Experimental knotless scleral fixation with expanded polytetrafluoroethylene (ePTFE): dynamics of anchoring strength in the early postoperative period

Abstract

Purpose

To investigate short-term scleral response to expanded polytetrafluoroethylene (ePTFE) sutures using the dynamics of its mechanical stabilization in an animal model.

Methods

Ten rabbits underwent unilateral lens extraction followed by implantation of two knotless scleral fixation techniques using ePTFE (Gore-Tex CV-8): Z-suture and chain-of-knots. Both techniques were implanted in each eye. At postoperative days 1, 5, 10, 20, and 30, two rabbits were analyzed per time point (n = 10), each contributing one independent dynamometric measurement per technique. The tensile force required to disinsert the implanted material from the sclera was quantified using a dynamometer.

Results

Dynamometric evaluation of tensile force of the Z-suture ranged from 0.12 to 0.32 N, whereas the chain-of-knots scleral fixation ranged from 0.07 to 0.27 N. Both techniques showed an increase in the force required to disinsert fibers from the sclera in the early postoperative period.

Conclusions

In this rabbit model, knotless ePTFE scleral fixation demonstrated increasing early mechanical stabilization. The differences between the two fixation techniques may be caused by a combination of mechanical factors and tissue adaptation, although these findings remain exploratory. Both techniques may provide sufficient early fixation strength in an experimental setting and support further preclinical and clinical evaluation.

Introduction

During routine cataract surgery, the intraocular lens (IOL) is placed in a capsular bag. Congenital, traumatic, or iatrogenic damage may result in the absence of zonular support. When this occurs, other methods are required to stabilize the IOL. These approaches include anterior chamber IOL, prepupillary or retropupillary iris-claw IOL, iris-sutured posterior chamber IOL, scleral-sutured posterior chamber IOLs, and intrascleral haptic fixation [1].

Various suturing materials have been used, with 10 − 0 Prolene being among the most widely used; however, its use declined owing to a high incidence of complications, particularly suture breakage [2]. To improve long-term stability and reduce these risks, 10 − 0 Prolene was subsequently replaced by 9 − 0 Prolene [3]. Meanwhile, thicker Prolene 5 − 0 and 6 − 0 is used in flanged techniques [4].

Another material expected to provide high long-term strength and stability was based on polytetrafluoroethylene (PTFE). PTFE, later trademarked as Teflon, was accidentally discovered in 1938 by Roy J. Plunkett [5]. Its first medical application was reported in 1972 when it was introduced in the form of prostheses for use in arterial and venous systems [6]. Expanded PTFE (ePTFE) is widely used in the field of medicine. Sutures, implants, patches, and prostheses are used in various medical applications [7–9]. It has also been widely used in ophthalmic surgery. For example, it is used in managing childhood ptosis [10], muscle elongation in the surgical treatment of strabismus [11], as a graft material in ophthalmic plastic and reconstructive surgery [12], among others.

According to the package insert, using ePTFE sutures during ophthalmic surgery is considered an off-label application, possibly because the fibers are too thick and the needles are not adapted to eye microsurgery. Nevertheless, ePTFE sutures are commonly used for IOL scleral fixation and are reportedly well tolerated in short-term studies [13]. Although ePTFE scleral fixation is used clinically, there is a lack of preclinical information regarding the suture behavior within the sclera.

The aim of our research was to determine, using an animal model, the short-term response of the sclera to ePTFE suturing. A parallel study was focused on the immunohistochemical reaction of the sclera to ePTFE [14]. The aim of this study, as the second part of our research, was to determine the tensile force of ePTFE fibers that are not fixed to the sclera by a knot. Novelty lies in time-dependent dynamometric evaluation of knotless ePTFE stabilization.

The study was presented in a video awarded at the 42nd Congress of the European Society of Cataract and Refractive Surgeons in 2024 [15].

Methods

Group

Ten female rabbits aged 12–14 months without eye diseases were used in this study (n = 10). The study was conducted at the Laboratory of Preclinical Studies, Biomedical Center, Faculty of Medicine in Pilsen, Charles University, Czech Republic, in collaboration with the Department of Ophthalmology, Third Faculty of Medicine, Charles University and Královské Vinohrady University Hospital in Prague, Czech Republic. A minimal number of rabbits was used to verify the feasibility of the innovative experiment and minimize the suffering of laboratory animals. The rabbits were maintained under standard laboratory conditions. The management of the rabbits and surgical procedures is described in a related article that focuses on the histological section of the experiment. All efforts were made to minimize animal suffering. The experiments reported here were conducted following the European Union Guidelines for Scientific Experimentation on Animals and performed under the supervision of the Animal Welfare Advisory Committee of the Charles University Faculty of Medicine in Pilsen. This study was approved by the Animal Welfare Advisory Committee of the Ministry of Education, Youth, and Sports of the Czech Republic (approval ID MSMT-15715/2022-4) [14]. The animals were ordered from a certified supplier of laboratory animals, VELAZ s.r.o., Prague.

Surgery

Ten rabbits underwent unilateral lens extraction, followed by implantation of two types of scleral fixation using Gore-Tex CV-8. The animals were premedicated with intramuscular administration of medetomidine (0.25 mg/kg; Narcostart, ProduLab Pharma, The Netherlands) and ketamine (15 mg/kg; Narkamon, Bioveta, Czech Republic) and subsequently intubated with a supraglottic mask (V-gel, Docsinnovent, UK). General anesthesia was induced and maintained by inhalation of isoflurane (0.5%–3.0%; Isoflurin, Vetpharma Animal Health, Spain), while vital functions were continuously monitored [14].

Surgery was always performed in one eye (right). Conjunctival peritomy was performed, after which a main incision and a paracentesis incision were created. The lens was extracted using standard phacoemulsification, with irrigation-aspiration performed as needed.

Gore-Tex CV-8 was used in all the scleral fixation simulations. Two types of scleral fixation were used.

1. The Z-suture technique, where the surgical needle attached to the ePTFE fiber was left outside the eye, whereas the opposite free end of the fiber was threaded into a 27-gauge needle and inserted into the eye ab externo through a scleral tunnel made 2 mm behind the limbus using a needle. Subsequently, two radial zigzag sutures (1.5–2 mm long) were placed in the sclera using a surgical needle (Fig. 1a, b). The fibers were pulled until no loose external loops were visible.

2. The chain-of-knots technique, involving a sequence of approximately 20 simple knots, which was tied to two fibers, creating a loop at one end and two loose fibers at the other. Another ePTFE fiber was threaded with a 27-gauge needle, which was used to create an ab externo scleral tunnel 2 mm behind the limbus. The pre-prepared chain-of-knots was inserted into the eye, and the two loose ends were captured using a loop of fiber inserted through the scleral tunnel and externalized. The fibers were pulled until the knots were visible at the outer opening of the scleral tunnel (Fig. 1c, d).

Fig. 1.

Two types of scleral fixation: Z-Suture (a, b) and chain-of-knots (c, d). a) The free end of the ePTFE fiber was threaded into a 27-gauge needle and inserted into the eye b) Two Z-suture loops were applied intrasclerally from the outside c) The chain-of-knots is inserted into the eye and externalized through the scleral tunnel using a loop in a 27-gauge needle d) Chain-of-knots threaded through the scleral tunnel

Both techniques were performed twice on each operated eye in the four quadrants without intraocular lens implantation.

In both techniques, loose external fibers were trimmed. Finally, all incisions and conjunctival peritomies were sutured.

After the surgical procedure, the operated eye was treated prophylactically with antibiotics and a corticosteroid ointment (Maxitrol) and covered with a sterile patch (Cosmopor E, Hartmann, Germany). Ointments were continuously applied throughout the experimental period. The animals recovered from general anesthesia after receiving atipamezole (1.25 mg/kg; Narcostop, ProduLab Pharma, The Netherlands) intramuscularly and were subcutaneously administered meloxicam (0.4 mg/kg; Metacam, Boehringer Ingelheim, Germany) and tramadol (4 mg/kg; Tramadol Kalceks, Kalcex, Latvia). Subsequently, meloxicam was administered subcutaneously at 24-h intervals for 2 days, and tramadol (0.5 mg/mL) was orally administered at the same time via drinking water [14].

Sample collection

Samples were collected 1, 5, 10, 20, and 30 days post-surgery.

The animals were humanely euthanized by introducing the same general anesthesia as during the experimental surgery (i.e., ketamine + medetomidine) and continuously monitored. Subsequently, a peripheral intravenous catheter was inserted into one of the ear veins, through which pentobarbital (100–200 mg/kg; Exagon, Richterpharma, Austria) and then a 7.45% potassium chloride solution were administered until cardiac arrest was recorded.

Two samples of each type of ePTFE suture (Z-suture, chain-of-knots) were obtained from each operated eye; one sample of each type was fixed in formaldehyde solution and processed for histological examination, and one sample of each type of ePTFE suture was reserved for dynamometry testing. (Fig. 2) These samples were stored in Ringer’s solution, kept at a temperature of 5 °C, and processed within 24 h.

Fig. 2.

Sclera sample with chain-of-knots

Dynamometry

Scleral samples with ePTFE fibers were fixed on custom-designed attachment devices designed using Fusion360 and printed using a 3D printer. The attachment device with fixation surfaces corresponding to the curvature of the sclera simulated the natural orientation of the sclera (Fig. 3). These attachment devices were mounted on a motorized testing bench (horizontal force-measuring bench PCE-HTS 50, PCE Instruments) (Fig. 4). All fibers protruding from the outer surface of the sclera were trimmed. Fibers from the inside of the eye were gripped by either a hook (chain-of-knots) or a clamp (Z-suture) connected to a dynamometer (Mxmoonfree 5 N Digital Force Gauge), with a sensitivity of 0.01 N. The pulling force simulated the direction of traction in the frontal plane. No preload force was applied before testing. The fibers were automatically pulled at a constant speed of 2 mm/s until complete extraction from the sample. The maximum tensile force recorded during the measurement was used for analysis.

Fig. 3.

Details of the sclera sample fixation

Fig. 4.

Dynamometer on motorized testing bench with a clamp holding the fiber protruding from the fixed sclera sample

The measurements were performed at a temperature of 20 °C and a humidity of 50%. Valid dynamometry results were obtained for all samples.

Statistical analysis

Due to the limited number of samples per time point, non-parametric methods were applied. Data are presented as mean ± standard deviation (SD). The relationship between time and tensile strength was assessed using Spearman’s rank correlation coefficient, including 95% confidence intervals.

Differences between suture techniques at individual time points were explored using the Mann–Whitney U test. Given the small sample size, these comparisons should be interpreted as exploratory.

No formal correction for multiple testing was applied due to the pilot nature of the study. Five exploratory Mann–Whitney comparisons were performed, corresponding to comparisons between the two fixation techniques at each postoperative time point.

Results

Ten rabbits underwent unilateral lens extraction, followed by implantation of two types of scleral fixation using Gore-Tex CV-8. The Z-suture and chain-of-knots fixation techniques were used. No adverse post-operative reactions were observed. Tissue samples were obtained 1, 5, 10, 20, and 30 days post-surgery. One sample of each type of ePTFE suture (Z-suture, chain-of-knots) was obtained from each eye for the dynamometry. The tensile force required to disinsert the implanted material from the sclera was quantified using a dynamometer.

Thus, one independent dynamometric measurement per fixation technique was obtained from each animal, without averaging of measurements. Each force value reported in Table 1 therefore represents a single independent measurement from one animal.

Table 1.

Individual dynamometric measurements of tensile force (N) for Z-suture and chain-of-knots fixation

Time from surgery	Rabbit identification	Tensile force for Z-suture (N)	Tensile force for chain-of-knots (N)
Day 1	1a	0.15	0.07
	1b	0.12	0.09
Day 5	5a	0.13	0.11
	5b	0.12	0.09
Day 10	10a	0.22	0.16
	10b	0.32	0.20
Day 20	20a	0.26	0.25
	20b	0.25	0.15
Day 30	30a	0.24	0.16
	30b	0.22	0.27

Each rabbit is marked with the date of sample collection and its rank as of that date (a or b)Each sampling period includes two rabbits (n=2), each contributing one independent measurement for both fixation techniques

The results of the dynamometric measurements are presented in Table 1 and illustrated in Fig. 5. The tensile force of the Z-sutures ranged from 0.12 to 0.32 N, while for the chain-of-knots, the tensile force ranged from 0.07 to 0.27 N.

Fig. 5.

Graph showing the force required to pull an ePTFE suture out of the sclera as a function of sampling time. The plotted values represent individual measurements for rabbits according to Table 1

Tensile strength increased over time for both suture techniques. Spearman correlation analysis demonstrated a positive association between time and tensile strength for the Z-suture (ρ = 0.66, 95% CI [0.05–0.90], p = 0.039) and the chain-of-knots suture (ρ = 0.83, 95% CI [0.40–0.96], p = 0.003).

The Z-suture showed a rapid increase in tensile strength between days 5 and 10, followed by a plateau, whereas the chain-of-knots suture demonstrated a more gradual and continuous increase over time.

Exploratory comparisons between techniques at individual time points did not reveal statistically significant differences (Mann–Whitney U test, all p > 0.05), although the Z-suture tended to show higher values in earlier time points. Given the limited sample size, all statistical findings should be interpreted with caution.

Discussion

Conventionally, scleral fixation with an ePTFE fiber is secured by tying a knot. However, the aim of this study was not to assess the resistance of the knot, but rather the behavior of loose fibers and knots, highlighting how ePTFE adapts to the sclera. The present Z-suture configuration is conceptually related to the knotless intrascleral fixation technique originally described by Szurman et al. [16], although adapted here for experimental biomechanical evaluation. The chain-of-knots technique was originally described by Cendelin et al. [17]

The rabbit was selected as an experimental model because of its ocular response to cataract surgery, which closely resembles that of humans [18]. In addition, the parameters of its sclera are similar to those of humans, particularly the thickness (approximately 0.37–0.57 mm) [19]. From the surgeons’ experience during the experiment, creating a longer scleral tunnel in rabbits was challenging, as the needle often directly passed through the tissue. Meanwhile, in humans, it is easier to increase tunnel length, possibly because of differences in scleral elasticity and thickness at the tunnel site. The scleral thickness decreases toward the posterior pole. However, because of the shorter axial length of the rabbit eye, the scleral tunnel is located closer to the equator, where the sclera is thinner. Moreover, in rabbits, tunnels were created near the site of muscle attachment, where the sclera was assumed to be thinner, as in humans. The possibility of a longer tunnel in humans may provide greater suspension strength using a chain-of-knots. The lengths of the loops in the Z-suture were similar to those that could be used in humans. However, similar scleral thickness does not necessarily imply full biomechanical equivalence.

The lowest scleral fixation strength for both methods was 0.07 N. The shorter scleral tunnel and sample processing could have led to inconsistencies and lower values. Additionally, cleaning the scleral samples and trimming any protruding ends, together with the episcleral tissue, may have partially loosened the suspension.

Adequately determining the amount of force required to hold an IOL in place is challenging. The disinsertion force required to release 1 mm polypropylene (6 − 0) flanges from a 30-gauge tunnel was approximately 2.0 N. The breaking force of 10 − 0 polypropylene suture was approximately 0.5 N [20]. According to Buschmann, the force required to break all the zonular fibers and remove the entire lens from the eye ranges from 0.098 to 0.39 N [21].

Notably, scleral suspension corresponds more closely to the sectorial resistance of the zonule. Pflugk found a tensile force of approximately 4 g in 2 × 6 mm samples, which is equivalent to 0.039 N [22].

The forces acting on the IOLs, which correspond to those exerted by the lens on the suspension, were even smaller. The mechanical forces acting on the IOLs during the saccadic movements did not exceed 0.0008 N. When calculating forces using the lens weight, lens volume, and the acceleration of gravity, an acceleration of > 300G is required to overcome the lowest suspension force in our experiment [23]. Therefore, stabilizing the ePTFE scleral fixations without knotting seems adequate.

Tensile strength increased over time for both suture techniques. The findings indicate a possible temporal trend but, given the limited number of samples at each time point, these observations should be considered exploratory. The findings suggest different temporal patterns of mechanical stabilization: an early strength gain for the Z-suture and a slower but sustained strengthening for the chain-of-knots fixation. These differences may reflect an initial greater influence of mechanical factors and early narrowing of the tunnel at the Z-suture and progressive ingrowth of repair tissue between the loops in the chain-of-knots as shown in our parallel study [14]. The measured resistance likely reflects both biological tissue adaptation and mechanical characteristics of the fixation configuration.

The interaction between PTFE (or ePTFE) and the sclera has been addressed in only a few studies. Tawakol et al. [24] performed ocular surgery in rabbits by applying PTFE strips to buckle the sclera of 14 eyes. Histological examination 1 week post-surgery revealed only a minimal cellular reaction, with no evidence of foreign body giant cells. After 2 weeks, a mild inflammatory cell response was observed around the material, accompanied by increased fibrous tissue formation. By the sixth postoperative week, the fibrous tissue completely enveloped the PTFE band. No adhesions were observed between the PTFE band and the surrounding tissues in all the examined groups. [24]

Furthermore, Korobelnik et al. used ePTFE episcleral implants to encircle scleral buckling in rabbits. Surgery was performed in 12 eyes, with an observation period of 20–128 days. In two eyes, the period of implantation was less than 1 month; in two other eyes, the implant was combined with wide conjunctival erosion, while no significant complications were observed in eight eyes [25].

The histological part of our experiment describes the interaction between the ePTFE suture and the sclera. The ePTFE fibers showed a typical foreign body reaction in the sclera, accompanied by signs of early regeneration. The ingrowth of regenerative tissue into the knot chain and closure of the tunnel around a single ePTFE fiber may contribute to increased stability of the fixation in the sclera [14]. The increasing pull-out force is consistent with tissue adaptation described in the accompanying histological study, but does not directly demonstrate absence of tissue atrophy or necrosis within the present mechanical experiment. The strength of the scleral fixation observed in this experiment suggests that both techniques may provide sufficient early fixation strength in an experimental setting and support further clinical evaluation. Both techniques have also been described in clinical practice, including Z-suture fixation with ePTFE [26] and chain-of-knots techniques [17, 27]. An advantage of both methods is the minimal risk of conjunctival dehiscence, as the fibers are cut at the scleral level and buried.

Study limitations include the small exploratory sample size, cross-sectional study design, inability to fully distinguish inter-animal variability from temporal effects, and biomechanical differences between rabbit and human sclera. An additional limitation is the inter-animal variability observed at later postoperative stages. In particular, the chain-of-knots configuration at day 30 demonstrated divergent tensile values between individual specimens, with one sample exceeding not only its paired specimen but also most earlier measurements for the same technique. This finding may reflect biological variability in tissue healing and remodeling and further emphasizes the exploratory nature of the present pilot study. The absence of a control group using conventional knotted ePTFE can be explained by a completely different method of fixation. This group would illustrate the strength of the knot as opposed to the fixation of the sutures themselves.

In conclusion, an experiment with rabbits was conducted to assess the dynamics of the tensile stabilization of ePTFE scleral fixation without knotting. The aim of this study was to verify the behavior of loose fibers and knots in the sclera. The disinsertion values of the Z-suture and chain-of-knot ranged from 0.07 to 0.32 N, with a tendency to gradually increase. Compared with the human sclera, these values may be influenced by the different properties of the sclera at the tunnel site; however, they still yield values that indicate good stabilization. The observed increase in pull-out force is consistent with tissue adaptation described in the accompanying histological study; these findings further suggest possible temporal differences between fixation techniques. Both methods may provide sufficient early fixation strength in an experimental setting and support further preclinical and clinical evaluation.

Author contributions

Conceptualization: Veronika Hatlova, MD, Jiri Cendelin, MD, PhD; Methodology: Jiri Cendelin, MD, PhD, Veronika Hatlova, MD, Lenka Vankova, MD, Stepan Rusnak, MD, PhD, Pavel Klein, Ing, PhD, Vera Krizkova, MD, PhD; Formal analysis and investigation: Veronika Hatlova, MD, Jiri Cendelin, MD, PhD, Marketa Stefkova, MD, Lenka Hecova, MDWriting - original draft preparation: Veronika Hatlova, MD; Writing - review and editing: Jiri Cendelin, MD, PhD, Lenka Vankova, MD, Vera Krizkova, MD, PhD, Pavel Studeny, MD, PhD; Funding acquisition: Lenka Hecova, MD, Stepan Rusnak, MD, PhD; Supervision: Jiri Cendelin, MD, PhD, Stepan Rusnak, MD, PhD, Vera Krizkova, MD, PhD, Pavel Studeny, MD, PhD.

Funding

This study was supported by the Charles University Cooperation Program, Research Areas MED/DIAG, and Surgical Disciplines. This study was supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project FIND No. CZ.02.1.01/0.0/0.0/16_019/0000787. L.V. was also supported by Charles University SVV – 2025 260 773.

Data availability

The datasets supporting the conclusions of this article are included within the article.

Declarations

Ethics approval and consent to participate

The experiments reported herein were conducted in compliance with the European Union guidelines for scientific experimentation on animals and under the supervision of the Animal Welfare Advisory Committee of the Charles University Faculty of Medicine in Pilsen. This study was approved by the Animal Welfare Advisory Committee of the Ministry of Education, Youth and Sports of the Czech Republic (approval ID MSMT-15715/2022-4).

Consent for publication

Not applicable.

AI use declaration

No artificial intelligence was used in the preparation of this article, except for translation into English and consultation on the statistical analysis.

Competing interests

The authors declare no competing interests.