One of the most common complications after bilateral sagittal split ramus osteotomy (BSSRO) is neurosensory impairment of the inferior alveolar nerve (IAN) following injury to the nerve bundle during surgery. Such injuries may occur due to the variable anatomy of the inferior alveolar canal and buccolingual and its vertical orientation123. This is an important concern in mandibular osteotomy, since patients may experience variable degrees of neurosensory dysfunction for long periods of time; in some cases, dysfunction is permanent1. The incidence of IAN dysfunction is reported to range between 13%–100% immediately after surgery to 0%–85% after one year4. Surgeons aim to decrease the incidence of IAN dysfunction by improving surgical techniques and performing preoperative imaging such as panoramic views to avoid nerve bundling. However, if they fail, injury to the nerve can lead to complications such as drooling, lip biting, and thermal injury as well as patient dissatisfaction3567.

Therefore, preoperative radiographic examinations are of significant clinical value8. Although panoramic and lateral cephalography is the main method used, it is limited by inconsistencies in magnification of different parts of the image and its inability to exhibit all three-dimensions891011. On the other hand, cone-beam computed tomography (CBCT) can illustrate buccolingual depth without distortion and seems excellent for localizing the inferior alveolar canal (IAC) and adjacent structures810121314. Therefore, accurately determining the position of the IAN canal within the mandible with CBCT before surgery may reduce IAN dysfunction after BSSRO. Despite its clinical relevance, the impact of CBCT combined with routine imaging compared to routine imaging alone on post-surgical sensory disturbances has been examined in only one preliminary study2. Therefore, the purpose of this study was to evaluate the effects of preoperative imaging of the IAN canal using CBCT with routine imaging versus routine imaging alone (panoramic and lateral cephalography) on sensory disturbances perceived during the first year after surgery. The null hypotheses were that there would be no differences between groups, between sexes, between left and right sides, over time, or across ages.

This parallel randomized clinical trial comprised 43 patients (30 females and 13 males) with skeletal class III deformity needing mandibular setback surgery with or without simultaneous maxillary surgery who were treated at Buali Hospital in Tehran, Iran during 2014 to 2016. We included patients who were candidates for BSSRO due to mandibular setback less than 7 mm and excluded patients who had jaw asymmetry, pathologic conditions of the IAN, previous surgery in that region, any neurosensory dysfunction of the IAN, required genioplasty, and had medical conditions with associated neuropathy before surgery. We also excluded patients who were randomly assigned to the CBCT group but who did not consent to undergo CBCT imaging, although they received routine treatment. The protocol ethics were approved by the Islamic Azad University's Board of Ethics and Research according to the Helsinki declaration (No. 08-21-214), and informed consent was obtained from each patient. The trial was reviewed by and approved/registered at an international institute before beginning the study (RCT code: IRCT2014052217798N1).

This study was composed of 43 patients (86 sides) who were randomized into an experimental group of 21 patients (42 sides) (14 females and 7 males, with a mean and standard deviation age of 26.1±4.5 years) and a control group of 22 patients (44 sides) (16 females and 6 males, with a mean age of 25.4±2.7 years; range, 18-33 years). Mean mandibular setback extent was 4.00±1.45 mm and 4.59±0.91 mm (range, 1–7 mm) in the experimental and control groups, respectively. There was no significant difference between groups according to sex (Fisher exact test, P=0.747), age (unpaired t-test, P=0.492), or amount of movement (unpaired t-test, P=0.115). No adverse effects were identified.

There were significant differences between the control and experimental neurosensory scores measured 1 month and 6 months after surgery.(Table 1) There were significant differences between the right and left sides during the first, fourth, and last intervals.(Table 2)

Except during the first interval (7 days after surgery, Spearman rho=0.635, P<0.001), there were no significant correlations between neurosensory functions perceived by patients on their left and right sides at the second (rho=0.200, P=0.199), third (rho=−0.058, P=0.710), fourth (rho=0.039, P=0.805), or fifth (rho=−0.129, P=0.409) follow-up visit. The extent of displacement was not associated significantly with sensory function rate on left or right side for any of the five intervals (all rho values ranged between −0.221 and 0.179, all P-values >0.250).

The mixed-effects model (n=86) indicated significant effects for jaw side (P=0.003) and CBCT intervention (P=0.002; Fig. 2) but not age (P=0.617) or displacement extent (P=0.122).

On the left side (n=43), repeated-measures ANOVA detected significant effects for time (P=0.001) and CBCT intervention (P=0.005). On the right side (n=43), a significant effect was observed for time (P<0.001), while the effect of the intervention was marginally significant (P=0.066). The effect of mandibular displacement was not significant on either side (both P-values >0.280).

We found that performing BSSRO surgery while taking advantage of CBCT information can aid clinicians seeking to reduce neurosensory deficits more rapidly. Only one previous preliminary study by Aizenbud et al.2 had compared CBCT and routine imaging versus routine imaging alone before BSSRO surgery. In that study, there was a significant difference between two groups in terms of neurosensory function during the first postoperative year, and they observed positive effects of CBCT imaging on postoperative neurosensory function2. However, they did not detect differences between sides2. CBCT allowed the surgeon in the present study to observe data that are not available with panoramic imaging alone, such as the distance of the mandibular foramen from the sigmoid notch on the medial side of the ramus, which allowed for better preservation of the nerve bundle. We also increased the number of patients in our sample for better statistical analysis.

Our findings can be discussed considering more general research. Yamamoto et al.17 evaluated neurosensory disturbances after BSSRO in patients who had undergone preoperative transaxial CT scans of the IAN canal before surgery and evaluated postoperative neurosensory function with two-point discrimination, light touch discrimination, and a thermocryesthesiometer during the first year after surgery. They concluded that neurosensory dysfunction occurred for all cases in which the mandibular canal was in contact with the lateral cortical plate of the mandible, and that a marrow space 0.8 mm or less between the mandibular canal and external cortical bone was more likely to associate with neurosensory disturbance. Two of the most common causes of injury to the IAN during BSSRO are the variable anatomy of the canal and the relationship of the IAN to the external cortex of the mandible1819. Injury to the nerve bundle most commonly occurs during vertical buccal osteotomy, splitting of segments, screw placement or compression by rigid fixation or during dissection, and osteotomy of the medial side of the ramus572021. The IAN canal has a S-shaped course in the ramus and mandibular body. From its entrance to the mandibular foramen, it is close to the lingual cortex; in the region of the third molar, it is most lateral and closest to the buccal cortex; and in the first molar region, it approaches the lingual cortex and finally exits the mental foramen near the premolar region222324. In our study, we obtained CBCT data in the experimental group that allowed us to determine the exact anatomic position of the nerve bundle and secure it from injury during medial dissection, osteotomy, vertical buccal osteotomy, and splitting of segments. This allowed clinicians to minimize neurosensory dysfunction after BSSRO. The reason neurosensory function on the right side improved more rapidly than that on the left side might be due to better access by the surgeon to the right side, resulting in better handling of the tissues and less traction-induced damage. The only other study that has made this comparison found no significant differences between sides2, possibly due to small sample size or greater caution exercised during surgery. Future studies are needed for further evaluation of the effect of side, since it might differ from case to case depending on the dominant hand of the surgeon or other operational habits.

Neurosensory disturbances could be attributed to age4. In this study, we did not observe a link between age and neurosensory dysfunction despite a wide range of ages among our subjects (and age range might matter1), but this may reflect the relative youth of our patients rather than an actual lack of association. We did not find the extent of mandibular displacement to be associated with neurosensory deficits, which might be attributed to the low variations in displacement.

This study was limited by some factors. We did not use objective methods of neurosensory evaluation such as blink reflex, electroneurography, or other neurophysiological techniques in our analysis. It remains unclear whether objective or subjective evaluations are superior. A systematic review showed that the incidence of nerve dysfunction is higher after subjective evaluations than objective ones. After one year of evaluation, objective methods determined an incidence of neurosensory dysfunction of 12.8%, while subjective methods yielded an incidence of 18.4%25. Other studies also showed that subjective evaluations of neurosensory dysfunction were associated with greater frequency of detection of sensory impairment than objective methods52627. One study found that the results of subjective evaluations of neurosensory function were comparable to the results of two-point discrimination28. The most important evaluation factor is patient satisfaction, and good results of aesthetic surgery may be negatively affected by a neurosensory dysfunction. Therefore, we applied subjective evaluations of neurosensory function in the present study. Injury to the IAN can be avoided by applying surgical techniques such as intraoral vertical ramus osteotomy, which yields an incidence of neurosensory dysfunction that is lower than that of BSSRO but has limitations in cases of mandibular deficiencies that require advancement and does not allow rigid fixation that requires a period of maxillomandibular fixation262930. The use of the Lindemann bur might be traumatic, especially when the site of surgery is very close to the nerve31. However, this was a routine protocol and is also a limitation of many other high- and (especially) low-speed rotating instruments. Moreover, since the protocol was standardized for both groups, it could not affect study outcomes. Finally, the method of randomization used in this study was nonstandard, and we did not use more popular methods such as a random number table.

In this study, we found that CBCT imaging can allow a surgeon to perform more conservative procedures and therefore indirectly contributes to acceleration of improvement in neurosensory functions over a 1-year period after treatment. We also found that neurosensory disturbances following BSSRO were ameliorated faster on the right side, perhaps due to surgical convenience when operating on that side.