Background Endoscopic extended maxillary mega-antrostomy (EMMA) is a mucosal sparing technique that allows maxillary drainage by gravity, with a reported symptomatic nasolacrimal duct injury incidence of 0–4%, based on history alone. Injury to the nasolacrimal duct is known to cause epiphora, a complication that is rare but more often seen in this revision surgery. Objective The goal of this study was to determine the incidence of nasolacrimal system penetration during EMMA. We, in addition, sought to determine the minimal safe distance from the midpoint of the maxillary line (the “M” point) to the nasolacrimal system to avoid this injury. Methods Six cadaveric heads underwent bilateral Jones II test followed by EMMA. Measurements from the M point to the antrostomy were recorded. The Jones II test was then repeated to assess for penetration and/or injury of the nasolacrimal system. If no penetration occurred at the surgical limit of EMMA, then dissection was continued incrementally until penetration occurred. This measurement was recorded. Results Lacrimal duct violation was identified in 5 of 12 procedures (42%). Lacrimal duct penetration occurred at an average of 3.7 mm (range, 2–7 mm) posterior to the M point. Conclusion Subclinical lacrimal system injury is likely to occur during EMMA. These findings would indicate that maintaining a distance of >7 mm from the maxillary line could avoid this injury.