A study of human jaw movements deduced from scratches on occlusal wear facets

In a previous investigation, scratches on tooth wear facets provided information about functional movements of the mandible in the occlusal range. Depending on the type of facet observed, two or three main directions were found. The aim now was to answer two questions: (1) how long does it take for a newly formed scratch to disappear? and (2) do the observed orientations of the scratches change over a long period of time? Eleven wear facets from lower first molars were analysed in four subjects with good natural dentition. Scratches recorded on cellulose replicas were observed under an interference microscope. The same wear facets were recorded one, two, three and six months later. From the photographs of the replicas, a sample of scratches was selected at random and their direction was measured. In the first part of the study, scratches absent at the time of the first observation appeared one month later; 87% of the new scratches had disappeared after one month. Ninety-six percent of the same group of scratches had disappeared after two months. In the second part of the study the angle made by each scratch with the sagittal plane was recorded. It was found that: (1) jaw movements in their occlusal phase can take any possible direction and (2) the distribution of the observed scratches was not random; preferential orientations exist and those present on the working facets are different from those on the non-working facets. Similar orientations were identified 6 months later.