SAGES Magazine

THE SOUTH AFRICAN GASTROENTEROLOGY REVIEW 2021 | VOLUME 19 | ISSUE 2 | 12 REVIEW became mainstream. However, it remained a consideration only for the minority of patients who were fit enough to have extensive thoraco-abdominal surgery with a reasonable chance of perioperative survival. In this era, the focus therefore remained on improving the use of rigid stents, with the rigid oesophagoscope being the mainstay of luminal inspection for thoracic surgeons who were slow to adopt the flexible fibreoptic instrument. This led to the development of design modifications of stents related to the luminal shape, diameter, material, and the proximal and distal flanges. These tubes were designed to be placed following blind oesophageal dilatation with graduated gum elastic bougies. This would provide a diameter that would allow for delivery of the stent through the stricture via an oesophagoscope-delivered pulsion technique (Figure 2). The Procter-Livingstone tube developed in South Africa was one of the many designs used (Figure 1). 5 The Achilles heel of all these methods, however, was the high perforation rate of up to 20%, a complication that was more often fatal than not. 6 This obstacle was largely overcome with the advent of the flexible fibre-optic endoscope and the addition of fluoroscopy. This allowed inspection of the stricture and the ability to easily direct a flexible soft-tipped guidewire through the stricture while radiographically confirming its correct placement in the stomach. With a guidewire in place, safe gradual dilation was less likely to cause a perforation and led to the development of specific over-the-wire delivery devices 7 . The most recognised of these delivery devices was the Nottingham introducer, which was used in the 80s and early 90s to deliver amodifiedAtkinson tube 8 (Figure 1). This resulted in radiologists and gastroenterologists, often in combination, taking over the custodianship of oesophageal stent delivery from the surgeons. A noteworthy and original contribution was made by Didcott from Rhodesia, who in 1973, reported the development of a tubular dilator (stent), made from high tensile stainless steel, and an accompanying delivery device. 9 He used the stent in a case series of 9 patients with benign strictures who responded to slow continuous dilation. As time progressed and technology evolved, this was applied successfully in conjunction with brachytherapy for carcinoma of the oesophagus. 10 Following Didcotts dilator came SEMS, which were originally designed and prototyped by Swedish inventor Hans Wallstén in 1982. 11 He designed a self-expanding metal meshwork tube which was coined and marketed as the Wallstent. This was used post coronary angioplasty to prevent restenosis. The stent was first deployed in a human coronary artery in Toulouse University Hospital Luasanne in 1986. This design soon became the forerunner of an array of gastrointestinal stents used in the oesophagus, biliary tract and colon. The first documented placement of an oesophageal SEMS was by Lindeberg in 1991. 12,13 Thereafter followed the pivotal randomised controlled trial by Knyrim et al. published in 1993 that provided the evidence that led to SEMS replacing the previously used rigid protheses. This led to a new era in the stent evolution and opened the doorway to a multitude of new stent designs, materials, and uses. 14,15 Stent types Uncovered SEMS SEMS were initially produced from stainless steel but are now currently made from nitinol wire that has greatly improved their utility. Nitinol is a nickel titanium alloy with shape memory and super-elastic characteristics, which enables the development of variable designs, such as knitted or braided stents. Knitted (also called “hooked wire” or “D-type”) have wires which hook around each other, allowing for longitudinal compression. Braided (also known as “crossing wire” or “S-type”) are the traditional woven stent, where wires only cross each other, as they expand their length shortens. Metal stents can be laser- Figure 3. Examples of SEMS commonly used in South Africa cut whereby metal mesh cylinders are perforated to allow for more flexibility. They all exert self-expansive forces until they reach their maximum fixed diameter. Laser cut stents have less shortening than the other designs mentioned. Figure 3 shows typical examples of SEMS used in South Africa. They are all packaged in a compressed, constrained form attached to a delivery device with a central guide wire channel. 16 They come with variable flare or funnel designs at both ends to reduce the risk of migration. Lengths of oesophageal stents vary from 10 to 15cm, fully deployed diameters range from 18 mm to 30mm, and the delivery device diameter varies from 5.3 to 6.2mm (16 to 18 French). This is larger than the largest accessory channels of even large channel endoscopes and so they cannot be delivered through the scope. Some come with proximal or distal deployment mechanisms to aid in accurate placement. Stent characteristics that may guide a decision include the radial force (force with which the stent expands from its compressed state), the axial rigidity (resistance of the stent to flexion), flexibility (bendability), conformability (ability to stay bent without trying to return to a straight configuration), and the degree of stent shortening (amount a stent shortens as it expands to its nominal diameter). Furthermore, stent durability and removability need to be considered based on the pathology that is being managed. The following paragraphs expand on the currently available categories of stents, their design, uses and complications. SEMS can be inserted into a narrower opening than rigid stents, making pre-dilation for the majority of patients unnecessary, thereby reducing the risks of perforation or migration. They can be uncovered, fully covered or partially covered. Covered SEMS The main complication that arises from bare metal or uncovered SEMS is malignant and non-malignant ingrowth leading to a new stricture or stent occlusion in up to 36% of patients. 17 In order to overcome this, stent covers or linings were produced to prevent occlusion from tumour growth. 18 Different stent cover materials have been used including polyurethane (which degrades early in an acidic environment), external silicone membranes (which are relatively stable but result in reduced conformability), and expanded polytetrafluoroethylene (ePTFE) (which is less elastic but provides more resilience against tumour ingrowth). Combinations of silicone and ePTFE are also used to cover different parts of the stent. 16 Further complications include mediastinal pain during the first few days, bleeding, embedment and fistulation into neighbouring structures such as the bronchus or major vasculature, which can be life threatening if they occur. A complication that ismore commonwith the use of fully covered SEMS than with bare stents is migration (36%). This potentially results in secondary interventions for removal, as well as pain