Peripheral Edge Zone or Landing Zone

Peripheral Edge Zone or Landing Zone explained

Note: The msdtm has 4 edge positions with it’s 15.8 mm diameter, as illustrated here, they are referred to as:

St, Flat 1, Flat 2 and Flat 3.

Our 18 mm msdtm list only St and 1 flat part of this is the chord diameter and design which is why the 18 mm St has a much flatter landing zone than the 115.8 mm as a standard landing zone.

Important to Know

Each step flat lowers the lens close to 100 microns, i.e. 2 Flat lifts the landing zone by 200 microns thus the lens will lower closer to the cornea by 200 microns, while 3 flat is a step of 300 microns!

This is important to know so you can estimate or accurately determine the change in the clearance over the apex or the lowering in general.

For example, after the lens has settled for half an hr you have noticed you have a tear layer thickness of 150 microns but you have too much blanching, you will then order a 1 flat, if you did not compensate you will now observe a tear layer thickness of only 50 microns, which is too low. You needed to compensate 100 microns to be where you were.

Sag changes do not alter the power except when you compensate to the next sag group that may have a different central radius and that may require SAMFAP adjustment, please check our availability chart. 

TLZ Torodial Landing Zone

From time to time the scleral toricity is greater than anticipated often noticed by some blanching at the 3 and 9 o’clock position only. In that case we have available on loan or for purchase a TLZ fitting set!

This simple set will allow you to diagnose the amount of toricity and lens required.

Available in many sag choices the msdtm has the following options available inside the TLZ fitting set, all of them logically at 90º and 180º 

S/F1 (standard/Flat1)

S/F2 (standard/Flat2)

F1/F2 (Flat 1/Flat2)

Although flatter lenses are available they are rarely used.

Please remember the same rule applies for the TLZ as for the dperical lenses, calculated from the flat each step flatter equals a drop in actual height. Example: switching from a ST/F1 to a ST/F2 is a 100 micron central lowering and you just need to be aware and may need to compensate for the loss in tear layer thickness!