What is the BOZR?
BACK OPTIC ZONE RADIUS (BOZR): Radius of curvature of the central, optic zone of a hard contact lens; previously known as back central optic radius (BCOR).
What is EDGE clearance RGP?
Edge clearance refers to the gap between the front surface of the cornea and the back surface of the peripheral curves, and it is edge clearance that is observable with fluorescein.
What is back optic zone?
optic zone radius, back (BOZR) (It was formerly called the back central optic radius (BCOR or BC) or posterior central curve radius (PCCR).) If the optic zone is surrounded by a peripheral zone there will be a radius of curvature of a back peripheral zone (BPR).
How do you measure Hvid?
Remove an eye piece from the slit lamp and insert the reticle. Use the slit lamp and integrated reticle to obtain a direct horizontal measurement of the patient’s visible iris diameter. Reference the example image below; the corneal diameter or horizontal visible iris diameter measures 11.8mm.
What is prism ballast?
Prism ballasting in corneal GP lenses is commonly used to achieve rotational stability for patients who have a visually significant amount of residual astigmatism. Historically, in the manufacturing process, the prism was generated by offsetting the plastic button when it was cut.
What is Hvid and its normal value?
The average horizontal visible iris diameter (HVID) is 11.6mm to 12.0mm, but only 50% of patients fall within this range. This means that the other half of the patient population is wearing contact lenses that do not fit well and are either too large or too small for a patient’s eye.
What is the average Hvid?
The average HVID is 11.8mm. For every 0.2mm larger than 12.0mm, you should add 1.00D to the flat K value, resulting in a steeper lens.
When is Bitoric RGP used?
BITORIC LENSES Bitoric: Use this lens design when there is over 3.00D of prescription cylinder and most if not all of it is from corneal toricity. Bitorics aren’t hard to design when you think of them as two spherical designs on one lens; +0.25 flatter for the flatter K and +0.75 flatter for the steeper K.
When are Bitoric lenses used?
Bitoric candidates possess a high amount of corneal cylinder, which necessitates a toric back surface. Mandell suggests using a bitoric lens when a spherical lens exhibits decentration, excessive movement or an unacceptable fluorescein pattern (Figures 1 and 2).
What is peri ballast?
Peri-ballast designs are similar in concept to prism-ballast; the superior portion of the lens is thinned to produce a prism-like rotational stabilisation effect [1].
Why do we measure Hvid?
(Horizontal Visible Iris Diameter) If your patient appears to have a HVID that is larger or smaller than average (outside 11.6mm – 12.0mm), we advise measuring the patient’s HVID and incorporating this value into the initial design, as this will increase the probability that your first lens fitting will be successful.
How do you calculate Hvid?
Determination of Lens Specifications Accurate HVID can be measured in a few different ways: 1) manually estimate with hand-held rulers (Figure 1); 2) using the caliper tool on your topographer (Figure 2); and 3) using a contact lens of known diameter (Figure 3).
What is the difference between Hvid and Vvid in Optometry?
The horizontal visible iris diameter (HVID) is different and larger compared to the vertical visible iris diameter (VVID) (Figures 1 and 2). This is important data to take into consideration when fitting scleral lenses.