Author Topic: Different corneal sag between primary gaze and composite eye results  (Read 4096 times)

Forrest

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I found that the corneal sag height of 9.8mm chord length obtained by primary gaze topogarphery is different from that after composition with more lateral gaze topography. The difference can be as large as 10-15um. Of course, it is insignificant in scleral lens fitting, but very important in orthokeratology.

Then, I would like to know that which one is the more accurate sag height please?

Forrest

Grant

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Re: Different corneal sag between primary gaze and composite eye results
« Reply #1 on: March 12, 2013, 09:07:57 am »
Are you comparing the sag between central exam from a composite and the composite - or comparing a different primary gaze topography?   If you are comparing a primary gaze exam that is included in the composite then it is possible that the axes of measurement is not exactly aligned which will result in some differences in sag (which is measured relative to the axes) that will be more pronounced toward the periphery.   

In theory the composite exam should be more accurate then a single exam particularly in the periphery.  In general placido disc topographers are most accurate closest to the topography axis.  The composite combines several exams weighting the average at each point toward the topography closest to that point (and therefore most accurate).   This is however dependant on the accuracy of the registration process.

Forrest

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Re: Different corneal sag between primary gaze and composite eye results
« Reply #2 on: March 12, 2013, 02:14:01 pm »
Dear Grant,

Thank you for the illustration. Then I have a question. If I want to improve the accuracy of the sag height result, which of the following methods will be preferred please?

1. Take 5 primary gaze map and use the composite eye function to average them.

2. take one primary gaze and 4 peripheral gaze, then use the composite eye function to generate a big map.

Best regards,
Forrest