The lamina papyracea (orbital plate of the ethmoid) is the thin bony plate forming the medial wall of the orbit. It is the thinnest bone in the skull, measuring only 0.2-0.4 mm, and lies immediately lateral to the ethmoid air cells. The ethmoid labyrinth separates the orbit from the nasal cavity at this level, with the anterior and posterior ethmoid foramina transmitting the corresponding neurovascular structures across the superior part of the medial orbital wall.
The lamina papyracea is the most commonly fractured orbital wall and the primary route for orbital cellulitis spread from ethmoid sinusitis. In functional endoscopic sinus surgery (FESS), the lamina papyracea is the critical lateral landmark preventing inadvertent orbital fat prolapse and orbital penetration. The 24-36 mm rule for anterior and posterior ethmoidal artery positions guides safe surgical dissection. Blowout fracture medially through the lamina papyracea is less common than floor fractures but causes medial rectus entrapment with restricted abduction.
Ethmoid sinusitis erodes or passes through the thin lamina papyracea, allowing subperiosteal abscess formation between the lamina and the medial periorbita, producing proptosis, restricted and painful eye movement, and chemosis requiring urgent CT, IV antibiotics, and surgical drainage if no improvement within 24-48 hours.
Inadvertent penetration of the lamina papyracea during ethmoidectomy causes orbital fat prolapse into the operative field, recognised by the yellow lobular fat appearance, requiring immediate cessation of powered instrumentation and ophthalmological assessment for visual acuity and extraocular movement.
