Clinical indications for the diagnosis and treatment of functional posterior crossbite in pediatric population: a narrative review with clinical description

3.1 Aetiology

The development of FPXB involves a combination of dental-skeletal and neuromuscular functional factors. A small maxillary to mandibular intermolar dental width ratio stands out as the predominant variable associated with FPXB, with its origin potentially rooted in genetic or environmental influences. However, environmental factors appear to exert a more substantial influence on functional aspects during the developmental phase. Notably, conditions such as upper airway obstruction, characterized by hypertrophied adenoids or tonsils, and allergic rhinitis leading to mouth breathing, have demonstrated correlations with the initiation of posterior crossbites [21, 22]. These subjects often present an increased lower face height due to the adaptive changes in head posture caused by chronic oral breathing patterns [23]. Among environmental factors, non-nutritive sucking habits can contribute to the development of posterior crossbites [24, 25]; solid evidence would confirm that if sucking habits persist up to or beyond 48 months 7 up to 10 children develop anterior open bite, posterior crossbite or increased overjet, with posterior crossbite accounting for 29% of the malocclusions observed when sucking habit persists beyond 4 years of age [19].

Both functional and non-nutritive habits negatively influence the transverse growth of the maxilla (along the midpalatal suture) or the expansion of the alveolar processes and dento-alveolar arch at an early age, determining similar signs of malocclusion such as crossbite, anterior open bite and increased overjet. In this regard, factors such as tongue position and size, mouth breathing, non-nutritive sucking behaviors (such as digit or pacifier sucking), and jaw-posture habits play roles in the emergence of a posterior crossbite [24, 25, 26]. However, functional and non-nutritive habits can differently influence the vertical and sagittal skeletal facial growth patterns; for example, pacifier users seem to be more susceptible to developing dento-alveolar deformities, at least at the early stage, while oral breathers can develop greater lower face height with facial skeletal divergence and long-face syndrome (Fig. 1A–F). Such difference underlines the necessity for an extremely accurate differential diagnosis for planning the appropriate orthodontic treatment strategy, especially when they are in combination with other treatments. In case of oral breathing patterns, patients should be referred to the otolaryngologist to identify the site and the reason for increased airway resistance, and the orthodontic treatment should be coordinated with such information in order to avoid the risk for relapse; instead, pacifier users without significant skeletal disharmonies may benefit from interceptive orthodontic treatment supported by myofunctional therapy. In this regard, it should be considered to implement habit correction strategies. Ideally, these strategies should be applied before initiating orthodontic treatment.

Fig. 1.Example of different phenotypes of subjects affected by posterior crossbite and anterior open bite. (A–C) Oral breathers with skeletal higher facial angle and long-face syndrome. (D–F) Pacifier users with normal vertical facial growth developing dento-alveolar deformities.

Moreover, when an ectopic eruption occurs in the primary or permanent dentition, it can contribute to the development of crossbite. Ectopic eruption can occur when there is inadequate space in the dental arch for the tooth to emerge properly. This might happen due to factors such as crowding of teeth, small jaw size, or the presence of supernumerary teeth. When a tooth erupts in an abnormal position due to space constraints, it can potentially lead to crossbite by interfering with the normal alignment of opposing teeth [27, 28, 29].

It is important to point out that associations between nonnutritive sucking habits, airway obstructions, neonatal intubation and posterior crossbites do not necessarily suggest a cause-and-effect relation [20].

3.2 Diagnosis

Functional posterior crossbite (FPXB) is clinically characterized by a unilateral crossbite accompanied by a functional shift of the mandible toward the side affected by the crossbite. The lateral shift of the mandible in functional crossbite (FXB) leads to a skeletal (and often dental) midline deflection toward the side of the crossbite. It is often associated with transverse maxillary constriction with marginal ridges in line and the absence of simple dental crossbite while assessing inter-arch relationships in centric occlusion (CO). In this regard, FPXB differs from bilateral crossbite (CB) in the degree of severity of maxillary constriction (skeletal or dento-alveolar) since FXB exhibits a lesser maxillary to mandibular width discrepancy [30].

In Functional Posterior Crossbite (FPXB), the side affected by the crossbite typically exhibits a partial or complete Class II molar relationship, while the non-crossbite side demonstrates a Class I relationship due to rotational mandibular closure (Fig. 2A–C). This condition is associated with an asymmetric condyle position, where the non-crossbite side is situated downward and forward in the fossa, while the crossbite side is centered within the fossa [4, 31]. In FPXB, an evident discrepancy exists between centric occlusion (CO) and centric relation (CR). Hence, it is crucial to conduct a differential diagnosis with a true unilateral crossbite, wherein CO and CR usually align. This differentiation can be readily established by assessing the transverse inter-arch relationship in CR, confirming the presence of functional mandibular asymmetry (FPXB) or intra-arch asymmetry (true unilateral posterior crossbite (UPXB)) causing the unilateral crossbite (Fig. 2D).

Fig. 2.Example of FPXB. (A–C) The side affected by the crossbite typically exhibits a partial or complete Class II molar relationship, while the non-crossbite side demonstrates a Class I relationship due to rotational mandibular closure. (A,D) Discrepancy exists between centric occlusion (CO) and centric relation (CR): This differentiation can be readily established by assessing the transverse inter-arch relationship in CR, confirming the presence of functional mandibular asymmetry (FPXB) or intra-arch asymmetry (true UPXB) causing the unilateral crossbite.

Additionally, extra-oral examination may reveal asymmetry of the mandibular chin concerning the facial midline, thereby diminishing facial aesthetics. Such distinction is important not only to establish the appropriate treatment but also to plan the appropriate biomechanics since, in case of maxillary expansion, the appliance should provide more expansion on the CB side in true UPXB whereas conventional symmetric appliance framework is generally adequate for treating both FPXB and bi-lateral crossbite (Fig. 3A–D) [10].

Fig. 3.Differential diagnosis between FPXB and true UPXB. (A,B) FPXB featuring mild maxillary constriction and symmetric morphology of the palate. (C,D) true UPXB featuring maxillary constriction with significant reduction of the hemilateral diameter on the crossbite side.

Despite the recent enhancement in digital radiology and 3D imaging systems that can provide a detailed analysis of anatomical structures, the diagnosis of FPXB is still based mainly on clinical inspection. Concerning the radiological dataset, it is important to adequate the radiation exposure to the clinical necessity for diagnosis and treatment plan, according to the A.L.A.R.A. (As Low As Reasonably Achievable) and A.L.A.D.A. (As Low As Diagnostically Acceptable) principles [32]. Some indications for the use of 3D image examinations in these cases could be the management of impacted canines, transpositions, or ectopic and impacted teeth [33]. In this regard, in the absence of relevant significant skeletal disharmonies or suspicious oral breathing patterns, the cephalogram cannot be prescribed.

3.3 Indications for early treatment

3.3.2 Compensatory inter-arch asymmetry

The available evidence suggests that FPXB determines an asymmetric position of the condyles in the glenoid fossa when compared to subjects with normal occlusion [4, 31], with restoration of symmetry following early treatment [39, 40]. Instead, untreated posterior crossbites in adult patients result in alterations in mandibular symmetry and rotational position relative to the cranial base [41, 42, 43]. For this reason, as previously mentioned, early treatment of FPXB is advocated to avoid the persistency of the condylar asymmetry during the growing stage which could interfere with mandibular function and bone remodeling, and could muscle, skeletal, and joint adaptations in crossbites primarily occur during early development [44]. In this regard, early correction of FPXB by expanding the maxilla has been demonstrated to establish condyle and dental symmetry [39] and realign mandibular rotation [4].

Most of the studies analyzing the asymmetry in subjects with FPXB had been focused on the mandible [31, 45]. On the contrary, few studies have focused on the maxillary morphology and reported that FPXB in mixed dentition could be associated with asymmetric development of the maxillary arch [10, 46]. In this regard, recent evidence would suggest that FPXB can be associated with an asymmetric pattern of development of the maxilla [10]. Using the digital surface analysis technique, it was found that FPXB in mixed dentition could be associated with a symmetric contraction of the basal bone and asymmetry of the alveolar processes, with the crossbite side being narrower than the non-crossbite side. Such asymmetry may develop as an adaptive and compensatory process aiming at invalidating functional deviation caused by the mandibular shift toward the crossbite side (Fig. 4A–F) [47].

Fig. 4.Example of digital procedure used to assess palate morphology. (A–C) Palate segmentation. (D,E) Palate model mirroring. (F) Superimposition of the original model with the mirrored model with deviation analysis. Using this analysis, recent studies have confirmed that subjects with FPXB present a symmetric mild constriction of the palate with asymmetric development of the alveolar processes, being that the CB side is narrower than the non-CB side due to functional occlusal adaptations. Green color = range of tolerance, blue-tone colors = negative values (constriction), red-tone colors = positive values (expansion).

This means that subjects with FPXB often present symmetric skeletal constriction of the palate with asymmetric morphology of the alveolar process. This recent evidence is of great clinical relevance when considering the future application of orthodontic biomechanics in permanent dentition procedures (see treatment section), and introduces the risk of maxillary asymmetry as a factor for establishing early treatment. In this regard, even more recent evidence suggested that maxillary asymmetry improves one year after treatment of FPXB with rapid maxillary expansion [47]. Thus, although the primary goal of treating FPXB is the correction of the malocclusion and of the associated altered mandibular function, recent evidence would suggest that the early treatment of this condition is important to favor a more harmonious and symmetric development of the palate.

3.3.3 Facial aesthetics

In the era of social media and network communication, facial and smile attractiveness represents an essential factor supporting self-esteem and social relationships, even among young individuals [48]. Being focused on facial attractiveness, laypersons are increasingly becoming familiar with concepts of facial and soft-tissue aesthetics. This new awareness explains why specific skeletal malocclusions are consistently perceived as aesthetically displeasing by patients or their caregivers, potentially causing varying degrees of distress based on individual values and self-confidence [49, 50]. In the case of FPXB, the lateral shift of the mandible generates the asymmetry of the lower facial third upon closure, leading to lower midline discrepancy and chin deviation toward the side affected by the crossbite (Fig. 5A–D). For this reason, it is not rare that parents or caregivers seek orthodontic consultation after having noticed the appearance of mandibular asymmetry. In this regard, although functional adaptations represent the primary indications for early treatment of FPXB, the awareness of aesthetic deterioration from parents or caregivers could represent an important factor for requiring orthodontic consultation.

Fig. 5.Lower facial asymmetry (mandibular shift) in a young female featuring FPXB. (A) Extra-oral frontal view. (B–D) Intra-oral photographs.

3.4 Treatment options

Concerning treatment in the primary dentition, selective grinding is considered helpful to correct FPXB, as long as there are no signs of transverse maxillary arch constriction, i.e., inter-canine width differential >3.3 mm and no discrepancy in intermolar width (<1 mm) [19]. In this regard, posterior crossbites with an intermolar arch width differential exceeding one millimeter will necessitate upper-arch expansion through the use of a removable or fixed appliance, even supported by selective grinding. The prevalent and effective expansion appliances employed in the primary dentition encompass the removable upper expansion plate with a jackscrew and fixed lingual arch appliances. Despite numerous studies exploring the impacts of correcting posterior crossbites, the clinical significance of most outcomes remains inconclusive. For mild forms of unilateral posterior crossbite linked to a functional shift, selective grinding of premature contacts in primary teeth stands out as the sole clinically validated treatment approach for correction in the primary dentition. Another approach, developed by Prof. Pedro Planas, consists of the additive technique involving the use of crowns for managing crossbite in deciduous teeth, a method known as Planas Direct Tracks (PDTs) [53]. PDTs work by creating inclined planes on the teeth using composite resin, which guides the mandible into a corrected position during function. This approach helps modify the child’s chewing cycle and encourages bilateral chewing, thus correcting the malocclusion over time. It is a cost-effective and relatively simple technique that can be performed by general dentists, making it accessible even in public health settings [54].

Since the main cause of FPXB is a mild constriction of the maxilla, the routine treatment of this condition requires the increment of the upper arch diameters, in the form of skeletal or dento-alveolar expansion, depending on the etiology of the malocclusion.

Concerning dento-alveolar expansion, quad-helix or bi-helix appliances can represent a valid treatment option since they do not require the patient’s compliance. Generally, it is sufficient to gently press the appliance framework between anterior rings using a three-peak plier to obtain a symmetric dento-alveolar expansion (Fig. 6A–D). However, this procedure can be accomplished at their chairside by the orthodontist or pediatric dentist. Alternatively, a removable appliance including a jackscrew can be a valid alternative (Fig. 7A,B). In this case, parents or caregivers can perform the appliance activation at home; the turning frequency of the jack-screw should be set to 1 activation each every fifth to seventh day to favor dento-alveolar adaptation, avoiding the risk for appliance displacement occurring with faster activations. The appliance must be made with well-fitting clasps to prevent displacement. The removable appliance can be designed with an asymmetric cut of the resin which introduces different anchorage support between both sides, in case of necessity for slight asymmetric expansion. Nevertheless, caution should be used when opting for a removable appliance since poor compliance may result in a relapse of the previous expansion and lower success rates [55].

Fig. 6.Example of quad-helix appliance and activation for dento-alveolar expansion. (A,B) Extra-oral activation. (C) Drawing showing the appropriate site to apply bends. Generally, it is sufficient to gently press the appliance framework between anterior rings using a three-peak plier to obtain a symmetric dentoalveolar expansion. (D) Intra-oral view.

Fig. 7.Removable appliance including jackscrew can be a valid alternative. (A) Occlusal view. (B) Inner view.

To obtain skeletal expansion at the midpalatal suture, fixed expanders, in the form of Haas or hyrax designs are generally used (Fig. 8A,B). The resin included in the Hass expander could be helpful to mitigate the transmission of forces to the dentition due to the support of the palatal vault. In the mixed dentition stage, a slow expansion protocol is sufficient to generate skeletal expansion of the maxilla. For this reason, the rate of expansion is a quarter turn of the screw every two or three days and the estimated time to correct the crossbite is 6–12 weeks.

Fig. 8.Designs of fixed maxillary expanders used for opening mid-palatal suture. (A) Haas expander. (B) Hyrax expander.

If crossbite was not intercepted previously in the mixed dentition, rapid maxillary expansion is advocated in the early permanent dentition stage, due to its ability to induce a more significant degree of skeletal expansion and result in less dental tipping compared to other slower expansion protocols. The recommended rate of expansion is 1–2 quarter revolutions of the screw per day, and the anticipated duration for correcting the crossbite ranges from 2 to 6 weeks. Patients should be informed that an initial midline maxillary diastema will be created. Throughout the retention phase of treatment, this diastema will gradually close, often involving dental tipping as transeptal fibers approximate the central incisors. The usage of customized computer-aided design and computer-aided manufacturing (CAD-CAM) molar bands can be helpful in increasing the efficiency of the crossbite correction since it is possible to include an occlusal metal bite-block that allows to start of the expansion from the CR, that is, without the of presence occlusal interferences (Fig. 9A–D). Moreover, rapid maxillary expansion may also be performed in the late mixed dentition, as in this stage the median palatine suture may not be completely ossified (variations between patients). In fact, it has been demonstrated that there are different stages of maturation of the median palatine suture and that the most advanced stages of ossification are reached at ages often subsequent to those of the late mixed dentistry [56].

Fig. 9.Example of customized CAD-CAM molar bands for maxillary expansion. (A–C) Occlusal metal bite-block can help increase the efficiency of the crossbite correction since it is possible to start the expansion from the CR without the of presence occlusal interferences. (D) Occlusal view of metal bites block included into the customized bands.

Figs. 10,11,12,13 describe a full case of a child affected by FPXB treated with a maxillary expander featuring customized CAD-CAM molar bands.

Fig. 10.Extra-oral photographs of a male child presenting FPXB. (A) Frontal view in the rest position. (B) Frontal view while smiling. (C) Right profile view.

Fig. 11.Intra-oral photographs of a male child presenting FPXB. (A) Right lateral view. (B) Frontal view. (C) Left lateral view. (D) Upper occlusal view. (E) Lower occlusal view.

Fig. 12.Intra-oral photographs of the post-expansion stage. (A) Right lateral view. (B) Frontal view. (C) Left lateral view.

Fig. 13.Intra-oral photographs of the final retention stage. (A) Right lateral view. (B) Frontal view. (C) Left lateral view. (D) Upper occlusal view. (E) Lower occlusal view.

Both slow and rapid maxillary expansion require overexpansion, reaching a stage where the lingual cusps of the upper molars make contact with the buccal cusps of the lower molars. The appliance should be retained in situ for an extended duration, ranging from 4 to 6 months, and at least equivalent to the time necessary for correcting the crossbite. In cases where a screw functions as the active mechanism, stabilization can be achieved using a ligature wire or composite material to prevent relapse. However, although rapid maxillary expansion results in overall increases in maxillary arch widths, it has been demonstrated that some of the width gains achieved will be reduced during the fixed appliance treatment, and significant relapse will be reasonably observed over the long term, particularly in the inter-canine width [57]. Since at this stage (permanent dentition) patients could feature a consolidated adaptive compensatory asymmetry of the alveolar processes [10, 47, 58], it is possible that after completion of the expansion protocol, they reach overexpansion at the non-CB side and insufficient correction of the fossa-cusp relationship at the CB side. For this reason, the asymmetric design of ancillary components can be integrated into the appliance framework to increase the anchorage at the non-CB side and favor more dentoalveolar expansion at the CB side, net of skeletal expansion occurring at the midpalatal suture (Fig. 14A–D).

Fig. 14.Example of extreme alveolar adaptation to the FPXB in a young female in permanent dentition. In this sub-optimal clinical condition, it is possible to include a resin pad on the non-CB side to favor dissipation of the forces exerted on the dentition during maxillary expansion, to recover a symmetric morphology of the alveolar processes (more expansion on the CB side). (A) Pre-treatment condition (asymmetry). (B) Expander in place with asymmetric design of the resin pad. (C) Post-expansion stage. (D) Post-retention stage with symmetric palate morphology almost recovery.

Given the multifactorial nature of the transverse skeletal discrepancy in the upper maxilla, before initiating maxillary expansion, it is crucial to adopt a multidisciplinary approach involving the otolaryngologist and speech therapist. As the pediatric patient is still in an early developmental stage, eliminating the etiological factors responsible for maxillary constriction may allow for natural width development once the causative factors are removed. In such circumstances, since the primary goal of the orthodontist is to eliminate occlusal interferences, they may opt for minimal expansion in conjunction with occlusal grinding in the deciduous dentition. Subsequently, palatal expansion could be considered if the removal of etiological factors proves insufficient in restoring the width growth of the upper maxilla.

Another group of approaches includes skeletal anchorage systems which are advanced orthodontic solutions that use mini-implants or mini-screws to provide a stable and reliable anchor for various dental movements. Bone anchorage options for treating posterior crossbite include the hybrid Hyrax and micro-implant-assisted rapid palatal expansion (MARPE). Common outcomes of bone-anchored treatments include consistent disruption of the circummaxillary suture, greater forward displacement of the maxilla, fewer adverse effects from tooth-borne expansion, reduced clockwise rotation of the mandibular plane, and suitability for postpuberty or more mature patients [59]. These devices are particularly effective in promoting larger and more successful skeletal growth in adolescents compared to traditional dental-anchored expanders. However, the placement of mini-screws is a minor surgical procedure, which can be intimidating for children, and complications such as infection, implant failure or irritation could occur. Moreover, skeletal anchorage systems can be more expensive compared to traditional orthodontic appliances. Finally, in younger patients, alternative treatments such as maxillary expanders anchored on deciduous teeth can achieve effective and stable results without the need for skeletal anchorage. This more conservative approach respects natural dental development and avoids the risks associated with more aggressive skeletal interventions, making it more appropriate and safer for children [60, 61].

Tables 1 and 2 summarize the key aspects of various treatment options for managing posterior crossbite in children, highlighting their indications, advantages, and disadvantages to aid in treatment planning and decision-making.

Figs. 15,16,17 show the decisional algorithm involved in the clinical management of unilateral posterior crossbite in growing individuals.

Fig. 15.Decisional algorithm for treating unilateral posterior crossbite in the deciduous dentition. CO-CR: centric occlusion-centric relation; FPXB: functional posterior crossbite; UPXB: unilateral posterior crossbite; UA: upper arch.

Fig. 16.Decisional algorithm for treating unilateral posterior crossbite in the mixed dentition. CO-CR: centric occlusion-centric relation; FPXB: functional posterior crossbite; UPXB: unilateral posterior crossbite; UA: upper arch; LA: lower arch.

Fig. 17.Decisional algorithm for treating unilateral posterior crossbite in the permanent dentition. CO-CR: centric occlusion-centric relation; FPXB: functional posterior crossbite; UPXB: unilateral posterior crossbite; UA: upper arch; LA: lower arch.