This X-ray film reveals a poor crown-to-root ratio for tooth #21 (right), the lower left first premolar.  The tooth exhibits 50% bone loss, adding roughly 5-7 mm to the clinical crown of what is actually anatomical root.  The fulcrum, existing somewhere immediately apical to the height of the bone, does not allow for any adjacent bone to avoid compression or tension, resulting in virtually complete widening of the PDL and a grim prognosis, due to secondary occlusal trauma.