The demographics of each sample group (i.e. 39 cases from Hadassah Ein Keren Hospital
and 24 from Barraquer Clinic) are detailed in Table 1.
Table 1. Demographics of the two studied groups
Group
| Ethnicity
| Mean Age
| Gender
| Laterality
| n
|
Hadassah Ein Keren Hospital, Jerusalem, Israel
| Jewish; Arabic
| 68.67 yo,
SD ± 10,25
Min: 43
Max: 85
| Male: 43.85%
Female: 56.41%
| Right: 58.97%
Left: 41.02%
| 39
|
Clínica Barraquer, Bogotá- Colombia
| Latin American - Hispanic
| 60.41 yo,
SD ± 12,14
Min: 37
Max : 81
| Male: 41.66 %
Female: 58.33%
| Right: 66.6 %
Left: 33.3 %
| 24
|
|
|
|
|
| Total = 63
|
yo: years old; SD: Standard Deviation; Min: Minimum; Max: Maximum, n: Number of eyes studied.
The pre and post-operative statuses of the assessed variables are summarized in Table 2.
Table 2. Variables included in the present study
Variable
| Mean
| Standard Deviation
| Minimum
| Maximum
|
PreOp VA (Logmar)
| 0.494
| 0.346
| 0.041
| 1.477
|
PostOp VA (Logmar)
| 0.101
| 0.1043
| 0
| 0.301
|
Flat K
| 42.99 D
| 1.61906832 D
| 39.38 D
| 46.81 D
|
Steep K
| 44.09 D
| 1.76891495 D
| 40.23 D
| 48.5 D
|
Mean K
| 43.54 D
| 1.62941283 D
| 40.08 D
| 47.2 D
|
L
| 26.94 mm
| 1.107 mm
| 25.22 mm
| 30.08 mm
|
PreOp: Preoperative; PostOp: Postoperative; VA: Visual acuity; K: Keratometry; L: Axial length; n = 63
Pathologic changes were detected in eight patients (12.698% of sample group). Five
of the eight patients presented uveitis, diplopia, pseudoexfoliation syndrome, peripheral
lesions requiring laser treatment, or choroidal neovascularization (12.5% incidence
each condition). In turn, three patients presented important atrophic, myopic changes
(37.5% incidence).
Of the tested equations, the most accurate was the SRK/T formula (MedAE = 0.352),
followed by T2 (MedAE = 0.381) and Holladay 1 (MedAE = 0.389) formulas (Table 3, Graph 1). Lin’s
correlation12 factor was used to analyze the MedAE of the three methods (Table 4).
Table 3. Summary of the prediction error in the present study
Formula
| MAE
| Standard Deviation
| Minimum
| Maximum
| MedAE
| ≤±0.50 D
| ≤±1.0 D
| >2.00D
|
SRK/T
| 0.418
| 0.327
| 0.003
| 1.359
| 0.352
| 71.42%
| 20.63%
| 7.93%
|
Holladay1
| 0.455
| 0.314
| 0.037
| 1.404
| 0.389
| 61.90%
| 31.74%
| 6.35%
|
T2
| 0.435
| + 0.328
| 0.014
| 1.389
| 0.381
| 69.84%
| 22.22%
| 7.94%
|
L: Axial length, MAE: Mean absolute error, MedAE: Median absolute error, T2: T2 formula, H2: Corneal height estimation using T2, n = 63
Table 4. Lin’s correlation coefficient of the median absolute error of the methods used in
the present study.
| T2
| HOLLADAY 1
|
SRK/T
| ρc = 0.9829
95% CI = 0.9720 to 0.9896
| ρc = 0.9537
95% CI = 0.9253 to 0.9715
|
T2
|
| ρc = 0.9575
95% CI = 0.9311 to 0.9739
|
ρc: Lin’s concordance correlation coefficient, 95% CI: 95% confidence interval. n= 63
A substantial correlation was found between the T2 and SRK/T formulas. Correlations
between the SRK/T and Holladay 1 formulas and between the Holladay 1 and T2 formulas
were also substantial, but with only moderate lower limits of the confidence intervals.
Analysis of Calculation Methods
Since the main difference between the T2 and SRK/T formulas is the estimation of H,
the behaviors of L and keratometry were analyzed in relation to the estimated H.
L is used without any modification in calculating H2, while an adjusted L (LCOR) is
required by the HSRK/T formula. A correlative analysis was performed between both
H-calculation methods and L, with the results being a very low correlation between
HSRK/T and L (Table 5) but a strong positive correlation between H2 and L (r = 0.808; p < 0.05).
Table 5. Correlation between different methods of Corneal Height estimation and associated
variables.
| Axial Length
| Mean Keratometry
|
HSRKT
| r = 0.224
p = 0.078
| r = 0.805
p < 0.01
|
H2
| r = 0.808
p < 0.01
| r = 0.265
p < 0.05
|
H2.2
| r = 0.425
p < 0.01
| r = 0.695
p < 0.01
|
HSRK/T: Corneal height estimation using SRK/T, H2: Corneal height estimation using
T2, H2.2: Corneal height estimation using the alternative T2 formula.
This finding is important for the following reasons: (1) it suggests that L has a
strong effect on the estimation of H calculated with the method included in the T2
formula; (2) it might explain the higher MedAE seen in highly myopic eyes with the
T2 formula; and (3) it indicates that LCOR may be why L has less impact in estimating
H with the SRK/T approach.
In summary, modifying the calculation of H in the T2 formula improves its accuracy,
resulting in a lower MedAE in eyes with normal L. However, the benefit of this adjustment
seems to be lost in longer eyes, probably due to the effect of L in estimating H.
On the other hand, the SRK/T formula seems to be less affected by extreme L, which
could be associated with the inclusion of LCOR in its design.
The second variable needed to calculate H is the keratometry. A strong positive relationship
was found between HSRK/T and average keratometry (r = 0.805, p < 0.05), but a negligible
correlation was found between H2 and average keratometry (r = 0.265, p < 0.05).
Corneal Height (H) The performed analyses suggest that the presence of LCOR reduces the impact of extreme
AL values in the estimation of H. Therefore, including the corrected AL in the T2
formula might improve its behavior in long eyes. A formula that might solve the SRK/T
cusp problem but that would also include LCOR was needed. The easiest way to complete
this task was using the second regression formula described by Sheard et al. in the
original paper on the T2 formula. This second regression formula was excluded from
the final T2 formula because of its slightly lower correlation.7 In the present study, this formula is termed H2.2 and is calculated as follows:
H2.2 = -11.980+ 0.38626 × LCOR + 0.14177 × K
Estimations of H using the H2.2 formula were compared with results obtained using
the HSRK/T and H2 formulas (Graph No. 2, Table 6). The H2.2 method reduced the mean H value and the reported range of values.
Table 6. Corneal Height estimation using three methods
| Minimum
| Maximum
| Mean
| Standard Deviation
|
HSRKT
| 3.5101
| 6.6086
| 4.2713
| ±0.5490
|
H2
| 3.7947
| 5.4057
| 4.3567
| ±0.3503
|
H2.2
| 3.6395
| 4.7624
| 4.0631
| ±0.23624
|
HSRK/T: Corneal height estimation using SRK/T, H2: Corneal height estimation using
T2, H2.2: Corneal height estimation using the alternative T2 formula, n = 63.
Statistically significant differences were found between the H2.2 and H2 formulas
(p < 0.005), as well as between the H2.2 and HSRK/T formulas (p < 0.005). A moderate
correlation was found between H2.2 and average keratometry (r = 0.695, p < 0.05),
and a low correlation was found between L and H2.2 (r = 0.425, p < 0.05).
These results suggest that the H2.2 formula might improve H estimations, reducing
the mean H, the range of extreme values, and the influence of very high keratometry
and L values.
When H2.2 was used to estimate IOL, the MAE and MedAE were respectively 0.433 and
0.3815 (Table 7).
Table 7. Prediction error applying T2 with the alternative corneal height estimation method
and optimization of axial length
</p><table class=table><tbody><tr> <td><p><b>Formula</b></p>
MAE
| Standard Deviation
| Minimum
| Maximum
| MedAE
| ≤±0.50 D
| ≤±1.0 D
| >2.00D
|
T2 using H2.2 alone
| 0.433
| ±0,01177
| 0.0032
| 1.3856
| 0,3816
| 69,84%
| 22,22%
| 7,93%
|
T2 using H2.2 and optimized L
| 0,425
| ±0,33182
| 0,0025
| 1,382
| 0,3648
| 68.25%
| 23.81%
| 0%
|
H2.2= Corneal height estimation according to the alternative T2 formula, Optimized
L: Adjustment of L according to Wang L et al. n=63.
While these results are only slightly better than T2 formula, a better estimation
of H in highly myopic patients is obtained.
Optimized Axial Length An additional approach to improve results of the T2 formula in highly myopic eyes
is to optimize calculations of L. Since H2.2 includes LCOR, the method described by
Wang L et al.13 for the SRK/T formula can be used directly. When this approach was tested, the MedAE
and MAE were even lower than obtained with H2.2 alone (Table 7).