Cephalometric Analysis

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CONTENTS

                                           INTRODUCTION

                                                    

Orthodontists, in their attempts to change dento-facial deviations from accepted norms, have adopted cephalometric measurement, a method employed in physical anthropology. Cephalometric radiography was introduced in to orthodontics during the 1930s.

                          Cephalometrics can be a useful diagnostic and evaluative tool for the Orthodontist, Pedodontist, Prosthodontist, the Oral Surgeon and the General Practitioner of Dentistry.

Cephalometry had its beginning incraniometry.For many years anatomists and anthropologists were confined to measuring craniofacial dimensions using the skull of dead individuals.

Craniometryis concerned with measuring of dead skull whereas Cephalometry is concerned with measuring the head inclusive of soft tissues, be it living or dead. However Cephalometry had its limitations due to the inaccuracies that resulted from varying thickness of soft tissues.

With the discovery of X-rays by Roentgen in 1895, Radiographic Cephalometrycame in to being which helped to obtain the skeletal measurements with sufficient accuracy and convenience.

DEFINITION

Cephalometricradiography is a standardized method of production of skull radiograph, useful in making measurements of cranium and oro-facial complex. The radiograph thus obtained is called cephalogram.

HISTORY

In the 16th century artists Durerand DaVincisketched a series of human faces with straight lines joining homologous anatomic structures. Variations in these lines highlighted the structural differences among the faces.

Much later anthropologists invented an instrument- the Craniostat, which helped in orienting dry skulls and facilitating standardized measurements.

The discovery of x-ray by William Roentgen provided a big boon for the studying skeletal measurement.

A.J.Pacini in 1922 presented the concept of standardized radiographic head images where subjects were positioned to the cassette with gauze bandages at a distance of 2 meters from the x-ray tube.

—  In 1931 H. Hofrathin Germany and B. Holly Broadbentin United Statessimultaneously published articles in which they had refined the technique using a  high powered x-ray machine and a head holder called  Cephalostat.

Uses

•         Cephalometrics helps in orthodontic diagnosis by enabling the study of skeletal, dental and soft tissue structures of the cranio-facial region.

•         It helps in classification of the skeletal and dental abnormalities.

•         It also helps in establishing facial type.

•         Cephalometrics helps in planning treatment for an individual.

•         It helps in evaluation of the treatment results by quantifying the changes brought about by treatment.

•         Cephalometrics helps in predicting the growth related changes and changes associated with surgical treatment.

•         It is a valuable aid in research work involving the cranio-dental facial region.

Types of cephalogram

n  Lateral cephalogram– this provides a lateral view of the skull.

n  Frontal cephalogram– This provides antero-posterior view of the skull.

TECHNICAL ASPECT

The cephalometric radiographs are taken using an apparatus that consists of an

—  X-ray source

—  A head holding device calledcephalostat. The cephalostat consists of

     -  Two ear rods that prevent the movement of the head in the horizontal plane.

     -  Orbital pointer which brings vertical stabilization where the pointer contacts the lower border of the left orbit.

     - Forehead clampprovides support for the upper part of the face which is positioned above the region of the nasal bridge.

—  Cassette holder

Cephalostat are of two types

1.     THE BROADBENT-BOLTON METHOD- utilizes two film holders and two X ray tubes  that are at right angles to each other in the same horizontal plane so that two images (lateral & PA) could be simultaneously obtained without moving the subject.

2.     THE HIGLEY METHOD- is used in most modern cephalostatswhere one x-ray source and film holder with a cephalostat capable of being rotated.

OBTAINING THE CEPHALOGRAM

The lateral projection

The patient’s head was centred in the cephalostat with the superior borders of the external auditory meatus resting on the upper parts of the two ear rods.The lowest point on the inferior bony border of the left orbit, indicated by the orbital marker, was at the level of the upper parts of the ear rods.Nose clamp was fixed at the root of the nose to support the upper face.

The mid sagittal plane of the subject’s head is conventionally placed at 5 feet (152.4 cm or 60 inches) and the subject film distance usually at 7 inches(18cm).Film is placed toward the left side of the subject. The patient’s head is placed with Frankfurt horizontal plane parallel to the floor and the subjects teeth in their usual occlusal position and the lips are left loose.

The central beam of the x-ray coincides with the transmeatal axis, i.e. with ear rods of the cephalostat

The Posteroanterior Projection

The head is rotated by 90 degrees so that the central ray bisects the                transmeatal axis.

FRANKFURT HORIZONTAL PLANE

FH plane was chosen because this approximates the natural head position. But the FHP also had its drawbacks as :

—  Some individuals show a variation of their Frankfurt Horizontal plane to the true horizontal to an extent of ± 10°. (Average – 7^o)

—  The landmarks to locate the FH plane, Orbitale&Porion, are difficult to identify on a cephalogram.

                             So an alternative to overcome this problem was to use a functionally derived NHPwhich is obtained by asking the subject to look at the image of their eyes in the mirror located at the eye level. Functionally derived NHP was more accurate but its reproducibility was less than FHP.

CEPHALOMETRIC LANDMARKS

Cephalometric landmarks are readily recognizable points on a cephalometric radiograph, representing certain hard or soft tissue anatomical structures (anatomical landmarks) or intersections of lines (constructed landmarks).

Ideal Requirements: -

—  Should be easily seen on the roentgenogram,

—  Be uniform in out line, and easily reproducible.

—  Should have significant relationship to the vectors of growth.

—  Should permit valid quantitative measurements of lines and angles projected from them.

—  Measurements should be amenable to statistical analyses.

Cephalometric landmarks can be of two types: -

1.    Hard tissue landmarks

2.    Soft tissue landmarks

HARD TISSUE LANDMARKS

There are various hard tissue landmarks which are used for quantitative analysis and measurements.

Following are some of the important hard tissue landmarks;

·        A-point (Point A, Subspinale):-The deepest (most posterior) midline point on the curvature between the anterior nasal spine and prosthion.

·        Anterior nasal spine (ANS):-The most anterior point on the maxilla at the level of the palate

·       Articulare (Ar):- constructed point representing the intersection of three radiographic images: the inferior surface of the cranial base and the posterior out line of the mandibular condyles.

·        B-point (Point B, Supramentale):-the deepest (most posterior) midline point on the bony curvature of the anterior mandible, between infradentale and pogonion.

·        Basion (Ba):-the most anterior inferior point on the margin of the foramen magnum in the midsagittal plane.

·       Bolton (Bo):-the highest points on the outlines of the retrocondylar fossa.

·       BROADBENT REGISTRATION POINT:-It is the midpoint of the perpendicular from the centre of sella to the Bolton plane

·        CONDYLION:-most postero-superior point on the condyle of the mandible.

·       Gnathion (Gn):the most anterior inferior point on the bony chin in the midsagittal plane.

·        Gonion (Go):the most posterior inferior point on theoutline of the angle of the mandible.

·        Incisor inferius (Ii):the incisal tip of the most labially placed mandibular incisor.

·        Incisor superius (Is):the incisal tip of the most labially placed maxillary incisor.

·        Infradentale (Id):- the mostsuperio-anterior point on the mandibular alveolar process between the central incisors.

·        Key Ridge (KR):-the lowermost point on the contour of the shadow of the ant wall of infratemporal fossa.

·       Menton (Me):-the most inferior point of the mandibular symphysis in the midsagittal plane.

·       Nasion (Na):- the intersection of the internasal and frontonasal sutures in the midsagittal plane

·       Orbitale (Or):-the lowest point on the inferior orbital margin

·        Pogonion (Pog):-the most anterior point on the contour of the bony chin in the midsagittal plane

·       Porion (Po):-the most superior point on the outline of the external auditory meatus (anatomic).

·        Posterior nasal spine (PNS):-the most posterior point on the bony hard palate in the midsagittal plane, the meeting point between inferior & superior surfaces of the hard palate at its posterior aspect.

—  Prosthion (Pr, Supradentale):-the most inferior anterior point on the maxillary alveolar process between the central incisors (crest of alveolar bone).

·        Pterygomaxillary fissure (PTM):-A bilateral teardrop-shaped area of radiolucency, the anterior shadow of which is the posterior surfaces of the tuberosities of the maxilla. The PTM point is the intersection of the inferior border of the foramen rotundum with the posterior wall of the Pterygomaxillary fissure.

·        Sella (S):- the geometric centre of the pituitary fossa (sellaturcica), determined by inspection – a constructed point in the midsagittal plane.

SOFT TISSUE LANDMARKS

1.     Glabella: -The most prominent point on the forehead.

2.     Pronasale: -The anterior point of the intersection between the nasal and frontal bones.

3.     Pronasale: -The prominent tip of the nose.

4.     Subnasale: - represents the base of the nose.

5.     Superior labial sulcus: - represents the midline point between Subnasale and labralesuperius.

6.     Labralesuperius: - represents the vermilion border of the upper lip.

7.     Labraleinferius: - represents the vermilion border of the lower lip.

8.     Inferior labial sulcus: - represents the midline point between labraleinferius and pogonion

9.     Pogonion: - represents the most anterior point on the chin.

Cephalometric planes

These planes are derived from at least 2 or 3 landmarks and are used for measurements, separation of anatomic divisions, and definition of anatomic structures of relating parts of the face to one another.The various cephalometric planes used are:

1.     Frankfurt Horizontal plane: -This plane is drawn from Porion to Orbitale. This name was given at the conference of anthropology held at Frankfurt in1885. This plane was used by used by Down.

2.     Sella-Nasion plane: -It represents the anterior cranial base. It can be accurately located on the radiographs. Cranial base undergoes little change after the age of 6-7 years but the N-point can drift either forwards or vertically giving rise to an error.

3.     Palatal plane: -plane passing through the ANS and the PNS.

4.     Occlusion plane: -It is the plane passing through the cusp tips of the upper and lower first molars and a point bisecting the overbite.

5.     Basion-Nasion plane: -This plane is used in the Rickett’s analysis.

6.     Mandibular plane angle: -Mandibular planes have been defined by various authors based upon their clinical experience and use in their cephalometric analyses.

·        Tweed described the mandibular plane as a linethat is a tangent to the inferior border of the mandible.

·        Down considered the mandibular plane to represent a line connecting the pointgonion and menton.

·        Steiner drew the mandibular plane by joining the points Gonion and Gnathion.

7.     Facial plane: -It extends from nasion to pogonion.

8.    Facial-axis: -It is the line joining sella to gnathion.

9.     A- Pog line: -line from point A on the maxilla to the pogonion on the mandible.

10.                       E plane: -most anterior portion of the soft tissue nose and the soft tissue chin

11.                        Ramal plane: –it represents the posterior border of the ramus.

TRACING TECHNIQUE

Tracing should be systematic. One should begin with a general inspection of the cephalogram and then locate and identify standard landmarks. This is followed by tracing the anatomic structures in a logical sequence, and finally constructing derived landmarks and lines. This is used to produce a visual representation of form by tracing specific features from the cephalometric radiograph. It involves placing a piece of acetate tracing paper over the radiograph and tracing the contours of five components:soft tissue, maxilla, mandible, cranial base and dentition.

1.     Step 1: Trace Soft Tissue Profile

2.     Step 2: Trace Cranial Base

3.     Step 3: Trace Maxilla

4.     Step 4: Trace Mandible

5.     Step 5: Trace Dentition

A Completed Tracing

STEPWISE TRACING TECHNIQUE

Step 1

Draw at least two plus shaped crosses on the top rightand left corners of the radiograph. These are drawnaway from any landmarks and are used to orient thetracing over the radiograph.

Step 2

Trace the soft tissue profile, external cranium, and thecervical vertebrae.

Step 3

These are followed by the tracing of the cranial base,internal border of cranium, frontal sinus, and ear rods(Moorrees recommends abandoning porion and

instead using the superior border of the head ofcondyle to define FH).

Step 4

Maxilla and related structures including the key ridges(which represent the zygomatic processes of themaxillary bone) and pterygomaxillary fissures arethen

traced. The nasal floor is also traced along withthe anterior and posterior nasal spines. The first molarand the most anteriorly placed maxillary incisor(including its root) are also traced.

Step 5

Finally the mandible, including the symphysis, thelower border of the mandible, the condyles and thecoronoid processes is traced. The first molars and the most anteriorly placed incisor tooth including its rootare to be traced. The mandibular canal may be tracedand is a t times used for super positioning serialradiographs.

Cephalometric analysis

The major use of radiographic cephalometry is in characterizing the patient’s dental and skeletal relationships.This led to the development of a number of cephalometric analyses to compare a patient to the population standards.

William. B. Downs in 1948 developed the first cephalometric analysis. Its significance was that it presented an objective method of portraying many factors underlying malocclusion. This was laterfollowed by other analyses by Steiner (1952), Tweed (1953), Ricketts (1958), Sassouni (1969), etc

DOWN'S ANALYSIS

Introduction: -

For us to be able to derive any meaningful conclusions from the study of cephalograms, it is essential to havecertain standards against which to compare the dataobtained after analyzing the patient's cephalogram.One of the first and also one of the most commonlyused data / analysis was provided by Down.

Down divided his analysis into two components - The skeletal component helped in defining theunderlying facial type and the dental component isused to establish if the dentition is placed normally inrelation to the underlying bony structures.

Down classified the face into four basic types: -

• Retrognathic- a regressive or retruded lower jaw.

• Mesognathic- an "ideal" or average position of thelower jaw.

•Prognathic- a protrusive lower jaw.

• True prognathism- a pronounced protrusion of thelower face.

According to Down, any of the above four basic facial types could possess a normal occlusion and aharmonious facial profile, in form and proportion. Thisdid not mean that ideal skeletal profiles could not ordid not have dental malrelationships.Down used the Frankfort-Horizontal plane as thereference plane; as it approximates a near level positionwhen the patient is standing in a posture of distant VISION.

Down's Control Group

The control group studied by Down was derived from20 Caucasian subjects, who ranged in age from 12 to17 years and were equally divided as to sex. Allindividuals possessed clinically excellent occlusions.

SKELETAL PARAMETERS

·       Facial Angle

The facial angle is used to measure the degree ofretrusion or protrusion of the lower jaw. The facialangle provides an indication of the degree of recessionor protrusion of the mandible in relation to the upperface. Facial angle is the inferior inside angle formedby the intersection of the facial line (Nasion-Pogonion)to the Frankfort Horizontal (FH) Plane. The mean reading for this angle is 87.8° (± 3.6°)

with a range of 82° to 95°.A prominent chin increases this angle, whereas a

smaller than average angular reading suggests aretrusive or retro positioned chin.

·       Angle of Convexity

The angle of convexity is formed by the intersection of line N-point A to point A-Pogonion. This angle measures the placement of the maxillary basal arch at its anterior limit (point A) relative to the total facial profile (Nasion-Pogonion). This angle is read in plus or minus degrees starting from zero. If the line Pogonion-point A is extendedand located anterior to the N-A line, the angle is read as positive. A positive angle suggests prominence of the maxillary denture base relative to the mandible. A negative angle of convexity is associated with prognathic profile or in other words a Class III profile. The range extends from -8.5° to +10°, with a mean of 0°.

·       A-B Plane Angle

Points A and B are joined by a line which when extended forms an angle with the line Nasion-Pogonion, this is called the A-B plane angle. The A-B plane is a measure of the relation of the anterior limit of the apical bases to each jaw relative to the facial line. Generally point B is positioned behind point A thus this angle is usually negative in value, except in Class III malocclusions or Class I occlusions with prominence of the mandible.

A large negative value suggests a Class Il facial pattern, which can be due to the retro-positioned chin or mandible or underdeveloped chin point or a prominent maxilla, i.e., point B located behind point A. The range extends from a maximum of 0° to a minimum of 9° with a mean reading of -4.6°.

·       Mandibular Plane Angle

The mandibular plane according to Down, is a"Tangent to the gonial angle and to the pogonion”.  The mandibular plane angle is established by relating the mandibular plane to theFrankfort Horizontal plane. High mandibular plane angles occur in bothretrusive and protrusive faces and are suggestive of unfavorable hyper divergent facial patterns or 'long face cases'. The range extends from a minimum of 17° to a maximum of 28° with a mean of 21.9°.

·       Y-(Growth) Axis

The growth axis is measured as an acute angle formed by the intersection of a line from sellaturcica to Gnathion with the Frankfort horizontal plane. This angle is larger in Class II facial patterns than in those with Class III tendencies. It indicates the degree of downward, rear ward or forward position of the chin in relation to the upper face. A decrease of the Y-axis in serial radiographs may be interpreted as a greater horizontal than verticalgrowth of the face or a deepening of the bite in orthodontic cases. An increase in the Y-axis is suggestive ofvertical growth exceeding horizontal growth of the mandible or an opening of the bite during orthodontic treatment. The Y-axis reading also increases with the extrusion of the molars (this is generally desirable when correcting malocclusions in horizontal growers. The range extends from a minimum of 53° to a maximum of 66° with a mean reading of 59.4°.

DENTAL PARAMETERS

·       Cant of Occlusal Plane

Down originally defined it as the line bisecting the overlapping cusps of the first molars and the incisal overbite. The Cant measures the slope of the occlusal plane to the Frankfort Horizontal plane. When the anterior part of the plane is lower than the posterior, the angle would be positive. Large positive angles are found in Class II facial patterns. A long mandibular ramus also tends to decrease this angle. The mean value is +9.3° with a range of +1.5° to+9.3°.

·       Inter-incisal Angle

The inter-incisal angle is established by passing a line through the incisal edge and the apex of the root ofthe maxillary and mandibular central incisors. The inter-incisal angle is relatively small in individuals whose incisors are tipped forward on the denture base, i.e. they are proclined.The mean value is 135.4°, with a range of 130° to150°.

·       Incisor Occlusal Plane Angle

This angle relates the lower incisors to their functioning surface at the occlusal plane. The inferior inside angle is read as a plus or minus deviation from the right angle. The positive angle increases as these teeth incline forward i.e. become proclined. The values are least in class II div. 2 cases where the incisors are retroclined. The mean value is 14.5° with a standard deviation of ±3.5° and a range of +3.5° to +20°.

·       Incisor Mandibular Plane Angle

It is formed by the intersection of the mandibular plane with a line passing through the incisal edge and apexof the root of the mandibular central incisor. The angle is positive when the incisors are tippedforward on the denture base, i.e. they are proclinedforward. The value increases as the proclinationincreases.The mean value is l.4°with a range of -8.5° to +5°.

·       Protrusion of Maxillary Incisors

It is measured as the distance between the incisaledge of the maxillary central incisor to the line from Point A to Pogonion. This distance is positive if the incisal edge is ahead of the point A-Pogonion line and negative if the incisal edge lies behind this line. It indicates the amount of maxillary dental protrusion. The mean value is +2.7 mm with a range of -1.0 to+5 mm.

STEINER’S ANALYSIS

This was developed by Cecil.C.Steiner in 1952 can be consideredas the first of the modern cephalometric analysis for two reasons:

1.     It displayed measurements in a way that emphasized not just the individual measurements but their interrelationship into a pattern.

2.     Specific guide for use of cephalometric measurements in treatment planning.

Steiner divided his analysis into three parts-skeletal,dental and soft tissues. Skeletal analysis entails relatingthe upper and lower jaws to the skull and to each other.The dental analysis entails relating the upper and lower incisor teeth to their respective jaws and to eachother. And the soft tissue analysis provides a meansof assessing the balance and harmony of the lowerfacial profile.

Steiner noted that landmarks such as Porion andOrbitale are not always easily identified on lateral cephalometric head films; hence, he elected to use theanterior cranial base (Sella to Nasion) as the line ofreference for his analysis. The advantage of using thesetwo midline points is that they are moved only aminimal amount whenever the head deviates from thetrue profile position. This remains true even if the headis rotated in the cephalostat.

·       Relating the Maxilla to the Skull

The angle SNA is formed by joining the lines S-Nand N-A.The mean reading for this angle is 82°. If the angular reading is more than 82°, it would indicate a relative forward positioning or protrusionof the maxilla. Conversely, should the reading be lessthan 82°, it would indicate a relative backward orrecessive location of the maxilla.

·       Relating the Mandible to the Skull

To assess whether the mandible is protrusive orrecessive relative to the cranial base, the SNB angle is read. The mean for this angle is 80°. If the angle is less than 80°, it is indicative of aretruded mandible. An angle greater than 80° degrees

suggests a prognathic or forwardly positionedmandible.

·       Relating the Maxilla to the Mandible

The angle ANB provides information on the relative positions of the jaws to each other. The ANB angle provides a general idea of the antero-posterior discrepancy of the maxillary to the mandibular apical bases. The mean reading for this angle is 2°. A reading greater than 2° indicates a Class II skeletal tendency. Angles less than 2° and readings of below zero (e.g.-1°,-2°etc) indicate that the mandible is located ahead of the maxilla, suggesting a Class III skeletal relationship.

·       Occlusal Plane Angle

This is the angle formed between occlusal plane and S-N plane. The mean reading for normal occlusions is 14°. Theangle is increased in long face or vertically growingindividuals and also skeletal open bite cases. It may be decreased in horizontally growing individuals orcases with a skeletal deep bite.

·       Mandibular Plane Angle

The mandibular plane is drawn between Gonion (Go) and Gnathion (Gn). The mandibular plane angle is formed by joining the mandibular plane to the anterior cranial base (S-N plane). The mean reading for this angle is 32°. Excessively high (vertical growers) or low (horizontal growers) mandibular plane angles are suggestive of unfavorable growth patterns and these may complicate treatment results.

The Dental Analysis

·       Maxillary Incisor Position

The maxillary incisor is related to the N-A plane bothby angular as well as linear measurements. The upperincisor to N-A reading in degrees indicates the relativeangular relationship of the upper incisor teeth,whereas the upper central incisor to N-A reading inmillimeters provides information on the relative forward or backward positioning of the incisor teeth to the N-A line.The upper central incisors should relate to the NAline in such a way that the most anteriorly placedpoint of its crown is 4 mm in front of the N-A line and its axial inclinationbears a 22° angle to the line.

·       Mandibular Incisor Position

The relative antero-posterior linear position and angulation of the lower incisor teeth is determined byrelating the most protruding incisor tooth to the N-B line. The most labial portion of the crown of the lowerincisor teeth should be located 4 mm ahead of the N-B line and the axial inclination of this tooth to the N-Bline should be 25°.

·       Inter-incisal Angle

The inter-incisal angle relates the relative position of the upper incisor to that of the lower incisor. If the angulation is more acute or less than the mean of 130°, then the anteriors are considered to be proclined. Conversely, if the angle is greater than 130° or more obtuse, the upper and/or lower incisors may require advancing anteriorly or correction of their axial inclinations.

·       Relation of lower incisor to chin

According to Holdaway, the distance between the labial surface of the lower incisor to the N-B line and the distance from the pogonion to the N-B line should be equal i.e. 4mm.This is called Holdaway ratio. A 2 mm discrepancy between these measurements is acceptable; a 3 mm is less desirable, but tolerable. If the difference between these dimensions exceeds 4 mm, however, corrective measures are generally indicated.

THE SOFT TISSUE ANALYSIS

The analysis laid emphasis on the soft tissue profile aswell as the underlying skeletal structure. The profilewas mainly affected by the chin, nose and the lips. The shape and posture of the lips is partially governedby the underlying dentition and thus can be modifiedorthodontically. The thickness of the tissue over thesymphysis and the nasal structure also contributes to the prominence of the lower face and attention shouldbe paid to the same when as it may camouflage the underlying malocclusion.

Steiner's S-line

According to Steiner, the upper lip in well balanced faces should touch a line extending from the soft tissue, contour of the chin to the middle of an "S" formed by the lower border of the nose. The lower lip should be 2 mm behind this line. This line is referred to as the "S-line". Lips located beyond this line tend to be protrusive in which case the teeth and/ or the jaws usually require orthodontic treatment to reduce their prominence. If the lips are positioned behind this line, it is generally interpreted that the patient possesses a "concave" profile. Orthodontic correction usually entails advancing the teeth in the dental arches to protrude the lips to approximate the S-line.

TWEED ANALYSIS

Tweed developed this analysis as an aid to treatment planning, anchorage preparation and determining theprognosis of orthodontic cases. At that time greatemphasis was laid on the placement of the mandibular incisors for the preservation of the orthodonticallyachieved results.

            Dr. Tweed established that prognosis could be predicted relatively accurately based on the configuration of the triangle.

The analysis consists of the Tweed's triangle formed by: -

1. Frankfort horizontal plane.

2. The mandibular plane.

3. The long axis of lower incisor.

The three angels thus formed are: -

1. Frankfort-Mandibular plane (FMA)

2. Lower incisor to mandibular plane (IMPA)

3. Lower incisor to Frankfort horizontal (FMIA)

The normal values for

FMA = 25°

IMPA = 90°

FMIA = 65

The Tweed analysis is primarily for clinical treatment planning and should not be considered acomplete analysis by itself.The ideal positioning of the lower incisors helps in thestability of the results achieved, thereby, indicating theprognosis of the case.

THE WITS APPRAISAL OF JAW DISHARMONY

The "Wits" appraisal of jaw disharmony employs justone measurement and is intended as a diagnostic aidwhereby the severity or degree of antero-posterior jawdisharmony can be measured on a lateral cephalometric head film. It is to be used as an adjuvant alongwith other analysis, mainly to reconfirm their results.

The ANB angle is the most commonly used reading for the appraisal of the horizontal disharmony of theface. Any change in the relative forward or backwardpositioning of nasion by virtue of an excessively long or short anterior cranial base (represented by the S-Nline) or a relative posterior or anterior positioning ofboth jaws within the skeletal craniofacial complex willdirectly influence the ANB reading.

Clockwise or counterclockwise rotation of the S-Nline (due to nasion or sellaturcica being positionedrelatively superiorly or inferiorly to each other) either

increases or decreases the SNA reading. Conventional analysis would suggest that the maxilla is positionedeither forward or backward to the craniofacial complex. Similarly, the rotational effect of the jawsrelative to the cranial reference plane would also affectthe ANB angle reading directly.

The Wits appraisal entails drawing perpendicularson a lateral cephalometric head film tracing frompoints A and point B onto the occlusal plane (which is drawn through the region of maximum cuspalinterdigitation). The points of contact on the occlusalplane from points A and Bare labeled AO and BOrespectively.

In skeletal Class II jaw case, point BO would be located well behind point AO (A positivereading) whereas in skeletal Class III jaw disharmonies the "Wits" reading would be negative withpoint BO being in front of point AO (negative reading).The more the "Wits" readings deviate from 1 mmin males and 0 mm in females, the greater thehorizontal jaw disharmony.

 Class II                            Class III

Limitation of cephalometrics

Sources Of Errors In Cephalometry
ERROR	CAUSES OF ERROR		HOW TO MINIMIZE THE ERROR
1.     Radiographic projection errors A.     Magnification:a certain amount of enlargement is seen in cephalometric radiographs. B.     Distortions: the head being 3 dimensional causes different magnifications at different depths of field. This may result in distortions. 2.     Errorswithin the measuring system Errors may occur in the measurement of the various linear and the angular measurements. 3.    Errors in landmark identification A.     Quality of radiographic image B.  Precision of landmark definition and reproducibility of landmark location C.     Operator bias		Magnification errors are because the x-ray beams are not parallel with all points of the object. Landmarks and structures not situated in the midsagittal plane are usually bilateral and may cause dual images in radiograph. Rotation of the patient’s head in any plane of space in the cephalostat may produce linear and angular distortions. Human error may creep in during the tracing and measurements. Poor definition of radiographs may occur due to use of fast films and intensifying screens although the radiation dose is reduced. Movements of the object, tube or the film may cause a motion blur Blurring of the radiograph may occur as a result of scattered radiation that fogs the film. Poor contrast of film may make differentiation between adjacent structures difficult. Errors may occur if the landmark is not defined accurately. This causes confusion in identification of a landmark. In general certain landmarks are difficult to identify such as porion. Variation has been observed in landmark identification between operators. The operators expectations can result in bias of the values.	By using a long focus-object distance and a short object-film distance. By use of angular rather than linear measurements. This error may be overcome by recording the midpoint of the two images. By standardization head orientation using ear rods, orbital pointer and forehead rest. The use of computerized plotters and digitizers to digitizers to digitize the landmarks and to carry out the various linear and angular measurements has proven to be more accurate and consistent. Recommended films should be used to avoid poor definition radiographs This can be avoided by stabilization of the object, tube and the film. By increasing the current, the exposure time is reduced, thus minimizing the possibility of motion bur. This can be reduced by the use of grids. Good contrast is obtained by using good films and use of adequate Kv level. Too high Kv results in poor contrast. Landmarks have to be accurately defined. Certain landmarks may require special conditions to identify which should be strictly followed. Good quality radiography and use of average values from multiple identification of the same landmark. It is advisable for the same person to identify and trace in patients who are subject to serial cephalometric studies. This can be overcome by randomizing the recorded measurements and by adopting a double blind study pattern.

Conclusion

Broadbent’s gave us a three dimensional analysis, i.e the lateral view and P-A view. The lateral view is easy to work with and the patient is also much more recognizable than in the frontal (P-A) view, especially with soft tissue enhancement.

REFERENCE

—  CONTEMPORARY BOOK OF ORTHODONTICS BY PROFITT

—  TEXT BOOK OF ORTHODONTICS – GURKEERAT SINGH

—  THE ART AND SCIENCE OF ORTHODONTICS BY BALAJI