CORRECTING DECENTRED LENS FITS
Learning Outcomes
After completing this module you should:
- Understand factors other than fit can be responsible for lens decentration
- Be able to interpret axial and tangential topography identifying poor fit patterns
- Identify corneal staining and its cause
- Understand the lens modifications that can resolve issues.
DECENTRED LENS FITS
Nocturnal lenses have a first lens fit success of over 90%. Patients are successful when the treatment zone has the correct power and is bullseye. Nocturnal lens quality and design give this high first fit success, poor or optimistic (high Rx or high cyl) patient selection and poor baseline topography can reduce success. The Nocturnal New Fit Guarantee allows two exchanges to alter the lens fit giving the best opportunity for success.
Low drop out is also a benefit of Nocturnal. Patients do not experience dryness discomfort associated with day contact lens wear and because corneas should not change shape from childhood 1,2 the treatment zone position should be stable for the lifetime of the patient once fitted. IF the treatment zone position does alter then we need to consider whether there could be corneal disease like keratoconus. Or is there an external cause.
Lenses generally form a treatment zone centrally on the cornea when the eye is closed creating a treatment zones that is bullseye given the corneal displacement. A poor lens fit ie loose or tight alignment zone may cause the treatment zone to not be centred, but there are also external factors other than lens fit can impact on centration. These should be considered during initial fitting, and especially if a patient had a successful fit initially that has changed with time.
Baseline corneal factors that can impact on treatment zone position and power are:
ANGLE ALPHA
The distance the visual axis is from the centre of the cornea. The visual axis is usually nasal to the corneal centre. The larger the angle and the higher the Rx this can impact on the quality of the correction.
CORNEAL DISPLACEMENT
The baseline peripheral corneal contours where the lens alignment zone will fit (7.0 to 10.0mm) are a strong indicator to where the treatment zone will form. The larger the displacement and the higher the Rx this can impact on the quality of the correction.
TOPOGRAPHY SIZE / QUALITY
If there is poor topography information out to the alignment zone (to 10.0mm) this can mean the initial lens will not fit as accurately and may need adjusted.
Topography assessment is covered in the Nocturnal Accreditation, Topography Guide and Topography Lecture.
External factors that can impact on treatment zone position and power correction are:
LID PRESSURE
Lid interaction may result in decentration that can not be resolved by altering the lens fit. Patients should be advised not to wear sleep masks when starting to wear night lenses.
LID ROUGHNESS
Conjunctival follicles or lid cysts can interact with the lens causing lens decentration. Always perform an examination of the lids. Lid issues will usually impact on lens comfort. Treatment of the lid issue should be done prior to altering the lens fit. If the cornea is healthy patients can continue to wear lenses if there are lid issues. Preserved contact lens solution as well as allergic reactions can be a cause of lid roughness.
SOLUTION TOXICITY
See Lenses & Care Products Module
Nocturnal Adjustments
Nocturnal lenses are adjusted with the adjustment forms. There are three components of lens design that can alter the fit.
BOZR- This is the power, and can be thought of just like adjusting a soft spherical lens power. If a patient has a rRx of -0.50 the BOZR can be adjusted by -0.50 (0.1mm).
Treatment Zone – This is the BOZR and reverse zones and controls how close the center of the lens sits to the cornea. If there is irregularity in the treatment zone, or apical touch then the treatment zone needs to be altered.
Lens Fit- When topography reveals a decentred lens the alignment zone can be altered to adjust the fit.
Each adjustment order form has instructions.
The most common adjustment is when a power change is needed. The Power Adjustment form enable a pair of lenses to be ordered with a power adjustment for each eye.
When the treatment zone is bullseye or with expected decentration but has some irregularity the Treatment Zone Adjustment Form enable the apical tear thickness and the power of a lens to be ordered.
When lenses show poor fit the Lens Fit Adjustment Form enables full alteration of the lens.
The Clinical support Form can be completed for unusual cases or if you are unsure of what adjustments you should make.
The following case example works through the process for altering a decentred lens fit.
CASE EXAMPLE
Baseline
Rx: -3.50D 6/5
Topography: Ks 8.00 x 7.90, HVID 11.6mm. The baseline cornea does not have any displacement.
Rx and topography values indicate the Px should achieve full correction.
At review
UVA: 6/7.5
SE: -0.75 6/7.5
rRx:-0.75/-0.50 x 110 6/7.5
The subtractive topography is shown. Matching the topography pattern to the patterns in the adjustment form indicates a smiley face / superior TZ and the alignment zone of the lens is flat.
When a lens is decentred it will not give the full power correction. So although there is a -0.75 residual Rx this will not be incorporated into the new lens. Correcting the TZ centration should improve the power correction if the TZ moves onto the visual axis.
This case highlights the importance of good quality baseline maps. The baseline map is a small capture with missing information from the vertical area due to the eye not being open fully. A full capture may have prevented this adjustment.
Topography is indicating the AZ of the lens is flat. Steepening the AZ 1 step should bring the fit into alignment and give a bullseye TZ.
ADJUSTMENT PROCESS
1.Select the Fit Adjustment Form when logged in to scotlens.com
2.The topography pattern diagram advises steepen the AZ 1 step. This is selected on the form.
3.No other alterations are needed. Apical tear thickness is selected as Unchanged.
4.No power change is input.
5.The form is submitted confirmation of the changes is emailed to you.
REVIEW
At review the patient has UVA 6/6.
TZ is now bullseye.
The TZ power can still be seen to have some asymmetry in the power graph below the map. But no further change is indicated because the visual outcome for this patient is optimum, the baseline map is poor quality with the top lid cropping the capture. This may be affecting the reliability of the subtractive.
ALTERNATIVE OUTCOMES
If the TZ had not moved to bullseye it may be the lids interacting with the lens. It is not always possible to improve the TZ position. In cases where this happens optimise the power for the patient.
References DECENTRED LENS FITS
- Friling R, Weinberger D, Kremer I, Avisar R, Sirota L, Snir M. Keratometry measurements in preterm and full term newborn infants. Br J Ophthalmol. 2004
- Lisa A. Jones, G. Lynn Mitchell, Donald O. Mutti, John R. Hayes, Melvin L. Moeschberger, Karla Zadnik; Comparison of Ocular Component Growth Curves among Refractive Error Groups in Children. Invest. Ophthalmol. Vis. Sci. 2005;46(7):2317-2327
CORRECTING DECENTRED LENS FITS
SUMMARY
After completing this module you should:
- Understand other factors can be responsible for lens decentration
- Be able to interpret axial and tangential topography identifying poor fit patterns
- Identify corneal staining and its cause
- Understand the lens modifications that can resolve issues.