To ascertain the cause of undercorrection of the medial palpebral fissure in blepharoptosis surgery.
Twelve upper eyelids of 6 Asian cadavers, 4 male and 2 female (average age at death, 77.5 years), were studied. After exposing the levator muscle by removing bone of the superior orbital rim, the levator muscle and medial and lateral horns of aponeurosis were observed.
The levator muscle was located in the anterolateral direction. Bifurcation angles of the medial horn were steeper than those of the lateral horn, but the lateral horn was wider than the medial horn (Student's t test; P < 0.0001). The inferior edge of the superior expansion of the aponeurosis extensively covered the lateral horn but only slightly covered the medial horn. Accordingly, most of the medial horn was constituted by only the thin structure of the middle and inferior expansion of the aponeurosis.
Because the medial horn is structurally weaker and less dynamic than the lateral horn, the lateral side of the aponeurosis is pulled more strongly than the medial, often resulting in medial undercorrection in blepharoptosis surgery. To prevent this, the medial part should be fixed more widely than the lateral.