The use of computerized tomography (CT) in the assessment of nerve injuries is very limited, although it has been used more recently for assessment of the inferior alveolar canal with regard to the position of the third molar. An evaluation of bone window attenuation images may indicate violation of the cortical outline of the inferior alveolar canal, either from the roots of the third molar, iatrogenic implant placement, or following facial trauma (e.g., posterior mandible fracture) (Fig. 11.3), but yields little information regarding the condition or integrity of the IAN itself or the neurovascular bundle. The use of soft tissue window CT images for the LN or IAN is compromised by very poor detail resolution that precludes its routine application in neural assessment. Furthermore, dental artifacts often pose severe limitations in obtaining accurate information regarding the position of the LN to the lingual cortex of the mandible in critical areas, even in the soft tissue window views, and despite the current use of high-resolution image acquisition.

In 1998, CT cone beam (CBCT) technology, previously used only in angiographic imaging, was employed in the United States as a potential imaging modality for the maxillomandibular complex. The presurgical evaluation of impacted mandibular third molar relationship to the IAN has gained popularity over conventional CT scanning and plain panoramic radiographs among oral and maxillofacial surgeons. The need for accurate imaging with the lowest possible dose of radiation (i.e., ALARA rule, as low as reasonably achievable) seems to be satisfied acceptably with this technology. CBCT provides the desirable three-dimensional representation of the anatomic location of interest, with minimal distortion compared to traditional plain films and by simpler acquisition compared to traditional CT systems. Similar to the panoramic radiograph, CBCT can be used for preoperative risk assessment in various dentoalveolar procedures such as third molar surgery or dental implants and preprosthetic surgery. A major limitation, of course, remains the inability to visualize the IAN itself (within the inferior alveolar canal), or the LN, since no accurate soft tissue information can be obtained with use of CBCT (Fig. 11.4).

Magnetic resonance imaging (MRI) is the method of choice for radiographic visualization of all cranial nerves (CN), and each nerve segment can be seen and examined in detail with specific MRI sequences. Due to the complexity of the course f the CNs and surrounding anatomic structures, detailed examination of the CNs is made possible only with careful planning and selection of the specific MRI technique. The imaging plane, coil selection, slice selection, in-plane resolution, and use of special techniques can be tailored based upon the individual CN, and the segment of interest so that the highest possible image quality may be obtained. The trigeminal nuclei (intra-axial), cisternal portion (preganglionic), and Meckel’s cave (intradural) segments contain both the motor and sensory components of the trigeminal nerve and can be visualized with high-resolution T1- or T2-weighted MRI images. At the anterior aspect of the Gasserian ganglion, the sensory root divides into the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions and each may be followed and examined separately based upon their known course peripherally. The course of the LN and IAN branches of the mandibular division, after exiting from foramen ovale, can be followed with high-resolution, contrast-enhanced, T1-weighted (T1W), or T1W three-dimensional, fast-filed echo (T1W 3D FFE) sequences in the axial, coronal, and sagittal, or parasagittal, planes. Although detailed information can be obtained with the use of MRI, routine presurgical evaluation of the route and integrity of the LN and IAN is not standard practice. Rather the MRI is employed as the preferred imaging modality for examination of the status of the CNs most commonly in the presence of a disease process or following brain or nerve injury.

Miloro et al. have used high-resolution MRI (HR-MRI) in an attempt to document the in situ position of the LN in the third molar region directly, without surgical manipulation or tissue distortion artifact as in the studies by Kisselbach and Pogrel. Ten patients (20 sides) without prior dental surgery were imaged using an HR-MRI sequence (PETRA-phase encode time reduction acquisition) that enabled direct visualization of the LN (Fig. 11.5). This study documented that the lingual nerve position, while variable, was indeed vulnerable during third molar surgery, and the LN was found to be superior to the lingual crest in 10 % of cases, and in direct contact with the lingual plate in 25 % of cases. Kress and colleagues have been able to image the IAN using T2-weighted MRI imaging to visualize the IAN in the in situ position as well as in cases of mandibular fractures (Fig. 11.6); yet there is still lack of detailed resolution of the fascicular anatomy of the IAN itself.

Several reports have described the use of ultrasonography (US) and high-resolution ultrasound technology primarily for the assessment of peripheral nerve lesions. This real-time advanced technology, with recently available high-resolution probes, can offer compound imaging without radiation and in a relatively inexpensive manner. Although US has not been employed or investigated as a potential preoperative risk assessment tool for the trigeminal nerve, it has been demonstrated to be valuable in identification and safe advancement of the needle in brachial plexus and sciatic nerve blocks. It would be reasonable though to anticipate limitations with the use of US in examination of the IAN in the third molar region due to the presence of bone and teeth that might affect the echogenic signal. Visualization and documentation of the course and integrity of the LN on the other hand should be relatively easy with US, requiring only minimal training and familiarity of the operator with the regional oral anatomy (Fig. 11.7).

The majority of current interest is in documenting the post-injury condition of the nerve by objective means, since the information gathered by clinical and radiologic examination could be useful in staging the degree of neural injury, determining the prognosis for recovery, and planning microneurosurgical intervention. With increased image resolution, the precise degree of architectural disruption of the nerve could be visualized, and surgical intervention could be planned accordingly. Additionally, this information could be used to document the exact location of the injury prior to surgical exploration for repair. For example, this documentation could help to avoid surgical nerve exploration in the third molar region if the injury occurred in the pterygomandibular space as a result of a mandibular block injection injury. The information may also be used to determine the size and extent of the neuromatous segment and the possible need for a nerve graft. Finally, radiologic techniques could be used to objectively monitor neurosensory progression, in conjunction with clinical examination, either after nerve injury or in the post-repair phases of neural recovery.