Paper 21

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Why Medial Rectus (MR) Recession is more Powerful Per Millimeter than Lateral Rectus (LR) Recession:
The Globe’s Rotational Axis is Not Where You Thought It Was

Robert A Clark  and Joseph L Demer
Stein Eye Institute
Los Angeles, CA

Introduction:  Surgical dosing tables specify greater LR than MR doses for horizontal strabismus, a difference unexplained by models that assume globe rotation about its center. We tested this assumption by measuring angular displacements of lenses and globe-optic nerve (ON) junctions during horizontal ductions.

Methods:  Eighteen orthophoric adults underwent high-resolution, axial orbital magnetic resonance imaging fixating targets in central gaze, abduction, and adduction. Lines connecting corneal apices through minor lens axes to retinas approximated clinical ductions. Globe centers were calculated from area centroids of the largest globe cross-sections omitting corneas. Lens and globe-ON junction rotations around globe centers were compared with clinical ductions.

Results: Globe-ON junctions rotated significantly less around globe centers than did lenses in abduction (20.6°±4.7° versus 27.4°±7.4°, p<10-15) and adduction (25.3°±6.7° versus 31.9°±8.3°, p<10-14). Both rotations differed significantly from clinical adduction (27.9°±8.3, p<0.007 for both), but only in abduction was globe-ON junction rotation significantly less than clinical abduction (28.6°±9.4°, p<10-8). True globe rotational center was 2.2±0.5mm nasal and 0.8±1.0mm posterior to geometric globe center and shifted farther medially and posteriorly during adduction. This location gives each mm of MR recession approximately 30% more trigonometric rotational effect than equivalent LR recession.

Discussion: The large nasal and posterior decentration of the globe’s rotational axis is likely caused by the globe-ON attachment and profoundly influences horizontal rectus action, explaining why effects of smaller MR recessions are equivalent to larger LR recessions.

Conclusion: The globe’s rotational axis is decentered nasally and posteriorly from globe center, augmenting MR recession and diminishing LR recession.

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