Comparison of Physical Properties of Polypropylene Sutures and Knot Method to Prevent the Slippage of Sutured Scleral Fixation

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Abstract

INTRODUCTION

Surgical treatments for inadequate capsular support for intraocular lens (IOL) implantation include anterior chamber IOLs, scleral-sutured posterior chamber IOLs (PCIOLs), iris-sutured PCIOLs, intrascleral sutureless PCIOLs, and four-flanged fixation of IOLs [1-5]. Among these, the scleral-sutured PCIOL method is the most long-standing and adaptable technique, applicable to various types of IOLs [6]. However, this method is associated with long-term complications such as IOL dislocation and suture exposure [7-11]. IOL dislocation can occur due to the breakage of the suture securing the IOL haptic or slippage of the knot securing the suture to the haptic, with reported incidence rates ranging from 6.1 to 27.9% [12-14]. The long-term safety of 10-0 polypropylene sutures is compromised because of spontaneous breakage [11]. To enhance the long-term safety of scleral-sutured IOLs, 9-0 polypropylene sutures are preferred over 10-0 polypropylene sutures [10,12].

Despite efforts to prevent suture exposure by burying the knot under a scleral flap, issues with suture exposure have still been reported [10,12]. Using thicker 9-0 sutures may increase the risk of exposure owing to larger and thicker knots. The Z-suture technique was introduced to avoid making a knot in the sclera and prevent suture exposure outside the conjunctiva [15,16].

In this study, we introduced a method to prevent knot slippage during scleral-sutured PCIOL surgery and demonstrated its clinical efficacy based on the physical properties of the sutures. Additionally, we compared the differences between using 9-0 and 10-0 sutures. We also examined the physical properties of a single fisherman’s knot, which we had previously introduced, allowing for easy reattachment in cases of suture breakage [17,18].

MATERIALS AND METHODS

Measurement of tensile strength

10-0 and 9-0 polypropylene sutures (Prolene™; Ethicon, Inc., Cornelia, GA, USA) were removed from their packages and fixed into a paper frame to be loaded into a material testing machine (Fig. 1). Knots around the haptic of a 3-piece IOL (Sensor AR40e; J&J, Santa Ana, CA, USA) were made under a surgical microscope using surgical instruments. The measured suture length was 2.5 cm.

Figure 1.

Instruments and representative sample used for tensile strength measurements. A universal testing instrument (Instron 5966 Load Frame, Model 2701-065 controller; Instron Corp., Norwood, MA, USA) equipped with a 10N tension/compression load cell (Model 2530-428, Instron Corp.) (A) and a paper frame for measurement (B).

Tensile testing was performed using a universal testing instrument (Instron 5966 Load Frame, Model 2701-065 controller; Instron Corp., Norwood, MA, USA) equipped with a 10N tension/compression load cell (Model 2530-428; Instron Corp.). Each end of the 10-0 and 9-0 polypropylene sutures was subjected to tensile forces at a separation rate of 10 mm/min until failure, which was defined as either breakage of the suture or complete knot slippage. The tensile strength at failure was recorded as the maximum tensile strength (N) measured at failure. Ten measurements were taken for each group to ensure statistical reliability.

The test groups included intact raw suture material for both 10-0 and 9-0 polypropylene sutures; a single fisherman’s knot tied with the 10-0 and 9-0 polypropylene sutures; and overhand knots tied one (Fig. 2), three (Fig. 3), and five times around the haptics using both 10-0 and 9-0 polypropylene sutures.

Figure 2.

An illustration demonstrating the method for tying an overhand knots around the haptic using a suture. (A) The first overhand knot is tied around the haptic. (B-D) Finally, two simple knots are added to secure the suture and prevent loosening. The blue circle represents the cross-section of the haptic.

Figure 3.

An illustration demonstrating the method for tying three overhand knots around the haptic using a suture. (A) The first overhand knot is tied around the haptic. (B, C) Two additional overhand knots are tied on the opposite side of the haptic. (D) Finally, two simple knots are added to secure the suture and prevent loosening. The blue circle represents the cross-section of the haptic.

Measurement of the suture thickness and area

To measure the thickness and area of the surgeon’s knot for different suture types, 10 knots were made using 10-0 and 9-0 polypropylene sutures on a styrofoam base. Images of the knots were taken at ×40 magnification using a microscope (Fig. 4). These images were analyzed using Image J^® software to measure the overall knot area and thickness at the thickest part of the knot, with each measurement repeated three times. Differences in the maximum tensile strength between the two suture types were compared using Student’s t-test and ANOVA, with statistical significance set at p < 0.05.

Figure 4.

Representative photographs showing the formation of a surgeon's knot using two different types of sutures. Microscope images of the surgeon’s knot made of 10-0 (A, C) and 9-0 (B, D) polypropylene sutures (×40). Area (A, B) and thickness (C, D) of the knot are measured using Image J software^®.

RESULTS

The maximum tensile strength of unaltered sutures was 0.47 ± 0.02 N for the 10-0 suture and 0.52 ± 0.13 N for the 9-0 suture. The 9-0 suture exhibited significantly high tensile strength (p = 0.007) (Fig. 5A). For the single fisherman’s knot, the maximum tensile strength was higher (0.50 ± 0.14 N) for the 9-0 suture than for the 10-0 suture (0.27 ± 0.05 N) (p < 0.001, Fig. 5B).

Figure 5.

Bar graphs comparing the mean maximum tensile strength of intact sutures and sutures with single fisherman's knots. Maximum tensile strength of intact 9-0 and 10-polypropylene sutures (A) and a single fisherman’s knot made of 9-0 and 10-0 polypropylene (B). *Indicates statistical significance at p < 0.05.

When measuring the maximum tensile strength of knots around the haptic of the IOL, the results for the 10-0 suture were as follows: 0.11 ± 0.08 N for one knot, 0.23 ± 0.10 N for three knots, and 0.33 ± 0.01 N for five knots. The tensile strength increased significantly with more knots (p < 0.05). For the 9-0 suture, the tensile strength was 0.06 ± 0.04 N for one knot, 0.31 ± 0.17 N for three knots, and 0.19 ± 0.11 N for five knots. Three and five knots showed significantly higher tensile strength than one knot, but three knots exhibited higher tensile strength than five knots (Fig. 6).

Figure 6.

Maximum tensile strength of overhand knots tied one, three, and five times around the haptics by 9-0 and 10-0 polypropylene sutures. *Indicates statistical significance at p < 0.05.

The thickness and area of the knot were measured in seven samples of the 10-0 sutures and nine samples of the 9-0 sutures without measuring failure (Table 1). The knot area was significantly larger for the 9-0 suture (41,188.5 ± 5,968.7 μm²) than for the 10-0 suture (21,062.4 ± 5,171.7 μm²) (p < 0.001). The knot thickness was significantly greater for the 9-0 suture (195.6 ± 35.9 μm) than for the 10-0 suture (160.1 ± 14.5 μm) (p = 0.021).

Table 1.

Area and thickness of surgeon’s knot measured using Image J^® software

DISCUSSION

Friction is the force that resists motion when the surface of one object meets that of another object. This force is primarily influenced by the texture of the surfaces and the force required to press them together [19,20]. When an object is pushed against a surface, the frictional force increases and often exceeds the weight of the object. The frictional force is independent of the contact area between the two surfaces. This implies that even if two heavy objects have the same mass but different shapes, the frictional force remains the same when dragged over the ground. However, this relationship changes when the surface area becomes extremely small, as the coefficient of friction increases, potentially causing the object to dig into the surface [20].

Suture materials are categorized as absorbable or non-absorbable, monofilament or multifilament, and natural or synthetic. Larger-diameter sutures often lead to weaker knots and more foreign material at the surgical site; therefore, the smallest diameter suture that can adequately hold the tissue should be used. Multifilament sutures are generally stronger and have better knot security owing to their higher friction coefficient [19]. The handling characteristics of sutures are linked to their intrinsic stiffness, with more pliable, smaller-diameter sutures being easier to handle than stiffer, larger-diameter sutures [19].

When multiple overhand knots are tied, the force between the suture and the IOL haptic increases. However, if the knots become loose, the force required to increase friction does not increase. When comparing the 10-0 and 9-0 sutures, we observed that the knots made with 9-0 sutures had gaps between the loops. Despite applying tension when tying the knot, the higher rigidity of the 9-0 sutures caused the knot to loosen easily (Fig. 7). This loosening explains why knots with five overhand knots had a lower maximum tensile strength than those with three overhand knots.

Figure 7.

Representative photographs illustrating the gaps of a surgeon's knot using two different types of sutures. Microscope images of the surgeon’s knot made of 10-0 (A, ×40) and 9-0 polypropylene sutures (B, ×40) showing that the gaps between the sutures are prominent in the 9-0 polypropylene sutures (arrows).

Balancing stability and surgical time should be considered when choosing the optimal surgical method. Although more overhand knots around the haptic can increase the stability by enhancing friction, they also prolong the procedure and may cause the knot to loosen. When using the 10-0 polypropylene sutures, five knots showed higher tensile strength than three knots; however, the drawback is that it lengthens the surgical time. Because the three overhand knots provide tensile strength similar to that of a single fisherman’s knot, they are believed to offer sufficient stability. Our findings suggest that three overhand knots of 9-0 polypropylene provide sufficient stability, while being more efficient than five knots when considering the surgical time. This is particularly important in surgical contexts where time is critical and minimizing the risk of knot loosening is essential.

Considering suture exposure and the risk of infection, knotting the suture to the sclera should be avoided [12,21,22]. Techniques such as creating thick Tenon’s capsules, scleral flaps, or grooves aim to minimize knot exposure but cannot completely prevent it [21,23]. Our study showed that knots made with 9-0 sutures are significantly larger and thicker than those made with 10-0 sutures, which increases the risk of exposure. Therefore, the Z-suture technique, a knotless method for transscleral suture fixation, is recommended for surgeries involving 9-0 sutures to prevent conjunctival exposure [15]. The Z-suture effectively prevents conjunctival exposure of 9-0 polypropylene sutures.

We previously introduced a knot method that can be used when a suture breaks accidentally during scleral fixation surgery [17]. Although this knot does not provide the same maximum tensile strength as an intact suture, it is sufficiently strong to maintain its integrity while passing through the sclera. This technique is clinically useful, particularly for 9-0 sutures, which show effective bonding strength even in cases of accidental breakage.

In summary, tying multiple overhand knots around the haptic effectively prevents knot slippage during long-term scleral-sutured IOL fixation, with three overhand knots being the most efficient, balancing both knot security and surgical time. For long-term stability, thicker 9-0 sutures are preferable to 10-0 sutures. However, owing to the increased risk of exposure to thicker surgeon knots, the Z-suture technique, which avoids knotting, is recommended for scleral fixation surgeries using 9-0 sutures.

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