REFRACTION NOTES

REFRACTION

JACKSON’S  CROSS-CYLINDER

• It is a combination of two cylinders of equal strength but opposite power with their axis at right angles to each other.
• Commonly used are  +/-0.25 and +/-0.50
• REFINEMENT OF AXIS-

- Always done first.

-Cross-cylinder is held with its handle along the axis of cyl in the trial frame [i.e at 45deg] first with -0.5D and then with +0.5D

-If the patient reports no change in V/A bet the two positions-axis of the cyl in the trial frame is correct.

-If the visual improvement is found in one of the pos,a ` ‘plus’correcting cylinder shud be rotated in the direction of the plus cyl componentof the cross-cyl & vice versa.

REFINEMENT OF CYLINDER POWER-

-Cross-cyl of +/-0.25 is placed with its axis along the axis of the cross-cyl in the trial frame first with the same sign & then with the opp sign

-In the first pos the cyl correction is enhanced by o.25D & in the second it is diminisned by the same amount.

-When the V/A does not improve in any of the pos,the power is correct

If V/A improves in any of the pos,a corresponding correction shud be made.                                       

                        TRANSPOSITION

Q.1.Perform the toric transposition-

+3.00/-1.00 × 180 to the base curve of +6D

Step 1-first transpose the cylinder, so that it is the same sign as base curve.—

+ 2.00 /+ 1.00 ×90

Step 2-The required spherical is obtained by subtracting BC power from spherical power.-

S = S - BC

+ 2 – [+6] = - 4DS

Step 3-specify axis of the BC.This is 90 deg. To axis of reqd cyl.

+ 6D × 180

Step4-Add the reqd cyl to BC power

C = C + BC

+1D + 6D= +7d ×90

Complete toric formula-

-4DS /+6D ×180 /+7D ×90

Q 2. -2.00 /+1.5 ×90 to BC of -6D

Step 1- -0.5 /- 1.5 × 180 →  cyl transposition

Step 2- -0.5 – [-6]= -0.5 + 6= 5.5DS→ reqd spherical

Step 3- -6D ×90→ axis of BC

Step 4- -1.5 + [-6]= -1.5-6= -7.5 ×180→ reqd cyl

Complete toric formula-

5.5DS /-6D ×90 /-7.5 ×180

        TORIC LENSES [spherocylindrical]

• These are curved lenses where one surface is spherical & the other is toroidal.
• Toric lenses are used wen a cyl is also present in the prescription.
• Spherical power is ground on the anterior  surface & the posterior surface is made toroidal.
• Such lenses do not  produce a single defined image bcos the principle meridians form separate line foci at right angles to each other
• Toric lens =  spherical power / cylindrical power
• Spherical equivalent = Sphere + ½  cylinder

                           PRISMS

USES-

DIAGNOSTIC PRISMS-

1.  PBCT- For phorias & tropias

2.  Prism bar reflex test [Krimski’s test]

3.  Maddox double prism test – for cyclophorias

4.  Prism vergence test-

*   For convergence- base-out prism-N- 50 PD [ < 20PD-convergence insuficiency]

*   For divergence- base –in-prism-N-5-7PD [< 5PD- divergence insufficiency]

5.  4 Dioptre prism test

6   Diplopia test for ARC

7.  Detection of malingering

    -If a prism is placed in front of the seeing eye,the eye will move    to regain fixation

THERAPEUTIC PRISMS-

1. T/T of phorias

Exercising prism- base towards deviation

Relieving  prism-base away from deviation

2. To relieve diplopia
3. T/T  of ARC
4. T/T of Eccentric fixation
5. T/T of microtropia [< 2deg]

OPTICAL INSTRUMENTS-

1. SL
2. Applanation tono
3. Keratometer
4. Ophthalmoscope
5. synoptophore
6. Binocular loupe
7. Prism bar
8. phorometers
9. Fundus camera
10. Exophthalmometer

                             

REFRACTIVE SURGERY

                     LASIK-

LASER IN –SITU KERATOMILEUSIS

DEF-

Combines the precision of excimer laser photoablation with the advantages of an intrastromal procedure that maintains the integrity of Bowman’s layer & overlying epith.

Can correct –Myopia upto -13D

• Astigmatism upto -6D

PRE-REQUISITES-

1. Adequate globe exposure
2. Corneal thickness- 450μ
3. Flap thickness- 180μ
4. Stromal bed thickness- 250μ
5. IOP – 65 mm Hg

TECH-

• Pilocarpine 1%-miosis→ to aid centration
• Topical anaesthesia
• Cleaning & draping
• Exposure- wire speculum
• Corneal marking-with gentian violet

• marker-inner ring [3mm]-coincides with the pupil-centration
• outer ring-[10.5mm]-aids placement of suction ring
• pararadial lines-join the 2 rings-for correct alignment

• Fixation of suction ring-fixed on the sclera with decentration towards the hinge.IOP is raised to 65 mmHg.checked with a Barraquer tono
• Corneal flap-

-Cor is moisetened with BSS

-Microkeratome head is inserted in yhe track on the suction ring

-Forward foot pedal is pressed-micro moves forward & cuts the flap

-Reverse foot pedal is pressed-mic returns to its original pos leaving behind a good hinged flap

*  Stromal ablation-

-Flap is retracted along the hinge & stromal bed is dried

-CUT WET & ABLATE DRY

*  Ablation-Done within 30 sec

*Irrigation done

*  Excess fld removed with a cellulose sponge

*  Flap is reposited

COMPLICATIONS-

INTRAOPERATIVE

A] During flap preparation-

1. Flap of variable & suboptimal thickness & diameter

2.Tear / hole

3.Cor perforation

4.Free flap instead of hinged flap

5.Loss of flap

6.Interface debris

B]  During laser ablation –

1.  Decentration of ablation-Glare & monocular diplopia

2.Ablation of hinge- prismatic effect

C] During flap reposition-

1.  Incorrect placement

2.  Wrinkling on reposition

POSTOPERATIVE-

1. SANDS OF SAHARA SYN-

Intrastromal / intralamellar keratitis

2. Epithelial ingrowths
3. Central islands-Localised cor elevation d/t intrastromal ablation
4. Undercorrection / Overcorrection
5. Induced astigmatism
6. Microstriae –d/t gross misalignment of flap- “ Thumbprint” sign
7. Infection-nontuberculous mycobacteria
8. Dry- eye as cor Ns are severed during flap creation
9. Ectasia if residual stromal bed is not 250 micron

                                LASEK

• Lasaer subepithelial keratomileusis
• Also k/as epilasek
• Combines LASIK + PRK