Monday, May 22, 2006

18

Prentice’s Rule


Prentice’s Rule determines how much deviation you get by looking off center of a lens. There is no prismatic power at the optical center of the lens. Deviation in prism diopters (PD) = h (cm) x F where F = power of the lens and h = distance from the optical center of the lens.

**NOTE: a plus lens is really 2 prisms stacked base to base and a minus lens is 2 prisms stacked apex to apex.


Question: A patient wearing glasses with these lenses, OD: +3.00, OS: -1.00, complains of vertical diplopia when reading. Both eyes are reading 5 mm down from the optical center. How much total prism is induced in this reading position?


Answer:

Use Prentice’s rule: PD = hF where h = distance from optical center in centimeters and F = power of the lens.

Therefore, in the right eye, 0.5cm x 3.00D = 1.5 prism diopters base up (inferior segment of a plus lens), and in the left eye, 0.5cm x 1.00D = 0.5 prism diopters base down (inferior segment of a minus lens). Total induced vertical prism is 2.0 prism diopters.


Question: What is the induced prism for an individual wearing +5.00D OU, when reading at the usual reading position of 2mm in and 8mm down from the optical center of his lenses?


Answer:

Use Prentice’s rule: PD = hF

Therefore, vertically 5.00D x 0.8 = 4PD BU per eye (inferior segment of a plus lens) and horizontally 5.00D x 0.2 = 1PD BO per eye (nasal segment of a plus lens)

Spectacles provide a prismatic effect in viewing strabismic deviations. A plus lens will decrease the measured deviation, whether it is esotropia, exotropia or hyper/hypotropia. A minus lens increases the measured deviation, whether it is esotropia, exotropia or hyper/hypotropia. The true deviation is changed by approximately 2.5% per diopter.

For example, an exotrope of 40 ∆ wearing -10.00D spherical glasses will measure 2.5 (10) = 25% more exotropia, for a total measured deviation of 50 ∆ XT.

**NOTE: the 3M mnemonic- Minus Measures More

Convergence (in prism diopters) required for an ametrope to bi-fixate a near object is equal to the dioptric distance from the object to the center of rotation of the eyes, multiplied by the subject’s intra-pupillary distance in centimeters.

Convergence ( ∆ ) = 100/working distance (cm) x Pupillary Distance (cm)

Question: What is the convergence required by an individual with a 60mm intra-pupillary distance when viewing an object at 40cm?


Answer:

Convergence ( ∆ ) = 100/working distance (cm) x Pupillary Distance (cm)

100/working distance (cm) = 100/40cm = 2.50D

Pupillary Distance (cm) = 6cm

Convergence ( ∆ ) = 2.50D x 6 = 15 prism diopters of convergence


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