ORIGINAL ARTICLES
The effect of maxillary first premolar extraction and incisor retraction on mandibular position: Testing the central dogma of "functional orthodontics" Percy E. Luecke II1" and Lysle E. Johnston, Jr.** San Antonio, Texas, and Ann Arbor, Mich.
It has been argued by a vocal coterie of disaffected dentists that premolar extraction, incisor retraction, and "backward-pulling" mechanics conspire to "distalize" the condyles and, pad passu, to produce craniomandibular dysfunction. Given the gravity of this conjecture, it seemed appropriate to test the predictions it generates in a sample of patients of the type most often said to be at risk: 42 "edgewise" patients with Class II, Division 1 malocclusions, treated in conjunction with the extraction of two maxillary first premolars. Regional and anterior cranial-base cephalometric superimpositions were used to quantify the individual components of the molar and overjet corrections, to measure both at the chin and condyles the mandibular displacement seen during treatment, and to examine the extent to which this displacement is related to the correction of maxillary incisor protrusion. Although the present patients underwent marked upper incisor retraction (on average, about 5 mm), lip retraction was much less pronounced, and 70% of the sample showed a net forward displacement of mandibular basal bone. Significantly, changes in condylar position were not correlated with incisor retraction, as the "functional orthodontists" would have it, but rather with the changes in the buccal occlusion and the growth of the maxilla. Thus, 30% of the patients who showed evidence of distal displacement were generally nongrowing patients who underwent more than average anchorage loss in the mandible and less than average loss in the maxilla. Regardless of the direction of basal displacement, however, condylar remodeling apparently served to stabilize the spatial position of surface landmarks (e.g., condylion), an observation that underscores the futility of using any type of serial radiograph to assess changes in condylar position in the growing, unimplanted patient. (AM J ORTHODDENTOFACORTHOP 1992;101:4-12.)
Upper first bicuspids are extracted in my practice. The only teeth which require extensive movement therefore are the six upper front teeth, which are forced back to close spaces and correct the facial outlines. During this process the buccal teeth are adjusted to an interdigitating occlusion which answers all the demands of mastication, and, what is more, the positions of the teeth are easily retained.
In Susan's case with her type of malocclusion or her type of problem when she went to see the orthodontist, no way should headgear and retraction of the upper front teeth back toward the tongue have occurred. This left Susan with a bite that her lower jaw now bites in a displaced position, and no way should any patient be left in that condition. This is a strong breach--this is dental negligence.
C.S. Casd
J. W. Witzig 2
Based on a thesis submitted by Dr. Luecke in partial fulfillment of the requirements for the degree of Master of Science, Department of Orthodontics, St. Louis University. Dr. Luecke was the recipient of an American Association of Orthodontists Award of Special Merit (1991), and his research was supported in part by the American Association of Orthodontists grant AAO1989-1 and by monies from the St. Louis University Orthodontic Alumni Association. The present manuscript was prepared during Dr. Johnston's tenure as William Evans Visiting Fellow, University of Otago, Dunedin, New Zealand. *Assistant Professor, Department of Orthodontics, School of Dentistry, University of Texas Health Science Center at San Antonio. **Professor and Chairman, Department of Orthodontics, St. Louis University Medical Center, St. Louis, Mo.; present address, Department of Orthodontics and Pediatric Dentistry, School of Dentistry, The University of Michigan, Ann Arbor, Mich. 8/1/31588
Over the past few years, a variegated assortment o f dentists campaigning under the banner o f "functional orthodontics" have assisted in the formulation o f a nettlesome and surprisingly popular legal theory that premolar extraction, extraoral traction, and Class II elastics combine to produce a distal displacement o f the condyle that ultimately leads to a variety o f debilitating craniomandibular disorders (CMD). 3-7 Although this improbable reincarnation of Costen's syndrome s is largely innocent o f support in the refereed literature, its proponents appear to have won the hearts, minds, and patients of a good many referring dentists.
Votume 10t Number 1
In the process, they have convinced a growing segment of the specialty that a willful failure to honor the tenets of their distal-displacement theory is directly responsible for the various adverse CMD judgments that currently terrorize the ranks. In increasing numbers, orthodontists have responded to this double-barreled economic threat by abandoning premolar extraction, 9 even though decades of literature argue that flaring and expansion, essentially the alternative suggested by the functional orthodontic counter-culture, is a highly unreliable answer to the problem of crowding and protrusion. Although this seemingly mean-spirited attack is reminiscent of La Rochefoucald's claim that "it is not enough to succeed; others must fail," its long-term success will turn less on its ability to strike fear in the heart of the unbeliever than on the joint troth of its component assertions: that premolar extraction causes distal displacement of the mandible and that this distal displacement in turn causes CMD. On the face of it, neither seems particularly likely, especially given that the traditional gnathologic criticism of conventional, specialist-only techniques is that they tend to cause mesial, rather than distal, mandibular displacement. Jo-~7 The functional orthodontists, however, reserve their most aggressive and pointed criticism for treatments that emphasize incisor retraction, a change that is commonly said to "lock" or"trap" the mandible in a retruded position. 4 Thus, despite a record of apparently successful service dating back to Bourdet and Hunter in the middle of the 18th century, maxillary first-premolar extraction treatments--and the orthodontists who use t h e m - - a r e apparently now at risk of being blamed for any subsequent functional misadventures that their patients may suffer. Given that a considerable proportion of the population--perhaps a half or more--will at one time or another display the signs and symptoms of CMD, regardless of treatment history,~s.~9 it is clear that this legal strategy has the potential to do great mischief to the rational, orderly practice of orthodontics. Unfortunately, the literature has little of significance to say about the imaginative concept of "trapped" mandibles; however, from an analysis of ancillary data, it is possible to conduct at least a preliminary examination of this first link in the functional orthodontists' hypothesized chain of disaster. Specifically, given the letter of the Witzig/Yerkes conjecture, one would expect to see a distal displacement of mandibular basal bone that is closely correlated with the retraction of the maxillary incisors. Such an outcome, however, clearly would run counter to the neurobiology of occlusion and the known effects of conventional fixed appliance treatment.
Premolar extraction and mandibular position
5
First, given the pronounced overjet of the usual Class II malocclusion, the incisors could undergo marked retraction without touching, much less trapping, anything. Indeed, it seems more likely that a patient's centric occlusal position would be determined by the cusps and fossae of the teeth that actually do occlude--the premolars and molars (in concert with the innervation of their periodontal ligaments and the muscles of mastication). Accordingly, one would expect that the forward growth of the midface (1 to 2 mm) and anchorage loss from reciprocal closure of the maxillary extraction spaces (2 to 4 mm) would combine to produce a marked mesial movement of the buccal occlusion that should in turn tend to produce an anterior functional shift of the mandible, including its condyles. 2°'2~ To a lesser extent, this tendency to forward displacement would be offset by allowing the mandible to "settle" 1 to 2 mm back-into a full step Class II occlusion and by mandibular anchorage loss. By way of explanation, it should be noted that, in Class II treatments, lower anchorage loss usually helps to correct the molar relationship and thus is thought of as mesial molar movement relative to basal bone. In contrast, maxillary premolar extraction treatments leave the molars in a Class II relationship. Thus, if the molar relationship is effectively fixed, any mandibular anchorage loss would, of necessity, take the form of a distal displacement of the mandible. In the end, however, it can be argued that mesial displacement would be the most probable net effect of all these treatment changes. Ultimately, however, reason and logic have their limits, especially at a time when the specialty appears to have developed what amounts to a siege mentality concerning the question i~f extraction; a more direct and timely approach is needed. Thus it will be the purpose of this article to examine cephalometrically the dental and skeletal changes produced by the treatment of Class II, Division I malocclusions in conjunction with the extraction of upper first premolars. In the process, we will examine the alternative hypothesis that this treatment tends to feature instead a mesial mandibular displacement caused by changes in the "position of the buccal segments, rather than by incisor retraction. PATIENTS AND METHODS
The present study employed the pretreatment and posttreatment records of 42 edgewise patients (20 male and 22 female) with Class II, Division 1 malocclusions treated in conjunction with the extraction of maxillary first premolars at the Department of Orthodontics, St. Louis University Medical Center. The average starting age was 15.3 years (range, 8.7 to 39 years), and the average treatment time was 19 months (range, 11 to 33 months). Selection was based solely
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Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. Januao" 1992
"Pitchfork" Analysis Cranial Base Registered at SE
(Max.)
iaxilla
Upper Molar
(U6)
(Ul)
I
Skeleton
Molar Relation
(ABCH)
(6/6)
-rl'
Upper Incisor
(/_6)
Over jet (1/I)
ower Incisor
Lower Molar
Mandible (Mand.)
M a x i l l a * M a n d i b l e -- A B C H ABCH + Upper Molar * Lower Molar = Molar Correction ABCH ÷ Upper Incisor * Lower Incisor = Overjet Correction
Fig. 1. "Pitchfork" analysis: anteroposterior skeletal and dental components of molar and overjet corrections. The growth/displacement of the maxilla and mandible are measured relative to cranial base (SE registration); the movements of the upper and lower molars and incisors are measured relative to basal bone (regional superimposition). All measurements are executed parallel to the mean functional occlusal ptane and are given signs appropriate to the nature of their contribution to the molar and overjet corrections. As a result, the algebraic sum of the various skeletal and dental changes equals the treatment change in the molar relationship and incisal overjet.
on the availability and adequacy of records and was in no way influenced by any measure of treatment outcome (e.g., cooperation).
Cephalometric technique Tile present analysis was designed to measure the individual anteroposterior dental and skeletal changes that sum to produce the molar and overjet corrections (Fig. 1). This general method has been described in detail elsewhere -'°'2~and uses Bjrrk's methods of cranial base, maxillary, and mandibular superimposition2-'~" to measure the movement of the buccal segments (measured at the mesial contact points of the first molars) and the incisors relative to basal bone and the displacement of the jaws relative to cranial base. These individual components of the molar and overjet corrections are measured parallel to the mean functional occlusal plane (MFOP; with the maxillae superimposed, the pretreatment and posttreatment occlusal planes are averaged by inspection)2~ and are given signs appropriate to their contribution: positive if they reduce the overjet or normalize the major occlusion; negative, if they make things worse. In this analysis, the contribution from mandibular growth/advancement (Mand.) is assessed with reference to D point, the center of the symphysis located by inspection on the initial tracing and then passed through to the final tracing by regional superimposition. Unfortunately, because of mandibular rotation, a
change in the position of D point parallel to the MFOP does not necessarily imply a like displacement at the head of the condyle. To generate a direct measure of condylar displacement during treatment, C point, the center of the pretreatment condyle (and thus presumably a landmark in "'basal" bone), and condylion (Co), the most posterosuperior point on the surface of the condyle were passed through to the posttreatment mandibular tracing by regional superimposition in the mandibular body (symphysis, molar germs) and condyle, respectively. Their displacement during treatment was then assessed with reference to a Cartesian coordinate system in which Frankfort Horizontal (FH; pretreatment/posttreatment average) served as the abscissa, and a perpendicular erected through SE point (the point at which the averaged outlines of the greater wings cross the planum sphenoidale) formed the ordinate. In addition, the extent to which the drift of surface landmarks produced an apparent change in condylar position was assessed by measuring sella to articulare and sella to condylion (S-Ar and S-Co). To generate individualized control data for the behavior of these two measures, expected (i.e., mean) increments were calculated for each subject by interpolation according to age, sex, and treatment time from the Michigan cephalometric standards. "-6To address the popular concept that premolar extraction/incisor retraction routinely produces "dished-in" profiles," upper and lower lip retraction was mea-
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Premolar extraction and nzandibular position
7
Number 1
I. Dental, skeletal, and soft tissue components o f the molar, overjet, and soft tissue treatment changes
Table
Range
Measure
Mean
SD
Maximum
Minimum
t score (Ho:~ = O)
Skeleton-jaw displacement relative to cranial base
Max. Mand. (D point) Net (ABCH)
-0.8 1.2 0.5
1.0 2.6 2.1
1.6 8.4 6.3
-3.3 -5.0 -5.1
1.5 1.0 1.8
0.0 3.4 2.3
-7.5 - 1.5 - 6.7
3.0 1.8 3.3
11.4 4.7 13".8
-2.8 -3.2 -0.6
10.9"* 4.4** 12.3"*
5.3 l 1.8
-5.4 0.7
-7.9** 15.3"*
-5.1"* 3.1"* 1.4
Dentition-molar movement relative to basal bone
U6 L6 Net
-3.2 0.6 - 2.7
- 14.2"*
3.7** -9.4**
Dentition-incisor movement relative to basal bone
UI LI Net
5.0 1.2 6.3
Total change--ABCH plus net 6; ABCH plus net 1
6/6 Overjet
-2.2 6.7
1.8 2.9
**P < 0.0l.
sured, both parallel to MFOP (maxillary superimposition) and perpendicular to Ricketts' E plane. Finally, from the standpoint of a best-fit cranial base superimposition, we measured change in the angulation of FH, the mandibular plane, and arbitrary "basal" mandibular and maxillary planes defined by pairs of fiducial landmarks established on the first tracing and then passed through to the second by detailed maxillary and mandibular regional superimposition.
Statistical analysis Common descriptive statistics (mean, standard deviation, range), as well as t statistics for the null hypothesis of no change during treatment (Ho" ~ = 0), were calculated for all measures in the present analysis. In addition, paired t tests were used to test the null-hypothesis that the changes in S-Ar and S-Co do not differ significantly from the norms inferred from the Michigan standards. 26 Product-moment coefficients of linear correlation (r) were calculated to estimate the strength of the relationship between mandibular displacement, measured both at the chin (D) and condyle (C), and changes in tooth position--the maxillary molars and the incisors, each measured relative both to maxillary basal bone (UI and U6) and to cranial base (U1 plus Max. and U6 plus Max.). Finally, because one-arch treatments encourage the mandible to settle back into a fully interdigitated Class II molar relationship, there obviously can be changes in mandibular position that are independent of tooth movement. As a result, partial correlation (r,y. ,) was used to examine the relationship between mandibular displacement and the movement of the molars and the incisors, with change in molar relation (6/6) held constant.
RESULTS Descriptive statistics for the various dental and skeletal components o f the molar and overjet changes, along with t scores testing the null hypothesis o f no change, are summarized in Table I. It m a y be noted that, except for apical base change (ABCH), all dimensions changed significantly during the course o f treatment; D point showed a f o n v a r d displacement that averaged 1.2 mm. For both the molar and the ovejet corrections, tooth movement was by far the most important component. Descriptive and inferential statistics f6r lip retraction, measures of mandibular displacement taken at the condyle, and angular measures o f facial divergence are presented in Tables II and III. Although our estimates of the change in the position of condylar basal bone (horizontal change in C point) suggest that it, too, is usually displaced anteriorly, a surface landmark (Co) tends to maintain its horizontal position or even move distally (see S-Co and S-Ar). The distal drift of the condylar surface implied by the significant increase in S-Co and S-Ar, however, does not differ significantly (i.e., P < 0.05) from the individualized norms for these measures inferred from the Michigan standards foruntreatedpersons: t = 0 . 9 0 a n d 1.86, respectively. Note also that the marked retraction of the upper incisors did not have a comparable impact on the lips. More to the point of this communication, the incisor changes were essentially unrelated to condylar displacement during treatment. From the correlation coefficients summarized in
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Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. January 1992
Table II. Condylar displacement (basal and surface) and lip retraction Range I S¢oge
Measure
Mean
SD
MaMnutm
0.7 -3.2
1.7 3.0
4.8 1.7
-2.2 - 12.2
-2.8** - 7.3**
-0.2 - 0.9 0.5 0.9
0.9 1.9 1. l 0.9
2.1 1.8 3.0 2.8
- 2.0 -7.9 - 1.5 -0.9
- 1.3 -3.5** 3.0** 6.4**
- 1.4 -2.4
2.3 2.0
4.2 2.2
-5.8 -7.1
-3.9** -7.8**
0. l - 1.4
2.2 2.0
5. l 2.7
-3.8 -6.4
0.3 -5.0**
Minimum
(Ho:8 = O)
Condylar displacement C point (center) Horizontal Vertical Condylion (surface) Horizontal Vertical Sella-Co Sella-Ar
Lip retraction Upper lip Along MFOP To E plane Lower lip Along MFOP To E plane
* P < 0.05. **P < 0.01.
Table III. Angular changes relative to anterior cranial base Range Angle Frankfort plane Maxillary base FOP Mandibular base Mandibular plane (tangent)
Mean 0.0 0.6 -0.1 0.6 ! .0
SD
Maxhnum
I. I 1.2 2.9 1.8 1.6
4.2 5.4 8.6 6.6 6.2
I
Minimum - 1.9 - 1.5 -5.4 - 3.3 - 2.5
t score (Ho:6 = O) - 0.0 l 3.14"* -0.15 2.20 3.99'*
**P < 0 . 0 l .
FOP, Functional occlusal plane.
Table IV, it may be seen that anteroposterior change in the position of the chin (D poin0 was related to two of the measures of incisor displacement relative to cranial base and to all four measures of change in upper molar position. In contrast, the displacement of condylar basal bone (C point) was related only to changes in the position of the maxillary molars. DISCUSSION
From the present data, it is clear that the majority of maxillary first-premolar-extraction patients undergo a mesial mandibular displacement (along with a slight opening rotation) during maxillary premolar-extraction treatment. Regardless of whether displacement of C point, D point, or the mean of the two serves as the criterion, only about 30% (13 of 42) of the present subjects gave evidence of any posterior shift (an average
of 1 to 2 mm distal for this small subsample). Moreover, from the correlation coefficients of Table IV, it is clear that variation in mandibular position was correlated not with incisor retraction, but, rather, with displacement of the buccal segments. Indeed, the coefficients of linear correlation for the relationship between mandibular displacement (measured both at condyle and at the symphysis) and changes in the position of the maxillary buccal segments were exceptionally high (up to - 0.9). Although movement of D point bore a statistically significant relationship both to molar movement and to incisor retraction, condylar displacement was correlated only with changes in the position of the molars and premolars. On the face of it, the present modest correlation between incisor retraction and symphyseal displacement would seem to support the functional orthodontic hypothesis. It should be noted, however, that
Vohtme 101 Number 1
Premolar extraction and mandibular position
9
Table IV. Correlation coefficients (r) for the relationship between mandibular displacement and treatment changes in molar and incisor position Displacement measured at Treatment change
C point
D point
-0.05 -0.06 - 0.20 - 0.22
-0.22 -0.35 - 0.42"* - 0.59**
-0.30 -0.38* -0.46** -0.51"*
-0.41"* -0.81"* -0.62** -0.90**
Incisor retraction UI UI U1 UI
to to to to
maxilla maxiltar[6/6 const~t cranial base cranial base 16/6 constant
Anchorage loss U6 to maxilla U6 to m a x i l l a [ 6 / 6 constant U6 Io cranial base U6 to cranial b a s e l 6 / 6 constant *P < 0.05.
**P < 0.01.
this relationship may reflect the impact of a third factor, the buccal segment changes and mandibular rotation that would of necessity precede or accompany varying degrees of incisor retraction. For example, greater-thanaverage incisor retraction might require extensive use of Class II elastics and headgear, both of which would tend to reduce maxillary anchorage loss, to increase mandibular anchorage loss, and to extrude the molars. In the present analysis, the resulting downward and backward rotation would be read as a posterior displacement of the chin, and the buccal segment changes, the only effects that actually correlated with condylar displacement, would at the same time tend to produce a posterior bodily displacement of mandibular basal bone. In any event, if the direction of displacement is related to the molars, rather than the incisors, a post hoc analysis should support this interpretation. For purposes of discussion, the present sample was subdivided according to the direction of mandibular displacement (based on the algebraic average of the C and D point displacement), and the various skeletal and dental components of the molar and overjet corrections contrasted by means of completely randomized t tests. It may be seen from the "pitchfork" diagrams of Fig. 2 that the two subgroups did not differ significantly in terms of incisor retraction. Instead, the patients whose mandibles appear to have been displaced distally were those whose maxillae showed little forward growth (including every patient who was 17 years or older at the outset) and/or who either lost less anchorage than average in the maxilla or more anchorage than average in the mandible. Once again, the present findings support the logically (and biologically) more compelling alternative hypothesis that change in the centric occlusal position of the mandible is a function of changes in
anteroposterior position of the occluding buccal segments, rather than the generally nonoccluding incisors. As noted in a recent review by Tallents and coworkers~9: To make an assumption that the condyle has been forced distally as a result of therapy, without appropriate pretreatment documentation, is untenable. To make the assumption that because the incisors are upright they are "over-retracted," and that this over-retraction produced changes in condyle position again is untenable without adequate pretreatment documentation. Unfortunately, the present data argue that "adequate documentation" may be difficult to come by. In contrast to the significant mesial displacement of condylar "basal" bone, the surface of the condyle (abstracted here by Co) tended to remain stationary or even drift posteriorly a few tenths of a millimeter, a change that is consistent with the normal, growth-related posterior movement of the glenoid fossa (S-Ar; S-Co). Presumably, this relative stability is a by-product of the same condylar adaptability that underwrites the action of contemporary functional appliances. 27Moreover, it has recently been reported that the radiographic outline of the condyle may be subject to "seasonal variation. '':8 Thus, if surface changes tend not only to produce cyclic modifications in the apparent form of the condyle, but also to mask the true nature of any bodily displacement, then it would seem a futile gesture to use any kind of conventional condylar radiograph to assess changes in condylar position in unimplamed, growing patients. Thus the present findings seem to support Wyatt's scheme for iatrogenic mandibular distal displacement: "As the maxilla [sic] is moved backward, the muscles of mastication will attempt to retract the mandible when the patient closes in to [sic] maximum intercuspation. ''~ It seems appropriate, therefore, to note that if distal
10
Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. Janua~" 1992
Displaced Distally i
0.0,4
[
D i s p l a c e d Mesially
I~
t
-2.27
I c - -0.55 - 1.63
-2.83 1.10
" TI'
-1.14
l ~5.72 5.9 1
[ -3.36 I - 1.94
c • 1.3o 1.39
Tl'o ,
/'°
~
4.71
7.09 1.00 P
2.54
1.67 N = 13 -
Difference ~L
N = 29
[ 1.18,*
1.39,
I c • -t.85.-
-3.02.*
-0.89
1.oi - 1.18
T i, /oo 0.76 °
4.21."
Fig. 2. Post hoc comparisons between subgroups defined according to presumed direction of mandibular displacement during treatment. Note that, in addition to the obligatory differences in the dispracement of the landmarks that were used to define the two subsamples (C and D), the "distal" and "mesial" groups differ only in terms of the growth of the maxilla and the displacement of the buccal segments. In other words, anteroposterior changes in the position of centric occlusion appear to be independent of incisor retraction. Statistical significance of the differences depicted in the third diagram: *P < 0.05; **P < 0.01.
displacement actually does cause CMD, only patients who show a ponderable distal displacement of the upper buccal segments would be at risk. Thus the functional orthodontists should denounce nonextraction treatments and instead embrace premolar extraction, because in the former the maxillary buccal segments are commonly moved distally, whereas in the latter they almost always come forward as the extraction sites are closed. 2°'2~ Indeed, in the present study, 41 of 42 subjects showed mesial movement of the upper buccal segments. This argument, of course, is advanced mostly out of exasperation, because in truth any treatment can probably be moaified to minimize displacement in any direction or misused to maximize it. It remains to be seen, however, whether a millimeter or so of mandibular displacement is actually a threat to the public health. If the refereed literature is any guide, there is precious little, if anything, in the way of support for this contention. 19.29-3oMoreover, one must consider the ultimate
fate of changes in mandibular position caused by orthodontic tooth movement. Given the unremarkable assumption that centric occlusion is determined by teeth, it should come as no surprise that orthodontic treatment can change the position of centric occlusion. But what happens to this treatment-induced mandibular displacement once the appliances are removed? Although the present data do not address this issue, it may be noted that, in the absence of appliances, it is usually thought that the jaws move the teeth as part of the process of dentoalveolar compensation. 3t For example, it is usual to interpret the crowded, retroclined lower incisors of the patient with a Class III malocclusion as a dentoalveolar comp6nsation for a pattern of mandibular overgrowth. Accordingly, few would say that the excess growth results from the mandible being "freed up" by the linguoverted incisors, because to do so would be to reverse our usual ideas of cause and effect. And yet in the next breath
Volume 101
Number 1
we are often quite willing to talk o f "trapped" mandibles in Class II, Division 2 malocclusions or to express concern that tooth movement might produce a permanent d i s t a l - - o r , if you are gnathologically oriented, m e s i a l - - d i s p l a c e m e n t o f the mandible. In other words, we fear that, even after appliances have been removed, the teeth are more likely to displace the jaws than they are to move through bone. The transience of both the posterior mandibular positions produced in so-called "centric relation" reconstructions 32 and the anterior displacement commonly produced by comprehensive orthodontic treatment 17 argues that the jaws will ultimately win out over the teeth. Despite marked maxillary incisor retraction measured parallel to MFOP, the upper lip, on average, showed only a little over a millimeter o f change; the lower lip, none. Both lips, however, showed a more pronounced retraction relative to E plane. Thus the present data imply that the normal growth o f the nose and chin may have a more profound long-term impact on the relative protrusion o f the lips than does the orthodontic retraction o f the incisors. This interpretation does not mean that treatment never produces marked lip retraction (one patient, for example, experienced about 6 m m o f upper lip retraction along MFOP); rather, it argues that the soft tissue changes produced by onearch extraction treatments are, on average, much less pronounced than we have been led to expect, a conclusion that echoes the sentiments o f a recent examination of four first-premolar-extraction treatments. 33 On balance, our data argue that the corpus of "functional orthodontic" thought concerning the evils o f premolar extraction is either dead wrong or grossly exaggerated; if adhered to, it m a y constitute a threat to the public health. SUMMARY
Regional cephalometric superimposition was employed to characterize the dental and skeletal changes seen in a sample o f 42 patients who had Class II, Division 1, maxillary first-premolar extraction and who were treated with the edgewise technique. Approximately 70% o f the present sample underwent varying degrees o f forward mandibular displacement; 30% o f the sample underwent distal. More significantly, this mandibular displacement was correlated not with maxillary incisor retraction, which averaged 5 mm, but rather with changes in the spatial position o f the buccal occlusion. Surface changes, however, apparently tended to keep the head of the condyle fixed in space, regardless o f the direction of basal displacement produced by treatment. Finally, although the present treatments produced marked incisor retraction, the soft tis-
Premolar extraction and mandibular position
11
sue profile appears to have been influenced more profoundly by the growth o f the nose and chin. Thus the present data fail to support the claim that premolar extraction and incisor retraction must, o f necessity, lead to unsightly profiles and distal mandibular displacement. REFERENCES 1. Case CS. The question of extraction in orthodontia. Reprint, AM J ORTHOD 1964;50:660-91. 2. Witzig JW. Deposition, Brimm v. Malloy lsic], Oakland County (MI) Circuit Court, Case No. 852 987 50, July 13, 1987. 3. Bowbeer GRN. Saving the face and the TM--Part 2. Funct Orthod 1986;3:9-39. 4. Spahl TJ, Witzig JW. The clinical management of basic maxillofacial orthopedic appliances. Vol. I: mechanics. Littleton:
PSG Publishing, 1987. 5. Wyatt WE. Preventingadverse effects on the temporomandibular joint through orthodontic treatment. AM J ORTHODDENTOFAC ORTHOP 1987;91:493-9. 6. Witzig JW, Yerkes I. Researchers question dogma of protruded maxilla: findings hint of improper orthodontic treatment. Dentist 1988;66:21, 23, 49. 7. Bowbeer GRN. The seventh key to facial beauty and TMJ health--Part 2: Proper condylar position. Funct Orthod 1990;7:4-32. 8. Costen JB. A syndrome of ear and sinus symptoms dependent upon disturbed function of the temporomandibular joint. Ann Otol Rhinol Laryngol 1934;43:1-15. 9. O'Connor B. Contemporary trends in orthodontic practice: a national survey lThesis]. St. Louis: Departmentof Orthodontics, St. Louis University Medical Center, 1990. 10. Chappione RC. Special considerations for adult orthodontics. J Clin Orthod 1976;10:535-45. 11. Roth RH. Functional occlusion for the orthodontist. Part I. J Clin Orthod 1981;15:32-51. 12. Roth RH. Functional occlusion for the orthodontist. Part III. J Clin Orthod 1981;15:174-98. 13. Roth RH, Rolfs DA. Functional occlusion for the orthodontist. Part II. J Clin Orthod 1981;15:100-23. 14. Roth RH, Gordon WW. Functional occlusion for the orthodontist. Part IV. J Clin Orthod 1981;15:246-65. 15. Williamson EH. Occlusion and TMJ dysfunction. J Clin Orthod 1981;15:333-50. 16. Williamson EH. Occlusal concepts in orthodontic diagnosis and treatment. In: Johnston LE Jr, ed. New visits in orthodontics. Philadelphia: Lea & Febiger, 1985. 17. Johnston LE, EICO Orthodontic Study Group of Ohio. Gnathologic assessment of centric slides in postretention orthodontic patients. J Prosthet Dent 1988;60:712-5. 18. Mohlin B, Pilley JR, Shaw WC. A survey of eraniomandibular disorders in 1000 12-year-olds. Study design and baseline data in a follow-up study. Eur J Orthod 1991;13:111-23. 19. Tallents RH, Catania J, Sommers E. Temporomandibularjoint findings in pediatric populations and young adults: a critical review. Angle Orthod 1991;61:7-16. 20. Johnston LE Jr. A comparative analysis of Class II treatments. In: McNamara JA Jr, Carlson DS, Vig PS, Ribbens KA, eds. Science and clinical judgment in orthodontics. Monograph 18, Craniofacial Growth Series. Ann Arbor: Center for ttuman Growth and Development, The University of Michigan, 1986:103-48.
12
Luecke attd Johnston
Am. J. Orthod. Dentofa¢. Orthop. January 1992
21. Johnston LE Jr, Lin S-S, Peng S. Anchorage loss: a comparative analysis. J CH Tween Int Foundat 1988;16:23-7. 22. Bj6rk A. Prediction of mandibular growth rotation. A.',iJ OR'moP 1969;55:585-99. 23. Bj6rk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Onhod 1983;5:1-46. 24. Bjrrk A, Skieller V. Postnatal growth and development of the maxillary complex. In: McNamara JA Jr, ed. Factors affecting the growth of the midface. Monograph 6, Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development, The University of Michigan, 1986:61-99. 25. Jenkins DH. Analysisoforthodonticdeformityemployinglateral cephalometric radi~raphy. AM J OgrHOl9 1955;41:442-52. 26. Riolo ML, Moyers RE, McNamara JA Jr, Hunter WS. An atlas of craniofacial growth. Mon~raph 2, Craniofacial Growth Seties. Ann Arbor: Center for Human Growth and Developmeut, The University of Michigan, 1974:202-3. 27. Johnston LE Jr. The curious case of the chimerical condyle. In: Graber LW, ed. Orthodontics. State of the art; essence of the science. St. Louis: CV Mosby, 1986:88-99. 28. Dibbets JMH, van der Weele LTh. Flattened condylar projection
29.
30.
31. 32. 33.
in children: reflection of seasonal growth? Eur J Orthod 1991;13:161-5. Rinchuse DJ. Counterpoint: preventing adverse effects on the temporomandibular joint through orthodontic treatment. AM J ORTHOD DENTOFACORTHOP 1987;91:500-6. Reynders RM. Orthodontics and temporomandibular disorders: a review of the literature (1966-1988). A.',t J ORI"~ODDEWrOFAC OR'mOP 1990;97:463-71. Solow B. The dento-alveolar compensatory mechanism: background and clinical implications. Br J Orthod 1980;7:145-61. Celenza F, The centric position: replacement and character. J Prosthet Dent 1973;30:591-8. Drobocky OB, Smith RJ. Changes in facial profile during orthodontic treatment with extraction of four first premolars. A.,~t J OR'mOP DErCrOFACOR'ntOP 1989;95:220-30.
Reprint requests to:
Dr. Lysle E. Johnston, Jr. Department of Orthodontics and Pediatric Dentistry School of Dentistry The University of Michigan Ann Arbor, MI 48109-1078
AAO MEETING CALENDAR
1992--St. Louis, Mo., May 9 to 13, St. Louis Convention Center 1993--Toronto, Canada, May 15 to 19, Metropolitan Toronto Convention Center 1994--Orlando, Fla., May 1 to 4, Orange County Convention and Civic Center 1995--San Francisco, Calif., May 7 to 10, Moscone Convention Center
(International Orthodontic Congress) 1996--Denver, Colo., May 12 to 16, Colorado Convention Center 1997--Philadelphia, Pa., May 3 to 7, Philadelphia Convention Center 1998--Dallas, Texas, May 16 to 20, Dallas Convention Center 1999~San Diego, Calif., May 15 to 19, San Diego Convention Center
The effect of maxillary first premolar extraction and incisor retraction on mandibular position: Testing the central dogma of "functional orthodontics" Percy E. Luecke II1" and Lysle E. Johnston, Jr.** San Antonio, Texas, and Ann Arbor, Mich.
It has been argued by a vocal coterie of disaffected dentists that premolar extraction, incisor retraction, and "backward-pulling" mechanics conspire to "distalize" the condyles and, pad passu, to produce craniomandibular dysfunction. Given the gravity of this conjecture, it seemed appropriate to test the predictions it generates in a sample of patients of the type most often said to be at risk: 42 "edgewise" patients with Class II, Division 1 malocclusions, treated in conjunction with the extraction of two maxillary first premolars. Regional and anterior cranial-base cephalometric superimpositions were used to quantify the individual components of the molar and overjet corrections, to measure both at the chin and condyles the mandibular displacement seen during treatment, and to examine the extent to which this displacement is related to the correction of maxillary incisor protrusion. Although the present patients underwent marked upper incisor retraction (on average, about 5 mm), lip retraction was much less pronounced, and 70% of the sample showed a net forward displacement of mandibular basal bone. Significantly, changes in condylar position were not correlated with incisor retraction, as the "functional orthodontists" would have it, but rather with the changes in the buccal occlusion and the growth of the maxilla. Thus, 30% of the patients who showed evidence of distal displacement were generally nongrowing patients who underwent more than average anchorage loss in the mandible and less than average loss in the maxilla. Regardless of the direction of basal displacement, however, condylar remodeling apparently served to stabilize the spatial position of surface landmarks (e.g., condylion), an observation that underscores the futility of using any type of serial radiograph to assess changes in condylar position in the growing, unimplanted patient. (AM J ORTHODDENTOFACORTHOP 1992;101:4-12.)
Upper first bicuspids are extracted in my practice. The only teeth which require extensive movement therefore are the six upper front teeth, which are forced back to close spaces and correct the facial outlines. During this process the buccal teeth are adjusted to an interdigitating occlusion which answers all the demands of mastication, and, what is more, the positions of the teeth are easily retained.
In Susan's case with her type of malocclusion or her type of problem when she went to see the orthodontist, no way should headgear and retraction of the upper front teeth back toward the tongue have occurred. This left Susan with a bite that her lower jaw now bites in a displaced position, and no way should any patient be left in that condition. This is a strong breach--this is dental negligence.
C.S. Casd
J. W. Witzig 2
Based on a thesis submitted by Dr. Luecke in partial fulfillment of the requirements for the degree of Master of Science, Department of Orthodontics, St. Louis University. Dr. Luecke was the recipient of an American Association of Orthodontists Award of Special Merit (1991), and his research was supported in part by the American Association of Orthodontists grant AAO1989-1 and by monies from the St. Louis University Orthodontic Alumni Association. The present manuscript was prepared during Dr. Johnston's tenure as William Evans Visiting Fellow, University of Otago, Dunedin, New Zealand. *Assistant Professor, Department of Orthodontics, School of Dentistry, University of Texas Health Science Center at San Antonio. **Professor and Chairman, Department of Orthodontics, St. Louis University Medical Center, St. Louis, Mo.; present address, Department of Orthodontics and Pediatric Dentistry, School of Dentistry, The University of Michigan, Ann Arbor, Mich. 8/1/31588
Over the past few years, a variegated assortment o f dentists campaigning under the banner o f "functional orthodontics" have assisted in the formulation o f a nettlesome and surprisingly popular legal theory that premolar extraction, extraoral traction, and Class II elastics combine to produce a distal displacement o f the condyle that ultimately leads to a variety o f debilitating craniomandibular disorders (CMD). 3-7 Although this improbable reincarnation of Costen's syndrome s is largely innocent o f support in the refereed literature, its proponents appear to have won the hearts, minds, and patients of a good many referring dentists.
Votume 10t Number 1
In the process, they have convinced a growing segment of the specialty that a willful failure to honor the tenets of their distal-displacement theory is directly responsible for the various adverse CMD judgments that currently terrorize the ranks. In increasing numbers, orthodontists have responded to this double-barreled economic threat by abandoning premolar extraction, 9 even though decades of literature argue that flaring and expansion, essentially the alternative suggested by the functional orthodontic counter-culture, is a highly unreliable answer to the problem of crowding and protrusion. Although this seemingly mean-spirited attack is reminiscent of La Rochefoucald's claim that "it is not enough to succeed; others must fail," its long-term success will turn less on its ability to strike fear in the heart of the unbeliever than on the joint troth of its component assertions: that premolar extraction causes distal displacement of the mandible and that this distal displacement in turn causes CMD. On the face of it, neither seems particularly likely, especially given that the traditional gnathologic criticism of conventional, specialist-only techniques is that they tend to cause mesial, rather than distal, mandibular displacement. Jo-~7 The functional orthodontists, however, reserve their most aggressive and pointed criticism for treatments that emphasize incisor retraction, a change that is commonly said to "lock" or"trap" the mandible in a retruded position. 4 Thus, despite a record of apparently successful service dating back to Bourdet and Hunter in the middle of the 18th century, maxillary first-premolar extraction treatments--and the orthodontists who use t h e m - - a r e apparently now at risk of being blamed for any subsequent functional misadventures that their patients may suffer. Given that a considerable proportion of the population--perhaps a half or more--will at one time or another display the signs and symptoms of CMD, regardless of treatment history,~s.~9 it is clear that this legal strategy has the potential to do great mischief to the rational, orderly practice of orthodontics. Unfortunately, the literature has little of significance to say about the imaginative concept of "trapped" mandibles; however, from an analysis of ancillary data, it is possible to conduct at least a preliminary examination of this first link in the functional orthodontists' hypothesized chain of disaster. Specifically, given the letter of the Witzig/Yerkes conjecture, one would expect to see a distal displacement of mandibular basal bone that is closely correlated with the retraction of the maxillary incisors. Such an outcome, however, clearly would run counter to the neurobiology of occlusion and the known effects of conventional fixed appliance treatment.
Premolar extraction and mandibular position
5
First, given the pronounced overjet of the usual Class II malocclusion, the incisors could undergo marked retraction without touching, much less trapping, anything. Indeed, it seems more likely that a patient's centric occlusal position would be determined by the cusps and fossae of the teeth that actually do occlude--the premolars and molars (in concert with the innervation of their periodontal ligaments and the muscles of mastication). Accordingly, one would expect that the forward growth of the midface (1 to 2 mm) and anchorage loss from reciprocal closure of the maxillary extraction spaces (2 to 4 mm) would combine to produce a marked mesial movement of the buccal occlusion that should in turn tend to produce an anterior functional shift of the mandible, including its condyles. 2°'2~ To a lesser extent, this tendency to forward displacement would be offset by allowing the mandible to "settle" 1 to 2 mm back-into a full step Class II occlusion and by mandibular anchorage loss. By way of explanation, it should be noted that, in Class II treatments, lower anchorage loss usually helps to correct the molar relationship and thus is thought of as mesial molar movement relative to basal bone. In contrast, maxillary premolar extraction treatments leave the molars in a Class II relationship. Thus, if the molar relationship is effectively fixed, any mandibular anchorage loss would, of necessity, take the form of a distal displacement of the mandible. In the end, however, it can be argued that mesial displacement would be the most probable net effect of all these treatment changes. Ultimately, however, reason and logic have their limits, especially at a time when the specialty appears to have developed what amounts to a siege mentality concerning the question i~f extraction; a more direct and timely approach is needed. Thus it will be the purpose of this article to examine cephalometrically the dental and skeletal changes produced by the treatment of Class II, Division I malocclusions in conjunction with the extraction of upper first premolars. In the process, we will examine the alternative hypothesis that this treatment tends to feature instead a mesial mandibular displacement caused by changes in the "position of the buccal segments, rather than by incisor retraction. PATIENTS AND METHODS
The present study employed the pretreatment and posttreatment records of 42 edgewise patients (20 male and 22 female) with Class II, Division 1 malocclusions treated in conjunction with the extraction of maxillary first premolars at the Department of Orthodontics, St. Louis University Medical Center. The average starting age was 15.3 years (range, 8.7 to 39 years), and the average treatment time was 19 months (range, 11 to 33 months). Selection was based solely
6
Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. Januao" 1992
"Pitchfork" Analysis Cranial Base Registered at SE
(Max.)
iaxilla
Upper Molar
(U6)
(Ul)
I
Skeleton
Molar Relation
(ABCH)
(6/6)
-rl'
Upper Incisor
(/_6)
Over jet (1/I)
ower Incisor
Lower Molar
Mandible (Mand.)
M a x i l l a * M a n d i b l e -- A B C H ABCH + Upper Molar * Lower Molar = Molar Correction ABCH ÷ Upper Incisor * Lower Incisor = Overjet Correction
Fig. 1. "Pitchfork" analysis: anteroposterior skeletal and dental components of molar and overjet corrections. The growth/displacement of the maxilla and mandible are measured relative to cranial base (SE registration); the movements of the upper and lower molars and incisors are measured relative to basal bone (regional superimposition). All measurements are executed parallel to the mean functional occlusal ptane and are given signs appropriate to the nature of their contribution to the molar and overjet corrections. As a result, the algebraic sum of the various skeletal and dental changes equals the treatment change in the molar relationship and incisal overjet.
on the availability and adequacy of records and was in no way influenced by any measure of treatment outcome (e.g., cooperation).
Cephalometric technique Tile present analysis was designed to measure the individual anteroposterior dental and skeletal changes that sum to produce the molar and overjet corrections (Fig. 1). This general method has been described in detail elsewhere -'°'2~and uses Bjrrk's methods of cranial base, maxillary, and mandibular superimposition2-'~" to measure the movement of the buccal segments (measured at the mesial contact points of the first molars) and the incisors relative to basal bone and the displacement of the jaws relative to cranial base. These individual components of the molar and overjet corrections are measured parallel to the mean functional occlusal plane (MFOP; with the maxillae superimposed, the pretreatment and posttreatment occlusal planes are averaged by inspection)2~ and are given signs appropriate to their contribution: positive if they reduce the overjet or normalize the major occlusion; negative, if they make things worse. In this analysis, the contribution from mandibular growth/advancement (Mand.) is assessed with reference to D point, the center of the symphysis located by inspection on the initial tracing and then passed through to the final tracing by regional superimposition. Unfortunately, because of mandibular rotation, a
change in the position of D point parallel to the MFOP does not necessarily imply a like displacement at the head of the condyle. To generate a direct measure of condylar displacement during treatment, C point, the center of the pretreatment condyle (and thus presumably a landmark in "'basal" bone), and condylion (Co), the most posterosuperior point on the surface of the condyle were passed through to the posttreatment mandibular tracing by regional superimposition in the mandibular body (symphysis, molar germs) and condyle, respectively. Their displacement during treatment was then assessed with reference to a Cartesian coordinate system in which Frankfort Horizontal (FH; pretreatment/posttreatment average) served as the abscissa, and a perpendicular erected through SE point (the point at which the averaged outlines of the greater wings cross the planum sphenoidale) formed the ordinate. In addition, the extent to which the drift of surface landmarks produced an apparent change in condylar position was assessed by measuring sella to articulare and sella to condylion (S-Ar and S-Co). To generate individualized control data for the behavior of these two measures, expected (i.e., mean) increments were calculated for each subject by interpolation according to age, sex, and treatment time from the Michigan cephalometric standards. "-6To address the popular concept that premolar extraction/incisor retraction routinely produces "dished-in" profiles," upper and lower lip retraction was mea-
Volume 101
Premolar extraction and nzandibular position
7
Number 1
I. Dental, skeletal, and soft tissue components o f the molar, overjet, and soft tissue treatment changes
Table
Range
Measure
Mean
SD
Maximum
Minimum
t score (Ho:~ = O)
Skeleton-jaw displacement relative to cranial base
Max. Mand. (D point) Net (ABCH)
-0.8 1.2 0.5
1.0 2.6 2.1
1.6 8.4 6.3
-3.3 -5.0 -5.1
1.5 1.0 1.8
0.0 3.4 2.3
-7.5 - 1.5 - 6.7
3.0 1.8 3.3
11.4 4.7 13".8
-2.8 -3.2 -0.6
10.9"* 4.4** 12.3"*
5.3 l 1.8
-5.4 0.7
-7.9** 15.3"*
-5.1"* 3.1"* 1.4
Dentition-molar movement relative to basal bone
U6 L6 Net
-3.2 0.6 - 2.7
- 14.2"*
3.7** -9.4**
Dentition-incisor movement relative to basal bone
UI LI Net
5.0 1.2 6.3
Total change--ABCH plus net 6; ABCH plus net 1
6/6 Overjet
-2.2 6.7
1.8 2.9
**P < 0.0l.
sured, both parallel to MFOP (maxillary superimposition) and perpendicular to Ricketts' E plane. Finally, from the standpoint of a best-fit cranial base superimposition, we measured change in the angulation of FH, the mandibular plane, and arbitrary "basal" mandibular and maxillary planes defined by pairs of fiducial landmarks established on the first tracing and then passed through to the second by detailed maxillary and mandibular regional superimposition.
Statistical analysis Common descriptive statistics (mean, standard deviation, range), as well as t statistics for the null hypothesis of no change during treatment (Ho" ~ = 0), were calculated for all measures in the present analysis. In addition, paired t tests were used to test the null-hypothesis that the changes in S-Ar and S-Co do not differ significantly from the norms inferred from the Michigan standards. 26 Product-moment coefficients of linear correlation (r) were calculated to estimate the strength of the relationship between mandibular displacement, measured both at the chin (D) and condyle (C), and changes in tooth position--the maxillary molars and the incisors, each measured relative both to maxillary basal bone (UI and U6) and to cranial base (U1 plus Max. and U6 plus Max.). Finally, because one-arch treatments encourage the mandible to settle back into a fully interdigitated Class II molar relationship, there obviously can be changes in mandibular position that are independent of tooth movement. As a result, partial correlation (r,y. ,) was used to examine the relationship between mandibular displacement and the movement of the molars and the incisors, with change in molar relation (6/6) held constant.
RESULTS Descriptive statistics for the various dental and skeletal components o f the molar and overjet changes, along with t scores testing the null hypothesis o f no change, are summarized in Table I. It m a y be noted that, except for apical base change (ABCH), all dimensions changed significantly during the course o f treatment; D point showed a f o n v a r d displacement that averaged 1.2 mm. For both the molar and the ovejet corrections, tooth movement was by far the most important component. Descriptive and inferential statistics f6r lip retraction, measures of mandibular displacement taken at the condyle, and angular measures o f facial divergence are presented in Tables II and III. Although our estimates of the change in the position of condylar basal bone (horizontal change in C point) suggest that it, too, is usually displaced anteriorly, a surface landmark (Co) tends to maintain its horizontal position or even move distally (see S-Co and S-Ar). The distal drift of the condylar surface implied by the significant increase in S-Co and S-Ar, however, does not differ significantly (i.e., P < 0.05) from the individualized norms for these measures inferred from the Michigan standards foruntreatedpersons: t = 0 . 9 0 a n d 1.86, respectively. Note also that the marked retraction of the upper incisors did not have a comparable impact on the lips. More to the point of this communication, the incisor changes were essentially unrelated to condylar displacement during treatment. From the correlation coefficients summarized in
8
Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. January 1992
Table II. Condylar displacement (basal and surface) and lip retraction Range I S¢oge
Measure
Mean
SD
MaMnutm
0.7 -3.2
1.7 3.0
4.8 1.7
-2.2 - 12.2
-2.8** - 7.3**
-0.2 - 0.9 0.5 0.9
0.9 1.9 1. l 0.9
2.1 1.8 3.0 2.8
- 2.0 -7.9 - 1.5 -0.9
- 1.3 -3.5** 3.0** 6.4**
- 1.4 -2.4
2.3 2.0
4.2 2.2
-5.8 -7.1
-3.9** -7.8**
0. l - 1.4
2.2 2.0
5. l 2.7
-3.8 -6.4
0.3 -5.0**
Minimum
(Ho:8 = O)
Condylar displacement C point (center) Horizontal Vertical Condylion (surface) Horizontal Vertical Sella-Co Sella-Ar
Lip retraction Upper lip Along MFOP To E plane Lower lip Along MFOP To E plane
* P < 0.05. **P < 0.01.
Table III. Angular changes relative to anterior cranial base Range Angle Frankfort plane Maxillary base FOP Mandibular base Mandibular plane (tangent)
Mean 0.0 0.6 -0.1 0.6 ! .0
SD
Maxhnum
I. I 1.2 2.9 1.8 1.6
4.2 5.4 8.6 6.6 6.2
I
Minimum - 1.9 - 1.5 -5.4 - 3.3 - 2.5
t score (Ho:6 = O) - 0.0 l 3.14"* -0.15 2.20 3.99'*
**P < 0 . 0 l .
FOP, Functional occlusal plane.
Table IV, it may be seen that anteroposterior change in the position of the chin (D poin0 was related to two of the measures of incisor displacement relative to cranial base and to all four measures of change in upper molar position. In contrast, the displacement of condylar basal bone (C point) was related only to changes in the position of the maxillary molars. DISCUSSION
From the present data, it is clear that the majority of maxillary first-premolar-extraction patients undergo a mesial mandibular displacement (along with a slight opening rotation) during maxillary premolar-extraction treatment. Regardless of whether displacement of C point, D point, or the mean of the two serves as the criterion, only about 30% (13 of 42) of the present subjects gave evidence of any posterior shift (an average
of 1 to 2 mm distal for this small subsample). Moreover, from the correlation coefficients of Table IV, it is clear that variation in mandibular position was correlated not with incisor retraction, but, rather, with displacement of the buccal segments. Indeed, the coefficients of linear correlation for the relationship between mandibular displacement (measured both at condyle and at the symphysis) and changes in the position of the maxillary buccal segments were exceptionally high (up to - 0.9). Although movement of D point bore a statistically significant relationship both to molar movement and to incisor retraction, condylar displacement was correlated only with changes in the position of the molars and premolars. On the face of it, the present modest correlation between incisor retraction and symphyseal displacement would seem to support the functional orthodontic hypothesis. It should be noted, however, that
Vohtme 101 Number 1
Premolar extraction and mandibular position
9
Table IV. Correlation coefficients (r) for the relationship between mandibular displacement and treatment changes in molar and incisor position Displacement measured at Treatment change
C point
D point
-0.05 -0.06 - 0.20 - 0.22
-0.22 -0.35 - 0.42"* - 0.59**
-0.30 -0.38* -0.46** -0.51"*
-0.41"* -0.81"* -0.62** -0.90**
Incisor retraction UI UI U1 UI
to to to to
maxilla maxiltar[6/6 const~t cranial base cranial base 16/6 constant
Anchorage loss U6 to maxilla U6 to m a x i l l a [ 6 / 6 constant U6 Io cranial base U6 to cranial b a s e l 6 / 6 constant *P < 0.05.
**P < 0.01.
this relationship may reflect the impact of a third factor, the buccal segment changes and mandibular rotation that would of necessity precede or accompany varying degrees of incisor retraction. For example, greater-thanaverage incisor retraction might require extensive use of Class II elastics and headgear, both of which would tend to reduce maxillary anchorage loss, to increase mandibular anchorage loss, and to extrude the molars. In the present analysis, the resulting downward and backward rotation would be read as a posterior displacement of the chin, and the buccal segment changes, the only effects that actually correlated with condylar displacement, would at the same time tend to produce a posterior bodily displacement of mandibular basal bone. In any event, if the direction of displacement is related to the molars, rather than the incisors, a post hoc analysis should support this interpretation. For purposes of discussion, the present sample was subdivided according to the direction of mandibular displacement (based on the algebraic average of the C and D point displacement), and the various skeletal and dental components of the molar and overjet corrections contrasted by means of completely randomized t tests. It may be seen from the "pitchfork" diagrams of Fig. 2 that the two subgroups did not differ significantly in terms of incisor retraction. Instead, the patients whose mandibles appear to have been displaced distally were those whose maxillae showed little forward growth (including every patient who was 17 years or older at the outset) and/or who either lost less anchorage than average in the maxilla or more anchorage than average in the mandible. Once again, the present findings support the logically (and biologically) more compelling alternative hypothesis that change in the centric occlusal position of the mandible is a function of changes in
anteroposterior position of the occluding buccal segments, rather than the generally nonoccluding incisors. As noted in a recent review by Tallents and coworkers~9: To make an assumption that the condyle has been forced distally as a result of therapy, without appropriate pretreatment documentation, is untenable. To make the assumption that because the incisors are upright they are "over-retracted," and that this over-retraction produced changes in condyle position again is untenable without adequate pretreatment documentation. Unfortunately, the present data argue that "adequate documentation" may be difficult to come by. In contrast to the significant mesial displacement of condylar "basal" bone, the surface of the condyle (abstracted here by Co) tended to remain stationary or even drift posteriorly a few tenths of a millimeter, a change that is consistent with the normal, growth-related posterior movement of the glenoid fossa (S-Ar; S-Co). Presumably, this relative stability is a by-product of the same condylar adaptability that underwrites the action of contemporary functional appliances. 27Moreover, it has recently been reported that the radiographic outline of the condyle may be subject to "seasonal variation. '':8 Thus, if surface changes tend not only to produce cyclic modifications in the apparent form of the condyle, but also to mask the true nature of any bodily displacement, then it would seem a futile gesture to use any kind of conventional condylar radiograph to assess changes in condylar position in unimplamed, growing patients. Thus the present findings seem to support Wyatt's scheme for iatrogenic mandibular distal displacement: "As the maxilla [sic] is moved backward, the muscles of mastication will attempt to retract the mandible when the patient closes in to [sic] maximum intercuspation. ''~ It seems appropriate, therefore, to note that if distal
10
Luecke and Johnston
Am. J. Orthod. Dentofac. Orthop. Janua~" 1992
Displaced Distally i
0.0,4
[
D i s p l a c e d Mesially
I~
t
-2.27
I c - -0.55 - 1.63
-2.83 1.10
" TI'
-1.14
l ~5.72 5.9 1
[ -3.36 I - 1.94
c • 1.3o 1.39
Tl'o ,
/'°
~
4.71
7.09 1.00 P
2.54
1.67 N = 13 -
Difference ~L
N = 29
[ 1.18,*
1.39,
I c • -t.85.-
-3.02.*
-0.89
1.oi - 1.18
T i, /oo 0.76 °
4.21."
Fig. 2. Post hoc comparisons between subgroups defined according to presumed direction of mandibular displacement during treatment. Note that, in addition to the obligatory differences in the dispracement of the landmarks that were used to define the two subsamples (C and D), the "distal" and "mesial" groups differ only in terms of the growth of the maxilla and the displacement of the buccal segments. In other words, anteroposterior changes in the position of centric occlusion appear to be independent of incisor retraction. Statistical significance of the differences depicted in the third diagram: *P < 0.05; **P < 0.01.
displacement actually does cause CMD, only patients who show a ponderable distal displacement of the upper buccal segments would be at risk. Thus the functional orthodontists should denounce nonextraction treatments and instead embrace premolar extraction, because in the former the maxillary buccal segments are commonly moved distally, whereas in the latter they almost always come forward as the extraction sites are closed. 2°'2~ Indeed, in the present study, 41 of 42 subjects showed mesial movement of the upper buccal segments. This argument, of course, is advanced mostly out of exasperation, because in truth any treatment can probably be moaified to minimize displacement in any direction or misused to maximize it. It remains to be seen, however, whether a millimeter or so of mandibular displacement is actually a threat to the public health. If the refereed literature is any guide, there is precious little, if anything, in the way of support for this contention. 19.29-3oMoreover, one must consider the ultimate
fate of changes in mandibular position caused by orthodontic tooth movement. Given the unremarkable assumption that centric occlusion is determined by teeth, it should come as no surprise that orthodontic treatment can change the position of centric occlusion. But what happens to this treatment-induced mandibular displacement once the appliances are removed? Although the present data do not address this issue, it may be noted that, in the absence of appliances, it is usually thought that the jaws move the teeth as part of the process of dentoalveolar compensation. 3t For example, it is usual to interpret the crowded, retroclined lower incisors of the patient with a Class III malocclusion as a dentoalveolar comp6nsation for a pattern of mandibular overgrowth. Accordingly, few would say that the excess growth results from the mandible being "freed up" by the linguoverted incisors, because to do so would be to reverse our usual ideas of cause and effect. And yet in the next breath
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we are often quite willing to talk o f "trapped" mandibles in Class II, Division 2 malocclusions or to express concern that tooth movement might produce a permanent d i s t a l - - o r , if you are gnathologically oriented, m e s i a l - - d i s p l a c e m e n t o f the mandible. In other words, we fear that, even after appliances have been removed, the teeth are more likely to displace the jaws than they are to move through bone. The transience of both the posterior mandibular positions produced in so-called "centric relation" reconstructions 32 and the anterior displacement commonly produced by comprehensive orthodontic treatment 17 argues that the jaws will ultimately win out over the teeth. Despite marked maxillary incisor retraction measured parallel to MFOP, the upper lip, on average, showed only a little over a millimeter o f change; the lower lip, none. Both lips, however, showed a more pronounced retraction relative to E plane. Thus the present data imply that the normal growth o f the nose and chin may have a more profound long-term impact on the relative protrusion o f the lips than does the orthodontic retraction o f the incisors. This interpretation does not mean that treatment never produces marked lip retraction (one patient, for example, experienced about 6 m m o f upper lip retraction along MFOP); rather, it argues that the soft tissue changes produced by onearch extraction treatments are, on average, much less pronounced than we have been led to expect, a conclusion that echoes the sentiments o f a recent examination of four first-premolar-extraction treatments. 33 On balance, our data argue that the corpus of "functional orthodontic" thought concerning the evils o f premolar extraction is either dead wrong or grossly exaggerated; if adhered to, it m a y constitute a threat to the public health. SUMMARY
Regional cephalometric superimposition was employed to characterize the dental and skeletal changes seen in a sample o f 42 patients who had Class II, Division 1, maxillary first-premolar extraction and who were treated with the edgewise technique. Approximately 70% o f the present sample underwent varying degrees o f forward mandibular displacement; 30% o f the sample underwent distal. More significantly, this mandibular displacement was correlated not with maxillary incisor retraction, which averaged 5 mm, but rather with changes in the spatial position o f the buccal occlusion. Surface changes, however, apparently tended to keep the head of the condyle fixed in space, regardless o f the direction of basal displacement produced by treatment. Finally, although the present treatments produced marked incisor retraction, the soft tis-
Premolar extraction and mandibular position
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sue profile appears to have been influenced more profoundly by the growth o f the nose and chin. Thus the present data fail to support the claim that premolar extraction and incisor retraction must, o f necessity, lead to unsightly profiles and distal mandibular displacement. REFERENCES 1. Case CS. The question of extraction in orthodontia. Reprint, AM J ORTHOD 1964;50:660-91. 2. Witzig JW. Deposition, Brimm v. Malloy lsic], Oakland County (MI) Circuit Court, Case No. 852 987 50, July 13, 1987. 3. Bowbeer GRN. Saving the face and the TM--Part 2. Funct Orthod 1986;3:9-39. 4. Spahl TJ, Witzig JW. The clinical management of basic maxillofacial orthopedic appliances. Vol. I: mechanics. Littleton:
PSG Publishing, 1987. 5. Wyatt WE. Preventingadverse effects on the temporomandibular joint through orthodontic treatment. AM J ORTHODDENTOFAC ORTHOP 1987;91:493-9. 6. Witzig JW, Yerkes I. Researchers question dogma of protruded maxilla: findings hint of improper orthodontic treatment. Dentist 1988;66:21, 23, 49. 7. Bowbeer GRN. The seventh key to facial beauty and TMJ health--Part 2: Proper condylar position. Funct Orthod 1990;7:4-32. 8. Costen JB. A syndrome of ear and sinus symptoms dependent upon disturbed function of the temporomandibular joint. Ann Otol Rhinol Laryngol 1934;43:1-15. 9. O'Connor B. Contemporary trends in orthodontic practice: a national survey lThesis]. St. Louis: Departmentof Orthodontics, St. Louis University Medical Center, 1990. 10. Chappione RC. Special considerations for adult orthodontics. J Clin Orthod 1976;10:535-45. 11. Roth RH. Functional occlusion for the orthodontist. Part I. J Clin Orthod 1981;15:32-51. 12. Roth RH. Functional occlusion for the orthodontist. Part III. J Clin Orthod 1981;15:174-98. 13. Roth RH, Rolfs DA. Functional occlusion for the orthodontist. Part II. J Clin Orthod 1981;15:100-23. 14. Roth RH, Gordon WW. Functional occlusion for the orthodontist. Part IV. J Clin Orthod 1981;15:246-65. 15. Williamson EH. Occlusion and TMJ dysfunction. J Clin Orthod 1981;15:333-50. 16. Williamson EH. Occlusal concepts in orthodontic diagnosis and treatment. In: Johnston LE Jr, ed. New visits in orthodontics. Philadelphia: Lea & Febiger, 1985. 17. Johnston LE, EICO Orthodontic Study Group of Ohio. Gnathologic assessment of centric slides in postretention orthodontic patients. J Prosthet Dent 1988;60:712-5. 18. Mohlin B, Pilley JR, Shaw WC. A survey of eraniomandibular disorders in 1000 12-year-olds. Study design and baseline data in a follow-up study. Eur J Orthod 1991;13:111-23. 19. Tallents RH, Catania J, Sommers E. Temporomandibularjoint findings in pediatric populations and young adults: a critical review. Angle Orthod 1991;61:7-16. 20. Johnston LE Jr. A comparative analysis of Class II treatments. In: McNamara JA Jr, Carlson DS, Vig PS, Ribbens KA, eds. Science and clinical judgment in orthodontics. Monograph 18, Craniofacial Growth Series. Ann Arbor: Center for ttuman Growth and Development, The University of Michigan, 1986:103-48.
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Am. J. Orthod. Dentofa¢. Orthop. January 1992
21. Johnston LE Jr, Lin S-S, Peng S. Anchorage loss: a comparative analysis. J CH Tween Int Foundat 1988;16:23-7. 22. Bj6rk A. Prediction of mandibular growth rotation. A.',iJ OR'moP 1969;55:585-99. 23. Bj6rk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Onhod 1983;5:1-46. 24. Bjrrk A, Skieller V. Postnatal growth and development of the maxillary complex. In: McNamara JA Jr, ed. Factors affecting the growth of the midface. Monograph 6, Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development, The University of Michigan, 1986:61-99. 25. Jenkins DH. Analysisoforthodonticdeformityemployinglateral cephalometric radi~raphy. AM J OgrHOl9 1955;41:442-52. 26. Riolo ML, Moyers RE, McNamara JA Jr, Hunter WS. An atlas of craniofacial growth. Mon~raph 2, Craniofacial Growth Seties. Ann Arbor: Center for Human Growth and Developmeut, The University of Michigan, 1974:202-3. 27. Johnston LE Jr. The curious case of the chimerical condyle. In: Graber LW, ed. Orthodontics. State of the art; essence of the science. St. Louis: CV Mosby, 1986:88-99. 28. Dibbets JMH, van der Weele LTh. Flattened condylar projection
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in children: reflection of seasonal growth? Eur J Orthod 1991;13:161-5. Rinchuse DJ. Counterpoint: preventing adverse effects on the temporomandibular joint through orthodontic treatment. AM J ORTHOD DENTOFACORTHOP 1987;91:500-6. Reynders RM. Orthodontics and temporomandibular disorders: a review of the literature (1966-1988). A.',t J ORI"~ODDEWrOFAC OR'mOP 1990;97:463-71. Solow B. The dento-alveolar compensatory mechanism: background and clinical implications. Br J Orthod 1980;7:145-61. Celenza F, The centric position: replacement and character. J Prosthet Dent 1973;30:591-8. Drobocky OB, Smith RJ. Changes in facial profile during orthodontic treatment with extraction of four first premolars. A.,~t J OR'mOP DErCrOFACOR'ntOP 1989;95:220-30.
Reprint requests to:
Dr. Lysle E. Johnston, Jr. Department of Orthodontics and Pediatric Dentistry School of Dentistry The University of Michigan Ann Arbor, MI 48109-1078
AAO MEETING CALENDAR
1992--St. Louis, Mo., May 9 to 13, St. Louis Convention Center 1993--Toronto, Canada, May 15 to 19, Metropolitan Toronto Convention Center 1994--Orlando, Fla., May 1 to 4, Orange County Convention and Civic Center 1995--San Francisco, Calif., May 7 to 10, Moscone Convention Center
(International Orthodontic Congress) 1996--Denver, Colo., May 12 to 16, Colorado Convention Center 1997--Philadelphia, Pa., May 3 to 7, Philadelphia Convention Center 1998--Dallas, Texas, May 16 to 20, Dallas Convention Center 1999~San Diego, Calif., May 15 to 19, San Diego Convention Center
