Spectacle Skills CH15

Question 1

A patient who had an eye exam last week comes into the office upset. He shows you
two prescriptions for glasses: the one your doctor issued last week and another from
an out-of-town doctor dated 2 months ago. “One of these has got to be wrong,” he
says. “Look how different they are!”

Prescription #1:
OD: –2.50 – 1.75 × 087
OS: –1.75 – 2.25 × 112

Prescription #2:
OD: –4.25 + 1.75 × 177
OS: –4.00 + 2.25 × 022

Your response to the patient would be to explain:
a) refraction is subjective and he probably gave different responses the last time
b) the other office’s prescription is incorrect
c) the prescriptions are merely written in different formats
d) the need for another appointment to recheck the measurement

Answer 1

c) The prescriptions have merely been transposed. Be sure to keep your explanation in terms that the patient can understand. You probably need not demonstrate the process, but could just tell the patient that the two offices just use different lens formats.

Question 2

Transposition applies to which of the following lenses?
a) spherical
b) spherocylindrical
c) prismatic
d) bifocal/trifocal add

Answer 2

b) A spherocylindrical lens combines a sphere and cylinder to correct for astigmatism. A
spherical lens does not need to be transposed. Prism or the segments of multifocal lenses are likewise unaffected by transposition.

Question 3

Transposition applies to which of the following refractive errors?
a) mixed astigmatism
b) simple myopia
c) simple hyperopia
d) mixed emmetropia

Answer 3

a) Patients with astigmatism (of any type: myopic, hyperopic, or mixed) have cylinder in
their prescription. Simple myopia and hyperopia are spherical errors, without any astigmatic/cylindrical correction. There is no such thing as mixed emmetropia; emmetropia itself is the lack of a refractive error.

Question 4

Transposition is used to:
a) convert prism so it is equal between the two eyes
b) give the best correction possible without using cylinder
c) calculate the power of an intraocular lens implant
d) convert plus cylinder to minus cylinder

Answer 4

d) Transposition is the mathematical formula used to change plus to minus cylinder format
and vice versa. (By the way, answer b actually involves using spherical equivalent, another
optical formula, but in this case the incorrect one.)

Question 5

Transposition is accomplished by:
a) adding axis and cylinder, keeping the sphere the same
b) adding sphere and cylinder, changing the cylinder sign, and keeping the axis the same
c) adding sphere and cylinder, and changing the axis 90 degrees
d) adding sphere and cylinder, changing the cylinder sign, and changing the axis 90 degrees

Answer 5

d) Transposition involves adding the sphere and cylinder powers algebraically, changing the cylinder power sign, and rotating the axis by 90. If the original axis is less than 90, it can
be rotated by adding 90 as in the following examples: original axis is 045, add 45 + 90 = 135; original axis is 002, add 2 + 90 = 092. If the original axis is more than 90, it can be rotated by subtracting 90 as in the following examples: original axis is 092, subtract 92 – 90 = 002; original axis is 169, subtract 169 – 90 = 079.

Question 6

Care must be taken to figure the transposition correctly, because an error:
a) may result in induced prism
b) may result in an incorrect prescription
c) may make it impossible to fit the lenses into the frame
d) may make the lenses thicker than they should be

Answer 6

b) If you goof, the patient will get the wrong prescription.

Question 7

Transpose +2.00 – 1.00 × 075:
a) +1.00 + 1.00 × 075
b) –2.00 + 1.00 × 165
c) +1.00 + 1.00 × 165
d) –2.00 + 1.00 × 165

Answer 7

c) Algebraically add sphere and cylinder: +2.00 + (–1.00) = +1.00, sphere. Change cylinder sign: +1.00. Rotate axis by 090: 90
+ 75 = 165. Answer: +1.00 + 1.00 × 165.

Question 8

Transpose –3.00 + 2.00 × 090:
a) + 3.00 – 2.00 × 180
b) –1.00 – 2.00 × 090
c) –1.00 – 2.00 × 180
d) +1.00 – 2.00 × 180

Answer 8

c) Algebraically add sphere and cylinder:
–3.00 + 2.00 = –1.00, sphere. Change cylinder
sign: –2.00. Rotate axis by 090: 90 + 90 = 180. Answer: –1.00 – 2.00 × 180.

Question 9

Transpose –1.25 – 6.25 × 173:
a) –7.50 + 6.25 × 083
b) –4.00 + 6.25 × 083
c) –1.25 – 7.50 × 063
d) -7.50 + 6.25 × 063

Answer 9

a) Algebraically add sphere and cylinder:
–1.25 + (–6.25) = –7.50. Change cylinder sign:
+6.25. Rotate axis by 090: 173 – 90 = 083. Answer: –7.50 + 6.25 × 083.

Question 10

Transpose Plano +3.75 × 027:
a) –3.75 – 3.75 × 117
b) +3. 75 – 3.75 × 117
c) –3.75 – 3.75 × 177
d) +3.75 sphere

Answer 10

b) Algebraically add sphere and cylinder: 0 [Plano] + 3.75 = +3.75. Change cylinder sign:
–3.75. Rotate axis by 090: 27 + 090 = 117. Answer: +3.75 – 3.75 × 117.

Question 11

Transpose +1.25 + 1.25 × 154:
a) +2.50 – 1.25 × 064
b) Plano – 1.25 × 064
c) Plano – 1.25 × 109
d) +1.25 – 2.50 × 109

Answer 11

a) Algebraically add sphere and cylinder: +1.25 + 1.25 = +2.50. Change cylinder sign:
–1.25. Rotate axis by 090: 154 – 90 = 064. Answer: +2.50 – 1.25 × 064.

Question 12

Transpose –0.75 + 4.25 × 001
a) +0.75 – 4.25 × 091
b) +0.75 – 3.50 × 091
c) –0.75 – 4.25 × 089
d) +3.50 – 4.25 × 091

Answer 12

d) Algebraically add sphere and cylinder:
–0.75 + 4.25 = +3.50. Change cylinder sign:
–4.25. Rotate axis by 090: 1 + 90 = 091. Answer: +3.50 – 4.25 × 091.