A new optical low coherence reflectometry device for ocular biometry in cataract patients.

BACKGROUND: A new commercially available optical low coherence reflectometry device (Lenstar, Haag-Streit or Allegro Biograph, Wavelight) provides high-resolution non-contact measurements of ocular biometry. The study evaluates the validity and repeatability of these measurements compared with current clinical instrumentation. METHOD: Measurements were taken with the LenStar and IOLMaster on 112 patients aged 41-96 years listed for cataract surgery. A subgroup of 21 patients also had A-scan applanation ultrasonography (OcuScan) performed. Intersession repeatability of the LenStar measurements was assessed on 32 patients. RESULTS: LenStar measurements of white-to-white were similar to the IOLMaster (average difference 0.06 (SD 0.03) D; p = 0.305); corneal curvature measurements were similar to the IOLMaster (average difference -0.04 (0.20) D; p = 0.240); anterior chamber depth measurements were significantly longer than the IOLMaster (by 0.10 (0.40) mm) and ultrasound (by 0.32 (0.62) mm; p<0.001); crystalline lens thickness measurements were similar to ultrasound (difference 0.16 (0.83) mm, p = 0.382); axial length measurements were significantly longer than the IOLMaster (by 0.01 (0.02) mm) but shorter than ultrasound (by 0.14 (0.15) mm; p<0.001). The LensStar was unable to take measurements due to dense media opacities in a similar number of patients to the IOLMaster (9-10%). The LenStar biometric measurements were found to be highly repeatable (variability < or = 2% of average value). CONCLUSIONS: Although there were some statistical differences between ocular biometry measurements between the LenStar and current clinical instruments, they were not clinically significant. LenStar measurements were highly repeatable and the instrument easy to use.

Subjective and objective performance of the Lenstec KH-3500 "accommodative" intraocular lens.

AIM: To determine whether eyes implanted with the Lenstec KH-3500 "accommodative" intraocular lenses (IOLs) have improved subjective and objective focusing performance compared to a standard monofocal IOLs. METHODS: 28 participants were implanted monocularly with a KH-3500 "accommodative" IOL and 20 controls with a Softec1 IOL. Outcome measures of refraction, visual acuity, subjective amplitude of accommodation, objective accommodative stimulus response curve, aberrometry, and Scheimpflug imaging were taken at approximately 3 weeks and repeated after 6 months. RESULTS: Best corrected acuity with the KH-3500 was 0.06 (SD 0.13) logMAR at distance and 0.58 (0.20) logMAR at near. Accommodation was 0.39 (0.53) D measured objectively and 3.1 (1.6) D subjectively. Higher order aberrations were 0.87 (0.85) microm and lower order were 0.24 (0.39) microm. Posterior subcapsular light scatter was 0.95% (1.37%) greater than IOL clarity. In comparison, all control group measures were similar except objective (0.17 (0.13) D; p = 0.032) and subjective (2.0 (0.9) D; p = 0.009) amplitude of accommodation. Six months following surgery, posterior subcapsular scatter had increased (p<0.01) in the KH-3500 implanted subjects and near word acuity had decreased (p<0.05). CONCLUSIONS: The objective accommodating effects of the KH-3500 IOL appear to be limited, although the subjective and objective accommodative range is significantly increased compared to control subjects implanted with conventional IOLs. However, this "accommodative" ability of the lens appears to have decreased by 6 months post-surgery.

Corneal pachymetry in normal and keratoconic eyes: Orbscan II versus ultrasound.

PURPOSE: To compare corneal thickness measurements using Orbscan II (OII) and ultrasonic (US) pachymetry in normal and in keratoconic eyes. SETTING: Eye Department, Heartlands and Solihull NHS Trust, Birmingham, United Kingdom. METHODS: Central corneal thickness (CCT) was measured by means of OII and US pachymetry in 1 eye of 72 normal subjects and 36 keratoconus patients. The apical corneal thickness (ACT) in keratoconus patients was also evaluated using each method. The mean of the difference, standard deviation (SD), and 95% limits of agreement (LoA = mean +/- 2 SD), with and without applying the default linear correction factor (LCF), were determined for each sample. The Student t test was used to identify significant differences between methods, and the correlation between methods was determined using the Pearson bivariate correlation. Bland-Altman analysis was performed to confirm that the results of the 2 instruments were clinically comparable. RESULTS: In normal eyes, the mean difference (+/- 95% LoA) in CCT was 1.04 microm +/- 68.52 (SD) (P>.05; r = 0.71) when the LCF was used and 46.73 +/- 75.40 microm (P =.0001; r = 0.71) without the LCF. In keratoconus patients, the mean difference (+/- 95% LoA) in CCT between methods was 42.46 +/- 66.56 microm (P<.0001: r = 0.85) with the LCF, and 2.51 +/- 73.00 microm (P>.05: r = 0.85) without the LCF. The mean difference (+/- 95% LoA) in ACT for this group was 49.24 +/- 60.88 microm (P<.0001: r = 0.89) with the LCF and 12.71 +/- 68.14 microm (P =.0077; r = 0.89) when the LCF was not used. CONCLUSIONS: This study suggests that OII and US pachymetry provide similar readings for CCT in normal subjects when an LCF is used. In keratoconus patients, OII provides a valid clinical tool for the noninvasive assessment of CCT when the LCF is not applied.

Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy.

PURPOSE: To evaluate lenses produced by excimer laser ablation of poly(methyl methacrylate) (PMMA) plates. SETTING: University research laboratory. METHODS: Two Nidek EC-5000 scanning-slit excimer laser systems were used to ablate plane-parallel plates of PMMA. The ablated lenses were examined by focimetry, interferometry, and mechanical surface profiling. RESULTS: The spherical optical powers of the lenses matched the expected values, but the cylindrical powers were generally lower than intended. Interferometry revealed marked irregularity in the surface of negative corrections, which often had a positive "island" at their center. Positive corrections were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when observing through ablated lenses whose power had been neutralized with a suitable spectacle lens of opposite sign confirmed that the surface irregularities of the ablated lenses markedly reduced contrast sensitivity over a range of spatial frequencies. CONCLUSION: Improvements in beam delivery systems seem desirable.

Changes in posterior corneal curvature after photorefractive keratectomy.

PURPOSE: To determine whether myopic ablation by excimer laser photorefractive keratectomy (PRK) affects only the anterior curvature of the cornea or whether changes also occur in the posterior corneal curvature. SETTING: Department of Optometry and Neuroscience, UMIST, and Optimax Laser Eye Clinic, Manchester, United Kingdom. METHODS: Sixteen patients who presented for correction of myopia in 1 eye by excimer laser PRK were followed for 3 months. Only newly presenting patients were recruited, and the untreated eyes were used as controls. The patients were examined at the initial visit (0 week) and 6 and 12 weeks post-PRK. Measurements included Orbscan topography and pachymetry, autokeratometry, and ultrasound pachymetry. RESULTS: The mean patient age of the 8 men and 8 women was 29.6 years +/- 8.6 (SD) (range 20 to 47 years). The attempted mean spherical equivalent correction was between -1.73 and -6.43 diopters. Anterior corneal curvature and corneal thickness in the treated eyes changed systematically in relation to the amount of ablation. Posterior corneal curvature steepened in relation to the dioptric power treated. There were systematic differences between the pachymetry values obtained with the Orbscan and the ultrasound pachymeter. CONCLUSIONS: The results suggest that after myopic PRK, the thinner, ablated cornea may bulge forward slightly to steepen both anterior and posterior curvatures. This may account for the regression toward myopia that is typically found in the first few days posttreatment. The forward bulging is similar to the corneal relaxation effects observed after radial keratotomy.

Factors that influence patient choice of contact lens or photorefractive keratectomy.

OBJECTIVE: To explore characteristics of patients and factors that influence their decision in selecting photorefractive keratectomy (PRK) and contact lenses. METHODS: A questionnaire was presented to 255 PRK and 240 contact lens patients, covering patient demographics, refraction, reasons for choosing PRK or contact lens, and advice from clinicians or friends/relatives. RESULTS: Mean patient age was 37 years for PRK and 26 years for contact lens (P < .0001). The magnitude of the mean spherical equivalent refraction in the better eye was 4.14 D for PRK and 3.26 D for contact lens. Advertising influenced 36% (n = 92) of PRK compared to 17% (n = 41) of contact lens patients. In the PRK group, 52% (n = 133) opted for PRK based on cost; in the contact lens group the cost of PRK was thought to be high by 68% (n = 163). Eighty-four percent (n = 214) of the PRK group were contact lens wearers; of these, 32% (n = 68) were intolerant to contact lenses and 13% (n = 28) were intolerant to lens care solutions. Fifty-six percent (n = 134) of the contact lens group were concerned about the long-term effects of PRK and 49% (n = 118) were afraid to undergo a surgical procedure. CONCLUSIONS: PRK patients were significantly older but had only marginally higher refractive error than contact lens patients. Many patients appeared to discontinue contact lens use due to tolerance problems.

Reduction in intraocular pressure after excimer laser photorefractive keratectomy. Correlation with pretreatment myopia.

PURPOSE: The authors relate the observed reduction in intraocular pressure (IOP) after excimer laser treatment to the degree of myopia treated. BACKGROUND: Intraocular pressure, measured by both Goldmann applanation and noncontact tonometry, has been reported to decrease after excimer laser photorefractive keratectomy (PRK). However, IOP readings after excimer laser PRK might be inaccurate as a consequence of changes in both the thickness and curvature of the cornea. METHODS: Baseline IOP readings were measured by noncontact tonometry in each eye of a group of 1320 patients at the time of their initial consultation. These were compared to readings obtained before treatment of the second eye, which took place a minimum of 4 months later. The untreated eyes served as controls. The paired Student's t test was used for statistical analysis. RESULTS: After PRK, a decrease was observed in the IOP of treated eyes that was related to the degree of myopia treated. A significant difference was observed between treated and untreated eyes (P < 0.0000). CONCLUSIONS: The IOP measured after PRK for myopia may be reduced because of changes in corneal thickness (absence of Bowman's membrane and central thinning) and topography. This is of particular relevance when monitoring the IOP of those patients who are given steroid drops to prevent regression. It also may be of importance in the management of any future glaucoma.