A retrospective analysis was made of 300
rigid contact lenses fitted in my practice to patients with
keratoconus, considering in particular, base curve, back optic
diameter, edge lift, sagital height, and their relationship. By
computer analysis of this data a lens set was designed based on
these findings. The New Zealand manufacturer of the lens was the Corneal Lens
Corporation NZ Limited.
Antonio Bassett traveled the world over
20 years ago studied and fitted some 2,700 cases of keratoconus
in Asia, Africa, Europe and India. He was amazed to see that even
in quite advanced cones the lack of scar tissue present, and
realized contact lenses, not keratoconus, caused scarring of the
cornea at the cone apex.
Having specialized in keratoconus over
the last 10 years I was being referred patients who were wearing
extremely ill fitting lenses which were causing irreversible
corneal scar tissue and therefore permanent loss of vision.
However, because of these patients' lowered corneal sensitivity
they were often without symptoms. There were several keratoconus
fitting sets available and some fitted well over a limited base
curve range. However, the practitioner who only fitted a few
cases a year was poorly equipped to know which set should be used
on which cases. I, therefore, decided to attempt to design a
keratoconus set which was useful over all stages of the disease
and could be easily used by any contact lens practitioner who
fitted rigid lenses.
To achieve this I set the following conditions for the set:
To satisfy these conditions for the fitting set, computer analysis of previous fitting showed:
The back optic
zone would decrease as the base curve steepened.
If the lens fitted is
not 8.7 mm then as diameter is increased or decreased,
back optic zone and secondary curve widths are also
increased or decreased in a fixed ratio to produce a
similar fitting pattern with fluorescein on the eye.
To keep pooling at the base of the cone to a minimum, this must decrease as base curve steepens.
To explain why this is necessary: in early keratoconus a large back optic zone can be used and it only produces a shallow acceptable tear pool at the base of the cone. However, if a large back optic zone is used on an advanced cone a deep unacceptable pool is created at the base of the cone,
which traps tears causing anoxia
and subsequent corneal edema resulting in epithelial loss
at the cone apex, discomfort and poorer acuity.
If however, a small optic zone is used on the same
advanced cornea, the pooling is significantly reduced.
The back optic zone has been set
at a maximum to provide the largest optic area
"allowable" at any specific base. In existing
sets a small optic zone was set over their entire base
curve range, therefore penalizing patients with early
keratoconus by providing inferior vision.
By maximizing the optic zone
this provides maximum good stable vision to any base.
However, this does cause a shallow pool at the cone base
which in my experience is acceptable in most instances
and does not compromise the corneal integrity.
If, however, in some cases where
the cone is small and steep, a deeper than acceptable
pool is created, then by reducing the overall diameter of
the lens the optic zone automatically decreases, hence
reducing the pooling.
Optimum edge lift
produces a fluorescein ring under the edge 0.5 to 0.7 mm
wide. Standard lift will fit 70% of Rose K lens fittings.
If, however, edge lift is inadequate (in approximately
20% of the time), then increased edge lift must be used.
This lens will then have secondary curves 1.0 mm flatter
than the standard lift. Because this reduces the
effective sag of the lens, the base curve is steepened
and the power increased to compensate for this loss. The
lab automatically makes these calculations.
If edge lift is
excessive(approximately 10% of the time), then decreased
lift must be used. This lens will have secondary curves
0.5 mm steeper than the standard lift lens. Again, the
lab will calculate changes in sag and power to compensate
for the effective sag increase.
Keratometer readings are
of little value in assessing whether a keratoconic cornea
will require a toric back surface as a large cylinder
measured by the keratometer (central 3 mm ) may not
extend outside of the cone. The only method of evaluation
is by fluorescein pattern.
If in the horizontal meridian
the edge lift is barely adequate, but in the vertical
meridian edge lift is excessive, then you may try:
e.g.: Best fit - 7.0
Toric lens required 7.3/6.7
To determine the power required, use a standard lens of the flattest meridian (i.e., 7.3 in the above example).
The Rose K Lens has been specifically
designed to be easy to fit. It offers a choice of three different
secondary curve combinations producing three different edge
lifts. The design can be made in a spherical back surface, a full
back surface toric, or a toric periphery only.
It is manufactured in quality materials
and in any overall diameter, but the relationship of the back
optic zone to the base curve and overall diameter is fixed. The
lens is designed to produce good tear exchange underneath the
lens, to keep overall thickness minimal and to maximize the optic
area for any base curve.
The
Rose K Lens
Lens design by : Mr Paul Rose, Optometrist.
Lens manufacturer : Corneal Lens Corporation N.Z. Ltd.
The Rose Keratoconus lens uses a decreasing BCOD as the base curve steepens. The ratio is set but is dependent on base curve and diameter. However there are three degrees of edge lift that can be ordered - standard, increased and decreased.
Because this lens design follows more
closely the corneal shape, the Rose K lens provides excellent
vision and comfort. The power of the trial set increases as the
base steepens, allowing more accurate power assessment.
The Rose K Set
| Base curve : | 5.3 to 7.8 (LARGE) 5.8 to 7.6 (SMALL) |
| Power : | Any |
| Centre thickness : | Minimum |
| Lens material : | A choice of quality materials. |
| Edge lift : | Standard, increased or decreased. |
For more details in the UK contact:
NOVA CONTACT LENSES
Unit 11 Avebury Court,
Mark Road
Hemel Hempstead,
Herts
HP2 7TA
ENGLAND
Tel:- 44 (0)1442-253456
Fax:- 44 (0)1442-253301
Email:- nova-cl@dircon.co.uk
Web:- novacontactlenses.com