A study reported in the Journal of Oral Implantology used 3 methods to measure the anterior loop of the mental nerve on 12 human cadavers—cone beam computed tomography (CBCT), a 3-dimensional stereolithographic (STL) model, and anatomy. Injury to the anterior loop of the mental nerve can cause sensory disturbance, most notably numbness or altered sensory perception. Reports on the length and location of the mental nerve vary widely between patients. One study found the anterior loop in 28% of the patients. However, another study reported the anterior loop to be present 88% of the time. Some clinicians recommend maintaining a safety margin of one mm between implants and the nerve, while others suggest as much as a 6-mm distance. Because of conflicting reports, a variety of methods have been used to detect and measure the anterior loop. It has been determined that panoramic and periapical radiographs do not provide information about the loop that is reliable enough for clinicians to use in placing implants. This study seeks to determine the accuracy of CBCT and STL in identifying and measuring the anterior loop. The CBCT was found to be accurate and reliable; however, the STL was found to significantly both overestimate and underestimate the anterior loop.
Thus, the authors make the following recommendations: (1) CBCT should be a prerequisite in identifying and measuring the anterior loop of the mental nerve for implant surgery, (2) a fixed distance from the mental foramen should not be used as a safety guideline; rather, the anterior loop itself should be located, (3) a safety distance of at least 2 mm from the anterior-most portion of the loop should be observed in implant placement, and (4) the STL model should be used with caution; at this time, the model has not been shown to be highly accurate in estimating the anterior loop.
(Source: Santana RR, et al. Accuracy of cone beam computerized tomography and a three-dimensional stereolithographic model in identifying the anterior loop of the mental nerve: a study on cadavers. Journal of Oral Implantology, 2012, Volume 38, Number 6, pages 669 to 676)