Wenmin Ye DDS, MSD; Greg Huang DMD, MSD, MPH
Posterior crossbites may increase the likelihood of developing signs and symptoms of TMD.
While the evidence may suggest alterations in EMG and bite force, it is difficult to determine if these changes are clinically significant and predispose patients to future TMD.
For children in the primary and mixed dentition, is posterior crossbite associated with functional changes of the masticatory muscles?
The authors searched the Medline database from January 1965 to February 2008. They also searched reference lists. The inclusion criteria were: 1) human studies; 2) primary and early mixed dentition with posterior crossbite; 3) randomized controlled trials, controlled clinical trials and prospective studies; 4) articles in English language. The authors assessed the following outcomes: bite force, electromyographic (EMG) activity of masticatory muscles, and signs and symptoms of temporomandibular disorders (TMD). Two reviewers independently assessed studies for inclusion, extracted the data and critiqued the study quality.
The authors identified 494 articles, 8 of which met the inclusion criteria and were classified as: 6 controlled clinical trials, 1 randomized clinical trial (RCT), and 1 prospective study. A quality evaluation of the included articles revealed a mix of low, medium and high quality studies. Five of the studies evaluated bite force, 2 of them reported EMG differences during mastication, and 4 studies evaluated TMD signs and symptoms. The authors’ review stated that these studies found children with posterior crossbite to have reduced bite force and asymmetrical muscle function during chewing or clenching, and a significant association between posterior crossbite and TMD symptoms.
Based on the literature that the authors retrieved, they concluded that posterior crossbite was associated with reduced bite force and increased risk for TMD in children. However, the evidence was generally weak, with most studies including only 10 – 30 subjects with crossbites.
Source of Funding:
Importance and Context:
Orthodontists routinely advise patients to correct unilateral posterior crossbite during the mixed or early permanent dentition, with the rationale that uncorrected crossbite may cause asymmetric maxillo-mandibular relationship, and/or alteration of the disk-condyle relationship. This review attempted to summarize the present literature regarding the functional changes in bite force and masticatory muscle activity associated with posterior crossbite.
Strengths and Weaknesses of the Systematic Review:
This systematic review had several limitations: 1) only 1 database was searched for articles published in only 1 language; 2) the lists of included and excluded studies supported by reasons were not provided; and 3) it appears that several studies may have been incorrectly classified. In fact, the study that was described as a randomized controlled trial was actually characterized by the original authors as a cross-sectional study. Several other studies categorized as controlled clinical trials by the systematic review authors appear to be more accurately described as cross-sectional studies (1-3). If cross-sectional studies were eligible for inclusion, the authors should have considered several other cross-sectional studies for inclusion (4-6). Additionally, most of the outcomes, such as bite force and EMG activity, are signs and not symptoms of TMD. It may be inaccurate to conclude that these functional changes will lead to TMD symptoms.
Strengths and Weaknesses of the Evidence:
The evidence is rather weak on this topic, with most studies actually being cross-sectional studies rather than clinical trials. Also, most of the studies enrolled less than 30 subjects with crossbites. Additionally, the authors failed to include 3 large, cross-sectional studies in the review (4-6).
Implications for Dental Practice:
Although the current literature provides limited evidence that posterior crossbite is related to changes in EMG and bite force, it does not provide strong evidence that posterior cross-bites lead to TMD symptoms in the mixed dentition. In fact, 2 large, cross-sectional studies did not find a relationship between posterior crossbite and TMD in young children (4,5). It is important to differentiate functional changes based on surrogate outcomes from clinical TMD signs and symptoms. REFERENCES  Alarcon JA, Martin C, Palma JC. Effect of unilateral posterior crossbite on the electromyographic acitivity of human masticatory muscles. Am J Orthod Dentofacial Orthop. 2000; 118:328-34.  Rentes AM, Gaviao MB, Amaral JR. Bite force determination in children with primary dentition. J Oral Rehabil. 2002; 29:1174-1180.  Castelo PM, Gaviao MB, Pereira LJ, Bonjardim LR. Masticatory muscle thickness, bite force and occlusal contacts in young children with unilateral posterior crossbite. Eur J Orthod. 2007; 29:149-156.  Keeling SD, McGorray S, Wheeler TT, King, GJ. Risk factors associated with temporomandibular joint sounds in children 6 to 12 years of age. Am J Orthod Dentofacial Orthop. 1994 Mar; 105(3): 279-87.  Farella M, Michelotti A, Lodice G, Milani S, Martina, R. Unilateral posterior crossbite is not associated with TMJ clicking in young adolescents. J Dent Res. 2007; 86(2): 137-41.  Thilander B, Rubio G, Pena L, de Mayorga C. Prevalence of temporomandibular dysfunction and its association with malocclusion in children and adolescents: an epidemiologic study related to specified stages of dental development. Angle Ortho. 2002; 72(2) 146-54.