Relation of the Vector Force Needed to Lift the Upper Eyelids and the Degree of Exophthalmos.

The aim of this study was to determine the relation of the vector force needed to lift the upper eyelids and the degree of exophthalmos (EX). In the 109 magnetic resonance imaging films, the degree of EX (the shortest distance from the cornea to the line connecting both lateral orbital rims), the anterior angle (θ, an angle formed from the lower margin of the upper eyelid-superior transverse ligament (STL)--with a parallel line connecting the supraorbital rim and the infraorbital rim in the sagittal film), the length from the STL to the upper eyelid margin (levator length [LL]), the thickness of the STL (WT), and the thickness the of levator palpebrae (LT) were measured.The average EX was 14.5 ± 2.35  mm. The average θ was 33.84 ± 2.15 degrees. The vector force needed to lift the upper eyelids (cos θ) was 0.83. The average LL was 21.0 ± 1.54  mm. The average WT was 1.07 ± 0.22  mm. The average LT was 1.69 ± 0.30  mm. There was a significant positive relationship between the EX and age (P = 0.022). The EX in those younger than 20 years (12.8 ± 2.06) was significantly lesser than that of the other age groups. There was no significant relationship between the EX and cos θ. However, there was a significant positive relationship between the EX and the LL. There was a significant positive relationship between LL and LT, and between LL and WT.The farther the eyeball protrudes, the longer the LL is needed. The longer the LL is, the thicker the levator muscle and STL.