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. 2016 Apr 25;93(5):521–533. doi: 10.1097/OPX.0000000000000820

FIGURE 3.

FIGURE 3

The trade-off relationship between oblique tilt angle and the lateral expansion and vertical displacement. Increasing the tilt angle of the base-apex axis increases the vertical shift (dashed curves) but reduces the lateral expansion (solid curves) for 36Δ (black curves) and 57Δ (blue curves) PMMA prisms. For small angles of tilt, the gain in vertical shift (gap reduction) is higher in magnitude than the loss in lateral expansion. To close the standard 12-mm interprism separation (equivalent to 33 degrees vertical gap between the prism expansion areas or 16.5 degrees for each prism segment), the 36Δ prisms require 56 degrees tilt angle (black open marker), which reduces the lateral expansion to only 11 degrees (black filled marker). However, the 57Δ oblique prisms can close the gap with just 34 degrees tilt (blue open marker) and still provide 25 degrees lateral expansion (blue filled marker).