Effect of Axial Length Measurement Method on Refractive Outcomes of Cataract Surgery: Real World Comparison of Partial Coherence Interferometry and Immersion Ultrasound. Nathaniel Knox Cartwright & Derek Tole Bristol Eye Hospital, Bristol, United Kingdom Financial Disclosure: none

Background Partial coherence interferometry (PCI) has become the clinical gold standard for axial length (AL) measurement but cannot be used in all eyes Intraindividual comparisons of PCI and immersion ultrasound (IUS) have found they result in postoperative refractive equivalence after cataract surgery This contrasts with the clinical situation for PCI users who only use IUS as a second line measurement method The purpose of this study was to compare refractive outcomes after cataract surgery when PCI is used for AL measurement with when IUS is used due to PCI failure

Cohort 2544 phacoemulsification and Akreos Adapt AO (Bausch & Lomb) IOL implantations No copathology Uncombined and uncomplicated surgery Postoperative subjective refraction Corrected distance visual acuity ≥6/12 (20/40) IOLMaster (software version 3, Carl Zeiss Meditec) keratometry performed in all PCI AL measurement attempted in all but IUS used if this was not possible 10 MHz

Optimisation Process Hoffer Q, Holladay 1 and SRK/T formulae entered in code into Freemat (http://freemat.sourceforge.net/) Optimised biometry formula constants calculated by 2 methods To give an overall mean deviation from predicted postoperative refraction of zero (zero method) As recommended in the IOLMaster manual, that is as the mean of the all personalised constants excluding those > 2 SD from overall mean (Zeiss method)1 1. IOL Master Software Version 3.xx User Manual.http://www.doctor-hill.com/iol-master/docs/iolmaster_3.pdf (downloaded 17/0/2008).

Partial coherence interferometry Comparison of Groups Immersion ultrasound Partial coherence interferometry p value (unpaired t-test) Number 224 (9.9 %) 2047 (91.9 %) Mean axial length/ mm 23.51 (SD ±1.64) 23.56 (±1.39) 0.61 Mean mean corneal power/ D 43.85 (±1.47) 43.80 (±1.51) 0.67 Mean intraocular lens power/ D 20.9 (±4.4) 20.6 (±3.9) 0.27 Postoperative spherical equivalent/ D -0.17 (±1.11) -0.10 (±0.87) 0.29

Results: Zero method Unpaired t-test: PCI V IUS all p>0.05 Hoffer Q predicted Anterior Chamber Depth Holladay 1 Surgeon Factor SRK/ T A Constant IUS 5.34 1.59 118.62 PCI 5.33 1.57 118.65

Results: Zeiss method Unpaired t-test: PCI V IUS all p<0.05 Hoffer Q predicted Anterior Chamber Depth Holladay 1 Surgeon Factor SRK/ T A Constant IUS 5.22 (SD ±0.27) 1.59 (±0.49) 118.60 (±0.67) PCI 5.22 (±0.27) 1.58 (±0.48) 118.61 (±0.68)

Zeiss method Zero method Hoffer Q Holladay 1 SRK/T PCI IUS Mean error -0.26 (±0.80) -0.34 (±0.75) -0.09 (±0.76) -0.18 (±0.70) -0.11 (±0.72) -0.21 (±0.67) Mean absolute error 0.44 (±0.77) 0.49 (±0.66) 0.33 (±0.69) 0.39 (±0.61) 0.31 (±0.66) 0.38 (±0.59) % ± 0.25 D 42.8 37.1 64.5 54.9 64.2 51.8 % ±0.50 D 74.4 70.1 85.3 82.6 87.8 83.9 % ± 1.00 D 93.5 91.1 94.6 92.4 95.5 92.9 Zero method Hoffer Q Holladay 1 SRK/T PCI IUS Mean error 0.00 (±0.83) 0.00 (±1.02) 0.00 (±0.82) 0.00 (±0.84) 0.00 (1.02) Mean absolute error 0.51 (±0.65) 0.58 (±0.84) 0.56 (±0.84) 0.52 (±0.66) % ± 0.25 D 36.3 37.5 37.7 36.1 35.6 35.3 % ±0.50 D 62.8 59.8 64.8 62.9 63.3 % ± 1.00 D 88.7 86.0 89.1 88.6 88.9 83.6

Conclusions Optimised biometry formula constants (BFC) varied according the method by which they were calculated When the Zeiss optimisation method was used, AL measurement by IUS following PCI measurement failure did not require use of different BFC and did not worsen refractive outcomes Additionally, the Zeiss optimisation method resulted in a lower mean absolute error than when the zero method is used