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

Abstract

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.

Keywords: analgesia; anesthesia; dentistry; horse; mandible.