Terminal chop: New technique for full thickness nuclear segmentation in mature hard cataract.

We describe an efficient technique, "Terminal Chop," for full thickness nuclear segmentation in mature hard cataracts. Terminal chop utilizes the principle of secondary rock breakage system with stress concentration to break these hard nuclei. In this technique consonant to drag picks, a specially designed chopper "Terminator" is used to initiate a unique dispersive mechanical force to create a full thickness nuclear crack (crack formation) at the weakest soft and thin equator, which automatically traverses through the center and to the equator on the other side. Lateral separation of both the instruments placed at the equator, propagates the initial full thickness nuclear crack (crack propagation), breaking the entire nucleus into two complete pieces including the posterior plate. The direction of splitting follows the cleavage plane in a more direct manner, thus requiring least fracture force, least manipulation and stress, much lower than compressive stress, causing minimal trauma, and highly satisfactory postoperative results.

Hard cataracts are virtually all nucleus that are very solid, firm, and resistant to pressure; not easily broken, bent, crushed, or pierced like a solid hard rock. The absence of a protective epinuclear layer, the paucity of cortex, the fragility of the capsule, and the laxity of the zonules, increase the risk of injury to the supportive structures of the lens during surgery. The dense bulky central nucleus of a hard cataract is still harder and unbreakable, requiring a great deal of endurance or effort to trench, impale and segment.

Techniques which are used currently, such as divide and conquer described by Gimbel,[1] and stop and chop made popular by Koch and Katzen,[2] need stressful extensive manipulation and a longer time of high-intensity phaco power to sculpt the nucleus and to obtain the pieces for emulsification. Horizontal chop,[3] the original nuclear chopping technique introduced by Nagahara[4] in 1993, exerts a greater shock or strain with crushing compressive forces to incise and fracture the hard nucleus and quite often results in incomplete nuclear segmentation and intact posterior plate, creating problems for the surgeon to complete the emulsification procedure.

We describe an efficient technique “Terminal Chop,” to crack and break the mature hard cataract into two complete segments including the posterior plate with much ease and which is also free from high-intensity shock and strain. In this technique, consonant to drag picks in rock excavation system, a specially designed chopper “Terminator” [Fig. 1] is used to initiate a unique dispersive mechanical force to create a full thickness nuclear crack at the equator which automatically traverses through the center of the nucleus, over to the equator on the other side, breaking the entire nucleus into two complete pieces.

Figure 1

Terminator, Rajendra Prasad's Chopper: Angulated smooth, olive tip with blunt wedge inner edge and broader flat posterior surface

Technique

After routine incisions, the anterior capsule is stained with trypan blue dye to enhance the visibility. In accordance to soft shell strategy[5] the dispersive ophthalmic viscosurgical device (OVD) is injected into the anterior portion of the capsule following which the anterior capsule is flattened with the injection of cohesive OVD just in front of it. A large central capsulorhexis with an intended 5.5-6.0 mm. diameter is made [Fig. 2a]. Hydrodissection is accomplished using a hydrodissection needle and confirmed by rotating the nucleus to certify that it is totally free inside the capsular bag.

Figure 2

(a-d) Technique of Rajendra Prasad's Terminal Chop: (a) 5.5–6.00 mm. diameter central capsulorhexis is made (b), short superficial trench sculpted distal to center; (c) phaco probe engaged at the distal end of the trench (d) phaco tip impaled, keeping the tip directed toward the equator within the superficial layers of the nucleus and firmly hold the nucleus

Once rhexis is complete a short superficial central trench of 1 mm × 1 mm × 1 mm is sculpted in the nucleus [Fig. 2b]. Sufficient to hold and engage the 15° phaco tip to achieve a proper plane of depth in the nucleus, with low vacuum sculpt settings. Then using the hyper pulse or burst mode with high vacuum settings of 50% power, 450 mmHg vacuum, and 35 ml/min aspiration flow rate the phaco tip is engaged at the distal end of the trench [Fig. 2c]. Then without penetrating deep into the center of the nucleus phaco probe is impaled superficially keeping the tip directed towards the equator, parallel to the pupillary plane to achieve firm hold of the nucleus [Fig. 2d] just within the equator.

While nucleus is firmly held in position, equator of the nucleus is slightly drawn within the capsulotomy edge, then under direct visualization a specially designed wedge-shaped blunt olive tip chopper “Terminator” similar to drag pick, is very simply passed around the lens equator by sliding into the space created within the capsulotomy edge and hooked around the equator adjacent and parallel to the phaco probe, [Fig. 3a]; chopper is then simply dragged just 1.5–2 mm through the open edge equator, of the nucleus to create a small groove [Fig. 3b]. But while creating the groove the wedge inner surface of the chopper, consonant to drag pick in rock excavation system, also generates tension and dispersive forces along the sides of the groove and initiates a full thickness nuclear crack [Fig. 3c]. Initial equatorial crack automatically traverses through the center and to the equator on the other side breaking the entire nucleus into two complete pieces.

Figure 3

(a) Nucleus firmly held in position with phaco tip, equator of the nucleus slightly drawn with in the capsulotomy edge. Under direct visualization equator of nucleus is hooked with terminator (b) drawing the chopper within the equator of the nucleus to create a small groove; (c) wedge inner edge and flat posterior surface of chopper generate full thickness nuclear crack (crack formation) (d) 90° lateral separation of chopper and phaco tip placed at the equator propagate nuclear segmentation (crack propagation)

For complete separation of nucleus, without making any horizontal excursion, chopper takes a lateral position with in the initial crack parallel and opposite to the phaco tip at the equator. Once both the instruments achieve firm grip of the nucleus at the equator, they are separated 90° laterally to propagate the nuclear segmentation from the equator to center and equator on the other side [Fig. 3d]. The force vector of 90° lateral separation is continued concentrically [Fig. 3d], until splitting the entire nucleus into two complete clean halves. Once the complete nuclear division is achieved, the nucleus is then rotated 90° and the same procedure is repeated to further chop the nucleus into multiple fragments. Each free lens fragment is then drawn by the phaco probe and emulsified [Video 1].

Discussion

Phacoemulsification of mature hard cataract LOCS grade IV and above has always been a challenge.[6] The greatest challenge is in breaking them down. Current surgical techniques [Table 1] frequently end up in extensive manipulation and a longer time of high intensity compressive phaco forces with incising, crushing, drilling, or sculpting maneuvers at the poles, the free nucleus face either to hold the nucleus deep at the center for chopping or to create a groove or trench for further segmentation of the nucleus. The stressful mechanics of fracture to break these solid hard nuclei, quite often do not succeed, resulting in incomplete nuclear segmentation and intact posterior plate.

Table 1

Key points of difference between Nagahara's, (horizontal) Chop and Rajendra Prasad's Terminal Chop

According to the mechanical rock excavation systems[78] and Griffiths theory of brittle fracture,[9] these brittle hard objects are stronger in compression but weaker in tensile strength, so it is always difficult to crush, incise, sculpt, and segment these hard materials with compressive forces but relatively easier to crack and break, with inside-out dispersive mechanical forces.

Cracking and breaking is quite easier if the mechanical forces are applied at the open edge or naturally weak points on the surface of these hard objects. Inside-out dispersive mechanical forces applied on either side of the preexisting crack, groove, depression, or naturally weak open edge breaks them at a stress that is much lower than the compressive stress.

Open edge and naturally weak point of a hard nucleus is its equator, which is quite thin soft and narrow, while center of the nucleus is very bulky hard leathery and unbreakable. Terminal chop utilizes the principle of secondary rock breakage system with stress concentration to break these unbreakable hard nuclei.[78] In this technique, similar to cracking and breaking solid hard rocks, a unique inside-out dispersive mechanical force is created to initiate a full thickness nuclear crack at the weakest soft and thin equator which traverses through the entire nucleus, breaking it into two complete pieces.

In terminal chop [Table 1], instrument tips move away from each other at the equator to propagate the initial crack into a complete full thickness fracture, which breaks them at a stress that is much lower than the compressive stress. In horizontal chop,[3] instrument tips move toward each other in the horizontal plane with stressful compressive out of plane shearing force to incise and then break the nucleus, while in vertical chopping[3], the two instrument tips move towards each other in the vertical plane to create in plane shearing force, requiring very high stress or compressive forces to fracture these solid hard nuclei.

In the unique mechanics of terminal chop, similar to drag tools in rock excavation system, chopper plays an important role, as critical instrument maneuvers to crack the nucleus are performed by the chopper along with the phaco probe. The chopper used in Terminal Chop is named as Terminator [Fig. 1]. Terminator is a specially designed chopper to safely hook, hold, stabilize, and initiate a crack at the equator of the nucleus. The mode of action of terminator is similar to drag tool, generating stress at the sides of initial groove in the direction of equator on the other side, parallel to the nuclear surface, creating full-thickness tensile fracture.

Terminator consists of a round knurled handle, and a 60° angled distal shaft, with a tip. The tip is approximately 1.50 mm in length and angled 85° in relation to the remaining distal portion, which helps in full-thickness hooking and firm holding of the nucleus. The inner surface of the tip is wide blunt wedge edge with the broader flat posterior surface, similar to the tip of drag pick, angled 60° to the axis, to create a nick and initiate crack at the periphery of the nucleus adjacent to the phaco probe.

Conclusion

“Terminal Chop” technique is an efficient, safe, simple, and swift procedure for full-thickness nuclear segmentation, giving consistent results, especially in hard mature cataracts. The principle of mechanical rock excavation with drag pick chopper systems could be safely used to break these solid mature hard cataracts. The main consideration is given to the mechanics used in chopping system, forces required to induce fractures, the energy consumed in breaking these solid hard nuclei and more importantly the tool used to generate these forces and initiate the crack in the nucleus.

Video Available on: www.ijo.in

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.