The McCannel and Seipser knots allow surgeons to place sutures in the tight confines of the anterior segment of the eye.
23,24 While these techniques have been invaluable in allowing iris fixation of IOLs and iris repair, suturing in the closed chamber eye is both time consuming and difficult.
25 The injectable shape memory suture described in this report allows these procedures to be done quickly and easily, and marks a departure from conventional intraocular suture techniques. In the present study we describe the short-term biocompatibility of a nitinol clip specifically designed for joining intraocular structures. The clip was engineered to simplify the techniques and shorten the surgical time of intraocular suturing. Our study demonstrated that the placement of the clip in the iris did not have any adverse effects on the eye, specifically the corneal endothelium and retina. The corneal thickness, corneal endothelial cell count and morphology, cellular layers of the retina, and electroretinogram were statistically similar to the control eyes receiving polypropylene (Prolene) suture. Further, the surgical time was significantly faster for the deployment of the shape memory clip.
Surgical options to correct aphakia or to treat patients without adequate capsular support include anterior chamber IOL, a scleral-fixated IOL or an iris-fixated IOL.
26,27 Anterior chamber IOL is the simplest surgical procedure to correct aphakia; however despite its open-loop modern design, they are commonly associated with complications such as corneal endothelial loss leading to corneal decompensation, iris sphincter erosion, glaucoma, chronic inflammation, and hyphema.
27 Some eyes may require a pupilloplasty to support an anterior chamber intraocular lens.
28 Suturing an IOL offers a more anatomically correct final position, by placing the IOL nearer to the nodal pole of the eye.
29 Nevertheless, either scleral or iris-fixated IOL techniques have potential disadvantages. Although severe late complications like retinal detachment, choroidal effusion, and hemorrhage have been described with both techniques, their incidence is quite low.
13 The more frequent complications are IOL decentration, tilting, slippage, and subluxation.
30 These late complications usually occur 2 to 5 years after the implantation, and are often related to suture slippage or failure.
13,29,30 Further, the amount of time required to tie a suture knot in the anterior segment of the eye can take an experienced surgeon 15 minutes or more. In comparison, the device and clip discussed in these experiments can be deployed in approximately 1 minute in an in vivo surgical setting.
There is no consensus about why the intraocular sutures fail in the long-term. Theories about suture biodegradation, ultraviolet light photodegradation, erosion of the knot, and rupture or cutting by the sharp edge of the positioning hole have been theorized.
29 Experts in the field have urged the development of new suture materials and techniques to avoid such complications. This is especially relevant for iris-fixated IOLs, in where clinicopathological studies have demonstrated that IOL haptics are frequently situated outside the ciliary sulcus, therefore proper long-term positioning relies largely on the integrity of the fixation sutures.
29
The potential use of clips for intraocular fixation has been suggested before. Tzu et al.
31 used titanium clips for vascular repair and cardiovascular procedures (AnastoClip VCS, LeMaitre Vascular Inc., Burlington, MA) to clip an intraocular lens to the iris. However, the device used in their study was not designed for its intraocular use, and could only be used with an “open sky” approach, involving the removal of the central cornea as during penetrating keratoplasty. The cartridge was very wide (3 mm) and each clip measures 0.9 mm in width. Our 30-gauge device allows a closed chamber approach through microincisions, making it more suitable for ophthalmological surgeries.
The clip used in this study is composed of nitinol, which is a metal alloy which has become widely used across many medical fields. Because it is a nickel-based alloy, there are concerns about the dissolution of nickel ions in the body and the possibility of inducing allergic, toxic, and carcinogenic effects.
32 –34
Nonetheless, modern manufacturing methods allow minimizing the risk by using surface coatings and surface modifications to improve biocompatibility.
35 The major concern with a nitinol intraocular implant is corrosion.
36 There are several methods to prevent this from occurring and involve forming a stable protective layer over the nitinol. This may be accomplished by chemical or electrochemical polishing, surface laser treatment, ion implantation titanium, or adding bioactive surfaces such as plasma fibronectin.
36,37 One of the most efficient ways to stabilize the alloy is by thermally oxidizing the material, to create a solid layer of titanium oxide TiO
2 on the surface.
38,39 This can be accomplished by simple autoclaving, which was done for the implants used in this study. Studies have shown that this effect can be further augmented by surface passivation with electropolishing.
38
Nitinol's superelasticity and shape memory make it useful for adjoining tissue. The long-term effectiveness of a nitinol clip for vascular anastomosis and arteriotomy repair has been demonstrated in several studies and its biocompatibility validated. The healing and pathologic response in such cases has been proven to be very well tolerated, as the clips were fully covered with an endothelial layer, had minimal inflammatory response, and an absence of tissue necrosis. Furthermore, the tensile strength of the alloy is 38 times the tensile strength of comparable polypropylene (Prolene) or 10-0 nylon suture.
16,17
The study has a few limitations that we would like to address. The small number of animals in the study affects the variability of the results. The absence of baseline measurements of specular microscopy and anterior chamber OCT diminishes our capability of analysis of changes over time. However, the study does evaluate the effect of nitinol compared with 10-0 polypropylene (Prolene) suture as a control.
In summary, our study demonstrated that nitinol is well tolerated in the eye and showed no toxicity in the short-term, even in eyes injected with multiple clips. Seventy days after the initial surgery, the animals exhibited no anatomic or physiologic changes in the cornea or retina. Additional studies are under way to further characterize the surgical utility and biocompatibility of shape memory alloy clips in ophthalmic surgery.
Supported by The Colorado BioScience Discovery Grant (JO).
The authors thank the Rocky Mountain Lions Eye Bank for their help in processing and handling tissue and Christopher McGriff who helped with the OCT images acquisition.