Lower mandibular 1st molar is the tooth that erupts in oral cavity at age of 6 years while all deciduous teeth are present in mouth.
Parents and kids normally don't acknowledge presence of 1st permanent toot and brushing habits are not properly developed at this,thats why developing caries in this tooth is more common.
To save tooth root canal treatment is done.
access cavity is the 1st step of RCT(root canal treatment).following are the important features of tooth to be known for preparation of access opening.
1: mandibular first molar has two roots ,mesial root and distal root 
Mesial root
2 canals and 1 foramen - 40.5%
2 canals and 2 foramina - 59.5%
In Distal root
1 canal and 1 foramen - 71.1%
2 canals and 1 foramen - 17.8% 
2 canals and 2 foramina - 11.1%
Length of root canal
M - 20.9 mm
D - 20.9 mmv
Steps for access
1. Initial penetration is made with a high speed diamond bur in the exact centre of the mesial pit, with
the bur directed toward the distal
2. Once the 'drop' into the pulp chamber is felt, change to the endo-Z bur and enlarge this for
exploration
3. Locate with endodontic explorer orifices of MB, ML and D canals (tension of explorer against the
walls of preparation indicates the amount and
direction of extension necessary, so remove further
direction of extension necessary, so remove further
tissue with endo-Z)
4. Use from inside to outside a round slow speed bur to ensure complete removal of the roof of the
pulp chamber
5. Final prep extends to the height of mesial cusps, explore carefully the presence of a 2nd distal canal,
if this is the case, the access will be square in shape; otherwise if 3 canals are present access shape
is more triangular, with its base toward the mesial side of the tooth
