INTRODUCTION

The head and cervical traits of the vertebral column are part of a functional biomechanical unit, the cranial cervical mandibular system. This system is made up of three main structures: TMJ, occipital atlas axis articulation, and hyoid bone with its suspensor system. These three structures are strictly interdependent, but joined together with the rest of the body (vertebral column) by muscles and ligaments. Consequently, it is not unreasonable to expect that cervical posture can be related to craniofacial morphology1–6 and nasorespiratory function.7–11

According to Solow and Tallgren,1 extended craniocervical posture is frequently associated with an increase of anterior facial height, a decrease of sagittal jaw dimensions, and a steeper inclination of the mandible. When the head is flexed (in relation to the cervical column), anterior facial height is shorter, sagittal jaw dimensions are larger, and the mandibular plane is flatter.

Marcotte2 also reported a significant correlation between mandibular position and head posture: people with a hollow facial profile showed a tendency to bend the head downward, while people with a convex profile showed a tendency to bend the head upward.

An association between Class II malocclusion and forward head posture (or forward cervical inclination combined with an extended craniocervical angle) was described by Rocabado et al3 as the stronger evidence they had observed in the relationship between head posture and malocclusion. Similar results were obtained by Capruso et al4 who showed that forward head posture was associated with a very high probability of skeletal Class II and hyperdivergency.

D'Attilio et al5 found that the lower part of the spinal column was significantly straighter in subjects in skeletal Class III than in subjects in skeletal Class I and skeletal Class II. They stated that the size and position of the mandible are two factors that are strongly related to cervical posture. Based on all of these results, it is reasonable that head posture should be considered an important element of orthodontic diagnosis.

In the literature, a rarely discussed aspect is the possible relationship between dental crowding and the posture of the head and the neck. Dental crowding can be described either as a dentoalveolar discrepancy between available space (the space offered by bone to distribute all of the teeth) and the space needed (the space that is equivalent to the mesial distal width of all of the teeth),12 or as lack of a correct dental alignment with anomalous dental inclination, position, or rotation.13 This occlusal condition has a multifactorial etiology and shows a wide incidence after eruption of the second permanent molar.14–21

AlKofide and AlNamankani22 examined whether a relationship exists between posture of the head and neck, and the presence of certain malocclusions. In their study, a relationship between crowding and head posture could only be found in subjects with upper arch crowding and cervical curvature and not with lower dental crowding.

In a previous study, Solow and Sonnesen6 showed a strong inverse correlation between internal craniocervical angles and dental crowding greater than 2 mm. In particular, subjects with dental crowding of more than 2 mm in the lower anterior segment of the dental arch had mean craniocervical angles 3° to 5° larger than subjects without crowding.

The aim of the present investigation is to test if any relationships could be found between head posture and late lower arch crowding.

MATERIALS AND METHODS

For this study, we analyzed the pretreatment records of 200 randomly selected patients treated in the Department of Orthodontics, University of Rome, Tor Vergata. One hundred forty-five subjects were not included in the sample because they did not satisfy the inclusion criteria. Fifty-five subjects (27 female, 28 male; 12–18 years of age) were selected on the basis of the following inclusion criteria:

• Complete permanent dentition (without taking into consideration the eruption state of the third molar);

• No previous orthodontic treatment;

• No TMJ or cervical spine disorders.

Lateral Cephalometric Radiographs

Teleradiographs were made before beginning the study. Lateral skull radiographs were taken using Proline Ceph CM (Planmeca). The x-ray source had a focus of 0.6 mm, and the exposure data were 72 kV and 32 mA for 1.2 seconds. The equipment had a fixed film to focus plane distance of 190 cm and a fixed film to midsagittal plane distance of 10 cm with a final enlargement of 10%. For all subjects, 24 × 30 cm films were used. All lateral skull radiographs were taken by the same operator with the subjects standing in orthoposition with the head in the natural head position (self-balanced position) as described by Sahin Sağlam and Uydas.23 The lateral radiographs had to include the first four cervical vertebrae.

The lateral cephalograms were traced on acetate paper. Seven reference points (Table 1 and Figure 1) were marked on acetate papers including four points in the craniofacial area and three points in the cervical column area. Six lines (Table 2 and Figure 1) were considered. The eight variables studied are listed in Table 3. The cervical reference line and the postural variables were traced according to Solow and Tallgren.24

Table 1.  Reference Points of the Cephalograms

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Table 2.  Reference Lines of the Cephalograms

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Table 3.  List of Variables

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RESULTS

The 28 subjects of the study group showed a mean dental crowding of −3.85 mm (DS = 1.60), while the 27 subjects of the control group showed a mean excess space of 1.09 mm (DS = 2.2). Means and standard deviations of the craniovertical, craniocervical, and craniohorizontal angles are shown in Table 4.

Table 4.  Mean, Standard Deviation, and Mean Difference of the Postural Variables in Subjects With and Without Dental Crowding^a

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The t-test showed differences of postural variables between the two groups (Table 4). The subjects with more than 2 mm of dental crowding had mean craniocervical angles that were 5° to 6° larger than the mean angles of subjects with space conditions smaller than 2 mm (P < .01). Furthermore, the means of the craniohorizontal angles in the study group were 3.5° to 4° smaller than those in the control group (P < .05). For the other angles, no statistically significant difference was found (P ≤ .1).

DISCUSSION

The present study shows a clear pattern of association between more than 2 mm of lower arch crowding (left to right first molar included) and extended craniocervical posture (expressed by an increase of NSL/ CVT [P = .002], NSL/OPT [P = .001], NL/CVT [P = .008], and NL/OPT [P = .002]) as the Solow and Sonnesen's6 results (Figures 2 and 3).

How can head posture be associated with lower arch malalignment? We can only hypothesize about this relationship. According to Proffit's equilibrium theory,25 the teeth and facial skeleton are submitted constantly to the action of “external” lip and cheek forces and to “internal” tongue forces, and these pressures influence tooth position and facial morphology. This influence depends more on the duration of application time than on the intensity of the forces: a light force that acts for a long time on the jaw can induce more modifications than a strong force that acts for a short time. Proffit25 stated that in dental skeleton modifications, a very important rule is played by “a long-term muscular activity: the resting pressure of the lips, cheeks, and tongue.”

The soft perioral tissue stretching hypothesis formulated by Solow and Kreiborg26 can explain how the resting muscular activity depends on the head posture in relation to the vertebral column. According to this hypothesis, the soft tissue layer (skin, muscles, and fascia) that covers the head and neck, stretches and relaxes itself in relation to the degree of extension or flexion of the head. In cases of long-term hyperextension of the head posture, these soft tissues stretch, creating a dorsal and caudal force against the teeth and skeleton (Figure 4). If this force is not balanced by an increase of tongue muscular activity, it can induce a dorsal and caudal restraint on facial development and a retroinclination of the incisors with a consequent loss of correct alignment. Normal head posture can induce relaxed soft tissues with consequent sagittal development and proclination of the incisors.

The association between head posture and lower arch crowding could also explain the reports of Linder-Aronson27 and Woodside et al.28 They showed that subjects with obstruction of the nasopharyngeal airway presented a greater irregularity index and reduced incisor inclinations relative to the subjects without nasal airway obstruction. Furthermore, they showed that after adenoidectomy, and with the return of nasal respiration, an increased inclination of the incisors resulted.

A reduction of the nasal airway can cause, by reflex, a hyperextension of the head to facilitate air passage. An increase of the craniocervical angles is demonstrated in children with adenoids,7 with enlarged tonsils,8 with nasal allergy,9 and in patients with obstructive sleep apnea.10

The experiment of Vig et al11 demonstrated that when the nasal airway is obstructed, internal craniocervical angles immediately increased 5°. The head rotated behind and the jaw turned down. When the obstruction was removed, the head returned to its normal position. In order to consider the soft tissue stretching theory a valid determinant of the association between dental crowding and head posture, it is important that the stretch determine a real dorsally directed force that can alter the equilibrium between the lips, cheeks, and tongue on the incisors. Many studies have reported the lip pressure on the incisors in rest conditions. Parfitt29 recorded a lip pressure on the incisors between 3 g/cm² and 5 g/cm², while Winders30 reported this pressure was 6 g/cm². Also, Thuer et al31 recorded lip pressure both on the upper and lower incisors and found large individual variation with mean values of 2.2 g/cm² for the upper incisors and 9.4 g/ cm² for the lower incisors.

Hellsing and L'Estrange32 measured these pressures in relation to head posture on 15 adult subjects with normal respiration. In conditions of normal head posture, the lip pressures were 3.5 g/cm² on the upper incisors and 8.5 g/cm² on the lower incisors according to the result of Thuer et al.31 During head extension, a significant increase of the lip pressure both on the upper incisors (between 0.8 g/cm² and 1.4 g/cm²) and on the lower incisors (between 1.17 g/cm² and 1.95 g/ cm²) was recorded. However, during head flexion the lip pressure decreased progressively.

Even though the lip pressure on the lower incisors increases in cases of extended craniocervical posture, in order to the equilibrium theory25 it is important to consider if a correlation exists between an increased the craniocervical angle and tongue forces. In 1981, Wood33 studied subjects with Class I occlusion and recorded the pressures of the anterior and posterior part of the tongue on the lower arch. When the subjects assumed a hyperextension of the head, the pressure decreased at the anterior part of the tongue.

In addition, Archer and Vig34 studied the modifications of anterior and posterior lingual pressures in relation to the head posture in 10 adult subjects with Class I malocclusion. They showed that the anterior lingual pressure on the lower arch decreased significantly when the subjects moved from a flexed to an extended head position.

All of these findings show that in cases of extended craniocervical posture, the equilibrium between lips, cheeks, and tongue on the lower incisors is altered. In fact, extended head posture creates a stretch of the oral soft tissues resulting in increased lip pressure and in decreased pressure of the anterior part of the tongue on the lower incisors. This “long-term” condition could modify the inclination of the lower incisors toward a lingual direction. In our opinion, it could be useful to improve this study by increasing the size of the sample group and by evaluating if a difference in inclination of incisors exists between patients with extended and flexed head posture.

CONCLUSIONS

• There is a statistically significant association between an increase of craniocervical angles and lower arch dental crowding.

• Thus, the head posture is another factor that could affect the occurrence of the dental crowding, an occlusal condition with a multifactorial etiology.