The extrusion arch has been suggested as a predictable one-couple appliance that can be used for the treatment of AOB without the need for patient compliance [10, 12]. No prospective clinical study on its use has been previously published and therefore, there was no attempt to include to the present study a different treated group of patients, i.e., spurs and/or palatal cribs. Our main objective of this prospective clinical study was to quantify the relative maxillary incisor and molar movements in AOB patients treated using these mechanics by means of lateral cephalograms and cast models. The hypothesis of the study was accepted since the extrusion arch achieved successful closure of AOB by maxillary incisor extrusion with predictable side effects of molar anterior tipping and incisor uprighting. Our hypothesis is that with an extrusion arch the correction of AOB occurs within 5 to 8 months due to the forced extrusion of maxillary incisors. No spurs or tongue crib were used in our patients in order to interrupt sucking and thrusting habits, which may allow normal vertical development at the anterior region by elimination of the tongue contact.
Extrusion arch mechanics produced a mean maxillary incisor extrusion of 1.94 mm during a mean treatment period of 7.79 months. Previous studies observed similar results with other types of appliances over a 12-month period [8, 9, 14, 16, 17]. The removable palatal crib (RPC) resulted in incisor extrusion ranging from 1.64 mm [8], 2.47 mm [9], and 2.98 mm [17], while the use of bonded spurs (BS) promoted changes of 1.50 mm [8], 2.35 mm [9], 3.16 mm [16], and 2.33 mm [17] when associated with a chincup (CC) [16].
In the current study, overbite increased 3.07 mm in 7.79 months, with a final mean overbite of 1.79 mm. A previous study observed 3.51 mm of overbite correction and a final overbite of 0.57 mm with the use of fixed palatal crib (FPC) and 3.88 mm of correction and 0.84 mm of final overbite with the FPC over a period of 1 year [15]. These minor differences can be explained by the shorter treatment time of the present study and the fact that the use of the palatal crib also helped with the postural reeducation of the tongue, which does not occur with the extrusion arch mechanics. On the basis of these findings, it could be recommended that the extrusion arch be supplemented with appliances designed to alter tongue positioning in patients with a tongue thrust habit after the closure of AOB is accomplished.
In addition to the extrusive vector, the one-couple mechanics produced an uprighting moment on the maxillary incisors [10, 12]. This effect is favorable in most cases of AOB, which are usually accompanied by incisor labial proclination. In the present study, there was significant palatal tipping of the maxillary incisors (− 6.15 ± 6.42°). A previous study [18] observed a non-significant palatal tipping of the incisors with the use of palatal cribs (− 1.77°) and spurs (− 4.1°). Also, in the current study, a significant increase of 5.57° in the interincisal angle was observed, due to the retroclination of the anterior teeth. Similarly, previous studies observed an increase in the interincisal angle ranging from 9.66° (palatal crib and chincup) [16], 3.34° (BS), 9.65° (FPC), 7.01° (RPC), and 4.25° (CC) [8].
As a consequence of incisor retroclination, a significant reduction (− 1.65 mm) of overjet was also observed. These results were in contrast to those of previous studies [15, 16] which showed no significant differences in the overjet in patients treated with the RPC and FPC [15, 16]. In addition to overjet reduction, arch perimeter and length were also significantly reduced (− 3.02 mm and − 2.23 mm, respectively). These results were similar to a previous study [15], where an arch perimeter reduction of 0.92 mm and 0.38 mm and 1.34 mm and 0.52 mm arch length reduction were reported, with the use of the RPC and FPC, respectively. Additionally, it was shown that the palatal crib produced arch perimeter and length reductions of 2.6 mm and 1.4 mm, respectively [19]. In the present study, the arch perimeter and length reductions were greater in magnitude compared to previous studies. The most likely explanation for this finding was that the extrusion arch exerted a force on the maxillary incisors located anterior to the center of resistance, thus resulting in a moment tending to upright the anterior teeth. The palatal crib or spurs, on the other hand, promoted only passive extrusion of the incisors due to the habit cessation and postural reeducation of the tongue and upper lip posture, thus allowing extrusive movement of the incisors without a significant decrease in arch length or perimeter.
During extrusive movement of the incisors, the periodontium (gingiva) may not follow the teeth, with a possible increase in the clinical crown height. A small, but statistically significant increase of 0.33 mm in the incisor clinical crown height was observed. However, this was not considered to be clinically significant because this minor increase does not compromise periodontal integrity. Also, this change may be expected since mixed dentition patients often exhibit an under development of the vertical position of the maxillary incisors prior to AOB closure. Unfortunately, no other previous study investigated the amount of vertical changes in relation to the gingiva and clinical crown height occurred during the use of an extrusion arch.
In contrast to the extrusive force vector on the incisors, an intrusive force was generated on the molars. This would be expected to occur as a side effect of the one-couple force system [12]. A non-significant intrusion of − 1.01 mm was observed at the molars. This was contrary to previous studies which observed molar extrusion while using CC [8, 14, 17], BS, and PC [12]. A more unfavorable movement of the maxillary molars was due to the counter-clockwise couple produced as part of the extrusion arch force system, which promoted anterior tipping of the molars [10, 12]. Significant maxillary molar mesial inclination of 11.49° was observed. This side effect also contributed toward the reductions in arch length and perimeter, even though a transpalatal bar was used in all patients. The reduction in arch length and perimeter might cause some loss of space for maxillary permanent canines’ eruption, which called for attention during the transition to the permanent dentition. Based on these findings, care should be taken in order to avoid these effects if mesial inclination or intrusion of the maxillary molars is not desired. Perhaps, bonding the posterior deciduous teeth and using a rigid buccal segment archwire to reinforce anchorage might prevent these side effects during use of the extrusion arch. In order to eliminate the mesial maxillary molar inclination, temporary anchorage device such as miniscrews to serve as indirect anchorage could also be used. Another way to reduce side effects on the maxillary molars would be to decrease the extrusive force of the extrusion arch. As mentioned before, our extrusion arch made of beta-titanium alloy 0.017 × 0.025-in. delivered 40–60 g of force on the anterior teeth, which can be considered by some authors too high. Uribe et al. [20] advocated lower forces (30–40 g) in order to minimize side effects on maxillary molars. Thus, Uribe et al. [20] indicated to insert the Connecticut intrusion arch upside-down, which is a preformed archwire made of nickel-titanium exerting 35 g of force in order to treat an AOB.
No significant changes in the transverse dimension were observed in the present study. This result was in contrast to that observed for palatal cribs, which showed a significant increase in the transverse dimension [15]. In the present study, a transpalatal bar might have maintained the intermolar distance during AOB closure.
Overall, the extrusion arch showed predictable results, closing the AOB in 100% of the patients. Overbite correction occurred mainly from maxillary incisor extrusion and retroclination. Mesial tipping of the maxillary molars due to the counter-clockwise couple produced by the appliance should be monitored.
Limitations
Some limitations of this study included the severity of the anterior open bite in the patients enrolled and the short-term evaluation period. It is important to recognize that the results reported were obtained when treating a sample of dental AOB malocclusions; thus, they cannot be extrapolated to skeletal AOB subjects. Long-term studies are also recommended to assess potentially increasing posterior crowding and stability of the results.