When a tooth has very little left to rebuild upon, pins can be used to help hold the filling on. Pins do have a certain level of risk attached to them. There could be a chance that the pin will drill too close to the nerve of the tooth, causing pain (and leading towards root canal treatment). Also pins can weaken the remaining tooth structure causing micro-fractures in the tooth. Some dentists refrain from using pins for these reasons. However, without pins, the biting forces on a tooth like this would most probably knock the filling off.