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Change in Bite Force and Electromyographic Activity of Masticatory Muscle in Accordance With Change of Occlusal Plane
J Oral Maxillofac Surg 70:1960-1967, 2012
Change in Bite Force and Electromyographic Activity of Masticatory Muscle in Accordance With Change of Occlusal Plane Min-Kyu Park, DDS, MS,* Sung-Min Cho, DDS,* Kyoung-In Yun, DDS, PhD,† and Je-Uk Park, DDS, PhD‡ Purpose: The purpose of the present study was to evaluate effects of occlusal plane on masticatory
function (biting force, masticatory muscle activity, biting efficiency) after bimaxillary orthognathic surgery. Patients and Methods: The subjects of the present study consisted of a group of 55 adults who had undergone bimaxillary surgery more than 6 months earlier. Lateral cephalographs, bite force, and electromyographic measurements of the anterior temporal [EMG(t)] and masseter muscles [EMG(m)] were recorded before and after bimaxillary surgery. Statistical analyses were performed. Results: In the increased occlusal plane group, the frequency of decreased EMG(t) was significantly high. The frequency of increased EMG(t) was also significantly high in the decreased occlusal plane group. A negative correlation was found between the postoperative occlusal plane angle and the biting force efficiency change. No significant difference was found between the group that moved from an abnormal to a normal range and the group that moved from a normal to an abnormal range. The occlusal plane change was significantly greater in the decreased EMG(t) group than in the increased EMG(t) group. Conclusions: The value of EMG(t) was related to the changes in the occlusal plane, and the biting efficiency was affected by the postoperative occlusal plane angle. However, normalization of the occlusal plane might not play a major role in masticatory function. © 2012 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 70:1960-1967, 2012 The success of orthognathic surgery is currently evaluated using various methods. For example, treatment modalities for patients with a temporomandibular disorder are evaluated as successful when the temporomandibular pain is relieved, and those for patients with a craniofacial deformity are assessed as successful when esthetic improvement is achieved. Also, in some patients, masticatory function, such as the number of positions of contact, will be considered important factors in evaluating the success of treatment.1 It is widely known that the growth and development of the craniofacial area are affected not only by genetic factors, but also by the surrounding tissues, *Graduate Student, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, Seoul, Korea. †Clinical Assistant Professor, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, St Paul’s Hospital, Seoul, Korea. ‡Professor, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, Seoul, Korea.
including the craniofacial muscles.2 However, when the craniofacial skeletal structure changes through surgery, the craniofacial muscles and masticatory functions can also be expected to change and adapt to the changed environment. Although the level of postoperative esthetic recovery in patients who have received surgery to achieve esthetic and functional recovery can be measured objectively using lateral cephalographs, the level of functional recovery is difficult to objectify. However, the maximum biting force, mastication muscle force, maximum mandibular movement range of motion, shape and timing of chewing cycle and biting force during the mastication Address correspondence and reprints to Dr Park: Department of Oral and Maxillofacial Surgery, Seoul St. Mary’s Hospital, No. 505 Banpo-dong, Secho-gu, Seoul 137-701, Korea, e-mail: jupark@ catholic.ac.kr © 2012 American Association of Oral and Maxillofacial Surgeons
0278-2391/12/7008-0$36.00/0 doi:10.1016/j.joms.2011.07.022
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PARK ET AL
interval, mandibular movement analysis, and electromyographic (EMG) measurements can be used to measure the function of muscles and mastication.1 Of these evaluation methods, the biting force can be used as a standard in direct measurement of the strength and intensity of the masticatory muscle and is actually used broadly to assess the state of the masticatory muscle. For evaluation of the biting force, various methods have been used, such as a comparison between the teeth contact force using photoocclusion,3 measurement of the biting force using the sound transmission method,4 the relationship between the biting force and muscle activity,5-7 the relationship between the biting force and the soft tissue profile,8-10 and the correlation between the teeth contact number and position.11,12 EMG, which is broadly used in functional analysis, was used first by Moyer13 in orthodontics. Since then, EMG studies on normal occlusion7,14 and correlation studies on the craniofacial structure and electromyography15,16 have been continuously conducted. However, studies have been seldom conducted on the changes in EMG before and after bimaxillary surgery. The treatment of a maxillofacial deformity often requires bimaxillary orthognathic surgery to achieve high-quality results in function, as well as in the esthetic aspects. Maxillary surgery in bimaxillary orthognathic surgery always brings changes to the occlusal plane, which in turn changes the functional occlusion. This results in changes in the function of the maxillofacial system. Because setting a new occlusal plane determines a new position for the maxilla, it should be planned carefully before surgery to accomplish stability, as well as optimal functional and esthetic results after surgery. Currently, although some studies have been conducted domestically and overseas on the esthetic changes before and after surgery or on the postoperative stability related to the occlusal plane,17-19 the studies lacked functional results. The purpose of the present study was to evaluate the effects of the occlusal plane on masticatory function. The present study also intends to provide assistance in establishing the diagnosis and treatment plans for orthognathic surgery by observing the functional changes in the maxillofacial system by obtaining lateral cephalographs, electromyograms, and bite force measurements before and after bimaxillary orthognathic surgery in patients who had changes in the occlusal plane.
Patients and Methods The Catholic University (South Korea) institutional review board approved the present study, and all patients participating in the study provided written in-
formed consent, in accordance with Korean laws on bioethics and safety and the Declaration of Helsinki. PATIENTS
The present study was designed as a prospective cohort study. The patients who underwent bimaxillary orthognathic surgery were included. The exclusion criteria were temporomandibular disorder, partial edentulous ridge, and 1-jaw surgery. The number of subjects with measurements for all EMG and bite force values, only the EMG value, and only the bite force value before and after surgery was 23, 9, and 23, respectively. Thus, the subjects included in the present study consisted of a group of 55 adults who had visited the oral and maxillofacial surgery department at the Catholic University Medical College (South Korea) and had undergone bimaxillary surgery with changes in the occlusal plane and follow-up for longer than 6 months (average 7.4, range 6 to 9). Of the 55 patients, 20 were men and 35 were women. Their average age was 26 years (Table 1). LATERAL CEPHALOMETRIC MEASUREMENT
Lateral cephalographs were taken after relaxing the lips with central occlusion. Using the V-Ceph program, version 4.0, on the obtained images, the occlusal plane angle was measured with an angle between the S–N plane and the occlusal plane (the line between upper incisal tip and mesiobuccal tip of the first molar).
Table 1. PATIENT GROUPS
Group Class I Class II Class III Maxillary advance plus mandibular setback Maxillary inferior repositioning plus mandibular setback Maxillary posterior impaction plus mandibular setback Maxillary posterior impaction plus mandibular advance Maxillary setback plus mandibular advance Maxillary setback plus mandibular setback Maxillary intrusion plus mandibular setback Maxillary canting correction plus mandibular setback Maxillary and mandibular canting correction
Total (n)
Male (n)
Female (n)
11 8 36
3 2 15
8 6 21
4
2
2
6
3
3
30
11
19
4
1
3
1
0
1
1
1
0
1
0
1
6
2
4
2
0
2
Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
BITE FORCE MEASUREMENTS
The maximum biting force was measured using a bite force measuring device (GM10 Occlusal ForceMeter, Nagano Keiki, Tokyo, Japan). The Frankfort horizontal (FH) line of the patient was made parallel with the ground, and the patient was put in a vertical position. The measuring device was positioned on the first mandibular molar, and the patient was instructed to bite 3 times repeatedly to obtain the average value.20 One patient had a missing left first mandibular molar, and the measurement was made at the second molar. The measurements were made at 6-month intervals before and after surgery. A total of 46 patients had the biting force measured both before and after surgery. ELECTROMYOGRAPHY
The subjects were placed in a comfortable vertical sitting position without a headrest. The part to be measured was wiped clean with an alcohol swab before placing the electrode. Elector-paste was liberally applied to the electrodes. The electrodes for the anterior temporal muscle were placed on the anterior part of the hair and the upper part of the eyebrows during maximum mastication, and the electrodes for the masseter muscle were placed on the central part and the accessory electrode was placed on the ear. Using the electromyography instrument (Synergy mobile type, 5-channel, Viasys Health Care Systems, Old Woking, UK), maximum mastication was repeated at central occlusion to check whether maximum activity was induced. The measurement was made 3 times and recorded. The final value was reported as microvolts, which corresponds to the root mean square of the electrical activity of a certain period. Muscle activation during maximum occlusion was calculated as an average of the root mean square of the bilateral anterior temporal and masseter muscles measured 6 months before and after surgery. A total of 32 patients were measured with muscle activation both before and after surgery. The calculation of the occlusion efficiency was as follows33: occlusion efficiency (EMG/bite-force slope) ⫽ [EMG(m) ⫹ EMG(t)]/biting force, where EMG(m) is the mean value of the EMG measurements of the bilateral masseter muscles and EMG(t) is the mean value of the EMG measurements of the bilateral anterior temporal muscles. This formula adds the EMG value of the anterior temporal muscle and the EMG value of the masseter muscle and divides the result with the biting force. The result indicates the muscle activation necessary to generate a biting force of 1 kg at the occlusal plane. Therefore, it can be interpreted that this value is the inverse proportion to the occlusion efficiency. A total of 23 patients were
measured with both occlusion efficiencies before and after surgery. STATISTICAL ANALYSIS
The measurements obtained were statistically analyzed at a significance level of P ⬍ .05 using the Statistical Package for Social Sciences, version 12.0, for Windows (SPSS, Chicago, IL). In the functional values, kilograms were the unit used for the biting force, microvolts for EMG, and microvolts per kilogram for occlusion efficiency. Each of the changes (postoperative value ⫺ preoperative value) in the biting force, EMG of the masseter muscle, EMG of the anterior temporal muscle, and occlusion efficiency are described as ⌬biting force, ⌬EMG(m), ⌬EMG(t), and ⌬occlusion efficiency, respectively. The t test and Mann-Whitney U test were first used for the parametric and nonparametric values, respectively, to compare the functional changes in the group that moved from the normal range to the abnormal range and the group that moved from the abnormal range to the normal range and to compare the occlusal plane angle and the occlusal plane angle changes between groups with increased and decreased functional values. Second, analysis of variance and the Kruskal-Wallis tests were used for the parametric and nonparametric tests, respectively, to compare the functional changes between the occlusal plane angle groups. Third, to investigate the correlation between the changes in the occlusal plane angles before and after surgery and the functional changes, the Pearson correlation coefficient was used for the parametric test and the Spearman correlation coefficient was used for the nonparametric test. Finally, the Pearson 2 test was used for the parametric test and Fisher’s exact test for the nonparametric test to analyze the frequency of the increase/decrease of the functional changes in the group with an increased/ decreased occlusion plan angle, the group with a postoperative occlusal plane angle over 22 (group 3) and below 22 (groups 1 and 2), and the group that moved to normal/abnormal range.
Results ANALYSIS OF FREQUENCY OF INCREASE AND DECREASE OF FUNCTIONAL VALUES IN THOSE WITH INCREASED AND DECREASED OCCLUSAL PLANE ANGLES
The patients were divided into those with a numerically increased occlusal plane angle (SN–OP) and those with a decreased occlusal plane angle, and the increase and decrease of the functional value were cross analyzed. The frequency of the EMG(t) increase was significantly high when the occlusal plane angle had
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PARK ET AL
Table 2. CROSS ANALYSIS OF OCCLUSAL PLANE ANGLE AND FUNCTIONAL MEASUREMENTS
Variable Biting force Decrease Increase EMG(m) Decrease Increase EMG(t) Decrease Increase Occlusion efficiency Decrease Increase
Decreased Occlusal Plane (n)
Increased Occlusal Plane (n)
5 (31.3) 11 (68.8)
14 (46.7) 16 (53.3)
7 (58.3) 5 (41.7)
15 (75.0) 5 (25.0)
3 (25.0) 9 (75.0)
13 (65.0) 7 (35.0)
5 (45.5) 6 (54.5)
7 (58.3) 5 (41.7)
P Value .312 .438 .028 .537
Abbreviations: EMG(m), mean value of electromyographic measurements of bilateral masseter muscles; EMG(t), mean value of electromyographic measurements of bilateral anterior temporalis muscles. Data in parentheses are percentages. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
decreased, and the frequency of EMG(t) decrease was significantly high when the occlusal plane angle had increased. Although no other significant differences were present, the frequency of the EMG(t) decrease was high in the group with an increased occlusal plane angle and the frequency of the biting force increase was high in the group with a decreased occlusal plane angle (Table 2). CORRELATION BETWEEN POSTOPERATIVE OCCLUSAL PLANE ANGLE, CHANGES IN OCCLUSAL PLANE ANGLE, AND FUNCTIONAL MEASUREMENT CHANGES
The postoperative occlusal plane angle and the occlusion efficiency change showed a significant negative correlation (r ⫽ 0.467, P ⬍ .05). This result suggested a
positive relationship between the occlusion efficiency and postoperative occlusal plane (Table 3). VARIOUS DISCUSSIONS ON FUNCTIONAL VALUE OF CATEGORIZED OCCLUSAL PLANE ANGLE GROUP
The occlusal planes were categorized into 3 groups (group 1, SN–OP ⬍14°; group 2, SN–OP 14° to 22°; and group 3, SN–OP ⬎22°) and the changes in, and states of, the functional values were analyzed using various methods. First, we compared the functional changes of each postoperative occlusal plane angle group. When the postoperative occlusal plane angles were categorized into 3 groups, the 3 groups showed no significant differences (Table 4). However, the biting force and occlusion efficiency showed improvement as the occlusal plane category increased. Second, we performed a functional analysis between the group that moved from the abnormal range (⬍14° or ⬎22°) to the normal range (14° to 22°; defined through analysis of normal occlusion) and the group that moved from the normal range to the abnormal range by comparing the functional changes between the 2 groups. The functional changes did not show any significant differences between the 2 groups (Table 5). We also performed an analysis of the frequency of the increase and decrease in function between 2 groups. Although all functional items showed no significant results, the frequency of an increased biting force in the group that moved to the normal range was high (n ⫽ 6, 85.7%; Table 6). COMPARISON OF POSTOPERATIVE OCCLUSAL PLANE ANGLE AND CHANGES IN OCCLUSAL PLANE ANGLE IN GROUPS WITH DECREASED AND INCREASED FUNCTION
The patients were categorized into groups with increased and decreased biting force, EMG, and occlusion efficiency using the t test. The 2 groups
Table 3. CORRELATION COEFFICIENT BETWEEN POSTOPERATIVE OCCLUSAL PLANE ANGLE, CHANGES IN OCCLUSAL PLANE ANGLE, AND FUNCTIONAL MEASUREMENT CHANGES
Variable Postoperative occlusal plane angle Correlation coefficient P value Changes in occlusion plan angle Correlation coefficient P value
⌬EMG(m)
⌬EMG(t)
⌬Occlusion Efficiency
0.232 .121
⫺0.252 .164
⫺0.174 .342
⫺0.467 .024*
⫺0.068 .652
⫺0.170 .353
⫺0.313 .081
⫺0.211 .333
⌬Biting Force
Abbreviations as in Table 2. *Statistically significant at P ⱕ .05. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
Table 4. COMPARISON OF FUNCTIONAL MEASUREMENT CHANGES AMONG 3 POSTOPERATIVE OCCLUSAL PLANE ANGLE GROUPS
Group 1 (⬍14°)
Group 2 (14°-22°)
Group 3 (⬎22°)
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
⌬Biting force ⌬EMG(m) ⌬EMG(t) ⌬Occlusion efficiency
7 5 5 4
⫺23.17 ⫾ 120.37 ⫺7.05 ⫾ 49.94 ⫺4.52 ⫾ 18.84 0.40 ⫾ 0.96
22 15 15 10
⫺1.84 ⫾ 63.17 ⫺3.07 ⫾ 60.20 6.94 ⫾ 53.32 0.12 ⫾ 0.60
17 12 12 9
45.87 ⫾ 86.92 ⫺52.09 ⫾ 64.70 ⫺12.43 ⫾ 37.03 ⫺0.94 ⫾ 1.42
.104 .215 .585 .068
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
showed no significant differences in the postoperative occlusal plane angle. The changes in the occlusal plane angle were significantly high in the group with decreased EMG(m), but no significant differences were observed for the other items (Table 7).
Discussion Measurement of the bite force, which was used in the present study as a method for functional evaluation of the maxillofacial system, was conducted with the clenching state of the first molar area. The first molar area exhibits the largest biting force and is similar to the biting force at the centric occlusion, according to the study by Ramfjord and Ash.21 Also, the measurement of the electromyogram was made using the anterior temporal and masseter muscles, which provide easy access to measuring the electromyogram and also show the most definite contraction during maximum occlusion. In addition, the anterior temporal and masseter muscles have greater reproducibility for the electrode positions and reattachment, making the muscles a good place to measure the electromyogram. Patients showed a large standard deviation, along with a large variation in the measured biting force and EMG, indicating that various factors, such as the mastication pattern, occlusal contact point, mastication muscle strength, psychological factors, and measure-
ment errors, affected the results. Therefore, a nonparametric analysis was used more often, even though our study had a sufficient number of samples. For a more exact statistical analysis, more samples, greater experience with measuring, and stricter standards for excluding subjects (exclusion of patients with temporomandibular disorders and those who are partial edentulous) are required. Although it is not used frequently, the occlusal plane angle is one of the factors measured in bilateral orthognathic surgery and has many clinical meanings. There are various standards and reference values in setting an occlusal plane, and according to Down22 and Rickettes,23 the mean of the occlusal plane in a person with normal occlusion is 8 ⫾ 4° from the FH plane, and according to Richard et al,24 the line that connects the midpoint of the maxillary incisal tip and the mediobuccal cusp of the upper first molar forms 11.2 ⫾ 4° with the FH plane. In Korea, Lee and Sohn25 reported that the angle is 12° ⫾ 4° with the FH plane, and Shin and Chang26 reported that the angle is 17.8° ⫾ 4.67° with the SN plane. In the present study, the normal range of the occlusal plane was set to 18° ⫾ 4° for the SN plane and categorized those with less than 14° as a low occlusal plane group and those with greater than 22° as a high occlusion plan group. An ideal occlusal plane is an important factor in determining the harmony between occlusion and the movements of the mandible and is expected to affect
Table 5. COMPARISON OF FUNCTIONAL MEASUREMENT CHANGES BETWEEN CROSSOVER GROUPS WITH OCCLUSAL PLANES IN NORMAL OR ABNORMAL RANGE
Group Moved to Abnormal Range
Group Moved to Normal Range
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
⌬Biting force ⌬EMG(m) ⌬EMG(t) ⌬Occlusion efficiency
7 5 5 3
⫺2.24 ⫾ 83.22 ⫺36.23 ⫾ 31.98 ⫺12.35 ⫾ 17.55 ⫺0.30 ⫾ 0.69
7 7 7 4
41.10 ⫾ 57.64 ⫺4.49 ⫾ 50.41 ⫺9.30 ⫾ 48.54 ⫺0.16 ⫾ 0.67
.280 .106 1.000 .400
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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Table 6. CROSS ANALYSIS BETWEEN CROSSOVER GROUPS WITH OCCLUSAL PLANES IN NORMAL OR ABNORMAL RANGE AND FUNCTIONAL MEASUREMENTS
Variable Biting force Decrease Increase EMG(m) Decrease Increase EMG(t) Decrease Increase Occlusion efficiency Decrease Increase
Moved to Abnormal Range
Moved to Normal Range
4 (57.1) 3 (42.9)
1 (14.3) 6 (85.7)
5 (100.0) 0 (0)
3 (42.9) 4 (57.1)
3 (60.0) 2 (40.0)
4 (57.1) 3 (42.9)
1 (33.3) 2 (66.7)
3 (75.0) 1 (25.0)
P Value .266 .081 1.000 .486
Abbreviations as in Table 2. Data in parentheses are percentages. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
the decision regarding the range of surgery, esthetics, and the level of relapse after surgery. However, studies on this have varied, and we are not yet equipped with an established theory.27 Only rarely have studies showed the functional relationships of the occlusal plane after orthognathic surgery. When newly setting the occlusal plane, we can maintain the occlusal plane angle before surgery with consideration of both the functional and the esthetic aspects, or use methods such as aligning the maxilla’s occlusal plane through autorotation of the mandible and intentionally changing the occlusion plan for FH or SN.28,29 When the occlusal plane angle increases,
problems can occur, including the loss of anterior and canine guidance and functional interference with the posterior teeth.1 Also, Kohno et al30 and Wolford et al19 reported that the loss of anterior guidance and occlusion interference with the posterior teeth can occur easily and the maxillofacial system can develop functional disorders when the occlusal plane angle is large or there is a steep occlusal curve. In addition, Jo et al31 reported that if the occlusal plane angle becomes steep up to an average of 6°, occlusion interference can occur during mandible movement and can negatively affect the maxillofacial system, resulting in functional disorders. According to Reyneke and Evans,32 a posterior inferior rotation, which makes the occlusal plane incline to the standard plane of the cranial base by lifting the posterior part of the maxilla, is more beneficial than the contrary and can improve the esthetic effects. They also reported that reduction of the occlusal plane angle results in increasing joint pressure until the muscle, soft tissue, and teeth have readapted.19 The biting force is a vector sum of the tension forces of all closed mouth muscles. From the level of activation of some muscles, we can estimate that an occlusal plane vertical to the vector of the muscle stimulation will be most effective. Because it is difficult to find the direction of the vector using a morphologic search, the occlusion efficiency value was proposed33 to understand how much the mastication muscles generate the biting force. That value was used in the present study. According to the results of our study, the postoperative occlusal plane angle and the occlusion efficiency value showed a significant negative correlation. Moving from the low occlusal plane group to the high occlusal plane group, the occlusion efficiency values became
Table 7. COMPARISON OF POSTOPERATIVE OCCLUSAL PLANE ANGLE AND CHANGE IN OCCLUSAL PLANE ANGLE BETWEEN INCREASED AND DECREASED FUNCTIONAL MEASUREMENT GROUPS
Biting Force Decrease
EMG(m)
Increase
Decrease
Increase
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
Postoperative occlusal plane angle ⌬Occlusal plane angle
19 19
19.82 ⫾ 4.94 1.92 ⫾ 3.38
27 27
20.81 ⫾ 4.96 0.65 ⫾ 4.25
.509 .287
22 22
21.32 ⫾ 5.50 1.44 ⫾ 3.86
10 10
18.45 ⫾ 5.11 0.85 ⫾ 3.55
.173 .686
EMG(t) Postoperative occlusal plane angle ⌬Occlusal plane angle
16 16
20.97 ⫾ 5.57 2.71 ⫾ 3.22
16 19.88 ⫾ 5.48 16 ⫺0.20 ⫾ 3.70
Occlusion Efficiency .578 .024*
12 12
22.38 ⫾ 5.52 1.33 ⫾ 4.03
11 18.87 ⫾ 4.73 11 ⫺0.24 ⫾ 3.88
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. *Statistically significant at P ⱕ .05. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
.117 .353
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EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
smaller, but not significantly. Because the smaller the occlusion efficiency value the more effective the occlusion, increasing the postoperative occlusion plan angle would be expected to provide more functional benefits. In the present study, the frequency of an EMG(t) decrease in the group with an increased occlusal plane angle and the frequency of an EMG(t) increase in the group with a decreased occlusal plane angle were significant. Also, the postoperative occlusal plane angle of the group with a decreased EMT(t) was larger than that of the group with an increased EMG(t), indicating that the occlusal plane angle has a large relationship with the EMG(t). The relationship between the increase in the occlusal plane angle and the decrease in the EMG(t) can be explained by the decrease in the muscle moment arm and that the direction of the muscle activation vector becomes more vertical as the occlusal plane angle increases. Using previous occlusal plane angles of those with normal occlusion as a reference, we estimated that functional improvement would occur when the occlusal plane angle was normal after surgery; however, the present study showed no significant differences. Therefore, whether a normal occlusal plane angle in normal occlusion should be set to an ideal angle in patients undergoing surgery is yet to be determined. A proper theory should be established through additional studies on the ideal occlusal plane angle. Although the functional recovery period should be considered in a functional analysis, opinions vary on the period necessary for functional recovery. Proffit et al34 reported that there were no changes in the maximum biting force for up to 1 year, and Youssef et al35 and Throckmorton et al36 concluded that the occlusal forces began to increase in the 6 months after surgery and continue for up to 3 years. Youssef et al35 also reported that the EMG activity of the masseter and temporal muscles increase in the 6 weeks after surgery and continue to increase for up to 3 years. Throckmorton et al36 reported that the maximum occlusal forces improved gradually according to the constant rate. They concluded that the postoperative bite forces were greater than their preoperative forces within 6 months. Seo et al20 reported that the maximum biting force and the maximum mouth opening significantly increased until the third month after sagittal split ramus osteotomy in orthognathic patients, and the muscle activation of the masseter muscle and the anterior temporal muscle had recovered to preoperative values or more by 6 months after surgery. That study tested not only the biting force 6 months after surgery, but also the level of muscle activation. In addition, Throckmorton et al36 concluded that male patients improved more quickly than female pa-
tients. A preliminary analysis showed that the functional change in the male patient was greater than that of the female patient, but the difference was not significant in our data. The mandibular length (the lever arm from condylion to pogonion) and gonial angle changes with mandibular surgery, and this change can affect the masticatory function. Choi and Kim37 reported on the relationship between the maximum bite force and facial skeletal pattern in normal occlusion subjects and concluded that the lever arm length and gonial angle did not have any significant correlation with the posterior bite force. However, it might be valuable to investigate the multiple relationships between the occlusal plane angle, lever arm length, gonial angle, and masticatory function after orthognathic surgery.
References 1. Fonseca RJ: Oral and Maxillofacial Surgery (vol 2). Philadelphia, PA, WB Saunders, 2000 2. Graber TM: Growth and Development. Philadelphia, PA, WB Saunders, 1972, pp 27-128 3. Dawson PE, Arcan M, Lundeen HC, et al: Attaining harmonic occlusion through visualized strain analysis. J Prosthet Dent 46:443, 1981 4. Maness W, Benjamin M, Podoloff R, et al: Computerized occusal analysis; a new technology. Quit Internat 4:287, 1987 5. Hosman H, Naeije M: Reproducibility of the normalized electromyographic recording of the masseter muscle by using the EMG recording during maximal clenching as a standard. J Oral Rehb 6:49, 1979 6. Manns A, Miralles R, Valdivia J, et al: Influence of variation in anteroposterior occlusal contacts on electromyographic activity. J Prosthet Dent 61:617, 1989 7. Ahlgren J, Sonesson B, Blitz M: An electromyographic analysis of the temporalis function of normal occlusion. Am J Orthod 87:230, 1985 8. Oyer R, Tsay P: A biomechanical analysis of craniofacial form and bite force. Am J Orthod 99:298, 1991 9. Ringquist M: Isometric bite force and its relation to dimensions of the facial skeleton. Acta Odontol Scand 31:35, 1973 10. Lee TW, Lee KS: Isometric bite force and its relation to craniofacial morphology. Korean J Orthod 21:185, 1991 11. MacDonald JWC, Hannam AG: Relationship between occlusal contact and jaw-closing muscle activity during tooth clenching; part 1. J Prosthet Dent 52:718, 1984 12. Riise C, Ericksson SG: A clinical study of the distribution of the occlusal tooth contact in the intercuspal position at light and hard pressure in adult. J Oral Rehabil 10:473, 1983 13. Moyers RE: An electromyographic analysis of certain muscles involved in temporomandibular movement. Am J Orthod 36: 481, 1950 14. Latif A: An electromyographic study of the temporalis muscle in normal persons during selected positions and movements of the mandible. Am J Orthod 43:577, 1957 15. Ingervall B: Facial morphology and activity of temporal and lip muscles during swallowing and chewing. Angle Orthod 46: 372, 1976 16. Lwe AA, Takada K: Associations between anterior temporal, masseter and orbicularis oris muscle activity and craniofacial morphology in children. Am J Orthod 86:319, 1984 17. Lee YJ, Sohn BW: A study on the postoperative stability of occlusal plane in class III orthognathic surgery patients. Korean J Orthod 30:643, 2000 18. Lee GY, Jang HJ, Lee SH: Clinical articles: Study on the changes of occlusal plane angle in mandible prognathism after orthognathic surgery. J Korean Assoc Maxillofac Plast Reconstr Surg 25:432, 2003
PARK ET AL 19. Wolford LM, Chemello PD, Hilliard F: Occlusal plane alteration in orthognathic surgery—Part I: Effects on function and esthetics. Am J Orthod Dentofac Orthop 106:304, 1994 20. Seo JC, Kim JR, Yang DK: A study on changes in the maximum bite force after orthognathic surgery. J Korean Assoc Oral Maxillofac Surg 22:121, 1996 21. Ramfjord SP, Ash MM: Occlusion (ed 3). Philadelphia, PA, WB Saunders, 1983, pp 128-174 22. Downs WB: Variations in facial relationships; their significance in treatment and prognosis. Am J Orthod 34:812, 1948 23. Rickettes RM: Cephalometric analysis synthesis. Angle Orthod 31:141, 1961 24. Richard K, Djeng SK, Ho CK: The relationships of upper facial proportions and the plane of occlusion to anatomic reference planes. J Prosthet Dent 61:727, 1989 25. Lee WY, Sohn BH: Cephalometric analysis of maxillofacial normals in Korean adults. Korean. J Orthod 14:135, 1984 26. Shin MC, Chang YI: A study on the vertical dysplasia in the skeletal class III malocclusion. Korean J Orthod 20:333, 1990 27. Hwang CJ, Lim SA, Moon JL: A study on the postoperative stability of hard tissue in orthognathic surgery patients depending on the difference of occlusal plane. Korean J Orthod 29: 239, 1999 28. Proffit WR, Bell WH: Open Bite: Surgical Correction of Dentofacial Deformities. Philadelphia, PA, WB Saunders, 1980, pp 1111-1113
1967 29. Epker BN, Fish LC: Dentofacial Deformities: Integrated Orthodontics and Surgical Correction. St Louis, MO, CV Mosby, 1986, p 400 30. Kohno S, Karasuyama H, Yoshida K, et al: A study on the causative mechanism of occlusal interference on the balancing side. J Jpn Prosthodont Soc 32:505, 1988 31. Jo BW, Ahn SH, Kim JP: The clinical study of the effect of occlusal planes on the stomatognathic system. J Korean Acad Prosthodont 33:10, 1995 32. Reyneke JP, Evans WG: Surgical manipulation of the occlusal plane. Int J Adult Orthodon Orthognath Surg 5:99, 1990 33. Okane H, Yamashina T, Nagasawa T, et al: The effect of anteroposterior inclination of the occlusal plane one biting force. J Prosthet Dent 42:487, 1979 34. Proffit WR, Turvey TA, Fields HW, et al: The effect of orthognathic surgery on occlusal force. J Oral Maxillofac Surg 47:457, 1989 35. Youssef RE, Throckmorton GS, Ellis E, et al: Comparison of habitual masticatory cycles and muscle activity before and after orthognathic surgery. J Oral Maxillofac Surg 55:699, 1997 36. Throckmorton GS, Buschang PH, Ellis E: Improvement of maximum occlusal forces after orthognathic surgery. J Oral Maxillofac Surg 54:1080, 1996 37. Choi WC, Kim TW: Relationship between maximum bite force and facial skeletal pattern. Korean J Orthod 33:437, 2003
Change in Bite Force and Electromyographic Activity of Masticatory Muscle in Accordance With Change of Occlusal Plane Min-Kyu Park, DDS, MS,* Sung-Min Cho, DDS,* Kyoung-In Yun, DDS, PhD,† and Je-Uk Park, DDS, PhD‡ Purpose: The purpose of the present study was to evaluate effects of occlusal plane on masticatory
function (biting force, masticatory muscle activity, biting efficiency) after bimaxillary orthognathic surgery. Patients and Methods: The subjects of the present study consisted of a group of 55 adults who had undergone bimaxillary surgery more than 6 months earlier. Lateral cephalographs, bite force, and electromyographic measurements of the anterior temporal [EMG(t)] and masseter muscles [EMG(m)] were recorded before and after bimaxillary surgery. Statistical analyses were performed. Results: In the increased occlusal plane group, the frequency of decreased EMG(t) was significantly high. The frequency of increased EMG(t) was also significantly high in the decreased occlusal plane group. A negative correlation was found between the postoperative occlusal plane angle and the biting force efficiency change. No significant difference was found between the group that moved from an abnormal to a normal range and the group that moved from a normal to an abnormal range. The occlusal plane change was significantly greater in the decreased EMG(t) group than in the increased EMG(t) group. Conclusions: The value of EMG(t) was related to the changes in the occlusal plane, and the biting efficiency was affected by the postoperative occlusal plane angle. However, normalization of the occlusal plane might not play a major role in masticatory function. © 2012 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 70:1960-1967, 2012 The success of orthognathic surgery is currently evaluated using various methods. For example, treatment modalities for patients with a temporomandibular disorder are evaluated as successful when the temporomandibular pain is relieved, and those for patients with a craniofacial deformity are assessed as successful when esthetic improvement is achieved. Also, in some patients, masticatory function, such as the number of positions of contact, will be considered important factors in evaluating the success of treatment.1 It is widely known that the growth and development of the craniofacial area are affected not only by genetic factors, but also by the surrounding tissues, *Graduate Student, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, Seoul, Korea. †Clinical Assistant Professor, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, St Paul’s Hospital, Seoul, Korea. ‡Professor, Department of Oral and Maxillofacial Surgery, Catholic University of Korea, Seoul, Korea.
including the craniofacial muscles.2 However, when the craniofacial skeletal structure changes through surgery, the craniofacial muscles and masticatory functions can also be expected to change and adapt to the changed environment. Although the level of postoperative esthetic recovery in patients who have received surgery to achieve esthetic and functional recovery can be measured objectively using lateral cephalographs, the level of functional recovery is difficult to objectify. However, the maximum biting force, mastication muscle force, maximum mandibular movement range of motion, shape and timing of chewing cycle and biting force during the mastication Address correspondence and reprints to Dr Park: Department of Oral and Maxillofacial Surgery, Seoul St. Mary’s Hospital, No. 505 Banpo-dong, Secho-gu, Seoul 137-701, Korea, e-mail: jupark@ catholic.ac.kr © 2012 American Association of Oral and Maxillofacial Surgeons
0278-2391/12/7008-0$36.00/0 doi:10.1016/j.joms.2011.07.022
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interval, mandibular movement analysis, and electromyographic (EMG) measurements can be used to measure the function of muscles and mastication.1 Of these evaluation methods, the biting force can be used as a standard in direct measurement of the strength and intensity of the masticatory muscle and is actually used broadly to assess the state of the masticatory muscle. For evaluation of the biting force, various methods have been used, such as a comparison between the teeth contact force using photoocclusion,3 measurement of the biting force using the sound transmission method,4 the relationship between the biting force and muscle activity,5-7 the relationship between the biting force and the soft tissue profile,8-10 and the correlation between the teeth contact number and position.11,12 EMG, which is broadly used in functional analysis, was used first by Moyer13 in orthodontics. Since then, EMG studies on normal occlusion7,14 and correlation studies on the craniofacial structure and electromyography15,16 have been continuously conducted. However, studies have been seldom conducted on the changes in EMG before and after bimaxillary surgery. The treatment of a maxillofacial deformity often requires bimaxillary orthognathic surgery to achieve high-quality results in function, as well as in the esthetic aspects. Maxillary surgery in bimaxillary orthognathic surgery always brings changes to the occlusal plane, which in turn changes the functional occlusion. This results in changes in the function of the maxillofacial system. Because setting a new occlusal plane determines a new position for the maxilla, it should be planned carefully before surgery to accomplish stability, as well as optimal functional and esthetic results after surgery. Currently, although some studies have been conducted domestically and overseas on the esthetic changes before and after surgery or on the postoperative stability related to the occlusal plane,17-19 the studies lacked functional results. The purpose of the present study was to evaluate the effects of the occlusal plane on masticatory function. The present study also intends to provide assistance in establishing the diagnosis and treatment plans for orthognathic surgery by observing the functional changes in the maxillofacial system by obtaining lateral cephalographs, electromyograms, and bite force measurements before and after bimaxillary orthognathic surgery in patients who had changes in the occlusal plane.
Patients and Methods The Catholic University (South Korea) institutional review board approved the present study, and all patients participating in the study provided written in-
formed consent, in accordance with Korean laws on bioethics and safety and the Declaration of Helsinki. PATIENTS
The present study was designed as a prospective cohort study. The patients who underwent bimaxillary orthognathic surgery were included. The exclusion criteria were temporomandibular disorder, partial edentulous ridge, and 1-jaw surgery. The number of subjects with measurements for all EMG and bite force values, only the EMG value, and only the bite force value before and after surgery was 23, 9, and 23, respectively. Thus, the subjects included in the present study consisted of a group of 55 adults who had visited the oral and maxillofacial surgery department at the Catholic University Medical College (South Korea) and had undergone bimaxillary surgery with changes in the occlusal plane and follow-up for longer than 6 months (average 7.4, range 6 to 9). Of the 55 patients, 20 were men and 35 were women. Their average age was 26 years (Table 1). LATERAL CEPHALOMETRIC MEASUREMENT
Lateral cephalographs were taken after relaxing the lips with central occlusion. Using the V-Ceph program, version 4.0, on the obtained images, the occlusal plane angle was measured with an angle between the S–N plane and the occlusal plane (the line between upper incisal tip and mesiobuccal tip of the first molar).
Table 1. PATIENT GROUPS
Group Class I Class II Class III Maxillary advance plus mandibular setback Maxillary inferior repositioning plus mandibular setback Maxillary posterior impaction plus mandibular setback Maxillary posterior impaction plus mandibular advance Maxillary setback plus mandibular advance Maxillary setback plus mandibular setback Maxillary intrusion plus mandibular setback Maxillary canting correction plus mandibular setback Maxillary and mandibular canting correction
Total (n)
Male (n)
Female (n)
11 8 36
3 2 15
8 6 21
4
2
2
6
3
3
30
11
19
4
1
3
1
0
1
1
1
0
1
0
1
6
2
4
2
0
2
Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
BITE FORCE MEASUREMENTS
The maximum biting force was measured using a bite force measuring device (GM10 Occlusal ForceMeter, Nagano Keiki, Tokyo, Japan). The Frankfort horizontal (FH) line of the patient was made parallel with the ground, and the patient was put in a vertical position. The measuring device was positioned on the first mandibular molar, and the patient was instructed to bite 3 times repeatedly to obtain the average value.20 One patient had a missing left first mandibular molar, and the measurement was made at the second molar. The measurements were made at 6-month intervals before and after surgery. A total of 46 patients had the biting force measured both before and after surgery. ELECTROMYOGRAPHY
The subjects were placed in a comfortable vertical sitting position without a headrest. The part to be measured was wiped clean with an alcohol swab before placing the electrode. Elector-paste was liberally applied to the electrodes. The electrodes for the anterior temporal muscle were placed on the anterior part of the hair and the upper part of the eyebrows during maximum mastication, and the electrodes for the masseter muscle were placed on the central part and the accessory electrode was placed on the ear. Using the electromyography instrument (Synergy mobile type, 5-channel, Viasys Health Care Systems, Old Woking, UK), maximum mastication was repeated at central occlusion to check whether maximum activity was induced. The measurement was made 3 times and recorded. The final value was reported as microvolts, which corresponds to the root mean square of the electrical activity of a certain period. Muscle activation during maximum occlusion was calculated as an average of the root mean square of the bilateral anterior temporal and masseter muscles measured 6 months before and after surgery. A total of 32 patients were measured with muscle activation both before and after surgery. The calculation of the occlusion efficiency was as follows33: occlusion efficiency (EMG/bite-force slope) ⫽ [EMG(m) ⫹ EMG(t)]/biting force, where EMG(m) is the mean value of the EMG measurements of the bilateral masseter muscles and EMG(t) is the mean value of the EMG measurements of the bilateral anterior temporal muscles. This formula adds the EMG value of the anterior temporal muscle and the EMG value of the masseter muscle and divides the result with the biting force. The result indicates the muscle activation necessary to generate a biting force of 1 kg at the occlusal plane. Therefore, it can be interpreted that this value is the inverse proportion to the occlusion efficiency. A total of 23 patients were
measured with both occlusion efficiencies before and after surgery. STATISTICAL ANALYSIS
The measurements obtained were statistically analyzed at a significance level of P ⬍ .05 using the Statistical Package for Social Sciences, version 12.0, for Windows (SPSS, Chicago, IL). In the functional values, kilograms were the unit used for the biting force, microvolts for EMG, and microvolts per kilogram for occlusion efficiency. Each of the changes (postoperative value ⫺ preoperative value) in the biting force, EMG of the masseter muscle, EMG of the anterior temporal muscle, and occlusion efficiency are described as ⌬biting force, ⌬EMG(m), ⌬EMG(t), and ⌬occlusion efficiency, respectively. The t test and Mann-Whitney U test were first used for the parametric and nonparametric values, respectively, to compare the functional changes in the group that moved from the normal range to the abnormal range and the group that moved from the abnormal range to the normal range and to compare the occlusal plane angle and the occlusal plane angle changes between groups with increased and decreased functional values. Second, analysis of variance and the Kruskal-Wallis tests were used for the parametric and nonparametric tests, respectively, to compare the functional changes between the occlusal plane angle groups. Third, to investigate the correlation between the changes in the occlusal plane angles before and after surgery and the functional changes, the Pearson correlation coefficient was used for the parametric test and the Spearman correlation coefficient was used for the nonparametric test. Finally, the Pearson 2 test was used for the parametric test and Fisher’s exact test for the nonparametric test to analyze the frequency of the increase/decrease of the functional changes in the group with an increased/ decreased occlusion plan angle, the group with a postoperative occlusal plane angle over 22 (group 3) and below 22 (groups 1 and 2), and the group that moved to normal/abnormal range.
Results ANALYSIS OF FREQUENCY OF INCREASE AND DECREASE OF FUNCTIONAL VALUES IN THOSE WITH INCREASED AND DECREASED OCCLUSAL PLANE ANGLES
The patients were divided into those with a numerically increased occlusal plane angle (SN–OP) and those with a decreased occlusal plane angle, and the increase and decrease of the functional value were cross analyzed. The frequency of the EMG(t) increase was significantly high when the occlusal plane angle had
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Table 2. CROSS ANALYSIS OF OCCLUSAL PLANE ANGLE AND FUNCTIONAL MEASUREMENTS
Variable Biting force Decrease Increase EMG(m) Decrease Increase EMG(t) Decrease Increase Occlusion efficiency Decrease Increase
Decreased Occlusal Plane (n)
Increased Occlusal Plane (n)
5 (31.3) 11 (68.8)
14 (46.7) 16 (53.3)
7 (58.3) 5 (41.7)
15 (75.0) 5 (25.0)
3 (25.0) 9 (75.0)
13 (65.0) 7 (35.0)
5 (45.5) 6 (54.5)
7 (58.3) 5 (41.7)
P Value .312 .438 .028 .537
Abbreviations: EMG(m), mean value of electromyographic measurements of bilateral masseter muscles; EMG(t), mean value of electromyographic measurements of bilateral anterior temporalis muscles. Data in parentheses are percentages. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
decreased, and the frequency of EMG(t) decrease was significantly high when the occlusal plane angle had increased. Although no other significant differences were present, the frequency of the EMG(t) decrease was high in the group with an increased occlusal plane angle and the frequency of the biting force increase was high in the group with a decreased occlusal plane angle (Table 2). CORRELATION BETWEEN POSTOPERATIVE OCCLUSAL PLANE ANGLE, CHANGES IN OCCLUSAL PLANE ANGLE, AND FUNCTIONAL MEASUREMENT CHANGES
The postoperative occlusal plane angle and the occlusion efficiency change showed a significant negative correlation (r ⫽ 0.467, P ⬍ .05). This result suggested a
positive relationship between the occlusion efficiency and postoperative occlusal plane (Table 3). VARIOUS DISCUSSIONS ON FUNCTIONAL VALUE OF CATEGORIZED OCCLUSAL PLANE ANGLE GROUP
The occlusal planes were categorized into 3 groups (group 1, SN–OP ⬍14°; group 2, SN–OP 14° to 22°; and group 3, SN–OP ⬎22°) and the changes in, and states of, the functional values were analyzed using various methods. First, we compared the functional changes of each postoperative occlusal plane angle group. When the postoperative occlusal plane angles were categorized into 3 groups, the 3 groups showed no significant differences (Table 4). However, the biting force and occlusion efficiency showed improvement as the occlusal plane category increased. Second, we performed a functional analysis between the group that moved from the abnormal range (⬍14° or ⬎22°) to the normal range (14° to 22°; defined through analysis of normal occlusion) and the group that moved from the normal range to the abnormal range by comparing the functional changes between the 2 groups. The functional changes did not show any significant differences between the 2 groups (Table 5). We also performed an analysis of the frequency of the increase and decrease in function between 2 groups. Although all functional items showed no significant results, the frequency of an increased biting force in the group that moved to the normal range was high (n ⫽ 6, 85.7%; Table 6). COMPARISON OF POSTOPERATIVE OCCLUSAL PLANE ANGLE AND CHANGES IN OCCLUSAL PLANE ANGLE IN GROUPS WITH DECREASED AND INCREASED FUNCTION
The patients were categorized into groups with increased and decreased biting force, EMG, and occlusion efficiency using the t test. The 2 groups
Table 3. CORRELATION COEFFICIENT BETWEEN POSTOPERATIVE OCCLUSAL PLANE ANGLE, CHANGES IN OCCLUSAL PLANE ANGLE, AND FUNCTIONAL MEASUREMENT CHANGES
Variable Postoperative occlusal plane angle Correlation coefficient P value Changes in occlusion plan angle Correlation coefficient P value
⌬EMG(m)
⌬EMG(t)
⌬Occlusion Efficiency
0.232 .121
⫺0.252 .164
⫺0.174 .342
⫺0.467 .024*
⫺0.068 .652
⫺0.170 .353
⫺0.313 .081
⫺0.211 .333
⌬Biting Force
Abbreviations as in Table 2. *Statistically significant at P ⱕ .05. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
Table 4. COMPARISON OF FUNCTIONAL MEASUREMENT CHANGES AMONG 3 POSTOPERATIVE OCCLUSAL PLANE ANGLE GROUPS
Group 1 (⬍14°)
Group 2 (14°-22°)
Group 3 (⬎22°)
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
⌬Biting force ⌬EMG(m) ⌬EMG(t) ⌬Occlusion efficiency
7 5 5 4
⫺23.17 ⫾ 120.37 ⫺7.05 ⫾ 49.94 ⫺4.52 ⫾ 18.84 0.40 ⫾ 0.96
22 15 15 10
⫺1.84 ⫾ 63.17 ⫺3.07 ⫾ 60.20 6.94 ⫾ 53.32 0.12 ⫾ 0.60
17 12 12 9
45.87 ⫾ 86.92 ⫺52.09 ⫾ 64.70 ⫺12.43 ⫾ 37.03 ⫺0.94 ⫾ 1.42
.104 .215 .585 .068
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
showed no significant differences in the postoperative occlusal plane angle. The changes in the occlusal plane angle were significantly high in the group with decreased EMG(m), but no significant differences were observed for the other items (Table 7).
Discussion Measurement of the bite force, which was used in the present study as a method for functional evaluation of the maxillofacial system, was conducted with the clenching state of the first molar area. The first molar area exhibits the largest biting force and is similar to the biting force at the centric occlusion, according to the study by Ramfjord and Ash.21 Also, the measurement of the electromyogram was made using the anterior temporal and masseter muscles, which provide easy access to measuring the electromyogram and also show the most definite contraction during maximum occlusion. In addition, the anterior temporal and masseter muscles have greater reproducibility for the electrode positions and reattachment, making the muscles a good place to measure the electromyogram. Patients showed a large standard deviation, along with a large variation in the measured biting force and EMG, indicating that various factors, such as the mastication pattern, occlusal contact point, mastication muscle strength, psychological factors, and measure-
ment errors, affected the results. Therefore, a nonparametric analysis was used more often, even though our study had a sufficient number of samples. For a more exact statistical analysis, more samples, greater experience with measuring, and stricter standards for excluding subjects (exclusion of patients with temporomandibular disorders and those who are partial edentulous) are required. Although it is not used frequently, the occlusal plane angle is one of the factors measured in bilateral orthognathic surgery and has many clinical meanings. There are various standards and reference values in setting an occlusal plane, and according to Down22 and Rickettes,23 the mean of the occlusal plane in a person with normal occlusion is 8 ⫾ 4° from the FH plane, and according to Richard et al,24 the line that connects the midpoint of the maxillary incisal tip and the mediobuccal cusp of the upper first molar forms 11.2 ⫾ 4° with the FH plane. In Korea, Lee and Sohn25 reported that the angle is 12° ⫾ 4° with the FH plane, and Shin and Chang26 reported that the angle is 17.8° ⫾ 4.67° with the SN plane. In the present study, the normal range of the occlusal plane was set to 18° ⫾ 4° for the SN plane and categorized those with less than 14° as a low occlusal plane group and those with greater than 22° as a high occlusion plan group. An ideal occlusal plane is an important factor in determining the harmony between occlusion and the movements of the mandible and is expected to affect
Table 5. COMPARISON OF FUNCTIONAL MEASUREMENT CHANGES BETWEEN CROSSOVER GROUPS WITH OCCLUSAL PLANES IN NORMAL OR ABNORMAL RANGE
Group Moved to Abnormal Range
Group Moved to Normal Range
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
⌬Biting force ⌬EMG(m) ⌬EMG(t) ⌬Occlusion efficiency
7 5 5 3
⫺2.24 ⫾ 83.22 ⫺36.23 ⫾ 31.98 ⫺12.35 ⫾ 17.55 ⫺0.30 ⫾ 0.69
7 7 7 4
41.10 ⫾ 57.64 ⫺4.49 ⫾ 50.41 ⫺9.30 ⫾ 48.54 ⫺0.16 ⫾ 0.67
.280 .106 1.000 .400
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
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PARK ET AL
Table 6. CROSS ANALYSIS BETWEEN CROSSOVER GROUPS WITH OCCLUSAL PLANES IN NORMAL OR ABNORMAL RANGE AND FUNCTIONAL MEASUREMENTS
Variable Biting force Decrease Increase EMG(m) Decrease Increase EMG(t) Decrease Increase Occlusion efficiency Decrease Increase
Moved to Abnormal Range
Moved to Normal Range
4 (57.1) 3 (42.9)
1 (14.3) 6 (85.7)
5 (100.0) 0 (0)
3 (42.9) 4 (57.1)
3 (60.0) 2 (40.0)
4 (57.1) 3 (42.9)
1 (33.3) 2 (66.7)
3 (75.0) 1 (25.0)
P Value .266 .081 1.000 .486
Abbreviations as in Table 2. Data in parentheses are percentages. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
the decision regarding the range of surgery, esthetics, and the level of relapse after surgery. However, studies on this have varied, and we are not yet equipped with an established theory.27 Only rarely have studies showed the functional relationships of the occlusal plane after orthognathic surgery. When newly setting the occlusal plane, we can maintain the occlusal plane angle before surgery with consideration of both the functional and the esthetic aspects, or use methods such as aligning the maxilla’s occlusal plane through autorotation of the mandible and intentionally changing the occlusion plan for FH or SN.28,29 When the occlusal plane angle increases,
problems can occur, including the loss of anterior and canine guidance and functional interference with the posterior teeth.1 Also, Kohno et al30 and Wolford et al19 reported that the loss of anterior guidance and occlusion interference with the posterior teeth can occur easily and the maxillofacial system can develop functional disorders when the occlusal plane angle is large or there is a steep occlusal curve. In addition, Jo et al31 reported that if the occlusal plane angle becomes steep up to an average of 6°, occlusion interference can occur during mandible movement and can negatively affect the maxillofacial system, resulting in functional disorders. According to Reyneke and Evans,32 a posterior inferior rotation, which makes the occlusal plane incline to the standard plane of the cranial base by lifting the posterior part of the maxilla, is more beneficial than the contrary and can improve the esthetic effects. They also reported that reduction of the occlusal plane angle results in increasing joint pressure until the muscle, soft tissue, and teeth have readapted.19 The biting force is a vector sum of the tension forces of all closed mouth muscles. From the level of activation of some muscles, we can estimate that an occlusal plane vertical to the vector of the muscle stimulation will be most effective. Because it is difficult to find the direction of the vector using a morphologic search, the occlusion efficiency value was proposed33 to understand how much the mastication muscles generate the biting force. That value was used in the present study. According to the results of our study, the postoperative occlusal plane angle and the occlusion efficiency value showed a significant negative correlation. Moving from the low occlusal plane group to the high occlusal plane group, the occlusion efficiency values became
Table 7. COMPARISON OF POSTOPERATIVE OCCLUSAL PLANE ANGLE AND CHANGE IN OCCLUSAL PLANE ANGLE BETWEEN INCREASED AND DECREASED FUNCTIONAL MEASUREMENT GROUPS
Biting Force Decrease
EMG(m)
Increase
Decrease
Increase
Variable
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
n
Mean ⫾ SD
n
Mean ⫾ SD
P Value
Postoperative occlusal plane angle ⌬Occlusal plane angle
19 19
19.82 ⫾ 4.94 1.92 ⫾ 3.38
27 27
20.81 ⫾ 4.96 0.65 ⫾ 4.25
.509 .287
22 22
21.32 ⫾ 5.50 1.44 ⫾ 3.86
10 10
18.45 ⫾ 5.11 0.85 ⫾ 3.55
.173 .686
EMG(t) Postoperative occlusal plane angle ⌬Occlusal plane angle
16 16
20.97 ⫾ 5.57 2.71 ⫾ 3.22
16 19.88 ⫾ 5.48 16 ⫺0.20 ⫾ 3.70
Occlusion Efficiency .578 .024*
12 12
22.38 ⫾ 5.52 1.33 ⫾ 4.03
11 18.87 ⫾ 4.73 11 ⫺0.24 ⫾ 3.88
Abbreviations: SD, standard deviation; other abbreviations as in Table 2. *Statistically significant at P ⱕ .05. Park et al. Effects of Postoperative Occlusal Plane on Masticatory Function. J Oral Maxillofac Surg 2012.
.117 .353
1966
EFFECTS OF POSTOPERATIVE OCCLUSAL PLANE ON MASTICATORY FUNCTION
smaller, but not significantly. Because the smaller the occlusion efficiency value the more effective the occlusion, increasing the postoperative occlusion plan angle would be expected to provide more functional benefits. In the present study, the frequency of an EMG(t) decrease in the group with an increased occlusal plane angle and the frequency of an EMG(t) increase in the group with a decreased occlusal plane angle were significant. Also, the postoperative occlusal plane angle of the group with a decreased EMT(t) was larger than that of the group with an increased EMG(t), indicating that the occlusal plane angle has a large relationship with the EMG(t). The relationship between the increase in the occlusal plane angle and the decrease in the EMG(t) can be explained by the decrease in the muscle moment arm and that the direction of the muscle activation vector becomes more vertical as the occlusal plane angle increases. Using previous occlusal plane angles of those with normal occlusion as a reference, we estimated that functional improvement would occur when the occlusal plane angle was normal after surgery; however, the present study showed no significant differences. Therefore, whether a normal occlusal plane angle in normal occlusion should be set to an ideal angle in patients undergoing surgery is yet to be determined. A proper theory should be established through additional studies on the ideal occlusal plane angle. Although the functional recovery period should be considered in a functional analysis, opinions vary on the period necessary for functional recovery. Proffit et al34 reported that there were no changes in the maximum biting force for up to 1 year, and Youssef et al35 and Throckmorton et al36 concluded that the occlusal forces began to increase in the 6 months after surgery and continue for up to 3 years. Youssef et al35 also reported that the EMG activity of the masseter and temporal muscles increase in the 6 weeks after surgery and continue to increase for up to 3 years. Throckmorton et al36 reported that the maximum occlusal forces improved gradually according to the constant rate. They concluded that the postoperative bite forces were greater than their preoperative forces within 6 months. Seo et al20 reported that the maximum biting force and the maximum mouth opening significantly increased until the third month after sagittal split ramus osteotomy in orthognathic patients, and the muscle activation of the masseter muscle and the anterior temporal muscle had recovered to preoperative values or more by 6 months after surgery. That study tested not only the biting force 6 months after surgery, but also the level of muscle activation. In addition, Throckmorton et al36 concluded that male patients improved more quickly than female pa-
tients. A preliminary analysis showed that the functional change in the male patient was greater than that of the female patient, but the difference was not significant in our data. The mandibular length (the lever arm from condylion to pogonion) and gonial angle changes with mandibular surgery, and this change can affect the masticatory function. Choi and Kim37 reported on the relationship between the maximum bite force and facial skeletal pattern in normal occlusion subjects and concluded that the lever arm length and gonial angle did not have any significant correlation with the posterior bite force. However, it might be valuable to investigate the multiple relationships between the occlusal plane angle, lever arm length, gonial angle, and masticatory function after orthognathic surgery.
References 1. Fonseca RJ: Oral and Maxillofacial Surgery (vol 2). Philadelphia, PA, WB Saunders, 2000 2. Graber TM: Growth and Development. Philadelphia, PA, WB Saunders, 1972, pp 27-128 3. Dawson PE, Arcan M, Lundeen HC, et al: Attaining harmonic occlusion through visualized strain analysis. J Prosthet Dent 46:443, 1981 4. Maness W, Benjamin M, Podoloff R, et al: Computerized occusal analysis; a new technology. Quit Internat 4:287, 1987 5. Hosman H, Naeije M: Reproducibility of the normalized electromyographic recording of the masseter muscle by using the EMG recording during maximal clenching as a standard. J Oral Rehb 6:49, 1979 6. Manns A, Miralles R, Valdivia J, et al: Influence of variation in anteroposterior occlusal contacts on electromyographic activity. J Prosthet Dent 61:617, 1989 7. Ahlgren J, Sonesson B, Blitz M: An electromyographic analysis of the temporalis function of normal occlusion. Am J Orthod 87:230, 1985 8. Oyer R, Tsay P: A biomechanical analysis of craniofacial form and bite force. Am J Orthod 99:298, 1991 9. Ringquist M: Isometric bite force and its relation to dimensions of the facial skeleton. Acta Odontol Scand 31:35, 1973 10. Lee TW, Lee KS: Isometric bite force and its relation to craniofacial morphology. Korean J Orthod 21:185, 1991 11. MacDonald JWC, Hannam AG: Relationship between occlusal contact and jaw-closing muscle activity during tooth clenching; part 1. J Prosthet Dent 52:718, 1984 12. Riise C, Ericksson SG: A clinical study of the distribution of the occlusal tooth contact in the intercuspal position at light and hard pressure in adult. J Oral Rehabil 10:473, 1983 13. Moyers RE: An electromyographic analysis of certain muscles involved in temporomandibular movement. Am J Orthod 36: 481, 1950 14. Latif A: An electromyographic study of the temporalis muscle in normal persons during selected positions and movements of the mandible. Am J Orthod 43:577, 1957 15. Ingervall B: Facial morphology and activity of temporal and lip muscles during swallowing and chewing. Angle Orthod 46: 372, 1976 16. Lwe AA, Takada K: Associations between anterior temporal, masseter and orbicularis oris muscle activity and craniofacial morphology in children. Am J Orthod 86:319, 1984 17. Lee YJ, Sohn BW: A study on the postoperative stability of occlusal plane in class III orthognathic surgery patients. Korean J Orthod 30:643, 2000 18. Lee GY, Jang HJ, Lee SH: Clinical articles: Study on the changes of occlusal plane angle in mandible prognathism after orthognathic surgery. J Korean Assoc Maxillofac Plast Reconstr Surg 25:432, 2003
PARK ET AL 19. Wolford LM, Chemello PD, Hilliard F: Occlusal plane alteration in orthognathic surgery—Part I: Effects on function and esthetics. Am J Orthod Dentofac Orthop 106:304, 1994 20. Seo JC, Kim JR, Yang DK: A study on changes in the maximum bite force after orthognathic surgery. J Korean Assoc Oral Maxillofac Surg 22:121, 1996 21. Ramfjord SP, Ash MM: Occlusion (ed 3). Philadelphia, PA, WB Saunders, 1983, pp 128-174 22. Downs WB: Variations in facial relationships; their significance in treatment and prognosis. Am J Orthod 34:812, 1948 23. Rickettes RM: Cephalometric analysis synthesis. Angle Orthod 31:141, 1961 24. Richard K, Djeng SK, Ho CK: The relationships of upper facial proportions and the plane of occlusion to anatomic reference planes. J Prosthet Dent 61:727, 1989 25. Lee WY, Sohn BH: Cephalometric analysis of maxillofacial normals in Korean adults. Korean. J Orthod 14:135, 1984 26. Shin MC, Chang YI: A study on the vertical dysplasia in the skeletal class III malocclusion. Korean J Orthod 20:333, 1990 27. Hwang CJ, Lim SA, Moon JL: A study on the postoperative stability of hard tissue in orthognathic surgery patients depending on the difference of occlusal plane. Korean J Orthod 29: 239, 1999 28. Proffit WR, Bell WH: Open Bite: Surgical Correction of Dentofacial Deformities. Philadelphia, PA, WB Saunders, 1980, pp 1111-1113
1967 29. Epker BN, Fish LC: Dentofacial Deformities: Integrated Orthodontics and Surgical Correction. St Louis, MO, CV Mosby, 1986, p 400 30. Kohno S, Karasuyama H, Yoshida K, et al: A study on the causative mechanism of occlusal interference on the balancing side. J Jpn Prosthodont Soc 32:505, 1988 31. Jo BW, Ahn SH, Kim JP: The clinical study of the effect of occlusal planes on the stomatognathic system. J Korean Acad Prosthodont 33:10, 1995 32. Reyneke JP, Evans WG: Surgical manipulation of the occlusal plane. Int J Adult Orthodon Orthognath Surg 5:99, 1990 33. Okane H, Yamashina T, Nagasawa T, et al: The effect of anteroposterior inclination of the occlusal plane one biting force. J Prosthet Dent 42:487, 1979 34. Proffit WR, Turvey TA, Fields HW, et al: The effect of orthognathic surgery on occlusal force. J Oral Maxillofac Surg 47:457, 1989 35. Youssef RE, Throckmorton GS, Ellis E, et al: Comparison of habitual masticatory cycles and muscle activity before and after orthognathic surgery. J Oral Maxillofac Surg 55:699, 1997 36. Throckmorton GS, Buschang PH, Ellis E: Improvement of maximum occlusal forces after orthognathic surgery. J Oral Maxillofac Surg 54:1080, 1996 37. Choi WC, Kim TW: Relationship between maximum bite force and facial skeletal pattern. Korean J Orthod 33:437, 2003