How does endodontic treatment save the tooth?
The endodontist removes the inflamed or infected pulp, carefully cleans and shapes the inside of the canal. Then fills and seals the space. Afterwards your dentist will place a crown or other restoration on the tooth to protect and restore it to full function. 

Will I feel pain during or after the procedure? 
Many endodontic procedures are performed to relieve the pain of toothaches caused by pulp inflammation or infection. With modern techniques and anesthetics, most patients report that they are comfortable during the procedure. 
For a few days after treatment, your tooth may feel sensitive, especially if there was pain or infection before the procedure. This discomfort can be relieved with over-the-counter or prescription medications. Your tooth may  feel slightly different from your other teeth for a period of time.
Why would I need an endodontic procedure repeated?
As occasionally happens with any dental or medical procedure, a tooth may not heal as expected after initial treatment for a variety of reasons:
Narrow or curved canals were not treated during the initial procedure. Complicated canal anatomy went undetected in the first procedure.
The placement of the crown or other restoration was delayed following the endodontic treatment. 
The restoration did not prevent salivary contamination to the inside of the tooth.
In other cases, a new problem can jeopardize a tooth that was successfully treated. For example:
New decay can expose the root canal filling material to bacteria, causing a new infection in the tooth.
A loose, cracked or broken crown or filling can expose the tooth to new infection. 
A tooth sustains a fracture.
What will happen during retreatment? 

The endodontist will reopen your tooth to gain access to the root canal filling material. In many cases, complex restorative materials—crown, post and core material—must be disassembled and removed to permit access to the root canals. After removing the canal filling, the endodontist can clean the canals and carefully examine the inside of your tooth using magnification and illumination, searching for any additional canals or unusual anatomy that requires treatment. The endodontist will fill and seal the canals and place a temporary filling in the tooth. 
Why would I need endodontic surgery?
  Surgery may be used in diagnosis. If you have persistent symptoms but no problems appear on your x-ray, your tooth may have a tiny fracture or canal that could not be detected during nonsurgical treatment. In such a case, surgery allows the endodontist to examine the entire root of your tooth, find the problem, and provide treatment. 
Sometimes calcium deposits make a canal too narrow for the instruments used in nonsurgical root canal treatment to reach the end of the root. If your tooth has this “calcification,” your endodontist may perform endodontic surgery to clean and seal the remainder of the canal.
Surgery may also be performed to treat damaged root surfaces or surrounding bone. 
Although there are many surgical procedures that can be performed to save a tooth, the most common is called apicoectomy or root-end resection. 
What is an apicoectomy?

The endodontist opens the gum tissue near the tooth to see the underlying bone and to remove any inflamed or infected tissue. The very end of the root is also removed. 
A small filling may be placed in the root to seal the end of the root canal, and a few stitches or sutures are placed in the gingiva to help the tissue heal properly. 
Over a period of months, the bone heals around the end of the root.
Are there other types of endodontic surgery?

Other surgeries endodontists might perform include dividing a tooth in half, repairing an injured root, or even removing one or more roots. 
Will the procedure hurt?

Local anesthetics make the procedure comfortable. You may feel some discomfort or experience slight swelling while the incision heals. This is normal for any surgical procedure. Your endodontist will recommend appropriate pain medication to alleviate your discomfort.
Your endodontist will give you specific postoperative instructions to follow. If you have questions after your procedure, or if you have pain that does not respond to medication, call the endodontist.
When can I return to my normal activities?

Most patients return to work or other routine activities the same day. 
Your dentist or endodontist is suggesting endodontic surgery because it is the best option for saving your own natural tooth.  Of course, there are no guarantees of success with any procedure. 
Practice Limited to Endodontics