Mixed Deck — All NOCE Topics Flashcards

Which contact lens care concern has been associated with certain multipurpose solutions and silicone hydrogel lens materials?

Answer: Decreased surface wettability due to preservative interaction

Certain preservatives (such as PHMB) in some multipurpose solutions can interact with silicone hydrogel lens surfaces, leading to decreased wettability and increased discomfort.

What does 'limbal clearance' refer to in the context of contact lens fitting?

Answer: The space between the lens edge and the corneal limbus

Limbal clearance is the space between the edge of a contact lens and the corneal limbus (corneoscleral junction), which is important for limbal vessel health and comfort.

When fitting a toric soft contact lens, which factor is MOST critical for achieving adequate visual correction?

Answer: Axis alignment and rotational stability

Proper axis alignment is critical for toric lenses because mislocation of even a few degrees from the intended axis can significantly reduce the cylinder correction.

A patient reports seeing color fringes around objects, especially in their peripheral vision. This phenomenon is most likely caused by which lens aberration?

Answer: Chromatic Aberration

Chromatic aberration occurs because lens materials refract different wavelengths (colors) of light at slightly different angles, causing them to focus at different points. This dispersion of light results in color fringing, which is most noticeable in higher power lenses and when looking through the periphery. It is directly related to the Abbe value of the lens material.

A patient who is a welder requires new safety glasses. Which frame material would be the most suitable choice due to its high durability, resistance to corrosion, and ability to withstand high temperatures?

Answer: Beryllium

Beryllium is a very strong, lightweight, and corrosion-resistant metal that can also withstand high temperatures, making it an excellent choice for demanding environments like welding. Zyl and Nylon are plastics that would not hold up well to the heat and potential sparks of welding. Monel, while a common metal for frames, is not as resistant to corrosion or high heat as beryllium.

A patient complains about annoying reflections on their lenses, especially during video calls and when driving at night. Which lens treatment is specifically designed to minimize these reflections and increase the amount of light passing through the lens?

Answer: Anti-reflective coating

Anti-reflective (AR) coating consists of multiple microscopic layers of metallic oxides applied to the lens surfaces. These layers use the principle of destructive interference to cancel out reflected light waves, allowing more light (up to 99%) to transmit through the lens, which enhances visual acuity and reduces distracting glare and reflections.

In a myopic eye, parallel light rays from a distant object come to a focus:

Answer: in front of the retina

Myopia, or nearsightedness, occurs when the eyeball is too long or the cornea has too much curvature. This causes parallel light rays from distant objects to converge and focus at a point in front of the retina, resulting in blurry distance vision.

What is the purpose of performing an 'over-refraction' during a contact lens fitting?

Answer: To refine the contact lens power while the lens is on the eye

Over-refraction is performed with the trial contact lens in situ to determine what additional power is needed to achieve best corrected acuity with that specific lens.

During the final inspection of a pair of eyeglasses using a manual lensmeter, the target is clear and centered, but the lens marking device seems to be misaligned. Which of the following is the MOST likely consequence of this misalignment?

Answer: The optical center will be incorrectly dotted on the lens.

The lens marking device is responsible for physically dotting the optical center (or prism reference point) on the lens surface after it has been located with the lensmeter's reticle. If the marking pins are misaligned, they will place the ink dots at a position that does not correspond to the true optical center found during measurement.

What is the minimum oxygen transmissibility (Dk/t) recommended for daily wear soft contact lenses to prevent corneal hypoxia?

Answer: 24 × 10⁻⁹

Holden and Mertz established that a minimum Dk/t of 24 × 10⁻⁹ is required for daily wear soft lenses to prevent significant hypoxic stress to the corneal epithelium.

A patient with a prescription of +8.00 D sphere was refracted at a vertex distance of 12 mm. If their new glasses are fit with a vertex distance of 8 mm, what is the compensated power that should be ordered?

Answer: +7.75 D

When a plus lens is moved closer to the eye, it loses effective power. To compensate for this, the ordered power must be decreased. The formula for effective power can be used: Fc = F / (1 - dF), where Fc is the compensated power, F is the original power, and d is the change in vertex distance in meters. The change is 12mm - 8mm = 4mm, or 0.004m. Since the lens moved closer, 'd' is positive. Fc = +8.00 / (1 - (0.004 * +8.00)) = +8.00 / (1 - 0.032) = +8.00 / 0.968 ≈ +8.26 D. The closest standard power is +8.25 D. However, the common rule is that for every 1mm closer a plus lens moves, you add approximately 0.12D per 8D of power. A simpler way to remember is that moving a plus lens closer to the eye results in a loss of plus power for the patient, so we must order a stronger lens. Moving a plus lens away from the eye increases its effective power, so a weaker lens would be ordered. In this case, moving the +8.00 D lens 4mm closer results in a perceived power that is weaker. To maintain the intended correction, a slightly stronger power must be ordered. Let's re-evaluate with the other common formula F(comp) = F / (1 + d*F). d = 0.004m. F(comp) = 8 / (1 + (0.004*8)) = 8 / 1.032 = +7.75 D. This indicates that to achieve an effective power of +8.00D at 8mm, a lens of +7.75D is needed. This is because moving a plus lens closer to the eye weakens its effect, so the patient needs less power in the lens itself. The wearer perceives less plus power, so the lens power must be reduced. Therefore, the compensated power is +7.75 D.

When initially selecting a base curve for a soft contact lens, which measurement is most commonly used as the starting point?

Answer: The average of both K readings

For soft contact lens fitting, the base curve is typically selected based on the average keratometry reading because soft lenses drape over the entire cornea.

A patient with low vision due to macular degeneration needs a device to help them read their mail and write checks. Which of the following aids would be most effective and practical for these specific tasks?

Answer: A desktop electronic video magnifier (CCTV)

A desktop electronic video magnifier (CCTV) is ideal for tasks like reading mail and writing checks because it provides significant, adjustable magnification and contrast enhancement. Crucially, it offers a large working distance between the camera and the task, allowing the user to comfortably place items underneath and write, which is difficult with spectacle-mounted or handheld magnifiers. A monocular telescope is for distance viewing, and large-print cards are not applicable to the tasks described.

A presbyopic patient with significant anisometropia complains of double vision and eye strain specifically when reading through their new bifocals. Which of the following specialized lens modifications is designed to correct this issue?

Answer: Slab-off (bicentric grinding)

Slab-off, also known as bicentric grinding, is a technique used to add base-up prism to the reading portion of one lens to correct for vertical imbalance at near. This imbalance is a common problem in anisometropic patients wearing multifocals, leading to symptoms like diplopia and asthenopia when reading.

A patient's pupillary distance (PD) is 62 mm. They have selected a frame with an 'A' measurement of 52 mm and a distance between lenses (DBL) of 18 mm. What is the total decentration required for this order?

Answer: 8 mm

First, calculate the Frame PD (also known as Geometric Center Distance or GCD) by adding the 'A' measurement and the DBL: 52 mm + 18 mm = 70 mm. Then, find the total decentration by subtracting the patient's PD from the Frame PD: 70 mm - 62 mm = 8 mm.

The U.S. Food and Drug Administration (FDA) regulates contact lenses, including non-corrective cosmetic lenses, with a specific classification. What is this classification and its primary implication?

Answer: Class II or Class III Medical Devices, which legally require a valid prescription from a licensed professional for purchase and dispensing.

The FDA classifies all contact lenses, whether for vision correction or cosmetic purposes, as Class II or Class III medical devices because they are placed directly on the eye and pose a potential risk to health. This classification means they cannot be sold over-the-counter and legally require a valid prescription from a licensed eye care provider for dispensing.

When comparing two finished lenses of the exact same power and size, which material property is the primary determinant of the lens's final weight?

Answer: Specific gravity

Specific gravity is the measure of a material's density relative to the density of water. For a given volume, a material with a lower specific gravity will be lighter. Therefore, when comparing two lenses of identical dimensions, the one made from the material with the lower specific gravity (e.g., Trivex at 1.11 g/cm³) will weigh less than one made from a material with a higher specific gravity (e.g., Crown Glass at 2.54 g/cm³).

What is the recommended replacement schedule for daily disposable contact lenses?

Answer: Discard after each single use

Daily disposable lenses are designed to be worn once and discarded at the end of the day, providing a fresh sterile lens with each use and minimizing deposit accumulation.

When using a manual lensometer, if the sphere lines come into focus at +2.00 D and the cylinder lines come into focus at +3.50 D, and the axis wheel is set to 045, what is the correct prescription in plus-cylinder format?

Answer: +2.00 +1.50 x 045

In a manual lensometer, the sphere power is the first reading that comes into focus. The cylinder power is the difference between the second reading and the first reading (+3.50 - +2.00 = +1.50 D). The axis is the reading on the axis wheel when the cylinder lines are in focus, which is 045. Therefore, the prescription is +2.00 +1.50 x 045.

Which of the following lens materials has the highest Abbe value, resulting in the least amount of chromatic aberration?

Answer: Crown Glass

Crown glass has an Abbe value of approximately 58-59, which is the highest among the options. A higher Abbe value indicates less chromatic aberration, meaning less color fringing and better optical clarity. Polycarbonate has a low Abbe value (around 29-31), Trivex is better (around 43-45), and high-index materials typically have lower Abbe values (32-36).