The copy right of this article belongs to Aravind eye Hospital
This article is being published here for purely academic reasons
Phacoemulsification in Vitretomized Eyes
Dr. Preethika Gandhi, Aravind Eye Hospital, Madurai
Determining the correct power of the IOL to be implanted in vitrectomized eyes is the most crucial step. Ability to predict IOL power with high degree of accuracy is the key for successful visual outcome. This is where previously vitrectomized eyes poses a problem, as PPV alters the refractive status of the eye, and it is challenging to identify the refractive shift post surgery.
All aspects of biometry are essential. Inaccurate keratometry by 0.50 diopters will cause final IOL power to be inaccurate by the same amount. Similarly, variation in measurement of axial length by 0.1mm will cause 0.3 diopters of IOL power variation. Thus minor errors in measurement cause cascading effect leading to post-operative refractive surprises.
IOL Master
IOL master, an ultra-high resolution optical biometry system, is in clinical practice for over a decade now. This combined biometry instrument gives measurement of axial length, keratometry, anterior chamber depth (ACD) and option to calculate IOL power from different formulae. The most important use of IOL Master is in ‘Optimisation’ of IOL power as per post-operative visual outcome.
IOL master employs the principle of Optical coherence biometry. It uses partially coherent infrared light beams of 780nm diode laser with coherence length of 130nm1. Laser light emitted is split up into two beams in a Michelson Interferometer. One mirror of interferometer is fixed and other is moved at constant speed making one beam out of phase with other. Both beams are projected in the eye and get reflected at cornea and retina. The light reflected from the cornea interferes with that reflected by the retina if the optical paths of both beams are equal. This interference produces light and dark band patterns which is detected by a photo detector. The signals are amplified, filtered and recorded as a function of the position of the interferometer mirror. An optical encoder is used to convert the measurements into axial length measurements.
One of the advantages of IOL master is able to measure axial length in different situations like phakic, pseudophakic or silicone filled eyes just by selecting the appropriate mode. By opening window of ‘AL Setting’ examiner can select the mode. In AL setting 9 different modes are available.
As a combined biometry instrument IOL master measures other aspects also. Central corneal power is measured as in automated keratometry. The IOL master requires 3 keratometer measurements to be taken. Only then, will a mean value be passed on to the IOL calculation. The corneal curvature results are displayed in mm or dioptres. Anterior chamber depth on the IOL master is interpreted as the distance between the anterior vertex of the cornea and the anterior vertex of the lens. IOL master can give easy measurements of WTW (White to White) distance. Measuring WTW distance is useful in calculation of phakic IOL.
Limitations
IOL master is an optical device, so very dense cataract, thick posterior capsular plaque, mature lens; presence of significant media opacities like corneal scars, vitreous haemorrhage limits its use. In cases of nystagmus or patients with unsteady eyes, IOL master fails to measure the axial length.
Immersion Technique
A 10 MHz ultrasound transducer probe is used. By the immersion technique, the ultrasound probe does not come into direct contact with the cornea,
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but instead, it uses a coupling fluid between it
and the probe, preventing compression through
fluid. Because there is no corneal compression, the
displayed result represents the true axial length
more closely. Measurement consistency from one
measurement to the next is often outstanding, due
to the lack of corneal compression and the fixed
position of the ultrasound probe over the surface
of the cornea.
Applanation Technique
A-scan biometry by applanation requires that the
ultrasound probe be placed directly on the corneal
surface. Even in the most experienced hands,
some compression of the cornea is unavoidable.
This typically being 0.14 mm – 0.28 mm.
Measurement taken by applanation method will
frequently show variability from one to the next,
as a result of inconsistent corneal compression,
and will be seen even under the most experienced
guidance. The way to avoid this is to change to the
immersion technique, as described above.
IOL Power Formulae
First Generation – SRK 1 and the Binkhorst
formula
Second Generation – SRK – 2
Third Generation – SRK T, Holladay, Hoffer – Q
Fourth Generation – Holladay 2, HAIGIS
The formula that we use in our Institute
belongs to the 3rd generation. For axial length
more than 22mm, SRK – T formula is used, and
for axial length less than 22mm, Hoffer – Qid
used.
SRK T Formula
The SRK T formula is a third generation formula,
described in 1990 by John Retzlaff and Donald
Sanders. It combines the benefits of both the
theoretical and regression formulae. The SRK T
formula uses theoretical and regression formulae.
The SRK T formula uses theoretical elements
like predicted post-operative anterior chamber
depth, retinal thickness adjusted axial length and
refractive indices of the cornea. The regression
element is primarily used to optimize the ‘A
Constant’. This formula works in eyes of normal
length and moderately long and very long eyes.
The SRK T formula has made the SRK 2 formula
obsolete since it combines all the advantages of the
SRK 2 formula and also enables one to optimize
the A – Constant.
Optimization is the process of making a
formula more predictable by refining and defining
the manufacturer’s lens constant. In simple terms,
optimization is achieved by analysing postoperative
outcomes with respect to the targeted
refraction for a given surgical technique and a
specified model or design of IOL as well as for a
given range of axial lengths. This optimization is
then added on to the “A Constant’ to make the
formula more predictable.
The Hoffer – Q Formula
Third generation formula was described by
Dr. Kenneth Hoffer in 1993.
P = f (A,K,Rx, pACD)
– A : axial length
– K : average corneal refractive power
(K-reading)
– Rx : refraction
– pACD personalized ACD (ACD –
constant)
This formula predicted IOL power as a
function of the axial length, average central corneal
power and the previous refraction of the patient.
The PACD – the personalized A constant was the
equivalent of the A-Constant in the SRK formula.
The construction of the Hoffer – Q formula was
such that it was extremely reliable in short eye
balls with an axial length of less than 22.0 mms.
Biometry in Silicone Oil Filled Eyes
It is problematic in deciding the power of IOL
to be used in silicone oil filled eyes, since the
echographically measured axial length of an eye
is greater in the presence of silicone oil. This is
because the speed of sound in silicone oil is slower
than in vitreous humour.
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Theoretically, the speed of sound in silicone
oil of viscosity 1000 centistokes compared with
the speed of sound in vitreous humour decreases
by a factor of 0.64.
It is therefore possible to calculate the true
depth of vitreous cavity (VCD oil x 0.64) = True
axial length.
(VCD oil x 0.64) = True axial length
AL without oil = ACD oil + LT oil + (VCD oil x
(sound vel oil / sound vel. Vit)]
AL without oil = Axial length of vitrectomized eye
(true axial length)
AL oil = measured axial length in the presence of
silicone oil
ACD oil = Anterior chamber depth in presence
of silicone oil
LT oil = Lens thickness in presence of silicone oil
Sound vel oil = velocity of sound in silicone oil
(1000 cs) 987 m/s
Sound vel vit = Velocity of sound in vitreous
1532m/s
SRK / T formula was used to compare the
measured power of IOL and the estimated power
of IOL.
Conversion factor of 0.71 is used when ACD,
LT, and VCD are not known. This conversion
factor can be used for silicone oil of viscosity 1300
centistokes.
Preferred Choice of Technique for IOL
Surgery in Vitrectomized Eyes
Manual extracapsular cataract extraction is
adversely affected by the alterations of previous
vitrectomy. Nucleus expression is affected by the
lack of vitreous support, and the result of this
manoeuvre can be the ejection of vitreous into the
anterior chamber through the zonules leaving the
nucleus insitu or even displacing it backwards2.
Hence traditional extra capsular extraction or
manual small incision cataract surgery using rigid
intra ocular lens are less frequently done these days as
surgeons prefer phacoemulsification using foldable
IOL implantation as it is associated with smaller
incision size, less induced astigmatism, quicker
visual rehabilitation and fewer complications
and moreover the intraocular pressure and fluid
dynamics can be controlled and minimizes the
risk of eye hypotony.
Hence we consider phacoemulsification as
the preferred surgical technique in vitrectomized
eyes. However post vitrectomized eyes must be
approached cautiously, anticipating the possibility
of hard lenses, weak or damaged zonules, unusual
anterior chamber depth fluctuations, intraoperative
miosis and occult tears in the posterior capsule.
Phacoemulsification
Smaller incisions have become the standard, with
phacoemulsification now being the method of
choice for most surgeons. Phacoemulsification
as a method to remove the cataractous lens was
first proposed more than 20 years ago. Nucleus
removal, formerly performed primarily in the
anterior chamber, is now performed in the posterior
chamber, decreasing damage to the corneal
endothelium. Improved wound construction
allows many wounds to be left unsutured, and
smaller wounds allow shorter recovery time and
greater intraoperative control and safety. Hence in
todays practice phacoemulsifcation is the preferred
technique for cataract surgeries.
Phaco Machine
The machine consists of console, foot pedal, hand
piece and their connections. The console consists
of a computer which controls all the functions
of the machine. The setting for the various
parameters, i.e. power, vacuum and flow rate are
fed in this. These settings represent the maximum
level of the parameter that will be achievable, the
further linear control is with the foot pedal. There
are two types of hand pieces; phaco hand piece
and irrigation, aspiration hand piece. The phaco
hand piece contains the piezoelectric crystal, which
is in contact with the tip. The tip is covered by
a silicon sleeve. The proximal end of hand piece
is connected to the console with an electric cord.
There are two more connections: one each for the
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irrigation tubing and for connecting the aspiration
system.
Foot pedal control is the most important
aspect of phaco. Though the foot pedal of each
machine may have a different design, it essentially
consists of main central part and sidekicks. The
main part of the foot pedal controls infusion,
aspiration and phaco power. The entire distance
that the foot pedal traverses is divided by 2
dentations into 3 excursions-I (irrigation only),
IA (infusion and aspiration) and IAP (infusion,
aspiration and phaco).
Phaco power is produced by the ultrasonic
vibrations of the Quartz crystal in the hand piece.
There may be 2 -6 crystals, 6 giving more stroke
length and more power. The frequency is variable
from 29-60 Hz in different machines. Higher
frequency ensures a better cutting action but more
heat is generated though, practically this does not
significantly affect the surgical outcome.
TIPS
The angulation of the tips may vary from 0 – 60°.
Tips with 60°, 45°,30°,15° and 0° angulation are
available. The more the angulation, the lesser the
holding power but the cutting power is more,
e.g.60° tip is a sharper tapered tip holding power.
The 45° tip has a very good cutting ability and
was very popular initially as the emphasis was
then on ‘Divide and Conquer’ in which trenching
(thus cutting ability) was more important than
occlusion. With the advent of aspiration phaco,
the most popular tip today is 30°. This has
adequate holding and cutting power and is useful
both for trenching and in chopping. The 15° and
0°angulated tips are better for holding but have a
poorer cutting action.
Fluidics
The fluidics of the machine refers to the integrated
functions performed by infusion and aspiration
systems by which a stable AC is maintained.
One of the major advantages that phaco has over
conventional cataract surgery is the fact that it
is performed in a closed chamber maintaining
ocular structures especially the cornea, iris and
posterior capsule. The infusion system consists
of a bottle, the height of which provides the
gradient for flow. The tubing from the bottle is
run through a pinch valve which is controlled
by the foot pedal. The infusion is gravity fed, 2
feet bottle height conforming to approximately
44mmHg (2ft =60cm = 600mm water column,
600/13.6=44mmHg). A bottle height of 3+ 1
maintains a safe IOP with sufficient fluid entering
the eye.
Raising the bottle height too much can have
undesirable effects due to the effect and (or),
due to the fact that the AC has a fixed volume
and trying to fill excess fluid results in zonular
stress leading to patient discomfort. Also, too
much fluid can lead to fluctuation of the lens iris
diaphragm resulting in irritation to the iris and
miosis. Another problem in raising the bottle
height may be repeated iris prolapse, especially
if the pupil is small and wound is large and there
will be unnecessary lavage of the cornea and
iris. The present computer assisted aspiration
systems together with improved tubing have made
phacoemulsification a swift and safe surgery. The
two functions of the aspiration system are lavage
of the anterior chamber and creation of a hold for
emulsification / crushing of the nucleus. Lavage
is governed by the flow rate and the hold is a
function of the vacuum generated by the system.
The aspiration systems consist of a pump that is
either flow based or vacuum based. The common
type of flow pump is the peristaltic pump. Venturi
is the prototype of a vacuum – based machine.
The incision for phacoemulsification is
either a sclera corneal or a clear corneal tunnel.
After construction, curvilinear capsulorrhexis of
about 5 – 5.5mm is made. Then the nucleus is
emulsified using the phaco hand piece. The most
popular methods of nucleotomy are “Divide and
Conquer”, Chopping” or a combination of the two
ie. “Stop and Chop”. In stop and chop we start
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with a trench as in Divide and Conquer, stop after
the first trench and then proceed with chopping
techniques. After nucleus emulsification, the
cortex is aspirated using the irrigation, aspiration
handpiece. A foldable intraocular lens is then
placed in the bag. After all the viscoelastic has
been aspirated, the anterior chamber is formed.
Challenges Encountered during
Phacoemulsification of Vitrectomized
Eyes
Cataract surgery in eyes that have undergone
vitrectomy is more difficult and presents a higher
incidence of complications than that performed
in non-vitrectomized eyes. The most important
anatomical feature that characterizes these eyes are
deep anterior chamber, lack of vitreous support
and possible damage to the posterior capsule or
zonular fibres.
In vitrectomized eyes, conjunctival scarring
may prohibit use of a limbal based conjunctival
flap. Small pupil, is a feature that is commonly
encountered. Likewise, sudden changes in anterior
chamber depth because of the disparity between
fluid flow and outflow, owing to the absence of
vitreous support is another worrying feature.
On entering the anterior chamber to commence
phacoemulsification, a characteristic sequence
of events occur. When the phacoemulsification
cannula is introduced and irrigation begins, the
anterior chamber will deepen significantly, the iris
aperture will enlarge, and the iris lens diaphragm
would move posteriorly with the iris surface,
assuming a concave configuration. The presence
of a deep anterior chamber has been noted by
Sneed et al3.
Weak zonules and unusual mobility and
flaccidity of the posterior capsule is another feature
commonly observed in vitrectomized eyes, hence
the risk of tearing the capsule during aspiration is
very common and so, should be dealt with utmost
care. According to Smiddy et al4, zonules may also
be preoperatively weak in the post vitrectomized
eyes. In that study, capsulotomy was done in a can
opener fashion, and they cautioned surgeons to
carefully inspect the lens for phacodonesis during
that type of construction of capsulotomy, It is to
be noted that routine use of capsulorrhexis reduced
stress on the zonules, and no inadvertent zonular
disinsertions occurred. In addition, excessive
hydrodissection was done to release all traction
on the zonules.
During some instances, substantial posterior
or lateral displacement of the lens occurs during
the anterior capsulotomy. Because of the lack of
support by the anterior vitreous gel, expression
of the nucleus may be unsafe in some cases.
The posterior capsule may be weak, leading to
disastrous complications like posterior dislocation
of the lens nucleus. Phacoemulsification does
not encounter generally those problems because
it provides better control of the fluid dynamics
and intraocular pressure minimizing the risk of
eye hypotony. Hence it is therefore the preferred
surgical technique in these eyes. Nevertheless,
there are many complications associated with
cataract surgery in vitrectomized eyes, which the
surgeon must be completely aware of.
Although Smiddy et al4 had described
the posterior movement of the lens in the
post vitrectomy patients, they had not drawn
attention to its relation to placement of the
phacoemulsification cannula. It is often difficult
to efficiently engage nuclear material with the
phacoemulsification cannula when the anterior
chamber deepened significantly under irrigation.
The deepening of the anterior chamber could
not be modulated by variation of the fluidic
parameters of the phacoemulsification instrument.
What was useful, however, was substitution of a
posteriorly angulated phacoemulsification tip. The
use of the tip allowed successful engagement of the
nucleus during posterior excursions as well as more
efficient delivery of ultrasonic power to emulsify
harder nuclei. On completion of construction of
the nuclear trough, cracking manoeuvres were
difficult due to the deep anterior chamber. In nonvitrectomized
eyes undergoing cataract surgery,
12 AECS Illumination
the abrupt occurrence of papillary mydriasis with
simultaneous deepening of the anterior chamber
is often an indication for an occult posterior
capsule rupture and impending dislocation of the
nucleus in the vitreous cavity. However, in the post
vitrectomized eyes, it is a normal occurrence and
not accompanied by posterior capsule rupture.
Posterior capsule plaque is a common
intraoperative finding, especially those patients
who have had intravitreal tamponade (like gas,
silicone oil) during PPV. Some surgeons try
polishing the surface of the posterior capsule, but
in those eyes where there is excessively mobile
posterior capsule, polishing is not done.
Postoperatively, the course of the patients was
unremarkable, except for the high incidence of
posterior capsule opacification and the need for
Nd-YAG capsulotomy was also very high.
Reference
1. Blankenship GW, Machemer R. Long term diabetic vitrectomy results; report of 10 year follow up.
Ophthalmology 1985; 92 : 503 – 506.
2. Blodi BA, Paluska SA. Cataract after vitrectomy in young patients. Ophthalmology 1997; 104 : 1092 –
95.
3. Sneed S, Parish RK II, Mandelbaum S, O’Grady G. Technical problems of extracapsular cataract
extractions after vitrectomy. Arch Ophthalmol 1986; 104 : 1126 -1127.
4. Smiddy WE, Stark WJ, Michels RG, et al. Cataract extraction after vitrectomy. Ophthalmology 1987; 94 :
483 – 487.
