Jotirnal of Oral Rehabilitation, 1991, Volume 18. pages 531-545
Orthognathic diagnosis and treatment planning: a cephalometric approach S.J. C H A C O N A S a n c f F . D .
F R A G I S K O S * UCLA School of Dentistry, Los
Angeles, California, and "University of Athens School of Dentistry, Athens, Greece
Summary
Cephalometric analyses have long been important diagnostic tools for the orthodontic specialist. Such analyses, as they pertain to adult skeletal problems and their consequent therapy, should also be a valuable adjunct for various dental specialties, e.g. the oral surgical clinician. However, because of the complexity of most analyses, it is difficult to glean the important values needed for most orthognathic cases. In this article an analysis is presented to aid the clinician in surgical diagnosis and treatment planning. Introduction
The art and science of cephalometrics is not new. Ever since Camper investigated prognathism craniometrically in 1791, anthropologists have been interested in the ethnographic determination of facial form and pattern. Anthropometries, or the 'measurement of man\ found the human skull a fertile source of information. By studying different ethnic groups, age groups and sexes, and by measuring the size of the various parts and recording variations in the position and shape of cranial and facial structures, it became possible to devise certain broad standards that were descriptive of the human head. As a specialized field of anthropometries, the study of the head became known as 'craniometries' or 'cephalometrics." The development of cephalometric analyses has enabled the orthodontist to study various skeletal and dental relationships that correlate radiographic measurements with clinical observations. Orthodontists now use cephalometric techniques to plan treatment, to monitor the patient during therapy, and to analyse growth and mechanotherapy after active patient care. As a result of these studies, much information pertinent to orthognathic surgical diagnosis, treatment planning and prognosis has become available. This article will review basic cephalometric landmarks and analyses, and suggest a method that may be applied to patients undergoing various orthognathic surgical procedures. Review of the literature
Cephalometry was reported in the literature virtually simultaneously by Broadbent (1931), an orthodontist, and Hofrath (1931), a prosthodontist. Broadbent's primary objective was to provide a technique for measurement of craniofacial growth changes, whereas Hofrath's idea was to evaluate the results of prosthodontic reconstruction. Correspondence: Dr Spiro J. Chaconas. School of Dentistry, University of Cahfornia, Los Angeles, CA 9(X)24, U.S.A.
531
532
S.J. Chaconas and F.D. Fragiskos
The clinical phase of ccphalometry was developed by the contributions of orthodontists such as Downs (1948), Riedel (1952), Stcincr (1953), Tweed (1954), and Ricketts (1975). Some attempts have been made by oral surgeons to incorporate various cephalometric analyses into their diagnosis and treatment planning of orthognathic surgical cases. However, the authors feel that this may not be done on a routine basis, largely because of a lack of confidence in the cephalometric technique, and the false theory that cephalometry is just a 'numbers game' without full justification for its use. More recently, articles have been published in the literature specifically aimed at enhancing the oral surgeon's understanding of the relationships between cephalometrics and orthognathic surgery. Stirrups et al. (1986) described a cephalometric analysis of the Lc Fort II osteotomy procedure. They used an analysis based on the sella-nasion line, a line between the anterior nasal spine and the posterior nasal spine, and the lower border of the mandible. Hiranaka and Kelly (1987) published an article describing the use of a computer-assisted cephalometric study to evaluate the stability of simultaneous orthognathic surgery on the maxilla and mandible. A cephalometric analysis, in which the soft tissue profile changes that accompany the advancement of the mandible were studied, has been recorded by Mommaerts & Mar.xer (1987). Cephalometric landmarks Because the method of radiographic cephalometry has been designed and developed mainly by orthodontists, it is almost exclusively used within this dental specialty. However, cephalometry can be a useful diagnostic tool in other specialties of dentistry as well. Unfortunately, 'cephalometric language' has remained primarily within the province of the orthodontists. It is therefore considered appropriate to present a brief review of the various landmarks and terms used. Broadbent (1931) and Hofrath (1931) developed the cephalometer, which holds the head to standardized radiographic views, so that developmental changes can be monitored longitudinally in the same individual (Fig. 1). A thorough understanding of the osteology of the eraniofacial complex is a prerequisite for learning the science of eephalometry. Certain skeletal and soft tissue landmarks are vital to a basic understanding of oral surgical cephalometrie analyses (Fig. 2). Only the lateral cephalometric view will be discussed, as this view usually provides sufficient information for diagnosis and treatment planning for the anteroposterior and vertieal problems which the oral surgeon encounters in his or her orthognathie treatment of patients. Description of landmarks Nasiori (N). This is the frontonasal suture, or junction of the frontal and nasal bones. Nasion is seen in profile as an irregular notch. The nasal bone is considerably less dense radiographieally than the frontal bone, and the suture can be readily followed even when the notch is not apparent. Porion (P). This is the most superior point of the external auditory meatus. Orbitale (O). This is the lowest point on the inferior bony margin of the orbit. Anterior nasal spine (ANS). This is the spinous process of the maxilla that forms the most anterior projection of the floor of the nasal cavity.
Orthognathic diagnosis and treatment planning
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Fig. 1. Cephalometric views showing skeletal tissues and detitition. Left panel: lateral view permits analysis of anteroposterior and vertical skeletal dysplasias, as well as discrepancies in tooth pt)sition. Right panel: frontal view pcrtnits analysis of width atid vertical skeletal dysplasias, as well as dental arch discrepancies.
Po'
Fig. 2. Lateral cephalometric landmarks used for orthognathic analysis. Lateral cephalometric analysis is more commonly used than frontal analysis because most skeletal discrepancies are in the anteroposterior and vertical dimensions.
534
5.7. Chaconas and F.D. Fragiskos
Point A (subspinale). This is an arbitrary measuring point taken at the innermost curvature from the anterior nasal spine to the crest of the maxillary alveolar process. Point A denotes the approximate junction between the basal or supporting maxillary bone and the alveolar bone (apical base). Pogonion (Po). This is the most anterior point on the symphysis of the mandible. Menton (M). This is the most inferior point on the symphysis of the mandible. Protruberance menti (PM) (suprapogonion). This is the point at which the profile of the mandibular symphysis changes from convex to concave. Xi Point (Xi). This is the geographical centre of the mandibular ramus. Pronasale (Pn). This is the most prominent or anterior point of the nose. Soft tissue pogonion (Po'). This is the most prominent or anterior point on the soft tissue chin. Cephalometric lines and planes. Once the landmarks or 'alphabet' of the cephalometric language have been learned, these points are connected to form the various lines and planes used for analysis (Fig. 3). Frankfort horizontal plane (FH). This facial plane connects the lowest points of the orbits (orbitale) and the superior points of the external auditory meatus (P).
Fig. 3. Pertinent cephalometric lines and planes used in orthognathic analysis.
Orthognathic diagnosis and treatment planning N-A
535
line. This is the line that connects nasion (N) and subspinale (point A).
Facial plane (NPo). Often history assigns labels incorrectly. Although it only connects two points, the facial line is often referred to as the facial 'plane'. However, it is an important reference linking nasion (N) and pogonion (Po). Mandibular plane (MP). This plane represents the lower border of the mandible. The line is drawn at a tangent to the lower border of the ramus posteriorly, and extends through the menton (M) anteriorly. Occlusal plane (OP). Traditionally this plane is represented by a line that bisects the first molar occlusion posteriorly and extends anteriorly to bisect the incisor occlusion. Recently, orthodontists have tended to bisect all of the posterior occlusion and to extend the line anteriorly through the incisor occlusion. This is therefore termed the 'functional' occlusal plane. ANS-Xi line. This line is drawn between the anterior nasal spine (ANS) of the maxilla to the point representing the geometric centre of the mandibular ramus (Xi). Corpus axis (Xi-PM). This line is drawn from the Xi point to the PM point, and represents the length of the body of the mandible. APo line. This line extends from point A on the maxilla to point Po on the mandible, and represents the maxillomandibular relationship of the patient. E line (Pn-Po'). This line connects the most anterior points of the soft tissue nose (Pn) and the soft tissue chin (Po'). Orthognathic cephalometric analysis As with the use of any other language, the 'words' (lines and planes) must be put together in order to give meaningful sentences (analysis) that make diagnostic and clinical sense. The cephalometric analysis described in this article has been developed in order to provide the dental clinician with the maximum information from the least number of measurements. The 'average' measurements that will be described here are those for a 'normal' Caucasian adult. Skeletal criteria Maxillary depth (NA to FH; average = 90 degrees) (Fig. 4) This angle gives an indication of the anteroposterior position of the apical base of the maxilla relative to a horizontal facial plane (FH). The angle is larger than normal in a skeletal Class 11 malocclusion because of a protracted midface. It is usually smaller than normal in a true Class 111 malocclusion and in cleft palate patients, particularly those who have had surgery at an early age to close the palate. In simple terms, a large angle indicates that the maxilla is protracted, and a small angle indicates that the maxilla is retracted. So, for example, the oral surgeon can use this measurement as a guide to determine the type of anteroposterior surgery of the maxilla that is required, and the amount of displacement that is needed in the upper jaw.
536
S.J. Chaconas and F.D. Fragiskos
Fig. 4. The maxillary depth is determined by the angle formed by the NA line with the Frankfort horizontal plane (FH).
FH
Po
Fig. 5. The mandibular depth is determined by the angle formed by the NPo line with the Frankfort horizontal plane (FH).
Mandibular depth (NPo to FH; average = 90 degrees) (Fig. 5) This angle gives an indication of the anteroposterior position of the most anterior point of the mandible. A Class II skeletal malocclusion with a retrognathic mandible would reveal a smaller-than-normal measurement. A larger-than-normal measurement would be indicative of a skeletal Class III malocclusion associated with a prognathic mandible. As in the previous case, the oral surgeon can utilize this measurement for the anteroposterior discrepancy of, in this case, the lower jaw. It would give the clinician an
Orthognathic diagnosis and treatment planning
537
indication of the need for surgical interventioti with regard to the matidible, and the amount of eorreetion that is needed. Maxillomandibular relationship (maxillary depth miritis mandibtdar depth) (average = 0 degrees) In an adult, the average maxillary and mandibular depth angles are 90°. Therefore a line drawn from N to Po should pass through point A in a normal adult patient. This would give the patient a straight skeletal and soft tissue profile (Fig. 6, eentre). If point A is anterior to NPo, the patient's profile will be eonvex (Fig. 6, left). This would be indieative of either a maxilla that is protraeted or a mandible that is retrognathie. If the skeletal convexity is severe, and eannot be treated orthodontically, then a surgieal approaeh may be warranted. The oral surgeon would then deeide, after studying the individual measurements of maxillary depth and mandibular depth, whieh jaw requires surgery. If point A is posterior to the NPO line, the maxillomandibular relationship is prognathic, and the profile is coneave (Fig. 6, right). This skeletal eoneavity may require surgical treatment, either by moving the maxilla forward, or by moving the mandible back, again depending on the severity of the maxillary depth and mandibular depth angles. MF angle (MFA) (MF to FH; average = 25 degrees) (Fig. 7) The only significance of this angle per se is that it provides an indication of the vertical height of the mandibular ramus. In a severe Class II, division 1 malocclusion, this angle is larger than normal because the mandible has not grown properly in all directions. This measurement also aids the dental surgeon in determining facial type (Fig. 8). The authors believe that it is important to determine the facial type before any proposed surgical intervention. The iarger-than-normal mandibular plane angle (high angle) is usually indicative of a dolichofacial pattern (Fig. 8c). Such a patient will have a long, narrow face and narrow dental arches. Because of the narrow nasal cavities and difficulty with nasal breathing, the patient will have mouth-breathing tendencies. To open the oropharynx, this type of patient may thrust the tongue forward. The
Fig. 6. Left: a convex profile associated with a Class II malocclusion. A protracted maxilla and/or retrognathic mandible is evident. Centre: straight profile of a Class I occlusion illustrates a harmonious maxillomandibular relationship. Right: a prognathic mandible and/or retracted maxilla produces a concave profile in a Class III malocclusion.
538
S.J. Chaconas and F.D. Fragiskos
FH
Fig. 7. The mandibular plane angle is formed by the mandibular plane (MP) and the Frankfort horizontal plane (FH).
n (a)
A (b)
A (c)
Fig. 8. Facial types (above) and dental arches (below) arc compared: (a) brachyfacial: (b) mesofacial; (c) dolichofacial.
mouth-breathing problem is likely to have contributed to the aetiology of the initial malocclusion, and will also cause instability after orthognathic treatment of this skeletal deformity. The oral surgeon should consult with an otolaryngologist prior to treatment for correction of the patient's mouth-breathing problem. The 'high angle' dolichofacial pattern is the most difficult problem to treat orthognathically, and care should be taken to diagnose and plan treatment correctly for this type of patient. Such
Orthognathic diagnosis and treatment planning
539
individuals often require mandibular advancements, and surgeons who have attempted this procedure are aware of the inherent relapse tendencies of such cases. The lower-than-normal mandibular plane angle patient (brachyfacial) is usually associated with a Class II, division 2 type of skeletal malocclusion (Fig. 8a). Such a patient would have a short, broad face with an accompanying wide dental arch. Lower face height (ANS-Xi-PM; average = 47 degrees) (Fig. 9) This is the angular measurement formed by the intersection of a line from ANS to Xi and PM. It indicates the presence of a skeletal open-bite, larger-than-normal angle (dolichofacial), or a deep-bite, smaller-than-normal angle (brachyfacial). This cephalometric measurement, together with phonetics, is useful for determination of the divergence of the oral cavity with regard to the vertical maxillomandibular relationship. The latter helps the oral surgeon to decide whether the open or deep bite is dental or skeletal in nature. If the lower face height measurement is significantly higher or lower than normal, then this is an indication that a surgical approach may be necessary. Dental criteria Occlusal plane (average = -3-5 mm to lip embrasure) (Fig. 10) This is a linear measurement for determination of the correct vertical position of the occlusal plane. The negative average measurement indicates that the occlusal plane is below the lip embrasure. If the occlusal plane is too far beneath the lip embrasure, excessive maxillary gum tissue will be visible. A high occlusal plane anteriorly may result in hidden maxillary teeth and excessive visibility of mandibular teeth at rest. The oral surgeon should therefore be familiar with this measurement prior to and during surgical manipulation of the jaw structures in order to provide a pleasing aesthetic result for the patient.
Fig. 9. Lower face height indicates divergence of oral cavity.
540
S.J. Chaconas and F.D. Fragiskos
OP N-/
Fig. 10, The occlusal plane (OP) is located 3 5mm below the lip embrasure (EM) in an aesthetically pleasing dentition.
Po
Fig. 11, The mandibular incisor protrusion is measured by the position of the lower incisor (mm) relative to the maxillomandibular line (APo),
Mandibular incisor protrusion (incisor to APo; average = +1 mm) (Fig. Jl) In orthodontics, as well as in orthognathic surgery, the position of the mandibular central incisor provides a key to treatment planning. Specifically, this measurement determines the anteroposterior position of the incisal edge of the mandibular central incisor relative to the line from point A on the maxilla to Po on the mandible. Inasmuch as the position of the APo line is an indication of the maxillomandibular relationship, this measurement relates the lower incisor to the maxilla and the mandible.
Orthognathic diagnosis and treatment planning
541
The oral surgeon should therefore not only be aware of the skeletal position of the upper and lower jaw structures, but should also know how the teeth relate to the maxillomandibular relationship. Furthermore, since many orthognathic surgery patients have significant dental crowding as well as their micrognathic condition, the clinican should be aware of the fact that every movement of the lower incisor forward, cephalometrically, will increase the dental arch length twofold. Conversely, backward movement of the lower incisor will decrease the dental arch length twofold. In addition, it must be borne in mind that all of the teeth in both dental arches are dependent on the position of the lower incisor. In other words, the rest of the teeth 'fall into place' like the pieces of a jig-saw puzzle. Profile analysis F line (lower lip to Fn-Fo' line: no average) (Fig. 12) This measurement relates the lower lip to a line from the tip of the nose (Pn) to the tip of the soft tissue of the chin (Po'). Although in a 9-year-old child with a normal soft tissue profile the lower lip is usually 2 mm behind the E line, there is no established mean for the adult patient. The prominence of the nose and chin affects this measurement, as does the position of the underlying teeth and skeleton. The surgeon should beware of any excessive repositioning of the jaw anteroposteriorly that may adversely affect the position of the lips as they relate to the 'E' line. If the nose and chin are too prominent, surgical alterations to these structures may have to be considered to produce a generally pleasing and aesthetic result.
b)
(
Orthognathic diagnosis and treatment planning: a cephalometric approach S.J. C H A C O N A S a n c f F . D .
F R A G I S K O S * UCLA School of Dentistry, Los
Angeles, California, and "University of Athens School of Dentistry, Athens, Greece
Summary
Cephalometric analyses have long been important diagnostic tools for the orthodontic specialist. Such analyses, as they pertain to adult skeletal problems and their consequent therapy, should also be a valuable adjunct for various dental specialties, e.g. the oral surgical clinician. However, because of the complexity of most analyses, it is difficult to glean the important values needed for most orthognathic cases. In this article an analysis is presented to aid the clinician in surgical diagnosis and treatment planning. Introduction
The art and science of cephalometrics is not new. Ever since Camper investigated prognathism craniometrically in 1791, anthropologists have been interested in the ethnographic determination of facial form and pattern. Anthropometries, or the 'measurement of man\ found the human skull a fertile source of information. By studying different ethnic groups, age groups and sexes, and by measuring the size of the various parts and recording variations in the position and shape of cranial and facial structures, it became possible to devise certain broad standards that were descriptive of the human head. As a specialized field of anthropometries, the study of the head became known as 'craniometries' or 'cephalometrics." The development of cephalometric analyses has enabled the orthodontist to study various skeletal and dental relationships that correlate radiographic measurements with clinical observations. Orthodontists now use cephalometric techniques to plan treatment, to monitor the patient during therapy, and to analyse growth and mechanotherapy after active patient care. As a result of these studies, much information pertinent to orthognathic surgical diagnosis, treatment planning and prognosis has become available. This article will review basic cephalometric landmarks and analyses, and suggest a method that may be applied to patients undergoing various orthognathic surgical procedures. Review of the literature
Cephalometry was reported in the literature virtually simultaneously by Broadbent (1931), an orthodontist, and Hofrath (1931), a prosthodontist. Broadbent's primary objective was to provide a technique for measurement of craniofacial growth changes, whereas Hofrath's idea was to evaluate the results of prosthodontic reconstruction. Correspondence: Dr Spiro J. Chaconas. School of Dentistry, University of Cahfornia, Los Angeles, CA 9(X)24, U.S.A.
531
532
S.J. Chaconas and F.D. Fragiskos
The clinical phase of ccphalometry was developed by the contributions of orthodontists such as Downs (1948), Riedel (1952), Stcincr (1953), Tweed (1954), and Ricketts (1975). Some attempts have been made by oral surgeons to incorporate various cephalometric analyses into their diagnosis and treatment planning of orthognathic surgical cases. However, the authors feel that this may not be done on a routine basis, largely because of a lack of confidence in the cephalometric technique, and the false theory that cephalometry is just a 'numbers game' without full justification for its use. More recently, articles have been published in the literature specifically aimed at enhancing the oral surgeon's understanding of the relationships between cephalometrics and orthognathic surgery. Stirrups et al. (1986) described a cephalometric analysis of the Lc Fort II osteotomy procedure. They used an analysis based on the sella-nasion line, a line between the anterior nasal spine and the posterior nasal spine, and the lower border of the mandible. Hiranaka and Kelly (1987) published an article describing the use of a computer-assisted cephalometric study to evaluate the stability of simultaneous orthognathic surgery on the maxilla and mandible. A cephalometric analysis, in which the soft tissue profile changes that accompany the advancement of the mandible were studied, has been recorded by Mommaerts & Mar.xer (1987). Cephalometric landmarks Because the method of radiographic cephalometry has been designed and developed mainly by orthodontists, it is almost exclusively used within this dental specialty. However, cephalometry can be a useful diagnostic tool in other specialties of dentistry as well. Unfortunately, 'cephalometric language' has remained primarily within the province of the orthodontists. It is therefore considered appropriate to present a brief review of the various landmarks and terms used. Broadbent (1931) and Hofrath (1931) developed the cephalometer, which holds the head to standardized radiographic views, so that developmental changes can be monitored longitudinally in the same individual (Fig. 1). A thorough understanding of the osteology of the eraniofacial complex is a prerequisite for learning the science of eephalometry. Certain skeletal and soft tissue landmarks are vital to a basic understanding of oral surgical cephalometrie analyses (Fig. 2). Only the lateral cephalometric view will be discussed, as this view usually provides sufficient information for diagnosis and treatment planning for the anteroposterior and vertieal problems which the oral surgeon encounters in his or her orthognathie treatment of patients. Description of landmarks Nasiori (N). This is the frontonasal suture, or junction of the frontal and nasal bones. Nasion is seen in profile as an irregular notch. The nasal bone is considerably less dense radiographieally than the frontal bone, and the suture can be readily followed even when the notch is not apparent. Porion (P). This is the most superior point of the external auditory meatus. Orbitale (O). This is the lowest point on the inferior bony margin of the orbit. Anterior nasal spine (ANS). This is the spinous process of the maxilla that forms the most anterior projection of the floor of the nasal cavity.
Orthognathic diagnosis and treatment planning
533
Fig. 1. Cephalometric views showing skeletal tissues and detitition. Left panel: lateral view permits analysis of anteroposterior and vertical skeletal dysplasias, as well as discrepancies in tooth pt)sition. Right panel: frontal view pcrtnits analysis of width atid vertical skeletal dysplasias, as well as dental arch discrepancies.
Po'
Fig. 2. Lateral cephalometric landmarks used for orthognathic analysis. Lateral cephalometric analysis is more commonly used than frontal analysis because most skeletal discrepancies are in the anteroposterior and vertical dimensions.
534
5.7. Chaconas and F.D. Fragiskos
Point A (subspinale). This is an arbitrary measuring point taken at the innermost curvature from the anterior nasal spine to the crest of the maxillary alveolar process. Point A denotes the approximate junction between the basal or supporting maxillary bone and the alveolar bone (apical base). Pogonion (Po). This is the most anterior point on the symphysis of the mandible. Menton (M). This is the most inferior point on the symphysis of the mandible. Protruberance menti (PM) (suprapogonion). This is the point at which the profile of the mandibular symphysis changes from convex to concave. Xi Point (Xi). This is the geographical centre of the mandibular ramus. Pronasale (Pn). This is the most prominent or anterior point of the nose. Soft tissue pogonion (Po'). This is the most prominent or anterior point on the soft tissue chin. Cephalometric lines and planes. Once the landmarks or 'alphabet' of the cephalometric language have been learned, these points are connected to form the various lines and planes used for analysis (Fig. 3). Frankfort horizontal plane (FH). This facial plane connects the lowest points of the orbits (orbitale) and the superior points of the external auditory meatus (P).
Fig. 3. Pertinent cephalometric lines and planes used in orthognathic analysis.
Orthognathic diagnosis and treatment planning N-A
535
line. This is the line that connects nasion (N) and subspinale (point A).
Facial plane (NPo). Often history assigns labels incorrectly. Although it only connects two points, the facial line is often referred to as the facial 'plane'. However, it is an important reference linking nasion (N) and pogonion (Po). Mandibular plane (MP). This plane represents the lower border of the mandible. The line is drawn at a tangent to the lower border of the ramus posteriorly, and extends through the menton (M) anteriorly. Occlusal plane (OP). Traditionally this plane is represented by a line that bisects the first molar occlusion posteriorly and extends anteriorly to bisect the incisor occlusion. Recently, orthodontists have tended to bisect all of the posterior occlusion and to extend the line anteriorly through the incisor occlusion. This is therefore termed the 'functional' occlusal plane. ANS-Xi line. This line is drawn between the anterior nasal spine (ANS) of the maxilla to the point representing the geometric centre of the mandibular ramus (Xi). Corpus axis (Xi-PM). This line is drawn from the Xi point to the PM point, and represents the length of the body of the mandible. APo line. This line extends from point A on the maxilla to point Po on the mandible, and represents the maxillomandibular relationship of the patient. E line (Pn-Po'). This line connects the most anterior points of the soft tissue nose (Pn) and the soft tissue chin (Po'). Orthognathic cephalometric analysis As with the use of any other language, the 'words' (lines and planes) must be put together in order to give meaningful sentences (analysis) that make diagnostic and clinical sense. The cephalometric analysis described in this article has been developed in order to provide the dental clinician with the maximum information from the least number of measurements. The 'average' measurements that will be described here are those for a 'normal' Caucasian adult. Skeletal criteria Maxillary depth (NA to FH; average = 90 degrees) (Fig. 4) This angle gives an indication of the anteroposterior position of the apical base of the maxilla relative to a horizontal facial plane (FH). The angle is larger than normal in a skeletal Class 11 malocclusion because of a protracted midface. It is usually smaller than normal in a true Class 111 malocclusion and in cleft palate patients, particularly those who have had surgery at an early age to close the palate. In simple terms, a large angle indicates that the maxilla is protracted, and a small angle indicates that the maxilla is retracted. So, for example, the oral surgeon can use this measurement as a guide to determine the type of anteroposterior surgery of the maxilla that is required, and the amount of displacement that is needed in the upper jaw.
536
S.J. Chaconas and F.D. Fragiskos
Fig. 4. The maxillary depth is determined by the angle formed by the NA line with the Frankfort horizontal plane (FH).
FH
Po
Fig. 5. The mandibular depth is determined by the angle formed by the NPo line with the Frankfort horizontal plane (FH).
Mandibular depth (NPo to FH; average = 90 degrees) (Fig. 5) This angle gives an indication of the anteroposterior position of the most anterior point of the mandible. A Class II skeletal malocclusion with a retrognathic mandible would reveal a smaller-than-normal measurement. A larger-than-normal measurement would be indicative of a skeletal Class III malocclusion associated with a prognathic mandible. As in the previous case, the oral surgeon can utilize this measurement for the anteroposterior discrepancy of, in this case, the lower jaw. It would give the clinician an
Orthognathic diagnosis and treatment planning
537
indication of the need for surgical interventioti with regard to the matidible, and the amount of eorreetion that is needed. Maxillomandibular relationship (maxillary depth miritis mandibtdar depth) (average = 0 degrees) In an adult, the average maxillary and mandibular depth angles are 90°. Therefore a line drawn from N to Po should pass through point A in a normal adult patient. This would give the patient a straight skeletal and soft tissue profile (Fig. 6, eentre). If point A is anterior to NPo, the patient's profile will be eonvex (Fig. 6, left). This would be indieative of either a maxilla that is protraeted or a mandible that is retrognathie. If the skeletal convexity is severe, and eannot be treated orthodontically, then a surgieal approaeh may be warranted. The oral surgeon would then deeide, after studying the individual measurements of maxillary depth and mandibular depth, whieh jaw requires surgery. If point A is posterior to the NPO line, the maxillomandibular relationship is prognathic, and the profile is coneave (Fig. 6, right). This skeletal eoneavity may require surgical treatment, either by moving the maxilla forward, or by moving the mandible back, again depending on the severity of the maxillary depth and mandibular depth angles. MF angle (MFA) (MF to FH; average = 25 degrees) (Fig. 7) The only significance of this angle per se is that it provides an indication of the vertical height of the mandibular ramus. In a severe Class II, division 1 malocclusion, this angle is larger than normal because the mandible has not grown properly in all directions. This measurement also aids the dental surgeon in determining facial type (Fig. 8). The authors believe that it is important to determine the facial type before any proposed surgical intervention. The iarger-than-normal mandibular plane angle (high angle) is usually indicative of a dolichofacial pattern (Fig. 8c). Such a patient will have a long, narrow face and narrow dental arches. Because of the narrow nasal cavities and difficulty with nasal breathing, the patient will have mouth-breathing tendencies. To open the oropharynx, this type of patient may thrust the tongue forward. The
Fig. 6. Left: a convex profile associated with a Class II malocclusion. A protracted maxilla and/or retrognathic mandible is evident. Centre: straight profile of a Class I occlusion illustrates a harmonious maxillomandibular relationship. Right: a prognathic mandible and/or retracted maxilla produces a concave profile in a Class III malocclusion.
538
S.J. Chaconas and F.D. Fragiskos
FH
Fig. 7. The mandibular plane angle is formed by the mandibular plane (MP) and the Frankfort horizontal plane (FH).
n (a)
A (b)
A (c)
Fig. 8. Facial types (above) and dental arches (below) arc compared: (a) brachyfacial: (b) mesofacial; (c) dolichofacial.
mouth-breathing problem is likely to have contributed to the aetiology of the initial malocclusion, and will also cause instability after orthognathic treatment of this skeletal deformity. The oral surgeon should consult with an otolaryngologist prior to treatment for correction of the patient's mouth-breathing problem. The 'high angle' dolichofacial pattern is the most difficult problem to treat orthognathically, and care should be taken to diagnose and plan treatment correctly for this type of patient. Such
Orthognathic diagnosis and treatment planning
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individuals often require mandibular advancements, and surgeons who have attempted this procedure are aware of the inherent relapse tendencies of such cases. The lower-than-normal mandibular plane angle patient (brachyfacial) is usually associated with a Class II, division 2 type of skeletal malocclusion (Fig. 8a). Such a patient would have a short, broad face with an accompanying wide dental arch. Lower face height (ANS-Xi-PM; average = 47 degrees) (Fig. 9) This is the angular measurement formed by the intersection of a line from ANS to Xi and PM. It indicates the presence of a skeletal open-bite, larger-than-normal angle (dolichofacial), or a deep-bite, smaller-than-normal angle (brachyfacial). This cephalometric measurement, together with phonetics, is useful for determination of the divergence of the oral cavity with regard to the vertical maxillomandibular relationship. The latter helps the oral surgeon to decide whether the open or deep bite is dental or skeletal in nature. If the lower face height measurement is significantly higher or lower than normal, then this is an indication that a surgical approach may be necessary. Dental criteria Occlusal plane (average = -3-5 mm to lip embrasure) (Fig. 10) This is a linear measurement for determination of the correct vertical position of the occlusal plane. The negative average measurement indicates that the occlusal plane is below the lip embrasure. If the occlusal plane is too far beneath the lip embrasure, excessive maxillary gum tissue will be visible. A high occlusal plane anteriorly may result in hidden maxillary teeth and excessive visibility of mandibular teeth at rest. The oral surgeon should therefore be familiar with this measurement prior to and during surgical manipulation of the jaw structures in order to provide a pleasing aesthetic result for the patient.
Fig. 9. Lower face height indicates divergence of oral cavity.
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S.J. Chaconas and F.D. Fragiskos
OP N-/
Fig. 10, The occlusal plane (OP) is located 3 5mm below the lip embrasure (EM) in an aesthetically pleasing dentition.
Po
Fig. 11, The mandibular incisor protrusion is measured by the position of the lower incisor (mm) relative to the maxillomandibular line (APo),
Mandibular incisor protrusion (incisor to APo; average = +1 mm) (Fig. Jl) In orthodontics, as well as in orthognathic surgery, the position of the mandibular central incisor provides a key to treatment planning. Specifically, this measurement determines the anteroposterior position of the incisal edge of the mandibular central incisor relative to the line from point A on the maxilla to Po on the mandible. Inasmuch as the position of the APo line is an indication of the maxillomandibular relationship, this measurement relates the lower incisor to the maxilla and the mandible.
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The oral surgeon should therefore not only be aware of the skeletal position of the upper and lower jaw structures, but should also know how the teeth relate to the maxillomandibular relationship. Furthermore, since many orthognathic surgery patients have significant dental crowding as well as their micrognathic condition, the clinican should be aware of the fact that every movement of the lower incisor forward, cephalometrically, will increase the dental arch length twofold. Conversely, backward movement of the lower incisor will decrease the dental arch length twofold. In addition, it must be borne in mind that all of the teeth in both dental arches are dependent on the position of the lower incisor. In other words, the rest of the teeth 'fall into place' like the pieces of a jig-saw puzzle. Profile analysis F line (lower lip to Fn-Fo' line: no average) (Fig. 12) This measurement relates the lower lip to a line from the tip of the nose (Pn) to the tip of the soft tissue of the chin (Po'). Although in a 9-year-old child with a normal soft tissue profile the lower lip is usually 2 mm behind the E line, there is no established mean for the adult patient. The prominence of the nose and chin affects this measurement, as does the position of the underlying teeth and skeleton. The surgeon should beware of any excessive repositioning of the jaw anteroposteriorly that may adversely affect the position of the lips as they relate to the 'E' line. If the nose and chin are too prominent, surgical alterations to these structures may have to be considered to produce a generally pleasing and aesthetic result.
b)
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