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3 Magnification Surgery

       Heidi Phillips

Introduction

      Surgery of exotic animals frequently involves small anatomic structures and the need for consistent and reliable exposure, illumination, precision, and clear focus while operating in a confined space. Using some form of magnification is recommended for most surgeries of small exotic animals (Beeber 2000; Bennett 2000b; Bennett and Lock 2000; Jenkins 2000b; Lock 2000; Mullen 2000; Mullen and Beeber 2000; Samour 2010). Technological advances in magnification surgery and refinement of microinstruments and microsuture enable the exotic animal surgeon to successfully perform technically demanding surgical procedures (Jarrett 2004; Bohan et al. 2010; Carr and Castellucci 2010). Magnification surgery enables visualization of detail and differences of anatomy, pathology, and tissue color and character not otherwise discernible (Jarrett 2004; Mungadi 2010; Al‐Benna 2011; Stanbury and Elfar 2011). Small, bleeding vessels are more readily identified for coagulation, minimizing hemorrhage, and improving outcome for many procedures (Bennett 2000b; Mungadi 2010).

Optics and Principles of Magnification

      Resolution is the ability of an optical system to discern detail in an object or distinguish two separate objects. The human eye is the limiting factor of many optical systems, its ocular resolution being 0.2 mm (Carr and Castellucci 2010). This means that people who observe two points closer together than 0.2 mm will see only one point.

      Magnification of an image is increased most easily by decreasing the distance between the eye and the object being imaged. The resolution limit of the unaided eye can be increased by close proximity to objects (Chang 2013). This is not always achievable in surgery as decreasing the distance between the surgeon and patient may not permit safe, aseptic manipulation of instruments and tissues or an ergonomic, comfortable, and sustainable posture for the surgeon (Bennett 2000a). Moreover, the healthiest human eye cannot refocus an image at distances closer than 10–12 cm (Carr and Castellucci 2010). Optical aids such as operating microscopes and surgical loupes can improve resolution by many orders of magnitude (Carr and Castellucci 2010). Optical aids permit safe magnification of tissues by increasing the size of the image of the object that is projected to the surgeon's retina.

Operating microscopes and commonly used surgical loupes achieve magnification using a two‐lens system: the objective lens and eyepiece lens (Figure 3.1). The objective lens, which is nearest the object being imaged, focuses light rays from the object to generate a real, inverted image. The eyepiece lens transforms this image to the magnified, virtual image seen by the surgeon (Carr and Castellucci 2010; Cordero 2014). Total magnification of the system is the product of the magnification afforded by both the objective and eyepiece lenses (Carr and Castellucci 2010).

      Although operating microscopes and surgical loupes utilize similar optical principles, they differ in how magnification is defined. The distance between the objective lens of operating microscopes and the objects to be operated is fixed, and all users achieve the same magnified image. The distance between the objective lens of a surgical loupe and object to be operated varies according to the surgeon's stature and posture; not all users achieve the same magnified image with the same loupe. As the working distance increases, the magnification power decreases. Loupe models are named according to magnification power, but specified magnification power can be achieved only at a specified distance. Users with longer working distances require higher‐power loupes than users with shorter working distances to achieve the same level of magnification (Chang 2014a).

To effectively utilize optical aids in surgery, the exotic animal surgeon must understand the principles of magnification, including focal length, depth of focus, working distance, and field of view particular to the optical aid (Pieptu and Luchian 2003). A surgeon