Patients

The STROBE Statement for improving the quality of observational studies was followed in reporting the present study ( http://www.strobe-statement.org ). A prospective cohort study was designed, and the clinical study protocol was approved by the local ethics board. The study population comprised all patients referred for implant surgery in the posterior mandible between 2000 and 2013. Thirteen consecutive patients (12 women and one man, ranging in age from 45 to 68 years) were selected for treatment with a distal fixed partial denture supported by dental implants placed in conjunction with lateralization of the IAN using the modified surgical procedure described below. The medical status and medications were recorded in each case. Patients with immunological diseases, uncontrolled diabetes mellitus, or other systemic contraindications were excluded from the study. All patients enrolled in this study were non-smokers. Smoking patients were not excluded.

Panoramic radiographs and computed tomography (CT) scans were obtained for all patients. In all cases, the CT scans revealed that the patients had less than 7 mm of bone height between the alveolar crest and the mandibular canal at the planned implant sites. There was 5–7 mm of available alveolar bone in all patients, but not at all implant sites. Patients were included in the study on the basis of a requirement for at least two implants, one of which was frequently placed posterior but close to the mental foramen. Three of the 13 patients had 7–10 mm of bone over the IAN canal in the mandible immediately behind the mental foramen, while the availability of bone was less than 7 mm in posterior areas.

All aspects of the operation were discussed with the patient, and all of the patients agreed to undergo surgery with the IAN lateralization technique. All were informed of the possibility of experiencing some postsurgical neurosensory disturbances, and written informed consent for the treatment was obtained in each case. A total of 27 dental implants were placed at the time of nerve lateralization. All of them were cylindrical type, measuring 13–15 mm in length and 3.8–4.1 mm in diameter.

Surgical procedure

The IAN repositioning surgical procedure was undertaken under local anaesthesia, obtained by infiltration of mepivacaine 1% containing 1:100,000 epinephrine. All patients received preoperative antibiotics (2 g amoxicillin 1 h before surgery) and were asked to rinse with 0.2% chlorhexidine gluconate antiseptic solution immediately prior to surgery.

The procedure was started with a midcrestal incision extending from the retromolar region to the mesial portion of the cuspid tooth region, where an oblique relaxing incision was made. A posterior relaxing incision was made in the retromolar region and then a full-thickness mucoperiosteal flap was reflected and the lateral surface of the mandible up to the inferior border of the bone and mental foramen was exposed. The osteotomy defined a rectangular bone window that was centred on the position of the IAN canal ( Fig. 1 ); this was made with an ultrasonic bone-surgery device (Piezosurgery, Mectron Medical Technology, Carasco, Italy) and two inserts in a bevelled shape (Piezosurgery OT7 and OP1). The OT7 insert with a thickness of 0.55 mm was used to remove the cortical bone plate, with the aid of chisels ( Fig. 2 ). The remaining trabecular bone and the thin bone wall of the canal were removed with the OP1 insert and bone curettes ( Fig. 3 ). The IAN bundle was dissected free and moved buccally with the aid of an elastic vessel loop ( Fig. 4 ). The surgical sequence for implant installation was then performed, and once the drilling was completed, each implant was inserted while the nerve bundle remained retracted. Nineteen Astra, six Straumann Standard Plus, and two Sweden & Martina implants were placed. When the implants were in position, the nerve was repositioned over the lateral aspect of the implants ( Fig. 5 ), with the placement of a double layer of collagen membrane between the nerve and the implants (Bio-Gide; Geistlich, Baden-Baden, Germany). Finally, the cortical bone plate was shaped to reduce its thickness at the expense of its inner surface, and was subsequently reintroduced over the bony window, restoring the continuity and contour of the mandible ( Fig. 6 ). In the first of the 13 patients, the bony plate was secured with a screw inserted in the upper border; however, in the remaining 12 patients the bony plate was simply stabilized under the sutured mucoperiosteal flap using resorbable sutures (3–0 Vicryl).

Osteotomy of the right mandible by piezoelectric surgery. The proximity of the mental nerve distal to the cuspid can be observed.

The buccal cortical bone is removed to gain access to the inferior alveolar nerve.

After cortical bone removal, cancellous bone is still present in the area posterior to the mental foramen.

The inferior alveolar nerve is exposed using hand instruments and Piezosurgery insert OP1.

The position of the inferior alveolar nerve after implant insertion can be observed.

At the end of surgery, the cortical plate is reintroduced into its original position. It can be fixed with a screw or simply stabilized under the mucoperiosteal flap.

The patients were given amoxicillin 500 mg every 8 h for 7 days and ibuprofen 400 mg every 8 h for 4 days. They were asked to follow a soft diet and to perform appropriate oral hygiene for 2 weeks. The sutures were removed 10 days after the operation ( Fig. 7 )

Postoperative panoramic radiograph showing the position of the implants in relation to the inferior alveolar nerve. The ends of the implants are positioned inferior to the canal. The limits of the osteotomy, but not the bony defect, can be observed.

Neurosensory evaluation

All surgeries were performed by the same surgeon (JCdV). Patients received weekly follow-up examinations during the first postoperative month, monthly examinations thereafter for a period of 6 months, and a further examination at 12 months after surgery. Self-reported subjective sensory changes were recorded at each time point, and objective sensory assessments were conducted by a single investigator (PB) at 3 months after surgery. This time point was selected because neuropraxia or Sunderland first-degree injury resulting in conduction blockade following nerve traction usually lasts no more than a few days to weeks. Axonotmesis or Sunderland second-degree injury is more severe, with Wallerian degeneration distal to the site of injury. Because the endoneurium remains undisrupted in both of these lesions, complete neurophysiological recovery normally occurs within 3 months of injury.

With the patient’s eyes closed, the examiner tested the innervation of the lower lip and chin. Light touch and pain tests were used to selectively discriminate between large myelinated quick-adapting A-alpha fibres, lightly myelinated A-delta fibres (which constitute 90% of A axons), and unmyelinated C fibres. A two-point discrimination test was used to selectively study large myelinated slow-adapting A-alpha fibres, which constitute the remaining 10% of A axons.

For the light touch test, patients were asked to identify whether the lower lip or the chin was being touched using a cotton-tipped applicator. Each patient reported when he/she felt a light touch, also indicating the exact location. Cotton swabs were used to assess a large area of the lower lip and chin, in order to map and measure any area in which no stimulus could be felt, but also providing an effective moving stimulus for the demonstration of dynamic mechanical allodynia. Each side was tested in blocks of 10 trials, and an overall correct response of 80% or higher was considered normal. Three sensitivity levels were recorded: (1) anaesthesia, (2) hypoesthesia, and (3) normal sensitivity.

The pain test was performed using a sharp instrument (sterile dental needle) to map the area of anaesthesia or hypoesthesia, ascertaining whether the stimulus was felt within the lower lip and chin area. The result was positive when instrument contact elicited pain.

Finally, the two-point discrimination test was done using sharp callipers by simultaneously touching the skin in two locations. The distance between the two points of the callipers was gradually increased until the patient could distinguish them as two separate points; the minimum distance reported was recorded. Lips can generally distinguish two points 2–4 mm apart, whereas the skin over the lower border of the mandible can only distinguish two points at 8–10 mm apart. A measurement of less than 15 mm was considered normal in this study, in accordance with Ferrigno et al.

No preoperative evaluation was performed with any of these three tests. As a control, the instruments were first applied in all cases to the lower lip and chin of the non-operated side. They were then applied to the operated side using the same pressure, and the results were compared between the two sides.

Statistical analysis

The Mann–Whitney test was used to analyze the significance of any difference in distances measured with the two-point discrimination test, in order to determine the recovery of nerve function after IAN lateralization. Spearman’s correlation coefficient was applied to estimate the significance of light touch effects, and Fisher’s exact test was applied to analyze the results of pain tests. The level of significance was set at 0.05. All tests were two-sided, and all calculations were done using PASW Statistics 18 software (SPSS Inc., Chicago, IL, USA).