Posterior fixation sutures: a revised mechanical explanation for the fadenoperation based on rectus extraocular muscle pulleys

PURPOSE: To determine the effect of the rectus extraocular muscle pulleys on the fadenoperation, an operation designed to fixate the posterior muscle belly to the underlying retroequatorial sclera. METHODS: First, duction into the field of action of the operated-on muscle was quantified retrospectively after fadenoperation. Magnetic resonance imaging was then performed prospectively after surgery to verify anatomic changes. Forced duction testing was performed prospectively during surgery before and after faden placement. Finally, computed tomography in a cadaver containing radiographic markers was performed prospectively to determine the effect of fadenoperation on the position of the medial rectus insertion relative to its pulley. RESULTS: Mean maximum adduction after medial rectus fadenoperation was 18 degrees (range, 10 to 25 degrees; 13 eyes). Fadenoperations combined with large medial rectus recessions restricted adduction more than fadenoperations combined with smaller recessions ( P = .019), but even fadenoperations without recessions substantially restricted adduction. Mean maximum abduction after lateral rectus fadenoperation was 40 degrees (range, 25 to 45 degrees; four eyes). Axial magnetic resonance imaging in two eyes demonstrated a smaller loss of muscle tangency to the globe during contraction than predicted by geometric models. Forced ductions in nine patients performed immediately after faden placement demonstrated a new mechanical restriction to duction toward the operated-on muscle. Cadaveric computed tomographic scans demonstrated posterior displacement of the medial rectus pulley during adduction after fadenoperation. CONCLUSIONS: Posterior fixation sutures do not significantly decrease muscle torque during contraction. Because posterior fixation sutures posteriorly displace the pulley sleeve during duction toward the operated-on muscle, the mechanical restriction after surgery probably represents the force deforming the pulley. This mechanical restriction may account for the limitation in duction seen after fadenoperation.