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GA Thomas, P Marchetto, R Greene, R Fechtner; Mechanical Analysis of a Patient-Operated Tonometer . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3426.
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Purpose: We have tested the accuracy and reproducibility of the pressure readings on a set of commercial tonometers (Proview, Bausch and Lomb). We chose this type of tonometer because it is designed to allow a patient to monitor his or her own intra-ocular pressure without anesthesia, and also because it is simple in its construction and appears potentially inexpensive. Methods: We have made measurements of the tonometers in controlled laboratory conditions using a robot arm with micrometer control to eliminate effects related to positioning of the device by the patient. We have measured the true pressure, P, applied to the tonometer using a calibrated, piezo-electric force meter, in place of the patient’s eye. In our initial studies, we have compared P with the nominal pressure readings on the tonometer, Pn, over a range 0<P<40 (mmHg) on 4 tonometers and checked the reproducibility of each. Our working hypothesis was that pressure readings would follow the ideal form P=k*Pn, with the calibration constant, k near a value 1.0, and that we would measure the value of k and its uncertainty dk. Results: We have found that the effective value of k is pressure dependent and roughly 3 to 5 at P below 15 (mmHg) in all of our devices, with the value of dk also pressure dependent and as large as 0.2 in this range of P. At higher pressures, P above 20 (mmHg), the value of k approaches a constant near 1, and the value of dk decreases by about a factor of 10. We interpret the results as indicating that the device acts like a spring moving inside a cylinder with friction, with an offset in the Pn scale and with a non-linearity. The frictional force per unit area, Pf, exerts a force on the spring, leading to irreproducibility. The scale of Pn is shifted by an offset, P0, of about 10 (mmHg), and there is a small non-linearity to the spring of the form, k2*Pn**2., where k2 is a constant. The pressure function is then described by the form: P= Pf +k*(Pn + P0)+ k2*(Pn + P0)**2 . Conclusion: These tonometers may be useful in clinical applications if the practitioner takes into account the functional relationship between the true and nominal pressures and the limitations on reproducibility. Supported in part by RPB, Inc. The Proview tonometers were donated by Bausch and Lomb.
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