• loss of reference point
• the fracture might not be limited to the undermined tooth structure only but may involve larger area or lead to crack which may indicate tooth extraction

• Because there are pulp tissues beyond the roof.
• to reduce stresses and consequent breakage of instruments used for cleaning & shaping
• to accomplish the root canal treatment safely and efficiently 
• we can't clean or negotiate all the anatomy of the tooth without deroofing

• safe ended diamond stones
• endo z burs.
• conventional diamond stones for experience practitioners.

• visual and instrumentation convenience.
• make a balance between mechanical and biological properties of root canal treatment

1. knowledge of the most likely internal anatomy of the tooth
2. preoperative radiograph
3. removal of undermined or weakened tooth structure
4. is the tooth restorable or not?
5. geometric design: small better than large to avoid overextension
6. pre-access analysis

• long access of the tooth: if the tooth is tilted, we might make cervical perforation if the tooth was treated as if it is in normal alignment.

Technical procedure:

• preserve peri-cervical dentin because it's very precious; it affects the longevity of the tooth and restoration
• convenience: "I can not treat what I cannot see"
• instrumentation convenience
• inspection of the floor and searching for the canals is the last step in access cavity preparation 

• access cavity in upper and lower incisors is triangular with apex cervically.
• access cavity in canines is oval inciso-gingivally.

• a video of access cavity preparation of a maxillary central incisor

• Access opening of Maxillary Canine step by step demo
  
  

• Two canals mostly or single canal, They might have different curves like dilacerations, double curves or bayonet so it requires modification in access cavity preparation and special concerns in instrumentation.
• access cavity preparation in premolars is oval buccolingually extending from buccal cusp tip to the base of the palatal cusp.
• if there is extra root (molarized premolar): the access cavity is modified to be triangular in shape and It's called T shaped access.
  

Avoid excessively widening the access cavity towards the mesial wall in the upper first premolar. There's a mesial concavity at the cemento-enamel junction, which increases the risk of a lateral coronal perforation. 

• occlusal surface is smaller than that of upper premolars, They may have anatomical variations, extra root(molarized premolar), extra canals, c-shaped canals. 
• when there is extra root canal, flaring may be extended up to the buccal cusp tip to be able to negotiate the lingual canals.

• Three canals: pulp chamber is triangular with its base buccally and the apex palatally, extending from buccal cusp tip toward the mesiobuccal groove and to the base of the mesiopalatal cusp.
• four canals: (90% of cases) quadrilateral configuration with additional extension mesialy to accommodate MB2.
  
  
  

• Mesiobuccal canal: under the mesiobuccal cusp tip.
• Distobuccal canal: in front of the mesiobuccal groove.
• palatal canal: under the base of the mesiopalatal cusp.

•  look for MB2 under the MB, slightly mesial if you draw a line between MB and P.

  Finding MB2 might sometimes be tricky due to a dentin triangle, which can be removed using:

  1. Ultrasonic Endo Tips

  2. Long Shank Low-Speed Burs

  3. Rotary Files like Wave One Gold with brushing motion

  4. Endo Explorer for safer, more controlled exploration

  5. Micro Openers—small files with probe-like handles for better visibility.

• mesial root: majority of cases have 2 canals.
• mesiobuccal canal: under mesiobuccal cusp tip.
• mesiolingual canal: 1 mm lingual to the central groove.
• distal root 1 or 2 canals
• access cavity configuration may be rectangular or triangular

• single root with c shaped canal morphology
• extra roots in lower 6 or 7 are called  In some cases, lower molars can have two distal roots instead of just one. This additional distal root is called radix endomolaris and paramolaris, and its orifice is located in the distolingual corner of the access cavity, as shown in the image.
  
  
  
• mid mesial canal may be present in the groove between mesiobuccal and mesiolingual and it may be  confluent with one or both of them(exiting in one foramen).
  
  
  

Access Cavity preparation tips and Tricks 

A lot of people think access cavity preparation is easy and doesn’t require any specific techniques or tricks, but then end up with a perforation. Here are a few tricks to help you make an access cavity without perforation:

Your boundary should be the CEJ (cementoenamel junction)

Here’s how to get it right:

1. Keep a periodontal probe handy to measure the length of the crown to the CEJ, as there can be slight variations between people. It’s generally about 12mm.
2. After you feel the “drop” with the round bur, switch to a safe-end stone or an Endo Z bur to continue removing the roof. Don’t rely solely on the round bur, as it’s easy to slip and cause bleeding.

The orifices are always at equal distances along the M-D (mesiodistal) line if we draw a line across the floor.

This means that if you locate the ML orifice and draw a line down the center of the floor, the MB orifice will also be the same distance from that line. This rule applies to all teeth except upper molars.

The orifice is located along a perpendicular line to the line dividing the floor in half.

If you have the access cavity open but still can’t find the canals, draw a perpendicular line on the middle of the floor and search in that direction, especially under the cusp tips. This will help you find the canals quickly without expanding the access cavity unnecessarily.

Tip: In lower molars, if you can’t find the orifices under each cusp tip, they’re likely closer to the buccal cusps, so avoid expanding toward the lingual side.

The orifice is always located at the junction between the floor and wall.

Always keep your instrument moving along the borders of the access cavity rather than exploring the middle. This is a key tip to avoid perforation.

The orifice is always located in the angles of the junction between the floor and wall.

This follows the previous rule, indicating that you should focus on the line angles rather than the edges of the access cavity.

Color change

The pulp chamber floor is usually darker than the walls. If you forget to measure to the CEJ, the color difference can help you know when you’ve reached the pulp chamber.