The first consideration of the buyer’s guide to surgical microscopes is what surgical discipline it will used for: neurosurgery, otolaryngology, ophthalmology or spine. Some models may be common among several disciplines and others may not. Eye surgery is the first one to come to mind.
The purpose of the surgical microscope is to enhance the surgeon’s view during the operation. Items such as focus, magnification and illumination are the usual factors to consider. However, there are additional factors: stability, foot print, integration with IGS, positioning, size, plus add ons such as video cameras, recorders, monitors, fluorescent cameras. Let’s start at the beginning for the buyer’s guide to surgical microscopes: the binoculars.
There are many types of binoculars. Fixed position or inclinable. With inclinables, a consideration is the degree of angulation you will need (160°, 180°, 200°). In addition, the field of view of each ocular is important. 10X offers a wide FOV while 12x offers a narrower FOV. Is the light pathway a set of mirrors or prisms. Prisms are much better at transmitting the image, but they are more expensive. The other thing is the IPD (inter-pupillary distance adjustment). Is it on a adjustable rod with sterilizable knobs or do your have to grab each ocular to adjust.
Here is where commonality among disciplines ends. Neurosurgery, otolaryngology, and spine microscopes all have variable focus. The microscope body does not move, rather the internal optical lenses move. As an example, that means you could have a focal range of 200 mm to 500 mm depending on the front lens. Many microscopes will have various variable focal lengths to choose from. Consideration for these focal lengths are the instrumentation used as well as arm reach needed.
For ophthalmology, a fixed focal length is used: 175 mm or 200 mm are the most common. Fine focusing can be adjusted by using a foot pedal. In this case, the entire microscope body moves up and down. In addition, most current model optics have apochromatic coated lenses for truer anatomical color.
There are basically three types of illumination sources used with surgical microscopy: halogen, xenon and LED. Neurosurgical, ENT and spine procedures will use 300 W xenon bulbs that generate a color temperature of around 5600 K (Kelvin) which is considered “daylight”. The advantage of this temperature range is that the anatomical structures color will be more truer. One characteristic of a 300 W xenon light source is it generates a lot of heat. If special light cables are not used, the heat can affect the skin of the patient.
On the other hand, for ophthalmology (and some ENT procedures), halogen or LED illumination is utilized. That’s because xenon contains the blue light spectrum which can cause permanent damage to the retina. The halogen color temperature is around 2700 K due to the fact it is a 150 W incandescent bulb. LED illumination is becoming used more in ophthalmology due to it’s greater homogenous lighting of the cornea alleviating “hot spots”. These LEDs usually have a “soft” filter that mimics the halogen temperature color simply because that is what the ophthalmologist is used to.
The buyer’s guide to surgical microscopes considers three basic types of floor stands: counter weight balanced arm, spring balanced swing arm, and friction balanced arm. Let’s take each in turn:
COUNTER WEIGHT BALANCED ARM: These types of floor stands (see image below) are mostly for neurosurgical or neuro-otology procedures. The floor stand arm has weights at one end to compensate for the weight of the microscope optics on the other end. When perfectly balanced, the surgeon is able to maneuver the optics via the handles ergonomically and effortlessly. This is especially important during neurosurgical cranial cases where constant re-positioning of the optics are necessary. Many of the manufacturers of counter balanced microscopes offer automatic or at least one touch balancing. The brakes are electromagnetic. When the brakes are released the optics should be free floating and when the brakes are engaged there should be no bouncing of the optics. The arm moves up and down and lateral movements are accomplished by rotating the floor stand tower on it’s base.
SPRING BALANCED SWING ARM: These type of floor stands are used many in spine surgical cases as well as ENT cases. They utilize a series of two arms (see image below) that maneuver laterally (the arm that is attached to the floor stand base) as well as up and down/laterally (the arm that holds the microscope optics). The arm is supported by a tension spring that is, in most cases, balanced by a motor. Electromagnetic brakes are employed as in the counter weight balanced arm.
FRICTION BALANCED ARM: These floor stands are usually used in clinic or in minor surgical procedures. They do not have motorized balancing, however some do have mechanical brakes.