Preoperative assessment including biometry

CHAPTER 9 Preoperative assessment including biometry



David Sculfor



Chapter outline


SOURCES OF ERROR IN KERATOMETRY 35

SOURCES OF ERROR IN AXIAL LENGTH MEASUREMENT 36

A-SCAN ULTRASOUND BIOMETRY 36

LASER INTERFERENCE BIOMETRY (LIB) 38

FINAL CHECKS FOR ALL BIOMETRY 38

POSTOPERATIVE TARGET REFRACTION 38

SUMMARY 39

REFERENCE 39

Biometry is a vital part of the cataract process because the effective IOL power largely controls the final refractive error. That refractive error is the ‘take home’ element of the surgery, and will remain virtually unchanged for the rest of the patient’s life. An incorrect IOL is a leading cause of successful litigation in ophthalmology1.


Increasingly biometry is being carried out by technical staff, but the implanting surgeon is answerable for incorrect biometry, not the technician. The surgeon needs a thorough understanding of the principles of biometry and common sources of error to be able to check the biometry for quality before surgery. Clearly the first step is to ensure that the technician is properly trained and accredited to do biometry, and experience should not be confused with competence.


Easy targets for litigation are the calibration and service records for the biometry equipment. While there may not be a problem with the machine, it does not look good if there is no record of these having been carried out. Calibration should be checked, ideally each day, using the reference piece supplied with most instruments, and a record kept. Schedule an annual service visit for a more detailed calibration check, together with an electrical safety test.



Sources of error in keratometry


A 1 diopter (D) error in keratometry will give a 1 diopter error in the final refractive outcome. Unfortunately there are a number of sources of such errors.


Keratometry errors that are difficult to control for include:



• Diurnal variation in corneal curvature

• Non-standard corneal index

• Unusual corneal profile (commonly a prolate ellipse).

Errors that can be avoided include:



• Incorrect corneal index when converting from millimetres radius to dioptric power

• Contact lens wear

• Poor fixation during keratometry

• Poor focusing or alignment

• Instrument not focused for user (Javal–Schiotz type)

• Refractive surgery.

The surgeon should ensure that all the instruments used in his or her practice have the same corneal index. Indices in common use range from 1.3315 to 1.338 (approximately equivalent to 42.77 D to 43.61 D). Which one is used is perhaps less important than that all the instruments in a practice or hospital use the same setting.


To the author’s knowledge there have been no large-scale published studies looking at how the cornea changes on removal of soft and gas-permeable lenses. The only safe recommendation is to wait until the readings stabilize before the biometry calculations are done, which for GP lens wearers could mean months. However experience suggests that the changes are relatively small, if unpredictable, and if the patient is unable to manage without contact lenses, then counsel the patient to expect the final refraction to change over time, and record that discussion in the notes.


One key measure is repeatability. The surgeon should insist that at least three readings are recorded, and with each meridian being considered separately they should be within 0.30 D of each other. In Table 9.1, the third K1 reading is more than 0.3 D different from the other two. Similarly, one of the K2 readings is different. They cannot all be right, so repeat readings should be taken, using a drop of saline on the cornea to improve wetting.



Table 9.1 Sources of error in axial length measurement

image


Sources of error in axial length measurement


A 1 mm error in axial length will give approximately 3 D of refractive error. It is important that surgeons are able to assess the quality of axial length readings to ensure that the commonest errors have been eliminated. To this end, the biometry printout should include a representative scan as well as the lens calculation results.



A-scan ultrasound biometry


See Figure 9.1.

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Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Preoperative assessment including biometry

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