Clark Robert A, Demer Joseph L
Department of Ophthalmology, University of California, Los Angeles, California; Stein Eye Institute, University of California, Los Angeles, California; David Geffen Medical School at the University of California, Los Angeles, California.
Department of Ophthalmology, University of California, Los Angeles, California; Stein Eye Institute, University of California, Los Angeles, California; Department of Neurology, University of California, Los Angeles, California; Neuroscience Interdepartmental Program, University of California, Los Angeles, California; Bioengineering Interdepartmental Program, University of California, Los Angeles, California; David Geffen Medical School at the University of California, Los Angeles, California.
Ophthalmology. 2018 Aug;125(8):1234-1238. doi: 10.1016/j.ophtha.2018.03.003. Epub 2018 Mar 30.
Tables typically recommend greater lateral rectus (LR) than medial rectus (MR) surgical doses for horizontal strabismus of any given magnitude, a difference unexplained by mechanical models that assume globe rotation about its center. We tested this assumption during horizontal ductions.
Prospective observational study.
Eighteen adult subjects with normal binocular vision.
Surface coil magnetic resonance imaging at 390 or 430 μm resolution was obtained using 2-mm-thick contiguous axial planes while subjects fixated targets in central, right, and left gaze. Angular displacements of lines connecting the corneal apex through the minor lens axis to the retina were measured to approximate clinical ductions. Globe centers were calculated from their area centroids. Apparent lens and globe-optic nerve (ON) junction rotations around the globe center were then compared with clinical ductions.
Apparent angular rotations of lenses and globe-ON junctions during horizontal ductions.
Globe-ON junctions appeared to rotate significantly less around globe centers than did lenses for abduction (20.6°±4.7° vs. 27.4°±7.4°, ± standard deviation (SD), P < 0.001) and adduction (25.3°±6.7° vs. 31.9°±8.3°, P < 0.001). Both rotations differed significantly from clinical adduction (27.9°±8.3°, P < 0.007), but only in abduction was globe-ON junction rotation significantly less than clinical abduction (28.6°±9.4°, P < 0.001). The true geometric globe rotational center was 2.2±0.5 mm medial and 0.8±1.0 mm posterior to the geometric globe center and was displaced farther medially and posteriorly during adduction. This eccentricity imbues each millimeter of MR recession with approximately 30% more trigonometric rotational effect than equivalent LR recession.
The medial and posterior eccentricities of the normal ocular rotational axis profoundly influence horizontal rectus action. The proximity of the globe's rotational axis to the MR shortens its lever arm relative to the LR, explaining why mechanical effects of smaller MR recessions are equivalent to larger LR recessions.
对于任何给定程度的水平斜视，表格通常推荐外直肌（LR）的手术剂量大于内直肌（MR），但假设眼球绕其中心旋转的力学模型无法解释这种差异。我们在水平转位过程中测试了这一假设。
前瞻性观察性研究。
18名双眼视力正常的成年人。
使用2毫米厚的连续轴向平面，以390或430微米的分辨率进行表面线圈磁共振成像，同时受试者注视中心、右侧和左侧注视目标。测量连接角膜顶点通过晶状体短轴至视网膜的线的角位移，以近似临床转位。通过其面积质心计算眼球中心。然后将晶状体和眼球-视神经（ON）交界处围绕眼球中心的明显旋转与临床转位进行比较。
水平转位过程中晶状体和眼球-ON交界处明显的角旋转。
眼球-ON交界处围绕眼球中心的旋转在外展时（20.6°±4.7°对27.4°±7.4°，±标准差（SD），P<0.001）和内收时（25.3°±6.7°对31.9°±8.3°，P<0.001）明显小于晶状体。两种旋转与临床内收（27.9°±8.3°，P<0.007）均有显著差异，但仅在外展时眼球-ON交界处旋转明显小于临床外展（28.6°±9.4°，P<0.001）。真正的几何眼球旋转中心位于几何眼球中心内侧2.2±0.5毫米和后方0.8±1.0毫米处，在内收时向内侧和后方移位更远。这种偏心使得每毫米的MR后徙比同等的LR后徙具有约30%更多的三角旋转效应。
正常眼球旋转轴的内侧和后方偏心对水平直肌的作用有深远影响。眼球旋转轴靠近MR缩短了其相对于LR的力臂，解释了为什么较小的MR后徙的力学效应等同于较大的LR后徙。
