EXPERIMENTAL PROCEDURE

7.1 PATIENT SAMPLE AND MEASUREMENT FREQUENCY

With the consultants' and patients' permission, consecutive

patients undergoing routine cataract extraction (with or without

IOL> were included in the study. The measurements, their

frequency and the purpose ot the study was explained to each

patient who then completed a torm of consent record, as shown on

the next page. Patient exclusion criteria were as follows:

a) Previous anterior segment surgery, for example trabeculectomy or penetrating keratoplasty where the endothelium may have

already been damaged so the cornea would be less likely to

respond in a normal manner. This is also true for cases of

traumatic cataract whose inclusion would have biased the grouping

as an lOL would be unlikely if they had iris damage.

b) other ocular pathology, for example glaucoma. Sustained

raised introcular pressure is known to have a deleterious effect

on endothelium and hence on corneal thickness response and

recovery. Corneal epithelial fragility is increased in diabetes

(O'Leary à Millodot 1981) therefore the corneal response to surgery would be abnormal.

c) co-operation difficulty with the measurements required, for

example Parkinsons disease or severe arthritis prohibiting a

d) Difficulty with travelling to attend appointments, for

example if a hospital ambulance or special escort was required.

e; Serious operative surgical complications. Although rare,

should these occur the cornea would be likely to respond in an

abnormal fashion.

During the study period approximately five thousand patients

were admitted to Moorfields Eye Hospital for cataract extraction.

As the study was carried out in the context of a routine service

clinic and the author was often on duty in alternative clinics,

the recruitment of patients occurred over a prolonged time. The

first patient was measured in January 1984, the last of this

sample in January 1988. this was unavoidable because the

research programme was subordinate to service commitments.

Furthermore, the protocol demanded measurements to be taken on

ten separate occasions, which was a further constraint on the

number of new patients who could be recrluted in a given period.

As Moorfields Eye Hospital is a postgraduate teaching hospital

there is inevitably a large number of surgeons performing

cataract surgery, unlike a district general hospital where the

surgery is carried out by the same group of surgeons every year.

At the time of this study two of the four surgeons were

consultant ophthalmologists and two were lecturers at the

Institute of Ophthalmology. By using more than one surgeon

patients could be recruited more rapidly. In an area where

factor, particularly In view of recruitment restraints already

mentioned. Ideally, measurements on patients operated upon by a

single surgeon would be less open to bias introduced by

variations of surgical technique, nevertheless the other factors

were thought to be more important. The results were aggregated as

one of the requirements of the surgeons for their co-operation

was that the outcomes of individual surgeons would not be

described in the study.

One hundred and seventy patients were randomly selected, of

which one hundred and forty were eligible and agreed to take part

(see form of consent record). The surgical techniques were

independently chosen by their surgeon. These were: using a

corneal incision and nylon sutures (group C), and using a limbal

incision and virgin silk sutures (group L). All the surgery was

performed using operating biomicroscopy with ECCE technique and

without detailed regard to preoperative astigmatism. Basic

measurements of average corneal power and the axial length of the

eye were noted to assist in the calculation of the power of the

lOL when required. A reverse-sloping incision was used, with a

similar range of incision size in the two groups (90 to 130

degrees). Corneal sections were sutured with five interrupted

10/0 monofilament nylon sutures, while limbal incisions were

sutured with five interrupted 8/0 virgin silk sutures. An example

of the type of intraocular lens used was: Coburn posterior

chamber lOL model 72UV, control number 171228, power +20.00D,

A control group ot thirteen non-operated fellow eyes in both

groups was monitored at the same visits as the operated eyes;

seven from patients undergoing corneal incision surgery, and six

from those undergoing limbal incision surgery. A further control

group of ten non-operated eyes of age matched volunteers from the

hospital staff was also monitored.

Measurement Frequency

Where possible, assessment visits were combined with routine

postoperative clinic visits to minimise patients' Inconvenience.

Measurements were carried out every 2 weeks to obtain a better

appreciation ot when the astigmatism changed. On the basis of

previous work (see Chapter 3), it had been established that In

the majority of cases, changes in astigmatism have stabilised by

3 to 6 months postoperatively, so the final measurement in this

study was made at the 24 week stage. Measurements were taken at

the following times: within 1 week preoperatively, within the

first few days postoperati v e l y , and postoperat ively at 2, 4, 6,

Ô, 10, 12, 16, and 24 weeks. Hence, visits for measurements extra

to the routine clinic visits of the first few days, 2, 6, 12 and

24 weeks, were only required on three to four occasions.

7.2 REFRACTION AND CORRECTED VISUAL ACUITY

A traditional method of refraction was carried out using a

combination of objective assessment by Keeler hand-held spot

retlnoscopy and subjective refraction using small aperture trial

(approximately 30 minutes) refraction was performed first so that

the patient could more easily concentrate and give accurate

subjective replies.

The first postoperative assessment was at one to three days

after the operation when the patients lacrimation and photophobia

had decreased. The internally illuminated Snellen test chart

position and the retraction method were kept the same for each

visit, as were room illumination and the back vertex distance of

the trial lenses. Visual acuity was measured as a Snellen

fraction with the best optical correction in the form of the

trial lenses correctly positioned relative to the pa t i e n t ’s

visual axis. Glare sources were avoided.

7. 3 KERATOMETRY PROCEDURE

Keratometry was performed using the circular illuminated

mires of the American Optical keratometer (Model No. 11705) and

the Bausch and Lomb keratometer (Model No. GH4389), ensuring the

same instrument was used for all assessment visits, and taking

the mean of three readings in each of the two principal meridia,

recentering and refocusing for each reading,

For the preoperative assessments, precautions were taken to

avoid unnecessary stimulation of accommodation in the operator.

For example, keeping both eyes open but suppressing the

unobserving eye, and carefully focusing the eyepiece on the

graticule by turning it from the positive direction. The

instrument was moved away from the patient then slowly moved

h