A Look at Ortho-k and Corneal Epithelium Health
This study investigates the use of
single-fit lenses for ortho-k and how they effect the corneal epithelium.
By Ruiduan Liao, Juanjuan Feng, Yongcong Chen, Jingwen Wang and Wenhui Zhu
orthokeratology (ortho-k) has been widely used to treat myopia in China. We
usually use only subjective refraction and keratometry readings to order
orthokeratology lenses for patients. Typically we do this without a detailed
fitting trial. Our goal is no lens refitting during the procedure. This
procedure was what we wanted to evaluate in this report because many
practitioners have hoped this procedure would work.
Figure 1. An example of
low-grade corneal staining.
Safety of the ocular surface is a great concern
for us. We wanted to know if individual corneal variation would result in lens
position variation or affect corneal shape during treatment. We also wondered if
overnight wear of ortho-k lenses had any side effects on corneal health as
assessed by corneal fluorescein staining.
investigated whether accelerated ortho-k using a single-fit lens for all
patients can reduce myopia and still maintain corneal epithelium health, by
analyzing the relationship between lens positions and corneal staining. We'll
share the details and the results of our investigation.
Up the Study
We performed a retinoscopy
examination in the mydriatic condition, subjective refraction and keratometry
and corneal topography to assess the corneal curvature and eccentricity value
for each of the 147 (51 male, 96 female) young myopic subjects (292 eyes), who
ranged in age from 10 to 30 years. Their subjective refractions ranged from
�1.00D to �9.00D (average in �4.26D) with astigmatism of less than �2.00D
and unaided visual acuities (VAs) from 20/40 to 20/2000. We then trial fit each
patient with diagnostic lenses that had a base curve (BC) limited to 3.50D
flatter than the subject's flat K for an efficient fit estimation.
then fit each subject with optimal four-curve reverse geometry design ortho-k
lenses (with a Dk of 58 and a 10mm diameter). All subjects wore the original
pair of lenses overnight (for eight to 10 hours) for 28 days and removed them
during the day. We measured and examined unaided VA, subjective refraction and
corneal topography and evaluated lens movement, lens position, fluorescein
pattern and corneal staining for each subject the day after we dispensed the
lenses and weekly thereafter.
the effect of ortho-k and the relationship between corneal staining and lens
position mainly according to the results of the first and the 28th days.
2. An example of higher-grade staining.
Examining the Results
the 292 eyes, 60 reached an unaided VA of 20/25+ after one night of wear. On the
28th day of observation, 219 eyes had reached 20/25 or better. We found corneal
staining in 131 treated eyes, among which 69 percent (90 eyes) had tight-fitting
lenses moving less than 1mm. Tight-fitting lenses were more likely to exhibit
staining. The subjective refraction decreased in average from the initial
�4.26D to �2.47D on the first day, and to �0.80D on the 28th day.
We graded corneal staining in our study according to the Schnider system: grade
0 = the absence of staining and shows a situation in which the cornea wets well;
grade 1 = diffuse light staining with no coalescence; grade 2 = light
coalescence but the absence of fluorescein penetration into the deeper layers of
the epithelium; grade 3 = marked coalescence with some deep penetration of
fluorescein into the deeper epithelial layers; grade 4 = complete coalescence
and extensive loss of epithelial cells.
saw a high incidence of corneal staining in our study, most of which was grade 1
(Figure 1) or grade 2 after one night of treatment. On the 28th day, among eyes
with corneal staining 36 percent were of grade 2 or grade 3. We did not observe
any grade 4 staining in our research (See Table 1 and Figure 2).
Incidence of Corneal Staining
|1st Day [eyes (%)]
|28th Day [eyes (%)]
The fluorescein pattern of the ideal lens fit is: base curve (BC) 4mm to 6mm
central touch, fitting curve (FC) 0.5mm to 1mm ring of fluorescein pooling,
alignment curve (AC) 1mm bearing and peripheral curve (PC) 0.2mm to 0.4mm ring
of fluorescein pooling around the edge. The lens centers well and exhibits 1mm
to 2mm of movement with each blink.
lens is considered decentered when it overruns the limbus by more than 0.5mm and
in these cases ring jam may appear on the Placido ring reflections on
The lens fit is tight when it
moves less than 0.5mm. An AC tight fitting lens central zone demonstrates
fluorescein pooling, while in the entire tight-fitting lens the FC and AC
fluorescein patterns are thin or even disappear completely. This may result in
lens imprinting on the cornea. (See Figure 3 for an example fluorescein pattern
of a tight-fitting lens.)
In our study some
lens positions changed during the therapy though they were initially ideal. We
found 79 lenses decentered on the 28th day (Table 2). The lens fit became tight
in 148 eyes. Some were AC tight fitting, while in most cases the entire lens fit
tight with lens fixation, central islands or ring jam (Table 3).
Corneal Staining vs. Lens Position on the 28th Day
|With staining [eyes (%)]
|Without staining [eyes (%)]
Corneal Staining vs. Lens Movement in the 28th Day
|| NORMAL MOVEMENT
|| TOTAL EYES
|With staining [eyes(%)]
|| 90 (61%)
|| 103 (73%)
|| 58 (39%)
accelerated ortho-k using one single-fit lens for each patient, the FC is 6.00D
to 12.00D steeper than BC for the reverse curve. The BC creates a force to
redistribute the epithelial cells from the center to the periphery during the
treatment process, which we believe accelerates the speed of the myopia
In China, ortho-k lens
designers do not observe patients' overall corneal shape directly, but refer
only to the central curve and subjective refraction. However, we believe the
corneal curvature (central and especially peripheral) varies in different races,
regions and individuals, as do lid structures, blink patterns and tear quality,
and these all affect lens position.
when lenses were designed and dispensed without considering these factors, or
the lenses can't be modified or refit as necessary, they could easily become
decentered or tight especially in cases of great reductions (>5.00D) or for
flat peripheral corneas.
We found corneal
staining in more eyes with decentered lenses than in those with centered lenses.
The difference in corneal staining incidence between the normal lens movement
group and the tight-fitting is significant. We speculate this occurs because the
lens fits improperly and/or the Dk of the materials is not high enough.
important in maintaining corneal health is supplying it with enough oxygen. The
tear film is the major source of oxygen for GP lens wearers. GP lenses supply
the cornea with oxygen by:
a. Tears pumped under lens with movement and b. Oxygen transmission through the
Note the bubbles under the BC zone and how the FC and AC widen and
sharpen on this tight-fitting lens.
||Figure 4. A
case of corneal indentation exhibited by corneal topography.
Therefore the exchange of tears under the lens
and the lens position are important in maintaining the corneal oxygen supply.
Research shows that while wearing GP lenses, the equivalent oxygen percentage (EOP)
can or may decline from 15 percent to six percent under closed eyes, which is
lower than the minimal physiologic needs of corneal oxygen supplies (10
In overnight wear therapy, the
oxygen from air is cut off, the secretion of tears decreases and blinks are
absent, leading to corneal hypoxia.
accelerated ortho-k using a single-fit lens for all patients, when the amount of
myopia reduction is large, and the lenses aren't refit according to changes in
corneal curvature during the therapy, the lens can easily change to a
We believe the direct
mechanical impairment from tight-fitting lenses and the toxic simulators of the
metabolic by-products in the hypoxic condition cause corneal lesions. Also,
perhaps even more seriously, tight fitting leads to lens fixation, which
destroys the structure and stability of the tear film. Tear exchange under the
lens is broken off, resulting in corneal edema and epithelium lesions caused by
corneal anoxia, after which corneal staining appears.
5. A case of a warped cornea exhibited by the placido
disk of corneal topographer.
tendency of obvious improved unaided VA and the decrease of the subjective
refraction shows that ortho-k is effective in reducing myopia. However the VA
improvement alone does not mean that the whole process of ortho-k treatment is a
Also, the lenses changed position
and corneal staining occurred during the procedure. Tight lens fitting is
related to corneal staining. We suggest that a fitting trial and refitting are
essential in maintaining a good visual outcome and corneal epithelium health. In
the design of accelerated ortho-k, using a single-fit lens for all, the BC is
4.00D to 5.00D flatter than the flat K. Generally it can reduce 2.00D to 5.00D
of myopia within a period of about two weeks to two months.
is an individual patient procedure. The lens position changes because of
individual corneal variations, and this changes corneal shape during therapy
even though it may have been optimal at first. So a detailed fitting trial and
subsequent refitting are essential in maintaining good visual outcomes and
corneal epithelium health.
recommend the use of a single lens for ortho-k treatment. Whether we should use
a higher Dk materials for overnight wear ortho-k treatment is also a serious
References are available
upon request. To receive references via fax, call (800) 239-4684 and request
document #96. (Have a fax number ready.)
Liao, Feng, Chen, Huang and Zhu are ophthalmologists at the First Affiliated
Hospital of Sun Yat-sen at the University of Medical Sciences in Guangzhou, P.R.
China, where Drs. Liao and Chen are also associate professor.
Contact Lens Spectrum, Issue: August 2003