The aim of the present study was to compare two methods used in inferior alveolar nerve (IAN) repositioning to evaluate their effect on the vitality of intact teeth anterior to the mental foramen. Nerve lateralization (NL) is defined as the lateral reflection of the IAN without incisive nerve transection; nerve transposition (NT) involves sacrifice of the incisive nerve. Twenty-one patients were included in this study. Vitality tests for the teeth anterior to the mental foramen, including pulse oximetry and electric pulp testing, were evaluated at 1 week prior to surgery and at 1 week, 1, 3, 6, and 12 months after surgery. Lower lip and chin neurosensory changes were also recorded at the same time intervals by static light touch test with a cotton-tipped applicator and two-point discrimination test with sharp callipers. Vitality tests were negative after the operation in the NT group, while all had normal values at 1 week prior to the operation. In the NL group, only two patients (20%) had negative test results at 1 week after surgery. Lip and chin neurosensory changes in the total transpositions (28 operations) were seen in 7.1% at 1 year after the operation. It appears that NL is a more physiological procedure than NT.
The reconstruction of atrophied posterior mandibular ridges with height deficiencies is possible through different routes, including onlay bone grafting, interposition bone grafting, vertical alveolar osteodistraction, nerve lateralization and transposition, and a combination of nerve repositioning and inlay bone grafting.
Inferior alveolar nerve (IAN) repositioning is indicated in implant dentistry in situations where the vertical distance between the ridge crest and the alveolar nerve is significantly reduced. A remaining 5–8 mm of bone is recommended in the literature for this procedure. Severe resorption of posterior mandibular bone will alter the crown/root ratio of a fabricated fixed dental prosthesis because of the increase in the clinical crown. In this situation, bicortical placement of implant fixtures that can be performed after nerve repositioning leads to better biomechanical conditions. Dental fixture implants are almost always placed simultaneously with the nerve repositioning procedure. Although short implants with a 1–2 mm safety margin between the end of the implant and the IAN can be used, the long-term prognosis after bone loss is compromised. Placement of dental implants lingual to the IAN is also possible in very limited cases.
Repositioning of the inferior alveolar neurovascular bundle from its bony canal within the mandible can be undertaken into two ways: (1) with preservation of the incisive nerve and lateralization of the inferior alveolar neurovascular bundle posterior to the mental foramen, which is called nerve lateralization (NL), and (2) nerve repositioning with the sacrifice of the incisive neurovascular bundle and mental foramen transposition, which is called nerve transposition (NT).
Nerve repositioning is not limited to implant dentistry. It has previously been used for the mental nerve to relieve the pressure from an overlying denture, the resection of a benign pathology of the posterior mandibular region, mandibulotomy to gain access to benign and malignant tumors that are located in the medial mandibular ramus area, therapeutic decompressions, and orthognathic surgeries. The same procedure has previously been reported for other nerves such as the optic nerve, suprascapular nerve, and facial nerves. In IAN transposition with incisive nerve transection, the vitality (blood perfusion and nerve supply) of the teeth anterior to the mental foramen has not yet been investigated. Thus, the present study was designed and conducted to evaluate the reinnervation and revascularization of intact teeth anterior to the mental foramen after NT and to compare this process with the results of NL to determine whether incisive nerve transection has any irreversible effect on the vitality of the teeth anterior to the mental foramen.
The IAN is the largest sensory branch of the mandibular nerve. It enters the mandible via the mandibular foramen on the medial side of the ramus and continues its course in the mandibular bone. Finally, in the mental foramen, it divides into two terminal branches – the mental and incisive nerves. The incisive nerve has four branches including a mucocutaneous branch, short and long periodontal branches, and a pulpal branch. In the bone, the IAN is accompanied by the inferior alveolar artery, which is a branch of the maxillary artery, and two or more small communicating veins that drain into the facial vein ( Figs. 1 and 2 ). Lymphatics are also present in the bony canal that encloses these elements.