. • Broadbent standardized source/target distance of 5 feet from the anode to midsagittal plane of the patient’s head. Hofrath used the same distance at 2 meters www.indiandentalacademy.com
The Bolton Room in the Anatomical Laboratory of the Medical School at Western Reserve University, Cleveland.
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• Had ear rods inserted into patient’s respective external auditory meatus,which became the machine porion • Also took frontal films by a second X-Ray head, keeping the patient and the Cephalostat stationary. • 1938 Allen Brodie made the significant clinical analysis of the clinical effects of the orthodontic treatment • 1941 Tweed presented Tweed Triangle • 1947 Margolis introduced Maxillofacial Triangle analysis • 1948 Downs introduced truly classical full scale www.indiandentalacademy.com Cephalometric analysis.
• 1953 Cecil Steiner introduced Steiner analysis • Later multiple analyses were introduced.
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Cephalometric equipment • Consists of an X-ray source and a head holding device called cephalostat Cephalostats are of two types • Broadbent –Bolton method (1931) – Utilizes two X-Ray sources – Two film holders – Subject not moved between the lateral and PA ceph • Higley method (1936) – One X-ray source and Film holder – Patient has to be repositioned in various projections – Used in modern cephalostats www.indiandentalacademy.com
• Cephalostat consists of two ear rods that prevent movement of the head in horizontal plane • Vertical stabilization is provided by an orbital pointer that contacts the lower border of the left orbit • The upper part of the face is supported by a forehead clamp positioned above the region of the nasal www.indiandentalacademy.com bridge
• The distance between X-ray source and the midsagittal plane of the patient is fixed at 5 feet (152.4 cm) • Thus the equipment helps in standardization using a constant head position which helps to compare serial radiographs • Natural Head Position (NHP) is a standardized and reproducible orientation of the head in space when one is focusing on a distant point at eye level. www.indiandentalacademy.com
• X-ray source – Small focal spot obtained by rotating anode – 75 KVp,12mA,0.5 second • X-Ray film – 8 X 10” film cassette – High speed film (400) – Used with intensifying screens – Along with a fixed or moving grid
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Conventions in taking Cephalograms
Lateral Cephalogram
Frontal Cephalogram www.indiandentalacademy.com
Cephalometric landmarks Types – Anatomic – Derived Hard tissue landmarks Soft tissue landmarks
Anatomic These landmarks represent actual anatomic landmarks of the skull.
Derived landmarks These are obtained secondarily from anatomic landmarks. www.indiandentalacademy.com
Criteria for landmark selection • Should be easily identifiable • Should be uniform in outline and reproducible • Should permit valid quantitative measurements of lines and angles projected from them.
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Unilateral landmarks in lateral cephalograms • • • • • • • • • • •
Nasion (Na) Anterior nasal spine (ANS) Subspinale(“A” point) Supramentale (“B” point) Pogonion (Pog) Gnathion (Gn) Menton (Me) Glabella Basion (Ba) Posterior nasal spine (PNS) Sella (S) www.indiandentalacademy.com
Bilateral landmarks Both left and right points are located and used, but some clinicians use the midpoint of the two. Following are the points– Orbitale (Or) – Gonion (Go) – Condylion (Co) – Articulare (Ar) – Pterygomaxillary fissure (PTM)
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Uses of Cephalometrics 1) Description
Cephalometrics helps in describing the morphology or growth pattern of a particular individual. This morphology or growth pattern can be compared to the standard or the ideal or may even be compared with patient’s own earlier cephalograms. 2) Diagnosis Cephalometrics in orthodontia is helpful in analyzing the nature of the problem and helps in clarifying it. For example it can help in differentiating face type and it can help differentiate whether the malocclusion is skeletal or dental. www.indiandentalacademy.com
3) Prediction
Cephalometrics to a certain extent can be useful in predicting future growth, the morphologic pattern expected and also helps in predicting the consequence of a particular treatment plan. 4) Treatment Planning By helping in the diagnosis and also in the prediction of craniofacial morphology it aids in developing a clear treatment plan.
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5) Evaluation of treatment results and prognosis
Recurrent cephalograms help in evaluation of results of the treatment. It helps the orthodontist to know whether the treatment is proceeding on expected lines or not.
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Errors and limitations I. Assumptions – Assumption of symmetry – Assumption of coordination of transmeatal axis – Assumption of occlusion – Adequacy of one projection II.Technique limitations A) Errors in projection – Positional errors – Magnification errors – Quality of the image. – Misalignment of cephalometer. www.indiandentalacademy.com
B) Errors in identification – Errors in landmark identification – Errors in calculation of linear and angular measurements. – Errors in superimposition III- Limitations of some common analysis.
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• Assumptions While taking a lateral cephalogram bilateral skeletal symmetry is often presumed. In daily clinical practice, nearly all faces have some degree of asymmetry. Facial asymmetry should be taken into account, otherwise more serious problems may be obscured by the method analysis of cephalogram or due to the neglect of orthodontist. To confirm asymmetry additional PA projection should be taken. www.indiandentalacademy.com
• Occlusal position – Conventional position of the mandible while taking cephalogram is habitual occlusal position. – In patients with an important functional element, this convention may be misleading. – Occlusal position of mandible can be confirm by two procedures • Use wax bites in both usual occlusal position and retruded contact position. • Use the postural position as well as an occlusal position for both lateral and PA projections. www.indiandentalacademy.com
• Orientation on transmeatal axis – The central ray is supposed to pass along the transmeatal axis. But external auditory meatus may be asymmetric as any other cephalic structures. • Adequacy of one planar projection – Often lateral cephalogram is presumed to be sufficient. To learn more, PA, Oblique and Basal views should also be utilized.
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Errors of projection- Results due to two dimensional depiction of three- dimensional structures. Since the rays that produce the image are not parallel and originate from small source. – Points not in the mid sagittal plane are distorted according to the laws of perspective • Angular measurements become too obtuse • Linear measurements are foreshortened – Side nearer to X-Ray source is enlarged more than side closer to film
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• Positioning of patient – Accurate positioning of patient in all three planes is difficult. – Head rotation around vertical axis can cause the angles to become obtuse. – Head rotation around sagittal axis causes differences in image position for bilateral structures.
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– Head rotation arround the transmeatal axis can change the profile of the patient on the cephalogram. – Patient should be positioned with Frankfort plane horizontal or in natural postural position with teeth in centric occlusion
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Effect of head posture on cephalometric sagittal angular measures BY Tng TT- AJODO 1993 • The purpose of this study was to quantify the effect of head posture changes on the commonly used sagittal angular measures SNA, SNB, and SNPogonion (SNPg). • The sample comprised 30 Chinese skulls. Cephalograms were taken in a purpose designed skull holder with the Frankfort plane horizontal and at +10 degrees, +20 degrees, +30 degrees, -10 degrees, -20 degrees, and -30 degrees. • A geometric grid was used to locate the deepest landmarks, and the landmarks were digitized followed by computer superimposition on the S-N line. www.indiandentalacademy.com
• Overall, the angles were underestimated by approximately 1 degrees. • In general, the angular differences and their standard deviations increased as the skull rotation increased with most of the differences for the SNB and the SNPg angles being clinically significant. • It was concluded that head posture needs to be standardized during cephalometry. Changes in posture significantly affect the location of some landmarks and the subsequent data obtained. www.indiandentalacademy.com
• Quality of the X-ray image – Depends upon • Image Contrast - Is general term that describes the range of densities on a radiograph.
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• Sharpness and Image Blurring –Sharpness is the ability of a radiograph to define an edge precisely. And blurring is the degree of unsharpness. Can be caused by• Image receptor blurring • Motion blurring • Geometric blurring.
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Image receptor blurring
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Motion Blurring- Image sharpness can be lost through movement of the film, subject or x-ray source during exposure. Can be prevented by keeping the exposure time as short as possible for a given kVp and mA.
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Geometric blurring
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Magnification • Magnification occurs because the beam diverges from the focal spot to the image plane • Magnification given by:
I SID M= = O SOD
• Largest when object close to focal spot. www.indiandentalacademy.com
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– There is 5-8% increase in size of the object, with subject to film distance varying from 812cm. – Important to control this by using same subject –film distance for all projections.
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Effect of cephalometer misalignment on calculations of facial asymmetry By- Ki-Heon Lee:AJODO 2007;132:15-27
• Authors evaluated errors introduced into the interpretation of facial asymmetry on posteroanterior cephalograms due to malpositioning of the x-ray emitter focal spot. • Methods: A representative dry skull with 22 metal markers was used to generate a series of PA images from different emitter positions by using a fully calibrated stereo cephalometer. www.indiandentalacademy.com
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• Conclusions of the studyMisalignment of the x-ray emitter focal spot introduces systematic errors into the interpretation of facial asymmetry on PA cephalograms. For misalignments of less than 20 mm, the effect is small in individual cases. However, misalignments as small as 10 mm can introduce spurious statistical findings of significant asymmetry when mean values for large groups of PA images are evaluated. www.indiandentalacademy.com
Errors of Landmark Identification Moyers in AJO 1979- There is no theory of cephalometries, only conventions.
The raw data within the cephalogram are points and curves representing anatomic shadows structured in a complicated fashion, both logicaily and biologically. Some cephalometric points lie on one curve (gonion), some on two curves (menton) or none (ear-rod "porion"). Some "points" are not points on the skull at all. Articulare, for example, the intersection of two shadows, is in fact a line looked at down its length. It is not a landmark or an intersection, for the curves used to define it never cross in space. www.indiandentalacademy.com
Landmarks and curves. • Any landmark is either "anatomic" or "extrernal. • Anatomic landmarks are true , biologic loci identified by some feature of the local morphology. Examples include tips of cusps, nasion. and sella turcica. • Other landmarks, however, not differentiated by local properties are defined implicitly by the maximum or minimum of some geometric property and so we call them external. Menton is the lowest point of the mandibular symphysis: pogonion is the most anterior point on the chin; condylion the most superior posterior point on www.indiandentalacademy.com the condyle
• As the mandible rotates, the positions of all extremal points on it are altered. Then such points cannot be located until an orientation, let us say the horizontal is fixed. • Now all the standard orientations are themselves operationally defined in terms of landmarks too. The Frankfort plane, for instance passes through the top of porion and the bottom of orbitale, but "top" and "bottom" are themselves defined in terms of the orientation of that one subject to earth. www.indiandentalacademy.com
• Malplacement of the horizontal orientation may be due to a mistake in positioning the subject, to inappropriateness of the landmarks for that subject, or perhaps because of disproportionate growth or asymmetry. This makes the whole scheme susceptible to a pernicious form of mismeasurment in which error in the orientation is propagated to affect the positions of all the orientation dependent landmarks instead. www.indiandentalacademy.com
Straight lines or what do we do about bulges?
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Studying growth changes • Growth itself is not compatible with the current cephalometric scheme, for as landmarks are carried along on a field of continuous spatial deformation, the change between them is not at all summarized by any set of distances and directions, however hallowed.
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• Growth is somewhere between the landmarks, not localized "at" them in any sense. • To understand the changes of growth and remodeling, we need to know how each land mark is being moved away from the others. • It would suffice even to know how each is carried away from those on either side changing local distances and directions simultaneously. • Such an analysis is necessarily formulated in terms of the curved form changes between landmarks where growth is altering the relationship of any pair of landmarks in a geometrically intricate fashion. www.indiandentalacademy.com
The reliability of head film measurements 1. Landmark identification By- Sheldon Baumrind AJO 1971
• The study was designed to check operator reliability in the identification of standard cephalometric landmarks. • n= 20 • Each of the 20 films were then "traced" by each of the five members of the first year postgraduate class at the University of California Division of Orthodontics. • 16 standard cephalometric landmarks were traced by each member. www.indiandentalacademy.com
Conclusions of the study• Even when one is replicating assessments of the same film, errors in landmark identification are too great to be ignored. • The magnitude of error varies greatly from landmark to landmark. • The distribution of error for most landmarks is not random but is, rather systematical in the sense that each landmark has its own characteristic and usually noncircular envelop of error. www.indiandentalacademy.com
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The reliability of head film measurements 2. Conventional angular and linear measures BySheldon Baumrind AJO 1971
•
Errors in angular and linear head film measurement are of only three types. (1) Errors of projection, (2) Errors of landmark location, (3) Mechanical errors in drawing lines between points on tracings and in measuring with ruler or protractor. www.indiandentalacademy.com
• There are three considerations ,which determine what impact the error in identification of a specific landmark will have on the linear and angular values involving that landmark. • The first is the actual magnitude of the error involved in identifying the specific landmark. • The second is the linear distance on the tracing between the point representing the landmark and the points representing the other landmarks with which it is connected in the process of computing a given measure. www.indiandentalacademy.com
• The third consideration is the direction from which the line segment between two landmarks intersects the envelope of error of each landmark, as the perimeter of the scattergram of error is properly termed.
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• That angular and linear measurements from head films contain considerable errors--errors which are far too great to be overlooked. • The magnitude of error differs greatly from measure to measure, so that it simply is not proper to treat all angular or linear measures as if they were of equal reliability. • Both the absolute values of errors and the variability among replicated estimates tend to be greater for angular measures than for linear measurements. www.indiandentalacademy.com
The reliability of head film measurements 3. Tracing superimposition By- Sheldon Baumrind •
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AJO 1976. Tracings of head films for twenty-five children in the late mixed-dentition stage with mean time interval between films very close to 2 years, was performed and superimposed independently by four trained judges. Four different methods of superimpositions were used. Superimposed on SN- Registered at sella. Superimposed on anterior cranial base Superimposed on palatal plane. www.indiandentalacademy.com Superimposed on mandibular border.
Conclusions of the study• Different types of anatomic plane superimposition are made with errors of different magnitudes and characters. • The findings also strongly imply that, for most land marks evaluated with respect to most superimpositions, rotational effects produce a larger portion of the total error of superimposition than do translational effects.
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Reducing errors – – – – – – –
Careful selection of analysis Clear understanding of point definition Good quality film and standardization Duplicate measurements Error calculation Care when interpreting results Automated computerised radiographic identification of landmarks
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Limitations of some of the commonly used analysis. – An analysis is misused if too rigid an application of mean value is made. The total range and variance are more important and practical. – Inappropriate application due to differences resulting from age, sex, race etc.
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Downs analysis(1948) 1. Difficulty in locating porion. 2. F-H plane may not be true horizontal Facial angle (N-Pg to F-H) 87.8± 3.6°
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• Steiner’s analysis (1953) – ANB angle not an accurate measurement. – Variation in SNA angle
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Shortcomings of ANB • Recognized by Jenkins(1956) – Significant effect of rotational and vertical jaw dimensions relative to cranial base – Used the functional occlusal plane as a reference base for measurement of jaw disharmony
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• Established the ‘a’ plane drawn through point A at right angles to the occlusal plane and then measured from the ‘a’ plane to point B, Gnathion, and the mandibular incisor edge. www.indiandentalacademy.com
• Harvold 1963 also used an occlusal plane and measured the A-B difference. • Taylor 1969 also pointed out the ANB angle did not always indicate true apical base relationship Relative forward or backward movement of nasion would likewise change the ANB reading, as would forward /backward positioning of maxilla and mandible.
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• Beatty 1975 reported that the ANB angle is not always an accurate method of establishing the actual amount of apical base divergence • An alternative to ANB angle-AXD angle where point X is formed by projecting point A onto a perpendicular to the SN line www.indiandentalacademy.com
• Ferrazzini 1976 demonstrated that ANB depended not only on the A-P relationship of the jaws but on the : – inclination of the palatal plane – Maxillary prognathism – Vertical facial dimension
• Said “ too much importance should not be given to the ANB angle, nor should it be considered the absolute measurement of A-P relationship of the jaws. Rather it should be judged always in respect to other variables” www.indiandentalacademy.com
• Binder 1979 showed that for every 5 mm anterior displacement horizontally ,the ANB angle changed 2.5 ° – A 5mm upward displacement of nasion altered the ANB angle 0.5 ° – Downward displacement of nasion changed the ANB angle 1°
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• Sperry et al 1977 concluded that antero-posterior dysplasia should be assessed relative to the cant of the occlusal plane ,and that true dental base discrepancies can be noted relative to the occlusal plane. • Rotberg et al 1980 correlated the Wits appraisal with ANB difference – When positive ANB is less than 4° ,Wits could be either positive or negative – ANB angle is between 4°-8° ,all Wits values were positive www.indiandentalacademy.com
• Roth and Martina 1982 recognized the ANB angle as an invalid measure of sagital skeletal disharmony because it’s affected by rotations and variations in the sagital and vertical jaw dimensions relative to the cranial base.
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• Bishara et al 1983 determined the changes in the ANB angle and Wits appraisal between 5 yrs of age and adulthood in men and women • Result : ANB angle changes significantly with age but Wits does not. • Concluded that both ANB and Wits should be used to help arrive at a more accurate diagnosis of A-P base relationship www.indiandentalacademy.com
• Jarvinen 1985 stated that “the use of the apical base should be replaced by a better method to determine sagital apical base difference” • Suggested Wits appraisal as a possible alternative.
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• Wits appraisal is a linear measurement and not an analysis per se. It is simply an adjunctive diagnostic aid that may prove useful in assessing the extent of A-P skeletal dysplasia and in determining the reliability of the ANB angle.
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Occlusal and Mandibular planes
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• Facial plane divergency – Use of mandibular plane alone for assessment of facial divergency is prone to error. The occlusomandibular plane angle is an additional guide to the degree of facial divergency.
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• Tweed’s analysis Tweed Triangle – IMPA=90° – FMA=25° – FMIA=65° Variation in any of these planes will change the angles of the facial triangle. E.g. steep mandibular plane angle- IMPA should not be 90°
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• Ricketts analysis (1960s) – Normative data for many measurements was based on unspecified samples collected by him. – He used chronological age instead of the development age
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Conclusion • The validity and reliability of single cephalometric measurements or groups of measurements ,in a description of craniofacial and dental variation are often questionable. • Each analysis has its advantages as well as shortcomings • Single patient film is compared to norm values. In case of any anatomic variation, values will not correspond to norms.
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• Variation in location of anatomic landmarks could result in incorrect conclusions from the analysis • Orthodontic diagnosis must be based on a comprehensive individual evaluation of each patient. The limitations ,as well as the advantages, of Cephalometry must be recognized
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References 1. Ki-Heon Lee. Effect of cephalometer misalignment on calculations of facial asymmetry.AJODO 2007;132:15-27 2. Sheldon Baumrind. The reliability of head film measurements 1. Landmark identification AJO 1971. 60;2:111-127 3. Sheldon Baumrind .The reliability of head film measurements 2. Conventional angular and linear measures By- AJO 1971.60; 5: 505-517. www.indiandentalacademy.com
4. Sheldon Baumrind The reliability of head film measurements 3. Tracing superimposition AJO 1976.70;6:617-644. 5. Robert E. Moyers. The inappopriateness of
conventional cephalonIetrics. AJO 1979.75;6:599-617. 6. J.A.Salzmann. Limitations of roentgenographic cephalometric cephalometrics. AJO 196.59;3:169-188.
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7. Tng TT.Effect of head posture on cephalometric sagittal angular measures. AJODO 1993 Oct;104(4):337-4 8. Alexender Jacobson. Radiographic Cephalometry.Page295-304..Quintessence Publication. 1995 9. White, haroah.Oral RadiologyPage 83-90. Mosby.2000 10. Oktay H. A comparison of ANB, Wits, AF-BF and APDI measurements. AmJ Orthod Denwfac Orthop 1991;99:122-126
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11. Wylie GA, Fish LC, Epker BN. Cephalometries: A comparison of five analyses currently used in the diag-nosis of dentofacia1 deformities. IntJ Adult Orthod Orthognathic Surg 1987;2:15--36. 12. Ghafari, Engel GA, Laster LL. Cephalometric superimposition on the cranial base: A review and comparison of 4 methods. Am] Orthod Dentofac Orthop 1987;91 :403-413.
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