TRADITIONAL GP SPHERICAL FITTING GUIDE
Traditionally, GP Lenses have to be fit using the technique of oneye
observation of diagnostic lenses. Diagnostic fitting requires the insertion
of lenses of known material and parameters followed by careful slit
lamp observation of lens movement and position. Once the appropriate
fitting relationship has been established, an over refraction is
performed to determine final lens power. Diagnostic lens fitting allows
the practitioner to observe the actual lens parameters.
The Base Curve
The primary curve on the posterior surface of a GP Lens is the
base curve. This single curve will determine the amount of lens bearing
and lens clearance in the area of the midperipheral cornea. Clinical
experience has taught us that optimum lens performance is achieved with
a base curve that provides:
An area of lens bearing or "contact point" midperipherally
along the horizontal meridian approximately 3.0 to 4.0mm from the center
of the cornea & Unobstructed movement of the lens along the vertical
meridian.
The areas of lens bearing at 3 and 9 o’clock keep the lens into position
along the horizontal meridian to restrict lateral movement allowing
the lens to remain centered on the cornea. Once this is accomplished,
the same base curve should permit unobstructed movement of the lens
along the vertical meridian with the blink. Based on these criteria,
it is easy to see why individuals with 1.00 to 2.00 of withtherule
astigmatism, are often ideal candidates for spherical GP Lenses.
Base Curve Selection
#1 Measure the central corneal curvature and identify
the Flat K
Example:
K’s = 43.00 @180 / 44.75 @ 90 
Flat K = 43.00 D 
#2 Calculate the amount of corneal astigmatism (difference between the
flat and steep keratometric readings)
Example:
K’s = 43.00 @ 180 / 44.75 @ 90 
Corneal Astigmatism = 1.75D 
#3 Select the base curve based on the amount of corneal cylinder and Flat
K:
Corneal Cylinder 
Base Curve 
0.00 to 0.75D 
0.25D Steeper than Flat K 
0.87 to 1.50 
0.50D Steeper 
1.62 to 2.25 
0.75D Steeper 
2.37 to 3.00 
1.00D Steeper 
3.12 & greater 
Toric design 
Base Curve Diagnostic Lens Evaluation
HORIZONTAL RELATIONSHIP
The ideal base curve relationship on a cornea with the "withtherule"
corneal astigmatism should exhibit alignment or slight apical clearance
across the horizontal meridian. This optimum fitting relationship is most
often accomplished with a base curve that is fit using the previous Corneal
Cylinder nomogram.
VERTICAL RELATIONSHIP
The vertical meridian should exhibit slight pooling of fluorescein at
12 and 6 o’clock thereby accomplishing the second fitting objective of
unobstructed lens movement along the vertical meridian.
LENS PERIPHERY
The fluorescein pattern should exhibit 360 degrees of peripheral clearance.
The sole function of the flatter spherical or aspheric periphery is to
provide clearance of the lens as it makes its excursion across the flatter
portions of the cornea with lateral gaze and blinking.
Rule
In the fitting of GP Lenses, it is helpful to
remember two rules as they apply to lens position and movement.
RULE 1A
Spherical lens which aligns or vaults the central cornea, will bear
upon the flattest portion of the midperipheral cornea. Or simply
stated, the lens will be "Tightest" where the cornea
is the "Flattest".
RULE 2A
GP Lens will always move in the direction of least mechanical resistance,
i.e., along the steeper meridian. Or simply stated, the lens will
be "Loosest" where the cornea is the "Steepest".

Low to Moderate AgainsttheRule Astigmatism
As previously outlined, optimum lens performance is best achieved when
a bearing zone is present midperipherally along the horizontal meridian
and the lens is allowed unobstructed movement along the vertical meridian.
In the case of againsttherule astigmatism, a lens fitted with central
alignment, i.e., on Flat K, will result in an area of maximum lens bearing
at 12 and 6 o’clock. This fitting relationship results in obstructed
lens movement along the flatter vertical meridian. The same base curve
relationship along the steeper horizontal meridian does not permit an
adequate bearing relationship at 3 and 9 o’clock. This results in lens
rocking along the horizontal meridian as well as nasal or temporal lens
decentration.
Some patients, with minimal degrees of againsttherule astigmatism,
can be successfully fitted with GP Lenses, by selecting a base curve
radius that is flat enough to permit some vertical lens movement. Horizontal
lens position may occasionally be accomplished by the strength of the
upper lid interaction, which can override the lens tendency to displace
nasal or emporal. Therefore, the anatomical structure of the low to
moderate againstthe rule cornea (2.50D or less) may not be amenable
to the fitting of a spherical GP Lens Lens. In most cases, these individuals
would be better served with toric GP Lenses.
Power Calculation by Over refraction
Perform a spherical refraction over the diagnostic lens to determine
the resultant lens power.
Example:
Diagnostic lens power 
2.00D 
Over refraction 
1.00 sphere 
Final lens power 
3.00D 
Power Calculation by Empirical Formula
Perform a spectacle refraction and place the prescription in minus cylinder
form.
Example:
Spectacle Rx plus cylinder 
7.75 +1.75 x 90 
Spectacle Rx in minus cylinder 
6.00 1.75 x 180 
If the spherical component of the minus cylinder Rx is greater than
+/ 4.00D, correct for vertex distance.
Example:
6.00D. spectacle power is = 
5.50D. at the corneal plane 
Add together the spherical component of the vertex corrected spectacle
Rx and the amount of power derived from the corneal cylinder/base curve
nomogram to obtain the final contact lens prescription.
If the base curve of the lens is fitted Steeper than Flat K, increase
the minus power of the contact lens by the same amount as the tear lens
power.
"SAM" (STEEPER ADD MINUS).
If the base curve of the lens is fitted Flatter than Flat K, decrease
the minus power of the contact lens by the same amount as the tear lens
power. "FAP" (FLATTER ADD PLUS).
Example:
Vertexed Spectacle Rx 
5.5 0D

Base Curve Steepened By 
0.5 0D

Final lens Power (SAM) 
6.0 0D

Determining Overall Lens Diameter
In the fitting of GP Lenses, the parameter most frequently overlooked
by practitioners is the appropriate selection of overall lens diameter.
The optimum lens diameter should be based on the size of the cornea
on which it rests, in that larger diameter corneas, 12.4mm, will require
a larger lens diameter and smaller diameter corneas, 11.2mm, will require
a smaller lens diameter. Corneal diameter can be measured through a
variety of techniques such as a slit lamp measuring ridicule or a hand
held millimeter ruler. In selecting the appropriate lens diameter, a
simple rule of thumb to follow is that the lens diameter should be 2.5mm
smaller than the horizontal visible iris diameter.
If you prefer a quicker method of diameter selection, use the nomorgram
below:
Base Curve 
Recommended Diameter 
39.00 to 43.87 
9.6 Diameter 
44.00 to 45.87 
9.3 
46.00 to 48.37 
9.0 
48.50 to 50.00 
8.7 