In the 1990s a number of authors have studied the refractive changes induced in the cornea as a result of the procedure.2-6 Several surgical modifications have been described, [7][8][9][10][11] each of which may theoretically influence the changes noted in the cornea following surgery. In 1995 we reported the intraocular pressure results following microtrabeculectomy, a small flap modification of the original design which we have utilised routinely since 1991.11 The scleral trapdoor at 2 × 2 mm is approximately one sixth of the area of the Cairns' procedure, and the 0.75 × 0.75 mm internal opening is just one seventh of the area of the original osteum. As a smaller sized operation theoretically should result in less surgical trauma, we performed a prospective study to investigate the changes in corneal curvature following microtrabeculectomy. We utilised vector analysis and vector decomposition techniques to determine the "with the rule" and "against the rule" changes induced in the cornea. Materials and methodsFollowing ethics committee approval and informed consent, a cohort of eyes requiring filtering surgery underwent microtrabeculectomy. None of the eyes had had previous ocular surgery. Full details of the surgical technique have been described elsewhere.11 Briefly, a limbus based conjunctival flap was fashioned commencing 4 mm from and exposing the limbus. SuYcient scleral cautery was then performed using a battery powered bipolar cautery (Mentor) over an area approximately 3 mm × 3 mm, suYcient to avoid haemorrhage during the procedure. A 2 mm × 2 mm scleral trap door was constructed, for this study centred at the 90 degree meridian, and an anteriorly sited 0.75 mm diameter internal sclerostomy was achieved with a Kelly punch (Storz). A small basal peripheral iridectomy was followed by two 10/0 nylon scleral trapdoor sutures and a running 10/0 gauge monofilament polyglycolic acid suture to the conjunctiva. Balanced salt solution (Alcon) and air were then injected into the anterior chamber via a paracentesis performed at the start of surgery. Postoperative medications routinely employed were atropine 1% drops three times a day and betamethasone 0.1% with neomycin sulphate 0.5% four times a day for the first week. At the 1 week review the atropine was stopped and the steroid/antibiotic reduced at the discretion of the clinician. All patients continued topical steroids for at least 1 month but for no longer than 3 months postoperatively. All surgery was performed by the same surgeon (SAV) who was experienced in the technique and no suture manipulations by laser or otherwise were used postoperatively.In order to provide the raw data for analysis, manual keratometry with a Javal/Schiotz keratometer and computerised corneal videokeratoscopy with the Eyesys corneal analysis system
Uncomplicated lamellar flap creation is responsible for systematic changes in corneal topography and induction of higher-order optical aberrations. Predictors of this response include stromal bed thickness, flap diameter, and total corneal pachymetry.
Dr. Claoué has a financial interest in the patent controlling the Fresnel prism IOL. Dr. Potgieter has no financial or proprietary interest in any material or method mentioned.
House, 1979, pp. 211. Cloth, $24.95. This book forms part three in the series of studies in Spatial Analysis stimulated by the theoretical and applied research conducted within the Netherlands Economics Institute (NEI), Rotterdam and published by Saxon House. Unlike the previous two volumes (Paelinck, Nijkamp: Operational Theory and Method in Region Economics, 1979; Klaasen, Paelinck, Wagenaar: Spatial Systems, 1979) this volume covers a specialized subject at a very advanced level, but in a poorly organized way. Assuming a lot of prior knowledge, it highlights some of the problems relating to model estimation and comparison. However, the book gives neither an introduction to the art of spatial model building nor a review of techniques and problems such as the calibration and estimation of such models.The book presents some brief general remarks about different estimation techniques including rather recent developments in the field, and applies these techniques to several specific models which are used by the NEI. These include a simple Keynesian income generating model, the interregional sectoral attraction model (developed in the Weberian or Loeschian tradition by Leo Klaassen), a simple threshold model, and a conventional shopping location model. After some development and discussion of the models, the log-likelihood-functions of the spatially autocorrelated error terms are derived. The book shows, with practical examples, how different specified models and error terms can be compared. The advantages of correctly specified error terms are clarified. In particular, the meaning of correct error specification within the context of spatial models is illustrated using appropriate examples.Clearly the way space enters into models should be central to the book. However, the reader is forced to search out the actual detail. The case of the income generating model demonstrates how space can be introduced to a conventional model by means of a spatial contingency matrix. In the KlaassenInterregional-Attraetion-Model the communication (including transportation) costs represent spatial interdependence and are considered in an input-outputframework. The shopping model is based on the spatial gravity concept. Surprisingly, problems of combining cross-sectional and time-series analysis (pooling) are not examined although an exhaustive bibliography on that subject is given. Another omission is the effect of different size and choice of regions on estimation and model comparison.A severe drawback of the book is that, although the mathematical treatment is sound, the technical analysis of problems is obscured by poor presentation of the subject. The purpose of mathematical developments is rarely explained, nor the meaning of the resulting formula. The notation is confusing and sometimes inconsistent. The text switches between a very high and a rather low level of sophistication.For example, whereas the meaning of biased estimation is explained, a Frobenius-root is assumed to be known. The text gives no help in generalizing...
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