Elastic hysteresis in human eyes is an age‐dependent value

Background: The elastic hysteresis phenomenon is observed when cyclic loading is applied to a viscoelastic system. The purpose of this study was to quantitatively evaluate elastic hysteresis in living human eyes against an external force. Design: Prospective case series. Participants: Twenty‐four eyes of 24 normal human subjects (mean age: 41.5 ± 10.6 years) were recruited. Methods: A non‐contact tonometry process was recorded with a high‐speed camera. Central corneal thickness, corneal thickness at 4 mm from the centre, corneal curvature and anterior chamber depth were measured. Intraocular pressure was also measured using Goldmann applanation tonometry and dynamic contour tonometer. Main Outcome Measures: Energy loss due to elastic hysteresis was calculated and graphed. Results: The mean central corneal thickness was 552.5 ± 36.1 µm, corneal curvature was 7.84 ± 0.26 mm and anterior chamber depth was 2.83 ± 0.29 mm. The mean Goldmann applanation tonometry‐intraocular pressure was 14.2 ± 2.7 mmHg and dynamic contour tonometer‐intraocular pressure was 16.3 ± 3.5 mmHg. The mean energy loss due to elastic hysteresis was 3.90 × 10⁻⁶ ± 2.49 × 10⁻⁶ Nm. Energy loss due to elastic hysteresis correlated significantly with age (Pearson correlation coefficient = 0.596, P = 0.0016). There were no significant correlations between energy loss due to elastic hysteresis and other measurements. Conclusion: Energy loss due to elastic hysteresis in the eyes of subjects was found to positively correlate with age, independent of anterior eye structure or intraocular pressure. Therefore, it is believed that the viscosity of the eye increases with age.     

Intraocular pressure measurement-comparison of dynamic contour tonometry and goldmann applanation tonometry

PURPOSE: The dynamic contour tonometer (DCT, Pascal tonometer, Swiss Microtechnology AG, Port, Switzerland) was recently introduced as a new method of intraocular pressure measurement, supposedly independent of corneal properties. In this study we analyzed the agreement and correlation of dynamic contour tonometry and Goldmann applanation tonometry (GAT) and investigated the influence of central corneal thickness (CCT) and corneal curvature. We also considered preferential patient groups for both methods. METHODS: In a prospective study of 100 eyes without glaucoma, intraocular pressure was measured using dynamic contour tonometry and Goldmann applanation tonometry, followed by measurements of central corneal thickness and corneal curvature. RESULTS: A clear correlation between dynamic contour tonometry and Goldmann applanation tonometry was found (r = 0.693; P < 0.001). Dynamic contour tonometry generally resulted in higher intraocular pressure measurements (median difference + 1.8 mm Hg, mean difference + 2.34 mm Hg). Unlike dynamic contour tonometry, Goldmann applanation tonometry was remarkably affected by central corneal thickness, but neither method was significantly influenced by corneal curvature. Bland-Altman graphs showed remarkable disagreement between dynamic contour tonometry and Goldmann applanation tonometry, which could be partially explained by the influence of central corneal thickness on Goldmann applanation tonometry. To obtain valid readings, dynamic contour tonometry required a more extensive selection of patients than Goldmann applanation tonometry. CONCLUSIONS: Dynamic contour tonometry seems to be a reliable method for intraocular pressure measurement, which unlike Goldmann applanation tonometry is not influenced by central corneal thickness. In clinical practice, advantages from dynamic contour tonometry can be expected for cooperative patients, outpatients, and patients with sufficient bilateral ocular fixation, whereas Goldmann applanation tonometry measurements are more reliable in case of patients with inadequate cooperation, poor vision, or nystagmus.     

Augeninnendruck und Hornhautdicke

OBJECTIVE: Corneal thickness and deformation seem to have a considerable influence on intraocular pressure measurement. Due to differences in the corneal deformation in either non-contact tonometry or applanation tonometry, both methods should be compared in the same patient group depending on central corneal thickness. METHODS: In 106 eyes of 55 patients (18 males, 37 females, age 17-89 years, mean 63.3 years) with glaucoma and central corneal thickness between 409 and 644 microm (Orbscan II pachymetry) intraocular pressure was measured in each eye with non-contact tonometry (Reichert AT550) and 30 min later with Goldman applanation tonometry. RESULT: Non-contact tonometry as well as applanation tonometry showed a positive correlation between measured intraocular pressure and corneal thickness. The steepness of the line of regression was 0.33 mmHg per 10 microm of corneal thickness in non-contact tonometry and 0.17 mmHg per 10 microm of corneal thickness in applanation tonometry. CONCLUSION: Independently of the large differences in individual pressure measurements between non-contact tonometry and applanation tonometry, we found higher IOP values with non-contact tonometry in thicker corneas as compared with applanation tonometry. In thinner corneas there was a better correspondence between both methods. Thus, it seems very likely that corneal rigidity increases with corneal thickness.     

Effect of corneal curvature and corneal thickness on the assessment of intraocular pressure using noncontact tonometry in patients after myopic LASIK surgery

PURPOSE: To evaluate the effect of corneal curvature and corneal thickness on the assessment of intraocular pressure (IOP) using noncontact tonometry (NCT) in patients after myopic LASIK surgery. METHODS: All patients who had myopic LASIK in a university-based eye clinic between February 2002 and May 2002 were retrospectively analyzed. Preoperative NCT was compared with postoperative NCT, postoperative corneal thickness, and postoperative corneal curvature. RESULTS: The difference between the mean preoperative NCT (15.46 +/- 2.50 mm Hg) and postoperative NCT (6.30 +/- 1.57 mm Hg) was significant (9.16 +/- 1.96 mm Hg, P < 0.010). Preoperative NCT significantly correlated with postoperative NCT (P < 0.001), postoperative corneal thickness (P = 0.006), and postoperative anterior corneal curvature (P < 0.010). CONCLUSIONS: Both corneal thickness and anterior corneal curvature affect IOP assessment in patients with myopic LASIK. Although correction formulas can be used to estimate the actual IOP, alternative methods should be investigated to assess IOP independent of corneal thickness and curvature.     

The relationship between central corneal thickness and Goldmann applanation tonometry

Background: The aim of this study was to investigate the relationship between Goldmann applanation tonometry and central corneal thickness in a large sample of healthy eyes. Method: Five hundred eyes of 500 subjects (253 women, 50.6 per cent and 247 men, 49.4 per cent) were analysed in a prospective healthy population study. Mean age of the sample was 31 ± 8 years. Goldmann applanation tonometry was carried out by one physician. Tonometric values were the mean of three consecutive readings. Subsequently, another physician carried out ultrasonic pachymetry with the DGH 2000 AP ultrasonic pachymeter (DGH Technology Inc, San Diego, USA). Ten measurements were made at the centre of the cornea of each eye. The lowest value was used for analysis. Results: Applanation tonometry and central corneal thickness were correlated (r = 0.184, p < 0.001). There was no significam correlation between corneal thickness and age (r = 0.083, p = 0.065), mean spherical equivalem refraction (r = 0.083, p = 0.065) or visual acuity (r = 0.036, p = 0.187). Conclusion: In normal eyes, there is no statistically significant correlation between changes of intraocular pressure and changes of central corneal thickness but they suggest a relationship between intraocular pressure and central corneal thickness. Goldmann applanation tonometry has a systematic error in accuracy of intraocular pressure readings of healthy eyes caused by its dependence on central corneal thickness. Measurement of corneal thickness by optometrists should be the first step in diagnosing intraocular pressure pathologies.     

New ways to measure intraocular pressure

PURPOSE OF REVIEW: In the last 10 years, several new means to measure intraocular pressure have emerged. This review covers recent findings concerning four new technologies: the ocular response analyzer, dynamic contour tonometry, rebound tonometry and the Proview phosphene tonometer. RECENT FINDINGS: The ocular response analyzer provides measurements of corneal biomechanics, including corneal hysteresis. Intraocular pressure readings from the ocular response analyzer have correlated well with Goldmann applanation tonometry and seem to be independent of corneal thickness in nonglaucoma patients; however, further studies are needed to determine whether this is true in glaucoma patients. Dynamic contour tonometry also appears to give pressure readings that are independent of corneal thickness. Rebound tonometry is convenient, can be used without topical anesthesia and appears to correlate well with Goldmann tonometry; however, pressure readings from rebound tonometry are not independent of corneal properties. Use of the Proview phosphene tonometer appears to decrease patient anxiety regarding their glaucoma; however, studies have not been supportive of its accuracy. SUMMARY: Dynamic contour tonometry provides intraocular pressure readings that are less dependent on corneal properties than Goldmann applanation tonometry. Rebound tonometry appears to correlate well with Goldmann tonometry and can be used without topical anesthesia.     

Theoretical basis of goldmann applanation tonometry

For more than 50 years Goldmann applanation tonometry has been the internationally accepted method for measuring intraocular pressure. In Goldmann applanation tonometry, however, some basic physical properties are oversimplified and the method has some flaws and restrictions. This paper is intended to promote the understanding of the methodological basis of Goldmann applanation tonometry and describes the most important factors influencing the measurement of intraocular pressure (e. g., corneal thickness, corneal radius, axial length and corneal morphology). Furthermore, the basic principles of other commonly used tonometer devices will be discussed. New developments are anticipated that will measure the true intraocular pressure more accurately than Goldmann applanation tonometry.     

Application of physical principles in the development of tonometry

A review of the development of applanation tonometry is undertaken from the perspective of the physical principles involved. Original articles that have contributed to this process are discussed. The result of this endeavour leads to a conclusion that corneal thinning procedures have no effect on original intraocular pressure. Also concluded is a methodology that would allow the direct measurement of original intraocular pressure.     

Intraocular pressure assessment after laser in situ keratomileusis: a review

This paper aims to review the current methods available for the measurement of intraocular pressure after myopic laser in situ keratomileusis for the correction of myopia. Searches were performed for studies that assessed or compared various methods of intraocular pressure assessment. There were 20 eligible studies that explored the use of pneumotonometry, pressure phosphene tonometry, rebound tonometry, dynamic contour tonometry, statistical modeling, mathematical formulae, ocular response analyzer and even measuring intraocular pressure on the nasal cornea. Our review shows that an ideal method would be one that is independent of corneal factors. Dynamic contour tonometry and pressure phosphene tonometry held promise in research settings. More studies need to be done to validate the new methods of intraocular pressure assessment, especially in glaucoma patients. It is important to empower laser in situ keratomileusis patients with knowledge of these difficulties and potential implications for the future.     

The noncontact tonometer. Clinical evaluation on normal and diseased eyes.

The measurement of intraocular pressure with a noncontact tonometer was correlated to a Goldmann tonometer in 20 normal persons, 20 glaucomatous patients, and 8 patients suffering from corneal diseases. In normal persons the error in measurement for the noncontact tonometer was related to their skill in fixation, and in eye patients to height of pressure and corneal state. Acceptable correlation was found between noncontact tonometry and Goldmann applanation tonometry when the cornea was normal and the pressure below 35 mmHg (Goldmann), otherwise noncontact tonometry was only a guide, and in the presence of corneal disease, unreliable. Good fixation reduced the method error. The standard deviation was 1.09 mmHg at poor fixation and 0.60 mmHg at good fixation. Repeated measurements on the same eye with noncontact tonometry did not alter the intraocular tension.     

Central corneal thickness and related factors in an elderly American Chinese population

Background: To assess central corneal thickness and related factors in an elderly American Chinese population residing in San Francisco. Design: Cross‐sectional community based study. Participants: American Chinese aged 40 years and older were enrolled using random cluster sampling and volunteer screening in the Chinatown district of San Francisco. Methods: The following data were obtained: central corneal thickness by ultrasound pachymetry, intraocular pressure by Goldmann applanation tonometry, axial length by A‐scan biometry, refractive status and corneal curvature by autorefractor. History of systemic and ocular diseases was collected via standard questionnaire. Main Outcome Measures: Central corneal thickness. Results: Of 311 eligible subjects, 274 consented to study participation, and 228 phakic eyes were analyzed. Mean corneal thickness was 524.1 ± 31.1 µm, 545.5 ± 30.9 µm and 538.9 ± 31.8 µm in the sampling cluster, volunteer group and all subjects, respectively. A multiple linear regression model showed corneal thickness to be negatively associated with age (standardized regression coefficient [SRC] = ?0.21; P = 0.016) and corneal curvature (SRC = ?0.19; P = 0.018) but positively correlated with intraocular pressure (SRC = 0.20; P = 0.023). Conclusions: The distribution of central corneal thickness among this American Chinese population is similar to that reported in studies from East Asia. The independent factors associated with thinner corneas included older age, lower intraocular pressure and greater corneal curvature. While descendents of Chinese immigrants in America have, on average, thicker corneas than their ancestors, this phenomenon is potentially impacted by the level of intraocular pressure.     

新型眼压计原理及其在临床中的应用

眼压测量是目前对青光眼进行疗效观察及随访的主要手段之一,眼压计是临床工作中测量眼压的重要工具,修氏眼压计和Goldmann 压平眼压计长期以来为国内外临床医生所广泛应用,近十年来,又出现了一些新型的眼压计,如动态轮廓眼压计（DCT）、I Care回弹眼压计、Tono-Pen眼压计、Diaton眼压计和Proview压眼闪光眼压计等。其原理各异,临床应用价值也众说纷纭。DCT与Goldmann 压平眼压计（GAT）有较好的相关性,且测量值不受角膜性状的影响,但其对于配合差者有较大误差。I Care回弹眼压计、Tono-Pen眼压计、Diaton眼压计和Proview压眼闪光眼压计均为便携式眼压计,测量时不需使用麻醉药,便于青光眼的筛查,但其临床应用较少,测量的准确性仍需进一步研究。（眼科,2012,21： 136-140）     

Continuous intraocular pressure measurement over several days with the Codman Microsensor--a case report]

BACKGROUND: In eyes with irregular corneal surface (e.g. following bullous keratopathy, irregular astigmatism, edema or scars and following perforating keratoplasty), applanation tonometry often cannot be performed or results do not correlate with the clinical findings. In these cases, intraocular measurement of intraocular pressure is necessary. By puncturing of the anterior chamber, single measurements can be done for a short time period. Data of the course of intraocular pressure for a long period of time can not be assessed by this method. We report on two patients whom we implanted a continuously measuring probe into the anterior chamber for up to 96 hours instead of puncturing the anterior chamber. PATIENTS AND METHODS: A neurosurgical micro sensor (CODMAN, Norderstedt) was placed into the anterior chamber via a 1.2 mm wide and 4 mm long scleral tunnel. The data were transmitted to the ICP Express Display Monitor (CODMAN, Norderstedt) and displayed. From there, the data were transmitted to the multifunctional monitor DINAMAP Plus 8720 (CRITIKON, Norderstedt). After analog-digital transformation, the data were recorded on a personal computer with Pentium processor for analysis (patient #1: one measurement per minute, patient #2: one measurement per 10 seconds). In the first patient, implantation of the probe was indicated by enormous deviation of applanation tonometric measurements (12 to 20 mmHg) from the measurement results with the finger tips (25 to 30 mmHg). Clinical findings correlated to the higher intraocular pressure. Due to a decompensation of the corneal transplant, a re-keratoplasty was necessary. Within this operation, the micor sensor for continuous measurement of the intraocular pressure was implanted. The probe was explanted the next day. In the second patient, an primary chronical open-angle glaucoma in both eyes was known. In 1997, corneal transplantation has been performed in both eyes due to corneal dystrophy. Postoperatively, intraocular pressure stayed high. Applanation tonometry gave measurements of 16 to 20 mmHg although the measurement results with the finger tips exceeded 30 mmHg. To find out the real intraocular pressure and to have a basis for a rational therapy, we implanted the intraocular measurement probe for five days to determine the intraocular pressure at night and day. After measuring the baseline values, the efficiency of several antiglaucomatous drugs was tested to find out the drugs with the highest effect to prescribe it to the patient after the removal of measuring probe. RESULTS: The intraocular measurement with the CODMAN micro sensor could confirm in both patients that the measurements by applanation tonometry were wrongly too low. The measurement results with finger tips were confirmed by the intraocular measuring. The data had essential implications for the patients. Meanwhile, in both patients pressure lowering surgery was performed. The probe did not cause intraocular problems (1 day respectively 5 days). An irritation of the anterior chamber did not appear. In the first patient, the measuring probe moved from its position with following external filtration. So the probe was explanted only the next day. A movement of the probe tip can be avoided be appropriate subconjunctival suture fixation. CONCLUSION: Continuous measuring and recording of the intraocular pressure may be indicated, if applanation tonometry gives unreliable or even wrong results. Via a long scleral tunnel, a water-proof implantation into the anterior chamber is possible. Because a postoperative irritation could not be seen, we think that the probe only causes a minor falsification of the intraocular pressure. The described pressure measuring system allows measuring intraocular pressure continuously and assessing the individual effect of different antiglaucomatous drugs. Before using the probe as routine procedure, some improvements are necessary, e.g. smaller tip of the probe. The transmission wire to the     

Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis

PURPOSE: To understand and quantify intraocular pressure (IOP) measurement errors introduced by corneal variables during applanation tonometry using a cornea biomechanical model. SETTING: Department of Ophthalmology, Biomedical Engineering Center, The Ohio State University, Columbus, Ohio, USA. METHODS: The model assumed an overall resultant pressure that was based on the summation of the applanation pressure, the true IOP, and the surface tension caused by the tear film to determine the final deformation of the corneal apex during IOP measurement. Corneal resistance was varied according to the cornea's biomechanical properties, thickness, and curvature, and the effect of each variable on the accuracy of IOP tonometry readings was examined quantitatively. RESULTS: The model demonstrated that tonometry readings do not always reflect true IOP values. They deviate when corneal thickness, curvature, or biomechanical properties vary from normal values. Based on the model, predicted IOP readings have a 2.87 mm Hg range resulting from the variation in the corneal thickness in the normal population and a 1.76 mm Hg range from the variation in the corneal radius of curvature. Considering that Young's modulus of the corneal varies from 0.1 to 0.9 MPa in the normal population, the model predicts tonometry IOP readings will have a range of 17.26 mm Hg because of the variation in this corneal biomechanical parameter alone. CONCLUSIONS: The simulation based on the model demonstrated quantitatively that variations in each corneal variable cause errors in tonometry IOP readings. The simulation results indicate that differences in corneal biomechanics across individuals may have greater impact on IOP measurement errors than corneal thickness or curvature.     

Measurement of intraocular pressure in LASIK and LASEK patients using the Reichert Ocular Response Analyzer and Goldmann applanation tonometry

PURPOSE: To determine the efficacy of the Reichert Ocular Response Analyzer (ORA) to measure intraocular pressure (IOP) following corneal laser refractive surgery. METHODS: Intraocular pressure was measured using Goldmann applanation tonometry preoperatively and 3 months following LASIK and LASEK for all levels of myopia and low levels of hyperopia. In LASIK eyes, 120-microm flaps were cut using the Hansatome XP. The ORA was used to measure Goldmann correlated IOP and corneal compensated IOP 3 months postoperatively. Postoperative central corneal thickness, hysteresis, and corneal curvature were assessed to determine their relationship with postoperative change in Goldmann applanation tonometry IOP. Efficacy of ORA IOP measurement was analyzed by comparing Goldmann applanation tonometry IOP with Goldmann correlated IOP. RESULTS: LASEK was performed on 35 eyes, and LASIK was performed on 90 eyes. In the LASIK group, mean Goldmann applanation tonometry IOP decreased 3.7+/-2.3 mmHg postoperatively (P=.00). Postoperative Goldmann applanation tonometry did not differ significantly (P=.06) from postoperative ORA Goldmann correlated IOP (10.2+/-2.1 mmHg). In the LASEK group, mean Goldmann applanation tonometry IOP decreased 3.9+/-2.3 mmHg (P=.00). Postoperative Goldmann applanation tonometry did not differ significantly (P=.6) from postoperative ORA Goldmann correlated IOP (10.7+/-2.5 mmHg). Postoperative decrease in Goldmann applanation tonometry IOP did not correlate with age, ablation depth, pre- and postoperative central corneal thickness or corneal hysteresis, or postoperative decrease in corneal curvature. CONCLUSIONS: Intraocular pressure measurements were similar using both the ORA Goldmann correlated IOP and Goldmann applanation tonometry following keratorefractive surgery.     

Impressions- und Applanationstonometrie bei pathologischen Hornhäuten im Vergleich mit der intraokularen Nadeldruckmessung

Purpose. During the past 4 years we have demonstrated in eyes with corneal pathology that applanation tonometry (Goldmann, Perkins) generally delivers falsely low measurements in comparison to intraocular needle tonometry. The aim of this study was to evaluate whether impression tonometry (Schioetz) is more precise than applanation tonometry in determinating the intraocular pressure in eyes with corneal disorders. Patients and methods. In 75 eyes with suspected glaucoma and various corneal disorders, we performed applanation tonometry and impression tonometry before intraocular needle tonometry. Applanation tonometry was repeated after impression tonometry to unveal a possible tonography effect. Intraocular needle tonometry was performed thereafter. Results. Applanation tonometry results were 4.1±5.3 mmHg below intraocular pressure as determined by intraocular needle tonometry. Impression tonometry results were also lower: 4.3±6.8 mmHg (5.5 g), 4.3±6.4 mmHg (7.5 g), and 4.8±7.0 mmHg (10.0 g). The differences between applanation tonometry and impression tonometry were statistically not significant. In contrast, all the differences between extraocular tonometry procedures and intraocular needle tonometry were statistically highly significant (P<0.001). Conclusion. In corneal pathology both, applanation tonometry and impression tonometry do not deliver reliable results on an average. Only intraocular needle-tonometry delivers reliable results in these eyes.     

Young's modulus in normal corneas and the effect on applanation tonometry.

OBJECTIVES: To determine the statistically normal range of corneal Young's modulus in young healthy eyes in vivo, and to establish if this variation has a clinically significant influence on intraocular pressure (IOP) measurement using applanation tonometry. METHODS: Central corneal curvature, central corneal thickness (CCT), and applanation IOP (Goldmann tonometer) were measured using standard clinical techniques in one eye of 100 normal human subjects (22.0 +/- 2.9 years) in vivo. The Orssengo-Pye algorithm was used to calculate the corneal Young's modulus of these experimental subjects, and to produce a theoretical model of potential errors in Goldmann applanation tonometry estimates of IOP due to variations of Young's modulus and CCT. RESULTS: Corneal Young's modulus was 0.29 +/- 0.06 MPa [95% confidence interval (CI) 0.17 to 0.40 MPa]. According to the Orssengo-Pye model, the relationship between Young's modulus and the error in applanation IOP is linear; the slope was 23 mm Hg per MPa. An increase from the minimum to the maximum value of the calculated limits of agreement (95% CI) of Young's modulus caused a variation in applanation IOP of 5.35 mm Hg. The anticipated error at the extremes of the limits of agreement (95% CI) of CCT was similar at 4.67 mm Hg. CONCLUSION: Physiological variations in corneal Young's modulus may cause clinically significant errors in Goldmann applanation tonometry estimates of IOP.     

Applanation tonometry in "normal" patients and patients after LASIK

BACKGROUND: Until now it was thought that morphological parameters of the eye such as corneal thickness, corneal curvature and axial length do not affect tonometry results. However, the aim of this study was to find out whether there actually is an influence of these parameters on applanation tonometry. PATIENTS AND METHOD: In this prospective study we examined 125 eyes of 125 normal patients with a corneal thickness of 568.8 +/- 43.79 microm, a corneal curvature of 7.72 +/- 0.27 mm and an axial length of 23.62 +/- 2.05 mm. Before performing a phacoemulsification, the anterior chamber was temporarily punctured. With a closed system the intraocular pressure (IOP) was manometrically set at 20, 35 and 50 mmHg using an H (2)O column. The IOP was then measured with a Perkins tonometer. With these patients we compared 102 eyes that had undergone LASIK due to a myopia of 6.3 +/- 2.17 D. Before and 6 months after surgery, IOD, k-values and central corneal thickness of these patients were measured. RESULTS: At all set pressure levels there was a highly significant correlation of measured IOP and corneal thickness. At all set pressure levels the measured IOP significantly depended on corneal thickness (r(2) = 0.78 - 0.83). After LASIK, IOP was reduced from 16.5 +/- 2.1 to 12.9 +/- 1.9 mmHg. There was a significant correlation between IOP and corneal curvature as well as corneal thickness (r(2) = 0.631; P < 0.001). The biomechanical characteristics of the cornea are changed so that the measured IOP has to be corrected by an additional 0.75 mmHg. CONCLUSION: Since corneal thickness does affect Goldmann applanation tonometry we recommend to use the "Dresden Correction Table" (Tab. ) to achieve the real IOP. Pressure measurements after LASIK are inaccurate because of a change in corneal biomechanics, corneal thickness and curvature and they should be corrected as follows: IOP (real) = IOP (measured) + (540 - CCT)/71 + (43 - K-value)/2.7 + 0.75 mmHg.     

兔眼中央角膜厚度与Perkins压平眼压关系的研究

目的探讨中央角膜厚度(CCT)与Perkins压平眼压的关系,建立CCT、真实眼压与Perkins压平眼压三者关系的数学模型和CCT对Perkins压平眼压的校正公式。方法健康新西兰大耳白兔32只,双眼中1只眼行准分子激光屈光性角膜切削术(PRK),另1只眼测得的数据对行PRK眼得出的数据进行验证。采用随机数字表法随机取1只眼,采用PRK,人为改变兔眼的CCT,建立不同CCT的活体眼模型,分别采用Perkins压平眼压计、A型超声角膜测厚仪、角膜曲率计测量术前、术后Perkins压平眼压、CCT、角膜曲率以及应用电子直接眼压计测量真实眼压,分别对术前和术后测量值做相关分析和多元线性回归分析,并对另1组未行PRK眼测得的真实眼压、Perkins压平眼压、CCT对实验组得出的公式进行验证。结果兔眼双眼Perkins压平眼压、CCT、角膜曲率无统计学差别,无论是术前还是术后Perkins压平眼压与CCT均显著相关(r=0.761P<0.01;r=0.829,P<0.01),与角膜曲率无关(r=0.098,P>0.05;r=0.260P>0.05)。对术前、术后Perkins压平眼压与CCT建立回归公式,曲线的斜率分别为0.066、0.053,Perkins压平眼压的改变与CCT的改变有关。对真实眼压Y与Perkins压平眼压、CCT三者的关系建立回归公式为Y=12.107+1.254X1-0.033X2(X1=Perkins压平眼压,X2=CCT)。结论CCT的改变影响Perkins压平眼压的测量值,临床上应根据CCT来校正Perkins压平眼压的测量值。     

PRK术后2年眼压临床分析

目的;探讨PRK术后2年的眼压变化情况及其相关影响因素。方法：应用非接触式眼压计（NCT）对接受PRK手术的121只术眼进行了2年的跟踪观察,术前按术前屈光度分为3组,I组：－6．0D以下;Ⅱ组：－6．25D－－9．0D;Ⅲ组：－9．25D以上。并对术眼不同阶段NCT测量值进行多元分析。结果：各PRK术后NCT眼压测量值均较术前明显降低,具有显著性差异;并且切削厚度与眼压差存在显著相关性;术后眼压发生有规律性变化与术后常规应用激素类药物有关。结论：PRK后NCT眼压测量值低于术前并发生有规律性变化,术后眼压与角膜厚度、角膜切削厚度、角膜前表面曲率改变及拂炎眼液的用药时间、次数有关,并且测量值受NCT工作原理的影响。