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L. A. Eckstein; A Novel Surgical Caliper Calibrated to Measure Arc-Length. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4866.
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To design and construct a novel surgical caliper calibrated to measure arc-lengths rather than chord-lengths on a spherical surface.
Currently available calipers permit measurement only of chord-lengths and therefore underestimate distances along the surface of the globe (arc-lengths). This limitation introduces systematic measurement errors, which exceed 0.5 mm for distances greater than 11 mm and increase non-linearly to 3.6 mm for distances of 20 mm. In general, these differences are clinically important for distances in excess of 9 mm for an average globe 24 mm in diameter. To eliminate this source of error, we designed and prototyped a caliper that measures arc-lengths on a globe.
Using a traditional (chord-length) caliper we marked a series of points on a globe from 10 to 20 mm posterior to the limbus in 1 mm incremements. This series of marks was repeated with the arc-length caliper prototype. Finally, the distance between the corresponding marks was measured and compared to theoretical calculations of the difference. Our results reveal close concordance between the predicted and the measured differences between chord-lengths and arc-lengths (mean, 0.17 +/- 0.19 mm; range -0.2 - 0.4 mm).
Used in a manner similar to existing instruments, the arc-length caliper minimizes systematic errors in the measurement of distances on a spherical surface resulting from the difference between arc-lengths and chord-lengths. Thus, by increasing measurement accuracy, the application of this novel device may improve surgical outcomes. We envision the caliper being used for strabismus surgery involving large muscle recessions, oncologic surgery involving placement of posterior plaques or clips, and other ophthalmic procedures.
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