Fig. 1
A surgical anatomy of the limbus
Anterior limbal line – corresponds to termination of the Bowman’s membrane
Mid limbal line – corresponds to the termination of the Descemet’s membrane(Schwalbe’s line)
Posterior limbal line – corresponds to the scleral spur
With this basic background of the surgical limbus, we will now elaborate on the steps of the MSICS.
Conjunctival Peritomy
The opening of the conjunctival flap in MSICS is done with forceps (Colibri/Pierce Hoskin) in the non-dominant hand and conjunctival scissors in the dominant hand of the surgeon. The flap is based toward the fornix. Initiation of the conjunctival flap is done at 10 clock hour. The conjunctiva has to be grasped just short of the limbus with forceps and a firm vertical traction1 exerted to create a conjunctival fold (Fig. 2).
Fig. 2
Illustrating vertical traction causing conjunctival fold away from the sclera
The cut from the conjunctival scissors should be vertical with the limbs of the scissors perpendicular to the scleral surface. Since the tip of the scissors is blunt, it would not injure the sclera (Figs. 2 and 3).
Fig. 3
Conjunctival/tenon opening with exposure of underlying sclera
Once the initial cut has been inflicted, the dissection of the conjunctiva and tenons can be done separately. Alternatively, sub-tenon plane of dissection can be directly sought. Blunt dissection of the conjunctiva is carried by initially inserting the blades (closed) beneath the tenon capsule. The tip has to be directed toward the limbus and blades opened to separate the tenons from the underlying sclera (Fig. 4).
Figs. 4, 5 and 6
Diagrams illustrating the position and direction of conjunctival scissors
Undue posterior dissection has to be avoided. The conjunctiva is then cut at the limbus flush with the cornea. During this step, the forceps should exert proper conjunctival/tenons traction to lift it away from the cornea and the blades of the scissors should be tangential to the cornea surface (Figs. 5 and 6).
Cautery
The purposes of using cautery before initiating a scleral incision:
Allow proper visualization of the instrument during tunnel creation which otherwise gets obscured by blood.
Prevent/minimize bleed into the anterior chamber intra- and postoperatively and sub-conjunctival hemorrhage postoperatively.
However, caution should be exercised since unjustified use of cautery results in more harm than the benefit. Wet-field bipolar cautery allows lateral distribution of heat over the scleral surface compared to unipolar/thermal ball point cautery and hence it is preferred than the latter. There are few points to consider when applying cautery:
Point cautery should be applied to scleral bleeders only. Avoid forceful rubbing of the cautery tip to the scleral surface.
Cautery of limbal “blue zone bleeders” has to be avoided.
Cauterizing without proper tenons fascia removal may cause inefficient cauterization. Moreover cauterized tenons will be difficult to separate from the sclera.
Once sclera incision and tunneling is done, cautery should not be applied. It may cause fish mouthing and subsequent wound leak.
Disadvantages of sclera cautery:
Scleral thinning and scleral necrosis can occur following excessive cautery.
Poor wound healing.
(These effects of cautery are also compounded by the relative avascularity of the sclera and postoperative steroid usage.)
In the late postoperative period, excessive cautery may result in higher induced astigmatism.
Scleral Incision
“What constitutes the essential elements of a self-sealing cataract incision?” There are three components of an ideal self sealing tunnel in MSICS:
External sclera incision – constructed by the blade/surgical knife
Sclerocorneal tunnel – constructed by tunnel blade/crescent knife
Internal corneal incision – created by keratome
In practical terms one can think about the globe as a double-walled structure, at least in the vicinity of the wound. For the purposes of a cataract incision, one wall is the roof of the tunnel and one is the floor of the tunnel. It is these two layers acting in a predictable manner when pressure is applied from within during reformation of the anterior chamber that results in closure of the wound. During the initiation of the incision, the blade has to be kept as perpendicular as possible to the scleral surface. An angled or slanted incision will cause more separation of the lips of incision and consequent wound sagging or gaping (Figs. 8, 9, 10, and 11).
Fig. 8
Diagram illustrating perpendicular position of the blade while making incision
Fig. 9
Non-sagging and closely apposed incision
Fig. 10
Blade held at an angle to the sclera
Fig. 11
An angled wound resulting in wound gape following surgical maneuvering
The characteristics of the external scleral incision include:
1.
Size
2.
Shape
3.
Location
4.
Depth
Size
The size of the incision on the sclera is titrated according to the density of the nucleus to be extracted.
Considering the average equatorial diameter of an adult lens (8.8–9.2) as 9 mm and sclera extensibility of 0.5–1 mm, nucleus of any size can be promptly delivered through the external incision. Harder cataracts have lesser epinucleus and are less yielding. Hence the size has to be appropriate lest there will corneal endothelial damage and/or nuclear fracture while delivery.
Shape (Figs. 12, 13, 14, 15, and 16)
Fig. 12
Smile incision
Fig. 13
Straight incision
Fig. 14
Frown incision
Fig. 15
Chevron incision
Fig. 16
Inverted batwing incision
The concept of arc length and chord length has to be explained to understand the effect of various shaped incisions.
Arc length is measured along the line of incision. Chord length is measured from point of initiation to the point of ending of the incision (end to end).
Various incision configuration which are used are as follows:
Smile incision: When the incision is made parallel to limbus, the inferior edge of the incision may fall back, which flattens the cornea in this meridian. If the incision is made at 12 o’clock, this incision flattens the vertical meridian of the cornea, causing against the rule astigmatism.
Straight incision: When straight incision is fashioned, there are no chances of the inferior edge falling back. Whatever astigmatism is produced by the straight incision is because of the instability of the central portion of the wound, which is much less than the smile incision.
However, although both these incisions open the wound better during nucleus delivery, they induce more astigmatism.
Frown/chevron/inverted bat wing:
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