الفهرس 
The A- scan ultrasound
Common errors in ultrasonic A scan includeOnly 14 pages are availabe for public view
 |  | from | 72 |  |  |
| | | from | 72 | | |
Abstract
Cataract is the leading cause of preventable blindness worldwide. Cataract extraction with implantation of an intraocular lens (IOL) is the most frequently performed ophthalmic surgical procedure worldwide. Accurate calculation of the IOL power for attaining the desired postoperative refraction remains a research issue.
Several factors affect the refractive outcome after cataract surgery, including axial length, keratometry, and lens formulas. Of these factors the preoperative axial length measurement is a key determinant in the choice of intra-ocular lens (IOL) power.
This study aimed to determine whether intraocular lens (IOL) power calculations for cataract surgery -as measured by postoperative refractive error- using IOL master are more accurate in improving postoperative outcomes than applanation ultrasounic biometry (AUS) in highly myopic versus emmetropic eyes. Traditionally, contact A-scan ultrasonography is used. This measures the time taken for sound to traverse the eye and converts it to a linear value (spikes) using a velocity formula. The distance between the corneal and retinal spikes gives the axial length of the eye. Keratometry reading (K1&K2) taken by manual keratometer.
The IOLmaster 700 device is a computerized biometry device consisting of an OCT system to measure distances within the human eye along the visual axis, a Keratometer system to measure the corneal surface.
32 eyes from patients scheduled for phacoemulsification were included in the study. A scan biometry was done to 16 of them where 8 eyes were myopic and 8 eye were
emmetropic. IOL master biometry was done to the other 16 eyes where 8 of eyes were highly myopic and 8 eyes were emmetropic.
Results show that the axial length measured using the IOL master was 0.34mm longer than that measured using A- scan which was statically insignificant (p=0.2112).
In the emmetropic group, the mean difference between the two methods was 0.99 mm, which was statically significant (P=0.00068). In the high myopic group, we found that the mean difference between the two methods was 0.311mm, which was statically insignificant (p=0.433).
The difference between between the mean absolute errors (MAE) measured by IOLM and the mean absolute errors (MAE) measured by A scan was 0.371 (P=0.0385) which was statistically significant improving a 0.561 D error (MAE measured by A-scan) to 0.19 D error (MAE measured by IOLmaster).
This reflects a 66% improvement in absolute postoperative refractive error with IOL master compared with applanation ultrasound.
We can conclude that The IOL master provides an accurate axial length measurement and results in accurate intraocular lens power calculation. However, IOL master has a failure rate, particularly in the presence of dense cataracts.