The proper direction of the lateral pterygoid muscles leads to contraction
over the two halves of the mandible, resulting in mandibular flexure (Zarone
et al., 2003). Simultaneous contraction of the lateral pterygoid
muscles during mandibular opening and protrusion exerts a medial forward and
downward traction of the condyles (Gates and Nicholls, 1981;
Weinmann and Sicher, 1955; DuBrul and
Sicher, 1954). Decreasing the mandibular arch width during maximum opening
and anterior movement has been reported in several studies, which hypothesize
that the lateral pterygoid muscles travel in a posterior-lateral direction and
pull the mandible forward when forcefully contracted. Adduction occurs when
the mandible is pulled forward and the condyles are pulled medially (Gates
and Nicholls, 1981; McDowell and Regli, 1961; Osborne
and Tomlin, 1964; Regli and Kelly, 1967; De
Marco and Paine, 1974; Fischman, 1990).
There is a linear relationship between the degree of opening and mandibular
flexure (Jiang and Ai, 2002). In addition, mandibular
flexure probably represents a destructive factor on the dentoalveolar system
in bruxers with periodontal problems. Moreover, clenching not only applies an
additional force on teeth but can also lead to mandibular flexure (Korioth
and Hannam, 1994).
Hobkirk and Schwab (1991) showed mandibular flexure
in patients with at least two implants. The results revealed that the implant
position change during mandibular flexure in the most open, protruded and lateral
movements, which may be effective in fixed and removable prosthetic impression
procedures and the occlusal bite records. However, Law et
al. (2011) in their literature review reported that the clinical significance
of mandibular flexure on the success of dental implant treatment is at this
time unclear. This deformation of the mandible may result in biomechanical and
prosthetic complications such as porcelain fractures, cement failures and open
contacts (Zarone et al., 2003; Chen
et al., 2000). Moreover, mandibular flexure may lead to postinsertion
sore spots at the posterior borders of complete and partial removable dentures
(Chen et al., 2000).
Impressions obtained with the mandible in a most open position will have the
location of the teeth recorded in a more lingual position than would be found
in the mandible at rest or in occlusion (Fischman, 1990),
which creates two clinical problems. First, centric relation records may not
exactly fit to the casts from impressions at the maximum open position. Second,
interferences may occur when crowns and bridges made on casts from impressions
at the most open position, are place on tooth intraorally.
Based on the importance of mandibular flexure, the effect of this phenomenon on the width and length of lower arch were investigated on a group of female students of Mashhad Dental School.
MATERIALS AND METHODS
Subjects were recruited from the undergraduate female students in Mashhad Dental School. Thirty-five students volunteered to participate in the study. The age of the participants ranged from 22 to 25 years old. Inclusion criteria included complete class I dentition and same weight and height. Subjects with malocclusion, history of trauma to the mandible, orthodontic therapy, temporomandibular disorders and missing teeth were excluded from the study. Informed consent was obtained from all subjects, using a written form approved by the Ethic Committee of Mashhad University of Medical Sciences.
Deformation of the mandible was studied by the Mitutoyo digital calipers (Mitutoyo Corporation, Tokyo, Japan) fixed on mandibular second molars and canines to record the lower arch width and length change in the most open position and occlusion. Posterior mandibular width change during mouth opening were recorded from the second molar buccal notch of one side to the second molar buccal notch of the opposite side (intermolar). In addition, mandibular width change in the anterior segment was recorded from the distal canine on one side to the distal canine on the opposite side (intercanine). For measuring mandibular length change during mouth opening, measurements were recorded from the distal aspect of the second molar to the distal aspect of the canine.
The main outcome measures were mandibular intermolar and intercanine width as well as right and left length change in the most open and closed mouth positions, which presented normal distribution and homogeneity of variances. Data were analyzed by one-way ANOVA and paired t-test at the 0.05 level of significance.
The results of mandibular deformation during mouth opening in each subject
are presented in Table 1. The quantity of deformation represented
the amount of arch width change at the intermolar (MM) and intercanine (CC)
regions as well as arch length change at canine to molar in the right-side (CMR)
and left side (CML) of the mandible. The range of changes at MM, CC, CMR and
CML regions were as follows, respectively: -1.35-2.60, -0.53-1.14, -3.30-5.02
and -2.70-3.32 mm.
||Intermolar (MM), intercanine (CC), canine to molar in the
right-side (CMR) and canine to molar in the left-side (CML) change during
|*Mean distortion values, Negative values represent the increase
of arch length or width
||Means and standard deviations for mandibular deformation obtained
on mouth opening
As shown in Table 2, the average difference for intermolar
and intercanine distance is positive (0.1894 for MM and 0.1671 for CC areas),
which suggest that medial flexure occurs in the posterior and anterior regions
during maximum opening compared to the mouth closed position. In addition, lower
arch length change between the maximum opening and closed mouth position at
both sides of the jaw is negative (-0.0519 for CMR and -0.1547 for CML areas),
suggesting that this dimension in the most open position is greater than the
length of the mandible in occlusion.
The mean measurements of dimensional change in four planes is analyzed using
one-way ANOVA, which showed that there is no statistically significant difference
between the values obtained in the four groups (p = 0.76).
The consideration of mandibular flexure during various types of jaw movements
is an important factor in determining the appropriate strategy for restorative
treatments of the lower jaw. Lateral pterygoid muscle contraction results in
mandibular flexure. It has been shown that mandibular medial flexure is more
obvious in women (Chen et al., 2000).
In the present study, the effect of the mandibular flexure on width and length deformation of lower arch in a group of female dental students was evaluated. To eliminate bias in the current study, all subjects were selected with class I complete dentition.
It is important to note that the present study is the first research which
evaluates mandibular deformation in both anterior (intercanine) and posterior
(intermolar) regions, while previous studies have assessed the mandibular flexure
only in the posterior area (Burch and Borchers, 1970;
Chen et al., 2000; De Marco
and Paine, 1974; Fischman, 1990; Gates
and Nicholls, 1981; McDowell and Regli, 1961; Regli
and Kelly, 1967). Also, the mandibular length changes have been investigated
for the first time in our study.
Posterior and anterior arch width changes in the most closed and maximum open
positions were investigated. The mean measurements of posterior and anterior
width deformation were 0.19 and 0.17 mm, respectively. These dimensions of the
mandible in the maximum open position were smaller than the closed mouth position.
The mandible exhibits elastic behavior under functional forces, which may be
due to its horseshoe shape, the different direction of masticatory muscles and
bone density (Korioth and Hannam, 1994; Misch,
2008). Burch and Borchers (1970) found that the
mandible changed by as much as 0.438 mm between maximum open and closed mouth
positions in the first molar region. A study by Chen et
al. (2000) demonstrated that the mandibular flexure in the first molar
region was 0.20-0.43 mm.
According to the results of the current study, the effect of mandibular flexure/tension
in anterior segment is less than posterior region. It can be explained by the
distance between the anterior segment of the mandible from masticatory muscles.
It has been suggested that the prostheses made in anterior region demonstrated
better prognosis (Misch, 2008). De
Oliveria and Emtiaz (2000) found a relationship between mandibular flexure
and the inconvenience of patients treated with implant.
Increasing in longitudinal dimension of the lower arch between canine and second molar during mouth opening was seen, which may be due to mandibular flexure. Bending the mandibular ramus inward would cause tension along the body of the mandible, which may lead to a minute elongation of the lower arch. The mean length change in right and left arches was 0.05 and 0.15 mm, respectively. Although, the mean measures of length change in the most open and closed positions were greater in left side, there were not statistically significant differences.
It is important to note that mandibular flexure during mouth opening may affect on occlusal stability, implant prognosis, crown and bridge fitness and removable denture stability. Therefore, impressions should be performed with the patient's mouth in a most closed position. Long span fixed dental prostheses also should be avoided.
In the further studies, evaluation of mandibular flexure on fixed and removable prosthetic failures as well as the effect of masticatory muscles and ramus shape in patients with parafunctional habits will be investigated.
It is concluded that mandibular arch width in the posterior and anterior regions decreases at the most open position compared to rest position. Also, Increasing in longitudinal dimension of the lower arch between canine and second molar in maximum open position was seen.
The research results given in this study were obtained from doctoral thesis by a grant (No. 86777) supported from the Vice Chancellor of Mashhad University of Medical Sciences, Iran.