January 02, 2020
3 min read

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.

Amar Agarwal
Amar Agarwal

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.

timber hitch knot
Figure 1. Animated description of a timber hitch knot. Animation depicts the loop formation on a log of wood. A rope is passed around the log of wood (a). One end of the rope is crossed over itself, and a loop is created (b). The rope is further tucked under itself and is looped around (c). The rope end is pulled, and the hitch engulfs the log circumference (d).

Source: Priya Narang, MS, and Amar Agarwal, MS, FRCS, FRCOphth


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.

Animation of SFT
Figure 2. Animation of SFT. The needle is passed through the proximal and distal portion of the iris tissue (a). A loop of suture is withdrawn after approximation of the proximal and distal portion of the iris tissue (b). The suture end is passed through the loop (c). The suture ends are pulled, and the loop slides inside, approximating and holding the iris tissue (d).
Mechanics of SFT pupilloplasty
Figure 3. Mechanics of SFT pupilloplasty. The image depicts the forces T2 and T1 exerted on the iris tissue. T2 is the frictional force that acts toward the center of the pupil, and T1 is the force exerted by the peripheral iris tissue. With four throws, T2 is greater than T1, hence the SFT loops holds its position, whereas when T2 is less than T1 with fewer than four throws, the SFT loop opens up.

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.

Disclosures: Agarwal and Narang report no relevant financial disclosures.