A male 25 years old presents to the dental clinic with black spot .He also gives the history of previous dental treatments of other teeth as well . Intraoral examination reveals pit and fissure caries with 17. The alignment of the rotary instrument shank axis in this patient should be:

The anatomic orientation of caries lesion formation in the pit and fissure areas of posterior teeth requires alignment of the rotary instrument shank axis (through proper positioning of the handpiece) so that it is parallel with the long axis of the tooth crown prior to initiation of the preparation. Correct alignment of the long axis of the shank limits the likelihood of iatrogenic removal, and thereby weakening, of adjacent healthy (occasionally referred to as “sound”) coronal tooth structure. Most proximal caries lesions associated with posterior teeth also require that the shank axis be aligned parallel with the long axis of the tooth crown. Caries lesion formation associated with the facial or lingual surfaces of the dentition require that the shank axis be aligned perpendicular to the external surface of the tooth where the lesion is located. Sturdevant operative dentistry  7th E D I T I O N, Page No:125