Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Ophthalmologists are microsurgeons, trained to operate with precise control and meticulous detail on one of the smallest organs in the body. As such, they rely on a variety of magnification instruments to operate. Operating microscopes can provide staggering levels of magnification and resolution for corneal, cataract and vitreoretinal procedures. However, many external, oculoplastics and pediatric procedures can also be performed with high quality surgical loupes.
Surgeons from a wide range of microsurgical subspecialties use surgical loupes on a routine basis in their practices. Loupes can provide up to six times magnification compared with operating microscopes, which provide up to 40 times magnification. While operating microscopes are usually necessary when performing procedures on structures that are less than 1-2 mm, loupes are less expensive, easier to use, and mobile alternatives for procedures requiring less magnification. Amongst the range of surgical subspecialists, Ophthalmologists depend relatively heavily on operating microscopes and surgical loupes.
Several factors should be taken into consideration when purchasing surgical loupes. First, resolution of the loupes is critically important. Resolution is the capacity to differentiate small structures from one another. Resolution depends heavily on the design and quality of the lenses used in the loupes, and is the primary factor in determining the overall quality of the loupe. Some lenses may provide excellent resolution across a portion, but not all, of the visual field. It is difficult to ascertain the resolution of the instrument without trying on the loupes and comparing them with other loupes. However, such trial and error is often necessary to ensure the best “fit” when purchasing surgical loupes.
Another important factor to consider is the width of the loupe field. Usually, ophthalmic surgeons will prefer a large field width, since a wider loupe field allows the surgeon to keep his or her instruments in perspective with the area of focus, thereby simulating the wide visual field of the naked eye. This facilitates instrument adjustment and shortens response times by the surgeon; in addition, it could reduce eye fatigue. The width of the field is also determined in part by the design and quality of the optics. However, it relates principally to the magnifying power of the lens, since a high power lens inevitably will have a smaller field width compared with a low power lens. Of course, the magnifying power of the surgical loupes is an important consideration as well. Ophthalmic surgeons usually prefer loupes magnifying between 2.5X to 4X.
Some procedures require more magnification than others, and a surgeon performing oculoplastics procedures may require loupes with different magnification compared with those surgeons performing pediatric strabismus procedures or scleral buckles. In addition, two surgeons performing the same procedure may prefer two separate magnifications to achieve maximum comfort. Unfortunately, no standard exists to measure magnification across surgical loupe manufacturers. Different manufacturers use different definitions when determining magnification, and ten to fifteen percent differences in magnification are not uncommon for a reported magnification level.
An often overlooked factor contributing to the overall quality of a surgical loupe is the instrument’s depth of field. The field depth is a measure of the loupe’s capacity to focus across a given distance. If the loupe field depth is long, the surgeon will be able to focus easily over a long range in the surgical field. Similarly, if the loupe field depth is short, the surgeon obtains a sharp image only over a narrow, restricted range. Less important factors in selecting surgical loupes are the weight of the instrument (most surgeons prefer lighter frames to minimize strain), and the working angle of the loupes.
The working angle, also referred to as the optical declination angle, is the angle to which the surgeon lowers his or her eyes in order to obtain the optimal working position through the loupes. The working angle can be measured by a protractor device, but most surgeons measure their angle by sitting in their optimal working position with their eyes closed, then opening their eyes to ensure that their loupes match their working angle. The Ophthalmologist should choose a working angle that maintains an excellent musculoskeletal position in order to avoid muscle stain of the head, neck, or back. Innovations in lens design and quality will continue to improve upon existing surgical loupes and expand the types of procedures that Ophthalmologists can perform without surgical microscopes.