Evaluation of the equine mental foramen block: cadaveric and in vivo injectate diffusion.

OBJECTIVE: To describe injectate diffusion for two equine mental foramen block techniques. STUDY DESIGN: Descriptive anatomic study. ANIMALS: A total of 12 equine heads and three live horses. METHODS: Equine heads were longitudinally sectioned to create 24 hemi-heads for testing two mental foramen block techniques (T1 and T2) and two injectate volumes (3 and 5 mL) of mixed dye and contrast medium. T1 needles were directed rostrocaudally into the mental foramen for 3 cm, and T2 needles were directed dorsolaterally to ventromedially into the foramen for 1 cm. Hemi-heads were randomly assigned one injection technique and volume. Radiographs evaluated needle tip positioning, distance traveled by injectate and injectate diffusion pattern. Specimens were dissected to measure the length of circumferential nerve staining. The more effective technique was tested in three live horses and evaluated via computed tomography. Summary statistics described results. RESULTS: Neither injection technique nor injectate volume had a significant impact on circumferential nerve staining. Circumferential nerve staining, median (range), was 15 (0-33) mm for T1 and 10 (0-42) mm for T2. Injectate diffusion patterns revealed that injectate was more likely to thread alongside the inferior alveolar nerve for T1 (9/12) and bolus around the rostral inferior alveolar nerve for T2 (9/12). Bolus diffusion patterns were associated with increased circumferential nerve staining ≥1 cm (9/24) when compared with thread patterns (6/24). Diffusion of injectate within the mandibular canal was greater with 5 than 3 mL. In vivo testing of T2 with 5 mL injectate resulted in decreased incidence of circumferential nerve staining ≥1 cm [median (range) 5 (0-14) mm]. CONCLUSION AND CLINICAL RELEVANCE: T2 created an injection diffusion pattern more likely to result in circumferential nerve staining ≥1 cm, but the low incidence of in vivo circumferential nerve staining ≥1 cm suggests that block efficacy may vary.

Anatomic analysis of the equine mental foramen and rostral mandibular canal using computed tomography.

OBJECTIVES: To characterize the anatomy of the mental foramen and determine associations with age, weight, sex and breed. STUDY DESIGN: Retrospective descriptive study. ANIMALS: Forty-one horses, 0.6-25.2 years and weighing 136-820 kg. METHODS: Computed tomography (CT) studies of equine heads performed over 5 years were evaluated in multiplanar and three-dimensional reconstruction. Measurements obtained were positioning of the foramen along the mandible, foramen height and width, foramen-canal angulation and rostral canal mineralization. Exploratory statistical analyses investigated associations between measurements and age, weight, sex and breed. RESULTS: Evaluation of 41 CT studies revealed foramen positioning was one-third of the horizontal distance of the incisor-premolar space from the second premolar and one-third of the vertical distance in the mandible from the dorsal surface of interproximal space at the level of the foramen. Age was negatively correlated to horizontal positioning [r = -0.42; 95% confidence intervals (CI), -0.64 to -0.13]. Mean foramen width was 6.4 (range, 2.3-17.1) mm and height 5.6 (range, 2.1-10.3) mm. Age and foramen height (r = 0.32; 95% CI, 0.01 to 0.57) and width (r = 0.30; 95% CI, -0.01 to 0.30) were positively correlated. Thoroughbred and Warmblood/Draft horses had increased foramina heights compared with Quarter horses [analysis of variance (anova)p = 0.02]. Males had wider foramina (95% CI, -2.66 to 0.07). Angulation of the foramen-canal was mean ± standard deviation 68.3 ± 12.8° (range, 37.9-105.6°) transverse plane, 28.3 ± 4.1° (range, 18.2-39.0°) saggital plane and 41.4 ± 8.45° (range, 22.2-58.6°) dorsal plane. Older horses had decreased mineralization of the rostral canal (anova, left p = 0.015, right p = 0.025). CONCLUSIONS: The size, shape, positioning and angulation of the mental foramen varies. Mineralization of the canal is decreased in older horses. CLINICAL RELEVANCE: This study improves understanding of mental foramen anatomy and complications with needle placement.