1. 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

2. 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

3. 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

4. Optimisation Process
Hoffer Q, Holladay 1 and SRK/T formulae entered in code into Freemat (
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. (downloaded 17/0/2008).

5. 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

6. 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

7. 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)
(±0.67)
PCI
5.22 (±0.27)
1.58 (±0.48)
(±0.68)

8. 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

9. 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