Securing Endografts for Abdominal Aortic Aneurysms with Flexible Shaft Technology

A new endovascular aneurysm repair device features S.S. White expertise

By Steve Grimes, S.S. White Technologies in collaboration with Medtronic

The aorta is the largest artery in the human anatomy and its failure can prove fatal. According to the Centers for Disease Control, aortic aneurysms were the primary cause of death for over 9,800 Americans in 2014 alone.[1] These aneurysms can be pernicious, often growing slowly and undetected.[2] When an aneurysm ruptures, blood leaks out of the aorta and can be life threatening.[3] If an Abdominal Aortic Aneurysm (AAA) or Thoracic Aortic Aneurysm (TAA) is detected, surgical treatment must be swift and precise. So, when an entrepreneurial company in California decided to develop a reliable anchor to secure endografts in Endovascular Aneurysm Repair (EVAR), it consulted with experts in flexible drive shaft technology to build an applicator that precisely and quickly deploys a sturdy anchor. 

Medical Device Entrepreneurs Consult Flexible Shaft Experts

Aptus Endosystems, co-founded by a doctor and a mechanical engineer, identified an issue in AAA treatment. After initially attaching an endograft to a vessel wall, grafts can slip, allowing blood to leak back into the aneurysm. In cases of rupture, this hardware failure renews the risk of a fatal outcome. A new anchor design could save lives and lay the foundation for a successful business.

The eventual design, dubbed the Heli-FX™ EndoAnchor™ System, is “kind of a small staple, like a spiral coil, not like the staple we use to staple papers,” says John Shah, Sales Manager at S.S. White Technologies, Inc. The company, born out of a dentist’s need for a maneuverable drill, is a global leader in flexible shaft engineering. S.S. White’s involvement with Aptus followed naturally from the basic mechanics of the anchor: to install itself and the endograft into the vascular wall, it must be twisted. However, using a solid shaft to provide the needed torque was an impossibility. “They needed a flexible shaft to maneuver the front end and distal end [of the anchor], and they can bend part of the flexible shaft where it is needed,” explains Shah. Much like a corkscrew into a cork, “they install the EndoAnchor^TM implant [the anchor] with an applicator—the flexible shaft drives it.”  

Engineering a Flexible Shaft for a Minimally Invasive Device

Recognizing the critical importance of a flexible application system, the Aptus team sought out S.S. White early in the product development process. “They have industry-leading expertise,” says Mohan Krishnan, PhD Senior Engineering Director, Supply Management at Medtronic Cardiac & Vascular Group, which acquired the technology in 2017. Indeed, Aptus would need such expertise. “Many of these cardiac procedures used to be very invasive—the surgeon would have to crack open the chest in order to implant a medical device,” Krishnan continues, “those days are disappearing and being largely replaced by minimally invasive ways to deliver and implant the medical device.” The trend toward minimally invasive solutions puts pressure on device manufacturers to develop smaller equipment, and offer, as in this case, more precise control. “[Aptus] approached us, and they initially laid out some specifications: this is the size, this is the length, this is how much it should bend,” Shah says. S.S. White creates flexible shafts for a wide variety of applications, from aerospace to automotive, and the company’s established medical business was well prepared to miniaturize its technology for AAA repair.

S.S. White met some of the specifications by adjusting the manufacturing process. For example, the fittings on the ends of the shaft are laser welded into place. However, “there are competing design constraints,” as Krishnan says, and the various specifications from Aptus threatened to counteract each other so completely that a workable design could prove impossible in less-skilled hands. The overall shaft had to fit inside the delivery catheter, so diameter must be limited. This catheter, of course, was designed to be as small as possible for a minimally invasive procedure. At the same time, the shaft had to transfer a specific amount of torque. “To really manage and balance size versus the torque performance characteristics,” Krishnan sums up, comprised the basic design challenge.

Flexible shafts consist of overlapping layers of wires wrapped in opposing directions around a central wire or mandrel. However, the relative simplicity of the technology’s basic mechanics masks the complex interrelations of each component. The size, weight, and material of wires, as well as the number of wires making up each layer, profoundly impacts how each shaft performs.

The first step toward finding an optimal balance for a flexible shaft design used to be simple trial and error. This process would delay not only the development process for a shaft, but could easily drag out an already lengthy development process for any medical device using a flexible shaft component. At the time, Aptus was a fledgling enterprise, and an overly drawn-out development process could have sunk not only the product but the company. Fortunately, S.S. White’s PERFLEXION™ software sped the process by taking all constraints into account and outputting the building blocks of an initial design. “It is like a recipe,” says Shah, “and from that we go and make it.” Of course, in addition to the basic parameters of size and torque, there are a variety of other considerations that go into a final design. Shah gives the example of DTTR, a flexible shaft characteristic representative of how smoothly the shaft rotates. A flexible shaft which fits perfectly and delivers the correct amount of torque is useless if it ‘jumps’ during operation. Additionally, the shaft would have to perform while turning in both the clockwise and counterclockwise directions.

Beyond Product Development: Scaling for Market

Ultimately, the Aptus team appreciated the engineering S.S. White delivered. “When a customer comes to them with a requirement that says, ‘here’s my idea or product and it needs to do X,Y, and Z,’” says Krishnan, “S.S. White is an industry leader in understanding that customer requirement, and then translating that into the actual construction of the shaft.” After completing clinical trials and earning FDA approval and a CE mark, the new technology attracted the eye of Medtronic, which acquired the company and the Heli-FX™ EndoAnchor™ System. However, S.S. White’s value to the project did not end with the conclusion of the product development phase, or with the product’s move to a new corporate home. Medtronic’s inventory of the system ran low immediately after the acquisition, as they were drawing from previous supplies. Unable to meet growing demand, Medtronic turned to S.S. White. “It was a lot of weekend and late-night calls and work that S.S. White had to do to get us more product,” recalls Krishnan.

Flexible Shaft Experts Make Innovative AAA Treatment Possible

For one medical device manufacturer intent on bringing an exciting new solution to market, S.S. White’s unmatched flexible shaft design capabilities played an instrumental role in successful product development. Now with a product line extension designed for applications in endovascular Thoracic Aortic Aneurysm (TAA) treatment.  the Heli-FX™ System is an evolving technology. With its complete product offerings now in full commercialization, S.S. White will continue to supply flexible shafts on a global scale and provide life-saving therapy for AAA and TAA patients.