Evaluation, prevention and surgical treatment of post-keratoplasty astigmatism with the use of computer assisted videokeratography.
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
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.