Sturm's conoid

sturm's conoid, astigmatism

1. S turm's conoid & its clinical application Dr samarth mishra

2. . STURM’S CONOID: It is an optical condition in which refractive power of cornea and lens is not the same in all meridians , therefore instead of single focal point there are two focal points separated by focal interval, this is called sturm’s conoid . The distance between two focal points is called sturm’s conoid interval .

3. . In a toric surface,one principal meridian is more curved than the second principle meridian. The principle meridian with minimum curvature , & therefore minimum power is called BASE curve of a toric lens. The configuration of rays refracted through a toric surface/astigmatic surface is called sturm’s conoid .

4. . At point A , the vertical rays [ V ] , the vertical rays are converging more than the horizontal rays [ H ]; so the section here is horizontal oval or an oblate ellipse .

5. . At point B ( first focus), the vertial rays have come to a focus while the horizontal rays are still converging & so they form a horizontal line. (horizontal line)

6. . At point C , the vertical rays are diverging and & their divergence is less than the convergence of the horizontal rays; so a horizontal oval is formed here. (horizontal oval)

7. . at point D , the divergence of vertical rays is exactly equal to the convergence of the horizontal rays from the axis. So here the section is a circle which is called the circle of least diffusion. (circle of least diffusion)

8. . At point E ,the divergence of vertical rays is more than the convergence of horizontal rays; so the section here is a vertical oval. (divergence of vertical rays>> convergence of horizontal rays)

9. . At point F ; ( second focus), the horizontal rays have come to a focus while the vertical rays are divergent. So a vertical line is formed here. (vertical line at second focus)

10. . Beyond f ( as at point G ) : both horizontal and vertical rays are diverging and so the section will always be a vertical oval or prolate ellipse. The distance between the two foci (B & F) is called the focal interval of sturm .

11. . the shape of bundle of the light rays at different levels in a sturm’s conoid is as follows:

12. . Etiology: Corneal causes . It occurs due to abnormality of curvature of cornea. (Most common cause of astigmatism.) E.g eyelid pressure, pterygium , corneal scars, corneal degeneration, keratoconus,mild corneal opacities .

13. . Lenticular causes: It is comparatively rare. It may be- Curvatural --- lenticonus Positional -----congenital tilting & traumatic subluxation of lens. Index ----developing cataract/ nuclear sclerosis/ index astigmatism.

14. . FOCUS OF STURM’S CONOID AC/TO THE TYPES OF ASTIGMATISM: REGULAR ASTIGMATISM : when the refractive power changes uniformly from one meridian to another( i.e there are two principal meridia ) The parallel rays of light are not focussed on a point but form two focal lines.

15. . Types: with the rule astigmatism : in this type the two principal meridia are placed at right angles to one another. But the vertical meridian is more curved than the horizontal. This is called “with the rule astigmatism” as similar condition exists normally ( the vertical meridian is normally rendered 0.25D more convex than the horizontal meridian by the pressure of eyelids.

16. . against the rule astigmatism : the horizontal meridian is more curved than the vertical meridian. oblique astigmatism : type of regular astigmatism where the two principal meridia are not the horizontal and vertical though these are at right angle. bioblique astigmatism : principal meridia are not at right angle to one another.

17. . Simple astigmatism : the rays are focused on the retina in one meridian and either in front(simple myopic astigmatism)/behind( simple hypermetropic astigmatism).

18. . Compound astigmatism: rays of light in both the meridia are focused either in front or behind the retina and the condition is labelled as compound myopic / compund hypermetropic astigmatism.

19. . Mixed astigmatism: condition wherein the light rays in one meridian are focused in front and in other behind the retina. Such patients have comparatively less symptoms as “circle of least diffusion “ is formed on the retina.

20. .

21. . .

22. . Clinical features : -blurring of vision - asthenopic symptoms. -tilting of head. -headache. -half closure of lids. -squinting. -burning and itching

23. . IRREGULAR ASTIGMATISM: Chr by an irregular change of refractive power in different media. Types: -corneal irregular astigmatism: corneal scars, keratoconus . - lenticular irregular astigmatism: d/t variable refractive index in diff. parts of crystalline lens. Seen during maturation of cataract. -retinal irregular astigmatism: d/t distortion of macular area.

24. . Clinical features: -defective vision -distortion - polyopia ( seeing multiple objects)

25. . surgical t/t of astigmatism: Incisional refractive procedure; Astigmatic keratotomy : -making transverse or arcuate cuts in the mid periphery perpendicular to the steepest corneal meridian. -incised meridian flattens while the meridian perpendicular to it steepens by nearly the same amount. - transverse or arcuate incision can be given.

26. . Limbal relaxing incision: - To correct mild(-1 to -2) astigmatism. -Incision made at limbus , so , optical quality of cornea is preserved. - easy and safe.

27. . Laser ablation corneal refractive procedures: Photoastigmatic refractive keratotomy: -uses a cylindrical rather than a spherical ablation pattern to remove a tissue in a chosen meridian. -axis of astigmatism should be marked with the patient seated, because it may shift when the patient reclines.

28. . Astigmatic epi -LASIK: preferred over astigmatic PRK. Astigmatic LASIK: astigmatismof 0.5 to 10.0D is amenable to correction with LASIK. Astigmatic C-LASIK : presently the best technique to treat corneal astigmatism

29. . MANAGEMENT OF POST-KERATOPLASTY ASTIGMATISM: SUTURE REMOVAL: -suture removal in steep meridia may improve a varying degree of both regular & irregular astigmatism. -near a tight suture ,the keratoscopic mires are closer together and may demonstrate a ‘V’ indentation vector.

30. . RELAXING INCISIONS: - arcuate incisions along the steeper meridian in the donor cornea 0.5mm central to the host-graft junction correct an astigmatism of 3.5-8.5D. -two relaxing incisions involving 70% of corneal depth are made 180 deg. apart.

31. . Relaxing incisions with compression sutures: -after making relaxing incisions , two or three 10-0 nylon sutures are applied at the graft host junction 90 deg. away from the steepest meridian.

32. . corneal wedge resection: -to correct an astigmatism of 10-20D before repeating the penetrating keratoplasty . -corneal wedge of 1.0-1.5mm wide base and 90 deg. in extent is made. -gap is sutured by five to seven deep interrupted 10-0 nylon or prolene sutures.

33. . Ruiz procedure: - if a corneal resection fails/ patient has a highly myopic spherical equivalent. - if significant anisometropia exists such as post- keratoplasty eye with more myopic eye. -deep horizontal keratotomy incisions are made with a guarded diamond blade in a ‘step ladder pattern’ along the axis of steepest corneal meridian. -it is imp. To ensure that the horizontal and radial incision donot intersect ( as this causes gaping and poor wound healing )

34. Ruiz procedure

35. . Toric IOL : Toric IOLs refer to astigmatism correcting intraocular lenses used at the time of cataract surgery to decrease post-operative astigmatism. Patient should have a visually significant cataract and astigmatism.

36. . The toric lenses currently available are designed to correct regular corneal astigmatism. Patients with irregular astigmatism do not fare as well.

37. THANK YOU