Self-expanding stents are a remarkable development. They come in a variety of sizes and designs intended for blood vessels, airways, and even intestines. They have just a few downsides. For instance, tissue tends to grow in and over bare metal designs blocking passageways that the stent was intended to help open. They may move out of place or migrate, and occasionally the pulmonary devices fatigue because coughing is so vigorous. Pulmonary stents are not as compliant with the anatomy as they could be, they may cause infections such as halitosis, and can be difficult to deploy and reposition. More than one such stent has required gruesome surgery to remove it.

But the drawbacks are not insurmountable. A few solutions presented by Boston Scientific's Claude Clerc in a recent presentation at the Cleveland Clinic are that most pulmonary stents are fully or partially covered to prevent tissue ingrowth, and several profiles on bare ends tend to prevent migration. Future generations, he adds, may be made of different materials that are more compliant, and drug coatings may solve the infection problems.

After several decades on the market, the shortcomings have been identified and a sufficient number of good ideas are available to get redesigns underway. Even the toughest nut to crack, a feature that allows repositioning a stent or removing it, has been successfully tested in several variations.

Stent design in a nutshell

Good stent design seems the impossible task: it can't be too strong or too weak, and the values for weak and strong differ from person to person. Clerc says the near-ideal stent will be available in 8 to 20-mm diameters and 20 to 80-mm lengths. It will undergo a minimum shortening when deployed, be compliant with the anatomy, and include features that keep it in place. Other useful design features include supporting an appropriate radial force and resisting fatigue. A stent should be biocompatible and removable. Furthermore, a good stent will resist infection and have mucocillary clearance, a characteristic that lets the patient cough up secretions such as mucus. In addition, the stent's delivery device should have a low profile, be flexible, and let surgeons place them with accuracy.

Clerc says stent dimensions and radial forces seem to have little or no influence on migration so some additional features may be necessary. He says it is still in debate as to the exact shape for the end of a fully covered esophageal stent that would best keep it from migrating, but three ideas seem prevalent. A conic end seems fair at preventing migration, a sharp-step end works better at preventing migration, and a progressive step is fair to good at halting movement. The later has the fewest adverse effects. The dimensions for these flares should be 5 to 8 mm larger than stent body.

Scales and spikes on stent ends have also been used to halt migration with some success. Clerc says scales, although difficult to evaluate, seem ineffective in esophageal stents, although research indicates that using more scales on a stent than have been tested may be more effective. Scales are effective in biliary stents. And in pulmonary stents, at least in tests on pigs, five of six smooth stents migrated whereas only one of six spiked stents did so. And while there are more extreme methods for holding stents in place, such as anchoring them with stitching through the neck as one Japanese surgeon has tried, acceptance of the technique may be limited.

Fatigue in stents is another problem. Literature studies show that airways are subject to large deformations during forced inspiration and expiration and coughing and even amplified in certain benign diseases. Studies on airway dynamics during coughing show a 33% maximum-diameter reduction on the trachea and 29% on the main bronchus. Even in healthy people, the cross section of an airway can decrease from 11 to 61% during forced inspiration and expiration, and up to 80% for tracheomalacia patients.

Pulmonary stents must sustain fatigue cycles that differ from other applications. Clerc says stent designers must know more about actual deformation of implanted stents under coughing and forced breathing often associated with malignant and benign diseases. And if that's not enough, they must ensure that test fixtures closely simulate actual conditions in terms of type of deformation, amount of deformation, and number of cycles. Clerc points to finite-element analysis as one analytical tool that can assist.

It's a numerical method that simulates complex loading on stent shapes that considers their maerials, structures, and cross sections. The simulations are CPU intensive because the analyses are nonlinear owing to their large deformations. What's more, the stress analyses and fatigue simulations depend on material models which need developing for new alloys.

Repositioning the stent

Cleveland Clinic surgeon Thomas Galdae says the Dumon stent, an older design made of polymer, has always been somewhat removable. But the procedure needed a rigid bronchoscope and it was not as neat as he would like.

In fact, just placing stents is still tricky because they shorten enough when deployed that they sometimes don't wind up where surgeons would like them. Galdae says that has given rise to the rule of thumb: It's better to be distal than perfect. That is, it is almost better to position a stent too far in so it can be pulled back somewhat as needed.

Stent manufacturers are listening. One recent stent design lets surgeons recapture (pull back into the deployment device) a stent to precisely reposition it if necessary. The Evolution Controlled Release Esophageal Stent, from Cook Medical, Bloomington, Ind., (cookmedical.com) may be the first of its kind, says the company. It adds that placing the stent precisely the first time may reduce the need for repeat procedures. The stent has been granted 510(k) clearance from the FDA and is intended for patients with esophageal cancer.

The company says the Evolution stent's retractable delivery system improves monitoring while placing it with a “point-of-no-return” indicator. Each squeeze of the stent's trigger-based introducer — a complex control handle — deploys or recaptures a proportional length of stent. A direction button lets surgeons switch from deployment to recapture tasks, and the “point-of-no-return” mark tells physicians when recapture is no longer available.

The stent uses dual-flanged ends to secure it and reduce the risk of migration after placement. The company says it is the only esophageal stent with internal and external silicone coatings to resist tumor ingrowth. The coatings also enhance patients' ability to swallow food normally instead of being fed through a tube. Galdae hints at hybrid stent with a similar feature that will soon be available. The market for gastrointestinal stents currently totals $200 million globally, more than 50% of which is in the U.S.

Developments in laser cutting

The wide variety of stent materials and their applications have kept laser manufacturers busy meeting new requirements and improving their cutting lasers and manufacturing equipment. “For example, peripheral stents require different designs and materials, particularly Nitinol with larger wall thickness and increased stent length and complexity,” says Stefan Quandt, Industry Manager at laser equipment manufacturer Rofin Sinar Inc., Plymouth, Mich., (rofin.com). This has lead to new requirements addressed by recent developments in stent laser-cutting equipment, particularly with regard to tooling, laser cutting strategies, and new choices for laser sources to match a stent's particular properties. “One goal of ours is to achieve highest cut precision and quality with limited post processing. Ongoing improvements in stent-cutting equipment have increased productivity as a function of cutting speed and maximizing yield,” says Quandt.

Besides an increased material thickness for specific applications, another quest is to increase the number of structures on any given stent surface. This leads to a requirement for narrower cut width. Recent laser developments, particularly adding the Disk Laser to the Rofin stent cutters, allow for the smallest cut width, about 10 microns (0.0004 in.) in material of 100-micron wall thickness or less.

Different types of lasers and their properties are aimed at cutting metal stents, says Quandt. For instance, lamp-pumped (Nd:YAG) lasers deliver a high peak power, fiber lasers provide a high frequency for fast cutting, and disk lasers produce the narrowest cut width. Disk and fiber lasers in particular provide improved cut-surface quality due to the higher frequency of their pulses.

Laser and equipment developments are under way to further reduce post-processing of laser cut stents, reduce the heat put into the stent structure during laser cutting, and provide laser tools for polymer stent structures.

Traditional 2-axis stent cutting (X and A) provides cuts towards the center of the tube and generates stent struts with a V-shaped profile. “Rofin introduced a patented 4-axis stent cutting system that allows off-center cuts. This feature allows shaping new stent geometries and placing holes with precise diameters into the tubes,” he says.

“Furthermore, sophisticated control functions and other proprietary features allow, for example, modifying the laser power or frequency while cutting with specific pulse shapes. Modifications on stent struts make it possible to include cutting grooves or blind holes that may provide additional surface areas on the stent,” adds Quandt.

He adds that industry requirements for smaller structures, improved cut quality, reduced post-processing, and optimized output all lead to continued developments in stent cutting equipment and laser sources.

Rating the features of a few materials

	Silicone	Polyurethane	ePTFE
Biocompatibility	Good	Good	Good
Resistance to degradation	Good	Good to fair, Depends on formulation	Good
Manufacturability	Fair	Fair	Fair
Radial force	Good	Good	Fair. Does not add radial force

A few ideas for preventing stent migration

Anti-migration feature	Flares	Bare end	Scales and spikes
Illustration
Pros	Atraumatic	Good anchoring Proven technology	Atraumatic
Cons	Lack of information on shape/sizing	Tissue ingrowth Somewhat difficult to remove	Lack of information on shape/sizing
Migration rate, %	17	0 to 5	No data

BSI's Claud Clerc says the amount of flare on a stent is still open to debate. Source: Claude Clerc, Boston Scientific