Refraction problems after refractive surgery

While refractive surgery such as radial keratotomy, epikeratophakia, and corneal relaxing incisions offer many potential benefits to patients, they can also generate optical problems such as overcorrection or undercorrection of the prior refractive error and variable vision. The authors offer suggestions for avoiding problems through proper patient selection; candid preoperative communication with the patient about possible difficulties and limitations; under-standing of the physiological changes that may occur; and optical techniques for alleviating postoperative vision problems.     

Microbiologic Examination of Bandage Contact Lenses Used after Corneal Collagen Cross-linking Treatment

Purpose: To investigate the agents of bacterial contamination of contact lenses after corneal collagen cross-linking (CCL), and to present the possible changes of ocular flora after riboflavin/ultraviolet A.

Methods: Seventy-two contact lenses of patients who underwent CCL and 41 contact lenses of patients who underwent photorefractive keratectomy (PRK) as control group were enrolled to the study. After 48?h of incubation, broth culture media was transferred to plates. Samples were accepted as positive if one or more colony-forming units were shown.

Results: There were positive cultures in 12 (16.7%) contact lenses in the CCL group and 5 (12.2%) had positive cultures in PRK group. Coagulase-negative staphlycocci (CNS) were the most frequent microorganism. Alpha hemolytic streptococci and Diphteroid spp. were the other isolated microorganisms.

Conclusions: Bacterial colonization can occur during and early after the CCL procedure in epithelial healing. To prevent corneal infections after the treatment, prophylactic antibiotics should be prescribed.     

放射状角膜切开术术后白内障手术及人工晶状体度数选择

目的探讨放射状角膜切开术（RK）术后白内障手术的诊治特点及人工晶状体度数选择的可靠性。方法回顾性分析我院2010
年3月~2013年6月RK术后的白内障患者8例（12只眼）,术前对白内障患者的视力、角膜曲率及眼轴长度进行检查,比较患者用
TOPCON-KR8800型自动角膜曲率计测量的3组角膜中央2.6 mm的角膜曲率值,选择较为低的K数值,选用经SRK T三元回归公
式自动计算的近正视化（-1.00D~-2.00D）的人工晶状体（后房型折叠人工晶状体）,行白内障超声乳化摘除联合人工晶状体植入术,
术后3月随访患者的视力、屈光状态。结果RK术后白内障患者行人工晶状体植入的12只眼术后视力均较术前提高：其中6只眼裸
眼视力达到0.8~1.0,另外6只眼裸眼视力达到0.4~0.6+,矫正视力0.6~1.0;术后屈光状态：6只眼接近正视（+0.75D~-1.00D）,另外6
只眼的屈光度为-1.00D~-2.50D,通过术后配镜,患者均能接受。结论对RK术后的白内障患者,施行白内障超声乳化吸出术+人工
晶状体植入术是可行的;比较患者的3组自动角膜曲率计测量的角膜中央2.6 mm的角膜曲率值,选择较为低的K数值,使用SRK T
公式计算人工晶状体的度数,预留-1.00~-2.00D的近视较为安全,避免>+3.00D的远视出现,也较为患者接受。
     

Cardinal points and generalizations

In the presence of astigmatism a focal point typically becomes the well‐known interval of Sturm with its pair of axially‐separated orthogonal line singularities. The same is true of nodal points except that the issues are more complicated: a nodal point may become a nodal interval with a pair of nodal line singularities, but they are not generally orthogonal, and it is possible for there to be only one line singularity or even none at all. The effect of astigmatism on principal points is the motivation behind this paper. The three classes of cardinal points are defined in the literature in a disjointed fashion. Here a unified approach is adopted, phrased in terms of rays and linear optics, in which focal, nodal and principal points are defined as particular cases of a large class of special structures. The special structures arising in the presence of astigmatism turn out to be described by mathematical expressions of the same form as those that describe nodal structures. As a consequence everything that holds for nodal points, lines and other structures now extends to all other special points as well, including principal points and the lesser‐known anti‐principal and anti‐nodal points. Thus the paper unifies Gauss’s and Listing’s concepts of cardinal points within a large class of special structures and generalizes them to allow for refracting elements which may be astigmatic and relatively decentred. A numerical example illustrates the calculation of cardinal structures in a model eye with astigmatic and heterocentric refracting surfaces.     

Clinical evaluation of keratometry and computerised videokeratography: intraobserver and interobserver variability on normal and astigmatic corneas

AIMS—To evaluate intra- and interobserver variability in measurements on normal and astigmatic corneas with keratometry and computerised videokeratography.
METHODS—Keratometric readings with the 10 SL/O Zeiss keratometer and topographic maps with the TMS-1 were obtained by two independent examiners on 32 normal and 33 postkeratoplasty corneas. Inter- and intraobserver coefficients of variability (COR) for measurements of steep and flat meridian power and location, in addition to the magnitude of astigmatism, were assessed.
RESULTS—Compared with TMS-1, the 10 SL/O keratometer showed a superior repeatability in measuring normal corneas (intraobserver COR for keratometry and TMS-1 respectively: 0.22 and 0.30 D for steep meridian power; 0.18 and 0.44 D for flat meridian power; 0.26 and 0.40 D for astigmatism; 5° and 26° for steep meridian location; 5° and 13° for flat meridian location). Astigmatism intraobserver COR (0.20 D and 0.26 D for the two observers) and interobserver COR (0.28 D) of the keratometer for normal corneas was very good and not affected by observers' experience. Repeatability of the TMS-1 on normal corneas was found to be: (a) observer related, and (b) astigmatism related. A novice observer showed a much greater COR (1.62 D for astigmatism, 30° for flat meridian location) compared with the experienced examiner (0.40 D for astigmatism, 13° for flat meridian location). Higher deviation scores were observed for corneas with higher astigmatism. For the postkeratoplasty corneas, again the keratometer achieved superior reproducibility (astigmatism interobserver COR 1.12 D for keratometry, 4.06 D for TMS-1; steep meridian location interobserver COR 10° for keratometry, 34° for TMS-1).
CONCLUSION—Keratometric readings are more reproducible than topographic data both for normal and postkeratoplasty corneas. The two instruments should not be used interchangeably especially on highly astigmatic corneas. For the TMS-1, users with the same level of experience should be employed in clinical or experimental studies.

Keywords: keratometry; computerised videokeratography; astigmatic corneas     

A Clinical Investigation of the Surgical Correction of Myopia by the Method of Fyodorov

The surgical correction of myopia using the method of Fyodorov known as radial keratotomy consists of 16 partial thickness, radial incisions in the cornea, which result in central flattening and peripheral bulging, reducing the degree of myopia. The purpose of this investigation was to determine: (1) the amount of myopia correctable; (2) the time required for stabilization of corneal curvature changes; (3) the degree to which the variables affect the results; (4) the surgical and postoperative complications; and (5) patient motivation and satisfaction. Preliminary results revealed a significant reduction of the myopia. The keratometry readings and refractive correction required appeared to stabilize by the third month. Fluctuating vision and increased glare were the most frequent complications encountered. The preliminary results of 20 cases followed for six months postradial keratotomy are reported.     

Extended Wear Contact Lenses for Myopes: A Follow-up Study of 400 Cases

The long-term effects of extended wear contact lenses (Perfilcon A [Permalens]) for myopia have been studied among 400 patients in a private clinic from December 1977 to September 1982. The drawbacks and possible complications were evaluated. With the exception of one case, no permanent modification of the cornea or ocular adnexa was found. The high rate of success among well-selected patients (without significant astigmatism or chronic infection of the eyelids) far outweighs the risks of radial keratotomy as it is known today. By and large patients opt for this method of optical correction once they are aware of its availability and remain with it despite the necessary frequent replacement of the lenses.     

New corneal shapes in keratorefractive surgery

The corneal shape change as measured by the Humphrey keratometer and the corneascope after radial keratotomy in 50 patients was compared. The Humphrey keratometer analyzes a unique curve fit algorithm of the aplanatic corneal surface and designates this comparison as a shape factor. Both the corneascope photographs and the Humphrey keratometer demonstrated central corneal flattening after successful radial keratotomy. In addition, the normal "plus" shape factor was routinely converted to a "negative" shape factor after the procedure. This is analogous to converting the aplanatic central cornea, which normally approximates the end of an ellipse, to a topography which appears in cross section as the side of an ellipse. This alteration in topography is directly correlated to the magnitude of cycloplegic refractive change seen with this operation. The corneascope map topography of the cornea confirms this alteration in curvature.