Metro-Seg Crescent Bifocal
The Metro-Seg Crescent Bifocal was designed to address the most desirable traits of presbyopic needs in a contact lens. These traits include a crescent shaped segment, gas permeable material, one piece construction, thin design, no image jump (monocentric), unlimited add power and a segment that self aligns.
The Metro-Seg is basically a prism ballast lens with a near crescent segment. This segment has a slight ledge at the top that tends to channel the segment into position with each blink.
Prism ballast is present to give the lens proper orientation and the amount of prism is outlined in the fitting procedure section. Segment height and prism may differ from what was common in PMMA bifocal fitting because of gas permeable characteristics.
Back surface design is biaspheric finish, which consists of a well blended aspheric secondary and peripheral curves that create corneal alignment without junctions.
FluoroPerm 60 (60-DK) is used as the lens material because of it's superior combination of wettability, permeability, comfort and deposit resistance. The FluoroPerm 60 material is a desirable option for the Metro-Seg in order to allow extra permeability due to the increase in thickness over standard GP (Oxygen Permeable) lenses.
Metro-Seg Crescent Bifocal is a suitable choice for patients who need +1.00D or more add power for near vision correction. Experienced wearers of gas permeable and PMMA lenses adapt very well to Metro-Seg lenses with proper lens specification transfer into Metro-Seg design.
Soft lens patients may resist the initial "feel" of Metro-Seg or other GP lenses, but can be easily motivated by increased visual acuity at both distance and near . These soft lens patients should be prepared for the difference in "feel" of a GP lens and understand that only a short adaptation period is necessary to be successful and comfortable.
Metro-Seg can also be made in base toric, front toric and bitoric design to accommodate those patients with high degrees of corneal astigmatism or residual astigmatism.
Poor patients for Metro-Seg Crescent Bifocal lenses are those who need little or no correction at near or those who need little distance correction. Those patients who need only near correction are the worst candidates even though they may persist. These patients usually lose motivation quickly and either give up lenses completely or wear only for special occasions.
Other poor candidates may be the ones who have tried other modes of bifocal contacts unsuccessfully. They expect to fail, therefore they lack motivation.
The ideal candidate will have a firm lower lid that meets the limbus during straight ahead gaze.
In order to have the near segment in place for a maximum of wearing time, an on "K" fit or flatter is desirable to allow the Metro-Seg to drop into position quickly.
The enclosed fitting guide is directed toward achieving optimum fitting results, but patient control and motivation are essential. As with spectacle bifocals, there is an adaptation period for the patient to become accustomed to the bifocal segment. For this reason initial lens adjustment is discouraged during the first two weeks. Lens changes made strictly upon patient response can lead to fitting failure.
Segment Height Determination
Lower lid configuration is the most important factor in determining segment height for the Metro-Seg. If the lower lid is at the limbus, a segment placement of 0.5mm below geometric center is used for plus lenses, and a segment placement of 0.8mm below geometric center is used for minus lenses. If the lower lid is above the limbus an even lower segment height must be used.
Patients with a lower lid position as high as 1.5mm above the lower limbus are successfully wearing the Metro-Seg bifocal. However, patients with lower lids that position below the limbus should be avoided. These patients must either tilt their heads upward and look down to achieve translation into the reading segment or they have to squint to lift the segment.
When evaluating the segment height, a low light setting on the slit lamp works well to keep the pupil to a size which will more closely match a pupil size under normal room illumination. Under normal illumination the segment height should be at or below the pupil margin. In most cases a 0.4mm below pupil margin is desirable.
Metro-Seg bifocals are usually manufactured without truncation because of potential excessive lens movement created during blink as the upper lid lifts the truncation. Initial truncation is only recommended for those patients who are already wearing truncated lenses or those with plus lenses. Truncation can be used to lower the segment height. Truncation can also be used to achieve translation on those patients whose lower lids are not firm enough to support the lens in downward gaze. In these cases a minus angle truncation is recommended. The truncation is angled in toward the base curve creating a ledge for the lid to catch.
For best results, fit the Metro-Seg with the alignment technique. The following charts are a base curve selection guide and an illustration of appropriate diameters.
|Corneal Cylinder||Base Curve Selection|
|Pl to 1.00||.25 flatter than K|
|1.12 to 2.50||on flat K|
|2.50 or more||consider toric base|
|K = 45.00/45.00||use 44.75 base curve|
|K = 45.00/47.00||use 45.00 base curve|
Note that the Metro-Seg can be fit successfully without the use of bifocal trial lenses.
Segment position should be generally .8mm below geometric center, or just below the pupil margin under normal lighting. Segment height may need to be adjusted to compensate for lower lid.
|1 1/4||+4.00 or more|
|1 1/2||+3.87 to Plano|
|2||Plano to -3.00|
|2 1/4||-3.12 to -5.00|
|2 1/2||-5.12 or more|
|40.00 - 40.87||9.6||7.8|
|41.00 - 41.87||9.4||7.8|
|42.00 - 42.87||9.4||7.8|
|43.00 - 43.87||9.4||7.8|
|44.00 - 44.87||9.2||7.8|
|45.00 - 45.87||9.2||7.6|
|46.00 - 46.87||9.2||7.6|
|47.00 - 47.87||9.2||7.6|
|48.00 - 48.87||9.2||7.6|
All secondary curves are bi-aspheric.
No truncation is necessary for orientation.
Fitting Different Types of Patients
Use single vision trial lenses to determine the best alignment for new patients. Do fluorescein evaluation for all patients using the #12 Wratten filter and the cobalt filter on the slit lamp to observe the lenses. Alternatively, designing the lens from "K"s and refraction is also possible with very good results.
Refitting GP (Oxygen Permeable) Patients
If the original GP lenses were fit with the alignment technique, duplicate the old base curve. If the original GP lenses were fit using the superior limbal or clearance technique, use the trial lenses to determine the best alignment fit as if this were a new patient.
Refitting PMMA Single Vision Wearers
Single vision PMMA wearers should be fit with single vision GP lenses before any attempt is made to fit the Metro-Seg bifocal. They should be allowed to adapt to the GP single vision lenses for 90 days before they invest in bifocals.
Refitting PMMA Bifocal Wearers
The PMMA bifocal wearer can be fit with the Metro-Seg immediately. These wearers have to adapt to gas permeability and can make the transition except for easier breakage of GP's. In choosing the lens specifications, duplicate the original PMMA bifocal, but use a slightly lower seg height.
Power change, diameter reduction, front surface polishing and edge re-polishing should be performed by the laboratory to avoid Metro-Seg lens damage. In office modification to the lens' secondary and bevel should be done with caution by an experienced person, as these changes can lead to a need for edge reshaping and edge thickness reduction. Power change modifications are possible through the laboratory, but may be limited according to the existing lens power. Consultation with the laboratory can determine if the lens power modification is possible.
Metro-Seg Crescent Bifocal fitting examinations should include fluorescein pattern evaluation using a yellow Wratten filter attached to the slit lamp or Burton lamp.
It is very important that the lens be centered on the cornea when evaluating alignment, as an off center lens may confuse the final determination of fitting results. The fluorescein pattern should show a green band at the lens edge and a relatively even green distribution over the remaining area, free of noticeable pool or bearing areas.
Ideally, the lens will move freely with the blink and drop quickly to a position of about 1mm below the center of the cornea. After the blink, the bifocal segment will center itself within 20 degrees of the inferior 6 o'clock position. Segment rotation of up to 20 degrees should not be considered a problem.
Extra patient tearing may be present for most patients during initial lens dispense, therefore changes should be avoided if possible at this stage. A more complete evaluation is possible after patient adaptation of one to two weeks.
The most common initial patient complaint is that the near segment interferes with their distance vision, even if the segment is in the proper position. The segment line should be at or below the pupil margin after the lens settles. The patient will see a slight inferior blur when the segment is in position, but this is normal and does not interfere with distance vision. Attempts to eliminate this slight inferior blur will usually cause the segment to be too low for proper translation. A patient's blink pattern may affect the fitting results if they give a half blink or if they are a poor blinker.
By following the fitting guide, problems will be kept to a minimum, but even with these efforts, some adjustments may be necessary. In addition to the following chart, laboratory consultation is available for problem solving. Most problems will be related to either lower lid position or lens movement, in which case, both should be observed closely before seeking a solution.
|Segment and lens high||reduce lens diameter|
|Segment high, but not lens||order lower segment or truncate|
|Segment and lens low||order large flatter lens|
|Segment low, but not lens||order higher segment|
|Lens rotating more than 20 degrees||reduce lens diameter or truncate|
|Slow lag time||order steeper lens, or reduce lens diameter, or add prism|
|Excessive lens movement||order steeper lens, possibly larger|
|3 & 9 Staining||flatter and larger lens|
|Poor translation||truncate lens|