Traditionally, facial asymmetry has been classified in many ways. Among these, the following categorization is the most feasible. According to Fonseca and Turvey’s facial asymmetry classification, facial asymmetry can be categorized into four types (Fig. 13.1) [1, 2].
However, in my opinion, one more category is needed: craniofacial asymmetry originating from asymmetry of the cranial base, which would be the most difficult facial asymmetry to manage. Therefore, I suggest this modified classification of facial asymmetry.
Pseudo facial asymmetry is not true facial asymmetry, despite the asymmetric positioning of the mandible. Occlusal interference, habitual posturing, condylar dislocation, or dystonia are potential causes. As a result, the condyle is displaced forward relative to centric relation (CR) in the fossa. Conversely, true asymmetry involves facial asymmetry that exists while the condyle is seated in its original temporomandibular joint (TMJ) CR. Therefore, in this situation, the asymmetry would not be corrected although the condyle was situated in a gentle superoposterior position (Figs. 13.2, 13.3).
Developmental asymmetry is the non-pathologic, non-syndromic development of facial asymmetry. Genetics, intrauterine molding, or natural growth variances may cause developmental facial asymmetry. Usually, the facial asymmetry is present at birth but is not identified until later. To be categorized as developmental facial asymmetry, TMJ-related pathology or symptoms should be absent. Both condyles may be approximately equal in size and shape and most patients demonstrate similar condylar proportions during growth.
Unilateral mandibular hyperplasia is a typical example of overdevelopmental facial asymmetry. The ipsilateral mandibular ramus, body, and parasymphysis, including the condyle, are enlarged while the contralateral mandible appears normal. Specifically, unilateral condyle hyperplasia is a good example of overdevelopmental facial asymmetry.
The etiologies of underdevelopmental facial asymmetry are diverse. This type of asymmetry can be categorized as congenital, acquired facial asymmetry, adolescent internal condylar resorption, or as a connective tissue disorder.
Congenital underdevelopmental facial asymmetry includes unilateral cleft lip and palate, hemifacial microsomia, and Treacher Collins syndrome. Acquired underdevelopmental facial asymmetry may result from trauma, infection, or ankylosis. Idiopathic condyle resorption (ICR) presents unilaterally and can also cause underdevelopmental facial asymmetry.
Traditionally, craniofacial asymmetry has not been described as a category of facial asymmetry in textbooks or in the literature. However, I strongly suggest this as a separate categorization because craniofacial asymmetry has quite unique features from other types of facial asymmetry and is the most challenging to correct. Some patients can present with a twisted face.