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Các nhạc cụ, trong đó yêu cầu các đối tượng sử dụng một tấm cắn cho sự ổn định và liên kết, thông thường phải mất thực hành để có được dữ liệu đáng tin cậy, và nó đã được rõ ràng nếu điều này mức độ đào tạo và khả năng tái đạt được. | Schachar s Theory of Accommodation 53 operated in a darkened room does not offer the usual accommodative stimulus but relies purely on defocus. The instrument which requires the subject to use a bite plate for stability and alignment generally takes practice to obtain reliable data and it was unclear if this level of training and reproducibility was achieved. Glasser and colleagues 12 have speculated that the possible restoration of near vision via scleral expansion could function via nonaccommodative mechanisms such as inducing multifocality of the crystalline lens. A number of patients in the phase I clinical trial of scleral expansion in the United States are now undergoing wavefront analysis to provide an objective measurement and assess mechanisms that may underlie improvement in near vision after this procedure. B. CONCLUSION There are few subjects in ophthalmology capable of generating as much lively debate as that of accommodation and presbyopia. The processes of accommodation and disaccom-modation are complex to say the least and involve changes in muscular lenticular and extralenticular components. At some time almost every one of these components has been proposed as a factor in the development of presbyopia. We have tried in this chapter to present a balanced view of Schachar s versus Helmholtz s theory of accommodation along with experimental evidence and arguments that have been espoused by proponents of both sides. In a number of key respects the proposed mechanisms are antithetical. The universal nature of presbyopia and the intense interest in its reversal justifies further research in this area to elucidate its pathophysiology. ACKNOWLEDGMENT Supported by the Midwest Corneal Research Foundation Inc. REFERENCES 1. Koretz JF. Accommodation and Presbyopia. In Albert DM Jakobiec FA eds. Principles and Practice of Ophthalmology Basic Sciences. Philadelphia Saunders 1994 270-282. 2. Schachar RA. Is Helmholtz s theory of accommodation correct Ann .
