The use of smaller diameter suture material and finer, sharper needles has made it preferable, if not necessary, to use magnification for strabismus surgery. Telescopes mounted on glasses frames or on a headband are extremely useful. The magnification may vary from 2.5X to 4.5X. The limiting factors in magnification include (1) surgeon's comfort, (2) restricted field size, (3) limited depth of focus, and (4) need for increased illumination.
For comfort, a properly fitted pair of spectacle frames with a wide elastic band behind the head connecting the temple pieces of the glasses frame works well (Figure 16). The surgeon soon becomes accustomed to the various restrictive factors associated with use of a magnifying device while enjoying the improved view. If the surgeon has presbyopia, he or she may choose to place a suitable add low in the spectacle lens to obtain a wider useful field of vision when looking at near, but away from the operative field and ‘around’ the loupes (Figure 17).
Figure 16 Operation telescopic loupes
Zeiss telescopic loupes mounted on a sturdy spectacle frame with an elastic head strap
A fiber optic head-mounted spot light provides superior illumination especially when working in a deep, narrow cavity.
How much magnification is best?
Field size decreases with increase in magnification. The trade-off should be arrived at by the surgeon through a trial and error method. Working distance is unique to the particular magnifying instrument used. This should be selected according to the surgeon's preference, but the working distance should not be too close. Depth of focus also should be determined by trial and error. Illumination may be improved by using an overhead operating room light that is properly adjusted. Some surgeons prefer additional light supplied by a head-mounted fiberoptic light (Figure 18).
A few surgeons use a floor-mounted or ceiling-mounted microscope for strabismus surgery. This technique provides excellent magnification and illumination; however, with this technique, the surgeon is even more severely restricted. Those surgeons who use a microscope for strabismus surgery are strong advocates for this technique. I suspect that once a surgeon has used the operation microscope and is reconciled to the trade off, it is difficult to go back to lower magnification. The optics and illumination of the operation microscope can be used to obtain videotapes of strabismus procedures. During these cases, the surgeon may elect to use a loupe for magnification while operating ‘around the microscope.’ Some surgeons who use the technique of anterior ciliary vessel salvage while recessing or resecting an extraocular muscle have recommended use of a microscope.