riving Growth Ocular Anatomy

Transcription

1 riving Growth Ocular Anatomy

2 Tears The tear film has three component layers: 1. The lipid layer-lies on top of the aqueous layer and is exposed to the air 2. The aqueous layer-lies on top of the mucin layer 3. The mucin layer-indirect contact with the corneal and conjunctival epithelium T The tear film is produced by the secretions of numerous glands and cells. No one structure provides all the components of the tear film. It protects and moistens the front surface of the cornea. It supplies oxygen and some nutrients to the cornea as well as filling in gaps between the epithelial cells to provide a smooth, regular refractive surface.

3 Cornea The cornea is a clear, transparent structure, referred to as the window or watch crystal of the eye. It has a complex surface that acts as a powerful lens. The cornea affords protection and forms the front of the anterior chamber. Most of the refraction of light takes place through the cornea. The cornea is avascular (contains no blood vessels). A plexus of fine capillaries surround the cornea and provide its complete blood supply. Injury, infection or inflammation of the cornea can produce scarring. The resultant opacity compromises visual acuity. The adult cornea is approximately 1 mm thick and 11.5 mm in diameter and has five distinct layers. It has a refractive power of about 44 diopters of plus power.

4 Keratometer

5 Taking the Reading 1. Focusing the instrument for the patient s eye: a. Looking through the eyepiece (E.P.), focus the instrument until the target mire is clear. b. Swivel the instrument (D), or move it up and down (E), until the cross-line is within the central focusing circle (lower right mire circle). c. Lock the instrument in place (D). d. sign, and focus sharply. Focus the central focusing circle (F) until it becomes single and sharp.

6 Slit Lamp Radiuscope

7 Phoropter

8 Contact Lens

9 Base Curve The term base curve refers to the radius of curvature of the central posterior surface of the lens. It may be expressed in diopters of surface power or in millimeters (mm) of curvature. Optical Zone The optical zone is the chord diameter over which the base curve extends on the posterior surface of the contact lens. It is described in millimeters (mm) of diameter.

10 Secondary Curve The secondary curve of a lens may also be referred to as the intermediate curve. It lies between the base curve and the peripheral curve on the posterior lens surface. The secondary curve is usually.70mm to 1.50mm flatter than the base curve radius (average 1.00mm). This allows the lens to better align the flatter portion of the cornea adjacent to the Optical Zone.

11 Peripheral Curve The peripheral curve is the outermost curve on the posterior lens surface. It s function is to conform to the flatter periphery of the cornea and to clear the limbal areas as the lens moves with the blink. The peripheral curve is noted as a radius curvature in mm., and a width in mm. The peripheral curve radius is generally dependent on the diameter of the lens- the smaller the lens diameter the steeper the radius and the larger the lens diameter the flatter the radius.

12 Anterior Optical Zone The anterior optical zone is the radius of curvature on the anterior lens surface which determines the refractive power of the lens. The curve may also be referred to as the power curve or the lenticular bowl. Like the posterior optical zone, the anterior O.Z. has both a radius of curvature (power) and a diameter which are measured in mm s.

13 Lenticular Flange A lenticular flange refers to the increase or decrease in anterior edge thickness to aid in lens positioning. An increase in edge thickness is used to help position low minus and most plus lens designs. The extra edge thickness creates greater lid interaction, thus allowing the upper lid to position the lens higher on the cornea. This type of lenticulation is often referred to as a myoflange design.

14 Prism Ballast A prism ballast lens design incorporates a thicker inferior portion to the periphery of the lens. This allows the upper lid to force the lens downward in the case of a high riding lens (known as the watermelon seed effect ). The increased lens thickness also allows the lower lid to force the lens upward during down gaze, as in the case of many bifocal contact lens designs Truncation A truncation is the removal of the inferior and/or superior portions of the lens to aid in meridional orientation and stabilization. This technique is commonly used in either toric or bifocal contact lens designs

15 Rigid Lens Design Considerations 1. Anatomical Considerations Lid position Location of corneal astigmatism 2. Base Curve (Optical Zone) Radius 3. Optical Zone Diameter 4. Secondary Curve Radius 5. Peripheral Curve Radius 6. Lens Diameter 7. Blend Curve 8. Edge Form 9. Center Thickness 10. Lens Power

16 Step 1. Anatomical Features Figure 1. The ideal lid position for an alignment fitting philosophy, below the superior limbus covering the superior one third of the cornea. Figure 2. A less than optimum upper lid position for alignment fitting. Note its position at or above the superior limbus. Figure 3. The ideal fitting dynamics are present in this 2-diopter withthe-rule astigmat fitted with alignment along the horizontal meridian. Figure 4. In against-the-rule astigmatism, a lens fit with alignment along the flat(vertical) meridian will tend to orient nasally or temporally against the action of the upper lid.????? Fig. 1 Fig. 2 Fig. 3 Fig. 4

17 Anatomical Features Two anatomical features must be evaluated before the actual fitting begins. The first is the position of the upper lid and the second is the location of the corneal astigmatism. These anatomical features will have significant effect on the lens design and fitting philosophy chosen.

18 Step 2 Base Curve (Optical Zone) Radius Figure & Base Curve (Optical Zone) Radius. In designing a contact lens from the inside out, the first parameter to be established will be the base curve(optical zone) radius. To maintain a true alignment fitting philosophy, it is necessary to select a base curve radius 0.50 diopter flatter than the flattest K reading regardless of the cornea toricity The alignment fitting philosophy is illustrated by superimposing a flatter radius lens (quarter) on to the cornea (nickel). Note the central alignment and lack of peripheral touch.

19 Step 3 Optical Zone Diameter Figure 10A and B. The principle that for any given corneal curvature there is one and only one optical zone diameter which will allow maximum corneal alignment is illustrated. Figure 10A represents a diopters (7.50mm) cornea which requires a smaller optical zone to achieve alignment. Figure 10B shows a flatter cornea diopters (8.23mm) requiring a larger optical zone diameter to achieve a maximum alignment relationship.

20 Step 4 Secondary Curve Radius and Width Figure 11. The secondary curve radius and width. Fig. 11 Fig. 12 Figure 12. The secondary curve is the adjustable portion of the rigid lens. It can be made wider for larger diameter lenses or made narrower for smaller diameter lenses.

21 Step 5 Peripheral Curve Radius and Width Figure 13. The peripheral curve radius and width. The different peripheral curve philosophies required in lenses fitted with interpalpebral techniques (left), versus superior limbal resting techniques are shown. Note the slightly steeper peripheral curve philosophy required in lenses fitted interpalpebrally versus the flatter peripheral curve philosophy present on lenses fitted with superior limbal resting techniques.

22 Step 6 Lens Diameter The main function of diameter is to stabilize the lens and ensure that the optical portion of the lens remains over the pupil. The diameter of a rigid contact lens is best determined by measuring the visible iris diameter and designing the lens 2.3mm smaller. Example: Visible iris diameter= 11.5 mm 2.3 mm rule Lens diameter= 9.2 mm For practical purposes, visible iris diameters may be classified as smallapproximately 11.0mm, requiring a small overall diameter of 8.7mm; medium 11.5mm, requiring a 9.2mm diameter lens; or large 12.0mm, requiring a 9.7mm diameter lens

23 Step 7 Blend Curve Blending is generally required at the junction between the secondary and peripheral curves. The blend should be heavy enough to remove any junction that exists between two curves. Generally, if you can see the junctions, the patient can feel them.

24 The edge profile is an important consideration when designing any rigid contact lens. In apical alignment or apical bearing philosophies, adequate lid interaction is mandatory to support and stabilize the lens to position the optics over the pupil. The opposite is true when fitting apical clearance interpalpebral lens designs. In this situation, all efforts should be made to minimize lid/lens interaction by thinning the anterior edge profile to limit edge mass. Step 8 Edge Profile

25 Step 9 Center Thickness Plano

26 Thank You!!!