Seitz B, Langenbucher A, Nguyen N X, Kus M M, Küchle M
Department of Ophthalmology, University of Erlangen-Nürnberg, Erlangen, Germany.
Ophthalmology. 1999 Apr;106(4):693-702. doi: 10.1016/S0161-6420(99)90153-7.
To assess the validity of corneal power measurement and standard intraocular lens power (IOLP) calculation after photorefractive keratectomy (PRK).
Nonrandomized, prospective, cross-sectional, clinical study.
A total of 31 eyes of 21 females and 10 males with a mean age at the time of surgery of 32.3 +/- 6.6 years (range, 24.4-49.5 years).
Subjective refractometry, standard keratometry, TMS-1 corneal topography analysis, and pachymetry were performed before and 15.8 +/- 10.4 months after PRK for myopia (n = 24, -1 .5 to -8.0 diopters [D], mean -5.4 +/- 1.9 D) or myopic astigmatism (n = 7, sphere -2.0 to -7.5 D, mean -4.4 +/- 1.9 D; cylinder -1.0 to -3.0 D, mean -1.9 +/- 0.7 D). The IOLP calculations were done using two different formulas (SRK/T and HAIGIS).
Keratometric power (K) and topographic simulated keratometric power (TOPO) as measured (Kmeas, TOPOmeas) and as calculated according to the change of power of the anterior corneal surface or according to the spherical equivalent change after PRK (Kcalc, TOPOcalc), IOLP for emmetropia, and postoperative ametropia for calculated corneal powers were assessed in a model.
After PRK, mean Kmeas and TOPOmeas were significantly greater (0.4-1.4 D, maximum 3.3 D) than mean KRcalc and TOPOcalc (P < 0.0001). On average, the relative flattening of the cornea after PRK was underestimated by 14% to 30% (maximum, 83%) depending on the method of calculation. The mean theoretical IOLP after PRK ranged from + 17.4 D (SRK/T, TOPOmeas) to +20.9 D (HAIGIS, Kcalc) depending on the calculation method for corneal power and IOLP calculation formula used. For both formulas, IOLP values using keratometric readings were significantly higher (>1 D) than IOLP values using topographic readings (P < 0.0001). The theoretically induced mean refractive error after cataract surgery ranged from +0.4 to +1.4 (maximum, +3.1) D. Corneal power overestimation and IOLP underestimation correlated significantly with the spherical equivalent change after PRK (P = 0.001) and the intended ablation depth during PRK (P = 0.004).
To avoid underestimation of IOLP and hyperopia after cataract surgery following PRK, measured corneal power values must be corrected. The calculation method using spherical equivalent change of refraction at the corneal plane seems to be the most appropriate method. In comparison with this method, direct power measurements underestimate corneal flattening after PRK by 24% on average. Use of conventional topography analysis seems to increase the risk of error. However, because this study is retrospective and theoretical, there is still a need for a large prospective investigation to validate the authors' findings.
评估准分子激光原位角膜磨镶术（PRK）后角膜屈光力测量及标准人工晶状体屈光力（IOLP）计算的有效性。
非随机、前瞻性、横断面临床研究。
共纳入21例女性和10例男性的31只眼，手术时平均年龄为32.3±6.6岁（范围24.4 - 49.5岁）。
对近视（n = 24，-1.5至-8.0屈光度[D]，平均-5.4±1.9 D）或近视散光（n = 7，球镜-2.0至-7.5 D，平均-4.4±1.9 D；柱镜-1.0至-3.0 D，平均-1.9±0.7 D）患者在PRK术前及术后15.8±10.4个月进行主观验光、标准角膜曲率测量、TMS - 1角膜地形图分析及角膜厚度测量。使用两种不同公式（SRK/T和HAIGIS）计算IOLP。
测量的角膜曲率屈光力（K）和地形图模拟角膜曲率屈光力（TOPO）（Kmeas、TOPOmeas），以及根据PRK术后角膜前表面屈光力变化或球镜等效度变化计算得出的（Kcalc、TOPOcalc），正视眼的IOLP，以及模型中计算角膜屈光力后的术后屈光不正情况。
PRK术后，平均Kmeas和TOPOmeas显著高于平均KRcalc和TOPOcalc（0.4 - 1.4 D，最大3.3 D）（P < 0.0001）。平均而言，根据计算方法不同，PRK术后角膜相对扁平度被低估14%至30%（最大83%）。根据角膜屈光力计算方法和IOLP计算公式的不同，PRK术后平均理论IOLP范围为+17.4 D（SRK/T，TOPOmeas）至+20.9 D（HAIGIS，Kcalc）。对于两种公式，使用角膜曲率读数的IOLP值显著高于使用地形图读数的IOLP值（>1 D）（P < 0.0001）。白内障手术后理论上诱发的平均屈光不正范围为+0.4至+1.4（最大+3.1）D。角膜屈光力高估和IOLP低估与PRK术后球镜等效度变化（P = 0.001）及PRK术中预期消融深度（P = 0.004）显著相关。
为避免PRK术后白内障手术时IOLP低估及远视，必须校正测量的角膜屈光力值。使用角膜平面屈光球镜等效度变化的计算方法似乎是最合适的方法。与该方法相比，直接屈光力测量平均低估PRK术后角膜扁平度24%。使用传统地形图分析似乎会增加误差风险。然而，由于本研究是回顾性和理论性的，仍需要大型前瞻性研究来验证作者的发现。
