The goal of this clinical trial is to compare bone healing after using apical tooth microsurgery to remove root end infection, either by use of rotary burs for cutting bone and root end, or by use of piezoelectric surgery for the same procedures.

The main questions it aims to answer are:

• Is there a difference in bone healing and reformation between the two surgical procedures ?
• Is there a difference in bone healing between cutting the bone with a rotary bur and cutting the bone with the 'bone window' technique that uses piezoelectric unit ?

The aim of this study is to prospectively compare postsurgical healing after the use of microsurgical technique to eliminate apical pathology, either by use of rotary burs for osteotomy and root resection, or by use of piezoelectric surgery for the same clinical procedures. Null hypothesis is that there is no difference in healing and buccal bone thickness reformation between the two surgical procedures Another purpose of the study is to prospectively evaluate healing after the application of a novel buccal bone preservation technique, called the 'bone window' technique. The technique will be applied in a subgroup of patients on maxillary and mandibular premolar and molar teeth with an intact buccal cortical bone and healing and buccal bone reformation will be assessed at follow up. Null hypothesis in this part, is that there is no difference in healing and buccal bone preservation between osteotomy with a bur and the 'bone window' technique.

Inclusion Criteria:

• Age 18 years and older consenting to the surgical procedure as well as agreeing to preoperative, postoperative and at least one follow-up CBCT evaluation after 12 months
• Noncontributory medical history (American Society of Anesthesiologists class I and II)
• A history of previous endodontic treatment with radiographic presence of apical periodontitis
• A true endodontic lesion: microsurgical classification A, B, or C according to Kim and Kratchman, 2006.
• Lesion size 10 mm or smaller in diameter measured on preoperative CBCT
• Coronal restoration should be present at the time of follow up examination.

Exclusion Criteria:

• Nonconsenting patients and patients younger than 18 years of age
• Medical history with American Society of Anesthesiologists class III to V
• Insufficient coronal restoration
• Nonrestorability or traumatized teeth
• Teeth with microsurgical classification D, E, or F according to Kim and Kratchman, 2006.
• Mobility I or higher
• Radiographic presence of nonapical root resorption
• Teeth with a vertical root fracture or coronal/ midroot perforation
• Lesion size larger than 10 mm in diameter measured on preoperative CBCT
• Use of bone graft material for regeneration

Only IPD used in the results publication

Beginning 3 months and ending 3 years after the publication of results

IPD will be shared with researchers outside the study team after their request. Request has to be made to the study's principal investigator with explanation of the reason / scientific purpose the IPD is requested. IPD will be offered for conducting meta-analysis, systematic review studies on pertinent research study topics to the study conducted. Raw data with pseudonyms and numbering of participants, protocol data, results data, statistical analysis will be offered. The principal investigator of the study maintain all raw data in numbered folders with no information that reveals the participant identity. Numbered folders with no personal information of participants, tables , statistical analysis, anonymized excel files will be sent by email to other researchers after they sign a confidentiality statement for use of the data