COMET Combined Microscopic and Endoscopic surgery
Introduction:
Endoscopic sinus surgery has grown in leaps and bounds. With better understanding of the mucociliary mechanisms in clearing the sinuses has made the endoscopic surgeon more conservative. In this surgical modality two optical systems are used surgically to treat nose and paranasal sinuses.
The optical systems used are:
1. Optical microscope
2. Nasal endoscope.
This combination of both optical systems for the same surgical procedure is called combined microscopic and endoscopic technique (COMET surgery).
The microscope commonly used in this surgical procedure is the Zeiss, with 300 mm objective lens. The optimal magnification preferred is 6 x.
Major parameters of optical systems used in this surgery:
1. Focal distance (working distance)
2. Depth of field (focus)
3. Numerical aperture
4. Field of view
5. Resolution
6. Brightness
7. Distortion
8. Protection
Focal distance: (working distance) is defined as the distance between the object and the first surface of the lens system. For optimal surgical results this distance should be really a comfortable one. The maximum working distance should not exceed the length of the operating surgeons arm. The working distance increases when the focal length of the objective lens is increased. As far as the microscope is concerned this is optimally 300 mm.
When using a microscope the head of the surgeon is held steady behind the eyepiece, the focus is adjusted by simply turning a knob provided for this purpose. Both the hands are incidentally free for purposes of manipulation.
In the case of an endoscope, the focal distance is fixed and cannot be adjusted. The working space is also limited. The head and one of the hand of the surgeon will have to move to keep the operating field in focus.
Depth of field (focus): is defined as the distance by which the image may be shifted longitudinally with reference to the retina of the viewer's eye without blur. This is actually the visible depth of the object. A microscope understandably provides a much larger field of depth, making the surgeon really comfortable. Where as in an endoscope the depth of field is shorter, and hence the endoscope should be moved constantly to increase its field of depth.
Numerical aperture: This is related to the depth of field (focus), the luminance and the resolution of the system. The numerical aperture of the microscope is smaller and hence more light is required to illuminate the target area. The resolution limit is also smaller than that of an endoscope. This aperture can be varied and adjusted.
In an endoscope the numerical aperture is higher and the image hence looks brighter, with better finer details. However the numerical aperture is fixed and cannot be adjusted.
Field of view: In the case of microscope the field of view is more extended, and the surgeon is able to follow the movement of almost the whole of the surgical instrument, where as in the case of an endoscope, the field is much smaller and hence only the tip of the surgical instrument used will be visible.
Resolution: is defined as the minimal distance which is necessary to separate two points in a surgical field. In the case of a microscope, the resolution limit is smaller, but can be varied and adjusted. In the case of an endoscope the resolution is higher, giving finer details in a clear form. However the resolution is fixed in the case of an endoscope.
Luminance: Is defined as the brightness of an image. In the case of a microscope the luminance is smaller, but can always be increased by using a bright light. The endoscope has a higher luminance. Since the tip of the endoscope acts as the light source, surgical instruments with highly reflective surfaces whne held close may cause distortion and saturation of images.
Cleanliness / protection: This is vital in any surgery to prevent any post operative infections. In the case of a microscope all the optical systems are far away from the surgical field, and hence is ideally protected. The endoscope however must be place close to the operating field, hence is prone for staining, fogging and scratching.
Microscope / Endoscope Optical perfomances:
| Optical characteristics | Microscope | Endoscope |
|---|---|---|
| Focal length | 0.5 - 100 mm | 0.2 - 5 mm |
| Magnification | 5x - 5000x | 2x - 26x |
| Numerical aperture | 0.10 - 0.96 | 0.25 - 0.7 |
| Resolution power | <430 l p/mm | 250 - 400 l p/mm |
| Field depth | Few micrometers | Micrometers to nanometer |
| Field of view | 6 - 20 mm | 30 - 120 degrees (different scopes) |
| Distortion | Small | Large |
Ergonomics of a microscope:
The major advantage of a microscope is that it is rigid, self supported and positioned far away from the surgical field. The field of view is large, with a good depth of field. The major advantage being both the hands of the surgeon are left free for manipulation.
Ergonomics of an endoscope:
The endoscope provides good image clarity, and resolution. By changing the endoscope angular vision is possible. The major disadvantage is that one hand is occupied for manipulation of the endoscope, leaving only one hand for surgical instrumentation.
By judicious combination of both these excellent optical instruments ideal surgical results can be achieved in COMET surgery.
Procedure:
The surgery is ideally performed under general anesthesia. The patient is placed in supine position, with head slightly elevated and gently rotated towards the surgeon.
The surgeon should be standing, with the patient's nose right in front of his eyes.
General anesthesia is preferred because of its obvious advantages. The oropharynx should be packed with moist gauze. Before intubation the nasal cavity of the pateint is packed with gauze impregnated with 1% xylocaine with 1 in 100,000 adrenaline. This helps to shrink the nasal mucosa and the turbinates making visiblity of insides of nasal cavities easy.
1. Septoplasty is performed if necessary for access. If needed this step may be done under microscopy.
2. Management of middle turbinate comes next. Concha if present must be removed.
3. Unicinectomy is performed under microscopy.
4. Maxillary antrostomy: This includes removal of diseased mucosa, irrigation with warm normal saline. The maxillary sinus is visualised using
a angled endoscope. Residual disease from inside the sinus cavity should be removed.
5. Ethmoidectomy (partial or total according to situation)
6. Frontal sinusotomy (performed using endoscopes angled), with irrigation with normal saline.
7. Sphenoidotomy using angled telescopes. Irrigation with normal saline is performed.
8. Removal of inferior turbinate if necessary.
