With an estimated prevalence of approximately 30% and 15% for awake and sleep bruxism [1], respectively, these masticatory muscle activities are a clinically significant phenomenon in the adult population. Jaw closure, which is a prerequisite for the execution of jaw clenching and tooth grinding in the context of bruxism, is caused by three paired masticatory muscles: the temporal, the masseter, and the medial pterygoid muscle. The latter two, which attach their tendons to the tuberosities of the mandibular angle, contribute to about 65% of the intrinsic strength of the jaw-closing muscles [2].
The vertical masticatory forces exerted during the first mastication cycles and measured directly on individual teeth range from 20 N to 150 N depending on the food texture, with maximum forces of up to 250 N measured in single cases (chewing gummy bears). In contrast, the maximum voluntary biting forces between antagonistic molars are usually between 500 N and 700 N [3]. However, the force exerted during sleep bruxism can significantly exceed the amplitude of the maximum voluntary bite force during wakefulness [4].
Sustained bruxism may have clinical consequences by increasing the risk of developing clinical signs and symptoms, such as tooth wear [5], fatigue [6–7], pain of the masticatory muscles [7–8] or temporomandibular joints (TMJs) [9–10], anterior disk displacement [8] and TMJ clicking [6, 11], or masseter hypertrophy [12]. Current strategies for assessing the presence of bruxism are based on (a) self-report by the individual, (b) clinical examination, and (c) instrumental approaches such as electromyographic recordings, including polysomnography and/or audio/video recordings [13]. Together, they form a grading system with increasing likelihood of a valid diagnosis of bruxism. In contrast to anamnestic information and data obtained from the clinical examination, however, instrumental diagnostic procedures have the disadvantage that they are less frequently available, their use is associated with cost and time, and not every patient gives consent for this type of assessment.
In this situation, a previously neglected additional diagnostic observation may come into play: The flattening of the formerly rounded surfaces of the mandibular condyle and the posterior slope of the articular eminence. While this radiological sign has traditionally been associated almost exclusively with osteoarthrosis or osteoarthritis, it may also be the biological result of adaptive remodeling due to repetitive mechanical loading from compressive forces (e.g., jaw clenching) and thus represent nothing more than bony adaptation [14], due to the intimate relationship between function and form in biologic systems [15–16]. In fact, already in 1939 Molnár [17] stated that “[t]he masticatory system is certainly subjected to the greatest stress by the masticatory musculature […] The effects of this stress, which can be seen in a functional shaping according to the law of ‘functional adaptation,’ can be demonstrated particularly well in the lower jaw bone […]”.
On closer examination of panoramic radiographs, we noticed that an appreciable number of individuals diagnosed with bruxism had bony changes not only in the condylar area, but also in another part of the mandible, the mandibular angle. Here, an increase in bone apposition was often observed. Since, to our knowledge, no study has been conducted on that topic, we wanted to investigate whether the prevalence of these morphological alterations is higher among bruxers compared to a control group. Another aim was to classify the degree of these morphological changes.