INTRODUCTION

The etiology of intra- and extracapsular temporomandibular disorders (TMD) remains a subject of dispute, particularly with regard to the role of occlusion. Some studies on patient and nonpatient populations have found a significant correlation between malocclusions and signs and symptoms of TMD,1–6 whereas others have not been able to substantiate these results.7–10

Several investigators have reported that the morphology of craniofacial skeletons correlates with forms of jaw muscles.1112 It has been shown that the chewing pattern is influenced by the presence of certain malocclusions, and that there is a relationship between facial shape and the electrical activity in the chewing muscles.13 For example, activity in the masticatory muscles is generally higher in individuals with a rectangular face and lower in individuals with a large gonial angle, whereas hyperactivity in the middle and posterior temporalis muscle has been found on the affected side in patients with unilateral posterior crossbite.

The prevalence of occlusal factors in relation to TMD is common in the literature for various ethnic groups worldwide. However, only few studies have attempted to survey the occlusal factors in relation to masticatory muscle tenderness.1415

According to the Pancherz,16 children with Class II malocclusion exhibit an impaired muscle activity in comparison with children with normal occlusion. This is especially apparent with respect to the masseter muscle. Deep overbite and Class II, division 2 malocclusions are commonly cited factors in TMJ patient populations,1718 although the belief in their role is not accepted universally.19 Deep bite is commonly said to be a cause of condylar displacement and masticatory muscle pain.20–23 An increase in muscle tenderness has been reported associated with open bite in a study of children and young adults.15 Riolo et al15 indicated that the relative frequency of muscle tenderness for cusp-to-cusp boy subjects was consistently greater than that for the Class I and Class II boy subjects for all ages. It has also been reported that the muscle coordination during functional movements is different in Class III malocclusion.24

With these considerations in mind, the aims of this study were to

• Investigate the relationship between occlusal factors (such as Angle classification of molars, anterior and posterior crossbites, excessive overjet, open and deep bites, functional shift, and anterior crowding) and masticatory muscle tenderness among 10- to 19-year-old Turkish subjects.

• Identify possible sex differences between female and male subjects.

MATERIALS AND METHODS

In the present study, 716 individuals (355 male and 361 female subjects) were selected randomly from four different schools in Konya, Turkey. The distribution of age for all participants is shown in Table 1. The following inclusion criteria were used for the selection of the sample: 10 years of age or older; all permanent first molars erupted; and Caucasian.

TABLE 1.  Number of Female Subjects and Male Subjects by Age Group

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The examiners recorded the Angle classification bilaterally for molars, presence of anterior and posterior crossbites, excessive overjet, open and deep bites, functional shift, and severity of anterior crowding.

Anteroposterior molar relationship groups were established as (1) Class I, (2) Class II, division 1, (3) Class II, division 2, and (4) Class III on both sides. These classifications were mutually exclusive. The cases with the molars in one side being in Class I and on the other side in Class II or Class III relationship were excluded because of the difficulties of classification.

Crossbites were designated as either present or absent. Anterior and posterior crossbites were recorded as a single tooth or multiple teeth. No scissors bite was observed among all subjects. Overjet or the horizontal distance between the facial surface of the upper central incisors and the facial surface of the lower central incisors was recorded using a periodontal probe. Overjets exceeding six mm were recorded as extreme.

Similarly, overbites were established as (1) open bite (negative overbite), (2) deep bite (overbite greater than the lower incisor's crown height), and (3) overbite less than the total overlap of the lower incisor's crown.15

Functional shifts in the occlusion were recognized by deconditioning the subject's habitual biting pattern, thus making it possible to measure any functional shift of the mandible. In this way, any functional shift caused by an occlusal interference resulting in a reflex alteration in the movement pattern at the occlusal interface was detected. Deconditioning was accomplished using cotton rolls to separate posterior teeth for three minutes before any measurement of a functional shift was attempted. It was judged as a functional shift present or absent.

Crowding was estimated separately for the anterior segment of the maxillary and mandibular arches. Crowding was recorded as 0 = no crowding; 1 = mild (lack of space for less than half of the mesiodistal width of a tooth in a segment); 2 = moderate (lack of space for half or more of the width of one tooth but less than one tooth); and 3 = severe (lack of space for one tooth or more). Crowding was registered as being present if it was three mm and if it was at least one arch.

Symptoms of tenderness of the masticatory system were recorded on the basis of an anamnestic examination and a clinical examination. The tenderness with palpation of masseter and temporalis muscles and functional manipulation of lateral and medial pterygoid muscles was registered. Muscle tenderness of the anterior temporal, posterior temporal, and insertion of the temporal and superficial masseter was determined by palpation on both sides. Tenderness was graded as Score 0, if subject described no tenderness; Score 1, if the subject could feel the difference between the right and left sides or described little tenderness; Score 2, if described the palpation as painful; and Score 3, if pain gave rise to a palpebral reflex or guarding. Functional manipulation was carried out on the lateral and medial pterygoid muscle according to Okeson,25 and the findings were graded in the same way as for the masseter and temporal muscles. During functional manipulation, all the information needed was obtained by having the patient open wide, protrude against resistance, clench the teeth together, and then bite on a separator. If a muscle was a true source of pain, functional manipulation is helpful in identifying this source.

The examinations were carried out by two trained investigators. The interobserver variability between two trained investigators was low, and the examination error seemed negligible.26 Methods and criteria used in this study have been used as standard procedures in many other epidemiologic investigations and have been proved to be valid.26

All statistical analyses were performed using the SPSS software package (SPSS for Windows 98, version 10.0, SPSS Inc., Chicago, Ill). Differences in prevalence of masticatory muscle tenderness and occlusal factors and associations between masticatory muscle tenderness and occlusal factors were evaluated using the chi-square test (χ²). Comparisons between the sexes were also performed. Values of P < .05 were considered to indicate statistical significance.

RESULTS

The distribution of different occlusal factors in 10- to 19-year-old Turkish subjects is presented in Table 2. None of the subjects had severe masticatory muscle pain (Score 2, 3), whereas most of the scores were 1. The distributions of masticatory muscle tenderness according to the different occlusal factors are shown in Table 3.

TABLE 2.  Prevalence of Different Occlusal Factors Among 10- to 19-y-old Turkish Subjects

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TABLE 3.  Distribution of Masticatory Muscle Tenderness Among Different Occlusal Factors

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The prevalence of masticatory muscle tenderness (Score 1, mild tenderness) was 56.25% among 10- to 19-year-old Turkish subjects. In general, 59.83% of female and 52.67% of male subjects had tenderness of the masticatory muscles. It appeared that the functional shift in this sample was quite rare (approximately 1% of the cases), therefore it was excluded from the statistical analyses.

There was an association between Angle Class I and lateral pterygoid muscle tenderness and Angle Class III malocclusion and medial pterygoid muscle tenderness (P < .05). Associations were also found between anterior crossbite with medial pterygoid and temporal muscle tenderness (P < .05); anterior crowding with lateral pterygoid and temporal muscle tenderness (P < .05); excessive overjet with masseter and lateral pterygoid muscle tenderness (P < .05); open bite with medial pterygoid muscle tenderness (P < .05); and deep bite with lateral muscle tenderness (P < .05) (Table 4). In general, medial pterygoid (P < .01), lateral pterygoid (P < .05), and masseter muscle tenderness (P < .05) were high in female subjects (Table 5).

TABLE 4.  The Relationship Between Occlusal Factors and Masticatory Muscle Tenderness

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TABLE 5.  The Relationship Between Sex and Masticatory Muscle Tenderness

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Medial and lateral pterygoid muscle tenderness in Angle Class I cases and masseter and medial pterygoid muscle tenderness in Angle Class II, division 1 malocclusion cases were higher in female subjects (P < .05). In excessive overjet cases, masseter muscle tenderness (P < .05), in open-bite cases, medial pterygoid muscle tenderness (P < .05), and in deep-bite cases, masseter (P < .01) and medial pterygoid (P < .05) muscle tenderness were also higher in female subjects. Complete findings are presented in Table 6.

TABLE 6.  The Relationship Among Occlusal Factors, Gender, and Masticatory Muscle Tenderness

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DISCUSSION

The prevalence of different occlusal factors or masticatory muscle tenderness has been noted in several re-ports.1–1012141527353738 Some of the prevalence studies exhibit methodological limitations and had a lack of specific diagnostic criteria, small or self-selected samples, and lack of developmental age-specific data. A study with standardized examination techniques, well-defined diagnostic criteria, and a large representative sample would help to substantiate the prevalence of disorders in children.

In a study by Bush,27 nearly 300 dental students were examined for muscle tenderness and various occlusal contacts, and he reported only mild masticatory muscle tenderness. In the present study, none of the subjects had severe masticatory muscle pain (Score 2, 3), whereas most of the scores were 1, in accordance with Bush's findings.

Children with Class II malocclusion tended to have more masticatory muscle tenderness.28 In adults, an association has been demonstrated between muscle tenderness and Class II malocclusion.1 No relationship between muscle tenderness and Angle classification was found in one study.25 Another indicated that neither Class II nor cusp-to-cusp molar relationships were statistically associated with muscle tenderness, whereas a significant interaction was found between molar relation, sex, and age and the odds of having muscle tenderness.15 An autopsy study of young adults13 showed that Class III malocclusions were associated with deviation in the normal form of the temporomandibular joint components, and this was associated with masticatory muscle tenderness. Surprisingly, instead of Angle Class II malocclusion, we found an association between Angle Class I and lateral pterygoid muscle tenderness and Angle Class III with medial pterygoid muscle tenderness.

It has been suggested that undue forces are placed on the masticatory muscles as a consequence of an unfavorable incisor relationship.29 Studies have suggested that factors such as increased overjet and overbite and anterior open bite are likely to be predisposing factors rather than initiating factors in the production of masticatory muscle symptoms.29 However, Seligman et al30 found that overjet was not related to symptoms of TMJ sounds or to masticatory muscle tenderness in a young adult nonpatient population. In the present study, statistically significant relationships were found between increased overjet and masseter and lateral pterygoid muscle tenderness. Our results are not in agreement with the findings of Seligman et al.30 In our study, a statistically significant relationship was found between open bite and medial pterygoid muscle tenderness, in agreement with Riolo et al.15

Pullinger and Seligman31 found no associations between deep bite (overbite of five mm or more) and masticatory muscle tenderness, and our findings are not in agreement.31 We found a statistically significant relationships between deep bite and lateral pterygoid muscle tenderness.

According to Riolo et al,15 the relative frequencies of muscle tenderness were significantly lower among subjects with a functional shift (17%) than in those without (8%). A relationship between a functional shift and masticatory muscle tenderness was also found for both sexes and across all age groups in the same study.15 However, in the current study, no relationship was found between functional shift and masticatory muscle tenderness for both sexes.

Ahlgren and Posselt32 and Shaw13 have described an association between crossbite and occlusal interferences in children. Shaw13 reported that hyperactivity in the middle and posterior temporalis muscle was found on the affected side in patients with unilateral posterior crossbite. Egermark-Eriksson et al28 indicated that children with crossbite were found to have muscle tenderness more often than children without crossbite, and there does not seem to be any difference in risk between unilateral and bilateral crossbite or between crossbite in premolar and molar segments.33 Results of Egermark-Eriksson and Ingervall33 agree with findings in adults by Mohlin et al.19 However, no relationship was found between posterior crossbite and masticatory muscle tenderness in the present study. We found a statistically significant relationship between anterior crossbite and medial pterygoid and temporal muscle tenderness.

Rantanen34 found 24% of 2218 undergraduate students to have some type of TMD, or other disturbances of mandibular movements, with symptoms more common in girls (30%) than in boys (18%). Currently, it is widely understood that TMD is multifactorial in nature and is more prevalent in girls than in boys.335–38 Studies on the development of pain and tenderness in muscles or jaws also revealed higher frequencies in girls than in boys.38–40 Therefore, with respect to masticatory muscle tenderness, it is of a great importance to examine the sex differences in muscle tolerance in relation to the occlusal factors.

In general, our findings indicated that medial pterygoid muscle tenderness, lateral pterygoid muscle tenderness, and masseter muscle tenderness were higher in female than in male subjects. These results are in agreement with the previous epidemiologic studies' results.336–38 According to Riolo et al,15 the relative frequencies of muscle tenderness for cusp-to-cusp boy subjects were consistently greater than those for the Class I and Class II boy subjects for all ages. The girl molar classification groups showed no systematic relationship across age. In the present study, medial pterygoid muscle tenderness was higher in female subjects for both Angle Class I and Class II, division 1, and with open bite. Medial pterygoid muscle tenderness was found in subjects with excessive overjet, and masseter muscle tenderness was found in a higher ratio in female subjects. Masseter and medial pterygoid muscle tenderness was also higher in female subjects with deep bite. The highest prevalence of female subjects classified with some degree of muscle tenderness associated with different occlusal characteristics may be related to typical physiologic differences of the feminine sex, such as regular hormonal variations, muscular structure, and different characteristics of the conjunctive tissue. These matters need to be investigated fully.

CONCLUSIONS

The prevalence of masticatory muscle tenderness (Score 1, mild tenderness) was found in more than 50% of 10- to 19-year-old Turkish subjects. This percentage seemed to increase with age and become higher in the adult population.

Statistically significant relationships were found between masticatory muscle tenderness and all investigated occlusal factors except posterior crossbite and functional shift. It appears that short-term reversible therapy is adequate to resolve most muscle problems in most children. Certainly, studies are needed to confirm these findings. Studies that evaluate the effectiveness of treatments need to be controlled for each specific diagnosis (ie, masticatory muscle disorders vs disc interference disorders).

Statistically significant sex differences were found between different occlusal factors and masticatory muscle tenderness. Medial and lateral pterygoid muscle tenderness in Angle Class I cases and masseter and medial pterygoid muscle tenderness in Angle Class II, division 1 malocclusion cases were higher in female subjects. In excessive overjet cases, masseter muscle tenderness (P < .05), in open-bite cases, medial pterygoid muscle tenderness (P < .05), and in deep-bite cases, masseter (P < .01) and medial pterygoid (P < .05) muscle tenderness were also higher in female subjects. These results suggest that greater masticatory muscle tenderness in female subjects may contribute to the greater prevalence of TMD in them.

At this time, there is no scientific documentation that early correction of malocclusion will prevent masticatory muscle or temporomandibular joint disorders. Well-controlled longitudinal studies are needed in this area.