What are the external scleral and ocular landmarks for estimating the location of internal ocular structures?

External Scleral and Ocular Landmarks for Internal Structures

The external scleral landmarks for locating internal ocular structures are based on axial length-related measurements, with the electronics case and implant positioning determined by precise external fixation tables that correlate surface anatomy to underlying macular and retinal structures. 1

Key External Anatomical Landmarks

Rectus Muscle Insertions

• The four rectus muscles serve as primary external reference points for surgical planning and device positioning 1
• These muscles are isolated during 360° conjunctival peritomy to establish orientation for deeper structure localization 1

Superotemporal Quadrant Positioning

• The superotemporal quadrant is the standard external reference zone for positioning devices relative to internal structures 1
• Precise measurements from this external location correlate with optimal electrode array placement over the macula internally 1
• Axial length-related tables provide specific measurements from external scleral fixation points to ensure accurate positioning over internal retinal structures 1

Pars Plana Location

• The pars plana serves as the external landmark for accessing the vitreous cavity 1
• A 5.2 mm sclerotomy through the pars plana allows insertion of arrays while avoiding damage to internal structures like the ciliary body 1
• The sclerotomy should be directed perpendicular to the sclera to prevent ciliary body detachment, demonstrating the critical relationship between external approach angle and internal structure preservation 1

Internal Structure Correlation

Scleral Spur as Internal-External Bridge

• The scleral spur represents a critical anatomical landmark visible on both external examination (via gonioscopy) and internal imaging 1, 2
• The ciliary muscle method is the most successful and least variable approach for identifying the scleral spur position, with 81% success rate and superior intraobserver and interobserver variability compared to other methods 2
• Scleral spur identification is more difficult in narrow angle and brown eyes 2

Limbal Structures

• The perilimbal region contains the trabecular meshwork, which filters aqueous into Schlemm's canal 3
• This specialized scleral zone connects external corneal landmarks to internal anterior chamber angle structures 3

Posterior Segment Landmarks

• The lamina cribrosa at the posterior sclera permits optic nerve axon exit and can be visualized with enhanced depth imaging OCT 4, 3
• Short posterior ciliary arteries and their branches through emissary canals in the sclera were visualized in 86% of eyes, providing vascular landmarks 4
• The central retinal artery and vein locations were identified in all eyes, maintaining consistent relationships to external scleral anatomy 4

Clinical Implications

Surgical Planning Considerations

• External fixation measurements must be precisely calculated based on axial length to ensure optimal internal positioning, as this is identified as a critical component affecting electrode array placement over the macula 1
• Sclerotomy construction requires attention to wound geometry—straight incisions perpendicular to sclera prevent internal complications 1
• Long scleral passes with mattress sutures increase vector forces for wound closure, preventing hypotony from inadequate sclerotomy closure 1

Imaging Correlation

• Anterior segment OCT can visualize the relationship between external scleral landmarks and internal angle structures with 95.7% Dice coefficient accuracy 5
• The scleral spur position significantly affects angle parameter measurements—variability in scleral spur placement has large effects on angle parameters and iris concavity ratio 2

Common Pitfalls

• Assuming small amounts of wound oozing will resolve spontaneously—this should never be assumed and requires thorough checking for leakage 1
• Using curved or chevron-shaped sclerotomies instead of straight incisions, which causes cable and wound puckering 1
• Failing to account for axial length variations when translating external measurements to internal structure locations 1