Intra-corneal rings

Principle:
Barraquer‘ thickness law’: when you add material to the periphery of the cornea or remove an
equal amount of material from the central area, you achieve a resultant flattening effect. Vice
versa, when you add material to the center or remove it from the corneal periphery, you obtain a
steeped surface curvature.
- Intracorneal ring acts as tissue addition leading to a flattening in the cornea periphery.
- The thicker and the smaller diameter the device the higher is the corrective result obtained.
Indications:
1) Myopia:
- low levels (- 1.00 to - 3.00 D spherical equivalent)
- ≤ 1D astigmatism
- ≥ 21 years old
- documented stability of refract ion, as demonstrated by a change of <;0.50 D for at least 12 months
prior to the preoperative examination
2) Astigmatism:
- Progressive keratoconus
Contact lens intolerance
Corneal ectasia post excimer laser
- Corneal irregularities post-radial keratotomy
- Corneal irregularities post-penetrating keratoplasty
- Pellucid marginal degeneration
- Corneal irregularities post-trauma
- Myopia/astigmatism in thin corneas.
-


Contra-indications:
-
-
-
-
Refraction: unstable
Pupil: Patients with pupils >7 mm may be predisposed to low light sensitivity such as glare and
halos and should be appropriately advised
Cornea:
1) Non-clear central zone
2) Central thickness ≤ 480 µm, or peripheral thickness ≤ 570 µm
3) Thickness ≤ 450 µm at incision location
4) Mean K value ≥ 75 D or ≤ 40 D
5) Hydrops
6) Recurrent erosions, corneal dystrophy  predispose to future complications
7) History of herpetic keratitis
Systemic:
1) Collagen diseases
2) Uncontrolled diabetes
3) Severe atopy
4) Local or systemic active infection
Drugs "retard wound healing":
1) Sumatriptan
2) Amiodarone
3) Isotretinoin
Patient:
1) ≤ 21 years old
2) Pregnant or nursing

Rings used:
Material
External
diameter
Internal
diameter
Arc length
Thickness
Intrastromal Corneal Ring segments
"INTACS"
conventional
INTACS SK
PMMA
8.1 mm
7.4 mm
6.77 mm
150̊
0.25 - 0.45 mm
"0.025
increment"
6 mm


Cross section
Refractive
effect
hexagonal
-1 to -4 D
150̊
0.40 mm (for steep
K-value 57-62 D and
cylinder > 5 D)
0.45 mm (for steep
K-value > 62 D and
cylinder > 5 D)
oval
Ferrara ring segments
Keraring
segments
PMMA CQ-acrylic
5.6 mm
5.6 mm
4.4 mm
4.4 mm
160̊
0.15 – 0.35 mm
"0.025 mm increment"
90 - 210̊
0.15 – 0.35 mm
"0.05 mm
increment"
triangular
triangular
(6.0 mm for myopia up
to 7.00 D, 5.0 mm for
higher degrees of
myopia)
NB1: Both, Ferrara and Keraring segments, are triangular. The Prismatic effect, created by the
triangular shape of the ring intending to eliminate the halo phenomena and obfuscation, would be
expected due to the orthesis reduced diameter.
NB2:
- Use symmetric segments when the cone on the posterior float is within the central 3-5 mm optical
zone and the sphere power > cylinder power on the manifest refraction notated in positive cylinder.
- Use the asymmetric segments when the cone on the posterior float is located outside the 3 mm
geometric center and the cylinder power,> the sphere power on the manifest refraction notated in
positive cylinder.

Pre-operative evaluation:
1)
2)
3)
4)
5)
6)
7)
8)
9)

Corneal topography, posterior float
Pachymetry
Spherocylndrical power
Manifest refractive spherical equivalent (MRSE)
Best corrected visual acuity (BCVA) with spectacles
Uncorrected visual acuity (UCVA)
Keratometric (K) values
Slit-lamp examination of the anterior segment
Contact lens tolerance.
Technique:
-
Anesthesia: topical with oral sedation
Mechanical method:
1) Geometric center of the cornea is identified using the 11 mm zone marker and the center is
marked using a Sinskey hook
2) The ‘incision mark’ of the procedure marker is aligned 1 mm from the limbus, at the 9 o’clock
position for right eyes and at the 3 o’clock position for left eyes.
3) A superior-temporal radial 1.0 mm long incision is created to approximately 70% of the cornea
thickness on the steepest axis of the topographic map
NB: To identify the proposed incision location, draw a
“blue” line across the center of the posterior view in
the direction the cone is displaced. Then, draw a “red”
line perpendicular to the “blue" line making a plus sign
or “X” over the posterior view. The blue line
represents the flat meridian,(where the Intacs will be
placed),and the red line represents the steep
meridian,(where the incision will be placed).
4) The intrastromal tunnels are initiated using a pocketing hook (formerly a stromal spreader)
5) A glide blade is introduced into the incision to assess incision length and to verify the adequacy
of the pocket
-
6) The vacuum centering guide (VCG) is placed along with the procedure marker on the corneal
surface. The VCG and procedure marker are aligned with the geometric center of the cornea.
7) The KV 2000 vacuum system is started on the low setting and ramped up to the high setting
8) The Counter clockwise "CCW" & the Clockwise "CW" dissectors (corneal separators) are used to
create the intrastromal tunnels in the desired directions and the Intacs inserts are implanted
into the tunnels.
9) One or two 10-0 nylon sutures or tissue glue may be used to close the radial incision at the
corneal surface
Femtolaser method:
1) The disposable glass lens applanates the cornea to fixate the eye and helps to maintain the
precise distance from the laser head to focal point
2) Parameters: pulse duration 600 femtoseconds, the inner to outer diameter of the Intacs tunnel
set from 6.7 to 8.2 mm. Spot size is 1 µm and the energy is 6 µ J
3) After the original incision is reopened with Sinskey hooks, the Intacs segment is slightly rolled
superiorly to achieve the proper entering angle with the wound.

4) The leading edge of the segment is inserted into the pocket
5) Once half of the segment is in the tunnel, the grasp of the forceps is released and the
remainder of the segment is nudged into the tunnel with a Sinskey hook.
Post-operative care:
After surgery, patients receive antibiotic and corticosteroid eyedrops three times daily for 2 weeks. Patients are
instructed not to rub their eyes following surgery. A plastic shield is recommended at night for several weeks to
avoid eye rubbing.
 Complications:
1) epithelial defects at the keratotomy site
2) anterior and posterior perforations while creating the channel
3) extension of the incision toward the central visual axis or toward the limbus
4) shallow placement and/or uneven placement of the Intacs segments
5) infectious keratitis with the introduction of the epithelial cells into the channel during the channel
dissection
6) asymmetric placement
7) persisting incisional gap
8) decentration, and stromal thinning
9) corneal stromal edema around the incision and channel from surgical manipulation
10) under- or over-correction
11) ↓ corneal sensitivity
12) Neovascularization at incision site
-
-
NB:
One or two Intacs segments: when the keratoconus is peripheral (similar to pellucid marginal
degeneration), not central, it may be preferable to place a single segment instead of 2 segments.
However some doesn't recommend single segment use.
LASIK after Intacs: after ring segments have been removed from patients whose vision did not
improve to a satisfactory level (e.g. due to undercorrection or induced astigmatism), LASIK has been
performed with good success. The flap is created in a plane superficial to the previous ring segment
channel.