Knot mechanics important to success of SFT
The turns of the hitch press on each other and the iris tissue to produce self-locking units that can withstand the pull of the peripheral iris.
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Pupil reconstruction is an essential aspect to prevent photophobia and filter the amount of light that is needed in cases with traumatic mydriasis and atonic pupils. Surgical repair is of substantial benefit as it has the potential to reduce visual glare and light sensitivity.
The Ashley Book of Knots describes “knot” as a term that comprises hitches, bends, splices and true knots and knot tying as an important aspect of any scout program. The book illustrates the timber hitch method, which is used to attach a single length of rope to a log of wood or any cylindrical object (Figure 1). A proposed theory of hitches makes predictions that are correct when tested empirically. We described single-pass four-throw (SFT) pupilloplasty, which involves a single pass with four throws and has a self-retaining, self-locking mechanism created due to the helical structure with intertwining of sutures (Figure 2). Dr. Jack Holladay first suggested the resemblance of the helical loop of SFT to the timber hitch.
Source: Priya Narang, MS, and Amar Agarwal, MS, FRCS, FRCOphth
Mechanics
Friction prevents slipping of a knot, and to provide friction, there must be some pressure that occurs at a place within the knot, which is called the nip.
As demonstrated in Figure 3, if T2 is the force exerted toward the center and T1 is the force that acts to pull the iris tissue toward the periphery, the SFT loop will hold and stay in place if T2 is greater than T1. With four throws, T2 is greater than T1; hence, the SFT loop holds. With fewer than four throws, the SFT loop loosens and opens up as T2 is less than T1. In addition to this, the iris tissue does not exert pressure or force T1 that would lead to opening up of the loop. The application of four throws has been found to be optimal to hold the iris tissue in place. The addition of more throws only adds to the bulk of the suture material inside the eye, and theoretically more friction is created and more energy is needed to approximate and slide the loop internally.
Frictional force created also depends upon the surface area of the contact of suture with iris tissue and on the force created by turns, crossings and wrapping around of the loop on itself, which creates a pinching effect and adds strength to the hitch. The slippage of a hitch/knot can be mitigated by leaving plenty of rope at the working end of the knot and by tightening it as much as possible before loading. Similarly, in SFT, before the suture ends are cut, pulling both suture ends ensures tightening of the loop across the iris tissue. About 1 mm of suture end is left on either side to prevent slippage of the cut end inside the loop, which prevents it from loosening and eventually opening up. We have not reported any case of opening of the SFT loop when the optimal four throws are taken in situ.
SFT, which is similar to the timber hitch, is reproducible and easy to learn and emulate. The different turns of the hitch press on each other and the iris tissue to produce self-locking units that can withstand the pull of the peripheral iris.
- References:
- Ashley CW. The Ashley Book of Knots. New York: Doubleday; 1944:12.
- Narang P, et al. Eur J Ophthalmol. 2017;doi:10.5301/ejo.5000922.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: aehl19c@gmail.com; website: www.dragarwal.com.
- Priya Narang, MS, can be reached at Narang Eye Care & Laser Centre, Ahmedabad, India; email: narangpriya19@gmail.com.
Disclosures: Agarwal and Narang report no relevant financial disclosures.