The first picture above shows microleakage around an old filling. All fillings have a lifespan after which they are no longer functional. In this example, the old filling is intact, it is not broken but there is another problem. The seal between the filling and the tooth structure is longer exists. This can be seen as the dark staining between the filling and the tooth. This means that salivary fluids and bacteria have entered the tooth and will very likely cause decay underneath the filling. If this is not repaired and the decay progresses, this can then cause sensitivity, discomfort and possibly a tooth abscess.
The second picture shows the tooth repaired with a new filling. Notice the seal between the filling and the tooth. This new restoration will function comfortably for the patient for many years to come.