Astigmatism is a common complication after penetrating keratoplasty (PKP) that affects visual rehabilitation in a considerable number of patients. The scope of this thesis was to investigate different aspects of problem "post-PKP astigmatism" with the aid of computer assisted video-keratography (CAVK) through four parallel studies. First, the agreement and repeatability of CAVK were evaluated against the keratometer. A systematic bias of CAVK in measuring steeper principle meridians and higher amount of astigmatism on normal corneas was demonstrated. Measuring agreement between the two instruments on post-PKP corneas was worse than in normal corneas. CAVK repeatability was also found to be observer related as well as astigmatism related. A novice observer has shown larger variability in measurement, and higher deviation scores were seen for highly astigmatic corneas. Repeatability of keratometer measurements was found to be better for post-PKP corneas. The two instruments demonstrated clinically significant differences, both on normal and astigmatic corneas. It is concluded that they cannot be used interchangeably. A new qualitative classification system is proposed in this thesis, for post-PKP corneas. Twelve distinct topographic maps were recognised and some of these may form a continuum. The interobserver repeatability was tested to be high (91% after second review), a prerequisite for any potential clinical application. In post-PKP corneas, the incidence of irregular astigmatism was found to be about double that of regular astigmatism (59% vs. 30% respectively), with prolate and oblate patterns seen in equal proportions (21%). Regular astigmatic patterns were found to be associated with higher astigmatism. Following PKP, the healing process and suture manipulations cause a decrease of the regular astigmatic patterns (and a corresponding increase of the irregular astigmatic patterns) with time. In a prospective randomised study of 95 eyes, it was demonstrated with the aid of CAVK, that suture induced post-PKP astigmatism is not significantly different between a technique using a single continuous adjustable suture (SCAS) and a technique using a combination of interrupted and continuous sutures (lCS). Although SCAS offered an earlier visual stabilisation following PKP, it was also found to be associated with higher complication rates, in particular early loosening of the suture. In the final part of this study, a prospective randomised study of 31 eyes was conducted to assess the advantage of CAVK (as compared to keratometry) in planning asymmetric surgery with relaxing incisions and compression sutures for the surgical correction of high post-PKP astigmatism. The improvement in the results was found to be limited when CAVK was used, but this may be due to the fact that most of these corneas showed regular astigmatism preoperatively.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:389166 |
| Date | January 1997 |
| Creators | Karabatsas, Constantinos H. |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1983/997198ac-860a-48d6-a41f-6a9699cb2254 |
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