- Email: [email protected]
Cephalometric parameters affecting severity of anterior open bite
Copyright 9 Munksgaard1997 InteraationalJournalof
Int. J. Oral Maxillofac. Surg. 1997; 26:321-326 Printed in Denmark. All rights reserved
Oral&
MaxillofacialSurgery ISSN 0901-5027
Aestheticand reconstructivesurgery
Cephalometric parameters affecting severity of anterior
W. M. Tsang, L. K. Cheung, N. Samman Department of Oral and MaxillofaciatSurgery, University of Hong Kong, Hong Kong
open bite W. M. Tsang, L. K. Cheung, N. Samman: Cephalometric parameters affecting severity of anterior open bite. Int. J. Oral Maxillofac. Surg. 1997; 26: 321-326. 9 Munksgaard, 1997 Abstract. One hundred and four patients with anterior open bite (AOB) were subdivided by statistical means into three distinctly separate groups with AOB of different severity based on the extent of separation of the incisors in the vertical plane. Cephalometric evaluation of all subjects was completed using 24 skeletal and 12 dentoalveolar measurements, and the results were compared between the mild, moderate and severe groups of AOB. The findings suggest that the palatal plane, mandibular occlusal plane, upper anterior der~tal height and upper posterior dental height correlate significantly with the severity of AOB. In contrast, skeletal jaw relationship in the horizontal plane correlated only weakly with severity. This study further highlighted the steep mandibular occlusal plane as an important indicator of AOB severity in contrast to the maxillary occlusal plane.
Although anterior open bite (AOB) is a dental phenomenon, it is commonly associated with skeletal jaw discrepancies. Patients presenting with mild AOB may have normal facial balance, however in severe cases the facial proportions become elongated resulting in a long face syndrome. The conventional skeletal classification of AOB is primarily based on the horizontal plane only, yet the term "anterior open bite" means a vertical separation of the upper and lower anterior teeth when the posterior teeth are in occlusion. Hence, it is not surprising that many reports indicate a poor correlation between the skeletal classification and the extent of mOB 7,34. It would be more logical to grade severity of AOB according to vertical or angular parameters. A crucial aspect in the treatment of AOB is correct diagnosis of the contributing components of the deformity. If the AOB is primarily a dental condition due to the proclination of the
upper and lower incisors, simple treatment by orthodontic correction of the proclined teeth or by controlling obvious contributory factors such as thumb sucking may be successful. If the AOB is associated with underlying skeletal abnormalities, then any attempt at treatment by orthodontic extrusion of the teeth will inevitably fail. A surgical approach is currently favoured to achieve better stability9,13,25,but unfortunately many reports demonstrate that AOB is prone to relapse despite surgery4,19,20. This study aims to identify the skeletal parameters which correlate with the severity of the AOB in the vertical plane.
Material and methods The subjects of this study were chosen from over 800 patients presenting to the department of Oral and Maxillofacial Surgery, University of Hong Kong, for treatment of their
Key words: cephalometry;jaw abnormalities; dental occlusion. Accepted for publication 24 March 1997
dentofacial deformity. One hundred and four ethnic Chinese patients from this group, exhibiting anterior open bite, were identified and included in the study based on a minimum age of 15 years and a minimum separation of 1 mm between the upper and lower incisors when measured perpendicular to the Frankfort plane on the pretreatment cephalometric radiograph. Patients with history of trauma to the mandibular condyle, extracoronal restoration of the incisors, syndromes, or previous orthognathic surgery were excluded. All pretreatment cephalographs were traced by the first author. Landmarks used were mostly those of BJORK2 with some additions selected for this study. In total, 24 skeletal and 12 dentoalveolar features were measured and these are illustrated in Figs. 1 4 and defined in Tables 14. The range of severity of open bite, as noted by the degree of separation in mm between the incisors in a vertical plane perpendicular to the Frankfort plane, was recorded and subjected to cluster analysis to arrive at distinct categories of severity in the sample. Discriminant analysis was used to determine which combination of cephalometric variables distinguished most significantly be-
322
Tsang et al.
t
FH
"I"
i
; i',
1
i
I i
l PP
A
PP
OB
%%%
Fig. 1. Linear measurements of skeletal landmarks (see Table 1 for definitions).
Fig. 2. Linear measurements of dentoalveolar landmarks (see Table 2 for definitions).
FH
PP
Fig. 3. Angular measurements o f skeletal landmarks (see Table 3 for definitions).
Fig. 4. Angular measurements of dentoalveolar landmarks (see Table 4 for definitions).
Severity o f anterior open bite Table 1. Linear measurements of skeletal landmarks Name
Code
Anterior cranial base S-N Posterior cranial base S-AR Lower posterior facial height LPFH Posterior facial height PFH Upper posterior facial height UPFH Anterior facial height Upper anterior facial height Lower anterior facial height
AFH UAFH LAFH
Mandibular body length Sella to posterior nasal spine
GO-ME S-PNS
Definition Distance between sella and nasion Distance between sella and articulare Distance between articulare and gonion Distance between sella and gonion Distance between ethmoidal registration point and posterior nasal spine Distance between nasion and menton Distance between nasion and anterior nasal spine Distance between anterior nasal spine and menton Distance between gonion and menton Distance between sella an d posterior nasal spine
tween the categories of severity previously defined. Multiple linear regression was constructed to identify the cephalometric parameters determining the severity of open bite. Skeletal jaw relationships in the horizontal plane were also recorded as normal, i.e. orthognathic, retrognathic or prognathic. The relationship between the severity of open bite in the vertical plane and the skeletal jaw relationship in the horizontal plane was investigated by a 3• frequency table for men and women separately. The error of measurement was analysed by randomly selecting 40 radiographs, which were measured on two separate occasions four weeks apart. The systematic error was assessed by a paired t-test on the means of individual variables obtained from the measurements on the two separate occasions. The systematic and random errors were assessed by the coefficient of reliability and coefficient of repeatability 15,3~
Results There were 45 men and 59 women in the group with an age range of 15-45 years (mean 23). The degree of open
bite as defined in this study ranged from 1 to 11 mm, and the frequency distribution is shown in Fig. 5. The histog r a m suggested a trimodal distribution with superimposition of three distinct populations each with its own peak. Based on the cluster analysis, the best separation between the three subgroups of open bite, well illustrated by Fig. 6, was achieved using the K-mean method of clustering and the three categories of open bite were defined as follows: 1. Mild: A O B of 3.75 m m or less 2. Moderate: A O B between 3.75 m m and 6.85 m m 3. Severe: A O B over 6.85 m m In the sample, the mild group comprised 59 subjects (57%), the moderate group 32 subjects (31%) and the severe group 13 subjects (12%). The jaw relationship in the horizontal plane was n o r m a l in 45 subjects (43%), retrognathic in 14 subjects (14%), and prognathic in 45 subjects (43%). In the female open bite subjects, there was a weak but statistically sig-
323
nificant association between the mild category o f open bite and normal skeletal jaw relationship in the horizontal plane, and between the moderate category and the prognathic jaw relationship. The Spearman correlation coefficient was small (0.26) but significant (P=0.0426). However, in the male subjects, there was no significant association between the horizontal jaw relationship and the different categories of severity. Discriminant analysis identified the following variables as the best combination of cephalometric parameters to distinguish between the above three categories of severity of open bite: the mandibular occlusal plane, the overjet, the lower gonial angle, the upper anterior dental height, the upper posterior dental height, the palatal plane and the functional occlusal plane. Multiple linear regression was constructed for the total sample to identify the variables which best correlated with severity of open bite. Nine variables with P<0.0001 (except A N B with P=0.024) were identified as follows: palat~tl plane, mandibular occlusal plane, upper anterior dental height, lower anterior dental height, upper posterior dental height, overjet, lower anterior facial height, interincisal angle and the A N B angle. Error analysis Results of the paired t-tests on 35 variables of 40 radiographs measured twice indicated that the systematic error was not significant (lowest P value was greater, than 0.05). The coefficient of reliability of all variables was greater than 92% and the coefficient of repeatability
Table 2. Linear measurements of dentoalveolar landmarks Name
Code
Definition
Upper posterior dental height (UPDH)
U6-PP
Upper anterior dental height (UADH) Lower posterior dental height (LPDH)
U1E-PP L6-MP
Lower anterior dental height (LADH) Overjet
L1E-MP OJ
Overbite
OB
Upper 1st molar to sella in vertical plane
U6-SV
Perpendicular distance between mesiobuccal cusp tip of maxillary first molar to palatal plane Perpendicular distance between upper central incisal edge and palatal plane Perpendicular distance between mesiobuccal cusp tip of mandibular first molar to mandibular plane Perpendicular distance between lower central incisal edge and mandibular plane Distance between tip of lower central incisor to upper central incisal edge along a line parallel to Frankfort horizontal Distance between tip of lower central incisor to upper central incisal edge along a line perpendicular to Frankfort horizontal Distance between mesiobuccal cusp tip of maxillary first molar to sella along a line perpendicular to S-N* Distance between mesiobuccal cusp tip of maxillary first molar to Sella along a line parallel to S-N*
Upper 1st molar to sella in horizontal plane U6-SH * S-N: Sella-nasion line
324
Tsang et al.
12,
16 14
10.
12
E
8,
10
E
8
._= nn O '<
8 6 4 2 0
ya ,Y..,, "
"
"
"
2 0,
Std. Dev = 2.45 Mean = 4.03 N = 104.00 "
4
-2, N=
59
32
13
1.00
2.00
3.00
lo ,"1 "
"
'
' Oo
Mild, M o d e r a t e and S e v e r e
" Oo
AOB in mm Fig. 5. Histogram showingfrequency distribution of anterior open bite.
of all variables was smaller than 5.6~ and 3.7 mm. A comparison of the error between angular and linear measurements by independent t-tests indicated that angular measurements were subject to a significantly greater error than linear measurements.
Discussion The term "open bite" can be traced back to CARAVELLIin 18421, when it already referred to a distinct type of malocclusion. Many classifications have since been proposed but none of them have been universally accepted. SHmA29 classified open bite according to etiology into the developmental and acquired categories, but in the same year THOMA33 proposed a classification based on the site (anterior or lateral) and pattern (slanting or angulated). Later on, surgeons considered open bite from a treatment point of view and separated the skeletal from the nonskeletal or dental types 14,23. WORMS et al. 35 classified AOB by severity and extent of involvement into pseudo, true and compound types. The latest version is a comprehensive classification proposed by RICHARDSON26 combining etiological and skeletal considerations. There is also no universal definition for AOB. The term may refer to below-average overbite, an edge-to-edge occlusion, or a definite degree of gap between the upper and lower incisors23,31,32. KIM 18 used the occlusal plane to define AOB as a lack of coincidence between the maxillary and mandibular occlus01 planes, which fail to meet at the anterior teeth. In the present study, we considered the AOB more from a func-
Fig. 6. Box-plot of anterior open bite against severity.
tional viewpoint with a minimum threshold of a 1 mm gap at the front teeth, since patients will then be more likely to have chewing and speech problems and seek treatment. It was interesting that the cluster analysis showed three peaks in the sample distribution, thus indicating the three gradings of AOB from mild to severe, and enabling further analysis. Most previous studies to determine the cephalometric parameters of AOB compare patients with AOB to those with normal occlusion, deep overbite, or other skeletal deformities3,5,6,1~ 12,16,17,21,27,31 Many significant par-
ameters of AOB were noted, but some conflicting results have led to confusion. The conclusions from any one study might well have been true for that sample of patients, but often the study group composition was biased. No study looked at the intragroup distribution of AOB or made any attempt to determine the significant factors which contributed to the AOB being mild, as opposed to severe in extent. Lateral cephalography provides the traditional evaluation method in dentofacial diagnosis. We tested the correlation of the maxillo-mandibular relationship in the anteroposterior plane
Table 3. Angular measurements of skeletal landmarks
No./Code
Definition
1. SNA 2. SNB 3. N-S-AR 4. SN-MP 5. SN-FH 6. FH-FOP 7. FH-PP 8. S-AR-GO 9. GOA 10. UGOA 11. 12. 13. 14.
LGOA FH-MNOP FH-MXOP SN-PP
Angle between sella-nasion line and nasion-A point line Angle between sella-nasion line and nasion-B point line Angle between nasion-sella line and sella-articulare line Angle between sella-nasion line and mandibular plane Angle between sella-nasion line and Frankfort horizontal line Angle between Frankfort horizontal and functional occlusal plane Angle between Frankfort horizontal plane and palatal plane Angle between sella-articulare line and articulare-gonion line Angle formed by mandibular plane and a tangent to posterior border of ramus through articulare Angle between tangent to posterior border of ramus through articulare and line joining nasion and gonion Angle between mandibular plane and line joining nasion and gonion Angle between Frankfort horizontal and mandibular occlusat plane Angle between Frankfort horizontal and maxillary occlusal plane Angle between sella-nasion line and palatal plane
Table 4. Angular measurements of dentoalveolar landmarks
No/Code
Definition
1. 2. 3. 4.
Angle between sella-nasion line and long axis of upper incisor Angle between palatal plane and long axis of upper incisor Angle between long axis of upper and lower incisors Angle between mandibular plane and long axis of lower incisor
U1-SN U1-PP I-I LI-MP
Severity of anterior open bite with the severity grading of AOB. Out of the 18 possible combinations, only two showed weak correlation of significance. This finding, therefore, justifies a different approach to the assessment of AOB. The vertical linear and angular parameters were separately assessed by two independent methods of analysis. With the discriminant analysis, seven significant factors were noted, while with the linear regression analysis, nine significant factors were found. However, only five factors were common to both methods. These were: the palatal plane, the mandibular occlusal plane, the upper anterior dental height, the upper posterior dental height and the overjet. Although the overjet was also found to be a significant factor, it is a measurement in the horizontal plane and, as already mentioned, has only weak correlation. Of the remaining four factors with correlation to the severity of AOB, two were vertical linear parameters of the maxillary alveolar process, and two were angular parameters of the maxilla and mandible respectively. This finding highlights several points worthy of further consideration. AOB caused by dental abnormality is conventionally considered to be a mild form of open bite, whereas that which is associated with a skeletal abnormality constitutes the severe form. In this study, it was interesting to learn that, with increasing severity of AOB, the maxillary dentoalveolar vertical heights were the significant factors. Severity of AOB correlated with a direct proportional increase in the posterior dentoalveolar height, and was inversely proportional to the anterior dentoalveolar height. This combination of dentoalveolar height correlations influences the rotation of the maxilla, and is reflected by changes in the palatal plane. Therefore, the logical clinical tenet that anticlockwise rotation of the palatal plane, in addition to the absolute heights of the dentoalveolar segments, correlated well with the severity of AOB, has been confirmed by this study. The occlusal plane in AOB should be considered differently from non-AOB. Many studies measured the occlusal plane by bisecting the AOB, and this led to the wrong conclusion that AOB produces an increase in the occlusal plane 3,1~ On realizing this error, some researchers considered the occlusal plane separately for the maxillary
and the mandibular teeth and found that the significant factor was the mandibular occlusal plane and not the maxillary occlusal plane9,z4. This was not totally confirmed by ELLIS et al. 6, who showed that in AOB the maxillary occlusal plane was less steep, while the mandibular occlusal plane was steeper than in non-AOB. The maxillary occlusal plane may well be fiat in AOB and this is a common phenomenon, but it does not decrease with increasing severity of open bite. In this study, and in agreement with NAnotrM et al. 24 and Eran,ZLUs, the mandibular occlusal plane becomes steeper with increasing severity of AOB. This can be due to the combination of the downward direction of growth of the mandible and its anterior teeth, reflected by an increase in the gonial angle and a reverse curve of Spee.
Acknowledgments: The expert advice on statistical analysis of Dr Philip Yu, Department of Statistics, and Miss May Wong, Department of Periodontology and Public Health, University of Hong Kong, is gratefully acknowledged.
References 1. ANDERSONGM. Practical Orthodontics. 7th ed. St. Louis, MO: C.V. Mosby, 1948, chapter 2. 2. BJORKA. The face in profile: an anthropological x-ray investigation on Swedish children and conscripts. Svensk Tandl Tidskr 1947:40 (suppl. 5B). 3. CANGIALOSI TJ. Skeletal morphologic features of anterior open-bite. Am J Orthod 1984: 85: 28-36. 4. DENISONTF, KOKICHVG, SHAPmOPA. Stability of maxillary surgery in open bite versus non-open bite malocclusions. Angle O~thod 1989: 59: 5-10. 5. ELLISE, MCNAMARAJA. Components of adult Class III open bite malocclusion. Am J Orthod 1984: 42: 277-90. 6. ELLIS E, MCNAMARA JA, LAWRENCE TM. Components of adult Class II open bite malocclusion. J Oral Maxillofac Surg 1985: 43: 92-105. 7. ENLOWDH, KURODAT, LEWISAB. The morphological and morphogenetic basis for craniofacial form and pattern. Angle Orthod 1971: 41: 161-88. 8. ENUNLU N. Palatal and mandibular plane variations in open-bite cases with varying aetiology. Trans Europ Orthod Soc 1974: 165-71. 9. EPKERBN, FISHLC. The surgical orthodontic correction of Class III skeletal open-bite. Am J Orthod 1978: 73: 60118.
325
10. FROST DE, FONSECARJ, TURVEYTA; HALL DJ. Cephalometric diagnosis and surgical-orthodontic correction of apertognathia. Am J Orthod 1980: 78: 65769. 11. HAPAKFM. Cephalometric appraisal of the open-bite case. Angle Orthod 1964: 34: 65-72. 12. HARALABAKISNB, YIAGTZISSC, TOUTOLINTZAKISNM. Cephalometric characteristics of open bite in adults: a threedimensional cephalometric evaluation. Int J Adult Orthod Orthog Surg 1994: 9: 223-31. 13. HAYMONDCS, STOELINGAPJW, BLIJDORP PA, LEENENR J, MERKENSNM. Surgical orthodontic treatment of anterior skeletal open bite using small plate internal fixation. Int J Oral Maxillofac Surg 1991: 20: 223-7. 14. HINDS EC, KENT JN. Diagnosis and selection of surgical procedures in management of open-bite. J Oral Surg 1969: 27: 939-49. 15. HOUSTONWJB. The analysis of errors in orthodontic measurements.Am J Orthod 1983: 83: 382-90. 16. JARABAK JR. Open bite skeletal morphology. Fortschr Kieferorthop 1983: 44: 122-33. 17. JONESOG. A cephalometric study of 32 North American black patients with anterior open bite. Am J Orthod 1989: 95: 289-96. 18. KIMYH. Anterior open bite and its treatment with multiloop edgewise archwire. Angle Orthod 1987: 57: 290-321. 19. LANSELYCV, STIRRUPSDR, MOOS KE Relapse following surgical treatment of anterior open bite. Br J Oral Maxillofac Surg 1986: 24: 391-404. 20. LOPEZ-GAVITOG, LITTLERM, JOONOEPH DR. Anterior open-bite malocclusion: longitudinal 10-yearpost-retention evaluation of orthodontically treated patients. Am J Orthod 1985: 87: 175-86. 21. LOUFTY MS. Cephalometric evaluation of deep over-bite and anterior open-bite in Kuwait school children. Trans Europ Orthod Soc 1973: 281-5. 22. Lowr AA. Correlation between orofacial muscle activity and craniofacial morphology in a sample of control and anterior open-bite subjects. Am J Orthod 1980: 78: 89-98. 23. M m ~ I E. A review of anterior openbite. Br J Orthod 1978: 5: 21-7. 24. NAHOtrM HI, HOROWlTZSL, BENEDICTO EA. Varieties of anterior open-bite. Am J Orthod 1972: 61: 486-92. 25. REITZIK M, BARER PG, WAINWRIGHT WM, LIMB. The surgical treatment of skeletal anterior open-bite deformities with rigid internal fixation in the mandible. Am J Orthod Dentofacial Orthop 1990: 97: 52-7. 26. RICHARDSONA. A classification of open bites. Eur J Orthod 1981: 3: 289-96. 27. SASSOUNIV, NANDAS. Analysis of dento-
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facial vertical proportions. Am J Orthod 1964: 50: 801-23. 28. SCHENDEL SA, EISENFELD J, BELL WH. The long-face syndrome: vertical maxillary excess. Am J Orthod 1976: 70: 398408. 29. SHIRA RB. Surgical correction of openbite deformities by oblique sliding osteotomy. Oral Surg 1961: 19: 275-90. 30. STmRUPS DR. Guidance on presentation of cephalometry-based research studies. A personal perspective. Br J Orthod 1993: 20: 359-65.
31. SUBTELNYJO, SAKUDAM. Open-bite: diagnosis and treatment. Am J Orthod 1964: 50: 337-58. 32. SWINEHARTEW. A clinical study of openbite. Am J Orthod 1942: 28: 18-34. 33. THOMAKH. Y-shaped osteotomy for correction of open bite in adults. Surg Gynae Obst 1943: 77: 40. 34. TROUTEN JC, ENLOW DH, RABINE M, PrmLVS AE, SWEDLOWD. Morphological factors in open bite and deep bite. Angle Orthod 1983: 53: 192-211. 35. WORMS FW, MESKIN LH, ISAACSON RJ.
Open-bite. Am J Orthod 1971: 59: 58995.
Address:
Dr Nabil Samman Associate Professor Department of Oral and Maxillofacial Surgery University of Hong Kong 34 Hospital Road Hong Kong
Int. J. Oral Maxillofac. Surg. 1997; 26:321-326 Printed in Denmark. All rights reserved
Oral&
MaxillofacialSurgery ISSN 0901-5027
Aestheticand reconstructivesurgery
Cephalometric parameters affecting severity of anterior
W. M. Tsang, L. K. Cheung, N. Samman Department of Oral and MaxillofaciatSurgery, University of Hong Kong, Hong Kong
open bite W. M. Tsang, L. K. Cheung, N. Samman: Cephalometric parameters affecting severity of anterior open bite. Int. J. Oral Maxillofac. Surg. 1997; 26: 321-326. 9 Munksgaard, 1997 Abstract. One hundred and four patients with anterior open bite (AOB) were subdivided by statistical means into three distinctly separate groups with AOB of different severity based on the extent of separation of the incisors in the vertical plane. Cephalometric evaluation of all subjects was completed using 24 skeletal and 12 dentoalveolar measurements, and the results were compared between the mild, moderate and severe groups of AOB. The findings suggest that the palatal plane, mandibular occlusal plane, upper anterior der~tal height and upper posterior dental height correlate significantly with the severity of AOB. In contrast, skeletal jaw relationship in the horizontal plane correlated only weakly with severity. This study further highlighted the steep mandibular occlusal plane as an important indicator of AOB severity in contrast to the maxillary occlusal plane.
Although anterior open bite (AOB) is a dental phenomenon, it is commonly associated with skeletal jaw discrepancies. Patients presenting with mild AOB may have normal facial balance, however in severe cases the facial proportions become elongated resulting in a long face syndrome. The conventional skeletal classification of AOB is primarily based on the horizontal plane only, yet the term "anterior open bite" means a vertical separation of the upper and lower anterior teeth when the posterior teeth are in occlusion. Hence, it is not surprising that many reports indicate a poor correlation between the skeletal classification and the extent of mOB 7,34. It would be more logical to grade severity of AOB according to vertical or angular parameters. A crucial aspect in the treatment of AOB is correct diagnosis of the contributing components of the deformity. If the AOB is primarily a dental condition due to the proclination of the
upper and lower incisors, simple treatment by orthodontic correction of the proclined teeth or by controlling obvious contributory factors such as thumb sucking may be successful. If the AOB is associated with underlying skeletal abnormalities, then any attempt at treatment by orthodontic extrusion of the teeth will inevitably fail. A surgical approach is currently favoured to achieve better stability9,13,25,but unfortunately many reports demonstrate that AOB is prone to relapse despite surgery4,19,20. This study aims to identify the skeletal parameters which correlate with the severity of the AOB in the vertical plane.
Material and methods The subjects of this study were chosen from over 800 patients presenting to the department of Oral and Maxillofacial Surgery, University of Hong Kong, for treatment of their
Key words: cephalometry;jaw abnormalities; dental occlusion. Accepted for publication 24 March 1997
dentofacial deformity. One hundred and four ethnic Chinese patients from this group, exhibiting anterior open bite, were identified and included in the study based on a minimum age of 15 years and a minimum separation of 1 mm between the upper and lower incisors when measured perpendicular to the Frankfort plane on the pretreatment cephalometric radiograph. Patients with history of trauma to the mandibular condyle, extracoronal restoration of the incisors, syndromes, or previous orthognathic surgery were excluded. All pretreatment cephalographs were traced by the first author. Landmarks used were mostly those of BJORK2 with some additions selected for this study. In total, 24 skeletal and 12 dentoalveolar features were measured and these are illustrated in Figs. 1 4 and defined in Tables 14. The range of severity of open bite, as noted by the degree of separation in mm between the incisors in a vertical plane perpendicular to the Frankfort plane, was recorded and subjected to cluster analysis to arrive at distinct categories of severity in the sample. Discriminant analysis was used to determine which combination of cephalometric variables distinguished most significantly be-
322
Tsang et al.
t
FH
"I"
i
; i',
1
i
I i
l PP
A
PP
OB
%%%
Fig. 1. Linear measurements of skeletal landmarks (see Table 1 for definitions).
Fig. 2. Linear measurements of dentoalveolar landmarks (see Table 2 for definitions).
FH
PP
Fig. 3. Angular measurements o f skeletal landmarks (see Table 3 for definitions).
Fig. 4. Angular measurements of dentoalveolar landmarks (see Table 4 for definitions).
Severity o f anterior open bite Table 1. Linear measurements of skeletal landmarks Name
Code
Anterior cranial base S-N Posterior cranial base S-AR Lower posterior facial height LPFH Posterior facial height PFH Upper posterior facial height UPFH Anterior facial height Upper anterior facial height Lower anterior facial height
AFH UAFH LAFH
Mandibular body length Sella to posterior nasal spine
GO-ME S-PNS
Definition Distance between sella and nasion Distance between sella and articulare Distance between articulare and gonion Distance between sella and gonion Distance between ethmoidal registration point and posterior nasal spine Distance between nasion and menton Distance between nasion and anterior nasal spine Distance between anterior nasal spine and menton Distance between gonion and menton Distance between sella an d posterior nasal spine
tween the categories of severity previously defined. Multiple linear regression was constructed to identify the cephalometric parameters determining the severity of open bite. Skeletal jaw relationships in the horizontal plane were also recorded as normal, i.e. orthognathic, retrognathic or prognathic. The relationship between the severity of open bite in the vertical plane and the skeletal jaw relationship in the horizontal plane was investigated by a 3• frequency table for men and women separately. The error of measurement was analysed by randomly selecting 40 radiographs, which were measured on two separate occasions four weeks apart. The systematic error was assessed by a paired t-test on the means of individual variables obtained from the measurements on the two separate occasions. The systematic and random errors were assessed by the coefficient of reliability and coefficient of repeatability 15,3~
Results There were 45 men and 59 women in the group with an age range of 15-45 years (mean 23). The degree of open
bite as defined in this study ranged from 1 to 11 mm, and the frequency distribution is shown in Fig. 5. The histog r a m suggested a trimodal distribution with superimposition of three distinct populations each with its own peak. Based on the cluster analysis, the best separation between the three subgroups of open bite, well illustrated by Fig. 6, was achieved using the K-mean method of clustering and the three categories of open bite were defined as follows: 1. Mild: A O B of 3.75 m m or less 2. Moderate: A O B between 3.75 m m and 6.85 m m 3. Severe: A O B over 6.85 m m In the sample, the mild group comprised 59 subjects (57%), the moderate group 32 subjects (31%) and the severe group 13 subjects (12%). The jaw relationship in the horizontal plane was n o r m a l in 45 subjects (43%), retrognathic in 14 subjects (14%), and prognathic in 45 subjects (43%). In the female open bite subjects, there was a weak but statistically sig-
323
nificant association between the mild category o f open bite and normal skeletal jaw relationship in the horizontal plane, and between the moderate category and the prognathic jaw relationship. The Spearman correlation coefficient was small (0.26) but significant (P=0.0426). However, in the male subjects, there was no significant association between the horizontal jaw relationship and the different categories of severity. Discriminant analysis identified the following variables as the best combination of cephalometric parameters to distinguish between the above three categories of severity of open bite: the mandibular occlusal plane, the overjet, the lower gonial angle, the upper anterior dental height, the upper posterior dental height, the palatal plane and the functional occlusal plane. Multiple linear regression was constructed for the total sample to identify the variables which best correlated with severity of open bite. Nine variables with P<0.0001 (except A N B with P=0.024) were identified as follows: palat~tl plane, mandibular occlusal plane, upper anterior dental height, lower anterior dental height, upper posterior dental height, overjet, lower anterior facial height, interincisal angle and the A N B angle. Error analysis Results of the paired t-tests on 35 variables of 40 radiographs measured twice indicated that the systematic error was not significant (lowest P value was greater, than 0.05). The coefficient of reliability of all variables was greater than 92% and the coefficient of repeatability
Table 2. Linear measurements of dentoalveolar landmarks Name
Code
Definition
Upper posterior dental height (UPDH)
U6-PP
Upper anterior dental height (UADH) Lower posterior dental height (LPDH)
U1E-PP L6-MP
Lower anterior dental height (LADH) Overjet
L1E-MP OJ
Overbite
OB
Upper 1st molar to sella in vertical plane
U6-SV
Perpendicular distance between mesiobuccal cusp tip of maxillary first molar to palatal plane Perpendicular distance between upper central incisal edge and palatal plane Perpendicular distance between mesiobuccal cusp tip of mandibular first molar to mandibular plane Perpendicular distance between lower central incisal edge and mandibular plane Distance between tip of lower central incisor to upper central incisal edge along a line parallel to Frankfort horizontal Distance between tip of lower central incisor to upper central incisal edge along a line perpendicular to Frankfort horizontal Distance between mesiobuccal cusp tip of maxillary first molar to sella along a line perpendicular to S-N* Distance between mesiobuccal cusp tip of maxillary first molar to Sella along a line parallel to S-N*
Upper 1st molar to sella in horizontal plane U6-SH * S-N: Sella-nasion line
324
Tsang et al.
12,
16 14
10.
12
E
8,
10
E
8
._= nn O '<
8 6 4 2 0
ya ,Y..,, "
"
"
"
2 0,
Std. Dev = 2.45 Mean = 4.03 N = 104.00 "
4
-2, N=
59
32
13
1.00
2.00
3.00
lo ,"1 "
"
'
' Oo
Mild, M o d e r a t e and S e v e r e
" Oo
AOB in mm Fig. 5. Histogram showingfrequency distribution of anterior open bite.
of all variables was smaller than 5.6~ and 3.7 mm. A comparison of the error between angular and linear measurements by independent t-tests indicated that angular measurements were subject to a significantly greater error than linear measurements.
Discussion The term "open bite" can be traced back to CARAVELLIin 18421, when it already referred to a distinct type of malocclusion. Many classifications have since been proposed but none of them have been universally accepted. SHmA29 classified open bite according to etiology into the developmental and acquired categories, but in the same year THOMA33 proposed a classification based on the site (anterior or lateral) and pattern (slanting or angulated). Later on, surgeons considered open bite from a treatment point of view and separated the skeletal from the nonskeletal or dental types 14,23. WORMS et al. 35 classified AOB by severity and extent of involvement into pseudo, true and compound types. The latest version is a comprehensive classification proposed by RICHARDSON26 combining etiological and skeletal considerations. There is also no universal definition for AOB. The term may refer to below-average overbite, an edge-to-edge occlusion, or a definite degree of gap between the upper and lower incisors23,31,32. KIM 18 used the occlusal plane to define AOB as a lack of coincidence between the maxillary and mandibular occlus01 planes, which fail to meet at the anterior teeth. In the present study, we considered the AOB more from a func-
Fig. 6. Box-plot of anterior open bite against severity.
tional viewpoint with a minimum threshold of a 1 mm gap at the front teeth, since patients will then be more likely to have chewing and speech problems and seek treatment. It was interesting that the cluster analysis showed three peaks in the sample distribution, thus indicating the three gradings of AOB from mild to severe, and enabling further analysis. Most previous studies to determine the cephalometric parameters of AOB compare patients with AOB to those with normal occlusion, deep overbite, or other skeletal deformities3,5,6,1~ 12,16,17,21,27,31 Many significant par-
ameters of AOB were noted, but some conflicting results have led to confusion. The conclusions from any one study might well have been true for that sample of patients, but often the study group composition was biased. No study looked at the intragroup distribution of AOB or made any attempt to determine the significant factors which contributed to the AOB being mild, as opposed to severe in extent. Lateral cephalography provides the traditional evaluation method in dentofacial diagnosis. We tested the correlation of the maxillo-mandibular relationship in the anteroposterior plane
Table 3. Angular measurements of skeletal landmarks
No./Code
Definition
1. SNA 2. SNB 3. N-S-AR 4. SN-MP 5. SN-FH 6. FH-FOP 7. FH-PP 8. S-AR-GO 9. GOA 10. UGOA 11. 12. 13. 14.
LGOA FH-MNOP FH-MXOP SN-PP
Angle between sella-nasion line and nasion-A point line Angle between sella-nasion line and nasion-B point line Angle between nasion-sella line and sella-articulare line Angle between sella-nasion line and mandibular plane Angle between sella-nasion line and Frankfort horizontal line Angle between Frankfort horizontal and functional occlusal plane Angle between Frankfort horizontal plane and palatal plane Angle between sella-articulare line and articulare-gonion line Angle formed by mandibular plane and a tangent to posterior border of ramus through articulare Angle between tangent to posterior border of ramus through articulare and line joining nasion and gonion Angle between mandibular plane and line joining nasion and gonion Angle between Frankfort horizontal and mandibular occlusat plane Angle between Frankfort horizontal and maxillary occlusal plane Angle between sella-nasion line and palatal plane
Table 4. Angular measurements of dentoalveolar landmarks
No/Code
Definition
1. 2. 3. 4.
Angle between sella-nasion line and long axis of upper incisor Angle between palatal plane and long axis of upper incisor Angle between long axis of upper and lower incisors Angle between mandibular plane and long axis of lower incisor
U1-SN U1-PP I-I LI-MP
Severity of anterior open bite with the severity grading of AOB. Out of the 18 possible combinations, only two showed weak correlation of significance. This finding, therefore, justifies a different approach to the assessment of AOB. The vertical linear and angular parameters were separately assessed by two independent methods of analysis. With the discriminant analysis, seven significant factors were noted, while with the linear regression analysis, nine significant factors were found. However, only five factors were common to both methods. These were: the palatal plane, the mandibular occlusal plane, the upper anterior dental height, the upper posterior dental height and the overjet. Although the overjet was also found to be a significant factor, it is a measurement in the horizontal plane and, as already mentioned, has only weak correlation. Of the remaining four factors with correlation to the severity of AOB, two were vertical linear parameters of the maxillary alveolar process, and two were angular parameters of the maxilla and mandible respectively. This finding highlights several points worthy of further consideration. AOB caused by dental abnormality is conventionally considered to be a mild form of open bite, whereas that which is associated with a skeletal abnormality constitutes the severe form. In this study, it was interesting to learn that, with increasing severity of AOB, the maxillary dentoalveolar vertical heights were the significant factors. Severity of AOB correlated with a direct proportional increase in the posterior dentoalveolar height, and was inversely proportional to the anterior dentoalveolar height. This combination of dentoalveolar height correlations influences the rotation of the maxilla, and is reflected by changes in the palatal plane. Therefore, the logical clinical tenet that anticlockwise rotation of the palatal plane, in addition to the absolute heights of the dentoalveolar segments, correlated well with the severity of AOB, has been confirmed by this study. The occlusal plane in AOB should be considered differently from non-AOB. Many studies measured the occlusal plane by bisecting the AOB, and this led to the wrong conclusion that AOB produces an increase in the occlusal plane 3,1~ On realizing this error, some researchers considered the occlusal plane separately for the maxillary
and the mandibular teeth and found that the significant factor was the mandibular occlusal plane and not the maxillary occlusal plane9,z4. This was not totally confirmed by ELLIS et al. 6, who showed that in AOB the maxillary occlusal plane was less steep, while the mandibular occlusal plane was steeper than in non-AOB. The maxillary occlusal plane may well be fiat in AOB and this is a common phenomenon, but it does not decrease with increasing severity of open bite. In this study, and in agreement with NAnotrM et al. 24 and Eran,ZLUs, the mandibular occlusal plane becomes steeper with increasing severity of AOB. This can be due to the combination of the downward direction of growth of the mandible and its anterior teeth, reflected by an increase in the gonial angle and a reverse curve of Spee.
Acknowledgments: The expert advice on statistical analysis of Dr Philip Yu, Department of Statistics, and Miss May Wong, Department of Periodontology and Public Health, University of Hong Kong, is gratefully acknowledged.
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Address:
Dr Nabil Samman Associate Professor Department of Oral and Maxillofacial Surgery University of Hong Kong 34 Hospital Road Hong Kong