Читать книгу Surgery of Exotic Animals - Группа авторов - Страница 70

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 must properly manipulate the components of the optical system and surgical instruments ergonomically and in an ergonomic posture. (Carr and Castellucci 2010; Eivazi et al. 2015). The fundamental principles of optical magnification and components of the most commonly used optical systems are defined below:

Figure 3.1 Optics of magnification aids that use a two‐lens system. Note the focal length and working distance.

Source: Pieptu and Luchian (2003) and Cordero (2014).

Stereopsis: Perception of depth and three‐dimensional structure obtained by processing visual information delivered through the eyepieces to the surgeon's eyes. Stereopsis is achieved using an operating microscope or surgical loupe by manipulation of the eyepieces to accommodate the surgeon's interpupillary distance and visual deficits, also termed stereoscopy (Carr and Castellucci 2010; Socea et al. 2015).

Binocular head: Body of an operating microscope. Contains the eyepieces, lenses and/or prisms, and magnification changer (Figure 3.2). The binocular head may be straight, inclined, or inclinable (Carr and Castellucci 2010). Inclinable binoculars are adjustable through a wide range of angles to allow the surgeon comfortable head and neck posture and working position.

Eyepieces: Eyepieces contain the binocular lenses and/or prisms necessary for magnification. Eyepieces of operating microscopes are available in powers of 10× and 12.5× most commonly and contribute to total magnification achieved by the optical aid (Carr and Castellucci 2010). Eyepieces have rubber cups adjustable in height to accommodate surgeons wearing corrective eyeglasses and have diopter settings adjustable for each surgeon's vision deficits (Figure 3.3).

Figure 3.2 Binocular head–body of the operating microscope. Contains the eyepieces, lenses and prisms, and magnification changer.

Figure 3.3 Eyepieces contribute to the total magnification of an operating microscope. Note the rubber cups and diopter settings adjustable to an individual surgeon.

Focal length: A measure of how strongly a lens converges or diverges light. The distance from the center of the lens to the area on the lens where light rays originating from a point on the focused object converge (Pieptu and Luchian 2003; Cordero 2014) (Figure 3.1).

Interpupillary distance: The distance between the centers of the surgeon's pupils. Eyepieces of an operating microscope or surgical loupe are adjustable or customized to accommodate the interpupillary distance of the individual surgeon (Carr and Castellucci 2010; Mungadi 2010) (Figure 3.4).

Focal depth (depth of focus): Range of object position through which the object may be viewed at a set magnification level and remain in focus (Pieptu and Luchian 2003).

Working distance: The distance from the objective lens to the object. Working distance is dependent on the focal length of the lenses and ranges from 22 to 50 cm (Pieptu and Luchian 2003; Carr and Castellucci 2010; Cordero 2014) (Figure 3.1).

Figure 3.4 Eyepieces of an operating microscope are adjustable to accommodate the interpupillary distance of the individual surgeon.

Field of view: Extent of the operating field seen in focus through the optical system (Figure 3.5). Field of view changes with magnification level according to the formula: field of view diameter = 200/total magnification factor (Pieptu and Luchian 2003). The diameter of the field of view is inversely proportional to the level of magnification; the higher the magnification, the smaller the field of view (Carr and Castellucci 2010). In exotic animal surgery, it is possible at times to fit the entire patient into the field of view (Bennett 2000a).

Magnification changer of operating microscopes: A system of lenses between the objective and eyepiece lenses that allows for changing magnification manually by 3–6 steps at a time or for continuous adjustment of magnification (power zoom) (Carr and Castellucci 2010; Cordero 2014).

Magnification changer of surgical loupes: Interchangeable working distance optics available with some loupes can increase magnification power for special procedures (Chang 2015).