What are the locations of the equator, ora serrata, macula, and retina in relation to the scleral surface?

Anatomical Landmarks on the Scleral Surface

The equator is located approximately 13-14 mm posterior to the limbus on the external scleral surface, the ora serrata lies approximately 5-6 mm posterior to the limbus, the macula projects externally to the superotemporal quadrant (typically 3-4 mm temporal and 1 mm superior to the superior rectus muscle insertion), and the retina lines the entire internal surface of the globe from the ora serrata posteriorly to the optic nerve head. 1, 2

Equator Location

• The equator represents the widest circumference of the globe, positioned approximately 13-14 mm posterior to the limbus on the external scleral surface 2
• At this landmark, the sclera reaches its thinnest point at approximately 420 μm in humans, making it the optimal location for transscleral drug delivery compared to the limbus (500 μm) or posterior pole (860 μm) 3, 1
• The equatorial region's reduced thickness provides a critical surgical consideration for transscleral procedures and device positioning 1

Ora Serrata Location

• The ora serrata, the anterior termination of the retina, is located approximately 5-6 mm posterior to the limbus internally 3, 4
• This landmark represents a crescentic junction where the retina ends and the ciliary body begins 3
• The ora serrata is clinically significant as the site of retinal dialysis in traumatic retinal detachments, which is pathognomonic for ocular trauma 3, 4
• A cone-rich rim extends around the margin of the human retina at the ora serrata, though its functional significance remains debated 5

Macula Location on Scleral Surface

• The macula projects externally to the superotemporal quadrant of the globe, serving as the standard external reference zone for positioning devices relative to internal macular structures 1
• External fixation measurements must be precisely calculated based on axial length to ensure optimal internal positioning relative to the macula 1
• The four rectus muscles serve as primary external landmarks for surgical planning, with the superior rectus muscle providing a key reference point for locating the underlying macula 1

Retina Relationship to Sclera

• The retina lines the entire internal surface of the posterior globe, separated from the sclera by the choroid 3, 2
• The retina extends from the ora serrata anteriorly (5-6 mm posterior to the limbus) to the optic nerve head posteriorly 3
• The scleral thickness varies dramatically across its surface: 500 μm at the limbus, thinning to 420 μm at the equator, then thickening to 860-1000 μm near the optic nerve head 3, 1, 2

Critical Surgical Considerations

• Failing to account for axial length variations when translating external scleral measurements to internal retinal structure locations is a critical pitfall 1
• The pars plana, located approximately 3.5-4.0 mm posterior to the limbus, serves as the external landmark for accessing the vitreous cavity during vitreoretinal surgery 1
• In young eyes, the equator moves posteriorly with eyeball growth (approximately 1 mm over 2 months in animal models), while surgically recessed muscle insertions remain relatively stable 6

Clinical Implications for Drug Delivery

• The total scleral surface area measures approximately 16.3-17.0 cm² in human eyes, providing substantial area for transscleral drug delivery 2
• Less than 1/10⁸ of topically administered protein drugs reach retinal therapeutic targets due to barriers including the scleral stroma's glycosaminoglycan matrix and high water content (80%) 1
• The choroid functions as a drug clearance "sink" with extremely high blood flow (696±110 mg/min), rapidly removing drugs attempting to reach the retina from external scleral approaches 1