PART III PARAMETER RELATIONSHIPS

8. 10 PARAMETER RELATIONSHIPS

8.10.1 Parameter comparison

Analysis by multiple regression was used to investigate the

postoperative changes at stages 6 weeks and 16 weeks for HSF, VSF

and corneal astigmatism by PEK to find whether these were related

to preoperative corneal astigmatism (keratometry) values and so

might be predictable to some extent. No significant relationship

at p<0.05 level was found (see Table 8.16). There was also no

significant relationship found to the preoperative actual values

of HSF and of VSF. Postoperatively, although there was no

significant time effect demonstrated by either HSF or VSF because

of the variable results, (C) group did show a significant

difference between HSF and VSF changes, F=11.95, df=715,l,

p<0.01. <L) group did not do so. This indicated that a corneal

incision disturbed VSF to a greater extent than did a limbal

incision. Corneal astigmatism by PEK showed a significant

relation at p<0.01 level to corneal astigmatism by keratometry,

so either instrument could be used to measure central corneal

curvature. No significant relationship was found between

preoperative corneal thickness in whatever region, and HSF, VSF,

and corneal astigmatism whether by PEK or keratometry.

No significant difference was found between males and females

as regards the time effect on the parameters measured, nor on

6.10.2 Relation to the presence of lOL

As the presence ot an lOL was a possible factor in the corneal

response to the surgery, the results were re-analysed with regard

to whether an lOL was implanted (Table 6.17). Coincidently the

lOL and non-IOL group sizes were very similar; mean age with IÛL

(N=54> was 65.4 years, SD=+/-8.7, and the mean age without lOL

(N=51> was 56.5 years SD=+/-13.4. There was no significant

difference between the mean ages, although the surgeons were more

reluctant to Implant an lOL In the younger patients because at

that time little was known regarding the corneal tolerance to

several decades of lOL presence.

An imbalance was noted between the proportions of lOL/non lOL

and operation type; (C) and lOL N=35, (C) and non lOL N= 19, (L)

and lOL N=19, (L) and non lOL N=32. This tended to reflect the

clinical opinions of the surgeons in the study. More recently

improvements in lOL design and surgery techniques have led to

fewer cases of lOL Induced endothelial damage so lOLs can be

implanted into a wider variety of patients. If the original

proposal had been to study the influence of an lOL then more

balanced numbers would have been sought.

On examining the response of the corneal astigmatism (by

keratometry) , multiple regression showed no significant relation

at p<0.05 level to whether or not an lOL was present. The

exception was at 16 weeks when p=0.04 but only 6 eyes with lOLs

Multiple regression showed that the relation between

postoperative changes and preoperative values was not dependent

on the presence ot an lOL. The two exceptions to this were tor

patients with an lOL, at 6 weeks (p=0.03, N=7) and at 12 weeks

(p=0.03, N=7). At these stages there was an apparently

significant relation between changes and preoperative values, but

as the numbers were small no definite conclusion was drawn. At

these two stages, data was compared between patients with and

without lOLs. 't ' tests showed no significant difference between

these groups (8 weeks, t=0.66, p=0.51, 12 weeks, t=0.44, p=0.65).

Multiple regression on corneal astigmatism CPEK> changes found no

significant relation at p<0.05 level to the presence of an lOL.

For HSF, no significant time effect at p<0.05 was found either

for actual values or postoperative changes, whether or not an lOL

was present. On analysis of the VSF data by multiple regression,

no significant relation at p<0.05 level was demonstrated between

the time effect and the presence of an lOL. No influence was

apparent by the lOL on the relation of postoperative changes to

preoperative values.

Next, the influence of an lOL on the corneal thickness

response was considered. No significant influence on the time

effect or the amount of postoperative thickness change was

revealed. Table 8.18 lists the postoperative changes related to

their preoperative thickness. Table 8.19 lists the significance

presence ot an lOL. There are a tew occasions . when the lOL

appears to Influence this. For example, the iL) group superior

thickness changes are significantly related (at p<0.01> to

preoperative value In those patients without an lOL, except for

the 6 week stage when postoperative oedema is still somewhat

variable. This suggests that their postoperative response is more

predictable than that of lOL patients and that patients with an

lOL take longer to stabilise at a corneal thickness similar to

the original value. This feature is also demonstrated by the (L)

group inferior region changes and to a relatively lesser extent

by the other regions.

Generally, (L) group changes appear to stabilise nearer to the

preoperative value at a quicker rate than do those of the (C)

group. The latter present a more variable response such that the

changes at many stages show no significant relation to

preoperative value nor to the presence of an lOL. This is

understandable for the superior region is near the Incision

site, yet even the inferior region changes also show variability

and no significant relation to preoperative values. No previous

reports of the relation between postoperative thickness

CHAPTER 9