Insertion Tube Flexibility

As previously mentioned, the handling characteristics of the insertion tube are extremely important, particularly for colonoscopes. For easy insertion, the instrument must be capable of accurately transmitting all of the subtle movements and torque applied by the endoscopist. Any rotation that the endoscopist applies to the proximal portion of the shaft (torque) must be transferred to the distal tip of the instrument in a 1 : 1 ratio, although this capability is lost when the instrument is looped. The torquing ability of the instrument is facilitated by flat, spiral metal bands that run just under the skin of the insertion tube (see Fig. 3.2). Because these bands are wound in opposite directions, they lock against one another as the tube is torqued, accurately transmitting rotation of one end of the tube to the other. At the same time, gaps between these spiral bands allow the shaft to flex freely. The bands also give the insertion tube its round shape. Their stiffness prevents the internal components of the insertion tube from being crushed by external forces. These spiral bands are covered by fine strands of stainless steel wire, braided into a tubular mesh. A plastic polymer layer, typically black (or dark green on colonoscopes), is extruded over this wire mesh to create the smooth outer surface of the insertion tube. The polymer layer provides an atraumatic, biocompatible, and watertight surface for the insertion tube. It is usually marked with numbers to gauge the depth of insertion.

Experience has shown that a more rigid insertion tube is optimal for examining the fixed anatomy of the upper GI tract. The colon, with its tortuosity and freely moving loops, is best examined by a more flexible instrument. The instrument should be sufficiently floppy (nonrigid) to conform easily to the tortuous anatomy of the patient and to exert minimum force on the colon wall and attached mesentery. The instrument also should have sufficient column strength to prevent buckling when the proximal end of the instrument is pushed. In addition to its flexibility, the colonoscope should have sufficient elasticity to pop back into a straightened condition when it is pulled back; this aids in removing loops.

Obtaining the best combination of flexibility, elasticity, column strength, and torquing ability is the art and science of insertion tube design. Improvements in one of these characteristics often negatively affect one or more of the others. The final design is usually a compromise between these ideal characteristics, confirmed by months of clinical testing. To improve insertion further, the flexibility of both gastroscope and colonoscope insertion tubes typically varies from end to end. As Fig. 3.3 illustrates, the distal 40 cm of a colonoscope insertion tube is significantly more flexible than the proximal portion. This variation in flexibility is achieved by changing the formulation of the tube’s outer polymer layer as it is extruded over the wire mesh during manufacturing. As Fig. 3.4 illustrates, the extruder contains two types of resins, one significantly harder than the other. Initially, as the distal end of the insertion tube passes through the machine, a layer of soft resin is applied to the distal 40 cm of the wire mesh. This soft resin is gradually replaced by the hard resin within a transition zone near the middle of the tube. The proximal portion of the insertion tube (50 to 160 cm) is constructed of only the hard resin.¹ The end result is an insertion tube that has a soft distal portion for atraumatically snaking through a tortuous colon, with a stiffer proximal portion that is effective at preventing loop reformation in the portions of the colon that have already been straightened by the colonoscope.

Fig. 3.3 Graphic representation of insertion tube stiffness. Note the variation—the distal 40 cm is more flexible. Note extra stiffness when the instrument is in the “stiff” setting (dashed line) versus in the “soft” setting (solid line).

Fig. 3.4 Composition of insertion tube cover.

Adjustable Flexibility

Clinical experience has shown that endoscopists often disagree over what constitutes the ideal insertion tube. This disagreement may be due to differences in training, insertion technique, or past experience. In addition, some endoscopists have expressed a desire to change the characteristics of the insertion tube during the procedure, based either on insertion depth or on the patient’s anatomy, which has led to the development of an insertion tube with adjustable stiffness.²

Colonoscopes with adjustable stiffness have a tensioning wire that runs the length of the insertion tube (see Fig. 3.2). The amount of tension in this wire is controlled by rotating a ring at the proximal end of the insertion tube, just below the control section (Fig. 3.5). When the pull wire in this stiffening system is in the “soft” position, the stiffening system provides no additional stiffness to the insertion tube beyond that provided by the wire mesh and polymer coat. As Fig. 3.5 illustrates, when the control ring is rotated to one of the “hard” positions, an angled slot in the control ring pulls on the slide pin at the end of the pull wire, stretching the pull wire and placing it under heavy tension. This tension stiffens the coil wire that surrounds the pull wire and adds significant rigidity to the insertion tube. As Fig. 3.3 illustrates, although the base stiffness of the insertion tube is established by varying the mixture of hard and soft resins in the polymer base layer, the insertion tube can be stiffened further at will during the procedure by rotating the stiffness control ring. The variable stiffness mechanism does not run the entire length of the insertion tube so that the distal portion of the endoscope is not affected.

Fig. 3.5 Mechanism for stiffening a variable-stiffness colonoscope.

Distal Tip

Fig. 3.6 illustrates the components found in the distal tip of a typical end-viewing endoscope, such as a gastroscope or colonoscope. The larger of the circular glass lenses on the distal tip is the objective lens. This lens focuses a miniature image of the GI mucosa on the surface of a solid-state CCD image sensor. The image sensor sends a continuous stream of images back to the videoprocessor via a collection of very fine electrical wires. The objective lens and CCD unit are tightly sealed to prevent condensation from fogging the image and to protect the imaging system from damage if fluid were to enter the instrument accidentally. Light to illuminate the interior of the body travels through the instrument via fiberoptic illumination fibers. This light is evenly dispersed across the endoscope’s field of view via a light guide lens system.

Fig. 3.6 Colonoscope distal tip assembly.

Some endoscopes have a single illumination system (as shown in Fig. 3.6). Other endoscope models have two fiberoptic bundles and two light guide lenses to improve illumination on both sides of the biopsy forceps (e.g., snare) and to facilitate the packing of components within the insertion tube. The channel used for biopsy and suction exits close to the objective lens on the distal tip. The position of the biopsy channel relative to the objective lens determines how accessories appear in the image as they enter the visual field. On some instruments, the snare or biopsy forceps appears to emanate from the lower right corner of the image; on other instruments, these accessories enter the visual field from the lower left corner, and so forth. When planning difficult procedures, such as piecemeal polypectomy or hemostasis, it is crucial that the endoscopist know where the accessories will enter into his or her field of view.

The insertion tube also contains small tubes that carry air and water through the instrument (see Fig. 3.2). These tubes typically merge into a single tube a few inches from the distal tip (see Fig. 3.9 further on). This combined air/water tube connects to the air/water nozzle on the tip of the instrument. Under control of the endoscopist, water can be fed across the objective lens to clean it, or air can be fed from the nozzle to insufflate the GI tract. Some gastroscopes and colonoscopes have an additional water tube and a water-jet nozzle on the distal tip for washing debris from the mucosa (see Fig. 3.6).

Fig. 3.7 illustrates the components found in the distal end of a typical duodenoscope. Fig. 3.7A is a schematic cross section through the optical and illumination systems found in the distal tip of the duodenoscope. The objective lens for viewing the tissue is now located on the side of the distal end rather than on the very tip of the instrument. A prism is used to deflect the angle of view 90 to 105 degrees and to convert the instrument into a side-viewing endoscope. The illumination fibers are likewise steeply bent at the tip of the instrument, directing the light to emanate from the side. As in end-viewing instruments, an air/water nozzle positioned near the objective lens directs water across the lens to clean it, followed by air to blow away any remaining water droplets. Air from this nozzle is also used to insufflate the patient. All duodenoscopes have a forceps elevator to deflect actively the tip of any accessory passed through the channel. The elevator mechanism is shown in Fig. 3.7B. This elevator normally lies in a recess within the tip of the endoscope (lowered position). When the endoscopist wishes to raise the accessory up into the field of view, he or she operates a thumb control on the control section of the instrument (not shown). This thumb control pulls on the elevator wire, lifting the elevator out of its recess into a raised position, deflecting the tip of the accessory up into the field of view. In some instruments, such as the V-scope (Olympus Corp, Melville, NY), this elevator has a small groove that entraps a 0.035-inch guidewire to aid in wire stabilization during accessory exchanges.

Fig. 3.7 A and B, Duodenoscope distal tip assembly.

Bending Section and Tip Angulation

The distal tip of the insertion tube of the endoscope can also be manipulated by the endoscopist. The deflectable portion, referred to as the bending section, is constructed differently from the rest of the insertion tube. As Fig. 3.8 illustrates, the bending section is composed of a series of oddly shaped metal rings, each one connected to the ring on either side of it via a freely moving joint. These joints are constructed using a series of pivot pins, each one displaced from its neighbors by 90 degrees. One set of pivots allows the bending section to curl in the up-and-down direction. A second set allows the bending section to curl in the right-and-left direction. Together, they enable the bending section to curl in any direction. The direction of the curl is controlled by four angulation wires that run the length of the insertion tube (see Fig. 3.2). These four wires are firmly attached to the tip of the bending section at the 3 o’clock, 6 o’clock, 9 o’clock, and 12 o’clock positions. Pulling on the wire attached at the 12 o’clock position causes the bending section to curl in the up direction and achieves what endoscopists refer to as “up tip deflection.” Pulling on the wire attached at the 3 o’clock position causes right tip deflection. Pulling the other two wires causes down and left deflection. The endoscopist is able to pull on each of these wires in turn by rotating either the up-and-down or right-and-left angulation knobs. (For simplicity, Fig. 3.8 illustrates only the up-and-down angulation system.) Rotating the up-and-down and right-and-left knobs together produces a combined tip movement (e.g., upward and to the right) and allows the endoscopist to sweep the tip of the endoscope in any direction.

Fig. 3.8 A, Construction of bending section. B, Rotation of angulation knob caused deflection of bending section.