Examination of the Topographical Anatomy and Fetal Development of the Tendinous Annulus of Zinn for a Common Origin of the Extraocular Recti

Abstract

The aim was to clarify the topographical anatomy of the common tendinous ring for the four rectus muscles in both adults and fetuses. METHODS. We histologically examined the annular ligament for a common origin of the extraocular rectus muscles using 10 specimens from elderly individuals and 31 embryonic and fetal specimens. RESULTS. At 6 to 8 weeks, each rectus carried an independent long tendon, individually originating from the sphenoid. Notably, we found additional origins from the optic or oculomotor nerve sheath. At 12 to 15 weeks, the lateral, inferior, and medial recti muscles were united to provide a C-shaped musculofibrous mass that was separated from the superior rectus originating from the edge of the optic canal opening. Morphologic features at 31 to 38 weeks were almost the same as those at 12 to 15 weeks, but the long and thick common tendon of the three recti reached the sphenoid body in the parasellar area. In adults, a ring-like arrangement of the rectus muscles ended at a site 8.1 to 12.0 mm anterior to the optic canal opening and independent of the superior rectus origin, the lateral, inferior, and medial recti formed a C-shaped muscle mass. The united origins of the three recti changed to a fibrous band extending along the superomedial wall of the orbital fissure. CONCLUSIONS. Consequently, none of the specimens we examined exhibited an annular tendon representing a common origin of the four recti, suggesting that the common tendinous ring includes only medial, lateral, and inferior rectus muscles with the superior rectus taking its origin independently.

Examination of the Annular Tendon (Annulus of Zinn) as a Common Origin of the Extraocular Rectus Muscles: 2. Embryological Basis of Extraocular Muscles Anomalies

“…The upper part of the ring, known as the superior tendon of Lockwood, gives origin to the SR, the rest of the MR, and the upper head of the LR. 11 However, one of the most recent studies suggested an independent origin of the SR. 12 We also described a consistent accessory head of the MR near an origin of the SO or LPS. These interpretations were based on semiserial sagittal sections along the long axis of the orbit in seven late-stage fetuses.…”

“…Thus, the present results indicate a need to revise the location of the accessory head of the MR reported in our previous schematic diagram. 12 In fact, the accessory head of the MR passed along the lateral side (not the medial side) of the SO. Our revised diagram (Fig.…”

“…33 Indeed, the initial extraocular muscles can have transient origins from the optic or oculomotor nerve because of the delayed development of cartilage. 12 Tawfik and Dutton 19 reviewed such a muscle insertion to a nerve.…”

“…We examined histological sections from 25 late-stage fetuses, which included contralateral sides of 7 previously examined fetuses. 12 The mean gestational age was 32 weeks (range, 28-40 weeks) and the median crown-rump length (CRL) was 276 mm (range, 217-340 mm). All heads were cut along the midsagittal line, and a left or right half was used for analysis.…”

“…The university ethics committee approved this study (B08/374). Sagittal sections along the long axis of the orbit from another fetus (CRL, 276 mm), which was prepared for our recent study, 12 are shown for comparison.…”