1 The aim of this research was to provide a simple yet realistic model of the sino-nasal tissue as 2 a major requirement for developing more efficient endoscopic neurosurgery simulation 3 systems. Ex-vivo indention tests were performed on the orbital floor soft tissue of four sheep 4 specimens. The resulting force-displacement data was incorporated into an inverse finite 5 element model to obtain the hyperelastic mechanical properties of the tissue. Material 6 characterization was performed for

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... Polynomial, Yeoh, Mooney-Rivlin and Neo-Hookean 7 hyperelastic models, using a Sequential Quadratic Programming algorithm. Experimental 8 results indicated a relatively large elastic deformation, up to 6mm, during indentation test 9 with a considerable nonlinearity in the force-displacement response. All hyperelastic models 10 could satisfy the convergence criteria of the optimization procedure, with the highest 11 convergence rate and a close fittings accuracy associated with the Yeoh hyperelastic model. 12 The initial guess of the material constants was found to affect the number of iterations before 13 converging, but not the optimization results. The normalized mean square errors of fitting 14 between the model and experimental curves were obtained as 2.39%, 4.26% and 4.65% for 15 three sheep samples, suggesting that the Yeoh model can adequately describe the typical 16 hyperelastic mechanical behavior of the sino-nasal tissue for surgery simulation. Finite Element Method; SQP algorithm; Hyperelastic models; Sino-nasal tissue. 19 through the natural pathways of the nose and sinuses to visualize and access the surgery site 1 and permit excision or biopsy of the lesion using especial instruments. The surgery outcome 2 is obviously advantageous over the open surgery due to the reducing swelling, bleeding, and 3 discomfort, and also a faster recovery [1]. 4 Despite the progressive technological innovations, the ESSS requires many technical skills 5 [2,3]. In one hand, the anatomy of the sinus and nasal cavity is complex and there are many 6 vital neurovascular structures in its proximity that are in the risk of injury during surgery, 7 e.g., orbital content, carotid arteries, optic nerve, brain, and other intracranial tissues. On the 8 other hand, the restricted vision, the non-intuitive hand-eye coordination, and the use of a 9 single endoscope for both viewing and instrument manipulation in a narrow space with very 10 limited mobility increase the possibility of surgery mistakes that may lead to severe 11 consequences. It has been reported that neurosurgery ranks as the most liable specialty 12 amongst all the medical subspecialties to malpractice suits in the US, with over 19% facing a 13 claim each year [4]. 14 In order to perform a safe and effective surgery with no complications, the ESSS trainees 15 require special training programs to gain sufficient hand-eye co-ordination and instrument 16 manipulation skills [5,6]. Currently, the cornerstone of surgical education is learning through 17 observation which is increasingly challenged by legal and ethical concerns for patient safety, 18 as well as limited efficiency and high costs of operating room time [7,8]. An alternative 19 approach has been to practice the surgical procedure on animal or cadaver models; this is 20 again restricted by the fact that it needs special lab facilities and cannot be accomplished 21 repetitively due to the ethical and economic problems. The emerging field of surgical