title: Viscoelastic characterization of extraocular Z-myotomy.
abstract: OBJECTIVE
  Z-myotomy is an extraocular muscle (EOM) weakening procedure in which two incisions are made from longitudinally-separated, opposite EOM margins for treatment of strabismus. We examined the in vitro biomechanics of Z-myotomy using tensile loading.
  
  
  METHODS
  Fresh bovine rectus EOMs were reduced to 20 × 10 × 2-mm dimensions, and clamped in a microtensile load cell under physiological conditions. Extraocular muscles 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 was imposed at 100 mm/s, after which elongation was maintained for greater than 100 seconds during force recording at maintained deformation. Stress relaxation tests with nonincised specimens having widths ranging from 1 to 9 mm were conducted for viscoelastic characterization of corresponding equivalence to 20% to 80% Z-myotomy. Data were modeled using the Wiechert viscoelastic formulation.
  
  
  RESULTS
  There was progressively reduced EOM failure force to an asymptotic minimum at 60% or greater Z-myotomy. Each Z-myotomy specimen could be matched for equivalent failure force to a non-Z-myotomy specimen with a different width. Both tensile and stress relaxation data could be modeled accurately using the Wiechert viscoelastic formulation.
  
  
  CONCLUSIONS
  The parallel fiber structure results in low shear force transfer across EOM width, explaining the biomechanics of Z-myotomy. Z-myotomy progressively reduces force transmission to an asymptotic minimum for less than 60% surgical dose, with no further reduction for greater amounts of surgery. Equivalence to EOM specimens having regular cross-sections permits viscoelastic biomechanical characterization of Z-myotomy specimens with irregular cross-sections.
authors.affiliation: Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, Los Angeles, United States Department of Mechanical Engineering, University of California, Los Angeles, Los Angeles, United States.
authors.forename: Andrew
authors.initials: A
authors.lastname: Shin
authors.affiliation: Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, Los Angeles, United States.
authors.forename: Lawrence
authors.initials: L
authors.lastname: Yoo
authors.affiliation: Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, Los Angeles, United States Biomedical Engineering Interdepartmental Program, University of California, Los Angeles, Los Angeles, United States.
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Demer
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D002417:Cattle; D004548:Elasticity / Q000502:physiology*; D009801:Oculomotor Muscles / Q000502:physiology* / Q000601:surgery; D013508:Ophthalmologic Surgical Procedures; D013285:Strabismus / Q000601:surgery; D013314:Stress, Mechanical; D013718:Tensile Strength / Q000502:physiology*
keywords: biomechanics; extraocular muscle; strabismus surgery