Posterior inflection of weakened lateral rectus path: connective tissue factors reduce response to lateral rectus...

To determine why lateral rectus (LR) muscle recession has a variable effect on binocular alignment using magnetic resonance imaging (MRI).

Prospective, observational, interventional case series.

Posterior LR muscle path lengths from the orbital apex to first globe contact were determined by axial plane, surface coil MRI in eight patients with unilateral LR muscle palsy and in four patients before and after bilateral LR muscle recession.

Posterior paths of paretic LR muscles were 2.2 to 6.0 mm longer (mean, 3.4 mm; P = .0002) than normal contralateral paths. Each paretic LR muscle was sharply inflected laterally at a point in the anterior orbit corresponding to the histologic location of the LR muscle pulley sleeve. Every recessed LR muscle was 0.8 to 4.4 mm (mean, 2.4 mm; P = .0008) longer after surgery than before surgery, with less temporal deflection.

The LR muscle pulley suspension contributes to LR muscle tension, tightening the muscle belly by stretching it temporally when LR muscle tone is reduced. The increase in LR muscle path length resulting from temporal inflection offsets the effect of recession by up to 4 mm. Connective tissue action explains some response variability after LR muscle recession.