122 - Morphological Variation of the Infraorbital Canal in Horses

Poster Board Number: 122
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Abstract Body : A retrograde maxillary nerve block is a recently proposed anesthetic technique for equine orofacial surgeries of tissues that are innervated by the maxillary nerve. It involves passing a needle through the infraorbital foramen and caudally down the infraorbital canal to reach the maxillary nerve at the level of the maxillary foramen. Although the retrograde approach avoids the risk of collateral damage to periorbital structures associated with traditional approaches via the zygomatic arch, its reliability depends upon predictable variation in the anatomical structure of the infraorbital canal. To evaluate how the infraorbital canal varies as a function of age and laterality, the three-dimensional shape of the left and right infraorbital canals was reconstructed with visualization software 3D Slicer 5.2.1 from computed tomographic datasets of cadaveric horse heads (n=14) ranging in age from one-day-old to twenty-one years old. The shape of each 3D model was evaluated in MATLAB by measuring the distance between a cubic spline curve along the center of the canal and a straight line from the infraorbital to the maxillary foramen. If an infraorbital canal is straight and has no curvature, then the measured distance would be zero along its entire length, whereas a canal with curvature would result in a measured distance with a magnitude proportional to the degree of deviation from a straight line. Horses younger than three years of age have greater curvature of the infraorbital canal closer to the maxillary foramen, whereas horses older than three years of age have greater curvature of the canal closer to the infraorbital foramen. Furthermore, the relative position of the middle alveolar branch, which innervates the second premolar, shifts distally toward the infraorbital foramen during development. These changes are likely due to allometric growth of the skull. Within-age variation was also observed and is likely due to differences in timing of tooth eruption. Tooth eruption was also correlated with larger and more pronounced curvature of the infraorbital canal. Variation in the curvature of the left and right infraorbital canals was also observed in most specimens but was more pronounced in female horses. However, no statistically significant relationship was found between age, sex, or variation in laterality. These results suggest that the anatomy of the infraorbital canal varies somewhat unpredictably as horses age. Therefore, clinical procedures that presume a particular infraorbital canal morphology, such as retrograde maxillary nerve blocks, may be hampered by individual variation.