Preoperative Implant Site Analysis: CBCT, the Inferior Alveolar Canal, and At-Risk Anatomical Structures

Preoperative assessment of the implant site through CBCT (Cone Beam Computed Tomography) isn't a diagnostic formality but the clinical prerequisite for safe surgery. At-risk anatomical structures in implant surgery — the inferior alveolar canal in the mandible, the maxillary sinus and nasal floor in the maxilla, the mental foramen and its anterior branches — can be damaged by unplanned surgical maneuvers with permanent consequences for the patient. Documented adequate preoperative CBCT assessment is, in countries with more established dental case law, an essential safeguard for the clinician. Thorough understanding of the radiographic anatomy of at-risk structures — and its variants — is the skill that distinguishes passively reading a CBCT from actively planning surgery.

The inferior alveolar canal (IAN canal) is the critical anatomical structure for mandibular implantology. The canal's path from the mandibular lingula to its emergence at the mental foramen shows significant individual variability: the canal's vertical position relative to the residual alveolar crest ranges from 5 to 20 mm depending on the degree of post-extraction resorption. In severely atrophic mandibles (Cawood-Howell type V-VI), the canal can be less than 5 mm from the residual crest or even surface at the crest — a situation making standard implantology impossible without surgical nerve repositioning or the use of short implants. CBCT with voxel size ≤0.2 mm allows tracing the canal in multiplanar sections with ±0.5 mm accuracy, enabling precise measurement of available bone height above the canal and planning of the safety distance (standard: 2 mm between the implant apex and the canal's upper margin).

Anatomical variants of the mental foramen and accessory canals are frequently underestimated in implant planning. The anterior loop of the mental nerve — an anterior loop of the inferior alveolar bundle extending anterior to the foramen before emerging — is present in 17-88% of the population (enormous variability tied to study methodology and adopted definition) and extends 0.5-6 mm anterior to the foramen. Failing to account for this loop can lead to nerve damage during implant surgery in the premolar area, even when the implant apex is positioned "anterior" to the visible foramen. Accessory mental foramina — small aberrant canals emerging buccally or inferiorly to the main foramen — are present in 2-10% of patients and can be mistaken for the main foramen in two-dimensional planning. CBCT allows distinguishing and precisely mapping these foramina.

Intraosseous vascularization of the lateral maxillary sinus wall is relevant for lateral sinus lift surgery. The posterior superior alveolar artery (PSAA) and its intraosseous branches anastomose with the infraorbital artery, forming a vascular arcade at the lateral sinus wall level, visible on CBCT as an intraosseous canal 0.5-2.5 mm in diameter in 50-70% of patients studied by Mardinger et al. (J Periodontol, 2007). Damage to this vessel during lateral window preparation causes bleeding that compromises surgical visibility and control of the Schneiderian membrane. Planning the lateral window osteotomy must preoperatively identify the vascular canal's position — typically 16-19 mm above the alveolar crest — and place the window's upper border below this level.

Bone quality analysis on CBCT — through densitometric measurement in Hounsfield Units (HU) on CBCT reconstructions — is a growing application in implant planning, although with important methodological limitations. HU values in CBCT aren't directly comparable to multislice spiral CT values — the same bone tissue measured with different CBCT scanners can show HU values differing by 30-50% due to scatter radiation and beam hardening effects. HU measurement in CBCT is useful for relative assessment (comparing different areas within the same exam) but not for absolute bone density classification. The Lekholm-Zarb clinical classification (D1-D4), based on tactile sensation during surgical preparation, remains the clinical gold standard for intraoperative bone quality assessment.

The CBCT report prepared by the implant clinician — or by the maxillofacial radiologist in collaboration with the clinician — should systematically document: available bone height and width at the placement site, distance from the nearest at-risk structure with recommended safety margin, identification of anatomical variants (accessory canals, mental loop, sinus septa, intraosseous arteries), qualitative bone density assessment, and identification of incidental pathology (residual roots, osteolytic lesions, sinus asymmetries). This structured report — documented in the patient's chart — forms the basis for patient informed consent (who must be aware of at-risk structures present at their site) and for traceability of the planning in case of complications.