Surfaces. There are different surfaces on the teeth: mesial, occlusal, lingual, buccal, distal. The letters describe the surfaces where the filling was placed.
Descriptions. Excellent question.Both refer to restorations(fillings) that involve two surfaces of the tooth.An MO involves the mesial and occlusal surfaces of a back tooth.That is the chewing surface(occlusal) and the in between surface closest to the tooth in front of it.An ML is a mesial lingual restoration on a front tooth which involves the tongue side(lingual) and the in between side closest to the middle.
Dental Anatomy. MO = Mesio-occlusal, and ML = Mesio-lingual is terminology referring to the surfaces of the teeth.
The difference. The difference is mo is a 2surface filing for posterior teeth ml is 2 surface filing for anterior teeth.