Introduction

The growth of the mandible and maxilla hinge on sound development of the soft tissue function and the jaw function. Any condition interfering with the development of the functional matrices or directly affecting bone formation may cause changes in the growth (size) and development (morphology) of the mandible and maxilla, termed dentofacial deformity. By definition, acquired dentofacial deformities occur after birth. Dentofacial deformities may vary in expression and severity and often involve an element of skeletal and dental asymmetry.

Diseases causing disturbances in dentofacial growth and development may originate from local or systemic conditions and often involve soft tissue and/or the temporomandibular joint (TMJ). Various underlying pathologies may lead to a similar morphological expression of dentofacial deformity. In order to understand the underlying etiologies of acquired dentofacial and dental asymmetries, it is important to first understand the unique properties of TMJ development to appreciate how different conditions may impact dentofacial growth. This chapter, therefore, begins with a description of TMJ growth and development and its association with asymmetric dentofacial development.

The Temporomandibular Joint (TMJ) and Its Association with Asymmetric Dentofacial Development

The TMJ is a synovial joint with unique properties related to histology, anatomy, biology, and function (Owtad et al. 2013). These unique properties are important for normal TMJ growth and function but are also critical in situations in which acquired conditions interfere with normal dentofacial development.

Temporomandibular Joint (TMJ) and Mandibular Postnatal Growth and Development

In the long bones of the appendicular skeleton, the bone forms in the epiphysis situated near both ends of the bone, whereas the cartilage in the developing joint derives from the original hyaline cartilage model of the developing bone. Hyaline is a primary cartilage and consists mainly of type II collagen. During compression and inactivity, hyaline cartilage will remain largely intact and unaffected because of its high type II collagen content, whereas fibrocartilage will potentially degenerate because of its high type I collagen content. Furthermore, the fibrocartilage on the TMJ condyle serves as a basis for generating bone by appositional proliferation. Hence, the fibrocartilage is an important mandibular growth site. The TMJ condyle develops by remodeling, apposition, and resorption, based on cartilage proliferation, whereas the ramus and mandibular body grow and develop by periosteal appositional and resorptive activity. The bone‐forming activity related to the mandibular condyle is characterized as endochondral ossification. Endochondral ossification also occurs in the epiphyseal plates of the long bones. However, the complex process of the endochondral ossification is somewhat different between condylar growth and growth of the long bones because of the anatomical position of the calcifying zones (Enlow and Hans 1996).

Environmental and genetic factors impact the epigenetic expression of hormones, growth factors, and cytokines within the growth cartilage that orchestrates the condylar and ramus ossification. This intra‐articular position makes the condylar growth cartilage vulnerable to environmental changes such as compression (e.g. after orthognathic surgery or trauma) or pathological conditions (e.g. inflammation, infections, and disc displacements). Such environmental changes will affect the epigenetic expression within the condylar cartilage which may, in turn, impact mandibular morphology by interfering with “normal” dentofacial growth and development (Figure 4.1.1). When such a condition occurs in one side only or the timing of the condition is different for the right and left side, the result is a mandibular dentofacial asymmetry, which often has a secondary impact on maxillary growth and development.

Dentofacial asymmetry may also occur in skeletally mature patients if they experience local conditions leading to unilateral TMJ degeneration (e.g. arthritis or osteoarthritis). TMJ degeneration may change the mandibular position and cause dentofacial deformity and asymmetry (Tanaka et al. 2008).

Dental Occlusion and Acquired Skeletal and Dental Asymmetries

Dental occlusion is formed by a complex interplay of various morphological, physiological, genetic, and functional components. The contributing factors influencing occlusal development are listed in Table 4.1.1. Acquired conditions interfering with any of these factors may foster the development of malocclusion. Dental asymmetries are often caused by an underlying skeletal mandibular asymmetry. These asymmetries are often more pronounced in the lower third of the face involving primarily the mandible and, to some extent, the maxilla. Maxillary asymmetries are often secondary to mandibular asymmetries. Dentofacial deformity and skeletal asymmetries may lead to soft‐tissue asymmetries. A close relation and similarity exists between facial hard‐tissue and soft‐tissue asymmetries (Economou et al. 2018).

Acquired Dentofacial and Dental Asymmetries

The etiological conditions that can lead to acquired dentofacial deformity and asymmetry are listed in Table 4.1.2. These conditions may occur in both growing patients and in skeletally mature patients. Acquired asymmetries typically originate from dentofacial undergrowth/degeneration of one side compared with the contralateral side. However, in specific conditions, overgrowth may act as an underlying factor causing asymmetric development. Four general categories exist of acquired conditions leading to dentofacial deformity and asymmetry: (i) Autoimmune diseases and conditions, (ii) skeletal growth diseases, (iii) soft‐tissue conditions, and (iv) TMJ conditions (Pirttiniemi et al. 2009). The four general categories and their subcategories are listed in Table 4.1.2 and will be elucidated below. The table should be seen as an attempt to summarise acquired conditions leading to dentofacial asymmetry. It should be noted, however, that the division into subcategories is not mutually exclusive, as some of the subtypes are represented in more than one of the general categories.

Autoimmune Conditions

Autoimmune diseases may cause dentofacial deformity and asymmetry in growing individuals and skeletally mature patients alike. In growing patients, autoimmune diseases may impact dentofacial development through condylar growth reduction and deformity. Through involvement of the TMJs, conditions like juvenile idiopathic arthritis (JIA) and systemic lupus erythematosus (SLE) can impact dentofacial growth and development. Scleroderma and, to some degree, mixed connective tissue disease (MCTD) may cause the development of underlying skeletal asymmetry due to abnormal hardening and inflexible cutis in growing individuals. The common features of these conditions in growing patients are that an autoimmune reaction interferes with normal dentofacial growth and development through (i) inflammation of the synovial lining of the TMJ in close contact with the condylar growth cartilage, leading to growth reduction and degeneration; (ii) affection of the soft‐tissue matrices and normal joint function; (iii) a direct impact on the remodeling of the mandibular and maxillary bodies; and (iv) an indirect impact on maxillary development secondary to impaired mandibular growth (Tanaka et al. 2008; Arnett et al. 1996a, 1996b; Stoustrup et al. 2020).

Rheumatoid arthritis (RA), idiopathic condylar resorption (ICR), and osteoarthritis (OA) involving the TMJ are conditions that may lead to dentofacial deformity and asymmetry in skeletally mature subjects (Table 4.1.2). Idiopathic condylar resorption and OA have traditionally both been considered “low‐inflammatory” TMJ conditions with condylar degeneration related to micro trauma. The autoimmune category of acquired conditions in skeletally immature subjects also involves ICR. In fact, controversy reigns over the ICR condition which is, by some, perceived as a local form of OA with a juvenile or adolescent onset. Others consider ICR to be an isolated form of TMJ arthritis that is different from JIA. Finally, others consider that this isolated TMJ condition belongs to the TMJ conditions category as it is thought to occur due to mechanical functional overloading in predisposed individuals. The ICR condition is therefore listed in both the autoimmune category and the TMJ condition category of acquired conditions in Table 4.1.2.

Juvenile Idiopathic Arthritis (JIA)

JIA is the most common autoimmune disease in childhood affecting 12.8–15/100,000 children annually in the Nordic European countries (Berntson et al. 2003). A high risk exists of TMJ involvement where inflammation causes condylar deformity and impaired condylar and mandibular development. JIA most often presents with several joints involved. However, in rare cases, JIA may occur in a form involving only the TMJ(s). The inflammatory cytokine‐driven process influences normal endochondral ossification, changing the morphology of the condyle and affecting the normal development of the mandible and, secondarily, the development of the maxilla. JIA‐related dentofacial deformity should be seen as the outcome of a complex process involving decreased bone formation, TMJ dysfunction, and degeneration of TMJ components (Stoustrup et al. 2020) (Figure 4.1.2).

Unilateral TMJ involvement will often cause development of dentofacial asymmetry with a typical morphology consisting of a short condyle and ramus on the affected side and thereby mandibular rotation according to the z‐axis (Figure 4.1.3). The chin deviates toward the affected side, but the incisal dental midline will often deviate to the contralateral side since the point of rotation according to the z‐axis will be located between the pogonion and the incisal midpoint. Furthermore, the occlusal plane is canted upward to the affected side. Although asymmetry is more pronounced in cases with unilateral TMJ involvement, asymmetry may also be seen in bilateral cases because of a timely difference in involvement. Deformation of the joint components may cause suboptimal joint function giving rise to pain, decreased jaw mobility, and TMJ dysfunction in the form of a reduced mouth‐opening capability (Glerup et al. 2020; Stoustrup et al. 2018).