Shaft of Arthroscope

The shaft consists of number of lenses arranged longitudinally with spacers in between them. It should be noted that it is not one long lens buts a series on lenses. The first question that comes to mind is why do we need a long shaft and the second one is if we do need a long shaft, why don’t we have one long lens inside it? The shaft has to be long so as to enable the tip to be inside the joint while the eye-piece is sufficiently outside the body for the surgeon to see through it. Typically a 4mm arthroscope is available in lengths of 160mm to 175mm. The answer to second question is that one long lens is required because lens has a very peculiar feature that the longer it is, the greater is its weight and more expensive it is. Thus, in order to make the scope economically viable to sell and also easy to maneuver, we need a series of lenses. These lenses placed longitudinally are separated by spacer blocks that prevent the lenses from banging into each other and at the same time maintain the optical integrity. This combination of lenses and spacers in longitudinal fashion is called Rod Lens Technology. This technology enables the transmission and focussing of image onto the eye-piece satisfactorily at minimum possible cost.

Arthroscopes are available in varying shaft diameters. For big joints like knee and shoulder, Arthroscopes with shaft diameter of 4mm are used whereas for small joints like wrist and ankle, 2.7mm and 1.9mm shaft diameters are used. Smaller joints can accommodate smaller diameters and hence the variation in shaft specifications. Also note that the variation in diameter brings about difference in Field of View (FOV). Field of view is defined as the angle, within which the scope can receive light from an object. For any given endoscope, the FOV is constant. Typically, for a 4mm scope, the FOV is of around 110 - 120deg for different makes.