OPT506 - L04 - Baseline Measurements and Keratometry 2024-25 PDF

Baseline Measurements & Keratometry Dr. Asma Zahidi This lecture is based on the following topics from Y1: Anatomy and Physiology (OPT407) Cornea; Limbus Expected values in healthy humans; variation from age, gender, ethnicity Geometric and physical optics as well as vis...

Baseline Measurements & Keratometry Dr. Asma Zahidi This lecture is based on the following topics from Y1: Anatomy and Physiology (OPT407) Cornea; Limbus Expected values in healthy humans; variation from age, gender, ethnicity Geometric and physical optics as well as visual optics (OPT405 & 406) Clinical optometry skills Subjective/objective refraction BVD compensation Keratometry PD measurement Max / Min Pupillary diameter HVID / VPA One time code for 3 this lecture: Intended learning outcomes By the end of this topic, you should be able to: Use different instrumentation to measure ocular parameters for contact lens fitting Record the findings from the baseline measurements and keratometry correctly and accurately Interpret the findings from the baseline measurement of the ocular parameter  Information you need for lens selection History & symptoms Baseline measurements Keratometry Anterior eye assessment EYES CONTACT LENS Corneal diameter (HVID) Vertical Total diameter Palpebral of SCL and RGP Aperture (VPA) Pupil size Ocular parameters PD (mono / bino) Visible iris diameter (horizontal) Palpebral aperture (vertical) Minimum, normal and maximum pupil diameter Why do we need these parameters? PD – Trial frame HVID/VVID/ maximum & minimum Pup Ø – Total diameter of the CL Ocular parameter and CL Visible iris diameter HVID Soft contact lens totaldiameter SCL TD = HVID + 2mm Palpebral aperture Small PA (< 9mm) require smallTD RGP TD = HVID - 2mm Large PA (> 11mm) require large TD Ocular parameter and CL Pupil diameter Normal values: photopic 3-4mm mesopic 4-5mm PupØ ≤ 5mm are unproblematic for either SCL or RGP PupØ > 6mm SCL are first choice Minimum Maximum Habitual Ocular parameters What else might be of interest? Blink rate Lid tension Lid angles Corneal scleral profile (CSP) Ocular parameter and CL Blink rate Evaluation during the history and symptoms (why?) Normal: 10 to 12 per minute (every 5 to 6 seconds) High blink rate: symptomatic for dry eye If fitted with S C L or RGP showing good wettability, blink rate should go back to normal Low blink rate: stable tear film If too low, deposits on CL are more likely Tip: Use only CL with good wettability! Reduced blink rate leads to reduced movement and thus to decreased exchange of tear fluid. Use small diameters Ocular parameter and CL Lid tension Evaluation through a gentle lift of the upper and lower lid Classification: high; normal; low Too high lid tension can be a contraindication for CL wear Lid tension has an effect on: Movement of the lens Positioning of the lens Handling Significance for CL wear: Too high lid tension causes a decentration of an RGP (initially downwards, later upwards) Too low lid tension reduces the necessary movement of the lens on the cornea  Ocular parameter and CL Lid angle (position) The shape, position and height of the lids influences the choice of: The total diameter of the lens and thus the positioning of the lens on the cornea Handling Normal angle Small palpebral aperture Large palpebral aperture High lower lid position 1/5 of the cornea is Both lids cover parts of the Both lids do not cover the The lower lid covers significant covered by the upper lid cornea. cornea at all. portions of the cornea. The lower lid margin is inline with the inferior Unsuitable for RGP(high Suitable to a limited extend Unsuitable for RGP, as the lens limbus tension on the CL from for RGP (the upper lid must would be positioned on the both lids) cross the lens edge every lower lid margin, thus can cause Suitable for SCL andRGP blink which causes reflexes, blurred vision and Tip: use small TD discomfort). Increased risk to monocular double vision. lose the lens. Tip: use large TD Corneoscleral profile (CSP) C SP = describes the topographical shape between the cornea and the sclera Information you need for lens selection History & symptoms Baseline measurements Keratometry Anterior eye assessment EYES CONTACT LENS Central corneal BC radii (keratometry) BOZR Keratometry 2𝑎𝑦′ Principle 𝑟" = 𝑦 − 𝑦′ Two mires (T1 and T2) are imaged on the cornea The image Y’ (T1’ and T2’) is smaller, virtual and upright and positioned directly behind the corneal apex The imagesize of Y’ is dependent on the corneal radius and the distance of the device Types Zeiss-Bombe Zeiss CL-150 Rodenstock C-Mess Topcon OM-1 Topcon OM- 4 Keratometers Keratometers – reading the corneal curvature Keratometers – reading the meridian Keratometers – reading the meridian Javal-Shiotz Keratometer Review keratometry notes Y1 Throughout the procedure, adjust the focus and re-center the mires as needed Javal-Shiotz Keratometer Throughout the procedure, adjust the focus and re-center the mires as needed Bausch & Lomb Keratometer Review keratometry notes Y1 Throughout the procedure, adjust the focus and recenter the reticle as needed Recording your results Record each eye separately Remember that it is the power meridian being recorded (NOT the axis) Instrument usually show the radius in mm and power in diopters (D) RE: 7.85@175, 7.50@85 LE: 7.60@180, 7.70@90 Record the conditions of the mires. E.g.: Mires clear and regular Mires irregular and distorted OR use scales (available in your workbook) What problems can occur? Distorted mires corneal scar dry eye patient didn’t blink (high TF evaporation) No stable coincidence mires unstable tear film Eyes not opened wide enough Mires incomplete cable in front of the mires Very small radius Patient does not fixate properly (measured too far outside) Opposite eye not covered Tip: In case of unstable mire images, the administration of eye drops before the measurement can help. Central corneal radii Measurement area on the cornea approx. 3mm Measures of two different radii, which are orthogonal to each other (90°) One flat rflat @ One steep rsteep @ 0,52 mm What if the meridians are not 90° to each other??? Interpret corneal radii measurements r > 8.2 mm – flat cornea 7.4 < r < 8.2 mm - normal corneal radius range r < 7.4 mm - steep cornea r < 7.0 mm - indication for Keratoconus (further assessment necessary) Difference between the central radii (∆r = r flat – r steep) Indicates the toricity of the cornea (resulting in a corneal astigmatism (CA)) ∆r = 0.0 mm spherical cornea ∆r < 0.1 mm small CA CL fit – aim for flatter fit 0.1 < ∆r < 0.3 normal CA CL fit – parallel ∆r > 0.3mm high CA CL fit – aim for a steeper fit ∆r > 0.4mm very high CA CL fit – Back toric surface (RGP or SCL) Ocular astigmatism Ocular astigmatism can arise from the surfaces of the lens and cornea, but its usually the cornea - Corneal astigmatism Corneal astigmatism can be measured using a keratometer (more on this in Year 2) - Lenticular astigmatism Astigmatism on the anterior and posterior surfaces of the lens Calculated by taking the keratometer reading away from the total astigmatism found during refraction Astigmatism Principle Principle meridians are meridians are not perpendicular perpendicular Astigmatism With the rule (WTR) Against the rule (ATR) cornea more curved in the cornea more curved in the horizontal vertical meridian meridian r1 r1 Flattest curvature Steepest curvature r2> r1 r2 r2 r1> r2 Steepest curvature Flattest curvature Astigmatism decreases over first few years of life and changes with age Astigmatism tends to be with the rule when young and against the rule when older Astigmatism Oblique the steepest curve lies in between 120 and 150 degrees or 30 and 60 degrees Steepest curvature 120° 60° Steepest curvature 150° 30° Astigmatic conditions Total astigmatism (TA) The sum of the corneal and internal astigmatism. Internal astigmatism (IA) It occurs due to the posterior surface of the cornea and tilting or shape of the crystalline lens or refractive index change of the optical components. Corneal astigmatism (CA) When the cornea has unequal curvature or dioptric power on the the anterior surface. TA = IA + CA Astigmatic conditions REMEMBER! Power meridian = meridian of ‘greatest’ optical power (r (mm) @ (°)) Axis meridian = meridian of the ‘least’ optical power (cyl (D) x (°)) To describe a meridian you can either use r (mm) or the cyl (D) BUT always be aware that the axis between the two are different (90° to each other) Example: K= 7.80 @5, 7.75 @95 Rule of thumb: Power meridian: 0.25DC@95 Every 0.05mm difference Axis meridian: 0.25DC x 5 in curvature = 0.25D Astigmatic conditions TA = subjective refraction (in BVD = 0!) CA = Keratometry + calculation IA = TA – CA Significance for CL fitting – RGP or SCL? TA < IA: Spherical refractive error or total astigmatism is smaller than internal astigmatism (cornea and internal are compensating each other) →SCL TA > IA: Internal astigmatism is smaller than the total astigmatism, that means that the cornea has a significant contribution towards the TA (in ideal case ‘pure’ CA) →RGP Refreshing skills… Small radius indicates _______________________________ Large radius indicates _______________________________ Calculate the astigmatism of the following cornea: OS: 7.80mm at 180 / 7.60mm at 90 How will we use this measurement in contact lens practice? A patient with astigmatism… Prescription: OD: +2.00/-1.25 x 180 Keratometry results: OD: 7.80 @ 180 / 7.65 at@ 90 What is the corneal astigmatism? Does this coincide with the prescription? What will happen if you correct the corneal astigmatism? A patient with astigmatism… Prescription: OD: +2.00/-1.25 x 180 Keratometry results: Difference between flat meridian and OD: 7.80 at 180 / 7.65 at 90 steep meridian What is the corneal astigmatism? 0.75D (with the rule astigmatism) Does this coincide with the prescription? What will happen if you correct the corneal astigmatism? A patient with astigmatism… Prescription: Difference between rflat and OD: +2.00/-1.25 x 180 rsteep Keratometry results: OD: 7.80 at 180 / 7.65 at 90 What is the corneal astigmatism? 0.75D (with the rule astigmatism) Does this coincide with the prescription? Not fully: 0.50D of lenticular astigmatism What will happen if you correct the corneal astigmatism? Assessment of the corneal shape Corneal topographers Assessment of the corneal shape Orbscan Pentacam Colour coded maps Radius Sagittal maps Tangential maps Corneal power displays Other Pachymetry elevation Radius Power Elevation What did we learn today? Visible iris diameter SCL TD = HVID + 2mm RGP TD = HVID - 1.5mm Pupil diameter PupØ ≤ 5mm OK for SCL & RGP PupØ > 6mm SCL are first choice Palpebral aperture Small PA (< 9mm) small TD Large PA (> 11mm) large TD What did we learn today ? Reduced blink rate Use only CL with good wettability Use small diameters Lid tension High lid tension causes decentration of RGP Low lid tension reduces necessary movement of lens CSP The flatter the sclera in comparison to the cornea, the flatter the SCL needs to be Keratometry Identify the problems during keratometry and its cause References / Resources Gasson A and Morris J (4th Edition 2010) The Contact Lens Manual: A Practical guide to fitting. Butterworth-Heinemann Chapter 2: Instrumentation Phillips A.J. and Speedwell, L. (2006) Contact lenses. 5th edn. Elsevier Chapter 7: clinical instrumentation in contact lens practice Questions? Specialist Optometry Skills | OPT506 | Dr Daniela Oehring 50