AnnMarie Hipsley, PhD, DPT; David H. K. Ma, MD, PhD; Karolinne M. Rocha, MD, PhD; and Brad Hall, PhD
There are several available treatment options for presbyopes including spectacles, contact lenses, corneal surgery, or nonaccommodative intraocular lenses. Spectacles and contact lenses are the prevailing treatments.1 However, none of these options aim to restore true physiological dynamic accommodation. While these options are effective for treating the symptoms of presbyopia, there remains a need for a procedure that can restore dynamic accommodation and potentially rejuvenate ocular rigidity that occurs with age that may impact other physiological functions of the eye. Laser scleral ablation has the potential to fill this need.
Excimer lasers emit radiation in the far ultraviolet light spectra (0.19 to 0.35 μm), and have been used successfully in refractive surgery to correct refractive errors of the eye for almost 3 decades.2,3 Lasers in this wavelength allow precise removal of corneal tissue by means of photochemical laser tissue interaction breaking molecular bonds in the tissue and reshaping the cornea.4 Mid-infrared lasers (0.7 to 1000 μm) are frequently used in clinical applications because they remove tissue with less thermal damage to surrounding tissues.5–7 Near infrared lasers, such as erbium: yttrium-aluminum-garnet (Er:YAG) lasers, have several advantages over other mid-infrared lasers. Er:YAG lasers are solid-state lasers, using an erbium-doped yttrium-aluminum-garnet as the medium, and typically emit mid-infrared light with a wavelength of 2.94 μm (Figure 14-1). This wavelength is strongly absorbed by water as it coincides with the peak absorption of water (3.00 μm), giving it a unique clinical application that is more photo-mechanical than photothermal.8 When an Er:YAG laser beam is well absorbed at the target, it can cause rapid vaporization, or ablation, in both hard and soft tissues. Here we demonstrate a novel, minimally invasive, nonablative, laser eye therapy that is based on thermally inducing a microporation of the sclera in a mathematical matrix array using an Er:YAG laser. The Er:YAG laser wavelength of 2.94 μm is strongly absorbed in water, which is the major constituent of human soft tissue. A precisely controlled sequence(s) of subablative Er:YAG laser pulses are delivered to scleral tissue in order to achieve controlled heating of the collagen in the deeper microfibril layers, without overheating the tissue. Therefore, tissue removal is very precise with virtually no collateral thermal damage due to the coincidence of Er:YAG wavelength to the absorption peak of water. All soft tissue, because of its high water content, is ablated very efficiently.8 For this reason, pulsed Er:YAG lasers have been used safely and successfully since the early 1990s in many medical laser surgery applications, such as cosmetic, dermatology, urology, and dental laser surgery.5–7 Er:YAG lasers (2.94 μm) have also been the preferred wavelength in the spectrum over CO2 lasers and other YAG lasers in various wavelengths because it is absorbed almost 10 million times more than visible light wavelengths, and 10 thousand times more than by the output of neodymium:YAG lasers decreasing thermal damage significantly (see Figure 14-1). Controlling thermal damage is of clinical significance when utilizing lasers as a treatment method in soft tissue, where an increased thermal damage zone has a known effect on wound healing, which imposes subsequent safety, efficacy, and stability implications.
Er = erbium; Ho = holmium; Nd = neodymium; Tm = thulium; YAG = yttrium-aluminum-garnet
Adapted from Welch AJ, Van Gemert MJ. Optical-Thermal Response of Laser-Irradiated Tissue. New York, NY: Plenum Press; 1995; Bashkatov A, Genina E, Kochubey V, Tuchin V. Optical properties of human sclera in spectral range 370–2500 nm. Optics and Spectroscopy. 2010;109(2):197-204.
A unique safety feature of the Er:YAG is that it has the lowest thermal diffusion than any other laser in the mid-infrared spectrum. This is beneficial because a low thermal diffusion time and a shorter laser pulse is needed for the thermal energy from the laser to propagate into scleral tissue.9 Table 14-1 compares the absorption, penetration, and thermal diffusion time in the sclera for 5 types of lasers. When compared to other wavelengths, Er:YAG has the highest absorption by 2 orders of magnitude. Additionally, the light penetration depth of Er:YAG is the lowest by 1 to 3 orders of magnitude. Compared to Er:YAG, Holmium:YAG and Thulium:YAG have 2 orders of magnitude higher penetration and 2 orders of magnitude lower absorption. The high absorption and low penetration are a unique safety feature of using the Er:YAG laser in the eye. Furthermore, this particular Er:YAG wavelength has very low spectral properties and therefore presents very low risk of light scatter of the laser beam to other tissues of the eye.
Laser scleral ablation procedures to treat presbyopia began as an improvement to a procedure known as anterior ciliary sclerotomy (ACS). ACS involved radial or spoke incisions, with a knife or radiofrequency blade, through sclera overlaying the ciliary muscle.10 The ACS procedure aimed to increase the space between the ciliary muscle and the lens, tightening the zonules and restoring dynamic accommodation.11 Accommodation was observed to improve slightly with ACS; however, long-term results suggest that the procedure was unsuccessful at restoring accommodation.11 Lin and colleagues argued that the rapid wound healing of the sclera following ACS was responsible for the poor long-term results, and proposed instead to use Er:YAG laser ablation in the sclera (radial sclerectomy).12,13 This procedure was termed laser presbyopia reversal (LAPR), and the results of LAPR were largely mixed. Both ACS and LAPR are no longer available, however, there were compelling results of effectiveness that still left unanswered questions about how scleral ablative therapies were affecting near and intermediate vision with a measurable effect.11–13
RESTORATION OF DYNAMIC ACCOMMODATION
The loss of dynamic accommodation with age is complex and not yet fully understood. Von Helmholtz14 argued that the reduction of accommodative ability in presbyopes was caused by the loss of elasticity of the lens substance. Conversely, Schachar argued that accommodative ability declines because of a decreasing gap between the lens perimeter and the ciliary ring with age.15 Recent evidence has also highlighted many extralenticular factors (primarily the zonules, choroid, and sclera) that influence the loss of accommodative ability with age.16,17 Using a computer-animated model, Goldberg also demonstrated the influence of extralenticular factors, specifically that the sclera moves during accommodation.18 Additionally, ocular rigidity has been correlated with a clinically significant loss of accommodation with age.19 Goldberg18 further demonstrated in his model the relationship between the forward and centripetal movement of the ciliary muscles, which is proportional to changes in the central optical power of the crystalline lens. Lenticular accommodation is also proportional to the change in distance between the ora serrata and the sclera spur landmarks of the ciliary muscle attachments.18,20–22
Based on these recently illuminated biomechanical factors in accommodation loss,18,21–24 Laser Anterior Ciliary Excision (LaserACE [Ace Vision Group]) is an eye laser therapy designed to reduce ocular rigidity and create compliance in the scleral tissue using a laser-generated matrix of microporations (micropores). As the connective tissues in the eye age, the collagen and elastin within continuously crosslink to form fibrils and microfibrils, which increases scleral stiffness.25,26 A rigid sclera compresses and exerts stress on underlying structures, leading to their biomechanical dysfunction, and specifically structures related to accommodation.27–30 LaserACE produces an uncrosslinking effect in scleral tissue, alleviating stress over key physiological anatomy that lies directly beneath the aging scleral tissue, such as the ciliary muscle and the accommodation complex. The procedure utilizes an Er:YAG laser (VisioLite) to create microporations in 3 critical zones (Figure 14-2) without touching any components or relative tissues of the cornea. The 3 zones are as follows18,21–24:
- The scleral spur at the origin of the ciliary muscle (0.5 to 1.1 mm from the anatomical limbus)
- The mid ciliary muscle body (1.1 to 4.9 mm from the anatomical limbus)
- Insertion of the longitudinal muscle fibers of the ciliary, just anterior to the ora serrata at the insertion zone of the posterior vitreous zonules (4.9 to 5.5 mm from the anatomical limbus)
The procedure uses a laser frequency of 10 to 30 Hz, a laser fluence of 30 to 50 mJ/cm2, and a spot size of 600 μm. Microporations are placed in a 5.0-mm x 5.0-mm matrix pattern in 4 oblique quadrants of the eye, using a fiber handpiece and near-contact 80-degree curved tip (see Figure 14-2). Each micropore has a depth to about 90% the depth of the sclera, the point that the blue hue of the choroid is just visible.
An overview of the LaserACE surgical procedure is shown in Figure 14-3. The surgery is performed bilaterally on the same day, with each eye taking approximately 10 to 15 minutes to complete. Prior to the procedure, topical antibiotics and anesthetics are used, as well as orally administered benzodiazepines. To protect the cornea, an opaque corneal shield is placed on the cornea for the duration of the procedure.
A collagen matrix powder (Collawound, Collamatrix) is applied directly over the scleral ablation matrices with a cannula. This degradable collagen matrix temporarily fills the microporations, preventing fibrosis. An 18.0-mm plano bandage scleral contact lens (methafilcon A) is used postoperatively to cover the ablation zones and hold the collagen in place. Topical antibiotics and steroids are used are used for 1 week postoperatively, followed by a steroid taper for 2 weeks.
In 2016, Ace Vision Group completed Institutional Review Board-approved phase III clinical trials, in Taiwan at the Chang Gung Memorial Hospital, investigating the visual performance of 26 patients up to 24 months postoperatively after LaserACE treatment. Uncorrected and corrected distance visual acuities were measured at near, intermediate, and distance (40 cm, 60 cm, 4 m) using standard Early Treatment Diabetic Retinopathy Study charts. Patient-reported satisfaction and patient-reported visual function were investigated using the Catquest 9SF Survey.31 Additionally, intraocular pressure (IOP) was measured using a pneumatic tonometer, and stereoacuity was measured using the Randot stereoscopic test.