Burstone/ dental implant courses by Indian dental academy

Source: AJO-DO on CD-ROM (Copyright Š 1998 AJO-DO), Volume 1984 Mar (224 - 237): Diagnosis and treatment planning of skeletal asymmetry with submental-vertical radiograph - Forsberg, Burstone, and Hanley -------------------------------Diagnosis and treatment planning of skeletal asymmetry with the submental-vertical radiograph Clifton T. Forsberg, B.S., D.D.S., Charles J. Burstone, D.D.S., M.S., and Kevin J. Hanley, D.D.S. Farmington, Conn.

The purpose of this study was to assess the reproducibility of landmarks visible in the submental-vertical radiographic projection. Ten subjects with at least a 2 mm apical base discrepancy as diagnosed by means of a posteroanterior headfilm were selected. Mean tracing and measurement error was found to be less than 1 mm for all landmarks. A system of patient orientation and radiographic technique is presented. The radiographs were made at 90 kVp, 15 mA, 1 second exposure, using Kodak Lanex X-D film with a speed of 600. A system of cephalometric analysis is presented for the assessment of skeletal asymmetry in the horizontal plane. This system of analysis uses landmarks within the cranial base, maxilla, and mandible to construct reference lines with which to assess asymmetry. These reference systems allow the assessment of asymmetry within each component part of the craniofacial complex as well as the relative relationship of these parts to one another. Examples are presented to demonstrate the use of this system in assessing skeletal asymmetry. These examples show how this system of analysis can be incorporated into the data base for a particular patient and how it can be useful in making treatment decisions for patients with skeletal asymmetries. This method lends itself to future incorporation into a three-dimensional computerized cephalometric analysis.

With the advent of orthognathic surgery, the orthodontist's role in diagnosis and treatment planning of cases involving skeletal disharmony has expanded greatly. since contemporary surgical procedures can alter the bones of the craniofacial complex, it is important that the orthodontist accurately assess the degree to which skeletal disharmony contributes to a given malocclusion before he or she formulates treatment objectives.

Lateral headfilms are routinely used to assess anteroposterior and vertical relationships within the craniofacial complex, but these films are of little use in the assessment of facial symmetry. Several methods of assessing asymmetry by means of posteroanterior (P-A) headfilms have been suggested in the past.1-9 Problems associated with the use of the P-A cephalometric view, especially the selection of valid midsagittal reference points, have led to the use of alternate cephalometric projections in the assessment of asymmetry.

Berger10 was the first to suggest using the submental-vertical (S-V) projection in cephalometrics to assess asymmetry. He used a best-fit line through crista galli, crista frontalis, vomer, tubercle of atlas, odontoid process, and crista occipitalis interior as a midsagittal reference line, but he did not test this reference line for reproducibility or validity.

Gilbert11 investigated the accuracy of the submental-vertical view with the film cassette oriented parallel to the Frankfort horizontal plane. Width factors were highly reproducible, but significant error was found in length determinations. No definitive cephalometric analysis was proposed.

In the assessment of asymmetry, the submental-vertical projection is potentially more useful than the P-A projection. The S-V projection allows utilization of anatomic landmarks on the cranial base, remote from the facial bones, for determination of the midsagittal axis. Pearson and Woo12 found an exceptional degree of symmetry in the sphenoid bone. Keith and Campion13 used the sphenoid bone as a fixed reference in comparing development of growth of the skull. Marmary and associates14 showed that the perpendicular bisector of a line joining the foramina spinosa was a reliable and accurate midline. The work of Moss and Salentijn15 concluded that the passage and location of neurovascular bundles during orofacial growth cannot be violated. These studies of the stability and homogeneity of location of basal foramina support the choice of the foramina spinosa as reference points in the construction of a midsagittal reference axis. Using these cranial base landmarks, Ritucci and Burstone16 developed a cephalometric system for assessment of asymmetry of the craniofacial complex. They showed this system to be reproducible in a sample of eleven dry skulls with intact adult dentitions in which there were minor or no occlusal discrepancies.

The purpose of this study is to evaluate skeletal asymmetry in a sample of patients presenting for orthodontic therapy and to develop, for clinical use, a system of analysis based on the methods of Ritucci and Burstone.

METHODS AND MATERIALS Three skull radiographs were selected at random. Each radiograph was traced three times on acetate paper. Each tracing was done at a separate sitting to promote an unbiased selection of landmarks. These tracings were used to assess the interobserver error.

Ten patients were selected on the basis of having at least a 2 mm apical base discrepancy as diagnosed by a standard P-A radiograph used in treatment planning. A submental-vertical radiograph of each subject was taken by the following technique: Each patient was examined from a frontal view. Right and left infraorbital rims were palpated, and orbitale was marked on each with water-soluble ink for reference.

Each patient was then placed in a Wehmer cephalostat and seated on a straight-backed chair with a pillow supporting the upper back and cervical area. By means of a bilateral infraorbital pointer, the patient's head was rotated posteriorly until the Frankfort horizontal plane became parallel with the film cassette (Fig. 1).

Head position was then fixed and the radiograph was taken. Kodak Lanex X-D film with a film speed of 600 was used with Kodak regular X-Omat screens and exposed for 1 second at 90 kVp and 15 mA. The cathode-to-earrod distance was a standard 60 inches, and the earrod-to-film distance was fixed at 16 cm.

In cases in which there was noted asymmetry of the external auditory meatus or of right and left orbitale, only the right infraorbital pointer was used. Mandibles of all patients were positioned in centric relation by clinical manipulation of the jaw.

Pencil tracings on acetate paper were prepared for each submental-vertical radiograph. Five radiographs were selected at random to determine tracing and measurement error. Each of these radiographs was traced three times at separate sittings, and the measurements were recorded.

The cephalometric analysis proposed by Ritucci and Burstone was used to assess asymmetry in the cranial base, zygomaxillary complex, and mandible. This system, involving anatomic structures, points, and intersections from which geometric constructions, reference planes, and axes can be constructed, is described below.

CRANIAL BASE REFERENCE SYSTEM The landmarks chosen for the analysis of cranial base symmetry are the right and left foramina spinosa, right and left lateral borders of the cranial vault, right and left middle cranial fossa, basion, opisthion, and right and left mandibular fossae of the temporal bone. Each is discussed below. (See Figs. 2 and 3.) The numbering corresponds to Fig. 3: 1. Foramina spinosa points (FSP)— The geometric center of each foramen spinosa (FS). The interspinosum line connects the right and left foramen spinosa points. The interspinosum axis is the perpendicular bisector of the interspinosum line.

2. Posterior cranial vault points (PCV)— The intersections of the lateral borders of the cranial vault with a line, parallel to the interspinosum line, which is drawn across the cranial vault at its section of greatest width.

3. Middle cranial fossa points (MCF)— The most anterior points relative to the interspinosum line, on each lesser wing of the sphenoid bone (LWS).

4. Basion (Ba)— The most anterior point, relative to the interspinosum line, on the border of the foramen magnum.

5. Opisthion (Op)— The most posterior point, relative to the interspinosum line, on the border of the foramen magnum.

6. Condylion anterioris (CA)— A point on the anterior of each condylar head which is chosen to represent the mandibular fossa of the temporal bone. The method by which this point is selected is described in the discussion of the mandible.

Measurements to assess bilateral symmetry within the cranial base were made relative to a coordinate axis system consisting of the interspinosum line, which serves as the x axis, and the interspinosum axis, which serves as the z axis. The intersection of the interspinosum line and axis is the zero point, or origin of the axis system. Each data point was assigned a pair of Cartesian coordinates (x,z), based upon its distance in millimeters, in the horizontal and transverse dimensions from the origin. The symmetry of paired and unpaired data points relative to the coordinate axis system can be assessed by means of these coordinates.

Analysis of the symmetry of the posterior cranial vault points, basion, and opisthion was limited to the medial-lateral dimension only. The symmetry of the middle cranial fossa points and the condylion anterioris points was analyzed mediolaterally and anteroposteriorly.

The symmetry of structures in the zygomaxillary complex and mandible relative to the cranial base was also assessed. The position of the condylar geometric midline, gonial geometric midline, mandibular dental midline, and coronoid process points, as well as all points used in the analysis of the zygomaxillary

complex, were analyzed relative to the interspinosum axis system of the cranial base. These points are described in the discussion of the zygomaxillary complex and mandible which follows.

Maxillary reference system The analysis of the zygomaxillary complex requires the tracing of the zygomaxillary arches, pterygomaxillary fissures, maxillary central incisors, maxillary first molars, and the vomer and involves the use of the reference points and lines (Figs. 2 and 4) that follow. The numbers below correspond to Fig. 4: 1. Pterygomaxillary fissure (PTM)— The most medial and posterior point of each pterygomaxillary fissure. The PTM line connects the right and left PTM points. The PTM axis is the perpendicular bisector of the PTM line.

2. Buccale (Bc)— The point on the internal surface of each zygomatic arch where the arch turns medially and directly starts upon a backward sweep.

3. Zygion points (ZP)— The intersections of the lateral borders of the zygomatic arches (ZA) with a line, parallel to the PTM line, which is drawn across the section of greatest bizygomatic width.

4. Anterior cranial vault points (ACV)— The points where the lateral borders of the cranium are intersected by a line connecting the right and left zygion points.

5. Angulare points (A)— The most anterior points, relative to the PTM line, of the triangular opacities present at the external orbital angle where the upper and lower orbital rims meet and the zygomatic arch inserts.

6. Anterior vomer point (AVP)— The intersection of the vomer (V) with a line connecting the right and left angulare points.

7. Posterior vomer point (PVP)— The intersection of the vomer with the PTM line.

8. Maxillary dental midline (MDM)— The point contact between the mesial surfaces of the crowns of the maxillary central incisors.

9. Maxillary apical base midline (MAB)— A point midway between the roots of the maxillary central incisors at a level which is one third of the distance from the apex of the tooth to the alveolar crest. This point is determined on the P-A radiograph and its position is then transferred to the S-V radiograph in its proper position relative to the dental midline.

Measurements to assess bilateral symmetry within the zygomaxillary complex were made relative to a coordinate axis system consisting of the PTM line, which serves as the x axis, and the PTM axis, which serves as the z axis. The intersection of the PTM line and axis is the origin, or zero point, from which all measurements are made. The bilateral symmetry of anterior and posterior vomer points, anterior cranial vault points, zygion points, maxillary apical base midline, and maxillary dental midline were analyzed along the x axis only. Buccale and angulare points were analyzed along both the x and z axes.

Mandibular reference system

The mandible is well visualized in the S-V radiograph. The lateral and medial borders of the mandibular body and ramus, condylar heads, coronoid processes, first molars, central incisors, and gonial angles are traced. The analysis of the mandible involves utilization of the reference points and lines that follow (Figs. 2 and 5). The numbers below correspond to Fig. 5: 1. Gonion point (Go)— The midpoint mediolaterally on the posterior border of each gonial angle (G).

Mandibular body lines are reference lines, which are constructed as follows. The midpoint mediolaterally of the mandibular body at the distal aspect of each mandibular first molar is determined. A line is drawn from this point through gonion point, extending posteriorly through the condyle on each side.

2. Condylion anterioris (CA)— The intersection of the mandibular body line with the anterior border of each condyle (C). Condylion line connects the right and left condylion anterioris points. Condylion axis is the perpendicular bisector of the condylion line.

3. Condylion posterioris (CP)— The intersection of the mandibular body line with the posterior border of each condyle.

4. Condylion lateralis (CL)— The tangent point to each lateral condylar border of a line drawn parallel to each mandibular body line.

5. Condylion medialis (CM)— The tangent point to each medial condylar border of a line drawn parallel to each mandibular body line.

6. Coronoid process point (CPP)— The most anterior point, relative to the condylion line, on each coronoid process (CP).

7. First molar point (FMP)— The most distal point in line with the central groove on each mandibular first molar.

8. Gonial geometric midline of the mandible (GGM)— The midpoint along an arc connecting the right and left mandibular body lines from gonion to gonion, which is located midway labiolingually within the anterior body of the mandible.

9. Condylar geometric midline (CGM)— The midpoint of the curve extending from condylion anterioris on one side to the contralateral point.

10. Mandibular dental midline (Mand. DM)— The point contact between the mesial surfaces of the crowns of the mandibular central incisors.

11. Mandibular apical base midline (Mand. AB)— A point midway between the roots of the mandibular central incisors at a level which is one third of the distance from the apex to the alveolar crest. The point is determined on the P-A radiograph, and its position relative to the dental midline is noted. The apical base midline is then transferred to the S-V tracing in its proper position relative to the dental midline.

Measurements to assess bilateral symmetry within the mandible were made relative to a coordinate axis system consisting of the condylion line, which serves as the x axis, and the condylion axis, which serves as the z axis. The intersection of the condylion line and axis is the zero point, or origin, from which all measurements are made. Each data point is assigned a pair of Cartesian coordinates. The symmetry of

condylion medialis, coronoid process, and first molar points was analyzed along both the x and z axes.

The gonial geometric midline, condylar geometric midline, apical base midline, and dental midline were analyzed along the x axis only.

Additional measurements were made as follows: 12. The linear distance along the mandibular body line from condylion anterioris to first molar point was measured bilaterally.

13. The position buccolingually of each first molar within the body of the mandible was assessed. A line, perpendicular to the buccal surface of each molar at the region of the mesiobuccal groove, was drawn to the lateral border of the mandibular body. The length of this line was measured, and the orientation of the tooth relative to the lateral border of the mandible was noted. If the buccal surface of the molar was lateral to the lateral border of the mandibular body, the distance was a negative number. All other positions were indicated by positive measurements.

The mean and standard deviation for the sample of ten patients was calculated for each landmark measured. The mean difference between coordinate values of all paired structures was calculated.

Calculations were made by means of the absolute value of each coordinate as well as coordinate values with signs. This permitted the assessment of symmetry of bilateral paired structures about constructed midlines. Standard deviations were calculated to evaluate the variance in degree and direction of asymmetry within the sample.

Measurement error and tracing error were calculated by taking five S-V radiographs at random, tracing each radiograph three times at separate sittings, and recording the measurements. The mean value was calculated for each landmark. The absolute deviations from these mean values were used to calculate the mean error for each landmark recorded. A one-way analysis of variance was also performed to assess the within-group variance of the measurements for each landmark. Values were rounded off to the nearest 0.01 mm.

RESULTS Interobserver tracing and measurement error for three radiographs from Ritucci's dry-skull study showed a mean error of less than 1 mm for all landmarks.

Intraobserver tracing and measurement error for this sample of patients (Table VI) shows that mean error was generally less than 1 mm for all landmarks.

Analysis of these ten subjects indicates that asymmetry within the various craniofacial landmarks was the rule rather than the exception. Asymmetry was noted in the cranial base, the zygomaxillary complex, and the mandible.

Cranial base structures exhibited varying degrees of asymmetry relative to the interspinosum axis (Table I). Calculated mean values for this sample indicate a left-side dominance for basion, opisthion, and posterior cranial vault points (Tables I and V). Middle cranial fossa points indicate a right-side dominance (Table V). Condylion anterioris points did not show significant dominance for calculated mean values.

Large standard deviations indicate that considerable variation does exist within the sample. Asymmetry in

the anteroposterior direction was also noted in this sample for paired points. Middle cranial fossa and condylion anterioris showed left-side dominance, being positioned more anteriorly than to the right (Table V).

Maxillary and mandibular structures measured relative to the interspinosum line and axis also showed asymmetry. For unpaired structures, all measured points showed a right-side dominance (Table II). Again, large standard deviations indicate significant variance in the sample. For paired structures, buccale, angulare, PTM point, and coronoid process point all showed right-side dominance (Table V). In the anteroposterior dimension, buccale, angulare, PTM point, and coronoid process point all showed the left side placed more anteriorly (Table V).

When zygomaxillary structures were measured relative to the PTM line and axis, maxillary dental midline, maxillary apical base midline, and anterior vomer point showed left-side dominance (Table III).

For paired structures, anterior cranial vault points showed left-side dominance, whereas buccale and angulare showed right-side dominance (Tables III and V). In the anteroposterior dimension, buccale and angulare were placed more anteriorly on the left side (Tables III and V).

When mandibular structures were measured relative to condylion line and axis, unpaired structures again all showed left-side dominance (Table IV). For paired structures, coronoid process point and mandibular first molar point showed right side dominance (Tables IV and V) . For the anteroposterior dimension, coronoid process point and mandibular first molar point were located more anteriorly on the left side (Tables IV and V).

It should be noted that these patterns of dominance are for mean values with relatively large standard deviations and obtained from a small sample of only ten patients.

Ritucci15 measured magnification and distortion, using metallic implants in his dry-skull study. He found this to average 11.5% for mandibular first molar implants and 8.8% for infraorbital implants. Direct measurements from the most lateral aspects of amalgam restorations of mandibular first molars were compared to radiographic measurements for five patients. Magnification distortion averaged 10.8% at the mandibular first molar level for this sample.

DISCUSSION Reliability of this method of cephalometric analysis is summarized in Table VI. Mean error measurement for all landmarks is less than 1 mm. The one-way analysis of variance demonstrates a within-group variance of less than 1 mm for all measurements with the exception of buccale and angulare when measured relative to the PTM line and axis. This higher variance for these two landmarks can be explained by the fact that sometimes the zygomatic arches are about the same width as the anterior cranial vault. When this situation occurs, the density of bone that results from superimposition of the zygomatic arch and the anterior cranial vault results in poor definition of these landmarks radiographically. This increased variance is noticed only when these landmarks are measured relative to the PTM line and axis.

The most medial extent of the pterygomaxillary fissure is also a difficult landmark to identify. The combination of these two factors accounts for the greater variance shown for buccale and angulare when measured relative to the PTM line and axis. With reference to the interspinosum line and axis, the within-

group variance for buccale and angulare was less than 1 mm. These findings indicate a degree of precision similar to that found in other cephalometric studies.

This cephalometric system gives us a reproducible method of assessing skeletal asymmetry in the horizontal plane. This analysis is useful when diagnosis of the severity of a skeletal asymmetry is crucial in determining when orthognathic surgery or orthodontic therapy would be the most efficient mode of treatment. With the advent of adult orthodontics and improvements in surgical procedures, it is most probable that surgical corrections of asymmetry will gain popularity in the future. The contemporary orthodontist is concerned with restoration and maintenance of optimal dentofacial esthetics and function in his patients. Proper diagnosis and treatment planning can greatly clarify the desired objectives of treatment. Correction of malocclusions associated with disharmony of the craniofacial skeleton has always presented difficult treatment problems and often disappointing esthetic results. This method of cephalometric analysis can greatly aid the orthodontist in the diagnosis and treatment planning of asymmetrical cases by defining the actual area of the craniofacial complex where the disharmony exists.

Appropriate treatment objectives can be formulated only when diagnosis has clarified the extent and location of the skeletal discrepancy. Since the mandible is a movable bone, it is important that the films be taken in centric relation16,17 The greater the accuracy in determining centric relation, the better will be the evaluation of mandibular asymmetry relative to the maxilla and cranial base.

A few examples demonstrate how this analysis can aid the decision-making process. Fig. 6 shows a tracing of a Patient L. S. from our sample. Constructed reference axes are labeled. This patient had a Class II, Division 1 malocclusion with a retrognathic mandible as diagnosed by a standard lateral headfilm. Facial examination revealed that the chin was to the right of the facial midline. Dental malocclusion showed 11 mm of incisal overjet, 9 mm Class II at the right molar, 13 mm Class II at the left molar, and a midline discrepancy of 3.5 mm. The patient was a 24-year-old man whose chief complaint was of retrognathia. The submental-vertical radiographic projection demonstrated the extent of the skeletal disharmony. Cranial base structures were fairly symmetrical within themselves, compared to the interspinosum line and axis, when viewed from left to right. When we examine the anteroposterior relationship, however, we see that condylion anterioris on the left side was located 5.5 mm more anteriorly than on the right side (Table I and Fig. 6.). This was highly significant because these points represent the position of the glenoid fossa and, therefore, mandibular position. The mandibular transcondylar axis was located 5 mm to the right of the cranial base midline. The condylar geometric midline was 9 mm to the right of this cranial base midline. This indicates that the mandible was also asymmetrical within itself, with the left body being longer. This added to the displacement of the chin to the right. Analysis of the maxillary points representing the posterior maxilla, PTM, and posterior vomer showed that the posterior maxilla was also positioned to the right relative to the cranial base midline (Table II). These findings demonstrated significant asymmetry at all levels of the craniofacial complex.

For the desired esthetic result, surgical intervention would be necessary in the maxilla, because of its positioning to the right, and in the mandible, because of the retrognathia as well as the positioning to the right side.

Patient M. D. was a 22-year-old woman with a chief complaint of maxillary dental protrusion. The dental relationship was Class II, Division 1 with a prognathic maxilla as diagnosed by a lateral headfilm. The left side was 3.5 mm Class II at the molar with a molar crossbite. The right side was 5 mm Class II at the molar. There was 12 mm of incisal overjet. An S-V radiograph revealed that Patient M. D. had significant

asymmetry of the cranial base as demonstrated by the measurements for middle cranial fossa points, posterior cranial vault points, and condylion anterioris points. Left condylion anterioris was located 4 mm anterior to right condylion anterioris relative to the interspinosum line and axis (Table I, Fig. 7). The posterior maxilla was positioned to the right side, as shown by PTM points and posterior vomer point (Table II). The zygomaxillary complex showed a right-side dominance relative to the interspinosum line and axis, as well as the PTM line and axis (Tables II and III). The mandible was relatively symmetrical within itself but was placed to the right because of the position of the glenoid fossa (Table IV). This was a case in which there was an asymmetry of the cranial base and maxilla with a fairly symmetrical mandible.

Surgical correction of the maxillary asymmetry would not alter the zygomatic area unless a LeForte III procedure was done. Asymmetry was not the chief problem in this case, so the orthodontic treatment option was chosen.

Patient T. F. was a 28-year-old man with a chief complaint of asymmetrical dental occlusion in that his dental midlines were not coincident. The skeletal relationship was normal, as revealed by a lateral headfilm. The right side had a Class I molar relationship and the left side had a Class II molar relationship. Dental midlines were off by 3 mm. The S-V radiograph revealed significant asymmetry of the cranial base (Table I). Condylion anterioris was placed 4.2 mm more anteriorly on the left than on the right side relative to the interspinosum line and axis (Table I, Fig. 8). When we look at measurements for the maxilla and zygomaxillary area, we see that in the right and left direction things were relatively symmetrical but that in the anteroposterior direction the left side was consistently more anteriorly placed (Table II). The maxillary landmarks revealed that, within the maxilla itself, structures were relatively symmetrical when measured to the PTM line and axis (Table III). The mandible was relatively symmetrical when measured to the condylion line and axis (Table IV). Patient T. F. had undergone serial extraction 18 years prior to presentation at our clinic, and this had allowed dental compensations to develop. The patient had a stable occlusion, and no treatment was recommended.

These examples demonstrate how useful the S-V radiograph can be in determining the actual area of asymmetry within the craniofacial complex. Reliable submental-vertical radiograph projections can be produced on any cephalometer using a bilateral infraorbital pointer, an exposure of 1 second, 90 kVp, 15 mA, and Kodak Lanex X-D film with a speed of 600. When carefully oriented S-V radiographs are taken, the method should prove to be repeatable and could be valuable in determining long-term effects of such treatment procedures as rapid maxillary expansion, orthopedic therapy, and surgical procedures to correct asymmetry.