Review of Small Gauge Vitrectomy: Progress and Innovations

Abstract

Purpose. To summarise the surgical advances and evolution of small gauge vitrectomy and discuss its principles and application in modern vitreoretinal surgery. The advent of microincisional vitrectomy systems (MIVS) has created a paradigm shift away from twenty-gauge vitrectomy systems, which have been the gold standard in the surgical management of vitreoretinal diseases for over thirty years. Advances in biomedical engineering and surgical techniques have overcome the technical hurdles of shifting to smaller gauge instrumentation and sutureless surgery, improving surgical capabilities and expanding the indications for MIVS.

Full text

Review Article
Review of Small Gauge Vitrectomy: Progress and Innovations
Shaheeda Mohamed,
1
Carl Claes,
2
and Chi Wai Tsang
1
1
Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital,
Kowloon, Hong Kong
2
Sint Augustinus Hospital, Wilrijk, Belgium
Correspondence should be addressed to Carl Claes; claes.md@skynet.be
Received 17 February 2017; Accepted 29 March 2017; Published 15 May 2017
Academic Editor: Ala Moshiri
Copyright © 2017 Shaheeda Mohamed et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Purpose. To summarise the surgical advances and evolution of small gauge vitrectomy and discuss its principles and application in
modern vitreoretinal surgery. The advent of microincisional vitrectomy systems (MIVS) has created a paradigm shift away from
twenty-gauge vitrectomy systems, which have been the gold standard in the surgical management of vitreoretinal diseases for
over thirty years. Advances in biomedical engineering and surgical techniques have overcome the technical hurdles of shifting to
smaller gauge instrumentation and sutureless surgery, improving surgical capabilities and expanding the indications for MIVS.
1. Introduction and History of Microincisional
Vitrectomy Surgery (MIVS)
Robert Machemer introduced pars plana vitrectomy (PPV)
in 1971. Prior to this, Kasner had described vitreous excision
removal under an open sky technique, using sponge and scis-
sors [1]. Machemer developed a closed system for vitreous
removal with control of intraocular pressure. His vitreous
infusion suction cutter (VISC) was 17-gauge (1.42 mm in
diameter), multifunctional, and utilized a 2.3 mm scleral inci-
sion [2, 3]. O’Malley and Heintz separated the components of
vitreous cutting, infusion, and illumination and developed
the first three-port 20-gauge(G) vitrectomy system in 1974.
Absorbable sutures were used to close the sclerotomies and
conjunctiva [4]. Improvements in electric and pneumatic
cutters led to 20G three-port vitrectomy becoming the stan-
dard technique for vitreoretinal surgery for over thirty years,
until the advent of microincisional vitrectomy surgery
(MIVS). In 1985, Machemer and Hickingbotham introduced
the first 20G trocar/cannula system to be inserted into the
sclerotomy, allowing for easier passage of instruments and
reduced traction at the vitreous base [5]. In 1990, De Juan
developed 25G instrumentation for use in paediatric eyes
[6]. Peyman then developed a 23G vitrectomy probe in
1990, primarily intended for vitreous and retinal biopsies
[7, 8]. In 2002, Fujii et al. introduced a 25G transconjunctival
vitrectomy system using microtrocars and cannulas and
popularized the widespread use of small gauge pars plana vit-
rectomy [9–11]. Eckardt later introduced 23G vitrectomy
instrumentation as an alternative to the 25G system [12],
and the trend toward yet smaller gauge instruments contin-
ued with the development of a 27G sutureless vitrectomy sys-
tem by Oshima in 2010 [13].
2. Advantages and Disadvantages of Small
Gauge Vitrectomy
Small gauge vitrectomy, with its smaller instrumentation
intended to be transconjunctival, self-sealing, and sutureless,
has theoretical advantages including decreased ocular trauma
and inflammation, decreased corneal astigmatism, reduced
operating times, faster postoperative recovery, increased
patient comfort, reduced conjunctival scarring, and conjunc-
tival preservation, especially in patients with prior or pending
glaucoma surgery [14–16]. In addition, smaller gauge vitrec-
tomy instruments are better suited to the narrower spaces of
paediatric eyes.
However, miniaturization of instruments limits instru-
ment diameter and lumen, with counterproductive effects
on instrument flexibility, efficiency, and performance. Initial
indications for small gauge vitrectomy were limited to those
not requiring extensive vitrectomy, membrane dissection,
Hindawi
Journal of Ophthalmology
Volume 2017, Article ID 6285869, 9 pages
https://doi.org/10.1155/2017/6285869

or phacofragmentation, due to initial issues with limited
instrument array and increased flexibility. Advances in
wound construction, instrumentation, fluidics, cutter tech-
nology, illumination, and wide-angle viewing systems
(WAVS) have overcome the handicaps of smaller gauge
instrument size and are discussed in detail as follows.
3. Trocar/Cannula System
Standard 20G vitrectomy surgery requires conjunctival inci-
sions and sclerotomies of 0.89 mm diameter. Smaller gauge
vitrectomy using transconjunctival trocar/cannula systems,
have reduced the scleral incision diameter to 0.64 mm for
23G, 0.51 mm for 25G, and 0.4 mm for 27G. The trocar/can-
nula system theoretically creates less traction on the vitreous
base during instrument entry and exit. However, a large
retrospective study by Rizzo et al. found similar incidence
of retinal detachment after sutureless 23G and 25G vitrecto-
mies and conventional 20G vitrectomy (1.7% versus 1.2%)
[17]. The once only placement of the cannulas maintains
the alignment between the conjunctiva and sclera and is less
traumatic to wound borders than the repeated insertion and
withdrawal of instruments through a 20G sclerotomy. It also
increases the chances of self-sealing sclerotomy closure and
minimizes the risk of suture-related inflammatory reaction,
or subsequent atrophy and thinning over the sclerotomy
site. Cannulas also allow for interchangeability of instru-
ment and infusion sites, allowing for improved access in
certain instances.
However, the cannula sleeve internal diameter limits the
radius of curvature of intraocular scissors, resulting in a
blunted curve and shorter blades, and renders them less effi-
cient for membrane cutting and dissection than their 20G
counterparts, forcing surgeons to use other methods [18].
The cannula sleeve may also slightly affect instrument rota-
tion and flexion during globe manipulation, as well as ante-
rior and peripheral access. Placing the sclerotomies closer
to the horizontal meridian reduces the need to rotate instru-
ments significantly for peripheral and superior access and
avoids displacement of the infusion as the eye is rotated
inferiorly [19].
4. Wound Construction
Wound construction in smaller gauge vitrectomy systems is a
critical step and affects whether the sclerotomy seals well at
the end of surgery. The thickness of the sclera in the area of
the pars plana is 0.8 mm. Early incisions were straight
(perpendicular to the sclera) in 25G systems, as better self-
sealing was expected with the smaller diameter incisions
[11]. This was changed to an angled (oblique) scleral incision
after studies showed better wound closure and reduced risk
of hypotony compared with straight incisions [20–22].
One- and two-step techniques of angled wound construction
have been described.
The original two-step technique for 23G vitrectomy, as
described by Eckardt, involves displacement of the con-
junctiva and stabilization of the eye with a pressure plate,
followed by use of a sharp angled MVR blade to create the
initial slit opening in the sclera followed by insertion of
the blunt trocar, onto which the cannula is mounted
[12]. This technique allows more consistent wound crea-
tion, but it may sometimes cause difficulty in finding the
initial point of trocar insertion. The modern one-step
technique involves entry by a sharp trocar with a mounted
cannula. Cannulas are quick to insert and easily removed
from the trocars without need for a second instrument,
but it may be necessary to apply a slightly higher pressure
to insert the microcannula at an oblique angle, which can
cause problems in eyes with recent corneal or scleral
wounds [19].
Additional modifications have been made to the one-step
angled technique to improve wound architecture. In general,
the longer (more oblique) the intrascleral path, the better the
wound apposition. In Zorro’s incision, the blade is inserted
obliquely at an angle of 10 to 15 degrees and enters the vitre-
ous without straightening [23]. Pollack improved on this by
suggesting a biplanar incision, where the trocar is inserted
at a 5-degree angle to the sclera until 50% scleral depth, and
then raised to a 30-degree angle to the sclera. Trocar entry
at 30 degrees produces a longer tunnel length of 1.414 mm,
compared with a tunnel length of 1.154 mm produced by tro-
car entry at 45 degrees. The 30% increase in tunnel length
results in more watertight closure [23]. Alternatively, the tra-
jectory may also be made very tangential to the sclera at
about 5 degrees and then tilted up to a more perpendicular
angle after the cut is made through the sclera in order to
avoid impaling the retina. Moreover, older blades created
chevron-shaped incisions with a tendency to gape, but newer
blades create flat, linear incisions [18].
The course of angled incisions can run perpendicular or
parallel to the limbus. Kwok et al. modified the original per-
pendicular incision by rotating the sclera tunnel by 90
degrees, making it parallel to the limbus [24]. Due to the ori-
entation of scleral fibres around the cornea, scleral incisions
made parallel to the limbus offer a theoretical benefit of dis-
placing the scleral fibres rather than cutting them, as in inci-
sions that run perpendicular to the limbus, facilitating more
rapid and superior sclerotomy closure [25, 26]. In addition,
scleral tunnels that run parallel to the limbus are less likely
to encroach on the lens or retina. Microincisional scleral tun-
nel entry radial to the limbus leaves more room for future
sclerotomies than conventional 20G incisions running paral-
lel to limbus, preventing coalescing of wounds in repeat
surgeries.
Conjunctival and scleral vessels should be avoided where
possible, to reduce postoperative subconjunctival haemor-
rhage. Conjunctival displacement from the scleral incision
has been proposed in order that the two incisions will not
be aligned after cannula withdrawal, and the conjunctiva will
cover the sclerotomy. It is intended to reduce the risk of post-
operative scleral wound contamination. However, Singh et al.
demonstrated that conjunctival displacement did not prevent
ocular surface fluid from entering sutureless 25G scleral inci-
sions in cadaver eyes [27]. Avoiding conjunctival displace-
ment in eyes with silicone oil fill may also prevent leaking
silicone oil from becoming trapped in the subconjunctival
and sub-Tenon’s space [28].
2 Journal of Ophthalmology

5. Valved Cannula System
Newer valved cannula designs remove the need for plugs and
consist of a cap-like silicone membrane mounted onto the
cannulas (DORC, Dutch Ophthalmic Research Corporation,
Zuidland, the Netherlands), or built into the cannula head
(Alcon, Fort Worth, Texas, US). They help maintain a closed
system, provide more stable intraocular pressure (IOP) con-
trol during instrument exchange, and reduce the amount of
infusion. High infusion flow can cause turbulence when
working with perfluorocarbon liquids, direct mechanical
trauma to the retina, ballooning of the retina if the infusion
is directed toward a retina break, or increased dehydration
if fluid-air exchange has already been performed. Valved
cannulas address the problem of high flow from the infusion
through open cannulas during instrument exchange due to
IOP compensation features, which can lead to a “fountain
effect”at the open cannulas and dislodge plugs, or cause
vitreous or retinal incarceration at the sclerotomies [29].
However, valved cannulas can lead to increased friction
between the instrument and the valve, and difficult entry
for soft or flexible tip instruments, such as the soft tip back-
flush cannula, or the diamond dusted membrane scraper
(DDMS). Entry of such instruments requires straight entry
at the centre of the valve aligned with the cannula direction,
or a second instrument to act as a glider displacing the valve
leaflet [30]. Other alternatives are cutting or removing the
soft tip and using newer retractable versions of flexible tip
instruments, such as the DDMS. A built-in valved cannula
design can also create intraocular pressure buildup during
air-silicone oil exchange, and venting extensions that allow
air to go through the valves have been introduced to prevent
this. For DORC valved cannula systems, the silicone caps can
be easily popped offto enable passage of soft-tipped instru-
ments, or to allow for venting.
6. Transconjunctival Sutureless 20G Entry
Cannulated and noncannulated transconjunctival suture-
less entries for 20G systems have also been developed to
allow use of the traditionally more rigid scissors, forceps,
and cutters and to allow hybrid 20G/25G or 20G/23G
approaches, such as dropped nucleus or intraocular for-
eign body (IOFB) removal. In 1996, Chen et al. introduced
self-sealing sclerotomies using scleral flaps for 20G vitrec-
tomy [31]. Single-step and two-step entry 20G transcon-
junctival cannulated systems (TCS) are commercially
available. Lafeta and Claes described a two-step entry for
20G valved TCS (DORC, Zuidland, the Netherlands) using
limbus-parallel 3.5 mm scleral tunnels made at a 10-degree
angle to the sclera with a bent stiletto, without conjuncti-
val displacement [32]. None of the eyes required suturing,
although it should be noted that 92% of eyes received air
tamponade. Only one eye developed hypotony (defined
as IOP less than 6 mmHg). Single-step beveled entry non-
valved 20G TCS (Synergetics, O’Fallon, MO) has been
reported by Kim et al. and Shah et al. However, 35% to
38% of eyes required suturing [33, 34].
7. Cannula Removal and Wound Closure
The self-sealing ability of a sclerotomy wound is affected by
wound architecture, scleral tunnel length, scleral elasticity,
wound apposition by residual vitreous, surface tension of a
gas bubble, and intraocular pressure. To facilitate approxi-
mation of the wound edges, the cannulas should be with-
drawn in a tangential trajectory. Infusion pressure can be
decreased prior to cannula removal to minimize vitreous
prolapse [19]. Infusion pressure may be activated to raise
internal pressure while concurrent external pressure on the
wound facilitates the angled incision tunnel to collapse and
close [32]. Some vitreous remnant may also plug the ports
to an extent during cannula removal. However, there is no
increased rate of retinal detachment attributable to this
[17]. Removal of the cannula over a nonhollow probe such
as a light pipe has been proposed as a means to decrease vit-
reous wick incarceration. However, competency of scleral
closure may be affected [35]. Partial fluid-air exchange may
help reduce wound leak from the sclerotomies until fibrin
seals the wounds, due to the increased surface tension of
gas compared to fluid [19]. However, better wound construc-
tion has obviated its routine use in MIVS. If a wound leak is
still detected at the end of surgery, absorbable sutures can be
placed, especially in the setting of leaking silicone oil. Leakage
from sclerotomies is more likely in highly myopic eyes with
low scleral rigidity, in eyes with scarred conjunctiva or sclera
from previous surgery, in Marfan’s syndrome [36], and in
young children [19].
8. Instrument Rigidity, Functionality, and Array
Rigidity of instruments is dependent on material, thickness,
diameter (gauge), and length [37]. As the trend toward
smaller gauge continued, problems with instrument array
and tool flexure arose. Initially, 25G vitrectomy was primarily
utilized in macular surgeries. As the range of instruments for
small gauge systems increased, surgeons applied 25G and
27G systems to cases requiring more extensive peripheral vit-
rectomy, and flexibility of the smaller cutter was a problem,
especially when using the instruments to affect eye rotation
for peripheral access and visualization. Hubschman et al.
demonstrated that 23G and 25G cutters were less stiffthan
20G cutters. Even within the same gauge group, cutter stiff-
ness varied due to differences in internal diameter among
25G and 23G vitrector probes [38]. Paradoxical movements
at the tip of thinner forceps can also occur since stress on
the shaft near the proximal end of the forceps can cause a
reverse movement of the distal end during attempted rota-
tion of the eye. Some surgeons stabilize the smaller gauge
instruments with an extra finger close to the sclerotomy to
reduce bending. Optimal positioning of the sclerotomies
close to the horizontal meridian, avoiding the supraorbital
rim and bridge of the nose, wide-angle viewing systems,
and scleral depression, all minimize the need for eye rotation
and problems related to tool flexure.
Newer generation 25G and 27G cutters, endoillumina-
tors, and laser probes are now stiffer, and newer forceps are
shorter to increase stiffness. However, shorter instruments
3Journal of Ophthalmology

may not be suited for use in highly myopic eyes with long
axial lengths. Oshima et al. shortened the 27G cutter from
32 mm to 25 mm, with similar rigidity to the 25G cutter,
but were still able to perform core and peripheral vitrectomy
in eyes with axial lengths ranging from 22 to 28 mm [13].
Tapered stiffening sleeves have also been developed as
another means to increase rigidity of the thinner 27G instru-
ments. Besides shortening, the radius of curvature of curved
instruments is also often blunted in order to accommodate
passage through the narrower internal diameter of the
cannulas. As a result, 25G curved scissors are less efficacious
than larger 20G scissors, and dissection of dense membranes
may need to be completed by other methods [23].
Due to the improvements in instrument stiffness, instru-
ment array in the smaller gauge systems has expanded
accordingly, as well as application to a wider range of surgical
indications, including simple and complex retinal detach-
ments, macular surgeries, tractional retinal detachments,
and stages 4 and 5 retinopathy of prematurity [39–45]. The
27G vitrectomy platform now has an extensive instrument
portfolio including valved trocars, light pipe, cutter, back-
flush brush, forceps, straight scissors, laser, and diathermy.
Phacofragmentomes for removal of dense dropped nuclear
fragments have traditionally been limited to 20G, but a
23G fragmentome has recently been introduced (DORC,
Zuidland, the Netherlands).
9. Fluidics of Vitrectomy
9.1. Infusion Flow Rates. Reduction in internal diameter of
the infusion cannula in smaller gauge systems increases fric-
tional forces and loss of pressure head and decreases volume
flow at the infusion tip entry into the eye, as per Poiseuille’s
law, which states that flow of an incompressible viscous fluid
is proportional to the fourth power of radius of the transmit-
ting tubing and inversely proportional to its length [29]. The
volume flow rate decreases by a factor of sixteen when the
inner tubing radius is reduced by half. In addition, the
volume flow rate is directly proportional to the pressure dif-
ferential and inversely proportional to the fluid viscosity.
Higher infusion pressures in the range of 40–50 mmHg
may be a way to compensate for this and allow higher flow
rates in smaller gauge systems, but may affect eyes with com-
promised ocular perfusion [19]. Infusion fluid can be infused
into the eye either by a gravity-fed system or a pressurized
system. In gravity-fed systems, infusion pressure, measured
in centimetres of water, is equivalent to the bottle height
above the eye. In vented gas-forced infusion systems, the
infusion bottle itself is pressurized and allows for rapid infu-
sion pressure control via console-controlled venting [29]. In
the Constellation system (Alcon, Fort Worth, Texas, US),
the infusion is pressurized within the console cassette, which
should ideally be placed at eye level. The integrated pressur-
ized infusion has internal, noninvasive sensors that con-
stantly measure flow into the eye through the infusion line
and cannula and integrate it through the microprocessor of
the computer. The resistance is measured during machine
priming. Ohm’s law for fluids is analogous to Ohm’s law
for electricity and states that pressure (gradient) is equal to
flow rate multiplied by resistance. Vitrectomy creates a pres-
sure gradient that the machine senses and compensates for
by increasing infusion. Infusion pressure can therefore be
adjusted according to the sensed flow rate to maintain the
desired IOP during surgery, and IOP compensation is accu-
rate to within 2 mmHg [37].
9.2. Cutter Flow Rates. Vitreous cutters developed based on
the VISC had different drive systems. The electric cutter
maintained a constant duty cycle (percentage of time the cut-
ter port is open relative to each cutting cycle) with increased
cut rate, but it was heavy and the electric motor in the hand-
piece led to easy muscle fatigue. The pneumatic cutter was
first reported by O’Malley and Heintz in 1975 [4]. Until quite
recently, pneumatic cutters employed a single pneumatic
pulse from a pneumatic energy source located in the machine
to close the cutter guillotine blade and relied on a spring to
open it to complete a duty cycle. Pneumatic cutters were
smaller and lighter, but as the mechanical properties of the
spring remain constant, as the cut rate increased, the inability
of the spring to keep up with the pneumatically driven clo-
sure increased the time the port is closed, thereby decreasing
the duty cycle [46].
Engineering advances led to newer dual pneumatic drive
cutters, which replaced the passive spring return phase with a
second pneumatic piston that actively pushes the guillotine
blade into the open position. This allowed a higher duty
cycle at ultrahigh-speed cut rates up to 7500 cuts per min-
ute and allowed surgeons to vary the duty cycle between
50% (50/50), less than 50% (shave mode), or more than
50% (core mode) [18]. The latest twin duty cycle (TDC)
cutter design on the Enhancing Visual Acuity (EVA) vitrec-
tomy system (DORC, Zuidland, the Netherlands) has a sec-
ond port in the internal guillotine blade of the pneumatic
cutter. The concept of a double-port cutter was originally
patented by Hayafuji more than 20 years ago. With two cut-
ting edges, it cuts both forward and backward, nearly elimi-
nating any port closed time, resulting in a 92% duty cycle
independent of cutting speed and allowing cut rates to be
doubled to reach 16,000 cuts per minute. With the smaller
27G cutters, increased cutting rate and duty cycle improve
cutting efficiency, without unduly increasing tractional
forces [47].
Cutter size, speed (cut rate), duty cycle, internal probe
diameter, and cutter geometry (including port diameter, dis-
tance between the port and tip), all affect its performance.
The internal diameter of vitrector probes has decreased from
0.52 mm for 20G, 0.36 to 0.39 mm for 23G, 0.26 to 0.29 mm
for 25G, and 0.20 mm for 27G systems. It should be noted
that the smaller gauge cutters may show some variability of
internal diameters [38]. Larger port diameters, such as in
the newer Ultravit 25G+ or 27G+ systems, allow higher flows
[29]. While the external diameter of the cutter handpiece has
dropped from 0.9 mm for 20G to 0.4 mm for 27G, the port
diameter of the 27G cutter still reaches 60% that of the 20G
probe. The ports of 23G, 25G, and 27G cutters are also signif-
icantly closer to the tip of the probe compared with 20G
cutters [18]. Smaller 25G and 27G cutters, with port openings
close to the tip, can get extremely close to the retina with
4 Journal of Ophthalmology

smaller sphere of influence on surrounding tissue [48].
This not only enhances safety during vitreous shaving over
mobile retina but also can allow the cutter to serve as a
dissection tool by enabling access to the very narrow tissue
planes during membrane dissection in diabetic tractional
detachments [49].
Flow rate through the cutter is influenced by the infusion
pressure, aspiration pressure, port diameter, internal diame-
ter of the probe, drive mechanism of the cutter, duty cycle,
and viscosity of the aspirated vitreous [50]. Adding to the
complex interactions, the vitreous itself is a heterogenous
substance that exhibits viscoelastic properties. It is elastic
and deformable, and its attachments to the retina require
the vitreous to be cut as it is aspirated in order to reduce trac-
tion on the retina. The vitreous is 98% water, and the rest is
composed of a matrix of collagen fibrils, hyaluronic acid, pro-
teins, and glycoproteins. Since it does not behave as a liquid,
other factors such as aspiration pressures, cut rates, and duty
cycle govern cutter flow rates in clinical settings [50]. Effi-
cient surgery requires the ability to control outflow through
the cutter to achieve high flow, such as during core vitrec-
tomy or induction of a posterior vitreous detachment, and
conversely, to enable low flow, such as during peripheral
vitreous shaving over a detached retina. In both situations,
high cut rates are desirable to reduce pulsatile traction on
the retina.
Higher flow rates in smaller gauge cutter systems can be
achieved by higher aspiration vacuums in the range of 400–
600 mmHg to counter the higher pressure head loss with
the smaller vitrector probe diameters. Duty cycles with a
longer port open time also result in higher flow rates for vit-
reous removal. With pneumatic cutters, the duty cycle
converges at 50% with increasing cut rate for both open
biased and closed biased duty cycles. However, it is important
to note that high cut rates reduce the “bite”size and thus the
effective viscosity of non-Newtonian fluids such as vitreous.
Flow rates and efficiency of vitreous removal can therefore
be maintained at high cut rates, and pulsatile traction is min-
imized [29]. Watanabe et al. have even recently reported
removal of dropped nuclear fragments using a 27G TDC
cutter and found that the cutter was able to maintain stable
suction power to hold the fragments at high cut rates [47].
Vitrectomy systems, such as Constellation (Alcon, Fort
Worth, Texas, US), have traditionally used Venturi pumps
to create vacuum because the older peristaltic pump designs
were constrained by slower rise times than Venturi pumps
due to pump inertia and inherently pulsatile flow resulting
from rotary compression of the flexible tubing [29]. How-
ever, due to advances in peristaltic pump design, some newer
vitrectomy machines offer both venturi and peristaltic
pumps. With venturi pump systems, the vacuum is set and
flow will vary according to viscosity of substances encoun-
tered by the cutter. High maximum vacuum can be set for
core vitrectomy and low maximum vacuum for peripheral
vitreous shaving. In peristaltic systems, it is the flow that is
set and vacuum varies to maintain flow with varying viscosity
of substances. Similarly, high flow rates can be set for core
vitrectomy and low flow rates for vitreous shaving. EVA
(Dorc, Zuidland, the Netherlands) has a flow-control
technology called VacuFlow Valve Timing Intelligence
(VTI) that combines computer-controlled operating pistons
and closure valves working in small-flow chambers to allow
the surgeon to have adjustable settings for both peristaltic
and venturi controls. There is some debate as to whether a
peristaltic pump system gives more control during vitreous
shaving over a detached retina [51]. However, low flow set-
tings, and automatic adjustment of vacuum and infusion
parameters to maintain constant flow, do minimize surge
turbulence at the port and traction on surrounding tissue
[29]. Furthermore, newer generation MIVS systems offer a
dual dynamic drive (3D) vitrectomy mode or a proportional
vitrectomy mode. The 3D vitrectomy mode allows for simul-
taneous linear control of cutting rate and vacuum pressures
to produce the resulting flow rate and enhancing efficiency.
As the surgeon presses the footpedal, he can change the set-
tings linearly from a preset start point for cutting and the vac-
uum to a preset endpoint. The proportional vitrectomy mode
allows for high fixed cut rates as the vacuum is varied linearly,
thereby reducing pulsatile traction and enhancing safety [18].
“Port-based flow limiting,”which describes the flow lim-
itations of cutter gauge size, port size, cut rate, and duty cycle,
can be seen as an advantage of smaller gauge systems. A
reduced flow rate and high cut rates reduce the average vitre-
ous fibre travel between cuts and therefore limit the traction
exerted on the vitreous and retina [29]. A closed biased duty
cycle and low flow reduce motion of the detached retina dur-
ing peripheral vitreous shaving and reduce postocclusion
surge after sudden elastic deformation of dense membranes
through the cutter port during membrane delamination with
the cutter in diabetic tractional detachments [29].
10. Illumination
The first light source for vitrectomy originated from an exter-
nal slit illuminator. In 1976, Peyman introduced endoillumi-
nation for 20G vitrectomy using a fibre optic inserted into the
vitreous cavity [52]. Coaxial and slit lamp transcorneal illu-
mination from the operating microscope produce scattered
light (glare), while endoillumination minimizes light reflec-
tions and light scattering from the viewing system lens,
cornea, lens, and vitreous [53]. Modes of endoillumination
include light pipes, chandelier lights, and illuminated
instruments.
Handheld light pipes allow techniques of focal bright illu-
mination, specular illumination where light shone at a critical
angle causes an almost transparent surface to glow, highlight-
ing surface irregularities, as well as retroillumination by
reflecting the endoilluminator offthe surface of the retina,
retinal pigment epithelium, choroid, sclera, or offthe cutter
[53]. Conventional halogen or metal halide light sources ini-
tially caused decreased illumination with the smaller gauge
light probes. Compared to 20G light pipes, 23G and 25G
endoilluminators had reduced light transmission due to
reduced surface area of the fibre optic by 40% and 50%,
respectively, and therefore required higher power sources.
Initially, high arc lamps (xenon and mercury vapour)
provided the high power output required for small gauge
endoilluminators [53]. Newer light emitting diode (LED)
5Journal of Ophthalmology

light sources provide up to 40 lumens without degradation of
light output, can last more than 10,000 hours, and are there-
fore particularly suited for smaller gauge endoillumination.
Moreover, newer light probes have wider cone angles and
allow better peripheral viewing with less probe angulation.
Some newer generation light probes offer more than 100
degrees divergence angle, compared to older generation
endoilluminators with illumination fields ranging from 50
to 80 degrees. Beveled sheath designs on the tips of some help
to minimize glare, while providing wide-angle illumination.
Chandelier light and illuminated instruments were devel-
oped to allow bimanual surgery. Illuminated picks provide
focal bright light at the surgical dissection site, allowing
clearer delineation of the surgical dissection planes. Illumi-
nated lasers, scissors, forceps, and infusions are also available.
Chandelier illumination is fixed in the sclera and provides
wide-angle diffuse lighting. Eckardt developed 25G “twin
light”chandelier illumination in 2003, introduced through
two sclerotomies to provide more homogenous lighting and
minimize shadows seen with single fibres [54]. Other 25G
endoilluminators include the Tornambe Torpedo (Insight
instruments, Stuart, FL) and the Awh 25G chandelier
(Synergetics Inc., St Charles, MO). Much brighter xenon
light sources, such as BrightStar (DORC, Zuidland, the
Netherlands), Photon Light Source (Synergetics Inc., St
Charles, MO), or integrated into vitrectomy machines such
as Constellation (Alcon, Fort Worth, Texas, US), allowed
the development of smaller gauge chandeliers. Oshima devel-
oped a self-retaining 27G chandelier endoilluminator in 2007
[55], and Eckardt then introduced a 27G twinlight chandelier
illumination system [56]. A 30G dual fibre chandelier system
in 29G cannulas (Synergetics Inc.,St. Charles, MO) is now
also available [57].
While chandelier lights produce superior video image
quality, the more distant diffuse fixed illumination may be
less helpful in identifying dissection planes at the surgical
point of interest and cause glare after fluid-air exchange.
Excessive use of diffuse illumination also reduces the ability
to see transparent structures such as the internal limiting
membrane (ILM), clear epiretinal membranes (ERM), and
the vitreous, compared to focal illumination from light pipes
[53]. Shadows cast by instruments in the path of the light
may impede visualization, and thermal buildup has also been
known to occur in the steadily illuminated chandelier [58].
Phototoxicity from high intensity light sources can be
reduced by starting with low intensities, lowering intensities
when switching from 25G to 20G endoilluminators, shorten-
ing exposure times to the macula, and maximizing working
distances between the tip of the endoilluminator and the ret-
ina [53]. Newer LED light sources, used in LEDStar (DORC,
Zuidland, the Netherlands) and integrated in EVA (DORC,
Zuidland, the Netherlands), have built-in adjustable yellow
filters to minimize phototoxicity.
11. Wide-Angle Viewing Systems (WAVS)
In conjunction with developments in MIVS, enhancements
in wide-angle viewing systems (WAVS) have improved pan-
oramic viewing of the surgical field and enhanced safety and
efficiency. They reduce the need for eye rotation, head repo-
sitioning, or scleral indentation and are particularly advanta-
geous when using the smaller gauge cutters. Most WAVS
consist of two components: an indirect ophthalmoscope lens
placed on or above the cornea and a prismatic stereo reinver-
ter that reinverts the image. WAVS are broadly classified into
contact and noncontact viewing systems. Contact WAVS
have a fixed field of view depending on the lens dioptric
power, whereas the field of view in noncontact systems varies
depending on the distance between the ophthalmoscope lens
and the cornea [59]. Two contact-based wide-angle lens sys-
tems, ClariVit and HRX (Volk Optical Inc), are available.
They provide approximately 10 degrees wider field of view
than noncontact systems and provide superior image quality
as they eliminate corneal aberrations and light reflections by
directly placing the lens on the cornea. However, an experi-
enced assistant is needed to hold the lens, and therefore more
surgeons prefer to use the noncontact systems. Noncontact
systems widely used include BIOM (Binocular Indirect
Ophthalmo Microscope; Oculus, Wetzlar, Germany), OFFISS
(Optical Fibre Free Intravitreal Surgery System, Topcon Med-
ical Systems, Oakland, NJ), Resight 700 (Carl Zeiss Meditec
AG, Jena, Germany), and Peyman-Wessels-Landers vitrec-
tomy lens (Ocular Instruments, Bellevue, WA) [60]. The
BIOM system is the most commonly used WAVS, is easily
adaptable to most microscopes, and is easily sterilized. The lat-
est version, BIOM 5, offers foot-pedal controlled focusing and
automatic image inversion. Newer lenses are wider in diame-
ter, are adapted to compensate for the optical properties of the
eye, and have variable back focal length optimized to focus on
a curved instead of a flat surface, allowing the concave fundus
surface to be in focus for the full extent of the retina from the
macula to the periphery with minimal distortion or aberra-
tion. However, flat planoconcave contact lenses still have
superior axial resolution and lateral resolution over wide-
angle systems for macular surgery [53].
12. Complications Associated with MIVS
12.1. Intraoperative. Rise in intraocular pressure to more
than 60 mmHg has been measured during insertion of the
trocar cannula complex [60], but newer sharper trocar blade
designs have improved ease of entry. Increased intraocular
pressure and globe deformation can open recent corneal or
scleral wounds, and placing sutures prior to port insertion
reduces the risk of wound gaping and hypotony [61]. Dis-
placement of the infusion cannula, during scleral indentation
and eye rotation, can lead to serous or haemorrhagic choroi-
dal detachment [62]. It can also occur in eyes with choroidal
edema, such as in redetachment surgeries [63]. The dislo-
cated infusion can be quickly moved to one of the other
two ports to repressurize the eye. Avoiding excessively long
scleral tunnels and placing the infusion closer to the horizon-
tal meridian prevent easy dislodgment by the inferior lid or
speculum. Cannula dislodgement can occur when instru-
ments are withdrawn from the eye as a result of increased
friction between the instrument and cannula wall, such as
when removing forceps or scissors without fully closing the
jaws [19]. Sclerotomies situated over areas of scleral thinning,
6 Journal of Ophthalmology

such as in eyes with repeat surgeries, may have reduced fric-
tion between the cannula and the sclera and predispose to
cannula dislodgement [23]. The dislodged cannula can be
mounted onto a trocar and reinserted through the same
scleral tunnel, or if it cannot be found, a new sclerotomy
can be made. Rarely, breakage and intraocular dislocation
of a segment of a cannula tip have also been reported [63].
Use of hybrid 20G/23G or 20G/25G systems, such as
during phacofragmentation, can create infusion/outflow
mismatch if care is not taken to raise infusion pressures
to match egress [64]. Entry site breaks are not common
in small gauge vitrectomy [65, 66]. Gentamicin retinal tox-
icity has also been reported when given subconjunctivally
in eyes undergoing small gauge sutureless vitrectomy and
should be avoided [67].
12.2. Postoperative. Wound sealing of the sclerotomies was
the main problem in the development of sutureless small
gauge vitrectomy systems. The hypotony is usually transient,
but can sometimes be severe, leading to choroidal detach-
ment or haemorrhage, hypotonous maculopathy, or gas
escape, and inadequate tamponade [19, 68]. Furthermore,
initial reports suggested higher rates of endophthalmitis.
This may be due to contamination from conjunctival flora,
ingress associated with postoperative hypotony, and vitre-
ous wick effect at unsutured sclerotomies [69]. India ink
passage has been demonstrated in eyes with unsutured
25G, 23G (straight or beveled), and 20G sclerotomies,
compared to no entry of India ink in eyes with sutured
sclerotomies [21, 70]. Kunimoto et al. reported an endoph-
thalmitis incidence of 0.23% for 25G PPV compared to
0.018% for 20G [71], and Scott et al. identified an endoph-
thalmitis incidence of 0.84% for 25G PPV compared to
0.03% for 20G in their cohort studies [69]. This may have
been related to variations in sclerotomy construction, as a
straight incision was found to have increased risk of endoph-
thalmitis compared with a beveled approach. A systematic
review by Govetto et al. did not find an increased risk of
endophthalmitis for microincisional vitectomy systems com-
pared to standard 20G vitrectomy [72].
13. Summary and Future Directions for MIVS
Significant strides in microincisional vitrectomy system flu-
idics, instrumentation, illumination, and viewing systems
have been made in recent years, and MIVS has all but
replaced 20G systems for a wide variety of vitreoretinal surgi-
cal indications. Retinal specialists have shifted away from
20G systems to smaller sutureless systems that have reduced
operative times, surgical trauma, inflammation, astigmatism,
and improved patient comfort, postoperative recovery times,
and patient satisfaction. This drives the quest toward even
smaller gauge systems, although this is tempered by the engi-
neering challenges, instrument tradeoffs, surgical learning
curves, availability, and, importantly, the higher costs. Care-
ful case selection and optimisation of surgical techniques
for small gauge systems are important for surgical success.
Conflicts of Interest
Dr. Claes is a consultant to Alcon.
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