The role of muscle function in internal derangements of the jaw joint

From a clinical perspective it is crucial to obtain insight in the normal and
disturbed dynamics of the lower jaw. Patients with the craniomandibular
dysfunction (CMD) syndrome often complain not only about pain in the
masticatory muscles or the jaw joint, but also about a disturbance in the
movement possibilities of the lower jaw. They may have problems opening or
closing the mouth or hear annoying sounds from the joints during jaw movements.
Generally, the disturbances of jaw movements are caused by aberrations in shape
or mutual position (internal derangements - ID) of the various structures of
the jaw joint. A common type of ID is a symptomatic hypermobility. It is more
prevalent with women than with men and occurs in individuals where the
mandibular condyle travels beyond the articular eminence during wide opening of
the mouth. This movement itself, however, does not necessarily lead to
symptomatic hypermobility as this occurs in about 50% of experimental subjects
without accompanying movement disturbances. Although this disorder is
relatively mild by nature, it can develop into a so-called 'open-lock', where
the subject is unable to close the mouth adequately. It is thus far unknown
which factors are involved in the development of the serious clinical variant.

The objective of the study is to discover if in patients with hypermobility of
the jaw joint, an 'open-lock' syndrome can come into existence as a consequence
of aberrant activation pattern of the masticatory musculature.

Jaw movements and accompanying activation patterns of the superficial
masticatory muscles will be registered for patients with a so-called
intermittent 'open-lock' syndrome (patients with a hypermobile jaw joint which
sometimes have and sometimes don't have problems closing the mouth). These
registrations will be compared with those of subjects that do have a
hypermobile jaw joint but do not suffer from an intermittent 'open-lock'
syndrome.
In order to enable determination of a causal relationship between muscle use
and movement problems biomechanical models will be constructed tailored to
these patients. They will be used to analyze the consequences of aberrant
muscle activation patterns for jaw movement possibilities and limitations in
the relevant subject. These will assist in assessing the boundary conditions in
terms of muscle activation for a smooth closing of the mouth after being opened
wide. Furthermore, it enables prediction of aberrant movements and movement
limitations caused by aberrant muscle activation patterns. The latter will be
verified using the obtained jaw movement and EMG registrations.
In order to construct biomechanical models tailored to the patients, their
relevant geometry (shape of the jaw joints, thickness and attachment sites of
the masticatory muscles) have to be assessed. This will be performed with
modern imaging techniques such as MRI and Cone Beam CT.

The burden is primarily associated with time.
The risks of registrations of jaw movement and of MRI are negligible.
The risks of registration with CT are only relevant if, apart from the proposed
study, the subjects have received or will receive radiation with an amount that
comes close to what is considered acceptable.