NEW TECHNIQUES FOR GORE-TEX SUTURE FIXATION OF IOLS

Yuri McKee, MD, MS

The use of expanded polytetrafluoroethylene (ePTFE; Gore-Tex; WL Gore & Associates) suture material and surgical implants has been instrumental in cardiovascular and general surgery for more than 40 years. Using ePTFE microsuture in ophthalmology for the suture fixation of intraocular lenses (IOLs) is an off-label technique first described by Dr. Kenneth Rosenthal in 1996.¹ Due to the occurrence of suture degradation and breakage in materials such as nylon and polypropylene, more robust suture material is desirable for the permanent suture fixation of IOLs. The ePTFE material confers several desirable features for ophthalmic suture. The material is approximately 50% air, allowing for fibroblasts and leukocytes to invade the material, resulting in an increase in strength over time. Monofilament ePTFE suture is nonabsorbable, does not undergo hydrolysis, and is completely hydrophobic; attributes that promote the long-term stability of the suture. It has one of the lowest coefficients of friction of any material, allowing for minimal tissue disruption as the suture is passed. The expanded version of PTFE allows for needle swedging on a near 1:1 ratio with the thread diameter.² This advantage was initially exploited in cardiovascular surgery to minimize bleeding from needle holes during vessel anastomosis, but is readily adoptable to ophthalmic surgery, especially in transscleral applications. Gore-Tex suture sizing differs from the standard US Pharmacopeia (USP) nomenclature because the diameter of the suture will change depending on the tension placed across it. The smallest Gore-Tex suture diameter available is CV-8, which roughly corresponds to a USP sizing of 7-0 suture diameter when properly tensioned. CV-8 untensioned mean suture diameter is 0.091 mm, while USP 7-0 suture is 0.050 to 0.069 mm. Further complicating this comparison is the fact that ePTFE suture material is somewhat flat as opposed to the round cross-section of traditional monofilament polymer suture. The suture knot-pull tensile strength for CV-8 Gore-Tex suture is 0.3 kg, equivalent to the strength of 6-0 monofilament polypropylene. Thus, monofilament ePTFE suture material has a higher strength to diameter ratio than polypropylene monofilament suture and is equivalent to that of braided polyester suture material.²

Two particular drawbacks exist in the ophthalmic usage of ePTFE suture. First, the usage is off-label, and therefore ophthalmic surgery–specific needle-suture combinations are not available. This deficiency has caused surgeons to improvise new techniques in order to use this suture material. Second, the package insert recommends at least 7 equally tensioned, flat square throws be used to secure the knot on Gore-Tex suture. While mandatory for cardiovascular indications such a chordae tendineae repair, this approach is not feasible in ophthalmic microsurgery. Fortunately, Gore-Tex suture in ophthalmic use is not under nearly the physical strain as when used in cardiovascular applications, so it is not necessary to secure the suture ends with more than 3 throws in this author’s opinion. Despite this, a 3-throw knot of ePTFE material is still much larger than that of 10-0 nylon or 9-0 polypropylene sutures commonly used in ophthalmic surgery. The bulky nature of ePTFE suture knots can pose a challenge in ophthalmic applications.

Improvised techniques in ePTFE suture-secured IOLs often involve removing the needle, which is not readily adaptable to ophthalmic surgery. The ePTFE suture thread can be passed through the sclera using 25-gauge microforceps or by using a 10-0 nylon suture passed through a 30-gauge hypodermic needle to act as a snare. Many ophthalmic surgeons have overcome the limitations of the available needle-suture combinations of CV-8 Gore-Tex suture with ingenious modifications to their technique.3,4

Solving the knot-sized problem has been a more difficult issue. Surgeons have generally used 3 basic approaches. First, the knot may be allowed to remain under the conjunctiva. The ePTFE material is very soft and pliable, and may not erode through conjunctiva as readily as stiffer nylon or polypropylene material. However, the risk of conjunctival erosion is still present, and this approach is probably the least popular due to long-term safety concerns. The second approach is to rotate the knot into the eye. This technique risks breaking the suture, which would require repeating nearly the entire surgical procedure. Additionally, the knot tends to be much larger than the sclerotomy through which it must be passed, making for difficult rotation and causing potential scleral leaks at the suture site. Most recently, the use of a Hoffman pocket has been described to bury the Gore-Tex knot within the sclera.⁵ While this approach is likely the safest, the size of the knot can make this elegant technique more challenging than the traditional approach with 9-0 polypropylene suture.

To solve the knot-size issue, a closer investigation into the chemical properties of PTFE can be instructional. PTFE has a melting point of 327°C. However, the initial decomposition temperature is 200°C. PTFE does not flow when melted, but instead behaves as a gel.⁶ This property is exploitable for our purposes. The aggregate effect of the repeating carbon-fluorine bonds (Figure 51-1)⁶ makes PTFE nonreactive with nearly all chemicals except the highly reactive alkali metals. Thus, the proximate application of relatively low levels of thermal stress is an easy way to modulate PTFE to our advantage. Although PTFE is nontoxic and very stable even at higher temperatures, care must be taken to avoid extreme thermal stress on the material. PTFE begins to rapidly decompose above 350°C. The decomposition byproducts contain several fluorocarbon gases, including hydrogen fluoride. Hydrogen fluoride is corrosive to human tissue and toxic in very low amounts. Hydrogen fluoride readily reacts with water vapor to create hydrofluoric acid, which may settle on ocular tissue causing immediate tissue damage. Although the amount of hydrogen fluoride gas formed would be infinitesimal and tissue damage is highly unlikely, it is still advisable to avoid exposing ePTFE suture to the temperature range of most medical cautery units if in close proximity to ocular tissue.

Exploiting the Thermal Properties of ePTFE

The technique described herein exploits the easily achievable melting point and lack of flow of PTFE. The ePTFE material can be easily melted in the proximity of a battery-powered, hand-held, disposable, low-temperature loop cautery. These loop cautery units are available from a variety of manufacturers in both low and high temperature models. In general, the tip of the low temperature model achieves 600°C to 700°C while the high temperature models achieve 1200°C to 2200°C. The high-temperature cautery is easily identified by the bright glowing red or white-hot filament that reaches extreme temperatures within 1 to 2 seconds. The low-temperature cautery tip maintains a neutral grey or dim red color despite the rapid heating. Even the low-temperature loop cautery is too hot for the safe melting of ePTFE for our purposes, so it is critical that the cautery loop tip be held a small distance away from the ePTFE material and slowly advanced until a reaction is noted in the material. Never allow the tip of any cautery to contact ePTFE or (other polymer material) because even the lowest energy cautery devices easily exceed the decomposition temperature of ePTFE. Upon any visible physical reaction of the ePTFE material, the heat source should be immediately withdrawn. Use of the high-temperature loop tip cautery is too unpredictable and may rapidly cause the breakdown of the inert ePTFE material into dangerous hydrogen fluoride gas. Therefore, only a low-temperature loop cautery held in proximity to ePTFE should be considered for this purpose.

The melting of ePTFE suture is used to seal the perimeter of a 30-gauge needle hole that has been placed near the end of the suture thread. Passing the swedged needle directly through the suture tail to create a lasso of ePTFE will cause the suture material to split under even minimal tension. However, if a 30-gauge needle is used to make a small hole near the terminus of the suture thread and the suture needle is passed through this hole, then the low-temperature cautery can be used to melt the ePTFE material into a gel around the needle. This creates a reinforced buttonhole near the end of the suture thread that can then be used to create a very strong ePTFE lasso that does not require any suture throws to secure the suture thread to an IOL haptic.

Variations of this Gore-Tex lasso technique are demonstrated in Case Studies 1 through 3 of this chapter. Case 1 is a wetlab demonstration of the core technique. First, the tail end of the CV-8 suture is flattened using a needle driver or forceps with tying platforms. Next, a 30-gauge needle is used to make a hole within 1 mm of the end of the suture tail. The suture needle is passed through this hole with no tension placed on the suture. A low-temperature loop cautery is placed 1 cm from the tail of the suture impaled on the needle. The cautery is slowly advanced until the slightest reaction of the ePTFE material is noticed, at which time the cautery tip is quickly withdrawn. The needle of the CV-8 suture is then pulled through the hole in the tail of the suture filament and a closed cinch loop is created with the suture.

In Case 3, this lasso loop is placed around the haptic of a 3-piece foldable IOL to elongate the haptic with the ePTFE material. To ensure that the ePTFE lasso is secured to the end of the polymer haptic, the same low-temperature cautery can be placed in proximity to the terminal haptic end in order to create a thermal-induced bulb or rivet on the haptic.7 Once again, the melting point of most polymers used for IOL haptics is below that of even low-temperature loop cautery, so the cautery should never be allowed to touch the haptic material and should be withdrawn immediately upon noticing a change in the shape of the haptic material. Once the CV-8 lasso is secured to the haptic, the needle can be passed in an intrascleral manner at the same plane and direction as an intrascleral haptic would be directed. Upon exiting the sclera, the needle direction can be reversed and passed parallel and posterior to the original scleral pass. This double passing of the ePTFE suture has proven secure in this author’s hands when the haptic length was insufficient for intrascleral fixation due to a large sulcus diameter. In Case Studies 1 and 3, the haptic material is made of polyvinylidene fluoride (PVDF; Figure 51-2),⁸ a material with similar chemical structure to PTFE. High-temperature decomposition of PVDF will cause the release of hydrogen fluoride gas and should be avoided.⁸

Case 4 explores a final idea demonstrated in the wetlab that exploits the thermal properties of ePTFE. The technique involves placing 4 to 5 holes 0.5 mm apart on the terminal 2 mm of the ePTFE suture. A low-temperature cautery is held in proximity to these holes to seal the edges and confer strength to the edges of the perforations. Once this suture is passed in a transscleral fashion to support an IOL, the tail of suture can be sewn into the sclera in a repetitive manner that imbibes a small bit of ePTFE suture and a small bite of sclera across several passes. This “sewing machine” technique creates tension across the suture and scleral bites that negates the need for multiple square tensioned throws to secure the suture tails in a knot. The resultant closure is flat and does not need to be rotated through a sclerotomy or buried under a flap. Any tension across this closure causes an increase in friction and a progressively more secure closure that will ultimately supersede the knot-pull tensile strength of the suture thread. Practically speaking, forces of this magnitude would never be encountered when using suture to secure an IOL in the absence of capsular support.

In the following case studies, the aforementioned techniques using the thermal properties of ePTFE suture are demonstrated in the wetlab and the surgical suite.

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