Outdated definition

Sir, I wholeheartedly agree with S. J. Davies that the definition of the term 'malocclusion' is outdated and in need of revision (BDJ 2007; 202: 519–520). As a professor I formulated the following definition for teaching orthodontic postgraduates: 'A malocclusion is a solution for a complex, adaptive system to remain in equilibrium'.¹

The complexity referred to above is twofold: first, structural complexity is in-built in the craniofacial system, which comprises teeth, bone, muscles, joints, soft tissues and functional spaces. Second, mathematical complexity is in-built in the craniofacial system due to any permutation or combination of genes being inherited, and expressed according to the functional genomics of the individual patient. Thus, a given malocclusion is a stable condition, being arrived at through (a series of) developmental compensation(s). The crowns of teeth are unique, however, in that once fully formed there is no developmental mechanism to change their size or shape. The form of the crowns can change minutely over a period of time through attrition, abrasion and erosion or more quickly through invasive dental interventions. Nevertheless, because of the unyielding properties of enamel, teeth will take up the space made available to them during the course their development ie eruption, and if this space is insufficient then a malocclusion arises. Due to temporo-spatial patterning, given appropriate gene-environmental interactions, teeth will align themselves following the curves of Monson and Spee because of tooth morphology, which has evolved over millennia. In modern societies, however, altered signalling means that teeth end up in positions in which they are not optimal in terms of either function or aesthetics.

In addition to complexity, however, the craniofacial system has adaptability. The framework of the jaws is built from bone which, due to its rich vascularity, has a high degree of plasticity. This means that the facial skeleton can undergo bone remodelling in response to functional stimuli. Thus, while the teeth are actively erupting, their roots and supporting bone are subject to functional stimuli, which mould the final outcome. For example, if an object is placed close to the eruptive pathway, the teeth are in effect deflected into a position different than that defined by temporo-spatial patterning alone. However, the teeth retain their pre-determined form while the enclosing bone is 'deformed' through remodelling. This means that the application of an appropriate signal that remodels the bone can result in the correct re-positioning of the teeth.

Despite the propensity for adaptability, the final arbitrator is stability. The craniofacial system needs to be in balance or equilibrium in accord with functional demands, for example sufficient space for the tongue during mastication, speech and swallowing. Thus, a developmental compromise may be reached with the teeth in a less than ideal position but in a state in which craniofacial homeostasis has been reached. According to the spatial matrix hypothesis,¹ the craniofacial system will adapt through developmental compensation until a new position of functional stability has been reached. Moreover, specific procedures or treatment protocols may modulate this state of developmental stability to the benefit or detriment of individual patients.