Manual Small Incision Cataract Surgery – Part 1
Everything you ever wanted to know about manual small incision cataract surgery – Part 1
Manual Small Incision Cataract Surgery: A, B: Frown-shaped external incision (green arrows), funnel-shaped tunnel (yellow arrows) and forward-curved internal incision (white arrows) are seen. The internal incision is longer than the external incision. C: straight- and D: L-shaped (black arrows) external incisions are seen.
Manual Small Incision Cataract Surgery (MSICS) is a form of extracapsular cataract extraction that combines the sutureless, self-sealing, small-incision, minimally astigmatic advantages of phacoemulsification with advantages of speed, ease of surgery, low cost and low dependence on machines. It is an effective alternative to phacoemulsification that is widely accepted and practised in the developing world. In experienced hands, the additional safety it offers with hard brown and black cataracts, especially in patients with borderline endothelial counts, makes it an essential skill for every surgeon to learn.
This article deals with wound construction in MSICS – an integral part of the surgery responsible for intra- and postoperative wound and anterior chamber stability.
ASTIGMATICALLY NEUTRAL INCISIONAL FUNNEL:
The incisional funnel was described by Paul Koch in 1991. Gills and Sanders further concluded that corneal astigmatism is directly proportional to the cube of the length of incision and inversely proportional to its distance from the limbus. Thus, the astigmatically neutral zone is funnel shaped. To remain astigmatically neutral, incisions closer to the limbus need to be smaller and conversely, longer incisions may be constructed further from the limbus.
EXTERNAL SCLERAL INCISION:
For adequate exposure and for facilitating nucleus expression later in the surgery, a superior rectus bridle suture is preferred to allow the eye to be rotated downwards. A fornix-based conjunctival flap is made and major bleeders cauterised. Extensive scleral cautery should be avoided to avoid postoperative scleral necrosis. An angled 2.8mm bevel-up crescent blade or a Bard-Parker (no 15) blade may be used for creating the incision. The location, shape, length, depth and distance from the limbus are all important parameters.
The depth of the incision is about half the thickness of the sclera. Shallow tunnels tend to gape and buttonholing is more likely. Excessively deep tunnels can enter the supraciliary space, cause scleral disinsertion or premature entry. If the depth is incorrect, the tunnel can be restarted at the correct depth or it may be abandoned and a new site chosen.
A blunt dissector or a dissector directed excessively upwards tends to cause buttonholing. A sharp dissector or one directed excessively downwards tends to go deep. The dissector should follow the curve of the globe during both forwards and sideways movement and should be just seen through the overlying sclera.
The chord length of the incision is generally about 6-7mm. However, denser nuclei may require longer incisions. Commonly used incisional shapes are straight and frown. The frown incision, described by Singer in 1991, is more stable than a linear one, which in turn is more stable than a limbus parallel incision. Beginners, however, may start out with a straight incision 2mm from the limbus and apply a few sutures at the end of surgery before transitioning to the most commonly used frown incision.
Straight incision induces more against-the-rule astigmatism, and sutures help close this incision securely, as well as making it less astigmatic. The distance from the limbus at the closest point of the frown incision is generally about 2mm, and at its furthest point about 4mm. The chevron-shaped incision was described by Pallin and consists of two linear incisions at 90 degrees to each other in the form of an inverted V. An L-shaped external incision with a large internal incision has also been described.
The tunnel is extended forwards as well as sideways using the crescent blade, making sure that the depth of the scleral tunnel is maintained uniformly throughout. This can be attained by initiating the tunnel at a uniform depth by first passing the crescent blade held horizontally along the entire length of the incision at the desired depth.
Once this horizontal step is created, it is extended forwards by swivelling the blade to and fro while holding the globe firmly. The globe should not be stabilised by holding the tunnel lip, to avoid compromising it. The correct depth of tunnel is indicated by the blade being just seen through the overlying sclera.
The tunnel is enlarged sideways to create pockets on either side that are broader than the external incision. As the cornea is steeper than the sclera, the crescent dissector is angled upwards at the limbus to avoid a premature entry. The tunnel is then extended forwards uniformly in the cornea to up to about 1.5-2mm. The total length of the tunnel from the external scleral incision to the internal corneal incision is therefore about 3.5-4mm and it has an inverted trapezoid configuration. The nucleus is able to mould itself through this funnel-shaped tunnel.
INTERNAL CORNEAL INCISION:
The internal corneal incision is responsible for the stability and self-sealing nature of the tunnel. Lack of an adequate corneal valve mechanism can result in a non-self-sealing incision and astigmatism induction. Coating the tunnel with viscoelastic facilitates passing a 2.8 or 3.2mm keratome smoothly up to the anterior-most end of the tunnel.
The anterior chamber is then entered into with the keratome and extended on either side up to the limbus, so that the forward-curving internal incision is larger than the external incision. The keratome is held parallel to the iris and care is taken not to allow the anterior chamber to shallow while extending the internal incision, in order to maintain a uniform internal lip throughout the extent.
Though an 8mm internal wound is generally sufficient, very large nuclei may rarely require an incision of up to 10mm. The inner lip creates a valvular mechanism that seals the incision and makes sutureless surgery possible. It prevents the iris from prolapsing out during surgery, as well as avoiding wound leak and endophthalmitis postoperatively. The corneal incision makes the tunnel multi-planar.
MSICS can cause some against-the-rule astigmatism, more so if the tunnel gapes or is compromised at the end of surgery. Though most surgeons prefer to do a superior MSICS, the incision may be placed on the steep corneal axis if desired.
Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India and can be reached at firstname.lastname@example.org.