Purpose: To evaluate effects of transposition on extraocular rectus muscle paths in middle and deep orbit.
Methods: The effect of various transposition procedures was assessed in five patients, using surface coil magnetic resonance imaging (MRI), performed with controlled gaze before and after surgery. Path changes were compared with those expected under conventional concepts of functional orbital anatomy, quantified by biomechanical modeling.
Results: Vertical rectus transpositions of 6 to 10 mm produced changes in muscle paths of 3 mm or less, assessed posterior to the equator of the globe. Lateral rectus transpositions as large as 10 mm resulted in almost no movement of muscle bellies. Conventional modeling predicted much larger changes.
Conclusion: The authors observed less movement of rectus muscle bellies relative to orbital walls than would be expected under the traditional assumption that transposed muscles follow the shortest path from origin to insertion. This implies that middle and deep orbital tissues (connective tissues and compartmentalized orbital fat) constrain the paths of rectus muscle bellies, preventing them from sliding freely to follow their transposed insertions. The authors propose that these tissues function as "pulleys" elastically coupled to the orbital wall, and that they are important determinants of extraocular muscle function.