title: Stress-relaxation and fatigue behaviour of synthetic brow-suspension materials.
abstract: Ptosis describes a low position of the upper eyelid. When this condition is due to poor function of the levator palpebrae superioris muscle, responsible for raising the lid, "brow-suspension" ptosis correction is usually performed, which involves internally attaching the malpositioned eyelid to the forehead musculature using brow-suspension materials. In service, such materials are exposed to both rapid tensile loading and unloading sequences during blinking, and a more sustained tensile strain during extended periods of closure. In this study, various mechanical tests were conducted to characterise and compare some of commonly-used synthetic brow-suspension materials (Prolene(®), Supramid Extra(®) II, Silicone rods (Visitec(®) Seiff frontalis suspension set) and Mersilene(®) mesh) for their time-dependent response. At a given constant tensile strain or load, all of the brow-suspension materials exhibited stress-relaxation or creep, with Prolene(®) having a statistically different relaxation or creep ratio as compared with those of others. Uniaxial tensile cyclic tests through preconditioning and fatigue tests demonstrated drastically different time-dependent response amongst the various materials. Although the tests generated hysteresis force-strain loops for all materials, the mechanical properties such as the number of cycles required to reach the steady-state, the reduction in the peak force, and the cyclic energy dissipation varied considerably. To reach the steady-state, Prolene(®) and the silicone rod required the greatest and the least number of cycles, respectively. Furthermore, the fatigue tests at physiologically relevant conditions (15% strain controlled at 6.5 Hz) demonstrated that the reduction in the peak force during 100,000 cycles ranged from 15% to 58%, with Prolene(®) and the silicone rod exhibiting the greatest and the least value, respectively. Many factors need to be considered to select the most suitable brow-suspension material for ptosis correction. These novel data on the mechanical time-dependent performance could therefore help to guide clinicians in their decision-making process for optimal surgical outcome.
authors.affiliation: Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK. Electronic address: j.kwon@ucl.ac.uk.
authors.forename: Kyung-Ah
authors.initials: KA
authors.lastname: Kwon
authors.affiliation: Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK.
authors.forename: Rebecca J
authors.initials: RJ
authors.lastname: Shipley
authors.affiliation: Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK.
authors.forename: Mohan
authors.initials: M
authors.lastname: Edirisinghe
authors.affiliation: Department of Materials Science and Metallurgy, Cambridge Centre for Medical Materials, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Rayment
authors.affiliation: Department of Materials Science and Metallurgy, Cambridge Centre for Medical Materials, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: Serena M
authors.initials: SM
authors.lastname: Best
authors.affiliation: Department of Materials Science and Metallurgy, Cambridge Centre for Medical Materials, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: Ruth E
authors.initials: RE
authors.lastname: Cameron
authors.affiliation: Moorfields Eye Hospital, UCL Institute of Ophthalmology, NIHR Biomedical Research Centre for Ophthalmology, 162 City Road, London EC1V 2PD, UK.
authors.forename: Tahrina
authors.initials: T
authors.lastname: Salam
authors.affiliation: Moorfields Eye Hospital, UCL Institute of Ophthalmology, NIHR Biomedical Research Centre for Ophthalmology, 162 City Road, London EC1V 2PD, UK.
authors.forename: Geoffrey E
authors.initials: GE
authors.lastname: Rose
authors.affiliation: Moorfields Eye Hospital, UCL Institute of Ophthalmology, NIHR Biomedical Research Centre for Ophthalmology, 162 City Road, London EC1V 2PD, UK.
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Ezra
mesh_terms: D005143:Eyelids / Q000502:physiology; D008422:Materials Testing; D009132:Muscles / Q000502:physiology; D019736:Prostheses and Implants; D013314:Stress, Mechanical