Gas Permeable Inferior Decentration: Terry R. Scheid, OD, FAAO
Background
EA was a long-term rigid corneal contact lens wearer referred due to visual and discomfort problems with her current two year old gas permeable (GP) lenses. She reported night vision difficulties and horizontal injection. The lenses were often difficult to remove at the end of the wearing day. There were no contributory medical factors.
Test Procedures, Fitting/Refitting, Design and Ordering
Lens Verification:
The present verified as:
Overall/Optical Zone Diameter(OAD/OZD): OU 9.2/7.6mm
Base Curve Radius(BCR): OD 7.30mm OS 7.40mm
Power: OU -2.00D
Center Thickness: OU .19mm
Peripheral curve radii/widths: Unknown OU
Visual Acuity (with Contact Lenses):
OD: 20/20 Plano DS over-refraction
OS: 20/20 Plano DS over-refraction
Biomicroscopy (with Lenses):
Her contact lenses were inferiorly decentered with the optical zone diameter bisecting her 6 mm pupils. The lenses momentarily were picked up by the upper eyelids but rapidly descended with the inferior lens edge at the lower limbus. Fluorescein pattern interpretation revealed slight apical clearance with minimal edge clearance.
Biomicroscopy (without Lenses):
Horizontal injection grade 1 was present in addition to moderate peripheral desiccation (3-9 staining) OU. The edge of the upper eyelid was positioned at the superior limbus with the lower eyelid positioned 1 mm below the inferior limbus.
Keratometry:
OD: 45.50 @ 180/ 46.00 @ 90 Mires regular and clear
OS: 45.00 @ 180/ 46.00 @ 90 Mires regular and clear
Manifest Refraction (Immediately After Contact Lens Removal):
OD: -1.50 – 0.75 x 180 20/20
OS: -1.50 – 0.75 x 180 20/20
New Lens Design Was As Follows:
OAD/OZD: OU 9.6mm/8.0mm
BCR: OD: 7.45mm OS: 7.55mm
Secondary Curve Radius/width (SCR/W): OD 8.90mm/0.5mm
OS: 9.00/0.5mm
Peripheral Curve Radius/width (PCR/W): OU 12.00mm/0.3mm
Power: OU -1.25D
Center Thickness: OU 0.15mm
Minus Lenticular: OU 8.1 mm optic cap
Lens Material: 1.09 specific gravity
The lenses moved well with the upper eyelid and had an alignment to slight apical touch fluorescein pattern. Good edge clearance was observed. The lenses were well centered in front of the pupils. The patient’s discomfort and injection complaints were resolved.
Discussions/Alternative Management Options
The initial lenses had a relatively thick center thickness of 0.19 mm. Typically, a -2.00D larger diameter lens can be designed at 0.16 mm, even thinner with today’s designs. In fact, unless moderate-to-high corneal toricity is present, it can be argued that almost all spherical minus power lenses should be ordered in an ultrathin design to improve centration by significantly reducing lens mass while also keeping edge thickness reduced as well.
Another important factor, when indicated, is lenticulation. Low powered minus lenses and all plus lenses can often benefit from minus lenticulation enabling a thicker edge for enhanced upper eyelid lens interaction and thinner center thickness design. High minus power lenses (i.e., typically ≥ 5.00) benefit from a plus lenticular to reduce edge thickness and overall mass. The anterior cap is typically designed 0.1 to 0.2mm larger than the posterior optic zone. The larger the cap, the thicker the lens, the smaller the cap the smaller the effective optical zone. The anterior minus lenticular peripheral radius of the carrier is designed 1 to 3mm flatter than the base curve radius. For corneal GP lenses the lenticular junction thickness should be, at minimum, 0.14 mm.
The lenses were fit with slight apical clearance. Selecting a flatter base curve radius will move the center of gravity forward enhancing movement on the vertical meridian on these with the rule corneas. Fitting the lens larger will aid centration and enhance a lid attached fit. However, if the upper eyelids are positioned above the limbus and / or are flaccid, a small diameter (often approximately 9.0mm) fit steeper than “K” for an interpalpebral fitting relationship is recommended to keep mass reduced and lid-lens edge interaction minimal.
The optical zone size can be increased due to the patient’s large pupils. Increasing the optical zone size more than 0.3 mm will have a tendency to increase apical clearance. As the fit already was slight apical clearance, a flatter base curve can be selected for the new design to enhance a lid attachment fitting.
Supplemental Readings
- Scheid T. Rigid Gas Permeable Contact Lenses: Evaluation and Fitting. In Scheid T. Clinical Manual of Specialized Contact Lens Prescribing. Butterworth-Heinemann, Boston Mass 2002: 1-19
- Mandell R. Contact Lens Practice (4th ed) Springfield ILL. Charles C Thomas 1998: 448- 455.
- Lowther G. Review of Rigid Lens Designs and Effects on Lens Fit. Int Contact Lens Clinic 1998: 378-379.
- Carney L, Mainstone J, Quinn T, Hill R. Rigid Lens Centration: effects of lens design and Material Density. Int Contact Lens Clinic 1996: volume 23: 6-11.
- Quinn T, Carney L. Controlling Rigid Lens Centration through Specific Gravity. Int Contact Lens Clinic 1992: 84-88.
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