INTRODUCTION

Posterior crossbite is one of the most frequent malocclusions encountered by orthodontists during the primary, mixed, and permanent dentition stages. Its prevalence in the primary and mixed dentition stages ranges from 7% and 23%.^1–6 Posterior crossbite can be of skeletal or dental origin, either affecting one side, where it is called unilateral posterior crossbite, or affecting both sides, where it is called bilateral posterior crossbite. The classical literature suggests that posterior crossbite in children should be treated as early as possible to promote bilateral condylar symmetry and provide a favorable environment to allow normal growth and development to occur.⁷

If a functional posterior crossbite (FPXB) is left untreated, it can have deleterious effects on the development and function of the temporomandibular joints (TMJs). Skeletal remodeling of the TMJs can occur over time so that the condyles become more asymmetrically positioned in their fossae, and facial asymmetry and mandibular midline deviation toward the crossbite side might persist. Subsequent adaptation of the neuromusculature to the acquired mandibular position can cause asymmetric mandibular growth, facial disharmony, and severe skeletal crossbite in the permanent dentition. Some authors suggested that neither mandibular structure asymmetry nor condylar asymmetry would develop if a posterior crossbite was left untreated.^8,9 Several studies documented that a more symmetrical condyle position was achieved after treating posterior crossbite.^10,11 One other study reported minor changes of condylar position following posterior crossbite correction.¹²

The influence of untreated posterior crossbite on craniomandibular asymmetry varies from none to considerable. This considerable variation may be due to variation in study design, sample size, research approaches, and radiographic techniques used to investigate the condyle-fossa relationship, thus producing disparate outcomes among these investigations.

Mandibular and condylar asymmetries are believed to be a common feature of temporomandibular dysfunction (TMD).¹³ A recent study suggested a correlation between posterior crossbite with mandibular deviation and some signs and symptoms of TMD.^14,15 However, the direct role of posterior crossbite as a contributing factor in TMD is still controversial.

Because spontaneous correction of posterior crossbite is rare, early treatment has been recommended.^16–18 Maxillary expansion is the treatment choice for FPXB because it resolves the transverse maxillary deficiency and allows the mandible to regain a normal centric-intercuspal position. Several appliances are available for the correction of FPXB, such as rapid maxillary expanders (RMEs), slow maxillary expanders (SMEs), and semirapid expanders. Studies on the effects of posterior crossbite correction on the condyle are limited, and the question of whether posterior crossbite correction affects the condyle negatively or positively has still not received a clear answer. Therefore, the aim was to determine whether correction of posterior crossbite compared with nontreated crossbite results in actual changes of TMJs in growing patients.

MATERIALS AND METHODS

This systematic review was done following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement¹⁹ and Cochrane Handbook for Systematic Reviews of Interventions.²⁰ The protocol was registered at the Evidence Based Center, Faculty of Dentistry, Cairo University. The steps of screening the articles according to the title and abstract and full text, extraction of data, and quality assessment were done independently by the first two authors. Any conflict between the two authors was resolved by the third author if they did not reach a decision after discussion.

RESULTS

Study Selection

The electronic and manual search initially identified 2423 relevant records. After removal of duplicate citations, a total of 1591 articles were screened by reading titles and abstracts, and 1576 studies were excluded. Subsequently, the remaining 15 studies were read in full text for eligibility assessment according to the inclusion criteria. The excluded studies^7,10,22–32 and reasons for their exclusion are summarized in Table 1. Finally, two studies were included in this review: Lippold et al. (2008)¹¹ and McLeod et al. (2016).¹² The details of studies' inclusion are shown in the PRISMA flow diagram (Figure 1).

Table 1.  Articles Excluded After Full-Text Evaluation Based on Eligibility Criteria

View Fullscreen

View Figure

• Download as Powerpoint
• Download Figure

Study Characteristics

Both included studies were prospective RCTs and conducted in university settings. Both studies reported the age for the two groups separately, and the samples in both studies consisted of males and females. RME using the hyrax appliance was used in both studies. The outcomes of interest were measured using cone-beam computed tomography (CBCT) in one study¹² and ultrasound in the other.¹¹ Table 2 summarizes the characteristics of the included studies.

Table 2 Summary Table of Characteristics of the Included Studies^a

View Fullscreen

Table 2 Extended

View Fullscreen

RoB Within the Included Studies

This review included two RCTs. According to the Cochrane Risk of Bias Tool, both studies were judged as having low RoB for random-sequence generation and blinding of participants and personnel. Regarding allocation concealment and blinding of outcome assessment domains, both studies were judged as having unclear RoB, as it was not mentioned whether allocation concealment and blinding of outcome assessors were done or not. For incomplete outcome data domain, the Lippold et al. (2008)¹¹ study was judged as having low RoB, while it was judged as unclear for the other study¹² because of the imbalance in numbers of patients who were recruited from another clinical research project²⁹ without giving any reasons for this imbalance and attrition. Both studies were judged as having unclear RoB regarding selective reporting domain as study protocols were not available to identify any other unreported outcome. Sample size was not calculated in the McLeod et al. (2016)¹² study, whereas no standard deviations, confidence intervals, or exact P values were mentioned in the Lippold et al. (2008)¹¹ study. Therefore, both studies were considered as having unclear RoB in other bias domains. Eventually, both studies were judged to have an overall unclear RoB. The details of RoB assessment are summarized in Figure 2 and Appendix 2.

View Figure

• Download as Powerpoint
• Download Figure

Quality of the Evidence

Overall, the evidence for the outcomes evaluated by the GRADE system was assessed as very low quality, suggesting very little confidence in the effect estimate. The true effect was likely to be substantially different from the estimate of effect. The reasons for this judgment are outlined in Table 3.

Table 3 GRADE of Evidence: Does Posterior Crossbite Correction Compared With Untreated Posterior Crossbite Control Result in Temporomandibular Joint Changes?

View Fullscreen

DISCUSSION

Although many studies^7,10,11,22 recommended maxillary expansion to correct posterior crossbite to place the condyles in a more symmetrical position to avoid true asymmetry of the TMJs in the future, other authors^12,30 thought that there was no correlation between posterior crossbite correction and condylar position. Moreover, some authors^7,33,34 thought that correction of posterior crossbite using SME was superior to RME in placing the condyles in a more symmetrical position. This assumption does not rely on strong evidence as no evidence exists denoting that one type of expansion is superior to the others regarding its effect on TMJs, improving condylar position, and providing a good environment for proper growth of the TMJ to avoid any mandibular and condylar asymmetry.

RCTs, along with meta-analyses, are considered to be at the top of the hierarchy of research design and are followed by CCTs. In this review, only RCTs and prospective CCTs were included because poorly designed trials may give misleading results. Large amounts of data obtained from a poorly designed study cannot compensate for its poor design; also, combining poor-quality studies with the more rigorously conducted ones may not be useful and can result in misleading estimates of the underlying true effect. This review included only studies with control groups consisting of untreated growing patients with posterior crossbite. The control group consisted of patients exhibiting the same baseline characteristics as the experimental groups, except for the variable applied to the latter.³⁵ This has the advantage of eliminating and isolating confounding variables and bias and ruling out imagined or random effects of the treatment. Control groups with normal occlusion were considered invalid because they did not share baseline characteristics with treatment groups; therefore, all studies that included patients without posterior crossbite as a control group were excluded. Inclusion of an untreated control group ensures that the effect on the TMJs, if any, was only the result of treatment and excludes the effect of growth. These studies dealt with growing patients and made sure that no TMJ changes occurred spontaneously.

Although a comprehensive literature search was conducted, a shortage of high-quality clinical trials was evident. The two included studies were assessed as having unclear RoB. Because of methodological heterogeneity and the noncomparability of original data of the included studies, performing a meta-analysis was not possible. Lippold et al. (2008)¹¹ used ultrasound to determine condylar differences between centric and habitual occlusion before and after maxillary expansion. They noted statistically significant differences between the control and treatment groups after expansion. The treated patients exhibited a distinct improvement manifested as significant reduction in the differences between centric and habitual occlusion. On the other hand, no significant changes were observed in the control group.

McLeod et al. (2016)¹² evaluated condylar positions three-dimensionally before and after maxillary expansion using CBCT. They noted that most condylar positional changes were not significant between treatment and control groups. There were only two measurements that differed significantly between the treatment and control groups: left posterior condylar pole-foramen magnum distance (mean difference = 1.6 mm) and left posterior condylar pole-ELSA (mean difference = 0.82 mm).

Although there were minor TMJ changes in both of the reviewed studies, which were statistically significant for some measurements, these changes may have little impact clinically, especially considering that there was an absence of correlation between these positional TMJ changes and any TMJ clinical signs. Moreover, these changes were evaluated shortly after correction of posterior crossbite and may have been a temporary effect as there was no long-term follow-up.

The validity of the technique used for assessment and outcome measurements is crucial as it can greatly affect the findings observed. As suggested by a recent review,³⁶ CBCT is now considered as the most diagnostic, informative, and accurate method for evaluating the hard tissue components of the TMJ, and although McLeod et al. (2016)¹² used three-dimensional (3D) CBCT imaging, their findings were analyzed using 2D measurements. More accurate results would have been achieved if 3D evaluation of the condyle and the joint spaces were used. To the contrary, Lippold et al. (2008)¹¹ used ultrasonography, which is not the most informative method of measuring the outcome, and that is why the authors were not able to determine the direction of condylar deviation. To overcome these methodological problems in future research, CBCT should be used together with 3D measurements of the TMJ. A comprehensive 3D CBCT analysis of the TMJ developed by Alhammadi et al. (2015)³⁷ could be used to standardize the analysis and measurements used in evaluating TMJ changes. This analysis provides descriptive measurements for condylar position in the three orthogonal planes and can be used also for assessment of joint spaces and condylar symmetry. When studies with a standardized technique and evaluation method become available, their results can be combined in a meta-analysis to increase power and improve estimates of the size of the effect.

CONCLUSIONS

• There is a very-low-quality evidence regarding the positional and dimensional effects of posterior crossbite correction by maxillary expansion on the TMJs.

• There is no evidence available regarding the long-term effect of posterior crossbite correction on the TMJs.

• In the available literature, there is an absence of well-designed RCTs that could answer the current systematic review question.