To find out whether the angulation of mandibular third molars is associated with the thickness of the bone at the site of impaction, and if so, which particular angulation affects the protective mechanism of the nerve, we retrospectively studied the thickness of lingual bone at the sites of impaction of 200 mandibular third molars in 149 patients using coronal, sagittal, and axial slices of cone-beam computed tomograms (CT). We measured the bone at the cementoenamel junction of the mandibular second molar, at the mid-root of the third molar, and at the apex of the root. Bone less than 1mm thick was defined as "thinning". We correlated these measurements with the angulation of the tooth based on the position of the second molar and the occlusal plane in 3 dimensions: vertical, mesiobuccal, and buccolingual. The primary outcome was the thickness of the bone around the third molar. A total of 102 teeth were on the left (51%), and 125 were angulated with an occlusal plane of <85° (63%). The mean (SD) thickness of bone at the cementoenamel junction of the second molar was 1.40 (0.87) mm, at mid-root 1.07 (1.03) mm, and at the apex 1.07 (1.30) mm. When the horizontal and mesioangular angulations of teeth were <85°, the thickness of bone at the mid-root differed significantly from that when the vertical and distoangular angulations were 85° or more (p<0.001). Correlations between the thickness of the bone and the buccolingual angulations were significantly associated with perforation of the bone at mid-root and apex (p<0.003). The bone around horizontal and mesioangular impactions was 3.6 times more likely to be "thin" than that at mid-root of vertical and distoangular third molars. A buccolingual angulation was also associated with perforation of the lingual cortex (p<0.003). As the bone was thinner at the mid-root of horizontally and mesioangularly impacted teeth, it seemed to compromise the integrity of the lingual plate, which is the natural protective barrier of the lingual nerve. These findings could be of prognostic value.