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Andrew Shin, Lawrence Hyun Yoo, Joseph L Demer; Viscoelastic Biomechanical Characterization of Z-myotomy, An Extraocular Muscle (EOM) Weakening Surgery. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4550.
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© ARVO (1962-2015); The Authors (2016-present)
Z-myotomy is a procedure in which two incisions of chosen width are made from opposite EOM margins that are separated along the EOM length, designed to treat strabismus by lengthening and weakening the EOM. We examined the biomechanics of Z-myotomy using tensile loading.
Fresh bovine rectus EOMs were reduced to the 20x10x2mm dimensions, and clamped in a custom micro-tensile load cell under physiological temperature and humidity. EOMs were elongated until failure following scissors incisions made from opposite sides, spaced 8 mm apart and each encompassing 0, 40, 50, 60, or 80% EOM width. Initial strain to 30% elongation, sampled using a precision strain gauge, was imposed at 100mm/s rate, after which elongation was maintained for >100 s during force recording at maintained deformation. Stress relaxation tests with non-incised specimens having widths ranging from 1 - 9mm were conducted for viscoelastic characterization of corresponding equivalence to 20 - 80% Z-myotomy. A 3- parameter viscoelastic model was employed for modeling.
Z-myotomy of EOMs progressively reduced force transmission to minimum failure strength at 60% or greater Z-myotomy. Each Z-myotomy specimen has equivalent failure force to a non-Z-myotomy specimen with a different width. Results of both tensile and stress relaxation experiments could be modeled accurately using the theoretical viscoelastic formulation.
The parallel fiber structure of EOMs results in low shear force transfer across EOM width, explaining the biomechanical effect of Z-myotomy. Z-myotomy <60% progressively reduces force transmission in the EOM, but Z-myotomy of ≥60 % reduces force transmission to minimum residual value. Equivalence to EOM specimens having regular cross sections permits viscoelastic biomechanical characterization of Z-myotomy specimens with irregular cross sections.
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