Gidon Ellis, Daniel Cohen, John A Bycroft and Jim Adshead
Q. What are the characteristics of the ‘ideal’ stent?
A. The ideal stent would have the following characteristics [1]:
Have good memory, with a configuration that prevents migration
Have excellent flow characteristics
Be radio-opaque
Be biologically inert (biocompatible)
Resist biofilm formation, encrustation and infection
Be made of a flexible material with a high tensile strength
Be easy to insert
Be easy to remove or exchange
Have a reasonable price
Be used with minimal complications
Q. What are the indications for stent insertion?
A. The indications can be divided into elective and emergency. Elective indications include protection of anastomosis (pyeloplasty, ureteric reimplantation), to overcome extrinsic ureteric compression, prior to chemotherapy to optimise renal function in obstructive uropathy and pre-operatively (gynaecological or colorectal surgery) to aid identification of the ureter. Emergency indications include relief of ureteric obstruction and management of ureteric trauma.
Q. What are the complications of ureteric stent placement?
A. As well as those of actual insertion, complications can be divided into common and rare, as described in Table 14.1 [1].
Table 14.1 Common and rare problems of ureteric stent
|
Common |
Rare |
|
Trigonal irritation |
Obstruction |
|
Haematuria |
Kinking |
|
Fever |
Ureteric injury/ureteric perforation |
|
Infection |
Stent misplacement |
|
Inflammation |
Stent migration |
|
Encrustation |
‘Missed’/forgothen stent |
|
Biofilm formation |
Tissue hyperplasia |
Q. What are ureteric stents made of? Why are they radio-opaque?
A. Ureteric stents are manufactured from a variety of polymers, such as polyurethane and styrene-ethylene-butylene (C-flex). The radio-opacity of stents is increased by coating them in metals such as bismuth and barium.
Silicone stents are also manufactured - these are stiffer and thus may cause more mucosal irritation but can be left in situ for up to 1 year (cf conventional polyurethane stents, which need to be changed every 6 months).
Stents are generally between 22 and 30 cm in length and are usually of the ‘double-pigtail’ variety. Sizes are generally 4.7 to 8 Fr.
Metallic ureteric stents are variably used for benign or malignant ureteric strictures, e.g. Memokath ureteric stent made of nickel-titanium memory-shape alloy (Nitinol).
Q. What are the different types of ureteric guidewires available?
A. Many forms of ureteric guidewires have evolved over the years. Most guidewires are in the order of 0.035-0.038 inches in diameter, and approximately 150 cm long. Various configurations exist, and commonly wires may be coated with PTFE (polytetrafluoroethylene) and have flexible tips of various lengths. Variations include hydrophilic wires (such as the Terumo wire), guidewires with a hydrophilic tip (e.g. The sensor wire) and stiff wires (e.g. The Amplatz Super Stiff).
Q. What are the various baskets available for ureteroscopic surgery?
A. A large number of ureteroscopic baskets are commercially available. ttey may either be ‘tipped’ or ‘flat wire’, as used in semi-rigid ureteroscopy, or ‘tipless’, as used in flexible ureterorenoscopy. The tipless variety may allow easier access using the flexible scope, and avoid trauma to the collecting system (easily inserted into renal calyx if necessary). Baskets are commonly made of nickel-titanium memory-shape alloy (Nitinol), and range in size, from about 2 to 3.2 Fr. Baskets are available that open in different ways, such as ‘parachute’ and ‘helical’. (You should be prepared to talk about the baskets that you use in your practice.)
Q. Describe how a modern telescope, as used in cystoscopy, works.
A. Originally, before the work of Prof. Harold Hopkins, telescopes consisted of fine lenses cemented into long metal cylinders separated by long airspaces. This was replaced by the ‘Hopkins Rod-Lens System’ in the 1960s. This system is still in place today, and involves a series of long glass rods in a metal cylinder separated by shorter ‘lenses’ of air. The advantages of this are as follows: durability, superior light passage and image quality, reduced diameter of instrument (permitting parallel access channels), colour reproduction and the ability to ‘document’ images with photography or video.
Light is transmitted by optic fibre bundles running from an external light source (note this is usually a halogen external light source, which emits ‘yellowish’ light - thus the need for white balancing; neon light sources are expensive but do not need white balancing).
Q. How does an optic fibre work? What are the two main applications in urology and how do they differ?
A. Optic fibres are flexible glass (or plastic) fibres that allow light to pass through them via a process termed total internal reflection. Optic fibres are grouped together in a parallel fashion and protected by external plastic sleeves.
The two main uses in urology are for
Transmission of a light source. ‘Light leads’ transmit light from an external source to endoscopes. These leads consist of non-coherent fibres, and are relatively inexpensive to produce.
Transmission of images. Image transmission (e.g. from a camera) relies upon coherent bundles of optic fibres. In this case, the orientation of the fibres at the proximal end must be the same as the orientation at the distal end to prevent image distortion.
Flexible cystoscopes and both semi-rigid and flexible ureterorenoscopes have traditionally used a fibre-optic system, although some newer scopes utilise a digital system. As well as a non-coherent bundle of fibres to transmit light from the external light source, a fibre-optic endoscope utilises a coherent glass fibre bundle, which transmits light back to the eye-piece of the scope in an ordered fashion. Light is transmitted via a process known as total internal reflection through many thousands of fibres and the resultant image can be visualised directly or via a camera-stack system.
In common with rigid scopes, a working channel allows the passage of irrigation and instruments into the patient, although this channel is often of a smaller calibre. A flexible endoscope has a deflecting tip, which moves in response to the deflecting lever controlled by the surgeon. These are connected by two control wires. Flexible scopes are expensive to purchase and maintain and susceptible to damage. As many of the elements of the flexible scope are not heat resistant these scopes cannot be sterilised but rather decontaminated only.
Semi-rigid ureteroscopes utilise fibre-optics encased in a metal sheath, and not a rod- lens system. This provides the surgeon with a rigid instrument while permitting certain flexibility and is ideal for operating in the ureter.
Digital endoscope systems utilise a chip at the distal end of the scope which captures and transmits a digital image. The image tends to be of a much higher quality and the light cable and camera are integrated within the system, removing the need for extra cables and a heavy camera-piece to be attached to the hand-piece of the scope. These instruments are more expensive and at the present time tend to be slightly larger diameter than fibre- optic devices but will no doubt play an important role in the future of endo-urology.
The development of disposable flexible ureterorenoscopes is currently being evaluated.
Q. How do we express the size/diameter of surgical instruments (e.g. cystoscopes, catheters etc.)?
A. The ‘French gauge’ is used (Fr). This was developed by Charrière in the nineteenth century. The French gauge corresponds to three times the diameter (in mm). For example, a 21 Fr cystoscope sheath has an external diameter of 7 mm.
Q. What are the approximate lengths, diameters and working channel configurations of the major endo-urological instruments? (You should be prepared to draw the internal configurations of endo-urological instruments.)
A. Semi-rigid ureteroscopes vary in size dependent on manufacturer and working channel configuration. It should be remembered that they use fibre-optics for image transmission rather than the rod-lens system of traditional rigid instruments, and hence have a relatively small diameter that usually obviates the need for formal ureteric dilatation. The working element is in the order of 34 cm long, with the tip approximately 7-10 Fr (i.e. about 3 mm diameter). If one working channel is present it is usually about 3.4 Fr; if two are present they are about 2.3 Fr each.
Flexible ureteroscopes (ureterorenoscopes) configurations vary dependent on age and model. The distal end of the instrument is less than 9 Fr, and modern instruments may be even smaller (5.4 Fr, i.e. <2 mm diameter). Lengths vary but are usually around 70-80 cm. Working channels are approximately 3.6 Fr, permitting passage of instruments such as biopsy forceps up to 3 Fr and LASER fibres. The endoscope may be inserted by means of a hydrophilic access sheath, placed over a guidewire. These sheaths are approximately 45 cm and 10-14 Fr. ttey may have dual lumens to permit parallel instrument passage.
Cystoscopes vary in size. Adult cystoscope sheaths are generally between 17 and 25 Fr, and approximately 30 cm long. The components of the cystoscope are the telescope (rod-lens), bridge, obturator and sheath. The telescopes themselves are angled for various procedures, and are generally 0° (for urethrotomy, etc.), 30° and 70° (for cystoscopy). Telescopes are colour coded with bands around the light-lead connector, for example green, red, yellow for 0°, 30° and 70°, respectively.
Resectoscopes again vary in size dependent on manufacturer and configurations. Common external sheath diameters are 26 and 28 Fr.