Comparative Analysis of Contact and Immersion Technique in Ultrasonographic Biometry

PURPOSE: To establish the accuracy of the newly released biometer Ocuscan RxP(R) (Alcon, USA) by comparison with the established Ultrasonic Biometer Model 820(R) (Allergan Humphrey, USA), and to compare the accuracy of contact and immersion biometries. METHODS: This is a prospective study involving 27 patients (40 eyes) who were scheduled for cataract surgery and had axial lengths measured with an Ocuscan RxP(R) biometer using both contact (Method 1) and immersion (Method 2) techniques. As a reference, a contact type Ultrasonic biometer 820(R) (Method 3) was also used. IOL(Intraocular Lens) power for the cataract surgery was calculated using this result. An axial length which would have caused no post-operative refractive error was reversely calculated from the difference of target diopter and post-operative refractive error. This length was compared with the axial lengths obtained via Methods 1, 2 and 3. RESULTS: The means and standard deviations for the measurement sets were compared. Methods 1 and 2 showed no significant difference (23.22+/-0.68, 23.24+/-0.69 mm, p=0.55). The axial length measured by Method 3 was 23.32+/-0.67 mm. The difference between the target refraction and post-operative refractive error was 0.29+/-0.60D. The axial length was reversely calculated from the difference (23.07+/-0.84 mm). The differences between the reversely calculated axial lengths and those of Methods 1, 2 and 3 were 0.15+/-0.31, 0.17+/-0.31 and 0.24+/-0.28 mm, respectively. CONCLUSIONS: Biometric results from Methods 1 and 2 caused less refractive error than did Method 3. The contact and immersion methods are both accurate for IOL power calculation if performed by a well-skilled examiner.

Determine a Proper Axial Length by Ultrasonic Biometry in Opaque Hydrophilic Acrylic Intraocular Lens

PURPOSE: To investigate the accuracy of measuring ultrasonic axial lengths using A-scan (Ocuscan(R), Alcon, USA) on opaque intraocular lenses after hydrophilic lens (ACRL-160(R), Ophthalmed, USA) implantation. METHODS: We measured axial length through ultrasonic biometry prior to intraocular lens exchange. Twelve eyes of 14 patients who had intraocular lens opacity following hydrophilic acryl lens implantation were examined in a clinical study. We compared pre-cataract operative axial lengths to pre-IOL exchange axial lengths. The pre-IOL exchange axial lengths were measured in aphakic and pseudophakic modes. In the pseudophakic mode, the ultrasound velocity through an IOL was set at a rate ranging between 1,500 m/sec to 2,200 m/sec. RESULTS: The pre-IOL exchange axial lengths in the pseudophakic mode at the rate of 1,600 m/sec in lens velocity were the closest to pre-cataract operative values (p=0.88). CONCLUSIONS: When pre-cataract operative axial length is known with a hydrophilic acrylic intraocular lens, previous values may be used for IOL exchange of an opacity patient. If not, however, the closest values to pre-cataract operative axial lengths may be obtained by setting the velocity in the pseudophakic mode to a lens velocity rate of 1,600 m/sec.