Fundamentals of Clinical Ophthalmology - part 2 pot

It has been recognised that if an incision is placed further from the optical axis, then it may be increased in width while remaining astigmatically neutral Figure 2.1.2 The need for a l

The surgeon can make a partial thickness

incision, as for extracapsular surgery, and then

use this as the first step in the construction of

either a tri- or biplanar incision for the phaco

hand piece The nucleus is sculpted so that the

surgeon can appreciate the difference between

the plastic cataract and the human lens

Following initial grooving, if the surgeon still

feels confident that the cataract is within his or

her ability, then the nucleus can be rotated and

further grooving performed If difficulties are

encountered then the phaco tip should be

removed from the eye, the incision opened, and

an extracapsular cataract extraction performed

Having sculpted three or four nuclei most

surgeons will feel confident to continue with

phacoemulsification and proceed to nuclear

cracking with quadrant removal The incision

should always be constructed to enable the

surgeon to perform an extracapsular extraction

at any stage should this become necessary

Case selection

Virtually all cataracts can be removed from

the eye using phacoemulsification The limiting

factor is not the machinery but the surgeon’s

skill It is important that the trainer and trainee

select appropriate cases together at the

preoperative assessment stage and arrange the

theatre list accordingly

There are a number of points to consider

when selecting cases (Box 1.1) The eye should

have a clear healthy cornea, a pupil that dilates

well, and a reasonable red reflex A deep-set eye

or prominent brow/nose can make access difficult

while learning Axial length should be considered

when selecting patients Hypermetropic short

eyes present problems with a shallow anterior

chamber, whereas myopic eyes have a deep

anterior chamber Patients with potential zonular

fragility such as those with pseudoexfoliation or a

history of previous ocular trauma should be

avoided, as should patients who will find it

difficult to lie still for an appropriate length of

time or who require awkward positioning on theoperating table

The team approach

Adequate training must be provided for allmembers of the team in the operating theatre

A surgeon learning phacoemulsification is highlydependent on the nurse who is setting up andcontrolling the machine For example, when thenurse fully understands how the phaco machineworks, the surgeon need only concentrate onthe operation However, trainees will find it lessstressful if they are familiar with how to set upthe tubing and hand pieces, and with selectingprogrammes for the phaco machine Thisshould be encouraged by the trainer at an earlystage on the learning curve and may be achieved

by the trainee acting as the scrub nurse,supervised by a member of the nursing staff.This is also an effective method of teambuilding

The team needs to have a full understanding

of how training is to proceed and the timeimplications for surgery This includes the nurses,the anaesthetist, and anaesthetic technicians.Each team member plays a role in the trainingprocess, and when the final piece of nucleusdisappears into the phaco tip at the end of thesurgeon’s first “complete phaco” the team shouldfeel that they have all shared in that success

9

Box 1.1 Case selection: The ideal training case

• Healthy cornea

• Full pupil dilatation

• Good red reflex

• Moderate cataract density

• Easy surgical access (for example, no prominent brow)

• Average axial length (for example, 22–25 mm)

• Lack of ocular comorbidity (for example, pseudoexfoliation)

• Able to lie still and flat under local anaesthesia

Trainer and trainee communication

Most cataract surgery takes place under local

anaesthetic and beginners need to be taught that

the patient beneath the drape is awake

Appropriate communication should be used

between the trainer and trainee It is particularly

important to repress the desire for expressions of

surprise or frustration

It may be appropriate to inform the patient

that a team of doctors is present at the operation

and that discussion or description of various

stages of the procedure may take place This will

help to prevent the natural anxiety that is

experienced by patients who feel that a “junior

doctor” is “learning” on their eye A useful

teaching technique is to use the first person, for

example “I rotate the nucleus now”, as an actual

instruction and to use a pre-agreed word to

indicate that instrument removal from the eye

is desired

References

1 Leaming D Practice styles and preferences of ASCRS

members: 1998 survey J Cataract Refract Surg 1999;

25:851–9

2 Desai P, Minassian DC, Reidy A National cataract surgery survey 1997–8: a report of the results of the

clinical outcomes Br J Ophthalmol 1999;83:1336–40.

3 Seward HC, Davies A, Dalton R Phacoemulsification:

risk/benefit analysis during the learning curve Eye

1993;7:164–8.

4 Sugiura T, Kurosaka D, Uezuki Y, Eguchi S, Obata H,

Takahashi T Creating a cataract in a pig eye J Cataract

Refract Surg 1999;25:615–21.

5 van Vreeswijk H, Pameyer JH Inducing cataract in

post-mortem pig eyes for cataract training purposes J Cataract

7 Maloney WF, Hall D, Parkinson DB Synthetic cataract

teaching system for phacoemulsification J Cataract

Refract Surg 1988;14:218–21.

10

Phacoemulsification is a significant advance in

cataract surgery that reduces postoperative

inflammation, with early wound stability,

resulting in minimal postoperative astigmatism

and rapid visual rehabilitation Most of these

advantages are directly attributable to the

sutureless small incision Accordingly, incision

construction is a key component of modern

cataract surgery In each of the steps of

phacoemulsification, the success of a subsequent

step is dependent on that preceding it The

incision may be viewed as the first step in this

process and hence is central to the overall

success of the procedure

In 1967 Kelman1 demonstrated that

phacoemulsification might allow surgical incisions

to be as small as 2–3 mm in width However,

the subsequent widespread introduction and

acceptance of intraocular lenses (IOLs)

constructed of rigid polymethylmethacrylate

necessitated an incision width of approximately

7 mm The advantage of a small

phacoemulsifi-cation incision, with low levels of induced

astigmatism, was therefore substantially reduced

It has been recognised that if an incision is placed

further from the optical axis, then it may be

increased in width while remaining astigmatically

neutral (Figure 2.1).2 The need for a larger

incision was therefore partly overcome by the

development of posteriorly placed scleral tunnel

incisions3 and innovative astigmatic suture

techniques.4The advent of lens implants with anoptic diameter of around 5 mm allowed thesescleral tunnels to be left unsutured, and suchincisions have been shown to be extremelystrong.5 The development of foldable lensmaterials has enabled the initial smallphacoemulsification incision to be retained.6Thishas made it possible for a self-sealing incision to beplaced more anteriorly, in the clear cornea,without increasing astigmatism or loss of woundstability Further development in hand piece

technology has seen a reduction in phaco tip

diameter and hence incision width Some lenses

can be inserted through these incisions that

measure less than 3 mm; however, it remains to be

seen whether this further reduction in wound size

confers a significant refractive advantage

Incision choice

The principal decision facing a surgeon is

whether to perform a scleral tunnel incision

(STI) or clear corneal incision (CCI) The

refractive implications of these incisions are dealt

with separately below, but there are several other

factors that may influence the choice of incision

The more anterior position and overall shorter

tunnel length of a CCI increases hand piece

manoeuvrability and allows the phaco probe

more direct access to the anterior chamber and

the cataract Furthermore, a CCI may be less

likely to compress the irrigation sleeve of the

phaco probe and hence reduces the risk of

heating the incision, or “phaco burn” However,

the tunnel of a CCI extends further anteriorly

than does that of a STI, and this may lead to

corneal distortion or striae from the phaco hand

piece It has been demonstrated that incisions

in which the tunnel width and length are

approximately the same (square or near square;

Figure 2.2a) are more resistant to leakage than

are those in which the width is greater than

the tunnel length (rectangular; Figure 2.2b).5

Hence, when a polymethylmethacrylate or

folding IOL that requires a larger incision is

used, the comparatively longer tunnel of a STI

may be more likely to provide a wound that can

remain unsutured

A STI requires a conjunctival peritomy and

cautery to the episclera This is time consuming

and in patients with impaired clotting, for

example those taking asprin or warfarin, it is best

avoided Disturbance of the conjunctiva may

also compromise the success of subsequent

glaucoma drainage surgery.7 In addition, if a

patient has a functioning trabeculectomy, then a

CCI avoids an incision of the conjunctiva and

the risk of damaging the drainage bleb Ofcourse, a scleral tunnel is a prerequisite whenperforming a phacotrabeculectomy

There is some evidence to suggest thatendothelial cell loss may be lower whenphacoemulsification is performed through aSTI8 and it may therefore be a preferabletechnique in patients with poor endothelialreserve, for example those with Fuchs’endothelial dystrophy or following a penetratingcorneal graft The possible need, identifiedbefore surgery, for conversion to an expressionextracapsular technique may also influence thechoice of incision In favour of an enlarged STI

is that it may be easier to express the nucleusand less detrimental to the endothelium.However, a CCI may be quicker and easier toenlarge, at the possible risk of greater, inducedastigmatism

Factors such as previous vitreous surgery, inwhich the sclera may be scarred, and disordersthat predispose to scleral thinning and conjuctivaldiseases, for example ocular cicatricalphemphigoid, all favour a CCI Histologicalanalysis has demonstrated that phacoemulsificationincisions placed in vascular tissue initiate anearly fibroblastic response and rapid healing ascompared with those in avascular cornealtissue.9 This may be relevant to patients forwhom rapid healing is advantageous (forexample children and those with mentalhandicap) and to patients with reduced healing(for example diabetic persons and those takingcorticosteroids)

Table 2.1 summarises the comparative

advantages of STIs and CCIs It has been

suggested that these advantages may be

combined by placing the incision over the

limbus.10 However, the disadvantage is that

bleeding still occurs and cautery may be

required

Incision placement

A STI is usually placed at the superior or

oblique (superolateral) position, which ensures

that the conjunctival wound is under the

patient’s upper lid Surgeon comfort and ease of

surgery are also factors in this decision, and

these same factors influence the choice of

position for a CCI Aside from the refractive

issues dealt with below, there may be a number

of other considerations when selecting the

placement of an incision

Access via a temporal approach is often easier

in patients with deep-set eyes or with a

prominent brow In these circumstances the use

of a lid speculum with a nasal rather than

temporal hinge may be helpful (Figure 2.3)

Pre-existing ocular pathology, such as peripheral

anterior synechiae, corneal scarring and pannus,

or the position of a trabeculectomy filtering bleb

may alter the selection of an incision site

Surgically induced astigmatism

Scleral and corneal incisions both cause somedegree of corneal flattening in the meridian (oraxis) on which they are performed, withcorresponding steepening in the perpendicularmeridian, termed “surgically induced astigmatism”

As previously stated, this effect is dependent onthe size of the incision and its proximity to thecentre of the cornea (Figure 2.1) Because a STI

is performed further from the optic axis itinduces less astigmatism than does a CCI ofequivalent width Various STI pregroove shapes

13

Table 2.1 Comparative advantages of scleral and corneal incisions

Incision type Advantages

Scleral tunnel incision Minimal induced astigmatism

Large sutureless incisions possible May be combined with trabeculectomy at single site Less endothelial cell loss

Rapid wound healing Safe if converted to large-incision extracapsular technique Phaco hand piece less likely to cause corneal striae and distort view Clear corneal incision Induced astigmatism may be used to modify pre-existing astigmatism

Reduced surgical time Less likely to compromise existing or future glaucoma filtration surgery

No risk of haemorrhage; cautery not required Reduced risk of phaco burn (shorter tunnel) Increased ease of hand piece manipulation Avoids conjunctiva in diseases such as ocular cicatricial pemphigoid Avoids sclera when scarred and/or thinned

Easy to convert to large-incision extracapsular technique

Figure 2.3 Lid speculum with nasal hinge (BD Ophthalmic Systems).

have been described that, by altering wound

construction, attempt to minimise surgically

induced astigmatism These include straight,

curved (limbus parallel), reverse curved (frown),

and V-shaped (chevron) incisions However,

none of these has been clearly identified as

inducing less astigmatism.11

The degree of induced astigmatic change and

its stability over time varies with the meridonal

axis on which the incision is placed Both STIs

and CCIs produce the least astigmatism when

they are placed on the temporal meridian and

most astigmatism when they are placed

superiorly.12–14 An oblique position has an

intermediate effect.15,16These findings reflect the

elliptical shape of the cornea and the greater

proximity of the superior limbus to its centre

The surgically induced astigmatism reported by

several authors using different unsutured

triplanar incisions at three months is

summarised in Table 2.2 Superiorly placed

incisions are also associated with an increase in

astigmatism over time and a change toward

“against the rule” (ATR) astigmatism, with a

steeper cornea in the 180º axis.17,18 This effect,

which is dependent on incision size, has been

attributed to the effect of gravity and pressure

from the lids

The meridian on which an incision is placed

is therefore an important factor in surgical

planning, particularly with reference to a

patient’s pre-existing keratometric or corneal

astigmatism It should be noted that the

spectacle refraction may be misleading because

lenticular astigmatism is negated by cataract

surgery With increased age the majority of thepopulation develop ATR astigmatism Hence, atemporally placed incision may reduce orneutralise this astigmatism In a few circumstancesthe incision may induce a small degree of “withthe rule” (WTR) astigmatism, with cornealsteepening in the 90° meridian Although it isgenerally preferable to undercorrect pre-existingastigmatism and avoid large swings of axis,19

WTR astigmatism is considered normal inyounger individuals and may confer someoptical advantage

Reducing coexisting astigmatism during phacoemulsification

Naturally occurring astigmatism may bepresent in 14–50% of the normal population20,21

and cataract surgery provides the opportunity tocorrect this astigmatism This improves patients’unaided vision after surgery, reducing theirdependence on spectacles and increasing theirsatisfaction In patients with moderate levels

of pre-existing astigmatism, a reduction inastigmatism without altering the axis may beachieved, by placing the incision on the steep

or “plus” meridian This is of particularimportance when using multifocal lens implants,where astigmatism may substantially reduce themultifocal effect.22 In these circumstances,modifying incision architecture may increasethe astigmatic effect of a CCI Langerman23

described a triplanar CCI with a deep (750 µm)pregroove that was intended to create a limbal

“hinge” and ensure a non-leaking incision

14

Table 2.2 Reported surgically induced astigmatism (SIA) in unsutured triplanar incisions at three months

Incision type Incision site Incision length (mm) SIA (dioptres) Reference STI Superior 3·2 0·63 ± 0·43 Oshika et al.14

5·5 1·00 ± 0·59 Oblique 3·2 0·37 ± 0·28 Hayashi et al.15

5·0 0·64 ± 0·39 CCI Superior 3·0–3·5 0·88 ± 0·66 Long and Monica 12

even if pressure was applied to its posterior lip

(Figure 2.4) The deep pregroove has been

noted to have a keratotomy or limbal relaxing

effect that induces more astigmatic change,

which is more pronounced as the incision length

increases.24

When attempting to reduce astigmatism by

incision positioning, it is important to ensure

that it is accurately placed on the steep meridian

A 30º error will simply alter the axis ofastigmatism without changing its power (ifattempting a full correction) Smaller errorsdecrease the effect of the incision and change theaxis of astigmatism, albeit less dramatically.Because torsional eye movement may occurdespite local anaesthesia, the steep axis, or areference point on the globe from which this axiscan be derived, should be identified or markedbefore anaesthesia The axis can also beconfirmed with intraoperative keratometry at thestart of surgery When placing an incision on thesteep meridian of astigmatism, there are somemeridia that may necessitate the surgeonadopting an unusual operating position oroperating with their non-dominant hand(Figure 2.5) In such cases it may be preferable

to use a standard phacoemulsification incision inconjunction with an incisional refractivetechnique or a toric lens implant It is relevant tonote that, when correcting astigmatism with anincisional technique, coupling changes theoverall corneal power and larger corrections maytherefore alter the IOL biometry calculation (see

Chapter 6) Table 2.3 suggests an approach to

modifying incision type and placement in order

to avoid increasing, and possibly reduce,

pre-existing keratometric astigmatism However,

surgically induced astigmatism varies with the

size of incision and from surgeon to surgeon,

and it may be necessary to adapt this guide on

the basis of an individual’s experience with their

preferred incision techniques

Several techniques exist for modulating high

astigmatism intraoperatively These include

astigmatic keratotomy, limbal relaxing incisions,

opposite CCIs, and toric IOL implantation

Irrespective of the technique used, the astigmatic

effect of the phacoemulsification incision also

needs to be taken into account (unless it is

astigmatically neutral) Corneal video topography

should be performed before any refractive surgery

is performed to exclude the presence of irregular

astigmatism from, for example, a corneal ectatic

disease This reaffirms the axis of astigmatism,

which should be identified or marked on the eye,

as discussed above The surgeon’s principle aim

should be to preserve corneal asphericity and

reduce high preoperative astigmatism while

maintaining its principal meridian

Limbal relaxing incisions are partial thickness

incisions at the limbus (the corneoscleral

junction) and have been advocated as an effective

and safe method of reducing astigmatism during

cataract surgery.25 Compared with astigmatic

keratotomy they have the advantage of better

preserving corneal structure with more rapid

visual recovery and less risk of postoperative glare

or discomfort They are also easier to perform

and do not require preoperative pachymetry The

incisions can be performed at the start ofphacoemulsification or after lens implantation(before removal of viscoelastic) With reference to

a suitable nomogram (Table 2.4) or softwareprogram, single or paired, 6- to 8-mm longincisions are made at the limbus centred on theaxis of corneal astigmatism They are typically550–600 µm deep, and preset guarded disposableblades are available that avoid the need for anadjustable guarded diamond blade Astigmatickeratotomy nomograms usually use degrees

of arc to define the incision length and requirespecial instrumentation With an optic zone of

12 mm (the corneal diameter), degrees of arcapproximate to millimeters (for example, ~60° =

~6 mm), and this conveniently allows the length

of a limbal relaxing incision to be marked alongthe limbus with a standard calliper OppositeCCIs also do not require new instrumentation ornew surgical skills.26The use of paired incisions(both on the steep meridian) increases theexpected flattening effect of a single CCI, and amean correction of 2·25 D has been reported(using 2·8 to 3·5-mm wide phaco incisions).Although simple to perform, opposite CCIsnecessitate an additional penetrating incision thatmay have greater potential for complications

16

Table 2.3 Unsutured small incision planning in relation to pre-existing astigmatism

Pre-exisiting keratometric astigmatism Incision type and position

+ 0·75 D ATRTemporal CCI (or STI)

Table 2.4 Limbal keratotomy nomogram

Astigmatism Incision type Length Optical zone (dioptres) (mm)

2–3 Two LRIs 6·0 At limbus

>3 Two LRIs 8·0 At limbus Modified Gills nomogram for limbal relaxing incisions (LRIs) to correct astigmatism with cataract surgery.

Modified from Budak et al.25

when compared with an alternative

non-penetrating incisional technique.27

Implantation of a toric IOL avoids the

potential complications of additional corneal

incisions and has no effect on corneal coupling

An example is the Staar foldable toric lens

implant, which is identical to current silicone

plate haptic lenses except on its anterior surface

there is a spherocylindrical or toric refracting

element.28 Like all toric lenses, this requires

accurate intraoperative alignment in order to

correct astigmatism and relies on the IOL

remaining centred Although plate haptic lenses

may rotate within the capsular bag immediately

after implantation, they show long-term rotational

stability as compared with loop haptic lenses.29

Early postoperative reintervention may therefore

be required with plate haptic toric lenses and the

ideal toric lens design remains to be identified A

toric IOL also has the disadvantage that the

astigmatic correction is limited to a narrow range

of powers

Incision technique

Scleral tunnel incision technique

A conjunctival peritomy is first performed

with spring scissors and forceps (Figure 2.6a)

This is approximately the same length as the

proposed final incision width, and should be

measured and marked using a calliper

beforehand The conjunctiva is blunt dissected

posteriorly to expose the sclera 2–3 mm behind

the limbus It is important that this is fully

beneath Tenon’s fascia If necessary, one or two

radial relieving incisions may be made at the ends

of the conjunctival wound to improve exposure

The minimum cautery required to achieve

haemostasis is applied to the exposed episcleral

vessels over the proposed incision site

The width of the incision should be marked

2 mm behind the limbus using a calliper The

first step of the incision is to create a straight

pregroove incision of around one third scleral

thickness in depth (Figure 2.6b) Care should be

taken not to cut too deeply and incise the ciliary

body This may be avoided by using a guarded

blade with a preset cutting depth of approximately

300 µm (Figure 2.7) Disposable blades with a

fixed cutting depth are widely marketed for this

purpose During this step, the globe can be

stabilised, and counter traction applied, by

forceps gripping the limbus near to the lateral

edge of the peritomy

In the second step a pocket or crescent blade

is used to create the scleral tunnel By pressing

on the posterior edge of the pregroove with the

flat base of the blade, its tip is placed into the

anterior aspect of the groove Initially this may

require the blade to be directed relatively

downward, but as soon as the tunnel is

commenced the heel of the blade should be

lowered to the conjunctival surface to ensure an

even lamellar dissection through the sclera into

the corneal plane The lamellar cut should proceed

smoothly and anteriorly, with a combination of

partial rotatory and side to side motions The

lamellar dissection is continued until the tip of

the pocket blade is just visible within clear

cornea, beyond the limbus (Figure 2.6c) The

tunnel can then be extended further laterally, to

the full width of the pregroove and the desired

incision width During creation of the scleral

pocket, counter traction can be improved by

gripping the sclera adjacent to the lateral edge of

the pregroove or its posterior lip Neither the

fragile anterior edge nor the roof of the tunnel

should be gripped If an extremely sharp pocket

or crescent knife is used, for example a diamondblade, then counter traction may not berequired

The final stage of the incision is thenperformed using a keratome blade, the width ofwhich is matched to the diameter of the phacotip Counter traction is now best provided either

by gripping the limbus directly opposite theincision with forceps or by using a limbalfixation ring Limited side to side motions mayfacilitate full entry of the blade, without damage

to the pocket Once the blade tip is visible inclear cornea, at the end of the tunnel, it is angledposteriorly The blade should enter the anteriorchamber directly, avoiding contact between itstip and the lens or iris The blade should beadvanced so that the full width of the bladeenters the anterior chamber (Figure 2.6d)

Clear corneal incision technique

Many techniques have been described thatproduce an effective self-sealing CCI This maymimic a triplanar STI, with the creation of apregroove, followed by a tunnel or pocket andthen entry into the anterior chamber Incontrast, a uniplanar or “stab” incision may beperformed with a keratome directly through thecornea A biplanar incision is made by firstcreating a pregroove into which the keratome

is placed A bi- or triplanar incision is morelikely to provide a reproducible self-sealingincision in terms of width, length, and overallconfiguration than is a uniplanar incision.Moreover, in the event of conversion to a non-phacoemulsification technique, enlargement of auniplanar incision may cause difficulty inachieving an astigmatically neutral woundclosure For these reasons, a uniplanar incision

is not recommended for surgeons with littleexperience in corneal tunnel construction If thelens nucleus is hard and a higher level ofultrasound power or phacoemulsification time isanticipated, then the anterior wound edge may

be prone to damage from either manipulation or

Figure 2.7 A disposable 300 µ m guarded blade for

pregroove incision (Beaver Accurate Depth Knife;

BD Ophthalmic Systems).

phaco burn, and in these circumstances an

incision with a pregroove may be favoured

(Figure 2.8)

Before commencing the incision, the

formation of a self-sealing paracentesis at the

limbus in the plane of the iris will allow

the anterior chamber to be filled with a viscoelastic

This provides a consistently firm eye on which

the incision may be performed If a pregroove is

used, then its dimensions should first be marked

with a calliper along the avascular limbus The

eye is stabilised using either a limbal fixation ring

or toothed forceps at the limbus adjacent to the

incision site Some surgeons prefer to grip the

paracentesis, which reduces the risk of a

subconjunctival haemorrhage The pregroove

incision is then made perpendicular to the

corneal surface, just inside the limbal vascular

arcade, with a depth of around one third of

corneal thickness (Figure 2.9a) The use of a

guarded blade with a preset depth of

approximately 300 µm ensures a consistent

depth The keratome is placed in the groove by

depressing its posterior lip with the base of the

blade flattened against the globe Counter

traction is now best provided by gripping or

supporting the limbus, directly opposite the

incision The path of the keratome through the

cornea is similar irrespective of whether a one ortwo step incision is used The blade is firstangled to create a lamellar dissection in thecorneal plane This is continued anteriorly

Figure 2.8 Clear corneal incision wound profiles

compared (a) Biplanar: detail of the anterior external

wound edge highlights the pregroove (b) Uniplanar:

the anterior external wound edge is less robust.

a)

b)

Figure 2.9 Microscope view and wound profile: steps in the construction of a biplanar clear corneal incision (a) Eye stabilised with a ring and pregroove performed with a diamond blade (b) Corneal tunnel and entry into the anterior chamber with a keratome.