Rigid lens principals and products Week 9 Rigid

Rigid lens principals and products Week 9

Rigid gas perm lens is a hard material that allows 02 to pass through the material to the cornea. It is smaller than a soft contact lens. It is fit differently and more precisely. It “reshapes” the cornea to eliminate the astigmatism. Soft CL VS gas perm CL

Gas perm lenses are made from SA (silicone acrylate) and FSA (fluorinated silicone acrylate). These material allow O 2 to permeate the lens to the cornea, reducing edema and increasing wear time for the patient. It must be fit more precisely because it reshapes the cornea. Hard lenses, PMMA (polymethylmethcryl ate) have poor O 2 transmission. 1 st generation “hard” lens. Long lasting and very durable. Rigid lens principals

Dk Value Diffusion coefficient is the value of O 2 permeability of the lens material. The higher the Dk value the higher the O 2 permeation to the cornea. A higher Dk value will lower the chance of corneal edema. Dk can be expressed in the center thickness (L)of the material by the equation Dk. L. This will define the O 2 transmissibility of the material through the center thickness. Rigid lens principals

Two corneal shapes Spherical Aspherical (toric) Spherical Aspherical 44. 00/44. 00@90 46. 00/42. 00@90 Rigid lens principals

Spherical Implies Cornea that this corneal has minimal topographic variation. The K readings will be very similar horizontally and vertically. There will be minimal corneal astigmatism. Aspherical Implies Cornea that the cornea has a variance in corneal curvature and power. K readings will be different horizontally and vertically. The more the readings vary, the more astigmatism there will be. This is corneal astigmatism. Rigid lens principals

Aspherical corneas will has corneal astigmatism. With the rule astigmatism Against the rule astigmatism Lenticular astigmatism Oblique astigmatism Rigid lens principals

Base Curve (BC) The primary curve of the lens. It has to be designed to contour the anterior corneal surface and tear film. Rigid lens principals

The BC’s relationship to the cornea is referred to as: Bearing Alignment Clearance These are adjectives used to describe how the CL fits on the cornea. Rigid lens principals

Bearing Suggests that the lens is fitting flatter on the cornea than the keratometric readings. If the lens is bearing centrally, it is flat centrally, pushing the tear film to the periphery. If the lens is bearing peripherally, there will be no tear film peripherally. Rigid lens principals

Clearance Suggests that the lens is fitting steeper on the cornea than the keratometric readings. If the lens has clearance centrally, the lens fits steeper than K centrally and the tears will pool in the center. Rigid lens principals

Alignment Suggests that the lens perfectly contours the cornea and the tear film is evenly distributed on the corneal surface. The lens will not be bearing or have clearance. Usually used for patients with little or no corneal astigmatism, fit on “K”. Rigid lens principals

The base curve is selected based on the “K” readings. Rigid lenses are fit in three ways. On “K” Steeper than “K” Flatter than “K” This is ALWAYS based on corneal astigmatism, not refractive astigmatism. When fitting any of these, use the flattest “K” readings taken for that eye. Example……. . OD = 39. 00/41. 75@90 You want to fitter steeper than “K”. (even though it is a flat cornea) Use 39. 00 as the starting “K” for this eye. Fit steeper than 39. 00 depending on the amount of corneal astigmatism. Rigid lens principals

On “K” (alignment) Flatter than “K” Implies that the curvature is equal to the flattest “K” reading. This method is appropriate for low cylinder patients. using a base curve flatter than the flattest”K” meridian. It will exhibit a bearing effect on the corneal cap allowing increased lid interaction. Bearing will cause a minus tear layer effect requiring a plus power compensation. (FAP) flatter, add plus. Rigid lens principals

Steeper than “K” Tear film layer Implies The using a base curve steeper than the flattest “K”. This allow central clearance of the lens. The lens will position centrally, along with an increase of tear film. This tear film will cause a plus lens effect, requiring a minus power compensation. (SAM) Steeper add minus. tear film created by fitting steeper or flatter than “K” re shapes the cornea and the tear layer fills in the gaps to help correct for the cylindrical power not put into the CL. Rigid lens principals

Tear film layer Fitting Flatter than “K” causes a minus tear layer effect. The tear layer is pushed to the periphery creating a minus lens effect. This will have to compensated with flatter add plus when figuring the lens power. Rigid lens principals

Tear film layer Fitting steeper than “K” will cause a plus tear layer effect. The tear layer will pool in the center of the lens and this will need to be compensated by steeper add minus to the power of the lens. Rigid lens principals

Determining diameter Horizontal visible iris diameter The CL diameter should be 2. 0 -2. 5 mm smaller than the visible iris diameter or 1. 2 – 1. 5 mm from the limbus to the edge of the lens. Normal rigid lens D is 8 to 10. 5 mm, much smaller than a soft CL. Rigid lens principals

Determining diameter Corneal Alignments fit, flatter than “K” Large diameters will be fit flatter than “K”. This is called corneal alignment fit or Korb method. Corneal alignment fit rides high on the cornea and fit UNDER the upper lid. Sometimes referred to lid attachment fit. FAP rule applies. Rigid lens principals

Determining diameter Apical fit, steeper than “K” Smaller diameter corneas are fit steeper than “K” This is called an Apical lens design or interpalpebral lens design and fits in between the lids, centrally on the cornea. SAM rule applies. Rigid lens principals

Determining diameter Don’t worry, there’s a chart for everything!!! Corneal alignment fit Flatter than “K” Apical Large Smaller diameter fit Steeper than “K” Rigid lens principals diameter

Let’s review what you learned today!!! Rigid lens principals

Rigid lens principals review

Rigid lens principals review

Bearing means? Alignment means? Clearance means? FAP means? Why do we need to use FAP? SAM means? Why do we need to use SAM? What type of fit is steeper than “K”? Is that for a large or small diameter corneal? What does on”K” mean? Rigid lens principals review

Great job! See you next week for rigid lens fitting. Test in 2 weeks. Rigid lens principals