INTRODUCTION

One of the possibilities for surgical correction of presbyopia is intracorneal implants (corneal inlays). These devices are implanted in the thickness of the cornea in the non-dominant eye and through different mechanisms of action improve near visual acuity, with only a slight decrease in visual acuity for far in the eye with implants. There are currently three types of implants with different mechanisms of action¹:

A. KAMRA (Acufocus), which increases the depth of focus;

B. FLEXIVUE (Presbia), which is a positive lens with a small central hole;

C. RAINDROP (Revision Optics), which increases the anterior curvature of the cornea.

In this chapter the Inlay Kamra will be presented.

KAMRA INLAY

Characteristics of the Kamra Inlay

The Kamra Inlay is a 3.8 mm diameter polyvinylidene fluoride disc with a center hole of 1.6 mm (Figure 1). Its thickness is only 5 microns and its color is black. It features a glossy face and a matte face. It has 8400 small holes randomly distributed to facilitate corneal metabolism.

Mechanism of action

The Kamra Inlay causes an increased depth of focus, enabling an improvement in visual acuity for near, minimally affecting distance visual acuity.

PATIENT SELECTION

Candidates for surgery with the Kamra implant are those presbyopic patients with refraction between -5.00 and +3.00.

Preoperative study

A complete ophthalmologic study should be performed. Exclusion criteria are any corneal pathology, especially any ectopic pathology, as well as presence of cataracts, glaucoma or retinal pathology.

Central corneal thickness must never be less than 500 microns. Special attention should be given to the study of the ocular surface. As it is known, in this age group dry eye has a high prevalence, and all appropriate measures must be taken to alleviate this problem before surgery.

As already mentioned, the Kamra Inlay is implanted in the non-dominant eye, so it is essential to define before the surgery the dominant eye. For this there are 2 tests:

1. Motor test: In this test the patient is asked to fix an object with both eyes open through a hole. Then each eye is covered alternately. If the image remains inside the hole the patient is using the dominant eye, otherwise the non-dominant will be used.
2. Sensory test: In this test, the refraction of the patient is placed in the test frame. Then a +1.50 sphere is added alternately in one eye and another. The eye in which this addition causes more discomfort is the dominant eye.

Sometimes the two tests are coincident which translates to a strong dominance of one eye. If the tests do not coincide, the sensorial test is valued more.

As will be seen later, in the surgical technique, the Kamra Inlay should be centered with the visual axis. Therefore, in the preoperative study, it is necessary to determine the visual axis and its relationship with the center of the pupil. For this, the ACUTARGET (SMI) is used, which gives the relative position of the center of the pupil and the first image of Purkinje. If the difference between the two images is less than 300 microns, the Kamra Inlay can be centered with the center of the pupil, otherwise the implant should be centered at an intermediate point between the two images.

SURGICAL TECHNIQUE

The Kamra Inlay is implanted in the cornea of ​​the non-dominant eye at a depth never lower than 200 microns. There are three possible surgical techniques, all needing a Femtosecond Laser.

Pocket Emmetropic Kamra (PEK)

As the name implies, this technique does not present any correction of ametropia and can only be used in patients with emmetropia or mild myopia (<-0.75). In this technique a pocket at 200 microns of depth is created with the Femtosecond Laser, usually with a temporal opening. After dissection of the pocket, the Kamra Inlay is inserted with the matte face facing upwards.

Concomitant Lasik Kamra (CLK) or SIM-Lasik

This technique is used when in addition to presbyopia there is ametropia that needs correcting. The limits of the correction may be those usually used in Lasik, but in these cases due to the greater thickness of the flap (200 microns) the correction is restricted between -5.00 and +3.00 D. After marking the visual axis, we perform a 200-micron flap with the Femtosecond Laser. The desired ablation is performed with the excimer laser, and the surgery is terminated by placing the Kamra Inlay under the flap, with the matte face up and centered with the prior marking. Note that the target of ablation should not be emmetropia, but -0.75.

Post Lasik Kamra (PLK)

This technique was initially developed for patients who had previously undergone Lasik³. A pocket is made at 200 microns deep, just as in PEK, and the Kamra Inlay is implanted.

As there were better results in the time of recovery and incidence of dry eye in the PEK compared to the CLK, it was suggested to use the PLK technique for presbyopic patients with ametropia. Thus, in these cases a flap of 90 to 110 microns is created with the Femtosecond Laser (classic LASIK flap), the ablation is performed with the excimer laser (target -0.75), and a pocket is created at a depth never less than 100 microns below the ablation plane. The flap and pocket can be performed in the same surgical time, or preferably in 2 times (between 8 and 30 days).

POST-OPERATIVE REGIMEN

The postoperative regimen following Kamra Inlay implantation is as follows:

1. Antibiotic eye drops (4x/day) for 8 days;
2. Corticoid eye drops (fluorometholone) 4x/day for 8 days, 3x/day during the 1^st month, 2x/day during the 2^nd month and 1x/day during the 3^rd month;
3. Artificial tears at least 3x/day for 3 months. Some patients require artificial tears for a lifetime.

This is the typical postoperative regimen. It can be changed in case of complications (see respective section).

RESULTS

The results will be analyzed in three aspects:

1. Distance binocular visual acuity: As the Kamra Inlay is only implanted in the non-dominant eye, binocular acuity for far is typically 10/10 and is not affected by surgery;
2. Visual acuity for far from the implanted eye: In most cases visual acuity without correction of the implanted eye varies between 7/10 and 10/10 the day after surgery. At 3 months, most of the implanted eyes reach 10/10 (Figure 3), as can be seen from the analysis of the first 50 cases.
3. Near binocular visual acuity without correction: After Kamra Inlay implantation, 92% of patients achieve near visual acuities of J2 or better (Figure 4). Typically, these patients read comfortably J2 reaching with difficulty J1.5 or even J1^4,5.

COMPLICATIONS

Intraoperative complications

Intra-operative complications related to the creation of flaps and/or pockets are rare and dependent on good calibration and handling of the Femtosecond Laser. Since they have no relation to the Kamra implant, they will not be discussed here. It should be added, however, that in case of intraoperative complications one should abort the surgery, not implanting the Kamra Inlay.

Postoperative complications

The main postoperative complications are as follows:

1. Dry eye: Dry eye syndrome is the most frequent complication. In reality, in many cases, it is not a complication, but an exacerbation of a pre-existing condition. In the age group of presbyopia, and markedly in females (greater number of candidates for the Kamra implant), dry eye has a high prevalence. The symptomatology is aggravated with the corneal surgery, namely with creation of flaps (especially deep). It is therefore necessary to combat the dry eye with the frequent use of artificial tears and to try to reduce the surgical aggression of the ocular surface preferring PEK and PLK in detriment of CLK.
2. Myopic Shift: Sometimes a few months after surgery patients report low visual acuity for distance. There is a myopic refractive error. The cause of this disturbance is once again the dry eye. The treatment is carried out by instituting more aggressive therapy with artificial tears.
3. Hyperopic shift: In this situation there is a low visual acuity for near. In more severe cases, a stromal haze may be seen around the implant. The treatment consists of topical corticosteroid therapy for at least 1 month. In most cases the problem is solved. However, if the haze persists or there is relapse, explanting the inlay is advised.

SATISFACTION

Patient satisfaction

The patient satisfaction rate is reasonable, with 1/3 of the patients (33%) very satisfied, and 47% moderately satisfied. This last group of patients will become more dissatisfied over time. The remaining 20% ​​were not satisfied despite uncomplicated surgery (Figure 5).

Causes of dissatisfaction

1. Inlay decentration. In the cases where the inlay is decentered, there is difficulty in seeing at distance and near and sometimes the sensation of a monocular "diplopia". A test should be performed with Acutarget and in case of a decentration, proceed to re-center the inlay⁶.
2. Inability to read quickly. Some patients, while achieving good visual acuity at close range, find reading at normal speed difficult. In these cases, good lubrication of the ocular surface and reading training is advised⁷.
3. No adaptation to monovision. Although the Kamra Inlay slightly impairs distance visual acuity from the implanted eye, there is an important component of monovision. Some patients do not stand it.
4. Photic phenomena. Some patients, although with good visual acuities for far and near, complain of halos and glare particularly while driving at night. Faced with a case of dissatisfaction, the cause should be investigated and, if possible, resolved. The patient should be advised to wait 3 months because some unpleasant symptoms tend to improve over time. If this is not the case, the explantation of the Inlay must be performed.

EXPLANTATION OF THE INLAY

The explantation of a Kamra Inlay should be performed in the following situations:

1. In the presence of a stromal reaction (haze) that does not respond to corticoid treatment or relapses after treatment;
2. In case of patient dissatisfaction.

The explantation is carried out without difficulty opening the pocket or raising the flap. If done up to 6 months after the implant, no signs of the inlay are observed in the cornea. If the removal is delayed, an imprint of the inlay is observed in the cornea, which takes a few months to disappear⁸.

CONCLUSIONS

As conclusions and take-home messages about Kamra Inlay, the following comments are highlighted:

1. Kamra Inlay allows you to improve vision by increasing the depth of focus, without significantly compromising the vision from afar;
2. The target patients are emmetropic presbytes or those patients with ametropia compatible with excimer laser treatment (-5.00 to +3.00 D);
3. The implantation is done in the non-dominant eye in a pocket (preferably) or under a flap at a depth never inferior to 200 microns;
4. The typical visual result is distance visual acuity in the implanted eye between 9/10 and 10/10 and binocular near visual acuity of J2;
5. Complications are uncommon and usually avoidable following the postoperative protocol;
6. Full satisfaction of the patients is obtained in about ⅓ of the cases;
7. Intolerance to monovision, photic phenomena and slowness in reading are the main causes of dissatisfaction;
8. Explantation is easy and, if necessary, should be done within 6 months after implantation.

In conclusion, Kamra Inlay is considered a possible method in the treatment of presbyopia, but it is not yet a method of total and definitive efficacy.

REFERENCES

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