Taken from the Angle Orthodontist on CD-ROM (Copyright Š 1997 Angle Orthodontist, Inc.), 1994 No. 4, 299 - 310: Commentary: Skeletal jaw relationships Martin Fine. -------------------------------COMMENTARY Commentary: Skeletal jaw relationships Martin Fine, BDS, MSc Cephalometrics in orthodontic practice is an established diagnostic tool employed by clinicians worldwide. Conventional cephalometrics has served orthodontic research and diagnosis since its standardization in 1931.1 It is only in recent times that conventional cephalometric analysis has become the subject of increased scientific scrutiny. The orthodontic literature is replete with different analyses based upon linear, angular and/or proportional measurement systems. When applied to cephalometrics, these systems have little rigorous theoretical backing and are based mainly upon convention.2 In fact, in six decades of cephalometric usage, there has been relatively little scientific progress in the measurement of cephalometric form or in the measurement of biological form in general. The problem areas in cephalometrics can be divided into the following: 1.)
Imaging difficulties: the reduction of a complex three-dimensional
craniofacial form into a two-dimensional projection is the first in a cascade of steps which results in the indiscriminate loss of information in cephalometry. 2.)
Datum point selection: in conventional cephalometics irregular two-
dimensional form is reduced to a handful of datum points. Limited numbers of datum points provide only a cursory description of craniofacial form,
yielding no data concerning the curvature of boundary outlines,3 resulting in further indiscriminate data loss. 3)
Measurement difficulties: the combination of the loss of the third
dimension and further reduction of data through the use of limited datum point arrays is compounded by their summarization through inappropriate measurement techniques. Linear and angular techniques or their respective ratios are inadequate for describing cephalometric form.4 Different combinations of datum points may produce the same angle5 or linear distance. Also, size and shape parameters cannot be discriminated from traditional linear and/or angular cephalometric dimensions. Thus a change in the facial angle or distance between gonion and condylion may reflect a size or shape change, or more likely varying combinations of size and shape changes. Conventional cephalometric analysis generally involves a univariate approach of comparing individual measurements with corresponding population means. This method is more appropriate for population studies than for individuals.6 In addition, the variable correlation between different conventional cephalometric measures renders them unsuitable for univariate statistical
analysis.7
Multivariate
techniques
are
better
suited
to
cephalometric analysis and allow comparison of an array of measurements as a whole as opposed to discrete parts. In addition, the use of multiple discrete measurements in conventional cephalometrics depends on their subjective analysis. It is difficult, if not impossible, for a clinician to recount the logical steps made in arriving at a cephalometric diagnosis from the array of measurements which make up a conventional analysis.8
If traditional cephalometrics is fraught with so many problems, how has it been possible for cephalometrics to produce any useful results? Conventional cephalometric measurements are probably correlated with more sophisticated forms of measurement to a greater or lesser degree. For example, a patient with a large mandible (even if differently shaped than a “normal” mandible) is likely to show increases in most linear measurements of the mandible. Similarly, a “long face” is usually associated with an increased vertical dimension. Dr. Lowe and coworkers have addressed the concerns about conventional cephalometrics by using a measurement technique (EFF) with a rigorous scientific basis well-suited to the task of measuring irregular biological forms. As opposed to the Finite Element Method (FEM, a different rigorously-based method of measuring biological form) EFF facilitates the measurement of outline form. They then analyzed the EFF data appropriately using multivariate statistical techniques. The difficulty with EFF (and FEM) is that its parameters are difficult to understand (when compared with the relatively simple conventional cephalometric measures). For example, we can all picture how the mandibular plane angle will change as the mandible rotates open. What will happen to EFF parameters in this scenario? At the present time we simply do not have enough knowledge to elucidate how EFF parameters might vary to reflect different skeletal morphological patterns. One could argue that multivariate analysis will take care of this uncertainty. However, it is important that the multivariate analysis be provided with appropriate variables that reflect the important data. For example, measurements of cranial base form are likely to be less important in orthodontic A-P skeletal diagnosis than those of maxillo-mandibular form.
This factor can be taken into account by the differential variable weighting, which can reduce misclassification in Cluster analysis.9–11 In fact, the decision to include or exclude a variable is in itself a form of weighting. This paper has taken steps to address fundamental problems in cephalometrics. This could lead to further research which will provide for more formal diagnostic techniques and therefore more logical objective treatment planning.
Taken from the Angle Orthodontist on CD-ROM (Copyright Š 1997 Angle Orthodontist, Inc.), 1994 No. 6, 447 - 454: Landmark identification error in posterior anterior cephalometrics Paul W. Major, Donald E. Johnson, Karen L. Hesse,... ---------------------ORIGINAL ARTICLE Landmark identification error in posterior anterior cephalometrics Paul W. Major DDS,MSc.,MRCD(C); Donald E. Johnson DDS,MSc; Karen L. Hesse BSc.,DDS; Kenneth E. Glover DDS,MSc.,MRCD(c) Abstract : This study was designed to quantify the intraexaminer and interexaminer reliability of 52 commonly used posterior anterior cephalometric landmarks. The horizontal and vertical identification errors were determined for a sample of 33 skulls and 25 patients. The results show that there is a considerable range in the magnitude of error with different horizontal and
vertical values. Interexaminer landmark identification error was significantly larger than intraexaminer error for many landmarks. The identification error was different for the skull sample compared to the patient sample for a number of landmarks. The relevance of knowing the identification error for each landmark being considered in a particular application was discussed. Key Words : Landmark identification error · Posterior anterior cephalometrics · Intraexaminer reliability · Interexaminer reliability Since the introduction of a standardized method for obtaining skull radiographs,1 cephalometrics has become one of the major diagnostic tools in orthodontics. The posterior anterior cephalogram contains diagnostic information not readily available from other sources. This information allows the practitioner to evaluate the width and angulation of the dental arches in relation to their osseous bases in the transverse plane; evaluate the width and transverse positions of the maxilla and mandible; evaluate the relative vertical dimensions of bilateral osseous and dental structures; assess nasal cavity width; and analyze vertical and/or transverse facial asymmetries.2–7 Regardless of the clinical or research application, it is critical to know the reliability of the reference landmarks. Baumrind and Frantz8 point out that there are two general classes of error associated with cephalometric measurements. The first class of errors are “projection” errors which arise from the geometry of the radiographic setup. The fact that the x-ray beam originates from a source which has a finite size leads to a penumbra effect or optical blurring.9,10 The x-ray beam diverges as it moves away from the source, which results in an overall magnification of the object being radiographed and a radial displacement of all points
which are not on the principal axis (central ray). The radiographic image is distorted as points closer to the film are magnified less than points farther from the film. The second general class of landmark errors may be termed “errors of identification,� and arise due to uncertainty involved in locating specific anatomic landmarks on the radiograph. The precision with which any landmark may be identified depends on a number of factors.8,11,12 Landmarks lying on a sharp curve or at the intersection of two curves are generally easier to identify than points located on flat or broad curves. Points located in areas of high contrast are easier to identify than points located in areas of low contrast. Superimposition of other structures, including soft tissue over the area of the landmark in question, reduces the ease of identification. Precise written definitions describing the landmark reduces the chance of interpretation error. Operator experience is an important factor since increased knowledge of anatomy and familiarity with the radiographic appearance of the subject reduces interpretive errors. A literature review concerning the reliability of landmark identification in posterior anterior cephalometrics revealed only one article, by El-Mangoury et al.,12 which determined the horizontal, vertical and radial variability of 13 landmarks. They found that each landmark had its own characteristic noncircular envelope of error, and that the variability is different in the horizontal and vertical directions. Unfortunately, the majority of posterior anterior cephalometric analyses use landmarks whose identification error has not been independently reported. The purpose of this study was to examine the reliability of posterior anterior cephalometric landmarks. Skeletal and dental landmarks to be investigated were chosen to include those most commonly used in published posterior
anterior cephalometric analyses,13,14–19 and those landmarks which can be recognized on the posterior anterior cephalogram.20,21,22 Landmark reliability for cephalograms taken both on dry skulls and living patients were identified and compared. Materials and methods A sample of 33 dry adult skulls from the University of Alberta collection with intact dentitions and no gross asymmetries were radiographed with a standardized technique. The source-to-film distance was a constant 160 cm and the distance from the middle of the earrods to the film was 17.5 cm. A sample of 25 adult patient posterior anterior cephalograms based on the absence of obvious skeletal or dental asymmetries, was chosen from consecutive orthodontic records taken at a private radiology facility. All patient cephalograms were taken using a Siemans OP10 x-ray machine with standardized exposure and head positioning with Frankfort Horizontal parallel to the floor. Source-to-earrod distance was 60 inches and earrod-tofilm distance was 5 inches. Landmarks were digitized directly off the radiographs using a GP6 Sonic Digitizer R in conjunction with an IBM-compatible computer and a custom program developed using Basic TM. An individual coordinate system was established for each radiograph by including two fiducial points which consisted of a pinhole placed on each radiograph at the superior and medial corner of both earrod markers. These two pinholes were digitized first which enable the digitization program to calculate the slope of the line between the two pinholes. This value was used as the X-axis of a cartesian coordinate system. The Y-axis was calculated as the line perpendicular to the X-axis originating at the midpoint of the line between the two pinholes. This coordinate system eliminated the orientation of the radiograph on the
viewbox as a variable. Fifty-two commonly used landmarks were then digitized including 36 bilateral skeletal landmarks. The following landmarks (Figure 1) were identified on each radiograph: A. Bilateral skeletal landmarks 1.
Greater Wing Superior Orbit (GWSO) - the intersection of the
superior border of the greater wing of the sphenoid bone and lateral orbital margin. 2.
Greater Wing Inferior Orbit (GWI0) - the intersection of the inferior
border of the greater wing of the sphenoid bone and the lateral orbital margin. 3.
Lesser Wing Orbit (LWO) - the intersection of the superior border of
the lesser wing of the sphenoid bone and medial aspect of the orbital margin. 4.
Orbitale (O) - the midpoint of the inferior orbital margin.
5.
Lateral Orbit (LO) - the midpoint of the lateral orbital margin.
6.
Medial Orbit (MO) - the midpoint of the medial orbital margin.
7.
Superior Orbit (SO) - the midpoint of the superior orbital margin.
8.
Zygomatic Frontal (ZF) - the intersection of the zygomaticofrontal
suture and the lateral orbital margin. 9.
Zygomatic (Z) - the most lateral aspect of the zygomatic arch.
10.
Foramen Rotundum (FR) - the center of foramen rotundum.
11.
Condyle Superior (CS) - the most superior aspect of the condyle.
12.
Center Condyle (CC) - the center of the condylar head of the condyle.
13.
Mastoid Process (MP) - the most inferior point on the mastoid
process. 14.
Malar (M) - the deepest point on the curvature of the malar process of
the maxilla. 15.
Nasal Cavity (NC) - the most lateral point on the nasal cavity.
16.
Mandible/Occiput (MBO) - the intersection of the mandibular ramus
and the base of the occiput. 17.
Gonion (G) - the midpoint on the curvature at the angle of the
mandible (gonion). 18.
Antegonial (AG) - the deepest point on the curvature of the antegonial
notch. B. Midline skeletal landmarks 1.
Crista Galli (CG) - the geometric center of the crista galli.
2.
Sella Turcica (ST) - the most inferior point on the floor of sella
turcica. 3.
Nasal Septum (NSM) - the approximated midpoint on the nasal
septum between crista galli and the anterior nasal spine. 4.
Anterior Nasal Spine (ANS) - the center of the intersection of the
nasal septum and the palate. 5.
Incisor Point (IPU) - the crest of the alveolus between the maxillary
central incisors. 6.
Incisor Point (IPL) - the crest of the alveolus between the mandibular
central incisors. 7.
Genial Tubercles (GT) - the center of the genial tubercles of the
mandible. 8.
Menton (ME) - the midpoint on the inferior border of the mental
protuberance. C. Bilateral dental landmarks 1.
Maxillary Cuspid (MX3) - the incisal tip of the maxillary cuspid.
2.
Maxillary Molar (MX6) - the midpoint on the buccal surface of the
maxillary first molar. 3.
Mandibular Cuspid (MD3) - the incisal tip of the mandibular cuspid.
4.
Mandibular Molar (MD6) - the midpoint on the buccal surface of the
mandibular first molar. To determine intraexaminer landmark reliability, each radiograph was digitized five times by the principle investigator. To avoid operator bias, radiographs were digitized randomly and no individual radiograph was digitized more than once in a day. The raw data was examined for any single digitization which differed from the average of the other four by greater than 10 mm. Digitization of that particular radiograph was repeated, effectively eliminating any instances where the wrong point was digitized by mistake. Deviations from each landmark mean value were analyzed to give the standard deviation of the mean, which was considered to be the landmark identification error in millimeters. To determine interexaminer landmark reliability each radiograph was digitized one time by each of four operators with graduate level training in cephalometrics. Each operator was provided with written descriptions and diagrams of the landmark location for reference during digitization procedures. Data analysis was completed using the procedure outlined for intraexaminer landmark reliability. The error of the method was established by repeated digitization of a precisely defined point which consisted of a pinhole in the radiograph. Results A. Reliability of the Method Reliability is a measure of the reproducibility or, in this case, the closeness of the recorded coordinates for each particular landmark. In estimating the reliability of the method, four contributing factors were identified. 1.
Radiograph (R) - differences in landmark position between individual
skulls or patients.
2.
Position (P) - differences between positions of different landmarks
within the same skull or patient. 3.
Side (S) - differences in landmark position between the left and right
sides of the skull or patient. 4.
Case (C) - differences between successive digitizations of the same
radiograph. The reliability of the method was calculated using generalizability theory which uses an analysis of variance to separate the total variance into its component parts. The total variance is made up of the variation due to each factor plus the variation due to all combinations of factors. Since reliability is a measure of how reproducible the method is in repeated trials, any variance between successive digitizations is considered undesirable. To calculate the general reliability of the method, variance due to case and any other variance in combination with case were subtracted from the total variance, then this value was divided by the total variance. where: R = reliability; VT = total variance; Vc = variance due to case; VcRP = variance due to case in combination with radiograph position. Because the sample was accepted on the criterion of good facial symmetry, the relative contribution of side as a variable was not considered in the estimation of reliability. The very high level of reliability [Rx(skull) = .9995, Ry(skull) = .9992, Rx(patient) = .9910, Ry(patient) = .9985] indicates that the relative contribution of multiple digitizations to the total variance is very low. B. Method error
The magnitude of error associated with the equipment (SDx = .13 mm, SDy = .10 mm) was very close to the Âą .1 mm accuracy of the digitizer claimed by the equipment manufacturer. C. Intraexaminer landmark error The error associated with the identification of each landmark was calculated for both the skull and patient samples (Tables 1 and 2). There was a wide variation in the amount of identification error between landmarks, as well as between the vertical and horizontal error for each particular landmark. Visual inspection of the results indicates that the identification errors for the skull and patient radiographs were similar, with the values generally larger for the patient radiographs where soft tissue became a factor. Landmark identification error for the skull sample and patient sample were compared using a Student Newman Keuls comparison of means (P<.05). Horizontal identification error was significantly greater in the patient sample for Landmark Mandible/Occiput (MB0). Vertical identification error was significantly greater in the patient sample for Landmark Maxillary Cuspid (MX3) and Crista Galli (CG). Vertical identification error was significantly greater in the skull sample for Landmark Zygomatic Frontal (ZF) and Nasal Septum (NSM). D. Interexaminer landmark error The landmark identification errors for a single examiner and four examiners were determined for a selected sample of 20 skull and patient radiographs (Tables 3 to 6). The results indicate that landmark identification error was generally larger when four examiners were used, with the error for the patient sample larger than the skull sample. A Student-Newman-Keuls comparison of means was used to compare the identification errors of each sample.
The results listed in Tables 3 to 6 show the comparison between groups. Horizontal interexaminer landmark identification error was significantly larger than the intraexaminer error for four landmarks in the skull sample, and 10 landmarks in the patient sample. Vertical interexaminer landmark identification error was significantly larger than the intraexaminer error for eight landmarks in the skull sample and 17 landmarks in the patient sample. Horizontal interexaminer landmark identification error was larger in the patient sample compared with the skull sample for landmarks Lateral Orbit (LO), Foramen Rotundum (FR) and Malar (M). Vertical interexaminer landmark identification error was larger in the patient sample compared to the skull sample for Landmarks Orbital (O), Condyle Superior (CS), Condyle Center (CC), Zygomatic Frontal (ZF), Foramen Rotundum (FR), Maxillary Cuspid (MX3), Crista Galli (CG) and Genial Tubercles (GT). Discussion There was a great deal of variability in the magnitude of horizontal and vertical landmark identification errors. This variability existed both within each landmark and between different landmarks. This is in agreement with the findings of other studies into landmark identification errors.8,11,12,23– 25 The range of values (in millimeters) for intraexaminer errors (0.28–2.23) was of similar magnitude as that reported by Vincent and West11 (0.31– 2.09) who also used five digitizations. The El Mangoury et al.12 study into Posterior Anterior Cephalometric landmark identification error reported a range of error of 0.42 to 1.74. Her study used patient radiographs and when the same landmarks were examined in this study, the range of error was of similar magnitude (0.37–1.10). The interexaminer identification errors showed a wide variation in magnitude in both horizontal and vertical dimensions. The range of values
(0.31â&#x20AC;&#x201C;4.79) was larger than in the intraexaminer portion of the study. This difference can be attributed to interpretive differences between operators. The study by El Mangoury et al.12 used only one operator and did not report interexaminer error. The Baumrind and Frantz8 study on lateral cephalograms used multiple operators and the range of error reported in their study was 0.34 to 3.71, which is similar to the range found in this study. The choice of landmarks used in any analysis will depend on the objective of the analysis. Knowledge of the landmark identification error in both the horizontal and vertical directions is essential in establishing a valid analysis. Landmarks with a large horizontal identification error should be avoided in transverse measurements. Similarly, landmarks with large vertical identification error should be avoided in measuring vertical structural relationships. Some landmarks will be useful for measurements in one dimension but not in the other. For example, landmark Nasal Septum (NSM) has a relatively small horizontal error (.49 in the skull sample) and large vertical error (2.82 in the skull sample). Caution must be exercised when comparing data taken from skull samples to patient samples. Most landmarks had similar identification errors but there were exceptions. Some landmarks may be quite useful in research trials where one examiner takes repeated measurements, but less useful for clinical diagnosis where differences in interpretation may be large. For example, landmark Zygomatic (Z) had a relatively small intraexaminer error in both the horizontal (0.29) and vertical (0.51) dimensions, but large interexaminer errors in both the horizontal (2.42) and vertical (3.49) dimensions. This particular landmark may be very useful in research but would have limited value as part of a clinical diagnostic analysis.
The clinical significance of the magnitude of landmark identification error will depend on the level of accuracy required. The landmark identification errors reported in this study represent the standard deviation of error. Landmarks with identification errors greater than 1.5 mm should probably be avoided and landmarks with identification error greater than 2.5 mm are inappropriate. The reliability of landmarks for dried skulls was compared to live patients. In general landmarks are less reliable on patient radiographs where soft tissue reduces hard tissue image sharpness. These differences should be kept in mind when applying data from dry skull studies to clinical settings. The basis of cephalometrics in orthodontic diagnosis includes the use of standardized and reproducible head position in relation to the x-ray source and film. The cephalostat earrods minimize rotation about the vertical and transverse axis. A third reference may be positioned against the nose to prevent rotation about the anterior posterior axis.1 Rotations of the head can potentially occur through soft tissue distortion or improper patient positioning. This study did not investigate the effect of head rotation on landmark identification. Conclusion The intraexaminer and interexaminer landmark identification errors associated with 52 posterior anterior cephalometric landmarks were presented. The magnitude of landmark identification error had a wide range with the horizontal error often being different from the vertical error. Some landmarks showed significantly different errors when taken from skull radiographs
versus
patient
radiographs.
Interexaminer
landmark
identification errors were generally larger and, in many cases, significantly larger, than intraexaminer errors. Many of the proposed posterior anterior
cephalometric analyses use landmarks which have an unacceptable magnitude of landmark identification error. Taken from the Angle Orthodontist on CD-ROM (Copyright © 1997 Angle Orthodontist, Inc.), 1987 No. 2, 168 - 175: Cephalometric Reliability A Full ANOVA Model for the Estimation of True and Error Variance Peter H. Buschang, Richar... ---------------------or variance has been method error. Depending on the design of the analysis, method error alone could produce inaccurate results ( BUSCHANG ET AL. 19844). Moreover, comparisons of error variance are difficult to interpret due to the lack of standardization. In contrast, the coefficient of reliability that is presented
Taken from the AJO-DO on CD-ROM (Copyright © 1997 AJO-DO), Volume 1958 Dec (901 - 905): Résumé of the workshop and limitations of the technique - Salzmann -------------------------------Ever since God created man in His image, man has been trying to change man into his image. Attempts to change facial appearance are recounted throughout recorded history. The question of what is a normal face, as that of what constitutes beauty, will probably never be answered in a free society. Orthodontists, in their attempts to change facio-oro-dental deviations from accepted norms, have adopted cephalometric measurement, a method long employed in physical anthropology. With the introduction of
roentgenography, it was inevitable that this procedure should be employed as a medium for the purpose of roentgenographic cephalometrics. Taken from the AJO-DO on CD-ROM (Copyright Š 1997 AJO-DO), Volume 1987 May (414 - 426): Normal radiographic anatomy and common anomalies in cephalometrics - Kantor and Norton -------------------------------Normal radiographic anatomy and common anomalies seen in cephalometric films Mel L. Kantor, D.D.S., and Louis A. Norton, D.M.D. Chapel Hill, N.C., and Farmington, Conn. Lateral and posteroanterior cephalometric radiographs are used routinely in the diagnosis and quantification of dentofacial anomalies that require orthodontic treatment. The anatomic information that these films contain is occasionally overlooked as the clinician prepares tracings and makes measurements. With the increase of the average age of the orthodontic patient population, there is greater likelihood of the presence of disease. This article describes some important features of normal radiologic anatomy of the head and neck so that a clinician can better recognize pathologic changes. Common pathologic findings and anatomic anomalies are also illustrated. (AM J ORTHOD DENTOFAC ORTHOP 1987;91:414-26.) During the course of evaluation and treatment, the orthodontist often takes cephalometric radiographs of the patient's skull. A mathematic analysis is usually done to help diagnose and quantify skeletal and dental malocclusions, make growth predictions, or monitor the patient's treatment progress. However, fortuitous findings must not be overlooked or ignored. The clinician should evaluate the skull radiographs for any abnormalities
that might be present. To assist the orthodontist with this responsibility, we will review normal radiographic anatomy of the human skull emphasizing a systematic approach to interpretation. Examples illustrating variations of normal anatomy that may be mistaken for pathosis are provided as well as examples of pathologic changes that are often overlooked. No attempt will be made to illustrate the full range and distribution of normal anatomy in this limited review. References dealing with this subject are cited. FILM INTERPRETATION The information content of a radiograph is a complex function of film/screen selection, technique factors, processing, and patient anatomy. The first three of these parameters can be controlled and should be optimized to ensure the best radiographic image with the least patient exposure. However, once a radiograph is processed the amount of information recorded in the image does not change, but the amount of information that can be retrieved from each image is greatly affected by the circumstances under which the film is viewed.1,2 Reduced ambient lighting, quiet surroundings, and the elimination of peripheral light improve visual acuity.3,4 Kundel and Nodine5 have described two modes of visual perception of radiographs. First is "global perception" resulting from rapid parallel processing of the entire retinal image by means of pattern recognition and rapid association with previously acquired visual concepts. The second is "analytic perception,'' which is based on the extraction of features from the incoming visual data and the use of logical rules to combine them in a meaningful way. This technique results in a gradual buildup of the perception.
They
suggest
that
experienced
radiologists
perceive
abnormalities in a global manner and that specific features are perceived secondarily. The experienced orthodontist can often rapidly scan a
cephalometric film and tell whether a patient has a dental or skeletal problem or a combination of the two and what part of the anatomy is contributing the most to the problem. The cephalometric analysis usually corroborates this global impression and quantitates a qualitative judgment. Christensen and associates6 evaluated the effect of search time on perception and found that obvious abnormalities are detected almost instantaneously but that the overall number of abnormalities identified increased as the viewing time increased. The number of visual images that are immediately recognizable is a function of experience and the analytic approach is necessary to evaluate those images that represent uncommon findings. Even the experienced radiologist can be seriously misled and draw the wrong conclusion if pattern recognition is the primary mode of radiographic interpretation.7 Bisk and Lee8 reviewed 513 lateral cephalometric head films, which represented the total population of the orthodontic practice of the senior author. Eighteen films (3.5%) were classified as having abnormalities or pathosis present as follows: enlarged adenoids— 5, failure of segmentation C4-CS— 1, impacted canine— 1, interstitial emphysema— 1, osteoma— 1, sinus polyp— 1, and sinusitis— 8. Because abnormalities occur infrequently, the orthodontist should carefully search the cephalometric films for features that would suggest disease and warrent further investigation. Nanda, Merow, and Martin9 reported four cases of significant abnormalities that were incidental findings: (1) a foreign object in the right nostril, (2) bilateral retention cyst in the maxillary sinuses, (3) unusual intrasellar cyst with a tooth or dermoid and, (4) multiple cysts of the jaws as part of the basal cell nervous syndrome. Although the first two observations had little impact on the patients' health, the latter two findings could have
had a serious negative effect on the patients' well-being if they had been overlooked. CRANIUM In evaluating the cranium, the method suggested by Meschan10 is recommended. 1. Calvarium and base. Initially, the size and shape of the calvarium and base should be evaluated. Gooding11 reviews some of the common morphometric indices available and concludes that they are most valuable for following changes once an abnormality has been identified and that "with experience normal craniofacial proportions at different age levels are appreciated, and deviation is recognized as an indication of intra-cranial abnormality.'' The calvarium is divided into three layers; the inner and outer tables are compact bone and the middle table is cancellous. Thickness varies widely in individuals and this will be demonstrated as varying radiodensities on the radiograph. The thickest part of normal vault should not exceed 1 cm, after which some degree of cerebral underdevelopment or systemic disease should be suspected.12 2. Lines, impressions, channels, and sutures. Examination of the inner surface of the calvarium will show numerous lines, impressions, and channels that reflect the structure of the brain and its meningeal covering (Fig. 1, A). a. Meningeal vessel grooves. The arteries and veins of the meninges are closely adapted to the inner able of the calvarium resulting in lines readily identifiable by their well-defined borders, smooth undulating course, and characteristic location. The middle meningeal vessels are usually the most
prominent; they begin at foramen spinosum and branch out, tapering along the way. b. Diploic vein channels. The diploic veins are contained in channels within the cancellous bone of the middle table or diplรถe. They will appear as radiolucent channels 2 to 3 mm wide, coursing in an irregular pattern over the calvarium; they do not appear to taper as the meningeal vessels do. When two or more of these veins anastomose, a diploic lake may be present. The diploic venous lakes are irregular, usually less than 2 cm in size, and have multiple diploic veins running into them. Awareness of the existence of diploic venous lakes and the observation of diploic channels associated with them will usually allow the clinician to recognize these for what they are and not mistake them for osteolytic lesions, such as bone metastasis, meningoceles, fibrous dysplasia or histiocytosis X.13 c. Sutures. The sutures form the articulation of the cranial bones. Many of the sutures are closed by the second year of life. The spheno-occipital synchondrosis begins to ossify at puberty; the coronal, lambdoidal, and sagittal sutures persist through early adulthood.10,14 Premature closure of the sutures may be a primary defect, a component of other known head and neck syndromes, or associated with metabolic, osseous, or hematologic disorders.15 Sutural widening is usually a result of increased intracranial pressure or destruction of bone at the suture margins. Observation of any of these findings warrants further studies and consultation with the patient's physician is recommended. The coronal, lambdoidal, and squamosal sutures can be seen on the lateral cephalograph; the sagittal and lambdoidal sutures and their junction, lambda, are seen on the posteroanterior (PA) cephalogram. The sutures appear as radiolucent serpentine lines in their anatomically expected location. Occasionally, there are small independent
bones that persist within a suture; these are called wormian bones and the lambda region is a common location for them (Fig. 1, B). Multiple wormian bones may be associated with cleidocranial dysplasia, cretinism, or osteogenesis imperfecta.13 It is important to recognize the radiolucent lines that represent the meningeal vessel grooves, the diploic vein channels, and the sutures, and to be able to distinguish them from fractures of the calvarium, especially given a history of recent trauma. d. Arachnoid (pacchionian) granulation impressions. The arachnoid granulations are an out-pocketing of the arachnoid membrane and subarachnoid space that may extend into the dural sinuses or the adjacent lacuna laterales. When found in the latter region, they may present as irregularly rounded, sharply radiolucent depressions of the inner table of the skull. They are most commonly found just lateral to the superior sagittal sinus, although they can be located in proximity to any of the dural sinuses.16 They may also calcify and this presentation will be described in a later section. e. Dural sinuses. The sinuses of the dura mater are the channels by which the blood from the cerebral veins, and some of the meningeal and diploic veins drain into the internal jugular veins. The superior sagittal, sphenoparietal, transverse, and sigmoid sinuses groove the inner table of the calvarium producing broad radiolucent channels. f. Convolutional markings. Also called digital markings or brain markings, the convolutional markings are impressions or thinning of the inner table of the calvarium caused by pressure from the convolutions or gyri of the growing brain. They are most prominent in the 3- to 12-year age group and tend to regress with age.17,18 Absence of these markings in the young or persistence into adulthood, especially when accompanied by neurologic
signs and symptoms or other cranial morphologic abnormalities, is a significant pathologic finding.19,20 g. Artifacts. If the patient's hair is particularly thick, wet, or pulled taut, it may cause linear streaks to appear over the calvarium (Fig. 1, C). 3. Calcification within the calvarium. There are a number of intracranial structures that may calcify in the absence of any disease. Reiskin7 has stressed the importance of multiple right-angle views for the localization and evaluation of these structures as a necessary component to distinguish between those structures that are normal or physiologic and those that are pathologic. Meschan20 has described the normal structures within the calvarium that may calcify. They can be summarized as follows: a. Pineal gland. The incidence of pineal calcification varies from 33% to 76% in the North American white population; there is a considerably lower incidence in Japanese (10%), Indians (8%), and Nigerians (5%). The size of the calcification averages 5 mm in length and 3 mm in height and width. When seen in the frontal projection, the pineal gland is a midline structure and a shift of 3 mm or more from midline is considered significant (Fig. 2, A). Numerous methods have been described to localize the pineal gland in the lateral radiograph; in general, it will be found above and slightly behind the petrous portion of the temporal bone (Fig. 2, B). Calcification of the pineal in children is not as common as in adults, but it is not a rare phenomenon. It may be observed in approximately 5% of white children under 10 years of age. b. The habenular commissure may calcify and it will appear as a C-shaped radiodensity located a few millimeters anterior to the pineal gland in about 30% of the adult population (Fig. 2, C).
c. Meningeal calcifications. The falx cerebri is calcified in approximately 7% of adults and is usually shown to best advantage in the frontal projection where it appears as a linear midline radiopacity (Fig. 2, D). Calcification of the arachnoid granulation appears as uniform radiopacities near the corresponding granulation impression in the calvarium. d. Petroclinoid ligament and diaphragma sellae. Calcification of the petroclinoid ligament occurs in approximately 12% of adults and appears as a radiopaque line extending from the posterior clinoid process to the petrous ridge. Calcification of the diaphragma sellae may give the appearance of a separate enclosed pituitary fossa. However, it must be remembered that we are only seeing a two-dimensional representation and, in fact, there is a space between the interclinoid calcifications to accommodate the pituitary stalk. Radiographically, this appearance is described as ''roofing" or ''bridging" of the sella (Fig. 2, E). In the absence of any clinical neurologic signs or symptoms, these calcifications may be considered normal; however, it is important to remember that many pathologic processes can be associated with these calcifications. A patient with a calcified pineal gland who is experiencing headaches, nausea, and vomiting should not be ignored; appropriate referral and follow-up are warranted. Once again, the patient's hairstyle may create artifacts that mimic real findings. For example, if the hair is gathered on the lateral surface of the skull into pigtails, it may resemble intracranial calcification on the lateral skull film (Fig. 2, F). 4. Size and shape of the sella turcica. The sella turcica is a saddle-shaped formation of the sphenoid bone in the middle cranial fossa. When viewed in the lateral radiograph, the anterior clinoid processes are usually
superimposed; the hypophyseal fossa appears as a single dense curved line that merges posteriorly with the posterior clinoid processes of the dorsum sellae. The clinoid process may range from short and rounded to long and pointed. Normal variants include (1) a middle clinoid process, (2) extension of the sphenoid sinus into the dorsum sellae, posterior clinoid process or anterior process, and (3) bridging as previously described. Because the sella turcica is a midline structure, the floor of the hypophyseal fossa usually appears as a single line. A double-contoured appearance may represent a variant of normal, an artifact of positioning, or a significant pathologic change.21,22 When viewed in the sagittal plane, the normal range for the greatest anteroposterior dimension is 5 to 16 mm (average 10.6 mm), and the depth as measured from a line between the anterior and posterior clinoid processes to the floor of the hypophyseal fossa ranges from 4 to 12 mm (average 8.1 mm).23 Significant variation in the size, area, or volume of the sella associated with a variation of two standard deviations in height and weight as compared to age-matched cohorts suggests a pituitary abnormality and the patient's physician should be alerted to this finding. Expansion or erosion of the borders of the pituitary fossa, especially if accompanied by neurologic findings such as headaches, blurred or double vision, or dizziness, is a significant finding and the patient should be referred for a thorough evaluation. The sella turcica is also seen in the PA view where it is superimposed over the superior aspect of the nasal cavity. In this view the floor of the sella is usually convex upward. PARANASAL SINUSES The paranasal sinuses develop as outpouchings of the mucous membrane of the fetal nasal cavity that extend into the maxillary, sphenoid, frontal, and ethmoid bones, and subsequently enlarge. In adulthood the sinuses
communicate with the nasal cavity through ostia, thus reflecting their common embryologic origin. The maxillary, sphenoid, and ethmoid sinuses begin to enlarge in utero and may occasionally be detected radiographically at birth. The frontal sinuses do not begin to pneumatize until the second year and are not usually visible on the radiograph until the sixth year.24 Hence, all four sets of paranasal sinuses should be evident in the average orthodontic patient. The variation in size of the normal sinus may be great. 1. Maxillary sinuses are seen in the PA, base, and lateral views. In the standard PA view, the petrous portion of the temporal bone is superimposed over the superior one third of the sinus. If disease is suspected, the best view of the maxillary sinuses in the frontal plane is obtained with a Water's projection. The lateral view will show the borders in the sagittal plane; however, the right and left sinuses will be superimposed and often indistinguishable. On films obtained in the erect position, soft-tissue swelling can usually be differentiated from free fluid in the sinus by the nature of the air-shadow interface. The air-fluid line will be straight and paraliel to the floor (Fig. 3, A); a soft-tissue swelling will produce a shadow that follows the bony contours or is convex (Fig. 3, B). Bone destruction is an important radiographic sign that requires biopsy and/or culture. 2. Frontal sinuses are seen to best advantage in the PA and lateral views. They vary greatly in size, are usually asymmetric, and may even be absent. An osteoma of the frontal sinus is not a rare finding (Fig. 4); it may be an isolated finding or part of a generalized process such as Gardner's syndrome.25,26 If osteomas are identified in association with the sinuses or anywhere else, inquiry into family history and examination of the skin for sebaceous cysts are required. The patient's physician should be informed of any positive findings.
3. Sphenoid sinuses appear as a single cavity in the sphenoid bone, inferior to the sella turcica in the lateral film. Although identifiable in the frontal projection, the superimposition of the nasal septum, lateral nasal wall, and the medial wall of the orbits makes evaluation difficult. The lateral extension of the sphenoid sinuses is easily seen on the base projection; it is known to vary greatly and, in the absence of any other pathologic findings, should be considered an insignificant incidental finding.27 4. The ethmoid sinuses, also known as the ethmoid air cells, form the medial wall of the orbit and the lateral wall of the upper half of the nose. The ethmoid sinuses are divided by numerous septa resulting in multiple compartments. Of the radiographic projections typically obtained for orthodontic treatment planning, the ethmoid sinuses are best seen on the lateral and base views. In the frontal view, they are seen as a radiolucency between the medial rim of the orbit and the nasal septum. When evaluating the paranasal sinuses, the integrity of the bony borders and adjacent structures and the degree of aeration must be established. In health, the thin mucous membrane lining is not visible on the radiograph. MASTOIDS The mastoid air cells communicate indirectly with the nasal cavity via the middle ear; however, embryologically they develop separately from the paranasal sinuses. Nonetheless, the radiographic appearances of air-filled cavities within the bone resemble the ethmoid air cells. The distribution and pneumatization of the mastoid air cells are extremely variable; the cells are located in the mastoid process and periauricular region and may extend as far forward as the zygomatic process of the temporal bone.28 CERVICAL SPINE
The upper vertebrae are often visible on the lateral and PA cephalometric radiographs. The atlas has no body or spinous process and has the form of a ring. The axis has the fundamental structure of the cervical vertebra with the addition of an upward projection called the dens or odontoid process. The dens occupies the space where the body of the atlas would have developed; it articulates with the posterior surface of the anterior arch of the atlas and provides a pivot around which the atlas and skull rotate. The body of the axis and the odontoid process have separate ossification centers23 and often do not fuse until age 12.20 Therefore, a transverse radiolucency at the base of the odontoid process in a young ambulatory patient with no history of trauma should not be mistaken for a fracture. The C-spine has a gentle curvature and is convex anteriorly when viewed from the side. This normal lordotic curve is position-dependent and can be altered as a result of failure to achieve natural head position when placing the patient in the cephalometric head holder or as a result of muscle spasm that causes the patient to posture the head in an effort to reduce pain and discomfort. Lines drawn along the anterior and posterior margins of the vertebral bodies should be practically parallel. A straight line drawn along the front of the odontoid process meets the anterior margin of the foramen magnum and lies approximately 1 mm behind and away from the posterior border of the anterior arch of the atlas. The normal dimension of the spinal canal ranges from 18 to 27 mm at the first cervical vertebra to 15 to 20 mm at the seventh cervical vertebra for children 15 years of age and less. For adults, the ranges are 16 to 30 mm and 13 to 24 mm, respectively.20 In the PA view, the lateral border of the vertebral body will be in alignment and the spinous
process will be visible. Frank displacement of a vertebra is a serious abnormality that demands further investigation (Fig. 5). The intervertebral disk is a fibrocartilaginous anulus with a gelatinous center and is not visible on a conventional radiograph. However, we can make inferential observations about the intervertebral disk by evaluating the surrounding anatomy. The intervertebral disk space appears as a radiolucency between the vertebral bodies defined by the relatively parallel inferior and superior cortical margins. If the cortical margins appear convergent or the disk space is narrowed, this may suggest a herniated disk. UPPER AIRWAY AND NECK The upper air passagesâ&#x20AC;&#x201D; the nasal cavity, oral cavity, pharynx, and larynx â&#x20AC;&#x201D; appear radiolucent on the skull film. When sufficiently thick, the soft tissues of the region will have an intermediate radiodensity between the airway and skeleton. The nasal air passages usually conform to the bony architecture as the mucosal lining of the nasal cavity is usually less than 1 mm thick and does not cast a radiographic shadow. Thickened membranes or linings can be seen as an intermediate density between bone and air with proper exposure factors. The cigar-shaped nasal conchae will be superimposed over the airway; this will be discussed in greater detail in the next section. The dimensions of the oral airway will vary depending on the position of the tongue. If the tongue is elevated, it may contact the soft palate and their radiographic shadows will merge. The palatine tonsils are situated between the palatoglossal and palatopharyngeal folds in the lateral fauces. These can sometimes be distinguished on the lateral film, especially if they are inflamed and enlarged (Fig. 6).
On the superior aspect of the posterior wall of the nasopharynx, there is a collection of lymphatic tissue (the nasopharyngeal tonsils or adenoids) that may be quite large in children. This is usually easy to identify on the lateral cephalometric film (Fig. 7). Changes in breathing patterns caused by hypertrophied adenoids may affect facial growth patterns.29,30 The lymphatic tissue tends to atrophy with age and will not be as prominent in adult patients. The opening of the eustachian tubes on the lateral wall of the nasopharynx just behind the inferior nasal conchae may be evident as a round, relatively radiolucent area.20 These structures are difficult to see, but may be discerned with certain anatomic and exposure factors. The soft palate separates the nasopharynx from the oropharynx. At rest, it extends from the posterior borders of the hard palate and arches inferiorly. In the lateral projection, the hyoid bone is seen just below the angle of the mandible. The thyroid, cricoid, and tracheal ring cartilage are usually not visualized but may on occasion have areas of calcification that appear on the radiographs. The epiglottis and the laryngeal folds are also seen. The prevertebral soft tissue and muscles can be seen separating the airway from the vertebral column. The retropharyngeal shadow at the line of C2 varies from 2 to 7 mm in children less than 15 years of age and from 1 to 7 mm in adults; the retrotracheal shadow at the level of the C6 varies from 5 to 14 and 9 to 22 mm, respectively.20 The soft-tissue shadow should have a smooth anterior outline. In the PA view, the lateral wall of the laryngopharynx and the larynx are seen; other parts of the airway are obscured by superimposition of bony structures. DENTOMAXILLOFACIAL COMPLEX Orthodontists are most familiar with the facial portion of the skull as this is the region they routinely treat. For our purposes we will consider the
dentomaxillofacial complex to include the orbits, nose, zygomatic arches, and jaws. The paranasal sinuses have been dealt with separately in a previous section. 1. Orbits. In the PA view, the rim of the orbit is seen as a smooth round radiopaque line. There are a number of structures that appear within the orbit and these should all be evaluated. The lesser wing of the sphenoid contributes to the floor of the anterior cranial fossa and is seen as a horizontal convex-down curvilinear radiodensity in the superior third of the orbit. From the region where this line intersects the superolateral border of the orbit, there is another linear radiopacity running downward and medially; this is called the innominate line and represents a cuvature of the greater wing of the sphenoid. The optic foramen is a round radiolucency near the medial orbital wall. The superior and inferior orbital fissures can be seen extending from this region in
lateral-upward
and
lateral-downward
directions,
respectively.
Occasionally, one can follow the path of the inferior orbital fissure as it becomes the inferior orbital canal and emerges on the front of the face as the infraorbital foramen. Just medial and slightly below the infraorbital foramen is a somewhat larger well-defined circular radiolucency; this is foramen rotundum through which the maxillary division of the trigeminal nerve passes as it leaves the skull base. This may be a region deserving careful scrutiny if the patient complains of pain over the area that this division innervates. The vertical position of the foramen will vary depending upon the tilt of the patient's head relative to the central ray of the beam. At the junction of the middle and medial thirds of the superior rim of the orbit, the supraorbital foramen may be seen as a small, round radiolucency (Fig. 8). In the lateral view, the superior and inferior walls of the orbit are seen.
Likewise, the posterior and anterolateral margins of the orbit are visualized; however, the superimposition of structures makes it difficult to distinguish left from right. The zygomaticofrontal and maxillofrontal sutures may be seen at the rim of the orbit and should not be mistaken for fractures. 2. The nose. In the PA view, the nasal septum, lateral walls, and conchae are easily defined. The nasal septum should be positioned at the midline; displacement from the midline may represent a congenitally deviated septum, prior trauma, or the presence of a pathologic process causing the displacement (Fig. 9). Extending medially from the lateral walls are the nasal conchae or turbinates. The inferior and middle conchae are usually seen, but the superior conchae may not be visualized. In the lateral views, the inferior conchae appear as a cigar-shaped radiopacity. Often the posterior extent of the conchae extends beyond the posterior border of the maxillary sinus, which makes it radiographically difficult to distinguish from an isolated radiopacity in the nasal cavity. If there is a question as to what this radiographic shadow represents, establishing continuity of the outer boundary of the radiopacity with the adjacent turbinate bone should confirm its identity. Should a question persist, the posterior nasopharynx can be visualized by indirect laryngoscopy using an angled mirror and proper lighting. 3. Zygomatic arches. The zygomatic process arises from the maxillary bone at the region of the first molar. The radiodensity, size, and shape of this structure are variable and the structure often takes on a different form, depending upon the angle of the directed x-ray beam. The zygomatic process may appear quite radiolucent if the maxillary antrum extends into it. The greater the extension of the maxillary sinus into the zygomatic process, the greater the contrast of the dark radiolucent air spaces and the sharply defined
cortical walls of the process. Seen in the lateral cephalogram, the corticated walls of the zygomatic process appear as a U-shaped radiopaque line known as a key ridge. The definition of the molar apices superimposed on the zygoma will vary with the amount of pneumatization that has occurred. If aeration is minimal, molar apical and maxillary sinus anomalies may be masked or ill-defined. 4. The jaws. Details of the teeth and their surrounding structures are difficult to see on skull films because of superimposition of anatomic structures and the inherent resolution limitation of screen film. Evaluation of the teeth and periodontium is best accomplished by a periapical film. Most orthodontists use these intraoral films in their diagnostic evaluations and treatment plans. Misinterpretations can present problems here also. For example, occasionally a double image of the lamina dura is seen that reflects the normal concavities and fluting of the roots or the superimposition of different roots of a multirooted tooth such as the maxillary first molar. Superimposition of the lingual root surface and periodontal ligament space of the first premolar onto the distal surface of the canine in the periapical film should not be mistaken for a vertical root fracture of the canine. Care should be taken to examine carefully for supernumerary teeth and evidence of small developing bud follicles. They can be of great consequence if the clinician is trying to move teeth into the space they occupy. If initially overlooked and subsequently noted on follow-up radiographs, they are a source of embarrassment at least, and iatrogenesis at worst (Fig. 10). The trabecular pattern of the anterior maxilla is fine, granular, and dense. The posterior maxilla shows a slightly less dense pattern with larger marrow spaces. The trabeculae of the anterior mandible are thicker than the maxilla, presenting a course pattern with large marrow spaces. The posterior
mandibular periapical trabeculae and marrow spaces are usually the largest in the jaws. These can be variable in size and mimic pathologic lesions. Changes in the density and pattern of the cancellous bone may result from inflammation, systemic disease, or tumors (Fig. 11). The mandibular symphysis frequently has a radiolucent line at the midline suture that disappears at about 1 year postpartum. If this radiolucency is found in older children or adults, it may suggest a fracture or cleft. The genial tubercles are the bony projections of attachment of the genioglossus and geniohyoid muscles. They often have a small radiolucent area in the center (the lingual foramen) that is the point of exit of mandibular nerve. Depending upon its size, this may be mistaken for incipient pathosis. The mental fossa is a depression found in the labial aspect of the mandible. The thinness of the hard tissue in this area may be mistaken for periapical disease of the incisors. similarly, the- mental foramen, located between the first and second premolars, can mimic periapical pathosis in this area. The mandibular canal forrns a dark linear radiographic shadow with thin superior and inferior opaque borders cast by its lamella boundaries. The molar teeth apices are frequently projected over this canal, giving the illusion of a discontinuous lamina dura surrounding these teeth. This is due to the localized overexposure caused by this radiolucent linear structure. Finally, the submandibular fossa is a depression on the lingual side of the mandible below the mylohyoid ridge that accommodates the submandibular gland. It will appear as a local radiolucency with scant or absent trabeculation. The anterior and posterior aspects of this radiolucency will blend into the surrounding bony pattern. SUMMARY
We have presented a review of certain aspects of normal radiographic anatomy, discussed range and distribution, and identified some common errors in diagnosis. Nonetheless, this review has covered only a small amount of the information available. It is up to clinicians through careful study of the films, by use of available reference material, and by consultation with colleagues in medical and dental radiology to constantly expand and improve their knowledge of normal radiographic anatomy. All radiographs of the head taken for orthodontic purposes should be considered skull films before they are thought of as cephalograms. By adopting this attitude, the orthodontist will be inclined to carefully review these films for significant deviations from normal and evidence of pathosis. Only after this responsibility has been met should cephalometric tracings or other morphometric analysis be done. The authors wish to thank Dr. Allan B. Reiskin for reviewing the manuscript, and providing helpful comments and suggestions. We also appreciate the expert assistance provided by the UNC School of Dentistry Learning Resources Center, especially Mr. Warren McCollum for the production of the illustrations and photography. Taken from the Angle Orthodontist on CD-ROM (Copyright Š 1997 Angle Orthodontist, Inc.), 1997 No. 2, 83 - 85: Making sense of cephalometrics Robert M. Rubin. -------------------------------EDITORIAL Making sense of cephalometrics Robert M. Rubin, DMD, MS In the 60-year history since the development of cephalometric radiology, literally undreds of methods of analysis have been proposed. Many of them
have contributed to a better understanding of the complexity of changes associated with facial growth. Some analyses have been useful in identifying how individual patients vary from norms that have been derived from large numbers of cohorts. Some cephalometric analyses and methods of superimposition are useful in monitoring the changes that are due to growth or to a combination of growth and treatment. Cephalometric measurements are also useful in descriptive communication. Just as Angleâ&#x20AC;&#x2122;s classification describes a specific relationship between the teeth in the maxilla and mandible, the Downsâ&#x20AC;&#x2122; facial angle communicates a picture
of
a
relationship
between
the
Frankfort
horizontal
and
nasion/menton. Each method of analysis is based on certain assumptions, some expressed and some implied. This essay examines several assumptions and evaluates their strengths and weaknesses. In addition, there are two different uses for assessment of the presenting patient. How does this patient vary from recognized norms? This information allows the practitioner to focus on where the patientâ&#x20AC;&#x2122;s anomalies exist, and allows him or her to plan for the achievable ideal for the patient. The second use of cephalometrics is to monitor changes due to growth or treatment, or their combination. I propose that some measurements may be well-suited for assessment but are poor choices for monitoring change. Similarly, some measurements are poor choices for assessment but are particularly well-suited for observing change. Failure to make this distinction has led to confusion in treatment and absence of clarity in communication in describing changes that occur with growth and/or treatment.
Almost every article in the orthodontic literature begins with a section describing the cephalometric system used for the evaluation that follows. It would be more efficient if each writer did not have to define the method of cephalometric assessment, as would be the case if there were agreement in our profession on the measurements and their uses. Precedent for adopting such agreements exists. In 1929, the worldâ&#x20AC;&#x2122;s anthropologists met and agreed on the definition of the Frankfort horizontal plane. Orthodontists were quick to adopt that definition, and that agreement has contributed to better communication in the anthropologic and orthodontic literature. Now, 60 years after its introduction, radiographic cephalometrics is overdue for an agreement on how we assess craniofacial morphology and how we monitor changes due to growth and/or treatment. Consider the following analogy: The orthopedic surgeon, noting growth of the femur, observes that the inferior epiphysial cartilage grows several millimeters. The neurosurgeon may note oppositional growth between the lumbar vertebrae. Neither of these physicians would suggest that the result of these increments of growth would drive the feet into the ground. They are in agreement that growth of the vertebrae or femur contributes to increased height. This agreement may not seem remarkable because the obviousness of it is so apparent. But, consider the possibilities if we lived in a weightless environment. In that environment confusion about describing the results of growth would be possible. Some would say that femoral growth carries the ankle down; others would say it carries the pelvis up. Neurosurgeons might describe vertebrae growth as moving the feet and head in opposite directions. This is the sort of confusion we have in craniofacial growth. The ease of identifying sella turcica led many cephalometric researchers to choose the line from sella to nasion as a key line of registration. It turns out
to be a relatively poor choice because of the confusion it engenders. With the head facing right, as is generally agreed in cephalo-metrics in this hemisphere, we consider growth to move skeletal landmarks to the right and down, away from sella. Confusion occurs when growth at the sphenooccipital synchondrosis is considered. Its proliferation, which often continues through puberty and can total more than 10 mm, carries the glenoid fossa and the mandible to the left—the opposite direction that condylar growth carries the mandible. This conflict is analogous to viewing femur growth as carrying the feet into the ground. Agronin and Kokich never mentioned the spheno-occipital synchondrosis in their report, “Displacement of the glenoid fossa: a cephalometric evaluation of growth during treatment.” (Am J Orthod Dentofac Orthop 1987;91:42-8.)
They
stated that during craniofacial growth, articulare is displaced posteriorly and inferiorly relative to the sphenoid bone. “The data support the premise that changes in the spatial orientation of the glenoid fossa and temporal bone may have an effect on mandibular position.” They were undermined by their assumptions! A more accurate description is that growth of the sphenooccipital synchondrosis carries the craniomaxillary complex superiorly and to the right, making the case more Class II and increasing facial height. There is a baseline available for viewing craniofacial growth that is analogous to using the ground for a baseline for somatic growth. That baseline is basion, the anterior edge of the foramen magnum. Using basion as the base (aptly named), all craniofacial growth is seen as movement away from the spinal cord, just as all skeletal growth is viewed as elevating the top of the head. A cephalometric analysis that uses this concept is the Coben basion horizontal analysis, first presented in 1955. Orienting on basion and
maintaining the Frankfort plane parallel to the horizon, all growth of the craniofacial skeletal is seen to carry structures to the right, away from the vertebrae column. Frankfort horizontal is a useful plane because it is believed to approximate the optic axis, the plane that appears to be kept level throughout life. This is important as it correlates the clinical appearance of the patient to his or her cephalometric analysis. Analysis based on sella-nasion may not relate well to the presenting patient if the anterior cranial base is steeply sloped. One problem with the Frankfort plane is that it is not suitable for serial evaluations. Coben handles this by using a constructed Frankfort on subsequent tracings, drawn tangent to porion and at the same angle to sellanasion as the original tracing. Some cephalometric measurements are excellent for assessmentâ&#x20AC;&#x201D;that is, the evaluation of the initial film to describe the problem. The same measurement may, however, be a poor choice for monitoring change because an element of it may be unstable. For example, upper incisor to occlusal plane is an excellent assessment of the torque of the incisor. The norm is 65 degrees. It is a poor choice to monitor torque achieved because its baseline, the occlusal plane, can change during treatment. To monitor upper incisor change it would be wiser to use upper incisor to sella-nasion. However, this is a poor choice for assessment as it is remote from the occlusion and independently related. A large upper incisor-sella-nasion angle can be due to a procumbent incisor or a flat anterior cranial base. It is not sufficient to rate a measurement as good or poor. It is important to rate it as good or poor for assessment, and good or poor for monitoring change. Table 1 shows some examples of commonly used cephalometric measurements and an appraisal of their usefulness.
This essay proposes that superimposition on basion with the Frankfort plane kept horizontal be adopted as the universal method of registration for evaluating overall craniofacial changes due to growth and/or treatment. Area superimpositions will, of course, still be necessary to determine the specific sites of the changes. Such an agreement would eliminate the need to preface every cephalometric study with an extensive section describing the method of superimposition. The reduction in journal space and in readerâ&#x20AC;&#x2122;s time would be an enormous savings for our specialty, and lead to a more efficient comparison of studies. Frequently, it is impossible to compare similar studies when different landmarks and methods of superimposition are used. In addition, a glossary of measurements should be developed that not only defines the measurement, but indicates if it is valid for assessment or for documenting change. I believe these two measures would contribute to increased clarity in our literature and enhanced coherence in the process of planning treatment and evaluating progress and posttreatment records. Orthodontics is marvelously complex. It is unnecessary to add to its complexity by promulgating confusing and fuzzy assumptions that impair accurate communication.